A comparison between amplitude sorting and phase-angle sorting using external respiratory measurement for 4D CT

Abstract

Respiratory motion can cause significant dose delivery errors in conformal radiation therapy for thoracic and upper abdominal tumors. Four-dimensional computed tomography (4D CT) has been proposed to provide the image data necessary to model tumor motion and consequently reduce these errors. The purpose of this work was to compare 4D CT reconstruction methods using amplitude sorting and phase angle sorting. A 16-slice CT scanner was operated in ciné mode to acquire 25 scans consecutively at each couch position through the thorax. The patient underwent synchronized external respiratory measurements. The scans were sorted into 12 phases based, respectively, on the amplitude and direction (inhalation or exhalation) or on the phase angle (0-360 degrees) of the external respiratory signal. With the assumption that lung motion is largely proportional to the measured respiratory amplitude, the variation in amplitude corresponds to the variation in motion for each phase. A smaller variation in amplitude would associate with an improved reconstructed image. Air content, defined as the amount of air within the lungs, bronchi, and trachea in a 16-slice CT segment and used by our group as a surrogate for internal motion, was correlated to the respiratory amplitude and phase angle throughout the lungs. For the 35 patients who underwent quiet breathing, images (similar to those used for treatment planning) and animations (used to display respiratory motion) generated using amplitude sorting displayed fewer reconstruction artifacts than those generated using phase angle sorting. The variations in respiratory amplitude were significantly smaller (P < 0.001) with amplitude sorting than those with phase angle sorting. The subdivision of the breathing cycle into more (finer) phases improved the consistency in respiratory amplitude for amplitude sorting, but not for phase angle sorting. For 33 of the 35 patients, the air content showed significantly improved (P < 0.001) correlation with the respiratory amplitude than with the phase angle, suggesting a stronger relationship between internal motion and amplitude. Overall, amplitude sorting performed better than phase angle sorting for 33 of the 35 patients and equally well for two patients who were immobilized with a stereotactic body frame and an abdominal compression plate.

Evaluation of the combined use of two different respiratory monitoring systems for 4D CT simulation and gated treatment

“…The 3D volumetric images at individual respiratory phases are obtained by sorting the oversampled images or sinograms based on the associated respiratory phase‐angle or amplitude from the breathing trace that is acquired simultaneously during scanning 12. In the treatment planning process, if a gated treatment plan is decided, the 4D images are combined with the respiratory motion data to select an appropriate gating window 10.…”

Real‐time 4D dose reconstruction for tracked dynamic MLC deliveries for lung SBRT

“…The phase‐binning algorithm is based on previous work by Lu et al 29 but was modified to allow for online binning. Bins are defined relative to the automatically detected extremal respiratory positions (“peaks” and “valleys”) in each respiratory cycle.…”

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

“…Strategies to reduce motion artifacts and improve 4D CT image clarity have been developed and investigated — for example, the use of audiovisual biofeedback to help improve the patient's respiratory regularity, 9 , 10 the application of different algorithms to improve retrospective sorting, 11 , 13 and postprocessing of data to improve image reconstruction (14) . However, most of these strategies rely on visual evaluation of the improvements and are, therefore, prone to human subjectivity.…”