Mohammed Ibrahim Abdoelgabbar Mohammed Supervisor: Dr ...

Radiological examination frequency and collective effective dose from pediatric computed tomography (CT) in Sudan

BY:

Mohammed Ibrahim Abdoelgabbar Mohammed

B. Sc. (mathematics and Physics) 2006 / University of Khartoum

A thesis Submitted in partial fulfillment of the requirements for the degree of Master of Science in Physics

Supervisor: Dr. Ibrahim Idris Suliman

Department of Physics

Faculty of Science

University of Khartoum

August 2011

Dedication

To

My family: mother, father, brother, and sisters

All my teachers who guided me

My sincere friends

My colleagues

And every one who love me

Acknowledgment

I would like to express my grateful thanks to Dr Ibrahim Idris suliman the head of SSDL –radiation safety institute – Sudan atomic energy commission for his guidance, help, advising, directing, and carefully review of the research.

And I would like to thank Dr.abdoelmonem Adam for his great help, also I would thank the staff of all CT departments that I had visit to do my research survey for their kind cooperation.

It is a great pleasure to acknowledge all those people who had given me tremendous help and support in completing this study

Finally, I am especially grateful to my family for their support though all my life stage

المستخلص

في انتباه خاص من ناحية الوقاية من االشعاعاستخدام االشعة المؤينة يحتاج التصوير الطبي لالطفال ب

استخدام االشعة المقطعية في التصوير الطبي يتسبب في ارتفاع قيمة .ية اصابتهم بالسرطان احتمال

. ومحاولة تقليلها ما امكن ذلك الجرعاتالجرعات االشعاعية للمرضي والجمهور لذلك البد من تقدير تلك

هذه الدراسة الي حساب العدد الكلي لفحوصات االشعة المقطعية لالطفال خالل السنة باالضافة الي تهدف

لحساب الجرعة الجماعية . شعة المقطعية لالطفالحساب الجرعة الجماعية الفعالة الناتجة عن استخدام اال

اء مسح علي عشر مستشفيات لحصر عدد الفحوصات التي تجري خالل والعدد السنوي للفحوصات تم اجر

دراسة سابقة لحساب من الجرعة الفعالة الناتجة عن تلك الفحوصات تم الحصول عليها.اليوم

حساب ومن ثم تم .لفحوصات االشعة المقطعية لالطفال الجرعات الفعالة

الجرعة الجماعية الفعالة الناتجة عن فحوصات لفعالة وعدد الفحوصات السنوي و آانت الجرعة الجماعية ا

حيث آانت البطن تكرارا آان فحص  اعلي الفحوصات )man.Sv196(المقطعية لالطفال هي االشعة

عن فحص البطن الذي آانت نسبته للجرعة الكلية نتجت فعالةاعلي جرعة جماعية ) .32(%نسبته المئوية

)%49(  .

 وهي الجرعة التي حددتها المصادر العلمية man.Sv500عن بكثير تقل man.Sv196هذه الجرعة

Abstract

The pediatric medical imaging using ionizing radiation requires special attention in terms of

radiation protection for risk of cancer. The use of CT in medical imaging cause the high value of

radiation doses to patients and the public for that to be a measure of dose and try to decrease as

much as possible. The study aimed to assess the frequency and collective effective dose for

populations from pediatric CT procedures in Sudan. The annual collective dose form pediatric

CT examination had been calculated by a survey done at 10 hospitals providing data of

examination frequency per month. The annual examination frequency and annual collective

effective dose had been calculated by multiplying the effective dose of each examination by the

frequency. The results were calculated and discussed providing that the annual collective

effective dose from pediatric CT examinations is (169 man.Sv). The highest percentage

examination frequency was for abdomen (32%).The highest percentage contribution to total to

the total dose from pediatric CT examination was for abdomen (49%).The calculated annual

examination frequency and annual collective effective dose had been compared with the results

of literature and other studies to evaluate the estimated values.

This value of dose (196 man.Sv) far less than (500man.Sv) the dose which is set by the scientific

resources.

1 

Contents Chapter One: .................................................................................................................................... 5 

GENERAL INTRODUCTION ........................................................................................................ 5 

1.1 INTRODUCTION ..................................................................................................................... 5 

1.2 BIOLOGICAL EFFECTS OF RADIATIONS .......................................................................... 6 

1.2.1 Stochastic effect: ................................................................................................................ 7 

1.2.2 Characteristics of stochastic effects: ................................................................................... 7 

1.2.3 Deterministic effect: ........................................................................................................... 7 

1.2.4 Characteristics of deterministic effects: ............................................................................. 7 

1.3 PRINCIPLES OF RADIATION PROTECTION: ..................................................................... 8 

1.3.1 Justification: ....................................................................................................................... 9 

1.3.2 Limit dose: .......................................................................................................................... 9 

1.3.3 Optimization: ...................................................................................................................... 9 

1.4 OBJECTIVES .......................................................................................................................... 10 

1.4.1 General objective: ............................................................................................................. 10 

1.4.2 Specific objectives: ........................................................................................................... 10 

1.5 LITERATURE REVIEW ........................................................................................................ 10 

1.6 THESIS OUTLINES ............................................................................................................... 12 

Chapter Two: ................................................................................................................................. 15 

Physics of Computed Tomography ................................................................................................ 15 

2.1 CT PRINCIPLES ..................................................................................................................... 15 

2.2 CT GENERATIONS ............................................................................................................... 16 

2.2.1 First CT Generation Data Collection: ............................................................................... 16 

2.2.2 Second-Generation CT Scanners: ..................................................................................... 17 

2.2.3 Third-Generation CT Scanners: ........................................................................................ 18 

2.2.4 Fourth-Generation CT Scanners ....................................................................................... 19 

2.3 RADIATION FIELD QUANTITIES (RADIOMETRIC) ....................................................... 21 

2.3.1 Fluence ............................................................................................................................. 21 

2.3.2 Energy Fluence: ................................................................................................................ 21 

2 

2.4 PHYSICAL QUANTITIES ..................................................................................................... 21 

2.4.1 Kerma and kerma rate: ..................................................................................................... 21 

2.4.2 Absorbed dose: ................................................................................................................. 22 

2.4.3 Exposure ........................................................................................................................... 22 

2.5 RADIATION PROTECTION QUANTITIES: ........................................................................ 22 

2.5.1 Organ and tissue dose: ...................................................................................................... 22 

2.5.2 Equivalent dose: ............................................................................................................... 23 

2.5.3 Effective dose: .................................................................................................................. 23 

2.5.4 Collective Effective dose: ................................................................................................. 23 

2.6 CT DOSIMETRY .................................................................................................................... 23 

2.6.1 CTDI: ................................................................................................................................ 23 

2.6.2 CTDI100 ............................................................................................................................. 24 

2.6.3 Weighted CTDIw .............................................................................................................. 25 

2.6.4 Normalized weighted CTDI ............................................................................................. 25 

2.6.5 Volume CTDIvol ................................................................................................................ 25 

2.6.6 Dose- length product (DLP): ............................................................................................ 26 

2.7 OTHER MEDICAL IMAGING MODALITIES: .................................................................... 27 

2.7.1 Mammography: ................................................................................................................ 27 

2.7.2 Fluoroscopy: ..................................................................................................................... 27 

2.7.3 Magnetic resonance image (MRI): ................................................................................... 28 

2.7.4 Ultrasound: ....................................................................................................................... 28 

2.7.5 X-ray (Radiography): ....................................................................................................... 28 

2.7.6 Dental radiography: .......................................................................................................... 29 

Chapter Three: ............................................................................................................................... 31 

Material and method ...................................................................................................................... 31 

3.1 ESTIMATION OF CT RADIOGRAPHY EXAMINATION FREQUENCIES: .................... 31 

3.2 COLLECTIVE DOSE PER EXAMINATION: ....................................................................... 33 

Chapter Four: ................................................................................................................................. 36 

Result and discussion ..................................................................................................................... 36 

4.1 RESULTS ................................................................................................................................ 36 

3 

4.2 DISCUSSION: ......................................................................................................................... 42 

Chapter Five: .................................................................................................................................. 49 

