Nadim Joni Shah

Structural MRI volumes from different field strengths may correlate across individuals yet differ systematically, complicating pooling and calibration. We assessed paired 3 T and 7 T MP2RAGE volumetry in 22 healthy right-handed male volunteers (of 32 initially processed; 10 excluded after FreeSurfer surface-reconstruction failure), using SynthStrip and SynthSeg on reconstructed UNI images. Volumes were normalised to total intracranial volume and evaluated for global tissue classes, subcortical structures, and mean hemispheric cortical composites. Analyses included paired Wilcoxon tests with false-discovery-rate correction, Spearman correlations, absolute-agreement ICC(2,1), Bland–Altman analysis, and an exploratory leave-one-out comparison of three linear calibration models evaluated with symmetric mean absolute percentage error (sMAPE). Rank-order association was high for global/subcortical measures (ρ = 0.804–0.965, all p < 0.001), but agreement was region-dependent. Field-specific TIV normalisation yielded four significant negative offsets (−1.00% to −2.20%), whereas the common-denominator/raw-volume analysis yielded significant positive differences in all seven regions (+1.20% to +4.35%). ICC(2,1) ranged from 0.696 to 0.980. On the %TIV scale, all calibration models reduced error in the four offset regions, but none did so for the hippocampus. Rank order was preserved more consistently than absolute agreement; offsets should be interpreted as scanner-protocol differences rather than field-strength effects alone.

INTRODUCTION: Isolated rapid eye movement (REM) sleep behavior disorder (iRBD) is a prodromal phase of Lewy body diseases and linked to a high risk for cognitive decline. Therefore, iRBD provides a suitable window for early intervention. METHODS: In the monocentric randomized controlled trial “ Cog nitive Tr aining & a Healthy, A ct i ve L ifestyle Program for People with Isolated R EM Sleep B ehaviour D isorder” (CogTrAiL‐RBD), the feasibility and behavioral effects of a 5‐week digital, adaptive, multidomain cognitive training alongside a module promoting a healthy lifestyle (INT) were compared to a waiting‐list control group (CON) in 82 individuals with polysomnography‐confirmed iRBD. The primary outcome was change in executive functions at 6 weeks post‐allocation (t1) and at 6‐month follow‐up (t2). RESULTS: For executive functions, we found positive intervention effects for the INT versus CON with a medium effect at t1 ( d  = 0.500, 95% confidence interval [CI]: 0.221–0.779) and a small effect at t2 ( d  = 0.397, 95% CI: 0.115–0.679). DISCUSSION: CogTrAiL‐RBD provides initial evidence for the feasibility and potential cognitive benefits of cognitive training within a lifestyle‐guidance framework in individuals with iRBD. Its suitability for dementia prevention warrants further investigation.

Echo-planar imaging (EPI), commonly employed in functional MRI (fMRI), is highly susceptible to magnetic field inhomogeneities, leading to pronounced geometric distortions in reconstructed images. These distortions can result in substantial structural discrepancies between EPI and anatomical images acquired using the magnetization-prepared 2 rapid acquisition gradient echoes (MP2RAGE) method, thereby making it challenging to achieve accurate co-registration and subsequent localization of functional mapping. This issue can be effectively addressed by employing an anatomical imaging sequence that exhibits distortion profiles identical to those in EPI, referred to here as MP2EPI (magnetization-prepared 2 EPI). While this approach enables effortless co-registration with functional scans, it also introduces geometric distortions into the anatomical reference imaging, which limits its utility for analyses that rely on morphometric measurements or atlas-based segmentation. Distortion in MP2EPI can be corrected using additional data acquired with the reversed phase-encoding (PE) direction, which, however, significantly increases total acquisition time. To overcome this limitation, this work presents a novel MP2EPI sequence that simultaneously acquires reversed PE data within a single MP2EPI acquisition, without increasing the overall scan time. The primary focus of the current work is the technical implementation and validation of this sequence in the context of submillimeter fMRI at 7T.

The BrainPET-7T is an ultra-high-field MRI-compatible insert designed for human brain PET imaging. It comprises 120 detector blocks, each containing 1634 LSO scintillation crystals arranged at a 2mm pitch across three shifted layers with heights of 9 mm, 8 mm, and 7 mm. The output faces of the bottom layers are optically coupled via a 1.1 mm thick light guide to a 12 × 12 silicon photomultiplier (SiPM) array, grouped into 6 × 6 digital SiPM dies. In this work, we present a comprehensive timing calibration for the scanner and evaluate the resulting coincidence time resolution (CTR). Timing skews were calibrated across all detector blocks, as well as for individual dies and crystals within each block. The results demonstrate a strong dependence of CTR on the crystal position relative to the underlying dies, as well as the crystal layer. For calibration, we used crystals in the bottom layer of one block as a reference, since these provide the best CTR. Against this reference, the CTR varies between (415.6 ± 0.8) ps for bottom layer crystals centrally above a die to (620 ± 6) ps for top layer crystals situated between dies. Using our calibration technique, we reduced the CTR by the calibrations from ≈750 ps to ≈600 ps for single block pairs within an energy window of 420 to 650 keV. The NEMA CTR obtained was approx. 630 to 650 ps for energy windows of 440 to 594 keV and 420 to 650 keV for the entire activity range. The noise equivalent count rate peak for 440 to 594 keV and a coincidence time window of 2.4 ns was 327 kcps at 16.6 kBqmL⁻¹.

