Table 1. Mechanisms of strain measurement for different methods and their possible applications.

Type	Approach	Mechanism of mechanical strain evaluation	Strain range	Test	Target tissue	Reference examples
Electro-based	Strain gauge	The deformation of tissues induces the electrical signal changes, which can be converted into strain values of the tissues.	10∼107µε	Discrete In vivo Ex vivo	Bone Cartilage	Takano et al. (1999),Szivek, Anderson & DeYoung (1997),Pintar et al. (1995) Rolf et al. (1997) Markolf et al. (1998)
	Strain transducers				Ligament Tendon Muscle
Light-based	Microscopy camera	The relative strain is assessed by comparing the images before and after the tissue deformation.	102∼106µε	Serial Ex vivo	Cartilage Ligament Tendon Nerve Blood vessel	Moo et al. (2018) Wright et al. (1996),Bartell et al. (2015) Teo, Dutton & Han (2010) Butler et al. (1990) Squire, Rogers & Edelman (1999)
US-based	Tissue Doppler imaging	The strain is calculated from US images of the tissues, according to the Doppler effect (frequency shift) of the reflected US incited by the deformation of tissues.	103∼106µε	Serial Ex vivo In vivo	Myocardial wall Gastric wall Vascular wall	Perk, Tunick & Kronzon (2007) O’Neill et al. (2007) Ling, Zheng & Patil (2007) Amundsen et al. (2006) Gilja et al. (2002) Liang, Zhu & Friedman (2008) Wilson, Press & Zhang (2009b) Bihari et al. (2013)
	US elastography	The strain of tissues is assessed by the correlation of the pulsed US echo signals in windows before and after tissue deformation.		Serial In vivo Ex vivo	Myocardial wall Gastric wall Vascular wall Cartilage Tendon Ligament
	Speckle tracking echocardiography	Strain is quantified from changed reflection US interference patterns in the US images during the deformation of the tissues.		Serial In vivo Ex vivo
Magnet-based	Tag tracking MRI	The applied magnetization tags in the tissues change with the deformation of tissues and strain messages can be extracted from the changed images of tags.	102∼106µε	Serial In vivo Ex vivo	Myocardium Bone Cartilage Tendon Ligament Liver Brain	Axel (1997) Al Nazer et al. (2008) Sutter et al. (2015) Sheehan & Drace (2000) Mannelli et al. (2012) Hirsch et al. (2013)
	Elastography MRI	Strain is assessed from changed signal patterns in MR images obtained from the tissues before and after their deformation.		Serial In vivo Ex vivo
CT-based	CT	Strain values are acquired from the changes of reconstructed 3D structure of tissues before and after deformation.	10∼104µε	Serial In vivo Ex vivo	Bone Cartilage Heart Calcified cartilage Blood vessel	Novitskaya et al. (2014) Halonen et al. (2014) Pierce et al. (2016) Boekhoven et al. (2014) Gustafsson et al. (2018)
TC for image processing	DIC	Strain is evaluated by tracking the subsets including markers or speckles on the surface of tissues.	102∼104µε	Serial Ex vivo	Bone Blood vessels Other tissues with marked surface	Hussein, Barbone & Morgan (2012) Sheehan & Drace (2000) Bey et al. (2002a) Cyganik et al. (2014)
	DVC	Strain is evaluated by tracking image subsets by tracking the natural pattern in the tissues.		Serial In vivo Ex vivo	All tissues with specific structure features	Bay (1995) McKinley, English & Bay (2003) Bay et al. (1999) Toh et al. (2006)