S. Park

Fast compressed sensing-based CBCT reconstruction using Barzilai-Borwein formulation for application to on-line IGRT

PURPOSEnCompressed sensing theory has enabled an accurate, low-dose cone-beam computed tomography (CBCT) reconstruction using a minimal number of noisy projections. However, the reconstruction time remains a significant challenge for practical implementation in the clinic. In this work, we propose a novel gradient projection algorithm, based on the Gradient-Projection-Barzilai-Borwein formulation (GP-BB), that handles the total variation (TV)-norm regularization-based least squares problem for the CBCT reconstruction in a highly efficient manner, with speed acceptable for routine use in the clinic.nnnMETHODSnCBCT is reconstructed by minimizing an energy function consisting of a data fidelity term and a TV-norm regularization term. Both terms are simultaneously minimized by calculating the gradient projection of the energy function with the step size determined using an approximate Hessian calculation at each iteration, based on the Barzilai-Borwein formulation. To speed up the process, a multiresolution optimization is used. In addition, the entire algorithm was designed to run with a single graphics processing unit (GPU) card. To evaluate the performance, the Shepp-Logan numerical phantom, the CatPhan 600 physical phantom, and a clinically-treated head-and-neck patient were acquired from the TrueBeam™ system (Varian Medical Systems, Palo Alto, CA). For each scan, in total, 364 projections were acquired in a 200° rotation. The imager has 1024u2009×u2009768 pixels with 0.388u2009×u20090.388-mm resolution. This was down-sampled to 512u2009×u2009384 pixels with 0.776u2009×u20090.776-mm resolution for reconstruction. Evenly spaced angles were subsampled and used for varying the number of projections for the image reconstruction. To assess the performance of our GP-BB algorithm, we have implemented and compared with three compressed sensing-type algorithms, the two of which are popular and published (forward-backward splitting techniques), and the other one with a basic line-search technique. In addition, the conventional Feldkamp-Davis-Kress (FDK) reconstruction of the clinical patient data is compared as well.nnnRESULTSnIn comparison with the other compressed sensing-type algorithms, our algorithm showed convergence in ≤30 iterations whereas other published algorithms need at least 50 iterations in order to reconstruct the Shepp-Logan phantom image. With the CatPhan phantom, the GP-BB algorithm achieved a clinically-reasonable image with 40 projections in 12 iterations, in less than 12.6 s. This is at least an order of magnitude faster in reconstruction time compared with the most recent reports utilizing GPU technology given the same input projections. For the head-and-neck clinical scan, clinically-reasonable images were obtained from 120 projections in 34-78 s converging in 12-30 iterations. In this reconstruction range (i.e., 120 projections) the image quality is visually similar to or better than the conventional FDK reconstructed images using 364 projections. This represents a dose reduction of nearly 67% (120∕364 projections) while maintaining a reasonable speed in clinical implementation.nnnCONCLUSIONSnIn this paper, we proposed a novel, fast, low-dose CBCT reconstruction algorithm using the Barzilai-Borwein step-size calculation. A clinically viable head-and-neck image can be obtained within ∼34-78 s while simultaneously cutting the dose by approximately 67%. This makes our GP-BB algorithm potentially useful in an on-line image-guided radiation therapy (IGRT).

Liver motion during cone beam computed tomography guided stereotactic body radiation therapy