Objective. Inter-crystal scattering (ICS) degrades spatial resolution and contrast by causing spurious lines of response in small-pitch, staggered three-layer pixelated LSO scintillation detectors used in a high-resolution BrainPET-7T insert. This work aimed to investigate the distribution of ICS events in our BrainPET-7T detector and understand its underlying mechanisms. Approach. Six physical approaches for computing probabilities to identify the first interaction positions of ICS events were studied using Geant4 simulations, focusing specifically on events with two interactions, which represent the most frequent ICS occurrence. Performance was evaluated using ideal interaction positions and energies directly from simulations, ground truth crystal-center positions with ideal energies, as well as crystal-center positions and energies determined by a maximum likelihood positioning (MLP) algorithm. Main results. A method based on Klein-Nishina probabilities, which were computed using geometric information (KN-G) and MLP-derived crystal-center positions and energies, achieved superior accuracy, particularly when the distance between successive interactions in Compton events was shorter than 6 mm and when incident angles were below 15 • . This advantage is partly attributable to crystal-center discretization, which masks a substantial portion of backscattering events. It reached a peak accuracy of 92.2 % at a distance of 19 mm. Significance. These findings demonstrate the feasibility of combining the KN-G method with the MLP algorithm to mitigate the ICS effects, with the potential to improve the image quality of the 7T BrainPET insert.

MRI of fixed tissue is an excellent way to study pathological changes caused by different diseases with great anatomical detail. It is, however, known that properties of tissue change with fixation. The aim of this study was to determine the variability of several quantitative MRI (qMRI) parameters in fixed brain tissue obtained from donors unaffected by neurological conditions and investigate the existence of quantitative parameters which vary little between specimens. We introduce a 3D method for high-resolution mapping of water content, T ₁ and T ₂ * relaxation times and parameters characterising magnetisation transfer and apply it at 3T to 7 whole, fixed human brains (3 male, 4 female, aged between 47 and 79 years, mean age 67 years). The qMRI parameters determined include relaxation rates T ₁ and T ₂ *, MT ratio and T ₁ and T ₂ * after MT. From these we can further derive semiquantitative MT parameters such as the exchange rate (k _trans ) and bound pool fraction (f _bound ). Correlations between these parameters are investigated. In addition, truly quantitative water content determined non-invasively with MRI is reported on whole human post mortem brains – to our knowledge, for the first time. Water content was found to have mean values of 73% for WM and 85% for GM with standard deviation below 2.5% over 7 brains, and thus a few percent units higher than in vivo (69% and 81%) and of comparable constancy.

The BrainPET-7T insert is designed for simultaneous human-brain PET imaging with ultra-high-field MRI. It offers a high sensitivity and signal-to-noise ratio by detector blocks of pixelated LSO scintillators with a height of 2.4cm, digital silicon photomultiplier readout, and enhanced data processing methods. This makes it particulary interesting for +--emitter applications, such as dual-isotope imaging, which rely on the detection of triple coincidences. We investigated the scanner sensitivity using the isotopes 124I and 22Na, in comparison to 18F. Count rate performance was assessed using 124I and 18F for different energy window settings with an analysis following the NEMA NU 2-2018 protocol. For the count rate determination, an adapted strategy for estimating the scatter fraction for +--emitters was applied. The peak double coincidence sensitivity for 18F measured at the center and off-center was between 10.16(8)% and 12.37(10)% for energy windows of 440 to 594keV and 420 to 650keV. The true triple sensitivities obtained at the isocenter were 3.13(9)% and 4.88(12)% for 124I and between 0.8662(23)% and 3.112(5)% for 22Na depending on the selected energy window and rejection rule. The maximum RNEC of the 18F-measurement was 276kcps at 16.3kBqmL-1, while the maximum Rtrue was 780kcps at 22.7kBqmL-1 for an energy window of 440 to 594keV. The scatter fraction ranged from 28% to 33% and the random fraction from 3% to 40%. In the case of 124I, the ratio of estimated true triples to doubles was ≈6%, with a higher random fraction of 27% for triples compared to 2.8% for doubles. These results demonstrate the potential feasibility of employing the BrainPET-7T insert for +--emitter applications.

Objective. In this study, we propose a fast implementation of a maximum likelihood positioning (MLP) algorithm to estimate the energy and identify the active scintillator pixel in staggered layer scintillation detectors for positron emission tomograph (PET). Approach. High processing speed of the MLP algorithm was achieved by developing an iteration-free implementation paired with additional computational optimizations. Further, the MLP was implemented for a staggered layer design with pixelated scintillators which enables the determination of the gamma’s depth of interaction. The pixelation of the scintillator facilitates an iteration-free formulation of the MLP algorithm. The efficacy of the algorithm optimization was tested on a scintillation detector block designed for an ultra-high field BrainPET 7 T, comprising three scintillator pixel layers. Calibration measurements, in combination with an automated calibration script, were used to obtain the expected counts of scintillation photons required in the MLP algorithm. Main results. Using single-instruction-multiple-data parallelization, multi-threading and optimized cache lines, a maximum processing speed of approximately 22.5 million singles per second was achieved on a platform with four Intel Xeon Platinum 8168 CPUs and 60 threads, encompassing all required processing steps. The automatic calibration failed for 1–15 individual scintillator pixels in approximately 10% of the 120 scintillation detector blocks, necessitating manual correction. After applying the energy correction to the positioned single events, an energy resolution of Δ E = 12 % ± 2 % FWHM was obtained for the entire scintillation block. Significance. The obtained energy resolution is very close to the energy resolutions measured for the individual scintillator pixels, proving that the MLP accurately identifies the scintillating pixel and that the energy correction method effectively compensates for the light collection variations of the SiPM array. The presented method is not limited to PET and can be used as fast positioning and energy determination method with scintillation detectors for PET, SPECT and gamma cameras.