PET performance evaluation of the small-animal Hyperion IID PET/MRI insert based on the NEMA NU-4 standard

Current Applications of PET/MR: Part I: Technical Basics and Preclinical/Clinical Applications

Positron emission tomography/magnetic resonance (PET/MR) imaging has gone through major hardware improvements in recent years, making it a reliable state-of-the-art hybrid modality in clinical practice. At the same time, image reconstruction, attenuation correction, and motion correction algorithms have significantly evolved to provide high-quality images. Part I of the current review discusses technical basics, pre-clinical applications, and clinical applications of PET/MR in radiation oncology and head and neck imaging. PET/MR offers a broad range of advantages in preclinical and clinical imaging. In the preclinic, small and large animal-dedicated devices were developed, making PET/MR capable of delivering new insight into animal models in diseases and facilitating the development of methods that inform clinical PET/MR. Regarding PET/MR's clinical applications in radiation medicine, PET and MR already play crucial roles in the radiotherapy process. Their combination is particularly significant as it can provide molecular and morphological characteristics that are not achievable with other modalities. In addition, the integration of PET/MR information for therapy planning with linear accelerators is expected to provide potentially unique biomarkers for treatment guidance. Furthermore, in clinical applications in the head and neck region, it has been shown that PET/MR can be an accurate modality in head and neck malignancies for staging and resectability assessment. Also, it can play a crucial role in diagnosing residual or recurrent diseases, reliably distinguishing from oedema and fibrosis. PET/MR can furthermore help with tumour characterization and patient prognostication. Lastly, in head and neck carcinoma of unknown origin, PET/MR, with its diagnostic potential, may obviate multiple imaging sessions in the near future.

Setup and characterisation according to NEMA NU 4 of thephenoPET scanner, a PET system dedicated for plant sciences

Objective.ThephenoPET system is a plant dedicated positron emission tomography (PET) scanner consisting of fully digital photo multipliers with lutetium-yttrium oxyorthosilicate crystals and located inside a custom climate chamber. Here, we present the setup ofphenoPET, its data processing and image reconstruction together with its performance.Approach.The performance characterization follows the national electrical manufacturers association (NEMA) standard for small animal PET systems with a number of adoptions due to the vertical oriented bore of a PET for plant sciences. In addition temperature stability and spatial resolution with a hot rod phantom are addressed.Main results.The spatial resolution for a22Na point source at a radial distance of 5 mm to the center of the field-of-view (FOV) is 1.45 mm, 0.82 mm and 1.88 mm with filtered back projection in radial, tangential and axial direction, respectively. A hot rod phantom with18F gives a spatial resolution of up to 1.6 mm. The peak noise-equivalent count rates are 550 kcps @ 35.08 MBq, 308 kcps @ 33 MBq and 45 kcps @ 40.60 MBq for the mouse, rat and monkey size scatter phantoms, respectively. The scatter fractions for these phantoms are 12.63%, 22.64% and 55.90%. We observe a peak sensitivity of up to 3.6% and a total sensitivity of up toSA,tot= 2.17%. For the NEMA image quality phantom we observe a uniformity of %STD= 4.22% with ordinary Poisson maximum likelihood expectation-maximization with 52 iterations. Here, recovery coefficients of 0.12, 0.64, 0.89, 0.93 and 0.91 for 1 mm, 2 mm, 3 mm, 4 mm and 5 mm rods are obtained and spill-over ratios of 0.08 and 0.14 for the water-filled and air-filled inserts, respectively.Significance.ThephenoPET and its laboratory are now in routine operation for the administration of [11C]CO2and non-invasive measurement of transport and allocation of11C-labelled photoassimilates in plants.

Effect of depth of interaction resolution on the spatial resolution of SIAT aPET

Objective.Spatial resolution is a crucial parameter for a positron emission tomography (PET) scanner. The spatial resolution of a high-resolution small animal PET scanner is significantly influenced by the effect of depth of interaction (DOI) uncertainty. The aim of this work is to investigate the impact of DOI resolution on the spatial resolution of a small animal PET scanner called SIAT aPET and determine the required DOI resolution to achieve nearly uniform spatial resolution within the field of view (FOV).Approach. The SIAT aPET detectors utilize 1.0 × 1.0 × 20 mm3crystals, with an average DOI resolution of ∼2 mm. A default number of 16 DOI bins are used during data acquisition. First, a Na-22 point source was scanned in the center of the axial FOV with different radial offsets. Then, a Derenzo phantom was scanned at radial offsets of 0 and 15 mm in the center axial FOV. The measured DOI information was rebinned to 1, 2, 4 and 8 DOI bins to mimic different DOI resolutions of the detectors during image reconstruction.Main results. Significant artifacts were observed in images obtained from both the point source and Derenzo phantom when using only one DOI bin. When accurate measurement of DOI is not achieved, degradation in spatial resolution is more pronounced in the radial direction compared to tangential and axial directions for large radial offsets. The radial spatial resolutions at a 30 mm radial offset are 5.05, 2.62, 1.24, 0.86 and 0.78 mm when using 1, 2, 4, 8, or 16 DOI bins, respectively. The axial spatial resolution improved from ∼1.3 to 0.7 mm as the number of DOI bins increased from 1 to 16 at radial offsets from 0 to 25 mm. Two DOI bins are required to obtain images without significant artifacts. The required DOI resolution is about three times the crystal width of SIAT aPET to achieve a uniform submillimeter spatial resolution within the central 60 mm FOV and resolve the 1 mm rods of the Derenzo phantom at both positions.

