A shoulder magnetic resonance imaging (MRI) scan is a non-invasive diagnostic procedure that uses a powerful magnetic field and radio waves to create detailed images of the shoulder joint. This technology is valued for its ability to visualize soft tissues, which are often the source of shoulder pain and instability. MRI provides contrast between different tissue types, allowing clinicians to inspect the intricate network of tendons, muscles, ligaments, and cartilage. The resulting images are cross-sectional slices, presented in various orientations (axial, coronal, and sagittal views), helping to pinpoint the exact location and extent of an issue.
Understanding Signal Intensity and Image Weighting
The visual language of an MRI is based on signal intensity, which refers to the brightness or darkness of a tissue on the image. Different substances emit varying signals, which are translated into shades of gray; bright tissues have high signal intensity, and dark tissues have low signal intensity.
The appearance of tissues changes depending on the specific imaging sequence, known as image weighting. The two primary sequences are T1-weighted and T2-weighted images, each emphasizing different tissue properties. T1-weighted images are preferred for displaying anatomical structure because fat appears bright, while fluid, such as joint fluid, appears dark.
T2-weighted images are superior for detecting inflammation and pathology because they make fluid appear bright. Areas of injury or disease often involve increased water content or edema, which stands out as bright white. Many shoulder MRI protocols use fat suppression with T2 weighting, which makes fat appear dark, enhancing the contrast of bright, fluid-filled pathology.
Identifying Normal Shoulder Anatomy in an MRI
A healthy shoulder MRI presents a predictable pattern of signal intensities across its various components. The cortical bone (the outer shell of the humerus, scapula, and clavicle) appears uniformly dark on all sequences. In contrast, the bone marrow inside the bone has a high fat content and appears bright on T1-weighted images.
The soft tissues of the shoulder, like the rotator cuff tendons, normally exhibit a dark signal intensity on both T1 and T2 images. This dark appearance signifies an intact, tightly packed structure of fibrous tissue. The rotator cuff tendons include:
- Supraspinatus
- Infraspinatus
- Subscapularis
- Teres minor
The glenoid labrum, a rim of fibrocartilage surrounding the shoulder socket, also appears uniformly dark and triangular or rounded in cross-section. Normal joint fluid within the shoulder capsule is dark on T1-weighted images and mildly bright on T2-weighted images. The hyaline cartilage lining the joint surfaces typically shows an intermediate gray signal.
Visualizing Common Pathologies
Injuries and inflammatory conditions cause a characteristic change in the MRI’s visual output, primarily by introducing a bright fluid signal where it should not be. A rotator cuff tear is identified by a bright white line or gap that disrupts the normally dark tendon on T2-weighted images. A full-thickness tear shows this bright fluid signal extending completely from the joint side to the bursal side of the tendon.
Inflammation and edema in soft tissues or bone marrow appear as a bright signal on T2-weighted, fat-suppressed images. This occurs when water accumulates in the injured tissue, such as a muscle strain or a bone bruise. The increased signal intensity highlights the exact location of the acute injury.
A labral tear is visualized as an abnormal bright signal extending into the substance of the dark labrum or separating it from the glenoid bone. This bright signal represents fluid that has seeped into the tear, indicating a disruption of the fibrocartilage structure.