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Derivation of fshim values for 2 channel MultiTransmit independent RF waveforms Disclosure Number: IPCOM000236214D
Publication Date: 2014-Apr-11
Document File: 5 page(s) / 178K

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The Prior Art Database



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Derivation of fshim values for 2 channel MultiTransmit independent RF waveforms


Evaluation of SAR for RF pulses with independent waveforms on two transmit channels (MultiTransmit) normally would require a full numerical evaluation of the coil and body model with the RF waveform. This is a serious computing challenge and hard to validate (from safety and compliance perspective). It is proposed to evaluate SAR by decomposing the RF pulse shape in a series of block pulses that can be considered as RF shimmed. The SAR for each individual shimmed block pulse (RF sample) can be retrieved from the validated SAR look up tables (LUT) used to assess SAR of a full RF pulse for the MultiTransmit product software. Assessing the LUT for each RF sample is much faster than a full FDTD simulation, and uses validated SAR safety values. The SAR per RF pulse is the B1+ weighted average over the samples in the independent waveform RF pulse.

For block shaped or simple sinc-shaped RF pulses the SAR can be expressed in terms of multiplication factors. In the event of a shimmed RF pulses in which different ports of the RF transmission coil are activated at adjustable relative phase and relative amplitudes, the shimming can be taken into account accurately in a further effective multiplier. In a more advanced approach, the RF waveform can be more complicated and built-up as successive block pulses that each may have a different RF shim (relative phases and amplitudes for the respective driving ports. The novel insight is that also for these more complicated RF shimmed pulses, the SAR may be expressed in the same manner, only the effective multiplier taking the shimming into account needs to be adapted. Accordingly, the SAR computation algorithm needs only marginal adaptation to be applied to the more complicated shimmed RF pulses.


The fshim formalism is extended from basic RF shimming to RF pulses with independent waveforms on the two RF channels. See Figure 1 for a conceptual overview of the approach to derive SAR for (shaped) RF pulses, shimmed RF pulses, and RF pulses with independent waveforms. The latter are considered as a time series of RF shimmed block pulses. This approach allows to maximally re/use existing SW code and GOAL-C RF object implementations, by extending the concept of (shimmed) RF pulses to replace fshim by the fpulse, while building on the proven safety concept of the relative SAR LUT introduced for TX systems.

Figure 1. Overview of the concepts to evaluate SAR for RF pulse shapes with increasing degrees of freedom.

The general waveform (4) is shown with a reduced number of samples to improve legibility. fpulse is

derived from RF waveform attributes B1+

sample, polarization Psample, and B1+, all supplied by the RF Pulse


Note: B1+

        sample is defined as the average B1+ in the slice (like for global RF shimming), while B1+ (= RF`name:b1) is defined to deliver the target flip angle in the target FOX (field...