Design and performance simulation studies of a breast PET insert integrable into a clinical whole-body PET/MRI scanner

Objective. Three different breast positron emission tomography (PET) insert geometries are proposed for integration into an existing magnetic resonance imaging (MRI) breast coil (Breast Biopsy Coil, NORAS MRI products) to be used inside a whole-body PET/MRI scanner (Biograph mMR, Siemens Healthineers) to enhance the sensitivity and spatial resolution of imaging inside the breast.Approach. Monte Carlo simulations were performed to predict and compare the performance characteristics of the three geometries in terms of the sensitivity, spatial resolution, scatter fraction, and noise equivalent count rate (NECR). In addition, the background single count rate due to organ uptake in a clinical scan scenario was predicted using a realistic anthropomorphic phantom.Main results. In the center of the field of view (cFOV), absolute sensitivities of 3.1%, 2.7%, and 2.2% were found for Geometry A (detectors arranged in two cylinders), Geometry B (detectors arranged in two partial cylinders), and Geometry C (detectors arranged in two half cylinders combined with two plates), respectively. The full width at half maximum spatial resolution was determined to be 1.7 mm (Geometry A), 1.8 mm (Geometry B) and 2.0 mm (Geometry C) at 5 mm from the cFOV. Designs with multiple scintillation-crystal layers capable of determining the depth of interaction (DOI) strongly improved the spatial resolution at larger distances from the transaxial cFOV. The system scatter fractions were 33.1% (Geometries A and B) and 32.3% (Geometry C). The peak NECRs occurred at source activities of 300 MBq (Geometry A), 310 MBq (Geometry B) and 340 MBq (Geometry C). The background single-event count rates were 17.1 × 10⁶cps (Geometry A), 15.3 × 10⁶cps (Geometry B) and 14.8 × 10⁶cps (Geometry C). Geometry A in the three-layer DOI variant exhibited the best PET performance characteristics but could be challenging to manufacture. Geometry C had the lowest impact on the spatial resolution and the lowest sensitivity among the investigated geometries.Significance. Geometry B in the two-layer DOI variant represented an effective compromise between the PET performance and manufacturing difficulty and was found to be a promising candidate for the future breast PET insert.

Mutual interferences between SIAT aPET insert and a 3 T uMR 790 MRI scanner

Objective.Dual-modality small animal PET/MR imaging provides temporally correlated information on two biochemical processes of a living object. An magnetic resonance imaging (MRI)-compatible small animal PET insert named Shenzhen Institutes of Advanced Technology (SIAT) aPET was developed by using dual-ended readout depth encoding detectors to simultaneously achieve a uniform high spatial resolution and high sensitivity at the SIAT. In this work, the mutual interferences between SIAT aPET and the 3 T uMR 790 MRI scanner of United Imaging was quantitatively evaluated.Approach.To minimize the mutual interferences, only the PET detectors and the readout electronics were placed inside the MRI scanner, the major signal processing electronic was placed in the corner of the MRI room and the auxiliary unit was placed in the MRI technical room. A dedicated mouse radio fRequency (RF) coil with a transmitter and receiver was developed for the PET insert. The effects of PET scanner on theB₀andB₁field of the MRI scanner and the quality of the MRI images were measured. The effects of MRI imaging on the performance of both the PET detectors and scanner were also measured.Main results.The electronic and mechanical components of the PET insert affected the homogeneity of theB₀field. The PET insert had no effect on the homogeneity ofB₁produced by the dedicated mouse coil but slightly reduced the strength ofB₁. The mean and standard deviation of the RF noise map were increased by 2.2% and 11.6%, respectively, while the PET insert was placed in the MRI scanner and powered on. Eddy current was produced while the PET insert was placed in the MRI scanner, and it was further increased while the PET insert was powered on. Despite the above-mentioned interferences from the PET insert, the MR images of a uniform cylindrical water phantom showed that the changes in the signal-to-noise ratio (SNR) and homogeneity as the PET insert was placed in the MRI scanner were acceptable regardless of whether the PET insert was powered off or powered on. The maximum reduction of SNR was less than 11%, and the maximum reduction of homogeneity was less than 2.5% while the PET insert was placed inside the MRI scanner and powered on for five commonly used MRI sequences. MRI using gradient echo (GRE), spin echo (SE) and fast spin echo (FSE) sequences had negligible effects on the flood histograms and energy resolution of the PET detectors, as well as the spatial resolution and sensitivity of the PET scanner.Significance.The mutual interference between the SIAT aPET and the 3 T uMR 790 MRI scanner are acceptable. Simultaneous PET/MRI imaging of small animals can be performed with the two scanners.

