The following relates to the detector arts. It finds particular application in conjunction with radiation detectors for time-of-flight positron emission tomography (TOF-PET) but may also find applicant in other nuclear medical imagers employing radiation transmission or radiopharmaceuticals, such as single photon emission computed tomography (SPECT) imagers and positron emission tomography (PET) imagers as well as planar x-ray imagers, radio-astronomy, detectors for high energy particles (e.g. Cherenkov radiation, synchrotron radiation, colorimetric detectors, etc.), and the like, and will be described with particular reference thereto. It will be appreciated that the invention may also be applicable to other radiation detector modalities, and in systems and methods employing radiation detectors.
In positron emission tomography (PET), a radiopharmaceutical is administered to the imaging subject, in which the radioactive decay events of the radiopharmaceutical produce positrons. Each positron interacts with an electron to produce a positron-electron annihilation event that emits two oppositely directed gamma (γ) rays. Using coincidence detection circuitry, a ring array of radiation detectors surrounding the imaging subject detect the coincident oppositely directed gamma ray events corresponding to the positron-electron annihilation(s). A line of response (LOR) connecting the two coincident detections intersects the position of the positron-electron annihilation event. Such lines of response are analogous to projection data and can be reconstructed to produce a two- or three-dimensional image. In time-of-flight PET (TOF-PET), the small time difference between the detection of the two coincident γ ray events is used to localize the annihilation event along the LOR (line of response).
The performance of PET systems is influenced by sensitivity, time resolution and response, and noise. PET radiation detector modules traditionally include an array of photomultiplier tubes (PMT's) optically coupled with a scintillator crystal using an intermediate light guide layer. Solid state photoelectric detectors such as digital silicon photomultipliers (SiPM) optically coupled to a pixelated scintillator have also been proposed. SiPMs are based on avalanche photodiodes (APDs) operating in the Geiger mode. They are characterized by an improved sensitivity to γ rays and are less sensitive to scattering effects; however, they are prone to dark counts, which do not originate photon absorption.
A time-to-digital convertor (TDC) outputs a timestamp associated with each detected radiation event. The timestamp is used by the coincidence detection circuitry to determine coincidence pairs and the corresponding LORs and by the time-of-flight measurement circuitry. Traditionally, TDCs consist of a coarse counter and a fine counter. The coarse counter is a digital counter configured to count the rising edges of the reference clock. When an event is detected, a switch at the input of the coarse counter is latched into a register as a part of the timestamp. The fine counter measures the time difference between detected event and the subsequent rising edge of the reference clock as the remaining part of the timestamp. The output is a timestamp with a temporal resolution typically less than 100 picoseconds.
However, an event may or may not be detected due to a phenomenon known as meta-stability. Meta-stability is an unstable state that persists for an indefinite period typically occurring in synchronous circuits with one or more asynchronous inputs. A flip-flop is one device that is susceptible to meta-stability under certain conditions. A flip-flop has two logic states, a change at the input causes the flip-flop to alternate between the states. However, if the input changes during the setup or hold times it may enter a meta-stable state between the two logic states. The meta-stable state eventually decays to one of the two logic states, but the decay time can be significant making exact time measurements difficult.
In the example of a TDC, the input is connected to a flip-flop that is latched in response to a detection signal generated by a photoelectric detector. If the detection signal occurs during a rising edge of the reference clock and as a result the flip-flop enters a meta-stable state, the event is not detected until the next rising edge of the reference clock. Meta-stability at the input of the TDC can seriously affect the precision of the timestamp, thus reducing the accuracy of coincidence detection which can introduce significant noise into images.
The solid state nature of SiPMs allows for the integration of digital TDCs close to the APDs, thus improving the timing resolution of the PET system. Flip-flops with shorter setup and hold times have been proposed; however, traditional TDCs implementations still suffer from meta-stability because of the circuit design.
The present application provides a new and improved timing circuit suitable for PET detectors or other electronics which overcomes the above-referenced problems and others.
