Biomedical Engineering Reference
In-Depth Information
A self-mixing based prototype with a miniaturized laser Doppler probe is also being
built in order to monitor blood flow changes in rat deep brain structures without causing
significant damage to the tissue [3]. In the self-mixing method, the monitor photodiode
at the rear face of the laser diode is used for signal detection; a single optical fibre is
therefore used for light emission and detection. Pigtailed laser diodes, with 785 and
1308 nm laser wavelengths and with single mode optical fibre are used. Standard single
mode optical fibres have 125 and 250
μm
of cladding and jacket diameters, respectively.
The probe consists of the stripped optical fibre inserted in a micro-needle with an outer
diameter of 260
μm
[3]. Measurements will be made in the rat brain hippocampus. As
commercial available probes have a 450
μm
diameter, the use of only one optical fibre
allows us to reduce the size of the probe to 58%.
2.2
Phantom and Simulation Models
Phantom Model.
The phantom model was built with the purpose to evaluate,
in vitro
,
the non-invasive flowmeter prototype response to moving fluid at different depths (as it
can be found in skin) [6]. The phantom consists of a Teflon microtube rolled around an
aluminium metal piece with a total of six layers. The inner and outer diameters of the
microtube are 0.3 and 0.76 mm, respectively. Commercial skimmed milk has been cho-
sen as a moving fluid because it has various components that act as scatterers, namely
carbohydrates, fat, and protein. Moreover, it does not sediment like microspheres, and it
has similar behaviour to intralipid solutions [7]. Finally, milk is easier for handling than
blood and, besides, it is cheaper. However, as milk is unstable, we use the same milk
solution for one day only. Milk is pumped in the microtubes with a motorized syringe
with different velocities: 1.56, 3.12, 4.68, 6.25, 7.78, and 9.35 mm/s. Different milk
solutions were used: 100% milk, and aqueous solutions with 50% and 25% of milk.
The first simulated model (presented in figure 1) consists of three main layers. The
first layer is composed of a set of objects equivalent to the six microtube layers and it
has a total depth of 5 mm. The two deeper layers mimic the aluminium plate and have
a thickness of 0.1 mm each one: one acts as a scatterer with isotropic semi-spherical
backscattering and the other is a totally reflecting layer. The laser light was considered
as a pencil beam shape and it was positioned at the top of the most superficial tube. A
parabolic profile was used for the milk flow simulations where the maximum velocity
is twice the mean velocity.
The simulations were made only for 635 nm laser light wavelength due to the ab-
sence of information concerning milk and Teflon optical properties for 785 and 830 nm
laser light. The milk optical properties used were published by Waterworth
et al.
[7],
where the refractive index for milk is 1.346, absorption (
μa
) and scattering coefficients
(
μs
) are 0.00052 and 52
mm
−
1
, respectively. The Teflon optical properties used were
published by Li
et al.
[8] where the refractive index is 1.367,
μa
= 0.001
mm
−
1
and
μs
=167
mm
−
1
, respectively. Henyey-Greenstein phase function was used with g = 0.90
for both components [7] [8].
Simulations were made for six different velocities for milk speed, three different milk
solutions and three different detection distances, which gives a total of 54 simulations,
with 5,000,000 photons detected in each simulation.
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