Volume Rendering - Maximum Intensity Projection

As opposed to direct volume rendering, which requires every sample value to be mapped to opacity and a color, maximum intensity projection picks out and projects only the voxels with maximum intensity that fall in the way of parallel rays traced from the viewpoint to the plane of projection.

This technique is computationally fast, but the 2D results do not provide a good sense of depth of the original data. To improve the sense of 3D, animations are usually rendered of several MIP frames in which the viewpoint is slightly changed from one to the other, thus creating the illusion of rotation. This helps the viewer's perception to find the relative 3D positions of the object components. This implies that two MIP renderings from opposite viewpoints are symmetrical images, which makes it impossible for the viewer to distinguish between left or right, front or back and even if the object is rotating clockwise or counterclockwise even though it makes a significant difference for the volume being rendered.

MIP imaging was invented for use in nuclear medicine by Jerold Wallis, MD, in 1988, and subsequently published in IEEE Transactions in Medical Imaging.

Surprisingly, an easy improvement to MIP is Local maximum intensity projection. In this technique we don't take the global maximum value, but the first maximum value that is above a certain threshold. Because - in general - we can terminate the ray earlier this technique is faster and also gives somehow better results as it approximates occlusion.