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S488
ESTRO 36 2017
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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
6
OncoRay, National Center for Radiation Research in
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
Results
MAD
global
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