Performance Evaluation of a PET of 7T Bruker Micro-PET/MR Based on NEMA NU 4-2008 Standards

Purpose: This study aimed to measure the performance evaluation of the Bruker sequential micro-positron emission tomography/magnetic resonance imaging (PET/MRI) scanner by following National Electrical Manufacturers Association (NEMA) NU 4-2008 standards’ protocol. The system consists of a high-performance silicon photomultiplier (SiPM) advanced technology detector and a continuous lutetium-yttrium oxyorthosilicate (LYSO) crystal. Methods: A 22Na (sodium-22) point source was utilized to assess the spatial resolution and system sensitivity, and the Micro-PET scatter phantom measurements were conducted to measure count rate measurements and scatter fractions (SF). A mouse-like Micro-PET image quality (IQ) phantom was utilized as a model to analyze the uniformity, recovery coefficient (RC), and spillover ratio (SOR). A small animal PET/MRI imaging study was performed in a rat. Results: We calculated the spatial resolutions of filtered back-projection (FBP), and used 3D-MLEM to reconstruct PET images at the axial center and ¼ of the axial field of view (FOV) in axial, radial, and tangential directions. The best observed spatial resolutions in both reconstructed images were obtained in the tangential direction, and the values were 0.80 mm in 3D-MLEM and 0.94 mm in FBP. The peak noise equivalent count rate (NECR) in the 358–664 keV energy window was 477.30 kcps at 95.83 MBq and 774.45 kcps at 103.6 MBq for rat and mouse-sized scatter phantoms, respectively. The rat and mouse-sized phantoms scatter fractions (SF) were 14.2% and 6.9%, respectively. Conclusions: According to our results, the performance characteristics of the scanner are high sensitivity, good spatial resolution, low scatter fraction, and good IQ, indicating that it is suitable for preclinical imaging studies.

NEMA characterization of the SAFIR prototype PET insert

Background

The SAFIR prototype insert is a preclinical Positron Emission Tomography (PET) scanner built to acquire dynamic images simultaneously with a 7 T Bruker Magnetic Resonance Imaging (MRI) scanner. The insert is designed to perform with an excellent coincidence resolving time of 194 ps Full Width Half Maximum (FWHM) and an energy resolution of 13.8% FWHM. These properties enable it to acquire precise quantitative images at activities as high as 500 MBq suitable for studying fast biological processes within short time frames (< 5 s). In this study, the performance of the SAFIR prototype insert is evaluated according to the NEMA NU 4-2008 standard while the insert is inside the MRI without acquiring MRI data.

Results

Applying an energy window of 391–601 keV and a coincidence time window of 500 ps the following results are achieved. The average spatial resolution at 5 mm radial offset is 2.6 mm FWHM when using the Filtered Backprojection 3D Reprojection (FBP3DRP) reconstruction method, improving to 1.2 mm when using the Maximum Likelihood Expectation Maximization (MLEM) method. The peak sensitivity at the center of the scanner is 1.06%. The Noise Equivalent count Rate (NECR) is 799 kcps at the highest measured activity of 537 MBq for the mouse phantom and 121 kcps at the highest measured activity of 624 MBq for the rat phantom. The NECR peak is not yet reached for any of the measurements. The scatter fractions are 10.9% and 17.8% for the mouse and rat phantoms, respectively. The uniform region of the image quality phantom has a 3.0% STD, with a 4.6% deviation from the expected number of counts per voxel. The spill-over ratios for the water and air chambers are 0.18 and 0.17, respectively.

Conclusions

The results satisfy all the requirements initially considered for the insert, proving that the SAFIR prototype insert can obtain dynamic images of small rodents at high activities (\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\sim$$\end{document}∼ 500 MBq) with a high sensitivity and an excellent count-rate performance.

Recent Technical Advances in Accelerating the Clinical Translation of Small Animal Brain Imaging: Hybrid Imaging, Deep Learning, and Transcriptomics

Small animal models play a fundamental role in brain research by deepening the understanding of the physiological functions and mechanisms underlying brain disorders and are thus essential in the development of therapeutic and diagnostic imaging tracers targeting the central nervous system. Advances in structural, functional, and molecular imaging using MRI, PET, fluorescence imaging, and optoacoustic imaging have enabled the interrogation of the rodent brain across a large temporal and spatial resolution scale in a non-invasively manner. However, there are still several major gaps in translating from preclinical brain imaging to the clinical setting. The hindering factors include the following: (1) intrinsic differences between biological species regarding brain size, cell type, protein expression level, and metabolism level and (2) imaging technical barriers regarding the interpretation of image contrast and limited spatiotemporal resolution. To mitigate these factors, single-cell transcriptomics and measures to identify the cellular source of PET tracers have been developed. Meanwhile, hybrid imaging techniques that provide highly complementary anatomical and molecular information are emerging. Furthermore, deep learning-based image analysis has been developed to enhance the quantification and optimization of the imaging protocol. In this mini-review, we summarize the recent developments in small animal neuroimaging toward improved translational power, with a focus on technical improvement including hybrid imaging, data processing, transcriptomics, awake animal imaging, and on-chip pharmacokinetics. We also discuss outstanding challenges in standardization and considerations toward increasing translational power and propose future outlooks.

