Single-photon emission computed tomography
Single-photon emission computed tomography | |
---|---|
technetium exametazime within a patient's brain | |
ICD-9-CM | 92.0-92.1 |
MeSH | D01589 |
OPS-301 code | 3-72 |
Single-photon emission computed tomography (SPECT, or less commonly, SPET) is a nuclear medicine tomographic imaging technique using gamma rays.[1] It is very similar to conventional nuclear medicine planar imaging using a gamma camera (that is, scintigraphy),[2] 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
Principles
Instead of just "taking a picture of anatomical structures", a SPECT scan monitors level of biological activity at each place in the 3-D region analyzed. Emissions from the radionuclide indicate amounts of blood flow in the capillaries of the imaged regions. In the same way that a plain X-ray is a 2-dimensional (2-D) view of a 3-dimensional structure, the image obtained by a gamma camera is a 2-D view of 3-D distribution of a radionuclide.
SPECT imaging is performed by using a gamma camera to acquire multiple 2-D images (also called
SPECT is similar to PET in its use of radioactive tracer material and detection of gamma rays. In contrast with PET, the tracers used in SPECT emit gamma radiation that is measured directly, whereas PET tracers emit positrons that annihilate with electrons up to a few millimeters away, causing two gamma photons to be emitted in opposite directions. A PET scanner detects these emissions "coincident" in time, which provides more radiation event localization information and, thus, higher spatial resolution images than SPECT (which has about 1 cm resolution). SPECT scans are significantly less expensive than PET scans, in part because they are able to use longer-lived and more easily obtained radioisotopes than PET.
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.
To acquire SPECT images, the gamma camera is rotated around the patient. Projections are acquired at defined points during the rotation, typically every 3–6 degrees. In most cases, a full 360-degree rotation is used to obtain an optimal reconstruction. The time taken to obtain each projection is also variable, but 15–20 seconds is typical. This gives a total scan time of 15–20 minutes.
Multi-headed gamma cameras can accelerate acquisition. For example, a dual-headed camera can be used with heads spaced 180 degrees apart, allowing two projections to be acquired simultaneously, with each head requiring 180 degrees of rotation. Triple-head cameras with 120-degree spacing are also used.
Cardiac gated acquisitions are possible with SPECT, just as with planar imaging techniques such as multi gated acquisition scan (MUGA). Triggered by electrocardiogram (EKG) to obtain differential information about the heart in various parts of its cycle, gated myocardial SPECT can be used to obtain quantitative information about myocardial perfusion, thickness, and contractility of the myocardium during various parts of the cardiac cycle, and also to allow calculation of left ventricular ejection fraction, stroke volume, and cardiac output.
Application
SPECT can be used to complement any gamma imaging study, where a true 3D representation can be helpful, such as tumor imaging, infection (
Because SPECT permits accurate localisation in 3D space, it can be used to provide information about localised function in internal organs, such as functional cardiac or brain imaging.
Myocardial perfusion imaging
This section needs additional citations for verification. (January 2014) |
Myocardial perfusion imaging (MPI) is a form of functional cardiac imaging, used for the diagnosis of
A cardiac specific radiopharmaceutical is administered, e.g., 99mTc-
SPECT imaging performed after stress reveals the distribution of the radiopharmaceutical, and therefore the relative blood flow to the different regions of the myocardium. Diagnosis is made by comparing stress images to a further set of images obtained at rest which are normally acquired prior to the stress images.
MPI has been demonstrated to have an overall accuracy of about 83% (
Functional brain imaging
This section needs additional citations for verification. (January 2014) |
Usually, the gamma-emitting tracer used in functional brain imaging is
Because blood flow in the brain is tightly coupled to local brain metabolism and energy use, the 99mTc-exametazime tracer (as well as the similar 99mTc-EC tracer) is used to assess brain metabolism regionally, in an attempt to diagnose and differentiate the different causal pathologies of dementia. Meta-analysis of many reported studies suggests that SPECT with this tracer is about 74% sensitive at diagnosing Alzheimer's disease vs. 81% sensitivity for clinical exam (cognitive testing, etc.). More recent studies have shown the accuracy of SPECT in Alzheimer's diagnosis may be as high as 88%.[4] In meta analysis, SPECT was superior to clinical exam and clinical criteria (91% vs. 70%) in being able to differentiate Alzheimer's disease from vascular dementias.[5] 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 recent review article 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.[6]
99mTc-exametazime SPECT scanning competes with
Applications in nuclear technology
In the nuclear power sector, the SPECT technique can be applied to image radioisotope distributions in irradiated nuclear fuels.
