Setting a range for Isovalue when evaluating Isosurface

In the function Isosurface you must set a Isovalue to determine the contour in which matlab will plot your function, or data points. I am trying to isolate the isovalue for a certain area in a set of MRI and CT images I have, as it is important to look at tumors that have a different value in gray scale than the brain. So I am wondering if there is a way to use isosurface or edit the isovalue so that instead of taken the value given and going to the upper bound of the data, to make it go from a range of let’s say 0.25 of the value to 0.5 of the total value.

Thank you in advance