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For a first characterization, we used the two KETEK-PM3325-WB SiPMs each equipped with a 3x3x5 mm${}^3$ LYSO scintillation crystal provided with the PETsys TOFPET2 ASIC Evaluation Kit. We changed the lower of two discriminator thresholds (D_T1) in the timing branch from vth_t1 = 5 - 30. The overvoltage was varied in a range of 1.25 V - 7.25 V. The ambient temperature was kept at 16{\deg}C. For all measurements, we performed an energy calibration including a correction for saturation. We evaluated the energy resolution, the coincidence resolving time (CRT) and the coincidence rate. At an overvoltage of 6 V, we obtained an energy resolution of about 10% FWHM, a CRT of approximately 210 ps FWHM and 400 ps FWTM, the coincidence rate showed only small variations of about 5%. To investigate the influence of the ambient temperature, it was varied between 12{\deg}C - 20{\deg}C. At 12{\deg}C and an overvoltage of 6.5 V, a CRT of approx. 195 ps FWHM and an energy resolution of about 9.5% FWHM could be measured. Observed satellite peaks in the time difference spectra were investigated in more detail. We could show that the location of the satellite peaks is correlated with a programmable delay element in the trigger circuit., Comment: This paper is under review with IEEE TRPMS. It has been presented in a talk at the PSMR 2018. This version of the manuscript was submitted as revision 2 to TRPMS after incrporating the comments of the reviewers. Only minor textchanges were made. Results, values and figures did not change

Physics & Imaging in Radiation Oncology : phiRO 17, 13-19 (2021). doi:10.1016/j.phro.2020.12.002 special issue: "Magnetic Resonance Imaging in Radiotherapy / Edited by Oliver Jäkel (Guest Editor) ; Jürgen Debus (Guest Editor) ; Ludvig Muren (Editor-in-Chief); Daniela Thorwarth (Editor-in-Chief)", Published by Elsevier Science, Amsterdam [u. a.]

EJNMMI Physics 8(1), 30 (2021). doi:10.1186/s40658-021-00370-x, Published by Springer Open, Heidelberg

In state-of-the-art positron emission (PET) tomography systems, application-specific integrated circuits (ASICs)are commonly used to precisely digitize the signals of analog silicon photo-multipliers (SiPMs). However, when operating PET electronics in a magnetic resonance (MR) system, one faces the challenge of mutual interference of these imaging techniques. To prevent signal deterioration along long analog signal lines, PET electronics with a low power consumption digitizing the signals close to the SiPMs are preferred. In this study, we evaluate the power consumption of the TOFPET2 ASIC. Its power consumption ranges from 3.6 to 7.2 mW/channel as a function of the input stage impedance and discriminator noise settings. We present an analytical model allowing to compute the power consumption of a given ASIC configuration. The configured input stage impedance and discriminator noise have an impact on the coincidence resolution time, energy resolution, and photon trigger level. Since the TOFPET2 ASIC delivers state-of-the-art performance with a power consumption similar or even lower than other ASICs typically used for PET applications, it is a favorable candidate to digitize the signals of SiPMs in future simultaneous PET/MR systems., 10 pages, 7 figures

In modern positron emission tomography (PET) readout architectures, the position and energy estimation of scintillation events (singles) and the detection of coincident events (coincidences) are typically carried out on highly integrated, programmable printed circuit boards. The implementation of advanced singles and coincidence processing (SCP) algorithms for these architectures is often limited by the strict constraints of hardware-based data processing. In this paper, we present a software-based data acquisition and processing architecture (DAPA) that offers a high degree of flexibility for advanced SCP algorithms through relaxed real-time constraints and an easily extendible data processing framework. The DAPA is designed to acquire detector raw data from independent (but synchronized) detector modules and process the data for singles and coincidences in real-time using a center-of-gravity (COG)-based, a least-squares (LS)-based, or a maximum-likelihood (ML)-based crystal position and energy estimation approach (CPEEA). To test the DAPA, we adapted it to a preclinical PET detector that outputs detector raw data from 60 independent digital silicon photomultiplier (dSiPM)-based detector stacks and evaluated it with a [ $^{18}$ F]-fluorodeoxyglucose-filled hot-rod phantom. The DAPA is highly reliable with less than 0.1% of all detector raw data lost or corrupted. For high validation thresholds (37.1 $\pm$ 12.8 photons per pixel) of the dSiPM detector tiles, the DAPA is real time capable up to 55 MBq for the COG-based CPEEA, up to 31 MBq for the LS-based CPEEA, and up to 28 MBq for the ML-based CPEEA. Compared to the COG-based CPEEA, the rods in the image reconstruction of the hot-rod phantom are only slightly better separable and less blurred for the LS- and ML-based CPEEA. While the coincidence time resolution ( $\sim$ 550 ps) and energy resolution ( $\sim$ 12.3%) are comparable for all three CPEEA, the system sensitivity is up to 2.5 $\times$ higher for the LS- and ML-based CPEEA.

