Gamma camera Guide, Meaning , Facts, Information and Description
A system of detectors, photo sensors and computers used in the recording of gamma ray emissions, scintillation, emanating from radioactive tracer elements present in an object.The system accumulates counts of gamma radiation, specifically of those gamma photons which are absorbed by a crystal in the camera, usually a large flat crystal of sodium iodide with thallium doping designed for this purpose, housed in a light sealed housing. The crystal fluoresces ( emits faint flashes of light) when the energy of gamma photons which reach the crystal are absorbed and then re-released as faint flashes of visible light. Some of the gamma photons pass completely through the crystal, thus are not detected.
Photodetectors behind the crystal sense the number florescent flashes and a computer sums the fluorescent counts. The computer in turn constructs and displays a two dimensional image of the relative spacial count density on a monitor. This image then reflects a distribution and relative concentration of radioactive tracer elements present which are emitting the gamma rays.
H. Anger, in the 1960s, developed the first gamma camera, frequently called the Anger camera and still widely used today in most designs. The Anger camera principle uses sets of vacuum tube photodetectors, generally each tube face about 3 inches in diameter and arranged in hexagon configurations, behind the absorbing crystal. An electronic circuit design, connecting the photodetectors, is wired to reflect the relative coincidence of light fluorescence as sensed among the members of the hexagon detector array. Spacial location of each single flash of fluorescence is reflected by different voltages within the interconnecting circuit array.
Because no one has ever devised a lens for gamma radiation, a lead collimator is usually placed in front of the crystal. It serves as sort of a multiple pinhole camera lens to improve the spatial resolution of the detection process, though also significantly decreasing the sensitivity of detection since most of the photons aimed at the camera face are absorbed by the collimator. The sides and back of the camera are thick walled lead to provide shielding from gamma photons arriving from source the camera is not aimed toward.
The best current camera system designs can differentiate 2 separate point sources of gamma photons located a minimum of 1.8 cm apart, at 5 cm away from the camera face. Spacial resolution decreases dramatically with greater distance from the camera face. Thus the precise spatial accuracy of the computer image is rather limited; a fuzzy image made up of many dots of detected but not precisely located scintillation.
SPECT cardiac imaging, as used in nuclear cardiac stress testing, is performed using Anger cameras, usually one, two or three of which are slowly turned in a circle, on a single axis of rotation, around the target organ.
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