<title>Surgically appropriate maximum intensity projections: quantization of vessel depiction and incorporation into surgical navigation</title>

Abstract

Integration of tomographic angiograms into neurosurgical navigation should decrease the probability of vascular injury and allow localization of vascular lesions. Information from angiograms is often presented using maximum intensity projections (MIPs), which provide a more intuitive presentation of three-dimensional vascular structures. Conventional MIPs involve the whole image volume during ray casting. Our goal was to construct surgically appropriate MIPs that excluded information contralateral to the operation site and to quantify the accuracy of vessel depiction using this new method. For each angiogram slice, the center of mass (COM) was calculated. Together, the COM coordinates formed a boundary plane that clipped the contralateral information from ray casting. A separate depth buffer was created to preserve three-dimensional information. MIPs were examined quantitatively using a mathematical model of the head containing vascular structures of known diameter. The vessel widths of the resulting MIPs were then measured and compared. To examine the spatial accuracy of MIP images, a vascular phantom was created, which had rigid vessels of known diameter and extrinsic fiducial markers to perform a physical to image space registration. Studies with the mathematical model showed that the vessels appeared smaller in MIPs than their actual diameters. This decrease is attributed to the statistical properties of the ray casting process that are affected by the pathlength. Studies with the vascular phantom show correct localization of the probe in tomographic and projective image space. From these studies, we concluded that additional methods for providing information concerning vessel proximity during surgical guidance should be investigated. Surgically appropriate MIPs provide comparable images to conventional MIPs; however, they allow more focus on the vascular structures in proximity to the target site.