This paper presents a novel THz optical design that allows the acquisition of THz reflectivity maps of in vivo cornea
without the need for a field flattening window and preliminary imaging results of in vivo rabbit cornea. The
system’s intended use is to sense small changes in corneal tissue water content (CTWC) that can be precursors for a
host of diseases and pathologies. Unique beam optics allows the scanning of a curved surface at normal incidence
while keeping the source detector and target stationary. Basic system design principles are discussed and image sets
of spherical calibration targets and corneal phantom models are presented. The presented design will enable, for the
first time, non-contact THz imaging of animal and human cornea.
KEYWORDS: Terahertz radiation, Tissues, Magnetic resonance imaging, Reflectivity, Skin, In vivo imaging, Imaging systems, Abdomen, Medical imaging, Water
Terahertz (THz) detection has been proposed and applied to a variety of medical imaging applications in view of its
unrivaled hydration profiling capabilities. Variations in tissue dielectric function have been demonstrated at THz
frequencies to generate high contrast imagery of tissue, however, the source of image contrast remains to be verified
using a modality with a comparable sensing scheme. To investigate the primary contrast mechanism, a pilot
comparison study was performed in a burn wound rat model, widely known to create detectable gradients in tissue
hydration through both injured and surrounding tissue. Parallel T2 weighted multi slice multi echo (T2w MSME)
7T Magnetic Resonance (MR) scans and THz surface reflectance maps were acquired of a full thickness skin burn in
a rat model over a 5 hour time period. A comparison of uninjured and injured regions in the full thickness burn
demonstrates a 3-fold increase in average T2 relaxation times and a 15% increase in average THz reflectivity,
respectively. These results support the sensitivity and specificity of MRI for measuring in vivo burn tissue water
content and the use of this modality to verify and understand the hydration sensing capabilities of THz imaging for
acute assessments of the onset and evolution of diseases that affect the skin. A starting point for more sophisticated
in vivo studies, this preliminary analysis may be used in the future to explore how and to what extent the release of
unbound water affects imaging contrast in THz burn sensing.
Terahertz (THz) imaging is a relatively new non-destructive analytical technique that is transitioning from established application research areas such as defense and biomedicine to studies of cultural heritage artifacts. Our research adopts a THz medical imaging system, originally designed for in vivo tissue hydration sensing, to acquire high contrast imagery of painted plaster samples in order to assess the ability of the system to image the Byzantine wall paintings at the Enkleistra of St. Neophytos in Paphos, Cyprus. The original 12th century paintings show evidence of later painting phases overlapping earlier iconography. A thin layer of lead white (2PbCO3·Pb(OH)2) underlies, in parts, later wall paintings, concealing the original painting scheme beneath. Traditional imaging modalities have been unable to image the underlying iconography due to a combination of absorption and scattering. We aim to use THz imaging and novel optical design to probe beyond the visible surface and perform in situ analysis of iconography beneath the lead white layer. Imaging results of painted plaster mock-ups covered with a thin layer of lead white and/or chalk, as well as of a painted wooden panel with obscured writing, are presented, and from these images sufficient contrast for feature identification is demonstrated. Preliminary results from the analysis of these mock-ups confirmed the utility of this technique and its potential to image concealed original paintings in the Enkleistra of St. Neophytos. The results encourage analysis of THz scattering within paint and plaster materials to further improve spatial resolution and penetration depth in THz imaging systems.
THz imaging system design will play an important role making possible imaging of targets with arbitrary properties
and geometries. This study discusses design consideration and imaging performance optimization techniques in THz
quasioptical imaging system optics. Analysis of field and polarization distortion by off-axis parabolic (OAP) mirrors
in THz imaging optics shows how distortions are carried in a series of mirrors while guiding the THz beam. While
distortions of the beam profile by individual mirrors are not significant, these effects are compounded by a series of
mirrors in antisymmetric orientation. It is shown that symmetric orientation of the OAP mirror effectively cancels this
distortion to recover the original beam profile. Additionally, symmetric orientation can correct for some geometrical
off-focusing due to misalignment. We also demonstrate an alternative method to test for overall system optics
alignment by investigating the imaging performance of the tilted target plane. Asymmetric signal profile as a function
of the target plane’s tilt angle indicates when one or more imaging components are misaligned, giving a preferred tilt
direction. Such analysis can offer additional insight into often elusive source device misalignment at an integrated
system. Imaging plane tilting characteristics are representative of a 3-D modulation transfer function of the imaging
system. A symmetric tilted plane is preferred to optimize imaging performance.
This paper presents novel a first pass on the thorough analysis of THz optical designs intended for image acquisition of
burn wounds in animal models. Current THz medical imaging research typically employs and fixed source detector
architecture coupled by a train of off-axis parabolic mirrors. When used individually, parabolic mirrors have near
diffraction limited focusing properties, extremely low loss, and are dispersion free. However, when a combination or train
of multiple parabolic mirrors are utilized geometric errors can be generated early in the train and exacerbated as the beam
propagates to the detector. These errors manifest as significant increases in spot size, asymmetries about the optical axis
in beam irradiance and polarization, and the generation of cross polarization components. This work presents a novel
configuration of off-axis parabolic mirrors designed to maximize the practicality of beam alignment and image acquisition.
Quasi-physical optics simulations of the optical performance are described and significant perturbations in polarization
symmetry were observed. The configuration can be described as in between two canonical parabolic mirror configurations.
