Ultrasonic absorption in soft and hard fetal tissues

Abstract

Although the use of ultrasound as a diagnostic modality in obstetrics has become ubiquitous, concern that harmful effects may occur under special circumstances continues. A physical mechanism by which biological effects could occur is the deposition of heat in the tissue as a result of the energy absorbed from the acoustic wave. Because the teratogenicity of hyperthermia has been well-established and concerns of more subtle effects of elevated fetal temperature exist, there is currently a concerted effort to provide manufacturers of medical diagnostic ultrasound instrumentation and clinicians with guidelines for the use of ultrasound during pregnancy. There is a need to continue developing heat transfer models, such that more accurate estimates of the temperature elevation upon exposure to ultrasound can be made. Included in such models would be the absorption properties of fetal soft tissue and fetal bone as a function of gestational age. The absorption of ultrasound in fetal tissues and the resulting temperature elevation are studied in this thesis. The temperature elevation in fetal mice exposed to 1 MHz ultrasound is measured, and the results are compared with analytical values of the temperature increase that are calculated using a simple heat transfer model. The transient thermoelectric method for measuring the absorption coefficient of liquids and soft tissues is also analyzed. The results provide an experimental guide for more accurate measurements of the absorption coefficient in fetal soft tissue. Finally, the absorption in fetal bone as a function of gestational age is considered. In the absence of specific knowledge regarding the acoustic propagation of fetal bone, from which the temperature increase might be calculated, the temperature rise resulting from exposure to ultrasound can be measured. The temperature elevation in fetal bone exposed to 1 MHz ultrasound is measured for a range of gestational ages. An equivalent heat source obtained from the measurements, which might be used in numerical and analytical calculations, is given.