|
Polarimetry is a well-established tool for
non-invasive material characterization and involves comparison
of the polarization states of light before and after the light
interacts with the material. The use of polarized light for
characterization and imaging of highly scattering media, such as
biological tissue, has been studied. The effect of scattering on
the polarization state of light has been found to be useful for
imaging of surface or subsurface structures in scattering media,
and for transmission imaging of deep structures19. It has also
been shown that the scattering parameters of turbid tissue,
including the scattering coefficient, can be determined from
diffusely scattered polarized light20.
As such, polarized difference imaging (PDI) is a proven
technology, well understood for its potential benefits across
many industries. It is a process designed to mimic the
functional characteristics underlying polarization vision in
nature. A first evaluation of the values of sensitivity and
specificity of polarimetric imaging in the degree of
polarization (DOP) mode for cervical cancer detection showed
that polarimetry gives significantly better specificity results
when compared to colposcopy. As far as sensitivity is concerned,
if polarimetry is not revealed better than colposcopy, at least,
it stays at the same levels. Colposcopy shows a positive
predictive value (PPV) of 84% and a negative predictive value (NPV)
of 19-29% versus polarimetry's PPV of 97% and NPV of 33-74% .
However, PDI has not been adopted as the dominant imaging
protocol under scattering conditions because 1) it has required
the capture of two images at discrete points in time, therefore
any changes in the environment are highlighted in the
computation of the difference image, creating noise, and 2)
linear polarization filters limit the amount and wavelength of
transmitted light.
Our lead product consists of a real-time polarized light (rPDI)
camera that could be used in a number of applications, from
guiding surgical excision of skin cancers to detecting cancers
(e.g. oral cancer) at very early stages. One important
biomedical application is non-invasive or minimally invasive
detection of precancerous and early cancerous changes in human
epithelium, such as dysplasia or carcinoma in situ. As such,
rPDI might also have clinical utility in in vivo elastic light
scattering measurements to diagnose high grade squamous
interepithelial lesions (HSIL) of the cervix, which are a
cervical cancer precursor14. rPDI has also potential clinical
utility in cardiovascular, dentistry, and ophthalmology
applications (e.g. microcirculation (Sidestream dark field (SDF) imaging
,Sidestream
Dark Field(SDF),Side stream dark field
imaging (SDF),Sidestream dark field imaging (SDF))assessment, early detection
of cavities, and early diagnosis of melanoma of the retina,
respectively).
The technology of rPDI relies on a potentially revolutionary
method of detecting two perpendicular polarization vectors of a
light ray simultaneously. For the first time, polarized light
can be separated from scattered light in real time. The camera
uses linearly polarized light and acquires parallel-and
perpendicular-polarized images, then the difference image
subtracts the background diffusely scattered light and yields an
image that unveils the fabric of the skin. Cancer disrupts the
complex pattern of this fabric, revealing the cancer margin.
1. Real-time Polarization Difference Imaging (rPDI) in Surgery
and Cancer Detection
More than 85% of cancers originate in the epithelium, and
epithelial cancers are preceded by a curable precancerous stage.
As such, early detection is paramount for the successful
treatment of this disease. If detected at precancerous stage,
95% of the cases have a complete recovery1. However, many forms
of precancerous changes are difficult to detect using
conventional techniques which require histological examination
of biopsies obtained from visible lesions or random surveillance
biopsies.
Biological tissues are optically inhomogeneous, birefringent,
and absorbing media2. Precancerous lesions are characterized by
increased nuclear size and nuclear and/or cytoplasmic ratio; and
the scattering from the epithelial layer of tissue can provide
information on nuclear morphology3.
Polarized light imaging has been shown4,5 to give relevant
information on the borders of skin cancers that are not visible
to the naked eye. Skin cancers typically originate in the
superficial regions of the skin (epidermal basement membrane),
where polarized light imaging is most effective. Quick
assessment of skin cancer margins before surgery could guide the
doctor during excision and reduce the surgery time and patient
discomfort. A number of polarized light camera systems have been
used in the clinic6-9, but routine use has been limited by such
factors as size, weight, cost, poor user interface, and far from
optimal quality of image. Commercially available systems10 are
useful for eliminating glare and shadows from the field of view
but do not provide information on the backscattered degree of
polarization and superficial light scattering. More complex
systems based on confocal microscopy(Sidestream dark field (SDF) imaging
,Sidestream
Dark Field(SDF),Side stream dark field
imaging (SDF),Sidestream dark field imaging (SDF))11-13 enable high
resolution, imaging the dermis to 500 mm but with much higher
equipment cost and limited portability.
