Original paperMethod of evaluating respiratory induced organ motion by vector volume histogram
Introduction
Respiratory induced organ motion is an important issue in radiotherapy treatment planning of the thoracic and upper abdominal regions. It produces the greatest movement in the caudal-cranial (CC) direction because the most important muscle used in inhalation is the diaphragm. Several authors previously reported respiratory induced organ motion as studied using several imaging modalities. The lungs, esophagus, liver, pancreas, breast, prostate, stomach, and kidneys, among other organs, are all known to move with breathing [1], [2], [3], [4], [5], [6], [7]. Most of these data only measured in limited ways: at several points, the centroid, or the edge of the organ. These are derived from the spatial characteristics of respiratory induced organ motion; however, this approach does not consider organ deformation.
In recent years, deformable image registration (DIR) has been developed as a technology useful in application to image-guided radiotherapy (IGRT) and adaptive radiotherapy (ART) in its ability to create a new deformed image. Applications for radiotherapy include dose accumulation with DIR, auto segmentation, four-dimensional (4D) dose accumulation, and 4D computed tomography (4D CT)-derived ventilation imaging [8], [9], [10]. An advantage of DIR in ART is the spatial mapping of corresponding locations between images, and this may be used for structure delineation on a second image when the set of structures is present on the first image. Thus, each phase of the 4D CT image dataset can be deformed to match the objective phase image. With DIR, both end-expiration and end-inspiration phase images may be used to evaluate organ motion due to respiration. Several researchers have investigated the motion and deformation of tumors and normal tissue using image registration techniques [11], [12]. The plotted trajectories, or ‘mesh’, were displayed to indicate their calculation results, and results from fixed landmark points, and deformable techniques were compared. Currently, there are no known reports of using quantitative information as a function of the vector-volume for characterizing respiratory induced organ motion.
Here, we propose a novel method for evaluating respiratory organ motion using DIR. This work presents a quantitative method that evaluates the vector with the location of pixels inside each organ. We also measured respiratory organ motion from the displacement of the centroid of the organ in two phases.
Section snippets
Methods and materials
We used the 4D CT datasets of the five patients from the DIR website (www.DIR-lab.com). This website provides this set of test data for the specific critical evaluation of DIR spatial accuracy performance in multiple clinical settings. For a detailed description of the datasets, we refer the reader to Castillo et al. [13], [14]. The 4D CT images were acquired over the entire thorax and upper abdomen at 2.5 mm slice spacing using a General Electric Discovery ST PET/CT scanner (GE Medical Systems,
Results
Fig. 1 shows a deformation vector image in the three planes in the left-right (LR), anterior-posterior (AP), and CC directions for Case 1. As observed, the deformation of the lower lung and upper abdomen in the CC direction is quite large. Organ motion in both the AP and LR directions is smaller than that observed along the CC axis. Deformation greater than 20 mm occurs mainly around the diaphragm. Fig. 2 shows VVHs in the LR, AP, and CC directions and the 3D vector for Case 1. Organ motion
Discussion
In most previous investigations, the magnitudes of the tumor and organ motions were simply calculated using the centroid or edge coordinates of the contour drawn on each phase [1], [2], [3], [4]. Feng et al. analysed intra-fraction pancreatic tumor motion using cine-magnetic resonance imaging (MRI) and characterized the motion of the borders of tumors rather than the position of a single point such as the centroid of the organ [4]. It is well known that respiration induces not only rigid
Conclusions
We introduced a method of evaluating respiratory induced organ motion using DIR. Our proposed VVH method could be easily used for the volumetric evaluation of respiratory induced organ motion. Moreover, the method of VVH proposes to extend the methodology to inter-fractional motion using CBCT with DIR.
Conflict of interest
None.
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