PROJECT ABSTRACT This grant will support the purchase of a next-generation Siemens 3 Tesla human MRI scanner for mental health research in the integrated PET-MRI facility at the Athinoula A. Martinos Center for Biomedical Imaging. The proposed instrument will replace an existing, 16+-year-old obsolete 3 Tesla MRI scanner that is reaching end- of-support status by the vendor and has seen diminishing usage over the last 5 years. We have recently secured a replacement for the 15+-year-old Siemens BrainPET prototype that currently resides in this obsolete 3T scanner. To make full use of the integrated capabilities of the PET-MRI system and expand our capacity for a continually growing base of users, we now seek to upgrade the 3T MRI scanner to a next-generation 3T system equipped with high-performance gradients up to 200 mT/m in strength and 200 T/m/s in slew rate. The proposed system exceeds the capabilities of current commercially available 3T MRI systems and will represent the first system of its kind to be integrated with a PET system, enabling investigations into the cellular-scale structure and circuitry of the human brain with unprecedented detail and molecular specificity. The instrument offers a substantial increase in performance for investigators at Mass General Brigham and the greater Boston area, justified on a shared-use basis by over 25 users and 30 NIMH- and NIH-funded projects in PET-MR. The breadth of supported mental health research spans translational neuroimaging, therapeutic neuromodulation, and cognitive, psychiatric, and basic neuroscience studies in mental disorders such as major depressive disorder, bipolar disorder, autism spectrum disorder, schizophrenia, obsessive-compulsive disorder, and Alzheimer’s disease. The system to be replaced uses outdated hardware and software and has required excessive servicing with downtime of up to 30%, placing substantial pressure on our other 3T systems, which are highly utilized and cannot accommodate our expanding user community. The next-generation 3T MRI features: higher performance gradients; enhanced RF architecture; higher density specialized coils; the latest standardized hardware and software for multi-site studies; faster computer enabling modern imaging techniques; integrated physiological monitoring; new sequence and application packages for accelerated, multi-contrast exams; and an up-to-date testbed for the unique technology development performed at the Martinos Center. The new instrument is supported at a high level by MGB leadership, the Radiology Department, and the Martinos Center, covering the allocation of space and costs for installation, operational support, and maintenance, including an exceptional financial commitment to cover the gap between the NIH instrumentation award and purchase price of the upgrade. By serving a highly productive interdisciplinary group of NIMH- and NIH-funded investigators, the proposed upgrade will offer a cutting-edge research resource tha...