A cavitation and free radical dosimeter for ultrasound

A simple and sensitive method is described for measuring the number of hydroxyl radicals (.OH) produced in aqueous solution by high intensity ultrasound (US). The method is based on the conversion of nonfluorescent terephthalic acid (TA) to highly fluorescent hydroxyterephthalate (HTA). The spectrofluorimeter is calibrated by use of a TA solution which has been exposed to a reference dose of cobalt-60. This allows fluorescence readings produced by ultrasound to be read directly in equivalent centiGrays (cGy). The limit of detection for OH is about 1.3 X 10(-9) M (equivalent to an exposure of about 0.5 cGy of cobalt-60). The threshold for transient cavitation (TC) in an air-saturated aqueous solution was found at 0.7 watts per cm2. Cavitation was consistently produced when TA was exposed in a soft vinyl chamber and only when the chamber was being rotated about an axis which was perpendicular to the US beam. The use of TA as a quantitative indicator of transient cavitation should be of value to the study of ultrasonic biological effects and dosimetry.