Compact pulse width modulation circuitry for silicon photomultiplier readout

The idea of a very sensitive positron emission tomography (PET) system covering a large portion of the body of a patient already dates back to the early 1990s. In the period 2000–2010, only some prototypes with long axial field of view (FOV) have been built, which never resulted in systems used for clinical research. One of the reasons was the limitations in the available detector technology, which did not yet have sufficient energy resolution, timing resolution or countrate capabilities for fully exploiting the benefits of a long axial FOV design. PET was also not yet as widespread as it is today: the growth in oncology, which has become the major application of PET, appeared only after the introduction of PET-CT (early 2000).The detector technology used in most clinical PET systems today has a combination of good energy and timing resolution with higher countrate capabilities and has now been used since more than a decade to build time-of-flight (TOF) PET systems with fully 3D acquisitions. Based on this technology, one can construct total body PET systems and the remaining challenges (data handling, fast image reconstruction, detector cooling) are mostly related to engineering. The direct benefits of long axial FOV systems are mostly related to the higher sensitivity. For single organ imaging, the gain is close to the point source sensitivity which increases linearly with the axial length until it is limited by solid angle and attenuation of the body. The gains for single organ (compared to a fully 3D PET 20-cm axial FOV) are limited to a factor 3–4. But for long objects (like body scans), it increases quadratically with scanner length and factors of 10–40 × higher sensitivity are predicted for the long axial FOV scanner. This application of PET has seen a major increase (mostly in oncology) during the last 2 decades and is now the main type of study in a PET centre. As the technology is available and the full body concept also seems to match with existing applications, the old concept of a total body PET scanner is seeing a clear revival. Several research groups are working on this concept and after showing the potential via extensive simulations; construction of these systems has started about 2 years ago. In the first phase, two PET systems with long axial FOV suitable for large animal imaging were constructed to explore the potential in more experimental settings. Recently, the first completed total body PET systems for human use, a 70-cm-long system, called PennPET Explorer, and a 2-m-long system, called uExplorer, have become reality and first clinical studies have been shown. These results illustrate the large potential of this concept with regard to low-dose imaging, faster scanning, whole-body dynamic imaging and follow-up of tracers over longer periods. This large range of possible technical improvements seems to have the potential to change the current clinical routine and to expand the number of clinical applications of molecular imaging. The J-PET prototype is a prototype system with a long axial FOV built from axially arranged plastic scintillator strips.This paper gives an overview of the recent technical developments with regard to PET scanners with a long axial FOV covering at least the majority of the body (so called total body PET systems). After explaining the benefits and challenges of total body PET systems, the different total body PET system designs proposed for large animal and clinical imaging are described in detail. The axial length is one of the major factors determining the total cost of the system, but there are also other options in detector technology, design and processing for reducing the cost these systems. The limitations and advantages of different designs for research and clinical use are discussed taking into account potential applications and the increased cost of these systems.

We introduce a re-configurable system design for positron emission tomography (PET) for purpose of maximizing the cost effectiveness of PET. In this new design concept, a PET system employs a small number of compact, flat-panel (cFP) PET detectors (which we believe will be made available in near future by us or others) to assume various scanner geometries so that the performance properties desired in various imaging applications can be met by a single PET system. For purpose of demonstration, we consider the design of a re-configurable small-animal system consisting of four cFP detectors that are approximately 8×8 cm2 in size and made of 10mm thick LSO. We consider four geometries for the scanner and conduct Monte-Carlo simulations to study their sensitivity, scatter fractions, random fractions, and noise-equivalent count rates. Our results indicate that, in terms of sensitivity and imaging field-of-view (FOV) the re-configurable system that we consider are very promising for small-animal imaging.

Recently, avalanche photodiodes (APDs) are used as photo detectors in positron emission tomography (PET) because they have many advantages over photomultipliers (PMTs) typically used in PET detectors. We have been developing a PET detector with a multi pixel APD for a small animal PET scanner. Previously, we succeeded in the identification of four-layer depth of interaction (DOI) with a position sensitive PMT coupled to the backside of a crystal array by only an optimized reflector arrangement. We adopt this method for the APD detector to replace the position sensitive PMT. While APDs have many advantages as photo detectors, the detector performance with them is affected by electrical noise because of their low internal gain. Since scintillation lights are shared among many pixels by the presented method, weaker signals in pixels far from the interacting crystals are strongly affected by the noise. To evaluate the possible use of the four-layer DOI detector with APDs using the light sharing method, we constructed a prototype DOI detector and tested its performance. The prototype detector consisted of four layers of a 6 x 6 array with Lu2(1-x) Y2x SiO5 (LYSO) (Lu: 98 %, Y: 2 %) (Proteus Inc., U. S. A.) crystals, and a pixelized APD with a 4x8 pixel array (S8550, Hamamatsu Photonics K.K. ). The size of each crystal element was 1.46 mm x 1.46 mm x 4.5 mm and all element surfaces were chemically etched. The experimental results indicated that the four-layer DOI detector with the multi pixel APD can be used as a small animal PET detector.

