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Category: Invited talks  Publish Time: 2014-05-16 10:00 


Dynamic thermal-wave coherence tomographies for NDT and Biomedical Imaging



Center for Advanced Diffusion-Wave Technologies (CADIFT), Department of Mechanical and Industrial Engineering, University of Toronto, Toronto, Ontario, M5S 3G8, Canada.

Abstract.Energy transport in diffusion-wave fields is gradient driven and therefore diffuse, yieldingdepth-integratedresponses with poor axial resolution. Traditional diffusion-wave techniques, limited by the physics of parabolic diffusion, can only produce depth-integrated planar images as they are unable to generate three-dimensional subsurface imaging. This talk will present two new imaging methods developed in the CADIFT for enabling parabolic thermal-wave fields to exhibit energy localization akin to propagating hyperbolic wave-fields. This approach when used with a mid-IR camera results indepth-selective(or depth-resolved) photothermal imaging which not only improves axial and depth resolution, but also allows for deconvolution of individual responses of superposed axially discrete sources, opening a new field of subsurface Photothermal Coherence Tomography (PCT) using thermal waves. In this talk I will present two novel thermal-wave imaging methodologies: Matched filter binary phase coded (BPC) PCT and truncated-correlation photothermal coherence tomography (TC-PCT). The physical principles of these methodologies and examples of imaging applications to engineering materials and biomaterials will be discussed.

1         Binary phase coded Photothermal Coherence Tomography

First, I will briefly introduce thermophotonic radar imaging principles and techniques using chirped or BPC modulation, methods which can break through the maximum detection depth/depth resolution limitations of conventional photothermal waves. Using matched-filter principles, BPC-PCT, a methodology enabling parabolic diffusion-wave energy fields to exhibit energy localization akin to propagating hyperbolic wave-fields will be described [1]. It allows for deconvolution of individual responses of superposed axially discrete sources, opening a new field: depth-resolved thermal coherence tomography. Several examples from dental enamel caries diagnostic imaging to metal subsurface defect thermographic imaging will be discussed. Fig. 1 shows BPC-TCT resolving the step of a sub-surface wedge sample.

2     Truncated-correlation Photothermal Coherence Tomography

Next, I will introduce our very recent development of truncated-correlation photothermal coherence tomography (TC-PCT) [2], which exhibits the highest degree of energy localization and image resolution in a parabolic diffusion wave field to-date. TC-PCT enables three-dimensional “crisp” visualization of subsurface features/discontinuities which is not otherwise possible with known optical or conventional photothermal imaging techniques. Examples to be presented include imaging of solids with intricate sub-structures, specifically, holes in steel, trabecular bone structure through cortical and soft tissue overlayers, structural changes in animal bones following demineralization induced bone loss (artificial osteoporosis), and burn depth profiles in tissues. As a consequence of its high axial resolution and nearly lossless character, TC-PCT exhibits sub-surface depth profilometric capabilities over several (~ 4) thermal diffusion lengths, well beyond those of today’s thermal-wave probes. From the perspective of biomedical laser safety, TC-PCT is maximum-permissible-exposure compatible.

Fig. 2 shows TC-PCT “crisp” images of a cortical – cancellous goat interfaces bone breaking through the diffusion resolution and depth barriers.

3     Conclusions

In addition to biomedical diagnostic imaging and engineering materials testing, the two novel non-ionizing modalities constitute photothermal analogs of optical coherence tomography and open the way for applications in diverse areas of research where three-dimensional sub-surface depth-profiling is required.

Figure 1:(a)cross section of a step wedge sample.(b)Conventional lock-in thermography phase.(c)BPC peak delay time, and(d)BPC phase images of the step wedge sample using 16-bit code at 3Hz (acquisition time = 5.33s).Figure 2.Non-ionizing tomography of bone. (a)Cross-sectional photograph of a goat rib-bone with soft tissue overlayer. Labels C and T refer to the cortical and trabecular regions, respectively.(b)The irradiated tissue surface.(c)  The photothermal volume tomogram observed from the tissue surface. (d)The tomogram observed from the trabecular bottom surface.(e)Cross-sectional TC-PCT corresponding to (a).


[1] N. Tabatabaei and A. Mandelis, “Thermal coherence tomography using match filter binary phase coded diffusion waves”Phys. Rev. Lett.107, 165901 (5 pages) (2011).

[2] S. Kaiplavil and A. Mandelis, “Truncated-Correlation Photothermal Coherence Tomography: “Crisp” sub-surface imaging breaking through the diffusion resolution and depth barriers”Nature Photonics (In press)