  MRI-linear accelerator (MRI-Linac) systems allow for daily tracking of MRI
changes during radiotherapy (RT). Since one common MRI-Linac operates at 0.35T,
there are efforts towards developing protocols at that field strength. In this
study we demonstrate the implementation of a post-contrast 3DT1-weighted
(3DT1w) and dynamic contrast enhancement (DCE) protocol to assess glioblastoma
response to RT using a 0.35T MRI-Linac. The protocol implemented was used to
acquire 3DT1w and DCE data from a flow phantom and two patients with
glioblastoma (a responder and a non-responder) who underwent RT on a
0.35T-MRI-Linac. The detection of post-contrast enhanced volumes was evaluated
by comparing the 3DT1w images from the 0.35T-MRI-Linac to images obtained using
a 3T-standalone scanner. The DCE data were tested temporally and spatially
using data from the flow phantom and patients. K-trans maps were derived from
DCE at three time points (a week before treatment: Pre RT, four weeks through
treatment: Mid RT, and three weeks after treatment: Post RT) and were validated
with patients treatment outcomes. The 3D-T1 contrast enhancement volumes were
visually and volumetrically similar (+/- 0.6-3.6%) between 0.35T MRI-Linac and
3T. DCE images showed temporal stability, and associated K-trans maps were
consistent with patient response to treatment. On average, K-trans values
showed a 54% decrease and 8.6% increase for a responder and non-responder
respectively when Pre RT and Mid RT images were compared. Our findings support
the feasibility of obtaining post-contrast 3DT1w and DCE data from patients
with glioblastoma using a 0.35T MRI-Linac system.
