The Industrial Material Institute (IMI) together with the Institute for Biodiagnostic (IBD) has developed its own optical
catheters for cardiovascular imaging applications. Those catheters have been used experimentally in the in vitro coronary
artery model of the Langendorff beating heart and in a percutaneous coronary intervention procedure in a porcine model.
For some catheter designs, non-uniform rotational distortion (NURD) can be observed as expected from past experience
with intra-vascular ultrasound (IVUS) catheters.
A two-dimensional (2D) coronary artery test bench that simulates the path into the coronary arteries has been developed.
The presence or absence of NURD can be assessed with the test bench using a custom-built cardiovascular Optical
Coherence Tomography (OCT) imaging system. A square geometry instead of the circular shape of an artery is used to
simulate the coronary arteries. Thereby, it is easier to visualize NURD when it is present. The accumulated torsion
induced by the friction on the catheter is measured along the artery path.
NURD is induced by the varying friction force that is balanced by the accumulated torsion force. The pullback force is
measured and correlated with NURD observed in the 2D test bench. Finally, a model is presented to help understanding
the mechanical constraint that leads to the friction force variations.
The advantages and limitations of using a beating heart model in the development of intravascular optical coherence tomography are discussed. The model fills the gap between bench experiments, performed on phantoms and excised arteries, and whole animal in-vivo preparations. The beating heart model is stable for many hours, allowing for extended measurement times and multiple imaging sessions under in-vivo conditions without the complications of maintaining whole-animal preparation. The perfusate supplying the heart with nutrients can be switched between light scattering blood to a nonscattering perfusate to allow the optical system to be optimized without the need of an efficient blood displacement strategy. Direct access to the coronary vessels means that there is no need for x-ray fluoroscopic guidance of the catheter to the heart, as is the case in whole animal preparation. The model proves to be a valuable asset in the development of our intravascular optical coherence tomography technology.
We previously presented a method to fabricate phantoms of normal coronary arteries. This method allows the
deposition of multiple layers on a tubular structure, each layer replicating optical and mechanical properties
of coronary artery layers. We now present an improved method to produce phantoms of arteries affected by
atherosclerosis. The method now includes techniques to introduce structures that mimics the OCT signature of
a calcification and of a lipid pool.
We developed coronary artery phantoms that should be of great use for intravascular optical coherence tomography. Our phantoms mimic the OCT signal profile of coronary arteries, show mechanical properties approaching those of real tissue, and are durable.
Optical coherence tomography (OCT) is emerging as a technology that can potentially be used for the detection and monitoring of early dental enamel caries since it can provide high-resolution depth imaging of early lesions. To date, most caries detection optical technologies are well suited for examining caries at facial, lingual, incisal and occlusal surfaces. The approximal surfaces between adjacent teeth are difficult to examine due to lack of visual access and limited space for these new caries detection tools. Using a catheter-style probe developed at the NRC-Industrial Materials Institute, the probe was inserted into the interproximal space to examine the approximal surfaces with OCT imaging at 1310 nm. The probe was rotated continuously and translated axially to generate depth images in a spiral fashion. The probe was used in a mock tooth arch model consisting of extracted human teeth mounted with dental rope wax in their anatomically correct positions. With this ex vivo model, the probe provided images of the approximal surfaces revealing morphological structural details, regions of calculus, and especially regions of early dental caries (white spot lesions). Results were compared with those obtained from OCT imaging of individual samples where the approximal surfaces of extracted teeth are accessible on a lab-bench. Issues regarding access, regions of interest, and factors to be considered in an in vivo setting will be discussed. Future studies are aimed at using the probe in vivo with patient volunteers.
