Lecture 05 Image Quality and Patient Dose

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RTC on RADIATION PROTECTION OF PATIENTS FOR RADIOGRAPHERS

Accra, Ghana, July 2011

Image Quality and Patient Dose

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Overview

• To become familiar with the factors that determine the image clarity and the way the image quality can be improved

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Imaging quality

• Efficient diagnosis requires • acceptable noise

• good image contrast

• sufficient spatial resolution

• These factors are linked

• “Objective” measurement of quality is difficult

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Factors affecting image quality

Image quality

Contrast

Distortion & artifact Noise

Blur or Unsharpness

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Image contrast

Low Contrast

Medium Contrast

High Contrast

Image contrast refers to the fractional difference in optical density of brightness between two regions of an image

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Some factors influencing contrast

• Radiographic or subject contrast• Tissue thickness

• Tissue density

• Tissue electron density

• Effective atomic number Z

• X Ray energy in kVp

• X Ray spectrum (filtration)

• Scatter rejection• Collimator

• Grid

• …

• Image contrast• The radiographic contrast

plus :

• Film characteristics

• Screen characteristics

• Windowing level of CT and DSA

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• Peak voltage value has an influence on the beam hardness (beam quality)

• It has to be related to medical question• What is the anatomical structure to be

investigated?

• What is the contrast level needed?

• For a thorax examination : 110 - 120 kV is suitable to visualize the lung structure

• However only 65 kV is necessary to see bone structure

Technique factors (1)

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Technique factors (2)

• The higher the energy, the greater the penetrating power of X Rays

• At very high energy levels, the difference between bone and soft tissue decreases and both become equally transparent

• Image contrast can be enhanced by choosing a lower kVp so that photoelectric interactions are increased

• Higher kVp is required when the contrast is high (chest)

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Technique factors (3)

• The mAs controls the quantity of X Rays (intensity or number of X Rays)

• X Ray intensity is directly proportional to the mAs

• Over or under-exposure can be controlled by adjusting the mAs

• If the film is too “white”, increasing the mAs will bring up the intensity and optical density

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Receptor contrast

• The film as receptor has a major role to play in altering the image contrast

• There are high contrast and high sensitivity films

• The characteristic curve of the film describes the intrinsic properties of the receptor (base + fog, sensitivity, mean gradient, maximum optical density)

• N.B.: Film processing has a pronounced effect on fog and contrast

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Image Contrast

• Difference in signal – pixel value, film density

High Low

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Video monitor

• The video monitor is commonly used in fluoroscopy and digital imaging • The display on the monitor adds flexibility in the choice

of image contrast

• The dynamic range of the monitor is limited (limitation in displaying wide range of exposures)

• Increased flexibility in displaying image contrast is achieved by adjustment of the window level or grey levels of a digital image

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Blur or lack of sharpness

• The boundaries of an organ or lesion may be very sharp but the image shows a lack of sharpness

• Different factors may be responsible for such a degree of “fuzziness” or blurring

• The radiologist viewing the image might express an opinion that the image lacks “detail” or “resolution” (subjective reaction of the viewer to the degree of sharpness present in the image)

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Resolution

• Smallest distance that two objects can be separated and still appear distinct

• Example of limits• Film/screen: 0.01 mm

• CT: 0.5 mm

• Other definition: “Point-spread” function• Characteristic of a “point” object

• Point object expected to be point in image

• Blurring due to imperfections of imaging system

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Factors affecting image sharpness

Image Unsharpness

GeometricUnsharpness

Object Unsharpness

Motion Unsharpness

Subject Unsharpness

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Resolution and Focal Spot Size

Penumbra

Appearance of imageMore blur Less blur

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Measuring Resolution

Line pair test object

One line pair

{

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Geometric blur

• If the focal spot is infinitesimally small, the blur is minimized because of minimal geometric bluntness

• As the focal spot increases, the blur in the image increases

Small focal spot Large focal spot

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Geometric blur

• Another cause of lack of geometric sharpness is the distance of the receptor from the object

• Moving the receptor away from the object results in an increased lack of sharpness

