ACOUSTIC EXPOSURE DETERMINATIONS IN DIAGNOSTIC ULTRASOUND - RESPONSIBILITIES OF RESEARCHERS STUDYING BIOLOGICAL EFFECTS OF ULTRASOUND

ACOUSTIC EXPOSURE DETERMINATIONS IN DIAGNOSTIC ULTRASOUND RESPONSIBILITIES OF RESEARCHERS STUDYING BIOLOGICAL EFFECTS OF ULTRASOUNDPaul L. Carson, Ph.D. University of Colorado Medical Center Denver, ColoradoWith the exception of those involved in de­ veloping standard methods for measuring acoustic intensity, researchers studying biological effects of ultrasound have the greatest responsibility for accurate measure­ ment of a variety of indicators of acoustic exposure. Classed among these researchers should be those involved in epidemiological studies designed to reveal any toxic effects of ultrasound at diagnostic intensities.As an example of researchers' responsibili­ ties, consider someone performing controlled experiments on a biological system. He must not only document the various average and peak acoustic intensity levels at which a given effect is observed, but also should provide enough other information to allow duplication of the experiment and analysis of possible mechanisms of the observed acoustic intervention in the normal biologi­ cal processes. Similarly, for epidemiologi­ cal studies, methods must be available for determining whether ultrasound is responsi­ ble for abnormalities observed in a group of patients who have received ultrasound exami­ nations. In epidemiological studies not involving elaborate patient randomization procedures, correlation of acoustic exposure with the incidence of such abnormalities would be the most convincing and simple method of establishing a causal relationship. However, use of an overly simple indication of acoustic exposure could be quite mislead­ ing. For example, use of only the examina­ tion time to indicate differences in exposure may implicate ultrasound erroneously, since conditions actually causing the observed abnormality may also necessitate longer examinations. In addition, different in­ struments produce different intensities, so exposure time alone would not correlate well with any biological damage which may be observed.Well, what quantities should one measure? Some existing and possible definitions which may aid this discussion are outlined below,and a valuable reference is Session k of Reference 1, in particular C. R. Hill's "Chairman's Summary."Acoustic Exposure1. Indication of the generated acoustic field.2. Exposure rate = acoustic intensity3. Exposure rate could also be defined by values of acoustic field quanti­ ties (in water) a. Pressure b. Displacement c. Velocity d. Acceleration e. Velocity gradientDos imetric Quant it IesInteraction of acoustic field with material of interest1 .2.3.Dose = energy absorbed per unit mass Temperature cycle of material Acoustic field quantities (in the mater i al)Is the often accepted definition of the acous­ tic exposure rate as being the average acous­ tic intensity (power per unit area) as good as one would like? Ideally the acoustic ex­ posure should refer to^ values of one or more measurable acoustic fifeld quantities such as pressure or particle displacement or a func­ tion of field quantities as is the case with intensity. These values would be reported at the point or region of interest in the bio­ logical system, and would be related simply to the biological effect of the acoustic field.* However, this ideal case is not-If an acoustic field quantity is used to denote exposure, its value in a small mass of reference medium, i.e. water, at the point of interest should be quoted. The value of a field quantity depends on the medium, and therefore its value in the medium is properly dosimetric variable rather than one of ex­ posure.116