Gd is an abbreviation for Gadolinium, a chemical which is sometimes injected into a person’s blood during an MRI. Gadolinium normally cannot pass from the bloodstream into the brain, however during an MS relapse the blood brain barrier is disrupted, allowing gadolinium to enter into the brain where it enters MS lesions, creating a bright spot on an MRI. It is therefore used to identify ‘active’ lesions in the brain to distinguish them from old lesions (scars) or repairing lesions.