Biomedical Engineering Reference
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and incorporated spectral priors as well. Optical-property recovery in simulation
was accurate within 15 %, even when anatomical data did not specify a tumor region
of interest.
10.5
Combining Diffuse Optical Imaging with Other
Modalities
10.5.1
Magnetic Resonance Imaging
Fusion of optical imaging techniques with MRI is an active field of research. Thanks
to its high soft tissue contrast and three-dimensional imaging capabilities, MRI
is the perfect candidate for optical multimodal integration. To date, the focus of
optical/MRI fusion has been mainly limited to derive high-resolution structural
information to guide the optical inverse problem or to validate the nature and
localization of the information collected by one of the stand-alone systems.
The chief difficulty in performing MRI-optical hardware fusion resides in
designing an optical instrument delivering and collecting light without interference
from the high magnetic field. Thus, typically, the optical instruments are designed
around long optical fibers (up to 10 m) to operate in a shielded environment outside
the MRI examination room. The fibers are delivered to the examination chamber
through a magnetically shielded conduit. The fibers need to be void of any magnetic
material for safety reasons and to avoid artifacts in the MRI data. Similarly, the fiber
holder should be compatible with MRI requirements but still provide high-precision
positioning of the optical fibers (
tenth of a millimeter). Such holder also holds
the MR coil, limiting the space available for optimal optode positioning. Moreover,
due to the MR bore size and the nature of clinical MR coils, the physical space
available to bring in the fibers is limited, leading to a reduced number of fibers that
can be efficiently coupled with the MR systems. This is especially limiting with
the detection fibers that are generally fiber bundles of 3-5-mm diameter for clinical
applications. Thus, current optical systems that are integrated with clinical MRI
are typically limited to 16 detection fibers (cf. Fig.
10.3
). Then 3D optical imaging
becomes difficult, and great care should be taken to position the optodes close to the
pathology to be imaged. This can be very challenging in clinical settings in which
the imaging session time is restricted, constraining the time allocated to position the
pathology within the plane of the optical fibers.
MRI-optical fusion has been first developed to benefit from the high-
resolution MR anatomical maps. Such anatomical images are typically acquired
with T1-weighted or T2-weighted spin-echo sequences and registration with optical
images performed via static fiduciary markers. The MR image structure can be
further segmented into the different types of tissues investigated. In the case of
optical mammography, MR allows to segment the main structure of the breast
(adipose and fibroglandular tissue) and potentially the lesion. Spatial correlation
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