Christoph Lerche

MR compatibility aspects of a silicon photomultiplier-based PET/RF insert with integrated digitisation

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 MRIs 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.

SiPM based preclinical PET/MR insert for a human 3T MR: first imaging experiments

Simultaneous PET/MRI is a hybrid imaging modality which promises to play an important role in the field of molecular imaging, as it combines the outstanding soft-tissue contrast of MRI with the metabolic and functional information of PET and MRI. In addition, the possibility for true simultaneous acquisition allows for improved 4D registration which in due course may lead to enhanced image quality and image quantification. The main technical challenges of simultaneous PET/MR are the MR-based attenuation correction and the development of an MR-compatible PET detector technology. Avalanche photo diode based detectors have been already successfully integrated into preclinical as well as human systems [1,2]. Low but noticeable interferences between PET and MRI have been reported so far. Unfortunately, these implementations do not offer the measurement of time of flight (TOF) information in the sub-ns range, which is one of the drivers for high quality clinical PET and has been state-of-the-art in clinical PET/CT for the last 5 years.

Towards Software-Based Real-Time Singles and Coincidence Processing of Digital PET Detector Raw Data

This paper presents a software-based singles and coincidence processing (SCP) architecture for a digital PET/MR system that is based on SiPM detectors with local digitization coupled to preclinical crystal arrays. Compared with traditional PET systems, our system outputs detector raw data of the individual detector elements via optical Gigabit Ethernet interfaces instead of singles or coincidences. The raw data contains the digitized timestamps, energies, and identifiers of triggered SiPM pixels (hits). Although this approach requires a high bandwidth for the detector data transmission system, the availability of detector raw data offers unique opportunities to employ more accurate and computationally complex, iterative algorithms, which can lead to PET images with higher quality and accuracy. In this paper, we evaluate a parallel software-based SCP for three different crystal position estimation approaches with regard to its real-time capabilities. The SCP receives detector raw data as input and outputs list-mode coincidence data. The investigated PET system features ten singles processing units (SPU), each equipped with two PET detector stacks and a Gigabit Ethernet interface to a data acquisition and processing server (Dell Poweredge R910 equipped with 4× Intel Xeon X7560@2.27 GHz CPUs and 256 GByte DDR3-RAM), allowing lossless real-time acquisition of the entire raw data stream. Using the detector raw data of three previously stored measurements, our results show that the throughput (in Mhits/s) of a center-of-gravity (COG)-based parallel SCP is nearly 4× higher on average than the estimated detector raw data output that is generated from an activity of 37 MBq in the iso-center of the detector ring. Under the same conditions, an iterative maximum-likelihood (ML)-based parallel SCP leads to a 6× higher throughput on average, while a Gaussian-based parallel SCP also results in a 13× higher throughput on average. Compared with a serial processing approach, the parallel implementations show speedups of up to 38× on average for the ML-based, 39× on average for Gaussian-based, and up to 34× on average for the COG-based parallelized SCP for the three previously-stored measurements.

PET performance evaluation of a pre-clinical SiPM based MR-compatible PET scanner

We have carried out a PET performance evaluation of an SiPM based scanner designed for fully simultaneous preclinical PET/MR studies. The PET scanner has an inner diameter of 20 cm with a crystal size of 1.3 by 1.3 by 10 mm. The crystals are read out using MR-compatible SiPMs to allow the PET scanner to be located within a Philips 3T Achieva MRI scanner. The spatial resolution of the system, measured using SSRB and 2D FBP is just under 2.4mm in the trans-axial and axial directions. The system sensitivity is 0.6% for a point source at the centre of the field of view. The true coincidence count rate shows no sign of saturating at 30 MBq, at which point the randoms fraction is 9%, and the scatter fraction for a rat sized object is approximately 23%. Artefact-free images of phantoms have been obtained using SSRB/FBP and iterative reconstructions. The current performance is limited because only one of three axial ring positions is currently populated with detectors, and limitations of the first-generation detector readout ASIC used in the system. The performance of the system as described is sufficient for imaging rat-sized animals and large organs within the mouse. We have demonstrated here the feasibility of using the system to investigate dynamic processes simultaneously in a mouse using PET and MR with a dual labeled PET/MR probe. Extrapolating from the current performance results we anticipate that population of all three detector rings and upgrading of the readout ASIC will result in an MR-compatible PET scanner with performance similar to that of state-of-the-art non-MR-compatible pre-clinical systems.

