Spect Final

8/4/2019 Spect Final

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ABSTRACT

Single photon emission computed tomography (SPECT) is a nuclearmedicine tomographic

imaging technique using gamma rays. It is very similar to

conventional nuclear medicine planar imaging using a gamma camera. However, it is

able to provide true 3D information. This information is typically presented as cross-

sectional slices through the patient, but can be freely reformatted or manipulated as

required.

The basic technique requires injection of a gamma-emitting radioisotope calledradionuclide) into the bloodstream of the patient. Occasionally the radioisotope is a

simple soluble dissolved ion, such as a radioisotope of gallium(III), which happens to

also have chemical properties which allow it to be concentrated in ways of medical

interest for disease detection. However, most of the time in SPECT, a marker

radioisotope, which is of interest only for its radioactive properties, has been attached to

a special radioligand, which is of interest for its chemical binding properties to certain

types of tissues. This marriage allows the combination of ligand and radioisotope (the

radiopharmaceutical) to be carried and bound to a place of interest in the body, which

then (due to the gamma-emission of the isotope) allows the ligand concentration to be

seen by a gamma-camera.

SPECT can be used to complement any gamma imaging study, where a true 3D

representation can be helpful. e.g. tumor imaging, infection (leukocyte) imaging,

thyroid imaging or bone imaging. SPECT imaging is performed by using a gamma

camera to acquire multiple 2-D images (also called projections), from multiple angles.

A computer is then used to apply a tomographic reconstruction algorithm to the multiple

projections, yielding a 3-D dataset. This dataset may then be manipulated to show thin

slices along any chosen axis of the body, similar to those obtained from other

tomographic techniques, such as MRI, CT, and PET.


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CONTENTS

1. INTRODUCTION 1

2. THEORY AND INSTRUMENTATION 7

3. THE GAMMA CAMERA 9

4. WORKING 13

5. ACQUISITION PROTOCOLS 15

6. TYPICAL SPECT ACQUISITION PROTOCOLS 16

7. SPECT IMAGE ACQUISITION AND PROCESSING 17

8. RECONSTRUCTION 21

9. SPECT SCAN 25

10. POSITRON EMISSION TOMOGRAPHY 26

11. PET SCAN 27

12. COMPARISON OF PET AND SPECT 28

13. ADVANTAGES 30

14. DISADVANTAGES 31

15. APPLICATION 32

16. MILESTONES 35

17. CONCLUSION 36

18. BIBLIOGRAPHY 37


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1. INTRODUCTION

Emission Computed Tomography is a technique where by multi cross

sectional images of tissue function can be produced , thus removing the effect of

overlying and underlying activity. The technique of ECT is generally considered

as two separate modalities. SINGLE PHOTON Emission Computed Tomography

involves the use single gamma ray emitted per nuclear disintegration. Positron

Emission Tomography makes use of radio isotopes such as gallium-68, when two

gamma rays each of 511KeV, are emitted simultaneously where a positron from

a nuclear disintegration annihilates in tissue.

SPECT, the acronym of Single Photon Emission Computed Tomography

is a nuclear medicine technique that uses radiopharmaceuticals, a rotating camera

and a computer to produce images which allow us to visualize functionalinformation about a patients specific organ or body system. SPECT images are

functional in nature rather than being purely anatomical such as ultrasound, CT

and MRI. SPECT, like PET acquires information on the concentration of radio

nuclides to the patients body.

SPECT dates from the early 1960 are when the idea of emission traverse

section tomography was introduced by D.E.Kuhl and R.Q.Edwards prior to PET,X-ray, CT or MRI. THE first commercial Single Photon- ECT or SPECT

imaging device was developed by Edward and Kuhl and they produce

tomographic images from emission data in 1963. Many research systems which

became clinical standards were also developed in 1980s.


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2. THEORY AND INSTRUMENTATION

Single Photon Emission Computed tomography or what the medical world

refers to as SPECT is a technology used in nuclear medicine where the patient is

injected with a radiopharmaceutical which will emit gamma rays. We seek theposition and concentration of radionuclide distribution by the rotation of a photon

detector array around the body which acquires data from multiple angles. The

radiopharmaceutical may be delivered by 1V catheter, inhaled aerosol etc. The

radio activity is collected by an instrument called a gamma camera. Images are

formed from the 3-D distribution of the radiopharmaceutical with in the body.

