It is not possible to diagnose ACL pathology using sagittal MR images acquired with fully or overextended knees.

It is therefore important that the knee is slightly flexed within the coil (see Chap. 3 for more details).

It is essential that the range of image acquisition covers the entire femoral condyles and the tibial plateau from the medial to the lateral edge in the axial image (Fig. 2.1).

In a typical adult male patient, more than 25 slices will be required at the slice thickness of 3 mm.

In old days, acquisition of MR images in diagonal slices was severely limited by foldover artifacts. In those circumstances, it was necessary to externally rotate the distal lower limb by 15–20° to visualize ACL, which runs diagonally across the intercondyloid fossa, in sagittal images.

Thanks to improvement in both hardware and software in MR imaging, this limitation has become less significant. However, one should be careful not to unnecessarily internally or externally rotate the knee to prevent distortion of ligamentous structures including cruciate and collateral ligaments (Fig. 2.2).

In sagittal acquisition, direction of phase encoding is usually anterior to posterior. In this case, however, images will be hindered by artifacts arising from blood flow in the popliteal artery and vein (Fig. 2.3). This can be prevented by setting the phase encoding direction to superior to inferior, but one should attempt to minimize foldover artifacts.

Although both T1- and T2-weighted images are automatically acquired at many institutions without much consideration, T1-weighted images do not have much value in delineating ligamentous and meniscal lesions.

Normal ligaments and menisci show low intensity signals, and thus proton density- or intermediate-weighted images will allow better contrast with the surrounding cartilage and joint fluid (Fig. 2.4).

Fast spin-echo (FSE) sequence requires much shorter acquisition time compare to conventional spin-echo (SE) sequence and enables acquisition of images with higher anatomical resolution. Thus, FSE is often utilized in knee MRI. However, one needs to be cautious regarding the following issues:

Echo train length (ETL) should be kept to the minimum to prevent occurrence of blurring of image.

Fat may be depicted as hyperintensity and may lower the contrast against meniscal lesions. This can be prevented by application of fat suppression.

To acquire proton density-weighted FSE images, ETL should be kept to the minimum (max 5 or 6).

Using proton density-weighted FSE sequences with water-highlighted technique (adding –90° pulse at the end of the echo train to forcefully recover vertical ­magnetization, such as DRIVE (Philips), FRFSE (GE), RESTORE (Siemens) and T2 Plus (Toshiba)), one can emphasize the T2-weighted contrast even with a relative short TR. Joint fluid will be depicted as hyperintensity and with better delineation of cartilage, ligaments, and menisci (Fig. 2.5).

The magic angle effect is a phenomenon that results in artifactual hyperintensity in structures with ordered collagen, such as tendons and ligaments. This is because when collagen is oriented at 55° to the main magnetic field, dipole-dipole interactions becomes zero, resulting in a prolongation of T2 relaxation time.

Care must be taken not to mistake this as a pathological finding such as a ligamentous tear.

Magic angle effect is particularly notable with short TE sequences such as T1-weighted, proton density-weighted, or T2*-weighted (which is based on a gradient-recalled echo sequence using a low flip angle) sequences (Fig. 2.6).

Magic angle effect can also affect the posterior horn of the lateral meniscus (Fig. 2.7).

Magic angle effect can be avoided by using a long TE. Therefore, if magic angle effect is seen in T2*-weighted images, it can be eliminated by using SE sequences with a long TE or T2-weighted FSE sequences (Fig. 2.8).

Erickson SJ, et al. The “magic angle effect”: background and clinical relevance. Radiology. 1993;188:23–5.