The Hyperion IID PET insert is the first scanner using fully digital silicon photomultipliers for simultaneous PET/MR imaging of small animals up to rabbit size. In this work, we evaluate the PET performance based on the National Eletrical Manufacturers Association (NEMA) NU 4-2008 standard, whose standardized measurement protocols allow comparison of different small-animal PET scanners. The Hyperion IID small-animal PET/MR insert comprises three rings of 20 detector stacks with pixelated scintillator arrays with a crystal pitch of 1 mm, read out with digital silicon photomultipliers. The scanner has a large ring diameter of 209.6 mm and an axial field of view of 96.7 mm. We evaluated the spatial resolution, energy resolution, time resolution and sensitivity by scanning a 22Na point source. The count rates and scatter fractions were measured for a wide range of 18F activity inside a mouse-sized scatter phantom. We evaluated the image quality using the mouse-sized image quality phantom specified in the NEMA NU4 standard, filled with 18F. Additionally, we verified the in-vivo imaging capabilities by performing a simultaneous PET/MRI scan of a mouse injected with 18F-FDG. We processed all measurement data with an energy window of 250 keV to 625 keV and a coincidence time window of 2 ns. The filtered-backprojection reconstruction of the point source has a full width at half maximum (FWHM) of 1.7 mm near the isocenter and degrades to 2.5 mm at a radial distance of 50 mm. The scanner’s average energy resolution is 12.7% (ΔE/E FWHM) and the coincidence resolution time is 609 ps. The peak absolute sensitivity is 4.0% and the true and noise-equivalent count rates reach their peak at an activity of 46 MBq with 483 kcps and 407 kcps, respectively, with a scatter fraction of 13%. The iterative reconstruction of the image quality phantom has a uniformity of 3.7%, and recovery coefficients from 0.29, 0.91 and 0.94 for rod diameters of 1 mm, 3 mm and 5 mm, respectively. After application of scatter and attenuation corrections, the air- and water-filled cold regions have spill-over ratios of 6.3% and 5.4%, respectively. The Hyperion IID PET/MR insert provides state-of-the-art PET performance while enabling simultaneous PET/MRI acquisition of small animals up to rabbit size.

Biomedical physics & engineering express 4(6), 065027 (2018). doi:10.1088/2057-1976/aae6c2, Published by IOP Publ., Bristol

Physics in medicine and biology 60(18), 7045-7067 (2015). doi:10.1088/0031-9155/60/18/7045, Published by IOP Publ., Bristol

We have carried out a PET performance evaluation a silicon photo-multiplier (SiPM) based PET scanner designed for fully simultaneous pre-clinical PET/MR studies. The PET scanner has an inner diameter of 20 cm with an LYSO crystal size of 1.3 by 1.3 by 10 mm. The axial PET field of view (FOV) is 30.2 mm. The PET detector modules, which incorporate SiPMs, have been designed to be MR-compatible allowing them to be located directly within a Philips Achieva 3T MR scanner. The spatial resolution of the system measured using a point source in a non-active background, is just under 2.3 mm full width at half maximum (FWHM) in the transaxial direction when single slice rebinning (SSRB) and 2D filtered back-projection (FBP) is used for reconstruction, and 1.3 mm FWHM when resolution modeling is employed. The system sensitivity is 0.6% for a point source at the center of the FOV. The true coincidence count rate shows no sign of saturating at 30 MBq, at which point the randoms fraction is 8.2%, and the scatter fraction for a rat sized object is approximately 23%. Artifact-free images of phantoms have been obtained using FBP and iterative reconstructions. The performance is currently limited because only one of three axial ring positions is populated with detectors, and due to limitations of the first-generation detector readout ASIC used in the system. The performance of the system as described is sufficient for simultaneous PET-MR imaging of rat-sized animals and large organs within the mouse. This is demonstrated with dynamic PET and MR data acquired simultaneously from a mouse injected with a dual-labeled PET/MR probe.