PURPOSEnUnderstanding motion characteristics of liver such as, interfractional and intrafractional motion variability, difference in motion within different locations in the organ, and their complex relationship with the breathing cycles are particularly important for image-guided liver SBRT. The purpose of this study was to investigate such motion characteristics based on fiducial markers tracked with the x-ray projections of the CBCT scans, taken immediately prior to the treatments.nnnMETHODSnTwenty liver SBRT patients were analyzed. Each patient had three fiducial markers (2u2009×u20095-mm gold) percutaneously implanted around the gross tumor. The prescription ranged from 2 to 8 fractions per patient. The CBCT projections data for each fraction (∼650 projections∕scan), for each patient, were analyzed and the 2D positions of the markers were extracted using an in-house algorithm. In total, >55 000 x-ray projections were analyzed from 85 CBCT scans. From the 2D extracted positions, a 3D motion trajectory of the markers was constructed, from each CBCT scans, resulting in left-right (LR), anterior-posterior (AP), and cranio-caudal (CC) location information of the markers with >55 000 data points. The authors then analyzed the interfraction and intrafraction liver motion variability, within different locations in the organ, and as a function of the breathing cycle. The authors also compared the motion characteristics against the planning 4DCT and the RPM™ (Varian Medical Systems, Palo Alto, CA) breathing traces. Variations in the appropriate gating window (defined as the percent of the maximum range at which 50% of the marker positions are contained), between fractions were calculated as well.nnnRESULTSnThe range of motion for the 20 patients were 3.0 ± 2.0 mm, 5.1 ± 3.1 mm, and 17.9 ± 5.1u2009mm in the planning 4DCT, and 2.8 ± 1.6 mm, 5.3 ± 3.1 mm, and 16.5 ± 5.7 mm in the treatment CBCT, for LR, AP, and CC directions, respectively. The range of respiratory period was 3.9 ± 0.7 and 4.2 ± 0.8 s during the 4DCT simulation and the CBCT scans, respectively. The authors found that breathing-induced AP and CC motions are highly correlated. That is, all markers moved cranially also moved posteriorly and vice versa, irrespective of the location. The LR motion had a more variable relationship with the AP∕CC motions, and appeared random with respect to the location. That is, when the markers moved toward cranial-posterior direction, 58% of the markers moved to the patient-right, 22% of the markers moved to the patient-left, and 20% of the markers had minimal∕none motion. The absolute difference in the motion magnitude between the markers, in different locations within the liver, had a positive correlation with the absolute distance between the markers (R(2) = 0.69, linear-fit). The interfractional gating window varied significantly for some patients, with the largest having 29.4%-56.4% range between fractions.nnnCONCLUSIONSnThis study analyzed the liver motion characteristics of 20 patients undergoing SBRT. A large variation in motion was observed, interfractionally and intrafractionally, and that as the distance between the markers increased, the difference in the absolute range of motion also increased. This suggests that marker(s) in closest proximity to the target be used.

Four-dimensional cone-beam computed tomography and digital tomosynthesis reconstructions using respiratory signals extracted from transcutaneously inserted metal markers for liver SBRT.

PURPOSEnRespiration-induced intrafraction target motion is a concern in liver cancer radiotherapy, especially in stereotactic body radiotherapy (SBRT), and therefore, verification of its motion is necessary. An effective means to localize the liver cancer is to insert metal fiducial markers to or near the tumor with simultaneous imaging using cone-beam computed tomography (CBCT). Utilizing the fiducial markers, the authors have demonstrated a method to generate breath-induced motion signal of liver for reconstructing 4D digital tomosynthesis (4DDTS) and 4DCBCT images based on phasewise and/or amplitudewise sorting of projection data.nnnMETHODSnThe marker extraction algorithm is based on template matching of a prior known marker image and has been coded to optimally extract marker positions in CBCT projections from the On-Board Imager (Varian Medical Systems, Palo Alto, CA). To validate the algorithm, multiple projection images of moving thorax phantom and five patient cases were examined. Upon extraction of the motion signals from the markers, 4D image sorting and image reconstructions were subsequently performed. In the case of incomplete signals due to projections with missing markers, the authors have implemented signal profiling to replace the missing portion.nnnRESULTSnThe proposed marker extraction algorithm was shown to be very robust and accurate in the phantom and patient cases examined. The maximum discrepancy of the algorithm predicted marker location versus operator selected location was < 1.2 mm, with the overall average of 0.51 +/- 0.15 mm, for 500 projections. The resulting 4DDTS and 4DCBCT images showed clear reduction in motion-induced blur of the markers and the anatomy for an effective image guidance. The signal profiling method was useful in replacing missing signals.nnnCONCLUSIONSnThe authors have successfully demonstrated that motion tracking of fiducial markers and the subsequent 4D reconstruction of CBCT and DTS are possible. Due to the significant reduction in motion-induced image blur, it is anticipated that such technology will be useful in image-guided liver SBRT treatments.

Ultra-Fast Digital Tomosynthesis Reconstruction Using General-Purpose GPU Programming for Image-Guided Radiation Therapy