A breast dedicated positron emission tomography (PET) scanner has been developed based on monolithic LYSO crystals coupled to position sensitive photomultiplier tubes (PSPMTs). In this study, we describe the design of the PET system and report on its performance evaluation. MAMMI is a breast PET scanner based on monolithic LYSO crystals. It consists of 12 compact modules with a transaxial field of view (FOV) of 170 mm in diameter and 40 mm axial FOV that translates to cover up to 170 mm. The patient lies down in a prone position that facilitates maximum breast elongation. Quantitative performance analysis of the calculated method for the attenuation correction specifically developed for MAMMI, and based on PET image segmentation, has also been conducted in this evaluation. In order to fully determine the MAMMI prototype's performance, we have adapted the measurements suggested for National Electrical Manufacturers Association (NEMA) NU 2-2007 and NU 4-2008 protocol tests, as they are defined for whole-body and small animal PET scanners, respectively. Spatial resolutions of 1.6, 1.8, and 1.9 mm were measured in the axial, radial, and tangential directions, respectively. A scatter fraction of 20.8% was obtained and the maximum NEC was determined to be 25 kcps at 44 MBq. The average sensitivity of the system was observed to be 1% for an energy window of (250 keV-750 keV) and a maximum absolute sensitivity of 1.8% was measured at the FOV center. The overall performance of the MAMMI reported on this evaluation quantifies its ability to produce high quality PET images. Spatial resolution values below 3 mm were measured in most of the FOV. Only the radial component of spatial resolution exceeds the 3 mm at radial positions larger than 60 mm. This study emphasizes the need for standardized testing methodologies for dedicated breast PET systems similar to NEMA standards for whole-body and small animal PET scanners.

The aim of the CaLIPSO project (French acronym for Liquid Ionization Calorimeter, Scintillation Position Organometallic) is to develop the proof of concept for a new gamma detector for Positron Emission Tomography (PET) scanner for small animals and human brain imaging. The objective is to achieve a spatial resolution about 1 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> without losing efficiency with respect to current PET systems. Moreover, excellent time resolution, important for coincidence selection, is also targeted. Such performances are possible thanks to the concept of double detection of the signal created by photoelectron from the photon conversion in trimethyl bismuth, a innovative liquid filling a PET cell. The ongoing work is focused on prototypes developments for light and ionization detection systems. In this conference record we discuss the results of the simulation for the optical prototype and its comparison to the data obtained during the detector tests. We also present the performances of the PET scanner simulated with GATE and a comparison with a high resolution PET system.

Positron emission tomography (PET) and optical imaging are two major techniques for molecular imaging. The PET scanner which enables simultaneous detection of optical imaging will be a powerful tool in molecular imaging. Here, we propose a new detector that detects both signals: annihilation photons for PET and photons for optical imaging. It is based on a four layered depth-of-interaction (DOI) PET detector. Since the detector must be set at a close distance to an object for optical imaging, the DOI information becomes important to obtain uniform spatial resolution in PET imaging over the field-of-view. The proposed PET/optical imaging detector consists of a scintillation crystal block and a position sensitive photo multiplier tube (PS-PMT). While the bottom of the crystal block is optically coupled to the PS-PMT, four side surfaces of the crystal block are covered with reflectors. A dichroic mirror is placed on the top of the crystal block. The dichroic mirror allows fluorescence photons (longer wavelength than 600 nm) to pass through while it reflects scintillation photons (wavelength around 450 nm) generated within the crystal block. Owing to the dichroic mirror, both scintillation photons originating inside the crystal block and fluorescence photons coming from outside the block can be detected by the same PS-PMT. To evaluate of the DOI-PET/ optical detector, we first measured the influence of the dichroic mirror. The results show that the dichroic mirror maintains DOI-PET detector performance without significant loss of fluorescence photons for optical imaging.

