l - Fixation device on head : it answers to very precise require-
ments: it must fit to all kinds of skulls on one hand, into the scanner on
the other end (diameter : 250 mm). It must not uncomber the stereotactic
frame, be light and atraumatic to be worn by an awake patient during the
various stages of the diagnostic investigation (Scanner first, then stereo-
tactic exploration 12 or 24 hours later). At last it must be radiolucent and
must not cause any artifact on the images.
It consists in a cross of light alloy in which four carbon fiber
rigid arms can slide, these arms fit the stereotactic frame and are closely
fastened on the skull with nails. Once on the patient's head, the device
remains fixed on the skull with a system of lock-nuts which ensure an ade-
quate pressure (controlled by electro-luminescent diodes), so that the de-
vice does not move at all. It remains firmly fixed to the patient's head
during both the C.T. Scanning and the stereotactic exploration.
2 - The referential device itself : whatever the shape of the
patient's skull it must occupy a fixed place during the C.T. Scanning and
the stereotactic exploration ; it must enable a simple transfer of co-ordi-
nates x, y, Z. It consists in a rigid radiolucent plastic cylinder which
can be fixed on the support previously described.
Fine copper-wires making isocele triangles are sticked on the cy-
linder on both sides of the head. Thus, each wire leaves on the scanner sli-
ces a small track visualized by a very dense point. The measurement of the
distances between the different points allows to compute the level and the
orientation of the cross-sectional plane which can then be replaced in the
stereotactic space (fig.5).
III - DESCRIPTION OF THE RADIOLOGICAL INFORMATION ANALYSIS SYSTEM (fig.6)
It consists in the analytical stereoplotter TRASTER (manufactured
by MATRA-OPTICS) connected to a minicomputer, able to manage a few periphe-
ral units (drawing table, graphic display terminal, automatically control-
led tool-holder, storage units) (fig.7)
1 - Stereoplotter TRASTER : it ensures two functions, the
viewing of the stereomodel and the making of accurate measurements in this
stereomodel.
The traster, which is well-known in the world of photogrammetry
[9) is being built in a special model to answer medical specific require-
ments.
It consists in 3 different subassemblies
a) a mechanical and optícal part
It ensures the x-y travel of the photo-carrier plates and the
viewing of the stereomodel by optical projection in polarized light on a
Screen. The stereoscopic examination of the screen is possible with a pair
of glasses (with polaroid filters crossed at 90 degrees) ; this system
allows several operators to observe the stereomodel together.
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