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Medical Image Processing Techniques
INTRODUCTION TO IMAGE PROCESSING
In electr ical engineering and computer science, imageproces sing is any form of signal processing for which the input isan image, such as photographs or frames of video; theoutput of imageprocessing can be either an image or a set of characteristics or
parametersrelated to the image.Most image-processing techniquesinvolve treating the image as a two-dimensionalsignal and
applying standard signal-processing techniques to it.Image processingusually refers to digital image processing, but optical and
analogimage processing are also possible.
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TYPICAL OPERATION
Among many other image processing operations are:
Euclidean geometry transformations such as enlargement, reduction, and
rotation
Color corrections such as brightness and contrast adjustments,quantization, or color translation to a different color space
Digital composit ing or optical composit ing (combinationof two or more images).Used in film-making to make a "matte"
Interpolation, demosaicing, and recovery of a full image from a
raw image formatusing a Bayer filter pattern
Image registration, the alignment of two or more images
Image differencing and morphing
Image recognit ion, for example, extract the text from theimage by using opticalcharacter recognition
Image segmentation
High dynamic range imaging by combining multiple images
Geometric hashing for 2-D object recognition with affine invariance
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APPLICATIONS
Further information: Imaging
Computer vision
Face detection
Feature detection
Lane departure warning system
Non-photorealistic rendering
Medical image processing
Microscope image processing
Morphological image processing
Remote sensing
Automated Sieving Procedures
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MEDICAL IMAGING
Medical imaging
refers to the techniques and processes used to create images ofthehuma n body ( o r pa r t s a nd f un c t i on t he r e o f ) f o r c l i n
i c a l p u rp o s e s o r m ed i c a l s c i en c e(including the study ofnormal anatomy and
physiology).A s a d i s c i p l i n e a n d i n i t s w i d e s t s e n s e , it i s p a r t o f b i o l o g i c a l i m a g i n g a n d i ncorporates radiology ( in the wider sense), nuclear medicine, investigative r
adiologicalsciences, endoscopy, (medical) thermrorgraphy,
medical photography and microscopy (e.g.for human pathologicalinvestigations). Measurement and recording techniques which are
not p r i m a r i l y d e s i g n e d t o p r o d u c e i m a g e s , s u c h a selectroencephalography (EEG),magnetoencephalography(MEG), Electrocardiography (ECG) and others, but which producedatasusceptible to be represented as maps (i.e. containing positional
information), can beseen as forms of medical imaging.
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IMAGING TECHNOLOGIES
Electron microscopy
Radiographic
Magnetic resonance imaging (MRI)
Nuclear medicine
Photo acoustic imaging
Breast Thermography
Tomography
Ultrasound
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CREATION OF THREE-DIMENSIONAL IMAGES
R e c e n t l y , t e c h n i q u e s h a v e b e e n d e v e l o p e d t o e n a b l e C
T, M R I a n d u lt ra s ou n d scanning software to produce 3Dimages for the physician. Traditionally CT and MRI
scans produced 2D stati c output on fi lm. To produ ce 3Dimages, many scans are made, th encombined by computersto produce a 3D model, which can th en be manipulated bythe physician. 3D ultrasounds are produced using a somewhat similartechnique.With the abi lity to vis uali ze i mportan t structu resin great detail, 3D visualizationmethods are a valuable resource
for the diagnosis and surgical treatment of many pathologies.It was akey resource for the famous, but ultimately unsuccessful attempt
by Singaporeansurgeons to separate Iranian twins Ladan andLaleh Bijani in 2003. The 3D equipment wasused previously for
similar operations with great success.Other proposed or developedtechniques include:
Diffuse optical tomography
Elastography
Electrical impedance tomography
Optoacoustic imaging
Ophthalmology
o
A-scano
B-scano
Corneal topographyo
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Optical coherence tomographyo
Scanning laser ophthalmoscopySome of these techniques are still at aresearch stage and not yet used in clinical routines.
NON-DIAGNOSTIC IMAGING
Neuroimaging has also been used in experimentalci rc ums tances to all ow people (especially disabled persons) to
control outside devices, acting as a brain computer interface.
