96 / 1096
S83
ESTRO 36
_______________________________________________________________________________________________
Conclusion
Despite standardized information and performing
simulation on the treatment machine, anxiety remains an
item that needs specific attention. Even with fraction
duration times of up to 75 minutes, only a single patient
perceived this as being unacceptably long.
OC-0162 Optimizing sequences for MRI-guided
radiotherapy in cranial and head and neck regions
W.W.K. Fung
1
, S.Y. Man
1
, J. Yuan
2
, L.H. FUNG
2
, W.P.
LUK
2
, G. Chiu
1
1
Hong Kong Sanatorium & Hospital, Department of
Radiotherapy, Happy Valley, Hong Kong SAR China
2
Hong Kong Sanatorium & Hospital, Medical Physics &
Research Department, Happy Valley, Hong Kong SAR
China
Purpose or Objective
MR sequences using parallel acquisition technique (PAT)
with increasing acceleration factors could reduce the scan
time for treatment verification, but with the cost of losing
image quality that could affect verification accuracy. This
study assessed the effect of different PAT factors on image
quality, scan time and fusion accuracy, thus choosing a
sequence which is clinically suitable for MRI-guided RT in
cranial (C) and HN regions.
Material and Methods
Ten healthy volunteers were set up in treatment position
using headrest and immobilization mask on the flat couch
of a 1.5T MRI-simulator (Siemens MAGNETOM Aera). High
resolution isotropic (1.05mm) 3D TSE T1W and T2W MR
sequences were acquired (MR-ref). Based on MR-ref, 11
low-resolution (isotropic 1.4mm) verification sequences
(MR-P
xy
s) were acquired with GRAPPA where acceleration
factors x and y were altered in respectively phase
encoding and slice encoding directions. Effective PAT
factor (PAT-f) equals x times y. Four therapists (2 seniors
& 2 juniors) performed two sets of fusions: MR-ref & MR-
P
xy
s and MR-ref & duplicated MR-ref (control set) for C and
HN region. Shift results (6DOF) were recorded. Survey was
given to observers for scoring the image quality. Logistic
and linear regression were used.
Results
The scan time for MR-ref were 301s and 330s, and for MR-
P
xy
s it ranged from 249s to 49s and 254s to 59s for T1W and
T2W images respectively when PAT-f increased from 3
(MR-P
31
) to 16 (MR-P
44
). Subjective analysis showed that
the scores of all verification series were lower than the
reference and decreased with increasing PAT-f. Image
quality decreased when reducing the scan time (Fig.1).
Significant reduction of image quality (p<0.05) occurred
when PAT-f reached 12 for T1W and 6 for T2W images.
Observers favored T1W over T2W images (p<0.0001).
Scores from senior observers were significantly better
than juniors (p<0.0001).
About 80% of absolute shift errors in all MR-P
xy
s were not
significantly different from MR-ref in all directions. No
specific trend between PAT-f and shift error was observed,
implying fusion accuracy could not be predicted by PAT-f
value. The errors of T2W were significantly smaller than
T1W in almost all of the shifts (p<0.05). Junior observers
performed significantly better than seniors when fusing
T1W images (p<0.05). For treatment verification purpose,
T1W MR-P
42
(88s) and T2W MR-P
22
(173s) were chosen since
they were the fastest sequences with comparable image
quality to MR-ref. However when considering fusion
errors, MR-P
44
for T1W (49s) and T2W (59s) images, with
an even shorter scan time, should be opted instead as its
verification accuracy had no significant difference in most
shift directions from MR-ref (Fig.2) despite significantly
degraded image quality.
Conclusion
The degradation of image quality by increasing PAT-f does
not necessarily affect the fusion accuracy. MR-P
44
with
acceptable verification accuracy and shortest scan time is
therefore proposed for MRI-guided RT in C and HN regions.
OC-0163 Online workflow for the First-in-Man study
on bone metastases at the MRI-linear accelerator
L.T.C. Meijers
1
, S.J. Hoogcarspel
1
, A.N.T.J. Kotte
1
,
C.N.N. Nomden
1
, G.G. Sikkes
1
, I.H. Kiekebosch
1
, E.N.
Groot de
1
, G.H. Bol
1
, B. Asselen van
1
, I.M. Jurgenliemk-
Schulz
1
, L.G.W. Kerkmeijer
1
, B.W. Raaymakers
1
1
UMC Utrecht, Radiotherapy department, Utrecht, The
Netherlands
Purpose or Objective
The first treatment on the MR-Linac in the UMC Utrecht
includes patients with bone metastases, the first-in-man
study. A fast workflow is required since all processes will
be performed in an online setting. The aim of the present
study is to test the feasibility of an online workflow for
these patients at the MR-Linac within a 30 minute time
limit.
Material and Methods
A workflow with offline and online procedures was
developed for the MR-Linac. To test this workflow, the
Alderson phantom was used, a human body shaped
phantom with tissue-equivalent material. The phantom
yields sufficient CT and MR contrast. The offline workflow
included acquisition of a reference CT scan and manual
delineation of the spinal cord and one lumbar vertebra
(vertebra = CTV, PTV-CTV margin = 5 mm). The online
workflow at the MR-Linac, shown in figure 1, included
setup of the patient (= Alderson phantom) and acquisition