5.1 CONCLUSION ........................................................................................................................ 49 

5.2 REFERENCE ........................................................................................................................... 50 

4 

Chapter one

5 

Chapter One:

General Introduction

1.1 Introduction

Computed tomography (CT) is a medical imaging modality employing tomography used for

diagnostic and treatment procedures. Digital geometry processing is used to generate a three-

dimensional image of the inside of an object from a large series of two-dimensional X-ray

images taken around a single axis of rotation. Computed tomography was originally known

as the "EMI scan" as it was developed at a research branch of EMI, a company best known

today for its music and recording business. It was later known as computed axial tomography

(CT scan) and body section CT produces a volume of data which can be manipulated,

through a process known as "windowing", in order to demonstrate various bodily structures

based on their ability to block the X-ray beam. Although historically the images generated

were in the axial or transverse plane, orthogonal to the long axis of the body, modern

scanners allow this volume of data to be reformatted in various planes or even as volumetric

(3D) representations of structures. Although most common in medicine, CT is also used in

other fields such as nondestructive materials testing. The use of CT with children has grown

substantially in the past few years. There is also an increased interest in keeping the radiation

dose to children from CT as low as is clinically practical. This article reviews the physical

aspects of CT . Separately and how CT is used in the context of CT to provide the practical

insight necessary to approach this issue. Understanding radiation dosimetry and its potential

for deleterious health effects, having knowledge of the magnitude of the effective dose and

the dose to specific organs from CT, and considering the role of CT in the context of CT will

allow the reader to reduce the radiation dose to the patient without compromising the quality

of the patient’s care. A CT or CAT scan is a diagnostic imaging procedure that uses a

combination of x-rays and computer technology to produce cross-sectional images (often

called "slices"), both horizontally and vertically, of the body [1]. A CT scan shows detailed

images of any part of the body, including the bones, muscles, fat, and organs. CT scans are

more detailed than general x-rays. CT scans also minimize exposure to radiation. In

conventional x-rays, a beam of energy is aimed at the body part being studied. A plate behind

the body part captures the variations of the energy beam after it passes through skin, bone,

muscle, and other tissue. While much information can be obtained from a regular x-ray, a lot

of detail about internal organs and other structures is not available. In computed tomography

CT, the x-ray beam moves in a circle around the body. This allows many different views of

6 

the same organ or structure, and provides much greater detail. The x-ray information is sent

to a computer that interprets the x-ray data and displays it in 2-dimensional form on a

monitor. While many images are taken during a CT scan, in many cases, the patient receives

less radiation exposure than with a single standard x-ray. CT scans may be done with or

without contrast. "Contrast" refers to a substance taken by mouth or injected into an

intravenous (IV) line that causes the particular organ or tissue under study to be seen more

clearly [1]. Contrast examinations may require you to fast for a certain period of time before

the procedure. Your physician will notify you of this prior to the procedure. CT scans may be

performed to help diagnose tumors, investigate internal bleeding, or check for other internal

injuries or damage. Computed tomography has a wide range of uses in medicine. It can show

cancers in different parts of the body, helping doctors’ measure its spread and apply targeted

treatments. Any abnormal growth or structures such as cysts, tumors, abscesses, kidney or

bladder stones, can be detected as well. CT scans can also be used to help assess areas

of trauma and identify any structural damage. In all cases, computed tomography can provide

for a detailed examination of the body to develop the best and most accurate treatment for the

patient. Computed tomography requires more radiation than traditional x-rays, and the more

detailed and complex the CT scan is, the more radiation exposure the patient receives.

However, for most patients, the risk of allowing a problem like cancer to continue unchecked

is worse than the risks from radiation exposure. Also, the quality of computed tomography is

much higher than a traditional x-ray. It allows for fast identification of things like internal

bleeding, which a traditional x-ray would not be able to detect. Some other problems with

computed tomography are the associated costs, but as technology advance the cost of these

procedures decreases [1].

While computed tomography can provide a lot of important information to a medical team,

CT scanning should be justified. The radiation it involves precludes pregnant women and the

dye that is injected requires nursing mothers to take precautions. Children should not get

them unless it is medically necessary, and even then, repeated exposure should be avoided as

much as possible. Some people cannot physically fit in the machines either, so

accommodations have to be made to aid these people. Also, some areas being scanned could

also be examined on an MRI, or magnetic resonance imaging, and thus are not worth the risk

of computed tomography scan. [1].

1.2 Biological effects of radiations The human body is made up of many organs, and each organ of the body is made up of

specialized cells. Ionizing radiation can potentially affect the normal operation of these cells.

7 

In this point, we will discuss the potential for biological effects and risks due to ionizing

radiation and put these potential risks into perspective when compared to other occupations

and daily activities. From the biological effects of radiation on human body, radiation effects

are generally divided into two categories [2]:

1.2.1 Stochastic effect:

Stochastic effects are those that occur by chance and consist primarily of cancer and genetic

effects. Stochastic effects often show up years after exposure. As the dose to an individual

increases, the probability that cancer or a genetic effect will also increases. However, at no

time, even for high doses, is it certain that cancer or genetic damage will result. Similarly, for

stochastic effects, there is no threshold dose below which it is relatively certain that an

adverse effect cannot occur. In addition, because stochastic effects can occur in individuals

that have not been exposed to radiation above background levels, it can never be determined

for certain that an occurrence of cancer or genetic damage was due to a specific exposure.

The severity of stochastic effects is independent of the absorbed dose. Under certain exposure

conditions, the effects may or may not occur. There is no threshold and the probability of

having the effects is proportional to the dose absorbed [2]

1.2.2 Characteristics of stochastic effects:

• Severity is independent of absorbed dose

• Threshold does not exist

• Probability of occurrence depends on absorbed dose

1.2.3 Deterministic effect:

Based on a large number of experiments involving animals and other researches, further

supplemented by theoretical studies, it was discovered that severity of certain effects on

human beings will increase with increasing doses. There exists a certain level, the

"threshold", below which the effect will be absent. This kind of effects is called

"deterministic effects".

1.2.4 Characteristics of deterministic effects:

• Damage depends on absorbed dose

• Threshold exists

The following are possible effects of radiation on cells:

8 

Cells are undamaged by the dose

Ionization may form chemically active substances which in some cases alter the structure of

the cells. These alterations may be the same as those changes that occur naturally in the cell

and may have no negative effect [3].

Cells are damaged, repair the damage and operate normally

Some ionizing events produce substances not normally found in the cell. These can lead to a

breakdown of the cell structure and its components. Cells can repair the damage if it is

limited. Even damage to the chromosomes is usually repaired. Many thousands of

chromosome aberrations (changes) occur constantly in our bodies. We have effective

mechanisms to repair these changes.

Cells are damaged, repair the damage and operate abnormally

If a damaged cell needs to perform a function before it has had time to repair itself, it will

either be unable to perform the repair function or perform the function incorrectly or

incompletely. The result may be cells that cannot perform their normal functions or that now

are damaging to other cells. These altered cells may be unable to reproduce themselves or

may reproduce at an uncontrolled rate. Such cells can be the underlying causes of cancers [3].

Cells die as a result of the damage

If a cell is extensively damaged by radiation, or damaged in such a way that reproduction is

affected, the cell may die. Radiation damage to cells may depend on how sensitive the cells

are to radiation. All cells are not equally sensitive to radiation damage. In general, cells

which divide rapidly and/or are relatively non-specialized tend to show effects at lower doses

of radiation then those which are less rapidly dividing and more specialized. Examples of the

more sensitive cells are those which produce blood. This system (called the hemopoietic

system) is the most sensitive biological indicator of radiation exposure. The relative

sensitivity of different human tissues to radiation can be seen by examining the progression

of the Acute Radiation Syndrome on the following pages [3]

1.3 Principles of radiation protection:

The main objective of radiation protection (RP) is to avoid the deterministic effects by

keeping doses below the relevant threshold and to reduce the probability of stochastic effect

as much as is reasonably achievable.

9 

In most countries a national regulatory authority works towards ensuring a secure radiation

environment in society by setting requirements that are also based on the international

recommendations for ionizing radiation (ICRP - International Commission on Radiological

Protection) .This shall be achieved by the following requirements [4].

