ESTRO 36 Abstract Book

S488 ESTRO 36 2017 _______________________________________________________________________________________________

Oncology, Dresden, Germany 7 Institute of Radiooncology, Helmholtz-Zentrum Dresden-Rossendorf, Dresden, Germany Purpose or Objective In neuro-oncology, 3 Tesla (3T) MRI is the current clinical standard for tumor localization, radiotherapy volume delineation and stereotactic (radio)surgery, sometimes complemented by amino acid PET imaging. With superior SNR and image resolution, anatomical 7T MRI can visualize micro-vascularization in glioblastomas potentially allowing improved target volume delineation. However, concerns about geometrical distortion (GD) with increasing field strength (B 0 ) are detrimental for applications of 7T MR in image-guided interventions. For high-precision treatment strategies, the spatial integrity of anatomical images needs to be warranted within ±1mm. The aim of the study was to evaluate B 0 - and sequence- related GD in clinically relevant 7T sequences and compare it to equivalent 3T sequences and CT images Material and Methods To quantify B 0 - and sequence-related GD in T1-GRE, T1- TFE, T2-TSE, T2-TSE FLAIR on 7T and 3T sequences, a dedicated anthropomorphic head-phantom (CIRS Model 603A) was used. The phantom is composed of bone-/soft- tissue equivalent materials and contains a 3D grid (3mm rods spaced 15mm apart). System-based distortion correction methods were applied to restore the gradient uniformity of 3T and 7T. For all CT and MR images, 436 points of interests (POIs) were defined by manual reconstruction of the 3D grid points in the respective images. GD was assessed in 3 ways. Firstly, global GD was estimated by the mean absolute difference (MAD global ) between the measured and the true Euclidian distances of all unique combinations of POIs, independent of location within the phantom. Secondly, local GD was assessed by MAD local between the measured and the true Euclidian distances of all POIs relative to the magnetic field isocenter. Thirdly, a distortion map was created by evaluating 3D displacement vectors for each individual grid point in 3T and 7T images ranged from 0.19−0.75mm and 0.27−1.91mm, respectively, and was more pronounced than in CT images. CT was not entirely free of GD with MAD global ranging from 0.14−0.64mm. B 0 -related GD was larger in 7T than in 3T MRI with MAD local ranging from 0.21-1.81mm and 0.11-0.73mm, respectively (p<0.05). MAD local increased with increasing distance from the magnetic isocenter and largest GDs were noted at the level of the skull (Fig. 1). MAD local was <1mm for all sequences up to 68.7mm from the isocenter. Sequence- related GD at 7T was prominent in T1-TFE and significantly differed from other 7T sequences (p<0.001). Figure 2 shows an anisotropic distribution of GD in T1-TFE with increasing GD along the frequency-encoding direction Results MAD global

radiotherapy.

PO-0894 Comparing the spatial integrity of 7T and 3T MR images for image-guided radiotherapy of brain tumors J. Peerlings 1,2 , I. Compter 1 , F.M. Janssen 1 , C.J. Wiggins 3 , F.M. Mottaghy 2,4 , P. Lambin 1 , A.L. Hoffmann 1,5,6,7 1 Maastricht University Medical Center+- GROW - School for Oncology and Developmental Biology, Department of Radiation Oncology - MAASTRO, Maastricht, The Netherlands 2 Maastricht University Medical Center+, Department of Radiology and Nuclear Medicine, Maastricht, The Netherlands 3 Maastricht Brain Imaging Center - Scannexus, Maastricht University, Maastricht, The Netherlands 4 University Hospital RWTH Aachen University, Department of Nuclear Medicine, Aachen, Germany 5 University Hospital Carl Gustav Carus at the Technische Universität Dresden, Department of Radiotherapy, Dresden, Germany

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