The thermal index: its strengths, weaknesses, and proposed improvements.

The thermal index (TI) has been used as a relative indicator of thermal risk during diagnostic ultrasound examinations for many years. It is useful in providing feedback to the clinician or sonographer, allowing assessment of relative, potential risks to the patient of an adverse effect due to a thermal mechanism. Recently, several shortcomings of the TI formulations in quantifying the risk to the patient have been identified by members of the basic scientific community, and possible improvements to address these shortcomings have been proposed. For this reason, the Output Standards Subcommittee of the American Institute of Ultrasound in Medicine convened a subcommittee to review the strengths of the TI formulations as well as their weaknesses and proposed improvements. This article summarizes the findings of this subcommittee. After a careful review of the literature and an assessment of the cost of updating the TI formulations while maximizing the quality of patient care, the Output Standards Subcommittee makes the following recommendations: (1) some inconsistencies in the current TI formulations should be resolved, and the break point distance should be redefined to take focusing into consideration; (2) an entirely new indicator of thermal risk that incorporates the time dependence not be implemented at this time but be included in continuing efforts toward standards or consensus documents; (3) the exponential dependence of risk on temperature not be incorporated into a new definition of the TI formulations at this time but be included in continuing efforts toward standards or consensus documents; (4) the TI formulations not be altered to include nonlinear propagation at this time but be included in continuing efforts toward standards or consensus documents; and (5) a new indicator for risk from thermal mechanisms should be developed, distinct from the traditional TI formulations, for new imaging modalities such as acoustic radiation force impulse imaging, which have more complicated pulsing sequences than traditional imaging.

Fetal thermal effects of diagnostic ultrasound.

Processes that can produce a biological effect with some degree of heating (ie, about 1 degrees C above the physiologic temperature) act via a thermal mechanism. Investigations with laboratory animals have documented that pulsed ultrasound can produce elevations of temperature and damage in biological tissues in vivo, particularly in the presence of bone (intracranial temperature elevation). Acoustic outputs used to induce these adverse bioeffects are within the diagnostic range, although exposure times are usually considerably longer than in clinical practice. Conditions present in early pregnancy, such as lack of perfusion, may favor bioeffects. Thermally induced teratogenesis has been shown in many animal studies, as well as several controlled human studies; however, human studies have not shown a causal relationship between diagnostic ultrasound exposure during pregnancy and adverse biological effects to the fetus. All human epidemiologic studies, however, were conducted with commercially available devices predating 1992, that is, with acoustic outputs not exceeding a spatial-peak temporal-average intensity of 94 mW/cm2. Current limits in the United States allow a spatial-peak temporal-average intensity of 720 mW/cm2 for fetal applications. The synergistic effect of a raised body temperature (febrile status) and ultrasound insonation has not been examined in depth. Available evidence, experimental or epidemiologic, is insufficient to conclude that there is a causal relationship between obstetric diagnostic ultrasound exposure and obvious adverse thermal effects to the fetus. However, very subtle effects cannot be ruled out and indicate a need for further research, although research in humans may be extremely difficult to realize.