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A Green's function approach to local rf heating in interventional MRI Christopher J. Yeung and Ergin Atalara)

Summary: A Green's function approach to local rf heating in interventional MRI
Christopher J. Yeung and Ergin Atalara)
Departments of Biomedical Engineering and Radiology, Johns Hopkins University School of Medicine,
Baltimore, Maryland 21205
Received 28 August 2000; accepted for publication 9 February 2001
Current safety regulations for local radiofrequency rf heating, developed for externally positioned
rf coils, may not be suitable for internal rf coils that are being increasingly used in interventional
MRI. This work presents a two-step model for rf heating in an interventional MRI setting: 1 the
spatial distribution of power in the sample from the rf pulse Maxwell's equations ; and 2 the
transformation of that power to temperature change according to thermal conduction and tissue
perfusion tissue bioheat equation . The tissue bioheat equation is approximated as a linear, shift-
invariant system in the case of local rf heating and is fully characterized by its Green's function.
Expected temperature distributions are calculated by convolving averaging transmit coil specific
absorption rate SAR distributions with the Green's function. When the input SAR distribution is
relatively slowly varying in space, as is the case with excitation by external rf coils, the choice of
averaging methods makes virtually no difference on the expected heating as measured by tempera-
ture change ( T). However, for highly localized SAR distributions, such as those encountered with
internal coils in interventional MRI, the Green's function method predicts heating that is signifi-
cantly different from the averaging method in current regulations. In our opinion, the Green's
function method is a better predictor since it is based on a physiological model. The Green's


Source: Atalar, Ergin - Department of Electrical and Electronics Engineering, Bilkent University


Collections: Engineering; Biology and Medicine