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This research investigated anisotropic scattering of ultrasonic and photoacoustic waves from tissues consisting of
transverse isotropic structures. Anisotropic scattering refers to the systematic variation in acoustic scattered energy. Take
tendon as an example, the maximum occurs when the arrangement of the transducer and fiber orientation is
perpendicular and minimum occurs when the arrangement is parallel. Experimental results indicate the apparent
integrated backscatter (AIB), which is widely adopted to compute the scattered energy, for photoacoustic as well as
ultrasonic waves decayed as the arrangement changed from perpendicular to parallel. The AIB decrement using
transducers with center frequency of 3.5 MHz, 5 MHz, and 20 MHz were 10.50 dB, 18.01 dB, and 20.98 dB,
respectively. Photoacoustic AIB decrement detected by transducers with center frequency of 3.5 MHz, 5 MHz, and 20
MHz were 7.63 dB, 15.54 dB, and 17.76 dB, respectively. It is shown that higher detection frequency resulted in a larger
decrement. A hypothesis is proposed to explain why photoacoustic waves are less affected by the fibrous tissue. In
ultrasonic scattering, incident direction for each scatterer were similar due to the relatively planar wavefront, hence the
signal amplitudes scattered at the transducer direction are also similar. In photoacoustic scattering, the spherical-like
wavefront causes different incident directions for different scatterers, therefore the variation of the signal amplitude
collected by the transducer increases, resulting in a lower correlation with the microstructure. In addition, the decrement
of backscattered energy decreased for a single scatterer when the incident wave was spherical. Experimental and
simulation results verified the hypothesis. The discovery implies that photoacoustic imaging has the potential to detect
tissues with transverse isotropic structure that may be overlooked by conventional ultrasound imaging.
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