Matt Faulkner

We introduce the hybrid broken-ray tomography (HBRT) for three-dimensional imaging of weakly scattering systems. The HBRT utilizes fluorescent contrast agents and combines the principles and advantages of the broken-ray tomography and the non-reciprocal broken-ray tomography introduced by us previously. The HBRT uses angularly resolved intensity measurements at the incident and fluorescence wavelengths to reconstruct the attenuation and scattering coefficients at the excitation wavelength anywhere within the sample, as well as the attenuation coefficient at the fluorescence wavelength and the contrast agent concentration in the regions of contrast agent accumulation. The principles of HBRT have been validated by Monte Carlo simulations.

Laser-driven MeV x-ray sources are attractive for high resolution radiography of large, dense objects given their small source size (< 200 micron) compared to traditional LINAC x-ray sources and have the additional benefit of ultrafast imaging due to their picosecond duration pulses. Work at the ALEPH facility at Colorado State University has demonstrated repeatable pulses of MeV x-rays used to conduct tomographic reconstruction.

The need for high-resolution MeV x-ray tomography to observe the three-dimensional structure of dense, large-sized objects is rapidly increasing for the non-destructive evaluation of critical additively manufactured parts, national security, and other applications. We report a demonstration of high-resolution MeV computed tomography of a dense, large object with a laser-driven x-ray source. A record detector-limited MeV radiograph resolution of as determined with the Bennett approximation of the point spread function was achieved by irradiating millimeter-thick tungsten targets with 300 TW femtosecond laser pulses at a 0.5 Hz repetition rate. A tungsten alloy step wedge spectrometer indicates that the peak of the x-ray emission is between 1 and 2 MeV, with an endpoint energy of 19 MeV. To illustrate the radiographic imaging capability of the system, a tomographic reconstruction of a nickel superalloy turbine blade () with sub-millimeter resolution was performed using 2160 individual radiographs. The small x-ray source size opens the prospect of extremely high-resolution tomographs of large, dense objects. This laser-driven approach has major advantages for non-destructive evaluation.

Optical methods of biomedical tomographic imaging are of considerable interest due to their non-invasive nature and sensitivity to physiologically important markers. Similarly to other imaging modalities, optical methods can be enhanced by utilizing extrinsic contrast agents. Typically, these are fluorescent molecules, which can aggregate in regions of interest due to various mechanisms. In the current approaches to imaging, the intrinsic (related to the tissue) and extrinsic (related to the contrast agent) optical parameters are determined separately. This can result in errors, in particular, due to using simplified heuristic models for the spectral dependence of the optical parameters. Recently, we have developed the theory of non-reciprocal broken-ray tomography (NRBRT) for fluorescence imaging of weakly scattering systems. NRBRT enables simultaneous reconstruction of the fluorophore concentration as well as of the intrinsic optical attenuation coefficient at both the excitation and the emission wavelengths. Importantly, no assumption about the spectral dependence of the tissue optical properties is made in NRBRT. In this study, we perform numerical validation of NRBRT under realistic conditions using the Monte Carlo method to generate forward data. We demonstrate that NRBRT can be used for tomographic imaging of samples of up to four scattering lengths in size. The effects of physical characteristics of the detectors such as the area and the acceptance angle are also investigated.