Hi linbin. I hope more people respond to you as I have to admit that I HATE treating any bursitis.....unless it is related to ongoing biomechanical fault (i.e. rather than a traumatic insult). So...if more folks respond....I may just learn something!!!
When it's due to trauma (like a direct blow to the area), I say I hate treating it because I find my options with treatment are so limited. As we all know, research supporting the use of u/s is limited however I'm sure most us have clinical experiences that support its use from time to time. Personally, however, I have never found u/s to be all that useful in treating bursitis. That being said, when I have seen and palpated those old, recurrent bursitis' that seem "gritty" and / or "boggy", I have used u/s with the thought that perhaps high intensity, continuous u/s may help in re-starting an inflammatory response, activating some cells and just generally getting the body doing something in the area to jump-start the healing process. (Don't know if it actually works; just based on theory and my thoughts...)
When a bursitis is due to biomechanical faults (repetitive stress type of injury), I think it's easier to treat because there is more to work with but the bursitis itself still may take some time to settle. So for an ischial bursitis, I may be wondering about the status of the gluts, hams,....then related to that the deeper stabilizing musculature that if not adequate may be putting more demand on gluts, hams. Related to that, Lsp and pelvis mobility and/or dynamic stability. From an ergonomic perspective...what does this person do during the day. Do they sit alot? Can you alter their chair so that they don't have direct pressure on the ischial tuberosity?