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An algorithm for determining the crystal pixel and the gamma ray energy with scintillation detectors for PET is presented. The algorithm uses Likelihood Maximisation (ML) and therefore is inherently robust to missing data caused by defect or paralysed photo detector pixels. We tested the algorithm on a highly integrated MRI compatible small animal PET insert. The scintillation detector blocks of the PET gantry were built with the newly developed digital Silicon Photomultiplier (SiPM) technology from Philips Digital Photon Counting and LYSO pixel arrays with a pitch of 1A¢Â€Â‰mm and length of 12A¢Â€Â‰mm. Light sharing was used to readout the scintillation light from the 30 AƒÂ— 30 scintillator pixel array with an 8 AƒÂ— 8 SiPM array. For the performance evaluation of the proposed algorithm, we measured the scanner's spatial resolution, energy resolution, singles and prompt count rate performance, and image noise. These values were compared to corresponding values obtained with Center of Gravity (CoG) based positioning methods for different scintillation light trigger thresholds and also for different energy windows. While all positioning algorithms showed similar spatial resolution, a clear advantage for the ML method was observed when comparing the PET scanner's overall single and prompt detection efficiency, image noise, and energy resolution to the CoG based methods. Further, ML positioning reduces the dependence of image quality on scanner configuration parameters and was the only method that allowed achieving highest energy resolution, count rate performance and spatial resolution at the same time.

In this work we present detailed characterizations of our preclinical high resolution PET/MR insert based on the Hyperion-IID platform. The PET/MR insert consists of a ring of 10 singles detection modules, each comprising 2x3 scintillation detector stacks. Each detector stack features a 30x30 pixelated LYSO crystal array with a height of 12 mm and a pitch of 1 mm, coupled via a slit 2 mm light guide to a digital SiPM tile. The PET performance is stable under a wide range of operating points. The spatial resolution is below 1A,mm and the CRT reaches 260 or 450 ps depending on trigger settings. The energy resolution is 12.6% FWHM. The characterization of the MR compatibility showed no relevant degradation in PET performance during MRI operation. On the MRI side, we observe a degradation in B0 homogeneity from a VRMS of 0.03 ppm to 0.08 ppm with active shimming, while observing only minor degradations in the B0 field. The noise floor is slightly increased by 2-15% without any observable dependence on the activity. The Z gradients induces an observable eddy current inside the PET inserts which can lead to ghosting artifacts for EPI sequences. However, we don't observe any visible image degradation for widely used anatomical imaging sequences such as gradient echo and turbo spin echo sequences. To prove the viability of our PET/MR insert for in vivo small animal studies, we successfully performed a longitudinal mouse study with subcutaneously injected tumor model cells. The simultaneously acquired PET/MR images provide a high level of anatomical information and soft tissue contrast in the MR layer together with a high resolution image of the FDG tracer distribution in the PET layer.

The combination of PET and MRI into a hybrid device is challenging since both systems might interfere with each other. Therefore, the study of the MR-compatibility of such a combined device is crucial to understand and solve potential interference phenomena. Interference problems, which typically occur and are reported by several groups, range from B0 distortion over Signal-To-Noise Ratio (SNR) degradation to gradient disturbances. In this work, we present the current system status and investigations on the interference between a fully digital PET insert and a 3T clinical MRI. The Hyperion-IID PET/MR insert is designed to fit into a clinical MRI system and consists of 10 PET modules which are equipped with up to 6 detector stacks whereas one of them employs a 4×4 digital SiPM array for photon detection. For MRI acquisition, the PET insert is equipped with a dedicated PET transparent Tx/Rx mouse RF-coil. To study the influence of the PET insert on the MR performance, SNR measurements using SE sequences as well as dedicated noise scans were performed. The B0 field distortion caused by the PET insert was studied with field mapping and spectroscopic methods. The influence on the PET performance due to MR operation was evaluated by acquiring PET data from different phantoms (point sources and structured phantoms) while applying demanding gradient and RF dominated stress tests as well as normal MR imaging sequences (SE, FFE, EPI and TSE sequences). All experiments were performed with a partly (one of three possible rings) and a fully populated version of the Hyperion-IID scanner. The presence of the PET detector causes MR performance degradation on an acceptable level. The same holds true for the PET performance. However, MR stress tests reveal some sensitivity of the PET’s electronic to gradient switching. Details about this study will be presented at the conference.

