Biomedical Engineering Reference
In-Depth Information
To probe deep tissue such as in optical mammography, source-detector separation
spans from a couple of centimeters to a dozen of centimeters, depending on
the configuration selected. Due to the strong attenuation of the signal over such
propagation distances, the spectral information is acquired sequentially by time
multiplexing the wavelengths as hyperspectral source power is limited by ANSI
standard and offers limited signal to noise ratio. The optical signal must then
be integrated over a second or more to acquire robust data. Thus, acquisition
time can be rather lengthy and fast hemodynamic monitoring becomes extremely
challenging.
If the majority of diffuse optical imaging systems developed are based on CW
techniques, they suffer from intrinsic disadvantages. First, monochromatic CW
techniques are unable to distinguish between the effects of absorption and scattering
[
20
]. Only recently has it been demonstrated that by using multiple wavelengths,
absorption and scattering contributions could be unmixed [
21
]. Though in practical
cases, such approach requires accurate estimation of background optical properties,
which is not attainable with a CW data set alone but circumvented by implementing
a few time-dependent channels [
42
,
61
].
Second, intensity data are extremely sensitive to the surface tissues compara-
tively to deeper regions. As a consequence, CW techniques are highly dependent
on the tissue-probe coupling. If such drawback can be attenuated in the case of
static systems with numerous optode measurements [
22
,
23
], it renders optical
examination based on handheld probe highly operator dependent and sensitive to
hemodynamic changes in the dermal capillaries.
10.3.2
Frequency Domain
To alleviate some of the drawbacks associated with CW techniques, numerous
efforts have been put forward to integrate systems using time-dependent source
amplitude. Technically, two distinct approaches using time-dependent sources are
feasible: frequency domain (FD) and time domain (TD). Frequency domain refers
to techniques based on a source whose amplitude is modulated in the MHz range (up
to 1 GHz) and detection performed at the same frequency. Such a technique provides
a set of two measurements: modulation amplitude attenuation and phase shift due to
light propagation. In turn, these data allow to effectively separate the contributions
of absorption and scattering [
24
,
25
]. A comprehensive review of the frequency
domain technology has been performed by Chance et al. [
26
].
The development of FD imagers for thick tissue has been strongly influenced
with the efforts of Carl Zeiss [
27
,
28
] and Siemens Medical Engineering [
29
]
in the mid-90s. The prototypes built by these companies were dedicated to the
topographical examination of the soft compressed breast. The breast was gently
compressed between two clear windows, and a multiwavelength source-detector
Search WWH ::
Custom Search