Which MRI sequences are required to diagnose posterior reversible encephalopathy syndrome (PRES)?

MRI Sequences Required for PRES Diagnosis

For diagnosing posterior reversible encephalopathy syndrome (PRES), you need T2-weighted FLAIR sequences and diffusion-weighted imaging (DWI) with apparent diffusion coefficient (ADC) mapping as the essential core sequences. 1

Core Required Sequences

T2-Weighted FLAIR (Fluid-Attenuated Inversion Recovery)

• FLAIR is the primary diagnostic sequence for PRES, showing hyperintense signal in affected brain regions, particularly the parieto-occipital white matter. 1, 2
• Preferably acquire as 3D sequences with maximum 3 mm slice thickness and 1 × 1 mm in-plane resolution for optimal lesion detection. 3
• FLAIR demonstrates the characteristic vasogenic edema pattern that defines PRES, with bilateral subcortical white matter involvement. 2, 4

Diffusion-Weighted Imaging (DWI) with ADC Mapping

• DWI with ADC is essential to distinguish vasogenic from cytotoxic edema, which is critical for PRES diagnosis and differentiating it from stroke. 2, 5, 6
• In PRES, lesions typically show hypointense or isointense signal on DWI (not bright) and hyperintense signal on ADC maps, confirming vasogenic rather than cytotoxic edema. 2, 5
• This pattern distinguishes PRES from acute infarction, where DWI is hyperintense and ADC is hypointense. 2, 6
• Studies show 81.8% of PRES cases demonstrate hypo- or isointensity on DWI, and 90.9% show ADC hyperintensity. 5

Standard T2-Weighted Sequences

• Conventional T2-weighted sequences (single or dual echo) help confirm posterior fossa lesions and provide complementary information to FLAIR. 3
• These sequences are useful for identifying the full extent of edema and atypical locations. 4, 6

Optional But Useful Sequences

T1-Weighted Sequences (Pre- and Post-Contrast)

• Gadolinium contrast is NOT required for routine PRES diagnosis but can be helpful to exclude alternative diagnoses such as infection, inflammation, or tumor. 1, 7
• When performed, contrast may show focal enhancement in approximately 25% of cases, though this is not a diagnostic feature. 6
• Pre-contrast T1 sequences are typically normal in acute PRES. 8

Susceptibility-Weighted Imaging (SWI) or Gradient Echo

• Consider adding SWI to detect hemorrhagic complications, which occur in approximately 9-15% of PRES cases. 4, 6
• Hemorrhage can manifest as microbleeds, petechial hemorrhage, or frank hematoma. 6

Key Imaging Locations to Evaluate

Typical Distribution

• Parieto-occipital lobes are involved in 72-100% of cases and represent the classic PRES pattern. 4, 5, 6
• Bilateral, symmetric involvement of subcortical white matter is characteristic. 2, 4

Atypical but Common Locations

• Frontal lobes: involved in 30-51% of cases. 4, 6
• Basal ganglia: affected in 22% of cases. 4
• Cerebellum: involved in 17-33% of cases. 4, 5, 6
• Temporal lobes: affected in 8-33% of cases. 4, 6
• Brainstem: involved in 9% of cases. 4
• Thalamus: affected in 4-13% of cases. 4, 5

Critical Diagnostic Pitfalls

Avoid Misdiagnosing Cytotoxic Edema

• If DWI shows restricted diffusion (bright signal) with low ADC values, consider alternative diagnoses such as acute infarction or irreversible injury rather than typical PRES. 2, 5
• Approximately 43% of cases may show some restricted diffusion, which can indicate areas at risk for permanent injury. 4

Don't Rely on CT

• CT is NOT the preferred modality for PRES diagnosis because it has limited sensitivity for detecting subtle white matter edema. 1
• CT is useful primarily to exclude intracranial hemorrhage acutely but should be followed by MRI for definitive diagnosis. 1, 7

Recognize Atypical Patterns

• Involvement of basal ganglia, brainstem, or corpus callosum does not exclude PRES—these "atypical" locations are now recognized as common variants. 2, 4, 5
• Asymmetric involvement can occur and should not dissuade you from the diagnosis if other features are consistent. 4, 6

Follow-Up Imaging Protocol

• Repeat MRI with the same sequences (FLAIR and DWI/ADC) to confirm reversibility, which is a defining feature of PRES. 2, 5, 6
• Clinical and radiological improvement typically occurs within days to weeks with appropriate treatment of the underlying cause. 5
• Complete resolution of imaging abnormalities confirms the diagnosis retrospectively. 2, 6