超高磁場MRIによる分子イメージング

Molecular imaging using high-field magnetic resonance imaging：current concepts

　はじめに

　3 Tesla（3T）超高磁場MRIは，神経科学領域において最も注目を集めている生体イメージング装置である。一昨年の薬事認可を受け，国内でも急速に普及しつつある。本稿では3T MRIの新しい分子イメージング手法について紹介し，神経科学領域に与えるインパクトについて言及する。

　The advent of high-field magnetic resonance imaging（MRI）provides novel molecular imaging techniques, which have not been available in conventional neuroimaging modalities. Myelin imaging using a short inversion-time inversion-recovery technique can depict myelin-density related contrast mimicking myelin-stained specimens by means of the synergic effects of proton density and T₁, T₂ relaxations. Iron imaging using a spin-echo technique can reveal the intracerebral iron content due to an apparent T₂ relaxation enhancement effect. Susceptibility-weighted imaging generated by T₂^＊ relaxation and localized phase shift allows the visualization of minute venous structures within the cerebral parenchyma as a result of the blood oxygen level dependent effect, and it may aid in the visualization of vascular and/or metabolic reserve capacities in acute and chronic ischemias. By using neuromelanin imaging, we can noninvasively assess the depletion and/or dysfunction of dopaminergic and noradrenergic systems. Amyloid imaging using amyloidophilic compounds is a promising technique for the preclinical diagnosis of Alzheimer disease in the near future. Molecular imaging techniques using high-field MRI are powerful tools for investigating functional morphology in the normal or abnormal central nervous system.