Part IV examines frequency-domain photon diffusion in a homogeneous medium enclosed by a “concave” circular cylindrical applicator or enclosing a “convex” circular cylindrical applicator, both geometries being infinite in the longitudinal dimension. The aim is to assess by analogical and finite-element methods the changes of AC amplitude, modulation depth, and phase with respect to the line-of-sight source–detector distance for a source and a detector located along the azimuthal or longitudinal direction on the concave or convex medium–applicator interface. By comparing to their counterparts along a straight line on a semi-infinite medium–applicator interface, for the same line-of-sight source–detector distance, it is found that: (1) the decay-rate of AC photon fluence is smaller along the azimuthal direction and greater along the longitudinal direction on the concave interface, (2) the decay-rate of AC photon fluence is greater along the azimuthal direction and smaller along the longitudinal direction on the convex interface, (3) the modulation depth along both azimuthal and longitudinal directions decays more slowly on the concave interface and faster on the convex interface, and (4) the phase along both azimuthal and longitudinal directions increases more slowly on the concave interface and faster on the convex interface.
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