Objective.Time-of-flight (TOF) resolution improves positron emission tomography (PET) by localizing annihilation events along the line of response, which reduces statistical noise and improves image quality. In open-geometry PET systems, improved TOF can compensate for limited angular sampling by reducing geometric distortions and reconstruction artifacts. This work quantitatively studies how TOF resolution and angular coverage together determine image quality in open-geometry PET.Approach.Using Monte Carlo simulations, we estimate the minimum angular coverage required to obtain reconstructions with minimal distortion for different TOF performance levels. An idealized PET scanner model is first used to isolate the effects of angular sampling and TOF, without detector-related blurring. Five angular coverages (60, 90, 120, 150and 180) and six coincidence timing resolutions (400, 200, 100, 75, 50 and 25 ps FWHM) are evaluated. Image quality is assessed using point sources, the Derenzo phantom and the NEMA image quality phantom, reporting spatial resolution, contrast recovery, background variability and geometric distortion quantified by ellipse fitting.Main results.The results show that TOF resolutions below approximately 100 ps are a key enabling factor for highly open PET geometries, substantially reducing limited-angle artifacts and allowing image quality to approach that of full-ring systems. The main trends are further validated using a realistic Monte Carlo model of the Siemens Biograph Vision PET/CT scanner.Significance.These results provide practical design guidance for next-generation open-geometry PET systems, showing that fast TOF performance can reduce required detector coverage while preserving image quality.

BACKGROUND:Panel detectors have the potential to provide a flexible, modular approach to Positron Emission Tomography (PET), enabling customization to meet patient-specific needs and scan objectives. The panel design allows detectors to be positioned close to the patient, aiming to enhance sensitivity and spatial resolution through improved geometric coverage and reduced noncollinearity blurring. Parallax error can be mitigated using depth of interaction (DOI) information. PURPOSE:One of the key questions the article addresses is: Do panel detectors offer viable clinical imaging capabilities, or does limited angular sampling restrict their utility by causing image distortions and artifacts? Additionally, this article explores the scalability of panel detectors for constructing scanners with a long axial field of view (LAFOV). METHODS:Monte Carlo simulations using GATE software were used to assess the performance of panel detectors with various DOI resolutions and Time-of-Flight (TOF) resolutions as fine as 70 ps. The 30 × $\times$ 30 cm panels comprised pixelated 3 × $\times$ 3 × $\times$ 20 mm LSO crystals. Simulations were run on large high-performance computing clusters (122,000 CPU cores). Open-source CASToR software was used for (TOF MLEM) image reconstruction. The image quality of the scanners was assessed using a range of phantoms (NEMA, Derenzo, XCAT, and a high-resolution brain phantom). The Siemens Biograph Vision PET/CT scanner served as the reference model. The performance of larger 120 × $\times$ 60 cm panels was also evaluated. RESULTS:Sensitivity increases over threefold when panel-panel distance is reduced from 80 to 40 cm. The noise equivalent count rate, unmodified by TOF gain, of the panel detectors matches that of the reference clinical scanner at a distance of approximately 50 cm between the panels. Spatial resolution perpendicular to the panels improves from 8.7 to 1.6 mm when the panel-panel distance is reduced, and 70 ps + DOI detectors are used instead of 200 ps, no-DOI detectors. With enhanced TOF and DOI capabilities, panel detectors achieve image quality that matches or surpasses the reference scanner while using about four times less detector material. These detectors can be extended for LAFOV imaging without distortions or artifacts. Additionally, improving TOF and DOI performance enhances contrast-to-noise ratios, thereby improving lesion detection. CONCLUSIONS:A compact 2-panel PET scanner can match the performance of conventional scanners, producing high-quality, distortion-free images. Its mobility and flexibility enable novel applications, including bedside imaging and intensive care unitdiagnostics, as well as imaging in positions such as sitting or standing. Furthermore, the modularity of panel detectors offers the potential to construct cost-effective, high-performance total-body imaging systems.