The purpose of this work is to demonstrate an ultra-fast reconstruction technique for digital tomosynthesis (DTS) imaging based on the algorithm proposed by Feldkamp, Davis, and Kress (FDK) using standard general-purpose graphics processing unit (GPGPU) programming interface. To this end, the FDK-based DTS algorithm was programmed “in-house” with C language with utilization of 1) GPU and 2) central processing unit (CPU) cards. The GPU card consisted of 480 processing cores (2 × 240 dual chip) with 1,242 MHz processing clock speed and 1,792 MB memory space. In terms of CPU hardware, we used 2.68 GHz clock speed, 12.0 GB DDR3 RAM, on a 64-bit OS. The performance of proposed algorithm was tested on twenty-five patient cases (5 lung, 5 liver, 10 prostate, and 5 head-and-neck) scanned either with a full-fan or half-fan mode on our cone-beam computed tomography (CBCT) system. For the full-fan scans, the projections from 157.5°–202.5° (45°-scan) were used to reconstruct coronal DTS slices, whereas for the half-fan scans, the projections from both 157.5°–202.5° and 337.5°–22.5° (2 × 45°-scan) were used to reconstruct larger FOV coronal DTS slices. For this study, we chose 45°-scan angle that contained ~80 projections for the full-fan and ~160 projections with 2 × 45°-scan angle for the half-fan mode, each with 1024 × 768 pixels with 32-bit precision. Absolute pixel value differences, profiles, and contrast-to-noise ratio (CNR) calculations were performed to compare and evaluate the images reconstructed using GPU- and CPU-based implementations. The time dependence on the reconstruction volume was also tested with (512 × 512) × 16, 32, 64, 128, and 256 slices. In the end, the GPU-based implementation achieved, at most, 1.3 and 2.5 seconds to complete full reconstruction of 512 × 512 × 256 volume, for the full-fan and half-fan modes, respectively. In turn, this meant that our implementation can process > 13 projections-per-second (pps) and > 18 pps for the full-fan and half-fan modes, respectively. Since commercial CBCT system nominally acquires 11 pps (with 1 gantry-revolution-per-minute), our GPU-based implementation is sufficient to handle the incoming projections data as they are acquired and reconstruct the entire volume immediately after completing the scan. In addition, on increasing the number of slices (hence volume) to be reconstructed from 16 to 256, only minimal increases in reconstruction time were observed for the GPU-based implementation where from 0.73 to 1.27 seconds and 1.42 to 2.47 seconds increase were observed for the full-fan and half-fan modes, respectively. This resulted in speed improvement of up to 87 times compared with the CPU-based implementation (for 256 slices case), with visually identical images and small pixel-value discrepancies (< 6.3%), and CNR differences (< 2.3%). With this achievement, we have shown that time allocation for DTS image reconstruction is virtually eliminated and that clinical implementation of this approach has become quite appealing. In addition, with the speed achievement, further image processing and real-time applications that was prohibited prior due to time restrictions can now be tempered with.

Motion-map constrained image reconstruction (MCIR): Application to four-dimensional cone-beam computed tomography

PURPOSEnUtilization of respiratory correlated four-dimensional cone-beam computed tomography (4DCBCT) has enabled verification of internal target motion and volume immediately prior to treatment. However, with current standard CBCT scan, 4DCBCT poses challenge for reconstruction due to the fact that multiple phase binning leads to insufficient number of projection data to reconstruct and thus cause streaking artifacts. The purpose of this study is to develop a novel 4DCBCT reconstruction algorithm framework called motion-map constrained image reconstruction (MCIR), that allows reconstruction of high quality and high phase resolution 4DCBCT images with no more than the imaging dose as well as projections used in a standard free breathing 3DCBCT (FB-3DCBCT) scan.nnnMETHODSnThe unknown 4DCBCT volume at each phase was mathematically modeled as a combination of FB-3DCBCT and phase-specific update vector which has an associated motion-map matrix. The motion-map matrix, which is the key innovation of the MCIR algorithm, was defined as the matrix that distinguishes voxels that are moving from stationary ones. This 4DCBCT model was then reconstructed with compressed sensing (CS) reconstruction framework such that the voxels with high motion would be aggressively updated by the phase-wise sorted projections and the voxels with less motion would be minimally updated to preserve the FB-3DCBCT. To evaluate the performance of our proposed MCIR algorithm, we evaluated both numerical phantoms and a lung cancer patient. The results were then compared with the (1) clinical FB-3DCBCT reconstructed using the FDK, (2) 4DCBCT reconstructed using the FDK, and (3) 4DCBCT reconstructed using the well-known prior image constrained compressed sensing (PICCS).nnnRESULTSnExamination of the MCIR algorithm showed that high phase-resolved 4DCBCT with sets of up to 20 phases using a typical FB-3DCBCT scan could be reconstructed without compromising the image quality. Moreover, in comparison with other published algorithms, the image quality of the MCIR algorithm is shown to be excellent.nnnCONCLUSIONSnThis work demonstrates the potential for providing high-quality 4DCBCT during on-line image-guided radiation therapy (IGRT), without increasing the imaging dose. The results showed that (at least) 20 phase images could be reconstructed using the same projections data, used to reconstruct a single FB-3DCBCT, without streak artifacts that are caused by insufficient projections.

