Self-calibrated surface acquisition for integrated positioning verification in medical applications

Abstract

This paper presents a novel approach for a position verification system in medical applications. By replacing the already existing cross line laser projectors with galvoor MEMS-based projectors and utilizing the surveillance cameras, a self-calibration of the system is performed and surface acquisition for positioning verification is demonstrated. The functionality is shown by analyzing the radii of calibration spheres and determining the quality of the captured surface with respect to a reference model. The paper focuses on the demonstration with one pair of camera and projector but can also be extended to a multi-camera-projector system, as present in treatment rooms. Compared to other systems, this approach does not need external hardware and is thus space and cost efficient. Introduction Nowadays, a wide range of medical applications demand accurate patient positioning for a successful treatment. While the positioning for X-ray imaging allows tolerances of several millimeters since typically rather big areas are imaged, the required accuracy for CT-imaging and especially classical radiation therapy, Volumetric Arc Therapy (VMAT), Intensity-Modulated Radiation Therapy (IMRT) and 3D Conformal Radiation Therapy (3D CRT) for cancer treatment is much higher. The goal of radiation therapy is to damage the cancer cells as much as possible, while keeping the amount of radiation within the surrounding tissue to an absolute minimum. The de facto standard procedure for patient positioning in radiation therapy is as follows: An initial CT scan is performed to gather anatomical data for the treatment. Here, markers are placed on the patients skin, which are later used to align the patient with the orthogonal line lasers in the treatment room. In a previous step, those line lasers are calibrated to directly intersect in the linear accelerators (linacs) ”isocenter”, the point where the beams of the rotating linac intercept and therefore the radiation intensity is at its peak. The calibration of the isocenter is done performing the Winston-Lutz test. Once the isocenter is calibrated and the patient aligned, the treatment is started. Typically, the initial CT scans outcome is used for several radiation therapy sessions, so are the markers. Fig. 1 shows a typical treatment room with patient couch, gantry, red room lasers for positioning and a test phantom. The importance of precise patient positioning and the potential of optical surface imaging technologies for both positioning and respiratory gating is becoming more and more clear and was recently confirmed and discussed by publications such as [1], [2] and [3]. This paper provides a new method of verifying the patients Figure 1. Radiation-therapy room with gantry and positioning lasers (red) position with respect to the linacs isocenter. A typical treatment room already consists of multiple cameras for surveillance and line lasers for calibration, isocenter visualization and patient positioning. By replacing those static line lasers with galvoor MEMS-based laser projectors and combining the laser projectors with the cameras to active stereo systems, multiple applications are imaginable, while the main functionality of positioning the patient manually with respect to the laser crosses and thus, the isocenter, is still provided: 1. The extrinsic calibration of the system is performed automatically, making the handling easy and self-verifying. 2. The patient’s position is acquired by scanning the surface using a shape reconstruction method based on light sectioning. 3. The position can then be matched to the data from the CTscan, giving a translation vector for shifting the position of the patient by adjusting the treatment bench. 4. Additionally, respiratory gating can be performed to increase the efficiency of the therapy and thus, protecting the surrounding tissue during treatment. Since no CT-scan data or with the Winston-Lutz test aligned set of projectors was present and respiratory gating is still considered to be future work, this paper focuses on the first two applications and demonstrates the functionality by means of a test setup consisting of one camera-projector-pair. Although the proposed self-calibrating projector-camera setup is applicable for shape acquisition in general when used as a laser-line based structured light system, this paper only focuses IS&T International Symposium on Electronic Imaging 2019 Photography, Mobile, and Immersive Imaging 2019 353-1 https://doi.org/10.2352/ISSN.2470-1173.2019.4.PMII-353 © 2019, Society for Imaging Science and Technology