4.2.1 - Patient Positioning

The patient is placed supine in the MR unit with the arm alongside of the body. It is important to maintain the humerus in the neutral position, so that on the axial localizer images, the bicipital groove is seen at the 12 o'clock position. Excessive internal rotation will create apparent redundancy of the anterior capsule, whereas excessive external rotation will oppose the subscapularis tendon closely to the adjacent anterior capsule and glenohumeral ligaments, limiting the ability to distinguish the latter structures. Shoulder imaging requires the use of off-center FOVs. In the case of large patients, the FOV must be defined at the outer limit of the bore, close to the magnet itself. At these extreme off-center locations, the local main magnetic field becomes inhomogeneous. For this reason, a phased array shoulder coil is recommended to maximize SNR. Higher SNR allows more accurate identification of the water and fat peaks, providing more robust fat suppression, particularly for off-center FOVs. At the Hospital for Special Surgery (HSS), a Medrad shoulder phased array coil is used for shoulder imaging. The Medrad shoulder phased array RF coil consists of two paddles: the posterior paddle has one coil loop, and the anterior paddle has two coil loops. Before positioning the patient, the posterior paddle is first placed in the patient support pad on the table. The patient is positioned with the coracoid process centered over the posterior coil as shown in Figure 4.2 A-C. The anterior paddle is then adjusted on the patient's shoulder until the horizontal hash marks are aligned with the posterior coil position marker.

	Figure 4.2 (A) Proper patient positioning for Medrad shoulder coil. The coracoid process should be centered over the posterior coil element. (B,C) Optimal centering for the anterior component of the phased array shoulder coil.