Positron Imaging Centre

The Current Birmingham Positron Camera

  

The current Birmingham Positron Camera is a Forte dual-headed gamma camera manufactured by Adac Laboratories (California). It was commissioned in August 1999 and consists of two heads on a motorised gantry which permits rotation about a horizontal axis, and adjustment of the face-to-face separation of the detectors from 250 to 800 mm.

Each head contains a single crystal of NaI(Tl) scintillator, 500x400mm2 and 16mm thick, optically coupled to an array of 55 photomuliplier tubes (49 76mm tubes and 6 50mm tubes).

Whereas in a conventional gamma camera the photomultipliers are all interconnected via a network of resistors or capacitors to generate the positional signals, in this system each photomultiplier is connected to a separate ADC and a single board computer in the head controls the 55 ADC channels. When a scintillation occurs in the crystal its centroid is determined via software; this is more flexible than an analogue circuit, resulting, for example, in less distortion near the edge of the crystal. The main benefit, however, is in count rate, since very fast pulses can be used and signals from different regions of the crystal can to some extent be processed in parallel, with the result that the dead-time per pulse is approximately 170 ns and each head can operate at a singles rate of over 2M cps.

The detectors have an energy resolution of better than 15% (FWHM of the 511 keV photopeak), sufficient to discriminate against photons scattered by more than 30°. The quantum efficiency of each head for detecting 511 keV photons is approximately 23% (full spectrum) or 16% using just photopeak pulses, while the coincidence resolving time is 7.5 ns. As a result of the improved efficiency and better timing, the new camera is able to operate at much higher rates than the original camera without intolerable rates of random coincidences. With a central point source, useful count rates of over 100k events/s can be achieved.

The spatial resolution of the camera (FWHM of the back projected image of a point source) is approximately 6 mm. As with the original camera, the data are recorded event by event on computer for subsequent processing.