Real-time compton scatter correction

Details Real-time compton scatter correction Real-time compton scatter correction

1999-05-14

2002-04-09

Hannaher, Constantine (Department: 2878)

Radiant energy

Invisible radiant energy responsive electric signalling

With or including a luminophor

C250S363020

Reexamination Certificate

active

06369389

ABSTRACT:

FIELD OF THE INVENTION
This invention is related to the art of nuclear medicine imaging and in particular to digital nuclear medicine systems.
BACKGROUND OF THE INVENTION
This application is an improvement of a co-pending unpublished U.S. patent application Ser. No. 08/562,375, filed Nov. 24, 1995, titled “An Advanced Nuclear Medicine System.”
FIG. 1
shows a block diagram of a generalized prior art nuclear medicine system
550
used to image a patient
504
. System
550
is used to generate images of radio-nuclide concentrations in patient
504
. Patient
504
is injected with a radio-pharmaceutical which usually forms at least a concentration
506
in portions of patient's
504
anatomy. Patient
504
is placed in an examination area (not shown) so that a scintillation detector crystal
500
can detect gamma radiation emitted by radio-pharmaceuticals in concentration
506
.
In nuclear medicine imaging, gamma rays emitted by radioactive materials are treated as a particle phenomenon. Each measured photon corresponds to one radiation event and the number of radiation events from a region reflects the concentration of the radio-active material in that region. However, the energy of the events indicates whether they have traveled directly from concentration
506
or have their origin in a different region of the anatomy and have been scattered.
As a result, nuclear medicine system design emphasizes filtering real radiation events from scattered events (whose origin is unknown). Due to the weak interaction between gamma-rays and matter and the desire to use low dosages of radioactivity, gamma-rays are not captured on film or with detectors such as used in X-ray CT systems. Gamma radiation emitted by the radio-pharmaceutical interacts with detector crystal
500
to produce minuscule flashes of light. Each radiation event generates one light flash in detector
500
. Several of a plurality of photomultipliers
502
detect this flash of light and generate an electrical current responsive to the intensity of light sensed by the individual photomultiplier. The contributions of photomultipliers
502
are added together to determine the amount of energy in the event and, hence, its validity. In addition, the location of the event is determined by analyzing the signals from each of photomultipliers
502
.
Each photomultiplier
502
has its own signal processing circuit. The electric current produced by each photomultiplier
502
is amplified by an amplifier
508
and is then delayed and shaped by a shaper/delayer
512
. The purpose of shaping the signal is to compress the signal. Most nuclear medicine systems are event triggered and event blocked. Thus, when an event is registered, the system processes it and no further events can be registered or processed while the first event is being processed. Compression shortens the time extent of an event so that processing time (integration) is shorter and the maximum event rate is higher.
Typically, shaper/delayer
512
is triggered only if the radiation event has a total energy which is within a specific wide energy window. Otherwise, the measured event is probably an uninteresting scattering event and is discarded. The outputs of all of amplifiers
508
are summed by an adder
510
. The sum calculated by adder
510
is used by a gating unit
514
to selectably trigger shaper/delayer
512
responsive to the sum. If the sum is within a preset range of values, gating unit
514
triggers shaper/delayer
512
to process the radiation event. It should be noted that the total energy of the event is approximately determined at this stage. Using the delay, full scale integration eventually starts only if the approximate energy falls within predefined limits.
An integrator
516
integrates the signal produced by shaper/delayer
512
to find the total energy associated with the radiation event detected by one specific photomultiplier
502
. An important result of the integration is noise reduction.
As in many measurement systems, even when there is no event being measured, there is a parasitic DC level, usually referred to as a base-line voltage. This base-line voltage is typically subtracted from the signal before integration. Otherwise, the integrated signal contains a large (unknown) contribution from the base-line. This process is called base line restoration.
The individual detector circuits are connected to a single event processing unit
519
. A sequencer
517
multiplexes the results from all of integrators
516
and passes them serially to event processing unit
519
.
Typically, the analog signal is converted to a digital signal after integration. Conversion of analog signals to digital signals is problematic for the short pulse durations typical of nuclear medicine imaging. In particular, analog to digital converters tend to:
(a) have relatively low resolutions;
(b) be non-linear in their response; and
(c) have response curves which vary between converters and, for a single converter, with time.
U.S. Pat. No. 5,371,362, the disclosure of which is incorporated herein by reference, discloses a base line measurement and correction system. The output signal of each photomultiplier is sampled by an analog-to-digital (A/D) converter and analyzed to determine the values of the base-line voltage between radiation events. The determined base line voltage is subtracted from the sampled signals prior to integration to yield base-line corrected signals. Also disclosed is the addition of a sliding scale voltage to the photomultiplier output signal. A sliding scale voltage is generated by the system responsive to the amplitude of the sampled signal. The sliding-scale voltage is added to the signal from the photomultipliers so that its amplitude is within the linear range of the analog to digital converter.
In the APEX system (Elscint LTD., Haifa, Israel), a sliding scale signal having a cycle time which is 64 events long is added to the analog signal before conversion. Each step of the sliding scale is equivalent to about one LSB (least significant bit) of the A/D converter. After A/D conversion, A digital value corresponding to the sliding scale analog value is subtracted from the digitized value of the energy of the event. The sliding scale is event driven, thus, the sliding scale signal is constant for the duration of each event and varies by one level between events.
FIG. 2A
shows an analog signal generated by photomultiplier tubes
502
and
FIG. 2B
shows a sliding scale signal as described herein.
Referring again to
FIG. 1
, event processing starts with determining the X-Y position of the radiation event on detector head
500
. Only strong signals are useful for this determination. Thus, a selector
518
selects only those integration results which are above a threshold. A normalizer
520
normalizes the selected results to make their sum a constant and a position calculator
522
uses the normalized results to perform Anger arithmetic and calculate the position of the radiation event in the plane of detector
500
.
Typically, some of the Anger arithmetic calculations are performed by an array of weighted resistors. These resistors are connected directly to photomultipliers
502
and calculate weighted sums of the signals from photomultipliers
502
.
Following positioning, the radiation events are corrected for linearity errors, energy errors and variable sensitivity errors by an event corrector
524
. Linearity errors are systematic errors in the position calculation by Anger arithmetic. Sensitivity errors are caused by detector
500
having a position dependent sensitivity, i.e., some portions of detector
500
naturally detect more events than others portions, even if all of detector
500
is receiving a uniform event flux. Energy errors are caused by non-detection of some of the light generated by an event, e.g., light passing through the spaces between photomultiplier tubes
502
. Thus, similar events are acquired by system
550
as having dissimilar energy levels. Usually, the events whose energy level is not within a position dependen

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