Doing the 6 scans (3 without and 3 with gadolinium) the big issue is table time.. Not so much the contrast. They can do the brain, T spine and L spine without, then inject and do the 3 studies all over again for with.. one dose of contrast. Bam. Done. It lasts for a while.
Time is the problem. especially with older machines. MRI involves putting you in a magnetic field, then hammering you with radio waves to energize your water molecules. This causes heating to you.. akin to a microwave. Not enough to cook/hurt, but if they do too much too fast you get warm and sweaty and uncomfortable. So there have to be pauses built into the scanning sequence. But you are laying there not moving.. MRI tables are not known for comfort. This takes time.
Next thing.. is while there is a big antenna in the "donut" that transmits the radio waves, to be able to really "read" the energy coming back off the body there has to be an antenna (or coil, in parlance) really really close to the body. So MRI of the brain they put a cage like coil around the head. MRI of the spine they have you lay on a mat with the coil built in that puts it right up against the area being imaged. Most of the time only one of these coils can be in play at one time (the head, or the brain, or a leg coil for instance and the table is set up differently for each one.. so.. pull you out, get you off the sled, reconfigure, put you back in, scan the other area.. etc..
So this comes down to the last factor. Laying on these tables isn't painful but its not comfortable. Most of us can emotionally power through a 30 minute scan but might have issues enduring 2-3 hours if needed for all the sequences on an older machine. A lot of folks are claustrophobic, and need something to "calm" them.. sedatives add a whole nother level of complexity to scanning... especially for long series of scans.. An oral versed might do fine for 30 mins or being stuffed in the magnet bore and staying still. But repeat dosing might be needed for complex series of scans. which means pulling out, giving more meds... sometimes it works, sometimes it doesn't.
So these private, for profit imaging centers have found that they make more money, have less delays and "lost" time if they dont subject their paying customers to hours of scans in one session. You might be a trooper and do just fine.. or you might be the one that cant hang.. and now they have you "done" after 30 minutes with the machine scheduled for 3 hours and nobody else lined up. You will be the one spending an hour or two on the MRI table, the question really is.. one long session or several short ones on different days.
If you find someplace that will do it all at once, be honest with yourself. You need to be NOT claustrophobic. With the medical equivalent of a football helmet/internal pads on your face. You need the discipline to lay perfectly still (and not fidget/squirm) for over an hour. You only end up getting one dose of contrast on one day and you get it all done.