Effect of emitted power on waveform intensity in transcranial Doppler.

This study assesses the problem of transcranial Doppler recording failure and seeks to determine the extent to which this can be ameliorated by increased emitted power. We hypothesized that waveform intensity is directly related to the rate and quality of successful recording and may be compared quantitatively among groups of patients. Among a large group of patients recorded at 800 mW/cm2 emitted power, intensity was strongest in white men, weakest in black women, and intermediate in black men and white women. It declined with age in women of either race, but not in men of either race. Analysis of the effect of emitted power on intensity predicted that significant numbers of waveforms recorded at 800 mW/cm2 could not be recorded at the current clinical standards of 100 mW/cm2, the difference being most pronounced in elderly black women. Temporal bone window thickness measured in a series of adult cadaver skulls was least in white men, greatest in black women, and intermediate in black men and white women. The findings of this study support the hypothesis that temporal bone window thickness is an important determinant of recording difficulty and suggest that increased emitted power can significantly increase successful recording, particularly in black and elderly patients. Increased power alone, however, cannot completely solve the recording problem within safe limits.