The hold up is usually almost entirely insurance related for most people. People in my family (including myself) have had non-emergency imaging done several times over the past few years. We've had experiences ranging from getting it done same day to taking over a month to get approval and scheduled. This is in DFW, where there's no shortage of MRI machines around.

Meanwhile there are imaging labs that can do walk-ins if you're willing to pay cash and they have the slack in their schedule, usually somewhat cheap compared to what they'll bill insurance (and if you have a HDHP, what you'll pay). They don't want those machines idle, a gap in the schedule is money they aren't making.