The Use of CT Density Changes at Internal Tissue Interfaces to Correlate Internal Organ Motion with an External Surrogate

Stewart Gaede, University of Western Ontario
Gregory Carnes, University of Western Ontario
Edward Yu, University of Western Ontario
Jake Van Dyk, University of Western Ontario
Jerry Battista, University of Western Ontario
Ting-Yim Lee, University of Western Ontario

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Published in: Phys. Med. Biol. 54 259-273. doi: 10.1088/0031-9155/54/2/006

Abstract

The purpose of this paper is to describe a non-invasive method to monitor the motion of internal organs affected by respiration without using external markers or spirometry, to test the correlation with external markers, and to calculate any time shift between the datasets. Ten lung cancer patients were CT scanned with a GE LightSpeed Plus 4-Slice CT scanner operating in a ciné mode. We retrospectively reconstructed the raw CT data to obtain consecutive 0.5 s reconstructions at 0.1 s intervals to increase image sampling. We defined regions of interest containing tissue interfaces, including tumour/lung interfaces that move due to breathing on multiple axial slices and measured the mean CT number versus respiratory phase. Tumour motion was directly correlated with external marker motion, acquired simultaneously, using the sample coefficient of determination, r(2). Only three of the ten patients showed correlation higher than r(2) = 0.80 between tumour motion and external marker position. However, after taking into account time shifts (ranging between 0 s and 0.4 s) between the two data sets, all ten patients showed correlation better than r(2) = 0.8. This non-invasive method for monitoring the motion of internal organs is an effective tool that can assess the use of external markers for 4D-CT imaging and respiratory-gated radiotherapy on a patient-specific basis.