Trusted Execution Environments (TEE) in vehicles.

Root cause 6: Certificate Management of Non-Vehicle ITS-S Types. Pseudonym
usage by RSUs is not explicitly specified in the ETSI ITS standard. Thus it
remains unclear if and how their usage is different from vehicles. Introducing
smartphones (VRU) as a type of ITS-S requires appropriate management of these
devices. Smartphones are consumer-owned generic platform as opposed to
dedicated purpose devices such as OBUs and RSUs, thus a different approach to
manage them will be required. Another open issue of smartphones is whether and
how they can be revoked and removed from ITS system, if and when it is
necessary.

Recommendations & research objectives:

  * •

Define how the pseudonym usage in RSU is different from vehicles (Sec. 7.5).
Define how smartphones as a type of ITS-S is managed in the ITS system,
including enrolment, verification, and authorization, issuance of
certificates, usage of pseudonyms, and how they are revoked from the system
(Sec. 7.6).

Root cause 7: RSU Infrastructure Dependency. Many functionalities discussed in
EU project use cases depend on RSUs and the infrastructure behind them, such
as distribution and reloading of certificates. However, ubiquitous
installation and availability of RSUs is an assumption, not a given condition.
How soon an RSU infrastructure will be deployed depends on a number of non-
technical factors, such as government policy on ITS and budget allocation. It
is likely a gradual process and varies from one region to another and one
country to another. ITS-capable vehicles likely find themselves in the
situation where RSU installation is either scarce or non-existent. Thus, being
overly dependent on RSUs is counterproductive to the deployment of V2X
technology.

Recommendations & research objectives:

  * •

Define alternative solutions to reduce dependency on the ubiquitous RSU
deployment in such a way that necessary functionalities can be fulfilled
independent from RSUs when it is necessary, while making effective use of RSUs
when they are available (Sec. 7.3).

Root cause 8: MNO Involvement and Dependency. Within the context of VRUs,
whether interaction is required between the ITS system and MNOs to manage
their access to the ITS system is unspecified in the ETSI ITS specification.
If required, the solution needs standardization so that it will be adopted by
all MNOs at international level.

Recommendations & research objectives:

  * •

Define if the ITS management system and MNOs need to interact with each other
to manage VRUs (smartphones) or not. This interaction between them includes
definition of message contents (Sec. 7.6). This involves coordination with
appropriate standard bodies such as 3GPP.

Table 6. LIST OF RECOMMENDED ACTIONS TO ETSI ITS SPECIFICATIONS
---
Num | Description | Section
1 |  | Specify possible differences in the privacy protection requirements for different ITS-S types.
---
6.2, 6.3
2 |  | Resolve conflicting requirements of privacy protection and including vehcile-identifiable
---
information in CAM and DENM.
6.4
3 |  | Resolve conflicting requirements of privacy protection and road safety.
---
6.1, 6.5
4 |  | Resolve privacy protection in unicast communication from both internal and external threats.
---
6.6
5 |  | Address the potential issue of real-time certificate validation.
---
7.1
6 |  | Standardise certificate change and reloading rule or policy.
---
7.2, 7.3
7 |  | Resolve the time gap associated with passive revocation.
---
7.4
8 |  | Specify certificate management of RSUs and VRUs.
---
7.5, 7.6
9 |  | Specify different use of certificates for different purposes.
---
7.7
10 |  | Address vulnerabilities of broadcast messages against passive observers.
---
8.1
11 |  | Establish guidances on the use of unicast mode.
---
8.2
12 |  | Establish guidance on the implementation of plausibility validation and misbehaviour detection.
---
9.1
13 |  | Investigate solutions for accurate detection of VRU movement to VAM.
---
9.2
14 |  | Consider possible notion of device type-dependent trust level.
---
10.1
15 |  | Analyse and establish scenarios of interconnecting multiple management systems.
---
10.2, 10.3

##  12\. Conclusion and Future Work

In this paper, we examined the EU standards for ITS, related US standards,
various V2X-related EU projects, and relevant research papers. We integrated
information from these sources, analyzed, and identified gaps in the security
aspects of vehicular communication, focusing on the ETSI ITS specifications.

