Biomedical Engineering Reference
In-Depth Information
defined by the user. For example, a strict binary thresholding can be
employed to discriminate gray from white matter. The beauty of this
approach is that alternate intensity-to-property maps can be identified by
the user, depending on problem requirements. Figure 15.13 shows an exam-
ple of mesh generation at several stages of completion, including wireframe
descriptions with and without internal boundaries which define model
geometry, and image-to-grid segmentations based on gray
white matter
intensity thresholding. The figure makes it clear that high resolution com-
putational models which faithfully capture patient-specific geometry and
tissue types can be routinely produced.
Utilizing preoperative MR data to estimate tissue elastic properties may
also become a reality in the near future. Computational models of tissue
deformation play an important role here as well. By imparting small
amplitude (10
m) displacement to tissue at low frequencies (100 Hz) syn-
chronized with specialized MR pulse sequences, it is possible to measure
3D displacement fields at the MR voxel level. These displacement pat-
terns are a complex synthesis of dilatational and shear waves that are
functions not only of the local tissue properties, but also of the boundary
conditions and stimulation forces that are applied. Hence, estimation
algorithms are needed to infer intrinsic tissue mechanical properties from
the MR-measured displacements. An example of a recently reported finite
element technique is illustrated in Figure 15.14.
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The reconstruction of an
elastic property map is based on a simulation, but is none-theless quite
impressive, especially given the fact that the synthetic measurements
have been corrupted with 10% added noise, yet detailed discrimination
of differences in modulus between gray and white matter is quite clear.
If this technology can be developed, it is quite conceivable that patient-
specific, spatially resolved mechanical property maps would be available for
modeling purposes in the OR to improve the ability to account for intraoper-
ative tissue motion through a model-based approach.
15.6.2
Intraoperative Data
Tracking of instruments and cortical surface movement in the OR can be
accomplished with a variety of tools. Figure 15.15 illustrates one option
which consists of an operating microscope attached to a ceiling-mounted
robotic platform which maintains knowledge of a coordinate system in the
OR. The realization in Figure 15.15 is the Surgiscope System manufactured
by Elekta AB (Stockholm, Sweden). It offers a number of precision tracking
functions including a continuous readout of microscope location and orien-
tation, memorization of a particular location (and orientation) which can
be returned to at any time, and laser beam positioning within the optics of
the microscope convergent at its focus. Coregistered digital photographs
can be acquired of the surgical field and used to capture cortical motion as
illustrated in Figure 15.16, which shows two photographs recorded at different
