When a person gets an mri, sometimes it's with contrast, sometimes not. why?

Vascular: Contrast is administered with ct and MRI to help reveal the blood flow around a lesion. For example a cancerous growth in the brain might show up as nothing or have a benign appearance without contrast, but due to increased blood flow around such a lesion, light up much better with contrast. Many other disorders show up better with contrast. Some disorders show up just as well without contrast.