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OPEN SOURCE SOFTWARE
Several open source software packages are available for performinganalysis of medicalimages:
ImageJ
ITK
DICOMWORKS
GemIdent
PROPRIETARY SOFTWARE
MIMViewer
SureVistaVision
Universal PACS
Simpleware ScanIP
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AN INNOVATIVE MEDICAL IMAGING ARCHITECTURE
Fig: Medical Imaging Technology Architecture
omputed tomography (CT)
is a medical imaging method employing tomographycr ea ted by
computer processing. Digital geometry processing is usedto gene ra te a th ree-dimensional image of the inside of an
object from a large series of two-dimensional X-rayimages takenaround a single axis of rotation.CT produces a volume of data which canbe manipulated, through a process known as"windowing", in order to
demonstrate various bodily structures based on their ability to block theX-ray/Rntgen b eam. Although historically the images
gen erat ed were i n the axia l o r transverse plane, orthogonalto the long axis of the body, modern scanners allow this
volumeof data to be reformat ted i n various planes or e venas volumetric (3D) representations of structures. Althoughmost common in medicine, CT is also used in other f ields,su ch as nondestructive materials testing. Another example is the
DigiMorph project at theUniversityof Texa s a t Aus t in whic h u ses a C T sca nn e r t
o s tu d y b io lo gi ca l a nd pa le on to lo gi ca lspecimens.
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TERMINOLOGY
The word "tomography" is d erived from the Greektomos
(slice) and
graphein(towrite). Computed tomography was originally known as the "EMI
scan" as it was developed ata research branch of EMI, a company bestknown today for its music and recording business.It was later known
ascomputed axial tomography
(CAT or CT scan) andbody sectionroentgenography
.
A l t h o u g h t h e t e r m " c o m p u t e d t o m o g r a p h y " c o u l d b e used to de sc r ib e p os i t r on emiss ion tomography and s ingle
photon emission computed tomography,in practice i tusually refers to the computation of
tomography from X-ray images, especially in older medicalliterature and smaller medical facilities.In MeSH, "computed axial
tomography" was used from 1977-79, but thecurrentindexing explicitly includes "X-ray" in the title.
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HISTORY
In the early 1900s, the Italian radiologist Alessandro Vallebonaproposed a method torepresent a single slice of the body on
the radiographic f i lm. This method was kno wna st o m o g r a p h y . T h e i d e a i s b a s e d o n s i m p l e p r i n c i p l e s
of p ro j ec t i ve ge om et ry : m ov in gsynchronously and inopposite directions the X-ray tube and the film, which are
connectedtogether by a rod whose pivot point is the focus; theimage created by the points on the focal plane appears sharper,while the images of the other points annihilate as noise. This is
onlymarginally effective, as blurring occurs only in the "x"
plane. There are also more complexdevices which can move in morethan one plane and perform more effective blurring.Tomography hadbeen one of the pillars of radiologic diagnostics until the late
1970s,when the availability of minicomputers and of thetransverse axial scanning method, this lastdue to th e wor k of
Godfrey Hounsfield and South African born Allan McLeodCormack,gradually supplanted it as the modality of CT.The first
commercially viable CT scanner was invented by Sir GodfreyHounsfield inHayes, United Kingdom at EMI Central Research
Laboratories using X-rays. Hounsfieldconceived his ideain 1967, and it was publicly announced in 1972. Allan McLeod
Cormack of Tufts Universi ty in Massachu settsindependently invented a similar process, and
bothHounsfield and Cormack shared the 1979 Nobel Prize in Medicine.
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The original 1971 prototype took 160 parallel readings through
180 angles, each 1apart, with each scan taking a little over five
minutes. The images from these scans took
2.5hours to be processed by algebraic reconstruction techniques on a la rge compu te r. Thescanner had a single
photomultiplier detector, and operated on the Translate/Rotate
pr inciple . I t has been c la imed that thanks to the success
of Th e B ea t l es , EM I c ou ld fu n d research and build early
models for medical use. The first production X-ray CT machine
(infact called the "EMI-Scanner") was limited to making
tomographic sections of the brain,
buta c qu i r e d t he i ma g e da t a i n a bou t 4 mi n u t e s ( s c a n n i
ng t wo ad ja ce nt s l i c es ) , an d th ecomputation time was
about 7 minutes per picture. This scanner required the use of a
water-filled Perspex ta nk with a pre-shap ed rubb er "head-
ca p" at the fr on t , which encl os ed th e patient's head. The water-
tank was used to reduce the dynamic range of the radiation reachingthe
detectors. The images were relatively low resolution, being composed of
a matrix of only8 0 x 8 0 p i x e l s . Th e fi rs t EM I-
S c a n n e r w a s i n s t a l l e d i n A t k i n s o n M o r l e y H o s p i t a l i n
Wimbledon, England, and the first patient brain-scan was made with it
in 1972In the U.S., the first installation was at the Mayo Clinic.
