Leptomeningeal Enhancement on Contrast CT
Contrast-enhanced CT is markedly inferior to MRI for detecting leptomeningeal enhancement and should not be relied upon for this diagnosis. While CT can occasionally detect frank nodular leptomeningeal deposits, it is nearly always unable to demonstrate the diffuse linear enhancement that characterizes most cases of leptomeningeal disease 1.
Why CT Fails to Detect Leptomeningeal Enhancement
The fundamental limitation is that CT cannot distinguish subtle meningeal enhancement from the adjacent high-density skull. In a prospective study of 30 cases of meningeal carcinomatosis, contrast-enhanced CT was equal to MRI only in detecting obvious nodules, but was nearly always unable to show diffuse meningeal enhancement applied to the inner table of the skull 1. This represents a critical diagnostic gap, as diffuse linear enhancement is the most common pattern of leptomeningeal metastases 2.
The Superior Performance of MRI
MRI with gadolinium contrast is the gold standard for detecting leptomeningeal enhancement, with sensitivity of 66-98% and specificity of 77-97.5%. 2, 3 The key advantages include:
- Post-contrast T1-weighted and FLAIR sequences are the most sensitive for detecting leptomeningeal disease 2, 3
- 3D T1 post-contrast images with 1mm slice thickness allow detection of small deposits that CT cannot resolve 2, 3
- MRI can visualize enhancement patterns including sulcal enhancement, linear ependymal enhancement, cranial nerve root enhancement, and nodular deposits coating the cauda equina 2
Clinical Implications
Brain and total spine MRI is the preferred modality over computed tomography scans due to superior resolution. 2 In practical terms, if you suspect leptomeningeal metastases based on clinical presentation (headache, cranial nerve palsies, radicular pain, cauda equina syndrome in a cancer patient), ordering a CT scan will miss the majority of cases 2.
Common Pitfall to Avoid
Do not accept a negative contrast-enhanced CT as ruling out leptomeningeal disease. If clinical suspicion remains high, proceed directly to contrast-enhanced MRI of the brain and complete spine 2, 3. The only scenario where CT provides diagnostic value is when it shows progressive ventricular enlargement over time, which may suggest CSF flow obstruction from leptomeningeal disease 4.
Recommended Imaging Protocol
When leptomeningeal metastases are suspected, obtain MRI with the following specifications:
- Field strength: At least 1.5-Tesla, preferably 3-Tesla 2, 3
- Brain sequences: Axial T1, axial FLAIR, axial diffusion, axial T2, 3D post-gadolinium T1, and 3D post-gadolinium FLAIR 2, 3
- Spine sequences: Post-gadolinium sagittal T1, sagittal T1 without contrast, sagittal T2 with fat suppression, and axial T1 with contrast in regions of interest 2, 3
- Slice thickness: 1mm for optimal detection of small deposits 2, 3
- Timing: Perform MRI before lumbar puncture to avoid nonspecific post-procedural meningeal enhancement 3, 5, 6