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Background: Morphological imaging using MRI is essential for brain tumour diagnostics. Dynamic susceptibility contrast (DSC) perfusion-weighted MRI (PWI), as well as amino acid PET, may provide additional information in ambiguous cases. Since PWI is not always performed as part of standard MRI in brain tumours, we explored whether maps of relative cerebral blood volume (rCBV) in brain tumours can be extracted from the early phase of PET using O-(2-18F-fluoroethyl)-L-tyrosine (18F-FET). Using a hybrid BrainPET/MRI scanner, PWI and dynamic 18F-FET PET were performed in 33 patients with cerebral glioma and in four patients with highly vascularized meningiomas. Based on the dynamic PET data in meningiomas, the time interval from 0 – 2 min p.i. was selected to best reflect the blood pool phase in 18F-FET PET. For each patient, maps of MR-rCBV, early 18F-FET PET (0-2 min p.i.) and late 18F-FET PET (20-40 min p.i.) were generated and coregistered. Volumes of interest were placed on the tumour (VOI-TU) and on the normal-appearing contralateral brain tissue (VOI-REF). The correlation between the tumour-to-brain ratios (TBR) of the different parameters was analyzsed. In addition, three independent observers evaluated the MR-rCBV and early 18F-FET maps (18F-FET-rCBV) for concordance in signal intensity, tumour extent and intratumoural distribution. Results: TBRs calculated from MR-rCBV and 18F-FET-rCBV showed a significant correlation (r = 0.89, p < 0.001), while there was no correlation between late 18F-FET PET and MR-rCBV (r = 0.24, p = 0.16) or 18F-FET-rCBV (r = 0.27, p = 0.11). Visual rating yielded widely agreeing findings or only minor differences between the MR-rCBV maps and 18F-FET-rCBV maps in 93 % of the tumours (range of three independent raters 91–94%, kappa among raters 0.78-1.0). Conclusion: Early 18F-FET-maps (0-2min p.i.) in gliomas provide similar information to MR-rCBV maps and may be helpful when PWI is not possible or available. Further studies in recurrent gliomas are needed to evaluate whether 18F-FET-rCBV provides the same clinical information as MR-rCBV.

ObjectiveThe positron range is a fundamental, detector-independent physical limitation to special resolution in positron emission tomography (PET) as it causes a significant blurring of the reconstructed PET images. A major challenge for positron range correction methods is to provide accurate range kernels that inherently incorporate the generally inhomogeneous stopping power, especially at tissue boundaries. In this work, we propose a novel approach to generate accurate three-dimensional (3-D) blurring kernels both in homogenous and heterogeneous media to improve PET spatial resolution.ApproachIn the proposed approach, positron energy deposition was approximately tracked along straight paths, depending on the positron stopping power of the underlying material. The positron stopping power was derived from the attenuation coefficient of 511keV gamma photons according to the available PET attenuation maps. Thus, the history of energy deposition is taken into account within the range of kernels. Special emphasis was placed on facilitating the very fast computation of the positron annihilation probability in each voxel.ResultsPositron path distributions of 18F in low-density polyurethane were in high agreement with Geant4 simulation at an annihilation probability larger than 10−2∼10−3 of the maximum annihilation probability. The Geant4 simulation was further validated with measured 18F depth profiles in these polyurethane phantoms. The tissue boundary of water with cortical bone and lung was correctly modeled. Residual artifacts from the numerical computations were in the range of 1%. The calculated annihilation probability in voxels shows an overall difference of less than 20% compared to the Geant4 simulation.SignificanceThe proposed method significantly improves spatial resolution for non-standard isotopes by providing accurate range kernels, even in the case of significant tissue inhomogeneities.

Physics in medicine and biology 67(23), 235004 (2022). doi:10.1088/1361-6560/aca1f3, Published by IOP Publ., Bristol

Simultaneous MR-PET/-SPECT is an emerging technology that capitalises on the invaluable advantages of both modalities, allowing access to numerous sensitive tracers and superior soft-tissue contrast alongside versatile functional imaging capabilities. However, to optimise these capabilities, concurrent acquisitions require the MRI antenna located inside the PET/SPECT field-of-view to be operated without compromising any aspects of system performance or image quality compared to the stand-alone instrumentation. Here, we report a novel gamma-radiation-transparent antenna concept. The end-fed J-shape antenna is particularly adept for hybrid ultra-high field MR-PET/-SPECT applications as it enables all highly attenuating materials to be placed outside the imaging field-of-view. Furthermore, this unique configuration also provides advantages in stand-alone MR applications by reducing the amount of coupling between the cables and the antenna elements, and by lowering the potential specific absorption rate burden. The use of this new design was experimentally verified according to the important features for both ultra-high field MRI and the 511 keV transmission scan. The reconstructed attenuation maps evidently showed much lower attenuation ( ∼ 15 %) for the proposed array when compared to the conventional dipole antenna array since there were no high-density components. In MR, it was observed that the signal-to-noise ratio from the whole volume obtained using the proposed array was comparable to that acquired by the conventional array which was also in agreement with the simulation results. The unique feature, J-shape array, would enable simultaneous MR-PET/-SPECT experiments to be conducted without unduly compromising any aspects of system performance and image quality compared to the stand-alone instrumentation.