As a tribute to the impact of this sys tem on medi cal imagi ng
the Mayo Clinic has an EMI scanner on display in
theRadiology Department.The fi rs t CT sys tem
that could make images of any part of the body and didnotrequire the "water tank" was the ACTA (Automatic Computerized
Transverse Axial)
scanner des ig ned b y R ob er t S . Led le y , D DS a t Geo rg e t o
wn Unive r s i ty . Th i s machine had 30 pho tomul t ip l i e r
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tubes as detectors and completed a scan in only 9
translate/rotate cycles,much faster than the EMI-scanner. It
used a DEC PDP11/34 minicomputer both to operateth e se rvo-
mechanisms and to acquire and process the images. ThePfizer drug companyacquired the prototype from the
university, along with rights to manufacture it. Pfizer then began
making copies of the prototype, calling it the "200FS" (FS
meaning Fast Scan), whichwere selling as fast as they could make
them. This unit produced images in a 256x256 matrix,with much better
definition than the EMI-Scanner's 80x80
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TOMOGRAPHY
A form of tomography can be performed by moving the X-raysource and detector during an exposure. Anatomy at the target
level remains sharp, while structures at differentlevel s ar e
blurred. By varying the extent and path of motion, avariety of effects can beobtained, with variable depth
of f ield and different degrees of blurr ingof 'out of plane'structures.Although largely obsolete, conventionaltomography is still used in specific situationssuch as dental imaging
(orthopantomography) or in intravenous urography.
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TOMOSYNTHESIS
Digital tomosynthesis combines digital image capture andprocessing with simpletube/detector motion as used in
conventional radiographic tomography. Although there aresom e
similari t ies to CT, i t is a separate technique. In CT, thesource/detector makes acomplete 360-degree rotation aboutthe subject obtaining a complete set of data from whichimages
may be reconstructed. In digital tomosynthesis, only a smallrotation angle (e.g., 40degrees) with a small number of discreteexposures (e.g., 10) are used. This incomplete set of data can bedigitally processed to yield images similar to conventional
tomography with a
limited depth of field. However, because the image processing isdigital, a series of slices
atd i f f e r e n t d e p t h s a n d w i t h d i f f e r e n t t h i c k n e s s e s c a n be r ec on s t r u c t ed f r o m t h e s a m eacquisition, saving both time
and radiationexposure .Because the da ta acqui red i s incomple te , tomo
sy nt he si s is un ab le t o of fe r th eextremely narrow slicewidths that CT offers. However, higher resolution detectors can
beused, allowin g very-high i n-plane resolu tion, ev en if theZ-axis resolution is poor. The primary interest in
tomosynthesis is in breast imaging, as an extension tomammography,where it may offer better detection rates with little extra
increase in radiationexposure .Reconst ruc t ion a lgor i thms for tomosynthes is ar e s ign i f i ca n t ly d i f f e r en t f ro mconvent iona l CT, becausethe conventional f i l tered back projection algorithm require
s acomplete set of data. I terative algorithms based upon
expectation maximization aremostcommonly used, but are extremely computationally int
ensi ve. Some ma nu fact ure rs have produced practical systemsusing off-the-shelf GPUs to perform the reconstruction.
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WORKING OF COMPUTED TOMOGRAPHY
Computed Tomography is a powerful nondestructive evaluation(NDE) technique for p ro d u ci n g 2 -D a n d 3 -D c ro ss -
s e c t i o n a l i m a g e s o f a n o b j e c t f r o m f l a t X -ra y im a ge s. Characteris t ics of the internal st ructure of an
object such as dimensi ons, shape, internaldefects, anddensity are readily available from CT images. Shown below is a
schematic of aCT system.
The test component is placed on a turntable stage that is between a
radiation sourceand an imaging system. The turntable and the imagingsystem are connected to a computer sothat x-ray images collected can be
correlated to the position of the test component. Theimaging systemproduces a 2-dimensional shadowgraph image of the specimen just like
afilm radiograph. Specialized computer software makes it possible toproduce cross-sectionalimages of the test component as if it was being
sliced.
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HOW A C.T SYSTEM WORKS
The imaging system provides a shadowgraph of an object ,with the 3-D structurecompressed onto a 2-D plan e. The
density data along one horizontal l ine of the image
isuncompressed and stretched out over an area. This informationby itself is not very useful, but when the test component is
rotated and similar data for the same linear slice is collectedandoverlaid, an image of the cross-sectional density of
the component begins to develop. Tohelp comprehend how thisworks, look at the animation below.In the animation, a single line of
density data was collected when a component was atthe startingposition and then when it was rotated 90 degrees. Use the pull-
ring to stretch outthe density data in the vertical direction. It canbe seen that the lighter area is stretched acrossthe whole region.