NEMA NU 4-2008 performance evaluation and MR compatibility tests of an APD-based small animal PET-insert for simultaneous PET/MR imaging

An avalanche photodiode (APD)-based small animal positron emission tomography (PET)-insert was fully evaluated for its PET performance, as well as potential influences on magnetic resonance imaging (MRI) performance. This PET-insert has an extended axial field of view (FOV) compared with the previous design to increase system sensitivity, as well as an updated cooling and temperature regulation to enable stable and reproducible PET acquisitions. The PET performance was evaluated according to the National Electrical Manufacturers Association NU4-2008 protocol. The energy and timing resolution’s full width at half maximum were 16.1% and 4.7 ns, respectively. The reconstructed radial spatial resolution of the PET-insert was 1.8 mm full width at half maximum at the center FOV using filtered back projection for reconstruction and sensitivity was 3.68%. The peak noise equivalent count rates were 70 kcps for a rat-like and 350 kcps for a mouse-like phantom, respectively. Image quality phantom values and contrast recovery were comparable to state-of-the art PET-inserts and standalone systems. Regarding MR compatibility, changes in the mean signal-to-noise ratio for turbo spin echo and echo-planar imaging sequences were below 8.6%, for gradient echo sequences below 1%. Degradation of the mean homogeneity was below 2.3% for all tested sequences. The influence of the PET-insert on the B 0 maps was negligible and no influence on functional MRI sequences was detected. A mouse and rat imaging study demonstrated the feasibility of in vivo simultaneous PET/MRI.

Initial results of a mouse brain PET insert with a staggered 3-layer DOI detector

Objective.Small animal positron emission tomography (PET) requires a submillimeter resolution for better quantification of radiopharmaceuticals. On the other hand, depth-of-interaction (DOI) information is essential to preserve the spatial resolution while maintaining the sensitivity. Recently, we developed a staggered 3-layer DOI detector with 1 mm crystal pitch and 15 mm total crystal thickness, but we did not demonstrate the imaging performance of the DOI detector with full ring geometry. In this study we present initial imaging results obtained for a mouse brain PET prototype developed with the staggered 3-layer DOI detector.Approach.The prototype had 53 mm inner diameter and 11 mm axial field-of-view. The PET scanner consisted of 16 DOI detectors each of which had a staggered 3-layer LYSO crystal array (4/4/7 mm) coupled to a 4 × 4 silicon photomultiplier array. The physical performance was evaluated in terms of the NEMA NU 4 2008 protocol.Main Results.The measured spatial resolutions at the center and 15 mm radial offset were 0.67 mm and 1.56 mm for filtered-back-projection, respectively. The peak absolute sensitivity of 0.74% was obtained with an energy window of 400-600 keV. The resolution phantom imaging results show the clear identification of a submillimetric rod pattern with the ordered-subset expectation maximization algorithm. The inter-crystal scatter rejection using a narrow energy window could enhance the resolvability of a 0.75 mm rod significantly.Significance.In an animal imaging experiment, the detailed mouse brain structures such as cortex and thalamus were clearly identified with high contrast. In conclusion, we successfully developed the mouse brain PET insert prototype with a staggered 3-layer DOI detector.

High throughput software-based gradient tree boosting positioning for PET systems

The supervised machine learning technique Gradient Tree Boosting (GTB) has shown good accuracy for position estimation of gamma interaction in PET crystals for bench-top experiments while its computational requirements can easily be adjusted. Transitioning to preclinical and clinical applications requires near real-time processing in the scale of full PET systems. In this work, a high throughput GTB-based singles positioning C++ implementation is proposed and a series of optimizations are evaluated regarding their effect on the achievable processing throughput. Moreover, the crucial feature and parameter selection for GTB is investigated for the segmented detectors of the Hyperion II^DPET insert with two main models and a range of GTB hyperparameters. The proposed framework achieves singles positioning throughputs of more than 9.5 GB/s for smaller models and of 240 MB/s for more complex models on a recent Intel Skylake server. Detailed throughput analysis reveals the key performance limiting factors, and an empirical throughput model is derived to guide the GTB model selection process and scanner design decisions. The throughput model allows for throughput estimations with a mean absolute error (MAE) of 175.78 MB/s.

SimPET: a Preclinical PET Insert for Simultaneous PET/MR Imaging

PURPOSE

SimPET/M7 system is a small-animal dedicated simultaneous positron emission tomography and magnetic resonance imaging (PET/MRI) scanner. The SimPET insert has been upgraded from its prototype with a focus on count rate performance and sensitivity. The M7 scanner is a 1-T permanent magnet-based compact MRI system without any cryogens. Here, we present performance evaluation results of SimPET along with the results of mutual interference evaluation and simultaneously acquired PET/MR imaging.

