LAUSR

F our decades following the earliest acquisition of magnetic resonance (MR) derived images of the brain, MR technology has developed into an essential diagnostic tool across a diverse range of clinical applications. Central to this evolution has been the development of human MR scanners with increasingly greater magnetic fields, most recently those operating at field strengths of 7 tesla (T) or more (up to 11.7T). These ultrahigh field MR scanners have the potential to permit neuroimaging with unprecedented detail, leading to better characterization of normal tissue and pathologic lesions and improved planning of treatment and monitoring of response. As of early 2019, at least 76 whole-body MRI systems of 7T or greater field strength have been installed around the world according to a database developed by Renzo Huber of the layer fMRI blog. The 7T Terra (Siemens Healthineers, Erlangen, Germany) system has become the first ultrahigh field system to receive 510(k) clearance for clinical imaging. Signal-to-noise (SNR) and tissue contrast, 2 considerations central to the utility of MRI, both scale proportionally with field strength (see Table 1). Previous comparative studies across high (3T) and ultrahigh field MRI (7T and above) have empirically demonstrated a potentially supralinear relationship between field strength and SNR. A consequence of this greater SNR than conventional scanners is that images at ultrahigh field can typically be acquired with greater resolution and better differentiation of fine anatomical structures. This greater sensitivity of ultrahigh field imaging may help to establish imaging of biomarkers for diseases such as brain tumors, Alzheimer disease, neuropsychiatric disorders such as depression, post-traumatic stress, and schizophrenia, and neurological disorders such as epilepsy and multiple sclerosis. As a leading-edge technology, ultrahigh field imaging is not without its technical challenges. Realizing the clinical potential of ultrahighfield neuroimaging requires addressing issues such as greater B0 and B1 inhomogeneity and specific absorption rates (SARs).