Gamma camera

A gamma camera is a medical imaging device used in nuclear medicine. It produces images of the distribution of gamma ray emitting 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 gamma photons that 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 scintillates in response to incident gamma radiation: when the energy of a gamma photon is absorbed, it emits a faint flash of light. Photomultiplier tubes (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 Anger developed the first gamma camera in the 1960s. His original design, frequently called the Anger camera, is still widely used today. The Anger camera uses sets of vacuum tube photomultipliers, 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. Spatial location of each single flash of fluorescence is reflected by different voltages within the interconnecting circuit array.

In order to obtain spatial information from the gamma emissions from an imaging subject some method of correlating the detected photons with the source point is required. The conventional method is to place a collimator over the detection crystal/PMT array. The collimator essentially consists of a sheet of lead with holes in it. This limits the source of photons detected by the crystal under individual holes to a cone with the point at the midline centre of the hole and extending from the collimator surface out to space. From this arrangement it is the case that the subject can be imaged. This is also one of the sources of blurring in the image. (Note: Lead does not totally attenuate gamma photons incident so there can be some crosstalk between holes).

The presence of the collimator attenuates most (>99%) of incident photons. Other methods of image localisation (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.

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.

References

• 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 & #6,552,349pt:Camera gama