Biomedical Engineering Reference
In-Depth Information
volume is best localized by localization of the overlying skin. The
normal method of localization would then be to adjust the patient's
position until a light field coincident with the radiation beam is
aligned with tattoos or semi-permanent marks on the skin, the marks
having been previously placed there on the basis of a CT study,
radiographs taken during simulation, or observation and palpation.
Skin marks are also employed for deeper tumors provided that the
patient immobilization is adequate and the tumor can reasonably be
expected to remain in a fairly constant relationship with the skin.
It is usual to employ laser beams directed along three orthogonal
directions. The patient location is adjusted until these beams align
with marks placed on the patient's skin at a previous simulation.
Recently, great progress has been made in stereo-photogrammetry
and devices are now commercially available that, using optical
methods featuring at least two digital cameras, can measure with
millimeter accuracy a patient's skin surface in 3D relative to, say, the
treatment couch top - and hence, to the treatment machine as a whole.
The location in space of the skin surface measured at one time (e.g.,
just before treatment) can
be quantitatively compared
with that measured at a
previous reference time
and the difference between
them quantified and, even,
corrected for by moving
and
possibly
rotating
the patient according to
computer-calculated correc-
tions. This method is fast
and practical. Figure 7.5
shows an example of such
a device and a surface image
obtained with it.
Localization based on bony
Figure 7.5. Top left: view of a treatment
room outfitted with two (for a more
complete view) stereo-photogrammetric
cameras (outlined), and, bottom right, a
projection view of a measured 3D skin
surface with computed surface contour
lines superimposed. Figure courtesy of
Vision RT, Ltd.
anatomy
It is common in high-
precision work to relate the
target volume to bony
landmarks rather than to
skin marks. In this case,