Because the emission sources are inside the body cavity, this task is for

more difficult than for X-ray, CT, where the source position and strength are

known at all times.

i.e. In X-ray, CT, the attenuation is measured not the transmission source.

To compensate for the attenuation experienced by emission photons from

injected tracers in the body, contemporary SPECT machines use mathematicalreconstruction algorithms to increase resolution.

Because SPECT acquisition is very similar to planar gamma camera

imaging, the same radiopharmaceuticals may be used. If a patient is examined in

another type of nuclear medicine scan but the images are non-diagnostic, it may

be possible to proceed straight to SPECT by moving the patient to a SPECT

instrument, or even by simply reconfiguring the camera for SPECT imageacquisition while the patient remains on the table.

To acquire SPECT images, the gamma camera is rotated around the

patient. Projections are acquired at defined points during the rotation, typically

every 36 degrees. In most cases, a full 360 degree rotation is used to obtain an

optimal reconstruction. The time taken to obtain each projection is also variable,

but 1520 seconds is typical. This gives a total scan time of 1520 minutes.

Multi-headed gamma cameras can provide accelerated acquisition. For

example, a dual headed camera can be used with heads spaced 180 degrees apart,

allowing 2 projections to be acquired simultaneously, with each head requiring

180 degrees of rotation. Triple-head cameras with 120 degree spacing are also

used.The gamma camera is made up of two or three massive cameras opposite to

each other which rotate around a centre axis, thus each camera moving 180 or

120 degrees respectively. Each camera is lead-encased & weighs about 500

pounds. The camera has three basic layers the collimator (which only allows the


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gamma rays which are perpendicular to the plane of the camera to enter), the

crystal and the detectors. Because only a single photon is emitted from the

radionuclide used for SPECT, a special lens known as a collimator is used to

acquire the image from multiple views around the body .The collimation of the

rays facilitates the reconstruction since we will be dealing with data that comes inonly perpendicular .At each angle of projection, the data will be back projectedonly in one direction.

When the gamma camera rotates around the supine body, it stops at

interval angles to collect data. Since it has two or three heads, it needs to only to

rotate 180 or 120 degrees to collect data around the entire body .The collected

data is planar. Each of the cameras collects a matrix of values which correspond

to the number of gamma counts detected in that direction at the one angle.Images

can be reprojected into a three dimensional one that can be viewed in a dynamic

rotating format on computer monitors, facilitating the demonstration of pertinentfindings to the referring physicians.


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To acquire SPECT images, the gamma camera is rotated around the patient.

Projections are acquired at defined points during the rotation, typically every 36

degrees. In most cases, a full 360 degree rotation is used to obtain an optimalreconstruction. The time taken to obtain each projection is also variable, but 15

20 seconds is typical. This gives a total scan time of 1520 minutes.

Multi-headed gamma cameras can provide accelerated acquisition. For example,

a dual headed camera can be used with heads spaced 180 degrees apart, allowing

2 projections to be acquired simultaneously, with each head requiring 180

degrees of rotation. Triple-head cameras with 120 degree spacing are also used.

Cardiac gated acquisitions are possible with SPECT, just as with planar imaging

techniques such as MUGA. Triggered by Electrocardiogram (EKG) to obtaindifferential information about the heart in various parts of its cycle, gated

myocardial SPECT can be used to obtain quantitative information about

myocardial perfusion, thickness, and contractility of the myocardium duringvarious parts of the cardiac cycle; and also to allow calculation of left ventricular

ejection fraction, stroke volume, and cardiac output.


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5. ACQUISITION PROTOCOLS

5.1. Planar Imaging

The simplest acquisition protocol is theplanar image. With planar imaging,

the detector array is stationary over the patient, and acquires data only from this

one angle. The image created with this type of acquisition is similar to an X-rayradiograph. Bone scans are done primarily in this fashion.

