Synopsis

Keywords: Physics & Engineering: RF Safety

The introduction of multi-channel parallel transmit (pTx) systems to mitigate RF inhomogeneities in UHF-MRI has significantly increased the safety management complexity and SAR prediction concepts has to be extended to satisfy the new requirements. In pTx systems, the resulting SAR distribution depends on the particular RF pulses (time-dependent amplitude and phase) used in the sequence.

This lecture will focus on RF safety in pTx systems and give an overview of: RF monitoring techniques, SAR supervision using Q-matrices and VOPs, safety factors and validation techniques.

Abstract

The introduction of multi-channel parallel transmit (pTx) systems to mitigate the RF inhomogeneity problem in ultra-high field MRI has significantly increased safety management complexity, and SAR prediction concepts have to be extended to satisfy the new requirements.

In single-channel transmit systems, the power deposited at a given location scales directly with the total applied power. The absorbed RF energy can be assessed via directional couplers (P_forward – P_reflected) and, together with the exposed body mass, related to a global SAR aspect (e.g. whole-body). Via numerical simulations, a proportionality factor (k-factor) can be determined between local SAR and applied power, thereby enabling local SAR control.

In pTx systems, the total electric field at a given location is the linear superposition of the individual E-fields driven by the independent transmit channels. Thus, the resulting SAR distribution depends on the particular RF pulses (time-dependent amplitude and phase) used in the sequence. Again, measurements of the forward and reflected powers on each channel allows for a subject-based assessment of global SAR aspects, but local SAR assessment is more complex. According to the IEC norm, RF transmit coils with multiple channels can have attributes of both local and volume transmit coils. The appropriate control of SAR depends on the use of the coil.

The Q-matrix approach separates the spatial and temporal dependencies, resulting in a quadratic form for SAR calculation: SAR(r,t) = u'(t)*Q(r)*u(t). This formalism yields one Q-matrix for each voxel in a simulated body model. The number of Q-matrices for local SAR (>10⁶) can be compressed to return Virtual Observation Points (VOPs). This reduced set of VOP matrices drastically eases their incorporation into pulse design algorithms as well as enables real-time SAR supervision based on measured waveforms. The cost of compression is that a certain amount of SAR overestimation must be accepted.

Safety factors account for uncertainties in the monitoring hardware, for anatomical variability, and for modelling errors in the numerical simulation model. Since safety factors for modelling errors rely on the accuracy of the numerical simulations, validation techniques evaluate the error in simulation models. Validation measurements can be performed in the scanner or on the workbench. Validation techniques using the MR imaging system considering a similar setup to the in-vivo situation.

Content:
This educational session will give an introduction to SAR control during MRI exams, with a focus on multi-channel transmit systems, and give an overview of validation techniques.

SAR in single-channel systems

• Global and local SAR calculations

SAR in multi-channel (pTx) systems

• Generalised SAR models, Q-matrix formulism
• Virtual Observation Points (VOP) algorithms
• SAR-constrained RF pulse design

Validation methods

• B₁⁺ mapping
• Thermometry
• S-Parameter
• Direct E- and H-field mapping
• Direct temperature measurements

Acknowledgements

No acknowledgement found.