This lighter area would indicate an area of less density in thecomponent because imagin g sys tems t ypically glow b righterwhen the y ar e st ruck with an incr eased amount of radiation.When the information from the second line of data is stretched
acrossand averaged with the first set of stretched data, itbecomes apparent that there is a less densearea in the upper
r ight quadrant of the component 's cross-section. Datacoll ect ed at moreangles of rotation and merged together will
further define this feature. In the movie below, aCT i m a g e o fa c a s t i n g i s p r o d u c e d . I t c a n b e s e e n t h a t t h e c r o s s -s ec t i on o f t h e c a s t i n g becomes more defined as the casting isrotated, X-rayed and the stretched density informationis added to theimage.In the image below left is a set of cast aluminum tensilespecimens. A radiographicimage of several of these specimens is
shown below right
CT slices through several locations of a specimen are shown in the set ofimages below.A number of s lices through the object can be
reconstructed to provide a 3-D viewof i n t e r n a l a n d e x t e r n a l s t r u c t u r a l d e t a i l s . A s s h o w n b
e l o w, t h e 3 - D i m a g e c a n t h e n b emanipulated and sliced invarious ways to provide thorough understanding of the structure.
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Since i ts introduction in the 1970s, CT has becomean impor tant tool in medical ima gin g t o s up pl em ent X-
r a y s a n d m e d i c a l u l t r a s o n o g r a p h y . A l t h o u g h i t i s s t i l lq u i te expensive, i t is the gold standard in the diagnosis of
a large num ber of diff erent dis easeentities. It has morerecently begun to also be used for preventive medicine or
screening for disease, for example CT colonography for patients with ahigh risk of colon cancer. Althougha number of institutions offer full-
body scans for the general population.
HEAD
CT scanning of the head is typically used to detect:1.bleeding, braininj ury and sku ll f ract ures2.bleeding due to
a ruptured/leaking aneurysm in a patient with a suddensevereheadache3.a blood clot or bleeding within the brain shortly
after a patient exhibits symptoms ofastroke4 . a s t r o k e 5 . b r a i n t u m o r s 6.enlarged brain
cavit ies in patients withhydrocephalus7.diseases/malformations of the skull8.diagnose diseases of the temporal bone on the side of the skull, which
may be causinghearing problems9.plan radiation therapy forcan cer of t he b rai n o r o ther t is sues10.guide the passage of a
needle used to obtain a tissue sample (biopsy) from the brain
CHEST
CT can be used for detecting both acute and chronic changes inthe internals of thelungs. It is particularly relevant here because
normal two dimensional x-rays do not showsuch defects. Forevaluation of chronic interstitial processes thin sections with
high spatialfrequency reconstructions are used -often scans are performed
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both in inspiration andexpiration. This special technique iscalled High Resolution CT (HRCT). HRCT is normally
done with thin section with skipped areas between the thinsections. Therefore it produces asampling of the lung and not
continuous images. Continuous images are provided inastandard CT of the chest.For detection of airspace disease
(such as pn eumonia) or cancer , relat ivelyt hi ck s e c t i o n s a n d g e n e r a l p u r p o s e i m a g e r e c o n s t r u c t i o
n t ec hn iqu es ma y be a de qu a te . CTangiography of thechest is also becoming the primary method for detecting
pulmonaryembolism (PE) and aort ic dissection, and requires accurately timed rapid injections of contrast (Bolus Tracking)
and high-speed helical scanners. Cardiac CTA is now being usedtodiagnose coronary artery disease.
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PULMONARY ANGIOGRAM
CT pulmonary angiogram
(CTPA) is a medical diagnostic test used todiagnose pulmonary embolism (PE). I t employs computed
tomography to obtain an image of the pulmonaryarteries.MDCT (multi detector CT) scanners give the optimum
resolution and image qualityfor th is tes t. Imag es are usua llytaken on a 0.625 mm slice thickness, al though 2 mm
iss u f f i c i e n t . 5 0 -1 0 0 m l s o f c o n t r a s t i s g i v e n t o t h e p a t i e n t a t a r at e o f 4 m l / s . T h e tracker/locator is placed at the level of the
Pulmonary Arteries, which sit roughly at the levelof the carina
This is done using bolus tracking.Example of a CTPA, demonstrating a saddle embolus (dark horizontal
line) occluding the pulmonaryarteries (bright white triangle)
CARDIAC
With the advent of sub second rotation combined with multi-slice CT(up to 64-slice),high resolution and high speed can be obtained at
the same time, allowing excellent imagingof th e corona ry
arteries (cardiac CT angiography). Images with an evenhi gher temp oral resolution can be formed using retrospective
ECG gating. In this technique, each portion of the heart isimaged more than once while an ECG trace is recorded. The
ECG is then used tocorrelat e t he CT dat a with thei rcorresponding phases of cardiac contraction. Once
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thiscorrelation is complete, all data that were recorded while theheart was in motion (systole)can be ignored and images can be made
from the remaining data that happened to be acquiredwh i l e t h eh e a r t w a s a t r e s t ( d i a s t o l e ) . I n t h i s w a y , i n d i v i d u a l
f r a m e s i n a c a r d i a c C Tinvestigation have a better temporalresolution than the shortest tube rotation time.Methods are available
to decrease this exposure, however, such asprospectivelydecreasing radiation output b ased on the
concurrent ly a cquired ECG. Th is can result in asignificantdecrease in radiation exposure, at the risk of
compromising image quality if thereis any arrhythmia during theacquisition. The significance of radiation doses in the
diagnosticimaging range has not been proven, although thepossibility of inducing an increased
cancer r i sk a c r os s a p opu l a t i on i s a sou r c e o f s i gn i f i c ant con cer n . Thi s p ot ent ia l r i sk mu st be weighed againstthe competing risk of not performing a test and potentially not
diagnosing asignificant health problem such as coronary arterydisease.Dual Source CT scanners, introduced in 2005, allow
higher temporal resolution byacquiring a full CT slice in onlyhalf a rotation, thus reducing motion blurring at high heartrates
and potentially allowing for shorter breath-hold time. This isparticularly useful for ill patients who have difficulty holding their
breath or who are unable to take heart-rate loweringmedication.
