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Tissue-equivalent phantoms recognition using hyperspectral imaging

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The incessant innovations of hyperspectral imaging (HSI) and data mining algorithms create necessity for developing reliable means of assessment and comparison. In medical applications of HSI, for instance, one of such means is tissue-equivalent phantoms. These phantoms are designed to mimic the spectral behavior of real living tissues. In this work, gel-based-phantoms were prepared with adjusted ingredients. The gel phantom's ingredients include India ink and Intralipid to provide absorption and scattering, respectively. Unlike visual assessment and photography, HSI was successful in identifying the various phantoms based on their spectral signature. In conclusion, we introduce a simple method of evaluating the performance of newly developed optical imaging techniques including HSI via affordable, inexpensive, and easy-to-make phantoms.
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Proceedings of the 11th ICEENG Conference, 3-5 April, 2018
6-BE
1
Military Technical College
Kobry El-Kobbah,
Cairo, Egypt
11th International Conference
on Electrical Engineering
ICEENG 2018
Tissue-Equivalent Phantoms Recognition Employing Hyperspectral Imaging
Ramy Abdlaty* and Shirley Deng **
Abstract
The incessant innovations, of hyperspectral imaging (HSI), and data mining algorithms,
express the necessity for developing reliable assessment and comparison means. In
medical applications of HSI, for instance, one of the means of assessment is tissue-
equivalent phantoms. These phantoms are designed to mimic the spectral behavior of
the real living tissues. In this work, gel-based-phantoms are prepared with altered
ingredients. The gel phantom’s ingredients include India ink, and Intralipid to provide
absorption and scattering respectively. Unlike visual assessment, and photography, HSI
for succeeded to identify the various phantoms based on its spectral signature. In
conclusion, we introduce a simple method to evaluate the performance of newly
developed optical imaging techniques including HSI via an affordable, inexpensive, and
easy to make phantoms.
1- Introduction
Until then, skin diseases diagnosis and treatment evaluation are inspected by visual
assessment, the gold standard [1], [2], via a dermatologist [3], [4]. However, visual
assessment was criticized in many studies to be subjective, qualitative, temporally
inconsistent, and invasive [5][10]. To overcome the critiques of visual assessment,
objective techniques were proposed for precise skin inspection/ treatment assessment.
The proposed objective techniques, in literature, include optical [11], and non-optical
techniques [12]. In optical techniques, there are two main approaches for skin
assessment; one of which is based on diffuse reflectance spectroscopic (DRS)
measurements while the other is based on color imaging. The spectroscopic-based-
approach is well-known for detecting the spectral signature of the skin’s symptoms by
high precision, inexpensive equipment [4]. Hence, it aids the dermatologist in
differentiating between visibly alike skin’s symptoms. Unfortunately, spectroscopic
measurements techniques need direct skin contact; which is hard to achieve in cases
including burns, and limited to a small area of inspection; therefore, it takes lengthy time,
and effort to judge a sizeable region of interest (ROI). Color imaging/ photography, unlike
spectroscopic measurements, is contactless and deals with a sizable ROI. Nonetheless,
* Egyptian Armed Forces.
** Faculty of Health sciences, McMaster University, Ontario, Canada.
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photography suffers from poor spectral resolution; due to its limitation of three bands: red,
green and blue. Far from spectral shortage, photography is a cumbersome process since
it requires firm conditions in order to achieve the expected efficiency such as illumination
consistency. Taken together, the limitations of the famous optical techniques for skin
assessment provoked the inspiration of spectral imaging.
Spectral imaging (SI) is a hybrid technique combining both the advantages of DRS and
photography and thus, it overcomes the limitations of each [13]. HSI is used for acquiring
multiple frames for an object of interest, likewise photography, however, each frame is
captured at distinct wavelength [14]. As thus, the object’s ROI total spectral radiance is
acquired in a 3-dimensional data format, two of which are spatial and the third is spectral
one. SI data analysis/ classification enables a precise estimation of the componential
analysis for object under test [15]. Several considerations were taken in order to
categorize SI such as the number of bands, the resolution, and the acquisition schemes.