The performance of three different pairs of off-axis parabolic mirror pairs coupled to the novel configuration are presented
herein.
In the absence of an imaging technique that offers a highly dynamic range detection of malignant tissue intra-operatively, surgeons are often forced to excise excess healthy tissue to ensure clear margins of resection. Techniques that are currently used in the detection of tumor regions include palpation, optical coherence tomography (OCT) elastography, dye injections, and conventional ultrasound to pinpoint the affected area. However, these methods suffer from limitations such as minimal specificity, low contrast, and limited depth of penetration. Lack of specificity and low contrast result in the production of vague disease margins and fail to provide a reliable guidance tool for surgeons. The proposed work presents an alternative diagnostic technique, ultrasound-stimulated vibro-acoustography (USVA), which may potentially provide surgeons with detailed intra-operative imagery characterized by enhanced structural boundaries and well-defined borders based on the viscoelastic properties of tissues. We demonstrate selective imaging using ex vivo tissue samples of head and neck squamous cell carcinoma (HNSCC) with the presence of both malignant and normal areas. Spatially resolved maps of varying acoustic properties were generated and show good contrast between the areas of interest. While the results are promising, determining the precision and sensitivity of the USVA imaging system in identifying boundary regions as well as intensities of ex vivo tissue targets may provide additional information to non-invasively assess confined regions of diseased tissues from healthy areas.
Terahertz (THz) hydration sensing continues to gain traction in the medical imaging community due to its unparalleled
sensitivity to tissue water content. Rapid and accurate detection of fluid shifts following induction of thermal skin burns
as well as remote corneal hydration sensing have been previously demonstrated in vivo using reflective, pulsed THz
imaging. The hydration contrast sensing capabilities of this technology were recently confirmed in a parallel 7 Tesla
Magnetic Resonance (MR) imaging study, in which burn areas are associated with increases in local mobile water
content. Successful clinical translation of THz sensing, however, still requires quantitative assessments of system
performance measurements, specifically hydration concentration sensitivity, with tissue substitutes. This research aims
to calibrate the sensitivity of a novel, reflective THz system to tissue water content through the use of hydration
phantoms for quantitative comparisons of THz hydration imagery.Gelatin phantoms were identified as an appropriate
tissue-mimicking model for reflective THz applications, and gel composition, comprising mixtures of water and protein,
was varied between 83% to 95% hydration, a physiologically relevant range. A comparison of four series of gelatin
phantom studies demonstrated a positive linear relationship between THz reflectivity and water concentration, with
statistically significant hydration sensitivities (p < .01) ranging between 0.0209 - 0.038% (reflectivity: %hydration). The
THz-phantom interaction is simulated with a three-layer model using the Transfer Matrix Method with agreement in
hydration trends. Having demonstrated the ability to accurately and noninvasively measure water content in tissue
equivalent targets with high sensitivity, reflective THz imaging is explored as a potential tool for early detection and
intervention of corneal pathologies.
Research in THz imaging is generally focused on three primary application areas: medical, security, and nondestructive
evaluation (NDE). While work in THz security imaging and personnel screening is populated by a number of different
active and passive system architectures, research in medical imaging in is generally performed with THz time-domain
systems. These systems typically employ photoconductive or electro-optic source/detector pairs and can acquire depth
resolved data or spectrally resolved pixels by synchronously sampling the electric field of the transmitted/reflected
waveform. While time-domain is a very powerful scientific technique, results reported in the literature suggest that
desired THz contrast in medical imaging may not require the volume of data accessible from time-resolved
measurements and that a simpler direct detection, active technique may be sufficient for specific applications. In this
talk we discuss an active direct detection reflectometer system architecture operating at a center frequency of ~ 525 GHz
that uses a photoconductive source and schottky diode detector. This design takes advantage or radar-like pulse
rectification and novel reflective optical design to achieve high target imaging contrast with significant potential for high
speed acquisition time. Results in spatially resolved hydration mapping of burn wounds are presented and future
outlooks discussed.
KEYWORDS: Terahertz radiation, Magnetic resonance imaging, Tissues, Skin, In vivo imaging, Reflectivity, Visualization, Injuries, Medical imaging, Natural surfaces
Terahertz (THz) imaging is an expanding area of research in the field of medical imaging due to its high sensitivity to
changes in tissue water content. Previously reported in vivo rat studies demonstrate that spatially resolved hydration
mapping with THz illumination can be used to rapidly and accurately detect fluid shifts following induction of burns
and provide highly resolved spatial and temporal characterization of edematous tissue. THz imagery of partial and
full thickness burn wounds acquired by our group correlate well with burn severity and suggest that hydration
gradients are responsible for the observed contrast. This research aims to confirm the dominant contrast mechanism
of THz burn imaging using a clinically accepted diagnostic method that relies on tissue water content for contrast
generation to support the translation of this technology to clinical application. The hydration contrast sensing
capabilities of magnetic resonance imaging (MRI), specifically T2 relaxation times and proton density values N(H),
are well established and provide measures of mobile water content, lending MRI as a suitable method to validate
hydration states of skin burns. This paper presents correlational studies performed with MR imaging of ex vivo
porcine skin that confirm tissue hydration as the principal sensing mechanism in THz burn imaging. Insights from
this preliminary research will be used to lay the groundwork for future, parallel MRI and THz imaging of in vivo rat
models to further substantiate the clinical efficacy of reflective THz imaging in burn wound care.
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