pULSE - pTx Ultra-fast Local SAR Estimation

pULSE is a project combining advanced mathematics and medical physics to target some of the most pressing issues in the current development of ultra-high field magnetic resonance imaging (MRI), namely fast and reliable estimation of SAR, a patient-safety relevant parameter, especially for systems with multiple simultaneous radio frequency (RF) transmission channels (parallel transmission, pTx) efficient and fast calculation of optimized RF waveforms (pulses) for pTx a generalized approach to find RF coil hardware geometries based on the ideal current distribution required for a specific target organ We will develop a mathematical simulation framework for time-efficient calculation of electromagnetic fields inside the human body, based on pre-calculated compressed Greens functions for arbitrary excitations of a large number of current dipoles surrounding the human body in a realistic MR scanner environment. With such a method at hand, the ideal current patterns around the body can be obtained for any target organ. This implies that novel dedicated RF hardware can be developed based on theoretical principles and is expected to outperform the classical approach of empirical design, leading to improved image quality. To proof the principle of the framework set up by this project, we will calculate the ideal geometry, construct, and implement a pTx-compatible RF coil prototype for human cardiac imaging at 7 Tesla and show its applicability and performance in vivo on healthy volunteers.

The pULSE project has helped develop computer simulation methods for safety assessment in Magnetic Resonance (MR). MR scanners operate at magnetic field strengths of 1.5 Tesla and above, with 7 Tesla being the highest field in clinically approved systems. With higher field strength, the magnetization increases, and higher signal can be obtained, which can be used to get clearer images or measure faster. In addition to the main magnetic field, MR needs radio waves to perform the measurements. As an unwanted side-effect, these radio waves also lead to heating of the tissue. The amount of energy they deposit in the body is described by the so-called "specific absorption rate (SAR)". SAR is safety-relevant because it can be used to estimate tissue heating. Increasing the field strength, also increases the frequency of these radio waves which leads to undesired non-uniformity of their distribution in the body, which translates into suboptimal image quality. As a countermeasure, parallel transmission systems (multiple sources sending radio waves at the same time), have been invented. This technique can create more uniform images, but has the disadvantage that the SAR distribution patterns in the human body become complicated and more difficult to calculate. The pULSE project contributed to this challenge in a number of ways: a mathematically efficient method to calculate the resulting SAR for a given parallel transmission pulse can now be calculated within milliseconds, once the distribution of the electric fields is known. This is useful for other mathematical optimization algorithms for the parallel transmission radio wave shapes that take into account the SAR that comes with them. In the pULSE project, such an optimization method was demonstrated. Using these simulation tools, a device (a so-called "radio frequency coil") for investigation of the metabolism of the heart muscle was first optimized in its geometry and properties, and the constructed and tested. The coil allows researchers to obtain 2-3 fold signal from phosphorus-containing molecules in the heart, which tell about the cardiac energy metabolism. This is particularly relevant for studies of ischaemic events and their consequences for the heart tissue. As a further impact of the project, FWF funding has contributed to the founding of two spin-offs at both the Medical University of Vienna and Vienna University of Technology, one of which is concerned with radio frequency coils and the other with finite element simulations.