What are the next steps for a patient with a pancreatic MRI showing an area with dropout signal on out-of-phase imaging and slight enhancement, suggestive of a potential pancreatic lesion?

Pancreatic Lesion with Signal Dropout on Out-of-Phase MRI

The finding of signal dropout on out-of-phase imaging with slight enhancement most likely represents a lipid-containing lesion such as a lipoma or focal fatty infiltration, but you must obtain a dedicated pancreatic protocol MRI with MRCP to fully characterize the lesion and exclude other pathology. 1, 2

Understanding the Signal Dropout Finding

• Signal dropout on out-of-phase (opposed-phase) imaging indicates the presence of both fat and water within the same voxel, which causes signal cancellation 3
• This finding is most consistent with benign entities like focal fatty infiltration, lipoma, or rarely a well-differentiated liposarcoma 3
• The "slight enhancement" you describe is atypical for pure fat and raises concern that this may represent a more complex lesion requiring further characterization 3

Recommended Next Step: Dedicated Pancreatic MRI with MRCP

You should proceed with a comprehensive pancreatic protocol MRI with MRCP as the definitive next imaging study. 1, 2

Why MRI with MRCP is the Optimal Choice:

• The American College of Radiology recommends MRI with MRCP as the preferred imaging modality for characterizing pancreatic lesions due to superior soft-tissue contrast and ability to demonstrate ductal communication 2
• MRI provides superior assessment of internal architecture, septations, and mural nodules compared to CT 2
• MRCP has up to 100% sensitivity for demonstrating communication with the pancreatic duct, which is crucial for distinguishing cystic neoplasms from pseudocysts 2
• MRI avoids radiation exposure, which is important if serial follow-up imaging becomes necessary 2

Technical Specifications for the MRI Protocol:

• Obtain sequences including T1-weighted in-phase and out-of-phase imaging, T2-weighted imaging, diffusion-weighted imaging, and dynamic contrast-enhanced sequences with late arterial/pancreatic phase (40-50 seconds) and portal venous phase (70 seconds) 1
• Include thin-slice 3-D MRCP acquisitions to evaluate ductal anatomy 2
• Use gadolinium-based contrast unless contraindicated by renal function (eGFR <30 mL/min/1.73m²) 1

Alternative: CT Pancreas Protocol if MRI Unavailable

If MRI cannot be performed due to contraindications (pacemaker, severe claustrophobia, patient instability), proceed with dual-phase contrast-enhanced pancreatic protocol CT 1, 2

• CT should include late arterial/pancreatic phase (40-50 seconds) and portal venous phase (70 seconds) with thin-slice acquisition (≤3 mm) 1
• CT is particularly useful for identifying calcifications within the lesion or background parenchyma 2
• However, CT has lower sensitivity (73.9-93.6%) for detecting internal septations and mural nodules compared to MRI (96.8% sensitivity) 2

Role of EUS-FNA: Reserved for Specific Scenarios

Do not routinely proceed to EUS-FNA based solely on the current MRI finding. 4, 2

When to Consider EUS-FNA:

• Only if the dedicated pancreatic MRI demonstrates worrisome features such as: 4
  • Cyst size ≥3 cm with enhancing mural nodules
  • Thickened/enhancing cyst walls
  • Main pancreatic duct dilation ≥5 mm
  • Solid component within a cystic lesion
• EUS-FNA should be performed when results will change clinical management, not for routine characterization 2
• The addition of EUS-FNA to diagnostic workup significantly alters management in 72% of patients with worrisome features 4

Critical Differential Diagnosis to Consider

Based on signal dropout with enhancement, your differential should include:

• Focal fatty infiltration (most likely if no mass effect) 3
• Lipoma (benign, should show no enhancement) 3
• Solid pseudopapillary tumor (can contain fat, typically in young women) 5
• Pancreatic neuroendocrine tumor with fatty degeneration (rare, would show avid arterial enhancement) 5
• Liposarcoma (extremely rare in pancreas, would show thick septations and nodular enhancement) 5

Common Pitfalls to Avoid

• Do not assume this is benign based on signal dropout alone - the presence of enhancement suggests tissue complexity that requires full characterization 3
• Do not proceed directly to biopsy without complete imaging - this risks misdiagnosis and unnecessary procedures, as illustrated by cases where presumed malignancies were actually inflammatory collections 6
• Do not rely on CT if MRI is available - CT has significantly lower soft-tissue contrast resolution and may miss important features 2, 3
• Do not order non-contrast imaging - contrast enhancement patterns are essential for characterization 1

Clinical Context That Modifies the Approach

If the patient has impaired renal function (eGFR <30 mL/min/1.73m²):

• Perform unenhanced MRI with MRCP, which remains highly diagnostic without gadolinium 1

If the patient has a history of pancreatitis:

• Consider pseudocyst or walled-off necrosis in the differential, though these typically don't show signal dropout 7
• MRCP is particularly valuable for demonstrating ductal communication 7

If the patient has diabetes mellitus:

• Consider non-malignant pancreatic collections in the differential, as these can mimic malignancy 6