Thermal safety of vibro-acoustography using a confocal transducer

Abstract

Vibro-acoustography (VA) is an imaging method that forms a two-dimensional (2-D) image by moving two cofocused ultrasound beams with slightly different frequencies over the object in a C-scan format and recording acoustic emission from the focal region at the difference frequency. This article studies tissue heating due to a VA scan using a concentric confocal transducer. The three-dimensional (3-D) ultrasound intensity field calculated by Field II is used with the bio-heat equation to estimate tissue heating due to ultrasound absorption. Results calculated with thermal conduction and with blood perfusion, with conduction and without perfusion and without conduction and without perfusion are compared. Maximum heating due to ultrasound absorption occurs in the transducer's near-field and maximum temperature rise in soft tissue during a single VA scan is below 0.05 degrees C for all three attenuation coefficients evaluated: 0.3, 0.5 and 0.7 dB/cm/MHz. Transducer self-heating during a single VA scan measured by a thermocouple is less than 0.27 degrees C.

2010 World Federation for Ultrasound in Medicine & Biology. Published by Elsevier Inc. All rights reserved.

Figures

Fig. 1

Diagram of the vibro-acoustography imaging system. The object vibrates at the beat frequency Δf = f1 − f2 and its acoustic emission is recorded by the acoustic hydrophone. The focal spot of two cofocused ultrasound beams moves over the object in a raster C-scan to form a two-dimensional (2-D) image.

Fig. 2

Temperature rise on transducer axial plane resulting from vibro-acoustography imaging of one pixel in a medium with 0.5 dB/cm/MHz attenuation. The transducer is located above the top of these images. (a) Heat field at the end of 0.1 ms ultrasound exposure. (b) Heat field after 10 s of thermal conduction (blood perfusion is neglected here).

Fig. 3

Center planes of the three-dimensional (3-D) heat field caused by a 40 mm × 40 mm vibro-acoustography scan in a medium of 0.5 dB/cm/MHz attenuation. The transducer is located above the top of these images. (a) Heat field calculated without conduction and without perfusion (Maximum temperature rise is 0.0349°C). (b) Heat field calculated with conduction and without perfusion (Maximum temperature rise is 0.0316°C). (c) Heat field calculated with conduction and with perfusion (Maximum temperature rise is 0.0312°C).

Fig. 4

Spatial maximum temperature rise during and after a vibro-acoustography scan (40 mm × 40 mm C-scan, 0.25 mm resolution) in a medium with 0.5 dB/cm/MHz attenuation. The VA scan is finished at about 52 s. Neglection of conduction and perfusion slightly overestimates heating during the scan.

Fig. 5

Center planes of the three-dimensional (3-D) heat field caused by a 40 mm × 40 mm vibro-acoustography scan in a medium of (a) 0.3 dB/cm/MHz attenuation and (b) 0.7 dB/cm/MHz attenuation. The transducer is located above the top of these images. Conduction and perfusion are neglected and the acoustic output of the transducer is fixed, leading to higher in situ Isppa in (a) than in (b). The maximum temperature is 0.0226°C in (a) and 0.0468°C in (b).

Fig. 6

Temperature rise at the transducer surface measured by a thermocouple during a single vibro-acoustography scan.