PET Detector Module Research Articles

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

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.

MRI compatibility study of a prototype radiofrequency penetrable oval PET insert at 3T.

To perform an MRI compatibility study of an RF field-penetrable oval-shaped PET insert that implements an MRI built-in body RF coil both as a transmitter and a receiver. Twelve electrically floating RF shielded PET detector modules were used to construct the prototype oval PET insert with a major axis of 440mm, a minor axis of 350mm, and an axial length of 225mm. The electric floating of the PET detector modules was accomplished by isolating the cable shield from the detector shield using plastic tape. Studies were conducted on the transmit (B1) RF field, the image signal-to-noise ratio (SNR), and the RF pulse amplitude for a homogeneous cylindrical (diameter: 160mm and length: 260mm) phantom (NaCl + NiSO4 solution) in a 3T clinical MRI system (Verio, Siemens, Erlangen, Germany). The B1 maps for the oval insert were similar to the MRI-only field responses. Compared to the MRI-only values, SNR reductions of 51%, 45%, and 59% were seen, respectively, for the spin echo (SE), gradient echo (GE), and echo planar (EPI) images for the case of oval PET insert. Moreover, the required RF pulse amplitudes for the SE, GE, and EPI sequences were, respectively, 1.93, 1.85, and 1.36 times larger. However, a 30% reduction in the average RF reception sensitivity was observed for the oval insert. The prototype floating PET insert was a safety concern for the clinical MRI system, and this compatibility study provided clearance for developing a large body size floating PET insert for the existing MRI system. Because of the RF shield of the insert, relatively large RF powers compared to the MRI-only case were required. Because of this and also due to low RF sensitivity of the body coil, the SNRs reduced largely.

Limited-angle TOF-PET for intraoperative surgical applications: proof of concept and first experimental data

The intraoperative gamma probe (IPG) based on single gamma-ray detection remains the current gold standard modality for sentinel lymph node identification and tumor removal in cancer patients. However, IPGs do not meet the <5% false negative rate (FNR) requirement, a key metric suggested by the American Society of Clinical Oncology (ASCO). We aim to reduce FNR by using time of flight (TOF) PET detector technology in limited angle geometry system by using only two detector panels in coincidence. For proof of concept, we used two Hamamatsu TOF PET detector modules (C13500-4075YC-12) featuring 12× 12 arrays of 4.14× 4.14× 20 mm3 LFS crystal pixels with 4.2 mm pitch and coupled one-one to silicon photomultiplier (SiPM) pixels. The measured detector coincidence timing resolution (CTR) was 271 ps FWHM for the whole detector. We 3D printed lesion phantom containing spheres 2–10 mm in diameter, representing lymph nodes, and placed it inside a 10-liter warm background water phantom. Experimental results showed that with subminute data acquisition, 6 mm diameter spheres could be identified in the image when a lesion phantom with a 10:1 activity ratio to background was used. The simulation results were in good agreement with the experimental data by resolving 6 mm diameter spherical lesions with a 60 second acquisition time in a 25 cm deep background water phantom with a 10:1 activity ratio. As expected, the image quality improved as the CTR improved in the simulation and with decreasing background water phantom depth or increasing lesion-to-background activity ratio in the experiment. With the results presented here, we concluded that using a limited angle TOF PET detector system is a major step forward for intraoperative applications in that lesion detectability is beyond what conventional gamma- and NIR-based probes could achieve.

Evaluation of high-resolution and depth-encoding PET detector modules based on single-ended readout with TOFPET2 ASIC

Evaluation of high-resolution and depth-encoding PET detector modules based on single-ended readout with TOFPET2 ASIC

