SPECT Imaging

Single-photon emission computed tomography (SPECT) is a nuclear medicine tomographic imaging technique using gamma rays. It is very similar to conventional nuclear medicine planar imaging using a gamma camera. However, it can provide 3-D information in the form of cross-sectional slice images of the patient.  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.

The imaging agent used in SPECT emits gamma rays, as opposed to the positron emitters (fluorine-18) employed in PET. 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 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 imaging 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.

SPECT-scanning
SPECT scanning

Brain Imaging

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 imaging agents for more selective targeting for 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.

Myocardial Perfusion Scan

Myocardial perfusion scan (MPI) is a nuclear medicine procedure that investigates 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.

Myocardial-perfusion-SPECT-scan
Myocardial perfusion SPECT scan

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 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 achievable while maintaining a patient dose under 3 mSv.