Bruno Gauthier

Non-contact biomedical photoacoustic and ultrasound imaging

The detection of ultrasound in photoacoustic tomography (PAT) usually relies on ultrasonic transducers in contact with the biological tissue through a coupling medium. This is a major drawback for important potential applications such as surgery. Here we report the use of a remote optical method, derived from industrial laser-ultrasonics, to detect ultrasound in tissues. This approach enables non-contact PAT (NCPAT) without exceeding laser exposure safety limits. The sensitivity of the method is based on the use of suitably shaped detection laser pulses and a confocal Fabry-Perot interferometer in differential configuration. Reliable image reconstruction is obtained by measuring remotely the surface profile of the tissue with an optical coherence tomography system. The proposed method also allows non-contact ultrasound imaging (US) by applying a second reconstruction algorithm to the data acquired for NCPAT. Endogenous and exogenous inclusions exhibiting optical and acoustic contrasts were detected ex vivo in chicken breast and calf brain specimens. Inclusions down to 0.3 mm in size were detected at depths exceeding 1 cm. The method could expand the scope of photoacoustic and US to in-vivo biomedical applications where contact is impractical.

Optical delay line using rotating rhombic prisms

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.

Intravascular optical coherence tomography on a beating heart model

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.

Surface inspection of hard to reach industrial parts using low-coherence interferometry

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.

Laser-ultrasonic inspection of steel slabs using Saft processing

The detection of inclusions or small defects located below the surface of cast slabs is addressed in this paper. The technique combines laser-ultrasonics and synthetic aperture data processing for inspection on descaled slabs. An improved synthetic aperture data processing (F-SAFT) performed in the Fourier domain which includes the control of the aperture as well as spatial interpolation is used. A further improvement in F-SAFT reconstruction is made by taking into account the inspected surface profile, previously or simultaneously measured by an optical surface profiler. Samples with wavy inspected surface and flat-bottom holes at different depths are tested to validate the correction method for surface profile. Industrials steel slab samples are then tested to confirm the reliability of the proposed laser-ultrasonic approach. The inspection time duration and the resolution limit are also discussed.

Ex vivo imaging of early dental caries within the interproximal space

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.

Characterization of Thin Layered Structures using Deconvolution Techniques in Time-Domain and Fourier-Domain Optical Coherence Tomography

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.

Using a piezoelectric fiber stretcher to remove the depth ambiguity in Optical Fourier Domain Imaging

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.

Artifact removal in Fourier-domain OCT with a rotating probe

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.

Precise surface profilometry based on low-coherence interferometry

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.