An integral component of is a device (mechanical, optical,
ultrasonic or magnetic) that allows points in three-dimensional space
to be localized and their spatial coordinates transmitted to a
computer. The Viewing Wand system installed in the OR at the MNI uses
a mechanical localizer, the FARO 
arm, shown in arm. It is a six
degree of freedom articulated arm constructed from aluminum. A probe,
consisting of a 200 mm long cylinder with a thin shaft at its end, is
attached to a probe holder at the end of the arm. The articulated arm
is passive, meaning that the surgeon (or an assistant) manipulates it
and holds it in a desired position. The analogue signals produced by
the six angle potentiometers located at the joints, are digitized by
an analogue-to-digital (A/D) converter interfaced to a PC, which
transforms the digitized readings of the joint potentiometers into a
set of six numbers representing the position and the orientation of
the probe holder with respect to a coordinate system attached to the
base of the arm (FARO). Further calculations, which depend on
the dimensions of the probe used, must then be performed to determine the
position of the tip of the probe (see Chapter 4 for
more details).
