The output display standard: has it missed its target?

Abstract

Diagnostic ultrasound is the safest medical imaging modality for the exposed patient and this has contributed to it being established as the method of choice for fetal examinations. The clinical benefits of obstetric ultrasound are generally recognized and, for about three decades, one or more ultrasound examinations during pregnancy have been offered to millions of pregnant women worldwide. A number of follow-up studies of children exposed to ultrasound in utero provided reassuring evidence that prenatal ultrasound examination does not cause adverse effects with regard to malformations, childhood malignancy, neurological abnormalities or abnormal growth1. The reported weak association between prenatal ultrasound exposure and non-right-handedness2 needs further evaluation but does not refute the fact that there is at present no evidence which would contradict the safe use of ultrasound in pregnancy. This was supported by a recently published prospective study demonstrating that multiple ultrasound examinations during pregnancy did not have any adverse effect on the postnatal growth, behavior and neurological development of children followed up to the age of 8 years3. However, highenergy ultrasound can induce biophysical effects when passing through tissue, for example, thermal effects and mechanical stress, causing cavitation. Consequently, some authorities, such as the Food and Drug Administration (FDA) in the USA, set upper limits for the output from ultrasound equipment: the estimated in-situ intensity must not exceed 720 mW/cm2. In Europe, no such general regulation of the output levels is given. It is the user’s responsibility to control the output energy and to use the equipment in a safe manner. In order to facilitate ultrasound safety assessment, in the early 1990s, the American Institute of Ultrasound in Medicine and National Electrical Manufacturers Association introduced the ‘output display standard’ (ODS) using biophysical indicators for real-time display of safety information during scanning. The FDA adopted the ODS and issued regulations demanding the ODS information to be provided by the manufacturers. The definitions of these biophysical indicators and the requirements for their display are now included in an International Standard4. The ODS indicators comprise two types of biophysical index: the thermal index (TI) and mechanical index (MI)5. The indices are calculated for the given machine settings on the basis of tissue models and their acoustic properties. The TI is an estimate of the tissue temperature rise in degrees centigrade (◦C), which might be possible under ‘reasonable worst-case conditions’. For particular examination situations, three types of TI have been defined: soft tissue (TIS) and bone tissue (TIB) thermal index, and TI for cranial examinations (TIC). The MI attempts to indicate the probability of non-thermal effects occurring within the tissue. According to the ODS, for equipment that can at certain machine settings produce output energy giving TI or MI ≥ 1.0, the indices should be displayed if they exceed 0.4. The MI and TI are only rough estimates of possible effects and should not be understood as giving more than guidance to the operator. Nevertheless, the ODS is considered at present to be the best way of providing safety information. Several national and international professional bodies – societies of medical ultrasound – issue regularly statements with recommendations for the safe use of diagnostic ultrasound. The safety statements can be found in the official publications6 and on the websites