From: karel@cv.ruu.nl (Karel Zuiderveld)
Subject: Re: volume rendering (long)
Date: Tue, 18 Aug 1992 08:25:17 GMT
Organization: University of Utrecht, 3D Computer Vision Research Group
Keywords: MIP

In <1992Aug17.212447.17833@cs.wayne.edu> sem@siva.cs.wayne.edu (Rekha Manjakuppam) writes:

| "
| "Has anyone heard of MIP(Maximum Intensity Projection). This is
| "technique used in medical area wherein images, obtained from magnetic
| "resonance scanners, and are 3D data sets are processed to obtain 3D
| "projections of the arterial system. This method probably got its name
| "from the fact that the contrast of the arteries and other tissues is
| "good in the set of images. 

A perhaps more appropriate newsgroup for your question would be
sci.med.physics or sci.image.processing. 

MIP is perhaps the simplest 3D visualization algorithm in medical
image processing. From a state-of-the-art article I wrote a 
few months ago:

"3.8 Transmission-oriented methods

The use of algorithms for surface display causes problems when the
original data does not contain clear surface boundaries, as in SPECT
and ultrasound imaging. In such cases, transmission-oriented display
is often considered a useful alternative, of which X-ray projection
and brightest voxel display are the most frequently used techniques.
   ....
Brightest voxel display (also referred to as Maximum Intensity
Projection, MIP for short) display the maximum value encountered along
each ray cast into the volume. This technique is currently popular for
the display of Magnetic Resonance Angiography (MRA) volume data, where
each voxel value represents the amount of local blood flow.
Unfortunately, MIP presents an inconsistent spatial perspective to the
user; high intensity values obscure other features of interest and
lead to irregular depth positions of hte projected voxels. There is a
clear need for modified MIP algorithms; an example is the Closest
Vessel Projection which limits the projection range to the lumen of
the detected vessel closest to the projection plane. Unfortunately,
such algorithms tend to rely on preprocessing steps that mark
interesting voxels - a non-trivial classification problem on account
of the significant amount of noise in the volume data.

Transmission-oriented methods suffer from the same problem as any ray
casting method: The required geometrical transformation between object
and screen space implies resampling of the volume data. For reasons of
speed, the resampling step is often restricted to nearest neighbour
interpolation, which can give disturbing aliasing artifacts. Another
disadvantage of these methods is their computational burden. Since in
principle each voxel of the volume data contributes to the output
image, the ray casting process is time consuming in constrast with
surface display, where techniques such as octree encoding and bounding
volumes can be applied to skip irrelevant subvolumes. In some
applications, however, preprocessing algorithms are capable of
reducing the amount of calculations by identifying relevant
structures. An example is the use of greylevel thresholds to identify
potential blood vessels in MRA volume data.

For a good appraisal of the 3D structure, it is essential of
dynamically change the viewpoing (e.g., by continuously rotating the
3D object in front of the observer). "

It's a fairly standard algorithm in the field of medical image
processing - see e.g. "Volume visualization", A. Kaufman, IEEE
Computer Society Press, 1990 for more technical info. I assume that in
clinical journals as Radiology case studies can be found.

Karel

-- 
Karel Zuiderveld                            E-mail: karel@cv.ruu.nl
3D Computer Vision - Room E.02.222          Tel:    (31-30) 506682/507772
Academisch Ziekenhuis Utrecht               Fax:    (31-30) 513399
Heidelberglaan 100, 3584 CX Utrecht, The Netherlands