More on SAR and B1+rms

Firstly, thanks everyone who viewed or shared my online SAR and B1+rms calculator. Various comments made on the MRI Safety Facebook page made me realise further explanation is required.

RF induction and SAR in MRI, MRI from Picture to Proton
RF induction and SAR in MRI

The SAR calculator is based upon a similar equation to that shown above. This itself is a highly idealised approximation to reality, so the calculator will give you a qualitative understanding of how parameters interact. The equation above assumes that the body is a uniformly conducting sphere(! – r is the radius), that the B1 field is perfectly uniform and that the RF pulse waveform is rectangular with the same amplitude for every pulse in the sequence- obviously  all gross simplifications. The duty cycle D is the RF pulse length(t) times the number of pulses per TR divided by TR.

What happens if you enter the wrong patient weight?

Whatever patient weight (and height for some scanners) you enter, the scanner will adjust the RF transmit power to get the appropriate B1 that corresponds to the desired flip angle. This depends upon the “coil loading” – itself highly dependent upon body and coil geometry. The patient weight you enter will affect the scanner’s estimation of SAR (not the actual SAR). Given that SAR is RF power (watts) divided by weight (kg), too high a weight value entered will allow the RF transmitter to operate at higher power, giving the possibility of exceeding the actual SAR limit. Entering a patient weight that is too low will reduce the limit of RF power and may result in limitations to your scanning because the scanner thinks it is exceeding the SAR limit. As SAR is a key patient safety parameter, you should always enter the correct patient weight.

Why do some scanners make you enter patient height as well as weight?

This is because they use a slightly more realistic (but still very approximate) body model. This can improve the estimation of induced electric field and will change the estimated volume of tissue. The SAR and B1+ rms calculator only changes the latter. The advantages are that a better SAR estimation is achieved, but also that it may become slightly harder to hit the SAR limit, allowing more leeway in your scanning protocol.

If SAR increases with the square of B0, why does B1+rms not also change?

B1+rms is a time-weighted metric for the B1 field which determines the flip angle. The flip angle depends upon the gyromagnetic ratio (for protons 42.57 MHz/T), the RF pulse amplitude, waveform and duration. None of these are field or frequency dependent.

I hope these explanations aid your understanding of a complex topic, but bear in mind that the actual physics of RF power deposition is very complicated and that accurate estimations of SAR require sophisticated electro-magnetic modelling beyond the capabilities of the scanner’s SAR monitor (or this blog!)

Reference MRI from a Picture to Proton chapter 20: But is it safe?

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