What are the ways to visualize tomographic images?
In section mode, images can be visualized as axial, sagittal, coronal, or oblique sections, or even as curved sections (also known as multiplanar reformations [MPRs]). Curved MPRs are commonly used to analyze vascular structures on CT or MR angiography.
In volume mode, images can be visualized with the following renderings ( Figure 4-3 ):
- • Maximum intensity projection (MIP): A 2D rendered image is displayed in which the gray scale intensity assigned to each pixel is the maximum of the intensities of all voxels encountered along the projection axis. This is most commonly used when objects of interest are the brightest in the image (e.g., enhancing vessels and osseous or calcified structures).
- • Minimum intensity projection (MinIP): A 2D rendered image is displayed in which the gray scale intensity assigned to each pixel is the minimum of the intensities of all voxels encountered along the projection axis. This is most commonly used when objects of interest are the darkest in the image (e.g., lungs, air-filled tracheobronchial tree, and air-filled bowel).
- • Surface-shaded display (SSD): A 2D rendered image of an object surface is displayed. This is infrequently used given the availability of volume rendering techniques.
- • Volume-rendered (VR): A 2D rendered image of an object volume is displayed in which each voxel is assigned a brightness level or color and an opacity value ranging from 0% to 100%, which can be adjusted interactively as desired. The viewpoint is often external to the object of interest, although it can also be located internal to the object of interest to create virtual endoscopic (also called endoluminal or “fly through”) images. Virtual endoscopy is commonly used in virtual CT colonoscopy to visualize the large bowel and also in virtual CT bronchoscopy to visualize the tracheobronchial tree.