Enhanced truncated-correlation photothermal coherence tomography with application to deep subsurface defect imaging and 3-dimensional reconstructions

Abstract

Photothermal diffusion-wave imaging is a promising technique for non-destructive evaluation and medical applications. Several diffusion-wave techniques have been developed to produce depth-resolved planar images of solids and to overcome imaging depth and image blurring limitations imposed by the physics of parabolic diffusion waves. Truncated-Correlation Photothermal Coherence Tomography (TC-PCT) is the most successful class of these methodologies to-date providing 3-D subsurface visualization with maximum depth penetration and high axial and lateral resolution. To extend the depth range and axial and lateral resolution, an in-depth analysis of TC-PCT, a novel imaging system with improved instrumentation, and an optimized reconstruction algorithm over the original TC-PCT technique is developed. Thermal waves produced by a laser chirped pulsed heat source in a finite thickness solid and the image reconstruction algorithm are investigated from the theoretical point of view. 3-D visualization of subsurface defects utilizing the new TC-PCT system is reported. The results demonstrate that this method is able to detect subsurface defects at the depth range of ∼4 mm in a steel sample, which exhibits dynamic range improvement by a factor of 2.6 compared to the original TC-PCT. This depth does not represent the upper limit of the enhanced TC-PCT. Lateral resolution in the steel sample was measured to be ∼31 μm.

Matched-Filter Thermography

“…A significant breakthrough was achieved in the field by Tabatabaei and Mandelis' introduction of thermal coherence tomography (TCT) [22,26], which used the binary phase coding pulse compression technique (i.e., a narrow-band phase encoding technique) to optimize the thermal-waves' matched-filter responses in order to resolve overlaying absorbers in a diffusive field. Application of matched-filtering in a thermography system using pulsed excitation was first demonstrated by Kaiplavil and Mandelis [40] in 2011 and was later optimized to yield a highly depth-resolved thermal-wave tomographic imaging modality, termed truncated-correlation photothermal coherence tomography (TC-PCT) [41][42][43][44]. Some other fundamental advancements in the field of TWR include: incorporation of Barker [45] and Golay [46,47] code pulse compression techniques and performance optimization using spectral shaping [48].…”

3D Dental Subsurface Imaging Using Enhanced Truncated Correlation-Photothermal Coherence Tomography

“…Building on the high sensitivity of 808 nm TPLI to dental caries, we present here the first application of the recently developed enhanced Truncated Correlation-Photothermal Coherence Tomography (eTC-PCT) 26 thermophotonics-based 3D imaging technology to dental imaging. This technology is an evolution of the original TC-PCT method 27 , which has been shown to be capable of monitoring bone mineral loss 28,29 .…”

Non-invasive in-vivo 3-D imaging of small animals using spatially filtered enhanced truncated-correlation photothermal coherence tomography

“…eTC-PCT axial resolution is determined by W T . A longer W T will compile the data from more recorded frames for CC calculation of each pixel and thus results in higher contrast and SNR, at the cost of reducing axial resolution 14 .…”

“…The LFM excitation chirp signal controlls the diode laser (Jenoptic JOLD- 120-QPXF-2P) through a laser driver (PCO-6131, Directed Energy, Colorado, USA), and is recorded using a high-speed data acquisition module (NI PCI-6281) for synthesizing the reference chirp. The laser beam is passed through a collimator (F22SMA-B, Thorlabs Inc., New Jersey, USA), and a diffuser (ED1-C20, Thorlabs Inc., New Jersey, USA) to become collimated, expanded, and homogenized 14 .…”

“…It involves the detection of photothermal waves through emitted thermal infrared (IR) photons (Planck radiation) from tissues, captured by a mid-IR (MIR) camera. Enhanced Truncated-Correlation Photothermal Coherence Tomography (eTC-PCT) 14 is a novel imaging method based on the photothermal effect, which has achieved superior penetration depth (4 mm in a steel sample) and axial resolution in industrial materials compared to conventional thermal imaging technologies. It also achieves a maximum lateral resolution of 16 μm, which is twice as fine as the original TC-PCT 15 imaging.…”