Practical workflow for arbitrary non-circular orbits for CT with clinical robotic C-arms

Abstract

Non-circular orbits in cone-beam CT (CBCT) imaging are increasingly being studied for potential benefits in field-of-view, dose reduction, improved image quality, minimal interference in guided procedures, metal artifact reduction, and more. While modern imaging systems such as robotic C-arms are enabling more freedom in potential orbit designs, practical implementation on such clinical systems remains challenging due to obstacles in critical stages of the workflow, including orbit realization, geometric calibration, and reconstruction. In this work, we build upon previous successes in clinical implementation and address key challenges in the geometric calibration stage with a novel calibration method. The resulting workflow eliminates the need for prior patient scans or dedicated calibration phantoms, and can be conducted in clinically relevant processing times.

Fully automatic online geometric calibration for non‐circular cone‐beam CT orbits using fiducials with unknown placement

“…[1][2][3][4][5][6][7][8] In recent years, non-circular scan orbits have been increasingly investigated for potential benefits such as increased field of view (FOV), [9][10][11][12] improved image quality and/or dose reduction, 7,[12][13][14][15] collision avoidance, 14,16 accom-modating upright patient positions [17][18][19] and reduced metal artifacts. 10,[20][21][22][23][24][25][26][27][28] While studies have demonstrated sufficient reproducibility for standard circular scanning orbits, [29][30][31][32] current implementations of non-circular orbits often result in a less predictable geometry due to factors such as gantry jitters, sagging, and manual data acquisition, and so forth. 8,12,[24][25][26][27]33,34 Since an accurate CBCT geometry is critical for the image quality of 3D reconstructions, we require a geometric calibration method that can address these challenges in practical implementations of non-circular or...…”

“…10,[20][21][22][23][24][25][26][27][28] While studies have demonstrated sufficient reproducibility for standard circular scanning orbits, [29][30][31][32] current implementations of non-circular orbits often result in a less predictable geometry due to factors such as gantry jitters, sagging, and manual data acquisition, and so forth. 8,12,[24][25][26][27]33,34 Since an accurate CBCT geometry is critical for the image quality of 3D reconstructions, we require a geometric calibration method that can address these challenges in practical implementations of non-circular orbits. Specifically, the method should require no auxiliary instruments, accommodate non-circular geometries, provide frameby-frame geometric parameters to address frame-wise jitters,and be fast enough for the intraoperative workflow.…”

“…Fiducial-based methods localize the 2D coordinates of fiducial markers from the projection images and perform calibration-related calculations only on the coordinates. 26,35,36,40,42,[46][47][48][49][50][51] These operations are typically less computationally intensive than image-based methods, but the existing methods typically assume accurate localization of fiducials in projection images, which may be difficult when the background is complex. Methods using a known target assume that the 3D structure of the imaging target is precisely known a priori, for example, from a digital model or prior scans, as is the case with aforementioned dedicated calibration phantoms and 3D-2D registration methods.…”

“…Methods using a known target assume that the 3D structure of the imaging target is precisely known a priori, for example, from a digital model or prior scans, as is the case with aforementioned dedicated calibration phantoms and 3D-2D registration methods. Conversely, methods using an unknown target are suitable for fiducials placed in an unknown configuration and for cases where a patient has not previously been scanned or presents major anatomical changes from prior scans, but these methods typically require precise localization of fiducials in projection images prior to the geometric calibration, which is practically challenging when the fiducials and patient are simultaneously imaged, 26,42,52 or optimization of 3D image metrics. 45 Some newer methods are addressing some of these intrinsic limitations.…”

“…In our previous investigations of practical implementation of non-circular orbits, we tested an imagedbased 3D-2D registration method 33 and a fiducial-based method. 26,42 The 3D-2D registration method required no fiducials,but it required a prior scan of the imaging target and high computation time. The fiducial-based method used radiopaque beads placed directly on the imaging target with an unknown configuration and was fast.…”