Integrated, dynamic B0 and flip-angle shimming using multi-coil shim arrays - Project Summary/Abstract 7T MRI systems are installed worldwide more and more rapidly and are currently being evaluated in clinics for a number of conditions such as detection of small focal cortical dysplasia, brain tumors and multiple sclerosis (MS) lesions. However, the signal-to-noise and resolution advantage of 7T MRI is counterbalanced by increased heterogeneity of the static B0 and transmit B1+ fields that degrade image quality, which in turn slows clinical translation for example for patients with temporal lobe epilepsy or MS lesions in the lower part of the brain/cerebellum. Parallel transmission (pTx) is a popular solution to the problem of inhomogeneous B1+ but is costly (~$100k per transmit channel x 8 or 16 channels) and leads to complex specific absorption rate (SAR) management requiring lengthy electromagnetic simulations that need to be validated. B0 field inhomogeneities close to the sinuses and ear canals can be greatly reduced using multi-channel (MC) shim arrays that place dozens of small loops close to the head and can be rapidly be switched on/off, thus allowing dynamic (slice-by- slice) shimming which outperforms global brain shimming with conventional spherical harmonics shim coils. Despite the potential of pTx and MC shim arrays, those state-of-the-art technologies are not being quickly translated to the clinics because of the cost and complexity of combining and operating them. In fact, most 7T systems currently operate in single channel mode with conventional spherical harmonics shim coils, and therefore do not address head-on the dual problems of B0 and B1+ field inhomogeneities, thus likely yielding sub-optimal image quality that may affect the perceived clinical value of 7T MRI. We propose a bioengineering research program leveraging the field control capability of MC shim arrays to homogenize the flip-angle at 7T, thus yielding an integrated solution for B0 and flip-angle shimming that bypasses the need for pTx. We develop pulse design strategies that use the degrees-of-freedom of MC shim arrays for flip-angle control while minimizing power requirements and SAR. This includes general spokes and kT-point pulses where the gradient blips are replaced by MC shim array blips with more general spatial distributions, as well as optimal control of RF, gradient and MC shim current waveforms based on a direct optimization of Bloch dynamics. We use those approaches to design “universal” pulses that can be played quickly without preparation. We simulate and build an MC shim array integrated with the RF coil (“AC/DC” design) that is optimized for simultaneous B0 and flip-angle shimming performance, which we evaluate in healthy subjects and MS patients.
