Abstract
In the AAPM Report 80,( 1 ) the imaging modality of 4D CT and respiration‐correlated CT was declared a “promising solution for obtaining high‐quality CT data in the presence of respiratory motion”. To gather anatomically correct data over time, the existence of correlation between the internal organ movement and an external surrogate has to be assumed. For the in‐house evaluation of such correlation, we retrospectively analyzed 21 four‐dimensional computer tomography (4D CT) scans of five patients, out of which the artifacts experienced in three patients are shown here. To provide context and a baseline for the analysis of patient motion, a real‐tissue liver phantom was used with a solid water block and liver tissue. The superior–inferior motion of fiducials in phantom and patients was correlated to the recorded anterior–posterior motion of an external surrogate marker on the chest. The use of a solid water block yielded a measurable correlation coefficient of 0.98 or better using a sinusoidal animation pattern. With sinusoidally‐animated liver tissue, the minimum correlation observed was 0.96. Comparing this to retrospective patient data, we found three cases of a change in the correlation coefficient, or simply a low correlation. The source of this low correlation was investigated by careful examination of the breathing traces and the CT‐phase assignments used to reconstruct the datasets. Consequences of nonregular breathing are elaborated on. We demonstrate the impact of wrong phase assignments and missing image information in the 4D CT phase sampling processes. We also show how daily patient‐based correlation analysis can indicate changes in breathing traces, which can be significant enough to decrease, or completely eliminate, previously existing correlation.PACS numbers: 87.57.‐s, 87.57.Q‐, 87.57.cp, 87.57.N‐, 87.55.Qr
Highlights
In today’s radiation therapy programs, the use of 4D CT is the state-of-the-art solution to detect and address patient breathing-induced tissue motion
Clinical process and patient selection Our clinical SBRT protocol requires one 4D CT for treatment planning purposes and a 4D CT done prior to every SBRT fraction.[8] for most of the SBRT patients treated at our facility, four to six 4D CT datasets have been acquired over the course of treatment
We were able to localize the center of a stationary ball bearings (BBs) from all CT phases with 0.1 mm standard deviation, at 1.25 mm slice thickness with the phantom, and 0.3 mm with 2.5 mm slice thickness in patient scans
Summary
In today’s radiation therapy programs, the use of 4D CT is the state-of-the-art solution to detect and address patient breathing-induced tissue motion. 165 Szegedi et al.: 4D CT image acquisition errors in SBRT of liver well as accurately calculate and display doses. This data can only be obtained if proper correlation exists between the surrogate and various organs in the first place. Recent studies have focused on the correlation validity of 4D CT information, with Beddar et al[2] showing that this correlation generally exists for a cohort of eight patients, with noted variations over the breathing cycle. It is important to note that the accuracy of the 4D CT data itself, used to “measure” and study the correlation, is vulnerable to errors and artifacts due to irregular breathing patterns.[1,3] It is reasonable to anticipate that a measured correlation might deteriorate if highly irregular breathing is encountered
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
