Implementation and Clinical Evaluation of a Fetal ALARA Ultrasound System
The US Food and Drug Administration (FDA) provides guidelines for acoustic output to ensure that ultrasound remains a safe imaging modality used widely in applications such as obstetrics. However, even within these limits, additional acoustic output that does not lead to improved image quality represents an unnecessary risk to patient safety. Ultrasound manufacturers and professional societies advise users to observe the ALARA (As Low As Reasonably Achievable) principle with regard to acoustic exposure, but studies have shown that most ultrasound users do not adjust the transmit power. In this study, an adaptive ultrasound tool was implemented to automatically adjust acoustic exposure in response to real-time image quality feedback based on lag-one coherence (LOC). LOC is the average correlation between backscattered echoes received on neighboring array elements. Previous work has shown that LOC is predictive of local signal-to-noise ratio and sensitive to incoherent acoustic clutter and temporally-incoherent noise. LOC as a function of MI was assessed in seven volunteers. While imaging the placenta or the fetal abdomen, the system swept through 18 transmit voltages that correspond to MIs ranging from 0.08 to 1.21 and automatically selected the optimum MI based on reaching 95% of the maximum LOC. In this study, the optimal MI values were between 0.48 and 0.89, depending on the acoustic window. For all acquisitions, we saw a steady increase that approached an asymptote in LOC with increasing MI. Contrast, contrast-to-noise ratio (CNR), and LOC followed similar trends. These results suggest that maximum image quality can be achieved with acoustic output levels lower than the FDA limits and an automated tool can be employed in real-time to find this optimal MI for specific imaging conditions. Our results support the feasibility of an automated, LOC-based implementation of the ALARA principle with regard to acoustic exposure for obstetric ultrasound.
