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ABSTRACT
SPECT, the acronym for Single Photon Emission Computed
Tomography, is a nuclear medicine imaging modality, giving information
about a patient s specific organ or body system. The patient is
injected with a radiopharmaceutical, which will emit Gamma rays. The
radio activity is collected by an instrument called gamma camera and
the image is reconstructed. SPECT is used to make three dimensional
images of the heart, to perform brain studies and for skeletal
scintigraphy.
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 functional information about a
patient s 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 patient s 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 1980 s.
2. Single photon emission computed tomography (SPECT)
What is SPECT?
SPECT is short for single photon emission computed tomography.
As its name suggests (single photon emission) gamma rays are the
sources of the information rather than X-ray emission in the
conventional CT scan.
Why SPECT?
Similar to X-ray, CT, MRI, etc SPECT allows us to visualize
functional information about patient s specific organ or body system.
How does SPECT manage us to give functional information?
Internal radiation is administrated by means of a
pharmaceutical which is labeled with a radioactive isotope. This
pharmaceutical isotope decays, resulting in the emission of gamma rays.
These gamma rays give us a picture of what s happening inside the
patient s body.
But how do these gamma rays allow us to see inside?
By using the most essential tool in Nuclear Medicine-the Gamma
Camera. The Gamma Camera can be used in planner imaging to acquire a 2
-D image or in SPECT imaging to acquire a 3-D image.
How are these Gamma rays collected?
The Gamma Camera collects the gamma rays emitted from the
patient, enabling to reconstruct a picture of where the gamma rays
originated. From this we can how a patient s organ or system is
functioning.
3. THEORY AND INSTRMENTATION
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 the position 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 mathematical reconstruction algorithms to increase
resolution.
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 and weighs about 500 pounds .The camera has three basic layers
the collimator (which only allows the 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 in only perpendicular .At each angle of
projection, the data will be back projected only 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 pertinent findings to the referring
physicians.
4. THE GAMMA CAMERA
Once a radiopharmaceutical has been administered, it is
necessary to detect the gamma ray emissions in order to attain the
functional information. The instrument used in nuclear medicine for the
detection of gamma rays is known as gamma camera(fig 4.1).
Fig. 4.1 Parts of Gamma Camera
The components making up the gamma camera are 1. Camera Collimator
2. Scintillation Detector
3. Photomultiplier Tube
4. Positron Circuitry
5. Data Analysis Computer
4.1 Camera Collimator
The first object that an emitted gamma photon encounters after
exiting the body is the collimator. The collimator is a pattern of
holes through gamma ray absorbing material, usually lead or tungsten
that allows the projection of the gamma ray onto the detector crystal.
The collimator achieves this by only allowing those gamma rays
traveling along certain direction to reach the detector; this ensures
that the position on the detector accurately depicts the originating
location of the gamma ray.
4.2 Scintillation Detector
In order to detect the gamma photon we use scintillation
detectors. A Thallium-activated Sodium Iodide [NaI (TI)] detector
crystal is generally used in Gamma cameras. This is due to this
crystal s optimal detection efficiency for the gamma ray energies of
radionuclide emission common to Nuclear Medicine. A detector crystal
may be circular or rectangular. It is typically 3/8 thick and has
dimensions of 30-50 cm. A gamma ray photon interacts with the detector
by means of the Photoelectric Effect or Compton Scattering with the
iodide ions of the crystal. This interaction causes the release of
electrons which in turn interact with the crystal lattice to produce
light, in a process known as scintillation. Thus, a scintillation
crystal is a material that has the ability to convert energy lost by
radiations into pulses of light.
The basic scintillation system consists of:
1. Scintillator 2. Light Guide
3. Photo Detector
Fig. 4.2 Basic Scintillation System
4.3 Photomultiplier Tube Only a small amount of light is given off from the
scintillation detector. Therefore, photomultiplier tubes are attached
to the back of the crystal. At the face of a Photomultiplier tube (PMT)
is a photocathode which, when stimulated by light photons, ejects
electrons. The PMT is an instrument that detects and amplifies the
electrons that are produced by the photocathode. For every 7 to 10
photons incident on the photocathode, only one electron is generated.
