|
Magnetic resonance angiography (MRA) is a classification of MRI techniques that produce images of vascular structures. By design, MRA methods enhance contrast between flowing blood and stationary tissue. In bright blood techniques, vascular contrast is maximized using gradient echo sequences with flow compensated magnetic field gradients and short TE values, reduced TR and an imaging slice or slab oriented to maximize blood in-flow. Unlike the direct projection images formed by the transmitted x ray beam in conventional angiography, many of the MRA techniques generate three-dimensional image data and are subsequently processed into projection format. The most popular post-processing algorithm for this purpose takes advantage of the high vascular contrast of MRA data and simply chooses the brightest pixel intensity along lines of projection through the three-dimensional data set to map onto a two- dimensional surface. This method is called the maximum intensity projection (MIP) and produces high contrast images in which the anatomical arrangement of vascular structures can be easily appreciated. However, while MIP processing makes the anatomy readily apparent, it has a non-physical intensity behavior and does not have the relationship to density that is familiar to the reader of radiographic films. In addition, the MIP tends to underestimate vessel width and overestimate the extent of stenosis. A number of alternative projection algorithms as well as surface and volume rendering techniques have been proposed to overcome the drawbacks of the MIP, but the MIP has remained the most used method because of its high vascular contrast and S/N, ease of implementation, robustness, and speed. With this in mind, these qualities were some of the prerequisites of a new projection method, the weighted intensity summation projection (WISP) technique. In the WISP projection, intensity is related to the vessel dimension by computing a line integral over the projected thickness. High vascular contrast is maintained by using an intensity weighting function designed to minimize the contribution from stationary tissue, limit the contribution of the brightest structures, and retain low intensity vascular features found at vessel edges and in small diameter vessels. The weighting function is completely parameterized by the intensity histogram of the projected volume and requires no optimization on the part of the user for individual studies.
|
