Conventional CBCT Research Articles

SU-D-17A-02: Four-Dimensional CBCT Using Conventional CBCT Dataset and Iterative Subtraction Algorithm of a Lung Patient

Purpose: The Iterative Subtraction Algorithm (ISA) method generates retrospectively a pre-selected motion phase cone-beam CT image from the full motion cone-beam CT acquired at standard rotation speed. This work evaluates ISA method with real lung patient data. Methods: The goal of the ISA algorithm is to extract motion and no- motion components form the full reconstruction CBCT. The workflow consists of subtracting from the full CBCT all of the undesired motion phases and obtain a motion de-blurred single-phase CBCT image, followed by iteration of this subtraction process. ISA is realized as follows: 1) The projections are sorted to various phases, and from all phases, a full reconstruction is performed to generate an image CTM. 2) Generate forward projections of CTM at the desired phase projection angles, the subtraction of projection and the forward projection will reconstruct a CTSub1, which diminishes the desired phase component. 3) By adding back the CTSub1 to CTm, no motion CBCT, CTS1, can be computed. 4) CTS1 still contains residual motion component. 5) This residual motion component can be further reduced by iteration. The ISA 4DCBCT technique was implemented using Varian Trilogy accelerator OBI system. To evaluate the method, a lung patient CBCT dataset was used. The reconstruction algorithm is FDK. Results: The single phase CBCT reconstruction generated via ISA successfully isolates the desired motion phase from the full motion CBCT, effectively reducing motion blur. It also shows improved image quality, with reduced streak artifacts with respect to the reconstructions from unprocessed phase-sorted projections only. Conclusion: A CBCT motion de-blurring algorithm, ISA, has been developed and evaluated with lung patient data. The algorithm allows improved visualization of a single phase motion extracted from a standard CBCT dataset. This study has been supported by National Institute of Health through R01CA133539.

Comparative study of Puricelli’s panorametry in conventional panoramic radiography and cone beam computed tomography panoramic reconstruction

Objectives: to compare the reproducibility of Puricelli’s panorametry on conventional panoramic radiography and on CBCT reformatted panoramic images, acquired in MIP mode. This study also compared linear and angular measurements between both exams. Methods: were evaluated 54 pairs of conventional panoramic radiography and CBCT panoramic MIP reconstructions. panorametry was manually traced, in both films, by three examiners. The originally proposed linear and angular, dental and skeletal measurements where tabulated and compared between imaging methods. Inter-examiner reproducibility was tested by intraclass correlation coefficient, and the differences between measurements of both methods were analyzed by Student’s t test. Results: the inter-examiner correlation was high for both the conventional panoramic radiography and CBCT panoramic MIP reconstructions (0.86±0.07 and 0.88±0.07, respectively). Total magnification of pano-ramic radiograph, for the studied measurements, was of 4.3%, being vertically higher (12%) and to the horizontal inferior line (11.5%) comparing to the angular measurements of straight lines (6.7%) and to the horizontal superior lines (1.9%). The conventional panoramic radiography, in comparison to the CBCT, presented lower values for most angular measurements. Conclusions: Puricelli’s panorametry presents a high degree of reproducibility in both conventional panoramic radiography and CBCT reformatted panoramic images. Conventional panoramic radiography presents augmented linear and angular skeletal measurements and reduced dental angular measurements when compared to CBCT reformatted panoramic images. Conventional panoramic radiographs seem to present a consistent augmentation of the right side of the mandibular image.

WE-G-141-08: Simultaneous Reduction of Radiation Dose and Scatters for CBCT by Using Collimators

Purpose: On-board CBCT has been widely available in radiotherapy clinic for target localization. However, the extra radiation dose is always a concern for its frequent use. Additionally, the relatively large scatters in CBCT often impair the image quality. By using collimators/grids, some of the X-rays are stopped from reaching the patient and the detectors, hence both the scatters and the patient dose are simultaneously reduced. We show in this work that the data can still be reconstructed without any noticeable artifacts. We further investigate the relationship between the CBCT image quality and the amount of beams being blocked by collimators. Methods: A CBCT system with collimators was simulated following the typical geometry used in clinic. Different collimator designs were tested by varying the size and the number of the collimator holes, and at the same time, the ratio between transmitted beams to blocked beams was varied from 10 to 1/10, resulting in hundreds of different simulation scenarios. The simulated projections for lung and pelvis phantoms were reconstructed using the total variation technique. The image quality was examined by root mean square errors (RMSEs) and compared with the conventional CBCT image. Results: The CBCT image quality increases as the amount of beams passing through the collimators increases, and decreases as the size of the collimator holes increases while keeping the beam transmission rate constant. With ultra-high resolution collimators, the RMSEs were comparable to the conventional CBCT image until the beam transmission rate is reduced below 20%. Conclusion: Both the scatters and radiation dose can be dramatically reduced for CBCT by using collimators, while the image quality remains similar to the conventional CBCT. With high resolution collimators, the patient dose reduction can easily reach 1/4 of the conventional CBCT.

