Current and potential renal applications of contrast-enhanced ultrasound

The combination of microbubble technology and complementary ultrasound techniques has resulted in the development of contrast-enhanced ultrasound (CEUS) and, although initial clinical applications largely focussed on the liver, these are now becoming more diverse. With regard to the kidney, it is a safe, well-tolerated, and reproducible technique, and in selected cases, can obviate the need for computed tomography or magnetic resonance imaging. A clear advantage is the absence of nephrotoxicity. With respect to the current and potential renal applications, it is a useful technique in the evaluation of pseudotumours, acute pyelonephritis, renal tumours, cystic lesions, vascular insults, and renal transplantation. It may also be of value for monitoring the kidney following anti-angiogenic treatment or nephron-sparing interventional techniques for renal tumours. Assessment of microvascular perfusion using time–intensity curves is also likely to have further far-reaching applications in the kidney as well as other organs.

Introduction

Conventional ultrasound (US), which is safe, inexpensive, and almost universally available, is often the first-line investigation for renal disease. It incorporates the assessment of renal size, cortical thickness, and echogenicity, besides detection of obstruction, masses, stones, and vascular abnormalities. When images are non-diagnostic, further investigation with computed tomography (CT) and/or magnetic resonance imaging (MRI) is often undertaken to clarify, confirm, or refute the ultrasonic findings.

Although an accurate diagnosis is mandatory, it is important that the relatively high dose of radiation to patients at CT be considered. The effective dose from abdominal CT ranges from 6–24 mSv,1, 2 and it is estimated that 1.5–2% of all cancers may be attributable to radiation from CT examinations.³ In addition, patients with renal lesions are often scanned in multiple phases and may require follow-up studies. Furthermore, the impact of iodinated contrast agents in patients with potential renal impairment must be borne in mind. With respect to magnetic resonance imaging (MRI), there are occasionally issues related to patient tolerance⁴ and safety,5, 6 in particular the risk of nephrogenic systemic fibrosis in patients with estimated glomerular filtration rates (eGFR) less than 30 ml/min exposed to MRI contrast agents.

A safe alternative can now be considered. Contrast-enhanced ultrasound (CEUS), which employs microbubble contrast agents and complementary harmonic pulse sequences to demonstrate parenchymal perfusion, can be a helpful problem-solving tool in a number of clinical scenarios. The advantages of CEUS are well-documented and include its safety,7, 8, 9 simplicity, patient tolerance, lack of radiation, and real-time multiplanar imaging capability allowing prompt diagnosis. Clearly the role of this technique will depend on the patient’s clinical history and presentation remembering that some clinical issues still remain better addressed with cross-sectional imaging. CEUS, like all techniques, has limitations and these reflect the limitations of ultrasound, e.g., patients with large body habitus and poor imaging with conventional ultrasound are unlikely to gain benefit from a CEUS examination and may be more suitable for CT and/or MRI.

The vast majority of CEUS work has focused on the liver with studies demonstrating substantial improvement in detection and characterization of focal liver lesions compared with conventional ultrasound10, 11, 12, 13 with sensitivity and specificity approaching that of CT and MRI.14, 15 This review will focus on current and potential uses of CEUS with regard to the kidney. A number of such indications are well delineated in the European Federation of Societies for Ultrasound in Medicine and Biology (EFSUMB) guidelines¹⁶ (Table 1). However, it is important to remember that despite the above guidelines, the product licence for SonoVue, used in our practice, currently does not extend to renal applications and such studies are performed on an individual patient basis at the radiologist’s discretion.