Benefits and challenges of employing elevated acoustic output in diagnostic imaging

Abstract

The acoustic output levels used in diagnostic ultrasonic imaging in the US have been subject to a de facto limitation by guidelines established by the USFDA in 1976, for which no known bioeffects had been reported. These track-3 guidelines link the Mechanical Index (MI) and the Thermal Index (TI) to the maximum outputs as of May 28, 1976, through a linear derating process. Subsequently, new imaging technologies have been developed that employ unique beam sequences (e.g., harmonic imaging and ARFI imaging) which were not well developed when the current regulatory scheme was put in place, so neither the MI nor the TI takes them into account in an optimal manner. Additionally, there appears to be a large separation between the maxima in the track-3 guidelines and the acoustic output levels for which cavitation-based bioeffects are observed in tissues not known to contain gas bodies. In this presentation, we summarize the history of and the scientific basis for the MI, define an output regime and specify clinical applications under consideration for conditionally increased output (CIO), review the potential risks of CIO in this regime based upon existing scientific evidence, and summarize the evidence for the potential clinical benefits of CIO.