Rohrbach Daniel, Silverman Ronald H, Chun Dan, Lloyd Harriet O, Urs Raksha, Mamou Jonathan
Lizzi Center for Biomedical Engineering, Riverside Research, New York, NY, USA.
Department of Ophthalmology, Columbia University Medical Center, New York, NY, USA.
Transl Vis Sci Technol. 2018 Apr 11;7(2):21. doi: 10.1167/tvst.7.2.21. eCollection 2018 Apr.
Mapping of epithelial thickness (ET) is useful for detection of keratoconus, a disease characterized by corneal thinning and bulging in which epithelial thinning occurs over the apex. In prior clinical studies, optical coherence tomography (OCT) measurements of ET were systematically thinner than those obtained by 40-MHz high-frequency ultrasound (HFU) where a constant speed of sound () of 1636 m/s was used for all corneal layers. The purpose of this work was to study the acoustic properties, that is, , acoustic impedance (), and attenuation () of the corneal epithelium and stroma independently using a scanning acoustic microscope (SAM) to investigate the discrepancy between OCT and HFU estimates of ET.
Twelve unfixed pig corneas were snap-frozen and 6-μm sections were scanned using a custom-built SAM with an F-1.08, 500-MHz transducer and a 264-MHz bandwidth. Two-dimensional maps of , , and with a spatial resolution of 4 μm were derived.
SAM showed that the value of in the epithelium (i.e., 1548 ± 18 m/s) is substantially lower than the value of in the stroma (i.e., 1686 ± 33 m/s).
SAM results demonstrated that the assumption of a constant value of for all corneal layers is incorrect and explains the prior discrepancy between OCT and HFU ET determinations.
The findings of this study have important implications for HFU-based ET measurements and will improve future keratoconus diagnosis by providing more-accurate ET estimates.
上皮厚度（ET）测绘对于圆锥角膜的检测很有用，圆锥角膜是一种以角膜变薄和膨出为特征的疾病，其上皮变薄发生在角膜顶端。在先前的临床研究中，光学相干断层扫描（OCT）测量的ET系统性地比40兆赫高频超声（HFU）测量的结果更薄，在HFU测量中，所有角膜层都采用1636米/秒的恒定声速（）。这项工作的目的是使用扫描声学显微镜（SAM）独立研究角膜上皮和基质的声学特性，即声速（）、声阻抗（）和衰减（），以调查OCT和HFU对ET估计值之间的差异。
将12只未固定的猪角膜速冻，使用定制的SAM和F-1.08、500兆赫换能器以及264兆赫带宽扫描6微米厚的切片。得出了空间分辨率为4微米的、和的二维图谱。
SAM显示上皮中的值（即1548±18米/秒）明显低于基质中的值（即1686±33米/秒）。
SAM结果表明，假设所有角膜层的值恒定是不正确的，这解释了先前OCT和HFU测定ET之间的差异。
本研究结果对基于HFU的ET测量具有重要意义，并将通过提供更准确的ET估计值来改善未来圆锥角膜的诊断。
