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S919
ESTRO 36 2017
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EP-1703 Rapid prototyping phantom using LEGO® for
MRI distortion correction in MR guided radiation therapy
S. Neppl
1,2
, M. Reiner
1
, M. Peller
3
, C. Belka
1
, K. Parodi
2
,
F. Kamp
1
1
LMU Munich, Department of Radiation Oncology,
Munich, Germany
2
LMU Munich, Department of Medical Physics, Garching
b. München, Germany
3
LMU Munich, Department for Clinical Radiology,
Munich, Germany
Purpose or Objective
An accurate geometry is critical for the use of MR images
for dose calculation in MR guided radiation therapy.
Therefore, we designed an easily adjustable distortion
phantom based on LEGO® technic parts to detect and
correct geometrical inaccuracies caused by magnetic field
inhomogeneities.
Material and Methods
The designed phantom consists of LEGO® technic beam
parts with 13 holes, rectangular beam parts with 3+4 holes
("L shape" for stability) and pins to connect 2 or 3 beam
parts. The holes within the beam parts have a diameter of
4.85 mm and a center distance of 7.99 mm. The phantom
has a size of 24x24x22 cm³ and is placed in a container
filled with water (Fig. 1). The LEGO® parts do not give a
signal and are therefore not visible on the MR image in
contrast to their water-filled holes. The MR images were
acquired on a SIEMENS Magnetom® Aera with a clinically
used T1 weighted MPRAGE (Magnetization Prepared RApid
Gradient Echo) sequence with an isotropic voxel size of
1.5 mm, TE = 1.98 ms, TR = 1900 ms, TI = 900 ms and a
flip angle of 8°. An automatic hole detection of the Lego
parts was developed with the Image Processing Toolbox™
of Matlab® 2016a. First the beam parts are segmented
with an adaptive threshold and then a search for circular
structures is performed within the beam mask. The
centers of the recognized circles are iteratively corrected
to the brightest position within the circle. The exact
reference hole positions are imported from the CAD model
of
the
phantom.
Figure 1: Photo of the designed LEGO® phantom in
water. This phantom consists of 270 "beam 13 parts"
(grey), 85 "beam 3+4 parts" (black), long pins (blue) and
short pins (black).
Results
A very customizable MRI distortion correction phantom
was developed. The beam parts are correctly detected in
the inner part of the images. At the edges of the phantom
the distinction to the air outside the container and a low
contrast leads to undetected holes. The phantom should
hence not be placed at the bottom, but in the center of
the water container. More than 90 % of the holes within
the recognized beam parts are correctly located by the
algorithm (Fig. 2). The main reason for detection failures
are the connection pins, which are often filled with air
bubbles (no MR signal). The detection rate could be
improved using prior knowledge of the shape of the beam
parts,
available
from
the
CAD
model.
Figure 2: An exemplary snippet of a phantom MR slice.
The blue circles visualize the recognized holes on the
MR image and the red dots show the exact positions
given by the CAD model.
Conclusion
A cheap and extremely customizable phantom was
designed using LEGO® technic parts to correct MR images
for geometric distortion. For fast prototyping, the size of
the phantom can be easily adapted. The hole positions can
be extracted correctly and with a high detection rate from
the MR images. The shift from their nominal position to
the detected position can be used to create a distortion
map.
EP-1704 Breast tumour bed contouring: influence of
surgical clips assessed on the same imaging.
D. Ciardo
1
, M. Leonardi
1
, A. Morra
1
, G. Fanetti
1
, D.
Damaris
1
, S. Dicuonzo
1
, V. Dell'Acqua
1
, R. Ricotti
1
, F.
Cattani
2
, R. Cambria
2
, G. Baroni
3
, R. Orecchia
4
, B.
Jereczek-Fossa
1
1
European Institute of Oncology, Department of
Radiation Oncology, Milan, Italy
2
European Institute of Oncology, Unit of Medical Physics,
Milan, Italy
3
Politecnico di Milano, Dipartimento di Elettronica-
Informazione e Bioingegneria, Milan, Italy
4
European Institute of Oncology, Department of Medical
Imaging and Radiation Science, Milan, Italy
Purpose or Objective
To evaluate the inter-observer variability in the
contouring of tumour bed after lumpectomy in breast-
conservative surgery patients.
Material and Methods
We retrospectively analysed the planning computed
tomography (CT) of 47 patients who underwent external
radiotherapy (ERT) post-lumpectomy and intra-operative
radiotherapy (IORT). Three to 5 surgical clips were
positioned to delineate the excision cavity. The CT
acquired for ERT planning was modified to obtain a virtual
CT scan with hidden clips: in particular, clips were blurred
using a fully automated MATLAB script that replaces each
voxel of the clip with a value obtained from a normal
pseudo-random distribution with mean value and variance
corresponding to the mean value and variance of an outer
ring of the clip itself. Two expert and 2 junior radiation
oncologists contoured the tumour bed on the original CT
and on the processed CT, thus obtaining 376 contours. For
each patient, and for all possible pairwise combination of
contours, the center of mass distance (CMD), the overlap