B ‐ mapping and B inhomogeneity correction at high field THÈSE NO 6582 ( 2015 ) ÉCOLE POLYTECHNIQUE FÉDÉRALE DE LAUSANNE

Magnetic resonance imaging is a modern imaging technique that allows anatomical images used in many medical diagnosis to be acquired in a completely non-invasive way. MR images acquired at the highest strength of the main magnetic field B0 are of interest since they highly benefit from the increased signal to noise ratio which is proportional to B0. At ultra high field strengths (B0 ≥ 7 Tesla) images with more contrast and higher resolution can thus be obtained, opening new insights into the understanding of organ structures and disease evolutions. One of the main challenges of ultra high field magnetic resonance imaging is that the wavelength of magnetic resonance radiations, which is inversely proportional to B0 starts to be equal to or shorter than the typical organs of interest. At such wavelength, the transmit magnetic field B 1 used to manipulate the magnetization in magnetic resonance imaging is subject to constructive and destructive interferences and becomes position dependent. This inhomogeneity in the B 1 field leads to signal and contrast variations in the anatomical images which are prone to misinterpretation. The B 1 inhomogeneity can be so intense that regions void of signal are observed where structural information is lost. This thesis is about measuring and correcting the inhomogeneous B 1 field at 7 Tesla. To be able to correct the B 1 inhomogeneity, it is necessary to measure it first. An appropriate B 1 -mapping sequence should provide accurate measurements in a wide range of B + 1 values in a short amount of time since the acquisition of the B 1 distribution can be considered as an adjustment step before the acquisition of anatomical images. Moreover, parallel transmission, a promising tool providing more flexibility for the B 1 inhomogeneity correction, requires the acquisition of an individual B 1 profile for each element of an N channel array. Those profiles show large variations with intense B 1 values close to the transmitting elements and low values further distant. The acquisition of such profiles justifies the needs for a fast mapping sequence providing accurate measurements over a large range of B 1 values. According to these criteria, the SA2RAGE sequence, which is based on the combination of two three-dimensional gradient echo images, was developed. The sequence parameters were optimized in order to provide the largest range where the B 1 field is measured precisely. A typical three-dimensional 64x64x48 with 3x3x4mm3 B 1 map can be acquired in less than 2min with the SA2RAGE sequence. The next challenge was to correct the B 1 inhomogeneity observed across the brain at 7 Tesla. RF shimming, where the amplitudes and phases of the different elements of a parallel transmit array are optimized to generate an homogeneous B 1 distribution over the region of interest, was used as a first approach. The B 1 inhomogeneity correction provided by the standard RF shimming process being rapidly limited, another approach, where a specific set of amplitudes and phases is optimized for different slices along the image, was investigated. Compared to standard RF shimming, the slice based approach provided a 21% improvement.