X-ray Computed Tomography InstrumentPerformance Evaluation, Part I: Sensitivity to Detector Geometry Errors

Abstract

X-ray computed tomography (XCT), long used in medical imaging and defect inspection, is now increasingly used for dimensional measurements of geometrical features in engineering components. With widespread use of XCT instruments, there is growing need for the development of standardized test procedures to verify manufacturer specifications and provide pathways to establish metrological traceability. As technical committees within the American Society of Mechanical Engineers (ASME) and the International Organization for Standardization (ISO) are developing documentary standards that include test procedures that are sensitive to all known error sources, we report on work exploring one set of error sources, instrument geometry errors, and their effect on dimensional measurements. In particular, we studied detector and rotation stage errors in cone-beam XCT instruments and determined their influence on sphere center-to-center distance errors and sphere form errors for spheres located in the tomographically reconstructed measurement volume. We developed a novel method, called the single-point ray tracing method, that allows for efficient determination of the sphere center-to-center distance error and sphere form error in the presence of each of the different geometry errors in an XCT instrument. In Part I of this work, we (1) describe the single-point ray tracing method, (2) discuss optimal placement of spheres so that sphere center-to-center distance errors and sphere form errors are sensitive to the different detector geometry errors, and (3) present data validating our method against the more conventional radiograph-based tomographic reconstruction method. In Part II of this work, we discuss optimal placement of spheres so that sphere center-to-center distance errors and sphere form errors are sensitive to error sources associated with the rotation stage. This work is in support of ongoing standards development activity within ASME and ISO for XCT performance evaluation.

Tolerance to geometrical inaccuracies in CBCT systems: A comprehensive study

“…− Geometrical phantom 1: A synthetic phantom comprising 18 spheres of 15 mm diameter (voxel size 0.134 mm isotropic), to enhance certain effects in the visual evaluation. − Geometrical phantom 2: A synthetic phantom comprising 35 spheres of 15 mm diameter distributed in five slices (voxel size 0.134 mm isotropic), designed for quantitative evaluation of undesirable effects at different points of the FOV, similar to the one proposed by Muralikrishnan 19 .…”

X-ray Computed Tomography Instrument Performance Evaluation, Part II: Sensitivity to Rotation Stage Errors

“…The negative sign of the correlation arises because moving the rotation axis closer to the source has the same effect as moving the detector away from the source; i.e., both result in an increase of the magnification factor. Figure 7(a) in Part I [1] showed the lines that are most sensitive to this error source. These are diagonal lines joining spheres farthest apart from each other and passing through the center of the measurement volume P.…”

“…At the initial angular position of the object (index angle of 0°), spheres A, C, E, and G form a plane parallel to the detector, while spheres B, D, F, and H form a plane perpendicular to the detector. Dimensional measurements of these spheres are most sensitive to the different detector geometry errors discussed in Part I [1]. These spheres are also sensitive to several rotation stage geometry errors.…”

“…Currently, standards groups within the American Society of Mechanical Engineers (ASME) and the International Organization for Standardization (ISO) are discussing test procedures to include in documentary standards for performance evaluation of X-ray computed tomography (XCT) instruments. In Part I [1] of this two-part study, we recommended optimal placement of spheres in the measurement volume so that sphere center-to-center distance errors and sphere form errors are most sensitive to geometry errors associated with the detector. We presented a novel method, called the single-point ray tracing method, that efficiently estimates these sensitivities.…”

“…(a) Eight of the 125 spheres form four face diagonals and four body diagonals that can be used to capture all detector geometry errors as shown in Part I[1] and some rotation stage errors. (b) Eight additional spheres forming four face diagonals can be used to detect the remaining rotation stage errors.…”