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Optic Visualization of Auricular Nerves and Blood
Vessels: Optimisation and Validation
Eugenijus Kaniusas, Giedrius Varoneckas, Benedikt Mahr, and Jozsef Constantin Szeles
Abstract—Auricular blood vessels can be visualized by transillumination of the auricular tissue. The optimisation and validation of the optic visualization are the main subject of this
work. Since blood vessels and nerve fibers can be found along
one another, the transillumination reveals locations of auricular
nerves to unaided human eye. The visualization of the nerves is
needed by physicians to precisely insert needles into the auricle
for electrical stimulation of auricular nerves. The stimulation is
applied to relieve chronic pain and normalize vital physiological
parameters. Theoretical approaches are shortly considered which
are related to light absorption coefficients of different auricular
tissue types and coefficient changes over wavelength. The theory
predicts optimal optical contrasts if green and blue colors of light
are applied. An experimental validation has been carried out
using a novel transillumination device, a finger thimble, among
young and elderly, male and female; in total 22 volunteers. Complementary experimental approaches have confirmed theoretical
reasoning and have been used to optimize the optical contrast and
applied color mixture even further.
Index Terms—Blood vessels, ear, electrical stimulation, nerves,
optical absorption, optical contrast, pain, visualization.
I. I NTRODUCTION
T
HE EFFICIENT finding of auricular nerves is an important issue within the scope of their electrical stimulation
for pain therapy [1]. Here, an optical approach is presented
based on transillumination to disclose the neural network in
the auricle, whereas theoretical and experimental issues will be
considered.
It is important to note that the present study represents
a technically extended methodological and experimental approach with respect to our published data in [2]. In particular,
theoretical background on the optical visualization of auricular
nerves and blood vessels has already been reported in first
experimental results from 3 volunteers. In contrast, the given
paper introduces a novel transillumination device, a finger
thimble, which can be used in a clinical environment, designed
Manuscript received July 15, 2010; revised December 3, 2010; accepted
February 24, 2011. Date of publication July 7, 2011; date of current version
September 14, 2011. This work was supported by national cooperations with
Vita + Stimulants GmbH and Ingeborg Weiser & Co GmbH in Austria.
The Associate Editor coordinating the review process for this paper was
Dr. Miodrag Bolic.
E. Kaniusas and B. Mahr are with the Institute of Electrodynamics, Microwave and Circuit Engineering / E354, Vienna University of Technology,
1040 Vienna, Austria (e-mail: [email protected]).
G. Varoneckas is with the Sleep Medicine Centre, Klaipeda University
Hospital, Mechatronics Science Institute, Klaipeda University, 91225 Klaipeda,
Lithuania (e-mail: [email protected])
J. C. Szeles is with the Department of Surgery, Medical University of Vienna,
1090 Vienna, Austria (e-mail: [email protected]).
Digital Object Identifier 10.1109/TIM.2011.2159314
specifically for visualization of blood vessels. In addition, experimental optimization of light wavelength went a significant
step further. Two different auricular locations were checked
and it has been evaluated if a mixture of different wavelengths
would be more appropriate for increasing the optical contrast
to reveal the vessel’s network. Last but not least, an extensive
validation of the finger thimble was carried out in a larger group
of 22 volunteers; revealing also some qualitative differences
in visualization quality between different ages and genders.
The particular sample size was explicitly considered during
calculations of the statistical relevance of sample differences.
Actually, parallel proliferation of neural and vascular networks in human tissue serves the mutual needs of local information processing and local supply of nutrients [3]. Thus optical
visualization of blood vessels may serve the needs of nerve
visualization; the former is practically more feasible due to a
larger thickness of blood vessels (in auricle roughly < 500 μm)
in comparison with that of nerves (< 10 μm).
The visualized blood vessels indirectly disclose closely
aligned nerves. The vessels are easily discernible while the
nerves are indistinguishable to unaided human eye. The indirect
visualization of locations of nerve fibers is needed for the electrical stimulation of auricular nerves [1], usually the afferent vagal branch. Visualizing the neural network in the auricle should
support the physician to apply stimulation needles at nerve’s
locations. The resulting therapeutic effects include reduction of
acute and chronic pain perception, leading to a reduced intake
of pain-relieving medications [4]–[6]; improved sleep quality
[7], and reduced obesity in patients [8].
