B1+-mapping and B1+ inhomogeneity correction at high field

Magnetic resonance images acquired at the highest strength of the main magnetic field B0 are of interest since they highly benefit from an increased signal to noise ratio. 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 MR imaging is that the wavelength of MR radiations starts to be shorter than the typical organs of interest. At such wavelength, the transmit magnetic field B1+ used to manipulate the magnetization in MR imaging is subject to constructive and destructive interferences and becomes position dependent. This inhomogeneity in the B1+ field leads to signal and contrast variations in the anatomical images which are prone to misinterpretation. This thesis is about measuring and correcting the inhomogeneous B1+ field at 7 Tesla. To be able to correct the B1+ inhomogeneity, it is necessary to measure it first. An appropriate B1+-mapping sequence should provide accurate measurements in a wide range of B1+ values in a short amount of time since the acquisition of the B1+ distribution can be considered as an adjustment step. The SA2RAGE sequence was developed according to these criteria, allowing a typical three-dimensional B1+ map to be acquired in less than 2min. The next challenge was to correct the B1+ inhomogeneity observed across the brain at 7 Tesla. To obtain results of high quality, RF pulses were designed to generate the desired magnetization profile. It was already known that kT-point pulses designed in the small tip angle (STA) approximation provided substantial B1+ inhomogeneity correction. In this thesis, a methodology expressing the Bloch equations in a linear form was developed for the design of kT-point pulses beyond the STA regime. Excitation, inversion and refocusing pulses were designed and significant improvements were observed in the associated magnetization profiles when compared to the results found in the STA regime. The last part of the thesis was dedicated to the design of kT-points for a turbo spin echo (TSE) sequence in order to remove the effect of the B1+ inhomogeneity on T2-weighted imaging at 7 Tesla. In the first approach, a kT-point pulse was designed in the STA regime to make the excitation profile as homogeneous as possible. It was demonstrated that a symmetric kT-point pulse designed in the STA regime still generates an homogeneous excitation profile for flip angles as high as 120°. A unique symmetric kT-point pulse was designed in the STA regime and used to replace all the original hard pulses of a TSE sequence (static design). By adding parallel transmission, anatomical images largely devoid of artifacts resulting from the common B1+ inhomogeneity at 7 Tesla were acquired. To be able to acquire T2-weighted images with signal and contrast homogeneity by using a more efficient TSE sequence protocol, a specific kT-point waveform was optimized for each pulse of the TSE sequence (dynamic design). It was demonstrated that, although at a cost of an increase of the specific absorption rate, the dynamic outperforms the static kT-point design in terms of signal and contrast homogeneity obtained in the acquired T2-weighted images. The use of dynamic kT-points to obtain such a quality in T2-weighted imaging is thus promising for clinical applications at ultra high field.

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