The issues and gaps we have identified are significant. They include
conflicting and undefined specification in the standards. System level aspects
need further definition, e.g. interworking of multiple management systems
across multiple jurisdictions and countries. The security management of VRUs
(smartphones) as a new ITS-S type is missing. Without addressing these
aspects, the vehicular communication in reality can very well be unreliable,
insecure, and unusable, ultimately leading to accidents, injuries, or loss of
properties. Leaving details to implementation-specific solutions can lead to
varying degree of effectiveness among implementations. To ensure uniform
operation and effectiveness, further research and standardization is needed.
In this respect, the future work is to address these gaps and issues to define
possible solutions.

As a conclusion, the security solution in the ITS standards solely based on
PKI leaves a number of areas to reconsider. Additional approaches and
solutions are required to ensure vehicular communication is indeed secure so
that the overall objective to make roads safer and reduce road accidents can
be achieved rather than providing a new target for cyberattacks. The
fundamental nature of cyber-physical system, such as vehicular communication,
is that a sub-optimal system can cause physical damage in reality. In order to
avoid such losses, all relevant security and privacy aspects of vehicular
communication need to be addressed.

###### Acknowledgements.

This work was supported in part by CyberSecurity Research Flanders with
reference number VR20192203. This work was also supported in part by the
Research Council KU Leuven C1 on Security and Privacy for Cyber-Physical
Systems and the Internet of Things with contract number C16/15/058. In
addition, this work was supported in part by the Flemish Government through
the imec Netsec project, through the EIT Health RAMSES project, through the
Smart Highways SErVo project, and by the European Commission through the
Horizon 2020 research and innovation programme under grant agreement
H2020-SC1-FA-DTS-2018-1-826284 ProTego and MSCA-ITN-814035 5GhOSTS.

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# Homodyne Time-Domain Acousto-Optic Imaging for Low-Gain Photodetectors

Ahiad Refael Levi  Andrew and Erna Viterbi Faculty of Electrical Engineering,
Technion – Israel Institute of Technology, Technion City 32000, Haifa, Israel
<EMAIL_ADDRESS>Yoav Hazan  Andrew and Erna Viterbi Faculty of
Electrical Engineering, Technion – Israel Institute of Technology, Technion
City 32000, Haifa, Israel  Aner Lev  The Israel Center for Advanced Photonics
(ICAP), Yavne 81800, Israel  Bruno G. Sfez  The Israel Center for Advanced
Photonics (ICAP), Yavne 81800, Israel  Amir Rosenthal  Andrew and Erna Viterbi
Faculty of Electrical Engineering, Technion – Israel Institute of Technology,
Technion City 32000, Haifa, Israel

###### Abstract

Acousto-optics imaging (AOI) is a hybrid imaging modality that is capable of
mapping the light fluence rate in deep tissue by local ultrasound modulation
of the diffused photons. Since the intensity of the modulated photons is
relatively low, AOI systems often rely on high-gain photodetectors, e.g.
photomultiplier tubes (PMTs), which limit scalability due to size and cost and
may significantly increase the relative shot-noise in the detected signal due
to low quantum yields or gain noise. In this Letter, we have developed a
homodyne AOI scheme in which the modulated photons are amplified by
interference with a reference beam, enabling their detection with low-gain
photodetectors. We experimentally demonstrate our approach with a silicon
photodiode, achieving over a 4-fold improvement in SNR in comparison to a PMT-
based setup.

††journal: ol

Deep-tissue optical imaging is generally performed by illuminating the tissue
over a large area and using the reemitted diffused light to form an image
representative of the optical properties of the tissue [1]. By using
tomographic illumination and detection patterns, combined with optimization-
based inversion algorithms, depth-resolved imaging may be performed, as been
demonstrate in the field of diffuse optical tomography (DOT) [2, 3]. However,
purely optical techniques such as DOT are inherently limited in their spatial
resolution due light diffusion, mathematically characterized by an ill-
conditioned inverse problem.

Acousto-optics imaging (AOI) [4, 5] can improve the resolution of purely
optical techniques of deep-tissue imaging by using ultrasound (US). In AOI,
the tissue is both illuminated with a coherent laser and insonified with an
ultrasound transducer, leading to a pressure-induced refractive-index
modulation and vibrations of the optical scatterers in the insonified regions
[6]. As a results, light that travels through the insonified regions
experiences a phase modulation with the same frequency as that of the
ultrasound wave. Because of the high coherence of the laser, the reemitted
light exhibits a speckle pattern in which the intensity of each speckle grain
is temporally modulated with the ultrasound frequency. The modulation depth of
the speckle pattern may be measured using various methods [7], enabling a
localized detection of light in the tissue. AOI may be performed with a
focused US beam that scans the imaged region [8] or a set of plane US waves
[9], enabling the visualization of the light fluence rate within the tissue
with acoustic resolution.