We present results obtained with a positron emission tomography (PET) coincidence setup provided by the group of Prof. Peter Fischer from the University of Heidelberg (Lehrstuhl fur Schaltungstechnik und Simulation, Institut fur Technische Informatik). The setup employs two facing detector blocks which are built from an LYSO matrix of 30 × 30 × 10mm3 in size with a crystal pitch of 2.5 mm read out in a one-to-one readout scheme by FBK RGB-HD SiPMs with a SPAD size of 25 μm. Each SiPM channel is digitized by a single input channel of a PETA5 (Position-Energy-Timing ASIC). Four PETA5 chips, each with 36 input channels, are mounted on one of the ceramic detector PCBs which incorporate a liquid cooling maze. We used multiple 22Na point sources for calibration and evaluation purposes. In the calibration and result extraction, all available channels were taken into account simultaneously. The time-to-digital-converter (TDC) bins were calibrated and the noise pedestal for each channel was determined. The energy was calibrated and corrected for saturation using the 511 keV and the 1275 keV peaks, that were identified in the noise-pedestal-corrected ADC-value spectrum. Time offsets of all readout crystals were calibrated using coincident and qualified singles. The distribution of the TDC bin sizes was evaluated to have a mean of 50.2 ps and a standard deviation of approximately 20.2 ps. We determined the energy resolution of the setup for qualified coincidences as 13.0% and the coincidence resolution time as approximately 310 ps.

Our group has built the MRI compatible PET insert Hyperion IID, which is based on digital silicon photomultipliers [digital photon counters (DPCs)]. We characterized its performance for two different scintillator configurations already in previous publications. For this paper, we reused the raw detector data that were used in previous publications and stored for offline analysis. We reprocessed these data in order to improve the timing performance of the PET scanner. Different crystal delay and time walk calibration and correction methods are evaluated with respect to the coincidence resolving time (CRT) of the scanner. For all applied correction methods, we report CRTs both for a preclinical and a clinical scintillator configuration for all DPC trigger schemes and several energy windows. The newly obtained results were compared with the previously published values, and the additional benefit of using a time walk correction was evaluated. Previously published CRTs could be improved using a refined version of the original crystal delay calibration method without applying a walk correction by 6.3%–10.9% for a wide energy window ranging from 250–625 keV, and we could show an additional improvement of up to 2.6% by adding a walk correction. Using trigger scheme 1 and a very narrow energy window around the photopeak (500–520 keV), we could reach CRT values on system level of 208/240 ps for the clinical/preclinical scintillator configuration.

The combination of Positron Emission Tomography (PET) and Magnetic Resonance Imaging (MRI) into a single device is being considered a promising tool for molecular imaging as it combines the high sensitivity of PET with the functional and anatomical images of MRI. For highest performance, a scalable, MR compatible detector architecture with a small form factor is needed, targeting at excellent PET signal-to-noise ratios and time-of-flight information. Therefore it is desirable to use silicon photo multipliers and to digitize their signals directly in the detector modules inside the MRI bore. A preclinical PET/RF insert for clinical MRI scanner was built to demonstrate a new architecture and to study the interactions between the two modalities.The disturbance of the MRI's static magnetic field stays below 2 ppm peak-to-peak within a diameter of 56 mm (90 mm using standard automatic volume shimming). MRI SNR is decreased by 14%, RF artefacts (dotted lines) are only visible in sequences with very low SNR. Ghosting artefacts are visible to the eye in about 26% of the EPI images, severe ghosting only in 7.6%. Eddy-current related heating effects during long EPI sequences are noticeable but with low influence of 2% on the coincidences count rate. The time resolution of 2.5 ns, the energy resolution of 29.7% and the volumetric spatial resolution of 1.8 mm(3) in the PET isocentre stay unaffected during MRI operation. Phantom studies show no signs of other artefacts or distortion in both modalities. A living rat was simultaneously imaged after the injection with (18)F-Fluorodeoxyglucose (FDG) proving the in vivo capabilities of the system.