• N.B.: The smaller the focal size and closer the contact between the object and the film (or receptor), the better the image quality as a result of a reduction in the geometric sharpness

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Lack of sharpness in the subject

• Not all structures in the body have well-defined and separate boundaries (superimposition essentially present in most situations)

• The organs do not have square or rectangular boundaries

• The fidelity with which details in the object are required to be imaged is an essential requirement of any imaging system

• The absence of sharpness, in the subject/object is reflected in the image

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Lack of sharpness due to motion (1)

• Common and understandable blur in medical imaging

• Patient movement :• uncooperative child

• organ contraction or relaxation

• heart beating, breathing etc.

• Voluntary motion can be controlled by keeping examination time short and asking the patient to remain still during the examination

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Lack of sharpness due to motion (2)

• Shorter exposure times are achieved by the use of fast intensifying screens

• N.B.: Faster screens result in loss of details (receptor sharpness)

• Further, the use of shorter exposure time has to be compensated with increased mA to achieve a good image

• This often implies use of large focal spot (geometric sharpness)

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Distortion and artifacts

• Unequal magnification of various anatomical structures

• Inability to give an accurate impression of the real size, shape and relative positions

• Grid artifact (grid visualized on the film)

• Light spot simulating microcalcifications (dust on the screen)

• Bad film screen contact, bad patient positioning (breast)

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Noise

• Defined as uncertainty or imprecision of the recording of a signal

• Impressionist painting: precision of object increases with number of dots

• X Ray imaging: when recorded with small number of X- photons has high degree of uncertainty,more photons give less noise

• Other sources of noise:• Grains in radiographic film• Large grains in intensifying screens• Electronic noise of detector or amplifier

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Noise

Decreasing radiation intensity

Increasing noise

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Contrast & Noise

No

ise Contrast

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Radiography Issues

• Correct positioning• Improves diagnosis and reduces retakes

• PRE-exposure collimation• Minimises unnecessary tissue dose

• With CR/DR, there is a temptation to post-exposure (electronically) collimate – RESIST THIS!!

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Summary

• Different technical and physical factors may

influence the image quality by impairing the

detection capability of the anatomical structures

useful for diagnosis (increasing the image

unsharpness)

• Some factors depend on the receptor, some others are

more related to the radiographic technique

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Patient dose assessment

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Overview

• To become familiar with the patient dose assessment and dosimetry instrument characteristics.

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Parameters influencing patient exposure

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Essential parameters influencing patient exposure

Tube voltageTube currentEffective filtration

Exposure time

Field size

Kerma rate[mGy/min]

[min]Kerma[Gy]

[m2]

Area exposureproduct [Gy m2 ]

}}

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Factors in conventional radiography: beam, collimation

• Beam energy• Depending on peak kV and filtration• Regulations require minimum total filtration to absorb

lower energy photons• Added filtration reduces dose• Goal should be use of highest kV resulting in acceptable

image contrast

• Collimation• Area exposed should be limited to area of CLINICAL

interest to lower dose• Additional benefit is less scatter, better contrast

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Factors in conventional radiography: grid,patient size

• Grids• Reduce the amount of scatter reaching image receptor• But at the cost of increased patient dose• Typically 2-5 times: “Bucky factor” or grid ratio

• Patient size• Thickness, volume irradiated…and dose increases with

patient size• Except for breast (compression), no control• Technique charts with suggested exposure factor for

various examinations and patient thickness helpful to avoid retakes

• Use of AEC exposure

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Factors affecting dose in fluoroscopy

• Beam energy and filtration• Collimation• Source-to-skin distance

• Inverse square law: maintain max distance from patient

• Patient-to-image intensifier• Minimizing patient-to- I I will lower dose• But slightly decrease image quality by increased scatter

• Image magnification • Geometric and electronic magnification increase dose

• Grid• If small sized patient (les scatter) perhaps without grid

• Beam-on time!

