Indications for CEUS

Although considered "off-label", renal applications have quickly gained traction in Europe, primarily focusing on the characterization of indeterminate renal lesions when CT or MRI findings are inconclusive. It is particularly beneficial in patients with contraindications to iodinated or gadolinium-based contrast agents, such as those with renal insufficiency or a history of adverse contrast reactions. Other applications include monitoring lesion progression and guiding minimally invasive procedures like biopsy and ablation [1, 3, 5].

CEUS proves most useful in patients who require dynamic assessment of vascularity and perfusion without exposure to ionising radiation or nephrotoxic agents. It is especially advantageous in differentiating cystic from solid lesions and evaluating complex cystic structures where traditional imaging may provide equivocal findings [1, 2, 4].

Unique features and advantages of CEUS

CEUS offers several advantages over CT and MRI. It provides real-time imaging with superior temporal resolution, allowing precise assessment of vascularity and perfusion dynamics. Importantly, CEUS can differentiate between enhancing solid components and non-enhancing cystic portions, which is crucial in characterising indeterminate lesions. Unlike CT and MRI, CEUS has no reliance on potentially nephrotoxic contrast agents, making it safe for patients with renal dysfunction [1-3, 5].

Another key advantage is its sensitivity to subtle perfusion changes, which aids in detecting small septa or nodules in complex cystic lesions that may otherwise be overlooked. This enhanced sensitivity allows for better detection of malignancy features while reducing unnecessary invasive procedures in certain cases [3, 4].

Limitations of CEUS

Despite its numerous advantages, CEUS has certain limitations. Its heightened sensitivity, particularly in identifying enhancing septa, can sometimes lead to overclassification of lesions, falsely upgrading benign cysts to indeterminate categories. This may cause unnecessary follow-up imaging or interventions [4].

Other limitations include operator dependency and the learning curve associated with interpretation. Additionally, CEUS may be less effective in patients with obesity or extensive bowel gas, which can obscure the acoustic window. Deep-seated lesions or lesions located near renal sinus structures are also challenging to assess. Furthermore, while CEUS offers robust information on perfusion, it lacks the comprehensive anatomical detail provided by CT and MRI, particularly in evaluating the extrarenal spread of disease [2-4, 6].

CEUS imaging of cystic renal lesions

Bosniak category I cysts: simple benign cysts

Bosniak category I cysts are simple, benign cysts characterised by their thin (<2 mm) walls, absence of septa, calcifications, or solid components, and homogeneity without contrast enhancement on CT or MRI. They are generally well-defined, anechoic lesions on ultrasound, with posterior acoustic enhancement and no vascularity. Although CEUS is not routinely required for these cysts, it can be used in challenging cases where conventional imaging is technically limited or ambiguous. CEUS findings, when used, typically confirm the lack of enhancement, reinforcing their benign nature and eliminating the need for further follow-up [2, 3].

• Management: No further follow-up is required.

Bosniak category II cysts: minimally complex benign cysts

On CT/MRI, these cysts display a few (1-3) thin (≤2 mm) smooth septa without measurable enhancement. CEUS demonstrates no enhancement of the thin (<2 mm) cyst wall or septa, confirming their benign nature [2, 3].

• Management: No further follow-up is required.
  

  Fig 1: Case 1 – Bosniak category II cyst. Axial CT image at the corticomedullary phase shows a cystic mass with few (≤3) thin (≤2 mm) enhancing septa.

Bosniak category IIF cysts: presumably benign cysts

These cysts exhibit smooth minimally thickened (3 mm) septa or walls with potential enhancement on CT/MRI. CEUS helps confirm subtle enhancement of these minimally thickened (2-3 mm) structures, sometimes more clearly than CT/MRI, aiding in accurate classification [2, 3].

• Management: Active surveillance with periodic imaging.
  

  Fig 2: Case 1 – Bosniak category IIF cyst. (A) B-mode ultrasound reveals an intrarenal cyst with at least one minimally thickened (2-3 mm) smooth septum. (B) CEUS examination demonstrates enhancement of this minimally thickened septum.

