Effect of RF Pulse Sequence on Temperature Elevation for a Given Time-Average SAR.

In calculations of temperature increase during MRI, it is typically assumed adequate to consider the Specific energy Absorption Rate (SAR) levels averaged over an entire repetition time (TR) rather than explicitly consider the heating (as it occurs in reality) during the RF pulses only. Here we investigate this assumption with numerical calculations of SAR and temperature increase for a human head in a volume coil at 64 MHz and 300 MHz during three very different pulse sequences, each having a TR of 200 ms and a time-average whole-head SAR of 3.0W/kg, as well as with semi-analytical calculations considering a gradient-echo sequence in a segment of tissue with SAR of 10W/kg delivered in a 1ms pulse with TR of up to 5000 ms. While it is possible to calculate a temporal effect of specific pulse sequence on temperature, the difference between pulse sequences is so small and so transient that it should typically be adequate to consider only the time-average SAR in each TR.