Advanced ultrasound applications in the assessment of renal transplants: contrast-enhanced ultrasound, elastography, and B-flow

Abstract

Ultrasound is routinely used as the first imaging exam for evaluation of renal transplants and can identify most major surgical complications and evaluate vascularity with color Doppler. Ultrasound is limited, however, in the detection of parenchymal disease processes and Doppler evaluation is also prone to technical errors. Multiple new ultrasound applications have been developed and are under ongoing investigation which could add additional diagnostic capability to the routine ultrasound exam with minimal additional time, cost, and patient risk. Contrast-enhanced ultrasound (CEUS) can be used off-label in the transplant kidney, and can assist in detection of infection, trauma, and vascular complications. CEUS also can demonstrate perfusion of the transplant assessed quantitatively with generation of time-intensity curves. Future directions of CEUS include monitoring treatment response and microbubble targeted medication delivery. Elastography is an ultrasound application that can detect changes in tissue elasticity, which is useful to diagnose diffuse parenchymal disease, such as fibrosis, otherwise unrecognizable with ultrasound. Elastography has been successfully applied in other organs including the liver, thyroid, and breast; however, it is still under development for use in the transplant kidney. Unique properties of the transplant kidney including its heterogeneity, anatomic location, and other technical factors present challenges in the development of reference standard measurements. Lastly, B-flow imaging is a flow application derived from B-mode. This application can show the true lumen size of a vessel which is useful to depict vascular anatomy and bypasses some of the pitfalls of color Doppler such as demonstration of slow flow.

Contrast‐Enhanced Ultrasound for the Evaluation of Renal Transplants

“…With CEUS, peripheral and wedge-shaped areas of hypoenhancement are suspicious for underlying perfusion defects. 46 Additionally, CEUS can show areas of acute cortical necrosis, which will appear as diffusely diminished enhancement of the renal cortex. 46,47 Less commonly, CEUS has been used to evaluate the transplant urinary tract, particularly in patients with recurrent urinary tract infections.…”

“…46 Additionally, CEUS can show areas of acute cortical necrosis, which will appear as diffusely diminished enhancement of the renal cortex. 46,47 Less commonly, CEUS has been used to evaluate the transplant urinary tract, particularly in patients with recurrent urinary tract infections. Vesicoureteral reflux is a potential etiology of recurrent urinary tract infections in patients with renal transplants and is typically assessed with voiding cystourethrography or radionuclide voiding cystography.…”

“…Grayscale and color Doppler US can be insensitive for the detection of early ischemia. With CEUS, peripheral and wedge‐shaped areas of hypoenhancement are suspicious for underlying perfusion defects 46 . Additionally, CEUS can show areas of acute cortical necrosis, which will appear as diffusely diminished enhancement of the renal cortex 46,47 …”

Evolving Medical Imaging Techniques for the Assessment of Delayed Graft Function: A Narrative Review

“…Figure 5A to D shows representative CEUS images from a normal renal transplant. 77 Upon the administration of the microbubble contrast bolus, a normal transplant will demonstrate enhancement of the arteries within the first 20 seconds ( Figure 5A ). In the early parenchymal phase (20-45 seconds), the cortex shows enhancement first while the renal pyramids appear hypoechoic ( Figure 5B ).…”

Evaluation of the Graft Kidney in the Early Postoperative Period

“…6,15,16 Similarly, shear wave elastography (SWE) provides a quantitative measure of the stiffness of the graft kidney, and this has also been shown to correlate with parenchymal causes of graft dysfunction in a few studies. [17][18][19] The kidneys are ideal organs for evaluation by CEUS because of their high perfusion. Second, the transplanted kidney lies superficially in the iliac fossa, making an ultrasound (US) assessment easier and more accurate.…”