High resolution light diffraction tomography: nearfield measurements of 10 MHz continuous wave ultrasound

Light diffraction tomography is an ultrasound measurement method that offers possibilities to make high spatial resolution measurements. The aim of this study was to investigate the spatial resolution of light diffraction tomography by characterizing the complex pressure and phase patterns 0.5-1 mm from the surface of a 5 mm diameter 10 MHz ultrasound transducer. Three different transducers were measured with a detection width of 5 microm, sample distances of 20-40 microm and 50-100 projections/180 degrees. They were excited with a 10 Vpp CW-signal. To verify the results the optical measurements were compared with 75 microm diameter hydrophone measurements and with theoretical calculations. The light diffraction tomography results show very good agreement with the hydrophone measurements and pressure peaks separated 160 microm were resolved. Comparison with the theoretical calculation shows that small distortions, caused by defects in the matching layer or in the piezo-electric disc, disturb the symmetric ring-pattern characteristic of an ideal piston source.