Unlike conventional MRI techniques, it relies mainly on the motion of water molecules rather than the spin-proton density, T1 value or T2 value of the tissue. Diffusion of water molecules in tissue is a random thermal motion of water molecules in tissue (both intracellular and extracellular), and the direction and magnitude of diffusion are influenced by biofilm and macromolecules in tissue. MRI can control the magnetization state of water molecules in living tissue without affecting its diffusion process. Basic concepts: 1.Diffusion: Brownian motion. 2.Diffusion coefficient (D): the speed of molecular diffusion motion, is the range of random diffusion motion of water molecules per unit time, square mm/s. The diffusion coefficient of water molecules in different tissues is different, and it depends on the environment in which the water molecules are located. 3. Apparent diffusion coefficient (ADC): describes the speed and range of diffusive motion of molecules in different directions in DWI. Since the MR image itself cannot distinguish the signal attenuation caused by various reasons (such as thermal gradient, interproton interaction), the ADC value is used instead of D. The ADC value is calculated mainly based on the change of the signal intensity on the diffusion-weighted image. ADC=(LnS1/S2)/b2-b1b1 and b2 are the two applied diffusion sensitivity factors, S1 and S2 are the applied diffusion signal intensity of the tissue at the same site after the application of the diffusion-sensitive gradient field (b1, b2). The sensitivity of each MR imaging sequence (e.g., SE, GRE, EPI sequences) to diffusion motion performance is an indicator of the ability to detect diffusion motion. s/mm2. b value is related to the applied diffusion-sensitive gradient field strength, duration and interval. The diffusion sensitivity of water molecules in MRI increases with increasing b-value, but the image signal-to-noise ratio decreases accordingly . The current b-value commonly used in cranial DWI is about 1000s/mm2. 5. Isotropic diffusion. The ideal environment, the water molecules in all directions of the diffusion rate are synchronized, that is, the diffusion coefficient is the same, after a period of time their trajectory in a sphere. 6, anisotropic diffusion. In the human body, the diffusion of water molecules by a variety of factors (for example, the brain white matter fibers, due to the presence of myelin sheath, water molecules parallel to the white matter fibers when easy to diffuse, perpendicular to the white matter fibers when diffusion is limited), in all directions are not equal, after a period of time the impact of its local environment to diffusion movement in all directions is not synchronized, this diffusion movement is expressed as anisotropy. 7.Tensor. Used to represent a series of three-dimensional vector entities within the tension, this concept is derived from physics and engineering. 8, eigenvectors and eigenvalues. 9, the average diffusivity. MRI voxel within the average value of the diffusion amplitude in each direction, representing the size or degree of diffusion of water molecules within a voxel. 10.Fractional anisotropy. 11, relative anisotropy. 12.Volume ratio. DWI can observe the diffusion characteristics of water molecules. To increase the sensitivity of diffusion, the diffusion-sensitive gradient needs to be applied. Diffusion sensitive gradient can be fused with any pulse sequence. The diffusion-sensitive gradient can significantly increase the sensitivity of the sequence to the Brownian motion of water molecules, but it is also sensitive to other types of motion such as head motion. To freeze the macroscopic motion, a single excitation sequence is often used.