Locally Optimized B1 Field for MRI Systems

A current challenge for high static field magnetic resonance imaging (MRI) is the non-uniformity of the radio-frequency (RF) excitation B1 field. At high fields, the required B1 field has a relatively short wavelength: as a result, standing waves of the B1 field are created, thus causing a non-uniform excitation distribution and, ultimately, affecting the quality of the final MRI images. Use of transmit arrays can improve the uniformity of the magnetic field B1. However, it is not always possible to achieve a uniform B1 field while keeping low Specific Absorption Rate (SAR) levels, which is necessary to not overheat the patient. It is possible to obtain strong uniform fields and more easily control the power deposition in the body by focusing on a localized volume. We present an analytical method able to optimize the transmit efficiency of the transmit array, maximizing the field in the region of interest and minimizing the generated power, thus reducing overheating risks. In addition we present a new method to estimate the temperature increase due to RF power absorption solving the Pennes’ bioheat equation. Our assumption is that the heat conductivity term in the equation acts like a low-pass filter on the temperature distribution. Thus, starting from the simulated SAR distribution, this method can provide an accurate calculation of the temperature increase in a short time: compared to other numerical methods, it acts on the whole data matrix and not pixel by pixel making calculations even more than sixty times faster.