1.3.1 Justification:

No unnecessary use of radiation is permitted, which means that the advantages must

outweigh the disadvantages.

And also the justification is defined as: No practice or source within a practice should be

authorized unless the practice produces sufficient benefit to the exposed individuals or to

society to offset the radiation harm that it might cause; that is: unless the practice is justified,

taking into account social, economic and other relevant factors.“ In proposed and continuing

practices, the justification of practice must be such that the work uses radiation because it

gives benefit (or gain) to the exposed individuals or to society that exceeds radiological risk.

Justification in intervention provides more benefit in comparison to if there were no

intervention.[4]

1.3.2 Limit dose:

Is the normal exposure of individuals shall be restricted so that neither the total effective dose

nor the total equivalent dose to relevant organs or tissues, caused by the possible combination

of exposures from authorized practices, exceeds any relevant dose limit specified, except in

special circumstances provided for in the Standards. Dose limits shall not apply to medical

exposures from authorized practices. Each individual must be protected against risks that are

far too large through individual radiation dose limits [4].

There are different categories of dose limits for:

• Radiation workers

• Members of the public

• Trainees of radiation

• Planned special exposures; and female pregnant workers

1.3.3 Optimization:

In relation to exposures from any particular source within a practice, except for therapeutic

medical exposures, protection and safety shall be optimized in order that the magnitude of

individual doses, the number of people exposed and the likelihood of incurring exposures all

be kept as low as reasonably achievable, economic and social factors being taken into

10 

account, within the restriction that the doses to individuals delivered by the source be subject

to dose constraints. Radiation doses should all be kept as low as reasonably achievable. [4]. 1.4 Objectives

1.4.1 General objective:

1. To assess public exposure to ionizing radiation from medical use, specifically from

CT procedures.

1.4.2 Specific objectives:

1. To assess the examination frequency of paediatric CT procedures.

2. To estimate the collective effective dose from pediatric CT examinations.

3. To estimate the collective effective dose from each examination and specify the

contribution to total collective effective dose from pediatric CT procedures.

1.5 Literature review

A study for assessing the frequency of examinations and contribution of each medical

imaging procedure for pediatric had been done in Germany to estimate the percentage change

in collective effective dose for pediatric in Germany [5]. A nation-wide survey of exposure

practice in pediatric CT was conducted in Germany during the period from September 2005

until May 2006 on behalf of the ministry for environmental protection, conservation and

nuclear safety. The survey was based on questionnaires that were first sent to 1640 users of

CT scanners installed in hospitals and private practices, asking for the frequencies of five

types of examinations, subdivided into five age groups. In a subsequent second survey, a

selected number of 72 users, responsible for about two thirds of annual pediatric CT

examinations reported in phase 1, were asked for detailed dose-relevant data (scanner data,

scan protocols, examination-related data and examination frequencies)[5]. These data were

used for individualized dose assessment, depending on the type of scanner and age of the

patient. With return rates of 40 % in the first part and 58% in the second part of the survey,

representative results could be obtained for the five most frequent types of pediatric CT

examination. The most essential findings were: the percentage of pediatric CT examinations

was in the order of only 1% of all CT and thus much smaller than elsewhere (e.g. 6.5% for

USA). The most frequent type of examination was brain (52%), followed by chest (17%) and

entire abdomen (7%) other types of examination were quite rare (less than 5%). The age

distribution for pediatric CT examinations was almost uniform: (0 to 5 years: 40%, 6 to 10

11 

years: 28%, 11 to 15 years: 32%). Based on the results of this survey, proposals have been

made for diagnostic reference levels that refer to the third quartiles of the observed dose

distributions. In addition, such a way that the doses resulting from their scan protocol settings

could be benchmarked against the proposed reference dose values [5].

Brenner and Hall published an article in 2007 by which stated that up to 2% of all carcinomas

in the USA could be secondary to CT radiations [7]. Advances in science and technology

showed that the use of CT has its own indications [7]. The same protocol should be followed

in different hospitals as well. Basic principles of radiation protection should be monitored. As

much as possible, both technician and radiologist must be present during computerized

tomography for children, and MRI and ultrasound should be replaced if possible [7].

The other study had been done by Institute of Public Health in Romania for survey of

diagnostic pediatric radiology and the resulted effective dose. The purpose of their study was

to update the annual frequency of X-ray examinations and the pattern of pediatric radiology

in 2000 year in Romania [8]. Also, to assess in terms of effective dose the magnitude of

pediatric patient exposure during conventional X-ray examinations, selected by their high

frequencies or their relatively high doses delivered to patient. The annual effective doses

from all medical examinations for the average pediatric patients were: 0.85 mSv for 0 years

old , 0.53 mSv for 1 year old, 0.56 mSv for 5 years old , 0.72 mSv for 10 years old and 0.74

mSv for 15 year old. The resulting annual collective effective dose was evaluated at 872

man.Sv, with the largest contribution of pelvis and hip examinations. However, this value

could be much larger because the CT annual use increased in 2000 y up to 3.1% of total

examinations from a negligible one of 0.1% in previous survey. According to their study, the

pediatric X-ray examinations represented 12% of all medical procedures performed in 2000 y

in Romania. Current pattern of pediatric radiology shows a mean increase of CT

frequency of 3.1% of total annual X-ray procedures. The annual effective doses from

conventional X-ray procedures ranged from 0.53 mSv (age subgroup 5 y) to 0.85 mSv (new

born babies). The annual collective dose of paediatric patients from conventional X-ray

procedures was 872 manSv with the highest contribution of pelvis, hip and spine

examinations [8].

In Belgium, a study was performed to evaluate the examination protocols used for common

CT procedures of pediatric patients at different hospitals in order to determine whether

adjustments related to patient size are made in scanning parameters, and to compare patient

doses with proposed reference levels. Three pediatric hospitals and one non-pediatric hospital

participated in the study [9]. Weighted CT dose-index (CTDIw), dose–length product (DLP)

12 

and effective dose (E) were evaluated for three patient ages (1 year, 5 years and 10 years) and

three common procedures (brain, thorax and abdomen). CTDIw and DLP values higher than

the reference levels were found for all types of evaluated examination. E ranged from 0.4

mSv to 2.3 mSv, 1.1 mSv to 6.6 mSv, and 2.3 mSv to 19.9 mSv for brain, thorax and

abdomen examinations, respectively. All centers but one adapted their protocols as a function

of patient size. However, no common trend in the selection of protocol was observed. Some

centers divided the whole range of patient size into only two/three groups by age, while

others classified, the patients into six groups by weight. It was also observed that some

centers used the same mAs for the total range of patient sizes and decreased the pitch factor

for small children, which resulted in higher doses. They envisage the importance of careful

selection of technical scan parameters. They also conclude that, If CT parameters used for

pediatric patients are not adjusted on the basis of examination type, age and/or size of the

child, then some patients will be exposed to an unnecessarily high radiation dose during CT

examinations [9].

1.6 Thesis outlines

Chapter one:

This chapter includes the object and general introduction and the benefit of this study, and also literature review and previous study had been done on the field of the study to get more information about basic concepts and methods of assessing the public exposure , and biological effects of radiation and the radiation protection principle are mentioned in brief notes.

Chapter two:

This chapter describes the physics of computed tomography (CT) considering CT dosimetry and the units, and also the CT principles and generations and properties of each generation, and radiation physics and radiation quantities and units were reviewed in this chapter.

Chapter three:

This chapter explores the materials and methods used in this study and the survey for collecting data information about examination frequency and estimate the collective effective dose from pediatric CT.

Chapter four:

Chapter four based on the presentation of obtained results in tables and figures, and includes the discussion of the results compared with the literature and international studies.

Chapter five:

This chapter introduces the conclusion which had been figured out from this study.

13 

14 

Chapter Two

15 

Chapter Two:

Physics of Computed Tomography

2.1 CT principles

The use of CT in children has grown substantially in the past few years. There is also an

increased interest in keeping the radiation dose to children from CT as low as is clinically

practical. In previous years there were huge number of researches and studies were done at

many countries in the world to identify the operation of each radio diagnostic modalities in

collective effective dose.

