Multimodal Diffuse Optical Imaging - Biomedical Optical Imaging Technologies: Design and Applications - page 351

10.3 Instrumentation

Optical examination of thick tissue based on transmitted light through large organs
has been documented as far back as the early nineteenth century [ 11 ]. However, the
diagnostic potential of diffuse optical imaging was systematically investigated only
in the 1970s thanks to the introduction of NIR sources and detectors developed
for optical telecommunications. Following the work of Gros et al. [ 12 ]which
demonstrated that optical contrasts in the breast are spectrally dependent [ 13 ], many
technical improvements were achieved in the development of multispectral systems
[ 14 ] in the 1970s.
However, in the 1980s and early 1990s, numerous efforts have been produced to
design new instrumental techniques that offer high resolution (on the millimeter
scale or below) [ 15 ]. This focus has changed since the recognition that diffuse
optical imaging techniques do not compete with other clinical anatomical imaging
modalities in terms of spatial resolution. The current efforts of the diffuse optical
imaging community center on designing instruments that are sensitive to functional
changes, that are relatively cost-effective, and that can be deployed on the bedside
or in the primary care physician office [ 16 ].
Three major experimental techniques exist in the NIR diffuse optical imaging
field. These three techniques are categorized by the time dependence of the source
intensity impinging on the tissues and are referred to as continuous wave (CW),
frequency domain (FD), and time domain (TD) photon migration. In this section, we
review these alternative technological approaches and discuss their relative merits
and disadvantages.

10.3.1 Continuous Mode

Uses of light sources that continuously shine (or CW) on the specimen have been
the first historically to be employed [ 11 , 17 , 18 ]. CW systems encompass instruments
based on a light source that emits at a constant intensity but also systems in which
sources are modulated at low frequency ( kHz). The latter are the most common
instruments as phase-locked detection techniques allow source multiplexing with
high sensitivity. CW techniques are attractive techniques due to the availability
of high-power (up to watts) and spectrally narrow sources over a broad range of
NIR wavelengths and at a low cost. Also very sensitive affordable detectors with
high quantum yield (number of electron pairs generated per absorbed photon) are
available in this spectral range. The two main detectors commonly employed are
avalanche photodiodes (APD) and photomultipliers (PMT) [ 19 ]. PMTs are the most
sensitive detectors with the drawback of reduced dynamic range and higher costs
compared to APDs.