I. Clinical significance of T1-weighted and T2-weighted: In T1-weighted images: the shorter the tissue T1, the stronger the signal; the longer the tissue T1, the weaker the signal. For example, fat is strong signal, while e, s and F are low signal; In T2-weighted image: the shorter the tissue T2, the weaker the signal (black); the longer the tissue T2, the stronger the signal (white). If the fluid and edema are strong signal, while the muscle is gray-black. Second, normal meniscus MRI performance 1, lateral: anterior and posterior horn morphology, similar in size; 2, medial: posterior horn is wider than anterior horn, at least in 2-3 levels can be seen in the anterior and posterior horn is separated; Third, normal anterior cruciate ligament MRI performance Normal anterior cruciate ligament in each sequence shows low signal (elderly people due to fiber degeneration, can appear in the ligament medium signal). Normally the ACL is flat (because the knee is in extension during the examination and the ligament is lax when the knee is flexed). The posterior cruciate ligament is low signal in coronal, cross-sectional and sagittal planes; cross-sectional plane: the cross-section becomes progressively thinner; sagittal plane: the ligament is convex and bowed backwards with smooth edges; coronal plane: at the posterior part of the knee, vertical direction, due to the fact that the cut surface is located in the lower part of the bow; at the anterior and middle of the knee, the posterior cruciate ligament is round-like in cross-section. V. Accuracy of MRI in diagnosing meniscal tears The literature reports that the accuracy rate is 80-100% with arthroscopy and 95% with 3D MRI.