Ultrasound-based imaging methods of the kidney—recent developments

In recent years, several novel ultrasound (US)-based techniques have emerged for kidney diagnostic imaging, including tissue stiffness assessment with elastography, Ultrasensitive Doppler techniques, and contrast-enhanced ultrasonography to assess renal microvascularization. Renal elastography has become available with the development of noninvasive quantitative techniques, following the rapidly growing field of liver fibrosis diagnosis. With the increased incidence of chronic kidney disease, noninvasive diagnosis of renal fibrosis can be of critical value. However, it is difficult to simply extend the application of US elastography from one organ to the other due to anatomic and technical issues. Today, renal elastography appears to be a promising application that, however, still requires optimization and validation. New ultrasensitive Doppler techniques improve the detection of slow blood flow and can be used alone or after administration of US contrast agents. These microbubble-based agents are extremely well tolerated and can be administered even in cases of impaired renal function. Despite the lack of approval, they improve the characterization of atypical renal masses, complex cystic renal masses, and peripheral vascular disorders. Dynamic contrast-enhanced US is based on quantification of the signal intensity from region of interest and mathematical fits of the time-intensity curves. Perfusion-related parameters can be extracted for the monitoring of vascular changes in the renal parenchyma and in tumors in order to evaluate drug response. This estimation of renal perfusion depends on many parameters that should be kept constant for follow-up studies, and, when possible, an internal reference should be used to normalize the measurements.