Abstract
Purpose :
The acoustic reflectivity of tissue is an indicator of its physical properties, especially density variation with respect to its surround and internally. Because ultrasound biomicroscopy (UBM) transducers are tightly focused, reflection amplitude varies strongly with range with respect to its focal zone. Qualitatively, we see increased reflectivity of zonules in Marfan's syndrome, but cannot quantify this because of this non-linear dependence of reflectivity with depth. Our aim was to determine the possibility of compensating for the focal properties of the UBM in order to objectively quantify the reflectivity of ocular structures such as zonular fibers in UBM images.
Methods :
We measured reflectivity from a specular reflector as a function of depth on a Quantel Absolu UBM and then produced a spline function characterizing reflectivity with depth. We then applied an inverse function to UBM images to compensate for sensitivity as a function of depth. The function was then applied to uniform tissue mimicking phantom comprised of 25 μm diameter graphite particles suspended in gelatin and a series of UBM images of 8 eyes of 6 subjects with 3 or more scans at different ranges to assess reflectivity of zonular fibers.
Results :
While the compensated reflectivity with depth in the phantoms was near log-linear, indicative of the attenuation coefficient, the particles were only resolved within the 2.5 mm focal zone. The range of axial distance to the zonules in repeated measurements in clinical subjects averaged 0.66±0.40 mm. Mean zonule reflection amplitude (arbitrary units) was higher (122.4±27.4) in amplitude-corrected images versus uncorrected images (111.3±25.4), but their coefficients of variation were nearly identical (.1184 versus .1188).
Conclusions :
The sensitivity compensatory function allows a more objective assessment of attenuation in a uniform phantom, but the inverse function cannot correct for the poor focusing outside the focal region. The function cannot compensate for loss of resolution of fine structure such as zonules that are of particular interest in Marfan’s disease. We conclude that compensation for the sensitivity function alone is not adequate to assess reflectivity at different ranges, ultimately requiring beamforming using either an annular or linear array where focal depth is controlled synthetically.
This abstract was presented at the 2024 ARVO Annual Meeting, held in Seattle, WA, May 5-9, 2024.