Objective evidence of the stimulation effects was also established on the basis of vital physiological parameters. An
increase of blood flow velocity and oxygenation [9], improved
heart rate variability and blood perfusion [10] have been reported. From a theoretical point of view, the beneficial effects
of stimulating auricular nerve receptors may involve closing of
complex gate mechanisms in the spinal cord, which prevent
pain-related action impulses from reaching the brain and thus
block the perception of pain. An activation of inhibitory pain
control systems may also be involved, in parallel to a stimulated
release of neurotransmitters, e.g., endorphins.
The clinical efficiency of the stimulation depends on a precise and specific positioning of the needle in the auricle. In addition, different therapeutic approaches require different nerve
endings, parasympathetic (e.g., vagal) endings or sympathetic
endings [11]. Thus, an immediate and comfortable overview of
auricular blood vessels and the corresponding nerves is required
before the electrode needles are applied.
0018-9456/$26.00 © 2011 IEEE
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Currently, the nerves are found by a punctual measurement
of the electrical impedance of the auricular skin using a pencillike pointer, before the electrode needles are inserted [12]. The
local conductivity increases in the region of the nerves and their
supporting blood vessels. The level of impedance is signalized
acoustically to the physician. However, the spatial resolution of
this approach is highly limited offering only a restricted orientation for the physician. In addition, varying skin conditions of
the auricle, e.g., fat or dry, have a disadvantageous impact on
the measured impedance level. Moreover, the auricular tissue is
mechanically stressed during impedance measurements, which
is uncomfortable for the patients.
Further noninvasive approaches are available for vascular
imaging, e.g., classical capillary microscopy [13]. This method
offers a high resolution of vascular bed, but suffers from a low
probing depth of the light beam in the tissue and a limited
flexibility in portable applications; thus being inappropriate for
the visualization of the auricular vessels in vivo.
Novel transillumination of the relatively thin auricular tissue
is presented here to facilitate electrode needle positioning. The
blood vessels are optically revealed since the vessels exhibit
a specific light absorption [14], [15], which indirectly identifies the location of the nerves. Theoretical optimisation and
experimental validation, in addition to the results from [2],
yield relatively large contrast in the auricular region and offer a
comfortable view of the vascular and therefore neural network
to the physician.
II. T HEORETICAL C ONSIDERATIONS
Since the visualization is intended to help the unaided human
eye, but not an electronic camera, only the visible light should
be considered for the transillumination. To be precise, only
those spectral regions of visible light should be used in which
the light absorption and scattering by blood vessels differ as
much as possible from the absorption and scattering of the
surrounding tissue, respectively. Thus spectral regions of light
should be found showing a high optical contrast.
The light absorption coefficient μa from the Beer Lambert
law
I = I0 · e−μa ·x
(1)
describes the light absorption strength, whereas I is the transmitted light intensity, I0 the incident light intensity, and x
the coordinate along which the light propagates in the tissue.
Actually, the ratio 1/μa is the light penetration depth at which
I has fallen by 1/e.
The coefficient μa varies strongly over wavelength and tissue
type, as summarized in Fig. 1(a). In the spectral range of visible
light, compare Fig. 1(b), μa of blood vessels dominates the
absorption. The blood dominance with respect to the surrounding nearly bloodless tissue tends to be more prominent for
blue and green color, indicating a high optical contrast at these
wavelengths. For larger wavelengths, μa of blood decreases
implying problems in getting a high contrast. In particular, for
the red color, μa has already fallen to less than one tenth of μa
for the green or blue color.
Fig. 1. (a) Light absorption coefficient μa of different biological media
as a function of light wavelength λ. Data on oxyhemoglobin and deoxyhemoglobin of the blood was taken from [16], water [17], auricular cartilage [18],
epidermis/dermis, subcutaneous fat, and muscle [16]. (b) Approximate luminous intensity L of the used RGB color LEDs at forward current I of 20 mA
[compare Fig. 2(b)].
Light scattering is by far the more dominant for tissue-photon
interaction than light absorption. Therefore, collimated light
becomes diffuse already at a low penetration depth of a few
millimeters. However, the scattering coefficient does not vary
as strong as the absorption coefficient over the wavelength
[14]. Thus the scattering phenomena cannot be applied for the
aforementioned contrast enhancement by optimising the light
spectrum.
