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Encyclopedia > Single photon emission computed tomography

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 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. Nuclear medicine is a branch of medicine and medical imaging that uses unsealed radioactive substances in diagnosis and therapy. ... Tomography is imaging by sections or sectioning. ... Diagrammatic cross section of a gamma camera detector A gamma camera is an imaging device, most commonly used as a medical imaging device in nuclear medicine. ...

Contents

Principles

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 image is a 2-D view of 3-D distribution of a radionuclide. Radiography is the creation of radiographs, photographs made by exposing a photographic film or other image receptor to X-rays. ... A radionuclide is an atom with an unstable nucleus. ...


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 dataset. This dataset may then be manipulated to show thin slices along any chosen axis of the body, similar to those obtained from other tomographic techniques, such as MRI, CT, and PET. The mathematical basis for tomographic imaging was laid down by Johann Radon. ... The mri are a fictional alien species in the Faded Sun Trilogy of C.J. Cherryh. ... CT apparatus in a hospital Computed tomography (CT), originally known as computed axial tomography (CAT or CT scan) and body section roentgenography, is a medical imaging method employing tomography where digital geometry processing is used to generate a three-dimensional image of the internals of an object from a large... Image of a typical positron emission tomography (PET) facility Positron emission tomography (PET) is a nuclear medicine medical imaging technique which produces a three dimensional image or map of functional processes in the body. ...


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. A radiopharmaceutical is a radioactive pharmaceutical. ...


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 provide accelerated acquisition. For example, a dual headed camera can be used with heads spaced 180 degrees apart, allowing 2 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 MUGA. Triggered by 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. MUGA scan (Multiple Gated Acquisition scan) is noninvasive test to evaluate the function of the heart. ... ECG may also refer to the East Coast Greenway Lead II An Electrocardiogram (ECG or EKG, abbreviated from the German Elektrokardiogramm) is a graphic produced by an electrocardiograph, which records the electrical voltage in the heart in the form of a continuous strip graph. ... The ejection fraction (EF) or, more correctly, left-ventricular ejection fraction (often abbreviated LVEF) is a measure of how much blood the left ventricle of the heart pumps out with each contraction. ...


Application

SPECT can be used to complement any gamma imaging study, where a true 3D representation can be helpful. E.g. tumor imaging, infection (leukocyte) imaging, thyroid imaging or bone imaging. White Blood Cells is also the name of a White Stripes album. ...


Because SPECT permits accurate localisation in 3D space, it can be used to provide information about localised function in internal organs. E.g. functional cardiac or brain imaging.


Myocardial perfusion imaging

Myocardial perfusion imaging (MPI) is a form of functional cardiac imaging, used for the diagnosis of ischemic heart disease. The underlying principle is that under conditions of stress, diseased myocardium receives less blood flow than normal myocardium. MPI is one of several types of cardiac stress test. Ischaemic heart disease is a disease characterized by reduced blood supply to the heart. ... Myocardium is the muscular tissue of the heart. ... A cardiac stress test is a medical test performed to evaluate relative arterial blood flow increases to the left ventricular heart muscle during exercise, as compared to resting blood flow rates (i. ...


A cardiac specific radiopharmaceutical is administered. E.g. 99mTc-tetrofosmin (Myoview™, GE healthcare), 99mTc-sestamibi (Cardiolite®, DuPont). Following this, the heart rate is raised to induce myocardial stress, either by exercise or pharmacologically with adenosine, dobutamine or dipyridamole (aminophylline can be used to reverse the effects of dipyridamole). The chemical structure of adenosine Adenosine is a nucleoside comprised of adenine attached to a ribose (ribofuranose) moiety via a β-N9-glycosidic bond. ... Dobutamine is a beta adrenergic agonist. ... Dipyridamole is a drug that inhibits platelet aggregation and causes vasodilation. ... Aminophylline is a drug combination that contains theophylline and ethylenediamine in 2:1 ratio. ...


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. As the radionuclide redistributes slowly, it is not usually possible to perform both sets of images on the same day, hence a second attendance is required 1-7 days later (although, with a Tl-201 myocardial perfusion study with dipyridamole, rest images can be acquired as little as two-hours post stress). However, if stress imaging is normal, it is unnecessary to perform rest imaging, as it too will be normal – thus stress imaging is normally performed first.


MPI has been demonstrated to have an overall accuracy of about 83% (sensitivity: 85%; specificity: 72%) [1], and is comparable (or better) than other non-invasive tests for ischemic heart disease, including stress echocardiography. The gold-standard test, however, remains invasive cardiac catheterization. The sensitivity of a binary classification test or algorithm, such as a blood test to determine if a person has a certain disease, or an automated system to detect faulty products in a factory, is a parameter that expresses something about the tests performance. ... The specificity of a binary classification test with respect to a given class is the probability that the test correctly classifies case not belonging to that class. ... It has been suggested that Transesophageal_echocardiogram be merged into this article or section. ... Cardiac catheterization (heart cath) is the insertion of a catheter into a chamber or vessel of the heart. ...


