24-11-2010, 11:44 AM
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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.
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 leadencased 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).
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
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
Single photon emission computed tomography
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.
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 leadencased 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).
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
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