Regarding the number of bands, for example, SI is one of three: multispectral imaging
(MSI, usually uses less than 10 bands) [16][18], hyperspectral imaging (HSI, between
10 and 1000 bands) [13], [19], [20], or ultraspectral imaging (USI, 1000 or more bands)
[21]. In terms of the data acquisition, tunable filter-based-SI is common scheme for the
spectral scanning scheme. Two filters are most commonly used in HSI: acousto-optic
tunable filter (AOTF), and liquid crystal tunable filter (LCTF). AOTFs are distinguished by
higher spectral resolution, while LCTFs are recommended for achieving high quality
images. The use of tunable filters, in visible and near infrared spectra, entails the use of
broadband light sources such as tungsten-halogen or xenon light sources. Charge-
coupled device (CCD)/ complementary metal oxide semiconductor (CMOS) are the
challenging technologies for image detection in HSI. In terms of applications, HSI has a
successful role in several research fields including medicine [22]. Nevertheless, HSI
system is not handy for purchasing. Therefore, custom-made HSI systems is low-cost to
build but requires more effort to be validated and tested. To give an example, phantoms
with tissue like properties, provided that being longstanding, and easy to reproduce, are
fruitful way of optical innovative techniques assessment.
Tissue-equivalent phantoms (TEP) captivated the attention of research studies; since it
is convenient in simulating real tissues [23][27]. Typically, TEP fabrication is dependent
on wax, resin, and agarose as the base materials combined with either; milk, blood,
Intralipid, or yeast suspension. To mimic the natural absorbers/ diffusers in biologic
tissues, ink and polystyrene particles are added to the mixture [23], [25], [27][29]. TEP
can be customized to mimic the spectral signature of certain tissue in a minute band such
as red (633 nm) [30], [31] or a wider portion of the spectrum like near-infrared [32]. In this
work, we test a custom-made AOTF-HSI platform, originally designed by our group for
skin erythema assessment, by using visually alike, but physically different TEP. The test
results aid in evaluating the effectiveness of the developed AOTF-HSI system for a real
assessment regarding radiotherapy induced erythema in skin cancer treatment.
2- Materials and Methods
2.1- Hyperspectral Imaging Instrument
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Conveniently, HSI setup uses a wide-band light source, to illuminate the object of interest.
The light source, used here, is a set of two commercial tungsten-halogen lamps with total
power of 300 watt. The reflected photons back from the illuminated ROI, within the field
of view (FOV) of the instrument, are collected and directed toward an optical filter
positioned ahead of an image detector to scan through the successive spectral bands of
operation. To give more details, Figure 1 displays the schematic diagram for the
developed AOTF-HSI optical setup in the current work. In the lead, there is a zoom lens
(Cannon EF-S 55-250 mm f/4-5.6 IS) installed in order to collect the reflected photons.
These photons, within the lens FOV, are focused to form the object image in its focal
plane. The aforesaid plane is adjusted to be at the focal plane for the second lens in the
HSI optical setup. The second lens is the leading lens of an optical relay, formed of two
typical achromatic lenses, acting as image transporter from the first focal plane to the
detector screen. The optical relay lenses are achromatic in order to avoid the chromatic
aberration. In between the relay lenses, a polarizing beam splitter (PBS) is installed. The
PBS function is to modify the randomly polarized input beam of photons into two
orthogonally polarized rays. One ray, of the two orthogonal rays, is transmitted through
the PBS while the second is perpendicularly reflected.
