Answer to Question #776 Submitted to "Ask the Experts"

My answer is based on the assumption that the inquiry is about diagnostic ultrasound devices. Recent regulatory change has modified medical diagnostic device ultrasound acoustic output levels. Until 1992, the U.S. Food and Drug Administration (FDA) regulations (510[k Guide])limited the acoustic outputs of Diagnostic Ultrasound (DUS) devices in various application-specific categories to those used in 1976. Following extensive cooperative efforts between the American Institute of Ultrasound in Medicine, National Electrical Manufacturers' Association, a large number of other professional societies whose membership uses diagnostic ultrasound, and the FDA, these regulations were amended. Spatial peak temporal average (SPTA) acoustic outputs of all DUS devices could be increased from 94 to 720 mW cm ^-2—a 7.7- fold increase—except the use category for ophthalmology (which remained unchanged). In exchange for these allowable higher acoustic outputs, the FDA required the manufacturers to have two on-screen safety indexes: (1) the Mechanical Index (MI), a quantity indicating the potential for the occurrence of ultrasound-induced inertial cavitation, and (2) the Thermal Index (TI), a quantity indicating the potential for thermal bioeffects, respectively. The TI is numerically equivalent to the reasonable worst-case U.S.-induced temperature rise (DT oC) anticipated in the insonated tissue. Thus, manufacturers have two options. They can produce DUS devices under the old regulations and not have to include the on-screen indexes, or they can produce acoustically more powerful units and include the on-screen indexes. The FDA's regulation (the 510(k) Guide) can be accessed on the Web as a PDF file entitled: Information for Manufacturers Seeking Marketing Clearance of Diagnostic Ultrasound Systems and Transducers. There are four aspects pertinent to the matter of quality assurance in this area. First, the FDA relies upon the manufacturers to adhere to the federal regulations; thus, the manufacturer must provide to the FDA measures of the acoustic outputs of their devices. Second, there is no mandate for checking on acoustic outputs with product sale or subsequent use. Third, the TI and the MI are the only indicators the health care provider has of the acoustic outputs of the devices. The algorithms used to calculate these indices are from generalized formulae, thought to be conservative—for example, to overestimate the output as represented by the displayed TI and MI values. However, the Output Display Standard recognizes, for instance, the possibility for inaccuracy in estimating actual DUS-induced temperatures. These factors may impact quality assurance aspects of the exam itself; DUS exams are thought to be remarkably "safe" on the basis of epidemiological literature and generalized absence of any apparent deleterious patient outcomes from such exams. However, all of the epidemiological literature has been derived from use of "older" (that is, relatively low output, pre-510(k)-amendment) DUS devices. Fourth, there is a growing possibility of further relaxing or completely eliminating present FDA regulations on the premise that greater acoustic outputs would improve diagnostic capability under certain circumstances.

Morton W. Miller, PhD, School of Medicine and Dentistry