5.2. Planar Dynamic Imaging

Since the camera remains at a fixed position in a planar study, it is possible to

observe the motion of a radiotracer through the body by acquiring a series of

planar images of the patient over time. Each image is a result of summing dataover a short time interval, typically 1-10 seconds. If many projections are taken

over a long time, then an animation of the tracer movement can be viewed and

data analysis can be performed. The most common dynamic planar scan is to

measureglomerular filtration rate in the kidneys.

5.3. SPECT Imaging

If one rotates the camera around the patient, the camera will acquire views of

the tracer distribution at a variety of angles. After all these angles have been

observed, it is possible to reconstruct a three dimensional view of the radiotracer

distribution within the body. This is explained in the section of reconstruction.

5.4. Gated SPECT Imaging

As the heart is a moving object, by performing a regular SPECT of the heart, the

end image obtained will represent the average position of the heart over the time

the scan was taken. It is possible to view the heart at various stages of its

contraction cycle however, by subdividing each SPECT projection view into a

series of sub-views, each depicting the heart at a different stage of it's cycle. In

order to do this, the SPECT camera must be connected to an ECG machine whichis measuring the heart beat.


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7. SPECT IMAGE ACQUSITION & PROCESSING

Single photon emission computer tomography has its goal determination

of the regional concentration of radionuclide with in a specific organ as a

function of time. The introduction of radio isotope TC-99m by Harpen ,which

emits a single gamma ray photon of energy 140 KeV & has a half life of about

six hours signaled a great step forward for SPECT since this photon is easily

detected by gamma cameras . However, a critical engineering problem involving

the collimation of this gamma rays prior to entering the gamma camera have to

be solved before SPECT could establish itself as a viable imaging modality

Single photon emission computed tomography requires collimation ofgamma rays emitted by the radiopharmaceutical distribution within the body

Collimators for SPECT imaging are typically made of lead. They are about 4 to 5

cms thick and 20 by 40 cm on its side. The collimators contain thousands of

square, round or hexagonal parallel channels through which gamma rays are

allowed to pass. Typical low-energy collimators for SPECT weigh about 50 lbs,

but high energy models can weigh above over 200 lbs. Although quiet heavy,

these collimators are placed directly on top of a very delicate single crystal of a

NaI contain within every gamma camera.

Any gamma camera so occupied with a collimator is called an angle

camera after it is invented. Gamma rays traveling along a path that coincides

with one of the collimator channels will pass through the collimator unabsorbed

and interact with the NaI crystal creating light. Behind the crystal, a grid of photo

multiplier tubes collects the light for processing. It is from the analysis of thislight signals that SPECT images are produced .Depending on the size of anger

cameras whole organs such as heart and liver can be imaged. Large anger

cameras are capable of imaging the entire body and are used, for example, for

bone scans.

For the gamma rays emitted by radiopharmaceuticals typical for SPECT,there are two important interactions with matter. The first involves scattering of

the gamma ray off electrons in the atoms and molecules (DNA) within the body.

This scattering process is called Compton scattering. Some Compton scattered

photons are deflected outside the Anger cameras field of view and are lost to the

detection process. The second interaction consists of a photon being absorbed by

an atom in the body with an associated jump in energy level (or release) of an

electron in the same atom. This process is called the photoelectric effect and was

discovered for the interaction of photons with metals by Einstein, who received

the Nobel Prize for this discovery. Both processes result in a loss or degradation

of information about the distribution of the radiopharmaceutical within the body.


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The second process falls under the general medical imaging concept of

attenuation and is an active research area.

Attenuation results in a reduction in the number of photons reaching the

Anger camera. The amount of attenuation experienced by any one photondepends on its path through the body and its energy. Photons which experience

Compton scattering loose energy to the scatterer and are therefore more likely to

be scattered additional times and eventually absorbed by the body or wide-angle

scattered outside the cameras field of view. In either case, the photon (and the

information it carries about the distribution of the radiopharmaceutical in the

body) is not going to be detected and is thus considered lost due to attenuation.