On behalf of the HYPER Image project, a Silicon photomultiplier (SiPM) based preclinical PET insert for a commercial human 3 T MRI scanner was built. In this contribution we report on the stability of imaging performance of the PET scanner and MR hardness and compatibility. From data sets that were acquired during the last 7 months we extracted SiPM gain values and their annual drift, the mean energy resolution and the energy resolution drift, spatial resolution and spatial resolution drift, and photo peak position and their annual drift. Further, a point source and a hot rod phantom was imaged fully simultaneously with the MRI scanner and the PET scanner. No interference between either modality was observed.

Combining Positron Emission Tomography (PET) with Magnetic Resonance Imaging (MRI) results in a promising hybrid molecular imaging modality as it unifies the high sensitivity of PET for molecular and cellular processes with the functional and anatomical information from MRI. Digital Silicon Photomultipliers (dSiPMs) are the digital evolution in scintillation light detector technology and promise high PET SNR. DSiPMs from Philips Digital Photon Counting (PDPC) were used to develop a preclinical PET/RF gantry with 1-mm scintillation crystal pitch as an insert for clinical MRI scanners. With three exchangeable RF coils, the hybrid field of view has a maximum size of 160 mm $\,\times\,$ 96.6 mm (transaxial $\,\times\,$ axial). 0.1 ppm volume-root-mean-square B $_{0}$ -homogeneity is kept within a spherical diameter of 96 mm (automatic volume shimming). Depending on the coil, MRI SNR is decreased by 13% or 5% by the PET system. PET count rates, energy resolution of 12.6% FWHM, and spatial resolution of 0.73 mm $^{3}$ (isometric volume resolution at isocenter) are not affected by applied MRI sequences. PET time resolution of 565 ps (FWHM) degraded by 6 ps during an EPI sequence. Timing-optimized settings yielded 260 ps time resolution. PET and MR images of a hot-rod phantom show no visible differences when the other modality was in operation and both resolve 0.8-mm rods. Versatility of the insert is shown by successfully combining multi-nuclei MRI ( $^{1}{\rm H}/^{19}$ F) with simultaneously measured PET ( $^{18}$ F-FDG). A longitudinal study of a tumor-bearing mouse verifies the operability, stability, and in vivo capabilities of the system. Cardiac- and respiratory-gated PET/MRI motion-capturing (CINE) images of the mouse heart demonstrate the advantage of simultaneous acquisition for temporal and spatial image registration.

In this work, we present an initial MR-compatibility study performed with the world's first preclinical PET/MR insert based on fully digital silicon photo multipliers (dSiPM). The PET insert allows simultaneous data acquisition of both imaging modalities and thus enables the true potential of hybrid PET/MRI. Since the PET insert has the potential to interfere with all of the MRI's subsystems (strong magnet, gradients system, radio frequency (RF) system) and vice versa, interference studies on both imaging systems are of great importance to ensure an undisturbed operation. As a starting point to understand the interference, we performed signal-to-noise ratio (SNR) measurements as well as dedicated noise scans on the MRI side to characterize the influence of the PET electronics on the MR receive chain. Furthermore, improvements of sub-components’ shielding of the PET system are implemented and tested inside the MRI. To study the influence of the MRI on the PET performance, we conducted highly demanding stress tests with gradient and RF dominated MR sequences. These stress tests unveil a sensitivity of the PET's electronics to gradient switching., Highlights • We successfully operated a fully digital PET insert inside a clinical 3T MRI. • The main noise source is common mode noise from the PET's power supply. • Improvements of the power supply's shielding show a significant improvement. • Stress tests unveil a sensitivity of the PET's electronics to gradient switching.

