Parallel Transmission in Liver MRI at 7T: Initial Results

Xiaoping Wu¹, Sebastian Schmitter¹, Edwards J. Auerbach¹, J. Pfeuffer², Michael Hamm³, Kamil Ugurbil¹, P-F. Van De Moortele¹

The feasibility of torso imaging at 7T has been demonstrated in several organs. However, to consider successful clinical applications, fast, robust and practical methods must be developed to address the severe transmit B1 (B1+) inhomogeneity present at such high magnetic field. B1 shimming proves to be a successful approach to address this issue, especially in organs of a limited size such as the prostate. However, obtaining a uniform B1+ field at 7T using B1 shimming over the longest dimension of large organs such as the liver is extremely challenging, if not impossible, and such B1 shim solutions typically come at the cost of very low RF efficiency yielding SAR values beyond acceptable limits. Parallel transmission (pTX) has much larger degrees of freedom to address B1+ inhomogeneity and has been shown to be able to produce spatially homogeneous excitation in the human head. This technique, however, has not yet been demonstrated in body imaging at 7T, where mapping B1+ on multiple channels in the presence of respiratory motion is an additional challenge. Here, we report the first liver images obtained using 3D spoke pTX RF pulses for flip angle homogenization at 7T.