A method for improving the treatment and/or examination of vessel walls
through fluid, such as blood, functions by identifying the points in time
when the catheter is closest to the vessel wall or farthest from the
vessel wall. Identification of this relative location enables improved
spectral readings in larger vessels. In short, instead of trying to
overcome motion (e.g., by centering the catheter), this approach takes
advantage of motion by identify times when the catheter is closer to the
vessel wall, in order to gather more useful spectral information or
improve the efficacy of the treatment of the vessel walls. In the
specific example, the invention is used for near infrared (NIR)
spectroscopy. In some embodiments, the catheter head is designed to
induce relative movement between the head and the vessel walls.