Maximum likelihood based positioning and energy correction for pixelated solid state PET detectors

As part of a preclinical MR compatible small animal PET scanner, a compact block detector for gamma-ray detection has been developed. The block detector consists of a LYSO crystal array with 22 × 22 pixels and an array of 8 × 8 Silicon photomultiplier. An intermediate light guide enables positioning by light-sharing. Due to dark noise and variations of the photomultiplier gain and the light collection efficiency, the crystal identification and energy computation using the center of gravity method is erroneous. We propose an alternative positioning scheme that is based on maximizing the likelihood of the scintillation events. The method directly gives the index of the active crystal pixel and allows also to correct the registered gamma-ray energy. First studies show that the all-over energy resolution for one block detector is enhanced from about 28% to 15%. For the used data set, energy correction with the presented method clearly outperforms energy correction based on the center of gravity method.

Data Processing for a High Resolution Preclinical PET Detector Based on Philips DPC Digital SiPMs

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 D preclinical PET insert which uses Philips Digital Photon Countings (PDPC) digitial SiPMs. The gamma detectors use LYSO scintillator arrays with 30 ×30 crystals of 1 ×1 ×12 mm3 in size coupled to 4 ×4 PDPC DPC 3200-22 sensors (DPC) via a 2-mm-thick light guide. These self-triggering sensors are made up of 2 ×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 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.

Initial PET Performance Evaluation of a Preclinical Insert for PET/MRI with Digital SiPM Technology

Abstract Hyperion-IID is a positron emission tomography (PET) insert which allows simultaneous operation in a clinical magnetic resonance imaging (MRI) scanner. To read out the scintillation light of the employed lutetium yttrium orthosilicate crystal arrays with a pitch of 1 mm and 12 mm in height, digital silicon photomultipliers (DPC 3200-22, Philips Digital Photon Counting) (DPC) are used. The basic PET performance in terms of energy resolution, coincidence resolution time (CRT) and sensitivity as a function of the operating parameters, such as the operating temperature, the applied overvoltage, activity and configuration parameters of the DPCs, has been evaluated at system level. The measured energy resolution did not show a large dependency on the selected parameters and is in the range of 12.4%–12.9% for low activity, degrading to  ∼13.6% at an activity of  ∼100 MBq. The CRT strongly depends on the selected trigger scheme (trig) of the DPCs, and we measured approximately 260 ps, 440 ps, 550 ps and 1300 ps for trig 1–4, respectively. The trues sensitivity for a NEMA NU 4 mouse-sized scatter phantom with a 70 mm long tube of activity was dependent on the operating parameters and was determined to be 0.4%–1.4% at low activity. The random fraction stayed below 5% at activity up to 100 MBq and the scatter fraction was evaluated as  ∼6% for an energy window of 411 keV–561 keV and  ∼16% for 250 keV–625 keV. Furthermore, we performed imaging experiments using a mouse-sized hot-rod phantom and a large rabbit-sized phantom. In 2D slices of the reconstructed mouse-sized hot-rod phantom (∅ = 28 mm), the rods were distinguishable from each other down to a rod size of 0.8 mm. There was no benefit from the better CRT of trig 1 over trig 3, where in the larger rabbit-sized phantom (∅ = 114 mm) we were able to show a clear improvement in image quality using the time-of-flight information. The findings will allow system architects—aiming at a similar detector design using DPCs—to make predictions about the design requirements and the performance that can be expected.

Advances in Clinical PET/MRI Instrumentation.

In 2010, the first whole-body PET/MRI scanners installed for clinical use were the sequential Philips PET/MRI with PMT-based, TOF-capable technology and the integrated simultaneous Siemens PET/MRI. Avalanche photodiodes as non-magneto-sensitive readout electronics allowed PET integrated within the MRI. The experiences with these scanners showed that improvements of software aspects, such as attenuation correction, were necessary and that efficient protocols combining optimally PET and MRI must be still developed. In 2014, General Electric issued an integrated PET/MRI with SiPM-based PET detectors, allowing TOF-PET. Looking at the MRI components of current PET/MR imaging systems, primary improvements come from sequences and new coils.

Software-Based Real-Time Acquisition and Processing of PET Detector Raw Data

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 leastsquares (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 [18F]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 ± 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 LSand ML-based CPEEA. While the coincidence time resolution (~550 ps) and energy resolution (~12.3%) are comparable for all three CPEEA, the system sensitivity is up to 2.5× higher for the LS- and ML-based CPEEA.

Sensitivity Encoded Silicon Photomultipliers (SeSPs): A novel detector design for uniform crystal identification

In this paper, we present the idea of a novel detector design that uses the encoding of the sensitivity instead of a light guide for crystal identification. This scheme fully exploits the siPM feature of being composed by small independent elements that can be connected to form the desired geometry. Different 1D and 2D sensor structures have been designed and the first production has been launched at FBK. Investigation on position accuracy and energy resolution of the prototype sesPs are presented using an existing 64-element SiPM array with a pitch of 250µm pitch and 1.35mm height. These results show that the identification of small crystals (1mm pitch) with the SeSP concept is feasible.