The following text is largely excerpted from the article: Three-dimensional Volume Rendering of Spiral CT Data: Theory and Method. Calhoun PS, Kuszyk BS, David G. Heath DG, et al. (Radiographics. 1999;19:745-764.)
Advantages of surface rendering include superior speed and flexibility in image rendering. Many of today's graphics computers are optimized for the display of surface models. Applications in surgical planning (eg, for maxillofacial reconstruction) take advantage of this capability that allows surface models to be interactively repositioned and manipulated. In addition, for purposes of measurement a derived surface can be displayed with subvoxel precision with the "marching cubes" technique mentioned earlier. This capability has positive implications for the visualization of small structures such as cranial sutures. In general, surface-rendered images have the clearest volume depth cues of all 3D images. A common criticism of surface rendering is that the surface is derived from only a small percentage (less than 10% by some estimates) of the available data. In addition, surface rendering is not adequate for the visualization of structures that do not have naturally well-differentiated surfaces.
Maximum Intensity Projection
MIP is a 3D rendering technique that evaluates each voxel along a line from the viewer's eye through the volume of data and selects the maximum voxel value, which is then used as the displayed value. MIP is also widely available in commercial 3D software packages. The clinical utility of MIP has been evaluated extensively, and MIP has proved to be particularly useful in its original application: creating angiographic images from CT and magnetic resonance (MR) imaging data. However, Schreiner et al. have shown that different versions of the MIP algorithm can produce very different images. This suggests that 3D rendering algorithms such as MIP and volume rendering should be thought of as families of related image processing techniques rather than as single entities.
MIP has a number of related artifacts and shortcomings that must be taken into account to interpret the rendered images properly. The displayed pixel intensity will represent only the material with the highest intensity along the projected ray. A high-intensity material such as calcification will obscure information from intravascular contrast material. This limitation can be partially overcome with use of nonlinear transfer functions or, more practically, through volume editing. Volume editing can take the form of a preprocessing step (eg, section-by-section ["slab"] editing) or an interactive process (eg, "sliding-slab" MIP). Use of the highest intensity value also increases the mean background intensity of the image, in effect selecting the "noisiest" voxels and thereby decreasing the visibility of vessels in enhancing structures such as the kidney and liver. MIP images are typically not displayed with surface shading or other depth cues, which can make assessment of 3D relationships difficult. Also, volume averaging (the effect of finite volume resolution) coupled with the MIP algorithm commonly leads to MIP artifacts. A normal small vessel passing obliquely through a volume may have a "string of beads" appearance because it is only partially represented by voxels along its length. The severity of these artifacts depends on the resampling filters used. Despite its limitations, MIP usually has superior accuracy compared with surface rendering in CT angiography and produces relatively reproducible results with different operators.
Three-dimensional Volume Rendering
As commonly implemented, 3D volume rendering takes the entire volume of data, sums the contributions of each voxel along a line from the viewer's eye through the data set, and displays the resulting composite for each pixel of the display. Incorporation of information from the entire volume can lead to greater fidelity to the data; however, much more powerful computers are required to perform volume rendering at a reasonable speed. Differences in implementation of various volume rendering techniques result in varying quality and utility among applications. Nevertheless, volume rendering is useful in a wide variety of applications ranging from seismic data display to wind tunnel testing and is now being incorporated into commercially available software packages for medical imaging. With wider availability and improved cost-to-performance ratios in computing, volume rendering is likely to enjoy widespread use in the medical community.
The quality of data generated from modern medical scanners continues to improve. The evolution from conventional to spiral CT has advantages for 3D volume rendering. Tube heating limited the earliest spiral CT scanners to acquisition of thin sections through a small area of the body or thick sections through a larger area. Newer scanners allow longer acquisition times, resulting in larger volumes of very high resolution data. Advances in spiral CT scanner technology allow high-quality vascular imaging as well as improvements in a wide range of traditional imaging applications including higher sensitivity and specificity in detection of lung nodules and liver lesions. These high-resolution data are ideal for 3D volume rendering. It is now possible to achieve optimal contrast enhancement, which allows new imaging techniques such as 3D CT angiography. With spiral CT, axial images are reconstructed at arbitrary intervals, which also improves the quality of multiplanar and 3D reconstruction.
