Calculating SAR and Temperature Change

By David Carpenter,

in Human Tissue Using the FDTD Method

Remcom Inc.

Introduction

The effect of RF radiation from cell phones, implanted medical transmitters, and MRI systems is

a subject of ongoing concern for manufacturers, regulatory bodies, and users of such devices. RF

energy can cause heating in the tissue, which is quantified as the specific absorption rate (SAR) of

the electromagnetic energy and the resulting rise in temperature in the tissue. For product design-

ers, it has become standard practice to calculate the SAR of new devices to ensure that they gener-

ate a level below the limits set by the IEEE and other regulatory bodies.

The FDTD (finite-difference time-domain) modeling technique is the preferred method for mak-

ing SAR calculations. This differential formulation allows users to divide the model space into

very small cells, which provides excellent resolution of tissue in the human body. Other computa-

tional methods are unable to model the tissue structure to a resolution that is accurate enough for

such calculations.

Remcom's XFDTD is an FDTD-based 3-D electromagnetic solver that is used by manufacturers of

cell phones, implantable RF transmitters, and MRI devices to compute SAR values in their new

products. In addition to SAR measurement capabilities, the Bio-Pro version of XFDTD also features

an optional temperature rise module, which allows users to determine the effect of the SAR on

human tissue.

To complement XFDTD, Remcom also offers HiFi human and animal models and the VariPose

modeling package. With VariPose, the human body can be repositioned to produce an FDTD hu-

man body mesh model. Tissue resolutions of 10, 5, 3, 2, and 1 mm are available from the VariPose

models. XFDTD is then capable of discretizing selected areas (or the whole model space) into

smaller cell sizes by use of adaptive meshing.

XFDTD can also be used to determine the performance of an antenna under design, both as a

stand-alone device and in situ in the human body. Antenna efficiency, VSWR, S-parameters, im-

pedance, far-zone gain pattern, near-zone fields, partial patterns, and antenna diversity are some

of the measurements available to antenna designers.

Implanted RF Device Example

The following is an example of how Remcom's XFDTD software can be used to compute the fields

and SAR effects from an implanted device in a human body. In this example, XFDTD is used to

compute the input impedance, radiation gain pattern, and SAR of a patch antenna embedded

inside a human body. The patch antenna is 19.2mm (across-chest direction) by 32 mm (head-to-

foot direction) with a substrate of lossless dielectric of permittivity 9.5. The patch is 2 mm thick

with the ground plane and patch radiator the same dimensions. The patch feed is offset from the

center of the patch in the vertical (long) dimension.

Initial calculations are for the patch antenna in free space. In anticipation of using a 2 mm hu-

man body mesh, the patch is initially meshed with 2 mm FDTD cells. A slice through the mesh

showing the feed location as a green dot is displayed in Figure 1. All calculations are at 2.45 GHz.

For more information go to www.remcom.com