This electron from the cathode is focused on a dynode which absorbs
this electron and re-emits many more electrons. These new electrons are
focused on the next dynode and the process is repeated over and over in
an array of dynodes. At the base of the photomultiplier tube is an
anode which attracts the final large cluster of electrons and converts
them into an electrical pulse.
Each gamma camera has several photomultiplier tubes arranged in
a geometrical array. The typical camera has 37 to 91 PMT s.
4.4 Positron Circuitry The positron logic circuits immediately follow the
photomultiplier tube array and they receive the electrical impulses
from the tubes in the summing matrix circuits (SMC). This allows the
position circuits to determine where each scintillation event occurred
in the detector crystal.
4.5 Data Analysis Computer
Finally in order to deal with the incoming projection data and
to process it into a readable image of the 3D spatial distribution of
activity with in the patient, a processing computer is used. The
computer may use various different methods to reconstruct an image,
such as filtered back projection or iterative construction.
5. SPECT IMAGE ACQUSITIONAND 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
of gamma 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 this light 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. 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 photon depends 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
camera s 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 camera s field of view. Such photons however
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 of detector response, geometric spatial
resolution and sensitivity of the collimator, intrinsic spatial
resolution and sensitivity of the Anger camera, energy resolution of
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. System resolution 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.
5.1 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 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 of collimator used. The system spatial
resolution also depends on the type of collimator 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 is always 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 be made 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
have been 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 human extremities, such as the wrist and thyroid
gland, in addition to research animals such as rats.
5.2 COMPUTERS IN RADIOLOGY AND 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 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 appear in the late 1970s. Both of these
new imaging modalities required a computer. In the 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 medicine s 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.
5.3 IMAGE ACQUISITION
Nuclear medicine images can be acquired in digital format using
a SPECT scanner. The distribution of radionudide in the patient s 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 patient s 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.
6. RECONSTRUCTION
The most common algorithm used in the tomographic
reconstruction of clinical data is the filtered back projection method.
Other methods also exist. 1. Data Projection 2. Fourier Transform of Data 3. Data filtering 4. Inverse transform of the Data
5. Back projection
6.1 Data Projection As the SPECT camera rotates around a patient, it creates a
series of planar images called projections. At each stop, only photons
moving perpendicular to the camera face pass through the collimator. As
many of these photons originate from various depths in the patient, the
result is an overlapping of all tracer emitting organs along a
specified path. A SPECT study consists of many planar images acquired
at various angles. The fig 6.1(a)displays a set of projections taken of
a patient s bone scan.
Fig. 6.1 (a) Data Projection of Bone Scan
After all projections are acquired, they are subdivided by
taking all the projections for a single, thin slice of the patient at a
time. All the projections for each slice are then ordered into an image
called a sinogram as shown in fig 6.1(b). It represents the
projection of the tracer distribution in the body into a single slice
on the camera at every angle of the acquisition.
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 ©
Fig. 6.1 © Reconstruction of Sinogram
6.2 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 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.
6.3 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 .some typical filters used are
Hanning filter, Butter worth filter, Low pass cosine filter, ZWeiner
filter etc .Regardless of the filter used, the end result is to display
a final image that is relatively free from noise and is pleasing to the
eye. The fig. 6.3 depicts three objects reconstructed without a filter
true (left), without a filter noisy (middle) and with a Hanning filter
(right).
Fig. 6.3 Reconstruction of objects using Filters
6.4 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 the one dimensional inverse Fourier transform. Data
at this point is similar to the original fig. 6.4 (a) sonogram expect
it is smoothed as shown in fig. 6.4 (b).
(a) inverse transform of the data (b) Sinogram of inverse
transform
Fig. 6.4
6.5 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, 7. ADVANTAGES OF SPECT
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.
8. DISADVANTAGES OF SPECT
1. Since lead collimator is used, it introduces defects in
scanning. Only 1out of 1000 photons emitted hits the detector and
contributes to image reconstruction.
