Therapeutic potential metric for diagnostic transducers
The goal of this work is to develop a 'therapeutic potential' metric and experimental protocol to define the expected efficacy of commercial, diagnostic transducers for thermal therapy. Temperature rises at the transducer face were measured using thin-film thermocouples (TFTs) on a tissue mimicking phantom for sequences with moderate acoustic output as an indicator of the risk of damage to the transducer during therapeutic applications. Several measurements were then taken to evaluate the focal heating of the transducer and compared with thermocouple measurements. The acoustic power was measured near the surface of the transducer. Spatial peak and spatial average intensities were measured in the focal plane at low system voltages to avoid nonlinear effects. Finally, ARFI imaging displacements within ±25% of the focus in a tissue-mimicking phantom were evaluated. Four transducers were compared using these protocols (two curvilinear arrays, one phased array, and one two-dimensional array). Without focal gain considerations, acoustic power is an inaccurate predictor of focal heating. ARFI displacement cannot easily be used to estimate focal heating, primarily due to confounding mechanical phenomena. After normalizing each measurement by the ultrasound system input, measurement device properties, focal configuration, and temporal properties, the therapeutic potential metric can most efficiently and accurately be defined by maximizing the ratio of the spatial peak intensity at the focus to the heating at the transducer face. The ratio of spatial average intensity or temperature at the focus to surface temperature can be used as alternative or additional metrics. The ID, phased array was determined to have the highest therapeutic potential for focal depths near 3.75 cm. © 2007 IEEE.
