interp() (and variants) seems to work fine for what you are talking about. Many medical imaging applications like cubic spline, bi-cubic, but all the popular methods are used really. Generally, radiologists prefer the utmost Lossless methods that are possible. 1st of all, the possibility of any artifacts will (and should) automatically disqualify a method. In medical imaging we would rather see a low resolution illustration, including low SNR or even machine artifact included if that is the case, rather than the possibility of any artificial influence of an algorithm onto the data. You must be careful while we still have scanners at low resolutions (as compared to other technology). You can have an influence that shows in only 1 pixel or voxel on the acquired data, and, that thing will turn into a seemingly large "significant" area by over-interpolating.
imtool is not the most efficient way to observe images. I would suggest imshow() or similar methods in Matlab, with your own optimization.
