ADAC Forte positron camera

The Forte camera is a dual-headed gamma camera manufactured by Adac Laboratories (California), first commissioned in August 1999. The camera functions to amplify the small amounts of radiation from emitted gamma rays into an electrical signal that can be detected and measured. The two heads can rotate about a horizontal axis and the adjustment range of the face-to-face separation of the detectors is from 250 to 800mm.

Each head contains a single crystal of thallium activated sodium iodide [ NaI(TI)] which scintillates (emits photons) when interacting with gamma radiation. Each crystal is 590x470mm² and 16mm thick and is optically coupled to an array of 55 photomultiplier tubes (49, 76mm tubes and 6, 50mm tubes).  The photomultiplier tubes convert the emitted photons into an electrical signal and amplify it.

Each photomultiplier is connected to a separate analogue to digital converter (ADC) which converts the generated voltage into a number for the computer. A single on-board computer in the camera head controls the 55 ADC channels. When a scintillation event occurs the centroid position is determined via software comparing relative light intensity in each PMT. This is more flexible than an analogue circuit resulting in less distortion near the edge of the crystal.

The main benefit to this system is in the count rate. 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 170ns. Each head can operate at a data transfer rate of over 2,000,000 bytes per second (Bps). Each camera heads maximum capacity of detection is around 100,000 events per second.

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


Photgraph of ADAC Forte positron camera with dimension labels

Diagram showing front and side views  of ADAC Forte positron camera with dimensions