What do the -30 to +60mm markings mean in this MRI scan image?
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They are marking distance from a reference plane.
In the images you provided, you are looking at horizontal slices through a brain. The legends are indicating that each plane has that vertical distance from a reference point (if the subject were standing); the positive numbers are dorsal to that reference and the negative numbers are ventral.
A common reference point for MRI images (to define as [0,0,0]) is the midpoint of a line through the pre-auricular point, though ultimately it is an arbitrary distinction.
If the actual reference point is important to you, you will need to verify the reference coordinate system used in the study you are looking at. It's also possible you are looking at coordinates transformed to some reference brain rather than real-life coordinates for that particular subject, but again, whether that matters or not depends on why you care. If you are doing research across subjects, you probably want to know reference coordinates; if you are doing brain surgery on a particular patient you want real-life coordinates.