For a patient with a mass in the left leg and knee pain, and potential renal impairment or allergy to gadolinium-based contrast agents, is an MRI with or without contrast the best initial diagnostic step?

MRI for Left Leg Mass and Knee Pain: Contrast Recommendation

For a patient with a mass in the left leg and knee pain, order MRI without IV contrast as the initial study, which is usually appropriate and avoids gadolinium-related risks in patients with potential renal impairment. 1

Initial Imaging Algorithm

Start with plain radiographs of the affected area. 1 The American College of Radiology recommends radiographs as the initial imaging study for soft-tissue masses, as they can identify calcifications, bone involvement, and help characterize the lesion. 1

• For deep masses in the leg (below the deep fascia), radiographs are usually appropriate as the first step. 1
• Radiographs can reveal bone destruction, periosteal reaction, or soft tissue calcifications that narrow the differential diagnosis. 1

When Radiographs Are Nondiagnostic

If radiographs are nondiagnostic or show only nonspecific findings, proceed to MRI without IV contrast. 1 The ACR guidelines explicitly state that MRI without IV contrast is usually appropriate for further evaluation of soft-tissue masses when initial imaging is nondiagnostic. 1

Why MRI Without Contrast Is Preferred Initially

• MRI without contrast provides excellent soft tissue characterization and can distinguish solid from cystic masses, evaluate marrow involvement, and assess the relationship to neurovascular structures. 1
• Contrast is not required for initial characterization of most soft-tissue masses unless there is specific concern for vascular lesions, spontaneous hemorrhage, or when differentiating inflammation/infection from neoplasm. 1
• Gadolinium carries significant risks in renal impairment, including nephrogenic systemic fibrosis (NSF), a devastating and potentially fatal fibrosing disorder affecting skin and systemic organs. 2, 3, 4

When to Add Contrast

MRI without and with IV contrast is indicated if:

• Initial noncontrast MRI suggests a vascular mass or spontaneous hemorrhage. 1
• There is concern for infection, inflammation, or neoplasm requiring enhancement patterns for diagnosis. 1
• The noncontrast study is indeterminate and contrast would change management. 1

Critical Contraindications to Gadolinium

Avoid gadolinium-based contrast agents in patients with:

• Severe renal impairment (GFR <30 mL/min/1.73 m²) due to high risk of nephrogenic systemic fibrosis. 2, 3, 4
• Acute kidney injury of any severity, as NSF risk is markedly elevated. 2
• End-stage renal disease on dialysis, though aggressive hemodialysis immediately after exposure may reduce risk if contrast is absolutely necessary. 2
• Known gadolinium allergy or previous NSF. 2

Gadolinium Safety Considerations

• NSF is associated with larger doses, multiple exposures, and specific non-ionic linear chelates. 2
• The condition causes progressive severe fibrosis of skin and organs, leading to significant disability and increased mortality. 2
• No effective treatment exists for NSF—avoidance is the only reliable prevention strategy. 2
• Even moderate renal impairment (not just dialysis patients) carries risk, affecting a population 20 times larger than dialysis patients. 4

Practical Approach for Your Patient

1. Order plain radiographs of the left leg and knee first. 1
2. If radiographs are nondiagnostic, order MRI left leg without IV contrast. 1
3. Check renal function (creatinine, GFR) before considering any contrast study. 2, 3
4. Only add gadolinium contrast if the noncontrast MRI is indeterminate AND the patient has normal renal function (GFR >30 mL/min/1.73 m²). 1, 2

Common Pitfalls to Avoid

• Do not reflexively order "with and without contrast" for all soft-tissue masses—noncontrast MRI is diagnostic in most cases. 1
• Do not assume gadolinium is safe without checking renal function—even moderate impairment increases NSF risk. 4, 5
• Do not use gadolinium as a substitute for iodinated contrast in renal failure—this outdated practice has been abandoned due to NSF risk. 3, 4
• Do not order CT with contrast as the initial study unless the patient is MRI-incompatible or has metal hardware limiting MRI evaluation. 1