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Illumination Optimisation for Freehand Reflection-Mode Photoacoustic Imaging

DD Birtill, M Jaeger, N Bush, A Gertsch, JC Bamber, in collaboration with M Frenz, University of Bern, Switzerland; M Fournelle, R Lemor, Fraunhofer Institute for Biomedical Technology, Saarbrucken, Germany; L Masotti, El.En. SpA, Florence, Italy; H Wittig, tp21 GmbH, Saarbrucken, Germany

Source of funding: Institute of Cancer Research, CRUK, EPSRC, EU FP6 Marie-Curie, Zonare Medical Systems Inc

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 3D 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 (Figure 10). The 3D PSF of a reflection mode PA 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 PA scan head in the elevation 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 optical scattering in tissue. The background noise was found to be in influenced by the illumination geometry, and may have been associated with PA clutter generated by absorption in the background medium. The angle of, and distance separating, the 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. 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.

Fig.10.  (a) Clinical scan head, and the adjustable illumination variables. (b) Apparatus for PA imaging and scanning the graphite rod tip for point spread function (PSF) measurement. (c) Orthogonal slices through a measured 3D radiofrequency PA PSF, intersecting at the peak-amplitude voxel (top-left: axial-lateral plane, top-right: lateral-elevational plane, ). Also shown is the axial (A-line) profile through the peak voxel (bottom right).