ABDOMINAL AND PELVIC
CT is a sensitive method for diagnosis of abdominal diseases. It is used
frequently todetermine stage of cancer and to follow progress. It is also auseful test to investigate acuteabdominal pain,Renal stones, appendicitis,
pancreatitis, diverticulitis, abdominal aorticaneurysm, and bowelobstruction are conditions that are readily diagnosed and assessed
withCT. CT is also the first line for detecting solid organ injury aftertrauma
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EXTREMITIES
CT is often used to image complex fractures, especially onesaround joints, because of itsability to reconstruct the area of
interest in multiple planes. Fractures, ligamentous injuriesand
dislocations can easily be recognised with a 0.2 mm resolution.
ADVANTAGES AND HAZARDS
ADVANTAGES OVER TRADITIONAL RADIOGRAPHYThere are several advantages that CT has over traditional 2D medical
radiography.CT completely eliminates the superimposition ofimages of s tru ctu res ou tsi de t he a rea of interest. Because ofthe inherent high-contrast resolution of CT, differences between
tissuesthat differ in physical density by less than 1% can bedistinguished.Data from a single CT imaging procedure
consist ing of eit her mu ltiple conti guous or on ehelical scancan be viewed as images in the axial, coronal, or sagittal planes,
depending onthe diagnostic task. This is referred to as multiplanarreformatted imaging.CT is regarded as a moderate to high radiation
diagnostic technique. While technicaladvances have improvedradiation efficiency, there has been simultaneous pressure toobtainhigher-resoluti on imagi ng and use more comp lexscan techniques, bot h of which requ irehigher doses of
radiation. The improved resolution of CT has permitted thedevelopment of new investigations, which may have advantages;
compared to conventional angiography for example,CT angiography avoids the invasive insertion of an arterial
catheter and guidewire;CT colonography (also known as virtualcolonoscopy or VC for short) may be as useful as a b a r i u m
e n e m a f o r d e t e c t i o n o f t u m o r s , b u t m a y u s e a l o w e r ra d i a t i on d o s e. C T VC i s increasingly being used in the UK
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as a diagnostic test for bowel cancer and can negate theneed for acolonoscopy.The gr eat ly increa se d ava ila bi lit y of CT,
together with i ts va lue for an increasin gnumber ofconditions, has been responsible for a large rise in popularity. So
large has beenthi s ri se th at, in t he most rec entcomprehensive survey in the United Kingdom, CT
scansconsti tuted 7% of al l radiologic examinations, butcont ribu ted 47% of th e to ta l co ll ec ti vedose from medical X-ray
examinations in2000/2001.The rad ia t i on do se fo r a pa r t i cu la r s tu dy de pe
nd s on mu lt ip le fa ct or s : vo lu me scanned, patient build,number and type of scan sequences, and desired resolution
and imagequality. Additionally, two helical CT scanning parametersthat can be adjusted easily and thathave a profound effect on radiation
dose are tube current and pitch.Safety Concerns.T h e i n c r e a s e d u s e o f C T s c a n s h a s b e e n t h e g r e a t e s t
in two f i e ld s : sc re eni ng of ad ul t s (screening CT of
the lung in smokers, virtual colonoscopy, CT cardiac screening
and whole- body CT in asymptomatic patients) and CT imaging
of children. Shortening of the scanningtime to around one second,
eliminating the strict need for subject to remain still or be sedated,is one
of the main reasons for large increase in the pediatric population
(especially for thedia gnos is of app endi citi s). CT scans o f
children have b een est imated to p roduce n on-negligible
increases in the probability of lifetime cancer mortality leading
to calls for the useof reduced curr ent sett ings f or C T sc ans
of children. These calculations are based on theassumption
of a l inear relat ionship b etween radiation dose and cancer r isk; this claim iscontroversial , as some but not al l
evidence shows that smaller radiation doses are
le ssharmful. Estimated lifetime cancer mortality
risks attributable to the radiation exposure froma CT in a 1-year-
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old are 0.18% (abdominal) and 0.07% (head)an order of magnitude
higher than for adultsalthough those figures still represent a
small increase in cancer mortalityover the background rate. In the
United States, of approximately 600,000 abdominal and headCTexaminations annually performed in children under the age of
15 years, a rough estimateis that 5 00 of these individuals
might ult imately die from cancer at tr ibutable to the
CTradiation . The additional risk is still very low (0.35%)
compared to the background risk of dying from cancer (23%).
