The use of CT density changes at internal tissue interfaces to correlate internal organ motion with an external surrogate

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.

Advances in 4D radiation therapy for managing respiration: Part I – 4D imaging

“…1). An image-based internal surrogate is identified from the reconstructed image, including body area [14], analysis of various metrics in regions of interest [15, 16], and image registration [17]. Data-based surrogates are formed by identifying a consistent anatomical feature or implanted fiducial marker in the raw data (sinogram), and using this to sort the raw projections into bins.…”

The 4th Annual Ontario Thoracic Cancer Conference at Niagara-on-the-Lake

“…Based on evaluation of dose–volume histogram parameters, respiratory gating reduces the amounts of normal lung and liver that receive a significant dose. However, optimal techniques are still being investigated to correlate the use of external marker motion with internal tumour or organ motion 8. Despite the uncertainties of dose distribution and organ tracking, the use of respiratory gating is a promising strategy to aid in dose escalation, in the avoidance of critical structures influenced by respiration, and in the delivery of intensity-modulated radiation treatments.…”

Non-rigid image registration of 4D-MRI data for improved delineation of moving tumors

“…In this context, 4D computed tomography (CT) is commonly used as the basis for treatment planning [5]. External [6] or internal [7,8] respiratory surrogates can be acquired simultaneously during free-breathing CT imaging to retrospectively sort the acquired data into different respiratory phases (bins) and hence to avoid motion artifacts [9,10]. Magnetic resonance imaging (MRI) is an alternative method to acquire 4D data sets.…”