The fibrous structures in the shoulder are highly organized tissues with normally low signal on all pulse sequences. These structures include the joint capsule, glenohumeral ligaments, rotator cuff tendons, and the labrum. When there is disruption of the organization structure because of tendinopathy or tear, the signal intensity increases. Unfortunately, there are confounding factors that may cause artifactually increased signal intensity in the absence of pathology. These are discussed in more depth elsewhere in this article.
Articular cartilage is intermediate in signal intensity on T2 and spin echo sequences. Fluid appears as high in signal intensity on T2-weighted and short tau inversion recovery imaging, which is a fluid-sensitive sequence. Normal musculature is intermediate in signal intensity on all pulse sequences. Increased T1 signal may be seen with fatty atrophy and increased T2 signal may be seen with edema.
Normal cortical bone is dark on all pulse sequences because of the lack of mobile protons, whereas the marrow space usually is T1 hyperintense because of fat content. Heterogenous areas of low T1 signal may be seen with red marrow conversion. This finding is common particularly in patients who have systemic disease and increased red blood cell turnover. Examples include smokers and patients who have chronic obstructive pulmonary disease or renal insufficiency. The marrow of patients who have hematologic disorders, such as multiple myeloma, may have an identical appearance.
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