1. A surgical guidance system comprising: a storage medium for storing pre-operative image data associated with a patient; a surface topology imaging and optical position measurement system comprising: an optical projection device configured to project optical radiation onto an exposed surface of the patient, such that backscattered optical radiation is suitable for optical surface topology detection; an optical source having a wavelength selected to illuminate a set of fiducial markers provided on a movable instrument; two or more cameras, wherein at least one of said two or more cameras is configured for imaging the backscattered optical radiation, and wherein at least two of said two or more cameras are configured for imaging said set of fiducial markers when illuminated; and a surgical guidance controller operatively connected to said surface topology imaging and optical position measurement system and said storage medium, wherein said surgical guidance controller includes a processor configured to: control said optical projection device to illuminate the exposed surface and obtain, based on one or more images from said at least one camera, topological image data associated with the exposed surface; and control said optical source to illuminate said set of fiducial markers and obtain, from said two or more cameras, positional image data associated with said set of fiducial markers; determine a position and an orientation of said movable instrument relative to the exposed surface; and register said topological image data, and said position and orientation of the movable instrument to said pre-operative image data; wherein said surgical guidance controller is configured to control said optical projection device and said optical source such that operation of said optical projection device and said optical source are temporally interleaved, and such that the backscattered optical radiation is imaged during operation of said optical projection device, and said set of fiducial markers are imaged during operation of said optical source.