Automatic Geometry Calibration of X-Ray Equipment for Image Guided Radiotherapy

Purpose: A growing number of commercial health centers employ particleradiation for external tumor treatment. Allowing highly accuratedose delivery onto the carcinogen tissue these techniques are ofgreat benefit for the patients, but also require high precision concerningthe patient alignment. Position correction approaches comparing X-rayimages of the current patient position to a reference CT series allowfinal treatment set-up preciseness of less than 1 mm in the tumorarea. However, image based patient alignment leads to inaccuraciesif the geometric model for the equipment is not defined properlyor up-to-date. Especially the accuracy of X-ray based systems withdigital flat panels mounted in a rotating gantry suffers from changesof the geometry of the radiographic axes in different gantry positions.Even though the changes of the gantry structure usually are in thesub-millimeter range the effects are visible in the X-ray imagesacquired for the position correction. If these effects are not modeledin the process of DRR rendering both types of images cannot reallybe compared, which results in a significant error in the mutual informationbased automatic position correction approach. To cover all sourcesof geometric deviation, the calibration has to be performed for severalgantry angles and possibly for a set of different snout positions.To enable accurate and reliable patient alignments using image guidedprocedures, we propose an approach for the automatic calibrationof the geometric models of the X-ray equipment and show how the calibratedgeometry is used in a patient alignment application. Methods: The calibration is performed every six months. We use a specialdesigned calibration phantom consisting of a rigid body with a numberof equally distributed metallic spheres that can easily be recognizedin X-ray images, even from different acquisition angles. The routineitself can be described in several steps. First the phantom is placedon the treatment table with the central sphere located at the isocenterand its axes collinear to the axes of the treatment device. In thenext step images are acquired for each beamline at a given set ofgantry angles and snout positions. As the device geometry may deviatedepending on the rotation direction, each gantry angle has to beapproached from two sides. For each X-ray image a model of the metalspheres is computationally projected onto the flat panel plane, usingthe assumed geometric set-up of X-ray tube and panel. The real spherepositions are segmented from the image by a template matching method.By minimizing the offsets between the assumed and the real positionsall nine degrees of freedom (panel shifts and rotations, tube shifts)of the modeled beamline geometry can be adapted to the real geometry.During the alignment correction, the device set-up is given by thetreatment plan. Suitable sets of calibration values are chosen fromthe calibrated configurations and interpolated to minimize the geometricerror for the current device setting. The corrected beamline is finallyutilized in the computation of DRRs, which are matched to X-ray imagesto determine the patient alignment.Results: Tests have been performed in three different gantry constructionsusing Alderson phantom data for different body parts (head, thorax,pelvis). During the beamline calibration aberrations of the initialgeometry could be determined, reaching up to 5 mm panel shift alongthe main radiographic axis. Without calibrated beamlines the patientalignment procedure was able to achieve accuracies of about ±3.0mm. With calibrated beamlines the patient alignment accuracy couldbe increased dramatically so that deviations of less than ±0.5 mmfrom the perfect alignment could be determined. Conclusions: Through geometry calibration of the beamlines the accuracyfor the patient alignment can be raised dramatically, whereas theinitial set-up of the geometry was often not acceptable. Our calibrationprocedure could reduce patient alignment errors and can additionallyserve as an indicator for the geometric accuracy of the treatmentdevice. Comparison of patient alignment results for calibrated andun-calibrated X-ray equipment shows that an adequate calibrationroutine is indispensable.     «

Purpose: A growing number of commercial health centers employ particleradiation for external tumor treatment. Allowing highly accuratedose delivery onto the carcinogen tissue these techniques are ofgreat benefit for the patients, but also require high precision concerningthe patient alignment. Position correction approaches comparing X-rayimages of the current patient position to a reference CT series allowfinal treatment set-up preciseness of less than 1 mm in the tumorarea. However, image base...     »