Positron emission tomography (PET) detectors that use a dual-ended-scintillator readout to measure depth-of-interaction (DOI) must have an accurate DOI function to provide the relationship between DOI and signal ratios to be used for detector calibration and recalibration. In a previous study, the authors used a novel and simple method to accurately and quickly measure DOI function by irradiating the detector with an external uniform flood source; however, as a practical concern, implementing external uniform flood sources in an assembled PET system is technically challenging and expensive. In the current study, therefore, the authors investigated whether the same method could be used to acquire DOI function from scintillator-generated (i.e., internal) radiation. The authors also developed a method for calibrating the energy scale necessary to select the events within the desired energy window. The authors measured the DOI function of a PET detector with lutetium yttrium orthosilicate (LYSO) scintillators. Radiation events originating from the scintillators' internal Lu-176 beta decay were used to measure DOI functions which were then compared with those measured from both an external uniform flood source and an electronically collimated external point source. The authors conducted these studies with several scintillators of differing geometries (1.5 × 1.5 and 2.0 × 2.0 mm(2) cross-section area and 20, 30, and 40 mm length) and various surface finishes (mirror-finishing, saw-cut rough, and other finishes in between), and in a prototype array. All measured results using internal and external radiation sources showed excellent agreement in DOI function measurement. The mean difference among DOI values for all scintillators measured from internal and external radiation sources was less than 1.0 mm for different scintillator geometries and various surface finishes. The internal radioactivity of LYSO scintillators can be used to accurately measure DOI function in PET detectors, regardless of scintillator geometry or surface finish. Because an external radiation source is not needed, this method of DOI function measurement can be practically applied to individual PET detectors as well as assembled systems.

During particle therapy irradiation, positron emitters with half-lives ranging from 2 to 20 min are generated from nuclear processes. The half-lives are such that it is possible either to detect the positron signal in the treatment room using an in-beam positron emission tomography (PET) system, right after the irradiation, or to quickly transfer the patient to a close PET/CT scanner. Since the activity distribution is spatially correlated with the dose, it is possible to use PET imaging as an indirect method to assure the quality of the dose delivery. In this work, we present a new dedicated PET system able to operate in-beam. The PET apparatus consists in two 10 cm × 10 cm detector heads. Each detector is composed of four scintillating matrices of 23 × 23 LYSO crystals. The crystal size is 1.9 mm × 1.9 mm × 16 mm. Each scintillation matrix is read out independently with a modularized acquisition system. The distance between the two opposing detector heads was set to 20 cm. The system has very low dead time per detector area and a 3 ns coincidence window, which is capable to sustain high single count rates and to keep the random counts relatively low. This allows a new full-beam monitoring modality that includes data acquisition also while the beam is on. The PET system was tested during the irradiation at the CATANA (INFN, Catania, Italy) cyclotron-based proton therapy facility. Four acquisitions with different doses and dose rates were analysed. In all cases the random to total coincidences ratio was equal or less than 25%. For each measurement we estimated the accuracy and precision of the activity range on a set of voxel lines within an irradiated PMMA phantom. Results show that the inclusion of data acquired during the irradiation, referred to as beam-on data, improves both the precision and accuracy of the range measurement with respect to data acquired only after irradiation. Beam-on data alone are enough to give precisions better than 1 mm when at least 5 Gy are delivered.

Comparing positron emission tomography (PET) systems which have different features is not straightforward. To address this, we propose to image the same object with all considered PET systems using a fixed scan time, and reconstruct from each scan an image at the same predefined spatial resolution. With such resolution matched reconstructions, the images should be identical except for their noise. Therefore, the PET system that produces the image with the lowest variance has the best performance. An analytical model is described to compute this variance, assuming that the PET system is (approximately) cylindrical, the imaged object is a uniform cylinder centered in the field of view, and the variance is only computed at the center of the reconstructed image. The model takes into account the solid angle covered by the detectors, the detector stopping power, the time-of-flight (TOF) resolution, the scatter fraction and the spatial resolution of the system, the attenuation and diameter of the cylinder and the desired spatial resolution of the reconstructed image. The inverse of this variance can be considered as the effective sensitivity of the system. This effective sensitivity can be calibrated based on the NEMA line source sensitivity. As a performance metric for scanning long objects, the minimum sensitivity achieved over the object is computed, with an optimal number of bed positions and optimal overlap between them. The influence of the spatial and TOF resolution on the effective sensitivity is verified with simulations.