In positron emission tomography (PET) systems, light sharing techniques are commonly used to readout scintillator arrays consisting of scintillation elements, which are smaller than the optical sensors. The scintillating element is then identified evaluating the signal heights in the readout channels using statistical algorithms, the center of gravity (COG) algorithm being the simplest and mostly used one. We propose a COG algorithm with a fixed number of input channels in order to guarantee a stable calculation of the position. The algorithm is implemented and tested with the raw detector data obtained with the Hyperion-II $^{\rm D}$ preclinical PET insert which uses Philips Digital Photon Counting’s (PDPC) digitial SiPMs. The gamma detectors use LYSO scintillator arrays with $30 \times 30$ crystals of $1 \times 1 \times 12~\hbox{mm}^3$ in size coupled to $4 \times 4$ PDPC DPC 3200-22 sensors (DPC) via a 2-mm-thick light guide. These self-triggering sensors are made up of $2 \times 2$ pixels resulting in a total of 64 readout channels. We restrict the COG calculation to a main pixel, which captures most of the scintillation light from a crystal, and its (direct and diagonal) neighboring pixels and reject single events in which this data is not fully available. This results in stable COG positions for a crystal element and enables high spatial image resolution. Due to the sensor layout, for some crystals it is very likely that a single diagonal neighbor pixel is missing as a result of the low light level on the corresponding DPC. This leads to a loss of sensitivity, if these events are rejected. An enhancement of the COG algorithm is proposed which handles the potentially missing pixel separately both for the crystal identification and the energy calculation. Using this advancement, we show that the sensitivity of the Hyperion-II $^{\rm D}$ insert using the described scintillator configuration can be improved by 20–100% for practical useful readout thresholds of a single DPC pixel ranging from 17–52 photons. Furthermore, we show that the energy resolution of the scanner is superior for all readout thresholds if singles with a single missing pixel are accepted and correctly handled compared to the COG method only accepting singles with all neighbors present by 0–1.6% (relative difference). The presented methods can not only be applied to gamma detectors employing DPC sensors, but can be generalized to other similarly structured and self-triggering detectors, using light sharing techniques, as well.

The full potential of simultaneous PET and MRI image acquisition, such as dynamic studies or motion compensation, can only be explored if the data of both modalities are temporally synchronized. These hybrid imaging systems are often realized as custom made PET inserts for commercially available MRI scanner. Unfortunately, the standard MRIs do not always offer easily programmable synchronization outputs, nor can they be modified. Here, we demonstrate a simple method for temporal synchronization that does not require a connection to the MRI. It uses the normally undesired effect of induced voltages on the PET electronics by switching MRI gradients. In our preclinical PET/RF inserts the PET detector electronics are located in RF-tight housings between the gradient coil and the RF coil. Unlike RF fields, gradient fields penetrate the housings due to their relatively low frequencies. A dedicated coil system was made from traces and vias on the PET detector board. It captures induced voltages from the switching gradients and a time-stamped gradient trigger message is added to the PET data stream. Temporal alignment between PET and MRI data can be made at the beginning of each MRI sequence by detecting its first gradient or by recognizing its preparation phase. From then on, the MRI time can be translated to the PET time. Detecting the preparation phase of an MRI sequence was tested with a standard survey sequence (for brain) using the PET/RF insert “Hyperion I” inside a Philips 3T Achieva MRI. The three crusher gradient pulses (slew rate = 25 mT/m/ms) used during noise level determination can clearly be recognized in the data from the sensors. As a first application the sensors were used to synchronize PET and MRI time to facilitate MRI based PET motion compensation of a deforming breathing phantom.

In recent years, several groups have presented combined PET/MRI devices for clinical and preclinical research. Using new technologies like SiPMs, the performance of the systems improved over the years allowing a detailed exploration of the relatively new field of hybrid imaging modality. First presented at the NSS/MIC 2012, our group built a PET/MRI insert based on PDPC’s digital SiPMs (DPC). It is to date the only combined PET/MRI system using this technology which allows the performance evaluation of DPCs on system level. The Hyperion-IID PET/MR insert consists of 10 PET modules. Each PET module comprises 6 detector stacks whereas one of them consists of an interface board with local FPGA and a sensor tile. The latter employs a 4x4 array of sensor dies, each consisting of 2x2 DPCs. For photon detection, a 30x30 LYSO crystal array (1mm pitch, 12mm length) is glued using an intermediate 2mm light guide on top of each sensor tile. Measurements were performed using 22Na point sources (point like, activity = 1.5MBq) for different scenarios: the position dependence of the mentioned parameter was tested by moving a point source along the z axis. Different sensor configurations (trigger settings, validation thresholds, etc.) as well as temperature conditions were applied to characterize the influence of these parameters on the performance. We use an Anger algorithm and a Maximum Likelihood algorithm for crystal identification. We measure 12.3% (FWHM) energy resolution and a CRT of about 470ps and 520ps (FWHM) for trigger schemes 2 and 3. No dependence on the axial position is observed and no difference between the measurement inside and outside the magnetic field occurs. We measure for both B0 field scenarios an effective sensitivity of about 3.1%. Only minor effects due to the operation inside the B0 field can be observed. More results and details about this study will be presented at the conference.

