Can patella tendonitis show up in x-rays or mri months after it is healed and all activity back to normal?

Sometimes: Occasionally, in bad cases you can see a bony traction spur or fragmentation at the inferior pole of the patella. Mri would also show it, but mriis better for diagnosing acute patellar tendinitis.

Hard to say: The patellar tendon is a soft tissue structure and so is not usually visible on x-ray, but sometimes with chronic tendinitis, you can see calcifications in the tendon and this can be seen on the xray. In a similar fashion, even if the symptoms of tendinitis have resolved, there are sometimes chronic changes that can still be seen on mri, although certainly not always. Thank you for the question.

Yes: Chronic patella tendonitis shows a thickened patella tendon seen best on sagittal and axial MRI scans but can occasionally be identified on x-rays if severe.