The Specific Absorption Rate (SAR) is a limiting factor to all MRI-scans. Especially at ultra-high magnetic fields (≥ 7 Tesla), it imposes a significant constraint in the design of pulse sequences....Show moreThe Specific Absorption Rate (SAR) is a limiting factor to all MRI-scans. Especially at ultra-high magnetic fields (≥ 7 Tesla), it imposes a significant constraint in the design of pulse sequences. Due to interpatient variability and the complicated structure of human anatomy, it is difficult to accurately determine the exact SAR-distribution for individual patients. Computational simulations using high-resolution human body models can be used to estimate the SAR, but such models are not available for individual patients in a clinical setting. Here, a method for developing a personalized model for estimating SAR in the head using parallel transmission at 7 Tesla is proposed based on clustered segmentation of tissues. We found that by segmenting all the tissues in the head into fat, cerebrospinal fluid (CSF), grey matter, and bone, the peak-SAR can be determined with an error of less than 2.8 % of the overall peak-SAR. This result is shown to be reproducible for subjects of different ages and genders. Methods for the automated segmentation of this mapping in individual patients based on T1w-images, quantitative T1-mapping, and ultra-short TE-scans are proposed and tested experimentally. Using the proposed method, it should be possible to operate scanners closer to the true SAR-limits due to improved estimations of the actual patient-specific SAR.Show less
The medical technique of Magnetic Resonance Imaging (MRI) is barely available in developing countries because of its high cost and the strong requirements on infrastructure. To address this problem...Show moreThe medical technique of Magnetic Resonance Imaging (MRI) is barely available in developing countries because of its high cost and the strong requirements on infrastructure. To address this problem, we are developing a permanent magnet-based head scanner that is affordable (<50,000 EUR) and portable. Here, we report on the first observations of magnetic resonance in our custom magnet array with a field strength of 59 mT. Using custom made volume coils, we observe using Hahn echo (Spin echo) and CPMG pulse sequences. We discuss the step towards 2D imaging using rotating spatially encoding magnetic fields (rSEMs) and show simulations that indicate this is feasible in our setup. Finally, we discuss the technical challenges that still have to be overcome to turn this prototype into a diagnostic device for those in need.Show less
B0 magnetic field non-uniformity is the cause of a large amount of image artifacts in MRI. B0 inhomogeneities arise due to magnetic susceptibility differences between tissues. In particular, the 9...Show moreB0 magnetic field non-uniformity is the cause of a large amount of image artifacts in MRI. B0 inhomogeneities arise due to magnetic susceptibility differences between tissues. In particular, the 9 ppm magnetic susceptibility difference between air and tissue generate disturbances in the B0 main field near the skin. We study the B0 passive shimming approach of covering the skin with a susceptibility-matching material from both an experimental and a mathematical viewpoint. In the experimental study, a lightweight and simple to shape pyrolytic graphite composite foam is used to compensate for the field inhomogeneities in the region of the neck. We experimentally demonstrate that the pyrolytic graphite foam improves the uniformity of the static field in a phantom and in vivo at 3T. In the numerical study, we aim for a design of a neck shim which efficiently homogenizes the B0 field while being practically implementable. We propose a level set optimization method as an approach to find the optimum design for a neck shim. Simulations prove that the proposed method is able to solve the topological optimization problem while preserving the imposed constraints.Show less