Objective. The positron range is a fundamental, detector-independent physical limitation to spatial resolution in positron emission tomography (PET) as it causes a significant blurring of underlying activity distribution in the reconstructed images. A major challenge for positron range correction methods is to provide accurate range kernels that inherently incorporate the generally inhomogeneous stopping power, especially at tissue boundaries. In this work, we propose a novel approach to generate accurate three-dimensional (3D) blurring kernels both in homogenous and heterogeneous media to improve PET spatial resolution. Approach. In the proposed approach, positron energy deposition was approximately tracked along straight paths, depending on the positron stopping power of the underlying material. The positron stopping power was derived from the attenuation coefficient of 511 keV gamma photons according to the available PET attenuation maps. Thus, the history of energy deposition is taken into account within the range of kernels. Special emphasis was placed on facilitating the very fast computation of the positron annihilation probability in each voxel. Results. Positron path distributions of 18F in low-density polyurethane were in high agreement with Geant4 simulation at an annihilation probability larger than 10−2 ∼ 10−3 of the maximum annihilation probability. The Geant4 simulation was further validated with measured 18F depth profiles in these polyurethane phantoms. The tissue boundary of water with cortical bone and lung was correctly modeled. Residual artifacts from the numerical computations were in the range of 1%. The calculated annihilation probability in voxels shows an overall difference of less than 20% compared to the Geant4 simulation. Significance. The proposed method is expected to significantly improve spatial resolution for non-standard isotopes by providing sufficiently accurate range kernels, even in the case of significant tissue inhomogeneities.

The glutamate and γ-aminobutyric acid neuroreceptor subtypes mGluR

Currently, the metabotropic glutamate receptor 5 (mGluR5) is the subject of several lines of research in the context of neurology and is of high interest as a target for positron-emission tomography (PET). Here, we assessed the feasibility of using [11C]ABP688, a specific antagonist radiotracer for an allosteric site on the mGluR5, to evaluate changes in glutamatergic neurotransmission through a mismatch-negativity (MMN) task as a part of a simultaneous and synchronized multimodal PET/MR-EEG study. We analyzed the effect of MMN by comparing the changes in nondisplaceable binding potential (BPND) prior to (baseline) and during the task in 17 healthy subjects by applying a bolus/infusion protocol. Anatomical and functional regions were analyzed. A small change in BPND was observed in anatomical regions (posterior cingulate cortex and thalamus) and in a functional network (precuneus) after the start of the task. The effect size was quantified using Kendall’s W value and was 0.3. The motor cortex was used as a control region for the task and did not show any significant BPND changes. There was a significant ΔBPND between acquisition conditions. On average, the reductions in binding across the regions were - 8.6 ± 3.2% in anatomical and - 6.4 ± 0.5% in the functional network (p ≤ 0.001). Correlations between ΔBPND and EEG latency for both anatomical (p = 0.008) and functional (p = 0.022) regions were found. Exploratory analyses suggest that the MMN task played a role in the glutamatergic neurotransmission, and mGluR5 may be indirectly modulated by these changes.

Building on the success of positron emission tomography (PET)/CT scanners, a hybrid imaging technology, combining PET with the versatile attributes of magnetic resonance imaging (MRI), has been in use for over ten years. At the Forschungszentrum Julich, one of the four prototypes available worldwide, combining a commercial 3T MRI with a newly developed BrainPET insert, was installed in 2008, allowing simultaneous data acquisition with PET and MRI. Here, we report on our ten-year-long experience of the methods and technology developed through this paper with numerous patients and volunteers. In the course of our research, a number of different radiotracers, such as [18F]Fluoroethyltyrosine, [11C]Flumazenil, [11C]Raclopride, and [15O]H2O, have been applied while simultaneously measuring several different MR sequences. The PET data obtained with the BrainPET has demonstrated higher image quality and superior resolution capability compared to the stand-alone PET scanner, ECAT Exact HR+. Furthermore, the simultaneous acquisition of MR images alongside BrainPET scans, using different sequences, showed an excellent quality without any relevant artifacts and provided useful complementary information. Our experiences with the hybrid 3T MR-BrainPET indicate a promising basis for further developments of this unique technique, allowing simultaneous brain imaging in a clinical setting.

