Figure 4.

Imaging results of the phantom with an optical thickness of 37.5. (a) Illustration of the phantom on a YZ plane showing the plane of the absorbing objects, indicated by the gray line, buried in the middle of the phantom. (b) Cross-section of the phantom on the central XY plane. The two absorbing inclusions (Obj 1 and Obj 2) had similar dimensions: x = 1.5 mm, y = 6.0 mm, and z = 0.8 mm, while the whole sample's dimensions were x = y = 60.0 mm and z = 7.5 mm. (c) Comparison of normalized UOT and TRUE images of the sample with the 3.5 MHz transducer. The fitted TRUE image coincides with the fitted UOT² image, which is computed by the mathematic square of the UOT image signal strength. The coefficients of determination R² of the TRUE 3.5, UOT 3.5, and UOT² 3.5 fits are 0.95, 0.93, and 0.90, respectively. Again, the results verify that the TRUE signal is proportional to the square of the UOT signal (UOT²), consistent with the square law⁴. (d) Normalized TRUE images with the 3.5 (black cross) and 1.0 (green circles) MHz transducers. The R² value of the 1.0 MHz fit is 0.90. In (c) and (d), the symbols represent experimental data, while the solid curves represent Gaussian fits.