Medical Imaging(Nuclear and Radiation)

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MEDICAL

IMAGINGMohd Aidil UbaidillahD20091035132


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WHAT IS MEDICAL IMAGING

Medical imaging is the technique and process usedto create images of the human body (or parts andfunction thereof) for clinical purposes (medicalprocedures seeking to reveal, diagnose or examine

disease) or medical science (including the study ofnormal anatomy and physiology). Althoughimaging of removed organs and tissues can beperformed for medical reasons, such procedures

are not usually referred to as medical imaging, butrather are a part of pathology.


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HISTORY

Radiology began as a medical sub-specialty in firstdecade of the 1900's after the discovery of x-raysby Professor Roentgen.

The development of radiology grew at a good paceuntil World War II.

Extensive use of x-ray imaging during the secondworld war, and the advent of the digital computer

and new imaging modalities like ultrasound andmagnetic resonance imaging have combined tocreate an explosion of diagnostic imagingtechniques in the past 25 years.

Over the past 100 years, the technologicaladvances of x-ray tubes, power generation,


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DEVELOPMENT

Film Cassettes For the first fifty years of radiology, the primary

examination involved creating an image byfocusing x-rays through the body part of interest

and directly onto a single piece of film inside aspecial cassette.

In the earliest days, a head x-ray could require upto 11 minutes of exposure time. Now, modern x-

rays images are made in milliseconds and the x-raydose currently used is as little as 2% of what wasused for that 11 minute head exam 100 years ago.

Further, modern x-ray techniques (both anal of film

screen systems and digital systems, describedbelow) have significantly more spatial resolution


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Fluorescent Screens

The next development involved the use fluorescent

screens and special glasses so the doctor could seex-ray images in real time.

This caused the doctor to stare directly into the x-ray beam, creating unwanted exposure to

radiation. In 1946, George Sc hoenander developedthe film cassette changer which allowed a series ofcassettes to be exposed at a movie frame rate of1.5 cassettes per second.

By 1953, this technique had been improved toallow frame rates up to 6 frames persecond by using a

special"cut film changer."


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Contrast Medium

A major development along the way was the

application of pharmaceutical contrast medium tohelp visualize organs and blood vessels with moreclarity and image contrast.

These contrast media agents (liquids also referred

to as "dye") were first administered orally or viavascular injection between 1906 and 1912 andallowed doctors to see the blood vessels, digestiveand gastro-intestinal systems, bile ducts and gallbladder for the first time.


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Image Intensifier

In 1955, the x-ray image intensifier (also called I.I.)was developed.it is an imaging component whichconverts x-rays into a visible image (real timeimage).

By the 1960's, the fluorescent system (which hadbecome quite complex with mirror optic systems tominimize patient and radiologist dose) was largelyreplaced by the image intensifier/TV combination.

Together with the cut-film changer,the image Intensifier opened the way

for a new radiologic sub-specialtyknow as angiography to

blossom and allowed the

routine imaging of blood


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Nuclear Medicine

Nuclear Medicine studies (also called radionuclide

scanning) were first done in the 1950s usingspecial gamma cameras.Nuclear medicine studiesrequire the introduction of very low-levelradioactive chemicals into the body. Theseradionuclides are taken up by the organs in the

body and then emit faint radiation signals whichare measured or detected by the gamma camera.

Positron Emission Tomography or PET

It is a nuclear medicine scan that uses crosssectional data and reconstructs it as an image,much like CT scanning, but can see specificproblems better such as brain tumors, and theheart and lungs.

The most recent innovation in PET has lung cancer


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Single-photon emission computed

tomography (SPECT) a 3D tomographic technique that uses gamma

camera data from many projections and can bereconstructed in different planes.

A dual detector head gamma camera combinedwith a CT scanner, which provides localization offunctional SPECT data, is termed a SPECT/CTcamera, and has shown utility in advancing the

field of molecular imaging. In most other medical imaging modalities, energy

is passed through the body and the reaction orresult is read by detectors. In SPECT imaging, the

patient is


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Ultrasound Scanning

In the 1960's the principals of sonar (developedextensively during the second world war) wereapplied to diagnostic imaging.

