Do MRI scans usually require an injection of contrast?

Depends on diagnosis. Usually MRI scans of the spine and joints (shoulder, knees, etc) do not need a contrast injection. Contrast is given for spine MRI scans to search for tumors and postoperative scarring. Mri scans of the brain, abdomen, pelvis can be performed with a contrast injection to help find a tumor, but these scans can also be performed without injection. Breast MRI should be performed with contrast.
Not always. See radiologyinfo.Org. Your doc will order contrast if needed.
Maybe. It depends on your symptoms and the disease or injury your Doctor thinks you may have. In my MRI practice, less than 33% of all MRI studies require contrast injection.
Sometimes. It all depends on the region being scanned and the indication for the study. Generally speaking contrast is used when looking for tumor and infection. It is also good for highlighting vessels, but is not always necessary with mri.