RESUMO
Pulsed Doppler ultrasound examination of the fetal cerebral circulation may cause potentially harmful temperature elevations in brain tissue immediately beneath the insonated segment of the skull. This study measured the effect of variations in cerebral blood flow on ultrasonic heating of the cerebral cortex of anaesthetised, neonatal pigs. Wide and narrow ultrasound beams were used. Pulsed ultrasound exposures were delivered in 90 s bursts at 5.8 micros pulse length, pulse repetition frequency 8 kHz and centre frequency 3.5 MHz. Studies were performed with the target at the focus of a fixed, stationary beam of 0.3 cm -6 dB beam width (narrow beam) and I(spta) 1.4 W/cm(2) (n = 11), or with the target in the near field of a fixed, stationary beam of 1.6 cm -6 dB beam width (wide beam) and I(spta) 3.6 W/cm(2)(n = 5). The 90 s ultrasound exposures were performed under three different conditions of ambient cerebral blood flow: baseline (during normocarbic, normoxic conditions), increased (during hypercarbic, hypoxic conditions) and absent (postmortem). Cerebral blood flow was measured using the radiolabelled microsphere technique. In the narrow beam studies, cerebral blood flow during baseline was 34 +/- 4 ml/min/100 g, rising to 109 +/- 32 ml/min/100 g during the increased phase (p < 0.001); in the wide beam studies baseline flows were 29 +/- 9 ml/min/100 g, whereas flows in the increased phase were 128 +/- 32 ml/min/100 g (p < 0.001). There was no difference in the heating curves for normal, increased and absent cerebral blood flow for exposure to the narrow beam, when mean temperature increases of 1.5 degrees C at 90 s were recorded in each case (p > 0.21, power > 0.8). However, the heating curves for the wide beam were significantly different for the three rates of blood flow with mean temperature increases of 1.9 degrees C (normal flow), 1.7 degrees C (increased flow) and 2.4 degrees C (no flow) recorded at 90 s (p < 0.05).