Hybrid RCD-WTA Multiplexer for MR-compatible PET Detector

A hybrid multiplexer readout circuit one dimensional resistive charge division (1D-RCD) chain and winner takes all (WTA) circuit was proposed for MR-compatible PET detector modules. To prove the effectiveness of the circuit, three different readout circuits including WTA, 2D-RCD and proposed hybrid RCD-WTA multiplexer were evaluated for 4 × 4 LYSO-GAPD PET detector. Various parameters were precisely measured and quantitatively characterized. While there were considerable differences observed in rise time, fall time, pulse width and amplitude of analog output waveforms for each circuit, the detector performance was quite similar in crystal identification and energy resolution. However, pincushion distortion was observed in the flood histogram of 2D-RCD, and spatial linearity was relatively degraded. The coincidence time resolutions were 487 ps, 955 ps, and 676 ps for WTA, 2D-RCD, and hybrid RCD-WTA multiplexer, respectively. These results demonstrated that proposed hybrid multiplexer could effectively reduce the number of analog electronic channels from N² to only 2 for an N × N photosensor array, while maintaining intrinsic performance of multichannel PET detector requiring energy, position and time information for incidence photons.

Development and initial characterization of a high-resolution PET detector module with DOI

Organ-dedicated PET scanners are becoming more prevalent because of their advantages in higher sensitivity, improved image quality, and lower cost. Detectors utilized in these scanners have finer pixel size with depth of interaction (DOI) capability. This work presents a LYSO(Ce) detector module with DOI capability which has the potential to be scaled up to a high-resolution small animal or organ-dedicated PET system. For DOI capability, a submodule with one LYSO block detector utilizing PETsys TOFPET2 application-specific integrated circuit (ASIC) was previously developed in our lab. We scaled up the submodule and optimized the configuration to allow for a compact housing of the dual-readout boards in one side of the blocks by designing a high-speed dual-readout cable to maintain the original pin-to-pin relationship between the Samtec connectors. The module size is 53.8 × 57.8 mm2. Each module has 2 × 2 LYSO blocks, each LYSO block consists of 4 × 4 LYSO units, and each LYSO unit contains a 6 × 6 array of 1 × 1 × 20 mm3 LYSO crystals. The four lateral surfaces of LYSO crystal were mechanically ground to W14, and the two end surfaces were polished. Two ends of the LYSO crystal are optically connected to SiPM for DOI measurement. Eight LYSO blocks performance including energy, timing, and DOI resolution is characterized with a single LYSO slab. The in-panel and orthogonal-panel spatial resolution of the two modules with 107.4 mm distance between each other are measured at 9 positions within the field of view (FOV) with a 22Na source. Results show that the average energy, timing, and DOI resolution of all LYSO blocks are 16.13% ± 1.01% at 511 keV, 658.03 ± 15.18 ps, and 2.62 ± 0.06 mm, respectively. The energy and timing resolution of two modules are 16.35% and 0.86 ns, respectively. The in-panel and orthogonal-panel spatial resolution of the two modules at the FOV center are 1.9 and 4.4 mm respectively.

High-Resolution Depth-Encoding PET Detector Module with Prismatoid Light-Guide Array.

Depth-encoding detectors with single-ended readout provide a practical, cost-effective approach for constructing high-resolution and high-sensitivity PET scanners. However, the current iteration of such detectors uses a uniform glass light-guide to achieve depth encoding, resulting in nonuniform performance throughout the detector array due to suboptimal intercrystal light sharing. We introduce Prism-PET, a single-ended-readout PET detector module with a segmented light-guide composed of an array of prismatoids that introduce enhanced, deterministic light sharing. Methods: High-resolution PET detector modules were fabricated with single-ended readout of polished multicrystal lutetium yttrium orthosilicate scintillator arrays directly coupled 4-to-1 and 9-to-1 to arrays of 3 × 3 mm silicon photomultiplier pixels. Each scintillator array was coupled at the nonreadout side to a light-guide (one 4-to-1 module with a uniform glass light-guide, one 4-to-1 Prism-PET module, and one 9-to-1 Prism-PET module) to introduce intercrystal light sharing, which closely mimics the behavior of dual-ended readout, with the additional benefit of improved crystal identification. Flood histogram data were acquired using a 3-MBq 22Na source to characterize crystal identification and energy resolution. Lead collimation was used to acquire data at specific depths to determine depth-of-interaction (DOI) resolution. Results: The flood histogram measurements showed excellent and uniform crystal separation throughout the Prism-PET modules, whereas the uniform glass light-guide module had performance degradation at the edges and corners. A DOI resolution of 5.0 mm full width at half maximum (FWHM) and an energy resolution of 13% FWHM were obtained in the uniform glass light-guide module. By comparison, the 4-to-1 coupled Prism-PET module achieved a DOI resolution of 2.5 mm FWHM and an energy resolution of 9% FWHM. Conclusion: PET scanners based on our Prism-PET modules with segmented prismatoid light-guide arrays can achieve high and uniform spatial resolution (9-to-1 coupling with ∼1-mm crystals), high sensitivity (20-mm-thick detectors and intercrystal Compton scatter recovery), good energy and timing resolutions (using polished crystals and after applying DOI correction), and compact size (depth encoding eliminates parallax error and permits smaller ring-diameter).

