The cooling effect of liquid flow on the focussed ultrasound-induced heating in a simulated foetal brain.

There is a need to investigate the thermal effects of diagnostic ultrasound (US) to assist the development of appropriate safety guidelines for obstetric use. The cooling effect of a single liquid flow channel was measured in a model of human foetal brain and skull bone heated by a focussed beam of simulated pulsed spectral Doppler US. Insonation conditions were 5.7 micros pulses, repeated at 8 kHz from a focussed transducer operating with a centre frequency of 3.5 MHz, producing a beam of -6 dB diameter of 3.1 mm at the focus and power outputs of up to 255 +/- 5 mW. Brain perfusion was simulated by allowing distilled water to flow at various rates in a 2 mm diameter wall-less channel in the brain soft tissue phantom material. This study established that the cooling effect of the flowing water; 1. was independent of the acoustic source power, 2. was more effective close to the flow channel, for example, there was a marked cooling at a distance of 1 mm and negligible cooling at a distance of 3 mm from the channel; and 3. initially increased at low flow rates, but further increase above normal perfusion had very little effect.

Ultrasound-induced heating in a foetal skull bone phantom and its dependence on beam width and perfusion.

The cooling effect of single and multiple perfusing channels has been measured in a model of human foetal skull bone heated by wide and narrow beams of simulated pulsed spectral Doppler ultrasound (US). A focussed transducer operating with a centre frequency of 3.5 MHz, that emitted pulses of 5.7 micros duration with a repetition frequency of 8 kHz, was used. This produced a beam of power 100 +/- 2 mW with -6 dB diameters of 3.1 mm and 7.8 mm at 9 cm and 6 cm, respectively, from the transducer face. Arterial perfusion was simulated by allowing distilled water to flow in a large single channel or a grid of fine channels near the heated bone target. This study has established that: 1. perfusion-induced cooling is significantly enhanced when the bone phantom is heated by a wide rather than a narrow beam; 2. irrespective of the US beam width, a grid of small channels is more effective in cooling a heated bone target than a single larger diameter channel with the same volume flow rate; 3. the measured temperature rise and rate of temperature rise support the prediction of inverse proportionality to the US beam width; and 4. the perfusion time constants determined in our phantom model are 2 to 30 times larger than that assumed for the thermal index (TIB) algorithm.