Predicting error in rigid-body point-based registration

Guidance systems designed for neurosurgery, hip surgery, and spine surgery, and for approaches to other anatomy that is relatively rigid can use rigid -body transformations to accomplish image registration. These systems often rely on point-based registration to determine the transformation, and many such systems use attached fiducial markers to establish accurate fiducial points for the registration, the points being established by some fiducial localization process. Accuracy is important to these systems, as is knowled ge of the level of that accuracy. An advantage of marker-based systems, par ticularly those in which the markers are bone-implanted, is that registrati on error depends only on the fiducial localization error (FLE) and is thus to a large extent independent of the particular object being registered. Th us, it should be possible to predict the clinical accuracy of marker-based systems on the basis of experimental measurements made with phantoms or pre vious patients. This paper presents two new expressions for estimating regi stration accuracy of such systems and points out a danger in using a tradit ional measure of registration accuracy. The new expressions represent funda mental theoretical results with regard to the relationship between localiza tion error and registration error in rigid-body, point-based registration. Rigid-body, point-based registration is achieved by finding the rigid trans formation that minimizes "fiducial registration error" (FRE), which is the root mean square distance between homologous fiducials after registration. Closed form solutions have been known since 1966, The expected value [FRE2] depends on the number N of fiducials and expected squared value of FLE, [F LE2], but in 1979 it was shown that [FRE2] is approximately independent of the fiducial configuration C. The importance of this surprising result seem s not yet to have been appreciated by the registration community: Poor regi strations caused by poor fiducial configurations may appear to be good due to a small FRE value. A more critical and direct measure of registration error is the "target reg istration error" (TRE), which is the distance between homologous points oth er than the centroids of fiducials, Efforts to characterize its behavior ha ve been made since 1989, Published numerical simulations have shown that [T RE2] is roughly proportional to [FLE2]/N and, unlike [FRE2], does depend in some way on C. Thus, FRE, which is often used as feedback to the surgeon u sing a point-based guidance system, is in fact an unreliable indicator of r egistration accuracy. In this work we derive approximate expressions for [TRE2], and for the expe cted squared alignment error of an individual fiducial, We validate both ap proximations through numerical simulations. The former expression can be us ed to provide reliable feedback to the surgeon during surgery and to guide the placement of markers before surgery, or at least to warn the surgeon of potentially dangerous fiducial placements; the latter expression leads to a surprising conclusion: Expected registration accuracy (TRE) is worst near the fiducials that are most closely aligned! This revelation should be of particular concern to surgeons who may at present be relying on fiducial al ignment as an indicator of the accuracy of their point-based guidance syste ms.