There are no hard and fast rules about how any results are given, although with MRI and CT scans, you almost never get the scan result immediately. The images normally need to be examined by a consultant radiologist who will write a report. This inevitably leads to a delay.
Once the results are known, many consultants like to give the news, good or bad, in a face-to-face consultation. This allows you to ask any follow up questions you might have, and even if cancer is ruled out, the patient presumably still has symptoms that need to be addressed. In practice, the appointment times are often decided and sent to the patients even before the results are back. All this means that you can't really draw any conclusions from the timing of results, and you can't assume that if the results are good, you'll get a sudden phone call or letter telling you. I can't say that never happens, but it has never happened to me.