The basis for the optical techniques is to detect biochemical
and morphological features that are concurrent with precancerous
conditions. Examples of optical spectroscopy methods are elastic
light scattering, fluorescence, optical coherence tomography and
Raman spectroscopy. Fluorescence and Raman spectroscopy are
primarily sensitive to biochemical changes, while light
scattering and optical coherence tomography are primarily
sensitive to morphological changes. And while there has been
extensive work done on polarization imaging2, 5, 6, 15, none was
real time. Therefore, even subtle changes at cellular levels
were only translated as noise and might have interfered with
proper diagnoses.
Recently developed and developing technologies have greatly
improved tumour detection and surgical planning, but none of the
currently available tools provide a real-time intraoperative
assistance that is highly sensitive and specific, time efficient
and cost effective.
2. rPDI Significance in Microcirculation
On the other hand, orthogonal polarization difference imaging
would enable the direct visualization of the microcirculation (Sidestream dark field (SDF) imaging
,Sidestream
Dark Field(SDF),Side stream dark field
imaging (SDF),Sidestream dark field imaging (SDF))of
man without the use of imaging enhancing dyes. Microcirculatory
function is the main prerequisite for adequate tissue
oxygenation and thus organ function. Its purpose is to transport
oxygen and nutrients to tissue cells, ensure adequate
immunological function and, in disease, to deliver therapeutic
drugs to target cells.
to the tissues takes place, and consists of arterioles,
capillaries, and venules. The main cell types comprising the
microcirculation(Sidestream dark field (SDF) imaging
,Sidestream
Dark Field(SDF),Side stream dark field
imaging (SDF),Sidestream dark field imaging (SDF)) are the endothelial cells lining the inside of
the microvessels, smooth muscle cells (mostly in arterioles),
red blood cells, leukocytes, and plasma components in blood. The
structure and function of the microcirculation (Sidestream dark field (SDF) imaging
,Sidestream
Dark Field(SDF),Side stream dark field
imaging (SDF),Sidestream dark field imaging (SDF))is highly
heterogeneous in different organ systems. In general, driving
pressure, arteriolar tone, hemorheology, and capillary patency
are the main determinants of capillary blood flow16.
The importance of abnormalities observed in the microcirculation(Sidestream dark field (SDF) imaging
,Sidestream
Dark Field(SDF),Side stream dark field
imaging (SDF),Sidestream dark field imaging (SDF))
of hypertensive subjects is being increasingly recognised. These
microvascular changes may be central to the development of
end-organ damage brought about by hypertension, including
ischaemic heart disease. The primary function of the
microcirculation is to supply oxygen and nutrients to myocardial
tissue, and it also has an important role in regulating coronary
blood flow. Some 70-90% of the overall peripheral resistance of
the circulatory system arises at the level of the
microcirculation17 .
A number of technological advances during the last years have
enhanced the image quality of the microcirculation. In summary:
The nailfold videocapillaroscopy, a non-invasive examination
that includes a microscope with an epiillumination system, came
afterward, but its major disadvantage is the restricted area
available for investigation namely the nailfold capillary bed.
The orthogonal polarization spectral (OPS) imaging technique,
where subsurface scattered light allows the visualization of the
microcirculation(Sidestream dark field (SDF) imaging
,Sidestream
Dark Field(SDF),Side stream dark field
imaging (SDF),Sidestream dark field imaging (SDF)), was the next non-invasive exam, but it still
presents some drawbacks such as suboptimal capillary
visualization and image blurring due to red blood cell
movements. Excessive probe pressure modifies red blood cell
velocity. There is suboptimal imaging of capillaries due to
motion-induced
image blurring by movements of OPS device, tissue and/or flowing
red blood cells. Sidestream dark field (SDF) imaging (Sidestream
Dark Field(SDF),Side stream dark field
imaging (SDF),Sidestream dark field imaging (SDF))is the newest tool for
microcirculatory research. Illumination is provided by
concentrically placed light-emitting diodes to avoid image
blurring and to enhance image contrast. It represents a simple
and non-invasive imaging technique, with low cost, good
portability and high sensitivity that provides fine, well¬defined images. Microcirculation can also be studied
through laser Doppler flowmetry (LDF) or reflectance-mode
confocal-laser-scanning microscopy (RCLM). However, LDF cannot
show microcirculatory vessels and high cost of RCLM can be an
inconvenience.
We are proposing an improved version of polarization difference
imaging technology: real-time polarization difference imaging (rPDI).
rPDI consists of a system where optical components are designed
to be matched for optimal imaging performance (e.g., specific
wavelength, refractive index, field of view), and includes a
customized polarization beam splitter geometry and coatings
specifically optimized for imaging applications. As mentioned
above, rPDI splits the incoming beam of light into 2 orthogonal
planes at the same instant, based on polarization content only.