Abstract Background Mobile upright PET devices have the potential to enable previously impossible neuroimaging studies. Currently available options are imagers with deep brain coverage that severely limit head/body movements or imagers with upright/motion enabling properties that are limited to only covering the brain surface. Methods In this study, we test the feasibility of an upright, motion-compatible brain imager, our Ambulatory Motion-enabling Positron Emission Tomography (AMPET) helmet prototype, for use as a neuroscience tool by replicating a variant of a published PET/fMRI study of the neurocorrelates of human walking. We validate our AMPET prototype by conducting a walking movement paradigm to determine motion tolerance and assess for appropriate task related activity in motor-related brain regions. Human participants (n = 11 patients) performed a walking-in-place task with simultaneous AMPET imaging, receiving a bolus delivery of F18-Fluorodeoxyglucose. Results Here we validate three pre-determined measure criteria, including brain alignment motion artifact of less than <2 mm and functional neuroimaging outcomes consistent with existing walking movement literature. Conclusions The study extends the potential and utility for use of mobile, upright, and motion-tolerant neuroimaging devices in real-world, ecologically-valid paradigms. Our approach accounts for the real-world logistics of an actual human participant study and can be used to inform experimental physicists, engineers and imaging instrumentation developers undertaking similar future studies. The technical advances described herein help set new priorities for facilitating future neuroimaging devices and research of the human brain in health and disease.

The collaboration of Yale, the University of California, Davis, and United Imaging Healthcare has successfully developed the NeuroEXPLORER, a dedicated human brain PET imager with high spatial resolution, high sensitivity, and a built-in 3-dimensional camera for markerless continuous motion tracking. It has high depth-of-interaction and time-of-flight resolutions, along with a 52.4-cm transverse field of view (FOV) and an extended axial FOV (49.5 cm) to enhance sensitivity. Here, we present the physical characterization, performance evaluation, and first human images of the NeuroEXPLORER. Methods: Measurements of spatial resolution, sensitivity, count rate performance, energy and timing resolution, and image quality were performed adhering to the National Electrical Manufacturers Association (NEMA) NU 2-2018 standard. The system's performance was demonstrated through imaging studies of the Hoffman 3-dimensional brain phantom and the mini-Derenzo phantom. Initial 18F-FDG images from a healthy volunteer are presented. Results: With filtered backprojection reconstruction, the radial and tangential spatial resolutions (full width at half maximum) averaged 1.64, 2.06, and 2.51 mm, with axial resolutions of 2.73, 2.89, and 2.93 mm for radial offsets of 1, 10, and 20 cm, respectively. The average time-of-flight resolution was 236 ps, and the energy resolution was 10.5%. NEMA sensitivities were 46.0 and 47.6 kcps/MBq at the center and 10-cm offset, respectively. A sensitivity of 11.8% was achieved at the FOV center. The peak noise-equivalent count rate was 1.31 Mcps at 58.0 kBq/mL, and the scatter fraction at 5.3 kBq/mL was 36.5%. The maximum count rate error at the peak noise-equivalent count rate was less than 5%. At 3 iterations, the NEMA image-quality contrast recovery coefficients varied from 74.5% (10-mm sphere) to 92.6% (37-mm sphere), and background variability ranged from 3.1% to 1.4% at a contrast of 4.0:1. An example human brain 18F-FDG image exhibited very high resolution, capturing intricate details in the cortex and subcortical structures. Conclusion: The NeuroEXPLORER offers high sensitivity and high spatial resolution. With its long axial length, it also enables high-quality spinal cord imaging and image-derived input functions from the carotid arteries. These performance enhancements will substantially broaden the range of human brain PET paradigms, protocols, and thereby clinical research applications.