2. A blurring effect is caused due to the gamma particles
penetrating the collimator walls and opaque objects.
3. Spatial resolution is limited 4. Attenuation compensation is not possible due to multiple
scattering of electrons 9. SPECT APPLICATIONS
1. Heart Imaging SPECT 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 Imaging This 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 of receptors 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. 3. SPECT imaging is specially used to differentiate between
infarct and ischemic. Infarct is an area of necrosis in the tissue or
the organ resulting from obstruction 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.
4. Tumor detection SPECT 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 Scans Bone 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. 6. SPECT is superior to other imaging modalities in detecting
subtle instances of Spondylolysis and assessing the degree of injury
activity. SPECT is also used in diagnosing Alzheimer s disease, for
performing lung perfusion, abdominal and pelvic scanning and in
diagnosing epilepsy. Radionuclide scans with increased imaging
techniques such as SPECT have become safe well- established and highly
effective diagnostic tools in sports medicine. 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. This increases 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 surrounding the
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 and oxygen. 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.
11. COMPARISON OF PET AND SPECT
Fig. 11 SPECT imaging is inferior to PET because of attainable
resolution and sensitivity. Different radionuclide is used for SPECT
imaging that emits a single photon rather than positron emission as in
PET. Because a single photon is emitted from the radio nuclides used
for SPECT, a special lens known as a collimator is used to acquire the
image data from multiple views around the body. The use of collimator
results in a tremendous decrease in the detection efficiency as
compared to PET. For Positron Emission Tomography, collimation is
achieved naturally by the fact that a pair of detected photons (gamma
rays) can be traced back to their origin since they travel along the
same line after being produced. In PET, there might be as many as 500
detectors that could see a PET isotope at any one time where as in
SPECT; there may be only one or three collimators. 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.
PROS CONS
SPECT 1. Afford able Price
2. Large clinical practice 1. Limitation of spatial resolution
2. Blurring effect with higher energy tracers
PET 1. Good spatial resolution 1. Costly
2. Tracers required are of short half-lives, hence requires cyclotrons
and particle generators nearby itself
12. 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 detect and 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.
13. BIBLIOGRAPHY 1. Xiaochuan Pan; Chien-Min Kao; Sidky, E.Y.; Yu Zou; Metz, C.E.;
/spl pi/-scheme short-scan SPECT and image reconstruction with
nonuniform attenuation Nuclear Science, IEE Transactions on , Volume:
50 Issue: 1 , Feb. 2003 Page(s): 87 -96.
2. R.S.Khandpur, Handbook of Biomedical Instrumentation.
3. Dr .M. Armugam, Biomedical instrumentation. 4. Steve Webb, Principles of Medical Imaging. 5. John.G.Webster,Medical Instrumentation, Application and
design. 6. nucmed.bidmc.harvard. Edu
7. pumbed.com 8. cti-pet.com
9. healthimaging.com CONTENTS
1. INTRODUCTION 1
2. SINGLE PHOTON EMISSION COMPUTED TOMOGRAPHY 2
3. THEORY AND INSTRUMENTATION 3
4. THE GAMMA CAMERA 5
5. SPECT IMAGE ACQUISITION AND PROCESSING 9
6. RECONSTRUCTION 15
7. ADVANTAGES OF SPECT 20
8. DISADVANTAGES OF SPECT 21
9. SPECT APPLICATION 22
10. POSITRON EMISSION TOMOGRAPHY 24
11. COMPARISON OF PET AND SPECT 25
12. CONCLUSION 27
13. BIBLIOGRAPHY 28
ACKNOWLEDGEMENT
I extend my sincere gratitude towards Prof . P.Sukumaran Head
of Department for giving us his invaluable knowledge and wonderful
technical guidance
I express my thanks to Mr. Muhammed kutty our group tutor and
also to our staff advisor Ms. Biji Paul for their kind co-operation and
guidance for preparing and presenting this seminars.
I also thank all the other faculty members of AEI department
and my friends for their help and support.