For high throughput purpose, a mirror followed by a half-wave plate (λ/2) are installed on
the reflecting side of the PBS. The reason for installing the mirror and the half-wave plate
is to modify the reflected ray in both direction and polarization. By this modification, the
two rays of the input beam of photons become not only polarized but also matching the
the AOTF polarization, and injected within its acceptance angle. The incorporated AOTF
(Gooch & Housego TF625-350-2-11-BR1A), here, is a noncollinear configuration. The
AOTF crystal is made of tellurium oxide crystal (TeO2 ) with an input aperture of 11 x 12
mm window. The filter is operating in the spectral range of 450-800 nm. An 8-channel
digital frequency synthesizer (Gooch & Housego MSD040-150-0.2ADM-A5H-8 x 1) drives
the AOTF device with a radio frequency (RF) signal ranges from 40-150 MHz The RF
driver is capable of simultaneously emitting 8 separate frequencies through the TeO2
crystal, by which 8 spectral bands are diffracted. A set of control commands are used to
adjust the tuning RF signal. These commands are sent through a universal serial bus
protocol via a work station. This work station controls not only the AOTF device, but also
the image detector. The image detector (Ximea MQ042rg-CM Enhanced-IR), employed
in the optical setup, is a monochromatic complementary metal oxide semiconductor
(CMOS) camera, with a sensor size of 11.24 mm2. The whole components are well fixed
to a metal platform to ensure the stability and rigidity of the HSI system during both
movement and operation.
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Figure 1: A schematic diagram displays hyperspectral imaging (HSI) system
configuration used for AOTF and LCTF comparison. The optical components used to
build this setup are: zoom lens, square aperture, two achromatic (collimating and
focusing) lenses, flat mirrors; PBS: polarizing beam splitter; half-wave plate, and a
camera for capturing images, and computer for system control and data storage. All the
former components are tightly fixed to a metal platform to ensure the setup’s stability and
portability.
2.2-Tissue-equivalent phantom preparation
For reliability and marketing purposes, the newly developed optical instrumentation
including HSI need to be tested and characterized prior operation. One sort of
instrumentation testing is via the use of TEP’s. These phantoms are characterized by a
typical range of tissue’s optical properties. The easiest way of developing phantoms are
the aqueous suspensions, which are typically composed of Intralipid and India ink [24].
Although liquid phantoms are intensely adaptable, it is hard to include heterogeneities.
Therefore, several hydrogel-based phantoms mediated by agar, or gelatin offered the
opportunity to form distinct shapes and formats of phantoms.
Gel-based-phantoms, here, were constructed with identified concentrations of an
absorber (water soluble India ink) and scatterer (intralipid). The gel format of the
phantoms is controlled by using agar (R9012HR, Cedarlane Laboratories, Burlington, ON,
CA) solution by diluting agarose powder with a weight ratio of 1.0%. To verify the efficacy
of India-ink contributions used to prepare the phantom construction, samples of altered
ink solutions were prepared, displayed in Figure 2-(a), and characterized by two ways.
The first way is using a custom-made setup, shown in Figure 2-(b), composed of a broad
band light source, pinhole, objective lens, sample, and a fiber based spectrophotometer
(NEWPORT Model OSM400, Irvine, Ca, USA). The setup used a broad band light source
to illuminate the sample via a pinhole to clear the undesired optical noise. An objective
lens is placed at one focal length from the pinhole to to collimate the beam of light toward
the sample. The collimated beam, used to illuminate the ink samples is shaped using a
square aperture. The transmitted light is detected and measured via an optical fiber,
400 µm, and 0.22 NA, Thorlabs, Newton, NJ, USA) connected to a computer controlled
Proceedings of the 11th ICEENG Conference, 3-5 April, 2018
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spectrophotometer. The second way of verification is using a high standard calibrated
spectrometer (GENYSISTM 10S UV-Vis, Thermo Fisher Scientific, Waltham, Ma, USA).
Dilutions of the absorber component were done using a stock solution of water soluble
India ink as shown in Figure 2. Toward constructing distinct phantoms, the level of optical
absorption value (mm-1) is changed in each phantom by increasing the ink incrementally
while holding the reduced scattering at a constant value.
