Abstract
Abstract: :
Purpose: Current guidelines for ultrasound exposure in ophthalmic applications specify thresholds that are lower than in any other medical specialty. The primary reason for this standard is concern for formation of cataracts as a consequence of thermal or other potential damage mechanisms (e.g., cavitation). Damage to the lens is especially of concern due to the high acoustic absorption coefficient of collagen and the lack of perfusion, which would otherwise reduce heating. Since reduction of diagnostic power levels has some degree of reduction of diagnostic capability, conformance to the standard is not cost–free. In this study we investigate damage thresholds for lens and cornea damage in the rabbit eye at 7.5 MHz, a frequency typical of current Doppler ultrasound systems. Methods: Dutchbelt rabbits were anesthetized and the eye gently proptosed through a latex membrane. The exposed globe was immersed in a 35oC bath of normal saline, which provided acoustic coupling. We then used a focused ultrasound transducer (80 mm aperture) operating at 7.5 MHz to expose either the cornea or lens at 4 levels (up to 2500 W/cm2 at the focal point) for periods of 1, 5 and 10 minutes. We observed the eye during exposure and recorded observations. Following exposures, we examined the eye under a slitlamp. Each experiment was performed six times, with three eyes enucleated immediately post–treatment, and three after 3 days and examined by light microscopy. Results: With the focal zone in the lens, at an intensity of 1900 W/cm2, cataracts developed in all cases (mean time=28+17 sec). At 1200 W/cm2, cataracts developed in 50% of lenses, typically after 3–4 minutes. With the ultrasound focused on the cornea, diffuse cataracts formed in all cases at 2500 W/cm2 (mean time=2.8+0.3 minutes). At 1900 W/cm2, small diffuse cataracts were seen in 3 of 18 experiments, with no cataracts at lower intensities. The corneas themselves showed relatively small changes at the highest exposures, including uptake of fluorescein and epithelial defects Conclusion: The current standard of 28 W/cm2 (derated spatial peak pulse average) is well below the 1200 W/cm2 minimum found to cause cataracts in the rabbit model. The relatively mild effects upon the cornea may be related to efficient thermal diffusion in the presence of acoustic streaming from this thin structure.
Keywords: imaging methods (CT, FA, ICG, MRI, OCT, RTA, SLO, ultrasound) • anterior segment • cataract