Newer high-performance time-of-flight (TOF) positron emission tomography (PET) systems have the capability to preserve diagnostic image quality with low count density, while maintaining a high raw photon detection sensitivity that would allow for a reduction in injected dose or rapid data acquisition. To assess this, we performed quantitative and visual assessments of the PET images acquired using a highly sensitive (23.3cps/kBq) large field of view (25-cm axial) silicon photomultiplier (SiPM)-based TOF PET (400-ps timing resolution) integrated with 3T-MRI in comparison to PET images acquired on non-TOF PET/x-ray computed tomography (CT) systems. Whole-body 2-deoxy-2-[18F]fluoro-D-glucose ([18F]FDG) PET/CT was acquired for 15 patients followed by whole body PET/magnetic resonance imaging (MRI) with an average injected dose of 325±84MBq. The PET list mode data from PET/MRI were reconstructed using full datasets (4min/bed) and reduced datasets (2, 1, 0.5, and 0.25min/bed). Qualitative assessment between PET/CT and PET/MR images were made. A Likert-type scale between 1 and 5, 1 for non-diagnostic, 3 equivalent to PET/CT, and 5 superior quality, was used. Maximum and mean standardized uptake values (SUVmax and SUVmean) of normal tissues and lesions detected were measured and compared. Mean visual assessment scores were 3.54±0.32, 3.62±0.38, and 3.69±0.35 for the brain and 3.05±0.49, 3.71±0.45, and 4.14±0.44 for the whole-body maximum intensity projections (MIPs) for 1, 2, and 4min/bed PET/MR images, respectively. The SUVmean values for normal tissues were lower and statistically significant for images acquired at 4, 2, 1, 0.5, and 0.25min/bed on the PET/MR, with values of -18±28% (p < 0.001), - 16±29% (p = 0.001), - 16±31% (p = 0.002), - 14±35% (p < 0.001), and - 13±34% (p = 0.002), respectively. SUVmax and SUVpeak values of all lesions were higher and statistically significant (p < 0.05) for 4, 2, 1, 0.50, and 0.25min/bed PET/MR datasets. High-sensitivity TOF PET showed comparable but still better visual image quality even at a much reduced activity in comparison to lower-sensitivity non-TOF PET. Our data translates to a seven times reduction in either injection dose for the same time or total scan time for the same injected dose. This "ultra-sensitivity" PET system provides a path to clinically acceptable extremely low-dose FDG PET studies (e.g., sub 1mCi injection or sub-mSv effective dose) or PET studies as short as 1min/bed (e.g., 6min of total scan time) to cover whole body without compromising diagnostic performance.

The purpose of this study was to do a feasibility study on a microstrip transmission line (MS) RF coil for a positron emission tomography (PET) insert in a 7 Tesla human MRI system. The proposed MS coil integrated the RF shield of the PET detector as the ground conductor of the coil. We called the integrated module "MS PET coil." A single-channel MS PET coil was developed with an integrated RF-shielded PET detector module. For comparison, we also studied a conventional MS coil with a single-layer ground conductor. A lutetium fine silicate (LFS) scintillation crystal block (14 × 14 × 4-layer) with a silicon photomultiplier (Hamamatsu Photonics K.K., Shizuoka, Japan) and a front-end readout circuit board were mounted inside the shield cage of the MS PET coil. The MS PET coil was studied with and without PET detectors. All three coil configurations were studied with a homogeneous phantom in a 7T MRI system (Siemens Healthineers, Erlangen, Germany). PET data measurements were conducted using a Cesium-137 radiation point source. The MR images were similar for the MS coil and the empty MS PET coil, as well as for the cases of MS PET coil with and without PET measurements. Compared to the empty MS PET coil (without PET detector and cable RF shield), decreases in SNR, increases in image noise and RF power, and a slight decrease in resonance frequency were seen for the case of the MS PET coil with the detector and cable shield. Differences in the PET energy histograms or in the crystal identification maps with and without MRI measurements were negligible. Both the MRI and PET performances of the MS PET coil showed responses that matched the MS coil responses. The performance variations of MRI data with and without PET measurement and PET data with and without MR imaging were negligible.