The principles of CT were first developed by Radon in 1917. Radon's treatise proved that an

image of an unknown object could be produced if one had an infinite number of projections

through the object. Although the mathematical details are beyond the scope of this text, we

can understand the basic idea behind tomographic imaging with an example taken from

radiography. With plain film imaging, the three-dimensional (3D) anatomy of the patient is

reduced to a two-dimensional (2D) projection image. The density at a given point on an

image represents the x-ray attenuation properties within the patient along a line between the

x-ray focal spot and the point on the detector corresponding to the point on the image.

Consequently, with a conventional radiograph of the patient's anatomy, information with

respect to the dimension parallel to the x-ray beam is lost [10].

A single transmission measurement through the patient made by a single detector at a given

moment in time is called a ray. A series of rays that pass through the patient at the same

orientation is called a projection or view. There are two projection geometries that have been

used in CT imaging. The more basic type is parallel beam geometry, in which all of the rays

in a projection are parallel to each other. In fan beam geometry, the rays at a given projection

angle diverge and have the appearance of a fan. All modern CT scanners incorporate fan

beam geometry in the acquisition and reconstruction process. The purpose of the CT scanner

hardware is to acquire a large number of transmission measurements through the patient at

different positions. The acquisition of a single axial CT image may involve approximately

800 rays taken at 1,000 different projection angles, for a total of approximately 800,000

transmission measurements. Before the axial acquisition of the next slice, the table that the

patient is lying on is moved slightly in the cranial- caudal direction (the "z-axis" of the

scanner), which positions a different slice of tissue in the path of the X-ray beam for the

acquisition of the next image [23].

16 

In many ways CT scanning works very much like other X-ray examinations. X-rays are a

form of radiation—like light or radio waves—that can be directed at the body. Different body

parts absorb the X-rays in varying degrees. In a conventional x-ray exam, a small burst of

radiation passes through the body, recording an image on photographic film or a special

image recording plate. Bones appear white on the x-ray; soft tissue shows up in shades of

gray and air appears black. With CT scanning, numerous x-ray beams and a set of electronic

x-ray detectors rotate around the patient. At the same time, the examination table is moving

through the scanner, so that the x-ray beam follows a spiral path. A special computer program

processes this series of pictures, or slices of the body, to create two-dimensional cross-

sectional images, which are then displayed on a monitor.[10].

CT imaging is sometimes compared to looking into a loaf of bread by cutting the loaf into

thin slices. When the image slices are reassembled by computer software, the result is a very

detailed multidimensional view of the body's interior. Refinements in detector technology

allow new CT scanners to obtain multiple slices in a single rotation. These scanners, called

"multislice CT" or "multidetector CT," allow thinner slices to be obtained, resulting in more

detail of the body, in a shorter period of time.

Modern CT scanners are so fast that they can scan through large sections of the body in just a

few seconds. Such speed is beneficial for all patients especially children, the elderly and

critically ill. For some CT exams, a contrast material is used to enhance visibility in the area

of the body being studied [10].

2.2 CT generations

2.2.1 First CT Generation Data Collection:

The first commercial scanner invented by Hounsfield, was introduced in 1973. This scanner

acquired data with an x-ray beam collimated to a narrow “pencil” beam directed to a single

detector on the other side of the patient; the detector and the beam were aligned in a scanning

frame. A single projection was acquired by moving the tube and detector in a straight-line

motion (translation) on opposite sides of the patient Fig (2.1).

17 

Fig (2.1)

Fig (2.1) shows the first-generation CT scanner, which used a parallel x-ray beam with

translate-rotate motion to acquire data.[10]

To acquire the next projection, the frame rotated 1°, and then translated in the other direction.

This process of translation and rotation was repeated until 180 projections were obtained. The

earliest versions required about 4.5 minutes for a single scan and thus were restricted to

regions where patient motion could be controlled (the head). Since procedures consisted of a

series of scans, procedure time was reduced somewhat by using two detectors so that two

parallel sections were acquired in one scan. Although the contrast resolution of internal

structures was unprecedented, images had poor spatial resolution and very poor z-axis

resolution ( 13-mm section thickness) Fig (2.2).

Fig (2.2)

Fig (2) shows CT image of the head obtained with an early CT scanner. The scan plane

resolution is on the order of 3 mm for a field of view of 25 cm with an 80 × 80 matrix and a

z-axis resolution of approximately 13 mm. [10].

2.2.2 Second-Generation CT Scanners:

The main impetus for improvement was in reducing scan time ultimately to the point that

regions in the trunk could be imaged. By adding detectors angularly displaced, several

18 

projections could be obtained in a single translation. For example, one early design used three

detectors each displaced by 1°. Since each detector viewed the x-ray tube at a different angle,

a single translation produced three projections. Hence, the system could rotate 3° to the next

projection rather than 1° and had to make only 60 translations instead of 180 to acquire a

complete section Fig (2.3). Scan times were reduced by a factor of three. Designs of this type

had up to 53 detectors, were ultimately fast enough (tens of seconds) to permit acquisition

during a single breath hold, and thus were the first designs to permit scans of the trunk of the

body. Because rotating anode tubes could not withstand the wear and tear of rotate-translate

motion, this early design required a relatively low output stationary anode X-ray tube. The

power limits of stationary anodes for efficient heat dissipation were improved somewhat with

the use of asymmetrical focal spots (smaller in the scan plane than in the z-axis direction), but

this resulted in higher radiation doses due to poor beam restriction to the scan plane.

Nevertheless, these scanners required slower scan speeds to obtain adequate x-ray flux at the

detectors when scanning thicker patients or body parts [10].

Fig (2.3)

Fig (2.3) shows the second-generation CT scanner, which used translate-rotate motion to

acquire data. [10].

2.2.3 Third-Generation CT Scanners:

Designers realized that if a pure rotational scanning motion could be used, then it would be

possible to use higher-power, rotating anode x-ray tubes and thus improves scan speeds in

thicker body parts. One of the first designs to do so was the so-called third generation or

rotate-rotate geometry. In these scanners, the x-ray tube is collimated to a wide, fan-shaped x-

ray beam and directed toward an arc-shaped row of detectors. During scanning, the tube and

19 

detector array rotate around the patient fig (4), and different projections are obtained during

rotation by pulsing the x-ray source or by sampling the detectors at a very high rate. The

number of detectors varied from 300 in early versions to over 700 in modern scanners. Since

the slam-bang translational motion was replaced with smooth rotational motion, higher-

output rotating anode x-ray tubes could be used, greatly reducing scan times. One aspect of

this geometry is that rays in a single projection are divergent rather than parallel to each

other, as in earlier designs. Beam divergence required some modification of reconstruction

algorithms, and sampling considerations required scanning an additional arc of one fan angle

beyond 180°, although most scanners rotate 360° for each scan. Nearly all current helical

scanners are based on modifications of rotate-rotate designs. Typical scan times are on the

order of a few seconds or less, and recent versions are capable of sub second scan times.

Fig (2.4)

Fig (2.4) shows the third-generation CT scanner, which acquires data by rotating both the x-

ray source with wide fan beam geometry and the detectors around the patient. Hence, the

geometry is called rotate-rotate motion. [10]

2.2.4 Fourth-Generation CT Scanners

This design evolved nearly simultaneously with third-generation scanners and also eliminated

translate-rotate motion. In this case, only the source rotates within a stationary ring of

detectors Fig (2.5).

20 

Fig (2.5)

Fig (2.5) shows the fourth-generation CT scanner, which uses a stationary ring of detectors

positioned around the patient. Only the x-ray source rotates with wide fan beam geometry,

while the detectors are stationary. Hence, the geometry is called rotate-stationary motion. The

x-ray tube is positioned to rotate about the patient within the space between the patient and

the detector ring. One clever version, which is no longer produced, moved the x-ray tube out

of the detector ring and tilted the ring out of the x-ray beam in a wobbling motion as the tube

rotated. This design permitted a smaller detector ring with fewer detectors for a similar level

of performance. Early fourth-generation scanners had some 600 detectors and later versions

had up to 4,800. Within the same period, scan times of fourth-generation designs were

comparable with those of third-generation scanners. One limitation of fourth-generation

designs is less efficient use of detectors, since less than one-fourth are used at any point

during scanning. These scanners are also more susceptible to scatter artifacts than third-

generation types, since they cannot use anti scatter collimators. CT scanners of this design are

no longer commercially available except for special-purpose applications.