(a)
(b)
Figure 2: (a) Dilutions of the India ink component, used in developing the gel-based
phantoms, were done using a stock solution of water soluble India ink. (b) A custom-made
optical setup for measuring the absorbance of varying solution of water soluble India ink.
In preparation process of TEP, the agar powder (0.2 gram) is added to 9.8 mL of distilled
water, and then heated in a water path (85 oC) for 50 minutes. In parallel, 8.6 mL of water,
1.2 mL of 20% stock Intralipid solution, and 0.2 mL of India ink are mixed and poured into
a 10 mL test tube. Once the agarose solution becomes transparent, the mixture
containing the Intralipid is also put in the water path for 10 minutes. After then, the two,
10 mL test tubes are combined in a 50 mL test tube to be well mixed. At this point, fast
work is critical because the gel will rapidly solidify once it is removed from the heated
water path. Finally, pour the 20 mL mixed solution into the 20 mL phantom to form the
shape shown in Figure 3. The whole process of phantom construction was carefully
considered in each stage to achieve the required reliability.
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Figure 3: Tissue equivalent phantom (TEP’s) prepared with same ingredients (Agarose,
Intralipid, India ink, and distilled water) except the India ink concentrations. The ink
concentrations percentages are from left to right and from down to up are (0.0, 0.1, 0.2,
0.3 0.4, 0.5%) respectively.
2.3- Optical measurements
The optical measurement implemented, here, commenced with verifying the accuracy of
preparing samples of water soluble India ink. Five samples (0%,0.1%,0.2%,0.3%, and
0.4%) were used in the verification process. The measured light intensities before and
after inserting the samples were recorded. Based on Beer-Lambert’ s law, displayed in
Eq. (1), absorption was computed and plotted. Based on the former measurement, TEP,
with altered India ink concentration, were prepared. Afterwards, TEP were illuminated by
a wide band light source and imaged by the developed HSI device. An entire datacube
were recorded for each TEP. Instantaneously, each image cycle for a set of TEP’s, was
followed by capturing two datacubes. The first cube of images is taken for a white
standard reflectance target (Labsphere-SRS-99), while the second datacube is for the
sensor dark noise. The latter cube is recorded while the camera shutter and all the room
lights are turned off. The reflectance.
  
(1)
3- Experiments and Results
The prepared samples of ink solutions were characterized in terms of light transmittance
through samples. The results of the computed light intensity demonstrated an agreement
with the amounts of the India ink added to each sample. as shown in Figure 4. Based on
the attained results for sole India ink sole soluble solutions, the TEP’s were prepared in
the lab for multiple times. Other than India ink, the agarose and intralipid ratios were
adjusted to achieve the humidity and the stability of the phantoms. The stability of the
phantoms is of great importance in the imaging process since the employed HSI device
is, unlike microscope, horizontally imaging. The captured data is stored in the form of data
sets/ cubes for each phantom combined with the dark and the white standard target
cubes.
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Figure 4: the computed light absorption based on beer-Lambert’s law for the prepared
India ink soluble water solutions by (a) custom-made spectrometric measurement, and
(b) high standard calibrated spectrometer (GENYSISTM 10S UV-Vis, Thermo Fisher
Scientific, Waltham, Ma, USA).
The TEP’s datacubes were processed: first, by detector noise removal via subtracting the
dark cubes. Afterwards, the TEP’s datacubes are normalized to the formerly mentioned
white standard reflectance target datacube, in order to accommodate for uneven
illumination effects and the spectral dependence of the imaging setup applying Eq. (2).
Where is the normalized reflectance,  is the measured back reflected intensity of
the white standard, and  is the reflectance of the TEP. The value for is plotted
against one wavelength (~ 700 nm) to show how the reflectance of the prepared
phantoms are interrelated together as shown in Figure 5.
