Cao Zhang

Polyenergetic known-component reconstruction without prior shape models

Purpose: Previous work has demonstrated that structural models of surgical tools and implants can be integrated into model-based CT reconstruction to greatly reduce metal artifacts and improve image quality. This work extends a polyenergetic formulation of known-component reconstruction (Poly-KCR) by removing the requirement that a physical model (e.g. CAD drawing) be known a priori, permitting much more widespread application. Methods: We adopt a single-threshold segmentation technique with the help of morphological structuring elements to build a shape model of metal components in a patient scan based on initial filtered-backprojection (FBP) reconstruction. This shape model is used as an input to Poly-KCR, a formulation of known-component reconstruction that does not require a prior knowledge of beam quality or component material composition. An investigation of performance as a function of segmentation thresholds is performed in simulation studies, and qualitative comparisons to Poly-KCR with an a priori shape model are made using physical CBCT data of an implanted cadaver and in patient data from a prototype extremities scanner. Results: We find that model-free Poly-KCR (MF-Poly-KCR) provides much better image quality compared to conventional reconstruction techniques (e.g. FBP). Moreover, the performance closely approximates that of Poly- KCR with an a prior shape model. In simulation studies, we find that imaging performance generally follows segmentation accuracy with slight under- or over-estimation based on the shape of the implant. In both simulation and physical data studies we find that the proposed approach can remove most of the blooming and streak artifacts around the component permitting visualization of the surrounding soft-tissues. Conclusion: This work shows that it is possible to perform known-component reconstruction without prior knowledge of the known component. In conjunction with the Poly-KCR technique that does not require knowledge of beam quality or material composition, very little needs to be known about the metal implant and system beforehand. These generalizations will allow more widespread application of KCR techniques in real patient studies where the information of surgical tools and implants is limited or not available.

Experimental investigation of compliant wall deformation in a turbulent channel flow

Interaction of a compliant wall with a turbulent channel flow is investigated by simultaneously measuring the time-resolved, three-dimensional flow field using tomographic PIV and the two-dimensional surface deformation using Mach-Zehnder interferometry. The friction Reynolds number is Reτ  = 2300, and the Young’s Modulus of the wall is 0.93 MPa, resulting in a ratio of shear speed to centerline velocity (U 0) of 6.8. The wavenumber-frequency spectra of deformation contain a non-advected low-frequency component and advected modes, some traveling at U 0 and others at 0.72U 0. The wall dynamics is elucidated by correlating the deformation with flow variables, including the 3D pressure distribution. The pressure-deformation correlations peak at y/h~0.12 (h is half channel height), the elevation of Reynolds stress maximum in the log-layer. Streamwise lagging of the deformation behind the pressure is caused in part by phase-lag of the pressure with decreasing elevation, and in part by material damping predicte...

Simultaneous Measurements of 3D Flow and Wall Deformation in a Compliant Wall Turbulent Channel Flow

This study focuses on the interaction of a turbulent channel flow at Reτ=2310 over a flat, compliant boundary made of PDMS (Polydimethylsiloxane). Two noninvasive optical techniques, namely tomographic PIV (TPIV) and Mach-Zehnder Interferometry (MZI), are integrated to perform simultaneous measurements of the 3D flow field and the corresponding surface deformation. The measurements are performed in a refractive index-matched facility, where the working fluid is aqueous solution of sodium iodide (NaI). The TPIV measurement volume is 30×10×10 mm3 in the streamwise, wall-normal and spanwise directions, respectively. The MZI phase evaluation and unwrapping algorithms have been developed, calibrated and implemented. Preliminary results show qualitative correlation between wall deformation and flow structures near the wall.Copyright

Time-Resolved Simultaneous Measurements of Flow Field and Surface Deformation of Turbulent Channel Flow Over a Compliant Wall

As an initial step in our effort to investigate the interaction of a turbulent channel flow with a compliant wall, this paper focuses on the measurement techniques. Two noninvasive optical techniques, namely tomographic PIV (TPIV) and Mach-Zehnder Interferometry (MZI), are integrated to simultaneously measure the time-resolved, wall-normal deformation of the compliant transparent wall and the 3D velocity field of a turbulent channel flow above it. The two systems utilize the same laser, but different cameras. The paper provides a description of the optical setup, detailed information about calibration of the MZI system, as well as sample combined 3D velocity distributions and wall deformations. The measured wall deformation can be decomposed into low frequency structure modes, and higher frequency features that appear to advect with the flow.Copyright