PROCEDURES

Following NEMA NU 4-2008 standard, we evaluated the performance of the SimPET system. The M7 MRI compatibility of SimPET was also assessed by analyzing MRI images of a uniform phantom under different PET conditions and PET count rates with different MRI pulse sequences. Mouse imaging was performed including a whole-body ¹⁸F-NaF PET scan and a simultaneous PET/MRI scan with ⁶⁴Cu-NOTA-ironoxide.

RESULTS

The spatial resolution at center based on 3D OSEM without and with warm background was 0.7 mm and 1.45 mm, respectively. Peak sensitivity was 4.21 % (energy window = 250-750 keV). The peak noise equivalent count rate with the same energy window was 151 kcps at 38.4 MBq. The uniformity was 4.42 %, and the spillover ratios in water- and air-filled chambers were 14.6 % and 12.7 %, respectively. In the hot rod phantom image, 0.75-mm-diameter rods were distinguishable. There were no remarkable differences in the SNR and uniformity of MRI images and PET count rates with different PET conditions and MRI pulse sequences. In the whole-body ¹⁸F-NaF PET images, fine skeletal structures were well resolved. In the simultaneous PET/MRI study with ⁶⁴Cu-NOTA-ironoxide, both PET and MRI signals changed before and after injection of the dual-modal imaging probe, which was evident with the exact spatiotemporal correlation.

CONCLUSIONS

We demonstrated that the SimPET scanner has a high count rate performance and excellent spatial resolution. The combined SimPET/M7 enabled simultaneous PET/MR imaging studies with no remarkable mutual interference between the two imaging modalities.

Design and system evaluation of a dual-panel portable PET (DP-PET)

BACKGROUND

Integrated whole-body PET/MR technology continues to mature and is now extensively used in clinical settings. However, due to the special design architecture, integrated whole-body PET/MR comes with a few inherent limitations. Firstly, whole-body PET/MR lacks sensitivity and resolution for focused organs. Secondly, broader clinical access of integrated PET/MR has been significantly restricted due to its prohibitively high cost. The MR-compatible PET insert is an independent and removable PET scanner which can be placed within an MRI bore. However, the mobility and configurability of all existing MR-compatible PET insert prototypes remain limited.

METHODS

An MR-compatible portable PET insert prototype, dual-panel portable PET (DP-PET), has been developed for simultaneous PET/MR imaging. Using SiPM, digital readout electronics, novel carbon fiber shielding, phase-change cooling, and MRI compatible battery power, DP-PET was designed to achieve high-sensitivity and high-resolution with compatibility with a clinical 3-T MRI scanner. A GPU-based reconstruction method with resolution modeling (RM) has been developed for the DP-PET reconstruction. We evaluated the system performance on PET resolution, sensitivity, image quality, and the PET/MR interference.

RESULTS

The initial results reveal that the DP-PET prototype worked as expected in the MRI bore and caused minimal compromise to the MRI image quality. The PET performance was measured to show a spatial resolution ≤ 2.5 mm (parallel to the detector panels), maximum sensitivity = 3.6% at the center of FOV, and energy resolution = 12.43%. MR pulsing introduces less than 2% variation to the PET performance measurement results.

CONCLUSIONS

We developed a MR-compatible PET insert prototype and performed several studies to begin to characterize the performance of the proposed DP-PET. The results showed that the proposed DP-PET performed well in the MRI bore and would cause little influence on the MRI images. The Derenzo phantom test showed that the proposed reconstruction method could obtain high-quality images using DP-PET.

Development and Initial Results of a Brain PET Insert for Simultaneous 7-Tesla PET/MRI Using an FPGA-Only Signal Digitization Method

In study, we developed a positron emission tomography (PET) insert for simultaneous brain imaging within 7-Tesla (7T) magnetic resonance (MR) imaging scanners. The PET insert has 18 sectors, and each sector is assembled with two-layer depth-of-interaction (DOI)-capable high-resolution block detectors. The PET scanner features a 16.7-cm-long axial field-of-view (FOV) to provide entire human brain images without bed movement. The PET scanner early digitizes a large number of block detector signals at a front-end data acquisition (DAQ) board using a novel field-programmable gate array (FPGA)-only signal digitization method. All the digitized PET data from the front-end DAQ boards are transferred using gigabit transceivers via non-magnetic high-definition multimedia interface (HDMI) cables. A back-end DAQ system provides a common clock and synchronization signal for FPGAs over the HDMI cables. An active cooling system using copper heat pipes is applied for thermal regulation. All the 2.17-mm-pitch crystals with two-layer DOI information were clearly identified in the block detectors, exhibiting a system-level energy resolution of 12.6%. The PET scanner yielded clear hot-rod and Hoffman brain phantom images and demonstrated 3D PET imaging capability without bed movement. We also performed a pilot simultaneous PET/MR imaging study of a brain phantom. The PET scanner achieved a spatial resolution of 2.5 mm at the center FOV (NU 4) and a sensitivity of 18.9 kcps/MBq (NU 2) and 6.19% (NU 4) in accordance with the National Electrical Manufacturers Association (NEMA) standards.