SU‐FF‐T‐337: Patient‐Specific Treatment Planning System for BNCT Based On Dose Calculation Using MCNP

Purpose: A patient‐specific treatment planning system for BoronNeutron Capture Therapy (BNCT) based on dose calculation using MCNP, called BTPS, was developed at Hanyang University, Korea. To facilitate treatment planning for BNCT, overall procedures of the BTPS were designed based on a graphical user interface (GUI) and matched to clinical treatments of BNCT. Appropriate tools automatically associated with MCNP were built‐in. To provide optimized irradiation time in treatment by re‐calculating absorbed doses with measured boron concentrations and irradiatingneutron flux, various tools, which can decrease computation time, were employed into the BTPS. Method and Materials: The GUI based BTPS was built with a C++ Builder, and works on Windows. The BTPS reconstructs a 3D voxel phantom from patients images, and simulates treatment environments including the delineated target, irradiation field, and boron concentrations in tumor/normal tissues with easy manipulations through the GUI. MCNP input with the treatment environments are automatically generated for calculating the absorbed dose or neutron fluxes in the voxel phantom. Results: Computation time for calculating the absorbed dose and neutron fluxes in a voxel phantom (17×18×23 cm of 1 cm3 voxels, boron concentration of 10 ppm in each voxel, and histories of 10 millions) was about 5 minutes by using accelerated tallying techniques in MCNP and a parallel computing system (Pentium IV, 48 nodes). The results from the MCNP run are automatically imported. Total absorbed dose or neutron fluxes are calculated with irradiatingneutron flux and treatment time, and displayed as an isodose contour or dose volume histogram. Conclusion: Since the BTPS facilitates patient‐specific treatment planning for BNCT with efficient computation time, it is expected that the BTPS is applicable to clinical treatments of BNCT. Conflict of Interest: Research sponsored by the SRC/ERC program (R11‐2000‐067‐01001‐0) and the long‐term research and development program (M20505050003‐05A0905‐00310) of MOST/KOSEF.

SU‐FF‐J‐41: Comparison of Various Respiration Measurement Methods for 4D Radiotherapy

Purpose: To find the best method corresponding with respiratory target motion, ten patients respiratory patterns were measured by various methods simultaneously. Respective respiration monitoring methods were compared with fluoroscopic target motion during simulation. Method and Materials: A respiration monitoring system using thermocouple was developed to measure patients respiration. Conventional spirometer and home made thermocouple were connected to a mouse piece to measure the patients respiration simultaneously. A respiration acquisition program was built by using Labview 7.0 (National Instruments, Austin, TX), which acquire respiration signals and display its patterns. A fluoroscopic target tracking program was built by using IDL 6.1 (Research Systems, Inc, Boulder, CO). Ten patients with lung or livercancer participated in this study. Fluoroscopic movies were captured during acquisition of their respiration patterns. At the same time their skin motion was measured by using Real‐time Position Management® (RPM®, Varian, Palo Alto, CA) system. Respiratory patterns from spirometer, thermocouple, and RPM® system were compared with fluoroscopic target motion respectively. Its relationships were evaluated as correlation coefficient. Results: Comparing each correlation coefficient for spirometer, thermocouple, and RPM®, skin motion detection is the most correspondent with fluoroscopic target motion. However, respiration monitoring methods with spirometer or thermocouple also correlate well (more than 0.9). Conclusion: Respiratory pattern depends on a patient and his/her conditions. The relationship between thermocouple and fluoroscopic target motion could be enhanced by correlating respiratory signal with target motion. Respiration monitoring methods with spirometer or thermocouple, and skin motion detection are feasible to monitor the target motion for applying 4D radiotherapy.

SU-GG-T-541: Dosimetric Impact of Daily Setup Variations during Spine Radiosurgery

To examine the dosimetric stability against geometrical variations in the treatment plan of spine radiosurgery, the positioning data of CBCTimage guidance for 28 patients were used to make a comparison between the treatment plans before and after image guidance. The dose distribution in treatment plans were recalculated with the same IMRT fluence map and the CTimage shifted by the correction values. Some dosimetric parameters were employed to verify the tumor coverage and the spinal cord dose in the treatment plans. The Target volume which receive more than 95% of prescription dose(D95) for tumors and the equivalent uniform dose (EUD )for spinal cord were calculated to fish out the dosimetric variations. The relation between the tumor coverage and the geometric variations presents to be quadratic in all directions, while the EUD of spinal cord are linearly increased by geometrical variations in anterior‐posterior direction in which tumors are located in the view of spinal cord. Since there is no margin at the interface of tumor and spinal cord in most cases, the dosimetric effect of geometrical variation for spinal cord is significant. The tolerance dose of spinal cord in radiosurgery have to be considered very carefully with this dosimetric stability and the geometrical uncertainties.

