Andre Salomon

MRI-Based Attenuation Correction for Hybrid PET/MRI Systems: A 4-Class Tissue Segmentation Technique Using a Combined Ultrashort-Echo-Time/Dixon MRI Sequence

Accurate γ-photon attenuation correction (AC) is essential for quantitative PET/MRI as there is no simple relation between MR image intensity and attenuation coefficients. Attenuation maps (μ-maps) can be derived by segmenting MR images and assigning attenuation coefficients to the compartments. Ultrashort-echo-time (UTE) sequences have been used to separate cortical bone and air, and the Dixon technique has enabled differentiation between soft and adipose tissues. Unfortunately, sequential application of these sequences is time-consuming and complicates image registration. Methods: A UTE triple-echo (UTILE) MRI sequence is proposed, combining UTE sampling for bone detection and gradient echoes for Dixon water–fat separation in a radial 3-dimensional acquisition (repetition time, 4.1 ms; echo times, 0.09/1.09/2.09 ms; field strength, 3 T). Air masks are derived mainly from the phase information of the first echo; cortical bone is segmented using a dual-echo technique. Soft-tissue and adipose-tissue decomposition is achieved using a 3-point Dixon-like decomposition. Predefined linear attenuation coefficients are assigned to classified voxels to generate MRI-based μ-maps. The results of 6 patients are obtained by comparing μ-maps, reciprocal sensitivity maps, reconstructed PET images, and brain region PET activities based on either CT AC, two 3-class MRI AC techniques, or the proposed 4-class UTILE AC. Results: Using the UTILE MRI sequence, an acquisition time of 214 s was achieved for the head-and-neck region with 1.75-mm isotropic resolution, compared with 164 s for a single-echo UTE scan. MRI-based reciprocal sensitivity maps show a high correlation with those derived from CT scans (R2 = 0.9920). The same is true for PET activities (R2 = 0.9958). An overall voxel classification accuracy (compared with CT) of 81.1% was reached. Bone segmentation is inaccurate in complex regions such as the paranasal sinuses, but brain region activities in 48 regions across 6 patients show a high correlation after MRI-based and CT-based correction (R2 = 0.9956), with a regression line slope of 0.960. All overall correlations are higher and brain region PET activities more accurate in terms of mean and maximum deviations for the 4-class technique than for 3-class techniques. Conclusion: The UTILE MRI sequence enables the generation of MRI-based 4-class μ-maps without anatomic priors, yielding results more similar to CT-based results than can be obtained with 3-class segmentation only.

Simultaneous Reconstruction of Activity and Attenuation for PET/MR

Medical investigations targeting a quantitative analysis of the position emission tomography (PET) images require the incorporation of additional knowledge about the photon attenuation distribution in the patient. Today, energy range adapted attenuation maps derived from computer tomography (CT) scans are used to effectively compensate for image quality degrading effects, such as attenuation and scatter. Replacing CT by magnetic resonance (MR) is considered as the next evolutionary step in the field of hybrid imaging systems. However, unlike CT, MR does not measure the photon attenuation and thus does not provide an easy access to this valuable information. Hence, many research groups currently investigate different technologies for MR-based attenuation correction (MR-AC). Typically, these approaches are based on techniques such as special acquisition sequences (alone or in combination with subsequent image processing), anatomical atlas registration, or pattern recognition techniques using a data base of MR and corresponding CT images. We propose a generic iterative reconstruction approach to simultaneously estimate the local tracer concentration and the attenuation distribution using the segmented MR image as anatomical reference. Instead of applying predefined attenuation values to specific anatomical regions or tissue types, the gamma attenuation at 511 keV is determined from the PET emission data. In particular, our approach uses a maximum-likelihood estimation for the activity and a gradient-ascent based algorithm for the attenuation distribution. The adverse effects of scattered and accidental gamma coincidences on the quantitative accuracy of PET, as well as artifacts caused by the inherent crosstalk between activity and attenuation estimation are efficiently reduced using enhanced decay event localization provided by time-of-flight PET, accurate correction for accidental coincidences, and a reduced number of unknown attenuation coefficients. First results achieved with measured whole body PET data and reference segmentation from CT showed an absolute mean difference of 0.005 cm in the lungs, 0.0009 cm in case of fat, and 0.0015 cm for muscles and blood. The proposed method indicates a robust and reliable alternative to other MR-AC approaches targeting patient specific quantitative analysis in time-of-flight PET/MR.

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.

Performance evaluation of a prototype Positron Emission Tomography scanner using Digital Photon Counters (DPC)

We show performance results of a prototype Positron Emission Tomography scanner based on digital SiPMs, or Digital Photon Counters (DPC), developed by the Philips Digital Photon Counting unit. The scalability of the DPC technology is demonstrated by an excellent system coincidence timing resolution of 266 ps FWHM and an energy resolution of 10.7 % FWHM. Even while using 4 mm × 4 mm × 22 mm LYSO crystals, the spatial resolution is close to 2.4 mm. Although not optimized yet, the image homogeneity is 5.8 %. We show that the system performance is maintained even at highest count rates encountered in PET scans.

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.

