Imaging characteristics of gamma cameras

The qualification procedures of gamma cameras are recorded in the NEMA-NU1-2007 standardIt provides an accurate description of the subjects of measurements, the ways of evaluation and the facts that need to be provided to characterize the camera. The imaging characteristics determined by NEMA:

• Intrinsic (collimator-less) spatial resolution
• Intrinsic energy resolution
• Intrinsic uniformity
• System resolution (extrinsic, with a collimator)
• Intrinsic linearity
• Spatreg error (multiple window spatial registration)
• Count rate

The evaluation of a measurement carried out by applying a capillary or a point source, during which the full width at half maximum of a point response or a line spread function is observed, can be used to determine spatial resolution.

Figure 1.: Evaluation of a system resolution measurement using a capillary, LEHR collimator

The linearity test has to be performed using phantoms that consist of line sources, during evaluation differential and absolute linearity are distinguished. The uniformity test has to be carried out using a homogeneous source (a point source placed sufficiently far), we distinguish between differential uniformity, which is calculated based on a certain pixel and its direct neighbours, and integral uniformity, which takes the whole image into account.

Figure 2.: Uniformity test

The term count rate refers to the maximum activity that the system can image when the activity is increased. The count rate of the camera is finite because the fall time of the NaI crystal is ~230 ns and the base width of the signal is ~1 μs. In practice, however, this value does not characterize the system correctly, since the camera will not provide a good image below the maximal activity presented this way due to the pile-up phenomenon.

Figure 3.: Count rate curve, NEMA test

Pulse pile-up means that signals signals are placed on the higher energy tail of the photopeak in the energy spectrum, which cannot be separated by the system. This phenomenon typically occurs over 200 kcps and it degrades the image of the point source as if activity were present between the sources as well.

Figure 4.: Typical pulse pile-up over 200 kcps

Figure 5.: The pile-up degrades the image