Renal Elastography for the Assessment of Chronic Kidney Disease

*Flaviu Bob*

#### **Abstract**

For the assessment of chronic kidney disease, point shear wave elastography (pSWE) and shear wave speed imaging (2D-SWE) are suitable, but the use of elastography in the assessment of the kidneys is more difficult compared to the use in other organs, because of the complex architecture of the kidneys, characterized by a high anisotropy and also by the limited size of the renal parenchyma, where the measurements are performed. Despite the difficulties of renal elastography, the reproducibility of the method is good. Kidney shear wave speed values are influenced mainly by age and gender, while in chronic kidney disease, renal stiffness is sometimes decreased in more advanced disease and is not influenced mainly by the progression of fibrosis. There are studies proving that a decreased renal blood flow is associated with a decrease in kidney shear wave speed, the fact that could explain why patients with CKD tend to have lower kidney stiffness. Elastography is a real-time imaging method that could be useful in the assessment of the kidneys, but more extensive studies and even some improvements of the processing algorithms of raw data of elastography machines seem to be needed to implement the use in clinical practice.

**Keywords:** chronic kidney disease, kidney shear wave speed, renal stiffness, point shear wave elastography, shear wave speed imaging

#### **1. Introduction**

Chronic kidney disease (CKD), a progressive disease, with high morbidity and mortality, therefore associated with increased health costs, is becoming a public health problem because of the increasing incidence and prevalence. For the diagnosis of CKD, biochemical markers are used—glomerular filtration rate, estimated from the level of serum creatinine and urinary albumin/creatinine ratio. For the assessment of the progression of CKD histology can often be helpful, and different new biomarkers are emerging as important tools as well [1].

The use of imagistic methods in the early diagnosis, or to assess the progression of CKD is very limited. Conventional ultrasound is helpful in diagnosing cystic kidney diseases, which represent a small proportion of the causes of CKD. Regarding the most frequent etiologies of CKD (diabetes mellitus, arterial hypertension, glomerular diseases, or chronic tubulointerstitial diseases) information provided by ultrasound

is of limited help. Using conventional ultrasound we can quantify the renal size and parenchymal thickness, both decreasing in advanced stages of CKD, when due to the progression of fibrosis, the echogenicity of the renal cortex is increasing [2].

The increased echogenicity, observed by the investigator is however not quantifiable using conventional ultrasound, being therefore subjective. An ultrasound-based method that has proven its utility in the assessment of fibrosis in different other organs (liver in both, diffuse [3, 4] or focal lesions [5], spleen [6, 7], thyroid [8, 9] or prostate [10]), by measuring the stiffness of the tissue is elastography.
