I think these are creases from your sheets or bed clothes. I am surprised your mother or wife hasn't told you what these marks are!

These are depressions.... dermographia are elevations of swelling due to histamine release (allergy).

If these itch it is because of soap residues in the sheet or your clothes. Marks like these typically fade when you remove the
item causing the depressions. It can take an hour or more.

Long term depressions may indicate pitting edema, which means the tissue under the skin is holding water for some reason. But your back does not look swollen or fat so that is very unlikely!

Most slender people have vertebra that show ...it means nothing serious.

I do not think this is from any MRI. I think you have become hyperaware of everything you ignored before your injury. Since attention to medical diagnosis you have become overly concerned over normal phenomena. You know I hate say this but guys tend to react like this when they get their first "serious" illness. Women are used to all sorts of pain/symptoms migraines, edema everything uncomfortable when they start their periods. Males just don't have that experience which starts at 12 or 13 for women!