Until around 1990, CT technology had evolved to deliver scan plane resolutions of 1–2

lp/mm, but z-axis resolution remained poor and inter scan delay was problematic due to the

stop-start action necessary for table translation and for cable unwinding, which resulted in

longer examination times. The z-axis resolution was limited by the choice of section

thickness, which ranged from 1 to 10 mm. For thicker sections, the partial volume averaging

between different tissues led to partial volume artifacts. These artifacts were reduced to some

extent by scanning thinner sections. In addition, even though it was possible to obtain 3D

images by stacking thin sections, inaccuracy dominated due to involuntary motion from scan

to scan. The step like contours could be minimized by overlapping of CT sections at the

expense of a significant increase in radiation to the patient. Also, the conventional method of

section-by-section acquisition produced misregistration of lesions between sections due to

21 

involuntary motion of anatomy in subsequent breath holds between scans. It was soon

realized that if multiple sections could be acquired in a single breath hold, a considerable

improvement in the ability to image structures in regions susceptible to physiologic motion

could result. However, this required some technological advances, which led to the

development of helical CT scanners. [10].

2.3 Radiation field quantities (radiometric)

2.3.1 Fluence

Fluence is the flux (either particle or radiative flux) integrated over time. For particles, it is defined as the total number of particles that intersect a unit area in a specific time interval of interest, and has units of m–2 (number of particles per meter squared). Fluence can also be used to describe the energy delivered per unit area [11]. Suppose dN particles pass through an infinitesimal area da. Then, the particle fluence is:

Ф = ………….. [2.1]

2.3.2 Energy Fluence:

The energy Fluence (ψ) is the quotient dR by da where dR is the radiant energy incident on a sphere of cross-sectional area da. Thus:

ψ = ……………………………. [2.2]

The unit is J/m2 [11].

2.4 Physical quantities

2.4.1 Kerma and kerma rate:

The kerma K is the quotient dEtr by dm, where dEtr is the sum of the initial kinetic energies of all the charged particles liberated by uncharged particles in a mass dm of material, thus:

K= dEtr / dm …………………………… [2.3]

The SI unit of Kerma is J/kg or Gy Gy can be used for any type of radiation. Kerma rate K: is quotient dK by dt, where dk is the increment of kerma in the time interval dt thus:

K=dK / dt …………………………… [2.4]

The unit is Jkg-1 s-1 .If the special name gray is used, the unit of kerma is (Gy/s) [11].

22 

2.4.2 Absorbed dose:

Absorbed dose is a physical quantity to measure the radiation energy absorbed by unit mass of substances. Under normal circumstances, the larger the absorbed dose, the larger will be the hazard,

The absorbed dose D is the quotient dE by dm, where dE is the mean energy imparted to matter of mass dm, thus:

D = ……………………… [2.5]

The Units: RAD (Radiation Absorbed Dose) 1 rad = 100 ergs/g’ Gray (Gy) SI unit used to measure absorbed dose is the gray (Gy).1 Gy = 100 rad

2.4.3 Exposure Exposure is the quantity most commonly used to express the amount of radiation delivered to a point. The conventional unit for exposure is the roentgen (R), and the SI unit is the coulomb per kilogram of air (C/kg):

X = ……………………….. [2.6]

1 R = 2.58 x 10-4 C/kg , 1 C/kg = 3876 R

The reason exposure is such a widely used radiation quantity is that it can be readily measured. All forms of radiation measurement are based on an effect produced when the radiation interacts with a material. The specific effect used to measure exposure is the ionization in air produced by the radiation.[11]. 2.5 Radiation protection quantities:

2.5.1 Organ and tissue dose:

The mean absorbed dose in a specific tissue or organ is given by the symbol DT .It is equal to the ratio of the energy imparted, ET, of the tissue or organ thus:

DT =ET/mT ……………………. [2.7]

The mean absorbed dose in a specific tissue or organ is sometimes simply referred to as the organ dose [20]

23 

2.5.2 Equivalent dose:

The equivalent dose is the absorbed dose averaged over a tissue multiplied by the appropriate radiation-weighting factor.

HT=WR*DT ……………………. [2.8]

The unit: J/kg. The special name for the unit of equivalent dose is sievert (Sv). The radiation-weighting factor WR allows for difference in the relative biological effectiveness of the incident radiation in producing stochastic effects at low doses in tissue or organ T .For x-ray energies used in diagnostic radiology, WR is taken to be unity. [11]

2.5.3 Effective dose:

The effective dose E is the sum of all equivalent doses of external and internal irradiation, absorbed by all tissues and organs, regarding weighting factors of the tissues and organs, describing whole body exposure.

E = wT1 × H1 + wT2 × H2 + ................... [2.9]

E = ∑TWT*HT ………………….. [2.10]

wT = tissue or organ T weighting factor describing particular tissue or organ T sensibility for radiation. The unit is J/kg the special name is sievert (Sv). 2.5.4 Collective Effective dose:

It Describes radiation exposure of populations to different sources of radiation, If a measure of the radiation exposure in a population is desired, the collective effective dose can be calculated:

S = ∑ i Ei Ni ………………… [2.11]

where Ei is the mean effective dose to population subgroup i

Ni is the number of individuals in population subgroup. The Units: is J/kg the special name is (man Sv) [11].

2.6 CT dosimetry

2.6.1 CTDI:

The main dosimetric quantity used in CT is the computed tomography dose index (CTDI),

which is defined as the integral of the dose profile along a line parallel to the axis of rotation

for a single scan, divided by the nominal slice thickness [12].

24 

CTDI = ∞∞ dz …………………. [2.12]

where: D (z) = the radiation dose profile along the z-axis

N = the number of tomographic sections imaged in a single axial scan. This is equal to the

number of the data channels used in particular scan. The value of N may be less than or equal

to the maximum number of data channels available on the system, and T = the width of the

tomographic section along the z-axis imaged by one data channel .In multiple – detector –row

(multi -slice) CT scanner, several detector elements may be grouped together to from one

data channel [12]. In single –detector –row (single -slice) CT, the z-axis collimation (T) is the

nominal scan width. The CTDI may be assessed free in air or in phantoms, and the

measurement may be done with ionization chambers.

In X-ray diagnostics, only parts of the human body are exposed during an examination, and

due to the relatively low energy of photons, the energy deposition is very inhomogeneous.

Furthermore, different kinds of tissues and organs have different sensitivities for radiation. In

order to take these circumstances into account, The CTDI is the primary dose measurement

concept in CT. [12].

2.6.2 CTDI100

Represents the accumulated multiple scan dose at the center of a 100 mm scan and

underestimates the accumulated dose for longer scan length.

CTDI100 = dz …………………. [2.13]

In the case of CTDI100, the integration limits are 50 mm, which corresponds to the 100

mm length of the commercially available pencil ionization number. [12].

The pencil chamber of active length ℓ is not really measuring exposure (X), or air kerma, but

rather the integral of the single rotation dose profile D (z). Although the exposure (or air

kerma) Meter may convert the charge collected into an apparent exposure reading in

roentgens (R) (or air kerma reading in milligray [mGy]), the measured value, called the

“meter reading,” actually Represents the average exposure (or air kerma) over the chamber

length ℓ, That is:

25 

Meter reading = ℓ ℓ/

ℓ/ dz = .ℓ ℓ/

ℓ/ dz …………….. [2.14]

where f is the f-factor (exposure-to-dose conversion factor, D = f · X).

The dose distribution within the body cross section imparted by a CT scan is much more

homogeneous than that imparted by radiography, but is still somewhat larger near the skin

than in the body center.[12].

