Time progression and depth dependence of high frequency AIBS in ex-vivo porcine corneas

Background, Motive & Objective: Ex-vivo corneal tissue is prone to edema (swelling from fluid intake), an important concern for transporting this tissue for transplantation. Edema is also an issue for all ex-vivo cornea experiments. While gross changes in thickness are easy to determine, subtle structural changes can also occur and may impact suitability for transplant. This investigation proposes to use high-frequency apparent integrated backscatter (AIBS) to monitor these subtle changes as ex-vivo corneal tissue is exposed to edema inhibiting fluid. Statement of Contribution/Methods: Within a couple days of sacrifice, whole porcine eye globes were embedded in gelatin up to the corneal/scleral junction. Specimens were imaged with a confocal scanning acoustic microscope using a single element ultrasonic transducer (53 MHz center frequency, 31 MHz bandwidth, 1.67 f#). A solution of 25% glycerol and 10% dextran-40 was used to stabilize corneal hydration and couple the ultrasound into the tissue. Images were captured about every ten minutes for over 5 hours after fluid introduction. Laterally averaged AIBS was calculated over bandwidth with respect to a reference spectrum of a reflected signal from a fused silica optical flat. While some gross measures of corneal backscatter have been previously reported, this is the first detailed experiment exploring time progression and depth dependence and therefore examining subtle changes that are occurring. Results: In the epithelium AIBS increased from -31 to -29 dB after the first 11 minutes. It then decreased gradually over the next 108 minutes to -34 dB. Over the final 205 minutes it stabilized at -33 dB. In the anterior stroma AIBS gradually increased from an initial value of -56 to -50 dB over 119 minutes. It stabilized at -50 dB during the remaining 205 minutes. At mid stroma it gradually increased from an initial value of -59 to -55 dB over the first 119 minutes and then stabilized at about -55 dB for the remaining - - time. In posterior stroma it changed little, varying somewhat between -62 and -60 dB. Initial corneal thickness was 0.942 mm which decreased to 0.920 mm by 140 minutes, remained stable at this thickness for 119 minutes and then gradually increased to 0.936 mm during the last 64 minutes. As a control we observed that other ex-vivo porcine tissues such as oral mucosa do not undergo edema and have stable AIBS. Discussion and Conclusions: Systematic changes in AIBS of ex-vivo porcine cornea were measured as functions of both time and depth and shown to be sensitive to changes in corneal tissue. Previous research on edema and corneal backscatter postulated that an AIBS increase might be caused by disruption of the orderly spatial arrangement of fibrillar collagen. These more detailed results indicate that this relationship is more complicated than can be explained by edema alone. For example, AIBS in the anterior stroma increases with decreasing fluid content as indicated by corneal thinning. Also, at times AIBS in anterior stroma and epithelium move in opposite directions.