The process involves placing a small device called

a transducer, against the skin of the patient nearthe region of interest, for example, the kidneys.

This transducer produces a stream of inaudible,high frequency sound waves which penetrate into

the body and bounce off the organs inside. The transducer detects sound waves as they

bounce off or echo back from the internalstructures and contours of the organs. These

waves are received by the ultrasound machine andturned into live pictures with the use of computers


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Ultrasound representation ofUrinary bladder (black butterfly-like shape) and hyperplasticprostate


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Digital Imaging Techniques

Digital imaging techniques were implemented in

the 1970's with the first clinical use andacceptance of the Computed Tomography or CTscanner, invented by Godfrey Hounsfield.

Analog to digital converters and computers were

also adapted to conventional fluoroscopic imageintensifier/TV systems in the 70's as well.

Angiographic procedures for looking at the bloodvessels in the brain, kidneys, arms and legs, and

the blood vessels of the heart all have benefitedtremendously from the adaptation of digitaltechnology.


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Benefits of digital technology to all x-ray systems:

less x-ray dose can often be used to achieve thesame high quality picture as with film

digital x-ray images can be enhanced andmanipulated with computers

digital images can be sent via network to otherworkstations and computer monitors so that manypeople can share the information and assist in thediagnosis

digital images can be archived onto compactoptical disk or digital tape drives savingtremendously on storage space and manpowerneeded for a traditional x-ray film library

digital images can be retrieved from an archive at


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Computed Tomography (CT)

CT imaging (also called CAT scanning for ComputedAxial Tomography) was invented in 1972 by GodfreyHounsfield in England.

Hounsfield used gamma rays (and later x-rays) anda detector mounted on a special rotating frametogether with a digital computer to create detailedcross sectional images of objects.

Hounsfield's original CT scan took hours to acquire asingle slice of image data and more than 24 hoursto reconstruct this data into a single image.

Today's state-of-the-art CT systems can acquire asingle image in less than a second and reconstructthe image instantly.


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An original head-only CT

scanner from 1974

A CT scan image showing aruptured abdominal aorticaneurysm


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Magnetic Resonance (MR)

MR principals were initially investigated in the

1950s showing that different materials resonatedat different magnetic field strengths.

Magnetic Resonance (MR) Imaging (also know asMRI) was initially researched in the early 1970s and

the first MR im aging prototypes were tested onclinical patients in 1980.

MR imaging was cleared for commercial,clinicalavailability by the Food and Drug Administration

(FDA) in 1984and its use throughout the U.S.

has spread rapidly since.

Countless scientists have been involved in

the innovation of ma netic resonance.The


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HOW ITS WORK?

In nuclear medicine imaging the use of multi-detector systems for total-body, brain and heartscanning have recently gained increasingpopularity.

There is a strong development of the imagingsystems with a large number of tiny crystals madeby the newly developed high dense and fastresponding materials.


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METHODSPrinciples of image detection

single large scintillation crystal with a largenumber of photo multiplier tubes (PMT):

In the first method the gamma ray strikes a largecircular or rectangle thin crystal and the inducedscintillation light is distributed between the PMTs(Fig. 1A) according to their viewing spatial angles.

The x and y positions of PMTs are weighted by theirelectric signal responses from all PMTs and the Xand Y coordinates of the scintillation cloud strikingthe array of PMT and the corresponding energy iscomputed (Anger type of gamma camera).

The intrinsic spatial resolution of the imagingdevice strongly depends on the crystal thickness,


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Large number of tiny scintillation crystalswith position sensitive photo-multiplier tube(PSPMT):

In the second method (Fig. 1B) the gamma ray isabsorbed in a tiny crystal and all the inducedscintillation light is collected by a small area ofposition sensitive photo multiplier tube which

converts the incident light in a very thin layer intoa charge or current which is then converted todigital E (energy) signal.(electron to electric signal)

Each small sensitive area of PSPMT provides also

corresponding spatial coordinates X and Y for theparticular exposed crystal.

