Ultrasound. Update on thermal bioeffects issues

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.

Low-intensity pulsed ultrasound stimulates a bone-forming response in UMR-106 cells

Low-intensity (<100 mW/cm(2)) pulsed ultrasound (US) is an established therapy for fracture repair. In both animal and human trials, such US has been shown to facilitate fresh fracture repair and initiate healing in fractures with repair defects. However, the mechanism by which US achieves these outcomes is not clear. One possible mechanism is the direct stimulation of bone formation. To investigate this hypothesis, the current study investigated the mRNA response of isolated bone-forming cells (UMR-106 cells) to a single 20-min dose of low-intensity pulsed US. Using a novel US-cell coupling method, US was found to stimulate expression of the immediate-early response genes c-fos and COX-2 and elevate mRNA levels for the bone matrix proteins ALP and OC. These findings suggest that low-intensity pulsed US has a direct effect on bone formation. This may contribute to the beneficial effect of low-intensity pulsed US on fracture repair.

Ultrasound-induced temperature increase in guinea-pig fetal brain in utero: third-trimester gestation

Temperature increase was measured at various depths in the brain of living fetal guinea pigs during in utero exposure to unscanned pulsed ultrasound at ISPTA 2.8 W/cm2. Mean temperature increases of 4.9 degrees C close to parietal bone and 1.2 degrees C in the midbrain were recorded after 2-min exposures. When exposures were repeated on the same sites in each fetus after death, the corresponding mean temperature increases were 4.9 degrees C and 1.3 degrees C, respectively. Cerebral blood perfusion had little cooling effect on ultrasound-induced heating in the guinea pig fetus of 57-61 days gestational age.