Feasibility of Using Crystal Geometry for a DOI Scintillation Detector.

We have used simulations and measurements to investigate the feasibility of using slanted scintillator crystal geometries as means to provide depth-of-interaction (DOI) information for a pixelated gamma ray imaging detector. The simulations were performed to estimate the fraction of scintillation light detected by the photodetector as a function of interaction location along the height of crystals with different geometries. In addition, physical measurements of the light output for these crystal geometries were obtained from individual crystals coupled to a solid state photodetector (Philips digital-SiPM DPC-3200). In agreement with previous work, we found a change in light output in the slanted region of the crystals compared to the rectangular region. The results from this study indicate the potential of using slanted crystals to gather DOI information based on light output changes as a function of the location of interaction. An examination of the measured energy spectra for the geometries evaluated here, suggests that for BGO crystals somewhere between 2 or 3 DOI bins could be implemented. Based on these results, we conceived a design for a DOI detector module that consists of two slanted crystals, each read-out by separate SiPM pixels.

A New Pulse Pileup Rejection Method Based on Position Shift Identification.

Pulse pileup events degrade the signal-to-noise ratio (SNR) of nuclear medicine data. When such events occur in multiplexed detectors, they cause spatial misposition, energy spectrum distortion and degraded timing resolution, which leads to image artifacts. Pulse pileup is pronounced in PETbox4, a bench top PET scanner dedicated to high sensitivity and high resolution imaging of mice. In that system, the combination of high absolute sensitivity, long scintillator decay time (BGO) and highly multiplexed electronics lead to a significant fraction of pulse pileup, reached at lower total activity than for comparable instruments. In this manuscript, a new pulse pileup rejection method named position shift rejection (PSR) is introduced. The performance of PSR is compared with a conventional leading edge rejection (LER) method and with no pileup rejection implemented (NoPR). A comprehensive digital pulse library was developed for objective evaluation and optimization of the PSR and LER, in which pulse waveforms were directly recorded from real measurements exactly representing the signals to be processed. Physical measurements including singles event acquisition, peak system sensitivity and NEMA NU-4 image quality phantom were also performed in the PETbox4 system to validate and compare the different pulse pile-up rejection methods. The evaluation of both physical measurements and model pulse trains demonstrated that the new PSR performs more accurate pileup event identification and avoids erroneous rejection of valid events. For the PETbox4 system, this improvement leads to a significant recovery of sensitivity at low count rates, amounting to about 1/4th of the expected true coincidence events, compared to the LER method. Furthermore, with the implementation of PSR, optimal image quality can be achieved near the peak noise equivalent count rate (NECR).

A DOI Detector With Crystal Scatter Identification Capability for High Sensitivity and High Spatial Resolution PET Imaging.

A new phoswich detector is being developed at the Crump Institute, aiming to provide improvements in sensitivity, and spatial resolution for PET. The detector configuration is comprised of two layers of pixelated scintillator crystal arrays, a glass light guide and a light detector. The annihilation photon entrance (top) layer is a 48 × 48 array of 1.01 × 1.01 × 7 mm3 LYSO crystals. The bottom layer is a 32 × 32 array of 1.55 × 1.55 × 9 mm3 BGO crystals. A tapered, multiple-element glass lightguide is used to couple the exit end of the BGO crystal array (52 × 52 mm2) to the photosensitive area of the Position Sensitive Photomultiplier Tube (46 × 46 mm2), allowing the creation of flat panel detectors without gaps between the detector modules. Both simulations and measurements were performed to evaluate the characteristics and benefits of the proposed design. The GATE Monte Carlo simulation indicated that the total fraction of the cross layer crystal scatter (CLCS) events in singles detection mode for this detector geometry is 13.2%. The large majority of these CLCS events (10.1% out of 13.2%) deposit most of their energy in a scintillator layer other than the layer of first interaction. Identification of those CLCS events for rejection or correction may lead to improvements in data quality and imaging performance. Physical measurements with the prototype detector showed that the LYSO, BGO and CLCS events were successfully identified using the delayed charge integration (DCI) technique, with more than 95% of the LYSO and BGO crystal elements clearly resolved. The measured peak-to-valley ratios (PVR) in the flood histograms were 3.5 for LYSO and 2.0 for BGO. For LYSO, the energy resolution ranged from 9.7% to 37.0% full width at half maximum (FWHM), with a mean of 13.4 ± 4.8%. For BGO the energy resolution ranged from 16.0% to 33.9% FWHM, with a mean of 18.6 ± 3.2%. In conclusion, these results demonstrate that the proposed detector is feasible and can potentially lead to a high spatial resolution, high sensitivity and DOI PET system.

NEMA NU-4 performance evaluation of PETbox4, a high sensitivity dedicated PET preclinical tomograph.

