Continuous Phoswich™ Scintillators and Detectors

       RMD's Imaging Technology Group has developed a unique, first-of-its-kind, advanced family of phoswich scintillators and related detector technology, capable of providing continuous depth-of-interaction (DOI) information in a monolithic scintillator for gamma interactions in PET, SPECT and other gamma-sensing systems used in medical imaging, security and other applications.

        Termed Continuous Phoswich™ scintillators (CPS™) and detectors (CPD™), this breakthrough development in radiation detection, measurement and imaging is applicable in clinical medical imaging, pre-clinical small animal imaging, security applications such as baggage and cargo scanning, and industrial imaging. RMD's CPS material, in addition to its Continuous Phoswich capability, has been designed and shown to provide high light output, high speed, excellent energy resolution, insignificant afterglow and no hysteresis.

        Continuous Phoswich detection adds a third dimension to the localization of a scintillation event within a scintillator, thereby enabling the more accurate localization of the origin of the positron annihilation or gamma emission within the patient, small animal or other scan subject, in order to produce images with higher spatial and contrast resolution, free of radial elongation and parallax effects that result in degraded image quality, constrain system design or require greater system complexity in an effort to reduce these effects.

        With its inherent DOI determination capability, our Continuous Phoswich scintillator may be made thicker than a conventional scintillator, in order to increase sensitivity without compromising spatial resolution. With DOI determination, the traditional and often difficult imaging tradeoff between spatial resolution (favoring a thin sensor) and detection efficiency (favoring a thick sensor) no longer applies, since DOI determination decouples the previously fundamental relationship between these two important considerations. Increased sensitivity leads to faster imaging (e.g., for improved gated cardiac imaging or reduced scan time), reduced patient or subject dose (through greater collection efficiency), greater collection efficiency for short-lived isotopes, and/or improved imaging of brief phenomena (e.g., bolus passage, tracer washout or tracer decay).

        With CPS and CPD technology, utilizing conventional photodetectors and straightforward modifications to conventional front-end electronics, the position of event occurrence along a selected axis in the Continuous Phoswich scintillator is determined. This DOI information is then used in electronics and/or software to adjust X-Y position information in SPECT and gamma camera imaging and, in the case of PET, line-of-response (LOR) location and time-of-flight (TOF) information.

       In PET*, RMD's CPS and CPD technology eliminates or renders inconsequential the radial elongation effect of so-called "off-axis" events (that is, event coincidence detected by scintillators that do not share a common long axis), improves the timing resolution for "on-axis" events, and can even yield better off-axis than on-axis spatial resolution.
              (* Positron Emission Tomography.)

        As seen in an accompanying figure, PET spatial resolution for on-axis events is determined by scintillator crystal cross-sectional area and achievable event timing (TOF) resolution, with both conventional scintillators and CPS/CPD technology. However, for off-axis events with conventional scintillators, the radial elongation effect degrades spatial resolution increasingly with increasing distance between the event LOR and the central scan axis (since PET scintillator length is greater than its width), while event timing resolution actually improves somewhat with this increased distance.

        CPS/CPD technology determines the DOI of an event in a scintillator, and allows the straightforward determination, in front-end detector electronics and/or software, of the event LOR. As a result, the radial elongation error for off-axis events is comparable to or even less than that for on-axis events. CPS/CPD technology also allows radially longer PET scintillators (with single photodetectors) for increased overall sensitivity, and allows lines of response (LORs) that are farther from the central scan axis for improved utilization of the PET scanner bore and smaller minimum angles between pairs of coincidence detectors.

       Historically, most previous attempts to develop useful phoswich detection technology have been aimed at PET, where limited improvement over non-DOI coincidence sensing has been offered by often cumbersome and expensive solutions, that at best still supply only 1 or 2 bits of event depth information. Such phoswich detectors have utilized a variety of designs, including (typically) discrete combinations of multiple materials, sometimes double the number of photodetectors otherwise required, sometimes wavelength-shifting materials, and often significantly more complex electronics than otherwise required, to provide only 1 or 2 bits of DOI information. This often comes at the expense of light loss and reduced energy resolution, and at considerably greater system complexity and expense than standard non-phoswich scintillators and detectors.

        In SPECT* and other pinhole and related gamma imaging, CPS and CPD technology eliminates or renders inconsequential the parallax effects encountered in single-photon imaging with a gamma camera with pinhole, multi-pinhole or other focused collimation, including many forms of SPECT and gamma camera imaging.
              (* Single-Photon Emission Computed Tomography.)

        As seen in an accompanying figure, for pinhole, multi-pinhole and related collimators used in SPECT, and for some limited-angle tomography and planar imaging, the parallax effect causes degradation of resolution that increases radially from the point on the scintillator closest to the focal point of the radiation source (no parallax), to the edge of the scintillator (greatest parallax error).

        CPS/CPD technology allows the straightforward correction, in front-end detector electronics and/or software, of interaction event position in the Continuous Phoswich scintillator to a virtual thin plane, regardless of the actual scintillator thickness.

        RMD's CPS and CPD materials and devices are undergoing active test and evaluation, on an ongoing basis, at RMD and at the Center for Gamma Ray Imaging (CGRI) at the University of Arizona (Tucson, AZ), for use in PET, SPECT and other gamma imaging devices and systems.