The kinetic analysis of $^{{18}}\text{F}$ -FET time-activity curves (TAC) can provide valuable diagnostic information in glioma patients. The analysis is most often limited to the average TAC over a large tissue volume and is normally assessed by visual inspection or by evaluating the time-to-peak and linear slope during the late uptake phase. Here, we derived and validated a linearized model for TACs of $^{{18}}\text{F}$ -FET in dynamic PET scans. Emphasis was put on the robustness of the numerical parameters and how reliably automatic voxel-wise analysis of TAC kinetics was possible. The diagnostic performance of the extracted shape parameters for the discrimination between isocitrate dehydrogenase (IDH) wildtype (wt) and IDH-mutant (mut) glioma was assessed by receiver-operating characteristic in a group of 33 adult glioma patients. A high agreement between the adjusted model and measured TACs could be obtained and relative, estimated parameter uncertainties were small. The best differentiation between IDH-wt and IDH-mut gliomas was achieved with the linearized model fitted to the averaged TAC values from dynamic FET PET data in the time interval 4–50 min p.i.. When limiting the acquisition time to 20–40 min p.i., classification accuracy was only slightly lower (-3%) and was comparable to classification based on linear fits in this time interval. Voxel-wise fitting was possible within a computation time $\approx ~{1}$ min per image slice. Parameter uncertainties smaller than 80% for all fits with the linearized model were achieved. The agreement of best-fit parameters when comparing voxel-wise fits and fits of averaged TACs was very high (p < 0.001).

Translational Psychiatry 11(1), 60 (2021). doi:10.1038/s41398-020-01160-2, Published by Nature Publishing Group, London

The PET data were acquired using a 3T hybrid MR-BrainPET insert system (SIEMENS, Erlangen, Germany) in list mode. The coincidences were corrected for random events using the delayed window technique with VRR, dead time, attenuation and scattered coincidences (single scatter simulation – SSS method), and physical decay. An adapted NEMA phantom, without lung insert and with six sphere inserts, was used. One insert was filled with 11C and the others contained cold (no activity) water contrasting in a warm background cylinder. Coincidence data were acquired during eight isotope half-lives, giving 163 minutes total acquisition time for 11C, which has a half-life of T1/2 = 20.35 minutes. The ratio of the activity concentration between the sphere (Hot with 28 mm of nominal internal diameter) and the background region (BG) was 1.85:1. This value is frequently found for the ratio of activity concentrations in the grey matter cortex (GM) and cerebellar grey matter (reference region) during the steady-state condition in [11C]ABP688 studies. A cold transmission scan of the adapted NEMA phantom using 68Ge sources was acquired in a Siemens ECAT Exact HR+ PET scanner. This acquisition (2 bed positions, 20 minutes of transmission, reconstructed with OSEM 2D – 6 iterations and 16 subsets and a 256 × 256 matrix) was used to create the attenuation map for the phantom used in the 11C decay study.

Background: Iterative image reconstruction algorithms are widely used in positron emission tomography (PET). However, they are known to contribute to quantitation bias, which is particularly pronounced during dynamic PET studies such as neuroreceptor binding studies with 11C-labelled radiotracers where count rates become low towards the end of the examination. This problem is relevant in case simultaneous PET/MR studies which apply a bolus-infusion protocol to allow the multimodal comparison between control or resting state and stimulation effects in a single session of e.g. 60 min, i.e. 3 half-lives of 11C. A quantitation bias may interfere with stimulation related changes. In this study, we evaluated the impact of the 3D ordinary Poisson OSEM (3D OP-OSEM) on quantitation accuracy reconstructions and the subsequent propagation into binding potential values using a decay study of a 11C filled phantom and a human brain data set. To evaluate the reconstruction bias, we tested different reconstruction framing schemes and propose a framing scheme that keeps the counts per time frame constant over the full acquisition time. We also compared the vendor’s 3D OP-OSEM image reconstruction method to an in-house developed reconstruction (PRESTO toolkit). Results: In general, a bias for low and high activity concentration regions was observed in the range of ± 3% and - 3% to 5%, respectively. Using the alternative proposed framing scheme, a lower bias was achieved for regions with low activity concentration using PRESTO, and a stabilization of the mean bias for the binding potential was achieved at around 5% with the vendor’s reconstruction throughout the relevant activity curve time interval. Conclusions: The bias in activity concentration propagated into the binding potential values leading to a mean bias of around 5%, thus allowing the detection of changes in binding values during equilibrium of > 5%.