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ABSTRACT
SPECT, the acronym for Single Photon Emission Computed
Tomography, is a nuclear medicine imaging modality, giving information
about a patient s specific organ or body system. The patient is
injected with a radiopharmaceutical, which will emit Gamma rays. The
radio activity is collected by an instrument called gamma camera and
the image is reconstructed. SPECT is used to make three dimensional
images of the heart, to perform brain studies and for skeletal
scintigraphy.
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 functional information about a
patient s 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 patient s 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 1980 s.
2. Single photon emission computed tomography (SPECT)
What is SPECT?
SPECT is short for single photon emission computed tomography.
As its name suggests (single photon emission) gamma rays are the
sources of the information rather than X-ray emission in the
conventional CT scan.
Why SPECT?
Similar to X-ray, CT, MRI, etc SPECT allows us to visualize
functional information about patient s specific organ or body system.
How does SPECT manage us to give functional information?
Internal radiation is administrated by means of a
pharmaceutical which is labeled with a radioactive isotope. This
pharmaceutical isotope decays, resulting in the emission of gamma rays.
These gamma rays give us a picture of what s happening inside the
patient s body.
But how do these gamma rays allow us to see inside?
By using the most essential tool in Nuclear Medicine-the Gamma
Camera. The Gamma Camera can be used in planner imaging to acquire a 2
-D image or in SPECT imaging to acquire a 3-D image.
How are these Gamma rays collected?
The Gamma Camera collects the gamma rays emitted from the
patient, enabling to reconstruct a picture of where the gamma rays
originated. From this we can how a patient s organ or system is
functioning.
3. THEORY AND INSTRMENTATION
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 the position 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 mathematical reconstruction algorithms to increase
resolution.
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 and weighs about 500 pounds .The camera has three basic layers
the collimator (which only allows the 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 in only perpendicular .At each angle of
projection, the data will be back projected only 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 pertinent findings to the referring
physicians.
4. THE GAMMA CAMERA
Once a radiopharmaceutical has been administered, it is
necessary to detect the gamma ray emissions in order to attain the
functional information. The instrument used in nuclear medicine for the
detection of gamma rays is known as gamma camera(fig 4.1).
Fig. 4.1 Parts of Gamma Camera
The components making up the gamma camera are 1. Camera Collimator
2. Scintillation Detector
3. Photomultiplier Tube
4. Positron Circuitry
5. Data Analysis Computer
4.1 Camera Collimator
The first object that an emitted gamma photon encounters after
exiting the body is the collimator. The collimator is a pattern of
holes through gamma ray absorbing material, usually lead or tungsten
that allows the projection of the gamma ray onto the detector crystal.
The collimator achieves this by only allowing those gamma rays
traveling along certain direction to reach the detector; this ensures
that the position on the detector accurately depicts the originating
location of the gamma ray.
4.2 Scintillation Detector
In order to detect the gamma photon we use scintillation
detectors. A Thallium-activated Sodium Iodide [NaI (TI)] detector
crystal is generally used in Gamma cameras. This is due to this
crystal s optimal detection efficiency for the gamma ray energies of
radionuclide emission common to Nuclear Medicine. A detector crystal
may be circular or rectangular. It is typically 3/8 thick and has
dimensions of 30-50 cm. A gamma ray photon interacts with the detector
by means of the Photoelectric Effect or Compton Scattering with the
iodide ions of the crystal. This interaction causes the release of
electrons which in turn interact with the crystal lattice to produce
light, in a process known as scintillation. Thus, a scintillation
crystal is a material that has the ability to convert energy lost by
radiations into pulses of light.
The basic scintillation system consists of:
1. Scintillator 2. Light Guide
3. Photo Detector
Fig. 4.2 Basic Scintillation System
4.3 Photomultiplier Tube Only a small amount of light is given off from the
scintillation detector. Therefore, photomultiplier tubes are attached
to the back of the crystal. At the face of a Photomultiplier tube (PMT)
is a photocathode which, when stimulated by light photons, ejects
electrons. The PMT is an instrument that detects and amplifies the
electrons that are produced by the photocathode. For every 7 to 10
photons incident on the photocathode, only one electron is generated.
