With MRIcron, choose Azimuth=90 (or 270 depending on if you want to remove the left or right hemisphere), Elevation - 0, and then View/Cutout with X=500..1000 (or 0..500), Y=0..1000, Z=0..1000

This is easier with MRIcroGL
 Display/Render
 Display/Left (or Display/Right)
 In the Tool Panel, go to the Clipping panel and adjust the Depth slider to 50%, and the Azimuth to 25% (or 75%). By the way clicking on the text "Depth" and "Azimuth" advances the respective slider 25%, making it easy to put in the precise value. MRIcroGL will create much nicer renderings than MRIcron.

p.s. Surf Ice can also create midline images, using the same steps as MRIcroGL