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Commissioning of a 4D dynamic MRI motion phantom on a 3T MR scanner: motion validation study

Schneider, S.; Dolde, K.; Engler, J.; Hoffmann, A.; Pfaffenberger, A.



Systems for integrated magnetic resonance guided radiation therapy (MRgRT) provide real-time and on-line MRI guidance for unequalled targeting performance of moving tumours and organs at risk. The clinical introduction of such systems requires dedicated methods for commissioning, routine machine quality assurance (QA) and verification of gated and adaptive treatment plans. The aim of the study was to develop a commissioning protocol and method for automatic quantification of target motion and geometrical fidelity using a 4D dynamic MRI motion phantom.

Materials & Methods:

The CIRS MRI-LINAC Dynamic Phantom (Model 008M) was positioned on a flat tabletop overlay (Medibord Ltd) in a 3.0T MR scanner (Ingenuity TF PET/MR, Philips) using an in-house constructed base plate for a quick and reproducible setup. The torso-shaped phantom body, which was filled with mineral oil (Marcol Blend, Philips), includes 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 simulates 3D organ motion patterns. The geometric image distortion was determined as the mean and maximum Euclidean distance (∆mean, ∆max) between grid points in 3D spoiled gradient echo (THRIVE) MRI- and CT scans (Siemens Somatom Definition AS) after registration by means of the central fiducial. Sinusoidal 1D/2D/3D and a volunteer navigator scan-based 1D target motion pattern were evaluated using 2D cine MRI balanced turbo field echo sequences at a temporal resolution of 1.5-2 Hz. The motion tracking was performed with different pre-set frequencies (0.1-0.2 Hz) and amplitudes (up to 40/10/10 mm in IS/AP/LR direction). The target’s centre-of-mass motion pattern was reconstructed by an in-house developed automated image tracking algorithm implemented in MATLAB.

The base plate enabled a reproducible setup with a deviation of <1 mm in all directions. The geometric distortion increased with distance from the phantom’s centre predominantly in anterior direction, with ∆mean=0.56+/-0.28 mm and ∆max=1.5 mm in the THRIVE sequence relative to the CT scan. The frequencies of the reconstructed motion patterns from 2D cine MRI agree to the pre-set values within 2%; the reconstructed amplitudes showed a maximum deviation to the pre-set amplitudes of <0.8 mm in AP/LR direction and <0.4 mm in IS direction.


A method and protocol for commissioning a 4D MRI dynamic motion phantom on a 3.0T 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 pre-set motion parameters could be extracted with sufficient spatio-temporal accuracy from 2D cine MRI in all motion directions. The method developed can be used for routine QA tests of spatio-temporally resolved MRI data in MRgRT.

Keywords: Phantom; Motion; MRI

  • Lecture (Conference)
    6th MR in RT Symposium 2018, 30.06.-03.07.2018, Utrecht, Niederlande


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