Evaluation of the PETsys TOFPET2 ASIC in multi-channel coincidence experiments

BACKGROUND

Aiming to measure the difference in arrival times of two coincident γ-photons with an accuracy in the order of 200ps, time-of-flight positron emission tomography systems commonly employ silicon photomultipliers (SiPMs) and high-resolution digitization electronics, application specific integrated circuits (ASICs). This work evaluates the performance of the TOFPET2 ASIC, released by PETsys Electronics S.A. in 2017, dependent on its configuration parameters in multi-channel coincidence measurements.

METHODS

SiPM arrays fabricated by different vendors (KETEK, SensL, Hamamatsu, Broadcom) were tested in combination with the ASIC. Scintillator arrays featuring different reflector designs and different configurations of the TOFPET2 ASIC software parameters were evaluated. The benchtop setup used is provided with the TOFPET2 ASIC evaluation kit by PETsys Electronics S.A.

RESULTS

Compared to existing studies featuring the TOFPET2 ASIC, multi-channel performance results dependent on a larger set of ASIC configuration parameters were obtained that have not been reported to this extend so far. The ASIC shows promising CRTs down to 219.9 ps in combination with two Hamamatsu S14161-3050-HS-08 SiPM arrays (128 channels read out, energy resolution 13.08%) and 216.1 ps in combination with two Broadcom AFBR-S4N44P643S SiPM arrays (32 channels read out, energy resolution 9.46%). The length of the trigger delay of the dark count suppression scheme has an impact on the ASIC performance and can be configured to further improve the coincidence resolution time. The integrator gain configuration has been investigated and allows an absolute improvement of the energy resolution by up to 1% at the cost of the linearity of the energy spectrum.

CONCLUSION

Measuring up to the time-of-flight performance of state-of-the-art positron emission tomography (ToF-PET) systems while providing a uniform and stable readout for multiple channels at the same time, the TOFPET2 ASIC is treated as promising candidate for the integration in future ToF-PET systems.

Performance evaluation of the mouse version of the LabPET II PET scanner

The LabPET II is a new positron emission tomography technology platform designed to achieve submillimetric spatial resolution imaging using fully pixelated avalanche photodiodes-based detectors and highly integrated parallel front-end processing electronics. The detector was designed as a generic building block to develop devices for preclinical imaging of small to mid-sized animals and for clinical imaging of the human brain. The aim of this work is to assess the physical characteristics and imaging performance of the mouse version of LabPET II scanner following the NEMA NU4-2008 standard and using high resolution phantoms and in vivo imaging applications. A reconstructed spatial resolution of 0.78 mm (0.5 μ l) is measured close to the center of the radial field of view. With an energy window of 350 650 keV, the system absolute sensitivity is 1.2% and its maximum noise equivalent count rate reaches 61.1 kcps at 117 MBq. Submillimetric spatial resolution is achieved in a hot spot phantom and tiny bone structures were resolved with unprecedented contrast in the mouse. These results provide convincing evidence of the capabilities of the LabPET II technology for biomolecular imaging in preclinical research.

Energy spectra due to the intrinsic radiation of LYSO/LSO scintillators for a wide range of crystal sizes

PURPOSE

Most detectors in current positron emission tomography (PET) scanners and prototypes use lutetium oxyorthosilicate (LSO) or lutetium yttrium oxyorthosilicate (LYSO) scintillators. The aim of this work is to provide a complete set of background energy spectra, due to the scintillator intrinsic radioactivity, for a wide range of crystal sizes.

METHODS

An analytical model, developed and validated in a previous work, was used to obtain the background energy spectra of square base cuboids of different dimensions. The model uses the photon absorption probabilities of the three gamma rays (88, 202, and 307 keV) emitted following the beta decay of ¹⁷⁶ Lu to ¹⁷⁶ Hf excited states. These probabilities were obtained for each crystal size considered in this work from Monte Carlo simulations using the PENELOPE code. The probabilities are then used to normalize and shift the beta spectrum to the corresponding energy value of the simultaneous detection of one, two, or three gamma rays in the scintillator. The simulated cuboids had side lengths of 5, 10, 20, 30, 40, 50, and 60 mm and crystal thickness T = 2.5, 5, 10, 15, and 20 mm. From these results a complete set of energy spectra, including intermediate dimensions, were obtained. In addition, LYSO and LSO were compared in terms of their analytical background energy spectra for two crystal sizes. The analytical spectra were convolved using a variable Gaussian kernel to account for the energy resolution of a typical detector. A parameterization of the photon absorption probabilities for each gamma ray energy as a function of the cuboid volume to surface area ratio was obtained.

RESULTS

A data set of L(Y)SO background energy spectra was obtained and is available for the reader as 2D histograms. The model accurately predicts the structure of the energy spectra including the relative peak and valley intensities. The data allow visualizing how the structure evolves with increasing crystal length and thickness. Lutetium yttrium oxyorthosilicate and LSO present very similar background energy spectra for the range of sizes studied in this work and therefore the data generated can be confidently used for both scintillator materials. The filtered spectra showed a variable shift in the main peaks, depending on crystal size, alerting that to achieve a correct detector calibration using the background spectrum is not straight forward and requires precise data analysis and measurements. In addition, we found that square base L(Y)SO cuboids with same volume to surface area ratio have background spectra with the same structure.

