Design and Development of 7-T MR-Compatible PET Systems for Human Neuroimaging and Particle Physics
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allen-msallen-phd-physics-2026-thesis.pdf
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Author(s)
Allen, Magdelena S.
Advisor(s)
Catana, Ciprian
Hen, Or
Date Issued
May 2026
Publisher
Massachusetts Institute of Technology
Abstract
Background. Noninvasive imaging techniques are essential for studying human brain function in vivo. Positron Emission Tomography (PET) offers unsurpassed molecular sensitivity and specificity, and a wide range of radiotracers are available for studying neurotransmitters, receptors, and neuromodulators in humans. However, PET has drawbacks to spatial resolution and system sensitivity. Magnetic Resonance Imaging (MRI) offers exquisite tissue contrast and can measure hemodynamics on functional timescales, but hemodynamics are an indirect proxy for neuronal activity and cannot directly measure molecular processes. The tools to precisely measure dynamic neurochemical-specific brain activation on the timescales of cognitive processes are not yet in hand. Advances in PET instrumentation for higher sensitivity and spatial resolution, particularly when integrated with MRI, can address this challenge by enabling simultaneous measurement of neurochemical dynamics and functional hemodynamics. As high-performance gamma cameras, PET systems also provide opportunities for fundamental physics studies. Searches for symmetry violation beyond the Standard Model are of particular interest, as the vast matter-antimatter asymmetry observed in the Universe is unexplained, and can be pursued by measuring positronium decays to photons. PET/MR technology offers a unique platform for these experiments with the ability to precisely measure gamma-rays as well as perform in high magnetic field environments.
Methods. The Human Dynamic NeuroChemical Connectome scanner comprises the NeuroSphere, a high-spatiotemporal-resolution dedicated brain PET insert, integrated with the Siemens MAGNETOM Terra.X 7-T MRI for simultaneous functional imaging. The NeuroSphere achieves high intrinsic sensitivity through high-efficiency detectors, novel spherical geometry with high solid-angle coverage, and time-of-flight capability. The insert was designed for 7-T MR compatibility including custom electronics, support gantry, RF head coil, and electromagnetic interference (EMI) shielding. The PET readout configuration was optimized, and a scalable calibration and correction pipeline was developed to characterize crystal separation, energy resolution, depth-of-interaction (DOI) encoding, and timing performance. PET/MR compatibility was evaluated through mutual interference measurements and assessment of EMI shielding strategies. In parallel, a dedicated experimental configuration was
developed for particle physics measurements at 7 T, leveraging the modular NeuroSphere PET architecture. A custom data processing pipeline was implemented to study ortho-positronium (oPs) decays to three photons, including measurement of oPs lifetime in plastic scintillator and tests of CP invariance via decay asymmetries.
Results. The NeuroSphere PET insert was designed and assembled, with sensitivity simulated at > 30%, an order of magnitude improvement over most commercial systems and a factor of 2-3 improvement over state-of-the-art dedicated brain PET systems. The novel PET module demonstrated high detection efficiency, high intrinsic spatial resolution, clear crystal separation, DOI encoding capability, and competitive energy and timing resolution following DOI-based corrections. Several EMI shielding methods were evaluated for shielding efficiency
on the bench and noise attenuation in the MR environment, and shielding schemes were
designed and manufactured for the PET insert. Preliminary mutual interference tests were
encouraging. The custom physics experimental setup, supported by GATE simulations,
enabled measurement of oPs lifetime and decay kinematics. The measured decay asymmetry
is consistent with zero, placing a constraint on charge conjugation-parity (CP) symmetry
violation at the level of 2 x 10^−2.
Significance. A next-generation PET insert for 7-T MRI has been built, leveraging novel technologies to advance the sensitivity of brain PET imaging and operate in the high-field environment. The system is currently undergoing full-scale system testing and is expected to enable studies of neurochemical dynamics in humans on functional timescales. The PET module achieved competitive performance despite system constraints for MR integration and
high sensitivity. The system architecture was also adapted for a custom physics measurement setup and was used to constrain CP violation by measuring positronium decays under an applied magnetic field, the first measurement of its kind with PET/MR instrumentation. These results demonstrate the potential of the NeuroSphere PET system for advanced biomedical imaging and particle physics investigations.
MIT Department
Massachusetts Institute of Technology. Department of Physics
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