This study confirms that MRI can be used to show specific abnormalities in patients with AC compared to healthy control subjects 
. The specific abnormalities that were associated with AC include: (i) thickening of the CHL; (ii) thickening of the rotator interval; (iii) marked obliteration of subcoracoid fat triangle; and (iv) synovitis-like abnormalities around the long biceps tendon.
AC, a clinical condition with painful restriction of shoulder movement, is a common clinical problem that causes major functional morbidity and pain, more commonly in women over 40 years of age 
. The natural history of AC generally follows two courses. Some patients respond to conservative measures such as medications, local injections, and physical therapy. Other patients cannot tolerate the pain or the restrictions on motion and elect surgical intervention. Although there is no agreement regarding the etiology of AC, there is agreement about the observed pathologic changes: inflammation combined with a fibrotic reaction leading to thickening, contraction and subsequent adherence of the capsule, synovium and surrounding ligamentous structures 
. The initial stages of AC have a predominance of pain, with gradually increasing joint stiffness brought on by ongoing synovial inflammation and capsular fibrosis. In the later stages, as the inflammatory phase passes, capsular fibrosis becomes the predominant pathologic finding 
. Histologic and immunocytochemical studies have shown active fibroblastic proliferation accompanied by transformation of fibroblasts to myofibroblasts, a situation thought to initiate contracture of the coracohumeral ligament component of the rotator interval in the early stages of idiopathic AC 
. As the rotator interval shortens in the mediolateral and craniocaudal directions, the relative motion of the anterior margin of the supraspinatus tendon and the cranial margin of the subscapularis tendon becomes restricted and external rotation range diminishes.
It has been suggested that the CHL and rotator interval are of central importance in the development of AC 
. The CHL at the rotator interval is one of the primary structures restraining external rotation, which has particular clinical significance in AC 
. Indeed, Mengiardi et al found that a coracohumeral ligament thickness greater than 4 mm on sagittal oblique MR arthrographic images was associated with AC in 142 cases 
. Consistent with this result, the CHL in the rotator cuff interval was significantly thickened in our patients with AC. However, in patients with recalcitrant chronic AC, the contracted CHL was shown to be the primary lesion and, furthermore, release of the contracted structures relieved pain and restored motion at the shoulder 
. Omari et al also described that the CHL was transformed into a tough contracted band in 25 patients with primary AC and that the histology of these contractures consisted of a dense matrix of type III collagen populated with fibroblasts and myofibroblasts 
. When the contracture of CHL in patients was excised, pain subsided and shoulder function was restored. Taken together, the pathologic changes observed at the CHL by may be helpful for diagnosing AC by MRI.
There is evidence that the rotator interval is important for detecting and quantitatively assessing AC 
. The rotator interval is defined as a triangular structure with the base of the triangle as the coracoid process, the apex as the intertubercular groove, the inferior border as the superior aspect of the subscapularis tendon, and the superior border as the anterior aspect of the supraspinatus tendon. This triangular space contains the biceps tendon, superior glenohumeral ligament, the glenohumeral capsule, and the CHL 
. In addition to thickening of the CHL on MRI, we also found that the rotator interval capsule was thickened in AC compared with control subjects. MRI investigations of AC have previously shown thickening of the rotator interval capsule and exuberant synovitis surrounding the coracohumeral ligament, which may enhance after intravenous gadolinium injection 
. Lee et al also found that with AC there was significant thickening of the joint capsule at the axillary recess using MR arthrography 
. The results of our investigation agree with those of Carrillon, Lefecre-Colau and others, who found thickening of the joint capsule and synovial membrane at the rotator interval to be helpful in diagnosing AC 
. Moreover, in 17 of 22 patients, Mengiardi et al found an inflammatory obliteration of the subcoracoid fat triangle with AC 
, which we also observed in partial or complete form in this investigation. The complete obliteration of this fat triangle was specific to the diagnosis of frozen shoulder or AC. This subcoracoid triangle sign is easy to assess on sagittal oblique images and, thus, is helpful for daily routine clinical work. Taken together, these data show that both thickening of the rotator interval capsule and obliteration of the subcoracoid fat triangle on MRI were useful features for diagnosing AC.
In the present MRI study, we also showed that synovitis-like abnormalities around the long biceps tendon were markedly more frequent in patients with AC compared to control subjects. Synovitis-like abnormalities at the articular surface of the subscapularis tendon or at supraspinatus tendon were not significantly different when comparing AC patients with control subjects. In contradiction with these results, Bernard et al found synovitis-like abnormalities at the superior border of the subscapularis tendon were significantly more common in patients with frozen shoulder, while synovitis-like abnormalities around the long biceps tendon were not significantly different from findings in the control group 
. Other investigators have also shown synovitis-like abnormalities at the subscapularis tendon in patients with AC 
. Therefore, additional studies are needed to better define the role of these synovitis-like abnormalities in the diagnosis of AC.
There are two limitations in this study. On the one hand, all the patients with AC were not classified into early or late stages because of clinical staging of AC was reported to associate with synovial inflammation and capsular fibrosis 
. On the other hand, all the clinical diagnosing patients with AC were not treated and confirmed by the arthroscopy or surgery.
Despite these limitations in our study, some results show that the MR imaging may prove valuable for assisting diagnosis of AC of shoulder joint with a highly sensitive and specific detection.