In vivo assessment of the mechanical properties of crystalline lenses in a rabbit model using ultrasound elastography: Effects of ultrasound frequency and age

Abstract Purpose The occurrence of presbyopia and cataract is closely related to changes in the mechanical properties of the crystalline lens. There are no established methods so far for in vivo assessment. By introducing ultrasound elastography, we proposed group velocity (V g ) of an induced shear wave as a new biomarker to characterize the mechanical properties of the lens in our previous study. Here, we investigated the effect of the ultrasound frequency on measurement accuracy and validated the results with a conventional ex vivo compression testing. We also demonstrated a change trend in V g and its correlation with age in a rabbit model. Methods Eight New Zealand white rabbits were fed normally from the fourth to seventh month. An ultrasound elastography system was developed to measure V g in vivo on every eye once per month. The performances when using a high-frequency (L22-11v) and low-frequency (L11-4v) probe were compared. Rabbits were sacrificed after in vivo measurements by the end of the seventh month and this was followed by ex vivo ultrasound measurements and conventional compression tests on the extracted lenses. Results The results demonstrated that there were no significance differences in V g between measurements with high-frequency (USE-HF) and low-frequency (USE-LF) probes in the same month-age group. The mean V g and the standard deviation of four rabbits that were 7 months old were 2.37 ± 0.24  m/s, 2.36 ± 0.25 m/s, 2.43 ± 0.26 m/s and 2.44 ± 0.38 m/s, with USE-HF for ex vivo and in vivo measurements and USE-LF for ex vivo and in vivo measurements, respectively. There were no significant differences (p > 0.05) and they were all in agreement with the results of compression tests, which was 16.16 ± 1.84 kPa in Young's modulus. The results also showed that V g increased with age. In combination with the results of our previous study, V g showed a relatively sharp increase from 2 to 5 months, while it had a slight increase from 5 to 7 months. Conclusions The USE-HF and USE-LF has comparable accuracy in V g measurements while USE-HF had an advantage regarding better spatial resolution. The change trend of V g was in accord with the growth phase of New Zealand white rabbits, which usually results in sexual maturity at 5 months old. This implies that V g can be used as a biomarker parameter for evaluating the mechanical properties of the lens undergoing physiological changes.