Okotie Gregory, Duenwald-Kuehl Sarah, Kobayashi Hirohito, Wu Mon-Ju, Vanderby Ray
Department of Orthopedics and Rehabilitation, University of Wisconsin-Madison, Madison, WI 53705, USA.
J Biomech Eng. 2012 Feb;134(2):024504. doi: 10.1115/1.4006116.
Strain is an essential metric in tissue mechanics. Strains and strain distributions during functional loads can help identify damaged and pathologic regions as well as quantify functional compromise. Noninvasive strain measurement in vivo is difficult to perform. The goal of this in vitro study is to determine the efficacy of digital image correlation (DIC) methods to measure strain in B-mode ultrasound images. The Achilles tendons of eight male Wistar rats were removed and mechanically cycled between 0 and 1% strain. Three cine video images were captured for each specimen: (1) optical video for manual tracking of optical markers; (2) optical video for DIC tracking of optical surface markers; and (3) ultrasound video for DIC tracking of image texture within the tissue. All three imaging modalities were similarly able to measure tendon strain during cyclic testing. Manual/ImageJ-based strain values linearly correlated with DIC (optical marker)-based strain values for all eight tendons with a slope of 0.970. DIC (optical marker)-based strain values linearly correlated with DIC (ultrasound texture)-based strain values for all eight tendons with a slope of 1.003. Strain measurement using DIC was as accurate as manual image tracking methods, and DIC tracking was equally accurate when tracking ultrasound texture as when tracking optical markers. This study supports the use of DIC to calculate strains directly from the texture present in standard B-mode ultrasound images and supports the use of DIC for in vivo strain measurement using ultrasound images without additional markers, either artificially placed (for optical tracking) or anatomically in view (i.e., bony landmarks and/or muscle-tendon junctions).
应变是组织力学中的一个重要指标。功能负荷期间的应变和应变分布有助于识别受损和病理区域，并量化功能损害。体内无创应变测量很难进行。这项体外研究的目的是确定数字图像相关（DIC）方法在测量B型超声图像应变方面的有效性。切除八只雄性Wistar大鼠的跟腱，并在0%至1%应变之间进行机械循环。为每个标本采集了三段电影视频图像：（1）用于手动跟踪光学标记的光学视频；（2）用于DIC跟踪光学表面标记的光学视频；（3）用于DIC跟踪组织内图像纹理的超声视频。在循环测试期间，所有三种成像方式都同样能够测量肌腱应变。对于所有八条肌腱，基于手动/ImageJ的应变值与基于DIC（光学标记）的应变值呈线性相关，斜率为0.970。对于所有八条肌腱，基于DIC（光学标记）的应变值与基于DIC（超声纹理）的应变值呈线性相关，斜率为1.003。使用DIC进行应变测量与手动图像跟踪方法一样准确，并且DIC跟踪超声纹理时与跟踪光学标记时同样准确。本研究支持使用DIC直接从标准B型超声图像中存在的纹理计算应变，并支持使用DIC在不使用额外标记（无论是人工放置（用于光学跟踪）还是解剖可见（即骨标志和/或肌腱-肌肉连接处））的情况下，利用超声图像进行体内应变测量。