Optical coherence tomography was used to collect cross-sectional images of glass powder beds consisting of microspheres with diameters ranging from 8 to 175 µm. Images were formed by a collection of individual interferogram envelopes that give the backscattered light amplitude as a function of the optical path in the glass powder bed. The diameter distribution, for microspheres located near the surface of the beds, is obtained by appropriate peak distance measurements on threshold-selected envelopes after having performed the surface profilometry. The measured distributions are in good agreement with those obtained by laser diffraction. When considering the whole powder volume, the evaluation of the mean light penetration depth inside the powder beds proves to be a useful approach to evaluate the mean particle diameter, although no information is obtained on the actual particle size distribution in this case. Two simplified models are introduced to understand the linear relationship observed between the penetration depth and the mean particle size.
We present here the implementation of a fiber-based common-path interferometer for Swept Source Optical Coherence Tomography (SS-OCT). A common path configuration is often a suitable approach for increasing the stability of the measurements. Optical fibers are sensitive to temperature and some other mechanical perturbations which compromise absolute accuracy measurements. A common-path configuration provides a mean to
define a reference at the probe location and, thereby, to compensate for the optical path length perturbations. Additionally, in SS-OCT, we have to deal with autocorrelation noise and the mirror image artifact due to the
computation of the Fourier transform. Thus, our common-path implementation also includes acousto-optics
modulators to remove the depth degeneracy as it has already been done when using a "traditional" interferometer configuration. The efficiency of our system is validated by comparing images acquired with a "traditional" SS-OCT configuration and with our common path SS-OCT configuration.
This paper reports the study of a Swept Source Optical Coherence Tomography (SS-OCT) setup to remove
depth degeneracy for measurements performed with a rotating probe. One of the main drawbacks of SS-OCT
is its inability to differentiate positive and negative depths. Some setups have already been proposed to remove
this depth ambiguity by introducing a modulation by means of
electro-optic or acousto-optic modulators. For
cross-sectional imaging, we developed a setup that uses a piezoelectric fiber stretcher to generate a periodic
phase shift between successive A-scans, thus introducing a transverse modulation. The depth ambiguity is then
resolved by performing a Fourier processing in the transverse direction before processing the data in the axial
direction. This approach can also be applied to a rotating probe with a cylindrical geometry by introducing
phase shifts between A-scans belonging to successive rotations or between successive B-scans. In the later case,
the depth degeneracy is removed by first performing a Fourier processing along the cylindrical axis. We validate
this approach by processing images acquired with our catheterized probe based on a rotating fiber and fitted
with a GRIN lens and a prism at the tip.
A Gouy phase shift is acquired each time a beam goes through a focal point. It is a common practice in optical coherence tomography (OCT) systems to focus the light on a sample to obtain a good transverse resolution in addition to the axial resolution provided by the coherence gating. In presence of chromatic aberration, the position of the focal point is wavelength dependent. This leads to an enhanced wavelength dependency of the Gouy phase shift for measurements performed in the focal region of the sample arm. This affects the positioning of the envelope of the interferogram and can thus have a strong impact on the axial position accuracy as measured by an OCT system using the maximum of the envelope. Chromatic aberration not only leads to a Gouy phase shift, but also to an additional wavelength-dependent term related to the transverse phase distribution over the
aperture of the collecting optics. This last term brings a contribution that is of the same order as the Gouy phase
shift. We demonstrated these effects in time-domain OCT in a previous paper [Lamouche et al., Optics Comm., Vol. 239, 297 (2005)]. This problem is revisited with Fourier-domain OCT by performing measurements with a swept-source OCT system. This allows to confirm that the effect observed is not interferometer dependent but purely optical in origin. It also provides a clear experimental confirmation that the effect is related to chromatic aberration. This work is of interest for OCT measurements that require accurate relative position measurements in the focal region.
