Number Of Phase Bins Research Articles

Performance studies of four-dimensional cone beam computed tomography

Four-dimensional cone beam computed tomography (4DCBCT) has been proposed to characterize the breathing motion of tumors before radiotherapy treatment. However, when the acquired cone beam projection data are retrospectively gated into several respiratory phases, the available data to reconstruct each phase is under-sampled and thus causes streaking artifacts in the reconstructed images. To solve the under-sampling problem and improve image quality in 4DCBCT, various methods have been developed. This paper presents performance studies of three different 4DCBCT methods based on different reconstruction algorithms. The aims of this paper are to study (1) the relationship between the accuracy of the extracted motion trajectories and the data acquisition time of a 4DCBCT scan and (2) the relationship between the accuracy of the extracted motion trajectories and the number of phase bins used to sort projection data. These aims will be applied to three different 4DCBCT methods: conventional filtered backprojection reconstruction (FBP), FBP with McKinnon–Bates correction (MB) and prior image constrained compressed sensing (PICCS) reconstruction. A hybrid phantom consisting of realistic chest anatomy and a moving elliptical object with known 3D motion trajectories was constructed by superimposing the analytical projection data of the moving object to the simulated projection data from a chest CT volume dataset. CBCT scans with gantry rotation times from 1 to 4 min were simulated, and the generated projection data were sorted into 5, 10 and 20 phase bins before different methods were used to reconstruct 4D images. The motion trajectories of the moving object were extracted using a fast free-form deformable registration algorithm. The root mean square errors (RMSE) of the extracted motion trajectories were evaluated for all simulated cases to quantitatively study the performance. The results demonstrate (1) longer acquisition times result in more accurate motion delineation for each method; (2) ten or more phase bins are necessary in 4DCBCT to ensure sufficient temporal resolution in tumor motion and (3) to achieve the same performance as FBP-4DCBCT with a 4 min data acquisition time, MB-4DCBCT and PICCS-4DCBCT need about 2- and 1 min data acquisition times, respectively.

MO‐D‐BRC‐11: Performance Study of Filtered Back‐Projection (FBP) and Prior Image Constrained Compressed Sensing (PICCS) Based 4D Cone Beam CT: The Effects of Gantry Rotation Time and the Number of Phase Bins on Tumor Motion Delineation

Purpose: To study how the performance of 4D Cone Beam CT varies with gantry rotation time and the number of phase bins when two different reconstruction methods, Filtered Back‐Projection (FBP) and Prior Image Constrained Compressed Sensing (PICCS) are used. Methods: Numerical simulation studies were performed using a hybrid phantom. The phantom consists of in vivo human subject data and simulated lung cancer tumors with motion. The data acquisitions were simulated at 1, 2, 3, and 4 minutes. After data were sorted into different number of phase bins (5, 10, and 20), 4D CBCT images were reconstructed using FBP and PICCS methods. Motion trajectory of the tumor was then extracted using a free‐form deformable registration method. The root mean square error (RMSE) was used to characterize the deviation of motion trajectory from the ground truth. Results: (1) When gantry rotation time becomes longer, FBP‐4DCBCT images show reduced streak artifacts and improved accuracy in extraction of tumor motion profile (RMSE=5.4 mm, 2.5 mm, 1.5 mm, and 0.5 mm for 1, 2, 3, and 4 minutes gantry time respectively, for in‐plane motion); (2) PICCS‐4DCBCT enables streak‐free images starting at 1‐minute gantry rotation while FBP‐4DCBCT requires more than 4 minutes to achieve the comparable accuracy in motion trajectory (RMSE=0.2 mm for PICCS‐ 4DCBCT at 1‐minute gantry rotation and RMSE=0.5 mm for FBP‐4DCBCT at 4‐minute gantry rotation); (3) For in‐plane motion, when the number of phase bins increases, the RMSEs increase for FBP‐4DCBCT, while they are much more stable for PICCS‐4DCBCT. Conclusions: (1) To accurately extract tumor motion profiles, the number of phase bins should be more than 10. (2) PICCS‐4DCBCT enables accurate extraction of tumor motion profile using 1‐minute gantry rotation. In contrast, more than 4 minutes are required for FBP‐4DCBCT to achieve acceptable accuracy for extraction of tumor motion profiles.

SU‐GG‐J‐142: Investigation of MIP PET in Motion‐Encompassing Methods to Account for Respiratory Motion in Radiotherapy

Purpose: Radiation treatment planning with PET/CT has been shown to be beneficial and leads to smaller variations in target delineation than using CT only. This study investigates the 3D maximum intensity projection (MIP) PET images derived from respiratory‐gated 4D PET as a complement to the MIP CT derived from a 4D CT dataset and their use in treatment planning as a means to account for respiratory motion in free‐breathing radiotherapy for lung and abdomen tumors. Methods and materials: 21 PET/CT patient datasets with 31 lesions were acquired. The patient respiratory motion was monitored by the Real‐time Position Management system. PET listmode data were phase‐binned into 5 frames and reconstructed. 3D MIP PET images were derived from the 5‐bin 4D PET datasets. Tumor volumes were determined by an automatic segmentation method and compared for the MIP PET and static PET images. The tumor motion amplitude was determined by measuring the centroid in phase‐gated images. The impact of tumor size and motion amplitude were investigated. In addition, PET data were binned into 10 frames to study the noise change in MIP PET. The comparison between the MIP PET and MIP CT is under study. Results: Compared to static PET, the tumor volumes in the MIP PET increased by 10% 14%, with a maximum increase of 44%. The volume increases are larger for the smaller tumors sizes and larger motion amplitudes. The image noise of the MIP PET is higher than in static PET and increases with number of phase bins. Conclusion: The MIP PET has a potential to complement MIP CT to inform the radiation treatment planning for motion‐encompassing radiotherapy by accounting for respiratory motion.