Respiratory motion can degrade PET image quality and various correction approaches have been investigated. A novel method, applicable to simultaneous PET-MR is to measure the motion with the MR and correct the PET data with the known motion fields. This study investigates the effectiveness of motion correction for PET-MR acquisition with phantom studies. It incorporates the improved motion estimation or a simultaneously acquired MR with the realism of a respiratory driven motion and the deformability of the PVA cryogel phantom. PVA cryogel has been used for motion modeling because of its elastic properties. Motion is transferred with a hydraulic system to the MR room, inside the PET insert. The MR-compatible PET insert uses silicon photomultiplier photodetectors and acquisition is capable of being synchronized with the MRI. Spatial registration between PET and MR is essential for accurate motion correction using MRI derived motion fields. We have performed simultaneous PET and MR acquisition of a non-rigid continuously deformable phantom PET and MR visible phantom. Calibration procedures were used to ensure that PET and MR acquisitions were accurately aligned both spatially and temporally. The final step of this study is the motion correction by using known techniques such as Motion Compensated Image Reconstruction (MCIR) and Reconstruct-Transform-Average (RTA). We have demonstrated that the temporally and spatially correlated PET and MR datasels can be used to motion correct the PET acquisition

For a high resolution PET/MRI insert, we are using pixelated scintillator arrays with 1 mm pitch in combination with digital SiPM arrays. The pitch of the digital SiPM arrays is 4 mm. Therefore, a light-sharing technique is used to identify individual crystals. Information from several sensors, a cluster, is needed to identify the crystal in which a gamma photon has been converted to scintillation photons. We present first evaluations of the neighbor logic (NL) of the PDPe DLS 3200-22 dSiPM. The NL allows the use of strict trigger validation criteria while delivering the complete light distribution on the sensor tile. Measurements are performed at ~10°C sensor temperature. We compare the measurement with enabled NL to a standard triggering scheme which needs to use low trigger validation thresholds in order to achieve the same performance. Positioning of clusters is performed using a center-of-gravity algorithm. We show that the NL trigger scheme allows to achieve a better energy resolution and improved crystal identification. Furthermore, we show that the NL allows to recover more clusters containing light distributions with less missing relevant information while reducing the noise level in the data by more than one order of magnitude.

We developed the world's first MRI compatible PET detector based on digital silicon photomultiplier technology (digital SiPM). In this work we present our stack design and initial measurement results. The design of the detector stack considers all aspects of MRI compatibility and allows for the first time to operate digital SiPMs inside an MRI. The detector stack is composed of two layers. The top layer contains an 8 x 8 channel digital SiPM array with 4 mm pitch. The bottom layer hosts an FPGA for data acquisition and configuration as well as a circuit to control and monitor the bias voltage. We tested the detector stack in combination with an L YSO scintillator array containing 30 × 30 crystals with a pitch of 1 mm and 12 mm length. We build up highly integrated detector modules and tested the detector stack in a clinical 3 Tesla Philips Achieva MRI system. Our measurements indicate that the performance of the digital SiPM is unaffected by the MRI system even under extreme conditions. The presented detector stack is used to build up the Hyperion IID preclinical simultaneous PET/MRI insert for rodent studies within MEC and ForSaTum project.

Improving PET performance towards higher resolution and better sensitivity resulted in a continuous trend - especially in solid state PET and PET/MR - of an increasing number of channels to be read out by the detector electronics. Building complete PET systems with these detectors calls for a modular and self-triggered architecture with distributed processing power that is already in the detector module able to condense the data stream to corrected and gated singles.

The development of a highly integrated and MRI-compatible PET detector remains as one of the main challenges towards a truly simultaneous PET/MR scanner. Here we present the world's first PET/MR scanner design "Hyperion IID" with fully digital silicon photomultiplier technology (dSiPMs). It is based on the HYPERImage Insert "Hyperion I" using analog SiPMs demonstrated by Schulz et a!. in [1] and [2]. The new architecture promises outstanding PET performance and increased MRI compatibility as well as improvements in workflow.

The integration of PET and MR imaging requires a novel type of highly integrated PET detector. To cope with geometric constraints and MR compliance a very compact detector stack was built within the HYPERImage consortium. This allows a four side buttable detector module design with a low dead space in between. The scintillation light coming from a LYSO array is converted in a SiPM sensor tile with a high packing fraction and a high photo detection efficiency to provide sub-ns time-of-flight timing resolution. The analog signals coming from the SiPM elements are digitized close to the sensor to minimize potential crosstalk. A custom mixed-signal ASIC was integrated on a 64 channel sensor stack which is powered and controlled by an FPGA interface board. The complete sensor stack is assembled and characterized to extract the PET relevant parameters, in particular energy, timing and spatial resolution for clinical and pre-clinical PET/MR applicationsB.