2.6.3 Weighted CTDIw

The CTDI varies across the field of view (FOV). For example, for body CT imaging, the

CTDI Is typically a factor or two higher at the surface than at the center of the FOV. The

average CTDI across the FOV is estimated by the Weighted CTDI (CTDIw) where:

CTDIw = CTDI100, center + CDTI 100, edge …………… [2.15]

The values of 1/3 and 2/3 approximate the relative areas represented by the center and edge

values, The CTDIw is a useful indicator of scanner radiation output for a specific kVp , and

also represents the average absorbed radiation dose over the x and y directions at the center

of the scan from a series of axial scans.[12].

2.6.4 Normalized weighted CTDI

nCTDIw is the normalized CTDIw per radiographic exposure (mGy(mAs)-1) in the head or

body phantom for the setting of nominal slice thickness and applied potential used for an

examination .nCTDIw is determined for a single slice as in serial scanning.

nCDTIw = ………………. [2.16]

Where C is the tube current x the exposure time (mAs) for single in serial (axial) scanning or

per rotation in helical scanning [12].

2.6.5 Volume CTDIvol

To represent dose for a specific scan protocol, which almost always involves a series of

scans, it is essential to take into account any gaps or overlaps between the x-ray beams from

consecutive Rotations of the x-ray source. This is accomplished with use of a dose descriptor

known as the Volume CTDIw or (CTDIvol), which:

CTDIvol = CTDIvol …………… [2.17]

where I is the table increment per axial scan (mm).Since pitch is the ratio of the table travel

per rotation (I) to the total nominal beam width (N×T)

26 

Pitch = ………………… [2.18]

Pitch: was defined as the ratio of the table traveled per x-ray rotation tube to the slice width

(which was typically, but not always equal to the beam collimation. A pitch less than 1.0

indicates overlap of radiation beams, and pitch greater than 1.0 indicates gaps between the

radiation beams.

CTDIvol = × CTDI w……….. [2.19]

The CTDIvol provides a single CT dose parameter, based on a directly and easily measured

quantity, which represents the average dose within the scan volume for a standardized

(CTDI) Phantom. The SI units are milligray (mGy).

CTDIvol is a useful indicator of the dose to a standardized phantom for a specific exam

protocol, because it takes into account protocol-specific information such as pitch. Its value

may be displayed prospectively on the console of newer CT scanners, although it may be

mislabeled on some systems as CTDIw[12].

2.6.6 Dose- length product (DLP):

Dose Length Product is a measure of the dose of ionizing radiation that is incident on patients

in X-ray computed tomography To better represent the overall energy delivered by a given

scan protocol, the absorbed dose can be integrated along the scan length to compute the

Dose-Length Product (DLP), where:

DLP (mGy.cm) = CTDLvol (mGy) ×scan length (cm) ……….[2.20]

The DLP reflects the total energy absorbed (and thus the potential biological effect)

attributable to the complete scan acquisition.The implications of over ranging with regard to

the DLP depend on the length of the imaged Body region. For helical scans that are short

relative to the total beam width, the dose efficiency (With regard to over ranging) will

decrease. For the same anatomic coverage, it is generally more dose efficient to use a single

helical scan than multiple helical scans. The DLP (mGy.cm) values for the axial scan format

calculated using the following equation:

DLP = ∑ nCTDIw. T . N . C i …………… [2.21]

where nCTDIw is normalized CTDI per radiographic exposure (mGy (mAs)-1), і represents

each scan sequences forming part of an examination and N is the number of slice ,each of

thickness T(cm) and radiographic exposure C (mAs).[12].

27 

2.7 Other medical imaging modalities:

2.7.1 Mammography:

Mammography is a specific type of imaging that uses a low-dose X-ray system to examine

breasts. A mammography exam, called a mammogram, is used to aid in the early detection

and diagnosis of breast diseases in women. An x-ray (radiograph) is a noninvasive medical

test that helps physicians diagnose and treat medical conditions. Imaging with x-rays involves

exposing a part of the body to a small dose of ionizing radiation to produce pictures of the

inside of the body. X-rays are the oldest and most frequently used form of medical

imaging.[3].

Mammography is performed to screen healthy women for signs of breast cancer. It is also

used to evaluate a woman who has symptoms of a breast disease, such as a lump, nipple

discharge, breast pain, dimpling of the skin on the breast, or retraction of the nipple.

Screening mammograms are improving the detection of early breast cancer, when it is more

likely to be curable. Most but not all organizations recommend women began breast cancer

screening at age 40 and have repeat mammograms every 1 to 2 years.

All women over age 50 should have a screening mammogram every 1 to 2 years. [3].

2.7.2 Fluoroscopy:

Fluoroscopy is a type of medical imaging that shows a continuous x-ray image on a monitor,

much like an x-ray movie. It is used to diagnose or treat patients by displaying the movement

of a body part or of an instrument or dye (contrast agent) through the body. During a

fluoroscopy procedure, an x-ray beam is passed through the body. The image is transmitted

to a monitor so that the body part and its motion can be seen in detail.[3] .

Uses: Fluoroscopy is used in many types of examinations and procedures. Some examples

include

• Barium x-rays and enemas (to view movement through the GI tract)

• Catheter insertion (to direct the placement of a catheter during angioplasty or

angiography)

• Blood flow studies (to visualize blood flow to organs)

• Orthopedic surgery (to view fractures and fracture treatments)

Fluoroscopy is a type of x-ray procedure, and it carries the same types of risks as other x-ray

procedures. The radiation dose the patient receives varies depending on the individual

procedure. The two major risks associated with fluoroscopy are

28 

• radiation-induced injuries to the skin and underlying tissues (“burns”), and

• The small possibility of developing a radiation-induced cancer some time later in

life.[3].

2.7.3 Magnetic resonance imaging (MRI):

Magnetic resonance imaging (MRI) is a noninvasive medical test that helps physicians

diagnose and treat medical conditions. MRI uses a powerful magnetic field, radio frequency

pulses and a computer to produce detailed pictures of organs, soft tissues, bone and virtually

all other internal body structures. The images can then be examined on a computer monitor,

transmitted electronically, printed or copied to a CD. MRI does not use ionizing radiation (x-

rays).[3]. Physicians use the MR examination to help diagnose or monitor treatment for

conditions such as: Tumors of the chest, abdomen or pelvis and certain types of heart

problems.

2.7.4 Ultrasound: Ultrasound is cyclic sound pressure with a frequency greater than the upper limit of human

hearing. Although this limit varies from person to person, it is approximately 20 kilohertz

(20,000 hertz) in healthy, young adults. The production of ultrasound is used in many

different fields, typically to penetrate a medium and measure the reflection signature or

supply focused energy. The reflection signature can reveal details about the inner structure of

the medium [22]. Ultrasound is not limited to diagnosis, but can also be used in screening for

disease and to aid in treatment of diseases or conditions. The ultrasound used for diagnose

disease such as: cardiology, blood vessels, abdominal structures, Knee joint, and neck [22].

2.7.5 X-ray (Radiography):

Radiography (X-ray) is a Nondestructive Testing (NDT) method that examines the volume of

a specimen. Radiography (X-ray) uses X-rays and gamma-rays to produce a radiograph of a

specimen, showing any changes in thickness, defects (internal and external), and assembly

details to ensure optimum quality in your operation. A heterogeneous beam of X-rays is

produced by an X-ray generator and is projected toward an object. According to the density

and composition of the different areas of the object a proportion of X-rays is absorbed by the

object. The X-rays that pass through are then captured behind the object by a detector (film

sensitive to X-rays or a digital detector) which gives a 2D representation of all the structures

superimposed on each other. In tomography, the X-ray source and detector move to blur out

structures not in the focal plane. [3]

29 

2.7.6 Dental radiography:

Dental X-ray examinations provide valuable information that helps your dentist evaluate your

oral health. With the help of radiographs (the term for pictures taken with X-rays), your

dentist can look at what is happening beneath the surface of your teeth and gums. As X-rays

pass through mouth they are mostly absorbed by teeth and bone because these tissues, which

are called hard tissues, are denser than cheeks and gums, which are called soft tissues. When

X-rays strike the film or a digital sensor, an image called a radiograph is created.

Radiographs allow the dentist to see hidden abnormalities, like tooth decay, infections and

signs of gum disease, including changes in the bone and ligaments holding teeth in place.[3].

