Purpose or Case Report: Detection of leptomeningeal metastasis (LM) is critical to staging and prognosis of childhood CNS cancers like medulloblastoma and ependymoma. ¹ Though CSF examination is the historical gold standard for diagnosis, technological advances have earned MRI a central role in metastasis detection; recent work finds MRI more predictive of survival than CSF analysis. ^2-4 However, not all sequences are created equal for detection of tumor in the CSF, and pediatric MR imaging presents additional unique challenges such as patient motion, acoustic noise and scan time reduction. In this exhibit, we discuss the strengths and weakness of common sequences for LM detection; technical alternatives for reduction of motion, acoustic noise and scan time; and present a suggested targeted imaging protocol based on current best imaging practice.

With visual examples, we will discuss:
1. Signal characteristics of LM and role of complementary sequences for detection
2. Optimizing scan planes and slice thickness for tumor/metastasis location and patient size
3. Strengths and weaknesses of T1 FLAIR, SE, FSE/TSE, gradient echo and ultrafast spoiled gradient echo (VIBE/FAME/LAVA/THRIVE) sequences in terms of time, resolution, SNR and CSF artifact
4. Utility and optimization of post-contrast FLAIR, DWI, TrueFISP/bSSFP, and subtraction images for metastasis detection
5. Reducing imaging time: targeted sequences, k-space undersampling (HASTE, partial Fourier imaging), parallel imaging
6. Reducing acoustic noise: lowering bandwidth, longer echo spacing, modified gradient wave forms, alternate encoding (PETRA, SWIFT, zero-TE)
7. Reducing motion artifact: 2D vs. 3D, non-Cartesian acquisition schemes, motion correction

References:
1. Engelhard HH, Corsten LA. Cancer Treat Res 2005;125:71-85
2. Maroldi R, Ambrosi C, Farina D. Eur Radiol 2005;15:617-626
3. Pang J, Banerjee A, Tihan T. Journal of neuro-oncology 2008;87:97-102
4. Terterov S, Krieger MD, Bowen I, et al. J Neurosurg Pediatr 2010;6:131-136

Figure: Spine imaging at 3 Tesla. (a) Axial post-contrast T1-weighted VIBE images reconstructed at 3mm/0 gap show leptomeningeal metastasis to excellent advantage, without CSF pulsation artifact which obscures the same metastasis on axial (b) T1 TSE (3mm/0.3mm gap) with contrast. Metastasis was confirmed by CSF analysis.
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