The spatial resolution strongly depends on the sizeof the crystals and on the thickness and material of

the septa. Each crystal represents a pixel in the


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If gamma ray strikes the crystal too close to thereflector cover or to the optical fiber (crystal isinside optical fiber) then some of the ionizationdoesnt contribute to the scintillation light andtherefore the energy signal is smaller. Because of

this, a definite volume close to the edge, is notuseful and is treated as scattered.

For approximately 20 % of absorbed gamma rays,the reduction of the scintillation light will be

present. Some of the signals coming from these 20% will still be included in the lower part of thephoto peak but some will be lost.

The conclusion is that there is no meaning of using

thinner sized crystals than 1-2 mm depending on


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Large semiconductor crystal with array oftiny n-p sensitive areas:

In the third method (Fig. 1C) the incident gammaray is absorbed in the region of p-n junctionregion of the semiconductor crystal and a largenumber of electron-hole pairs are created.

Their number (approximately 3 - 5 eV/electron-holepair is spent on average) is proportional to theenergy of the gamma ray and is nearly ten timesgreater than the quantity of the scintillation light(approximately 30 eV per ionization).

For the same factor the energy resolution is better.The efficiency of the late developed (cadmium zincTelluride) CZT crystal is even better than for NaI.The intrinsic spatial resolution of the CZT gammacamera is considerably better and is about 2 - 3


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Picture 1. Comparison between CZT camera and NaI Anger camerabreast scans.


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EFFECTS OF MEDICAL IMAGING

Radiation damages the cell by damaging DNAmolecules directly through ionizing effects on DNAmolecules or indirectly through free radicalformation.

Deterministic effects:

such as cell killing, can be more immediate andhave a threshold above which severity increaseswith radiation dose. However, the threshold is notnecessarily the same in each individual or tissue.While healing may ensue, necrosis and fibroticchanges in internal organs, acute radiationsickness, cataracts, and sterility may also occur.For acute deterministic effects, large doses are

usually required,


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hereditary effects:

Radiation damage to the gonads during thereproductive period of life produces mutations tothe gametes. Inherited diseases can encompass arange of mild disorders to serious consequences,

including death or severe mental defects. However,no human population studies have shownhereditary effects from typical background ionizingradiation doses.


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SAFETY/PRECAUTION

Physicians should limit ionizing radiation examsv especially CT scans, which produce a massive

radiation dose.

Physicians must consider the consequences of

ionizing radiation in ordering radiology exams

v This involves things like selecting the rightprotocols, being sure that the examination is theright examination at the right dose for the right

patient, and keeping the dose as low as possiblebut adequate enough to get the results that areneeded to make an accurate diagnosis

Equipment needs to be regularly calibrated and all

safety features must be in working order.


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Patients Must Be Proactive About the Imaging TestsThey Undergo

v Patients should monitor their own exposure tomedical radiation and make their physician awarewhen a test is being ordered that they might haveundergone the same test in the past

Physicians should consider for patients who arereferred for additional imaging whether there is analternative imaging test that involves less radiationexposure than what is being ordered

v The amount of radiation you get is very muchrelated to your body habitus; if you are thinner,you will get a lower dose of radiation


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CONCLUSION

An imaging-based trial will usually be made up ofthree components:

A realistic imaging protocol. The protocol is anoutline that standardizes (as far as practicallypossible) the way in which the images are acquiredusing the various modalities (PET, SPECT, CT, MRI).It covers the specifics in which images are to bestored, processed and evaluated.

An imaging centre that is responsible forcollecting the images, perform quality control andprovide tools for data storage, distribution andanalysis It is important for images acquired at

Medical Imaging(Nuclear and Radiation)

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