However, if these statistics are extrapolated to the current
number of CT scans, the addit ional rise in cancer mortality could
be 1.5 to 2%. Furthermore, certainconditions can require children
to be exposed to multiple CT scans. Again, these calculationscan
be problematic because the assumptions underlying them could
overestimate the risk.CT scans can be performed with different
settings for lower exposure in children,although these
techniques are often not employed. Surveys have suggested that
currently,many CT scans are performed unnecessarily.
Ultrasound scanning or magnetic resonanceimaging is
alternatives (for example, in appendicit is or brain imaging)
without the r isk of radiation exposure. Although CT scans
come with an ad dition al risk of cancer (i t can beestimated
that the radiation exposure is the same as standing 2.4km away
from the WWIIatomic bomb blasts in Japan), especially in
children, the ben efits t hat s tem from th eir useoutweighs
the risk in many cases. Studies support informing parents ofthe risks of pediatricCT scanning ADVERSE REACTIONS TO
CONTRAST AGENTS
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Because contrast CT scans rely on intravenously administered contrastagents in order to provide superior image quality, there is a lowbut non-negligible level of risk associatedwith the contrast
agents themselves. Many patients report nausea and discomfort,
includingwarmth in the crotch which mimics the sensation ofwetting oneself. Certain patients mayexperience severe and
potentially life-threatening allergic reactions to the contrast dye.Thecontrast agent may also induce kidney damage. The risk of this
is increasedwith pa t i e n t s w ho ha ve p r e e x i s t i n g r e na l i n su f f i c i e nc y, pr ee xi st in g di ab et es , or re du ce dintravascular volume. In
general, if a patient has normal kidney function, then the risks
of contrast nephropathy are negligible. Patients with mild kidney impai rm en t are usu allyadvised to ensure full hydration for
several hours before and after the injection. For moderatekidneyfailure, the use of iodinated con trast should be avoided;
th is ma y m ean us in g analternative technique instead of CT e.g.MRI. Perhaps paradoxically, patients with severerenal failurerequiring dialysis do not require special precautions, as theirkidneys have solittle function remaining that any further damage
would not be noticeable and the dialysis willremove the contrast agent.
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MAGNETIC RESONANCE IMAGING
Magnetic Resonance Imaging(MRI) ,o r
nuclear magnetic resonance imaging(NMRI),
is primarily a medical imaging technique most commonlyused in radi ology to visualize the internal structure and
function of the body. MRI provides much greatercontrast between the different soft tissues of the body
than computed tomography (CT) does, makingit especially usefulin neurological (brain), musculoskeletal,
cardiovascular, and oncological(cancer) imaging. Unlike CT, it uses
no ionizing radiation, but uses a powerful magnetic fieldto align thenuclear magnetization of (usually) hydrogen atoms in water in
the body. Radiofrequency (RF) fields are used to systematicallyalter the alignment of this magnetization,causing the hydrogen
nuclei to produce a rotating magnetic field detectable by thescanner.This signal can be manipulated by additional magneticfields to build up enough informationto construct an image of the
body.Magnetic Resonance Imaging is a relatively new technology. Thefirst MR image was publ ished in 1973 an d th e fi rs t cross-
sectional image of a l iving mouse was publishedinJ a n u a r y 1 9 7 4 . T h e f i r s t s t u d i e s p e r f o r m e d o n h u
m a n s w e r e p u b l i s h e d i n 1 9 7 7 .
Bycomparison, the first human X-ray image was taken in1895.Magnetic Resonance Imaging was developed from
knowledge gained in the study of nuc lear magneti c resonance.In its early years the technique was referred to as
nuclear magnetic resonance imaging (NMRI). However, as theword nuclearwas associated in the
public mind with ionizing radiation exposure it is generallynow referred to simply as MRI.Sci ent is ts st ill us e t he te rm
NMRI when discussing non-medical devices operating on
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thesame principles. The term Magnetic Resonance Tomography (MRT)is also sometimes used.
