A Model for Gastrointestinal Tract Motility in a 4D Imaging Phantom of Human Anatomy

Abstract

<h3>Purpose/Objective(s)</h3> Gastrointestinal (GI) tract motility is one of the main sources for intra/inter-fraction variability and uncertainty in validating deformable image registration (DIR) algorithms. We present an implementation of four modes of GI motility in the 4D extended cardiac-torso (XCAT) digital phantom. <h3>Materials/Methods</h3> We consider motility modes that exhibit large amplitude changes in the diameter of the GI tract and may persist over timescales comparable to online adaptive planning and radiotherapy delivery. The following modes are modeled: peristalsis, rhythmic segmentation, high amplitude propagating contractions (HAPC), and tonic contractions. The models are intended for the organ of origin (i.e., peristalsis in esophagus and small intestine; HAPCs in colon) but can be applied to any user-specified GI tract organ. Peristalsis and rhythmic segmentations are modeled by traveling and standing sinusoidal waves. HAPCs and tonic contractions are modeled by traveling and stationary Gaussian waves. Spatial and temporal dispersion are implemented by exponential functions. The model is applied at the control points of the nonuniform rational B-spline (NURBS) surfaces defined in the reference XCAT phantom. The reference NURBS surfaces are dynamically updated at each timepoint based on the user-defined parameters shown in Table 1. GI motility is combined with the respiratory and cardiac motion available in XCAT. Cine MRI acquisitions from 50 fractions of 10 patients treated in a 1.5 T MR-Linac were analyzed to estimate default parameters for each model. <h3>Results</h3> We demonstrate the ability to generate realistic 4D MRI images that simulate four modes of GI motility combined with respiratory and cardiac motion. All modes of motility, except tonic contractions, were observed in the analysis of our cine MRI acquisitions. Peristalsis was the most common. Default parameters estimated from cine MRI data or from literature (if not observed in our measurements) are used as initial values for simulation experiments. <h3>Conclusion</h3> The digital phantom provides realistic models to aid in multimodality imaging research. The addition of GI motility will further contribute to the development, testing, and validation of DIR algorithms for MR-guided radiotherapy.