The present invention relates generally to a PET (positron emission tomography) scanning technique and more specifically to a PET scanning technique that facilitates accurate quantitative determination of tracer material in small bodies/tissue/organ samples in a PET scanner or assayed using a counter/detector.
Positron emission tomography (PET) relies on the detection of the annihilation of positrons emitted from a radionuclide (e.g. F-18, I-124, Ga-68) with electrons present in the surrounding medium. Under normal circumstances the standard assumption is that activity distribution of interest is embedded in tissue of more-or-less homogeneous density (typically that of water). Positrons are emitted from the radionuclide with a certain spectrum of energies, which determine the distribution of distances that a positron may travel through surrounding medium before annihilating with an electron.
In some situations, however, the distribution radionuclide may be in close enough proximity to a medium of density significantly different from that of water (for example, air). In such cases, some positrons have a significant probability of reaching a lower density medium such as air, and hence their probability of annihilating in close proximity to their point of emission is drastically reduced. These positron which reach air can be considered as “lost” to measurement. This situation is possible for small amounts (on the order of a gram or less) of tissues whose activity (based on a PET radionuclide) is measured in test tubes, and for xenograft tumors embedded under the skin of animals. This effect has been observed for PET radionuclides with relatively high-energy positron-emission spectra, such as I-124. Therefore, a method of properly accounting for the loss of positron-electron annihilation events is necessary in order to improve the estimates of PET radionuclide activity and activity concentrations.