Are appointments necessary? Is there a good time to arrive?

I think it depends. For me. I just needed an Xray and had no appointment. I walked in and surprisingly didn't wait long at all.

Yes an appointment is necessary. My PCP prescribed the MRI. I was contacted by MRI Associates for scheduling the appointment.

For xrays they do accept walk ins. If you prefer to make an appointment thats okay too. MRI and CT do need to be scheduled.