In accordance with one aspect, a timing circuit with a first and second TDC is presented. The first TDC is configured to output a first timestamp based on a first reference clock signal, and the second TDC is configured to output a second timestamp based on a second reference clock signal. A circuit outputs a corrected timestamp based on the first and second timestamp.
In accordance with another aspect, a method for assigning a timestamp is presented. A first and second reference clock signals are generated, in which the reference clock signals are non-synchronous. A trigger signal is received in response to a detected event. A first timestamp is determined based on a temporal relationship between the trigger signal and the first reference clock signal, and a second timestamp is determined based on a temporal relationship between the trigger signal and the second reference clock signal. A corrected timestamp is outputted based on the first and the second timestamps.
One advantage is that time resolution of timing circuits are improved.
Another advantage resides in redundancy.
Still further advantages of the present invention will be appreciated to those of ordinary skill in the art upon reading and understand the following detailed description.
The invention may take form in various components and arrangements of components, and in various steps and arrangements of steps. The drawings are only for purposes of illustrating the preferred embodiments and are not to be construed as limiting the invention.
With reference to
The radiation tomography scanner 8 includes a plurality of radiation detector modules 10 oriented to receive radiation from an imaging region 12. The radiation detector modules 10 are arranged in several adjacent rings along an axial direction; however, other arrangements of radiation detector modules can be used. Typically the radiation detector modules 10 are housed within a housing 14 of the tomography scanner 8 and thus are not visible from the outside. Each ring is comprised of hundreds or thousands of radiation detector modules 10. In some scanners, only a single ring of radiation detector modules 10 is provided, in others, up to five or more rings of radiation detector modules 10 are provided. It should be appreciated that detector heads can be used in place of the detector ring structure shown in
Each radiation detector module 10 typically includes a scintillator crystal disposed adjacent to the examination region. The scintillator crystal absorbs the γ ray (e.g. 511 keV in PET scanners) to generate a scintillation of optical photons. The photons are detected at the opposite end of the scintillator crystal by an array of photoelectric detectors such as photomultiplier tubes, photodiodes, SiPMs, or the like. In another embodiment, the scintillator crystal is a pixelated scintillator constructed from a plurality of optically isolated scintillator crystals, each coupled to a single photoelectric detector. Upon detection of photons, the photoelectric detector outputs a signal, or a plurality of signals if multiple photoelectric detectors are viewing the scintillation event, that is indicative of a detected radiation event. Each photoelectric detector is operatively connected to a trigger unit 20 that monitors the photoelectric detector output for the signal. If a signal is detected the trigger unit generates a trigger signal for the timing circuit 22, so as to timestamp the detected radiation event.
With reference to
At the input of each TDC, a storage element 36, 37, e.g. a flip-flop, latch, or the like, is latched when the trigger signal is present. If the input is stable the switch will latch at the subsequent rising edge of the reference clock. However, if the trigger signal is received at the input during a meta-stable region, i.e. during the setup or hold times, the switch may become meta-stable and the trigger signal will not be latched until the next rising edge of the reference clock leading to a significant increase in the timestamp error.
To reduce timing errors stemming from meta- stability at the input each TDC is synchronized to a unique reference clock. The first TDC 30 is synchronized to a first reference signal and the second TDC 31 is synchronized to a second reference signal. In one embodiment (
With reference again to
With reference again to
The invention has been described with reference to the preferred embodiments. Modifications and alterations may occur to others upon reading and understanding the preceding detailed description. It is intended that the invention be constructed as including all such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof.
This application claims the benefit of U.S. provisional application Ser. No. 61/181,705 filed May 28, 2009, which is incorporated herein by reference.
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/IB2010/051647 | 4/15/2010 | WO | 00 | 11/7/2011 |
Publishing Document | Publishing Date | Country | Kind |
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WO2010/136910 | 12/2/2010 | WO | A |
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61181705 | May 2009 | US |