The symbiosis of neuronal activities and glucose energy metabolism is reflected in the generation of functional magnetic resonance imaging (fMRI) and electroencephalography (EEG) signals. However, their association with the balance between neuronal excitation and inhibition (E/I-B), which is closely related to the activities of glutamate and GABA (γ -aminobutyric acid) and the receptor availability (RA) of GABAA and mGluR5, remains unexplored. This study investigates these associations during the resting state (RS) condition using simultaneously recorded PET/MR/EEG (trimodal) data. Glucose metabolism and neuroreceptor binding availability (non-displaceable binding potential (BPND)) of GABAA and mGluR5 were found to be significantly higher and closely linked within core resting-state networks (RSNs). The neuronal generators of EEG microstates and the fMRI measures were most tightly associated with the BPND of GABAA relative to mGluR5 BPND and the glucose metabolism, emphasising a predominance of inhibitory processes within in the core RSNs at rest.

Simultaneous trimodal positron emission tomography/magnetic resonance imaging/electroencephalography (PET/MRI/EEG) resting state (rs) brain data were acquired from 10 healthy male volunteers. The rs‐functional MRI (fMRI) metrics, such as regional homogeneity (ReHo), degree centrality (DC) and fractional amplitude of low‐frequency fluctuations (fALFFs), as well as 2‐[18F]fluoro‐2‐desoxy‐d‐glucose (FDG)‐PET standardised uptake value (SUV), were calculated and the measures were extracted from the default mode network (DMN) regions of the brain. Similarly, four microstates for each subject, showing the diverse functional states of the whole brain via topographical variations due to global field power (GFP), were estimated from artefact‐corrected EEG signals. In this exploratory analysis, the GFP of microstates was nonparametrically compared to rs‐fMRI metrics and FDG‐PET SUV measured in the DMN of the brain. The rs‐fMRI metrics (ReHO, fALFF) and FDG‐PET SUV did not show any significant correlations with any of the microstates. The DC metric showed a significant positive correlation with microstate C (r (s) = 0.73, p = .01). FDG‐PET SUVs indicate a trend for a negative correlation with microstates A, B and C. The positive correlation of microstate C with DC metrics suggests a functional relationship between cortical hubs in the frontal and occipital lobes. The results of this study suggest further exploration of this method in a larger sample and in patients with neuropsychiatric disorders. The aim of this exploratory pilot study is to lay the foundation for the development of such multimodal measures to be applied as biomarkers for diagnosis, disease staging, treatment response and monitoring of neuropsychiatric disorders.

Simultaneous PET/MR/EEG (Positron Emission Tomography – Magnetic Resonance – Electroencephalography), a new tool for the investigation of neuronal networks in the human brain, is presented here within the framework of the European Union Project TRIMAGE. The trimodal, cost-effective PET/MR/EEG imaging tool makes use of cutting edge technology both in PET and in MR fields. A novel type of magnet (1.5T, non-cryogenic) has been built together with a PET scanner that makes use of the most advanced photodetectors (i.e., SiPM matrices), scintillators matrices (LYSO) and digital electronics. The combined PET/MR/EEG system is dedicated to brain imaging and has an inner diameter of 260 mm and an axial Field-of-View of 160 mm.It enables the acquisition and assessment of molecular metabolic information with high spatial and temporal resolution in a given brain simultaneously. The dopaminergic system and the glutamatergic system in schizophrenic patients are investigated via PET, the same physiological/pathophysiological conditions with regard to functional connectivity, via fMRI, and its electrophysiological signature via EEG. In addition to basic neuroscience questions addressing neurovascular-metabolic coupling, this new methodology lays the foundation for individual physiological and pathological fingerprints for a wide research field addressing healthy aging, gender effects, plasticity and different psychiatric and neurological diseases.The preliminary performances of two components of the imaging tool (PET and MR) are discussed. Initial results of the search of possible candidates for suitable schizophrenia biomarkers are also presented as obtained with PET/MR systems available to the collaboration.

Accurate scatter correction is essential for qualitative and quantitative PET imaging. Until now, scatter correction based on Monte Carlo simulation (MCS) has been recognized as the most accurate method of scatter correction for PET. However, the major disadvantage of MCS is its long computational time, which makes it unfeasible for clinical usage. Meanwhile, single scatter simulation (SSS) is the most widely used method for scatter correction. Nevertheless, SSS has the disadvantage of limited robustness for dynamic measurements and for the measurement of large objects. In this work, a newly developed implementation of MCS using graphics processing unit (GPU) acceleration is employed, allowing full MCS-based scatter correction in clinical 3D brain PET imaging. Starting from the generation of annihilation photons to their detection in the simulated PET scanner, all relevant physical interactions and transport phenomena of the photons were simulated on GPUs. This resulted in an expected distribution of scattered events, which was subsequently used to correct the measured emission data. The accuracy of the approach was validated with simulations using GATE (Geant4 Application for Tomography Emission), and its performance was compared to SSS. The comparison of the computation time between a GPU and a single-threaded CPU showed an acceleration factor of 776 for a voxelized brain phantom study. The speedup of the MCS implemented on the GPU represents a major step toward the application of the more accurate MCS-based scatter correction for PET imaging in clinical routine.