Bosniak category III cysts: indeterminate cystic lesions 

CT/MRI findings include smooth thickened (≥4 mm), or irregular, enhancing septa or walls, often making them indeterminate for malignancy. CEUS typically shows clear enhancement of thick (≥4 mm) or irregular (> 3 mm) walls or septa, aiding in better visualisation of perfusion patterns [2, 3].

• Management: Biopsy or surgical consideration based on patient risk factors and imaging findings.
  

  Fig 3: Case 2 – Bosniak category III cyst. (A) T2-weighted axial MRI showcases a complex lesion with both hypointense and hyperintense components. Axial MRI images of unenhanced (B), postcontrast (C) and subtraction (D) fat-saturated T1-weighted sequences unveil at least one enhancing smooth thick (≥4 mm) septum.
  

  Fig 4: Case 2 – Bosniak category III cyst. B-mode ultrasound (A) and CEUS examination (B) reveal an indeterminate cystic lesion with multiple (≥4) thick (≥4 mm) enhancing septa.

Bosniak category IV cysts: likely malignant cystic tumours 

These cysts are characterised by soft-tissue nodules arising from the wall or septa, with measurable enhancement on CT/MRI. CEUS provides valuable confirmation by demonstrating perfusion within the nodular component, either nodules with obtuse margins (≥4 mm), or with acute marges of any size, strongly indicative of malignancy [2, 3].

• Management: Surgical resection due to high suspicion of malignancy.
  

  Fig 5: Case 3 – Bosniak category IV cyst. Axial CT images at the non-contrast (A), corticomedullary (B) and nephrogenic (C) phases show a focal enhancing nodule, i.e., a convex soft-tissue protrusion (≥4 mm) with acute margins, arising from the cystic wall (arrowhead).
  

  Fig 6: Case 3 – Bosniak category IV cyst. Axial T2-weighted MRI image demonstrates a large markedly hyperintense cystic component and solid wall nodule that is hypointense to the renal cortex (arrowhead). Contrast administration was contraindicated.
  

  Fig 7: Case 3 – Bosniak category IV cyst. (A,B) B-mode ultrasound shows a soft-tissue protrusion (≥4 mm) of the cystic wall with acute margins (arrowhead). (C) CEUS examination reveals enhancement of this wall nodularity (arrowhead), indicating a likely malignant cystic tumour.

CEUS imaging of solid renal lesions 

On CT/MRI, solid renal lesions may present as non-enhancing or heterogeneously enhancing masses with variable signal intensities [2]. CEUS offers superior temporal resolution, allowing detailed assessment of vascular patterns and differentiating between hypervascular and hypovascular lesions. This dynamic information can assist in diagnosing renal cell carcinoma (RCC) and guiding biopsy decisions [1, 5, 6].

• Management: CEUS findings suggesting malignancy typically warrant surgical resection or biopsy confirmation.
  

  Fig 8: Case 4 – Solid kidney lesion. Axial CT images at the precontrast (A) and early arterial (B) phases show a non-enhancing, slightly heterogeneous exophytic mass at the inferior pole of the right kidney.
  

  Fig 9: Case 4 – Solid kidney lesion. Coronal images demonstrate a renal mass that is hypointense at T2-wheighted sequence (A) and heterogeneously hyperintense at unenhanced fat-saturated T1-weighted MRI (B). Postcontrast (C) and subtraction (D) fat-saturated T1-weighted sequences show questionable arterial enhancement.
  

  Fig 10: Case 4 – Solid kidney lesion. (A,C) B-mode ultrasound shows an echogenic and slightly heterogenous kidney mass with a 45 mm diameter and a well-defined margin, adjacent to a simple cortical cyst. CEUS images at 0 min (B) and 2 min 30 sec (D) show heterogeneous enhancement of the solid nodule, which is concerning for renal cell carcinoma.