INTRODUCTION:Sentinel lymph node biopsy (SLNB) is a standard practice for staging cutaneous melanoma. High false-negative rates have an increased interest in adjunctive techniques for localizing SLNs. Mobile gamma cameras (MGCs) represent potential tools to enhance SLNB performance. METHODS:An institutional review board approval was obtained for this study (ClinicalTrials.gov ID NCT01531608). After obtaining informed consent, 20 eligible melanoma patients underwent 99mTc sulfur colloid injection and standard lymphoscintigraphy with a fixed gamma camera (FGC). A survey using a 20 cm square MGC, performed immediately preoperatively by the study surgeon, was used to establish an operative plan while blinded to the FGC results. Subsequently, SLNB was performed using a gamma probe and a novel 6 cm diameter handheld MGC. RESULTS:A total of 24 SLN basins were detected by FGC. Prior to unblinding, all 24 basins were identified with the preoperative MGC and the operative plan established by preoperative MGC imaging was confirmed accurate by review of the FGC images. All individual sentinel lymph nodes were identified during intraoperative MGC imaging, and in 5/24 (21%) cases, surgeon-reported additional clinically useful information was obtained from the MGC. CONCLUSIONS:Preoperative MGC images provide information consistent with FGC images for planning SLNB and in some cases provide additional information that aided in surgical decision-making.

The recently observed FLASH effect related to high doses delivered with high rates has the potential to revolutionize radiation cancer therapy if promising results are confirmed and an underlying mechanism understood. Comprehensive measurements are essential to elucidate the phenomenon. We report the first-ever demonstration of measurements of successive in-spill and post-spill emissions of gammas arising from irradiations by a FLASH proton beam. A small positron emission tomography (PET) system was exposed in an ocular beam of the Proton Therapy Center at MD Anderson Cancer Center to view phantoms irradiated by 3.5 × 10 10 protons with a kinetic energy of 75.8 MeV delivered in 101.5 ms-long spills yielding a dose rate of 164 Gy s −1 . Most in-spill events were due to prompt gammas. Reconstructed post-spill tomographic events, recorded for up to 20 min, yielded quantitative imaging and dosimetric information. These findings open a new and novel modality for imaging and monitoring of FLASH proton therapy exploiting in-spill prompt gamma imaging followed by post-spill PET imaging.

We demonstrate the first ever recorded positron-emission tomography (PET) imaging and dosimetry of a FLASH proton beam at the Proton Center of the MD Anderson Cancer Center. Two scintillating LYSO crystal arrays, read out by silicon photomultipliers, were configured with a partial field of view of a cylindrical poly-methyl methacrylate (PMMA) phantom irradiated by a FLASH proton beam. The proton beam had a kinetic energy of 75.8 MeV and an intensity of about 3.5 × 10 10 protons that were extracted over 101.5 ms-long spills. The radiation environment was characterized by cadmium–zinc–telluride and plastic scintillator counters. Preliminary results indicate that the PET technology used in our tests can efficiently record FLASH beam events. The instrument yielded informative and quantitative imaging and dosimetry of beam-activated isotopes in a PMMA phantom, as supported by Monte Carlo simulations. These studies open a new PET modality that can lead to improved imaging and monitoring of FLASH proton therapy.

The NeuroEXPLORER (NX) is a dedicated human brain PET/CT imager with fine spatial resolution, high sensitivity, and continuous head motion correction capability. The performance of the scanner was evaluated based on the NEMA NU 2-2012 standard. With FBP reconstruction, the average radial/tangential spatial resolutions were 1.89 and 2.27 mm, and the axial spatial resolutions were 3.03 and 3.34 mm in FWHM, corresponding to radial offsets of 1-, and 10-cm from the center, respectively. The measured system TOF resolution was 236 ps and the energy resolution was 10.5%. The sensitivities were 46.0 and 47.6 kcps/MBq at the center and 10 cm radial offset, respectively. The peak noise-equivalent count (NEC) rate was 1,300 kcps at an activity concentration of 57.5 kBq/mL, and the average scatter fraction was 35%. The maximum count rate error at peak NEC rate was < 5.0%. The contrast recovery coefficients from the NEMA IQ phantom varied from 75.7% (10-mm sphere) to 92.1% (37-mm sphere), and the background variability (BV) varied from 3.3% to 1.4% with a hot sphere-to-background concentration ratio of 4:1. With TOF and DOI-rebinning OSEM reconstruction (5 iterations, 10 subsets) the transverse and axial spatial resolutions at 1-cm from the center were further improved to 1.22 and 1.77 mm, respectively. OSEM reconstruction resolved 1.6 mm hot rods of a mini-Derenzo phantom using ≥ 8 iterations and 10 subsets with 0.6 mm voxels.