The skin μa [Fig. 1(a)] does not contribute to the optical
contrast, since the skin covers both blood vessels and surrounding tissue; however, the intensity of the incident light should
be sufficiently high to penetrate the skin. The values of μa
of adipose and muscle tissue are not relevant, because both
are hardly present in the auricle. Additionally, the cartilage is
prominent in the auricle, increasing slightly the contrast for the
green color [Fig. 1(a)], along with the advantage of a relatively
high luminous intensity of the green component of the light
emitting diode (LED) [Fig. 1(b)].
III. E XPERIMENTAL S TUDY
Besides theoretical considerations, a novel transillumination
device [Fig. 2(a)] was developed in addition to the original
set-up from [2]. It serves for optimization and validation of
the light spectrum in terms of getting a high optical contrast
in vivo, additionally to its aimed comfortable use in a clinical
environment.
A. Methods
As illustrated in Fig. 2(a), an autonomous battery powered
device was established for the transillumination of the auricular
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Fig. 3. Upper ear regions transilluminated with green LED at I = 20 mA, i.e.,
at constant luminous intensity (compare Fig. 1). (a) Regionally concentrated
illumination from volunteer with a relatively thick auricle. (b) More widely
spread illumination from another volunteer with a relatively thin auricle.
Fig. 2. (a) The finger thimble in application. (b) Design of the finger thimble
with a capacitive switch on the input side and 12 RGB color LEDs on the output
side with I as the forward current.
tissue. The hardware modules are imbedded into silicon which
is molded in the shape of a finger thimble. The shape of the
casting mold was established by a 3D printing technology to
facilitate its flexible design and comfortable use of the thimble.
As an advantage, operating of the thimble requires only one
finger, other fingers being free to hold or turn the auricle
according to the needs of the physician.
According to Fig. 2(b), the thimble is activated by a capacitive switch when putting it on the middle finger. The switch
turns on 12 elliptically arranged RGB LEDs of type LRTB
G6TG by OSRAM, which are connected in parallel and are
located on the top of the thimble. The integrated controller
offers a practical possibility to vary the color intensity of the
LEDs and even to build compositions of the different colors.
The peak emission of the LED was around 470 nm for blue,
520 nm for green, and 630 nm for red [Fig. 1(b)]. The luminous
intensity was set by adjusting forward LED current to 20 mA.
It should be noted that the luminous intensities of the different
color LEDs strongly differ for a constant current, as shown in
Fig. 1(b).
The ear was illuminated from the back by placing the matrix
of the LEDs close to the auricular skin at the backside. The
half-elliptical arrangement of the LEDs [Fig. 2(a)] facilitates
a convenient, purposeful, and large-area transillumination of
the auricular regions, compare Figs. 3 and 4(a). The resulting
images were recorded from the front side by a digital photo
camera (Sony DSC-H7, 8m pixel), mimicking unaided human
eye of the physician. The images were stored in JPEG format.
The statistical differences between the RGB channels of all
volunteers were assessed by the unpaired, independent samples
t-test for equal means, varying variance, and error probability of
0.05 for rejecting the null hypothesis. The statistical relevance
of the t test was quantitatively verified with respect to the given
sample size. That is, the necessary sample size was calculated,
which is sufficient to reach to above error probability while
sample means are compared. The standard deviations of compared samples were assumed to be identical and equal to the
larger deviation (worst case assumption). Thus the statistical
difference was taken as established if the null hypothesis was
rejected and the actual sample size was equal or larger than the
necessary sample size.
The investigations were performed on 22 volunteers, thirteen
males and nine females of nearly balanced mean age 36 and
37, respectively. A group of young people was established
including seven males and five females of mean age 25 with the
oldest volunteer being 29 years old; the adult group included
the remaining volunteers of mean age 50 with the youngest
volunteer of age 36. The young men of mean age 25 were also
qualitatively compared with adult women of mean age 53.
The color of the illumination was varied in between blue,
green, red, and green-blue. The images were recorded from the
upper part of the auricle (around scapha region) and the middle
part (around cavity of conchae). Three images were recorded
per color and per auricle location, which totals in 24 images
per volunteer. For the statistical analysis, the sharpest image
was chosen from the three images and derived intensities were
averaged over both auricular regions.
B. Results
Fig. 3 demonstrates two images in case of the transillumination by the green LED at a constant luminous intensity. In both
cases, the network of vessels is clearly recognizable in the transilluminated ear region. While in Fig. 3(a) the transilluminated
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Fig. 4. (a) Upper ear region transilluminated with green LED. (b) Zoomed region with a circular vessels arrangement. (c) Automatically traced contour revealing
vessels locations and arrangements.
