Literatures Reviews

GJ Vella, VF Humphrey, FA Duck, SB Barnett. The cooling effect of liquid flow on the focussed ultrasound-induced heating in a simulated foetal brain. Ultrasound in Med & Biol 2003; 29: 1193–1204,

The aim of this paper was to study the cooling effect of fluid flow during ultrasound exposure of a test phantom designed to simulate human foetal skull and adjacent brain. In this well designed, and executed, study, a narrow 3.5 MHz pulsed ultrasound beam (5.7 µs pulse length; 8 kHz prf; 20–255 mW power) was used to expose a bone phantom that had similar thermal and acoustic properties to human foetal bone. Flow was through 2 mm diameter wall-less channels. This diameter was chosen as being typical of intracerebral vessels in the human foetus (1–3 mm). Temperatures were measured using 50 µm thermocouples attached proximally and distally to the bone phantom in the ultrasound beam. It was found that the percentage cooling produced by the water was independent of source power, and its effect decreased with distance from the vessel, being negligible at 3mm. The amount of cooling was found to increase with flow rate up to a level at which it saturated despite an increase in flow. It was therefore concluded that flow must be sufficiently slow to allow enough time for heat to be conducted across the surrounding tissue and thus convected away from bone. This has also been observed in animal studies.

Temperature rises of the order of 2.5 C were measured for 100mW exposures. These were typically reduced by about 12% at the plateau of flow rate.

It is difficult to fully assess the clinical relevance of this study, except to say that temperatures indicated by the TIC may be reduced by a small amount when the ultrasound exposure is near large blood vessels, as might be expected. This is good from the safety viewpoint, but it is best to assume that such an effect will be negligible when doing a risk assessment.