HOW MRI WORKS
The body is largely composed of water molecules which eachcontain two hydrogennuclei or protons. When a person goes
inside the powerful magnetic field of the scanner, themagneticmoments of these protons align with the direction of the field.A radio
frequency electromagnetic field is then briefly turned on, causingthe protonsto alter their alignment relative to the field. When thisfield is turned off the protons return tothe original magnetization
alignment. These alignment changes create a signal which can
bedetected by the scanner. The frequency the protons resonate atdepends on the strength of themagnetic field. The position of protonsin the body can be determined by applying additionalmagnetic fields
during the scan which allows an image of the body to be built up.These arecreated by turning gradients coils on and off which
creates the knocking sounds heard duringan MR scan.Di seasedtissue, such as tumors, can be detected because the protonsin di ffer en ttissues return to their equilibrium state at differentrates. By changing the parameters on thescanner this effect is used
to create contrast between different types of body tissue.Contrastagents may be injected intravenously to enhance the appearanceof bloodvessels, tumors or inflammation. Contrast agents may
also be directly injected into a joint inthe case of arthrograms, MRimages of joints. Unlike CT, MRI uses no ionizing radiation andis
generally a very safe procedure. Patients with some metal implants,cochlear implants, andcardiac pacemakers are prevented from
havin g an MRI sca n due to e ffect s o f the st ron gmagnetic field
and powerful radio frequencypulses.M R I i s u s e d t o i m a g e e v e r y p a r t o f t h e b o d y ,
a n d i s p a r t i c u l a r l y u s e f u l f o r neurological conditions,for disorders of the muscles and joints, for evaluating tumors,
andfor showing abnormalities in the heart and blood vessels
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THREE-DIMENSIONAL (3D) IMAGE RECONSTRUCTION
THE PRINCIPLE
Because contemporary MRI scanners offer isotropic, or n ear isotropic, res olu tion,display of images does not need to be
restricted to the conventional axial images. Instead, it is possible for asoftware program to build a volume by 'stacking' the individual
slices one ontop of the other. The program may then display thevolume in an alternative manner.3D RENDERING TECHNIQUES
SURFACE RENDERING
A threshold value of greyscale density is chosen by theoperator (e.g. a level
t ha tc o r r e s p o n d s t o f a t ) . A t h r e s h o l d l e v e l i s s e t , u s i n gedge de tec t ion image process inga lgor i thms. From this, a
3-dimensional model can be constructed and displayed onscreen.Multip le models c an b e const ructed from vari ous
di ffe ren t th resholds , a llowi ng di ffe ren tcolors to represent eachanatomical component such as bone, muscle, and cartilage. However,theinterior structure of each element is not visible in this mode of operation.
VOLUME RENDERING
S u r f a c e r e n d e r i n g i s l i m i t e d i n t h a t i t w i l l o n l y d i s p l ay s u r f a c es wh i c h m e e t a threshold density, and will only display
the surface that is closest to the imaginary viewer. Involume rendering,
transparency and colors are used to allow a better representationof thevolum e to be sh own in a single im age - e.g. t he bon es
of the pelvis could b e displayed assemi-transparent,so that even at an oblique angle, one part of the image
does not concealanother.
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IMAGE SEGMENTATION
Where different structures have similar threshold density, it can becomeimpossible toseparate them simply
by adjusting volume rendering parameters. The solutionis calledsegmentation, a manual or automatic procedure that can
remove the unwanted structures fromthe image.
COMPUTED TOMOGRAPHY VERSUS MRI
A computed tomography (CT) scanner uses X-rays, a type ofionizing radiation, toacquire its images, making it a good tool for
examining tissue composed of elements of ahigher atomicnumber than the t issue surrounding them, such as bone and
ca lc if ic at ions (calcium based) within the body (carbon basedflesh), or of structures (vessels, bowel). MRI,on the other hand,
uses non-ionizing radio frequency (RF) signals to acquire its images andis best suited for non-calcified tissue, though MR images can
also be acquired from bones andteeth as well as fossils.CT may beenhanced by use of contrast agents containing elements of a higher
atomicnumber than the surrounding flesh such as iodine orba rium. Cont rast agen ts for M RI ar ethose which have
paramagnetic properties, e.g. gadolinium and manganese.Both CT andMRI scanners can generate multiple two-dimensional
cross- sect ions (slices) of tissue and three-dimensionalreconstructions. Unlike CT, which uses only X-ray
attenuation to generate image contrast, MRI has a long list ofproperties that may be used togenerate image contrast. By
variation of scanning parameters, tissue contrast can be
alteredand enhanced in various ways to detect different features.MRIcan generate cross-sectional images in any plane (including
oblique planes). Inthe past, CT was limited to acquiring images in theaxial (or near axial) plane. The scans usedto be called Computed
Axial
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Tomography scans (CAT scans). However, the developmentof multi-detector CT scanners with near-isotropic resolution,
allows the CT scanner to produceda ta that can beretrospectively reconstructed in any plane with minimal
loss of imagequality.For purposes of tumor detection andident if icat ion in the brain, MRI is general lysu pe r i or .
