High count rate gamma camera system

Details High count rate gamma camera system High count rate gamma camera system

2000-06-01

2003-06-10

Epps, Georgia (Department: 2873)

Radiant energy

Invisible radiant energy responsive electric signalling

With or including a luminophor

C250S363040, C250S363070

Reexamination Certificate

active

06576907

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to radiation-imaging apparatus employing “gamma cameras” having coordinate computation circuitry and energy summation circuits for computing the location and the energy of “events” in such systems, and in particular to high count rate gamma camera systems. This computed information is used to provide images based on the energy and location of the events.
BACKGROUND OF THE INVENTION
Since the invention of the gamma camera by Anger, scientists have been attempting to improve the count rate of the camera and the camera's resolution at the higher count rates when used in single photon emission (SPE) imaging systems. Anger gamma camera type systems are now also used in high count positron emission tomographic (PET) systems, using multi-head gamma cameras and coincidence circuits.
The count rate of the camera, that is the number of impingements that can be recorded by the camera per unit time, is a function of the dead time of the camera. The dead time of the camera is a time during which the system processes a single event and is not available to process a succeeding event. The term “event” as used herein means the impingement of a scintillator by radiation stimuli that surpasses a given threshold and cause a scintillation and a consequent electrical signal from light sensors, such as photo-multiplier tubes (PMTs), coupled to the scintillator. Related to the dead time, but separately defined therefrom is “pulse pile-up”. A pulse pile-up is a second scintillation that occurs within the light collection time of a first scintillation. In the case of pulse pile-up, the computation system treats the two pulses as one; and thereby computes the energy and location of both scintillations together, which results in an erroneous location and energy. In yet the erroneous computation understandably degrades the image and thus the rejection of pulse pile-up events significantly improves the image. However, the blanket rejection of pulse pile-up events by prior art systems lowers the count rate.
Anger type gamma cameras have been used in single photon emission computerized tomographic systems (SPECT) and planar systems for many years. More recently, Anger cameras have been used in PET systems. In both cases, the introduction of relatively high-speed detection electronics and computer systems for image acquisition and processing has made it even more desirable that the count rate of the Anger type camera be increased. In PET systems, the detection of two gamma rays in different gamma camera heads in coincidence is used to enable computation of imaging information. See, for example, a paper presented by G. Muehllehner et al, entitled “Performance Parameters of a Positron Imaging Camera”, published in the I.E.E.E. Transactions on Nuclear Science, volume NS-23, No. 1, pp 528-537 (February 1976). See also a paper entitled “Performance Parameters of the Longitudinal Tomographic Positron Imaging System” by A. M. J. Paans et al, in Nuclear Instruments & Methods, vol. 192, pp 491-500 (February 1982).
Higher count rates of usable output signals are achieved by decreasing dead time. Rejecting pile-up events improves the image but lowers the count rate. Among the systems used in the past for increasing the count rate have been the use of means for reducing the dead time of the cameras. More particularly, in the past, among the ways for reducing the dead time has been truncation of the pulse provided by the PMTs of the scintillation camera. See, for example, U.S. Pat. No. 4,455,616, the contents of which are hereby incorporated by reference.
Also, in the past, gamma camera images have been improved by, among other things, determining the region in the crystal within which a light event occurs, and coupling to the coordinate computing circuitry only photo-detectors immediately adjacent to the light event. Thus, in the past, it has been known to connect PMTs that are immediately adjacent to the light event to coordinate computation circuitry. See, for example, U.S. Pat. No. 4,100,413, the contents of which are hereby included herein by reference.
Another prior art method for allegedly increasing count rate has been utilization of more than one integrator for each scintillation detector channel enabling the system to collect more than one event per detector at a time in either PET or SPECT mode. (See, for example, U.S. Pat. No. 5,586,637). One of the problems with the system of that patent is that no pile-up rejection is used. The patent contends that no pile-up rejection is required. The multiple integrators, however, do not solve pile-up problems that occur within selected clusters. The pile-up events that are not rejected are contaminated, and their use adversely affects the image. The decision of prior art systems is to discard the pile-up events without taking into account the spatial distance between the events causing the pile-up. This discarding of pile-up events significantly decreases the count rate of the cameras.
SUMMARY OF THE INVENTION
It is an aspect of some embodiments of the present invention to take into account both the spatial and time separation between events that are presently considered pile-up events. Preferably, this enables utilization of previously discarded events.
In some embodiments of the invention, multiple events that occur within the dead time of the event detector (&tgr;
1
) ns are considered contaminated and are rejected. In some embodiments, events that are monitored by different clusters of PMTs and occur after &tgr;
1
ns are used. Also, events monitored by the same cluster and separated by at least the integration time of the detector (&tgr;
2
ns ns) are used.
According to alternate embodiments of the invention, events that are spatially separated can be used even if they are almost simultaneous.
According to an aspect of some embodiments of the present invention, a system of dynamic cluster selection is used. More particularly, a quick-Anger computation is performed in order to quickly obtain coarse X-Y coordinates. The X-Y coordinates may be normalized with the energy that is also quickly obtained and used to select a cluster of PMTs adjacent to the event for processing the signals initiated by the event. In some embodiments, the selection of the cluster is accomplished using a look-up table (LUT), wherein the address is the location defined by the coarse X-Y coordinates and the output selects the PMTs of the cluster. In some embodiments, the PMTs immediately adjacent to the light event location are directly selected by switching circuitry. The cluster will contain PMTs that are proximate to the event and the outputs of which will be used in the Anger computations. To accomplish this, one or more array of analog switches may be activated so that only the light sensors such as PMTs contained in the selected cluster are connected to the regular coordinate computation circuitry. One of the advantages of the dynamic cluster selection is the resultant improvement in homogeneity of the image. Optionally, the connection to the regular coordinate computation circuitry includes a pile-up rejector associated with the region monitored by the cluster.
In some embodiments, all multiple events occurring within a time frame of less than &tgr;
1
ns of each other are rejected. Multiple events, wherein one of the events is within the selected region and one of the events occurs outside of the scope of the selected cluster of PMTs are not considered pile-up events; even though the events occur within less than &tgr;
2
ns of each other, both events are used. The event outside the selected region does not interfere with the Anger computation of the current event within the selected region. In this way the system's dead-time is reduced significantly and the count rate is increased meaningfully; since, a plurality of different clusters are used to process the events within a given time frame and counts formerly rejected are now used. The detection and rejection of pile-up events, within the selected regions, is performed using event

Affiliated with

Also associated with

Rating

High count rate gamma camera system does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with High count rate gamma camera system, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and High count rate gamma camera system will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-3124209