In nuclear medicine, images of internal structures or functions of the body are acquired by using one or more gamma cameras to detect radiation emitted by a radio pharmaceutical that has been injected into the patient's body. A computer system controls the gamma cameras to acquire data and then processes the acquired data to generate images. Nuclear medicine imaging techniques include single photon emission computed tomography (SPECT) and positron emission tomography (PET). SPECT imaging is based on the detection of individual gamma rays emitted from the body, while PET imaging is based on the detection of gamma ray pairs resulting from electron-positron annihilations and emitted in coincidence with each other. Accordingly, PET imaging is sometimes referred to as coincidence imaging. Nuclear medicine imaging systems, which are sometimes referred to as gamma camera systems, include dedicated SPECT systems, dedicated PET systems, and systems having dual PET/SPECT capability. Gamma camera systems with dual PET/SPECT capability are available from ADAC Laboratories of Milpitas, Calif.
Gamma camera detectors typically include a number of photomultiplier tubes (PMTs), which provide electrical outputs in response to scintillation events occurring within a scintillation crystal. Electronic circuitry generally processes the output of each PMT. Accordingly, each PMT and its associated electronics may be referred to as a "channel". The output of a PMT may therefore be referred to as a PMT "channel signal". The channel signals are amplified and integrated to determine the total amount of energy associated with a given scintillation event. Scintillation events are represented as pulses in the channel signals.
One problem associated with gamma camera systems is pulse pile-up. Pulse pile-up is the occurrence of two scintillation-based pulses so close together in time that they overlap. Pulse pile-up distorts energy information and contributes to deadtime losses and can therefore cause inaccuracies in the imaging process. Pulse pile-up may be categorized into two different types: pre-pulse pile-up and post-pulse pile-up. Pre-pulse pile-up refers to the situation in which a pulse of interest is overlapped by the "tail" (trailing portion) of a preceding pulse. Post-pulse pile-up refers to the situation in which a subsequent pulse occurs before integration of the pulse of interest has been completed.
In certain prior art gamma camera systems, the problem of post-pulse pile-up has been addressed by causing integration of all channels to stop in response to each pile-up event. The partially integrated channel signals are then corrected for pre-pulse pile-up, post-pulse pile-up, or both, using an appropriate technique. This approach is disadvantageous, however, because valuable energy and position information is lost when integration of all channels is stopped, even though the pile-up may not have affected all channels. This approach becomes particularly problematic in the context of a system capable of both PET and SPECT imaging. For example, a dual PET/SPECT system which employs a single, contiguous NaI (Tl) crystal in each camera may experience ultra high counting rates (e.g., five to six million events per second) when the system is operated in PET mode. At such high counting rates, stopping integration of all channels, even for a very brief period of time, causes a substantial loss of information.
In addition, certain pre-pulse pile-up correction and pulse tail extrapolation techniques use an exponential approximation of the tail of a pulse. However, such an approach generally relies upon precise knowledge of when the beginning of the pulse occurred and tends to be computationally intensive.
What is needed, therefore, is a gamma camera system, which overcomes these and other disadvantages of the prior art.