Hi. I am a radiologist. If they thought it was a kidney stone, why did your doctor order an MRI? Most kidney stones do not show up on MRI at all. My go to exam for kidney stones is CT. Ultrasound can work in thin patients and kids, avoiding radiation, but CT is much better for both kidney stones and masses/cysts, especially in larger people. Ultrasound has a hard time adequately seeing detail in the kidneys in your average size American. MRI is good for looking at kidney masses, but it is usually a second line study for problem solving. Sometimes we will incidentally see a mass in the kidney on a spine or liver MRI. This is what we "affectionately" call an "incidental-oma". Fortunately, most of these end up being benign kidney cysts (not cancer), which are very common.
Glad that they thought that it was a cyst that they could follow. For the CT did they take some scans, inject iodine stuff into your vein and then take some more scans?

Sorry, but I have to disagree with your comment about ultrasound being best to evaluate cysts. Ultrasound is fine if the person is thin. However, detail in the kidneys in larger patients can be very difficult to discern and about all you can see, especially in the left kidney, is an indistinct blob, even with an experienced tech and a good quality machine. If I see a simple cyst on CT, US or MR, I leave it alone without any followup or anything. But it has to meet specific criteria to be called simple. After that, there are guidelines, called Bosniak Categories (seriously) that we follow to figure out how to proceed.
I don't disagree with your comment about lawyers and frivolous law suits being a factor.

Yup. There are different kinds of contrast or "dyes", by different companies. The ones that are "non-ionic" have less of an effect but they all have some effect. The sensation is largely due the intravenous contrast hitting the certain parts of the brain and can cause nausea and vomiting. Can also effect chemoreceptors in the body causing other sensations. The degree of sensation can also be determined by how fast it is injected into your vein. A faster injection (5cc/second) will have a more intense sensation than a slower injection (2cc/second). The speed is determined by the scan protocol prescribed for your study and how good the IV access is in your arm.
The bright light warning refers to laser lights that the CT scanner emits (think laser cannon mod on STTNG, or a laser level you might use to hang multiple pictures on the wall) so the technologist can be certain you are centered on the table. It might be hard to read the warning if you have already been blinded, but the lawyers tell the manufacturers that they have to put it there anyway. =) (Joking here. Please don't start a lawsuit.)