Four‐dimensional computed tomography: Image formation and clinical protocol

Abstract

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 CT data acquisition (4D Computed Tomography) is needed. 4DCT can be accomplished by oversampled CT data 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 CT data 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 2.5 mm 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.

A dosimetric comparison of stereotactic body radiation therapy techniques for lung cancer: robotic versus conventional linac‐based systems

“…CT data are reconstructed with a slice thickness of 2 mm. Scans are acquired with an X‐ray tube rotation time of 0.5 s. These parameters are close to those reported in the study by Rietzel et al ( 30 ) who concluded that the accuracy of 4D CT images is one CT slice thickness. Although additional setup error may be introduced in the process of matching the CBCT to DRR generated by average image of 4D CT, the total uncertainties can be limited within few millimeters.…”

Analysis of the advantage of individual PTVs defined on axial 3D CT and 4D CT images for liver cancer

“…The use of 4D CT scanning is not only able to determine the intrafractional tumor motion, 6 , 8 but also to eliminate respiration motion artifacts 9 , 11 . Moreover, the size of PTVs of lung cancers can be reduced using the technique of 4D CT compared with the technique of 3D CT 5 , 12 , 13 .…”

An automated method for comparing motion artifacts in cine four‐dimensional computed tomography images

“…The time stamps of the reconstructed CT images and the measured respiratory signal of the patient are retrospectively matched. The reconstructed images are sorted either by the phase (2) or by the displacement, (3) which are then stacked to create a three‐dimensional (3D) image of the patient for each image bin. A 4D image set is then reconstructed by viewing the 3D images in sequence for each image bin.…”