THE IMPORTANCE OF RAY PATHLENGTHS WHEN MEASURING OBJECTS IN MAXIMUM INTENSITY PROJECTION IMAGES

It is important to understand any process that affects medical data. O nce the data have changed from the original form, one must consider th e possibility that the information contained in the data has also chan ged, In general, false negative and false positive diagnoses caused by this post-processing must be minimized, Medical imaging is one area i n which post-processing is commonly performed, but there is often litt le or no discussion of how these algorithms affect the data. This stud y uncovers some interesting properties of maximum intensity projection (MIP) algorithms which are commonly used in the post-processing of ma gnetic resonance (MR) and computed tomography (CT) angiographic data, The appearance of the width of vessels and the extent of malformations such as aneurysms is of interest to clinicians, This study will show how MIP algorithms interact with the shape of the object being project ed, MIP's can make objects appear thinner in the projection than in th e original data set and also alter the shape of the profile of the obj ect seen in the original data, These effects have consequences for wid th-measuring algorithms which will be discussed. Each projected intens ity is dependent upon the pathlength of the ray from which the project ed pixel arises. The morphology (shape and intensity profile) of an ob ject will change the pathlength that each ray experiences, This is ter med the pathlength effect, In order to demonstrate the pathlength effe ct, simple computer models of an imaged vessel were created, Additiona lly, a static MR phantom verified that the derived equation for the pr ojection-plane probability density function (pdf) predicts the project ion-plane intensities well (R(2) = 0.96), Finally, examples of project ions through in vivo MR angiography and CT angiography data are presen ted.