Ingmar Graesslin

A specific absorption rate prediction concept for parallel transmission MR

The specific absorption rate (SAR) is a limiting factor in high‐field MR. SAR estimation is typically performed by numerical simulations using generic human body models. However, SAR concepts for single‐channel radiofrequency transmission cannot be directly applied to multichannel systems. In this study, a novel and comprehensive SAR prediction concept for parallel radiofrequency transmission MRI is presented, based on precalculated magnetic and electric fields obtained from electromagnetic simulations of numerical body models. The application of so‐called Q‐matrices and further computational optimizations allow for a real‐time estimation of the SAR prior to scanning. This SAR estimation method was fully integrated into an eight‐channel whole body MRI system, and it facilitated the selection of different body models and body positions. Experimental validation of the global SAR in phantoms demonstrated a good qualitative and quantitative agreement with the predictions. An initial in vivo validation showed good qualitative agreement between simulated and measured amplitude of (excitation) radiofrequency field. The feasibility and practicability of this SAR prediction concept was shown paving the way for safe parallel radiofrequency transmission in high‐field MR. Magn Reson Med, 2012.

Toward individualized SAR models and in vivo validation

The specific absorption rate (SAR) is a limiting constraint in sequence design for high‐field MRI. SAR estimation is typically performed by numerical simulations using generic human body models. This entails an intrinsic uncertainty in present SAR prediction. This study first investigates the required detail of human body models in terms of spatial resolution and the number of soft tissue classes required, based on finite‐differences time‐domain simulations of a 3 T body coil. The numerical results indicate that a resolution of 5 mm is sufficient for local SAR estimation. Moreover, a differentiation between fatty tissues, water‐rich tissues, and the lungs was found to be essential to represent eddy current paths inside the human body. This study then proposes a novel approach for generating individualized body models from whole‐body water‐fat‐separated MR data and applies it to volunteers. The SAR hotspots consistently occurred in the arms due to proximity to the body coil as well as in narrow regions of the muscles. An initial in vivo validation of the simulated fields in comparison with measured B1‐field maps showed good qualitative and quantitative agreement. Magn Reson Med, 2011.

Specific absorption rate reduction in parallel transmission by k-space adaptive radiofrequency pulse design

The specific absorption rate (SAR) is an important safety criterion, limiting many MR protocols with respect to the achievable contrast and scan duration. Parallel transmission enables control of the radiofrequency field in space and time and hence allows for SAR management. However, a trade‐off exists between radiofrequency pulse performance and SAR reduction. To overcome this problem, in this work, parallel transmit radiofrequency pulses are adapted to the position in sampling k‐space. In the central k‐space, highly homogeneous but SAR‐intensive radiofrequency shim settings are used to achieve optimal performance and contrast. In the outer k‐space, the homogeneity requirement is relaxed to reduce the average SAR of the scan. The approach was experimentally verified on phantoms and volunteers using field echo and spin echo sequences. A reduction of the SAR by 25–50% was achieved without compromising image quality. Magn Reson Med, 2011.

Detection of RF unsafe devices using a parallel transmission MR system.

The permanent presence of devices (pacemakers) inside a patient, or the need to use other devices (catheters), for diagnosis and treatment, usually represents a contraindication for a magnetic resonance examination. To help overcome this problem, a novel and noninvasive magnetic resonance system‐based concept is proposed to detect potentially unsafe radio frequency (RF) conditions of such devices to ensure patient safety.

Comprehensive RF safety concept for parallel transmission MR.

The goal of this study is to increase patient safety in parallel transmission (pTx) MRI systems. A major concern in these systems is radiofrequency‐induced tissue heating, which can be avoided by specific absorption rate (SAR) prediction and SAR monitoring before and during the scan.

SAR in Parallel Transmission

Parallel transmission bears the potential of compensating B1 fleld inhomogeneities induced by wave propagation efiects in (ultra) high fleld whole body MR imaging. However, with increasing fleld strength, the RF power deposition and the associated local speciflc absorption rate (SAR) represent an important attention point with respect to patient safety. This paper presents simulations of a 3T whole body eight-channel transmit/receive body coil loaded with a human bio-mesh model. Phantom SAR simulations were carried out and validated by temperature measurements. A good correlation between SAR simulations and measured temperature was obtained, so that the FDTD method can be considered to be a valuable tool in determining (local) SAR for patient safety in multi-channel transmission MRI systems.