3D Cone-beam CT Images Research Articles

TU‐C‐351‐04: Cone Beam CT Acquisition During Volumetric Arc Radiotherapy Delivery: Correction for Induced Artifacts

Purpose: To evaluate the feasibility of kV cone beam CT (CBCT) acquisition simultaneously with volumetric modulated arc therapy (VMAT) delivery, and to test a method to correct for degradation of image quality due to VMAT delivery. Method and Materials: A commercial CBCT system was modified to enable simultaneous CBCT acquisition with VMAT delivery. CBCT scans of an image quality phantom were acquired during VMAT delivery while varying the VMAT parameters. Dose rate, energy, and field size of the VMAT delivery, and phantom size, were varied to evaluate the effect on image quality. The mean and standard deviation of the signal in a known location was quantified both in the raw 2D projection images and also in the reconstructed 3D CBCT images. A nonlinear filter was tested to remove structural artifact and noise. An analytical scatter correction model was developed and used to remove scatter generated by the VMAT beam. Results: Structural artifact was reduced in the CBCT projections with a nonlinear filter. Scatter generated from the VMAT delivery varied with field size and dose rate, and minimally with phantom size. An analytical scatter model was constructed based on the VMAT fluence (field area times dose rate) for each CBCT projection, and applied to reduce scatter per projection. Applying the model improved uniformity from 7.9% to 3.0% and improved the contrast to noise ratio from 0.97 to 1.84. Conclusion: Megavoltage scatter, and its per projection variation, is the largest component contributing to degradation of CBCT image quality during VMAT delivery. The degradation was reduced with a scatter model based on the VMAT delivery. A secondary component was structural artifact related to the repetition rate of the megavoltage beam and the readout mechanism of the kV detector. Conflict of Interest: Research sponsored by Elekta, Inc.

Deformable Model and Four-Dimensional Reconstruction for Onboard CBCT

Respiration-correlated or 4D cone-beam CT imaging has recently been shown clinically feasible, and highly desirable for target localization in stereotactic radiotherapy of lung cancer. Due to the slow rotation speed of the Linac gantry and limited clinical time, the image quality of current 4DCBCT is significantly inferior to the diagnostic 4DCT. This work investigates a new technique to improve 4DCBCT imaging.

TH-E-M100F-07: Amplitude Gated Breath-Hold Treatment for Upper Abdominal Lesions with On Board Imaging Guidance

Purpose: To apply on-boardmdash;image (OBI) guided amplitude gating for breath-hold treatment and to assess treatment margin adequacy by analyzing the isocenter placement based on orthogonal 2D kV radiographs and 3D cone-beam CT (CBCT) acquired under amplitude-gated breath-hold. Method and Materials: 25 patients with liver, pancreas, bile duct, or adrenal lesions were treated by using amplitude-gated breath-hold technique during the last 18 months. 2D orthogonal kV and 3D CBCT images were acquired under amplitude-gated breath-hold and matched to the corresponding DRRs and planning CT images, respectively, for isocenter placement. A total of 438 sets of 2D kV images and 70 sets of CBCT images were analyzed. The margin of 95% probability, which warrants that the target will be within the treatment fields with a probability of 95%, was used to quantify the margin reduction of using image-guidance for the breath-hold treatment. Results: The average isocenter shifts based on 2D kV OBI matching over the breath-hold laser alignment were 0.31 ± 0.27 cm, 0.28 ± 0.28 cm and 0.29 ± 0.28 cm along the lateral (LR), longitudinal (SI) and vertical (AP) directions, respectively. After the patients were moved to the isocenter based on 2D images, the additional shifts based on 3D CT-CBCT matching were 0.28 ± 0.28 cm, 0.30 ± 0.37 cm and 0.32 ± 0.27 cm along LR, SI, and AP directions, respectively. The 95% probability margins of daily 2D OBI matching with respect to the 3D CBCT matching were 0.6 cm, 1 cm and 0.9 cm along LR, SI and AP directions. If image guidance technique was not used, the required margins of 95% probability increased to 1.1, 1.3 and 1.2 cm along the LR, SI and AP directions. Conclusion: Amplitude gated breath-hold technique is useful for the treatment of upper abdominal lesions. Image-guidance technique substantially reduced treatment margin.

Bilateral Distomolars in the Maxilla: Computed Radiographic and Cone-beam CT Findings

Distomolar (also known as the fourth molar) is an abnormality in the number of teeth. This finding is usually present on one side of the maxilla, and is comparatively rare as a bilateral finding. We encountered a case of bilateral maxillary distomolars on pantomography. The image reading reports of pantomography and cone-beam CT were created using the reporting system of our radiology department. As we observed panoramic computed radiography, we identified bilateral impacted third molars in the maxilla, with distomolars adjacent to each. Supernumerary teeth displayed normal tooth morphology with regard to crowns and roots but were slightly smaller than the third molars. On cone-beam CT, distomolars were found behind the third molar on both right and left sides of the maxilla. Distomolars were completely impacted, and parts of the roots were near the maxillary sinus on 3D cone-beam CT images.