Highly multiplexed SiPM signal readout for brain-dedicated TOF-DOI PET detectors

PurposeWe investigated the highly multiplexed readout of SiPM signals that are useful in developing brain-dedicated PET detectors with DOI-capable crystal blocks and large-area SiPM arrays. MethodsThe PET detector module used in this study was equipped with a two-layer relative-offset DOI crystal block and a 2 × 2 array of 16-channel SiPMs. The lower crystal-layer consisted of a 14 × 14 array of 1.78 × 1.78 × 8 mm3 LSO crystals and the upper crystal-layer consisted of a 13 × 13 array of 1.78 × 1.78 × 12 mm3 LSO crystals. The energy and position information was obtained via signals from the 8 × 8 resistive charge division multiplexing circuit. The timing performance was evaluated with varying multiplexing ratios (i.e. 16:1, 32:1, and 64:1) via first-order analog high-pass filtering. ResultsFor three different multiplexing schemes, all LSO crystals with two-layer DOI information were clearly resolved and yielded good energy resolutions of 10.5 ± 1.0% (upper) and 12.1 ± 1.7% (lower). The 16:1 multiplexing yielded an optimal timing performance with average CRT values of 325 ps FWHM (upper) and 342 ps FWHM (lower); however, the timing performances were maintained almost constant even for 64:1 multiplexing with average CRT values of 336 ps FWHM (upper) and 347 ps FWHM (lower). ConclusionsThe highly multiplexed SiPM signal readout via the first-order analog high-pass filtering could be an attractive solution to develop brain-dedicated PET scanners, effectively decreasing the burden of DAQ systems with moderate compromise in terms of TOF and DOI performances.

An FPGA-Based Fast Linear Discharge Readout Scheme Enabling Simultaneous Time and Energy Measurements for TOF-PET Detectors

The readout electronics in time-of-flight positron emission tomography (TOF-PET) detector modules conventionally contain a separate timing circuit and a separate energy measurement circuit. For channel-by-channel readouts, the front-end electronics will be unacceptably large and must rely on application specific integrated circuit (ASIC) chips. In this paper, we present a novel field programmable gate array (FPGA)-based readout scheme that unifies the time and energy measurements with only one analog amplifier per channel in addition to the FPGA. By constructing a simple and effective fast linear discharge circuit, one time-to-digital converter (TDC) per channel can implement time and energy readouts simultaneously with a short measurement dead time. To evaluate the performance of the new approach, a 16-channel PET detector module consisting of LYSO crystals coupled with a silicon photomultiplier (SiPM) array was constructed connecting to the readout electronics. Using a reference detector with a 164.7-ps time resolution to perform 22Na coincidence measurements, the average energy, and time resolution among 16 channels were measured as 12.44% and 372.6 ps, respectively. The typical measurement dead time is approximately 300 ns. The test results show the simplicity and feasibility of the proposed readout scheme, which is particularly useful for laboratory instrumentation.