To optimize the signal-to-noise ratio (SNR) of the measurement, parallel
detection of uncorrelated speckle grains is required, which is often performed
by digital cameras. However, the low time resolution of cameras introduces two
difficulties that limit their use in vivo. First, it complicates the
distinction between the effect of US modulation and speckle decorrelation, as
both these phenomena are faster than the camera frame-rate [10]. Second, it
limits the use of US bursts, for which the acoustic time of flight may be used
for depth sectioning.

In order to overcome speckle decorrelation and enable the use of acoustic
bursts, time-domain AOI (TD-AOI) may be used, in which the optical detection
is performed with detectors that operate at a higher bandwidth than that of
the US transducer [11, 12, 13]. The immunity of TD-AOI to speckle
decorrelation has facilitated in vivo demonstrations in small animals [14] as
well as clinical testing for the diagnosis of osteoporosis [15]. An additional
advantage of TD-AOI is its ability to locally quantify blood flow at different
depths from the spectral broadening of the AOI signal [16, 17] — a capability
that has been demonstrated for monitoring cerebral blood flow [18].

Because of the low intensity of the US-modulated light, TD-AOI is
conventionally performed with photodetectors with a high internal gain, e.g.
photomultiplier tubes (PMTs) that use the avalanche effect to multiply the
current created by a single photon [19, 13, 17]. However, despite the high
gain, this approach may lead to a lower SNR in the AOI measurement. First, the
avalanche is an inherently stochastic process in which the gain varies
randomly, thus increasing the relative shot noise in the signal. Second, in
the case of PMT, the relative shot noise is further increased by the low
quantum yield of the detector. Third, the cost and complexity of high-gain
detectors limits their scale-up to multi-element arrays, required for high-SNR
operation.

In this work, we have developed a new approach for TD-AOI in which the
reemitted light is not detected directly, but is rather interfered with a
reference beam in a homodyne configuration. The interference leads to an
optical amplification of the US-modulated light, enabling its detection with
low-gain photodetectors. While homodyne detection often requires controlling
the exact phase difference between the signal and reference arms [20], no such
control is required in our scheme due to the stochastic nature of the AOI
signal. We have experimentally tested our approach with a silicon photodiode
(PD), demonstrating over a 4-fold increase in SNR in comparison to a PMT, in
agreement with the theoretical prediction.

Assuming that the detection is performed over time scales faster than the
speckle decorrelation time, the field of a single speckle may be represented
by

$E_{i}=E_{bkg,i}e^{j\omega t}+E_{AO,i}e^{j\omega t}e^{j\left(2\pi
f_{US}t+\phi_{i}\right)},$ (1)

where $\omega$ is the angular frequency of the light, $f_{US}$ is the US
frequency and $\phi_{i}$ is a random phase distributed: $\phi_{i}\sim
U[0,2\pi]$ that represents the phase difference between the modulated and
unmoldulated parts of the field. $E_{bkg,i}$ and $E_{AO,i}$ are the amplitudes
of the unmodulated and modulated fields, respectively, where it is assumed
that $|E_{bkg,i}|\gg|E_{AO,i}|$.

In conventional TD-AOI, the measured optical power consists of spatial
integration over $N$ speckles, which are statistically independent and
identically distributed, leading to the following expression:

$\begin{split}P^{N}&\propto\sum_{i=1}^{N}\left|E_{bkg,i}e^{j\omega
t}+E_{AO,i}e^{j\omega t}e^{j\left(2\pi f_{US}t+\phi_{i}\right)}\right|^{2}\\\
&=\sum_{i=1}^{N}I_{bkg,i}+2\sum_{i=1}^{N}\sqrt{I_{bkg,i}I_{AO,i}}cos\left(2\pi
f_{US}t+\phi_{i}\right).\end{split}$ (2)