Positron Emission Tomography (PET) combined with Magnetic Resonance Imaging (MRI) as a hybrid imaging modality is about to become the next-generation imaging technique in the field of molecular imaging. The integration of PET detectors into an MR-gantry enabling simultaneous acquisitions with unaffected performance of PET and MRI is challenging, as PET detectors need to be unaffected by the MR operation, RF-silent (low RF emission) and compact. Especially the RF silence of a fully digital PET detector is demanding, as the digital detection and data acquisition architecture may produce electromagnetic (EM) field emission which can result in noise artifacts in MR images. The RF fields emitted by PET detectors, which may be coupled into the MRI RF coil, are therefore unwanted and are from an MRI point of view disturbances considered as noise. A conventional way to overcome RF emission is to use thick RF shielding which however leads to MR image artifacts due to eddy currents distorting the MR image. In this paper, we present investigations of novel interference reduction techniques which were implemented by solely modifying firmware designs used in FPGAs of our MR-compatible PET modules used in the PET Insert Hyperion-IID while keeping the entire hardware untouched. The principles apply on a more fundamental level namely the EM field coupling mechanism to the RF receive coil. We propose to reduce the coupling by shifting the clocking frequencies and by applying clock phase patterns of the PET sensors, leading to an optimization of the EM field emission with the aim for as little as possible RF shielding. The initial results presented in this paper demonstrate how our flexible PET architecture can be used to reduce the noise coupled into the MRI receive chain. Measurements performed with our near-field scanner in the lab and with the MRI confirm, that the frequency shifting approach can be applied to successfully reduce the noise coupled into the MRI receive chain. Least noise was measured at the Larmor frequency with the PET sensors clocked at 160MHz and 100 MHz.

In this paper, we propose an FPGA-based Data Acquisition and Control Architecture (DACA) with distributed processing capabilities for future PET-systems suitable for simultaneous PET-MRI operation, which can be configured to future detector geometries and crystal configurations of any preclinical and clinical PET system. Key-aspects for such an architecture are a scalable and modular infrastructure with common communication blocks for every processing node, a common interface for processing pipelines, bypass of raw detector data and a system-wide message-based data routing with modular addressing scheme. We present general conceptual aspects and implementations for the preclinical PET-Insert Hyperion-I as proposed by Schulz et at. and Hyperion-UD as proposed by Weissler et al., followed by initial throughput measurements of a PET-module, which demonstrates a linear response behavior of our FPGA-based architecture within the aimed data rates of up to 930Mbits/s.

PURPOSE In order to integrate electronic devices into a magnetic resonance imaging (MRI) scanner, shielding of the electronics with respect to the radio frequency (RF) transmit and receive system of the MRI scanner is required. Furthermore, MRI uses time-varying low-frequency magnetic fields for spatial encoding, i.e., the gradient magnetic fields. Time-varying magnetic fields induce eddy currents in all conductive elements. The eddy currents result in opposing magnetic fields, which can cause distortions of the magnetic resonance (MR) image. As shielding of lower frequencies is not feasible in this respect, an ideal shielding element should be transparent for gradient magnetic fields while providing a high RF shielding effectiveness. Furthermore, it should offer a low susceptibility to prevent distortion of the main magnetic field of the MRI. In this work, we characterize the aforesaid shielding parameters of different shielding samples. METHODS We developed a nuclear magnetic resonance (NMR) probe to measure the magnetic fields to quantify the field distortions time-resolvedly. The relative distortion was introduced as a proportionality constant relating the eddy-current-inducing field changes and the field distortions. The relative distortion was measured in the frequency range from 0 to 10 kHz for all shielding samples using the NMR probe. We characterized the shielding effectiveness of the samples in the frequency range from 1 to 150 MHz using a network analyzer. We conducted all measurements with three different materials, two carbon fiber composites and copper, each in various thicknesses. RESULTS The relative distortion of the magnetic fields induced by the carbon fiber composites samples was at least a factor of seven lower than the copper sample. A linear dependency on the sample thickness was measured for the main field distortion, the relative distortion and the shielding effectiveness. The relative distortion was roughly independent of the gradient frequency contrary to the shielding effectiveness, highly depending on the RF frequency. CONCLUSIONS We presented a very sensitive method to characterize the distortion of the main field distortion and the gradient transparency using an NMR probe. We analyzed different shielding materials regarding the main field distortion, the gradient transparency, and the shielding effectiveness. From the tested materials, we identified a carbon fiber composite with the lowest distortion on the MRI.

