Neurological Surgery 脳神経外科35巻9号

2007年09月発行

総説

Fiber tractographyと脳神経外科手術

著者： 橋本直哉¹ 吉峰俊樹¹

所属機関： ¹大阪大学大学院脳神経外科学

ページ範囲：P.869 - P.879

Ⅰ．はじめに

　MR拡散テンソルイメージング（magnetic resonance diffusion tensor imaging：DTI）が神経線維束の描出を目的として，近年研究目的だけでなく臨床の場面で応用されている．DTIは基本的に拡散強調画像（diffusion weighted imaging：DWI）の延長線上にあるが，DWIが水分子の「拡散能」の評価をもっぱらとするのと比較して，DTIは「拡散の異方性（an-isotropy）」を評価することが可能であり，このため白質における神経線維走行の方向性がより精密に決定できる．この概念が発表されたのは比較的新しく，1990年にMoseleyら³⁹⁾が白質における髄鞘が存在するための拡散異方性を，「anisotropic water diffusion in cat brain」として発表した．その後，1994年にはBasserら⁹⁾が異方性を追跡（tracking）することの原理となるanisotropic water diffusion in image voxelについて記述した．

　この稿で述べる神経線維のtractography（fiber tractography：FT）はDTIから得られた情報の表示方法のひとつであり，特定の機能的な神経線維束単位を描出するものである．これは，米国Johns Hopkins大学のMoriらにより確立され³⁷⁾，後に多くの研究者が発展させ³²⁾，現在も基礎，臨床を問わずに研究が行われている．特に，脳神経外科領域では活発に研究が行われ，日常診療への応用の報告も増加している．

　従来から脳神経外科手術，特にeloquent領域の手術では，術前にfunctional MRI，magneto-encephalography（functional MEG）や，positron emission tomography（PET）による皮質機能領域の検討を行い，ナビゲーションシステムや電気生理学的モニタリング，最近では5-aminolebulinic acid（5-ALA）によるchemical navigationなどのマルチモダリティーを駆使して，神経脱落症状を回避しつつ外科手術の成績向上が目指されてきた．しかし，これらの手法は脳機能の把握という点では皮質機能にとどまり，白質の機能的神経線維束の情報を得ることは困難であった．これらの白質情報は近傍の手術を行う上で極めて重要であることは言うまでもない．われわれは，脳神経外科手術にマルチモダリティーの一環としてFTを活用できないかを検討し，その有用性や問題点について報告してきた^16,24)．臨床での使用に際しては，あくまでMRI情報をもとに，計算しつくして作り上げられた画像にすぎず，in vivoでの検証は不十分であることを強調した．このことは現在でも変わっておらず，昨今のFTの汎用には少し警鐘を鳴らす必要があるかもしれない．

　この稿では，脳神経外科診療，手術に応用する上で必要なDTI-FTの基本的原理，方法を解説し，脳神経外科手術への応用の現状について述べる．これまでに明らかになったFTの有用性やピットフォールについて考察し，電気生理学的モニタリングによるFTのvalidationや神経科学に寄与する面について，今後の展望を含めて解説する．

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