Development and first use of a novel cylindrical ball bearing phantom for 9-DOF geometric calibrations of flat panel imaging devices used in image-guided ion beam therapy

Abstract

Image guidance during highly conformal radiotherapy requires accurate geometric calibration of the moving components of the imager. Due to limited manufacturing accuracy and gravity-induced flex, an x-ray imager's deviation from the nominal geometrical definition has to be corrected for. For this purpose a ball bearing phantom applicable for nine degrees of freedom (9-DOF) calibration of a novel cone-beam computed tomography (CBCT) scanner was designed and validated. In order to ensure accurate automated marker detection, as many uniformly distributed markers as possible should be used with a minimum projected inter-marker distance of 10 mm. Three different marker distributions on the phantom cylinder surface were simulated. First, a fixed number of markers are selected and their coordinates are randomly generated. Second, the quasi-random method is represented by setting a constraint on the marker distances in the projections. The third approach generates the ball coordinates helically based on the Golden ratio, ϕ. Projection images of the phantom incorporating the CBCT scanner's geometry were simulated and analysed with respect to uniform distribution and intra-marker distance. Based on the evaluations a phantom prototype was manufactured and validated by a series of flexmap calibration measurements and analyses. The simulation with randomly distributed markers as well as the quasi-random approach showed an insufficient uniformity of the distribution over the detector area. The best compromise between uniform distribution and a high packing fraction of balls is provided by the Golden section approach. A prototype was manufactured accordingly. The phantom was validated for 9-DOF geometric calibrations of the CBCT scanner with independently moveable source and detector arms. A novel flexmap calibration phantom intended for 9-DOF was developed. The ball bearing distribution based on the Golden section was found to be highly advantageous. The phantom showed satisfying results for calibrations of the CBCT scanner and provides the basis for further flexmap correction and reconstruction developments.

Implementation of dosimetry equipment and phantoms at the MedAustron light ion beam therapy facility

“…Thimble ionization chambers with intermediate collection volumes such as Semiflex type chambers (PTW, Freiburg) can also be used for dosimetry purposes, for instance, for output factor measurements. In order to check the integrity of the ionization chambers, x‐ray images of all chambers were acquired in a newly developed Imaging Ring System available at MedAustron, which is a single source dual energy tube and flat panel detector, integrated with a robotic patient positioning system. The ICs were placed as close as possible to the x‐ray source in order to increase the magnification.…”

An EPID‐based method to determine mechanical deformations in a linear accelerator

“…The presence of the head mount accessory was not necessary in this scenario since the beam collimation is not of interest. In this case, the problem of CT geometric calibration has already seen complete solutions in the form of 9 DOF calibrations . The addition of the kV isocenter verification would use the same setup as the one described in this work, and the resulting images could be analysed with the same software with minimal modifications.…”

“…In this case, the problem of CT geometric calibration has already seen complete solutions in the form of 9 DOF calibrations. 14,15 The addition of the kV isocenter verification would use the same setup as the one described in this work, and the resulting images could be analysed with the same software with minimal modifications. However, sequential kV and MV imaging would add time to the QA procedure.…”

A novel bone suppression algorithm in intensity‐based 2D/3D image registration for real‐time tumor motion monitoring: Development and phantom‐based validation

“…A set of CT scans was acquired on a Philips Brilliance Big Bore CT scanner (Philips Medical Systems, Inc., Cleveland, OH, USA) using the following settings: 120 kVp, 50 mAs, and slice thickness of 0.75 mm. Additionally, X‐ray images of a CC tumor motion of 50 mm with a 2 mm step size were acquired on the MedPhoton ImagingRing 39 (MedPhoton GmbH, Salzburg, Austria) at PA and LAT imaging angles (120 kVp, 25 mA, and 20 ms). All CT and X‐ray image acquisitions were performed with and without the ribcage inside of the ARDOS phantom to obtain a ground truth scenario for BS.…”