Much of the current activity in optical coherence tomography aims at increasing the image resolution. Nowadays,
two kinds of OCT techniques are available. The first approach is the Time-Domain OCT (TD-OCT) which
usually relies on a moving part into the reference arm to probe the sample in depth. The second approach is
the Fourier-Domain OCT (FD-OCT) in which the signal is acquired as a function of the wavelength and the
depth profile of the sample is obtained by Fourier transform. Theoretically, in both techniques, the resolution is
limited by the central wavelength of the source and by its full width at half maximum. Nevertheless, it is shown
in this paper that this resolution may be improved by using deconvolution technique based on Wiener filtering
and Autoregressive Spectrum Extrapolation (ASE). In our experiment, thanks to deconvolution an improvement
of a factor up to 4 is obtained in TD-OCT and about 2 in FD-OCT. As an illustration, the approach is applied
to TD and FD-OCT measurements of the profile of a carbon-epoxy composite to evaluate the performance in
determining the thickness of the upper layer within a resolution better than that provided by the conventional
processing of the OCT envelope.
As in conventional time-domain optical coherence tomography (OCT), speckle is inherent to any Optical Fourier
Domain Imaging (OFDI) of biological tissue. OFDI is also known as swept-source OCT (SS-OCT). The axial
speckle size is mainly determined by the OCT resolution length and the transverse speckle size by the focusing
optics illuminating the sample. There is also a contribution from the sample related to the number of scatterers
contained within the probed volume. In the OFDI data processing, there is some liberty in selecting the range of
wavelengths used and this allows variation in the OCT resolution length. Consequently the probed volume can
be varied. By performing measurements on an optical phantom with a controlled density of discrete scatterers
and by changing the probed volume with different range of wavelengths in the OFDI data processing, there is an
obvious change in the axial speckle size, but we show that there is also a less obvious variation in the transverse
speckle size. This work contributes to a better understanding of speckle in OCT.
Speckle is inherent to any Optical Coherence Tomography (OCT) imaging of biological tissue. It is often seen as
degrading the signal, but it also carries information about the tissue microstructure. One parameter of interest is
the speckle size. We study the variations in speckle size on optical phantoms with different density of scatterers.
Phantoms are fabricated with a new approach by introducing silica microspheres in a curing silicon matrix,
providing phantoms with a controlled density of scatterers. These phantoms are also solid, deformable, and
conservable. Experimental results are obtained with Time-Domain OCT (TD-OCT). Modeling is performed
by simulating a phantom as a random distribution with of discrete scatterers. Both experimental results and
modeling show that the speckle size varies when there are few scatterers contained within the probed volume,
the latter being defined by the coherence length and the spot size of the focusing optics. As a criterion to
differentiate tissues, the speckle size has the same sensitivity as the contrast parameter that is studied in Ref. 1.
This work also contributes to a better understanding of speckle in OCT.
This paper reports the study of an Optical Fourier Domain Imaging (OFDI) setup for optical coherence tomography.
One of the main drawbacks of OFDI is its inability to differentiate positive and negative depths. Some
setups have already been proposed to remove this depth ambiguity by introducing a modulation by means of
electro-optic or acousto-optic modulators. In our setup, we implement a piezoelectric fiber stretcher to generate
a periodic phase shift between successive A-scans, thus introducing a transverse modulation. The depth
ambiguity is then resolved by performing a Fourier treatment in the transverse direction before processing the
data in the axial direction. It is similar to the B-M mode scanning already proposed for Spectral-Domain OCT1
but with a more efficient experimental setup. We discuss the advantages and the drawbacks of our technique
compared to the technique based on acousto-optics modulators by comparing images of an onion obtained with
both techniques.
We propose a robust and efficient delay line using an ensemble of rotating rhombic prisms. Delay lines relying on
rotating elements provide fast and stable operation. Optical systems using rhombic prisms are quite easy to align
since these prisms are efficient even when slightly misaligned. Optical delay lines with a single rotating element
usually have a poor duty cycle and show large nonlinearity in the variation of the optical path lengh with the
angular position. Our delay line improves over existing technology by using off-centroid rotation and reinjection.
Off-centroid rotation allows the use of multiple prisms and, by optimizing the conditions of operation, the duty
cycle is increased and the nonlinearity is decreased. The duty cycle and repetition rate are further increased by
reinjecting the incoming ray towards the delay line when it is not first intercepted by the prism ensemble. We
have designed and built such a delay line using five prisms. The experimental device was tested at 2000 delay
scans per second and provided a duty cycle larger than 80% with about 5% nonlinearity. Higher delay scan rates
are easily achievable with this technology. The delay line was introduced in a time-domain optical coherence
tomography system and example of imaging of biological tissue is provided.