At 14OKeV, Compton scattering is the most probable interaction of a gamma ray

photon with water or body tissue. A much smaller percentage of photons are lost

through the photoelectric interaction. It is possible for a Compton scattered

photon to be scattered into the Anger cameras field of view. Such photonshowever do not carry directly useful information about the distribution of the

radiopharmaceutical within the body since they do not indicate from where

within the body they originated. As a result, the detection of scattered photons in

SPECT leads to loss of image contrast and a technically inaccurate image.

Acquiring and processing a SPECT image, when done correctly, involves

compensating for and adjusting many physical and system parameters. A

selection of these include: attenuation, scatter, uniformity and linearity ofdetector response, geometric spatial resolution and sensitivity of the collimator,

intrinsic spatial resolution and sensitivity of the Anger camera, energy resolutionof the electronics, system sensitivity, image truncation, mechanical shift of the

camera or gantry, electronic shift, axis-of-rotation calibration, image noise,

image slice thickness, reconstruction matrix size and filter, angular and liner

sampling intervals, statistical variations in detected counts, changes in Anger

camera field of view with distance from the source, and system dead time.

Calibrating and monitoring many of these parameters fall under the general

heading of Quality Control and are usually performed by a Certified Nuclear

Medicine Technician or a medical physicist. Among this list, collimation has the

greatest effect on determining SPECT system spatial resolution and sensitivity,

where sensitivity relates to how many photons per second are detected. Systemresolution and sensitivity are the most important physical measures of how well a

SPECT system performs. Improvement in these parameters is a constant goal of

the SPECT researcher. Improvement in both of these parameters simultaneously

is rarely achieved in practice.

7.1 Collimation

Since the time a patient spends in a Nuclear Medicine department relates

directly to patient comfort, there exists pressure to perform all nuclear medicine

scans within an acceptable time frame. For SPECT, this can result in relatively

large statistical image noise due to a limited number of photons detected within


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the scan time. This fact does not hinder our current clinical ability to

prognosticate the diseased state using SPECT, but does raise interesting research

questions. For example, a typical Anger camera equipped with a low-energy

collimator detects roughly one in every ten-thousand gamma ray photons emitted

by the source in the absence of attenuation. This number depends on the type ofcollimator used. The system spatial resolution also depends on the type ofcollimator and the intrinsic (built in) resolution of the Anger camera. A typical

modem Anger camera has an intrinsic resolution of three to nine millimeters.

Independent of the collimator, system resolution cannot get any better than

intrinsic resolution. The same ideas also apply to sensitivity: system sensitivity isalways worse than - and at best equal to intrinsic sensitivity.

A collimator with thousands of straight parallel lead channels is called a

parallel-hole collimator, and has a geometric or collimator resolution that

increases with distance from the gamma ray source. Geometric resolution can bemade better (worse) by using smaller (larger) channels. The geometric

sensitivity, however, is inversely related to geometric resolution, which means

improving collimator resolution decreases collimator sensitivity, and vice versa.

Of course, high resolution and great sensitivity are two paramount goals of

SPECT. Therefore, the SPECT researcher must always consider this trade-off

when working on new collimator designs. There have been several collimator

designs in the past ten years which optimized the resolution/sensitivity inverse

relation for their particular design.

Converging hole collimators, for example fan-beam and cone-beam havebeen built which improve the trade-off between resolution and sensitivity by

increasing the amount of the Anger camera that is exposed to the radionudide

source. This increases the number of counts which improves sensitivity. More

modem collimator designs, such as half-cone beam and astigmatic, have also

been conceived. Sensitivity has seen an overall improvement by the introduction

of multi-camera SPECT systems. A typical triple-camera SPECT system

equipped with ultra-high resolution parallel-hole collimators can achieve a

resolution (measured at full-width half-maximum (FWHM) of from four to seven

millimeters. Other types of collimators with only one or a few channels, called

pin-hole collimators, have been designed to image small organs and humanextremities, such as the wrist and thyroid gland, in addition to research animals

such as rats.