This electron from the cathode is focused on a dynode which absorbs
this electron and re-emits many more electrons. These new electrons are
focused on the next dynode and the process is repeated over and over in
an array of dynodes. At the base of the photomultiplier tube is an
anode which attracts the final large cluster of electrons and converts
them into an electrical pulse.
Each gamma camera has several photomultiplier tubes arranged in
a geometrical array. The typical camera has 37 to 91 PMT s.
4.4 Positron Circuitry The positron logic circuits immediately follow the
photomultiplier tube array and they receive the electrical impulses
from the tubes in the summing matrix circuits (SMC). This allows the
position circuits to determine where each scintillation event occurred
in the detector crystal.
4.5 Data Analysis Computer
Finally in order to deal with the incoming projection data and
to process it into a readable image of the 3D spatial distribution of
activity with in the patient, a processing computer is used. The
computer may use various different methods to reconstruct an image,
such as filtered back projection or iterative construction.
5. SPECT IMAGE ACQUSITIONAND 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
of gamma 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 this light 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. 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 photon depends 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
camera s 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 camera s field of view. Such photons however
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 of detector response, geometric spatial
resolution and sensitivity of the collimator, intrinsic spatial
resolution and sensitivity of the Anger camera, energy resolution of
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. System resolution 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.
5.1 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 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 of collimator used. The system spatial
resolution also depends on the type of collimator 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 is always 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 be made 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
have been 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 human extremities, such as the wrist and thyroid
gland, in addition to research animals such as rats.
5.2 COMPUTERS IN RADIOLOGY AND 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 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 appear in the late 1970s. Both of these
new imaging modalities required a computer. In the 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 medicine s 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.
5.3 IMAGE ACQUISITION
Nuclear medicine images can be acquired in digital format using
a SPECT scanner. The distribution of radionudide in the patient s 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 patient s 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.
6. RECONSTRUCTION
The most common algorithm used in the tomographic
reconstruction of clinical data is the filtered back projection method.
Other methods also exist. 1. Data Projection 2. Fourier Transform of Data 3. Data filtering 4. Inverse transform of the Data
5. Back projection
6.1 Data Projection As the SPECT camera rotates around a patient, it creates a
series of planar images called projections. At each stop, only photons
moving perpendicular to the camera face pass through the collimator. As
many of these photons originate from various depths in the patient, the
result is an overlapping of all tracer emitting organs along a
specified path. A SPECT study consists of many planar images acquired
at various angles. The fig 6.1(a)displays a set of projections taken of
a patient s bone scan.
Fig. 6.1 (a) Data Projection of Bone Scan
After all projections are acquired, they are subdivided by
taking all the projections for a single, thin slice of the patient at a
time. All the projections for each slice are then ordered into an image
called a sinogram as shown in fig 6.1(b). It represents the
projection of the tracer distribution in the body into a single slice
on the camera at every angle of the acquisition.
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 ©
Fig. 6.1 © Reconstruction of Sinogram
6.2 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 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.
6.3 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 .some typical filters used are
Hanning filter, Butter worth filter, Low pass cosine filter, ZWeiner
filter etc .Regardless of the filter used, the end result is to display
a final image that is relatively free from noise and is pleasing to the
eye. The fig. 6.3 depicts three objects reconstructed without a filter
true (left), without a filter noisy (middle) and with a Hanning filter
(right).
Fig. 6.3 Reconstruction of objects using Filters
6.4 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 the one dimensional inverse Fourier transform. Data
at this point is similar to the original fig. 6.4 (a) sonogram expect
it is smoothed as shown in fig. 6.4 (b).
(a) inverse transform of the data (b) Sinogram of inverse
transform
Fig. 6.4
6.5 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, 7. ADVANTAGES OF SPECT
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.
8. DISADVANTAGES OF SPECT
1. Since lead collimator is used, it introduces defects in
scanning. Only 1out of 1000 photons emitted hits the detector and
contributes to image reconstruction.