Because of this, and the fact that all of the available light at
the chosen wavelength reaches the polarization beam splitter,
there is a cleaner separation between the 2 polarization planes
-resulting in a more accurate image under lower light
conditions. Also, the resulting image Μ̯̽ΙνApplications of rPDI
technique could include skin microcirculatory evaluation and
allow dermatological diagnoses of skin pathologies.
Below is a table comparing different imaging technologies,
including rPDI.
Nailfold videocapillaroscopy (NVC), orthogonal polarization
spectral (OPS) imaging, sidestream dark field imaging (SDF)(Sidestream
Dark Field(SDF),Side stream dark field
imaging (SDF),Sidestream dark field imaging (SDF)),
reflectance-mode confocal-laser-scanning microscopy (RCLM),
laser Doppler flowmetry (LDF), and real-time polarization
imaging are compared concerning contrast image reconstruction,
portability, user-friendly capability and cost.
3. rPDI Application in Dentistry
Dental caries, or tooth decay, is a pathological process of
destruction of tooth structure by oral microorganisms, which can
lead to tooth loss if untreated. In coronal caries, lesions
begin in the enamel and cause demineralization of the enamel.
This demineralization changes the scattering properties of the
enamel, resulting in chalky or "white spot" lesions visible when
the caries occurs on smooth, unstained enamel surfaces. If the
carious lesion is detected before it reaches the dentin,
remineralization is still possible. After the carious lesion has
reached dentin, however, inflammation of the pulp occurs,
requiring a filling and leading to serious tooth decay and
eventual tooth loss if untreated. Restorative dentistry is most
effective when the progression of caries is detected early
before it reaches the dentin.
Current techniques for diagnosing caries are visual inspection,
mechanical probing(Sidestream dark field (SDF) imaging
,Sidestream
Dark Field(SDF),Side stream dark field
imaging (SDF),Sidestream dark field imaging (SDF)) with a sharp dental explorer, and
radiographic imaging. The tooth can be tactilely and visually
explored to determine the presence of indicators of tooth decay
such as surface irregularities, crevices, or discoloration.
However, the practice of probing all accessible tooth surfaces
with a sharp explorer is coming under increased scrutiny since
it can further damage enamel already weakened by decay and may
also cause cross-contamination between teeth. As tooth decay
primarily affects the region of calcium below the tooth surface,
detection of caries before significant damage occurs in the
tooth is very difficult.
By the time caries is evident under visual and tactile
examination of the tooth, the disease is usually in an advanced
stage, requiring a filling and occasionally leading to tooth
loss. As a consequence of conservative diagnoses and treatment,
there are false positives leading to unnecessary drilling and
placement of restorations in healthy teeth. Currently there is
no accurate device for determining whether restorations are in
need of replacement, resulting in enormous costs from the
unnecessary replacement of good restorations and complications
such as root canals from not replacing defective or aged
restorations.
Radiography is often used for detection of cavities, since it
provides integrated views of tooth structure that in certain
orientations can isolate carious lesions. The sensitivity of
radiographic systems, however, is limited by visible changes in
film density, making identification of small carious or
precarious regions difficult. Since radiographs are two
dimensional, precisely locating the position of such decay is
impossible. Moreover, due to the orientation of the x-ray
imaging, only interproximal lesions (between the teeth) are
easily detected, while early occlusal lesions (top of the
tooth), are difficult to detect. In addition, radiography uses
harmful ionizing radiation.
Given the disadvantages of current detection techniques, a need
exists for a device that provides safe, early diagnosis of
caries. rPDI applies the technique of polarimetry to image
dental hard tissue and detect the presence of caries based on
the depolarization of incident light. rPDI also has the
potential for detection of oral cancer in mouth, and the
monitoring of gum healing after dental surgery.
4. rPDI Application in Ophthalmology
Age-related macular degeneration (AMD) is the leading cause of
blindness in the adult population of the United States 21. In
AMD, the diseased tissues lie beneath the highly reflective
vitreoretinal interface, plus many layers of other retinal
cells, making it difficult to visualize the early changes in
this disease. As a result, the reported incidence of subretinal
changes(Sidestream dark field (SDF) imaging
,Sidestream
Dark Field(SDF),Side stream dark field
imaging (SDF),Sidestream dark field imaging (SDF)) both in patients with AMD and in aged normal subjects is
much higher in histologic studies than in clinical and
epidemiologic studies. Histology has been used to show that the
early stages of AMD and the development of drusen too small to
be detected clinically26–30. Widespread or diffuse subretinal
changes have been conclusively shown to occur long before donors
have reached the age associated with clinical disease26,29.