7.2 Computers In Radiology & Nuclear Medicine

Nuclear medicine relies on computers to acquire, store, process and

transfer image information. The history of computers in radiology and nuclear

medicine is however relatively short. In the 1960s and early 1970s, CT and

digital subtraction angiography where introduced into clinical practice for the

first time. Digital subtraction angiography used computers to digitally subtract


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from a standard angiogram the effects of surrounding soft-tissue and bone, thus

improving the image for diagnosis. Computed tomography relied on computers

to digitally reconstruct sectional data using various reconstruction algorithms

such as filtered back projection. The work horse of the CT unit was the

computer; without it CT was impossible. SPECT and MRI first began to appearin the late 1970s. Both of these new imaging modalities required a computer. Inthe case of MRI, the computer played a major role in controlling the gantry and

related mechanical equipment. In the SPECT case, as in CT, image

reconstruction had to be done by computer. Nuclear medicines reliance on

computers also has its roots in high-energy particle physics and nuclear physics.Both of these disciplines rely on statistical analysis of large numbers of photon

(or other particle) counts, collected and processed by a computer.

7.3 Image Acquisition

Nuclear medicine images can be acquired in digital format using a SPECT

scanner. The distribution of radionudide in the patients body corresponds to the

analog image. An analog image is one that has a continuous distribution of

density representing the continuous distribution of radionuclide amassed in a

particular organ. The gamma ray counts coming from the patients body are

digitized and stored in the computer in an array or image matrix. Typical matrix

sizes used in SPECT imaging are 256x256, 128x128, 128x64 or 64x64. The third

dimension in the array corresponds to the number of transaxial, coronal or

sagittal slices used to represent the organ being imaged. A typical SPECT

scanner has a storage limit of 16 bits per pixel.

Once a SPECT scan has been completed, the raw data image matrix is

called projection data and is ready to be reconstructed. The reconstruction

process puts the data in its final digital form ready for transmission to another

computer system for display and physician analysis.


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The most com

clinical data is the filt

1. Data Proje

2. Fourier Tra

3. Data filteri

4. Inverse tra5. Back proje

8.1 Data Projecti

As the SPECT

images called projecti

the camera face pass tfrom various depths

emitting organs along

images acquired at

taken of a patients bo

F

After all proje

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each slice are then or

6.1(b). It represents t

single slice on the ca

18

8. RECONSTRUCTION

on algorithm used in the tomographic

red back projection method. Other meth

tion

nsform of Data

g

sform of the Datation

n

amera rotates around a patient, it creates

ons. At each stop, only photons movin

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a specified path. A SPECT study consis

arious angles. The fig 6.1(a)displays a

ne scan.

ig 6.1(a):Data Projection of Bone Scan

tions are acquired, they are subdivided

, thin slice of the patient at a time. All t

dered into an image called a sinogram

e projection of the tracer distribution i

era at every angle of the acquisition.

reconstruction of

ds also exist.

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he projections for

as shown in fig

the body into a


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Fig.6.1(b):Sinogram

The aim of reconstruction process is to retrieve the radiotracer spatial

distribution from the projection data is shown in fig. 6.1 (c)

Fig.6.1(c):Reconstruction of Sinogram

8.2 Fourier Transform Of Data

If the projection sonogram data were reconstructed at this point, artifacts

would appear in the reconstructed images due to the nature of the subsequent

back projection operation. Additionally, due to the random nature of the

radioactivity. There is an inherent noise in the data that tends to make the

reconstructed image rough. In order to account for both of these effects, it is

necessary to filter the data. We can filter it directly in the projection space, which

means that we convolute the data by some sort of smoothing kernel.

Convolution is computationally intensive.Convolution in tyhr spatial

domain is equivalent to a multiplication in the frequency domain. This means

that any filtering done by the convolution operation in the normal spatial domain


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can be performed by a simple multiplication when transformed into the

frequency domain.

Thus we transform the projection data into the frequency space where by

we can more efficiently filter the data.

8.3 Data Filtering

Once the data has been transformed to the frequency domain, it is then

filtered in order to smooth out the statistical noise. There are many different

filters available to filter the data and they all have slightly different

characteristics. For instance, some will smooth very heavily so that there are not

any sharp edges, and hence will degrade the final image resolution .other filters

will maintain a high resolution while only smoothing slightly.