TU‐B‐201B‐04: Four‐Dimensional Cone‐Beam Computed Tomography and Digital Tomosynthesis Using Motion Signals Extracted from Fiducial Marker Inserted for Liver Cancer Radiation Therapy

Purpose: To reconstruct both phase‐wise and amplitude‐wise sorted 4D digital tomosynthesis (DTS) and 4D cone‐beam CT(CBCT)images based on breathing signals extracted from transcutaneously inserted metal markers for livercancer IGRT. Methods and Materials: The reconstruction process consists of 4 stages where 1) features of metal marker from selected region of interest (ROI) of its position is extracted, followed by 2) generating breath induced marker motion signal based on its position and predicting the missing signal through “profiling” of pre‐acquired breathing signal, 3) undergo amplitude‐wise and phase‐wise sorting with acquired projection data, and finally 4) reconstruct 4DCBCT and 4DDTS images.Results: In half‐fan geometry, the metal markers may be absent in up to 18.5% of the total X‐ray projections taken, which can be successfully replaced by signal “profiling” method that we propose here. With this, 4DDTS and 4DCBCT images were reconstructed. Comparing the reconstruction results between phase‐wise and amplitude‐wise sorting of 4DDTS and 4DCBCT images, motion artifacts of fiducial marker were less in amplitude‐wise than phase‐wise reconstructed images due to lesser amount of residual motion at each state of sorting process, thus more ideal for use in 4D image guidance. Conclusions: We have proposed the use of fiducial markers imaged at CBCT projections to acquire breath induced motion signal of liver to generate 4DDTS as well as 4DCBCT images. To the best of our knowledge, utilizing breath induced motion signal using metal markers for 4D imaging applications has never been attempted. Proposed method is advantageous compared to other methods in ways that 1) it does not require external gating system and 2) amplitude as well as phases‐wise sorting is selectively achievable. The proposed method can be utilized as intrafractional target motion verification process to improve precision of on‐line image guided radiotherapy for livercancer patients.

SU‐FF‐T‐186: Dosimetry in An IMRT Phantom Designed for a Remote Auditing Program

Purpose: Accurate delivery of the most‐up‐to‐date treatment techniques, such as intensity‐modulated radiation therapy(IMRT), is essential. An anthropomorphic phantom was designed and constructed for a remote‐audit program that allows evaluation of an institutions accuracy for deliveringIMRT. The dosimetric characteristics in the phantom were investigated. Method and Materials: The phantom has the shape of a cylinder with one target and three organ‐at‐risks (OARs) inside. The target and OARs were shaped analogous to those of nasopharynx cancer patients. TLD holders were inserted inside the target and the OARs for absolute dose measurements. The phantom allows measurements with ion chamber (IC) at the TLD locations, so that inter‐comparison between two dosimeters was possible. For the measurement of relative dose distribution, two film slots were orthogonally placed near the center of the phantom, which also enables inserting inhomogeneities near the target. Measurements with TLDs and ICs were done for four cases. The first was an anterior one‐port 6MV X‐ray (Primus, Siemens, USA) delivery; the second used the same beam geometrically, but with inhomogeneities inserted, and in the third case three ports of beams were distributed in equi‐gantry angle, and the fourth was an IMRT without inhomogeneities. For case 1–3, theoretical predictions were computed by using Monte Carlo code. Results: For anterior one port X‐ray delivery to a homogeneous/ inhomogeneous phantom, the deviation between the IC and TLD measurements, ranged from 1.7% to 2.3%. For three‐port beam delivery, the range was also similar. For IMRTdelivery, the differences were 0.1–2.8%. The differences between the TLD measurements and MC predictions ranged from −0.14–1.8%. Conclusion: The TLD measurements in the developed phantom agreed with IC and MC results within acceptable differences. The developed phantom with TLDdosimeters are feasible to be used for remote monitoring of IMRT. Supported by KFDA Grant No 05102eui BangPum and 06112eui BangAn300.