TH‐C‐103‐05: A Novel Method to Improve Single Phase Reconstruction Quality From Conventional CBCT Dataset

Purpose: to present a method to generate retrospectively a pre‐selected motion phase cone‐beam CT image from the full motion cone‐beam CT acquired at standard rotation speed. Materials and Methods: the method consists of subtracting from the full CBCT all of the undesired motion phases and obtain a motion de‐blurred single‐phase CBCT image CTnm. This is accomplished by means of a subtraction CBCT (CTs) added to the full CBCT (CTm). CTs is obtained as follows: i) we sort the CBCT projections into each phase bin k according to the known motion trace; ii) compute the digitally reconstructed radiographs (DRR) from the full reconstruction at the gantry angles correlated to bin k; iii) generate a set of subtraction projections for phase k by subtracting the DRR from the original projection at the same gantry angle; v) reconstruct CTs from this single‐phase subtraction projection set; vi) add CTs to CTm to obtain CTnm. A qualitative assessment of the image quality improvement was performed using a simple shapes phantom. To quantitatively study the method, we used the image quality modules of a Catphan phantom, evaluated in a static scan and under anterior‐posterior motion. Results: The single phase CBCT reconstruction generated by our method successfully isolates the desired motion phase from the full motion CBCT, effectively reducing motion blur. It also shows improved image quality, with reduced streak artifacts with respect to the reconstructions from unprocessed phase‐sorted projections only. Conclusions: A novel CBCT motion de‐blurring algorithm has been developed and tested with phantom data. The algorithm allows to improve visualization of a single phase motion extracted from a standard CBCT dataset. NIH R01CA133539

SU‐E‐J‐04: On‐Treatment Volumetric Imaging During Respiratory Gated VMAT

Purpose: To obtain on‐treatment volumetric patient anatomy during respiratory gated VMAT. Methods: On‐board imaging device integrated with Linacs offers a viable tool for obtaining patient anatomy during radiation treatment delivery.In this study, we acquired beam‐level kV images during gated VMAT treatments using a Varian TrueBeam\texttrademark STx Linac. These kV projection images are triggered by a respiratory gating signal and can be acquired immediately before treatment MV beam on at every breathing cycle during delivery. Because the kV images are acquired with an on‐board imaging device during a rotational arc therapy, they provide the patient anatomical information from many different angles or projection views (typically 20– 40). To reconstruct the volumetric image representing patient anatomy during the VMAT treatment, we used a compressed sensing method with a fast first‐order optimization algorithm. The conventional FDK reconstruction was also used for comparison purposes. The method was tested on a dynamic anthropomorphic physical phantom as well as a lung patient. Results: The reconstructed volumetric images for a dynamic anthropomorphic physical phantom and a lung patient showed clearly visible soft‐tissue target as well as other anatomical structures,with the proposed compressed sensing‐based image reconstruction method. Compared with FDK, the compressed sensing method leads to a ∼2‐and 3fold increase in contrast‐to‐noise ratio around the target area in the phantom and patient case,respectively. Conclusion: The proposed technique provides on‐treatment volumetric patient anatomy, with only a fraction (<10%) of the imaging dose used in conventional CBCT procedures. This anatomical information may be valuable for geometric verification and treatment guidance, and useful for verification of treatment dose delivery, accumulation, and adaptation in the future.