These findings, together with the extremely gradual progression
of early stages, suggest that age-related maculopathy (ARM) and
AMD may begin during middle age, if not sooner. Accurate
assessment and improved quantification of early stages of the
disease is of growing importance for studies of affected family
members and evaluating the effectiveness of proposed earlier
treatments, such as vitamin supplementation31. because of the
relatively low contrast of small drusen and other subretinal
deposits, especially in the presence of age-related decreases in
lens transparency and pupil size. In recent years, infrared
imaging has been used to improve the visibility of subretinal (Sidestream dark field (SDF) imaging
,Sidestream
Dark Field(SDF),Side stream dark field
imaging (SDF),Sidestream dark field imaging (SDF))changes such as drusen and hyperpigmentation in both middle-aged
and elderly subjects without clinical signs of disease32,33 .
There is excellent spatial correspondence of drusen appearing on
infrared images with the drusen that appeared in images produced
by other methods34. These advanced digital imaging techniques
have been used successfully, with large numbers of patients in a
clinical setting35, 36 . Thus, although infrared confocal
imaging has been shown to be superior to color fundus imaging
(Miura M, et al. IOVS 2001;42:ARVO Abstract 3800)37, the
standard method for most clinical studies remains color fundus
photography because of the lack of reliable commercial
instrumentation.
Detection of disease by imaging is a problem of determining the
most relevant properties of the tissue of interest and then
optimizing the technique. A simple model of the optical
properties of the healthy human retina and RPE consists of a
stack of layers containing well-ordered and contiguous cells and
their processes38, but this model fails to describe a retina
with ARM membrane thickens24, 27, 37-41. It becomes less
ordered, and as a result there is an overall increase in light
scattering from this layer32. On a finer scale there are
features such as drusen, hyperpigmentation, pigment clumping,
and eventually atrophy or exudation. These changes occur beneath
the brighter neural layers of the retina, which return a high
proportion of light, which has been either specularly(Sidestream dark field (SDF) imaging
,Sidestream
Dark Field(SDF),Side stream dark field
imaging (SDF),Sidestream dark field imaging (SDF)) reflected
or singly scattered. Thus, in the early stages of the disease,
the total amount of scattered light returned to the detector is
relatively low, and standard imaging modalities do not provide a
high-contrast image of early changes, even with near infrared
light and confocal imaging. Sampling multiply
scattered light and eliminating the directly backscattered light
causes the deeper layers and pathologic tissues to be viewed in
increased contrast32–34,36,42–43.
To form an image using multiply scattered light, the primary
method has been to minimize the directly backscattered light in
a confocal scanning laser ophthalmoscope (SLO)44 by the use of
an annular aperture and infrared illumination32,45. Other
techniques have also been developed to form an image from
multiply scattered light—for instance, by displacing the light
source out of alignment with the confocal aperture, as in
multiply scattered light tomography36. Studies have tested the
use of polarimetry for forming a retinal image from
predominantly multiply scattered light and compared the
resultant contrast to images formed from directly backscattered
light and have found that the depolarized light image produced a
3.4 times higher contrast of drusen and subretinal changes(Sidestream dark field (SDF) imaging
,Sidestream
Dark Field(SDF),Side stream dark field
imaging (SDF),Sidestream dark field imaging (SDF)) than
the parallel polarized light images48. Light that is singly
scattered off superficial layers retains polarization, whereas
light that has been scattered multiple times becomes
depolarized46,47. Thus, by using rPDI to image scattering, it
get past a limitation of the laser scanning, namely back/specular
reflections from the cornea and lens.
Moreover, Q5 hypothesizes that when rPDI is used at a wavelength
that is absorbed by melanin, it would (1) help pick up the
hyperpigmentation of the diseased tissue, (2) help detect (and
guide surgical resection of) melanoma of the retina, and (3)
help image microcirculation (Sidestream dark field (SDF) imaging
,Sidestream
Dark Field(SDF),Side stream dark field
imaging (SDF),Sidestream dark field imaging (SDF)) in the retina, since the same
wavelength is absorbed by blood vessels. At present, the use fluorescein angiography results in mild to severe side effects
ranging from headaches and vomiting to death in people injected
with the sodium fluorescein dye49.
References:
37. Ishiko S, Akiba J, Horikawa Y, Yoshida A. Detection of
drusen in the fellow eye of Japanese patients with age-related
macular degeneration using scanning laser ophthalmoscopy.
Ophthalmology. 2002;109:2165–2169.
38. Delori FC, Pflibsen KP. Spectral reflectance of the human
ocular fundus. App Opt. 1989;28:1061–1077. ……etc. |