PETbox4 is a new, fully tomographic bench top PET scanner dedicated to high sensitivity and high resolution imaging of mice. This manuscript characterizes the performance of the prototype system using the National Electrical Manufacturers Association NU 4-2008 standards, including studies of sensitivity, spatial resolution, energy resolution, scatter fraction, count-rate performance and image quality. The PETbox4 performance is also compared with the performance of PETbox, a previous generation limited angle tomography system. PETbox4 consists of four opposing flat-panel type detectors arranged in a box-like geometry. Each panel is made by a 24 × 50 pixelated array of 1.82 × 1.82 × 7 mm bismuth germanate scintillation crystals with a crystal pitch of 1.90 mm. Each of these scintillation arrays is coupled to two Hamamatsu H8500 photomultiplier tubes via a glass light guide. Volumetric images for a 45 × 45 × 95 mm field of view (FOV) are reconstructed with a maximum likelihood expectation maximization algorithm incorporating a system model based on a parameterized detector response. With an energy window of 150-650 keV, the peak absolute sensitivity is approximately 18% at the center of FOV. The measured crystal energy resolution ranges from 13.5% to 48.3% full width at half maximum (FWHM), with a mean of 18.0%. The intrinsic detector spatial resolution is 1.5 mm FWHM in both transverse and axial directions. The reconstructed image spatial resolution for different locations in the FOV ranges from 1.32 to 1.93 mm, with an average of 1.46 mm. The peak noise equivalent count rate for the mouse-sized phantom is 35 kcps for a total activity of 1.5 MBq (40 µCi) and the scatter fraction is 28%. The standard deviation in the uniform region of the image quality phantom is 5.7%. The recovery coefficients range from 0.10 to 0.93. In comparison to the first generation two panel PETbox system, PETbox4 achieves substantial improvements on sensitivity and spatial resolution. The overall performance demonstrates that the PETbox4 scanner is suitable for producing high quality images for molecular imaging based biomedical research.

Readout of the Optical PET (OPET) Detector.

The design of an imaging system capable of detecting both high-energy γ-rays and optical wavelength photons is underway at the Crump Institute for Molecular Imaging. This system will noninvasively image small animal models in vivo for the presence of positron emission tomographic (PET) and optical signals. The detector will consist of modules of multichannel photomultiplier tubes (MC-PMT) coupled to arrays of scintillator crystals. The MC-PMT will detect both the photons produced due to bioluminescence and the photons generated by the interaction of γ-rays within the crystals. The long wavelength photons produced through bioluminescence are only slightly attenuated by these crystals and are detected directly at the photocathode of the MC-PMT, resulting in signals of small (5-10 mV) short (~15 ns) pulses. In contrast, annihilation (511 keV) γ-rays interacting in the scintillator crystal send large bursts of photons to the PMT, and result in pulses that can be as large as 500 mV and > 200 ns duration. The processing of pulses with such different characteristics in a single circuit requires significant alteration of the standard pulse processing circuitry used in PET scanners. In this paper, we discuss the requirements of such a circuit and show the results of implementation of one design using single and multiple channel PMTs.

Detector Concept for OPET-A Combined PET and Optical Imaging System.

The design of an imaging system capable of detecting both high-energy γ-rays and optical wavelength photons is underway at the UCLA Crump Institute for Molecular Imaging. This system, which we call optical PET (OPET), will be capable of non-invasively and repeatedly imaging small animal models in vivo for the presence of PET and optical signals. In this study, we describe the physical principles behind the operation of the OPET imaging system and discuss the design concept for one of the detector modules. Additionally, we demonstrate the operation of an initial prototype detector module for simultaneous detection and imaging of annihilation radiation and single optical photons emanating from separate sources. These results indicate that the construction of an imaging system based on this detector technology is feasible.

Hospitalwide medication policies and standards.

The development and composition of a hospitalwide medication policies and standards manual are described. Medication policies and procedures developed independently by individual hospital departments and services at a 789-bed private teaching institution created problems related to consistency, the approval process, accreditation standards, and retrievability. Therefore, a joint nursing-pharmacy task force was formed to create a master document containing medication policies and standards for the entire institution. The manual also contains departmental medication-related procedures, the formulary of approved drugs, and key drug information. Its format allows for periodic updating and ease of use by nurses, pharmacists, physicians, and other health care professionals. It meets the Joint Commission on Accreditation of Healthcare Organizations (JCAHO) requirement of a collaborative framework to ensure optimal medication-use outcomes. All medication-related procedures in individual departments must comply with the new policies and standards. A hospitalwide medication policy manual, created through multidisciplinary collaboration, made the policies and procedures consistent and more accessible and met JCAHO and other regulatory requirements.

How satisfying is the practice of internal medicine? A national survey.

OBJECTIVE: To survey members of the American College of Physicians about their level of satisfaction and sources of dissatisfaction with practice, the extent to which their satisfaction is reflected in their counseling of students about careers in internal medicine, the prevalence of their concerns about patients' access to care, and their attitudes toward changes in the health care system. DESIGN: A questionnaire survey of a random sample of members of the College. PARTICIPANTS: Members (2254) of the College were surveyed; 1446 (64%) responded and 1290 (57%) of the responses were usable. MEASUREMENTS AND MAIN RESULTS: More than 80% of respondents were satisfied with their relationships with patients, professional challenges, and opportunities to interact with colleagues. Only about half were satisfied with their potential income, and most were dissatisfied with their autonomy or loss of control over clinical decision making. Major sources of concern were administrative burdens, the threat of malpractice litigation, loss of income, and loss of clinical autonomy. Forty percent of internists say that they discourage students from careers in internal medicine, and only 39% would once again pursue such a career. "Pain in the practice" seems generalized among internists: logistic regression analysis failed to show any specific groups who were most dissatisfied or concerned. With regard to access, 18% of internists had many patients without health care insurance; 69% had some patients without coverage; and the majority (61%) had some patients who lost their insurance because of changing jobs or location. Assuring universal access to care received the highest priority rating of a set of criteria for health care reform. CONCLUSIONS: There is growing dissatisfaction with the practice of internal medicine, primarily related to concerns over loss of clinical autonomy, the increase in administrative burdens, the potential loss of income, and the threat of malpractice litigation. Although physicians are concerned about patients' access to care, proposals to improve access should be examined for their effects on the major sources of physicians' dissatisfaction with practice.