# Renal Ultrasound Elastography: A Review of the Previous Reports on Chronic Kidney Diseases
## 1 Introduction  
The number of patients with chronic kidney disease (CKD) is increasing worldwide [1]. CKD is a risk factor for cardiovascular disease and the incidence rate of cardiovascular events increases with the progression of CKD [2]. Furthermore, CKD is progressive in nature, leading to end-stage renal disease [3]. Therefore, the early detection and management of CKD is needed. The clinical assessment of CKD relies on the glomerular filtration rate, which is estimated by the serum creatinine levels in most cases and urinary protein levels or albumin excretion rates. However, because the kidney has a compensatory mechanism in response to nephron loss, serum creatinine or urinary protein levels are not accurate indices for detecting early changes in CKD patients. Another issue requiring improvement in the clinical practice in patients with CKD is that renal pathological changes such as glomerulosclerosis, tubular atrophy, and interstitial fibrosis can be assessed only by a renal biopsy. A renal biopsy is also necessary to assess the alterations in transplanted kidneys; thus, it is an undisputed procedure for assessing renal damage and determining treatment strategies in the clinical practice of nephrology. However, a renal biopsy should be performed with a careful consideration of its benefits and potential complications such as hemorrhages and a loss of renal function. Several studies have investigated the potential benefits of serum or urinary markers in predicting renal histological changes or renal outcomes [4; 5; 6; 7]. Additionally, a few methods to reduce the potential risk of renal biopsies have been proposed [8]. As there is no established method for assessing pathological changes without a renal biopsy, it has been clinically challenging to detect early changes in kidney injuries and to non-invasively assess the pathological changes.  
Ultrasound elastography is a non-invasive technique for assessing tissue elasticity on the basis of the pathological and physiological principle that solid tumors or fibrotic tissues have different elasticities compared to normal tissues. Ultrasound elastography provides useful information and is convenient and non-invasive. Moreover, it has been validated in many clinical settings including liver disease, breast cancer, thyroid or parathyroid diseases, and pancreatic cancer [9; 10; 11; 12; 13; 14]. Ultrasound elastography is a well-established and widely used technique for the non-invasive assessment of liver fibrosis in chronic liver disease [15; 16]. Furthermore, the elastography value obtained using this technique correlated positively with the degree of hepatic fibrosis in patients with chronic hepatitis caused by hepatitis B and C virus infections [17]. In a meta-analysis, [PERSON] et al. demonstrated that ultrasound elastography had a good diagnostic performance in non-viral hepatic diseases especially severe fibrosis [18]. For a better understanding of this technique, we have reviewed and summarized the clinical evidence for renal ultrasound elastography and the technical aspects necessary for its practical application.