Four‐dimensional cone‐beam computed tomography using an on‐board imager

On-board cone-beam computed tomography (CBCT) has recently become available to provide volumetric information of a patient in the treatment position, and holds promises for improved target localization and irradiation dose verification. The design of currently available on-board CBCT, however, is far from optimal. Its quality is adversely influenced by many factors, such as scatter, beam hardening, and intra-scanning organ motion. In this work we quantitatively study the influence of organ motion on CBCT imaging and investigate a strategy to acquire high quality phase-resolved [four-dimensional (4D)] CBCT images based on phase binning of the CBCT projection data. An efficient and robust method for binning CBCT data according to the patient's respiratory phase derived in the projection space was developed. The phase-binned projections were reconstructed using the conventional Feldkamp algorithm to yield 4D CBCT images. Both phantom and patient studies were carried out to validate the technique and to optimize the 4D CBCT data acquisition protocol. Several factors that are important to the clinical implementation of the technique, such as the image quality, scanning time, number of projections, and radiation dose, were analyzed for various scanning schemes. The general references drawn from this study are: (i) reliable phase binning of CBCT projections is accomplishable with the aid of external or internal marker and simple analysis of its trace in the projection space, and (ii) artifact-free 4D CBCT images can be obtained without increasing the patient radiation dose as compared to the current 3D CBCT scan.

TH‐C‐ValB‐07: Enhanced 4D CBCT Imaging for Slow‐Rotating On‐Board Imager

Purpose: Four‐dimensional (4D) cone‐beam CT (CBCT) is currently obtained by respiratory phase binning of the acquired projections followed by independent CBCT reconstruction of the rebinned projections for each phase. Due to the significantly reduced number of projections in a single phase bin, the quality of 4D‐CBCT images is severely degraded. In this work, we develop a technique to combine the data from all phases for greatly enhanced 4D CBCT. Method and Materials: 4D‐CBCT data were acquired with “extremely‐slow‐gantry‐rotation” protocol (1 deg/sec) on a Varian Acuity system. The respiratory phase binning was achieved by automatic texture matching of the CBCT projections, and 4D‐CBCT images were subsequently obtained by Feldkamp reconstruction of the phase‐binned data. Finally, to enhance the quality, the 4D‐CBCT images were superimposed via deformable registration based on maximum mutual information and BSpline model. The method was quantitatively evaluated with numerical and physical phantom experiments. Three clinical cases are also being investigated. Results: The numerical and physical phantom studies showed that 4D CBCT resulted in less motion artifacts than 3D CBCT, however, the view‐aliasing artifacts are visible in 4D CBCT images due to limited number of projections being used for reconstructing each phase, this led to large fluctuation of the CT numbers in any uniform region and generally reduced contrast‐to‐noise ratio (CNR). In comparison, the proposed 4D CBCT technique scientifically improved the CT number accuracy, the image uniformity, and the CNRs. Conclusion: A novel technique for 4D CBCT imaging with a slow‐rotating onboard imager has been developed. It integrates information of different respiratory phases to achieve an enhanced image quality. The improvement on the CT number accuracy is extremely important to image‐guide radiation therapy for online treatment planning and off‐line dose verification.

Linac-integrated 4D cone beam CT: first experimental results

A new online imaging approach, linac-integrated cone beam CT (CBCT), has been developed over the past few years. It has the advantage that a patient can be examined in their treatment position directly before or during a radiotherapy treatment. Unfortunately, respiratory organ motion, one of the largest intrafractional organ motions, often leads to artefacts in the reconstructed 3D images. One way to take this into account is to register the breathing phase during image acquisition for a phase-correlated image reconstruction. Therefore, the main focus of this work is to present a system which has the potential to investigate the correlation between internal (movement of the diaphragm) and external (data of a respiratory gating system) information about breathing phase and amplitude using an inline CBCT scanner. This also includes a feasibility study about using the acquired information for a respiratory-correlated 4D CBCT reconstruction. First, a moving lung phantom was used to develop and to specify the required methods which are based on an image reconstruction using only projections belonging to a certain moving phase. For that purpose, the corresponding phase has to be detected for each projection. In the case of the phantom, an electrical signal allows one to track the movement in real time. The number of projections available for the image reconstruction depends on the breathing phase and the size of the position range from which projections should be used for the reconstruction. The narrower this range is, the better the inner structures can be located, but also the noise of the images increases due to the limited number of projections. This correlation has also been analysed. In a second step, the methods were clinically applied using data sets of patients with lung tumours. In this case, the breathing phase was detected by an external gating system (AZ-733V, Anzai Medical Co.) based on a pressure sensor attached to the patient's abdominal region with a fixation belt. The comparison of the reconstructed 4D CBCT images and the corresponding 4D CT images used for the treatment planning provides the required information for the calculation of possible setup errors. So, a repositioning of the patient is feasible even though the patient moves due to respiration. In addition to the external signal, the position of the diaphragm in the cranial-caudal direction could be extracted from each projection. Both independent sources of information show a very good agreement of the phase and even the amplitude of the movement and the external signal respectively. This suggests the usability of such a system for a gated dose delivery approach. However, more studies involving patients with different incidences have to be carried out to confirm these first results.