Fig. 5. Intensity I of the different color channels along the cross-section line in the above grey image for a single (upper) auricular region from a single volunteer.
The illumination is given by (a) blue LED, (b) green LED, and (c) green-blue LED.
region is rather small because of a relatively thick auricle, it is
more wide-spread in Fig. 3(b) due to a relatively thin auricle.
Fig. 4 shows another example with a zoomed transilluminated region [Fig. 4(b)]. For demonstration purposes, the contour of the zoomed region was traced by standard contrasting
tools, the result of which is given in Fig. 4(c).
As expected from the theoretical considerations and described in [2], the blue and green LEDs have shown subjectively
better optical contrast to unaided human eye than red LED
when blood vessels were visualized. In terms of a further
optimization based on the results in [2], Fig. 5 demonstrates
the effect of the green-blue LEDs.
Actually, Fig. 5 demonstrates the impact of the blood vessels
on the different color channels of the captured images. The
transillumination of the ear was varied among the blue LED,
green LED, and green-blue LEDs. The grey images (top images
in Fig. 5) show the analyzed excerpts (of size 100 × 100 px)
of the recorded images including vessels formations from the
same ear region and from the same volunteer. The depicted
cross-section line shows the path along which the vessels are
crossed and the intensities in the different color channels are
plotted.
During application of the blue or green LED [Fig. 5(a) and
(b)], the highest intensity and variance for the blood vessel
regions are given in the blue or green channels, respectively.
The vessel regions can be recognized in the grey images in
combination with the corresponding cross-section line. That is,
the blue or green light is absorbed more by the vessel than by its
surrounding tissue, which corresponds fully to the theoretical
considerations from above, compare Fig. 1(a). On the other
hand, the application of the green-blue LEDs [Fig. 5(c)] yields
intensity deflections in both channels, blue and green.
Since the discussed intensity change in the RGB color channels over the blood vessel’s regions (Fig. 5) determines the
applicability of the respective spectral range, the standard deviation of the channels intensity was assessed as a quantitative
statistical measure. Fig. 6 demonstrates the behavior of the
deviation for the RGB color channels considering excerpts of
the captured images with vessels contained (as grey images in
Fig. 5). Fig. 6 concerns two images per volunteer, each from the
different auricular region, which totals in sample size of 44.
As indicated by the means of the deviation in Fig. 6, the
application of the blue and green LED yields an increased
deviation for the blue and green channel, respectively. The
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Fig. 6. Statistical properties of the standard deviation s (= variability) of the intensity I in the color channels RGB considering all captured images (excerpts)
with vessels involved (compare Fig. 5). The illumination is given by (a) green LED, (b) blue LED, and (c) green-blue LEDs combination. The statistical differences
between the channels are indicated by asterisks “∗ ,” based on t-test with a sufficient sample size of 44 (two regions per volunteer) to reach error probability
p < 0.05.
Fig. 7. Qualitative differences between women and men considering the standard deviation s (= variability) of the intensity I in the color channels RGB of
all captured images (excerpts) with vessels involved (compare Fig. 5). The illumination is given by green LED. (a) All women versus all men. (b) Adult women
versus young men.
increased deviation is statistically significant in comparison
with most neighboring channels, as indicated in Fig. 6 by
asterisks. It should be stressed that the indicated statistical
differences consider the limited sample size to reach the error
probability of 0.05. In case of the green-blue LEDs, the standard
deviation of the intensity is largest for the green channel; the
deviation of the green channel being even significantly different
from that of the blue channel.
Qualitative comparison of groups of male and female yields
some potential differences in view of limited sample size.
Fig. 7(a) indicates that males tend to exhibit slightly higher
amplitudes of the deviation than females if the green LED is
applied. A similar behavior was also recognized between the
groups of young and adult volunteers; the young volunteers
showed a trend to a higher deviation, particularly for the green
LED. Potential differences in between seem to increase, if
young men are compared with adult women, as shown in
Fig. 7(b).
It should be noted that no differences were observed between
the upper and middle auricular regions concerning the deviations of the RGB channels intensity. Furthermore, the optical
contrast for the unaided human eye was—in most cases—much
better as could be conveyed by the used standard photo camera
and depicted by printed figures in the given paper.
