Characterizing geometrical accuracy in clinically optimised 7T and 3T magnetic resonance images for high-precision radiation treatment of brain tumours

Abstract

Background and purpose: In neuro-oncology, high spatial accuracy is needed for clinically acceptable high-precision radiation treatment planning (RTP). In this study, the clinical applicability of anatomically optimised 7-Tesla (7T) MR images for reliable RTP is assessed with respect to standard clinical imaging modalities.

Materials and methods: System- and phantom-related geometrical distortion (GD) were quantified on clinically-relevant MR sequences at 7T and 3T, and on CT images using a dedicated anthropomorphic head phantom incorporating a 3D grid-structure, creating 436 points-of-interest. Global GD was assessed by mean absolute deviation (MAD_Global). Local GD relative to the magnetic isocentre was assessed by MAD_Local. Using 3D displacement vectors of individual points-of-interest, GD maps were created. For clinically acceptable radiotherapy, 7T images need to meet the criteria for accurate dose delivery (GD < 1 mm) and present comparable GD as tolerated in clinically standard 3T MR/CT-based RTP.

Results: MAD_Global in 7T and 3T images ranged from 0.3 to 2.2 mm and 0.2-0.8 mm, respectively. MAD_Local increased with increasing distance from the isocentre, showed an anisotropic distribution, and was significantly larger in 7T MR sequences (MAD_Local = 0.2-1.2 mm) than in 3T (MAD_Local = 0.1-0.7 mm) (p < 0.05). Significant differences in GD were detected between 7T images (p < 0.001). However, maximum MAD_Local remained ≤1 mm within 68.7 mm diameter spherical volume. No significant differences in GD were found between 7T and 3T protocols near the isocentre.

Conclusions: System- and phantom-related GD remained ≤1 mm in central brain regions, suggesting that 7T MR images could be implemented in radiotherapy with clinically acceptable spatial accuracy and equally tolerated GD as in 3T MR/CT-based RTP. For peripheral regions, GD should be incorporated in safety margins for treatment uncertainties. Moreover, the effects of sequence-related factors on GD needs further investigation to obtain RTP-specific MR protocols.

Keywords: Anthropomorphic phantom; Diametric spherical volume; Geometrical distortion; Neuro-oncology; Radiation treatment planning; Ultra-high field MRI.

Fig. 1

Overview of 3D data acquisition and analyses. XYZ-coordinates are determined on CT and MR images (1a.), and based on product characteristics of the CIRS’ phantom model 603A (1b.). Global MAD (2a.) is based on distances with 2 variable grid-intersection points (*example given for 1 intersection but applies for all points). Local MAD (2b.) is based on distances between magnetic field isocentre and 1 variable grid-intersection points. Displacements vectors (3a.) are determined between the measured and reference coordinates of each individual data-point and indicate the relative 3D geometrical distortion.

Fig. 2

Absolute differences in Euclidian distance between the measured and reference dataset (grey dot) relative to the unique distances found in the reference dataset, observed within CT (a), MP2RAGE (b), T2-SPACE (c), T2-SPACE FLAIR (d), T1-GRE (e), 3D TFE (f), T2-VISTA (g), T2-VISTA FLAIR (h), T1-FFE (i). The overall geometric distortion was quantified by MAD_Global (±SD) (blue square). The 95% confidence interval (CI₉₅) is shown as the dotted horizontal line. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 3

MAD_Local values (±SD) relative to the distance from the magnetic isocentre at 7T (a) and 3T (b) MRI, both relative to CT (black square). Presented 7T MR sequence include MP2RAGE (green circle), T2-SPACE (purple diamond), T2-SPACE FLAIR (orange downward triangle), and T1-GRE (blue upward triangle). The same colour- and shape-code was used for the equivalent 3T sequences, 3D TFE, T2-VISTA, T2-VISTA FLAIR, and T1-FFE, respectively. The dotted horizontal line represents the 1 mm-acceptability level required for spatially reliable RTP. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 4

Distortion maps of MP2RAGE on 7T MRI (a) and 3D TFE on 3T MRI (b) measured in the axial plane nearest the magnetic isocentre (Y = 0), in the coronal plane (Z = 0), and in the sagittal plane (X = 0).

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