Models and regulatory considerations for transient temperature rise during diagnostic ultrasound pulses

A new diagnostic ultrasound (US) technique, sometimes called radiation force imaging, produces and detects motion in solid tissue or acoustic streaming in fluids via a high-intensity beam. Current models for estimating temperature rise during US exposure calculate the steady-state rise, using time-averaged acoustic output, as the worst case for safety consideration. Although valid for very short pulses, this analysis might not correspond to a worst-case scenario for the longer pulses or pulse bursts, up to hundreds of ms, used by this newer method. Models are presented to calculate the transient temperature rise from these pulse bursts for both the bone at focus and soft tissue situation. It is shown, based on accepted time-temperature dose criteria, that, for the bone at focus case and pulse lengths and intensities utilized by these methods, temperature may increase to levels that raise safety concerns. Also, regulatory aspects of this modality are analyzed in terms of the current FDA acoustic output limits for diagnostic US devices.