This study is an investigation of the effects of hand position and frequency on SAR and temperature rise due to cellular phone use. The study presents calculations of SAR and temperature rise in both head and hand tissue for a range of frequencies and hand positions.

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VariPose^® will be used to move the right hand into four possible positions for holding cellular telephones near the ear as part of a study on the impact of hand position on the SAR and temperature rise caused by the phones. The phones in the hands will be simulated at four frequencies with simplified telephone models appropriate for each frequency. The results for each case will be compared to simulations of the phone by the head without any hand present.

The study will only consider the head and neck and a portion of the right arm starting from just above the wrist to the hand. The fingers and wrist of the hand will be positioned but the arm will not be repositioned to put the hand near the head since this can be performed easily in the XFdtd^® software. The skeleton in VariPose was adjusted as shown in Figure 1 to put the hand into four reasonable positions for holding a phone.

Following the execution of the VariPose program, the repositioned hand may be cropped from the body and saved as an object for loading into XFdtd. In Figure 2, one of the repositioned hands is shown. In Figure 3 the internal tissues in one of the repositioned hands can be seen in this view taken from VariPose.

In Figures 4 and 5, the four hand positions are shown holding a simplified cellular telephone. The positions are labeled one through four starting at the upper left of Figure 4. Position One has the hand grasping the phone between the thumb and the index finger. Position Two places the phone between the fingers and the base of the palm with the thumb tucked behind the phone. Position Three has the thumb on one side of the phone, slightly extended above the top of the phone body, with two fingers on the opposite side of the phone. Finally, Position Four has the phone gripped between the thumb and three fingers.

The phones and hands are placed next to the head so that the position of the phone remains constant for each hand position. In some cases the fingers touch the skin of the face. Examples of the different phones are shown in Figure 6-Figure 8 with hand position One. The phones operate at frequencies of 450MHz, 900MHz, 1800MHz, and 2450MHz. Geometries are also set up with the head and phone without the hand to gauge the impact of the presence of the hand

The tissue material parameters and thermal properties are set as appropriate for the four different frequencies. The SAR and temperature rise will be saved for each case. The projects are then simulated using the XFdtd software. The temperature rise computation used in XFdtd is based on Penne’s bio-heat equation as developed in

C. M. Collins et. al., “Temperature and SAR Calculations for a Human Head Within Volume and Surface Coils at 64 and 300 MHz,” J. Magnetic Resonance Imaging, vol. 19, pp. 650–656, 2004

The peak values of SAR and temperature rise are displayed as field images and comparative graphs in Figure 10 through Figure 25. See the coordinate reference guide in Figure 9 for a description of the image plane locations.

At 450MHz the variation between the different hand positions is less than 20% for SAR and temperature rise and the location of the peaks is fairly constant. The peak Average SAR is always located in the cerebro-spinal fluid (csf) near the brain while the peak temperature rise is always in the muscle of the jaw.

At 900MHz the hand position has some impact on the peak values and location. The average SAR peak values show significant variation depending on hand position – generally increasing with the hand present. The peak temperature rise is usually in the head and slightly higher with the hand present.

High 1g Average SAR values can occur in the hand at 1800MHz. In the head, the SAR values are only slightly higher or lower with the hand present versus the head alone. Temperature rise results follow the same trend as the SAR. At this frequency the results are much more sensitive to the hand position.

At 2450MHz the SAR and temperature rise are significantly higher in the hand than the head. There are wide variations in the peak values depending on the hand position. The SAR and temperature rise in the head are lower when the hand is present versus the head alone.

Conclusion

At low frequencies, the presence and position of the hand has little impact on SAR and temperature rise position or values. As the frequency increases, the position of the hand has a larger impact on the location and value of the peak temperature rise. Finally, at high frequencies, the peak values in the head decrease as more energy is absorbed by the hand and the peak is always located in hand.