# Renal Ultrasound Elastography: A Review of the Previous Reports on Chronic Kidney Diseases
## 2 Principles of Ultrasound Elastography  
Ultrasound elastography can be classified into two techniques based on the physical quantity measured [19]. One of these is strain elastography where the compression of the target tissue induces tissue displacement; the displacement in the same direction as the compression is quantified. The target is manually compressed using an ultrasound transducer or an acoustic radiation force impulse (ARFI) [20]. The manual compression method can be applied only to superficial organs such as the breast and thyroid [19] owing to the poor transmission of the compression. Thus, this method may not be suitable for organs located deeper, including the kidneys. An ARFI employs a short-duration push-pulse from the transducer that is used to induce the tissue displacement.  
The second technique is shear wave elastography (SWE). In SWE, tissue deformation induced by an ARFI or mechanical vibration by an ultrasound device generates a shear wave that propagates perpendicular to the push-pulse [21]. There are three approaches to generate a shear wave; point shear wave elastography (pSWE), two-dimensional shear wave elastography (2D-SWE), and one-dimensional transient elastography (1D-TE). pSWE uses an ARFI to generate the tissue displacement in a single focal location; the shear wave propagates perpendicular to the excitation and is then measured. The result of the pSWE is directly reported as the shear wave velocity (SWV) or converted to Young's modulus E according to the equipment. Two-dimensional shear wave elastography (2D-SWE) is a newly invented technique using an ARFI. Instead of a single focal location in pSWE, 2D-SWE generates shear waves from multiple focal zones. This enables a real-time monitoring of the SWV or Young's modulus in a relatively wide target region of interest (ROI) [19]. In contrast with the other two techniques, the 1D-TE technique uses physical vibrations from the transducer to generate the shear wave. The diagnostic accuracies for the tissue elasticity differ among the techniques. pSWE and 2D-SWE can accurately detect liver fibrosis compared to 1D-TE [19]. Although 1D-TE has been widely applied to the superficial organs such as the liver and thyroid, its usefulness seems to be limited in deeply located organs, especially the kidney. The tissue elasticity can be assessed by measuring the propagation speed of the shear waves. In general, the shear waves transmit faster in hard tissues and slower in soft tissues. The results of the tissue elastography can be reported as a shear wave speed (meters per second) or Young's modulus (kilopascals) according to the ultrasound system. The equations for these parameters were well-described in a previous study [19]. The SWV correlated positively with the stage of fibrosis in chronic liver disease [9; 21].  
In reporting the results of tissue elasticity, the quality of the measurement needs to also be addressed. It is recommended in ultrasound liver elastography to report the number of the required acquisitions with at least ten measurements and the interquartile range-to-median ratio [22].