  
  
(2)
350 400 450 500 550 600 650 700 750 800 850
-0.5
0
0.5
1
1.5
2
2.5
Wavelength (nm)
-Log(I/I0)
Water
0.1% Ink
0.2% Ink
0.3% Ink
0.4% Ink
350 400 450 500 550 600 650 700 750 800 850
0
0.2
0.4
0.6
0.8
1
Wavelength (nm)
Absorption
Water
0.1% ink
0.2% ink
0.3% ink
0.4% ink
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Figure 5: Computed TEP’s reflectance at the spectral band of the centered 696 nm
wavelength for 5 phantoms with gradual increase in India ink ingredients. A linear
trendline is drawn to display the error between the computed reflectance and the ideal
one.
4- Discussion
Past, current, and continued efforts are exerted to compose phantoms in order to mimic
human tissues’ behavior. These efforts are recorded in many studies in literature [33].
Literally, the aqueous solution of a scattering medium, including Intralipid, and a water-
soluble India ink is designated as one of the top popular sort of biological simulating
phantoms. However, this sort of phantoms suffers from the disadvantage of the
incapability of creating layered structures. To overcome the previously mentioned
disadvantage, agarose component is involved in synthesizing phantoms. The agar
component allowed the development of multilayered structures where absorbing and
scattering ingredients are included. Likewise, the phantoms prepared for this work can be
molded into any custom shape.
The work, implemented here, made use of the agar-based phantoms to assess the
sensitivity of HSI in detecting changes in embeded chromophores concentration within
the visible and the near infrared region (450:800nm). The variation of the ink dye
embeded in the phantoms, used here, mimics the process of the human skin optical
properties change due to the variation of its encompassed chromophores. To verify the
reliability of the optical properties of the phantoms, the preparation process was repeated
and revised provided that the absorbing and scattering components are accurately
speckled. The revised steps of preparation include checking the macroscopic
homogeneity of the phantom ingredients by measuring the transmittance as has been
done with the India ink in Figure 4.
The procedure we introduce, in this work, facilitates the development of a phantom which
is straightforward in manufacturing, usage, and molding in alternate formats. The former
advantages of the phantom make it useful in biomedical studies, for example, offering the
required in vivo environment, likewise contrast the light-transport models with empirical
measurements. To give this work more sense, the phantoms, used here, proved the
feasibility of employing HSI in a research study, currently in its way at McMaster
university, in order to diagnose the skin lesions treated via radiotherapy. Figure 5
0.4
0.45
0.5
0.55
0.6
0.65
0.7
Ph-0.1 Ph-0.2 Ph-0.3 Ph-0.4 Ph-0.5
NORMALIZED REFLECTANCE
TISSUE EQUIVALENT PHANTOMS
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illustrates the capability of HSI for differentiating between various TEP while visual
assessment is incapable.
5- Conclusion
In a nutshell, the study presented here in this paper tagged the AOTF-HSI, with the
capability of differentiating between TEP’s with dissimilar ingredients. As a consequence,
the decision of using HSI becomes more feasible in skin diseases diagnosis. The major
advantages of AOTF is summarized in the following points:
AOTF-HSI configuration is distinguished by:
a. Fine spectral resolution
b. Significant out of band suppression
c. Fast wavelength access, appropriate for video rate
d. High diffraction efficiency (90%)
e. Compact size
f. Simultaneous diffraction of 8-distinct wavelengths
Ultimately, we need to emphasize that this work is a preliminary step toward skin
erythema objective assessment. In general, HSI system robust design is application
based. For medical applications, as an example, if the biological tissue of interest has
unknown optical properties, or characterized by close spectral features, AOTF-HSI might
be the recommended option.
Aknowledgement
The ingredients of the tissue equivalent phantoms used in the experiments were kindly
provided by Professor Qiyin Fang and described by Sharon Goh.
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