The alignment of image-space to physical-space lies at the heart of all image-guided procedures. In intracranial
surgery, point-based registrations can be used with either skin-affixed or bone-implanted extrinsic objects called fiducial
markers. The advantages of point-based registration techniques are that they are robust, fast, and have a well developed
mathematical foundation for the assessment of registration quality. In abdominal image-guided procedures such
techniques have not been successful. It is difficult to accurately locate sufficient homologous intrinsic points in imagespace
and physical-space, and the implantation of extrinsic fiducial markers would constitute "surgery before the
surgery." Image-space to physical-space registration for abdominal organs has therefore been dominated by surfacebased
registration techniques which are iterative, prone to local minima, sensitive to initial pose, and sensitive to
percentage coverage of the physical surface.
In our work in image-guided kidney surgery we have developed a composite approach using "virtual fiducials."
In an open kidney surgery, the perirenal fat is removed and the surface of the kidney is dotted using a surgical marker. A
laser range scanner (LRS) is used to obtain a surface representation and matching high definition photograph. A surface
to surface registration is performed using a modified iterative closest point (ICP) algorithm. The dots are extracted from
the high definition image and assigned the three dimensional values from the LRS pixels over which they lie. As the
surgery proceeds, we can then use point-based registrations to re-register the spaces and track deformations due to
vascular clamping and surgical tractions.