Many types of equipment are designed to detect physical events such as particle-matter interactions. Event detection is widely used in scientific research and in medicine. An example of useful event detection equipment is a nuclear medicine camera, also referred to as a Gamma camera. Such cameras can aid in locating diseased tissue, such as tumours, in the body.
Some conventional nuclear medicine imaging systems have two or more detectors. The detectors are of some of these are planar and include an array of detector devices such as photo multiplier tubes (PMTs). The detectors arrays are positioned above different sides of a patient. Gamma cameras can operate in different modes. For example, some nuclear medicine cameras perform single photon emission computed tomography (SPECT) in which information from a single detector is used to produce information. Other nuclear medicine cameras perform positron emission tomography (PET) in which the detection of two scintillation events, one in each of two detectors that occur 180.degree apart, are used to compute imaging information. These instruments are called PET scanners. In a PET scanner, detectors detect scintillation events that result when each photon of a photon pair collides with a crystal. In common PET scanners, many detectors are arranged in a series of rings which surround the region of the patient's body being scanned.
Before a PET scan is performed, the patient is injected with a radio-pharmaceutical, such as Fluoro-deoxyglucose (FDG). The radio-pharmaceutical is labeled with fluorine-18 which emits positrons that interact with electrons in the body. As a result of the interaction, the positrons are annihilated and gamma rays, including photon pairs, result. Photon pairs leave the point of the interaction in directions of travel that are 180.degree. apart from each other. When a photon comes in contact with a crystal of a detector, a scintillation event occurs. The scintillation event is detected by a photo detector device of the detector creating analog information. The analog information is digitized and processed by electronics and software to produce image information about objects such as tumors in the body.
Typical PET scanners, include detectors with multiple devices such as PMTs. For various reasons, the propagation time of trigger signals indicating detection of events varies between PMTs. One factor contributing to propagation time variance is the fact the time taken for the two gamma rays to reach the detectors depends on the distance traveled by each, even though are created at the same time. Yet another factor is that PMTs vary physically in ways that affect their response times. Another factor is the variance in the length of cables used to carry signals associated with different PMTs. Yet another factor is crystal response time variance by area. If the trigger signal is received by processing hardware and software significantly later than the event detected, inaccuracies may result. Inaccuracies may include false detection indications, and images with poor resolution. Therefore, it is critical to calibrate the timing of trigger signals so that they portray, as accurately as possible, what is actually occurring in the tissue of the patient.
Proper calibration of trigger signals can be important in PET scanners. Commonly, the timing calibration is performed on PET systems by positioning a radioactive source between detectors and monitoring rates of coincident events. Normally, the sources are the same ones which are used to perform transmission scans which are used for attenuation correction in PET. These sources orbit around the patient close to the detectors. Since they are far from the center of the scanner, they are always much nearer one detector than the other. Therefore, the gamma ray must arrive at farther detector later than the one arriving at the nearer detector, since both travel at the speed of light. Prior methods of calibration are often time consuming, since the source is only between a particular pair of detectors for a very small fraction of the total time, and may be imprecise because the steps performed are not accurately repeatable.
Detector calibration is especially critical in PET. If the collision of one photon of a photon pair with one detector is not reported at almost the same time as the collision of the other photon of the photon pair with another detector, the coincident event will be missed. Usually a timing window is used to define the maximum time during which two gamma rays are detected, and are considered coincident. The width of this timing window, (normally denoted by the Greek letter tau, τ) is of the order of 5–10 nsec. If the timing window is too wide, it is more likely that a random coincidence will occur. A random coincidence occurs when gamma rays from two different annihilations are detected within the timing window. Random coincidences occur between two detectors, I, and J having count-rates NI and NJ respectively, at a rate given by:RIJ=2τNINJ 
Random coincidences are the main source of noise in PET studies performed at high count-rates. When the detectors are very well aligned the timing window may be narrowed, allowing higher activities to be administered, and shorter imaging times. Techniques currently exist for calibrating PET systems, but these techniques have several disadvantages. Current techniques are complex and not accurately repeatable. Current techniques perform the timing alignment using many sub-groupings of detectors. Initially two opposing groups are aligned, then the first of these groups is used to align a third, while the second group is used to align a forth. Subsequently, the third group is used to align a fifth group, and the forth group is used to align a sixth group and so on. This leads to a propagation of timing errors, since the fifth group and the sixth group are not aligned to primary reference. The timing window could be made narrower, if all detectors were aligned to a common reference source. In addition, current calibration operations take a relatively long time to perform.