BLURRING OF VESSELS IN SPIRAL CT ANGIOGRAPHY - EFFECTS OF COLLIMATIONWIDTH, PITCH, VIEWING PLANE, AND WINDOWING IN MAXIMUM INTENSITY PROJECTION

Purpose: Our goal was to examine the effects of collimation width (CW) , pitch, viewing plane, and windowing on the display of in-plane vesse ls in maximum intensity projection (MIP). Method: A theoretical concep t based on partial volume averaging of vessels was developed to descri be the contents of voxels (densities) in MIP and to derive cross-secti onal vessel diameters and blurring. To validate the concept and to des cribe the influence of pitch, a Plexiglas cone submerged in water was scanned with varying CW and pitch. Binary MIP with three representativ e window levels was chosen so that definitive vessel diameters could b e quantitated. Results: The theoretical concept correctly predicted vo xel contents and blurring for CW greater than or equal to 3 mm and low pitch. For high pitch, actual blurring was larger; however, for a giv en table speed, blurring of the cone decreased with pitch while increa sing with CW. Overall blurring was most effectively reduced by using a thin CW and the transverse viewing plane. In the transverse viewing p lane, the least blurring was found using binary MIP with a low window level. On the contrary, in the longitudinal viewing plane, blurring wa s minimized using a window level halfway between the density of the co ne and that of the surrounding water. Conclusion: For CW greater than or equal to 3 mm, blurring of in-plane vessels can be explained with a simple geometrical concept based on partial volume. For accurate disp lay, the transverse viewing plane should be used, a proper windowing m ust be chosen, and the CW should be kept below vessel size while raisi ng the pitch to cover a reasonable volume.