IV. C ONCLUSION
The transillumination of the auricle is considered for optic
visualization of auricular blood vessels and thus for visualiza-
tion of parallel auricular nerves. This indirect determination
of the nerves locations is needed for a precise application of
stimulating electrode-needles by a physician within the scope
of electrical nerve stimulation for a pain relieving therapy. The
introduced transillumination may help to apply the stimulating
technology more effectively, more precisely allocating nerve
endings and thus facilitating pain relief throughout correct
needle insertion.
The given study considers theoretical and experimental approaches, with the focus on both color optimisation and validation of a novel transillumination device in a balanced volunteer
group. The theory has proved the optimal optical contrast for
the blue and green color and, as a practical conclusion, to
be useful for the vessels localization by physician [Fig. 1(a)].
In contrast, the red color is absorbed by blood to a lesser
degree; the red light penetrates the tissue with little losses
and produces practically no optical contrast in between vessels and surrounding tissue for the unaided human eye of the
physician.
The introduced finger thimble illuminates a relatively large
auricular area which offers a comfortable overview of the neural
network. In addition, the thimble can be operated by only
one finger which is an advantage in comparison with a pen
shaped device. The results from the experimental part have
confirmed the theoretical part. The use of green-blue color
mixture has not yielded any significant improvements; the green
light seems to dominate the optical contrast (Figs. 6 and 7).
This may be because of the brighter LEDs for the green color
[Fig. 1(b)].
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Potential influence of gender and aging was observed for the
green color LED with respect to the optical contrast. In case of
aging, these qualitative differences may be due to degeneration
of auricular vascular structure with the vessels diameter being
reduced over age. The gender influence may be explained by
our experimental observations that women have often showed
a distributed network of tiny vessels in the auricle while men
have exhibited relatively thick vessels being less spread. In
consequence, the optical contrast tended to be slightly better
for men. However, the limited sample size does not allow for a
clear separation of gender and aging aspects.
The applicable restrictions of the study comprise different ear
thickness of the involved volunteers and thus varying intensity
of the transmitted light. The use of digital photo camera may
also have limited the color optimisation results, because of
both automatic color balance in the camera depending on the
total light intensity and lossy compression in JPEG images. In
addition, a light pressure applied to the backside of the ear may
have induced some uncertainties in the reported data.
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Univ. Missouri-Kansas, Kansas, MO, 1981.
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J. W. Valvano, and T. E. Milner, “Optical and thermal properties of nasal
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Eugenijus Kaniusas was born in Šiauliai, Lithuania,
in 1972. He graduated in control and automation
engineering from the Faculty of Electrical Engineering at the Vienna University of Technology (VUT),
Vienna, Austria, in 1997. He completed Ph.D. in
2001 and habilitated (venia docendi) in 2006 within
the field of bioelectrical engineering.
Currently he is Associate University Professor
and head of research group “Biomedical Sensing”
at the Institute of Electrodynamics, Microwave and
Circuit Engineering, VUT. He teaches and conducts
research on electric, acoustic, optic, and magnetoelastic sensors for biomedical
applications, biomedical signal processing, and wearable hardware/software
concepts for diagnostic/therapeutic biomedical devices.
R EFERENCES
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[6] H. Kager, R. Likar, H. Jabarzadeh, R. Sittl, C. Breschan, and J. C. Szeles,
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[10] E. Kaniusas, L. Gbaoui, J. C. Szeles, T. Materna, and G. Varoneckas,
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Giedrius Varoneckas is Professor and Chief Research Associate at the Mechatronics Science Institute at the Klaipëda University, Klaipëda, Lithuania,
and Head of Sleep Medicine Centre of the Klaipeda
University Hospital, Lithuania. His research activities cover psychophysiology of cardiovascular
system, clinical application of heart rate variability
analysis, assessment of sleep and operator functional
state.
Benedikt Mahr graduated in Biomedical Engineering at the Technical University of Applied Sciences in Vienna, Vienna, Austria, in 2010. He
wrote his theses at the Institute of Electrodynamics,
Microwave and Circuit Engineering at the Vienna
University of Technology. Currently he is working toward the Ph.D. degree at the Department of
Surgery in the General Hospital of Vienna, Vienna.
Jozsef Constantin Szeles graduated from the Faculty of Medicine at the University of Vienna, Vienna,
Austria, in 1991.
Since 1994, he has been surgeon and scientist at
the Department of Surgery at the Medical University
of Vienna. Since 2006, he is head of a special outpatient clinic for electrical auricular therapy. His research activities cover cultivation of endothelial cells
and production of vascular bio prostheses, NMR
microscopy, and long-term electrical stimulation of
auricular nerve endings.