Radiofrequency Power Absorption and Specific Absorption Rate in MRI – what are they, how to model them, and how to reduce them?

Emre Kopanoglu¹
¹CUBRIC, Psychology, Cardiff University, Cardiff, United Kingdom

Synopsis

Keywords: Physics & Engineering: RF Safety, Transferable skills: Safety, Physics & Engineering: Pulse design

This talk will introduce what RF power deposition is and what its implications within the context of MRI are, examine how it leads to temperature increase, and discuss why specific absorption rate (SAR) is used as a safety parameter. Then, we will review SAR limits and different SAR definitions applicable in various imaging scenarios. We will discuss how SAR is used in practice and will talk through how SAR is modelled while scrutinizing various potential pitfalls that might lead to mismatches between modelling and reality. Finally, we will investigate factors that contribute to SAR, and explore strategies to reduce SAR.

Magnetic resonance imaging (MRI) relies on the use of three magnetic fields; static magnetic field, gradient magnetic fields, and radiofrequency (RF) magnetic fields. The static magnetic field aligns spins with itself, thereby creating a steady-state. However, we cannot record the spins’ signal in the steady-state. To generate a recordable response, RF magnetic fields excite the spins away from the steady-state into a transient state. To excite the spins effectively, RF magnetic fields need to operate at the resonance frequency of the spins, i.e., Larmor frequency.

As characterized by Maxwell’s Equations, magnetic fields operating at nonzero frequencies generate electric fields and vice versa. Although this back-and-forth between magnetic and electric fields enable wave propagation, it has undesired consequences for MRI, as electric fields lead to power deposition in lossy media such as human and animal tissues. Such power deposition leads to temperature increase in tissues. We need to understand, model, control and limit temperature increase in tissues in order to minimize its implications.

The Specific Absorption Rate (SAR) is an intermediary parameter that relates the RF power deposition in tissues to the resulting temperature change [1]. Because temperature change cannot be measured experimentally with enough accuracy in real time, SAR is used as one of the primary safety parameters in MRI [2]. Consequently, in this talk, RF power deposition in MRI will mainly be discussed through SAR.

In this talk, we will start with an introduction of RF power deposition in MRI, examine how it leads to temperature increase [1], and discuss why SAR is used as a surrogate safety parameter. Then, we will review the International Electrotechnical Commission’s (IEC) limits on SAR, expanding on different SAR definitions provided for different applications and scenarios [2]. Subsequently, we will discuss how SAR is used in practice; be it on a pulse-designer’s workstation [3-5] or the scanner hardware’s RF watchdog module [6, 7]. We will talk through how SAR is modelled [8-13] and scrutinize various scan conditions that might lead to mismatches between modelling and reality [8, 14-21]. Finally, we will investigate contributing factors of SAR [22], and then take a step back to see the bigger picture by exploring strategies to reduce SAR in general [23-25].

At the end of this talk, audience members will have learned about:

what SAR is,
what are different SAR definitions and when to use them,
why SAR is important for MRI,
where to find SAR limits,
what are potential pitfalls when modelling SAR, and,
some strategies to consider to be able to reduce SAR in an MRI scan.

Acknowledgements

No acknowledgement found.

References

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Proc. Intl. Soc. Mag. Reson. Med. 32 (2024)