Oblique MR imaging of the anterior cruciate ligament based on three-dimensional orientation

Purpose: To investigate the three-dimensional (313) course of the anterior cruciate ligament (ACL) and determine the optimum planes for oblique full-length MRI of the ligament. Materials and Methods: Twenty-five healthy volunteers were examined. Axial proton density-weighted (PDw) images of the knees of 20 volunteers were obtained. 3D paths along the course of the ACL and corresponding tangents were constructed. Angles between these tangents and reference lines RFL-1 (the line connecting the posterior edges of the femoral condyles), RFL-2 (the line through the intercondylar joint space), and RFL-3 (the line connecting the anterior and posterior edge of the medial tibial condyle) were measured. These angles were used for oblique T2-weighted (T2w) MRI of the knees of the remaining five volunteers, and the number of slices that depicted the entire ACL was calculated. Results: The mean angles to the ACL were 74.0 degrees for RFL-1, 79.9 degrees for RFL-2, and 70.4 degrees for RFL-3. Full-length visualization of the ACL was demonstrated by 1.4 slices using the sagittal oblique plane prescribed for RFL-1, 2.4 slices using the sagittal oblique plane prescribed for RFL-2, and 1.4 slices using the coronal oblique plane prescribed for RFL-3. Conclusion: The ACL is best depicted using a sagittal oblique imaging plane angled at 80 degrees from a line through the intercondylar joint space.