Dirk Diehl

A High Field Gradient Magnet for Magnetic Drug Targeting

A magnet for Magnetic Drug Targeting was designed, built and tested. The magnet applies a magnetic force on ferrofluids coupled with therapeutic agents for medical and biological applications, e.g. for regional treatment of cancer. The application for cancer treatment requires a high magnetic field gradient in the application volume around and inside the tumor. Furthermore the magnet should have low mass and size combined with an appropriate geometry for easy handling and positioning near the patient, as well as a surface temperature below 40 degC. The gradient of the magnetic field, which determines the magnetic force for small single-domain ferromagnetic particles, ranges from 100 T/m at the pole tip to a minimum of 10 T/m in an application volume of (20 mm)3 and points inwardly to the center of the pole tip

SAR simulations for high-field MRI: how much detail, effort, and accuracy is needed?

Accurate prediction of specific absorption rate (SAR) for high field MRI is necessary to best exploit its potential and guarantee safe operation. To reduce the effort (time, complexity) of SAR simulations while maintaining robust results, the minimum requirements for the creation (segmentation, labeling) of human models and methods to reduce the time for SAR calculations for 7 Tesla MR‐imaging are evaluated. The geometric extent of the model required for realistic head‐simulations and the number of tissue types sufficient to form a reliable but simplified model of the human body are studied. Two models (male and female) of the virtual family are analyzed. Additionally, their position within the head‐coil is taken into account. Furthermore, the effects of retuning the coils to different load conditions and the influence of a large bore radiofrequency‐shield have been examined. The calculation time for SAR simulations in the head can be reduced by 50% without significant error for smaller model extent and simplified tissue structure outside the coil. Likewise, the model generation can be accelerated by reducing the number of tissue types. Local SAR can vary up to 14% due to position alone. This must be considered and sets a limit for SAR prediction accuracy. All these results are comparable between the two body models tested. Magn Reson Med 69:1157–1168, 2013.

Evaluation of MR-Induced Hot Spots for Different Temporal SAR Modes Using a Time-Dependent Finite Difference Method With Explicit Temperature Gradient Treatment

An investigation of magnetic resonance (MR)-induced hot spots in a high-resolution human model is performed, motivated by safety aspects for the use of MR tomographs. The human model is placed in an MR whole body resonator that is driven in a quadrature excitation mode. The MR-induced hot spots are studied by varying the following: the temporal specific absorption rate (SAR) mode (ldquosteady imagingrdquo, ldquointermittent imagingrdquo), the simulation procedure (related to given power levels or to limiting temperatures), and different thermal tissue properties including temperature-independent and temperature-dependent perfusion models. Both electromagnetic and thermodynamic simulations have been performed. For the electromagnetic modeling, a commercial finite-integration theory (FIT) code is applied. For the thermodynamic modeling, a time-domain finite-difference (FD) scheme is formulated that uses an explicit treatment of temperature gradient components. This allows a flux-vector-based implementation of heat transfer boundary conditions on cubical faces. It is shown that this FD scheme significantly reduces the staircase errors at thermal boundaries that are locally sloped or curved with respect to the cubical grid elements.