Yes...contrast is used as an enhancement tool and may/may not be used with a scan on order to better identify certain regions/areas of the body...it is added to a "protocol". Consider the "protocol" to be the entire scan from start to finish. The technician will have an established protocol that has been designed, in conjunction with the radiologist, for looking at a very specific area of the body...this protocol is then broken down into a number of "sequences"...these are the 1min-5 min segments when you are asked to lay very still and the machine is garnering images. Your scan may last 30 mins and would have been broken into 5-7 different sequences in order to garner an entire set of images from different planes and angles. Each protocol will have very different sequences that have been established to look at very specific regions, whether or not contrast is administered. In that way a "routine brain" MRI, while looking at very specific areas of the brain is missing an additional sequence or two designed to look specifically for MS lesions. And typically speaking, the MS scan is actually done without contrast.
To give you an example, our particular clinic offers 16 different kinds of brain scans...all with different protocols...some will use a "routine brain" as the foundation, however the added sequences make them a very different scan in the end (with or without contrast being administered). Also consider the fact that each image garnered is done so on a "slice" basis (literally consider this just as you would slicing a loaf of bread), these slices may have a distance of 2-5 mm between each image garnered. So you can imagine how many different images of the brain can be generated using different protocols and focusing on different areas of the brain. Although we may look at the brain as a relatively small region of the body, consider how many images could be garnered using such thin slices. Which is why a routine brain is not specific enough for an MS evaluation.
I hope this helps, please let me know if there is anything else that isn't entirely clear, very happy to offer my thoughts.