Fig.6.3: Reconstruction of objects using Filters

Some typical filters used are Hanning filter, Butter worth filter, Low pass cosine

filter,Weiner filter etc .Regardless of the filter used, the end result is to display a

final image that is relatively free from noise & is pleasing to the eye.The fig. 8.3

depicts three objects reconstructed without a filter true (left), without a filter

noisy (middle) and with a Hanning filter (right).

8.4 Inverse Transform of Data

As the newly smoothed data is now in the frequency domain, we must

transform it back into the spatial domain in order to get out the x, y, z

information regarding spatial distribution. This is done in the same type of

manner as the original transformation is done, expect we use what is called theone dimensional inverse Fourier transform. Data at this point is similar to the

original fig. 8.4 (a) sonogram expect it is smoothed as shown in fig. 6.4 (b)


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Fig.8.4(a):Inverse transform of the data Fig.8.4(b) Sinogram of inverse transform

8.5 Back Projection

The main reconstruction step involves a process known as Back

Projection. As the original data was collected by only allowing photons emitted

perpendicular to the camera face to enter the camera, back projection smears the

camera bin data from the filtered sonogram back along the same lines from

where the photon was emitted from. Regions where back projection lines from

different angles intersect represent areas which contain higher concentration of

radiopharmaceutical,

Back Projection


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9. SPECT SCAN

A SPECT Scan is capable of providing information about blood flow to tissue. It

is a sensitive diagnostic tool used to detect stress fracture, spondylosis, infection (e.g.

discitis), and tumor (e.g. osteoid osteoma). Analyzing blood flow to an organ (e.g. bone)

may help to determine how well it is functioning.

Similar to a PET Scan, a radionuclide is injected intravenously. Tissues absorb

the radionuclide as it is circulated in the blood. As a camera rotates around the patient, it

picks ups photons, the radionuclide particles. This information is transferred to a

computer that converts the data onto film. The images are vertical and/or horizontal

cross-sections of the body part and can be rendered into 3-D format.

PET Scans (Positron Emission Tomography) and SPECT Scans (Single Photon

Emission Computed Tomography) were first used in the 1970's for research. Now, some

30 years later, these non-invasive techniques have been adapted to diagnose disease in

humans.As part of the family of nuclear imaging techniques, PET and SPECT scans use

small amounts of radionuclides (radioactive isotopes) to measure cellular/tissue change.

Radionuclides are absorbed by healthy tissue at a different rate than tissue undergoing a

disease process. A deviation in normal rates of absorption may be an indication of

abnormal metabolic activity, which could lead to structural change (e.g. vertebra). X-

rays, CT Scans, and MRI can only image structure (e.g. anatomy), not function or

metabolism.


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10. POSITRON EMISSION TOMOGRAPHY (PET)

The distribution of activity in slices of organs can be obtained in a more

accurate way using PET. In the simplest PET camera two modified sophisticated

cameras called Anger cameras are placed on opposite sides of the patient. Thisincreases the collection angle and reduces the collection times which are the

limitations of SPECT .In PET, radiopharmaceuticals are labeled with positron

emitting isotopes. A positron combines rather quickly with an electron. As a

result the two gamma quanta are emitted almost in opposite directions .

In PET scanners, rings of gamma ray of gamma ray detectors surroundingthe patient are used. Each detector interacts electronically with the other detectors

in the field of view. When a photon arrives within a short time frame, it is clear

that a pair of quanta was generated and that these were created somewhere along

the path between the detectors. Conventional PET tomography makes use of

standard filtered back projection techniques used in computed tomography and

SPECT. Three dimensional PET scanning has increased sensitivity but also

noise. But since higher sensitivity permits lower radiation doses, the use is

justified.

PET is used to study the dynamic properties of biochemical processes. A

large part of the biological system consists of hydrogen, carbon, nitrogen andoxygen. With the help of a cyclotron it is possible to produce short lived

isotopes of carbon, nitrogen and oxygen that emit positrons. Examples of these

isotopes are 0-15, N-13, and C-11 with half lives of 2, 10, and 13 minutes

respectively. PET uses electron collimation instead of lead collimation.

