Technical opportunities and challenges in developing total-body PET scanners for mice and rats

Feasibility Study for a Microstrip Transmission Line RF Coil Integrated with a PET Detector Module in a 7T Human MR Imaging System

PURPOSE

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."

METHODS

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.

RESULTS

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.

CONCLUSIONS

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.

High spatial resolution PET detectors based on 10 mm × 10 mm linearly-graded SiPMs and 0.5 mm pitch LYSO arrays

Objective. Position-sensitive silicon photomultipliers (PS-SiPMs) are promising photodetectors for ultra-high spatial resolution small-animal positron emission tomography (PET) scanners. This paper evaluated the performance of the latest generation of linearly-graded SiPMs (LG-SiPMs), a type of PS-SiPM, for ultra-high spatial resolution PET applications using LYSO arrays from two vendors.Approach. Two dual-ended readout detectors were developed by coupling LG-SiPMs to both ends of the two LYSO arrays. Each LG-SiPM has an active area of 9.8 × 9.8 mm2. Both LYSO arrays consist of 20 × 20 arrays of 0.44 × 0.44 × 20 mm3polished LYSOs with a pitch of 0.5 mm. The performance of the two detectors was compared in terms of flood histogram, energy resolution, timing resolution, and depth-of-interaction (DOI) resolutions.Main results. Flood histograms showed clear identification of all LYSO elements except for some edge crystals due to the larger size of the LYSO arrays compared to the active area of the LG-SiPMs and the misalignment between LG-SiPMs and LYSO arrays in the assembled detectors. At a bias voltage of 37.0 V, the detectors utilizing the Tianle LYSO array and EBO LYSO array provided energy resolutions of 17.5 ± 2.2 and 18.6 ± 2.0%, timing resolutions of 0.75 ± 0.03 and 0.78 ± 0.03 ns, and DOI resolutions of 2.16 ± 0.15 and 2.31 ± 0.12 mm, respectively.Significance. The results presented in this paper demonstrate that the new generation LG-SiPMs are promising photodetectors for ultra-high spatial resolution small-animal PET scanner applications.

Performance evaluation of a small-animal PET scanner with 213 mm axis using NEMA NU 4-2008

BACKGROUND

Long-axis positron emission tomography (PET) has emerged as one of the recent research directions in PET due to its ability to significantly enhance sensitivity and counting performance for low-dose imaging, rapid imaging, and whole-body dynamic imaging.

PURPOSE

The PET system presented in this study is a long-axis animal PET based on lutetium-yttrium orthosilicate and silicon photomultiplier, designed for whole-body imaging in rats. It features a diameter of 143 mm and an axial length of 213.3 mm. This study evaluated the performance of this PET system in accordance with the National Electrical Manufacturers Association (NEMA) NU 4-2008 standards.

METHODS

The performance evaluation was conducted according to the NEMA NU 4-2008 standards in terms of spatial resolution, sensitivity, counting rate performance, scatter fraction (SF) and image quality. In addition, a rat imaging study was conducted to assess the imaging capability of this PET system.

RESULTS

The average energy resolution of the PET system was 12.87%, the average coincidence timing resolution was 751 ps. The FWHM of spatial resolution reconstructed by filtered back projection and 3D-OSEM-PSF algorithm at 5 mm radial offset from the axial center were 1.65 and 0.88 mm. The peak absolute sensitivity measured by a point source at the center of the field of view was evaluated as 6.71% (361-661 keV) and 10.31% (250-750 keV). For the mouse-like phantom, the SF was 11.0% and the peak noise equivalent counting rate (NECR) was 1193 kcps at 94.2 MBq (2.54 mCi). For the rat-like phantom, the SF was 26.8% and the NECR was 682.5 kcps at 78.6 MBq (2.12 mCi).

CONCLUSIONS

The performance measurement results demonstrate that this PET system exhibits high sensitivity and count rate performance, making it potential for high-quality whole-body dynamic imaging of rats.

Design and implementation of continuous bed motion (CBM) in Xtrim preclinical PET scanner for whole-body Imaging: MC simulation and experimental measurements

PURPOSE

Preclinical PET scanners often have limited axial field-of-view for whole-body (WB) scanning of the small-animal. Step-and-shoot(S&S) acquisition mode requires multiple bed positions (BPs) to cover the scan length. Alternatively, in Continuous Bed Motion(CBM) mode, data acquisition is performed while the bed is continuously moving. In this study, to reduce acquisition time and enhance image quality, the CBM acquisition protocol was optimized and implemented on the Xtrim-PET preclinical scanner for WB imaging.

METHODS

The over-scan percentage(OS%) in CBM mode was optimized by Monte Carlo simulation. Bed movement speed was optimized considering ranges from 0.1 to 2.0 mm s-1, and absolute system sensitivities with the optimal OS% were calculated. The performance of the scanner in CBM mode was measured, and compared with S&S mode based on the NEMA-NU4 standard.