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Factors affecting dose in CT

• Beam energy and filtration• 120-140 kV; shaped filters

• Collimation or section thickness• Post-patient collimator will reduce slice thickness imaged

but not the irradiated thickness

• Number and spacing of adjacent sections

• Image quality and noise• Like all modalities: dose increase=>noise decreases

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Factors affecting dose in spiral CT

• Factors for conventional CT also valid

• Scan pitch• Ratio of couch travel in one rotation divided by

slice thickness

• If pitch = 1, doses are comparable to conventional CT

• Dose proportional to 1/pitch

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Patient dosimetry methods

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Patient dosimetry

• Radiography: entrance surface dose ESD• Output factors

• Dose – area product (DAP)

• Fluoroscopy: Dose - Area Product (DAP)

• CT:• Computed Tomography Dose Index (CTDI)

• Dose – length product (DLP)

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From ESD to organ and effective dose

• Except for invasive methods, no organ doses can be measured

• The only way in radiography: measure the Entrance Surface Dose (ESD)

• Use mathematical models to estimate internal dose. • Mathematical models based on Monte Carlo

simulations• Dose to the organ tabulated as a fraction of the

entrance dose for different projections• Since filtration, field size and orientation play a role:

long lists of tables (See NRPB R262 and NRPB SR262)

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From DAP to organ and effective dose

• In fluoroscopy: moving field, measurement of Dose-Area Product (DAP)

• In similar way organ doses calculated by Monte Carlo modelling

• Based on mathematical model• Conversion coefficients estimated as organ doses

per unit dose-area product• Again numerous factors are to be taken into

account such as projection, filtration, …• Once organ doses are obtained, effective dose is

calculated following ICRP103

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Dose measurements: how to measure dose indicators ESD, DAP,CTDI…

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Measurements of Radiation Output

X Ray tube

Filter

Ion. chamber

Lead slabTable top

SDD

Phantom (PEP)

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Measurements of Radiation Output

• Operating conditions

• Consistency check

• The output as a function of kVp

• The output as a function of mA

• The output as a function of exposure time

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Measurement of entrance surface dose

TLD, solid statedosimeter or ion chamber

Includes backscatter (~30%)

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Dose Area Product (DAP)

Transmission ionization chamber

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Dose Area Product (DAP)

0.5 m1 m

2 m

Air Kerma:Area:Areaexposure product

40*103 Gy2.5*10-3 m2

100 Gy m2

10*103 Gy10*10-3 m2

100 Gy m2

2.5*103 Gy40*10-3 m2

100 Gy m2

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Calibration of a Dose Area Product (DAP)

Film cassette

10 cm 10 cm

Ionizationchamber

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Levels of Dosimetry

• Level 1 - published tables

• Level 2 - Monte Carlo tables using known data

• Level 3 - direct measurement of skin dose

• Level 4 - humanoid phantom measurements with TLD

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Level 1 - Published Dose Tables

• ICRP, NCRP and various books have tables of “typical” doses for various x-rays

• Tables show organ doses and sometimes effective dose

• The data is usually old, from x-rays made with slower film/screen systems than in current use

• Still useful as a guide

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Level 2 - Monte Carlo Systems

• Provide organ doses, and effective dose

• Use calculated data but with user entry of various parameters :• HVL, kVp

• skin dose or free-in-air exposure at skin distance

• FSD, field size and position

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Monte Carlo Systems

• Various computer programs and lookup tables, eg. TISSDOSE, XDOSE, PCXMC

• Most users do not know the actual values for input variables, so often must use assumed values

• HVL, kVp, FSD, field size easy to assume – kVp/mAs and field size not always easy to assume

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Typical Radiology Doses (Melbourne Data)

Procedure ESD (mGy) Eff. Dose (mSv)

Abdomen (AP) 2.5 0.35

Chest (PA) 0.15 0.023

Lumbar spine AP/Lat 3.2/4.0 0.23/0.11

Pelvis (AP) 1.7 0.29

Thoracic spine (lat.) 2.6 0.097

Mammography (4 views) 4.4 0.44

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XDose

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PCXMC

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ImPACT CT Dose

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Summary

• In this lesson we learned the factors influencing patient dose, and how to have access to an estimation of the detriment through measurement of entrance dose, dose area product or specific CT dosimetry methods.