Abstract
2D cine MR imaging may be utilized to monitor rapidly moving tumors and organs-at-risk for real-time adaptive radiotherapy. This study systematically investigates the impact of geometric imaging parameters on the ability of 2D cine MR imaging to guide template-matching-driven autocontouring of lung tumors and abdominal organs.Abdominal 4D MR images were acquired of six healthy volunteers and thoracic 4D MR images were obtained of eight lung cancer patients. At each breathing phase of the images, the left kidney and gallbladder or lung tumor, respectively, were outlined as volumes of interest. These images and contours were used to create artificial 2D cine MR images, while simultaneously serving as 3D ground truth. We explored the impact of five different imaging parameters (pixel size, slice thickness, imaging plane orientation, number and relative alignment of images as well as strategies to create training images). For each possible combination of imaging parameters, we generated artificial 2D cine MR images as training and test images. A template-matching algorithm used the training images to determine the tumor or organ position in the test images. Subsequently, a 3D base contour was shifted to the determined position and compared to the ground truth via centroid distance and Dice similarity coefficient.The median centroid distance between adapted and ground truth contours was 1.56 mm for the kidney, 3.81 mm for the gallbladder and 1.03 mm for the lung tumor (median Dice similarity coefficient: 0.95, 0.72 and 0.93). We observed that a decrease in image resolution led to a modest decrease in localization accuracy, especially for the small gallbladder. However, for all volumes of interest localization accuracy varied substantially more between subjects than due to the different imaging parameters.Automated tumor and organ localization using 2D cine MR imaging and template-matching-based autocontouring is robust against variation of geometric imaging parameters. Future work and optimization efforts of 2D cine MR imaging for real-time adaptive radiotherapy is needed to characterize the influence of sequence- and anatomical site-specific imaging contrast.
Highlights
Respiratory, cardiac and gastrointestinal induced motion causes substantial changes in patient anatomy over the course of a single radiotherapy fraction (Langen and Jones 2001, Seppenwoolde et al 2002, Bussels et al 2003, Kitamura et al 2003)
The following sections present the dependency of the localization accuracy on the different imaging parameters for the left kidney, gallbladder and lung tumor
As the centroid distance (CD) and Dice similarity coefficient (DSC) were found to be highly correlated (Pearson coefficient: −0.91 for kidney, −0.84 for gallbladder, −0.94 for lung tumor), we only show and discuss results using CD as accuracy metric
Summary
Respiratory, cardiac and gastrointestinal induced motion causes substantial changes in patient anatomy over the course of a single radiotherapy fraction (Langen and Jones 2001, Seppenwoolde et al 2002, Bussels et al 2003, Kitamura et al 2003) This may lead to underdosage of the tumor and additional radiation exposure of critical organs (Keall et al 2006). The volume of interest may deform, shift perpendicularly to the plane or move out of it entirely For this reason, others have investigated how to best position, orientate and utilize 2D cine MR images for tumor and organ localization (Bjerre et al 2013, Tryggestad et al 2013, Brix et al 2014, Ipsen et al 2016, Bourque et al 2018). These studies were mostly based on heuristic approaches, tested only a small number of imaging strategies and often lacked a 3D ground truth as it is intrinsically not available in 2D MR images
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