Novel analytical approaches for testing and evaluation of biomaterials using infrared imaging

Abstract

In recent decades, Infrared Imaging has been immersed as a non-invasive testing and imaging technique in the area of biomedical applications for revealing the surface, subsurface, and characteristic information about the anomalies hidden inside the test object under inspection. This becomes possible due to its capabilities like the whole field, non-contact, non-ionizing, fast, and operator-friendly behavior over the other non-invasion imaging techniques such as Radiographic Imaging, Ultrasonic Imaging, and Magnetic Resonance Imaging. The infrared Imaging technique involves the principle of mapping temporal thermal distribution over the test object surface to reveal its surface and subsurface anomalies in detail. It can be implemented passively and actively. As all the objects above absolute zero Kelvin temperature radiate infrared radiations, capturing those infrared radiations at the ambient conditions without the external thermal stimulus is known as passive thermography. In passive thermography, all the features of interest are always at higher or lower temperatures them that of its ambient. Moreover, if abnormal thermal profiles are observed over the test object surface, then that would be a potential indication of the presence of anomalies. Nevertheless, its drawback of not providing enough thermal contrast in case of deeper subsurface anomalies limits its applicability. Then to overcome the limitation associated with passive thermography the deeper subsurface anomalies are detected by active infrared thermography. In which a pre-defined heat stimulus has been imposed over the test object surface to produce enough thermal contrast between the feature of interest and its surrounding. The most widely used source of excitation in active infrared thermography for biomedical imaging applications is the optically excited mechanism. This is because of its advantages such as whole-field, remote and economical compared to other excitation mechanisms. Among various active infrared thermographic techniques, optically excited techniques such as Pulse Thermography (PT), Pulse Phase Thermography (PPT), and Lock-in Thermography (LT) techniques are conventionally used for biomedical imaging. Whereas due to their limitations like high peak power requirement in case of pulsed based thermographic techniques both in PT and PPT and repetitiveness of experiment in case of Lock-in thermography to get better resolution this, limits their applicability in biomedical applications. To overcome the limitations associated with the conventional thermographic techniques this thesis work has been introduced frequency-modulated schemes like Linear Frequency Modulated Thermal Wave Imaging (LFMTWI) and Digitized Frequency Modulated Thermal Wave Imaging (DFMTWI). In the LFMTWI technique, the heat source has been modulated in the time domain as a linearly frequency modulated manner where a band of frequencies (decided by test object thickness and the test resolution required) with significant magnitude for a limited duration imposed over the test object. This provides a continuous depth scanning in a single experimentation cycle to revels the information about the anomalies hidden at a different-different location inside the test object. Then, in order to increase the test resolution as well as sensitivity the linear frequency modulated input has been digitized and used as incident heat input for the DFMTWI technique. In this technique, the fundamental frequencies, as well as additionally higher harmonics with a significant amount that increases overall bandwidth has been probed inside the test object. This improves the overall pulse compression properties because compressed pulse width is inversely proportional to that of bandwidth due to which pulse will be compressed to a very narrow duration so, that better depth resolution as well the sensitivity can be achieved. In this thesis mainly the detection capabilities of the proposed modulation schemes such as LFMTWI and DFMTWI have been studied with the help of novel analytical approaches over the conventional pulse and periodically modulation-based techniques by taking correlation coefficient as a figure of merit for biomedical applications. Further, all the analytical approaches result for different thermographic techniques have been validated with the numerical results obtained by finite element-based simulation performed by COMSOL Multiphysics.