We proposed and tested a novel geometry for PET system design analogous to pinhole SPECT called the virtual-pinhole PET (VP-PET) geometry to determine whether it could provide highresolution images. Methods: We analyzed the effects of photon acolinearity and detector sizes on system resolution and extended the empiric formula for reconstructed image resolution of conventional PET proposed earlier to predict the resolutions of VP-PET. To measure the system resolution of VP-PET, we recorded coincidence events as a 22 Na point source was stepped across the coincidence line of response between 2 detectors made from identical arrays of 12 · 12 lutetium oxyorthosilicate crystals (each measuring 1.51 · 1.51 · 10 mm 3 ) separated by 565 mm. To measure reconstructed image resolution, we built 4 VP-PET systems using 4 types of detectors (width, 1.51-6.4 mm) and imaged 4 point sources of 64 Cu (half-life 5 12.7 h to allow a long acquisition time). Tangential and radial resolutions were measured and averaged for each source and each system. We then imaged a polystyrene plastic phantom representing a 2.5-cm-thick cross-section of isolated breast volume. The phantom was filled with an aqueous solution of 64 Cu (713 kBq/mL) in which the following were imbedded: 4 spheric tumors ranging from 1.8 to 12.6 mm in inner diameter (ID), 6 micropipettes (0.7-or 1.1-mm ID filled with 64 Cu at 5·, 20·, or 50· background), and a 10.0-mm outer-diameter cold lesion. Results: The shape and measured full width at half maximum of the line spread functions agree well with the predicted values. Measured reconstructed image resolution (2.40-3.24 mm) was 66% of the predicted value for 3 of the 4 systems. In one case, the difference was 12.6%, possibly due to underestimation of the block effect from the low-resolution detector. In phantom experiments, all spheric tumors were detected. Small line sources were detected if the activity concentration is at least 20· background. Conclusion: We have developed and characterized a novel geometry for PET. A PET system following the VP-PET geometry provides high-resolution images for objects near the system's high-resolution detectors. This geometry may lead to the development of special-purpose PET systems or resolution-enhancing insert devices for conventional PET scanners. PET has evolved from a research tool for studying neurologic and cardiac functions of humans (1) to a clinical diagnostic tool for cancer patients (2), particularly since the introduction of PET/CT technology (3). With the introduction of high-resolution animal PET scanners in the mid1990s (4,5), PET became a driving force behind molecular imaging through in vivo imaging of small animals using positron-emitting radionuclide-labeled biomolecules (6).Resolution of PET is limited by the positron range of the radionuclide, acolinearity of the annihilation g-rays, and intrinsic spatial resolution of the detectors. For whole-body PET scanners with large diameters, the blurring of image resolution due to the acolinearity effect is approximatel...

A PET block detector module using an array of sub-millimeter lutetium oxyorthosilicate (LSO) crystals read out by an array of surface-mount, semiconductor photosensors has been developed. The detector consists of a LSO array, a custom acrylic light guide, a 3 × 3 multi-pixel photon counter (MPPC) array (S10362-11-050P, Hamamatsu Photonics, Japan) and a readout board with a charge division resistor network. The LSO array consists of 100 crystals, each measuring 0.8 × 0.8 × 3 mm3 and arranged in 0.86 mm pitches. A Monte Carlo simulation was used to aid the design and fabrication of a custom light guide to control distribution of scintillation light over the surface of the MPPC array. The output signals of the nine MPPC are multiplexed by a charge division resistor network to generate four position-encoded analog outputs. Flood image, energy resolution and timing resolution measurements were performed using standard NIM electronics. The linearity of the detector response was investigated using gamma-ray sources of different energies. The 10 × 10 array of 0.8 mm LSO crystals was clearly resolved in the flood image. The average energy resolution and standard deviation were 20.0% full-width at half-maximum (FWHM) and ±5.0%, respectively, at 511 keV. The timing resolution of a single MPPC coupled to a LSO crystal was found to be 857 ps FWHM, and the value for the central region of detector module was 1182 ps FWHM when ±10% energy window was applied. The nonlinear response of a single MPPC when used to read out a single LSO was observed among the corner crystals of the proposed detector module. However, the central region of the detector module exhibits significantly less nonlinearity (6.5% for 511 keV). These results demonstrate that (1) a charge-sharing resistor network can effectively multiplex MPPC signals and reduce the number of output signals without significantly degrading the performance of a PET detector and (2) a custom light guide to permit light sharing among multiple MPPC and to diffuse and direct scintillation light can reduce the nonlinearity of the detector response within the limited dynamic range of a typical MPPC. As a result, the proposed PET detector module has the potential to be refined for use in high-resolution PET insert applications.

