The
Scintillation
Camera
Only two stationary gamma-ray imaging devices have been commercially successful: the Anger scintillation camera (manufactured by Searle Radiographics, Picker, Ohio Nuclear, and General Electric, among others), and the Multi-crystal scintillation camera (manufactured by Baird-Atomic).
The Anger scintillation camera is shown schematically in Figure 1 on the following page. The scintillations produced in the sodium iodide detector are “looked” at by an array of 19 or 37 photomultiplier tubes (PMT). The scintillation light produced by a gamma-ray interaction in the detector crystal is shared by the PMT in the array. The contribution of H.O. Anger was to devise an electronic circuit which would produce an image dot on the face of an oscilloscope; the dot’s location corresponds to the location of the gamma-ray interaction in the circular detector. The first problem to be solved in the scintillation camera is the determination of the energy of the gamma ray. In a single PMT device (such as the rectilinear scanner), the pulse output from the PMT is proportional to the gamma-ray energy deposited in the scintillation detector. The same situation holds in the 19-tube array: the pulse size produced by each PMT is proportional to the light seen by that PMT. The output pulses of the 19 tubes are added together algebraically in the SUM circuit to form a SUM pulse, which is proportional to the total gamma-ray energy deposited in the crystal. The SUM pulse is sent to the pulse height analyzer, which produces an output pulse (called the Z-pulse) when the system has detected a gamma ray of the proper energy. This Z pulse is sent to an oscilloscope, where it causes one dot to be written on the face of the oscilloscope. A time exposure of the dot flashes is obtained to produce a scintophoto. If no additional information is provided by the system, a series of dots will be produced in the center of the oscilloscope; in other words, no localizing information is provided.
The information regarding where the dot is to be written on the face of the oscilloscope is produced by the X-, Y-position circuit. This circuit compares the pulse height output of each PMT with the SUM pulse. The position circuit produces X- and Y-deflection voltages which are applied to the deflection plates of the image oscilloscope. The process is illustrated schematically in Figure 2 on the following page. (The X-deflection plates are omitted for simplicity).
Figure 1.
(figure available in print form)
Figure 2.
(figure available in print form)
The result is a one-to-one correspondence between the location of the gamma-ray interaction on the face of the crystal and the location of the dot on the face of the oscilloscope. A scintiphoto then is a time exposure of a number of gamma-ray events. The time required for the system to process a gamma-ray interaction, from the first scintillation flash to the writing of the dot, is called the dead time of the system. The dead time is an important consideration when dynamic studies are performed, since it limits the rate of data accumulation, which in turn may limit the statistical accuracy obtainable during a dynamic study. The dead time characteristics of a number of scintillation camera systems is shown in the table on the next page.
