When is the contrast needed in a head mri? if i prefer to skip it, will this be a good enough image to rule out brain legions for balance issues?

MRI: Contrast in the MRI increases the yield of vascular lesions and active inflammatory conditions that disrupt the "blood brain barrier." in the absence of renal diseases the contrast used in MRI is much more safer than iodine based contrast used for ct scans. The addition of contrast is also very useful in detecting active lesions in multiple sclerosis. Discuss with your doctor your concerns.

Tumor: Standard MRI will pick up most abnormalities. Contrast is given if such abnormalities are seen which can help determine how aggressive they are. However, if your plain MRI is normal, a contrast study is not generally needed.

Increase sensitivity: Contrast gives more information and increases the yield of the MRI scan. It will light up areas where the "blood brain barrier" has been disrupted- such as in infection, inflammation and certain types of tumors.