Here is an MRI image of a human head. Unlike xray tomography scans, which only measure gradients in density, MRI measurements depend on several different variables. For example, MRI scan can be used to measure gradients in temperature diffusion, longitudinal magnetic relaxation, transverse relaxation, blood oxgenation, simple water density and other properties.
For discussion, let's assume that a bright spot in the image shown above is used to measure properties that would indicate the location of a "tumor". In the image above we can only locate the relative position of the "tumor" in the 2-dimensional (2D) plane. If we view the sequence of 2D images as they rotate about a vertical axis we can than locate the "tumor" in 3-dimensional (3D) space. NOTE: you may need to download this file and play the image-sequence locally to get a smooth rotation, only then will you be able to view the data in 3D. Our ability to preceive with the "mind's eye" property gradients in 3D in real-time is analogous to a supercomputer calculating a filtered backprojection of 2D images into 3D space instantaneously. Truely an amazing accomplishment if you have ever tried to do this on a computer and then realize that the fastest computer in the world can not come close to what the human brain can do with the "mind's eye".
How is this possible? The answer is the supercomputer reconstruction algorithm is required to reconstruct 3D data from 2D images quantitatively ( e.g. 3-4 significant figues ) were as the mind's eye makes a relative qualitative measurement and accuracy is not an issue. Hence accuracy is traded-off for a more comparitive "visual" format.
This is accomplished by using translucent voxel volume elements. There are several visualization software packages that can create translucent voxels volume elements. Here we have posted a copy of the PV-Wave procedure file and a binary data set (filename: man_head.exe, 905625 bytes) located on our anonymous file server http://ftp.sv.vt.edu/pub/MRI-Head that were used to create this example. NOTE: the file "man_head.exe" must be renamed "man_head.dat" before you use the provided PV-Wave procedure file. You will notice that the procedure file is amazingly simple and is easily adapted to be used with other data sets.
This imaging was done initially by D. Carr using the Advanced Rendering Library by Visual Numerics Inc. VNI (previously Precision Visuals Inc.) The ARL library was extended by Dr. Shawn Javid.
A new Medical Radiography Home Page has been recently created that provides a vast amount of information.
Revised September 11, 1995