Key words: ultrasound, thermal effects, eye, standards
Limiting thermal effects during diagnostic ultrasound scans of the eye is critical to the safety of this procedure. Hence, quantifying possible thermal effects is an important part of any diagnostic ultrasound submission to FDA for marketing clearance. FDA's guidance document for diagnostic ultrasound submissions uses the concepts within the American Institute of Ultrasound in Medicine/National Electrical Manufacturers Association (AIUM/NEMA) Output Display Standard (ODS). The ODS requires that diagnostic ultrasound devices display thermal indices when these indices are above a certain level. These indices provide an estimate of the maximum steady-state tissue temperature that could be reached during an examination. To calculate these index levels, models have been developed for ultrasound impinging on uniform soft tissue, fetal bone, and adult skull bone. The soft tissue thermal index (TIS), however, may not be relevant with respect to eye exposure because the eye has structures, particularly the lens and orbital fat, whose acoustic and thermal properties differ significantly from those used to formulate the TIS.
Therefore, last year OST scientists began a theoretical study of ultrasound-induced temperature rise in the eye. This year the mathematical model was refined and expanded to include the clinically relevant frequencies of 7 MHz and 40 MHz, as well as the 10-MHz and 20-MHz cases modeled previously. For the three lower frequencies, transducer diameters of 0.5 cm and 1.0 cm and focal lengths ranging from 0.5 cm to 3.0 cm were used. For 40 MHz, the diameters were 0.2 cm and 0.4 cm, and the focal length range was 0.2 cm to 1.2 cm. Calculations were done assuming that the ultrasound beam both intersects the lens and that it does not intersect the lens, both of which can occur during an ultrasound exam.
Results were computed in terms of the ratio of the maximum temperature rise, MaxTR (as given by the theoretical model) to the TIS for a constant ultrasonic intensity. For the through-lens case it was found that the largest values of MaxTR/TIS tended to occur at 1) the lowest frequencies, 2) the lowest f-numbers (i.e., the ratio of focal length to diameter), and, most significantly from a safety standpoint, 3) the lowest calculated temperature rises. For each f-number, the maximum calculated temperature rise occurred in the lens and the overall maximum always occurred at the largest f-number.
For the around-lens case (7-20 MHz), the highest values of MaxTR/TIS occurred at 1) the lowest frequencies and 2) a focal length of 2.5 cm, which corresponds to the location of the eye-orbital fat boundary. For each f-number greater than 3.5 for the 0.5 cm transducers and greater than 1.7 for the 1.0 cm transducers, the maximum calculated temperature rise occurred in the orbital fat at the eye-orbital fat boundary. For the around-lens case (40 MHz), the highest values of MaxTR/TIS, all less than one, occurred at the lowest f-number. For each f-number, the maximum calculated temperature rise occurred in the sclera near the transducer. For this case also, the overall maximum calculated temperature rise always occurred at the largest f-number.
From this study it has been concluded that 1) in some cases, especially through the lens, the high MaxTR/TIS values occurred when the calculated temperature rise was low, so the high ratio often was insignificant from a safety standpoint; 2) in some cases when the calculated temperature rise was not low, the ratio was close to unity, and the TIS thus was an appropriate thermal index to use; and 3) in other cases, however, the TIS significantly underestimated the calculated temperature rise under conditions in which that temperature rise could be important. In this regard, the around-lens model produced more cases of concern than did the through-lens model. Thus, the development of a thermal index applicable to insonation of the eye is needed. In the interim, FDA guidance with respect to the regulatory output limits for eye exposures has been amended based on this study.