Optimising the illumination geometry of a clinical reflection mode photoacoustic scanner

Abstract

Clinical photoacoustic (PA) imaging relies on illuminating objects at depth. To do this, it is important to optimise the illumination geometry with respect to the sensitivity pattern of the acoustic receiver, taking optical scattering into account. The three-dimensional point spread function (3D PSF) measured at various depths as a function of the optimisation variables, is being explored to determine its usefulness for this purpose. The 3D PSF of a reflection mode photoacoustic scanner was measured by acquiring a series of PA images of the tip of a 0.25mm radius graphite rod placed at a depth of 2 cm, by translating the photoacoustic linear array transducer and illumination optics in the elevational direction. This was done for a series of angles and separations of the fibre optic illuminators, for a background medium of 1% intralipid, which simulates, to first order, the optical scattering that would be experienced in tissue. The background noise was found to be influenced by the illumination geometry, and may have been associated with PA clutter generated by absorption in the background medium. The angle of illumination and distance separating fibre optic illuminators were found to be weakly optimum at around 76 degrees and 15.5mm respectively, where the PSF amplitude passed through a weak maximum. As expected, the shape of the 3D PSF was found to be independent of illumination geometry. However, the combination of using the tip of a graphite rod as a point object, and plotting the 3D PSF as a means of locating the peak signal, appears to be a successful method of studying the effect of illumination variables on signal strength. Ultimately when complete, this optimisation should enable the clarity images at the depth of interest to be maximised.