SPECT imaging is used to obtain a myocardial perfusion scan (SPECT scan) to investigate the function of the heart muscle (myocardium). This technique evaluates heart conditions such as coronary artery disease (CAD) and the motion of heart chambers. The main function of the myocardium is evaluated for left ventricular ejection fraction (LVEF) of the heart. The scan is usually performed in conjunction with a cardiac stress test. Technetium-99m tetrofosmin (Myoview™) is a radiopharmaceutical used in nuclear medicine for cardiac imaging.
The nuclear medicine tomography technique single photon emission computed tomography (SPECT) uses gamma rays and is similar to conventional planar imaging using a gamma camera. SPECT can produce 3-D scans in the form of cross-sectional slice images of the patient for example brain imaging. Computer imaging systems can transform this information to produce the required image. The technique overall requires delivery of a gamma-emitting radionuclide into the patient, normally through injection into the bloodstream. Since the development of computed tomography in the 1970s, the radioisotope in the organ and/or tissue can be mapped by SPECT imaging.
Myocardial perfusion scan of the heart
The above SPECT scan using thallium-201 shows the rest images (bottom rows) and technetium-99m sestamibi for the stress images (top rows). These myocardial perfusion scans play a vital role in the non-invasive evaluation of coronary artery disease. This study identified some patients who had coronary artery disease and therefore, provided prognostic information about the risk of adverse cardiac events for the individual patient. New radionuclides are being developed for myocardial perfusion and this particularly includes rubidium-82. The aim is to decrease the radiation dose to the patient by a factor of 10 compared to technetium-99m. In the future, a complete myocardial perfusion exam may be achieved while maintaining a patient dose under 3 mSv.
The radiotracers used in SPECT emits gamma rays, whereas positron emitters (fluorine-18) employed in . These SPECT radiotracers include the technetium-99m which is a metastable nuclear isomer of technetium-99, indium-111, iodine-123 and thallium-201. Furthermore, gaseous xenon-133 has shown promise for diagnostic inhalation studies in evaluating pulmonary function and imaging for the lung. The detection of the xenon-133 gas is by usage of a gamma camera. These specialized cameras contain the scintillation detector, collimator, sodium iodide crystals, and several photomultiplier vacuum tubes.
Imaging cerebral blood flow is used to assess brain function. These studies use several SPECT radiopharmaceuticals and mostly are technetium-99m imaging agents. They include technetium-99m pertechnetate, technetium-99m-pentetate (Tc-DTPA), technetium-99m gluceptate (Tc-GH), technetium-99m exametazime (Tc-HMPAO) and technetium-99m bicisate (Tc-ECD).
All gamma radiation emitted from SPECT radiopharmaceuticals are detected by the usage of a rotating gamma camera around the patient to produce 3-D imaging. The images undergo various electronic transformations by taking into account the distribution of the radiotracer, the process of filtered back projection and other tomographic techniques.
The radioisotopes used in SPECT scanning have relatively long half-lives, for example, technetium-99m (t½ = 6 hours), indium-111 (t½ = 2.8 days), iodine-123 (t½ = 13.22 hours) and thallium-201 (t½ = 73 hours). The radionuclide technetium-99m prepared from molybdenum-99 by using the technetium-99m generator and used for a variety of nuclear medicine diagnostic procedures. These technetium-99m based imaging agents are relatively cheap compared to PET and fMRI imaging. The problem with SPECT scanning is the absence of good spatial resolution.
In addition, the radioactivity of the contrast agent may highlight some safety issues concerning the administration of radioisotopes to the patient. In some cases, the radionuclide is attached to a specific ligand to create a radioligand complex. This complex allows the radiopharmaceutical to be transported and localized in a particular part of the body. At this location, the radiopharmaceutical will emit radiation to be detected by a gamma camera to produce 3-D images.
Brain imaging forms a central part of nuclear medicine by providing physicians functional diagnostic information about the disease states of the central nervous system. The various imaging modalities employed to study the brain include computerized tomography (CT) and functional magnetic resonance imaging (fMRI). Radiopharmaceuticals used for planar imaging, single-photon emission computed tomography (SPECT) and positron emission tomography (PET). In addition, to hybrids of PET-CT, SPECT-CT and PET-MRI can be used as suitable scanning systems.
These hybrid imaging scanners provide anatomic structures and functional information increasing the diagnostic power to identify an appropriate treatment plan for an individual patient. Brain imaging tools have utilized radiopharmaceuticals and/or imaging agents which utilize SPECT and PET imaging. Overall, there continue to be new developments in PET brain imaging agents for more selective targeting of brain tumours, cognitive disorders and central motor disorders.
Currently, SPECT and SPECT-CT imaging studies of the brain play a primary role in patient diagnosis. These brain imaging modalities involve single-photon emitting agents to evaluate brain death, epilepsy, cerebrovascular disease, neuronal function and cerebrospinal fluid (CSF) dynamics.
Spatial resolution and detection sensitivity are both important factors that play a vital role in SPECT scanning and PET radiotracers.
Scanner Spatial resolution
Clinical SPECT 8-12 mm
Clinical PET 4-6 mm
Preclinical SPECT ≤1 mm
Preclinical PET 1-2 mm
The majority of clinical gamma cameras can produce a tomographic spatial resolution of approximately 10 mm. However, certain preclinical SPECT scanners can provide a submillimetre spatial resolution. Subsequently, further modifications using multi-pinhole systems can produce a spatial resolution below 1 mm. Furthermore, the clinical and preclinical PET scanners have a spatial resolution of 1-2 mm and 4-6 mm respectively. Developments in brain PET scanners have produced an improved spatial resolution of approximately 2.5 mm in the central field of view.
Several research groups have achieved a spatial resolution of less than 1 mm by using fine segmented lutetium orthosilicate (LSO) crystals leading to the concept of micro-SPECT as a diagnostic imaging tool.
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