where $I_{bkg,i}=|E_{bkg,i}|^{2}$ and $I_{AO,i}=|E_{AO,i}|^{2}$. Since the
phase $\phi_{i}$ is uniformly distributed over $2\pi$, one obtains that
$\mathbb{E}[P^{N}]=N\mathbb{E}[I_{bkg,i}]$ and
$Var[P^{N}]=N\mathbb{E}[I_{bkg,i}I_{AO,i}]$. Accordingly, the signal in AOI is
often calculated as the standard deviation of $P^{N}$ at the frequency
$f_{US}$ and is proportional to $\sqrt{NI_{bkg}I_{AO}}$, where
$I_{bkg}=\mathbb{E}[I_{bkg,i}]$ and $I_{AO}=\mathbb{E}[I_{AO,i}]$.

In the proposed homodyne scheme for TD-AOI, the field in Eq. 1 is interfered
with a reference beam, leading to the following expression for a single
speckle grain:

$E^{int}_{i}={E_{ref}e^{j\omega t}+E_{bkg,i}e^{j\omega t}+E_{AO,i}e^{j\omega
t}e^{j(2\pi f_{US}t+\phi_{i})}}.$ (3)

Assuming $I_{ref}\gg I_{bkg,i}$, the following expression for the power of N
grains is obtained by neglecting $I_{bkg}$:

$P^{N}_{hom}\propto
NI_{ref}+2I_{ref}\sum_{i=1}^{N}\sqrt{I_{AO,i}}cos\left(2\pi
f_{US}t+\phi_{i}\right).$ (4)

Accordingly, the magnitude of the AOI signal in Eq. 4, described by the
standard deviation of the expression, is given by $\sqrt{NI_{ref}I_{AO}}$, and
the average power is given by $NI_{ref}$.

As can be seen from the above analysis, the use of homodyne detection enables
one to amplify the AOI signal since $I_{ref}\gg I_{bkg}$, where the goal is to
achieve sufficient amplification such that shot noise becomes the dominant
noise factor even when low-gain photodetectors are used. In the shot-noise-
limited case, the noise is proportional to the square root of the average
power, leading to an SNR that is proportional only to $\sqrt{I_{AO}}$ and is
independent of all the other parameters in Eq. 2 and 4. Thus, when examining
only inherent noise in the optical signal, homodyne TD-AOI attains the same
SNR as conventional TD-AOI. Accordingly, the advantage of the homodyne
approach is that it enables the use of low-gain photodetectors, with
potentially lower cost and noise factors, without the cost of increasing the
inherent SNR of the optical signal before detection.

Assuming shot-noise-limited detection, the SNR at the output of the
photodetector is given by [21]:

$SNR_{det.}=\sqrt{\frac{\eta P}{2\hbar\omega F\Delta f}},$ (5)

where $\eta$ is the quantum efficiency, $P$ is the optical power, $F$ is the
noise factor, $\hbar$ is Planck’s constant, and $\Delta f$ is the measurement
bandwidth. The noise factor $F$ is a result of gain fluctuations that occur
when the gain is achieved via an avalanche process, and is typically smaller
than 1.5 [21]. In the case of a PD, whose gain is provided by a trans-
impedance amplifier, the gain may be considered constant, i.e. $F=1$.
Accordingly, when shot-noise-limited detection is assumed, the gain in SNR
achieved by a PD over a PMT may be expressed by

$G_{SNR}=\sqrt{\frac{\eta_{PD}F_{PMT}}{\eta_{PMT}}}.$ (6)

Using the values $\eta_{PD}=76\%$, $\eta_{PMT}=5\%$, and $F_{PMT}=1.4$, given
for the components used, a theoretical SNR gain of $G_{SNR}=4.6$ is obtained.

Fig. 1 shows the experimental setup used in this work to test the performance
of homodyne TD-AOI. The system’s optics is schematically divided into 2 sub-
system: (1) splitting module and (2) merging module. In the splitting module,
a linearly polarized CW laser (DL Pro 780, Toptica) with a linewidth of $50$
KHz and wavelength of $780$ nm is split into 2 branches by two half-wave
plates ($\lambda/2$) and a polarization beam splitter (PBS). The first
$\lambda/2$ plate angle is set such that most of the optical power is
reflected by the PBS into the phantom via a multi-mode fiber with a $62.5\mu
m$ core diameter, 2 m length that delivered $200$ mW to the phantom boundary.
The light transmitted through the PBS is used for the reference beam. To
optimize the reference beam power, it passes through a second motorized
$\lambda/2$ plate and a polarizer. Eventually, the reference beam is coupled
to a polarization-maintaining (PM) fiber with a length of 4 m.

