Commissioning of a 4D MRI phantom for use in MR‐guided radiotherapy

Abstract

Purpose: Systems for integrated magnetic resonance guided radiation therapy (MRgRT) provide real-time and online MRI guidance for unequaled targeting performance of moving tumors and organs at risk. The clinical introduction of such systems requires dedicated methods for commissioning and routine machine quality assurance (QA). The aim of the study was to develop a commissioning protocol and method for automatic quantification of target motion and geometric accuracy using a 4D MRI motion phantom. Materials and methods: The commissioning was performed on a clinically used 3 T MR scanner. The phantom was positioned on a flat tabletop overlay using an in-house constructed base plate for a quick and reproducible setup. The torso-shaped phantom body, which was filled with mineral oil as signal generating medium, included a 3D grid structure for image distortion analysis and a cylindrical thru-hole in which a software-controlled moving rod with a hypo-intense background gel and a decentralized hyper-intense target simulated 3D organ motion patterns. To allow for sequence optimization, MR relaxometry was performed to determine the longitudinal T 1 and transverse T 2 relaxation times of both target and background gel in the movable cylinder. The geometric image distortion was determined as the mean and maximum 3D Euclidean distance (D mean , D max ) of grid points determined by nonrigid registration of a 3D spoiled gradient echo MRI scan and a CT scan. Sinusoidal 1D/2D/3D motion trajectories, varying in amplitude and frequency, as well as an exemplary 1D MR navigator diaphragm motion pattern extracted from a healthy volunteer scan, were scanned by means of 2D cine MRI and 4D MRI. Target positions were automatically extracted from 2D cine MRI using an in-house developed software tool. Results: The base plate enabled a reproducible setup with a deviation of <1 mm in all directions. Relaxometry yielded T 1 /T 2 values for target and background gel of 208.1 AE 2.8/30.5 AE 4.7 ms and 871 AE 36/13.4 AE 1.3 ms, respectively. The 3D geometric image distortion increased with distance from the magnetic isocenter, with D mean = 0.58 AE 0.30 mm and D max = 1.31 mm. The frequencies of the reconstructed motion patterns agreed with the preset values within 0.5%, whereas the reconstructed amplitudes showed a maximum deviation to the preset amplitudes of <0.5 mm in AP/LR direction and <0.3 mm in IS direction. Conclusion: A method and protocol for commissioning of a 4D MRI motion phantom on a 3 T MR scanner for MRgRT was developed. High-contrast and geometrically reliable 2D cine MR images of the phantom's moving target could be obtained. The preset motion parameters could be extracted with sufficient spatio-temporal accuracy from 2D cine MRI in all motion directions. The overall 3D geometric image distortion of <1.31 mm within the phantom grid confirms geometric accuracy of the clinically utilized 3D spoiled gradient echo sequence. The method developed can be used for routine QA tests of spatio-temporally resolved MRI data in MRgRT.

Medical physics challenges in clinical MR-guided radiotherapy

“…To address these issues, Tijssen et al suggest several dedicated QA tests inferring, among others, image quality at different gantry positions and image quality during beam delivery and MLC movement, which is particularly important when performing imaging during irradiation [65]. Dedicated phantom set-ups and QA procedures for gated beam delivery are also subject of current research [32,173].…”

“…Moreover, dosimeters capable of accurate absolute dose measurement in 2D or even 3D in the presence of magnetic fields, e.g., film, dosimetric gel or 3D diode arrays, are required [176][177][178]. The use of various static and dynamic, geometrical and anthropomorphic phantoms for end-to-end tests is reported, including also dedicated end-to-end tests for stereotactic radiosurgery [179] and for motion management relying on MR guidance [173]. More recently, developments towards specific deformable QA phantoms are reported [175,180].…”

A strategy to prevent a temperature‐induced MRI artifact in warm liquid phantoms due to convection currents

“…MRI phantoms have been developed primarily as standard reference objects or calibration tools used to validate the accuracy of in vivo measurements. Phantoms are built for a wide range of applications including geometric distortion, 2-4 motion correction, [5][6][7] Abbreviations used: 2D, two-dimensional; 3D, three-dimensional; ADC, apparent diffusion coefficient; B 0 , static magnetic field; B 1 , applied magnetic field; BW, bandwidth; CPMG, Carr- relaxometry, 8-10 perfusion, [11][12][13] diffusion, [14][15][16] chemical characterization [17][18][19] and MR-guided therapeutics, [20][21][22] all of which attempt to model a standard in vivo environment.…”

Anthropomorphic phantom for deformable lung and liver CT and MR imaging for radiotherapy

“…Many imaging phantoms designed for 4D thorax imaging are simple geometric phantoms with moving targets such as commercial phantoms (e.g. QRM Sim4D + Thorax-Phantom (QRM GmbH, Moehrendorf, Germany), Respiratory Gating platform (Standard Imaging Inc. Middleton, WI, USA) and MODUS-QA quasar respiratory motion phantom (Modus Medical Devices Inc. London, CA)) or custom phantoms (Fattori et al 2016, Schneider et al 2019 generally driven by step motors. Others are anthropomorphic (Nioutsikou et al 2006, Followill et al 2007, Serban et al 2008, Steidl et al 2012, Mayer et al 2015, Hazelaar et al 2018, such as CIRS (Dynamic Thorax Phantom-CIRS Inc. Norfolk, VA, USA) but with only a moving target, these are not necessarily useful for validating all 4D imaging techniques.…”