RESULTS

The optimal trade-off between absolute sensitivity and uniformity of sensitivity profile was achieved at OS-50 %. In comparison to S&S mode with maximum ring differences (MRD) of 9 and 23, the calculated equivalent speeds in CBM(OS-50 %) mode were 0.3 and 0.14 mm s-1, respectively. In terms of data acquisition with equal sensitivity in both CBM(OS-50 %) and S&S(MRD-9) modes, the total scan time in CBM mode decreased by 25.9 %, 47.7 %, 54.7 %, and 58.2 % for scan lengths of 1 to 4 BPs, respectively.

CONCLUSION

The CBM mode enhances WB PET scans for small-animals, offering rapid data acquisition, high system sensitivity, and uniform axial sensitivity, leading to improved image quality. Its efficiency and customizable scan length and bed speed make it a superior alternative.

Evaluation of monolithic crystal detector with dual-ended readout utilizing multiplexing method

Objective.Monolithic crystal detectors are increasingly being applied in positron emission tomography (PET) devices owing to their excellent depth-of-interaction (DOI) resolution capabilities and high detection efficiency. In this study, we constructed and evaluated a dual-ended readout monolithic crystal detector based on a multiplexing method.Approach.We employed two 12 × 12 silicon photomultiplier (SiPM) arrays for readout, and the signals from the 12 × 12 array were merged into 12 X and 12 Y channels using channel multiplexing. In 2D reconstruction, three methods based on the centre of gravity (COG) were compared, and the concept of thresholds was introduced. Furthermore, a light convolutional neural network (CNN) was employed for testing. To enhance depth localization resolution, we proposed a method by utilizing the mutual information from both ends of the SiPMs. The source width and collimation effect were simulated using GEANT4, and the intrinsic spatial resolution was separated from the measured values.Main results.At an operational voltage of 29 V for the SiPM, an energy resolution of approximately 12.5 % was achieved. By subtracting a 0.8 % threshold from the total energy in every channel, a 2D spatial resolution of approximately 0.90 mm full width at half maximum (FWHM) can be obtained. Furthermore, a higher level of resolution, approximately 0.80 mm FWHM, was achieved using a CNN, with some alleviation of edge effects. With the proposed DOI method, a significant 1.36 mm FWHM average DOI resolution can be achieved. Additionally, it was found that polishing and black coating on the crystal surface yielded smaller edge effects compared to a rough surface with a black coating.Significance.The introduction of a threshold in COG method and a dual-ended readout scheme can lead to excellent spatial resolution for monolithic crystal detectors, which can help to develop PET systems with both high sensitivity and high spatial resolution.

The detection instrumentation and geometric design of clinical PET scanner: towards better performance and broader clinical applications

Positron emission tomography (PET) is a powerful medical imaging modality used in nuclear medicine to diagnose and monitor various clinical diseases in patients. It is more sensitive and produces a highly quantitative mapping of the three-dimensional biodistribution of positron-emitting radiotracers inside the human body. The underlying technology is constantly evolving, and recent advances in detection instrumentation and PET scanner design have significantly improved the medical diagnosis capabilities of this imaging modality, making it more efficient and opening the way to broader, innovative, and promising clinical applications. Some significant achievements related to detection instrumentation include introducing new scintillators and photodetectors as well as developing innovative detector designs and coupling configurations. Other advances in scanner design include moving towards a cylindrical geometry, 3D acquisition mode, and the trend towards a wider axial field of view and a shorter diameter. Further research on PET camera instrumentation and design will be required to advance this technology by improving its performance and extending its clinical applications while optimising radiation dose, image acquisition time, and manufacturing cost. This article comprehensively reviews the various parameters of detection instrumentation and PET system design. Firstly, an overview of the historical innovation of the PET system has been presented, focusing on instrumental technology. Secondly, we have characterised the main performance parameters of current clinical PET and detailed recent instrumental innovations and trends that affect these performances and clinical practice. Finally, prospects for this medical imaging modality are presented and discussed. This overview of the PET system's instrumental parameters enables us to draw solid conclusions on achieving the best possible performance for the different needs of different clinical applications.

Performance Comparison of DOI-Encoding PET Detectors Based on 1.1-mm Pitch BGO Arrays With Different Reflectors

Bismuth germanate (BGO)-based positron emission tomography (PET) detectors are potential candidates for low-dose imaging PET scanners, owing to the high stopping power and low background radiation of BGO. In this paper, we compared the performance of two dual-ended readout PET detectors based on 15 × 15 BGO arrays. Both arrays had the same 1.1 mm pitch but utilized different reflectors - barium sulfate (BaSO4) and enhanced specular reflector film (ESR) - for high-resolution PET applications. The detectors were constructed with Hamamatsu 13361-2050-08 SiPM arrays. Each BGO element had dimensions of 1.02 × 1.02 × 20 mm3. The lateral surfaces of the BGO elements were unpolished (saw-cut), while the two ends were polished. Flood histograms showed that the detector based on the BGO array with BaSO4 reflector had much better crystal identification and depth-of-interaction (DOI) resolution. Specifically, the energy, DOI, and timing resolutions for the detector using the BGO array with BaSO4 reflector were 19.8 ± 1.5%, 4.13 ± 0.48 mm, and 2.80 ± 0.23 ns, respectively. In contrast, the values obtained using the BGO array with ESR reflector were 20.9 ± 2.1%, 7.69 ± 1.92 mm, and 2.93 ± 0.20 ns, respectively.