Investigation of the planar reflector substitution method using the finite element method

Abstract

The backscatter coefficient (BSC) has been shown to be an indicator of tissue state in assessing tumour response to therapy, but it is limited in its clinical applicability by the difficulty in acquiring the appropriate reference spectrum. The accuracy of a BSC estimate made by the substitution method is influenced by the quality of the reference spectrum, which normalises the measurement to the source’s diffraction field and spectral characteristics. Among the reference objects employed for normalisation, a planar reflector has been popular. With the sample region at the focus of a focused source or the last axial maximum of an unfocused source, the reflector has been placed either at this depth or half this depth by different authors. This work explores the effect of planar reflector positioning on BSC estimation. FE models were constructed to simulate BSC measurements of virtual phantoms with various sources, and the estimates compared to scattering theory. The results indicate that unfocused sources provide more accurate BSC estimates when the reflector is placed at half the distance to the last axial maximum (d), while focused sources provide better estimates with the planar reflector at the focal plane, and that focal-plane reflector positioning is preferable for source radii-of-curvature ≤ d.