We have studied the feasibility of a low-dose megavoltage cone beam computed tomography (MV CBCT) system for visualizing the gross tumor volume in respiratory gated radiation treatments of nonsmall-cell lung cancer. The system consists of a commercially available linear accelerator (LINAC), an amorphous silicon electronic portal imaging device, and a respiratory gating system. The gantry movement and beam delivery are controlled using dynamic beam delivery toolbox, a commercial software package for executing scripts to control the LINAC. A specially designed interface box synchronizes the LINAC, image acquisition electronics, and the respiratory gating system. Images are preprocessed to remove artifacts due to detector sag and LINAC output fluctuations. We report on the output, flatness, and symmetry of the images acquired using different imaging parameters. We also examine the quality of three-dimensional (3D) tomographic reconstruction with projection images of anthropomorphic thorax, contrast detail, and motion phantoms. The results show that, with the proper choice of imaging parameters, the flatness and symmetry are reasonably good with as low as three beam pulses per projection image. Resolution of 5% electron density differences is possible in a contrast detail phantom using 100 projections and 30 MU. Synchronization of image acquisition with simulated respiration also eliminated motion artifacts in a moving phantom, demonstrating the system's capability for imaging patients undergoing gated radiation therapy. The acquisition time is limited by the patient's respiration (only one image per breathing cycle) and is under 10 min for a scan of 100 projections. In conclusion, we have developed a MV CBCT system using commercially available components to produce 3D reconstructions, with sufficient contrast resolution for localizing a simulated lung tumor, using a dose comparable to portal imaging.
In external beam radiotherapy, electronic portal imaging becomes more and more an indispensable tool for the verification of the patient setup. For the safe clinical introduction of high dose conformal radiotherapy like intensity modulated radiation therapy, on-line patient setup verification is a prerequisite to ensure that the planned dosimetric coverage of the tumor volume is actually realized in the patient. Since the direction of setup fields often deviates from the direction of the treatment beams, extra dose is delivered to the patient during the acquisition of these portal images which may reach clinical relevance. The aim of this work was to develop a new acquisition mode for the PortalVision aS500 electronic portal imaging device from Varian Medical Systems that allows one to take portal images with reduced dose while keeping good image quality. The new acquisition mode, called RadMode, selectively enables and disables beam pulses during image acquisition allowing one to stop wasting valuable dose during the initial acquisition of "reset frames." Images of excellent quality can be taken with 1 MU only. This low dose per image facilitates daily setup verification with considerably reduced extra dose.
Purpose: To develop the helical cone‐beam scanning capability on a simulator CT system and apply a recently developed backprojection‐filtration (BPF) algorithm to reconstruct exact 3D images from data obtained in this system. Method and Materials: Helical cone‐beam CT is an emerging technology with many advantages over conventional CT scans, such as fast volume coverage speed, efficient use of x‐ray power, and sufficient data for exact 3D image reconstruction. We developed the helical cone‐beam capability on a simulator CT device (Acuity, Varian Medical Systems), which includes a kV x‐ray source, a patient couch, and an amorphous silicon flat‐panel detector (Varian PaxScan 4030CB). We fabricated a small motorized leadscrew mechanism on the couch of the Acuity, which allows for longitudinal translation of the patient couch at a constant speed as the gantry rotates. We applied the BPF algorithm to reconstruct exact 3D images from data obtained in this system. Results: A CatPhan phantom was used for data acquisition. 683 projection data were collected during a one‐turn gantry rotation while the couch was translated longitudinally with a helical pitch of 187.2 mm. The x‐ray source was operated at 125 kVp and 560 mAs. Before projection data were used for reconstruction, a number of corrections were performed, such as bad pixel, dark field, flood field, detector sag. We also applied simple corrections for beam hardening and scattering. 3D images were reconstructed from the corrected data by use of the BPF algorithm. Conclusion: We have developed for the first time the helical scanning capability on a simulator cone‐beam CT system and performed a preliminary phantom study. The BPF algorithm was used to reconstruct exact 3D images from the helical cone‐beam data. Such approach can readily be extended to cone‐beam CT components on a LINAC to provide more accurate image representations of the patient.
In many implementations of cone-beam CT in radiotherapy for target positioning, it is not uncommon that the maximum allowable field of view (FOV) cannot cover the patient because of the limited size of the flat-panel detector. In this situation, the measurements will contain truncated projections, leading to significant artifacts in reconstructed images. Asymmetric conebeam configurations can be used for increasing the FOV size by displacing the detector panel to one side. From the data acquired with such an asymmetric configuration, the well-known algorithm developed by Feldkamp, Davis, and Kress (FDK) can be modified to reconstruct images. With increasing detector asymmetry, however, the modified FDK algorithm may produce significant aliasing artifacts. In this work, we develop a novel algorithm for image reconstruction in asymmetric cone-beam CT, which can generate images with improved numerical properties and allow for large detector asymmetry. We have employed the asymmetric configuration and the developed algorithm in a cone-beam CT system in radiotherapy to increase the FOV size. Preliminary phantom studies have been conducted to validate the asymmetric configuration and the proposed reconstruction algorithm.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
customersupport@researchsolutions.com
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.