30 

Chapter three

31 

Chapter Three:

Materials and methods

Materials and methods used in this study are described in this chapter. 3.1 Estimation of CT radiography examination frequencies:

To estimate CT examination frequency, a survey was conducted based on data collected

from totally 10 (4 public and 6 private) hospitals, as a representative sample for 51 of CT

units distributed in different hospitals around the country. The information about the number

of CT examinations per month in each CT room was obtained by a questionnaire distributed

to facility correspondent participating in the study. The information received included: the

average number of CT examination per month for specific examination, chest, abdomen,

pelvis, L.spine, brain, neck, head and other examinations. The average annual examination

frequency for each unit was calculated from the data mathematically. The average annual

number of paediatric CT examination was calculated by multiplying the average examination

per month per unit by the total number of CT scanners in Sudan at the time of the study (51

CT units in 2011). The percentage of examination frequency for each examination type to the

total number of examinations per year was also calculated. Information to each CT unit,

including manufacturer, model, and installation date was also obtained.

Table 3.1 shows the information about the participating Hospitals. Information with regard

to: manufacturer, model, and installation date for each CT unit were provided.

32 

Table (3.1) shows the surveyed hospitals and CT devices:

The hospital Manufacturer Model Installation

date

H1 Ibn Alhaitham TOSHIBA Station 4 slice 2010

H2 Police hospital SIEMENS Somatom sensation 16

slice

2005

H3 Modern medical

cent

GE CTE 2005

H4 Asia hospital GE CTE 2005

H5 Yestabshiroon TOSHIBA dual slice 2006

H6 Military hospital SIEMENS Single slice 2005

H7 Soba university TOSHIBA station 4 slice 2005

H8 Fedail SIEMENS Somatom sensation 16

slice

2008

H9 Al amal national

hospt.

TOSHIBA 64 aquiline 2011

H10 El nilline

diagnostic

SIEMENS Somatom emmotion 2006

33 

3.2 Collective dose per examination:

In order to calculate the collective effective dose to population per examination for all

examinations, effective dose per procedure for each projection had been obtained from the

literature and the results of previous studies on assessment of pediatric CT dose in Sudan.

The total patient collective dose from diagnostic radiological examinations, CED, in units of

man.Sv, is calculated according to the formula:

CED = ∑ i Ei Ni …………….. [3.1]

where Ei is the mean effective dose to patients from a particular examination type and Ni is

the corresponding number of examinations of that type performed each year.

The Units: is J/kg the special name is (man Sv). [20

34 

35 

Chapter four

36 

Chapter Four:

Results and discussion

4.1 Results

Results of the study on examination frequency and collective effective dose are presented in

this chapter. The survey included 10 hospitals of which six are public and four are private

hospitals. The complete data are provided for 10 CT units of 51 CT scanners currently in

operation in the country (≈ 20%). This number is considered to offer a fair representative

picture of pediatric CT practice in Sudan.

Table (4.1) shows the survey data gathered from 10 hospitals. The table includes the number

of examinations per month per X-ray room for the selected hospitals. As can be seen from the

table, the most common pediatric CT examinations are: Brain, Abdomen, and Chest, Pelvis,

and Lumbar spine examinations. According to this survey the examinations with highest

frequency is abdomen with 24 procedures per month and then the brain with 20 examination

per month and then the chest with17 examinations per month followed by lumbar spine with

only five examinations per month.

The percentage annual collective effective dose for each examination type had been

calculated to determine the contribution of each examination type to total annual collective

dose from pediatric CT examinations. The percentage examination frequency and the

percentage collective dose had been plotted into two figures, (7) and (8).

37 

Table (4.1) shows the survey data gathered from 10 hospitals and the average of

examinations in month.

average H10H9 H8 H7 H6 H5 H4 H3 H2 H1

Hospital

Exams

17

24

2

5

20

3

1

12

14

5

3

50

4

-

4

4

-

-

8

-

4

16

12

4

4

24

8

-

20

32

-

20

24

-

-

4

8

-

-

-

-

-

12

20

-

4

200

-

-

2

2

-

-

43

-

-

30

30

-

-

60

-

-

4

16

-

4

12

-

4

70

96

9

8

155

13

-

Chest

Abdomen

Pelvis

L .spine

Brain

Neck

Head 76 92 20 72 98 12 232 49 120 60 362Total

38 

Table (4.3) provides the annual frequency for surveyed units and for total CT units in Sudan

and the percentage of each examination, the calculated collective effective dose from each

examination, and the percentage contribution of each examination in the total collective

effective dose. Table (4.3) shows examination frequency and collective effective dose from

CT procedures in Sudanese hospitals and their percentages.

The examination was about chest, abdomen, brain, neck, pelvis and other parts of the body.

The highest effective dose per procedure is for abdomen and pelvis (7 mSv) and then lumbar

spine (6.6 mSv) followed by abdomen examinations (5.6 mSv), while the others have

approximately similar effective dose values. However, the highest collective effective dose

was observed for abdomen examinations because of it is high frequency and high effective

doses.

The annual number of examinations per scanner found in sudanese hospitals is more than the

number reported in other countries. The collective effective dose to sudanese population

estimated in this study is low compared to the collective dose reported in developed

countries. It is suspected that the reason the low number of CT units and hence the annual

number of the CT examinations. On the other hand, this case will contradict in the large

number of scanner or greater number scans per examination performed in developed

countries.

39 

Table (4.2): Examination frequency and collective effective dose from CT procedures in sudanese

hospitals and their percentages.

Examination

Average annual

number of

exams per room

Total number

of exams per

year

Exams freq

%

E/exam

(mSv)

Annual

CED

(man.Sv)

CED

%

Chest 204 10404 22 2.4 24 4

Abdomen 288 14688 32 5.6 82 49

Pelvis 24 1224 3 4.3 5 3

L .spine 60 3060 7 6.6 20 12

Brain 240 12240 26 2.3 28 17

Neck 36 1836 4 2.1 3 2

Head 12 612 1 1.2 1 1

Abd.pelvis 12 612 1 7 4 2

Others 36 1836 4 0.8 2 1

Total 912 46512 100 32.3 1500 100

40 

Fig (4.1) provides the percentage of examination frequency, which were gathered from 10

different hospitals in Sudan. According to the figure, the highest percentages is brain with

(32%) followed with abdomen with (26%) and then chest with (22%).

Fig (4.1)

Figure (4.1): the percentage contribution of each examination and the annual frequencies

from pediatric CT examination

22%

32%

3%

7%

26%

4% 1% 1%

4%

Annual examination frequency %

Chest

Abdomen

Pelvis

L .spine

Brain

Neck

Head

Abd.pelvis

Others

41 

Fig(4.2), provides the percentage of collective effective dose, which were calculated by

multiplying the the effective dose with frequency. According to the figure, the highest

percentages is abdomen with (49%) followed by brain with (17%) and then lumbarspine with

(12%).

Fig (4.2)

Figure (4.2): the percentage contribution of each examination in the collective effective dose

from CTexamination.

5%

54%

3%

13%

19%

2%1% 2%1%

Collective dose percentage % 

Chest

Abdomen

Pelvis

L .spine

Brain

Neck

Head

Abd.pelvis

Others

42 

4.2 Discussion:

The results obtained clearly show large differences in pediatric CT examinations frequencies

between hospitals for the same type of examination. Assessing annual collective dose is

important. According to the findings of this study, the annual number of pediatric CT

examinations in Sudan is estimated to be (46512) examinations resulting in annual collective

dose of (1500 man.Sv). The estimates of collective effective dose presented here are average

for the Sudan population from pediatric CT procedure. While effective dose for a given

examination have been used, collective does for a specific examination may vary

substantially due to some examination delivered much higher effective dose than others. The

trends in percentage examination frequency and collective effective dose are shown in Fig

(4.1) and Fig (4.2), respectively. It is evident that some single type of examinations gives

high contributions to collective effective dose, despite their low frequency. As can be seen

the highest contributors to annual collective dose are abdomen, followed by brain and

Lumbar spine.

