Multimodal Diffuse Optical Imaging - Biomedical Optical Imaging Technologies: Design and Applications - page 354

The majority of time domain optical imaging systems are based on time-
correlated single-photon counting (TCSPC) hardware, single-photon counting
PMTs, and pulsed laser diodes. Significant technological advances have been made
in these three areas in the last decade, leading to a widespread acceptance of time-
resolved techniques in various biophotonics fields. Recently, with the advent of
fast-gated CCD, time-gating techniques have been also employed. The benefit of
the time-gating CCD is the high spatial density offered by this detector. However,
the current CCD do not have an optimal quantum yield in the NIR window, and the
necessity to acquire multiple gates sequentially to reconstruct a TPSF can lead to
lengthy acquisition time.
Similarly to the Carl Zeiss and Siemens Medical Engineering system, prototypes
based on soft compression of the breast between two planar plates and raster
scan of multiwavelength source-detector pair in transmittance geometry have been
developed by the Physikalisch-Technische Bundesanstalt group in Berlin [ 37 ]
and Politecnico di Milano groups [ 38 ]. Both systems have implemented off-axis
detection channels to provide depth information necessary for tomographic imaging.
Advanced Research Technologies (ART) Inc., Montreal, developed a commercial
system based on the same technology and principles: SoftScan R

.Themain
difference between this commercial system and the academic system is that in the
ART system, the examination is performed in the prone position with the breast
gently compressed in a chamber filled with matching liquid and in the academic
system, the patient is in a supine position similarly to mammographic systems.
ART's system employs four pulsed laser diodes, a coaxial detection channel with
4 off-axis channels [ 39 ].
Tomographic systems that do not employ mechanical scans of the optodes but
rather sequential point excitation of the tissue based on source fiber multiplexing and
parallel detection with fiber bundles have been developed at the University College
of London (UCL) and at the University of Pennsylvania. The UCL system is a two-
wavelength (780 and 815 nm), 32-detection/excitation channel system [ 40 ], whereas
the UPenn system is a six-wavelength (690, 750, 780, 800, 830, and 840 nm) system
spatially multiplexed to 64 fiber positions and 16 detection channels acquiring the
signalinparallel[ 41 ] and has been designed for concurrent MRI examination. These
systems acquire data from multiple projections, allowing accurate tomographic
reconstructions.

10.3.4 Ad Hoc Instrument Design Optimization

Volumetric diffuse optical imaging is a model-based imaging technique whose
performance is significantly dependent on the accuracy of the light propagation
model, the reconstruction strategies, and the data sets collected. It is well established
that quantitative accuracy and image resolution are dependent on the number of
source-detector pairs employed but also on the number of wavelengths used to
probe the medium. To offer superior quantitative performances and to increase