Functional brain imaging

Usually the gamma-emitting tracer used in fuctional brain imaging is 99mTc-HMPAO (hexamethylpropylene amine oxime). 99mTc is a metastable nuclear isomer which emits gamma rays which can be detected by a gamma camera. When it is attached to HMPAO, this allows 99mTc to be taken up by brain tissue in a manner proportial to brain blood flow, in turn allowing brain blood flow to be assessed with the nuclear gamma camera. A nuclear isomer is a metastable or isomeric state of an atom caused by the excitation of a proton or neutron in its nucleus so that it requires a change in spin before it can release its extra energy. ...


Because blood flow in the brain is tightly coupled to local brain metabolism and energy use, the 99mTc-HMPAO 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 (mental testing, etc.). More recent studies have show accuracy of SPECT in Alzheimer diagnosis as high as 88% PMID 16785801. 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. PMID 15545324 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. There are other articles with similar names; see Dementia (disambiguation). ...


99mTc-HMPAO SPECT scanning competes with 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. Positron emission tomography (PET) is a nuclear medicine medical imaging technique which produces a three dimensional image or map of functional processes in the body. ... Pets and humans often contribute toward the happiness of the other in a pet relationship. ...


Reconstruction

Reconstructed images typically have resolutions of 64x64 or 128x128 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 artifacts. This page is about artifacts in science. ...


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 filtered back projection reconstruction algorithm. Iterative reconstruction is an alternative algorithm which is growing in importance, as it is less sensitive to artifacts and can also correct for attenuation). Filtered back projection is a reconstruction algorithm used to invert the Radon transform. ... Method or group of algorithms used to reconstruct 2D and 3D images from the projections of an object. ...


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. However, 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. CT apparatus in a hospital Computed tomography (CT), originally known as computed axial tomography (CAT or CT scan) and body section roentgenography, is a medical imaging method employing tomography where digital geometry processing is used to generate a three-dimensional image of the internals of an object from a large... In computer vision, sets of data acquired by sampling the same scene or object at different times, or from different perspectives, will be in different coordinate systems. ...


Further reading

  • Elhendy et al., Dobutamine Stress Myocardial Perfusion Imaging in Coronary Artery Disease, J Nucl Med 2002 43: 1634-1646
  • For anyone interested in the brain-imaging applications of SPECT then this is a great review, although the full-text article is not available online without a subscription to the journal. The following link provides a link to the abstract only, and you might be able to access the full article through a membership with a medical library. W. Gordon Frankle, Mark Slifstein, Peter S. Talbot, and Marc Laruelle (2005). "Neuroreceptor Imaging in Psychiatry: Theory and Applications". International Review of Neurobiology, 67: 385-440.

See also

Diagrammatic cross section of a gamma camera detector A gamma camera is an imaging device, most commonly used as a medical imaging device in nuclear medicine. ... Neuroimaging includes the use of various techniques to either directly or indirectly image the structure, function, or pharmacology of the brain. ... Functional neuroimaging is the use of neuroimaging technology to measure an aspect of brain function, often with a view to understanding the relationship between activity in certain brain areas and specific mental functions. ... This article or section does not cite its references or sources. ... Image of a typical positron emission tomography (PET) facility Positron emission tomography (PET) is a nuclear medicine medical imaging technique which produces a three dimensional image or map of functional processes in the body. ... There are very few or no other articles that link to this one. ...

External links

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 15
Myocardial perfusion scan - - - tetrofosmin; MIBI 700 180 60 128 x 128 30
Brain scan - - - HMPAO; ECD 555-1110 360 64 128 x 128 30
Tumor scan Iodine-123 159 13 hours MIBG 400 360 60 64 x 64 30
White cell scan Indium-111 171 & 245 67 hours in vitro labelled leucocytes 18 360 60 64 x 64 30

  Results from FactBites:
 
SPECT scan| Single Photon Emission Computed Tomography | Mayfield Clinic (773 words)
Before the SPECT scan, you are injected with a chemical that is radiolabled, meaning it emits gamma rays that can be detected by the scanner.
SPECT scanning is also useful for pre-surgical evaluation of medically uncontrolled seizures (Figure 1).
The radioisotopes typically used in SPECT to label tracers are iodine-123, technetium-99m, xenon-133, thallium-201, and fluorine-18.
Single photon emission computed tomography: Definition and Much More from Answers.com (1476 words)
The image is formed by a computer synthesis of data that is transmitted by single gamma photons emitted by radionuclides administered to the patient.
Single photon emission computed tomography (SPECT) is a nuclear medicine tomographic imaging technique using gamma rays.
SPECT imaging performed after stress reveals the distribution of the radiopharmaceutical, and therefore the relative blood flow to the different regions of the myocardium.
  More results at FactBites »

 
 

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