Brain and Head-and-Neck MRI in immobilization masks: a novel and practical setup for radiotherapy

Purpose or Objective
In radiotherapy (RT), it is essential to perform the MRI and
CT exams in treatment position. For this purpose and to
minimize inter/intra-fraction movement, thermoplastic
immobilization masks are used for brain/head and neck
(HN) RT. However, standard immobilization masks are
incompatible with diagnostic MR head/neck coils. As a
compromise, flexible surface coils are adopted despite
their technician dependent positioning and inferior signalto-noise-ratio
(SNR) compared to head/neck coils (2-
channel vs. the 17-channel for head/neck receive coil).
This leads to relatively poor image quality and
reproducibility. Here, we explore the feasibility and
performance of a new immobilization setup for brain/HN
RT, redesigned to fit into the diagnostic head/neck MR
coils thereby boosting MR image quality and
reproducibility.
Material and Methods
MR images were acquired on 2 volunteers using a 3T
Ingenia MRI (Philips Healthcare, Best, The Netherlands).
The standard and the new RT setups are presented in
Fig.1. Several comparative tests were performed.
 Image quality test: 3D T1w TFE, 3D T2w TSE
FLAIR and 2D T2w TSE scans were acquired for
brain, for neck 2D T1w TSE and 2D T2w TSE
mDIXON scans were acquired.
 SNR test: SNR maps were computed from a T1w
scan (2 dynamics, the second being a noise scan)
as in (Kellman P., et al. MRM (2005)54:1439-47).
 Motion restriction test: the maximum motion in
the feet-head/left-right directions was
estimated from 2D cine-MR T1w bFFE
acquisitions (300 dynamics) using Optical Flow
(Zachiu C., et al. Phys Med Biol (2015)60:9003–
29).
 Inter-fraction repositioning test: two high
resolution 3D T1w sequences were acquired for
each setup. Between acquisitions, the
immobilization mask was removed and the
subject was asked to move to mimic two
different MRI sessions. The mean and the
standard deviation of the subject movement
between each pair of MR acquisitions were
computed using Optical Flow as a proxy of the
reproducibility of inter-fraction repositioning.
Results
Brain images acquired with the standard RT coil setup had
an inferior diagnostic quality compared to the new setup,
especially for the FLAIR contrast (inferior sensitivity in
detecting small lesions) (Fig. 2). For neck regions, the
image quality was comparable between setups.
The proposed setup allowed between 2 and 3 times higher
SNR values for both brain/HN MRI (not shown – limited Fig.
number).
Comparable motion restriction in the feet-head/left-right
directions (maximum motion ≈ 1 mm) and comparable
repositioning accuracy (mean inter-fraction movement 1
mm ± 0.5 mm) were observed for the standard and the
new setup.
Conclusion
We integrated a fixation mask in the diagnostic head/neck
MR coils. The proposed setup has several advantages:
diagnostic image quality in RT treatment position, high
SNR, homogenous signal, restricted motion (1 mm) and
accurate inter-fraction repositioning. Translation of the
new setup to the treatment table will be investigated.