A Self-Normalization Reconstruction Technique for PET Scans Using the Positron Emission Data

Positron emission tomography (PET) image quality in both clinical and preclinical environments highly depends on an accurate knowledge of the detector hardware to correct for image quality degrading effects like gain, temperature, and photon detection efficiency variations of the individual crystals. In conventional PET systems some of these variations are typically corrected using a dedicated calibration scan in which the scanner performance for a well-known activity source is measured. We propose an alternative method for estimating the relative sensitivity of each detector pixel using the coincidences as well as the singles emission data of each PET scan. The overall idea is to compare the total sum of all measured single photons before coincidence processing in each crystal with a steadily low-frequent distribution that can normally be expected. Both the estimated activity and the estimated detector sensitivity are simultaneously improved by using an extended iterative reconstruction scheme. This way we ensure the use of an optimal calibration correction (with the exception of a global factor) for each data set, even if the scanner performance has changed between two scans. Data measured with a preclinical PET scanner (HYPERIon-I) which uses analog silicon photomultipliers in combination with a custom-made ASIC shows a significant increase of image quality and homogeneity using the proposed method.

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.

PET performance and MRI compatibility evaluation of a digital, ToF-capable PET/MRI insert equipped with clinical scintillators.

We evaluate the MR compatibility of the Hyperion-II(D) positron emission tomography (PET) insert, which allows simultaneous operation in a clinical magnetic resonance imaging (MRI) scanner. In contrast to previous investigations, this work aims at the evaluation of a clinical crystal configuration. An imaging-capable demonstrator with an axial field-of-view of 32 mm and a crystal-to-crystal spacing of 217.6 mm was equipped with LYSO scintillators with a pitch of 4 mm which were read out in a one-to-one coupling scheme by sensor tiles composed of digital silicon photomultipliers from Philips Digital Photon Counting (DPC 3200-22). The PET performance degradation (energy resolution and coincidence resolution time (CRT)) was evaluated during simultaneous operation of the MRI scanner. We used clinically motivated imaging sequences as well as synthetic gradient stress test sequences. Without activity of the MRI scanner, we measured for trigger scheme 1 (first photon trigger) an energy resolution of 11.4% and a CRT of 213 ps for a narrow energy (NE) window using five (22)Na point-like sources. When applying the synthetic gradient sequences, we found worst-case relative degradations of the energy resolution by 5.1% and of the CRT by 33.9%. After identifying the origin of the degradations and implementing a fix to the read-out hardware, the same evaluation revealed no degradation of the PET performance anymore even when the most demanding gradient stress tests were applied. The PET performance of the insert was initially evaluated using the point sources, a high-activity phantom and hot-rod phantoms in order to assess the spatial resolution. Trigger schemes 2-4 delivered an energy resolution of 11.4% as well and CRTs of 279 ps, 333 ps and 557 ps for the NE window, respectively. An isocenter sensitivity of 0.41% using the NE window and 0.71% with a wide energy window was measured. Using a hot-rod phantom, a spatial resolution in the order of 2 mm was demonstrated and the benefit of time-of-flight PET was shown with a larger rabbit-sized phantom. In conclusion, the Hyperion architecture is an interesting platform for clinically driven hybrid PET/MRI systems.

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.

First in situ TOF-PET study using digital photon counters for proton range verification

Positron emission tomography (PET) is the imaging modality most extensively tested for treatment monitoring in particle therapy. Optimal use of PET in proton therapy requires in situ acquisition of the relatively strong (15)O signal due to its relatively short half-life (~2 min) and high oxygen content in biological tissues, enabling shorter scans that are less sensitive to biological washout. This paper presents the first performance tests of a scaled-down in situ time-of-flight (TOF) PET system based on digital photon counters (DPCs) coupled to Cerium-doped Lutetium Yttrium Silicate (LYSO:Ce) crystals, providing quantitative results representative of a dual-head tomograph that complies with spatial constraints typically encountered in clinical practice (2  ×  50°, of 360°, transaxial angular acceptance). The proton-induced activity inside polymethylmethacrylate (PMMA) and polyethylene (PE) phantoms was acquired within beam pauses (in-beam) and immediately after irradiation by an actively-delivered synchrotron pencil-beam, with clinically relevant 125.67 MeV/u, 4.6  ×  10(8) protons s(-1), and 10(10) total protons. 3D activity maps reconstructed with and without TOF information are compared to FLUKA simulations, demonstrating the benefit of TOF-PET to reduce limited-angle artefacts using a 382 ps full width at half maximum coincidence resolving time. The time-dependent contributions from different radionuclides to the total count-rate are investigated. We furthermore study the impact of the acquisition time window on the laterally integrated activity depth-profiles, with emphasis on 2 min acquisitions starting at different time points. The results depend on phantom composition and reflect the differences in relative contributions from the radionuclides originating from carbon and oxygen. We observe very good agreement between the shapes of the simulated and measured activity depth-profiles for post-beam protocols. However, our results also suggest that available experimental cross sections underestimate the production of (10)C for in-beam acquisitions, which in PE results in an overestimation of the predicted activity range by 1.4 mm. The uncertainty in the activity range measured in PMMA using the DPC-based TOF-PET prototype setup equals 0.2 mm-0.3 mm.