Ultrasound still has a place in ophthalmology. Several new developments in the technology facilitate detailed visualization of the posterior segment.
The ultrasound frequencies currently used in ophthalmology are 8 to 12 MHz for contact B-scans with quantitative or non-quantitative A-scans, 20-MHz instruments to image the anterior and posterior segments, and 30- to 50-MHz ultrasonic "biomicroscopy" instruments. These are high frequency B-scans with sector scans of a segment but not the full anterior segment. In addition, a very high-frequency ultrasound instrument (Artemis, ArcScan) can scan the anterior segment and has optical alignment, full anterior segment, and digital signal processing, Dr. Coleman explained.
"Currently available ultrasound techniques can evaluate the cornea to the level of microns. By imaging the sulcus, ciliary body, and lens, the results are superior to OCT imaging," he said. "For retina specialists, this technology is useful for looking at the anterior segment, that is, the ciliary body and the area around the lens. This imaging approach is also useful to diagnose and follow patients with tumors to determine if treatment with an iodine or other radioactive plaque is necessary or, if used, was effective. However, the posterior segment cannot be visualized using high frequencies because of ultrasound absorption, thus we are limited to about 25-MHz resolution."
Several new developments, especially when linked together, can provide excellent images of the posterior segment.
He demonstrated the case of a 45-year-old woman with diabetes. This form of imaging was able to visualize vitreous floaters and membranes as well as changes in the liquefied vitreous. The technique is particularly useful in evaluating patients with diabetes who have vitreous detachment and debris, Dr. Coleman said.
"For the first time, we can image the entire lens, and the entire volume of the vitreous can be measured," Dr. Coleman said. "Changes in the vitreous outside the area of visualization with OCT can be measured.
"We believe that this technology will have great benefit in studies, such as volume measurements of the vitreous to determine the most appropriate quantity of biologics that can be dispersed," he said. In addition, the lens, its area, and volume can be evaluated for IOL power and placement determinations, Dr. Coleman added.
"I hope that we will also be able to further demonstrate that the catenary theory of accommodation of the lens is the appropriate model to address presbyopia," he concluded.