Magnetic Resonance Imaging (MRI) is a non-invasive diagnostic tool that provides detailed images of structures within the body, including the complex anatomy of the shoulder. MRI utilizes various sequence types - T1-weighted, T2-weighted, Proton Density (PD), Fat Saturation (Fat Sat), and Short Tau Inversion Recovery (STIR) - to visualize different tissue characteristics. This article elucidates the shoulder's MRI anatomy and the appearance of its structures in these different sequences.

MRI Sequences and Their Clinical Significance

T1-Weighted Imaging

T1-weighted (T1W) sequences provide high anatomical detail and are excellent for evaluating the morphology of the shoulder structures. In T1W images, fat appears bright, while water and most pathology appear dark. Muscle and tendons have an intermediate signal. This contrast is particularly useful for assessing fatty infiltration of muscles and marrow pathologies.

T2-Weighted Imaging

T2-weighted (T2W) sequences offer high sensitivity to fluid, with fluid appearing bright. They are beneficial for detecting joint effusion, bursitis, tendon tears, and other pathologies associated with increased fluid content. Muscles and tendons are darker on T2W images compared to T1W, but pathology in these structures often appears bright due to edema or fluid.

Proton Density Imaging

PD sequences are valuable for detecting subtle differences in soft tissue structures. In these images, fluid has a slightly higher signal intensity compared to T1W images but lower than T2W sequences. This balance provides an excellent contrast resolution for the evaluation of intra-articular structures like the glenoid labrum.

Fat Saturation

Fat Sat techniques suppress the signal from fat, making it appear dark. These sequences are used in combination with T2W or PD sequences to increase the conspicuity of pathology by eliminating the bright signal of fat. This is especially useful in the shoulder for highlighting edema or inflammatory changes.

Short Tau Inversion Recovery

STIR sequences also suppress fat signal but are more effective in the presence of inhomogeneous magnetic fields. Pathological conditions, such as bone marrow edema, are highlighted, making STIR images particularly useful for the detection of subtle bone lesions.

Shoulder MRI Anatomy

Rotator Cuff

The rotator cuff consists of four muscles and their tendons: the supraspinatus, infraspinatus, teres minor, and subscapularis. On T1W MRI, these tendons exhibit a low signal intensity due to their dense fibrous structure, while the muscles have intermediate signal intensity. On T2W and STIR images, tendons typically remain low in signal unless there is a tear or tendinopathy, in which case they may appear bright due to fluid accumulation.

Glenoid Labrum

The glenoid labrum is a fibrocartilaginous structure that deepens the glenoid cavity. On T1W images, the labrum is low in signal intensity, whereas on PD sequences, it is more distinctly defined against the bright fluid in the joint. On T2W images with Fat Sat, a normal labrum typically remains dark; however, tears can be identified as bright signal extending into the labrum.

Joint Capsule

The joint capsule is best visualized on T2W Fat Sat or STIR sequences as it is low in signal on T1W images. Distention or thickening of the capsule, which may occur with adhesive capsulitis, is best appreciated on T2W images.

Bone Marrow

Bone marrow in adults is predominantly fatty and hence is bright on T1W images. Any alterations in this pattern, such as in marrow edema or infiltrative lesions, appear dark on T1W and bright on T2W, PD Fat Sat, and STIR sequences.

Articular Cartilage

Cartilage has intermediate to low signal on T1W and high signal on T2W images. PD-weighted images with Fat Sat provide a good balance for cartilage evaluation, often used for detecting subtle chondral defects.

Bursae

The subacromial-subdeltoid bursa, when distended with fluid, appears bright on T2W and STIR images but is not typically visible on T1W images due to its fluid content being similar in signal to adjacent soft tissues.

Conclusion

MRI is an invaluable imaging modality for evaluating the complex anatomy of the shoulder and identifying pathologies. Each MRI sequence provides unique information based on the different signal characteristics of shoulder tissues. A thorough understanding of these sequences and the normal anatomy is crucial for accurate diagnosis and treatment planning.

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Morrison, W. B., & Sanders, T. G. (20