Biomedical Engineering Reference
In-Depth Information
The software may also allow some planning or even rehearsal of the sur-
gical procedure. Craniotomies and trajectories in neurosurgery can be
planned to avoid critical structures. In image-guided total hip replacement,
a computer-assisted orthopedic surgical procedure, implant size and orien-
tation and acetabular cup orientation can be defined by manipulation of the
preoperative images.
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In maxillofacial surgery, the surgeon may rehearse
the cutting and movement of segments of bone to achieve the desired surgical
outcome.
12.3.2
Point-Based Registration
The most common method used in commercial systems to perform registra-
tion is to find corresponding point landmarks in the images and on the
patient. Such points are generally termed fiducials. These may be anatomical
landmarks, skin-affixed markers, or bone-implanted fiducials. The algorithm
normally used is the orthogonal Procrustes solution described in Chapter 3,
Section 3.4.
Our experience suggests that anatomical landmarks can be found with a
FLE of 3 to 5 mm. Published data suggest that skin-affixed fiducials can be
accurate to around 2 mm if used with care, the main source of inaccuracy
being the fact that the skin can move.
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Bone-implanted markers are by far the
most accurate fiducials, allowing each marker to be located both in the image
and on the patient to within 0.7 mm.
2, 3
12.3.3
Contour Registration
Linear features of an object could be used to provide registration. These could
be specific geometric features such as lines of maximum curvature, water-
shed, or crest lines.
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Marking linear features with a tracked pointer would be
rather difficult, so such contour registration has only been proposed for use
in conjunction with an intraoperative imaging device (see Section 12.4.2).
12.3.4
Surface Registration
A further method implemented in most commercial systems is to match a
number of points on the surface of the patient's skin to the same surface
extracted from the preoperative scans. Physical points are marked by drag-
ging a hand-held pointer over the skin. Unfortunately, skin is soft and may
deform. Brainlab (Munich, Germany) uses a laser point light source swept
over the skin surface, which avoids skin distortion due to physical contact.
However, there may still be some deformation due to muscle movement or
changes in patient positioning between scanning and intervention. To get
accurate rigid registration it is therefore desirable to mark points on the bone
surface and match this to the bone surface extracted from CT. These surface
