Model-based, one-sided, time-of-flight terahertz image reconstruction

In the last decade, terahertz-mode imaging has received increased attention for non-destructive testing applications due to its ability to penetrate many materials while maintaining a small wavelength. This paper describes a model-based reconstruction algorithm that is able to image defects in the spray-on foam insulation (SOFI) used in aerospace applications that has been sprayed on a reflective metal hull. In this situation, X-ray based imaging is infeasible since only one side of the hull is accessible in flight. This paper models the object as a grid of materials, each section of which has a constant index of refraction. The delay between the transmission and reception of a THz pulse is related to the integral of the index of refraction along the pulse’s path, and we adapt computed tomography (CT) methods to reconstruct an image of an object’s index of refraction. We present the results of our reconstruction method using real data of the timing of THz pulses passing through a block of SOFI with holes of a known location and radius. The resulting image of the block has a low level of noise, but contains artifacts due to the limited angular range of one-sided imaging and due to the narrow beam approximation used in the forward model.