SCINTILLATION CAMERA WITH MULTICHANNEL COLLIMATORS.

Abstract

The scintillation camera is a sensitive electronic instrument for taking pie tures of the distribution of gamma-ray and positron-emitting isotopes in vivo. The pictures are similar to those obtained from mechanical scanners, but they are produced in much less time. No scanning is employed because the scintillation camera is sensitive to all parts of its field of view during the entire exposure time. To obtain an image of activity distribution a collimator first projects a gamma-ray image of the subject onto a scintillator. The instrument described here uses a single sodium iodide crystal 11% inches in diameter by 3@inch thick. Coupled to the crystal through an optical light guide is a close-packed hexagonal array of 19 multiplier phototubes. The phototubes view overlapping areas in the seintillator so that light from each scintillation divides among the 19 tubes. The combination of scintillator, light guide, and phototubes is called an image detector (1). The phototubes are connected to an analog computer that identi fies the X and Y coordinates and the brightness of each scintillation occurring in the crystal. All photopeak scintillations are reproduced on an oscilloscope as point flashes of light in the same relative positions in which they occurred in the scintillator. The flashes are photographed over a period of time, and an image of the subject results. To obtain the best combination of sensitivity and resolution for a given subject and radionuclide, the optimum collimation method should be used. A brief account of the three collimating methods—pinhole, multichannel, and positron coincidence—has been given (2). For positron emitters, coincidence collimation gives excellent sensitivity and resolution for both large and small subjects. For small subjects containing gamma-ray emitters, pinhole collimation is the method of choice. It is used to obtain high-resolution pictures of small subjects such as the thyroid gland. However, for large gamma-ray emitting subjects, such as the brain or liver, collimators with large numbers of parallel holes (2-9) give the best com bination of sensitivity and resolution. A drawing of this type of collimator is shown with the scintillation camera image detector in Figure 1.