Respiratory motion can introduce significant errors in radiotherapy. Conventional CT scans as commonly used for treatment planning can include severe motion artifacts that result from interplay effects between the advancing scan plane and object motion. To explicitly include organ/target motion in treatment planning and delivery, time-resolved CTdata acquisition (4D Computed Tomography) is needed. 4DCT can be accomplished by oversampled CTdata acquisition at each slice. During several CT tube rotations projection data are collected in axial cine mode for the duration of the patient’s respiratory cycle (plus the time needed for a full CT gantry rotation). Multiple images are then reconstructed per slice that are evenly distributed over the acquisition time. Each of these images represents a different anatomical state during a respiratory cycle. After data acquisition at one couch position is completed, x rays are turned off and the couch advances to begin data acquisition again until full coverage of the scan length has been obtained. Concurrent to CTdata acquisition the patient’s abdominal surface motion is recorded in precise temporal correlation. To obtain CT volumes at different respiratory states, reconstructed images are sorted into different spatio-temporally coherent volumes based on respiratory phase as obtained from the patient’s surface motion. During binning, phase tolerances are chosen to obtain complete volumetric information since images at different couch positions are reconstructed at different respiratory phases. We describe 4DCT image formation and associated experiments that characterize the properties of 4DCT. Residual motion artifacts remain due to partial projection effects. Temporal coherence within resorted 4DCT volumes is dominated by the number of reconstructed images per slice. The more images are reconstructed, the smaller phase tolerances can be for retrospective sorting. From phantom studies a precision of about for quasiregular motion and typical respiratory periods could be concluded. A protocol for 4DCT scanning was evaluated and clinically implemented at the MGH. Patient data are presented to elucidate how additional patient specific parameters can impact 4DCT imaging.
This work was supported in part by NCI-P01–21239. Part of this work performed (by ER) was under funding from the Deutsche Forschungsgemeinschaft in cooperation with Gessellschaft für Schwerionenforschung. Special thanks to members of the Department of Radiation Oncology, including Noah Choi, Christopher Willett, Karen Doppke, Nancy Ditullio, and Joanne Pacella. The authors also wish to acknowledge the technical support of General Electric Medical Systems and Varian Medical Systems.
I.A. Artifacts in scanning moving objects
I.B. Breath held scans
I.C. Fluoroscopy to assess motion
I.D. 4D computed tomography
II. MATERIAL AND METHODS
II.A. Data acquisition
II.A.1. RPM system
II.A.2. CTdata acquisition
II.A.3. Retrospective image sorting
II.B. Phantom measurements
II.C. Patient studies
III.A. Phantom measurements
III.A.1. Performance of the RPM-system: Recorded object motion
III.A.2. In-slice image formation
III.A.3. Resorted image volumes
III.A.4. Object motion and volume
III.B. Patient data
III.B.1. Comparison of 4DCT and breath hold CT scans
III.B.2. Respiration irregularities
III.B.3. Imaging respiratory motion
Data & Media loading...
Article metrics loading...
Full text loading...