A depth-of-interaction encoding PET detector module with dual-ended readout using large-area silicon photomultiplier arrays

The performance of a depth-of-interaction (DOI) encoding PET detector module with dual-ended readout of LYSO scintillator arrays using large-area SiPM arrays was evaluated. Each SiPM array, with a surface area of 50.2 × 50.2 mm2, consists of 12 × 12 C-series SiPMs from SensL (SensL, Inc). The LYSO array, with a total size of 46 × 46 mm2 and a pitch size of 1.0 mm, consists of a 46 × 46 array of 0.945 × 0.945 × 20 mm3 polished LYSO crystals, separated by Toray reflector. Custom front-end electronics were designed to reduce the 288 SiPM signals of one detector module to nine signals, eight for position information and 1 for timing information. Schottky diodes were used to block noise from SiPMs that did not detect a significant number of scintillation photons following a gamma interaction. Measurements of noise, signal, signal-to-noise ratio, energy resolution and flood histogram quality were obtained at different bias voltages (26.0 to 31.0 V in 0.5 V intervals) and at two temperatures (5 °C and 20 °C). Clear acrylic plates, 2.0 mm thick, were used as light guides to spread the scintillation photons. Timing resolution, depth of interaction resolution, and the effect of event rate on detector performance were measured at the bias voltage determined to be optimal for the flood histograms. Performance obtained with and without the noise-blocking Shottky diodes was also compared. The results showed that all crystals in the LYSO array can be clearly resolved, and performance improved when using diodes to block noise, and at the lower temperature. The average energy resolution, flood histogram quality, timing resolution and DOI resolution were 23.8% ± 2.0%, 1.54 ± 0.17, 1.78 ± 0.09 ns and 2.81 ± 0.13 mm respectively, obtained at a bias voltage of 30.0 V and a temperature of 5 °C using the diode readout method. The event rate experiments showed that the flood histogram and energy resolution of the detector were not significantly degraded for an event rate of up to 150 000 counts s−1.

Geometry optimization of electrically floating PET inserts for improved RF penetration for a 3T MRI system.

An electrically floating radio frequency (RF) shielded PET insert with individual PET detectors shielded by separate Faraday cages enables the MRI built-in body RF coil to be used at least as an RF transmitter, in which the RF field penetrates the imaging region inside the PET ring through the narrow gaps between the shielded PET detector modules. Because the shielded PET ring blocks more than 90% of the imaging region for the transmit field from the body RF coil, it is very challenging to obtain the required RF field inside a full-ring floating PET insert. In this study, experiments were performed on the dependence of RF penetrability on different geometric aspects of the shielded PET modules and PET rings to optimize the design parameters to obtain the required RF field inside the PET ring. We developed several prototype cylindrical full-ring PET inserts using completely enclosed empty RF shield boxes (considered as dummy PET modules). Considering the RF shield box, we conducted studies for different axial lengths (240 and 120mm) and heights (30 and 45mm) of the shield boxes. On the other hand, considering the PET ring geometry, we also performed studies on three different categories of PET rings: a long-ring insert (longer than the MRI phantom), a short-ring insert (shorter than the MRI phantom), and a two-ring insert that combined two short-rings. In each ring category, two different inter-shield box gaps (1 and 3mm) were considered. In the case of the two-ring insert, three different ring-gaps (5, 10, and 20mm) were studied. In total, 21 PET inserts were studied with an inner diameter (i.d.) of 210mm. To study the effect of ring diameter, another long-ring insert was studied for the 270mm i.d. Experiments were conducted for the transmit RF (B1 ) fields and signal-to-noise ratios of spin-echo and gradient-echo images using a homogeneous phantom in a 700mm bore-diameter 3T clinical MRI system. RF pulse amplitudes generated automatically by the MRI system were recorded for comparison. A PET insert with a 3mm inter-box gap was found to perform the best, at a level which is acceptable for PET imaging. In the case of an insert of multiple short-rings instead of one long-ring insert, the 5 and 10mm ring-gaps provided higher RF field penetration. Increasing the inter-box gap improved the RF field penetration, whereas a ring-gap that was too wide concentrated the field near the ring-gap region. Relatively reduced RF power was required for wider inter-box gap or ring-gap or larger shield box height. Moreover, the rectangular shield box outperformed the trapezoidal shield box. On the other hand, when we changed the inner or outer diameter of the PET ring by keeping the same transaxial width of the shield boxes, we did not see any noticeable variation. Our study results provide comprehensive guidance on the geometrical design aspects of RF-penetrable PET inserts for efficient RF penetration inside the PET ring. By choosing proper geometric design parameters, we could get the RF field that was similar to the MRI-only case.