X-rays from the eclipsing millisecond pulsar PSR J1740-5340 in the globular cluster NGC 6397

<i>Aims. <i/>The millisecond pulsar PSR J1740-5340 in the globular cluster NGC 6397 shows radio eclipses over ~40% of its binary orbit. A first Chandra observation revealed indications for the X-ray flux being orbit dependent as well. In this work, we analysed five datasets of archival Chandra data taken between 2000 and 2007 to investigate the emission across the pulsar's binary orbit.<i>Methods. <i/>Utilizing archival Chandra observations of PSR J1740-5340, we performed a systematic timing and spectral analysis of this binary system.<i>Results. <i/>Using a -test, the significance for intra-binary orbital modulation was found to be between 88.5% and 99.6%, depending on the number of phase bins used to construct the light curve. Applying the unbiased statistical Kolmogorov-Smirnov (KS) test did not indicate any significant intra-binary orbital modulation. However, comparing the counting rates observed at different epochs, a flux variability on times scales of days to years is indicated. The possible origin of the X-ray emission is discussed in a number of different scenarios.

An investigation of temporal resolution parameters in cine‐mode four‐dimensional computed tomography acquisition

The accuracy of four‐dimensional computed tomography (4DCT) imaging depends on temporal characteristics of the acquisition protocol—for example, the temporal spacing of the reconstructed images (also known as cine duration between images) and the gantry rotation speed. These parameters affect the temporal resolution of 4DCT images, and a single default acquisition protocol, as commonly used in most clinics, may be suboptimal for a subset of respiratory motion characteristics. It could lead to substantial inaccuracies in target delineation. The aim of the present study was to evaluate the interplay between parameters affecting temporal resolution and the accuracy of the resulting images.We acquired 4DCT images of cylindrical phantoms under repetitive motion induced by a translation platform. Acquisition settings varied with respect to temporal spacing, gantry rotation speed, and motion period of the phantoms. Reconstructed images were sorted into 10 phase bins and were compared to static phantom images acquired at corresponding positions of the respiration phase. Acquisitions with different temporal spacing did not play a significant role in the amount of motion observed in full‐cycle maximum intensity projection images. Target delineation accuracy at end‐of‐inhalation phase was observed to be constant up to a threshold in the value of the reconstruction interval, beyond which it varied arbitrarily. This threshold was found to be correlated with the number of phase bins and the motion period. No observable variations were noted with images from the end of exhalation when temporal spacing was varied. Target delineation accuracy was observed to be enhanced in acquisitions using faster gantry rotation speeds. An evaluation of the acquisition parameters needs to be performed depending on the period of the motion and limiting factors such as the availability of acquisition settings, X‐ray tube workload, image storage, and processing power.PACS numbers: 87.53.Xd, 87.57.‐s, 87.57.Gg, 87.59.Fm

WE‐E‐M100F‐02: Interplay Between Image Quality and Temporal Resolution in 4DCT Acquisition Protocols

Purpose: The accuracy and temporal resolution of 4DCT imaging depends on temporal characteristics of the acquisition protocol like gantry rotation speed and image reconstruction interval, in addition to the spatial and temporal characteristics of the motion itself. Therefore, a generic acquisition protocol might not be suitable for all patients. The aim of this study is to evaluate the interplay between the parameters affecting the accuracy and temporal resolution of 4DCT images. Methods and Materials: Several 4DCT images were acquired of cylindrical phantoms under repetitive motion induced by a translation platform. Acquisition settings were varied with respect to image reconstruction interval, gantry rotation speed and motion period of the phantoms. Reconstructed images were sorted into ten phase bins and compared to CT images of static phantoms at corresponding positions of the respiration phase. Results: Acquisitions with different image reconstruction intervals did not play a significant role in the amount of motion observed in full cycle maximum intensity projection (MIP) images. Single‐phase image integrity was observed to be constant up to a threshold in the value of reconstruction interval, beyond which the image integrity varied somewhat arbitrarily due to the reduced number of images. This threshold is correlated with the number of phase bins and the motion period. Furthermore, image integrity was observed to improve with decreasing gantry rotation periods. Conclusions: Image integrity as evaluated in this study demonstrates relative gains with respect to changes in the temporal resolution parameters of 4DCT acquisition. Due to observed differences, we report that suboptimal settings could result in dramatic target delineation inaccuracies. Optimization of acquisition parameters needs to be performed, assessing the period of motion and limiting factors such as the availability of acquisition settings, image storage and processing power. Conflict of Interest: Sponsored in part by GE Healthcare.

Spatial corrections of ROSAT HRI observations

X-ray observations with the ROSAT High Resolution Imager (HRI) often have spatial smearing on the order of 10 arcsec (Morse 1994). This degradation of the intrinsic resolution of the instrument (5 arcsec) can be attributed to errors in the aspect solution associated with the wobble of the space craft or with the reacquisition of the guide stars. We have developed a set of IRAF/PROS and MIDAS/EXSAS routines to minimize these effects. Our procedure attempts to isolate aspect errors that are repeated through each cycle of the wobble. The method assigns a 'wobble phase' to each event based on the 402 second period of the ROSAT wobble. The observation is grouped into a number of phase bins and a centroid is calculated for each sub-image. The corrected HRI event list is reconstructed by adding the sub-images which have been shifted to a common source position. This method has shown approx. 30% reduction of the full width half maximum (FWHM) of an X-ray observation of the radio galaxy 3C 120. Additional examples are presented.