Attenuation correction can be more accurately done in case of PET. The

resolution of PET is much better and uniform than SPECT.


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11. PET SCAN

Many physicians in fields including cardiology, neurology, and oncology use

PET Scanning. A PET image can map the biological function of an organ, can detect

subtle metabolic changes, determine if a disease is active or dormant, may be used to

determine if a tumor is benign or malignant (malignant tumors have classic metabolic

patterns), and may be used to stage certain types of cancer.

A PET Scan is an expensive test. PET facilities require sophisticated computer

equipment, a cyclotron, and highly trained specialists. A cyclotron is a machine - an

accelerator that propels charged particles (e.g. protons) using alternating voltage in a

magnetic field.

The test begins with the injection of a radionuclide (tracer) specific to the

function/metabolism to be investigated. Within a short period of time, the tracer collects

in the specific body area. The patient lies comfortably on the scanning table, while a

ring-shaped machine is properly positioned over the target body part. Detectors in the

350-degree ring pick up gamma rays emitted from internal body tissues. The computer

analyzes this data to produce cross-sectional images on film and/or a video monitor.

The images are often color coded according to the concentration of the tracer.

PET Scan of Human Brain


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New collimators are designed planar in one direction and concave in other which

improves the spatial resolution and reduces the non isotropic blur in SPECT.

So that the resolution and sensitivity can be improved much to that of PET .

Although SPECT imaging resolution is not that of PET, the availability of new

SPECT radiopharmaceuticals, particularly for the brain and head, and the

practical and economical aspects of SPECT instrumentation make this mode of

emission tomography attractive for clinical studies of the brain. The cost of

SPECT imaging is very low comparing to PET.


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13. ADVANTAGES

1. Better detailed resolution:superimposition of overlying structures is

removed.

2. Lesion contrast higher: small deep lesions may be seen as small

differences in radiopharmaceutical distribution and can be detected.

Hence resolution is improved.

3. Localization of defects is more precise and more clearly seen by the

inexperienced eye.

4. Extend and size of defects is better defined.

5. Images free of background.

6. Time required to form an image is very less.

7. SPECT had a positive predictive value for Alzheimers disease of 92%

8. A bone scan costs about one third to half as much as a CT or MRI

9. The radiation exposure from one SPECT study is 1/3th the level of

radiation from an abdominal CAT scan.


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15. APPLICATIONS

1.Heart ImagingSPECT has been applied to the heart for myocardial perfusion imaging.

The following figure is a myocardial MIBI scan taken under stress conditions.

Regions of the heart that are not being per fused will display as cooler regions.

2.Brain ImagingThis figure is a transverse SPECT image of the brain.The hot spots present

in the right posterior region are seen clearly using SPECT. SPECT examines

cerebral function by documenting regional blood flow and metabolism.

The SPECT and PET imaging modalities are especially valuable in brain imaging

as they make it possible to visualize and quantify the density of different types ofreceptors and transporters. The accurate assessment of the density of receptors or

transporters in the brain structure is quite challenging because of the small size of

these structures.


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3.Kidney/Renal ImagingSPECT imaging is specially used to differentiate between infarct and

ischemic. Infarct is an area of necrosis in the tissue or the organ resulting fromobstruction of the local circulation by a thrombus or embolus. Ischemic is a

condition of the localized anemia due to an obstructed circulation. Clinical

studies indicate that SPECT is more accurate at detecting acute ischemia than CT

scan. The following is a renal planar scan using MAG3 tracer (a glucose analog)

Renal Imaging

4.Tumor detectionSPECT can be used to detect tumors in cancer patients in the early stages

itself. Using this slicing method, we can remove any interference from the

surrounding area and detect disfuntionality of organs pretty easily. The

radioactive chemicals will distribute through the body. The distributions can be

traced and compared to that of a normal healthy body. Since this method is so

precise, doctors can detect abnormalities in the early stages of disease

development when it is more curable. SPECT has been proven alternative to PET

in distinguishing recurrent brain tumor from radiation necrosis.

5.Bone ScansBone scans are typically performed in order to assess bone growth and to

look for brain tumors.The tumors are the dark areas seen in the picture below.