First study of on‐treatment volumetric imaging during respiratory gated VMAT

To obtain on-treatment volumetric patient anatomy during respiratory gated volumetric modulated arc therapy (VMAT). On-board imaging device integrated with Linacs offers a viable tool for obtaining patient anatomy during radiation treatment delivery. In this study, the authors acquired beam-level kV images during gated VMAT treatments using a Varian TrueBeam™STx Linac. These kV projection images are triggered by a respiratory gating signal and can be acquired immediately before treatment MV beam on at every breathing cycle during delivery. Because the kV images are acquired with an on-board imaging device during a rotational arc therapy, they provide the patient anatomical information from many different angles or projection views (typically 20-40). To reconstruct the volumetric image representing patient anatomy during the VMAT treatment, the authors used a compressed sensing method with a fast first-order optimization algorithm. The conventional FDK reconstruction was also used for comparison purposes. The method was tested on a dynamic anthropomorphic physical phantom as well as a lung patient. The reconstructed volumetric images for a dynamic anthropomorphic physical phantom and a lung patient showed clearly visible soft-tissue target as well as other anatomical structures, with the proposed compressed sensing-based image reconstruction method. Compared with FDK, the compressed sensing method leads to a ≈ two and threefold increase in contrast-to-noise ratio around the target area in the phantom and patient case, respectively. The proposed technique provides on-treatment volumetric patient anatomy, with only a fraction (<10%) of the imaging dose used in conventional CBCT procedures. This anatomical information may be valuable for geometric verification and treatment guidance, and useful for verification of treatment dose delivery, accumulation, and adaptation in the future.

Respiratory triggered 4D cone‐beam computed tomography: A novel method to reduce imaging dose

A novel method called respiratory triggered 4D cone-beam computed tomography (RT 4D CBCT) is described whereby imaging dose can be reduced without degrading image quality. RT 4D CBCT utilizes a respiratory signal to trigger projections such that only a single projection is assigned to a given respiratory bin for each breathing cycle. In contrast, commercial 4D CBCT does not actively use the respiratory signal to minimize image dose. To compare RT 4D CBCT with conventional 4D CBCT, 3600 CBCT projections of a thorax phantom were gathered and reconstructed to generate a ground truth CBCT dataset. Simulation pairs of conventional 4D CBCT acquisitions and RT 4D CBCT acquisitions were developed assuming a sinusoidal respiratory signal which governs the selection of projections from the pool of 3600 original projections. The RT 4D CBCT acquisition triggers a single projection when the respiratory signal enters a desired acquisition bin; the conventional acquisition does not use a respiratory trigger and projections are acquired at a constant frequency. Acquisition parameters studied were breathing period, acquisition time, and imager frequency. The performance of RT 4D CBCT using phase based and displacement based sorting was also studied. Image quality was quantified by calculating difference images of the test dataset from the ground truth dataset. Imaging dose was calculated by counting projections. Using phase based sorting RT 4D CBCT results in 47% less imaging dose on average compared to conventional 4D CBCT. Image quality differences were less than 4% at worst. Using displacement based sorting RT 4D CBCT results in 57% less imaging dose on average, than conventional 4D CBCT methods; however, image quality was 26% worse with RT 4D CBCT. Simulation studies have shown that RT 4D CBCT reduces imaging dose while maintaining comparable image quality for phase based 4D CBCT; image quality is degraded for displacement based RT 4D CBCT in its current implementation.

Prospectively Gated CBCT for Volumetric Image Guidance in SBRT

Two techniques for PG-CBCT have been implemented on a traditional LINAC geometry, and shown effective in the suppression of respiratory motion artifacts. Relative to retrospective 4D-CBCT techniques, PG-CBCT enables reduction of the CBCT imaging dose by acquisition of projections only at desired phases. Based on this work, a continuous method is recommended for clinical applications due to current mechanical limitations in sequential gantry rotation. Upper: A conventional non-gated CBCT scanner reconstruction of the moving phantom. Lower: Reconstruction of the moving phantom with the continuous prospectively gated implementation for the materials section (left) and resolution section (right) of the CATPHAN 504. Arrows indicated the finest discernable resolution of 6 lp/mm which is only resolved in the gated reconstruction. Using a triggered LINAC based kilovoltage OBI, two general methods for PG-CBCT were implemented and crossevaluated: 1) a sequential method, in which gantry rotation and acquisition only occur during an active gating window, and 2) a continuous method, in which the gantry continuously rotates at a slow speed throughout all portions of the breathing cycle and the source pulsed on during valid gates. As currently no clinical system allows for such PG-CBCT acquisition, custom XML scripts in conjunction with the Developer Mode functionality of the Varian TrueBeam STx’s were utilized to implement the techniques.

