Biomedical Engineering Reference
In-Depth Information
LDF signals recorded from human skin lack in estimating the sampling depth. These
difficulties lead to ambiguities in the discrimination of the fraction of light scattered
from superficial and deeper blood microcirculation skin layers [3]. Besides this, com-
mercial available flowmeters use different signal processing algorithms and calibration
procedures making impossible the comparison of their results [2]. Concerning LDF
invasive measurements, the smallest commercial probes available (with 450
μm
diam-
eter) are too large for research studies in small organs of animals as rat brain, causing
damage in an extension that may negatively impact local measurements [3].
Monte Carlo methods are statistic methods used in stochastic simulations with appli-
cations in several areas as physics, mathematics, and biology. Monte Carlo simulations
of light transport are very helpful in photon propagation studies in turbid media, as
skin. They have been widely used in LDF area (see for example [4]). In Monte Carlo
methods, the light transport in turbid media is based on the simulation of the photon
trajectories, where separate photons travel through the tissues. Several phenomena as
scattering, absorption and refraction can be simulated based on the scattering functions,
Fresnel relations, etc.
We present herein Monte Carlo simulations results for validation of two new laser
Doppler flowmeter prototypes. These prototypes have been built in order to eliminate
two drawbacks existing in the LDF technique. The first prototype aims at giving depth
perfusion measurements information (non invasive prototype) for human skin. The sec-
ond prototype aims at reducing the size of LDF invasive probes for rat brain measure-
ments (invasive prototype). For the first prototype validation, Monte Carlo simulations
in a phantom consisting of moving fluid at six different depths are herein proposed.
Simulations in a skin model are also presented. Measurements made in the phantom
built and in the human skin are herein presented and compared with the simulation re-
sults. For the second prototype validation, Monte Carlo simulations are carried out on a
rat brain model paired with
in vivo
measurements. In what follows, we first present the
two prototypes, the three simulated models and the measurements protocols. Finally,
the Monte Carlo simulations are detailed and the computed signals are presented and
compared with the measurements.
2
Materials and Methods
2.1
Prototypes
Skin microcirculation is present in the dermis and is organized into two horizontal
plexuses: the most superficial is situated in the papillary dermis at 0.4 - 0.5 mm below
the skin surface; the second plexus is located at the dermal subcutaneous interface at 1.9
mm from the skin surface where arteriovenous anastomoses can be found [5]. A new
laser Doppler flowmeter prototype with depth discrimination capabilities is being built
in order to determine the sampling depth of the backscattered photons used to compute
the LDF signal [3]. This prototype is a non-invasive and multi-wavelength prototype
device, with 635, 785, and 830 nm laser wavelengths. The probe used is from Perimed
AB and has a central emitting fibre and collecting fibres located at 0.14 (F0.14), 0.25
(F0.25) and 1.20 (F1.20) mm from the emitting fibre [3].
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