IEEE transactions on radiation and plasma medical sciences : ITRPFI 3(3), 377-385 (2019). doi:10.1109/TRPMS.2018.2886525, Published by IEEE, New York, NY

In order to simplify the Point-Spread-Function (PSF) reconstruction framework, resolution modelling can be decoupled from the iterative reconstruction process by an additional data resampling step previous to reconstruction. We call the proposed algorithm RESampling between Projection spACEs (RESPACE). In this abstract, RESPACE is applied to resample the simulated projection data to 2D Generic Cylinder Model (GCM) projection data structure, which will be used for reconstruction afterwards. Theoretically, the proposed algorithm merges pre-calculated detection probability information and prior information into the resampled projection data by applying Bayes’ Theorem. In contrast to conventional projection data handling, RESPACE can make the iterative reconstruction isolated from any detection model or PSF modelling, ensuring the closed structure of normal non-PSF iterative algorithms. In this study, we implemented a 2D-PET simulation Monte Carlo framework, which has the same geometrical property as Siemens BrainPET transverse structure. Conventional MLEM, MLEM-PSF (both image space and projection space with shift-invariant kernel) and RESPACE are implemented and investigated. As figures of merit, Bias-Resolution curves demonstrate that RESPACE could achieve similar resolution and even better bias suppression performance as the PSF method with a shift-invariant kernel. Moreover, no significant visual difference is observed between images from PSF and RESPACE reconstruction. These results demonstrate that RESAPCE offers equivalent performance as the shift-invariant PSF method and this approach is an alternative resolution modelling method independent from the iterative reconstruction algorithm.

PET imaging is widely used to perform clinical and preclinical studies. A rather new area is the field of plant research. PET allows the dynamic imaging of 3D carbon transport and allocation within plants shoot and root systems of plants. Here, a new system called phenoPET has been developed within the German Plant Phenotyping Network (DPPN) using Digital Photon Counters (DPC) by Philips (digital silicon photo multipliers). Those are organized in horizontal rings forming a Field of View (FOV) of 180mm diameter and 200mm height.The detected hits are arranged in time frames, which are written to disc via USB 3:0 with a maximum transfer rate of 375MBs−1. Preprocessing and coincidence sorting are done offline. To enable high dynamic range phenoPET can drop frames which are not saved on disc when the transfer rate comes to a limit.To compare carbon transport of different plants quantitative images are required. A preliminary normalization and random correction are implemented. In this context, count rate correction and dead time correction are mandatory to avoid quantitative bias. In a decay experiment the dead time of the DPCs reduces the rate of singles by up to 10% at an radioactivity of about 100MBq placed close to the center of the FOV. At this activity about 70% of the frames are dropped leaving a maximum single rate of about 2:5 • 105 cps on each module. Further count rate correction factors for coincidences due to missing frames are presented.

One challenge for PET-MR hybrid imaging is the correction for attenuation of the 511 keV annihilation radiation by the required RF transmit and/or RF receive coils. Although there are strategies for building PET transparent Tx/Rx coils, such optimised coils still cause significant attenuation of the annihilation radiation leading to artefacts and biases in the reconstructed activity concentrations. We present a straightforward method to measure the attenuation of Tx/Rx coils in simultaneous MR-PET imaging based on the natural 176Lu background contained in the scintillator of the PET detector without the requirement of an external CT scanner or PET scanner with transmission source. The method was evaluated on a prototype 3T MR-BrainPET produced by Siemens Healthcare GmbH, both with phantom studies and with true emission images from patient/volunteer examinations. Furthermore, the count rate stability of the PET scanner and the x-ray properties of the Tx/Rx head coil were investigated. Even without energy extrapolation from the two dominant γ energies of 176Lu to 511 keV, the presented method for attenuation correction, based on the measurement of 176Lu background attenuation, shows slightly better performance than the coil attenuation correction currently used. The coil attenuation correction currently used is based on an external transmission scan with rotating 68Ge sources acquired on a Siemens ECAT HR + PET scanner. However, the main advantage of the presented approach is its straightforwardness and ready availability without the need for additional accessories.

