Evaluating the influence of machine-specific DRR parameters on the accuracy of X-ray simulation and orthopaedic 2D-3D reconstruction

Reyneke, Cornelius

Evaluating the influence of machine-specific DRR parameters on the accuracy of X-ray simulation and orthopaedic 2D-3D reconstruction

Thesis / Dissertation

2023

Abstract

In orthopaedics, two-dimensional-to-three-dimensional (2D-3D) reconstruction allows 3D bone structures, conventionally derived from 3D modalities such as computed tomography (CT), to be derived from 2D modalities such as X-ray imaging. Thus, clinical interventions such as implant design and postoperative evaluation can be made more accessible, less expensive and, in some cases, the dose of ionising radiation to the patient can be reduced. State-of-the-art approaches iteratively warp a deformable 3D model of the bone, simulate an X-ray projection image from it, and compare the result to a real X-ray target image, with the goal of minimising the disparity between the two. The X-ray simulation method includes implicit X-ray machine-specific calibration settings, which affect both the resulting geometry and intensity profile of simulated X-ray images. However, the importance of correct projection calibration is not adequately discussed in literature, despite the fact that X-ray machines vary significantly with regard to their imaging setup. Furthermore, the reconstruction inaccuracies resulting from projection miscalibration are not well understood. In this thesis, first, an extensive literature review of the 2D/3D reconstruction problem is conducted and a unified mathematical formulation is proposed. Next, the development of a digitally reconstructed radiograph (DRR) renderer for simulating X-ray images is reported. The DRR renderer was developed using a standard volume rendering framework, adapted to the unique requirements of X-ray imaging. The renderer can be calibrated to machine-specific parameters, of which two were the focus in-depth experimental investigations: the distance from the X-ray source to the imaged object (S2O distance), and modelled energy intensity of the X-ray source (incident energy). For each correctly calibrated parameter of a nominal value, a set of perturbed values were generated and used to render corresponding simulated X-ray images. The resulting rendering errors were then measured through comparison to a groundtruth X-ray image of an object with known geometry and density. The results demonstrated a 2D Dice overlap error of 0.4% for every 1 cm with which the S2O distance was offset, and a pixel intensity error of 1% for every 0.1 point offset of the incident energy coefficient. In a subsequent experimental investigation, the same two machine-specific parameters were studied to determine the influence of machine-specific parameters on 2D-3D reconstruction accuracy using a single image. First, having generated a set of random DRRs, each was viii used as a simulated registration target while offsetting the correctly calibrated parameters in the same fashion as for the first experiment. A 2D Dice overlap error of at least 0.3% was estimated for every 1 cm with which the S2O distance was offset. Similar experiments were performed on real X-ray images of dry-bone femurs as registration targets, leading to a 2D Dice overlap error of at least 0.4% for every 1 cm with which the S2O distance was offset. The incident energy parameter had no discernible impact on the resulting registration accuracy for either the simulated or real-world targets. As a whole, the experimental results demonstrate that machine-specific calibration has a noticeable impact on the accuracy of simulated X-ray images and, in the case of the S2O distance, also on the accuracy of 2D-3D reconstruction. An in-depth commentary on the work done, together with suggestions for future research, concludes the thesis. In summary, the presented work provides valuable insight into previously overlooked facets of orthopaedic 2D-3D reconstruction, and suggests that machine-specific calibration should be carefully considered when performing 2D-3D reconstruction.

Keywords

Human Biology

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Collections

PhD / Doctoral

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