DRAFT Recommended Practice for Measurements and ...

1/29/98 127 C95.3-1991 Revision — 2 nd Draft
10/98 Draft
provide a discrete set of values, at a given distance apart (resolution), and hence, the
calculation of 1-gram averaged SARs leads to the same difficulty, particularly if
anatomically-detailed phantoms are used. If regularly-shaped models are used the
problem of calculating the 1-gram SAR is lessened, but then the effects of the shape and
properties of the ear and other features, which definitely affect the SAR distribution, are
not considered.
D5.8 Generalized Multipole Technique (GMT).
During the 1980’s several groups developed what were later unified under the name
generalized multipole technique (GMT) [Ludwig, 1989]. GMT refers to methods which
approximate the unknown field in each domain by several sets of functions which, in
contrast to the method of moments, do not have singularities within their respective
domains or their boundaries.
The expansions are matched at discrete points on the boundary of the domains resulting
in an overdetermined system of equations with a dense matrix. The overdetermination
factor is typically between 2 and 10. The system is solved in the least squares sense,
usually with QR-factorization methods [Golub, et al., 1989].
Since the global expansion functions of the GMT are very smooth at the boundaries, the
accuracy close to the boundaries is very high, which is important for dosimetry
applications. The greatest advantage of the GMT, however, lies in the fact that the
residual errors resulting from the least squares technique can be employed to validate
the quality of the results [Kuster, 1992a]. Since the largest errors usually occur at the
boundaries, the accuracy of the entire solution can be precisely determined [Regli, 1993].
The GMT, therefore, leads to very reliable dosimetric assessments. Since the method is
closely related to other analytical methods, accurate simulation of scattering problems
ranging over many orders of magnitude in fields strength are possible.
The severe limitation of the GMT is the difficulty involved in simulating real-world
applications. In contrast to the method of moments, in which sequential basis functions
are equivalent to a compact current, a GMT expansion is equivalent to a current
distribution over the whole boundary of the domain. For geometrically complex bodies,
the selection and location of the origin of the expansion functions is not quite obvious and
requires considerable expertise.
The method is described in detail in [Hafner,1990]. Commercial software based on the
GMT is commercially available, including a graphic interface for the PC. The code has
been successfully applied to dosimetric studies [Kuster, 1992a, and Kuster, 1993], and to
antenna design [Tay and Kuster, 1994].
D5.9 Impedance Method.
To obtain a detailed view of the power deposition pattern resulting from time-varying
magnetic fields used in hyperthermia, a method of modeling portions of the human body
using an impedance network has been developed [Gandhi, et al., 1984]. The region of
interest is subdivided into a number of cells, each of which is then replaced by an
equivalent impedance, and currents induced in the resulting network due to the
prescribed magnetic filed are found by the application of circuit theory. This approach
allows very fine modeling of inhomogeneities in the human body, with cell sizes of 0.5 cm
or smaller possible. In addition, the individual cells are assumed to have anisotropic
electrical properties, and this allows accurate modeling of interfaces.
Copyright © 1998 IEEE. All rights reserved. This is an unapproved IEEE Standards Draft,
subject to change.