In the wake of recent advancements in scintillator, photodetector, and low-noise fast electronics technologies, as well as in fast reconstruction software, positron emission tomography (PET) scanners have seen considerable improvements in spatial resolution, time resolution, and absolute sensitivity. To continue this trend, we present a helmet type PET brain scanner design that combines high solid angle coverage and double-ended readout of 30 mm-thick scintillator crystals to achieve excellent absolute sensitivity, depth of interaction resolution, and time resolution. This scanner comprises 598 detector arrays, each with 8 × 8 Lu1.8Y0.2SiO5:Ce (LYSO:Ce) crystals with dimensions 3.005 × 3.005 × 30 mm3one-to-one coupled on either end to silicon photomultipliers (SiPMs). Our Monte Carlo simulations based in the platform Geant4 predict that this scanner would attain an absolute sensitivity to a 35 cm line source placed at the center of the radial field of view of (17.1 ± 0.1)%, a depth of interaction resolution of (3.99 ± 0.05) mm, and a coincidence time resolution of (198 ± 5) ps. Our simulations also predict radial, tangential, and axial spatial resolutions at the center of the field of view of 3.3 mm, 3.1 mm, and 3.3 mm, respectively. As this set of simultaneous parameters compares favorably to today's most advanced clinical PET scanners and other proposed designs, this scanner has a good chance of becoming a preferred tool for high quality brain imaging.

In this simulation study, we evaluate the performance of a limited angular coverage PET system consisting of two/four fast-timing 50 ps FWHM CTR flat-panel detectors made of 5–20 mm long pixelated lutetium oxyorthosilicate crystals. We studied image quality and count rates following the National Electrical Manufacturers Association standard, spatial resolution by imaging a Derenzo phantom and a hot rod, and investigated the sensitivity of different scanner designs. We demonstrated the possible use of such a scanner by imaging a human head and a torso of the extended cardiac-torso (XCAT) digital phantom. All the designs were compared to the reference scanner, based on Siemens Biograph Vision PET/CT scanner geometry. We show that good coincidence timing resolution (CTR) can compensate for lower detection efficiency or smaller angular coverage. Good image quality can be obtained with a simple limited-angle PET system without distortions or artefacts. Substantial degradation of the spatial resolution with increased crystal length is observed in the two-panel design due to the parallax error, but not in the four-panel design. The four-panel design simulated with a CTR of 50 ps FWHM is comparable to that of the current state-of-the-art clinical PET/CT scanner. Similar fast-timing limited-angle planar detectors could enable much less expensive total-body or single organ (dynamically selectable) imaging devices.

In this partial review and partial attempt at vision of what may be the future of dedicated brain PET scanners, the key implementations of the PET technique, we postulate that we are still on a development path and there is still a lot to be done in order to develop optimal brain imagers. Optimized for particular imaging tasks and protocols, and also mobile, that can be used outside the PET center, in addition to the expected improvements in sensitivity and resolution. For this multi-application concept to be more practical, flexible, adaptable designs are preferred. This task is greatly facilitated by the improved TOF performance that allows for more open, adjustable, limited angular coverage geometries without creating image artifacts. As achieving uniform very high resolution in the whole body is not practical due to technological limits and high costs, hybrid systems using a moderate-resolution total body scanner (such as J-PET) combined with a very high performing brain imager could be a very attractive approach. As well, as using magnification inserts in the total body or long-axial length imagers to visualize selected targets with higher resolution. In addition, multigamma imagers combining PET with Compton imaging should be developed to enable multitracer imaging.