An overview of the activities in low-coherence interferometry (LCI) and optical coherence tomography (OCT) at the Industrial Materials Institute are presented. An innovative optical delay line using rhombic prisms is described. A few industrial applications are described: volume loss in a wear test, combination of LCI with laser-induced breakdown spectroscopy, and modification of an existing rheometer to increase its precision. Preliminary results related to the use of speckle to differentiate tissues and materials in OCT are presented. The speckle dimension is shown to be sensitive to low density of scatterers. An additional parameter extracted from the autocorrelation of speckle is also presented.
Optical inspection tools based on low-coherence interferometry and specialized for hard to reach industrial parts are
presented. A common path configuration using optical fiber components is described. Small diameter probes originally
developed for biomedical applications have been specialized for industrial inspection. Probes that can be used with a
Cartesian surface scanning system or a cylindrical scanning system are presented. The probes include a reference that
makes absolute accuracy measurements easier. Characterization of the internal surface of a worn plasma torch electrode
has been realized using that technique. Surface profiling of the barrel of a gun was also performed.
Low coherence interferometry (LCI) can be used to measure the profile of industrial products, the measured sample being scanned under the LCI probe. Axial distance measurements are made using the light reflected by the surface and collected on the same optical axis used for its illumination. Therefore, when the transverse resolution is not an issue, only a narrow laser beam is required and hard-to-reach surfaces can be probed with axial accuracy in the range of 1μm. With other techniques such as triangulation the surface requires to be visible from another point-of-view and complex shapes often become inaccessible. When the LCI instrument is assembled with optical fibers, delicate instrumentation may be kept away from harsh environments and only a single optical fiber needs to get close to the measurement location. However, optical fibers are particularly sensitive to temperature and a reference is required to compensate for path length drifts. Furthermore, industrial mechanical displacement systems typically induce positioning errors much larger than the LCI instrument accuracy. One approach to circumvent these problems consists in measuring the location of another surface close to the region of interest. Such a reference surface is not always available and typically requires a second probe. We found a more practical approach by using an optical quality window located over the sample surface and moving with the sample. The laser beam from the LCI instruments travels across the window just before it reaches the samples surface. The window surface induces a first reflection (4% of the incident power) and its distance is measured by the LCI as well as the sample surface distance. Since the location of the window is fixed relative to the sample while the entire surface is scanned, out-of-plane movement of the motorized slide is compensated. High-resolution measurements are obtained by simply subtracting the window plane from the sample surface.
An optical method for on-line hot steel surface quality inspection is proposed. The method is based on the four laser sources illuminating the surface at grazing incidence from different directions. A CCD line array camera, located perpendicularly over the surface, captures the scattered light generated by each source separately. Surface defects induce shadow and bright field patterns which are related to each source direction and angle of incidence. Such a multiple source configuration is required for the discrimination of real surface defects from normal reflectivity and surface geometry variations. The patterns are digitized for each source separately and then combined using a simple and fast algorithm. The method was tested in our laboratory on cold steel billet samples. Characteristic signatures were obtained for most surface defects such as blisters, scale embedments, large cracks and pinholes. A scanning rate of several thousand lines per second is planned for the industrial prototype by selecting a different wavelength for each laser source and by using a four wavelength CCD line array camera.
A laser stereo triangulation device is described which permits easier observation of objects with specular surfaces. The device's two cameras are first calibrated with the light pattern. The calibration data are used to compute the unique and reversible function to mapping the image sensed by the second camera within the image plane of the first camera. Simple correlation techniques are used to extract the originally projected light pattern from the numerous artifacts. Because of its simplicity, the algorithm can be computed in real time at video rates to produced a clean image representative of the surface geometry. Experiments conducted using a laboratory prototype produce reliable results wih promise for practical applications.
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