There is described methods and apparatuses for positioning medical devices onto (or close to) a desired portion of the interior wall of an internal channel, such as for scan imaging, for photodynamic therapy and/or for optical temperature measurement. In one embodiment, a catheter assembly has a distal portion that can be changed from a configuration suitable for traversing the internal channel to another configuration suitable for scan at least a spiral section of the interior wall of an internal channel, such as an artery. In one example, the distal portion spirals into gentle contact with (or close to) a spiral section of the artery wall for Optical Coherence Tomography (OCT) scanning. The spiral radius may be changed through the use of a guidewire, a tendon, a spiral balloon, a tube or other ways.
