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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 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. This data set 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 magnetic resonance imaging MRI , X-ray computed tomography X-ray CT , and positron emission tomography PET.
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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. 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.
Projections are acquired at defined points during the rotation, typically every 3—6 degrees. In most cases, a full 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 degrees apart, allowing two projections to be acquired simultaneously, with each head requiring degrees of rotation. Triple-head cameras with degree spacing are also used. 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.
SPECT can be used to complement any gamma imaging study, where a true 3D representation can be helpful, such as tumor imaging, infection leukocyte imaging, thyroid imaging or bone scintigraphy. 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 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.
Fundamentals of Molecular Imaging | CIRC
A cardiac specific radiopharmaceutical is administered, e. Following this, the heart rate is raised to induce myocardial stress, either by exercise on a treadmill or pharmacologically with adenosine , dobutamine , or dipyridamole aminophylline can be used to reverse the effects of dipyridamole. 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. Attaching it to HMPAO allows 99m Tc to be taken up by brain tissue in a manner proportional to brain blood flow, in turn allowing cerebral blood flow to be assessed with the nuclear gamma camera. Because blood flow in the brain is tightly coupled to local brain metabolism and energy use, the 99m Tc-HMPAO tracer as well as the similar 99m Tc-EC tracer is used to assess brain metabolism regionally, in an attempt to diagnose and differentiate the different causal pathologies of dementia.
SPECT is more widely available, because the radioisotope used is longer-lasting and far less expensive in SPECT, and the gamma scanning equipment is less expensive as well.
Emission Tomography: The Fundamentals of PET and SPECT
While 99m Tc is extracted from relatively simple technetiumm generators , which are delivered to hospitals and scanning centers weekly to supply fresh radioisotope, FDG PET relies on FDG, which is made in an expensive medical cyclotron and "hot-lab" automated chemistry lab for radiopharmaceutical manufacture , and then delivered immediately to scanning sites because of the natural short minute half-life of Fluorine In the nuclear power sector, the SPECT technique can be applied to image radioisotope distributions in irradiated nuclear fuels. 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.
The number of projections acquired is chosen to be approximately equal to the width of the resulting images. Comments and suggestions may be submitted at any time for Agency consideration to, Robert Phillips, Ph. Comments may not be acted upon by the Agency until the document is next revised or updated. For questions regarding the use or interpretation of this guidance contact Andrew Kang, MD at or by e-mail at s. Additional copies are available form the Internet. Please use document number to identify the guidance you are requesting.
The purpose of this guidance document is to provide a detailed description of the information that should be included in a premarket k notification for an emission computed tomography diagnostic device or nuclear tomography system submitted to the Center for Devices and Radiological Health CDRH. This information is an elaboration of the general requirements contained in 21 CFR A number of legislative changes relating to the authority of the agency have occurred.
These changes have resulted in the adoption of new regulations and administrative procedures by CDRH which affect the k process. The Food and Drug Administration Modernization Act FDAMA of and a re-engineering effort have resulted in the development of a new k paradigm, which incorporates alternative approaches to demonstrating substantial equivalence in premarket notifications.
These approaches are intended to facilitate the marketing clearance of devices, such as SPECT or PET devices, for which recognized standards exist, and for cases in which the new device is a modification of a previously cleared product. Under the Medical Device Amendments to the Federal Food, Drug and Cosmetic Act of , all Emission Computed Tomography ECT and their accessory devices may be cleared by k process, when the device shows substantial equivalence to the legally marketed predicate devices.
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