Analog and digital signal processing method using multi-time-over-threshold and FPGA for PET.

The goal of this study was to develop an analog and digital signal processing method using multi-time-over-threshold (MTOT) and field programmable gate arrays (FPGAs) to extract PET event information by using the internal clock of FPGA (~350 MHz), without ADC and TDC. The PET detector modules were composed of a 4 × 4 matrix of 3 × 3 × 20 mm3 LYSO and 4 × 4 SiPM array. Output charge signals of PET detector modules were amplified and fed into four comparators to generate trigger signals. The energy of the detected gamma ray was calculated by integrating the digitized pulse and the arrival time was determined from the time stamp of the lowest trigger signal by FPGA. The data packet containing energy, time, and position information was stored in list mode on the host computer. The performance of analog and digital signal processing circuits using MTOT method and FPGA was evaluated by measuring energy and time resolution of the proposed method and the values were 19% and 900 ps, respectively. This study demonstrated that the proposed MTOT method consisting of only FPGA without ADC and TDC could provide a simple and cost-effective analog and digital signal processing system for PET.

An improved time over threshold method using bipolar signals

The time over threshold (TOT) method has been recently proposed as a signal processing method used to calculate time and energy information by measuring the pulse arrival time and pulse duration over a preset threshold. Although TOT has been reported as an effective method for front end readout in PET applications, it has several limitations, including its non-linearity, lower dynamic range, and a trade-off between energy resolution and coincidence resolving time (CRT). In this study, we propose a novel design we developed to improve performance with regard to these problems occurring in the conventional TOT by employing a bipolar signal and two comparators. Using a high frequency CR shaping filter, a detected signal was converted into a bipolar signal, and the positive pulse of the converted bipolar signal had a fast rising time, while the negative pulse had a linear slope. The bipolar TOT circuit was composed of a preamplifier, a CR shaping filter, and two comparators. The PET detector was composed of a single LYSO coupled with 4 × 4 SiPM arrays, a bipolar TOT circuit, and an FPGA based TDC. And this was constructed to evaluate the performance of the proposed bipolar TOT method. A 16-ch PET detector module consisting of a 4 × 4 array LYSO coupled to a 4 × 4 SiPM arrays, an Anger logic discretized positioning circuit, and a 4-ch bipolar TOT circuit was also constructed to evaluate the functionality of the bipolar TOT method for PET applications. The pulse height resolution and CRT were measured using both the bipolar TOT method and the conventional TOT method. While the bipolar TOT method provided a similar pulse height resolution (10.4% ± 0.1%), the integral non-linearity (1.4%) and CRT (168 ± 4 ps) measured using the bipolar TOT method were greatly improved compared to those (17.2% and 258 ± 15 ps, respectively) measured with the conventional TOT method. The positions of the crystals were clearly identified, as seen in the flood histogram acquired using the 4-ch bipolar TOT circuit. The measured average pulse height resolution and average CRT for the 16-ch detector module were 11.5% ± 0.2% and 516 ± 24 ps. The results obtained in this study indicate that the bipolar TOT method requiring a relatively small number of electronic components could effectively improve the CRT, linearity and dynamic range. Furthermore, they also demonstrated the extendibility allowing the development of a PET system that consists of a large number of detectors.

