What are the next steps for a patient with simple cysts in both kidneys, a partially distended urinary bladder, and a discrepancy in right kidney size between a prior computed tomography (CT) scan and a current renal ultrasound?

Management of Discrepant Renal Imaging Findings

Obtain a contrast-enhanced MRI of the abdomen to resolve the discrepancy between the CT and ultrasound findings regarding right kidney size. 1

Rationale for MRI as Next Step

The American College of Radiology guidelines establish MRI as the superior modality for characterizing indeterminate renal findings, with significantly higher specificity (68.1%) compared to CT (27.7%) for distinguishing between different types of kidney masses 2, 1. MRI without and with IV contrast is the most appropriate next imaging study to investigate the unexplained right kidney enlargement seen on prior CT but not confirmed on ultrasound. 1

Why MRI Over Other Modalities

• MRI excels at characterizing renal lesions <1.5 cm that may have been missed on ultrasound, and can detect subtle enhancement patterns that indicate solid masses versus complex cysts 2, 1
• MRI is more sensitive than ultrasound for detecting small renal masses and can better evaluate the internal architecture of cystic lesions 2
• MRI can definitively characterize the simple cysts already identified on ultrasound and determine if additional pathology exists 1, 3

What to Look For on MRI

The MRI should specifically evaluate:

• Confirmation or exclusion of true kidney enlargement (comparing measurements to prior CT) 1
• Detection of any solid masses that may have been obscured on ultrasound 2
• Characterization of the bilateral simple cysts using Bosniak classification criteria 3
• Assessment for enhancement patterns that would indicate malignancy (>15% enhancement threshold distinguishes cysts from solid tumors) 2
• Evaluation for complex features such as thick septa, nodularity, or wall thickening that ultrasound may have missed 3

Management of the Simple Cysts

The bilateral simple cysts identified on ultrasound require minimal intervention:

• Simple cysts meeting ultrasound criteria (sonolucent, posterior acoustic enhancement, thin walls) are benign and require no further imaging if truly simple 2
• However, ultrasound has limitations in detecting subtle complexity, and MRI may reveal additional features (septal thickening, enhancement) that could upgrade classification 3
• Simple cysts are extremely common and increase with age, typically remaining asymptomatic unless complicated by hemorrhage, infection, or rupture 4

Critical Pitfall to Avoid

Do not dismiss the CT finding of right kidney enlargement simply because ultrasound showed normal size. 1 This discrepancy represents an indeterminate finding that requires resolution:

• Ultrasound is operator-dependent and may underestimate kidney size or miss subtle parenchymal abnormalities 5
• The prior CT finding of enlargement could indicate early infiltrative disease, collecting system obstruction that has resolved, or a measurement artifact 5
• MRI will definitively resolve this discrepancy and provide superior tissue characterization 2, 1

If MRI is Contraindicated

Should the patient have contraindications to both iodinated CT contrast and gadolinium-based MRI contrast:

• Contrast-enhanced ultrasound (CEUS) with microbubble agents is an appropriate alternative with 95% specificity for classifying benign versus malignant masses 2
• CEUS has 95.2% accuracy for characterizing indeterminate renal masses compared to only 42.2% for unenhanced ultrasound 2
• Microbubble agents are not nephrotoxic and do not affect renal function, making them safe in patients with renal impairment 2

What NOT to Do

• Do not perform routine follow-up ultrasound without further characterization of the size discrepancy 1
• Do not obtain unenhanced CT, as it cannot adequately characterize renal masses or assess for enhancement 2
• Do not pursue renal biopsy for cystic lesions, as it has low diagnostic yield unless solid components are present 3
• Do not assume the simple cysts explain the prior enlargement without confirming no other pathology exists 1