A major benefit of the three-dimensional (3D) PET imaging technique in neuroscience, as well as in clinical applications, is that it offers the possibility of dynamically quantifying metabolic processes with a sensitivity of up to 10−12 mol L−1 for the tracer concentration. However, all positron emission tomographs provide biased data with complex dependencies, which means that to obtain quantitative activity distributions in 3D, it is necessary to make several corrections. For example, inhomogeneous detector efficiencies, photon attenuation, Compton scattering, and random coincidences need to be corrected. Furthermore, dynamic imaging represents a challenge, because a high temporal resolution requires short acquisition time frames with rather poor statistics of recorded events from the radioactive decay. Apart from the necessary corrections, the applied reconstruction method has an important impact on the achievable image quality in PET. In this respect, iterative reconstruction methods are becoming the state-of-the-art techniques as they offer superior image quality when compared to analytical methods. Although iterative reconstruction is associated with higher computational demand, the higher calculation effort can be moderated by using a range of optimisation strategies and has been further helped by the remarkable boost in computational resources over the last two decades.

Motion is a frequent problem in magnetic resonance-positron emission tomography (MR-PET) acquisitions, leading to significant degradations of the image quality. This chapter gives an overview of this issue and potential remedies. First, different ways of measuring the intra-scan motion are discussed. This is sub-divided into external device-based PET-based and MR-based motion detection and tracking. Given that MRI-based methods can be relatively fast, they lend themselves to retrospective as well as prospective correction; in retrospective correction the motion information is used to correct flawed k-space data after the scan is completed, i.e. during reconstruction or post-processing, whereas in prospective motion correction the motion information is used to correct the MRI measurement itself in real time while the scan is still running. The goal of prospective correction is to acquire data that are unaffected by any motion that occurs during the measurement. Thereafter, several different motion correction techniques are presented, which are able to counter the negative effects of motion in both MRI and PET.

For high-resolution, iterative 3D PET image reconstruction the efficient implementation of forward-backward projectors is essential to minimise the calculation time. Mathematically, the projectors are summarised as a system response matrix (SRM) whose elements define the contribution of image voxels to lines-of-response (LORs). In fact, the SRM easily comprises billions of non-zero matrix elements to evaluate the tremendous number of LORs as provided by state-of-the-art PET scanners. Hence, the performance of iterative algorithms, e.g. maximum-likelihood-expectation-maximisation (MLEM), suffers from severe computational problems due to the intensive memory access and huge number of floating point operations. Here, symmetries occupy a key role in terms of efficient implementation. They reduce the amount of independent SRM elements, thus allowing for a significant matrix compression according to the number of exploitable symmetries. With our previous work, the PET REconstruction Software TOolkit (PRESTO), very high compression factors (>300) are demonstrated by using specific non-Cartesian voxel patterns involving discrete polar symmetries. In this way, a pre-calculated memory-resident SRM using complex volume-of-intersection calculations can be achieved. However, our original ray-driven implementation suffers from addressing voxels, projection data and SRM elements in disfavoured memory access patterns. As a consequence, a rather limited numerical throughput is observed due to the massive waste of memory bandwidth and inefficient usage of cache respectively. In this work, an advantageous symmetry-driven evaluation of the forward-backward projectors is proposed to overcome these inefficiencies. The polar symmetries applied in PRESTO suggest a novel organisation of image data and LOR projection data in memory to enable an efficient single instruction multiple data vectorisation, i.e. simultaneous use of any SRM element for symmetric LORs. In addition, the calculation time is further reduced by using simultaneous multi-threading (SMT). A global speedup factor of 11 without SMT and above 100 with SMT has been achieved for the improved CPU-based implementation while obtaining equivalent numerical results.

Dynamic PET provides temporal information about the tracer uptake. However, each PET frame has usually low statistics, resulting in noisy images. Furthermore, PET images suffer from partial volume effects. The goal of this study is to understand the effects of prior regularisation on dynamic PET data and subsequent anatomical partial volume correction. The Median Root Prior (MRP) regularisation method was used in this work during reconstruction. The quantification and noise in image-domain and time-domain (time-activity curves) as well as the impact on parametric images is assessed and compared with Ordinary Poisson Ordered Subset Expectation Maximisation (OP-OSEM) reconstruction with and without Gaussian filter. This study shows the improvement in PET images and time-activity curves (TAC) in terms of noise as well as in the parametric images when using prior regularisation in dynamic PET data. Anatomical partial volume correction improves the TAC and consequently, parametric images. Therefore, the use of MRP with anatomical partial volume correction is of interest for dynamic PET studies.