Positron emission tomography (PET) is one of the most promising techniques for hadron therapy monitoring. This method uses imaging of the annihilation photons resulting from positron emitting isotopes produced during patient irradiation in hadron therapy. In order to maximize the sensitivity, in-beam PET is the preferred option, which requires a partial ring configuration. The usage of partial ring PET systems causes image artefacts and low quality images due to incomplete data. The inclusion of Time-of-flight (TO F) information into the reconstruction algorithm provides better image quality for PET systems. Therefore, a PET system with very good time and spatial resolution is necessary for hadron therapy monitoring. The Resistive-Plate-Chambers (RPCs), providing very good time and spatial resolution might be used for the quality control of hadron therapy. However, they have lower sensitivity than crystal-based systems. The objective of this study is the comparison of the performance of two TOF-PET systems, a RPC-based PET system currently under investigation by the TERA group and a commercially available crystal-based PET system, with respect to their feasibility of being applied to monitor cancer treatment with ion- and proton-beams. Various Monte-Carlo simulations were performed using GATE to evaluate the performance of both systems following NEMA protocols. Furthermore, a set of sources emulating the annihilation photons from β+-emitters produced after proton pencil beam irradiation on a PMMA target were obtained. These sources were employed to study the capability to reproduce the proton beam path and the distal edge position. Finally, we calculated the activity distribution produced by carbon ions on the basis of one field of a real treatment plan. The results obtained indicate that a RPC-based system has five times lower sensitivity, similar spatial resolution and about two times higher scatter fraction compared to the crystal-based system. The low sensitivity obtained by the RPC-based system is not sufficient to determine range deviations.

An individual readout of silicon photomultipliers (SiPMs) would enhance the performance of modern positron emission tomography (PET) systems. However, as it difficult to achieve in practice, a multiplexing readout of SiPM arrays could be performed instead. In this study, we characterized the performance of three PET detector modules utilizing three different SiPM models with active areas of 3 × 3, 4 × 4, and 6 × 6 mm2. Each SiPM array was coupled with a 4 × 4 LYSO crystal block. For SiPM multiplexing, we used a discretized positioning circuit to obtain position and energy information, and applied a first-order capacitive high-pass filter to enhance the time-of-flight measurement capability of the PET detector. The energy performance was similar among the three different SiPM arrays, with an energy resolution of 10%–11%. The best timing performance was achieved with the SiPM array with an active area of 6 × 6 mm2, which yielded a coincidence timing resolution (CTR) value of 401 ps FWHM when an analog high-pass filter was applied. We expect that, in combination with high-performance SiPM multiplexing techniques, the SiPM array with an active area of 6 × 6 mm2 can provide a cost-effective solution for developing a whole-body PET scanner.

Positron emission tomography (PET) has high sensitivity for imaging radioactive tracer distributions in subjects. However, it is not possible to image free radical distribution in a subject by PET. Since free radicals are quite reactive, they are related to many diseases, including but not limited to cancer, inflammation, strokes, and heart disease. The Overhauser enhanced magnetic resonance imaging (OMRI) is so far the only method that images free radical distribution in vivo. By combining PET and OMRI, a new hybrid imaging modality might be developed that can simultaneously image the radioactive tracer and free radical distributions. For this purpose, the authors developed a PET/OMRI combined system for small animals. The developed PET/OMRI system used an optical fiber-based PET system combined with a permanent magnet-based OMRI system. The optical fiber-based PET system uses flexible optical fiber bundles. Eight optical fiber-based block detectors were arranged in a 56 mm diameter ring to form a PET system. The LGSO blocks were located inside the field-of-view (FOV) of the OMRI, and the position sensitive photomultiplier tubes were positioned behind the OMRI to minimize the interference between the PET and the OMRI. The OMRI system used a 0.0165 T permanent magnet. The system has an electron spin resonance coil to enhance the MRI signal using the Overhauser effect to image the free radical in the FOV of the PET/OMRI system. The spatial resolution and sensitivity of the optical fiber-based PET system were 1.2 mm FWHM and 1.2% at the central FOV, respectively. The OMRI system imaged the distribution of a nitroxyl radical (NXR) solution. The interference between PET and OMRI was small. Simultaneous imaging of the positron radiotracer and the NXR solution was successfully conducted with the developed PET/OMRI system for phantom and small animal studies. The authors developed a PET/OMRI combined system with the potential to provide interesting new results in molecular imaging research, such as in vivo molecular and free radical distributions.

Small-animal positron emission tomography as a tool for neuropharmacology