CONCLUSIONS

We present the energy spectra of L(Y)SO crystal of different sizes which will be very useful for industry and research groups developing and simulating detectors for positron imaging applications in terms of calibration, quality assurance, crystal maps, detector fine gain tuning, background reduction and other applications using the long-lived ¹⁷⁶ Lu source. We analyzed the data produced in this work and found that crystal cuboids with equal volume to surface area ratio produce the same background energy spectra, a conclusion that simplifies its calculation and clarifies why the same energy spectrum is observed under different experimental setups.

Performance Evaluation and Compatibility Studies of a Compact Preclinical Scanner for Simultaneous PET/MR Imaging at 7 Tesla

We present the design and performance of a new compact preclinical system combining positron emission tomography (PET) and magnetic resonance imaging (MRI) for simultaneous scans. The PET contains sixteen SiPM-based detector heads arranged in two octagons and covers an axial field of view (FOV) of 102.5 mm. Depth of interaction effects and detector's temperature variations are compensated by the system. The PET is integrated in a dry magnet operating at 7 T. PET and MRI characteristics were assessed complying with international standards and interferences between both subsystems during simultaneous scans were addressed. For the rat size phantom, the peak noise equivalent count rates (NECR) were 96.4 kcps at 30.2 MBq and 132.3 kcps at 28.4 MBq respectively with and without RF coil. For mouse, the peak NECR was 300.0 kcps at 34.5 MBq and 426.9 kcps at 34.3 MBq respectively with and without coil. At the axial centre of the FOV, spatial resolutions expressed as full width at half maximum / full width at tenth maximum (FWHM/FWTM) ranged from 1.69/3.19 mm to 2.39/4.87 mm. The peak absolute sensitivity obtained with a 250-750 keV energy window was 7.5% with coil and 7.9% without coil. Spill over ratios of the NEMA NU4-2008 image quality (NEMA-IQ) phantom ranged from 0.25 to 0.96 and the percentage of non-uniformity was 5.7%. The image count versus activity was linear up to 40 MBq. The principal magnetic field variation was 0.03 ppm/mm over 40 mm. The qualitative and quantitative aspects of data were preserved during simultaneous scans.

Advances in Preclinical PET Instrumentation

In the light of ever-increasing demands for PET scanner with better resolvability, higher sensitivity and wide accessibility for noninvasive screening of small structures and physiological processes in laboratory rodents, several dedicated PET scanners were developed and evaluated. Understanding conceptual design constraints pros and cons of different configurations and impact of the major components will be helpful to further establish the crucial role of these miniaturized systems in a broad spectrum of modern research. Hence, a comprehensive review of preclinical PET scanners developed till early 2020 with particular emphasis on innovations in instrumentation and geometrical designs is provided.

Comments on the NEMA NU 4-2008 Standard on Performance Measurement of Small Animal Positron Emission Tomographs

The National Electrical Manufacturers Association’s (NEMA) NU 4-2008 standard specifies methodology for evaluating the performance of small-animal PET scanners. The standard’s goal is to enable comparison of different PET scanners over a wide range of technologies and geometries used. In this work, we discuss if the NEMA standard meets these goals and we point out potential flaws and improvements to the standard.For the evaluation of spatial resolution, the NEMA standard mandates the use of filtered backprojection reconstruction. This reconstruction method can introduce star-like artifacts for detectors with an anisotropic spatial resolution, usually caused by parallax error. These artifacts can then cause a strong dependence of the resulting spatial resolution on the size of the projection window in image space, whose size is not fully specified in the NEMA standard. If the PET ring has detectors which are perpendicular to a Cartesian axis, then the resolution along this axis will typically improve with larger projection windows.We show that the standard’s equations for the estimation of the random rate for PET systems with intrinsic radioactivity are circular and not satisfiable. However, a modified version can still be used to determine an approximation of the random rates under the assumption of negligible random rates for small activities and a constant scatter fraction. We compare the resulting estimated random rates to random rates obtained using a delayed coincidence window and two methods based on the singles rates. While these methods give similar estimates, the estimation method based on the NEMA equations overestimates the random rates.In the NEMA standard’s protocol for the evaluation of the sensitivity, the standard specifies to axially step a point source through the scanner and to take a different scan for each source position. Later, in the data analysis section, the standard does not specify clearly how the different scans have to be incorporated into the analysis, which can lead to unclear interpretations of publicized results.The standard’s definition of the recovery coefficients in the image quality phantom includes the maximum activity in a region of interest, which causes a positive correlation of noise and recovery coefficients. This leads to an unintended trade-off between desired uniformity, which is negatively correlated with variance (i.e., noise), and recovery.With this work, we want to start a discussion on possible improvements in a next version of the NEMA NU-4 standard.