The 11CO2 tracer is of high interest to study carbon transport in living plants. However, conventional PET images of plant leaves suffer from the poor positron annihilation probability in the targeted structures, thus giving strong bias in reconstructed images. Here, PET/MR hybrid scanner systems offer a new method to detect and quantify escaping positrons as 2D projections by exploiting the directed travelling of positrons parallel to the present B0 field of the MR magnet system. Due to the Lorentz force all emitted positrons propagate on spiral trajectories parallel to the B0 field with tiny gyroradii. The introduction of a thin absorber screen within the field-of-view of the PET scanner, forces the annihilation of almost all incoming positrons inside the absorber. The annihilation photon pairs can be registered as coincidences and thus directly indicate the positron impact point on the screen by interpolating the Lines-ofResponse onto the screen. In this way, 2D images are obtained and the amount of positrons leaving the source can be quantified dynamically.

In this paper we present first results of an implementation of a direct LOR PET image reconstruction for the Siemens mMR Biograph. The implementation is based on a speed-optimised version of the PET Reconstruction Software Toolkit (PRESTO) to obtain moderate reconstruction time on a single computer. Reconstructed images for Iida phantom data and FDG human brain data are compared to images of the available Siemens mMR Biograph PSF reconstruction. Images of the Siemens PSF reconstruction suffer from Gibbs artifacts whereas for the direct LOR reconstruction no evident Gibbs artifacts are observed. For this reason, the direct LOR reconstruction can effectively improve quantification accuracy due to the reduction of the bias evoked by applying physical PSF kernels.

The digitally generated timemarks from more than 2000 individual detector elements in the pheno PET scanner show individual time offsets. These offsets (or skews) are in the range of ± 1 ns while the actual time resolution of each element is below 250 ps. Thus, the time resolution of the entire system will be highly improved by correcting these offsets. This paper presents a precise correction method based on measurements of the LYSObackground radiation. The Coincidence Resolving Time for the entire scanner could be improved from 1.45 ns to less than 300 ps.

Within the German Plant Phenotyping Network (DPPN), we developed a novel PET scanner based on Philips Digital Photon Counters (DPCs, or dSiPMs = digital Silicon Photomultipliers). The scanner is dedicated for plant research and provides functional information on carbon transport within the plant. To this end the detector ring is oriented horizontally. It provides a Field-of-View of 18 cm dia. and 20 cm in height. The read-out electronics cluster hits from different photodetector pixels when they originate from the same scintillation event. These single events are written via USB 3.0 with up to 300 MB/s to the computer system. Crystal identification, energy discrimination and coincidence detection is realized in software. The spatial resolution in the center Field-of-View (CFOV) could be estimated to approx. 1.6 mm from measurements of a dedicated hot rod phantom. Preliminary sensitivity measurements result in a peak sensitivity of 4.04% (ΔE = 250-750 keV) in the CFOV and a Coincidence Resolving Time of 298 ps could be achieved.

In hybrid magnetic resonance-positron emission tomography (MR-PET) studies with the Siemens 3T MR-BrainPET scanner an instantaneous reduction of the PET sensitivity was observed during execution of certain MR sequences. This interference was investigated in detail with custom-made as well as standard clinical MR sequences. The radio-frequency pulses, the switched gradient fields and the constant magnetic field were examined as the relevant parameters of the magnetic resonance imaging (MRI) system as well as the air temperature within the PET detectors. Our investigation comprised the analysis of the analog PET signals, the total count rates, the geometric distribution of the count rate reduction within the BrainPET detector as well as reconstructed images. The fast switching magnetic field gradients were identified to distort the analog PET detector signals. The measured count rate reduction was found to be less than 3%, but only up to 2% in the case of echo planar imaging sequences, as applied in functional MRI. For clinical sequences routinely used in hybrid MR-BrainPET measurements, a correction method has been designed, implemented, and evaluated .

At present, a new hybrid imaging technology combining PET and MRI is emerging. The construction of hybrid scanners that allow simultaneous MRI and PET required the development of a new type of MR-compatible PET detector, which was first realized for studying small animals. The first industrial prototypes of such integrated MR-PET scanners for human brain studies have been installed recently. In this article, the feasibility of this new approach is demonstrated by first studies in brain tumor patients. Beyond time saving in patient management and optimal spatial overlay of anatomic and functional information, this technique offers new perspectives with regard to the simultaneous observation of physiological and biochemical processes. Functional brain activation and interaction of receptor ligands during special activation tasks or pharmacological intervention may be explored in humans. Thus, MR-PET opens new horizons in multiparametric neuroimaging.

