Single-photon emission computed tomography

thumb|260px|SPECT image (bone tracer) of a mouse [[Maximum intensity projection|MIP]]

alt=|thumb|Collimator used to collimate gamma rays (red arrows) in a gamma camera

Single-photon emission computed tomography (SPECT, or less commonly, SPET) is a nuclear medicine tomographic imaging technique using gamma rays. It is very similar to conventional nuclear medicine planar imaging using a gamma camera (that is, scintigraphy), but is able to provide true 3D information. This information is typically presented as cross-sectional slices through the patient, but can be freely reformatted or manipulated as required.

The technique needs delivery of a gamma-emitting radioisotope (a radionuclide) into the patient, normally through injection into the bloodstream. On occasion, the radioisotope is a simple soluble dissolved ion, such as an isotope of gallium(III). Usually, however, a marker radioisotope is attached to a specific ligand to create a radioligand, whose properties bind it to certain types of tissues. This marriage allows the combination of ligand and radiopharmaceutical to be carried and bound to a place of interest in the body, where the ligand concentration is seen by a gamma camera.

Principles

thumb|A Siemens brand SPECT scanner, consisting of two gamma cameras

SPECT and other nuclear medicine imaging technologies differ from radiography by imaging the amount of biological activity rather than the structure of the body. This is measured by detecting photon emissions from decaying radionuclides and then reconstructed into a 3-D image. These radionucleotides are bonded to carrier molecules that localize the function of interest .

SPECT imaging is performed by using a gamma camera to acquire multiple 2-D images (also called projections), from multiple angles . A computer is then used to apply a tomographic reconstruction algorithm to the multiple projections, yielding a 3-D data set . Partly from this, SPECT scanners are cheaper than PET scans because of the more durable and plentiful radioisotopes that are used, such as technetium-99m.

Because SPECT acquisition is very similar to planar gamma camera imaging, the same radiopharmaceuticals may be used. If a patient is examined in another type of nuclear medicine scan, but the images are non-diagnostic, it may be possible to proceed straight to SPECT by moving the patient to a SPECT instrument, or even by simply reconfiguring the camera for SPECT image acquisition while the patient remains on the table.

thumb|left|SPECT machine performing a total body bone scan. The patient lies on a table that slides through the machine, while a pair of gamma cameras rotate around her.

To acquire SPECT images, the gamma camera is rotated around the patient and is comparable with (or better than) other non-invasive tests for ischemic heart disease.

Functional brain imaging

Usually, the gamma-emitting tracer used in functional brain imaging is technetium (99mTc) exametazime. <sup>99m</sup>Tc, which has a six-hour half-life, is a metastable nuclear isomer that emits gamma rays detectable by a gamma camera In meta-analysis, SPECT was superior to clinical examination and clinical criteria (91% vs. 70%) in its ability to differentiate Alzheimer's disease from vascular dementias. This latter ability relates to SPECT's imaging of local metabolism of the brain, in which the patchy loss of cortical metabolism seen in multiple strokes differs clearly from the more even or "smooth" loss of non-occipital cortical brain function typical of Alzheimer's disease. Another review article, published in 2012, showed that multi-headed SPECT cameras with quantitative analysis result in an overall sensitivity of 84-89% and an overall specificity of 83-89% in cross-sectional studies and sensitivity of 82-96% and specificity of 83-89% for longitudinal studies of dementia.

<sup>99m</sup>Tc-exametazime SPECT scanning competes with fludeoxyglucose (FDG) PET scanning of the brain, which works to assess regional brain glucose metabolism, to provide very similar information about local brain damage from many processes Due to the irradiation of nuclear fuel (e.g. uranium) with neutrons in a nuclear reactor, a wide array of gamma-emitting radionuclides are naturally produced in the fuel, such as fission products (cesium-137, barium-140 and europium-154) and activation products (chromium-51 and cobalt-58). These may be imaged using SPECT in order to verify the presence of fuel rods in a stored fuel assembly for IAEA safeguards purposes, to validate predictions of core simulation codes, or to study the behavior of the nuclear fuel in normal operation,

or in accident scenarios.

Reconstruction

alt=|thumb|SPECT [[Radon transform|Sinogram]]In SPECT, the quality of reconstructed images is generally limited by low spatial resolution and sensitivity, which causes increased attenuation and the risk of artifacts due to its limitations

Older versions of SPECT technology used of MLEM algorithms or maximum likelihood expectation maximization algorithms. Moving forward to more modern day systems, the algorithms have improved to algorithms such as OSEM or the Ordered Subset Expectation Maximization algorithm. These algorithms are like MLEM but break the data down into subsets, which creates faster and higher quality image reconstruction .

Typical SPECT acquisition protocols

{| class="wikitable"

!Study!!Radioisotope!!Emission energy (keV)!!Half-life!!Radiopharmaceutical!!Activity (MBq)!!Rotation (degrees)!!Projections!!Image resolution!!Time per projection (s)

||Bone scan||technetium-99m||140||6 hours||Phosphonates / Bisphosphonates||800||360||120||128 x 128||30

||Myocardial perfusion scan||technetium-99m||140||6 hours||tetrofosmin; Sestamibi||700||180||60||64 x 64||25

||Sestamibi parathyroid scan||technetium-99m||140||6 hours||Sestamibi|||||||||||

||Brain scan||technetium-99m||140||6 hours||Tc exametazime; ECD||555-1110||360||64||128 x 128||30

||Neuroendocrine or neurological tumor scan||iodine-123 or iodine-131||159||13 hours or 8 days||MIBG||400||360||60||64 x 64||30

||White cell scan||indium-111 & technetium-99m||171 & 245||67 hours||in vitro labelled leucocytes||18||360||60||64 x 64||30

SPECT/CT

In some cases, a SPECT gamma scanner may be modified to operate with a conventional CT scanner to combine Single-photon emission computed tomography with Computed tomography to provide both the detailed physiology of the body, as well as the ability to show the structure of the body being scanned. By layering the SPECT and CT data, a more accurate image can be created. In addition to this, the addition of CT helps to correct attenuation in the SPECT imaging.

Quality control

The overall performance of SPECT systems can be performed by quality control tools such as the Jaszczak phantom.

References

External links

Human Health Campus, The official website of the International Atomic Energy Agency dedicated to Professionals in Radiation Medicine. This site is managed by the Division of Human Health, Department of Nuclear Sciences and Applications
National Isotope Development Center Reference information on radioisotopes including those for SPECT; coordination and management of isotope production, availability, and distribution
Isotope Development & Production for Research and Applications (IDPRA) U.S. Department of Energy program for isotope production and production research and development