The basic design of the most common type of gamma cameras used today was developed by an American physicist, Hal Anger, and is therefore sometimes called the Anger Camera, which stil have nuclear medical usage. It consists of a large diameter NaI(Tl) scintillation crystal which is viewed by a large number of photomultiplier tubes.
A block diagram of the basic components of gamma cameras is described. The crystal and PM Tubes are housed in a cylindrical shaped housing (commonly called the camera head) and a cross-sectional view of this is shown in the figure. The crystal can be between about 25 cm and 40 cm in diameter and about 1.25 cm thick. The diameter is dependent on the application of the device. For example, a 25 cm diameter crystal might be used for a camera designed for cardiac applications, while a larger 40 cm crystal would be used for producing images of the lungs. The thickness of the crystal is chosen, so that it provides good detection for the 140 keV gamma-rays emitted from 99mTc (which is the most common radioisotope used today).
Scintillations produced in the crystal are detected by a large number of PM tubes, which are arranged in a two-dimensional array. There is typically between 37 and 91 PM tubes in modern gamma cameras with microvision working. The output voltages generated by these PM tubes are fed to a position circuit which produces four output signals, called ±X and ±Y. These position signals contain information about where the scintillations were produced within the crystal. In the most basic gamma cameras design, they are fed to a cathode ray oscilloscope (CRO). We will describe the operation of the CRO in more detail below.
When an unblank pulse is generated by the PHA circuit, the electron beam of the CRO is switched on for a brief period of time so as to display a flash of light on the screen. In other words, the voltage pulse from the PHA circuit is used to unblank the electron beam of the CRO.
So, where does this flash of light occur on the screen of the CRO? The position of the flash of light is dictated by the ±X and ±Y signals generated by the position circuit. These signals, as you might have guessed, are fed to the deflection plates of the CRO so as to cause the unblanked electron beam to strike the screen at a point related to where the scintillation was originally produced in the NaI(Tl) crystal.
The gamma cameras can therefore be considered to be a sophisticated arrangement of electronic circuits used to translate the position of a flash of light in a scintillation crystal to a flash of light at a related point on the screen of an oscilloscope. In addition, the use of a pulse height analyser in the circuitry allows us to translate the scintillations related only to photoelectric events in the crystal, by rejecting all voltage pulses except those occurring within the photopeak of the gamma-ray energy spectrum.
The operation of fairly traditional gamma cameras was described. Modern designs are a good deal more complex, but the basic design has remained much the same as has been described. One area where major design improvements have occurred is the area of image formation and display. The most basic approach to image formation is to photograph the screen of the CRO over a period of time to allow integration of the light flashes to form an image on photographic film. A stage up from this is to use a storage oscilloscope which allows each flash of light to remain on the screen for a reasonable period of time.
The most modern approach is to feed the position signals into the memory circuitry of a computer for storage. The memory contents can therefore be displayed on a computer monitor and can also be manipulated (that is, processed) in many ways. For example, various colors can be used to represent different concentrations of a radiopharmaceutical within an organ.