Although the CT scanning examinations of chest accumulate frequency over 22%, its

contribution to collective dose contributes only 4 percent. This was correct when effective

dose were calculated based on tissue weighting factor recommended in ICRP 60. However,

according to the new ICRP recommendation the chest examinations give high effective doses

and hence, high collective doses because it contains organs with high tissue weighting factors

such as breast. The contribution of head and other examinations to annual effective dose is

very small much less than 1% as expected.

The survey on paediatric CT practice for 2010-2011 has revealed a number of interesting

details not previously known. Al-though the study is based on a limited number of hospitals,

the results of this survey can nevertheless be regarded as being nationally representative since

the vast majority of pediatric CT examinations in Sudan are carried out by these particular

hospital. Dose relevant data were collected for examinations of brain, chest, L.spine, neck;

head and abdomen. Which were identified in the first part of this survey as being the six most

frequent types. Other types of examination are quite rare and were therefore not taken into

account. Most pediatric CT examinations are restricted to the head region specifically brain

(roughly two thirds), whereas only one third are carried out in other examinations. Therefore,

not only is the number of pediatric CT examinations relatively small in Sudan, but also so is

the average dose per pediatric CT exam. The age distribution is relatively flat, i.e. similar

43 

frequencies are noted for all age groups (up to 1year, 1 to 5 years, and 6 to 10 years). The

majority of the scanners used for pediatric CT examinations are quite modern, i.e. spiral

scanners with solid-state detectors. Most of these have multi-slice capabilities, thus enabling

significantly reduced total scan times.

Patient doses resulting from examinations made with ADC are somewhat higher than from

those with manual adaptation of the dose settings. This can be explained in part by the

present characteristics of some ADC devices, where exposure settings cannot be made in

terms of dose, but rather in terms of image quality, i.e. noise settings. In addition, these

particular devices are designed to maintain the selected noise level with changes in almost

any exposure setting that has influence on the noise in the resulting image.

Therefore, a reduced slice thickness or a sharper reconstruction filter inevitably forces these

ADC devices to operate at an increased dose level. However, since changes in these

parameters often have a positive effect on other aspects of image quality, e.g. detail contrast,

the corresponding increase in noise need be compensated for only slightly or not at all.In this

context, the question arises how dose should be adapted appropriately to the size of the

patient.

In the past few years, a large number of papers have been published on this subject with

significantly differing recommendations: slight adaptation, intermediate adaptation and

strong adaptation. The ICRP has recently brought new recommendation, which will change

the overall set of tissue weighting factors: these changes would decrease effective doses for

procedures, which expose the chest. The collective effective doses presented here should be

used with caution when evaluating an individual procedure .In addition, the value presented

above for various examinations are averages given with the realization that for any

examination, the actual dose in practice may vary by an order of magnitude. The results of

this study were compared with results from other published data.

A study had been done in Germany by M.Galanski and H.D.Nagel in 2005/06 to estimate

public exposure from CT diagnostic radiology for pediatric. The survey was based on

questionnaires that were first sent to 1640 users of CT scanners installed in hospitals and

private practices, asking for the frequencies of the types of examination, subdivided into five

age groups [5]. In a subsequent second survey, a selected number of 72 users, responsible for

about two third of annual pediatric CT examinations reported in the previous study, were

asked for detailed dose-relevant data .These data were used for individualized dose

assessment, depending on the type of scanner and age of the patient .According to the survey

the collective effective dose in Germany as a result of pediatric CT examinations was very

44 

high. In Iran, a study was carried out to determine the average absorbed dose in Yazd

province by pediatric CT examinations [13], and to evaluate the potential risks per year by

these examinations. This study was conducted in Yazd CT centers during 2005-2006. The

examination frequencies from 3 CT scanners were collected from all types of examinations

[13]. The effective dose was determined by CT dose program (Impact CT patient dosimetry

calculator) [13]. The study found that the collective effective dose from pediatric CT

examinations in Yazd was (32.48 man.Sv). We note that the collective effective dose in Yazd

is less than CED in Sudan because the number of CT scan in Yazd was just 3 devices while

as 51 devices in Sudan. According to the ICRP risk factors, radiation dose from CT

examinations could lead to about 1.3 fatal cancers per year. Therefore, request for CT

examinations should be more justified.

Table (4.3) shows comparison of results of the collective dose from CT pediatric obtained in

this study with the collective effective dose obtained in Yazd (Iran)[13].

45 

Table (4.3) shows comparison of results of the collective dose from CT pediatric for

Sudan and Yazd.

Examination Collective dose (man.Sv) of

this study (Sudan )

Collective dose (man.Sv)

Yazd (2006)

Chest 24 5

abdomen 82 5

pelvis 5 8

brain 28 9

neck 3 1

L.spine 20 2

Total 162 30

Another study had been done by Yoshito Tsushima, and Ayako Taketomi-in Takahashi

Tsushima et al. in Japan.[14]. An estimated 235.4 patients per 1,000 populations undergo CT

examinations each year, in Japan and 50% of the patients were scanned in two or more

anatomical locations in one CT session. There was a large variation in effective dose among

hospitals surveyed, particularly in lower abdominal CT (range, 2.6-19.0 mSv). CT

examinations of the chest and upper abdomen contributed to approximately 73.2% of the

collective dose from all CT examinations. It was estimated that in Japan, approximately 29.9

million patients undergo CT annually, the study was done for the patient in the age range of

(0-19)year and the estimated annual collective effective dose in Japan was 277.4 *103man. Sv

[13], while as the estimated collective dose in Sudan is (1500 man.Sv) according to this

study.The annual effective dose per person for Japan was estimated to be 2.20 mSv. The

data-sheets were sent to all 126 hospitals and randomly selected 14 (15%) of 94 clinics in

Gunma prefecture which had CT scanner(s), along with CT scan protocols for each

institution surveyed. Age and sex specific patterns of CT examination and factors, which

were responsible for the variation in radiation exposure, were determined. The following

46 

table shows the estimated annual number of CT examinations and collective dose in Japan.

We see that the annual collective dose in Japan is very high in comparison with collective

doses in Sudan because of the huge number of the CT devices in Japan. [13]

Table (4.4) shows the estimated annual number of CT examinations and collective dose in

Japan.

CT examination Annual number of CT

examination*106

Annual collective dose

(man.Sv)

Head 9.1 23600

Face 1.4 2800

Neck 10.1 265

.Chest 12.5 120000

Abdomen 6.2 52200

Total 39.3 225100

47 

Table(4.5) : Comparison of collective doses to sudanese population with those reported in

other countries .The table shows that the collective dose in Japan is highest with (225100

man.Sv) followed by the collective dose in Sudan with (162man.Sv) and then the collective

dose in Yazd with only (30) man.Sv. [14]

Table (4.5) shows the comparison of collective doses per exam for Yazd , Japan and Sudan

examination collective dose in Japan

2010 (man.Sv)

Collective dose

(man.Sv) Yazd

(2006)

Collective dose (man.Sv)

of this study (Sudan )

Head 23600 9 28

Face 2800 - -

Neck 26500 1 3

Chest 120000 5 24

Abdomen 52200 5 82

Pelvis - 5 5

L.spine - 2 20

Total 225100 30 162

48 

Chapter five

Conclusion

49 

Chapter Five:   

5.1 CONCLUSION  

The annual collective effective dose from pediatric CT procedures has been calculated by using results of the survey of the frequency of 8 types of examinations from 10 hospitals and the data of the effective dose per examination from previous study on radiation dose from such examinations. The total annual collective effective dose from all CT procedures performed in the Sudan in both public and private hospitals is calculated to be (169 man.Sv). The study showed that the annual collective dose to population from pediatric CT examinations in Sudan are low compared to doses from such procedures to other nations in particular developed countries. The difference in the development of healthcare systems and usage of CT scanners in medical imaging were taken into consideration. The lowering in estimated collective dose reduces to the lower number of CT scanners and frequency of CT examinations. From comparison with international published data the annual collective dose from CT procedure in Sudan is much lower but it should be surveyed in specific time intervals because of the rapid increase of the number of CT scanners and the use in medical imaging that can be clearly observed from the survey results that most of the CT scanners installed in recent years and most of hospitals and medical centers going to have CT scanners in the near future.

50 

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