R2 I was referring to your earlier comments that you can find something but can't buy it even though it's not restricted. When the book says "Rarity in Edge of the Empire is a simple way of measuring how difficult an item can be to find on a scale of 1-10" I interpret that to mean that you found it for sale. If you can't buy it through legal means then it's a black market item and therefore restricted. Obviously if the GM wants to make a bunch of stuff ® it's well within their purvey. And just like they have a chart for modifying rarity based on location, the GM can also adjust rarity based on what he or she thinks would be available in the area.
Narcotics (vicodin, crack, etc) would all be restricted items with different levels of rarity. Some restricted items with legitimate uses would be available with the right licenses or on the black market if you don't. Much like a imperial peacekeeping license allows people to wear weapons and armor where they would normally be restricted.
A CAT or MRI machine is a massive machine that can costs hundreds of thousands. They do require licensing, but aren't really restricted or the license is easy to get for someone who can afford to buy the scanner anyway. I see it like buying a starship. You have to be licensed and register your ship, but it's a tiny cost that's included in the cost of the ship.
R2 knows a lot more about star wars lore than I do so I concede that there could very well be lore reason for the empire to automatically assume that someone with a bacta tank was a terrorist or criminal. As a less learned fan I wouldn't bat an eye if a ship had a portable bacta tank. If it's a larger ship, then it makes sense because it's the fastest way to heal major injuries. Maybe the owner just happens to be wealthy and can afford the best. It's a dangerous galaxy out there, maybe the owner of the ship is a bounty hunter or freelance security. Having a bacta tank is completely plausible to me. It might raise some questions and some extra scrutiny, but there are plenty of plausible reasons to have one from my layman's point of view.