Reconstruction
Reconstructed images typically have resolutions of 64×64 or 128×128 pixels, with the pixel sizes ranging from 3–6 mm. The number of projections acquired is chosen to be approximately equal to the width of the resulting images. In general, the resulting reconstructed images will be of lower resolution, have increased noise than planar images, and be susceptible to
Scanning is time-consuming, and it is essential that there is no patient movement during the scan time. Movement can cause significant degradation of the reconstructed images, although movement compensation reconstruction techniques can help with this. A highly uneven distribution of radiopharmaceutical also has the potential to cause artifacts. A very intense area of activity (e.g., the bladder) can cause extensive streaking of the images and obscure neighboring areas of activity. This is a limitation of the
Attenuation of the gamma rays within the patient can lead to significant underestimation of activity in deep tissues, compared to superficial tissues. Approximate correction is possible, based on relative position of the activity, and optimal correction is obtained with measured attenuation values. Modern SPECT equipment is available with an integrated X-ray CT scanner. As X-ray CT images are an attenuation map of the tissues, this data can be incorporated into the SPECT reconstruction to correct for attenuation. It also provides a precisely registered CT image, which can provide additional anatomical information.
Scatter of the gamma rays as well as the random nature of gamma rays can also lead to the degradation of quality of SPECT images and cause loss of resolution. Scatter correction and resolution recovery are also applied to improve resolution of SPECT images.[13]
Typical SPECT acquisition protocols
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 built to operate with a
Quality control
The overall performance of SPECT systems can be performed by quality control tools such as the Jaszczak phantom.[15]
See also
- Daniel Amen, psychiatrist who uses SPECT for diagnoses
- Functional neuroimaging
- Gamma camera
- Magnetic resonance imaging
- Neuroimaging
- Positron emission tomography
- ISAS (Ictal-Interictal SPECT Analysis by SPM)
References
- ^ SPECT at the U.S. National Library of Medicine Medical Subject Headings (MeSH)
- S2CID 250665467.
- PMID 12468513.
- S2CID 39518497.
- PMID 15545324.
- S2CID 36441907.
- ISBN 9155459447.
- ISBN 9514779754.
- S2CID 98426662.)
{{cite journal}}
: CS1 maint: multiple names: authors list (link - .
- .
- S2CID 119657086.
- ^ "D. Boulfelfel, R.M. Rangayyan, L.J. Hahn, R. Kloiber, Restoration of Single Photon Emission Computed Tomography Images". Retrieved 10 January 2016.
- PMID 18997051.
- ISBN 1449645372p.189
- Cerqueira M. D., Jacobson A. F. (1989). "Assessment of myocardial viability with SPECT and PET imaging". American Journal of Roentgenology. 153 (3): 477–483. PMID 2669461.
Further reading
- Bruyant, P. P. (2002). "Analytic and iterative reconstruction algorithms in SPECT". Journal of Nuclear Medicine 43(10):1343-1358.
- Elhendy et al., "Dobutamine Stress Myocardial Perfusion Imaging in Coronary Artery Disease", J Nucl Med 2002 43: 1634–1646.
- Frankle W. Gordon (2005). "Neuroreceptor Imaging in Psychiatry: Theory and Applications". International Review of Neurobiology. 67: 385–440. PMID 16291028.
- ISBN 978-1-85233-617-2.
- Jones / Hogg / Seeram (2013). Practical SPECT/CT in Nuclear Medicine. ISBN 978-1447147022.
- Willowson K, Bailey DL, Baldock C, 2008. "Quantitative SPECT reconstruction using CT-derived corrections". Phys. Med. Biol. 53 3099–3112.
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