The reemitted light from the phantom, is collected by a second MM fiber with
$600\mu m$ core diameter, 0.39 NA, and 2 m length and delivered to the merging
module. The distance between the two fibers on the phantom boundary was $5$ mm
on $X$ axis. Since the reemitted light is unpolarized, a linear polarizer is
used to achieve a polarization that matches that of the reference beam. Both
beams are merged into a single interfered beam by a 50:50 BS and then coupled
into a $200\mu m$ core diameter fiber. The merged light is delivered to a
silicon PD (FDS-02, Thorlabs) with a quantum efficiency of $76\%$. Both the PD
and the PMT were connected to identical custom-made electronics, including a
trans-impedance amplifier (Texas Instruments LMH-6626), a $2.5$ MHz passive
low pass filter, and a voltage buffer. A 14-bit digitizer (ATS-9416,
AlazarTech) was used to sample the voltage signal at a frequency of 20 $MHz$.

The technique is tested on a tissue-mimicking phantom made of silicone mixed
with $193nm$ $TiO_{2}$ particlesforming a reduced scattering coefficient
equals to $\mu_{s}^{\prime}=15cm^{-1}$ and speed of sound of $990$ m/s [19].
The ultrasound modulation is generated by a focused piezoelectric transducer
(Panametrics, A392S) with diameter of $38.1$ mm, focal length of $9.4cm$, a
Rayleigh length of $3.58$ cm, and a cross-section FWHM of $4.25$ mm. The
transducer is driven by an arbitrary function generator (Tabor, 8026),
amplified to a 48 V peak-to-peak amplitude (.4-1.8-50EU26, SVPA), leading to a
peak pressure of approximately $250$ kPa in the acoustic focus. The transducer
may be used to deliver a single acoustic pulse into the phantom, which
modulates different depths at different times, enabling depth-resolved mapping
of the AOI signal without scanning by using the time-of-flight principle [22].
In our implementation, we used a coded sequence of pulses, rather than a
single pulse, to maximize the acoustic modulation depth [19]. Our sequence was
based on a code with 251 elements, where each pulse had a single period with a
frequency of $f_{US}=1.25$ MHz, corresponding to an axial resolution of $1.1$
mm, a total imaging depth of $27.6$ cm and sqeuence repetition rate of $5$
kHz. In each measurement the signal was measured continuously and averaged
over $10^{4}$ repetitions (a total duration of $2.008$ s).

Figure 1: AOI system setup. US signal is generated by a function generator,
amplified and then fed into a piezoelectric transducer that project it into
the phantom. The CW laser beam (780 mn) is split to both illuminating the
tissue, and providing the reference beam for later interference. The reemitted
light collected from the tissue boundary with a MM fiber into the merging
optical setup, where it is being polarized and merged with the polarized
reference beam using a 50:50 beamsplitter. The interfered light coupled into a
MM fiber delivering it onto the PD. The photocurrent from the PD is converted
to voltage and filtered before being digitized.

To obtain optimal reference power, a preliminary calibration experiment was
performed. The transducer was positioned in a fixed coordinates on the $X-Y$
plane and $100$ nW of the reemitted light was coupled into the sensor MM fiber
(Fig. 1). In the first part of the experiment, the reference beam was blocked
and the reemitted light was directed towards a PMT (R5900U-20-L16, Hamamatsu)
with a gain of $2.7\times 10^{5}$. The PMT signal was used to calculate the
baseline SNR of the measurement for conventional TD-AOI. The measurement was
then repeated with the reference beam unblocked, i.e. in a homodyne
configuration, for different reference powers. For each reference beam
intensity, the SNR values were averaged over 30 measurements to minimize the
variations between measurements.

Fig. 2 presents the SNR gain of homodyne TD-AOI as a funciton of
$\gamma=P_{ref}/P_{re}$, where $P_{ref}$ and $P_{re}$ are the intensities of
the reference beam and the reemitted light respectively. The figure shows that
for low values of $\gamma$, the SNR gain is lower than 1, i.e. the homodyne
system achieves a lower SNR than conventional TD-AOI. The reason for this
result is that at low levels of $\gamma$, the detection with PDs is not shot-
noise limited, but rather dominated by additive noise from the detector.
Accordingly, the SNR gain of homodyne-AOI increases approximately linearly
with $\gamma$ until a maximum value of 4.17, achieved for $\gamma=1.5\times
10^{4}$, corresponding to reference power of 1.4 mW. For higher reference
powers, the SNR decreased due to saturation of the PD.