A pre-clinical PET insert for a clinical 3T MRI scanner has been developed to acquire PET images simultaneously with MRI data. We assessed the imaging performance of the MR scanner equipped with the PET insert by running spurious noise scans with the MR scanner and by measuring the image SNR with homogeneous standard MR phantoms of different sizes. Also the B1 field distribution map was measured. The noise scans on the MR scanner with the PET insert powered on showed that RF power is coupled from the PET insert into the MR acquisition system. With additional common mode blocking, the noise floor was raised by factor 1.6 when the PET insert was acquiring data compared to the case that it was switched off. MR image SNR was quantified as described in the corresponding NEMA standards publication. Without additional common mode blocking, image SNR is reduced by a factor between 1.5 and 2, depending on the size of the object. With additional common mode blocking, image SNR is reduced by a factor 1.3 only. The timing resolution of the PET scanner was also determined after twofold correction of the timestamps. Mean coincidence timing resolution is approximately 3 ns, the best observed coincidence timing resolution was approximately 2 ns and the worst observed coincidence timing resolution was approximately 6 ns.

Silicon based photon detectors make it nowadays possible to build highly integrated PET detectors for simultaneous PET/MRI. Although the operating principle of silicon photomultipliers is believed to be insensitive to the strong magnetic fields inside MRI machines, the construction of an MRI compatible detector has to cover considerably more aspects of MRI compatibility. In this paper we report on our development of an MRI compatible PET detector stack based on digital SiPMs for the Hyperion IID scanner. We developed an MRI compatible PET detector stack composed of two main assembly groups, called sensor tile and interface board. The sensor tile contains an array of 64 digital SiPM and is connected via two connector to the interface board. The interface board contains a Xilinx Spartan 6 FPGA for data collection, configuration and voltage control. We investigated the PET performance and MRI compatibility of the detector stack in combination with pixilated LYSO scintillator arrays with 1 mm pitch and 4 mm pitch inside a 3 Tesla Philips Achieva MRI system. We used MRI sequences with intensive gradient switching, field homogeneity and spurious noise scans to investigate PET/MRI interference effects. Additionally we build up laboratory setups with electromagnets and strong permanent magnets to study gradient induced effects and B0 dependencies in more detail. The detector stack is fully functional inside the B0 field and position histograms are undistorted. Energy histograms showed on average a 1 % upward shift of the 511 keV photopeak position caused by a shift of bias voltage supply. Initially observed effects of gradient switching on energy resolution and stability could be traced back to noise pickup of the voltage controllers. We couldn't observe any direct effect of the MRI environment on the digital SiPM itself. In fact MRI compatibility in practice is determined by proper design of the entire system.