The majority of all modern PET cameras use LSO and LYSO crystals which contain a small proportion of the radioactive isotope of Lutetium. The aim of this study is to obtain attenuation maps for dedicated head PET/MR scanners based on the background activity of the LSO scintillator. In particular, attenuation maps for the Tx/Rx coils in the PET Field of View have to be provided. Before reconstructing such an attenuation map, the scanners stability with respect to detector temperature and background count rate was examined. First image reconstructions of the 8-channel Tx/Rx head coil and a 3-rod phantom obtained with LSO background prove that the method is feasible as long as the overall scan time is sufficiently long.

Use of iterative algorithms to reconstruct three-dimensional (3-D) positron emission tomography (PET) data requires the computation of the system probability matrix. The pure geometrical contribution can easily be approximated by the length-of-intersection (LOI) between lines-of-response (LOR) and individual voxels. However, more accurate geometrical projectors are desirable. Therefore, we have developed a fast method for the analytical calculation of the 3-D shape and volume of volumes-of-intersection (VOI). This method provides an alternative robust projector with a uniformly continuous sampling of the image space. The enhanced calculation effort is facilitated by using several speedup techniques. Exploiting intrinsic symmetry relations and the sparseness of the system matrix allows to create an efficiently compressed matrix which can be precomputed and completely stored in memory. In addition, a new voxel addressing scheme has been implemented. This scheme avoids time-consuming symmetry transformations of voxel addresses by using an octant-wise symmetrically ordered field of voxels. The above methods have been applied for a fully 3-D, iterative reconstruction of 3-D sinograms recorded with a Siemens/CTI ECAT HR+ PET scanner. A comparison of the performance of the reconstruction using LOI weighting and VOI weighting is presented.

Head motion can significantly degrade image quality of static and dynamic Positron Emission Tomography (PET) of the human brain. One method to regain acceptable image quality in the presence of motion is to include the correction for motion in the reconstruction process. When applying motion correction, the PET data can be segmented into discrete parts of similar head position, referred to as frames. This framing of the data can reduce the computational overhead necessary for motion correction during the reconstruction process by reducing the number of discrete head positions which have to be accounted for. Here a framing algorithm is presented which minimises residual motion in the framed data, while taking full advantage of the additional information provided by Magnetic Resonance Imaging (MRI) in a simultaneous MR–PET acquisition. In the work presented here information on motion is derived from EPI sequences acquired simultaneously with the PET data. A comparison to images reconstructed with regular framing show a more clearly delineated cortex due to increased contrast between grey matter and white matter. This improvement in image quality is achieved as well as a reduction in the number of frames, thereby reducing the reconstruction time. Preliminary data indicates an efficient reduction of residual intra-frame motion compared to regular framing.

The BrainPET scanner from Siemens, designed as hybrid MR/PET system for simultaneous acquisition of both modalities, provides high-resolution PET images with an optimum resolution of 3 mm. However, significant head motion often compromises the achievable image quality, e.g. in neuroreceptor studies of human brain. This limitation can be omitted when tracking the head motion and accurately correcting measured Lines-of-Response (LORs). For this purpose, we present a novel method, which advantageously combines MR-guided motion tracking with the capabilities of the reconstruction software PRESTO (PET Reconstruction Software Toolkit) to convert motion-corrected LORs into highly accurate generic projection data. In this way, the high-resolution PET images achievable with PRESTO can also be obtained in presence of severe head motion.

Using magnetic resonance (MR) information in maximum a posteriori (MAP) algorithms for positron emission tomography (PET) image reconstruction has been investigated in the last years. Recently, three methods to introduce this information have been evaluated and the Bowsher prior was considered the best. Its main advantage is that it does not require image segmentation. Another method that has been widely used for incorporating MR information is using boundaries obtained by segmentation. This method has also shown improvements in image quality. In this paper, two methods for incorporating MR information in PET reconstruction are compared. After a Bayes parameter optimization, the reconstructed images were compared using the mean squared error (MSE) and the coefficient of variation (CV). MSE values are 3% lower in Bowsher than using boundaries. CV values are 10% lower in Bowsher than using boundaries. Both methods performed better than using no prior, that is, maximum likelihood expectation maximization (MLEM) or MAP without anatomic information in terms of MSE and CV. Concluding, incorporating MR information using the Bowsher prior gives better results in terms of MSE and CV than boundaries. MAP algorithms showed again to be effective in noise reduction and convergence, specially when MR information is incorporated. The robustness of the priors in respect to noise and inhomogeneities in the MR image has however still to be performed.

The new hybrid imaging technology of MR-PET allows for simultaneous acquisition of versatile MRI contrasts and the quantitative metabolic imaging with PET. In order to achieve the quantification of PET images with minimal residual error the application of several corrections is crucial. In this work we present our results on quantification with the 3T MR BrainPET scanner.