Sensors for Positron Emission Tomography Applications

Positron emission tomography (PET) imaging is an essential tool in clinical applications for the diagnosis of diseases due to its ability to acquire functional images to help differentiate between metabolic and biological activities at the molecular level. One key limiting factor in the development of efficient and accurate PET systems is the sensor technology in the PET detector. There are generally four types of sensor technologies employed: photomultiplier tubes (PMTs), avalanche photodiodes (APDs), silicon photomultipliers (SiPMs), and cadmium zinc telluride (CZT) detectors. PMTs were widely used for PET applications in the early days due to their excellent performance metrics of high gain, low noise, and fast timing. However, the fragility and bulkiness of the PMT glass tubes, high operating voltage, and sensitivity to magnetic fields ultimately limit this technology for future cost-effective and multi-modal systems. As a result, solid-state photodetectors like the APD, SiPM, and CZT detectors, and their applications for PET systems, have attracted lots of research interest, especially owing to the continual advancements in the semiconductor fabrication process. In this review, we study and discuss the operating principles, key performance parameters, and PET applications for each type of sensor technology with an emphasis on SiPM and CZT detectors—the two most promising types of sensors for future PET systems. We also present the sensor technologies used in commercially available state-of-the-art PET systems. Finally, the strengths and weaknesses of these four types of sensors are compared and the research challenges of SiPM and CZT detectors are discussed and summarized.

NEMA NU-4 2008 performance evaluation of Xtrim-PET: A prototype SiPM-based preclinical scanner

PURPOSE

Xtrim-PET is a newly designed Silicon Photomultipliers (SiPMs)-based prototype PET scanner dedicated for small laboratory animal imaging. We present the performance evaluation of the Xtrim-PET scanner following NEMA NU-4 2008 standards to help optimizing scanning protocols which can be achieved through standard and reliable system performance characterization.

METHODS

The performance assessment was conducted according to the National Electrical Manufacturers Association (NEMA) NU-4 2008 standards in terms of spatial resolution, sensitivity, counting rate performance, scatter fraction and image quality. The in vivo imaging capability of the scanner is also showcased through scanning a normal mouse injected with ¹⁸ F-FDG. Furthermore, the performance characteristics of the developed scanner are compared with commercially available systems and current prototypes.

RESULTS

The volumetric spatial resolution at 5 mm radial offset from the central axis of the scanner is 6.81 µl, whereas a peak absolute sensitivity of 2.99% was achieved using a 250-650 keV energy window and a 10 ns timing window. The peak noise-equivalent count rate (NECR) using a mouse-like phantom is 113.18 kcps at 0.34 KBq/cc with 12.5% scatter fraction, whereas the NECR peaked at 82.76 kcps for an activity concentration level of 0.048 KBq/cc with a scatter fraction of 25.8% for rat-like phantom. An excellent uniformity (3.8%) was obtained using NEMA image quality phantom. Recovery coefficients of 90%, 86%, 68%, 40% and 12% were calculated for rod diameters of 5, 4, 3, 2 and 1 mm, respectively. Spill-over ratios for air-filled and water-filled chambers were 35% and 25% without applying any correction for attenuation and Compton scattering effects.

CONCLUSION

Our findings revealed that beyond compactness, lightweight, easy installation and good energy resolution, the Xtrim-PET prototype presents a reasonable performance making it suitable for preclinical molecular imaging-based research.

Optimization of a depth of interaction encoding PET block detector for a PET/MRI insert

Preclinical positron emission tomography, combined with magnetic resonance imaging (PET/MRI), is increasingly used as a tool to simultaneously characterize functional processes in vivo. Many emerging preclinical applications, however, are limited by PET detection sensitivity, especially when generating short imaging frames for quantitative studies. One such application is dynamic multifunctional imaging, which probes multiple aspects of a biological process, using relationships between the datasets to quantify interactions. These studies have limited accuracy due to the relatively low sensitivity of modern preclinical PET/MRI systems. The goal of this project is to develop a preclinical PET/MRI insert with detection sensitivity above 15% (250-750 keV) to improve quantitation in dynamic PET imaging. To achieve this sensitivity, we have developed a detector module incorporating a 2 cm thick crystal block, which will be arranged into a system with 8 cm axial FOV, targeting mice and rats. To maintain homogenous spatial resolution, the detector will incorporate dual-ended depth-of-interaction (DOI) encoding with silicon photomultiplier (SiPM) based photodetector arrays. The specific aim of this work is to identify a detector configuration with adequate performance for the proposed system. We have optimized the SiPM array geometry and tested two crystal array materials with pitch ranging from 0.8 to 1.2 mm and various surface treatments and reflectors. From these configurations, we have identified the best balance between crystal separation, energy resolution, and DOI resolution. The final detector module uses two rectangular SiPM arrays with 5  ×  6 and 5  ×  4 elements. The photodetector arrays are coupled to a 19  ×  19 array of 1 mm pitch LYSO crystals with polished surfaces and a diffuse reflector. The prototype design has 14.3%  ±  2.9% energy resolution, 3.57  ±  0.88 mm DOI resolution, and resolves all elements in the crystal array, giving it sufficient performance to serve as the basis for the proposed high sensitivity PET/MRI insert.