Regional sensitivity comparison between optical and acousto-optic sensing

Abstract

Optical systems based on near infrared spectroscopy probe biological tissue oxygenation of a relatively large region. The acousto-optic (AO) method can tag photons by focused ultrasound in a region of interest within the tissue for potential localized oxygenation sensing. This study aimed to compare the regional sensitivity between the optical and AO sensing techniques. The regional sensitivity was defined as the amount of change observed in the measured signal in response to a small localized change in the optical absorption. In both reflection and transmission configurations, optical systems based on a single source and detector have been shown to be more sensitive to the absorption variation in the superficial region. The results demonstrate that the AO sensitivity region depends on the location of the ultrasound focal region as well as the distribution of the optical fluence. In the transmission mode, the optimal AO sensitivity region has been found to be in the ultrasound focal region. In the reflection mode, however, the optimal sensitivity region of AO does not always coincide with the location of the ultrasound focal region. Instead, the AO measurement is also sensitive to the local absorption change in regions between the ultrasound focal region and the optical probes. In general, the AO method can probe deeper into the phantom as compared to the optical measurements.