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Gamma camera

A gamma camera is an imaging device, most commonly used as a medical imaging device in nuclear medicine. It produces images of the distribution of gamma rayemitting radionuclides.

A gamma camera is a complex device consisting of one or more detectors mounted on a gantry. It is connected to an acquisition system for operating the camera and for storing the images.

The system accumulates counts of gammaphotonsthat are absorbed by a crystal in the camera, usually a large flat crystal of sodium iodide with thallium doping in a light-sealed housing. The crystal scintillatesin response to incident gamma radiation: when the energy of an absorbed gamma photon is released, a faint flash of light is produced. This phenomenon is similar to the photoelectric effect. Photomultipliertubes (PMT) behind the crystal detect the fluorescent flashes and a computer sums the fluorescent counts. The computer in turn constructs and displays a two dimensional image of the relative spatial count density on a monitor. This image then reflects the distribution and relative concentration of radioactive tracer elements present in the organs and tissues imaged.

Hal Angerdeveloped the first gamma camera in 1957. His original design, frequently called the Anger camera, is still widely used today. The Anger camera uses sets of vacuum tubephotomultipliers. Generally each tube has an exposed face of about 3 inches in diameter and the tubes are arranged in hexagon configurations, behind the absorbing crystal. The electronic circuit connecting the photodetectors is wired so as to reflect the relative coincidence of light fluorescence as sensed by the members of the hexagon detector array; all the PMTs which simultaneously detect the (presumed) same flash of light. Thus the spatial location of each single flash of fluorescence is reflected as a pattern of voltages within the interconnecting circuit array.

In order to obtain spatial information about the gammaemissions from an imaging subject (e.g. a person's heart muscle cells which have absorbed an intravenous injected radioactive, usually thallium-201 or technetium-99, medicinal imaging agent) a method of correlating the detected photons with their point of origin is required.

The conventional method is to place a collimatorover the detection crystal/PMT array. The collimator essentially consists of a thick sheet of lead, typically 1-3 inches thick, with thousands of adjacent holes through it. The individual holes limit photons which can be detected by the crystal to a cone; the point of the cone is at the midline center of any given hole and extends from the collimator surface outward. However, the collimator is also one of the sources of blurring within the image; lead does not totally attenuate incident gamma photons, there can be some crosstalk between holes.

Unlike a lens, as used in visible light cameras, the collimator attenuates most (>99%) of incident photons and thus greatly limits the sensitivity of the camera system. Large amounts of radiation must be present so as to provide enough exposure for the camera system to detect sufficient scintillation dots to form a picture.

Other methods of image localization (pinhole, rotating slat collimator with CZT (Gagnon & Matthews) and others) have been proposed and tested; however, none have entered widespread routine clinical use.

The best current camera system designs can differentiate two separate point sources of gamma photons located a minimum of 1.8 cm apart, at 5 cm away from the camera face. Spatial resolution decreases rapidly at increasing distances from the camera face. This limits the spatial accuracy of the computer image: it is a fuzzy image made up of many dots of detected but not precisely located scintillation. This is a major limitation for heart muscle imaging systems; the thickest normal heart muscle in the left ventricle is about 1.2 cm and most of the left ventricle muscle is about 0.8 cm, always moving and much of it beyond 5 cm from the collimator face. To help compensate, better imaging systems limit scintillation counting to a portion of the heart contraction cycle, called gating, however this further limits system sensitivity.

SPECTcardiac 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.

References

  • H. Anger. A new instrument for mapping gamma-ray emitters. Biology and Medicine Quarterly Report UCRL, 1957, 3653: 38. (University of California Radiation Laboratory, Berkeley)
  • Anger HO. Scintillation camera with multichannel collimators. J Nucl Med 1964 Jul;65:515-31. PMID 14216630
  • PF Sharp, et al, Practical Nuclear Medicine, IRL Press, oxford
  • D. Gagnon, C.G. Matthews, US Patent #6,359,279 & #6,552,349nl:Gammacamera

pt:Camera gama

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It uses material from the http://en.wikipedia.org/wiki/Gamma+camera Wikipedia article Gamma camera.

 
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