SU‐E‐J‐179: Clinical Application of 40o Very Limited Angle CBCT (VLA CBCT) for Target Localization of Lung Cancer Patient Who Is Treated with SBRT and ABC in Conventional Radiation Therapy

Purpose: 40o VLA CBCT is used to verify the target position of lung cancer patient treated with x‐ray Stereotactic Body Radiation Therapy (SBRT) and Automatic Breath Coordinator (ABC) using Elekta SynergyTM linear accelerator, and XVITM CBCT. Methods: Limited Angle CBCT (LA CBCT) with 40o scanning angles is determined by (1) the maximum time the patient can hold breath (15–20 seconds) under ABC technique, and (2) the minimum scan angles to reconstruct the target with minimum artifacts due to incomplete projections, and (3) the start and stop angles of the LA CBCT, which is determined by the location of the tumor as well as geometry of the lung. Based upon the phantom study, 40o CBCT (with S20 collimator and H&amp;N reconstruction option) starting from 160 to 200 is the optimum to reconstruct the target in coronal images. In the range of these angles, the target is minimally over‐shadowed by the vertebral bodies and mediastinum, thus the target can be determined in the CBCT images. Free‐breathing (FB) CBCT is taken to determine the maximum range of the target movements, and 4 sets of VLA CBCT are taken to investigate the reproducibility of the VLA CBCT procedure.Results: The target movements with FB CBFT are ∼1cm in lateral and AP direction, and 2.5cm in Sup/Inf direction. The average motions during 4 breadth‐hold procedures are less than 5mm, which is on the order of uncertainties in intra/inter user variation. Also, VLA CBCTˈs are taken every 3 fields of the treatment. Their movements are the same ranges. Conclusions: The VLA CBCT is very effective to evaluate the target movements in the middle of the lung and to acquire images (20 seconds for VLA CBCT vs 2minutes for conventional 360o CBCT). Also, patient dose is reduced from 3–4cGy with standard CBCTto&lt;0.1cGy.

MO-D-BRC-01: Dosimetric Evaluation of a Novel Cone-Beam CT (CBCT) Estimation Method Using Prior Information for Lung Cancer Patient Treatment

Purpose: To evaluate the dosimetric accuracy of a novel CBCT estimation method using prior information for use in adaptive radiotherapy (ART)‐based treatment of lungcancer patients. Methods: Patient planning CT or previous day's CBCTimages are used as the prior images. The new CBCTimages are considered as a deformation of the prior images. The deformation field is solved by meeting the data fidelity constraint and minimizing the deformation energy of the field. The new CBCTimages are then obtained by deforming the prior images according to the deformation field. Data for ten lungcancer patients were tested for estimation using 57 projections acquired over 60° and 360° scan angles. The new CBCT reconstructed from fully sampled projections were used as the ground truth. Geometric accuracy of the estimation was evaluated using tumor volume percentage error. Dosimetric accuracy was evaluated by comparing the PTV dose distribution in each set of images. Results: The tumor volume percentage error was 13.3% and 4.3% in CBCT estimated using 57 projections over 60° and 360° scan angles, respectively. PTV min., mean and max. doses in estimated CBCT agree with those in true CBCT within 2% and 1% for 60° and 360° scan angles, respectively. The percentage volume of PTV covered by prescription dose in estimated CBCT agrees with that in true CBCT within 4% and 1% for 60° and 360° scan angles, respectively. The isodose lines in the estimated CBCTimages also match well with those in the true CBCTimages. Conclusions: The proposed method is able to provide accurate guidance for plan adaptation in ART treatment of lungcancer patients using 57 projections acquired over 360° scan angle. This implies a 12‐fold reduction in imagingdose relative to conventional CBCT for similar image quality for target delineation and dose calculation in ART.

SU‐GG‐T‐348: Development of a Precise Position‐Controlled Moving Phantom

Purpose: Current commercially available moving phantoms are limited in their motions and accuracy of positions. The purpose of this study is to develop a moving phantom with highly accurate position control (error less than 0.2 mm) in 3 directions so that it can reproduce most of patients' tumor motions obtained by 4D‐CT. Method and Materials: The moving phantom consisted of two parts: thorax phantom and 3D‐axial control unit. The thorax phantom was made of transparent resin, which has an electron density of 1.04. The plaster bones were structured by ink‐jet printing method, and were embedded in the resin. The 3D‐axial control unit was designed to achieve any motion less than 10 cm/sec in x, y and z directions in the region of 150×350×250 mm which was a cavity in the thorax phantom. Two acryl hemispheres with a radius of 1 cm were set on the tip of the 3D‐axial control unit. The motion imitating respiratory tumor motion or sine curve was input, and the motion of the target was measured by a laser displacement gauge to evaluate the accuracy. The target motion was also captured by conventional CT and CBCT to evaluate the performance. Results: The target motion was as accurate as less than 0.1mm in the case of sine waves (1 cm, 12 bpm). In the case of patient's tumor motion (up to 7 mm, 7.5 bpm), although the motion was not modified with smoothing, the error was less than 0.2 mm. The conventional CT and CBCT images reflected the motion of the target. Conclusions: We developed a novel moving phantom with a high accuracy in position control. This moving phantom could be used for evaluation of 4DRT techniques.