Generic high resolution PET detector block using 12 × 12 SiPM array

With the advancement of the Silicon Photomultiplier (SiPM) technology, these devices are becoming the mainstream for use in high resolution PET detectors especially where the compatibility with magnetic field is critical. Large-area SiPM arrays are now widely used in high resolution PET detector modules. The challenges are emerged in the readout and timing circuitry by the increase of pixel density in the area. The application-specific integrated circuits (ASICs) are developed to readout arrays with standard anode/cathode outputs. SiPM detection principle, which produces dark current noise in the inactive pixels, makes the multiplexing techniques more challenging rather than the PMT signals. In this work a new generic PET detector block using SiPM arrays with three outputs (anode, cathode, and fast) is presented which is suitable for applications in small animal, brain, and clinical PET imaging. The block detector accepts a 12 × 12 array with both standard and fast output signals with an overall detector dimension of 50 × 50 mm2. A new version of the scrambled crosswire readout (SCR) method was implemented to reduce 144 ‘fast outputs’ to 9 tile signals and 144 standard outputs to 16 energy channels. The tile signals are also used to generate time pickoff information for timing resolution. We implemented time-to-digital converter (TDC) in Xilinx’s SPARTAN6 field programmable gate array (FPGA) by using the 64-tap delay line. This low cost chip performs all required processes for position, energy, time, and interface architecture. The attached 24 × 24 LYSO:Ce array with 2 × 2 × 10 mm3 pixels is imaged and pixels are resolved clearly in the room temperature. The measured energy resolution of the detector block after calibration for all crystal pixels is 12.4% at 511 keV. Also, the coincidence resolving time for two identical modules was measured at 1.85 ns.

Performance of a SiPM based semi-monolithic scintillator PET detector

A depth encoding PET detector module using semi-monolithic scintillation crystal single-ended readout by a SiPM array was built and its performance was measured. The semi-monolithic scintillator detector consists of 11 polished LYSO slices measuring 1 × 11.6 × 10 mm3. The slices are glued together with enhanced specular reflector (ESR) in between and outside of the slices. The bottom surface of the slices is coupled to a 4 × 4 SiPM array with a 1 mm light guide and silicon grease between them. No reflector is used on the top surface and two sides of the slices to reduce the scintillation photon reflection. The signals of the 4 × 4 SiPM array are grouped along rows and columns separately into eight signals. Four SiPM column signals are used to identify the slices according to the center of the gravity of the scintillation photon distribution in the pixelated direction. Four SiPM row signals are used to estimate the y (monolithic direction) and z (depth of interaction) positions according to the center of the gravity and the width of the scintillation photon distribution in the monolithic direction, respectively. The detector was measured with 1 mm sampling interval in both the y and z directions with electronic collimation by using a 0.25 mm diameter 22Na point source and a 1 × 1 × 20 mm3 LYSO crystal detector. An average slice based energy resolution of 14.9% was obtained. All slices of 1 mm thick were clearly resolved and a detector with even thinner slices could be used. The y positions calculated with the center of gravity method are different for interactions happening at the same y, but different z positions due to depth dependent edge effects. The least-square minimization and the maximum likelihood positioning algorithms were developed and both methods improved the spatial resolution at the edges of the detector as compared with the center of gravity method. A mean absolute error (MAE) which is defined as the probability-weighted mean of the absolute value of the positioning error is used to evaluate the spatial resolution. An average MAE spatial resolution of ~1.15 mm was obtained in both y and z directions without rejection of the multiple scattering events. The average MAE spatial resolution was ~0.7 mm in both y and z directions after the multiple scattering events were rejected. The timing resolution of the detector is 575 ps. In the next step, long rectangle detector will be built to reduce edge effects and improve the spatial resolution of the semi-monolithic detector. Thick detector up to 20 mm will be explored and the positioning algorithms will be further optimized.

