Introduction

Two widely used Whole Body (WB) SAR assessment methods (pulse energy method and calorimetry method) are described in NEMA standard MS-8¹, but only the pulse energy method can be used with a human subject. The critical aspect is current sensing flux loops placement near the Tx coil to measure coil power loss, which varies in different imaging conditions. RF power deposition in the subject P_sub is calculated as:
P_sub = P_in – P_{CPL_loaded} [1]
where P_in is Tx coil “net” input power, i.e. forward power minus reflected power, and P_{CPL_loaded} is
coil power loss with the presence of test subject. Furthermore,
P_{CPL_loaded} = P_{CPL_unloaded} x (V_sub/V_unloaded)² [2]
where V_sub and V_unloaded are B1 field induced voltage in the flux loop measured with the test subject and a lossless phantom respectively, and P_{CPL_unloaded} is the Tx coil net input power measured with the lossless phantom. The standard recommends using a sufficient number of flux loops distributed evenly to get a balanced sample of the coil currents. In this case, the sum of the squared voltage values from all flux loops need to be calculated in Equation [2], assuming all flux loops have same sensitivity:
V_sub² (with test subject) or V_unloaded² (with the lossless phantom)= ∑ V_{flux loop}² [3]
We use numerical EM modeling to show that using multiple flux loops evenly distributed around the birdcage coil and spaced from endring may not give accurate results. Instead, single flux loop at a strategically selected location can give more accurate results.

Methods

Numerical FDTD EM modeling was performed with a generic 3T WB Tx coil (16-rung high-pass birdcage, diameter 750mm, length 650mm. RF shield diameter 790mm, length 850mm.^2,3) with Sim4Life (v4.4, ZMT, Zurich, Switzerland). The coil was tuned to 128MHz for 3T MRI and driven with two voltage sources located on one end ring. Sixteen square flux loops (Figure 1) were positioned near the coil endring (Figure 2), per MS-8. The flux loops form a circular loop with 730mm diameter. Visible Man⁴ was modeled with three imaging conditions (head, abdomen, and foot), and the following computed: power deposition in the subject, coil input power and induced voltage in all flux loops recorded by 50 Ohm resistors.

Results

Table 1 shows the differences between the RF power deposition in Visible Man and the value calculated based on MS-8, using all 16 flux loops and Equations [1]-[3]. RF power deposition in Visible Man is also compared with values calculated with each individual flux loop’s voltage using Equation [1], assuming each flux loop provides a separate measurement (Figure 3).

Discussion

Table 1 shows that based on MS-8, pulse energy measurement with a number of flux loops spaced from birdcage coil’s endring and evenly distributed around the coil (all with same sensitivity) did not give accurate power deposition in Visible Man for the three imaging landmarks investigated. The error ranges from 14.7% underestimate for abdomen and 41.0% overestimate for head. We did not model flux loops placed between coil rungs in the center axial slice. Experiment setup like the modeling presented here with all flux loops outside coil imaging volume is more advantageous at reducing cable (used to transmit flux loop voltage signal) exposure to Tx coil fields and resultant inference with the measurement. It can also help reduce potential interaction between human subject (especially big subjects) and flux loops if the loops are placed at positions 4, 5, 13, and 14 in Figures 2 and 3 (i.e. 3 and 9 o’clock). For the three landmarks investigated, although gray curves in Figure 3 had different patterns, flux loops placed at side of the bore (e.g. 3 and 9 o’clock locations) generally give better results than other locations.

Conclusion

We used numerical EM modeling to investigate the impact of current sensing flux loop’s location on pulse energy SAR measurement accuracy. Results obtained with a human model at three imaging landmark positions suggest that a single flux loop placed at 3 or 9 o’clock and spaced from WB birdcage coil’s endring may give more accurate results.