H o w e v e r , i n t h e c a s e o f s o l i d t u m o r s o f t h e a b d o m e nan d ch es t , CT is of t en prefe r red due to less mot ion
art ifact . Furthermore, CT usually is more widelyavailable,faster , much less expensive, and may be less
lik ely t o r equ ire the pers on to be sed ated or anesthetized.MRIis also best suited for cases when a patient is to undergo the
exam several timessucce ssiv ely in the shor t t erm, beca use,un like CT, it does not expose t he p ati ent to thehazards of
ionizing radiation.
ECONOMICS OF MRI
MRI equipment is expensive. 1.5 tesla scanners oftencos t b etween $1 mi llion and$1.5 million USD. 3.0 tesla
scanners often cost between $2 million and $2.3 millionUSD.Construction of MRI suites can cost up to $500,000
USD, or more, depending on projectscope.
SAFETY
Death and injuries have occurred from projecti les createdby the magnetic field,although few compared to the
mill ions of examinations administered. MRI makes use
of powerful magnetic f ields which, though they have notbeen demonstrated to cause direct biological damage, can
interfere with metallic and electromechanical devices.Additional(small) risks are presented by the radio frequency
systems, components or elements of the
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MRI system's operation, elements of the scanning procedure andmedications that may beadministered to facilitate MRI imaging.Thereare many steps that the MRI patient and referring physician can
take to help reduce theremaining risks, including providing a full,
accurate and thorough medical history to the MRI provider.
EXPERIMENTAL EXAMPLE FOR C T
COMPUTED TOMOGRAPHY,HEAD
Computed axial tomography (CAT), computer-assisted tomography, computedtomography, CT, or body
section roentgenography is the process of using digital
processingto generate a three-dimensional image of the internals of anobject from a large series of two-d i m en s i o n a l x -
r a y i m a g e s t a k e n a r o u n d a s i n g l e a x i s o f r o t a t i o n . S om et i m es c on t r a s t materials such as barium (administeredorally or rectally) or intravenous iodinated contrastare used.
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INDIVIDUAL IMAGES OF CT
A volume rendering of this volume clearly shows the high densitybones.
Fig.,Bone reconstructed in 3DAfter using a segmentation tool to remove the bone, thepreviously concealed vessels cannow be demonstrated.
Medical Image Processing TechniquesFig.,Brain vessels
reconstructed in 3D after bone has been removed by segmentation
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CONCLUSION
Image processing is any form of signal processing for which the input isan image andthe output of image processing can be either an image or a
set of characteristics or parametersrelated to the image.Most image-processing techniques involve treating the image as a two-dimensionalsignal and applying standard signal-processing
techniques. Recently, techniques have beendeveloped to enable CT,MRI and ultrasound scanning software to produce 3D images for
the physician.Traditionally CT and MRI scans produced 2D staticoutput on film. To produce 3Dimages , ma ny sca ns
are made, and then combined by computers to produce
a 3D model,which can then be manipulated by thephysici an. 3D u ltras ounds are p rodu ced usi ng asomewhat
similar technique.Computer tomography is a medical imagingmethod employing tomography created by computer processing.
Digital geometry processing is used to generate a three-dimensionalimage of the inside of an object from a large series
of two-dimensional X-ray images takenaround a single axis of rotation.
(CT) is a powerful nondestructive evaluation (NDE) techniquefor producing 2-D and 3-D cross-sectional images of an object from flat
X-ray images.The Italian radiologist Alessandro Vallebona proposed a methodto represent a singleslice of the body on the radiographic film.This method was known as tomography. The firstcommerciallyviable CT scanner was invented by Sir Godfrey Hounsfield inHayes, UnitedKingdom at EMI Central Research Laboratories usingX-rays.MRI can generate cross-sectional images in any plane
(including oblique planes). Inthe past, CT was limited to acquiringimages in the axial (or near axial) plane. The scans usedto be called
ComputedAxial
Tomography scans (CAT scans). However, the developmentof multi-detector CT scanners with near-isotropic resolution,
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allows the CT scanner to produceda ta that can beretrospectively reconstructed in any plane with minimal
lo ss of imag equality.
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REFERENCES
1.http://www.ndted.org/EducationResources/Radiography/AdvancedTechniques
2.http:/ /www.picturenewsletter .com/index
3.http:/ /wikipedia.com/Imageprocessing/
4.http:/ /en.wikipedia.org/wiki/MRI
5 . h t t p : / / w w w . o r c h i d - t e c h . c o m /
6.http:/ /www.medicalimaging.org/
7.http://en.wikipedia.org/wiki/Magnetic_resonance_imaging/
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