分子擴散係指分子帶有能量使其產生之隨機運動，又稱為布朗運動，此運動隨著周圍環境限制使分子產生不同的分布機率。擴散磁振造影運用這項特性，呈現環境之微細結構，以生物體內豐富之水分子為天然追蹤劑偵測大腦內之微細構造，例如神經纖維完整性、神經纖維方向及組織特性等。透過擴散磁振造影非侵襲性的特點，此技術已漸漸應用於臨床診斷及神經科學領域。

Molecular diffusion is a physical process due to random translational motion, namely Brownian motion, of molecules with non-zero thermal energy. As the Brownian motion is subject to the surrounding environment, its tract and distribution will reflect the corresponding microstructures. Accordingly, one may use diffusion MRI of water molecules, abundantly supplied in biological tissues, as endogenous probes of subtle brain architecture, such as neuronal integrity, neuronal orientation, and tissue characteristics. Because of its non-invasive feature, diffusion MRI has been applied to the field of neuroscience and used for clinical diagnosis with increasing importance.