  Augmented reality has the potential to improve operating room workflow by
allowing physicians to "see" inside a patient through the projection of imaging
directly onto the surgical field. For this to be useful the acquired imaging
must be quickly and accurately registered with patient and the registration
must be maintained. Here we describe a method for projecting a CT scan with
Microsoft Hololens and then aligning that projection to a set of fiduciary
markers. Radio-opaque stickers with unique QR-codes are placed on an object
prior to acquiring a CT scan. The location of the markers in the CT scan are
extracted and the CT scan is converted into a 3D surface object. The 3D object
is then projected using the Hololens onto a table on which the same markers are
placed. We designed an algorithm that aligns the markers on the 3D object with
the markers on the table. To extract the markers and convert the CT into a 3D
object took less than 5 seconds. To align three markers, it took $0.9 \pm 0.2$
seconds to achieve an accuracy of $5 \pm 2$ mm. These findings show that it is
feasible to use a combined radio-opaque optical marker, placed on a patient
prior to a CT scan, to subsequently align the acquired CT scan with the
patient.