2. A blurring effect is caused due to the gamma particles
penetrating the collimator walls and opaque objects.
3. Spatial resolution is limited 4. Attenuation compensation is not possible due to multiple
scattering of electrons 9. SPECT APPLICATIONS
1. Heart Imaging SPECT 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 Imaging This 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 of receptors 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. 3. SPECT imaging is specially used to differentiate between
infarct and ischemic. Infarct is an area of necrosis in the tissue or
the organ resulting from obstruction 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.
4. Tumor detection SPECT 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 Scans Bone 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. 6. SPECT is superior to other imaging modalities in detecting
subtle instances of Spondylolysis and assessing the degree of injury
activity. SPECT is also used in diagnosing Alzheimer s disease, for
performing lung perfusion, abdominal and pelvic scanning and in
diagnosing epilepsy. Radionuclide scans with increased imaging
techniques such as SPECT have become safe well- established and highly
effective diagnostic tools in sports medicine. 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. This increases 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 surrounding the
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 and oxygen. 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.
11. COMPARISON OF PET AND SPECT
Fig. 11 SPECT imaging is inferior to PET because of attainable
resolution and sensitivity. Different radionuclide is used for SPECT
imaging that emits a single photon rather than positron emission as in
PET. Because a single photon is emitted from the radio nuclides used
for SPECT, a special lens known as a collimator is used to acquire the
image data from multiple views around the body. The use of collimator
results in a tremendous decrease in the detection efficiency as
compared to PET. For Positron Emission Tomography, collimation is
achieved naturally by the fact that a pair of detected photons (gamma
rays) can be traced back to their origin since they travel along the
same line after being produced. In PET, there might be as many as 500
detectors that could see a PET isotope at any one time where as in
SPECT; there may be only one or three collimators. 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.
PROS CONS
SPECT 1. Afford able Price
2. Large clinical practice 1. Limitation of spatial resolution
2. Blurring effect with higher energy tracers
PET 1. Good spatial resolution 1. Costly
2. Tracers required are of short half-lives, hence requires cyclotrons
and particle generators nearby itself
12. 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 detect and 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.
13. BIBLIOGRAPHY 1. Xiaochuan Pan; Chien-Min Kao; Sidky, E.Y.; Yu Zou; Metz, C.E.;
/spl pi/-scheme short-scan SPECT and image reconstruction with
nonuniform attenuation Nuclear Science, IEE Transactions on , Volume:
50 Issue: 1 , Feb. 2003 Page(s): 87 -96.
2. R.S.Khandpur, Handbook of Biomedical Instrumentation.
3. Dr .M. Armugam, Biomedical instrumentation. 4. Steve Webb, Principles of Medical Imaging. 5. John.G.Webster,Medical Instrumentation, Application and
design. 6. nucmed.bidmc.harvard. Edu
7. pumbed.com 8. cti-pet.com
9. healthimaging.com CONTENTS
1. INTRODUCTION 1
2. SINGLE PHOTON EMISSION COMPUTED TOMOGRAPHY 2
3. THEORY AND INSTRUMENTATION 3
4. THE GAMMA CAMERA 5
5. SPECT IMAGE ACQUISITION AND PROCESSING 9
6. RECONSTRUCTION 15
7. ADVANTAGES OF SPECT 20
8. DISADVANTAGES OF SPECT 21
9. SPECT APPLICATION 22
10. POSITRON EMISSION TOMOGRAPHY 24
11. COMPARISON OF PET AND SPECT 25
12. CONCLUSION 27
13. BIBLIOGRAPHY 28
ACKNOWLEDGEMENT
I extend my sincere gratitude towards Prof . P.Sukumaran Head
of Department for giving us his invaluable knowledge and wonderful
technical guidance
I express my thanks to Mr. Muhammed kutty our group tutor and
also to our staff advisor Ms. Biji Paul for their kind co-operation and
guidance for preparing and presenting this seminars.
I also thank all the other faculty members of AEI department
and my friends for their help and support.