Radiant energy – Invisible radiant energy responsive electric signalling – With or including a luminophor
Reexamination Certificate
1998-05-07
2001-10-30
Epps, Georgia (Department: 2873)
Radiant energy
Invisible radiant energy responsive electric signalling
With or including a luminophor
C250S369000, C327S336000, C702S194000
Reexamination Certificate
active
06310349
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to the field of radiation detection and imaging technology.
2. Description of Related Art
When a radiation particle (gamma ray, neutron, electron, etc.) is detected in a scintillation detector, the scintillation detector will emit light, which is then converted into an electronic signal by a photosensor (e.g., photomultiplier tube or photodiode). This electronic signal can then be received and processed by electronic circuits. In the period after radiation hits the scintillation detector, the scintillation light decays exponentially with a time constant &tgr; (the time when the light level decays to 37% of the onset level), as shown in FIG.
1
.
FIG. 1
shows energy output by two gamma ray particles over time. Since the total amount of light emitted by the scintillation detector represents linearly the energy deposited by the radiation particle in the detector, the area or integral under the curves in
FIG. 1
is a measure of the particle energy. As shown in
FIG. 1
, area
5
and area
10
define a measure of the particle energy of the gamma ray particles. Furthermore, the initial peak in the light level is also proportional to the radiation energy. Hence both the area
5
and peak V
1
in
FIG. 1
may be used to measure the energy of the gamma ray or radiation particle. Since the area under the curve (integral of light) includes many more light signals than the instantaneous peak light level, the integral (the total amount of light emitted) is generally used to measure the radiation energy.
As the radiation flux increases, it becomes increasingly likely that the next radiation particle may arrive at the detector while all previous events are still emitting light (FIG.
2
). In this case, the identity of each individual radiation particle will be lost, and several particles will merge into one large signal, as shown in FIG.
2
. In this case neither the peak level (V
1
or V
2
of FIG.
1
), nor the integral information (area
5
or area
10
of
FIG. 1
) can be used to separate or measure the energy of each particle. In these situations, the detection system will fail to respond properly because of erroneous measurement.
It is known that it takes a time period of approximately 4&tgr; to collect 98% of the scintillation light from each radiation excitation. Thus, if the next event arrives at time t>4&tgr;, the pile-up-energy error on the next event will be less than 2%. Hence, to keep pile-up error small, it is desirable to minimize the chance that two events (radiation excitations) will occur in a time period less than 4&tgr;. Since the time-lapse between two events is a random distribution (i.e., the time-lapse between two events is a random variable) centered about the “average arrival time”, it is generally practiced in the prior art to operate the detector so that the “average arrival time” is 10×(4&tgr;)=40&tgr;, to lower the random chance of having two events coming closer than 4&tgr;. With this 10דhead-room”, the probability that two events will come closer than 4&tgr; would be approximately 10% (using Poisson statistics). The head-room factor as a function of pile-up percentage is shown in Table 1 below:
TABLE 1
HEAD-ROOM FACTOR AND PILE-UPS
Head-Room Factor
5 times
10 times
15 times
20 times
pile-up/total (%)
18%
10%
6.5%
5%
Thus, a 10×head-room is a reasonable choice, and is generally practiced in the prior art. When coupled with a 4&tgr; light-collection time (system dead-time), such a prior art detector provides a measured-energy error (due to pile-up) of less than approximately 2% for approximately 90% of the time, and an energy measurement error (energy resolution) greater than 2% for approximately 10% of the time. This minimum 10×head-room (40&tgr;) timing requirement means that the maximum detection-rate should be less than 1/(40&tgr;) for the scintillation detector.
The present invention permits a scintillation detector system to operate at a much higher event-rate (count-rate) by obviating the 10×head-room factor without pile-up. The present invention maintains a greater event-rate with little sacrifice in the total amount of scintillation light collected, specifically at a 10 times higher radiation flux with little or no sacrifice in measurement accuracy. If the fraction of scintillation light collected can be reduced (i.e., if a user is willing to compromise measurement accuracy), the present invention allows the detector to count at count-rates approximately twenty times greater than conventional methods.
SUMMARY OF THE INVENTION
The present invention includes an apparatus for signal pile-up prevention, comprising a delay circuit for receiving, holding, and passing an incoming signal; a computation circuit for determining a weighted value of the incoming signal; a sampling circuit for receiving the weighted value. The sampling circuit passes the weighted value (which may be passed to an A/D converter) upon receipt of a triggering signal, which corresponds to receipt of a next incoming signal at a trigger circuit. In an exemplary embodiment, the computation circuit may comprise an amplifier, an integrator, and an adder. In an exemplary embodiment, the weighted value is a sum of an integrated value and an instantaneous value, and may be a substantially constant value.
An apparatus according to the present invention may also include a smoothing circuit connected to the circuit adapted to receive the incoming signal. The apparatus may also comprise a residual subtraction circuit for reducing the weighted value by a residual signal value. The sampling circuit discharges said weighted value upon input of the triggering signal.
The present invention may be used in connection with nuclear medicine applications, such as a PET or gamma camera, and may be used to determine both energy and position information. Such an apparatus comprises a plurality of delay circuits, a plurality of computation circuits, and a plurality of sampling circuits, wherein each of the delay circuits receives a different incoming signal from a different output of a gamma camera. The delay circuit, computation circuit, and sampling circuit comprise a pile-up prevention circuit.
Particular embodiments will comprise a plurality of pile-up prevention circuits, and may include a digital signal processor and fast trigger connected to each of the pile-up prevention circuits. Such an embodiment may also comprise an inter-zone detection circuit connected to the fast trigger and a multi-zone-trigger processor connected to said inter-zone detection circuit, capable of centroid averaging. An exemplary embodiment may have a plurality of fast triggers.
A method for preventing signal pile-up according to the present invention may comprise: delaying an incoming signal for a preselected time prior to passing the incoming signal; computing a weighted value of the incoming signal; and sampling the weighted value upon receipt of a triggering signal from the next radiation particle, thereby preventing signal pile-up. Computing may include amplifying the incoming signal to obtain an amplified signal, integrating the incoming signal to obtain an integrated signal, and adding the amplified signal and integrated signal to obtain the weighted value. This method thereby creates a variable signal collection time.
Another method according to the present invention may determine position and energy information of incoming signals without pile-up. Such a method may include: delaying at least one prenormalized position signal and a total energy signal; computing a weighted value for each prenormalized position signal and the total energy signal; sampling the weighted value for each prenormalized position signal and the total energy signal, upon receipt of a triggering signal from the next radiation particle. In an exemplary embodiment, the prenormalized signals and the total energy signal may be corrected by subtracting remnant values of all previous signals.
In yet another aspect, the pr
Li Hongdi
Wong Wai-Hoi
Board of Regents , The University of Texas System
Epps Georgia
Fulbright & Jaworski LLP
Hanig Richard
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