From the Guidelines
Epidermoid cysts on MRI typically appear as well-defined lesions with characteristic signal intensities that reflect their keratin content, and a key diagnostic feature is their high signal intensity on diffusion-weighted imaging (DWI) with corresponding low apparent diffusion coefficient (ADC) values, as noted in the context of differentiating them from other lesions like vestibular schwannoma 1.
MRI Findings
The MRI findings for epidermoid cysts include:
- On T1-weighted images, they usually appear hypointense to isointense compared to brain parenchyma.
- On T2-weighted images, they appear hyperintense.
- High signal intensity on diffusion-weighted imaging (DWI) with corresponding low apparent diffusion coefficient (ADC) values, which helps differentiate them from arachnoid cysts.
- FLAIR sequences show heterogeneous signal intensity, often with mild peripheral enhancement.
- Typically, they do not enhance with gadolinium contrast, though minimal rim enhancement may occasionally be seen.
- They commonly demonstrate restricted diffusion due to their dense keratin content.
- Often have irregular margins and may show a "dirty" heterogeneous appearance compared to the homogeneous appearance of arachnoid cysts.
- Frequently display a characteristic "shaggy" border and may show calcifications on susceptibility-weighted imaging.
Diagnostic Considerations
These imaging characteristics reflect the cyst's composition of desquamated epithelial cells, keratin, and cholesterol, which create their distinctive MRI appearance, as can be inferred from the discussion on the diagnostic workup of vestibular schwannoma and the role of MRI in identifying and characterizing such lesions 1. The use of specific MRI sequences like DWI and the analysis of signal intensities on different sequences are crucial for an accurate diagnosis.
From the Research
MRI Findings in Epidermoid Cysts
- On T1-weighted images, epidermoid cysts may demonstrate a heterogeneous signal, with 32% showing a heterogeneous signal and 91% having irregular limits 2.
- On T2-weighted images, epidermoid cysts typically show high signal intensity 3, 4, 5.
- FLAIR sequences can help differentiate epidermoid cysts from arachnoid cysts, with epidermoid cysts appearing heterogeneous and different from the void signal of CSF in 86% of cases 2, 4.
- Diffusion-weighted imaging (DWI) can also aid in differentiation, with epidermoid cysts showing restricted diffusion and hyperintensity on DWI images 2, 4, 5, 6.
- The apparent diffusion coefficient (ADC) values of epidermoid cysts are typically lower than those of arachnoid cysts, but higher than those of cerebral white matter 4, 6.
Location and Margin
- Epidermoid cysts can occur in various locations, including the intracranial region 2, 4, 6 and the extremities 3.
- On MRI, epidermoid cysts often appear as well-circumscribed lesions with a subcutaneous location in the extremities 3.
- In the intracranial region, epidermoid cysts may extend through the foramen of Pacchioni in some cases 2.
Signal Characteristics
- On T1-weighted images, epidermoid cysts may show iso-signal or slightly high signal intensity compared to surrounding tissues 3.
- On T2-weighted images, epidermoid cysts typically show high signal intensity, although some may exhibit heterogeneous signal characteristics 3, 4, 5.
- No enhancement is typically observed inside epidermoid cysts on contrast-enhanced MRI 3.