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and to establish the performance of the approach on a
much larger patient cohort.
PO-0903 Patient-induced susceptibility effects
simulation in magnetic resonance imaging
J.A. Lundman
1
, M. Bylund
1
, A. Garpebring
1
, C.
Thellenberg Karlsson
1
, T. Nyholm
1
1
Umeå University, Department of Radiation Sciences,
Umeå, Sweden
Purpose or Objective
The role of MRI is increasing in radiotherapy. A
fundamental requirement for safe use of MRI in
radiotherapy is geometrical accuracy. One factor that can
introduce geometrical distortion is patient-induced
susceptibility effects. This work aims at developing a
method for simulating these distortions. The specific goal
being to objectively identify a balanced acquisition
bandwidth, keeping these distortions within acceptable
limits for radiotherapy.
Material and Methods
A simulation algorithm based on Maxwell’s equations and
calculations of shift in the local B-field was implemented
as a dedicated node in Medical Interactive Creative
Environment (MICE), which is available as a free
download. The algorithm was validated by comparison
between the simulations and analytical solutions on digital
phantoms. Simulations were then performed for four body
regions using CT images for eight prostate cancer patients.
For these patient images, CT Hounsfield units were
converted into magnetic susceptibility values for the
corresponding tissues, and run through the algorithm.
Figure 1: Simulated normalized local B-field for one of the
patients [ppm].
Results
The digital phantom simulations showed good agreement
with analytical solutions, with only small discrepancies
due to pixelation of the phantoms. For a bandwidth of 440
Hz at 3 T, the calculated distortions in the patient-based
images showed maximal 95th percentile distortions of
0.39, 0.32, 0.28, and 0.25 pixels for the neck, lungs,
thorax with the lungs excluded, and pelvic region,
respectively. In order to accommodate other field
strengths and bandwidths, normalized displacement
values were also simulated for these body regions.
Table 1: Simulated displacement values normalized to
field strength and bandwidth [pixels * BW / B0]
Conclusion
The 95th percentile of the patient-induced susceptibility
distortions can be kept below 0.5 pixels for a 3 T system
and 440 Hz bandwidth. With the provided normalized
data, distortions for other field strengths and bandwidths
can be calculated. The developed simulation software can
also be used to quickly and easily estimate the
susceptibility-based distortions from a given series of
patient CT images that are converted into susceptibility
values, or directly from a susceptibility map.
PO-0904 Development of an MRI-protocol for
radiotherapy treatment guidance in gastric cancer
V.W.J. Van Pelt
1
, M.F. Kruis
1
, T. Van de Lindt
1
, L.C. Ter
Beek
2
, M. Verheij
1
, U.A. Van der Heide
1
1
Netherlands Cancer Institute Antoni van Leeuwenhoek
Hospital, Radiation Oncology, Amsterdam, The
Netherlands
2
Netherlands Cancer Institute Antoni van Leeuwenhoek
Hospital, Radiology, Amsterdam, The Netherlands
Purpose or Objective
Because of the superior soft-tissue contrast of MRI,
integration of MRI in pre-operative radiotherapy (RT) for
gastric cancer, is expected to improve the identification
of shape and position of the target volume. MRI of the
stomach is technically challenging due to respiratory,
cardiac and bowel motion. In this study we therefore
developed a scan protocol consisting of anatomical and
functional sequences for staging and target delineation
(TD), for treatment planning (TP) including motion
modeling and for intra-fraction motion monitoring (MM).
Material and Methods
For staging and TD we compared high resolution (HR) T2-
weighted (T2w) turbo spin echo (TSE) MRI, applying either
navigator or respiratory sensor triggering during the
exhale position of the diaphragm to reduce motion
artifacts. For TP, the feasibility of a fast 3D HR mDixon
with a large Field of View (FoV) within one exhale breath-
hold (BH) was evaluated. For motion modeling, a 4D T2w
MRI with retrospective self-sorting reconstruction was
tested for robustness
[1]
. For intra-fraction MM, 2D T1w
dynamic turbo field echo (TFE), fast field echo (FFE) and
TSE Cine-MRI with a refocusing pulse were compared. For
staging and treatment response monitoring, a single-shot
echo planar Diffusion Weighted Imaging (DWI) was tested
using b-values of 0, 200 and 800 s/mm², applying either
free-breathing (FB), BH, navigator or respiratory
triggering. For Dynamic contrast enhanced (DCE) MRI, FB
T1w spoiled gradient echo, 4D mDixon and 4D THRIVE with
keyhole technique were compared. Subtraction images