The development of SPECT has enhanced the contrast resolution of bone scans

by screening out overlying and underlying tissue. This results in increased

detection and localization of small abnormalities especially in the spine, pelvis

and knees. A bone scan typically costs about one third to half as much as a CT or

MRI.


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15. MILESTONES

Although the first instance of SPECT was when Kuhl and Edwards produced the

first tomographs from emission data in 1963, the history of SPECT detectors

begins earlier.

In the 1940's crude spatial information about radioactive source distributionswithin the brain were produced using a single detector positioned at various

locations around the head.

Ben Classen improved this method in the 1950's when he invented the rectilinear

scanner. This device produced planar images by mechanically scanning adetector in a raster-like pattern over the area of interest. By today's standards, this

technique required very long imaging times because of the sequential nature of

the scanning.

A pin-hole in lead was used to project a gamma ray image of the source

distribution in 1953 by Hal Anger. The image was projected onto a scintillating

screen with photographic film behind it. This technique required extremely long

exposure times because of the huge inefficiencies in the system (principally due

to losses in the film). The inefficiencies in the system resulted in extremely high

radiation doses to patients.

In the late 1950's, Anger replaced the film and screen with a single NaI crystal

and PMT array. This formed the basis for the "Anger Camera" which is now the

standard clinical nuclear imaging device. Modern Anger Cameras use a lead

collimator perforated with many parallel, converging or diverging holes instead

of the original pin-hole configuration.

Kuhl and Edwards were the first to present tomographic images produced using

the Anger Camera in 1963.

Everett, Fleming, Todd and Nightengale suggested the use of the Compton effectfor gamma-radiation imaging in 1977. This technique is currently in use in

astronomy. It's adaptation to SPECT is non-trivial because of the vastly different

source distributions and geometry involved.

The investigation of the Compton Camera for SPECT began in 1983. Manbir

Singh and David Doria proposed and experimented with a basic design usingsolid state detectors, performed an analysis of possible detector materials, and

produced a small prototype for testing.


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17. CONCLUSION

It is reasonable to speculate about a constant by perhaps a slower rate of

increase of clinical applications of SPECT. It is safe to conclude that SPECT has

reached the stage where it will be a valuable and also an unavoidable asset to the

medical world.

SPECT being a nuclear medicine imaging modality , it has all the

advantages and disadvantages of nuclear medicine can be highly beneficial or

dangerous on the application , so is SPECT .In spite of this , Today , nearly all

cardiac patients receive a planar ECT or SPECT as part of their work-up to detectand stage coronary artery disease . Brain and Liver SPECT scans are also a

leading application of SPECT. SPECT is used routinely to help diagnose and

stage cancer, stroke, liver disease, lungs disease and a host of other physiological

(functional) abnormalities.

Attenuation of the gamma rays within the patient can lead to significant

underestimation of activity in deep tissues, compared to superficial tissues.

Approximate correction is possible, based on relative position of the activity.However, optimal correction is obtained with measured attenuation values.

Modern SPECT equipment is available with an integrated x-ray CT scanner. AsX-ray CT images are an attenuation map of the tissues, this data can be

incorporated into the SPECT reconstruction to correct for attenuation. It alsoprovides a precisely registered CT image which can provide additional

anatomical information.


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18. BIBLIOGRAPHY

1. www.healthimaging.com

2. www.spect.com

3. en.wikipedia.org/.../Single_photon_emission_computed_tomography

4. www.physics.ubc.ca/~mirg/home/tutorial/tutorial.html

5. www.sciencedaily.com

6. R.S.Khandpur, Handbook of Biomedical Instrumentation.

7. Dr .M. Armugam, Biomedical instrumentation.

8. Steve Webb, Principles of Medical Imaging.

9. John.G.Webster,Medical Instrumentation, Application and design.

10.www.nucmed.bidmc.harvard. Edu

11.www.pumbed.com

12.www.cti-pet.com

13.www.diagnosticimaging.com

14.www.mayoclinic.com/health/spect-scan

15.Herman,GaborT.(2009).Fundamentals of Computerized Tomography:

Image Reconstruction from Projections (2nd ed.). Springer.

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