Figure 2: Reference beam intensity calibration. The curve shows the SNR gain
obtained with PD vs. the reference beam intensity normalized by the intensity
of the reemitted light from the phantom boundary ($\gamma=P_{ref}/P_{re}$).
Each point was averaged over 30 repetitive measurements. The curve introduces
an exponential rise that ends with a peak SNR gain of 4.16. The slow decrease
in SNR caused by the saturation of the PD.

(a)

(b)

Figure 3:  Fluence rate profiles measured with conventional TD-AOI using a PMT
(blue) and with our homodyne approach using a photodiode (red) along different
axes, shown in Fig. 1: (a) $z$ axis and (b) $y$ axis. The fluence rate
reconstruction from the photodiode-based measurement achieved the same spatial
profile for the light fluence rate, but with a noise level that was typically
10 dB lower than that of the PMT-based measurement.

Once the reference beam intensity was set to maximal SNR gain, the 1D profile
of the fluence rate inside the phantom was mapped in two directions. In the
$z$ dimension (Fig. 3a) the mapping was performed using the time-of-flight
principle, whereas in the $y$ dimension (Fig. 3b) the transducer was
mechanically scanned with a step size of 0.5 mm. The measurement was conducted
for both the homodyne and conventional TD-AOI setups, and the results are
presented in Fig. 3a and 3b for the $z$ and $y$ scans, respectively. In both
cases, the same spatial profiles were obtained for the AOI signals both the
technique. The asymmetry depicted in the fluence rate shown in Fig 3a, which
is consistent for both the PMT and the PD measurements, caused by geometric
imperfections of tilted fibers with relation to the US axis. In terms of
sensitivity, the noise level in the PMT-based measurement was higher by
approximately 10 dB than the one performed with a PD, leading to a difference
in penetration depths of over 1 cm in Fig. 3b.

The results of homodyne and conventional TD-AOI were also compared in the
frequency domain. Fig. 4 shows the normalized power spectra of the AOI signals
for the spatial positions $y=0$ mm and $z=45$ mm in which the strongest
signals were obtained (Fig. 3). The figure shows that both techniques led the
same spectral behavior, with a strong response at the US frequency
$f_{us}=1.25$ MHz, and weaker responses obtained at higher harmonics.

Figure 4: Normalized power spectrum measured in the spatial position in which
the AOI was maximal using conventional TD-AOI with a PMT (blue) and our
homodyne approach with a photodiode (red). A consistent decrease in noise
level measured in both cases in favour of photodiode. The 2nd and 3rd
harmonics of the US signal are visible for both techniques.

In conclusion, we have introduced a homodyne detection method for TD-AOI, in
which the light reemitted from the tissue is interferred with a reference
beam. The interference leads to an optical amplification of the AOI signal
that enables its detection by low-gain photodetectors. In this work, a silicon
PD was used in the homodyne scheme, achieving over a 4-fold enhancement in SNR
in comparison to conventional TD-AOI performed with a PMT, explained by the
higher quantum yield of the PD and its lower noise factor. The improved
sensitivity of homodyne TD-AOI enabled deeper penetration without distorting
the spatial or spectral behavior of the signals.

In addition to the SNR enhancement demonstrated in this work, our homodyne TD-
AOI has the advantage of being compatible with PDs, which are considerably
more compact and affordable than PMTs. Accordingly, homodyne TD-AOI is more
compatible with parallelized detection in which an array of photodetectors
simultaneously measure the AOI signal in uncorrelated speckle patterns,
facilitating a further improvement in SNR.

Funding This work has received funding from the Ministry of Science,
Technology and Space (3-12970) and from the Ollendorff Minerva Center.
Acknowledgments The authors would like to acknowledge Mr. Moshe Namer for his
significant contribution to this work. Disclosures The authors declare no
conflicts of interest. Data availability The data files generated during
and/or analyzed during the current study are available from the corresponding
author on a reasonable request.

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