Clinical utility of an endorectal MRI-guided prostate probe: preliminary examinations

Abstract Prostate cancer (PCa) is one of the most common cancer diseases in men in the western countries [1]. Besides the palpation, and the amount of prostate-specific-antigen’s (PSA) inside the blood, the current diagnostic imaging technologies are not appropriate. Early diagnosis defining the exact tumor location, spread and margins could make efficient targeted biopsies and image-guided surgery. A multimodal imaging technique containing a transmit-receive surface coil for anatomical MR imaging, a (S)PET detector module, consisting of silicon photomultipliers (SiPM), for functional imaging and an ultrasound (US) probe are placed as close as possible to the prostate designed as an endorectal tube to increase sensitivity and spatial resolution. All materials that are used are non-magnetic. Advantages of the SiPM are diversified, like non-sensitive to magnetic fields, higher gain (105–106) than standard avalanche photodiodes (APD), good timing properties and compactness. The PET detector should reach approximately 1mm3 spatial resolution together with 60ps FWHM Time-of-Flight resolution and a high efficiency to reduce scanning time and injected dose. A home-made transmit-receive coil surrounding the PET module improves signal-to-noise-ratio (SNR) with respect to standard coils will be present. The system will be used as a MRI-insert and be able to visualize anatomic and metabolic information together. The US-probe is guiding examination for correct overlapping of the multimodal images. This procedure will save time, costs and the need of co-registration. By combining all advantages of each system, it will necessarily update the non-invasive treatment of PCa. The system is adapted and tested to a 3 Tesla MR scanner called Trio A Tim system and Allegra system from the company Siemens healthcare with a larmor frequency of 123.2 MHz and an input of 50 Ω free from artifacts. First results on homogeneity of the transmit-receive coil will be presented. Preliminary measurements showing the proposed device is challenging but feasible.

MRI compatibility study of an integrated PET/RF-coil prototype system at 3 T

We have been working on the development of a PET insert for existing magnetic resonance imaging (MRI) systems for simultaneous PET/MR imaging, which integrates radiofrequency (RF)-shielded PET detector modules with an RF head coil. In order to avoid interferences between the PET detector circuits and the different MRI-generated electromagnetic fields, PET detector circuits were installed inside eight Cu-shielded fiber-reinforced plastic boxes, and these eight shielded PET modules were integrated in between the eight elements of a 270-mm-diameter and 280-mm-axial-length cylindrical birdcage RF coil, which was designed to be used with a 3-T clinical MRI system. The diameter of the PET scintillators with a 12-mm axial field-of-view became 255mm, which was very close to the imaging region. In this study, we have investigated the effects of this PET/RF-coil integrated system on the performance of MRI, which include the evaluation of static field (Bo) inhomogeneity, RF field (B1) distribution, local specific absorption rate (SAR) distribution, average SAR, and signal-to-noise ratio (SNR). For the central 170-mm-diameter and 80-mm-axial-length of a homogenous cylindrical phantom (with the total diameter of 200mm and axial-length of 100mm), an increase of about a maximum of 3μT in the Bo inhomogeneity was found, both in the central and 40-mm off-centered transverse planes, and a 5 percentage point increase of B1 field inhomogeneity was observed in the central transverse plane (from 84% without PET to 79% with PET), while B1 homogeneity along the coronal plane was almost unchanged (77%) following the integration of PET with the RF head coil. The average SAR and maximum local SAR were increased by 1.21 and 1.62 times, respectively. However, the SNR study for both spin-echo and gradient-echo sequences showed a reduction of about 70% and 60%, respectively, because of the shielded PET modules. The overall results prove the feasibility of this integrated PET/RF-coil system for using with the existing MRI system.

Development of a compact DOI–TOF detector module for high-performance PET systems

To increase spatial resolution and signal-to-noise ratio in PET imaging, we present in this paper the design and performance evaluation of a PET detector module combining both depth-of-interaction (DOI) and time-of-flight (TOF) capabilities. The detector module consists of a staggered dual-layer LYSO block with 2 mm × 2 mm × 7 mm crystals. MR-compatible SiPM sensors (MicroFJ-30035-TSV, SensL) are assembled into an 8 × 8 array. SiPM signals from both fast and slow outputs are read out by a 128-channel ASIC chip. To test its performance, a flood histogram is acquired with a 22Na point source on top of the detector, and the energy resolution and the coincidence resolving time (CRT) value for each individual crystal are measured. The flood histogram shows excellent crystal separation in both layers. The average energy resolution at 511 keV is 14.0 and 12.7% at the bottom and top layers, respectively. The average CRT of a single crystal is 635 and 565 ps at the bottom and top layers, respectively. In conclusion, the compact DOI–TOF PET detector module is of excellent crystal identification capability, good energy resolution and reasonable time resolution and has promising application prospective in clinical TOF PET, PET/MRI, and brain PET systems.