1120 Objectives: PET human brain imaging has evolved dramatically, with the availability of a vast array of specific radiotracers, leading to numerous imaging paradigms to measure a wide range of brain targets and to assess neurotransmitter and receptor dynamics. Brain-dedicated PET systems could offer important advantages over currently available whole-body PET systems in terms of sensitivity and resolution. However, state-of-the-art for dedicated brain PET has not progressed beyond the HRRT, designed over 20 years ago. Therefore, there is a compelling need to build next generation human brain PET systems that incorporate significant advances in sensitivity and effective spatial resolution. This is the goal of the NeuroEXPLORER (NX) project. Methods: Experience with >4500 human brain HRRT studies at Yale has shown that sensitivity is a major limiting factor, i.e., radioactivity images are often noisy, and the best possible resolution cannot be achieved due to limited counts. Our experience with the total-body uEXPLORER system at UC Davis has shown that ultra-high sensitivity enables high SNR for short scans and parametric imaging at high spatial resolution with no smoothing. Thus, our primary NX design goals are three-fold. 1) Ultra-high sensitivity, to be achieved by maximizing the coincidence acceptance angle, with a long axial field-of-view (aFOV) together with excellent sensitivity-boosting time-of-flight (TOF). 2) To achieve exceptional image resolution through reduction in the size of detector elements, depth of interaction (DOI) readout, and corrections for inter-crystal scatter (ICS). Significant improvements in spatial resolution are needed to image small structures, reduce the partial volume effect, and measure the input function (IF) in the carotid arteries. 3) Head motion is a critical limit to image resolution and quantification accuracy. Thus, our third NX design goal is continuous motion tracking and correction. Results: The NX design is a cylinder with diameter and aFOV of ~50 cm. The system consists of 5 complete detector rings, each with 20 modules. An additional 6th ring has 3 blocks removed from each side to accommodate shoulders, to place the brain in the center of the aFOV. The 18-mm deep LYSO crystals have an in-plane dimension of 1.5 mm, leading to a simulated resolution of 1.6-1.8 mm. These are organized in micro-blocks of 8 crystals read out by 4 SiPMs with energy resolution 10-fold higher effective sensitivity than the HRRT for the brain, with an even greater advantage for the carotids. To provide even higher resolution (~ 1mm) to measure the carotid IF, the NX can support additional ultra-high resolution detector panels. Head motion tracking is performed with a real-time stereovision system using structured light. Initial comparison to the Vicra (which uses a tool mounted on the subject’s head) is underway with existing scanners. To optimize reconstruction and quantification accuracy, high-resolution brain simulations and novel phantom configurations are being designed. Conclusions: A next-generation ultra-high performance human brain PET, the NeuroEXPLORER, has been designed. Following manufacturing and testing, we will apply human imaging paradigms to demonstrate the effectiveness of the NX: 1) showing the dramatic sensitivity increase compared to the HRRT, 2) leveraging high sensitivity to reliably measure uptake in small nuclei, and 3) opening new frontiers of imaging neurotransmitter dynamics. The ultimate goal is a fully functioning and characterized system that dramatically expands the scope of brain PET protocols and applications. Research support: U01EB029811

This paper presents a novel PET geometry for breast cancer imaging. The scanner consists of a 'stadium' (a rectangle with two semi-circles on opposite sides) shaped ring, along with anterior and posterior panels to provide high sensitivity and high spatial resolution for an imaging field-of-view (FOV) that include both breasts, mediastinum and axilla. We simulated this total-breast PET system using GATE and reconstructed the coincidence events using a GPU-based list-mode image reconstruction implementing maximum likelihood expectation-maximization (ML-EM) algorithm. The rear-panel is made up of a single layer of LSO crystals (3.2 × 3.2 × 20 mm3each), while the 'stadium'-shaped elongated ring and the anterior panel are made with dual-layered LSO crystals (1.6 × 1.6 × 6 mm3each). The energy resolution and coincidence resolving time of all detectors are assumed to be 12% and 250 ps full-width-at-half-maximum, respectively. Various sized simulated lesions (4, 5, 6 mm) having 4:1, 5:1, and 6:1 lesion-to-background radioactivity concentration ratios, mimicking different biological uptakes, were strategically located throughout a volumetric torso phantom. We compared system sensitivity and lesion detectability of the dedicated total-breast PET system to a state-of-the-art clinical whole-body PET scanner. The mean sensitivity of the total-breast PET system is 3.21 times greater than that of a whole-body PET scanner in the breast regions. The total-breast PET system also provides better contrast-recovery coefficients for lesions of all sizes and lesion-to-background ratios in the breast when compared to a reference clinical whole-body PET scanner. Receiver operating characteristics (ROC) study shows the area under the ROC curve is 0.948 and 0.924 for the total-breast system and the whole-body PET scanner, respectively, in the detection of 4 mm diameter lesions with 4:1 lesion-to-background ratio. This study demonstrates our novel geometry can provide an imaging FOV larger than conventional PEM systems to simultaneously image both breasts, chest wall and axillae with significantly improved lesion detectability in the breasts when compared to a whole-body PET scanner.