The combination of PET and MRI is a challenging task, since both imaging modalities might influence each other. For unaffected imaging performance, PET detectors need to be untouched by the MR operation (MR-compatible) and RF-silent. The latter requirement is demanding, as the digital Data Acquisition and Control Architecture (DACA) produce noise in the MRI receive chain and thus deteriorate the Signal-to-Noise Ratio (SNR). To reduce these MRI-SNR degradations, we propose “RESCUE”, a MR-synchronously gated PET data acquisition technique. By controlling the PET-modules’ digital data acquisition in terms of interrupting the digital SiPM (dSiPM) operation during the MR signal receive phases, potential RF interferences can be reduced. The concept and design of RESCUE are based on the DACA of Hyperion-IID using 960 dSiPMs from Philips which are spread over 10 PET modules, every module containing 6 detector stacks. 16 dSiPMs are populated on one detector stack and a local FPGA per stack is used to clock and configure the dSiPMs and acquire data from them. To interrupt the PET data acquisition when the MRI is in receive-mode, an MRI trigger signal is sent via the backbone of the PET insert to all PET modules. In the stacks, the dSiPM-operation is interrupted by gating the dSiPMs’ input clock signals. This clock switch-off needs to be performed at a point in time when no dSiPM cell recharge takes place, as sensor-damage might otherwise occur. We implemented FPGA firmware to stop the above described dSiPM operation and to restart it upon release of the gating. PET-data was acquired using gating-sequences and analysis of sensor data acquired before and after interruption showed that the sensors were working properly after reapplying their clock signals. More details and MRI SNR-measurement results to demonstrate the interference reduction obtained with RESCUE will be presented at the conference.

Combining PET and MRI into a hybrid device is challenging since both systems might influence each other. A typical interference problem of such a combined device is the distortion of the MRI’s B0 field distribution due to the material brought inside the MRI’s FOV which is in particular challenging for small-bore PET-systems. High field homogeneity is needed for a good MRI acquisition in general as well as in certain applications. Typically, active shimming using dedicated coils is applied to improve the field homogeneity. However, these techniques are limited especially for localized distortion profiles with higher-order characteristics caused by PET/MRI inserts. As a consequence, we are exploring the potential application of shimming on PET detector level (for the Hyperion-IID PET/MRI insert), meaning that the distortion profile caused by PET modules is compensated using additional magnetic materials (passive shimming) and DC coils (active shimming). To explore the technique, B0 field measurements have been performed using a whole-body phantom in combination with the MRI body coil. An FFE sequence was used to measure distortion maps of DC loops and small magnetic objects (capacitors, ferrites). These distortion maps served as input for a software framework which has been written to perform the field optimization. The implementation was verified by measurements and fits were performed to extract characteristic parameters of the tested objects. Finally, the implemented software framework was used to homogenize a measured distortion map produced by a single PET module by superimposing distortion corrections from additional simulated materials. The resulting superimposed distortion map showed a significantly improved B0 field map quality (reduced spectral width and improved homogeneity). The simulated susceptibility distribution will be applied on PET module level and tested in experiments. Results and details about this study will be presented at the conference.

2012, our group presented the Hyperion IID platform which uses PDPC’s digitial SiPMs (DPC). In this work we use the same platform equipped with scintillator dimensions closer to a clinical application. This allows an investigation of the time of flight (ToF) performance of the platform and its behavior during simultaneous MR operation. We employ LYSO crystal arrays of 4×4×10 mm³ coupled to 4×4 PDPC DPC 3200-22 sensors allowing a one-to-one coupling of crystals to readout channels. Six sensor stacks are mounted onto a singles processing unit in a 2×3 arrangement. Two units are mounted on a gantry with a diameter of 216 mm. The DPCs are cooled down to approximately 5-10 °C under operation. We disable 20% of the worst cells and use an overvoltage (OV) of 2.0V and 2.5V. To obtain the best time stamps we use the first photon trigger and employ data quality cuts to filter out crystal scatter events. A narrow energy window of 511±50 keV is used and a minimal light fraction of the main pixel of more than 65% is requested. Using a Na22 point source in the isocenter of the modules the coincidence resolution time (CRT) of the two modules is evaluated outside the MR and inside the MR using different MR sequences. Gradient stress tests with switching z-gradients are performed. Inside the B0 field at 2.0V overvoltage the energy resolution is 11.45% (FWHM) and the CRT is 250ps (FWHM). At 2.5V overvoltage, the energy resolution is 11.15% (FWHM) and the CRT is 240ps (FWHM). During the heavy z-gradient sequence the energy resolution is degraded by 4.1% at 2V and 9.2% at 2.5V. The degradation of the CRT is 25% at 2V and 52% at 2.5V OV. During standard TSE and EPI sequences the CRT and energy resolution is not influenced. The Hyperion IID platform proofs to deliver good timing performance outside and inside an MR. The CRT is not influenced (