A full-ring PET insert device should be able to enhance the image resolution of existing small-animal PET scanners. Methods: The device consists of 18 high-resolution PET detectors in a cylindric enclosure. Each detector contains a cerium-doped lutetium oxyorthosilicate array (12 · 12 crystals, 0.72 · 1.51 · 3.75 mm each) coupled to a position-sensitive photomultiplier tube via an optical fiber bundle made of 8 · 16 square multiclad fibers. Signals from the insert detectors are connected to the scanner through the electronics of the disabled first ring of detectors, which permits coincidence detection between the 2 systems. Energy resolution of a detector was measured using a 68 Ge point source, and a calibrated 68 Ge point source stepped across the axial field of view (FOV) provided the sensitivity profile of the system. A 22 Na point source imaged at different offsets from the center characterized the in-plane resolution of the insert system. Imaging was then performed with a Derenzo phantom filled with 19.5 MBq of 18 F-fluoride and imaged for 2 h; a 24.3-g mouse injected with 129.5 MBq of 18 F-fluoride and imaged in 5 bed positions at 3.5 h after injection; and a 22.8-g mouse injected with 14.3 MBq of 18 F-FDG and imaged for 2 h with electrocardiogram gating. Results: The energy resolution of a typical detector module at 511 keV is 19.0% 6 3.1%. The peak sensitivity of the system is approximately 2.67%. The image resolution of the system ranges from 1.0-to 1.8-mm full width at half maximum near the center of the FOV, depending on the type of coincidence events used for image reconstruction. Derenzo phantom and mouse bone images showed significant improvement in transaxial image resolution using the insert device. Mouse heart images demonstrated the gated imaging capability of the device. Conclusion: We have built a prototype full-ring insert device for a small-animal PET scanner to provide higher-resolution PET images within a reduced imaging FOV. Development of additional correction techniques are needed to achieve quantitative imaging with such an insert. Hi gh-resolution PET scanners dedicated to small-animal imaging have been developed by several research groups since the 1990s (1-11). Combining high-resolution and quantitative imaging capability, small-animal PET has been a driving force behind the development of molecular imaging that brings together scientists from different disciplines to study biologic effects at the molecular level (12,13). Smallanimal PET has also been adopted by the pharmaceutical industry to study pharmacokinetics and pharmacodynamics to accelerate development of new drugs (14). The increasing demand for small-animal PET has led to commercialization of several small-animal PET technologies (15). Current technologic research and development is focused on further improvement of the resolution or sensitivity of small-animal PET systems (16).Most commercial small-animal PET scanners use inorganic scintillators for g-ray detection, a proven technology that provides good image resoluti...

Virtual-pinhole PET (VP-PET) imaging is a new technology in which one or more high-resolution detector modules are integrated into a conventional PET scanner with lower-resolution detectors. It can locally enhance the spatial resolution and contrast recovery near the add-on detectors, and depending on the configuration, may also increase the sensitivity of the system. This novel scanner geometry makes the reconstruction problem more challenging compared to the reconstruction of data from a standalone PET scanner, as new techniques are needed to model and account for the non-standard acquisition. In this paper, we present a general framework for fully 3D modeling of an arbitrary VP-PET insert system. The model components are incorporated into a statistical reconstruction algorithm to estimate an image from the multi-resolution data. For validation, we apply the proposed model and reconstruction approach to one of our custom-built VP-PET systems – a half-ring insert device integrated into a clinical PET/CT scanner. Details regarding the most important implementation issues are provided. We show that the proposed data model is consistent with the measured data, and that our approach can lead to reconstructions with improved spatial resolution and lesion detectability.

We are developing an insert device that will improve image resolution within a smaller field-of-view for clinical wholebody PET scanners. We modified SimSET (Simulation System for Emission Tomography) to simulate the insert and a PET scanner. The system consists of two detector rings. The inner ring represents an insert (r = 153 mm) with high-resolution detectors using 10 mm thick LSO. The outer ring represents a PET scanner (R = 413 mm) with 25 mm thick LSO. Events were binned into three sets of sinograms assuming a 2.4 and 6.75 mm crystal-pitch for the insert and the PET scanner, respectively. The detectors in the insert are modeled as 1, 2, or 4 layers with different offset configurations to evaluate the corresponding system resolution with the depth-ofinteraction (DOI) effect. Results show that image resolution at 1 cm radial offset is improved from 5.6 mm full-width-at-half-maximum (FWHM) of the original PET scanner to 2.0 mm with the insert. At 12 cm offset, the resolution of the original system is 5.9 and 5.5 mm for radial and tangential directions, respectively. With the insert, the radial resolution is 5.0 mm FWHM for a single-layer detector design, but improves to 2.7 and 2.2 mm for 2-and 4-layer DOI detectors, respectively. Different offsets for multi-layer detectors have negligible effect on resolution. Sensitivity of the device is, assuming the insert has a 2 cm axial extend, estimated to be 3.3%, including coincidence events from the insert-alone and insert-to-scanner sinograms. In contrast, if the insert is used as a stand-alone microPET scanner, its sensitivity is 1.3%.Index Terms-Depth of interaction (DOI), head and neck imaging, high resolution imaging, Monte Carlo simulations, pseudo pinhole positron emission tomography (PET).