We describe the design and performance of BRPET, a novel dedicated breast PET (dbPET) scanner designed to maximize visualization of posterior regions of the breast. BRPET uses prone imaging geometry and a 12-module detector ring built from pixelated LYSO crystals coupled to position sensitive photomultiplier tubes (PSPMTs). Optical coupling via slanted plastic fiber optic light guides permits partial insertion of the crystals into the exam table’s breast aperture. Image quality testing procedures were adapted from the NEMA NU4-2008 protocol. Two additional phantom tests quantified the posterior extent of the usable volume of view (VoV). BRPET axial, radial, and tangential FWHM spatial resolutions at the isocenter were 1.8, 1.7, and 1.9 mm, respectively. The peak absolute system sensitivity was 0.97% using an energy window of 460–562 keV. The peak noise equivalent counting rate was 5.33 kcps at 21.6 MBq. The scanner VoV extends to within ~6 mm of the plane defining the location of the chest wall. A pilot human study (n = 10) compared the diagnostic performance of FDG-BRPET to that of contrast enhanced MRI (CEMRI), with biopsy as ground truth. Averaged over three expert human observers, the sensitivity/specificity for BRPET was 0.93/1.0, compared to 1.0/0.25 for CEMRI.

This paper presents a new high-sensitivity PET geometry for high fidelity MRI-compatible PET breast imaging which can scan both breasts simultaneously and have: high sensitivity and resolution; compatibility with MR-breast imaged volume; complete visualization of both breasts, mediastinum and axilla; and a modular design. Whereas contemporary dedicated x-ray and molecular breast imaging devices only scan one breast at a time, this approach relies on an unconventional PET geometry, and is able to provide a PET field of view (FOV) larger than that from dedicated breast MRI. The system geometry is evaluated with GATE Monte Carlo simulations of intrinsic system parameters. Various sized lesions (4-6mm) having [6:1 to 4:1] lesion:background radioactivity ratios mimicking different biological uptake are simulated, strategically located throughout a volumetric anthropomorphic torso. Dedicated breast PET (dbPET) imaging is compared with contemporary clinical PET. The dbPET system sensitivity is >6X greater than for contemporary whole-body PET. The novel, non-conventional system geometry allows for simultaneous dual-breast imaging, along with full medial and axillary imaging. Iteratively reconstructed full-volumetric images illustrate sharper visualization of 4mm lower uptake [4:1] lesions throughout the FOV compared with clinical PET. Image overlap between dedicated breast PET and MRI FOVs is excellent. Simulation results indicate clear superiority over conventional, high-sensitivity whole-body PET systems, as well as improved sensitivity over single-breast dbPET systems. This proposed system potentially facilitates both early detection and diagnosis, especially by increasing specificity of MRI, as well as visualizing tissue heterogeneity, monitoring therapeutic efficacy, and detecting breast cancer recurrence throughout the entire mediastinum.

