# Renal Ultrasound Elastography: A Review of the Previous Reports on Chronic Kidney Diseases
## 3 The Use of Renal Ultrasound Elastography  
Ultrasound elastography of the kidney has been investigated in renal malignant tumors, transplanted kidneys, and native kidneys in patients with kidney diseases. Although several reports have used strain elastography, most of the previous reports on the kidney were based on SWE. The basic technical procedure of renal ultrasound elastography in the kidney is similar to that of other organs; however, several aspects specific to performing SWE on the kidney have been reported. First, the target ROI must be carefully selected (Figure 1). [PERSON] et al. investigated the potential factors influencing the results of the ARFI measurements. The depth of the ROI affects the SWV [23; 24]. The push-pulse to induce shear waves tends to attenuate in deeply located tissues; thus, the elastography values correlate negatively with the depth of the ROI. Therefore, in contrast with the superficial organs such as the breast and thyroid, the depth of the ROI should be matched to the best extent. For this reason, it is preferable to perform SWE of the kidney with the patients placed in prone or unilateral positions. Most previous reports set the ROI at the renal cortex or parenchyma. It has been suggested that the measurement in the cortex is more reliable than in the medulla [25]. In patients with an impaired renal function, the kidneys show atrophic changes. The shortening of the kidney length and lessening of the parenchymal thickness in advanced kidney disease are widely recognized changes often observed by an abdominal ultrasound [26; 27]. Although a careful determination of the ROI is required, the SWV can be measured reproducibly even in a patient with end-stage renal disease with a thin parenchymal thickness [28]. Breathing also needs to be considered whilst measuring the renal elasticity. Breathing does not affect the SWV in the liver [29]. However, because the kidney moves along with the breath, the SWV should be measured whilst the breath is held. In addition, a compression with an applied ultrasound transducer affects the SWV in the liver and kidney [30; 31]. Therefore, the operator must carefully apply the transducer to minimize the pressure of the transducer to the best extent. Third, the complexity of the renal structure must be considered. The anisotropy of the target organ influences the SWE results. Shear waves spread faster and slower parallel and  
Figure 1: Image of a shear wave elastography measurement. Representative ultrasound image for the measurement of renal elasticity using a Siemens Acuson S2000 ultrasound system (Siemens, Erlangen, Germany) with convex probes (4C1, frequency range: 1–4 MHz) and a mechanical index of 1.7. The region of interest was set perpendicular to the renal capsule. perpendicular to the structure, respectively [32]. The renal parenchyma is anatomically divided into two compartments with different anatomical structures. The renal cortex contains glomeruli, proximal convoluted tubules, and distal convoluted tubules. The renal medulla contains the loop of Henle, collecting duct, and vasa recta. The structures in the medulla are organized and radially oriented whereas the structures in the cortex are unevenly distributed and oriented in a convoluted manner [33]. According to the principle of shear wave propagation, the elasticity values are low when the shear waves propagate perpendicular to the renal structure and parallel to the renal capsule. Therefore, the position of the ultrasound transducer and the direction of the kidney must be set under the same condition. Most previous studies on renal elasticity investigated renal parenchymal stiffness with the shear waves parallel to the renal capsule. The other factor that influences the SWE results is the viscoelasticity of the tissue. Shear wave generation is attenuated in tissues with a high fluid composition. Considering the complex anatomical structure of the kidney, the ROI should be placed in an area without vasculature or cysts.