高磁場MRIと神経疾患

Clinical Applications of 3.0 T Magnetic Resonance System in the Neuroradiological Field

はじめに

　わが国では，2003年2月に3 tesla（以下，T）MR装置が頭部領域に限って薬事承認を取得したが，2005年に入り全身用装置としての薬事承認が得られた。2005年11月には複数メーカーの3T MR装置の薬事承認が得られ，今後3T MR装置は臨床機として主力となることが予測される。鳥取大学では3T MR装置が導入されて約3年が経過し，現在までに10,000例以上の頭部領域の臨床例を経験している。この経験を基に，3T MRIにおけるそれぞれの撮像法の特徴をふまえて，神経疾患の診断に際しての臨床的有用性を中心に，問題点を含めて概説する。

Abstract

　Recently,the introduction of 3 tesla (3T) magnetic resonance (MR) system for more sophisticated clinical applications has yielded in important benefits,especially in neuroradiology. The aim of this article is to illustrate the practical scientific applications of the 3T system in the neuroradiological filed. From the clinical point of view,we focused on the usefulness of 3T system for the diagnosis of several neurological disorders,such as brain tumors,vascular lesions,hemorrhagic lesions,acute cerebral infarcts and degenerative diseases. The greatest advantage of high-field MR system is a higher signal to noise ratio. This higher spatial resolution can provide precise anatomical information for brain tumor itself and its surrounding structures. Diffusion weighted imaging (DWI) also benefits from the higher signal to noise ratio and offers useful information for tumor characteristics. Navigation system with diffusion tensor tractgraphy is also available for surgical operation of brain tumors. Parallel imaging enables to improve the quality of tractgraphy by reducing susceptibility artifacts. 3T TOF (time-of-flight)MR angiography(MRA)demonstrates superior depiction of intracranial aneurysms compared with that of 1.5T TOF MRA. 3T TOF MRA is also useful for the evaluation and follow-up of stenoocclusive diseases including moyamoya disease. Susceptibility weighted imaging (SWI)is a BOLD (blood oxygenation level dependent)-sensitive method for visualizing anatomical features such as small cerebral veins in high detail. Therefore,3T system has advantages for obtaining detailed,high spatial resolution images of the venous network. SWI is useful for detection of hemorrhagic lesions and early diagnosis of acute hemorrhagic infarcts. SWI also can detect embolus and evaluate functional changes showing dilatation of medullary veins in the area of acute cerebral infarcts. Neuromelanin imaging using 3T system can directly demonstrate the locus ceruleus and substantia nigra by the neuromelanin content and have the potential to become a powerful tool in Parkinson's disease and other neurodegenerative disorders with Parkinsonism. Hyperintense putaminal rim can be often observed in clinically normal subjects on fast spin echo T2-weighted images at 3T system. This finding should not be mistaken for multiple system atrophy. 3T MR system indeed offers new potential because of a substantial increase in signal intensity provided by the higher magnetic field. Routine neuroradiologic imaging would benefit from higher magnetic field. However,It is required that extended knowledge of clinical data in comparison with 1.5T system to elucidate the efficacy of 3T system in the neuroradiology.