What's subacromial bursa?

Normal structure. Its a fluid filled sack between the rotator cuff and the acromion. It resides below the acromion, hence "sub-acromial". It functions to help the rotator cuff glide smoothly below the acromial bone. When a sub acromial bone spur irritates it, you get subacromial bursitis, or impingement syndrome of the shoulder. This can be painful, and a risk factor for rotator cuff tears in the future. Its usually treated with anti-inflammatory meds, pt, and rest. If not better, a work up is warranted.

Related Questions

What does artifact on the proton axial sequence suggesting a longitudinal split mean on a MRI? What does fluid in the subdeltoid and subacromial bursa

Possible tear. From your question it sounds like the images show something along the length of a tendon that could be from artifact or a tear. Fluid in the subdeltoid and subacromial bursa are secondary findings of a rotator cuff tendon tear. Rotator cuff tendon tear can happen in along the grain of the tendon (a longitudinal split) or across the grain of the tendon which shows up as a discontinuous tendon.

Got an MRI today and my report said the following "inflammatory process of the subscapularis tendon and supraspinatus tendon with intrasubstance injury. Subacromial bursa fluid in sub deltoid." What does this mean? Do I have a tear?

No. The report does not specifically state that there is a muscle tear.
Bursal fluid. Indicates bursitis. Tendon inflammation is tendinitis. Intrasubstance injury may be a tear within the interior of the tendon, but it would not be treated with surgery, and could also be due to the inflammation.