Intracranial temperature elevation from diagnostic ultrasound

Tissues of the central nervous system are sensitive to damage by physical a gents, such as heat and ultrasound. Exposure to pulsed spectral Doppler ult rasound can significantly heat biologic tissue because of the relatively hi gh intensities used and the need to hold the beam stationary during examina tions. This has significant implications for sensitive neural tissue such a s that exposed during spectral Doppler How studies of fetal cerebral vessel s. Recent changes in the FDA regulation allow delivery of almost eight time s higher intensity into the fetal brain by ultrasound devices that incorpor ate an approved real-time output display in their design. In this situation , ultrasound users are expected to assess the risk/benefit ratio based on t heir interpretation of equipment output displays (including the thermal ind ex, TI) and an understanding of the significance of biologic effects. To as sist in the assessment of potential thermally mediated bioeffects, a number of conclusions can be drawn from the published scientific literature: the amount of ultrasound-induced intracranial heating increases with gestationa l age and the development of fetal bone; pulsed spectral Doppler ultrasound can produce biologically significant heating in the fetal brain; the rate of heating near bone is rapid, with approximately 75% of the maximum heatin g occurring within 30 s; blood flow has minimal cooling effect on ultrasoun d-induced heating of the brain when insonated with narrow focused clinical beams; the threshold for irreversible damage in the developing embryo and f etal brain is exceeded when a temperature increase of 4 degreesC is maintai ned for 5 min; an ultrasound exposure that produces a temperature increase of up to 1.5 degreesC in 120 s does not elicit measurable electrophysiologi c responses in fetal brain; for some exposure conditions, the thermal index (TI), as used in the FDA-approved output display standard, underestimates the extent of ultrasound-induced intracranial temperature increase.