Depth of interaction determination in monolithic scintillator with double side SiPM readout

BackgroundMonolithic scintillators read out by arrays of photodetectors represent a promising solution to obtain high spatial resolution and the depth of interaction (DOI) of the annihilation photon. We have recently investigated a detector geometry composed of a monolithic scintillator readout on two sides by silicon photomultiplier (SiPM) arrays, and we have proposed two parameters for the DOI determination: the difference in the number of triggered SiPMs on the two sides of the detector and the difference in the maximum collected signal on a single SiPM on each side. This work is focused on the DOI calibration and on the determination of the capability of our detector. For the DOI calibration, we studied a method which can be implemented also in detectors mounted in a full PET scanner. We used a PET detector module composed of a monolithic 20 × 20 × 10 mm3 LYSO scintillator crystal coupled on two opposite faces to two arrays of SiPMs. On each side, the scintillator was coupled to 6 × 6 SiPMs. In this paper, the two parameters previously proposed for the DOI determination were calibrated with two different methods. The first used a lateral scan of the detector with a collimated 511 keV pencil beam at steps of 0.5 mm to study the detector DOI capability, while the second used the background radiation of the 176Lu in the scintillator. The DOI determination capability was tested on different regions of the detector using each parameter and the combination of the two.ResultsWith both parameters for the DOI determination, in the lateral scan, the bias between the mean reconstructed DOI and the real beam position was lower than 0.3 mm, and the DOI distribution had a standard deviation of about 1.5 mm. When using the calibration with the radioactivity of the LYSO, the mean bias increased of about 0.2 mm but with no degradation of the standard deviation of the DOI distribution.ConclusionsThe two parameters allow to achieve a DOI resolution comparable with the state of the art, giving a continuous information about the three-dimensional interaction position of the scintillation. These results were obtained by using simple estimators and a detector scalable to a whole PET system. The DOI calibration obtained using lutetium natural radioactivity gives results comparable to the other standard method but appears more readily applicable to detectors mounted in a full PET scanner.

MR-compatibility of a high-resolution small animal PET insert operating inside a 7 T MRI

A full-ring PET insert consisting of 16 PET detector modules was designed and constructed to fit within the 114 mm diameter gradient bore of a Bruker 7 T MRI. The individual detector modules contain two silicon photomultiplier (SiPM) arrays, dual-layer offset LYSO crystal arrays, and high-definition multimedia interface (HDMI) cables for both signal and power transmission. Several different RF shielding configurations were assessed prior to construction of a fully assembled PET insert using a combination of carbon fibre and copper foil for RF shielding. MR-compatibility measurements included field mapping of the static magnetic field (B0) and the time-varying excitation field (B1) as well as acquisitions with multiple pulse sequences: spin echo (SE), rapid imaging with refocused echoes (RARE), fast low angle shot (FLASH) gradient echo, and echo planar imaging (EPI). B0 field maps revealed a small degradation in the mean homogeneity (+0.1 ppm) when the PET insert was installed and operating. No significant change was observed in the B1 field maps or the image homogeneity of various MR images, with a 9% decrease in the signal-to-noise ratio (SNR) observed only in EPI images acquired with the PET insert installed and operating. PET detector flood histograms, photopeak amplitudes, and energy resolutions were unchanged in individual PET detector modules when acquired during MRI operation. There was a small baseline shift on the PET detector signals due to the switching amplifiers used to power MRI gradient pulses. This baseline shift was observable when measured with an oscilloscope and varied as a function of the gradient duty cycle, but had no noticeable effect on the performance of the PET detector modules. Compact front-end electronics and effective RF shielding led to minimal cross-interference between the PET and MRI systems. Both PET detector and MRI performance was excellent, whether operating as a standalone system or a hybrid PET/MRI.