Basic MR Safety: SAR to Temperature, Power Deposition/Monitoring, Effects of RF Coils & Field Strength
Nicolas Boulant1

1NeuroSpin/CEA, Saclay, France


The advent of parallel transmission at high field has led to many studies aiming at quantifying more accurately the Specific Absorption Rate (SAR)/temperature aspects while understanding in more details the risks involved in MRI experiments. This talk gives a review of different techniques employed for SAR calculations, their validations and real-time supervision. Safety margins arising from each step of the evaluation chain are described and future directions for temperature evaluation are presented.


The shortening of the radiofrequency wavelength at high field engenders more rapid variations of the electromagnetic field and may cause local Specific Absorption Rate (SAR) hotspots. Although SAR limits have been handled quite efficiently throughout the years via numerical simulations and net power supervision in the case of single-transmit channel systems, safety management has certainly become more complex with parallel transmission (pTx), requiring more thorough studies to validate the computations while leaving significant room for B1 field inhomogeneity-mitigation. Within this context, this talk describes current calculation, uncertainties, supervision and validation techniques to control the SAR during MRI exams. Future directions to take into account temperature, instead of SAR, are presented.

Global and local SAR control in single-channel transmit systems

In single-channel transmit systems, a relatively conservative and experimental assessment of the global SAR can be performed. If forward and reflected powers indeed are measured, energy conservation dictates that the global SAR should not exceed the absorbed power divided by the mass of the subject. Peak 10-g SAR can then be deduced analytically for specific geometries1 or via numerical simulations. A safety k-factor can then be computed with k=max(SAR10g)/SARGlobal which thereby enables local SAR control via simple power monitoring. Alternatively, subject-based local SAR can be estimated with B1 map measurements and processing2,3, but it is not yet as mature as the widespread use of numerical simulations.

Global and local SAR control in multi-transmit channel systems

In pTx, the total electric field at a given location is the linear superposition of individual E-fields driven by N independent transmit channels. Yet again the measurement of the forward and reflected powers on each channel allows for a subject-based assessment of the global SAR by invoking energy conservation. With a quick calibration scan, global SAR thereby can be predicted for any arbitrary RF excitation4,5. Estimation of local SAR yet to date still necessitates the use of numerical simulations. Although experimental6 and computational7 efforts have been made towards the use of subject-based models for accuracy, the use of generic models combined with safety factors still prevails due to its simplicity of implementation. To take into account properly the coupling between coil elements, co-simulation should be used to reproduce the correct scattering matrix8. Alternatively, to achieve the same goal linear combinations of simulated basis field elements can be fitted to complex B1 measurements9,10 for known geometries. Resulting errors between simulated and measured transmit B1 fields can then be propagated to estimate 10-g SAR uncertainties11 or N2 individual MR thermometry experiments12 can be carried out to measure directly the so-called SAR Q matrices13. Another safety factor can be added to account for anatomic variability by simply simulating several models and positions14-17 and estimate the risks. Finally, additional safety factors can be enforced depending on the information and uncertainties returned by the monitoring hardware6. The number of Q-matrices, normally defined for each voxel, can be compressed to return Virtual Observation Points (VOPs)18 and drastically ease their incorporation in pulse design algorithms19,20 as well as real-time SAR supervision based on measured waveforms.

Temperature control in multi-transmit channel systems

Temperature, or even thermal dose21, is more relevant from the safety point of view than SAR. To what extent the local temperature rises however highly depends on the location of the SAR hotspots and the amount of perfusion. As shown by Massire and co-workers22, over thousands of random pTx RF pulses each saturating the IEC local SAR limits at 7T, in simulation very few posed problems in terms of temperature. It was also shown with some simulations performed at 10.5 T that the indirect link between temperature and SAR in fact could be exploited to gain further scan performance and increase safety23. At the cost of pre-simulations, the temperature calculation formalism was also shown to become strictly identical to the one used for SAR so that temperature VOPs likewise could be derived23. Great progress thus has been made from the numerical point of view to make temperature management in RF pulse design and exam supervision a reachable goal. Debates, however, still exist about the validity of the Pennes’ bioheat equation often used and alternate models have been proposed24,25. Experimental efforts have been made to characterize better the relationship between SAR and temperature25-29 but they remain limited in number due to the difficulty of performing non-invasive small temperature rise measurements in vivo. More work hence is needed to investigate bioheat models further. Nevertheless, it should be kept in mind that current SAR limits provided in the international guidelines, perhaps indeed of empirical value, are meaningful only if a thermal rationale supports them.


No acknowledgement found.


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Proc. Intl. Soc. Mag. Reson. Med. 26 (2018)