Medical Image Processing Techniques

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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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Medical Image Processing Techniques

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