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An estimation method for maximum local SAR with evaluation points selected by iterative SAR calculation under a restricted random condition
Takafumi Ohishi1 and Koji Akita2

1Research and Development Center, Canon Medical Systems, Yokohama-shi, Japan, 2Toshiba, Kawasaki, Japan

Synopsis

This paper proposes an estimation method for maximum local SAR in parallel transmission. In order to estimate maximum local SAR with fewer evaluation points, the evaluation points are selected by iterative SAR calculation under a restricted random initial input signal condition. It is shown that, using wider initial phase range of the input signal in selecting the evaluation points than that supposed in MRI scanning, maximum local SAR is estimated within desired tolerance with 10 evaluation points which are about 1/30 of those selected by a conventional method.

Introduction

While the RF frequency is high and the wavelength is short in high field MRI, local specific absorption rate (SAR) increases at several places in a human body. Therefore, appropriate local SAR control is important especially in parallel transmission (pTx). SAR calculation1,2 is necessary for the local SAR control, but it takes much time. There is a method to estimate the maximum local SAR with the local SAR evaluation points selected by the clustering algorithm.3 This method has the possibility to have more local SAR evaluation points than a method considering human body shape in selecting local SAR evaluation points.

Methods

Figure 1 shows local SAR evaluation point number with maximum local SAR extracted by iterative SAR calculation with human model Duke (Sime4Life, ZMT, Zurich, Switzerland) while randomly changing the initial input condition at every eight feeding points of a whole-body coil. The maximum local SAR occurs repeatedly in particular points. In consideration of this result, we propose a maximum local SAR estimation method with the evaluation points selected by iterative SAR calculation under a restricted random input signal condition to estimate the maximum local SAR with fewer evaluation points. Under the restricted random condition, the input signal amplitude is set within 0.5 to 1.0, 0.7 to 1.0 or 0.9 to 1.0, and the initial input signal phase is set within θn - 30 to θn + 30 degrees, θn - 60 to θn + 60 degrees or θn - 90 to θn + 90 degrees, where θn is the fixed value at each eight input ports and θ1 = 0, θ2 = 45, … , θ8 = 315. The local SAR evaluation points are selected as follows.

Step1: Set the range of input signal amplitude and initial phase, the total number of iterative SAR calculation and the overestimation value.

Step2: At first SAR calculation, the evaluation point with maximum local SAR is selected as an evaluation point for estimating maximum local SAR.

Step3: In the second and subsequent SAR calculations, the maximum local SAR is compared with the estimation value which is the sum of the overestimation value and the maximum value among the local SAR of evaluation points selected on previous calculations. If the maximum local SAR is higher than estimation value, the evaluation point with the maximum local SAR is selected.

Step4: End when the trial number of SAR calculation reaches the total number.

In each SAR calculation in Step 2 and Step 3, the amplitude and initial phase are randomly set to a different value within the range set at Step 1. The overestimation value is 10 % of the maximum local SAR in the worst case, where the electric fields generated by the input signals to eight ports are combined with the same phase.

Results and Discussion

Figure 2 shows the maximum local SAR estimation results after 5000 trials using the amplitude range as a parameter. Dots on the diagonal reflect that the actual maximum local SAR and the estimated maximum local SAR are equal, while dots below the diagonal mean that the maximum local SAR is underestimated. Dots between the diagonal and the upper line indicates that the maximum local SAR is estimated within the tolerance. From these results, it is found that all estimation values of maximum local SAR are within a tolerance determined by the overestimation value. It also can be seen that the amplitude range when selecting the evaluation point may be narrower than that of the supposed input signal.

Figure 3 shows the maximum local SAR estimation results after 5000 trials using the initial phase range as a parameter. There are many underestimation results when the initial phase range in selecting the evaluation point is narrower than that of the supposed input signal. From these results, the remitted random condition in selecting the maximum local evaluation points should be a wider initial phase range than that of the supposed input signal.

Figure 4 shows maximum local SAR estimation results where the restricted random condition in selecting maximum local SAR evaluation points is a wider initial phase range than that of the supposed input signal. All estimated maximum local SAR is within a desired tolerance. The total number of selected evaluation points is 10 while the total number of evaluation points using conventional method under the same condition is 323.

Conclusion

This proposed method can estimate maximum local SAR within desired tolerance with fewer evaluation points than the conventional method.

Acknowledgements

No acknowledgement found.

References

[1] O.P. Gandhi et al. Specific absorption rates and induced current densities for an anatomy-based model of the human for exposure to time-varying magnetic fields of MRI. Magnetic Resonance in Medicine. 1999;41(4):816-823

[2] C. M. Collins et al. Spatial Resolution of Numerical Models of Man and Calculated Specific Absorption Rate Using the FDTD Method: A Study at 64 MHz in a Magnetic Resonance Imaging Coil. J Magn Reson Imaging 2003;18(3):383–388.

[3] G. Eichfelder and M. Gebhardt, Local Specific Absorption Rate Control for Parallel Transmission by Virtual Observation Points, Magnetic Resonance in Medicine 2011; 66: 1468–1476.

Figures

Figure 1: The results of SAR calculation while randomly changing the initial input signal condition at each eight input ports of a whole-body coil. Each dot in fig. 1 indicates the SAR calculation trial number and local SAR evaluation point number.

Figure 2: Maximum local SAR estimation results using the amplitude range of input signal supposed in MRI scanning and amplitude range of input signal in selecting the maximum local evaluation points. This figure shows the actual maximum local SAR versus the estimated maximum local SAR.

Figure 3: Maximum local SAR estimation results using the initial phase range of the input signal supposed in MRI scanning and the initial phase range of the input signal in selecting the maximum local evaluation points. This figure shows the actual maximum local SAR versus the estimated maximum local SAR.

Figure 4: Maximum local SAR estimation results. The initial phase range of the input signal in selecting maximum local SAR evaluation points is θn -120 to θn +120. The initial phase range of input signal supposed in MRI scanning is (a) θn -15 to θn +15, (b) θn -30 to θn +30, (c) θn -45 to θn +45, (d) θn -60 to θn +60, (e) θn -75 to θn +75 or (f) θn -90 to θn +90. The amplitude range both in selecting maximum local SAR evaluation points and in MRI scanning is 0.9 to 1.0.

Proc. Intl. Soc. Mag. Reson. Med. 27 (2019)
4159