Session Item

August 30
10:30 - 11:30
Online Stream 2
Poster Highlights 21: Hyperthermia
Johannes Crezee, The Netherlands
Poster highlights
First results from the national network for quality assurance of MR images in RT in Denmark
Signe Winther Hasler, Denmark


First results from the national network for quality assurance of MR images in RT in Denmark

Signe Winther Hasler1,2, Jesper Folsted Kallehauge3,4, Rasmus Hvass Hansen5, Carl Magnus Nilsson6, Dennis Tideman Arp7, Henrik D. Nissen8, Jens M. Edmund9,10, Faisal Mahmood1,2

1Odense University Hospital, Laboratory of Radiation Physics, Department of Oncology, Odense, Denmark; 2University of Southern Denmark, Department of Clinical Research, Odense, Denmark; 3Aarhus University Hospital, Danish Centre for Particle Therapy, Aarhus, Denmark; 4Aarhus University, Department of Clinical Medicine, Aarhus, Denmark; 5Copenhagen University Hospital, Section for Radiation Therapy, Department of Oncology, Center for Cancer and Organ Diseases, Copenhagen, Denmark; 6Zealand University Hospital, Radiation Therapy Department, Næstved, Denmark; 7Aalborg University Hospital, Department of Medical Physics, Department of Oncology, Aalborg, Denmark; 8Vejle Hospital, Department of Medical Physics, Vejle, Denmark; 9Herlev and Gentofte Hospital, Radiotherapy Research Unit, Department of Oncology, Herlev, Denmark; 10University of Copenhagen, Niels Bohr Institute, Copenhagen, Denmark

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Purpose or Objective

It is known that MRI is not as geometric accurate as CT. In MRI-guided RT this can have implications for the accuracy of dose delivery. MRI is now available in all RT facilities in Denmark and to monitor image quality, a national network for quality assurance of MR images in RT in Denmark (NIMBUS) was founded in 2018. The aim of the network is to improve quality by performing and sharing measurements and knowledge across all centers in Denmark and in particular to secure image quality in national clinical trials. By identifying baseline performance for MR image quality and quantitative MRI on a national level, centers that deviate can be identified and action can be taken to improve quality. In this study, the geometric accuracy of different MR scanners in eight centers was evaluated.

Material and Methods

The steering committee of NIMBUS reached out to all RT centers in Denmark through an MRI QA workshop to launch the network. At the participating centers, a general MRI sequence (3D T1W GRE) was used to test geometric accuracy with adjustments in repetition time and echo time to accommodate differences in field strength. A large field of view phantom (Magphan RT 820) was used in a traveling-phantom setup for all acquisitions. The acquisitions were made in the period from March 2019 to December 2020.

Distortion analysis of the MRI scans was performed in the web-based software Smári Image Analysis Service (The Phantom Laboratory, NY, USA). The analysis output was distortions as a function of distance to iso-center as well as maximum distortion and mean of the 10% maximum distortions within a 200 and 350 mm diameter spherical volume (DSV). The mean of the 10% maximum distortions was reported as a robustness check of the maximum distortion.


Seven of eight Danish RT centers and one radiological department have participated in this study so far. In total, the centers had eight MRI scanners and two MR-linacs (Table 1) which were evaluated.

All maximum distortions were below 0.43 mm within 200 mm DSV and below 1.2 mm within 350 mm DSV, except for the Siemens Aera 1.5 T (Figure 1). The mean of the 10% maximum distortions was in general about 20% lower than the maximum reported values; for all scanners, except for the Siemens Aera 1.5 T, the mean of the 10% maximum distortions was less than 1 mm within the 350 mm DSV. No dependency on field strength was seen for the reported distortions. 


In general, the scanners at the participating centers showed maximum distortions below 0.5 mm within 200 mm from the iso-center of the scanner. The reason for the high level of distortion of the MRI scanner at one center has not yet been found, but the detection of an outlier proves the value of the national collaborative MRI quality assurance network. The next steps include analysis of clinical MRI sequences, other image quality metrics, independent distortion calculation, and a web-based quality assurance system for monitoring.