Fronto-orbital sphenoethmoidal fibrous dysplasia

Cranial fibrous dysplasias (FDs) are rare and comprise less than 1% of all primary bone lesions. They may produce cosmetic deformities, peripheral compressive cranial neuropathies, and compressive central neurologic manifestations. We describe an unusual presentation of a fronto-orbital sphenoethmoidal FD in a 32-year-old woman with conventional radiographic, CBCT, and MRI findings. In the head and neck examination, an asymmetry was noticed on the left side, without evidence of adenopathy, paresthesia, or motor nerve deficiency. Panoramic radiographs showed a radiopaque expanded bone in the region of the posterior maxillary sinus and orbita. Computed tomography with three-dimensional reconstruction demonstrated an expanding lesion of the cranial bones, involving the ethmoid and periorbital bone, producing a ground-glass appearance. After the radiologic examination, the patient was referred for surgery with a diagnosis of cranial FD and underwent a cranioplasty. The CT and MRI features were typical for FD, but physicians and dental professionals should be aware of this diagnosis, even if no symptom is apparent and the patient came in only for a routine dental examination. Maxillofacial radiologists should also consider that the signal intensity on both T1- and T2-weighted images and the degree of contrast enhancement on T1-weighted images depend on the amount and degree of bony trabeculae, cellularity, collagen, and cystic and hemorrhagic changes.

SU-FF-J-89: Evaluation of Five Fiducial Markers Using MVCT and CBCT

Purpose: To evaluate different fiducial markers using different imaging modalities to find the optimal choices of markers. Method and Materials: A pelvis phantom was constructed out of different plastics. Simulated prostate, femoral head, femur, and pubis bones are embedded in the phantom. The prostate has three removable plugs. These plugs are made to a tight press fit and can be removed. Plugs were machined that contained different fiducial markers. Two types of gold seed markers (both 1.2mm diameter), three types of coil markers (0.75 to 1.15mm diameter), and a carbon marker (1.0mm diameter) were evaluated. The phantom was scanned using conventional CT, Varian CBCT, and Tomotherapy MVCT with each of the different markers. For CBCT, two FOV settings were used. For MVCT, two different techniques were used. To help quantify the results, profiles through the markers were obtained. The maximum signal and FWHM were evaluated. Results: The fiducial markers were clearly visible on all scans with the exception that the carbon marker was not visible using MVCT as expected. The smaller diameter coil had noticeably less artifact than the larger coils and the gold seed. The carbon marker had minimal if any artifact. Neither FOV for CBCT or technique for MVCT had a significant effect on marker visibility.Conclusion: Given that the smaller coil had the least amount of artifact of the markers that were visible with all modalities it would seem to be an ideal choice for institutions that have multiple modalities. Smaller gold seeds may also have this property but were not included in this study. For institutions using only CBCT, the carbon marker would be the best choice.

Preliminary study of a phase-contrast cone-beam computed tomography system: the edge-enhancement effect in the tomographic reconstruction of in-line holographic images

A phase-contrast cone-beam computed tomography (PC CBCT) system is proposed for small-animal imaging that incorporates the in-line holography technique into CBCT system. Theoretical analysis shows that the in-line holographic image can be approximately transformed into line integrals of an object function composed of an attenuation term and a phase term. The Fresnel diffraction theory is applied to generate in-line holographic images along a circular orbit, and the Feldkamp-Davis-Kress algorithm is applied to reconstruct the object function. The proposed system was investigated using a numerical phantom, and the reconstruction was evaluated using the edge-enhancement factor and the relative reconstruction error. The reconstruction results show that all the structures in the numerical phantom are bounded with enhanced edges with negligible artifacts. These enhanced edges make the reconstruction visually sharper and clearer. The results show that while the relative reconstruction errors are very close to that of the conventional CBCT reconstruction, having a small cone angle, weak attenuation, small focal spot size, and high-resolution detector are preferred for a greater edge-enhancement effect.