Tang Junhua, Liu Jun
Department of Biomedical Engineering, Ohio State University, Columbus, OH 43210, USA.
J Biomech Eng. 2012 Sep;134(9):091007. doi: 10.1115/1.4007365.
Scleral biomechanical properties may be important in the pathogenesis and progression of glaucoma. The goal of this study is to develop and validate an ultrasound method for measuring cross-sectional distributive strains in the sclera during elevations of intraocular pressure (IOP).
Porcine globes (n = 5) were tested within 24 hs postmortem. The posterior scleral shells were dissected and mounted onto a custom-built pressurization chamber. A high-frequency (55-MHz) ultrasound system (Vevo660, VisualSonics Inc., Toronto) was employed to acquire the radio frequency data during scans of the posterior pole along both circumferential and meridian directions. The IOP was gradually increased from 5 to 45 mmHg. The displacement fields were obtained from correlation-based ultrasound speckle tracking. A least-square strain estimator was used to calculate the strains in both axial and lateral directions. Experimental validation was performed by comparing tissue displacements calculated from ultrasound speckle tracking with those induced by an actuator. Theoretical analysis and simulation experiments were performed to optimize the ultrasound speckle tracking method and evaluate the accuracy and signal-to-noise ratio (SNR) in strain estimation.
Porcine sclera exhibited significantly larger axial strains (e.g., -5.1 ± 1.5% at 45 mmHg, meridian direction) than lateral strains (e.g., 2.2 ± 0.7% at 45 mmHg, meridian direction) during IOP elevations (P's < 0.01). The strain magnitudes increased nonlinearly with pressure increase. The strain maps displayed heterogeneity through the thickness. The lateral strains were significantly smaller in the circumferential direction than the meridian direction at 45 mmHg (P < 0.05). Experimental validation showed that the ultrasound speckle tracking method was capable of tracking displacements at the accuracy of sub-micron to micron. Theoretical analysis predicted the dependence of the strain estimation SNR on the strain level, as well as signal processing parameters such as kernel size. Simulation results showed that ultrasound speckle tracking had a high accuracy for estimating strains of 1-5% and a high SNR for strains of 0.5-5%.
A new experimental method based on ultrasound speckle tracking has been developed for obtaining cross-sectional strain maps of the posterior sclera. This method provides a useful tool to examine distributive strains through the thickness of the sclera during elevations of IOP.
巩膜生物力学特性可能在青光眼的发病机制和进展中起重要作用。本研究的目的是开发并验证一种超声方法，用于测量眼内压（IOP）升高期间巩膜的横截面分布应变。
在猪死后24小时内对猪眼球（n = 5）进行测试。解剖后巩膜壳并安装到定制的加压室上。使用高频（55 MHz）超声系统（Vevo660，VisualSonics公司，多伦多）在沿圆周和子午线方向对后极进行扫描期间采集射频数据。将IOP从5 mmHg逐渐升高至45 mmHg。通过基于相关性的超声散斑跟踪获得位移场。使用最小二乘应变估计器计算轴向和横向方向上的应变。通过比较超声散斑跟踪计算出的组织位移与致动器引起的位移来进行实验验证。进行理论分析和模拟实验以优化超声散斑跟踪方法，并评估应变估计中的准确性和信噪比（SNR）。
在IOP升高期间，猪巩膜在轴向应变方面（例如，在45 mmHg时子午线方向为-5.1±1.5%）比横向应变（例如，在45 mmHg时子午线方向为2.2±0.7%）表现出明显更大的应变（P值<0.01）。应变大小随压力增加呈非线性增加。应变图在整个厚度上显示出不均匀性。在45 mmHg时，横向应变在圆周方向上明显小于子午线方向（P<0.05）。实验验证表明，超声散斑跟踪方法能够以亚微米到微米的精度跟踪位移。理论分析预测了应变估计SNR对应变水平以及诸如内核大小等信号处理参数的依赖性。模拟结果表明，超声散斑跟踪对于估计1 - 5%的应变具有高精度，对于0.5 - 5%的应变具有高SNR。
已开发出一种基于超声散斑跟踪的新实验方法，用于获取后巩膜的横截面应变图。该方法为在IOP升高期间检查巩膜厚度上的分布应变提供了一种有用的工具。