With the efforts under way to improve spatial resolution of the revolutionary Explorer family of imagers, the acute need to develop dedicated imagers for breast, prostate, heart, etc. may slowly disappear, except for some specialized cases in treatment guidance and monitoring, for example in proton therapy. It is in fact happening already. Part of the reason is the high cost of the dedicated systems but also an intriguing emerging opportunity that long axial length PET scanners can be equipped with magnifying inserts that can locally boost the resolution, as per the so-called virtual pinhole concept by Yuan-Chuan Tai from WashU, also called Zoom-in PET. However, the exception are the brain imaging scanners. The special geometry of the optimal helmet type designs for imaging of the brain still gives the opportunity to the brain PET imager developers to compete for the "best" system. We all want to produce good quality dynamic molecular PET brain images at low injected radiation doses (and... low cost). Several designs are being proposed as well as being built at this time in many places around the world. These designs mostly fall in two categories: 1) the mini-Explorer cylindrical type or 2) the compact helmet type, both with large angular brain coverage assuring high sensitivity. Due to the compact sizes of the helmet-type systems, in order to substantially benefit from t lie improved TOF performance, one needs to achieve better than 100 ps FWHM timing performance. In fact, 50 ps FWHM would be a very nice goal. Several groups are working on such concepts. In this race, any new ideas from the expert instrumentation community (not only the medical one) are highly encouraged, as a great impact is expected on brain imaging once such high-performance but also dissemination-ready (i.e., robust and economical) designs are developed. Ideally, the brain imagers of the next generation will have high sensitivity and high spatial resolution approaching the predicted physical limit (due to positron range plus non-collinearity of the two emitted annihilation photons), limited to about 1 mm FWHM. Interestingly, there is a known connection between spatial resolution and sensitivity in detecting small lesions or structures, through the Partial Volume Effect (PVE). The adversarial effect of poor resolution on the detection of small structures is the blurring of the signal with the background. Inversely, if there is not enough statistics (detected/recorded events) per reconstruction voxel, even the best spatial resolution will not bring the tomographic uptake signal above the noisy background.

Explorer brings revolution to medical imaging. The wealth of molecular/functional information provided by a single scan is overwhelming. Beyond the obvious issues of how to store and analyze this vast amount of data, and how to fuse the PET images from the almost 200cm long total-body PET imager with MRI images, the imaging scientists work on improving spatiotemporal resolution of the scanner. But how can we obtain better spatial and time (time of flight) resolutions at the same time? Efforts to push timing resolution down to 50 ps and potentially even down to 10 Ps were initiated. While attaining similar to 1 mm resolution in the total-body Explorer imager is not immediately practically possible or even justifiable, due to other limiting factors such as large amount of recorded coincident events ("statistics") necessary to produce good quality similar to 1 mm resolution images in the human body and not just as before in the small animal body, one of the high-resolution scenarios is to imagine and start planning magnifying attachments-inserts to the Explorer scanner and a dedicated very high-performing 1 mm resolution, 100 Ps or better TOF resolution and similar to 30% efficiency) compact brain imager. This (Explorer + Brain) Tandem PET scanner may be closer to the ideal optimal human PET imager, if there is small interference between the two imager components to image body and brain, respectively. Other options such as 100cm long extended Torso Explorer plus Brain Imager are also being discussed.

SPECT systems using pinhole apertures permit radiolabelled molecular spatial resolution, good energy resolution, and high sensitivity are required. We designed what we consider the “optimal” radionuclide detector system for this task. It should allow studying both detection of unstable atherosclerotic plaques and monitoring the effect of therapies. Using mice is particularly challenging in situations that require several intravenous injections of radiotracers, possibly for weeks or even months, in chronically ill animals. Thus, alternative routes of delivering the radiotracer in tail vein should be investigated. In this study, we have performed preliminary measurements of detection of atherosclerotic plaques in genetically modified mice with high-resolution prototype detector. We have also evaluated the feasibility of assessing left ventricular perfusion by intraperitoneal distributions to be imaged in vivo in small animals. Nevertheless, studying cardiovascular diseases in small animal models is very challenging, and in particular, submillimeter delivering of MIBI-Tc in healthy mice.