ESTRO 37 Abstract book

S281

ESTRO 37

Conclusion Two techniques for distortion assessment have been shown and an offline geometric distortion correction method implemented. Ongoing work is to provide a sequence independent on-line correction for gradient non linearity. PV-0534 Multi-Resolution radial MRI to Reduce IDLE time in pre-beam imaging on an MR-Linac (MR-RIDDLE) T. Bruijnen 1 , B. Stemkens 1 , J.J.W. Lagendijk 1 , C.A.T. Van den Berg 1 , R.H.N. Tijssen 1 1 University Medical Center Utrecht, Radiotherapy, Utrecht, The Netherlands Purpose or Objective The exquisite soft-tissue contrast of MRI makes MR-Linac (MRL) systems the ultimate tool for online adaptive radiation treatments. The acquisition time of a conventional 3D pre-beam imaging sequence is several minutes, which results in long idle times before the first clinical actions can be performed. An additional 4D-MRI to characterize motion and obtain a representative (mid- position) motion state, further increases the pre-beam imaging time up to 10 minutes. To overcome these problems we have developed a single, multi-resolution, multi-purpose, radial MR sequence that allows image reconstruction to commence during data acquisition. The resolution and dimensionality (3D or 4D) are flexible and optimized with respect to the amount of data acquired at Fig 1 illustrates the general concept of the proposed pre- beam imaging protocol. Imaging data are continuously acquired during free-breathing using a 3D golden angle stack-of-stars readout [1]. Radially sampled sequences are robust to motion-induced artefacts and inherently portray the time-averaged (blurred) position of the anatomy [2]. After limited acquisition time we utilize the multi-resolution feature of golden angle radial sampling to interchange temporal and spatial resolution to obtain a first low resolution blurred image (3x3x4mm). This volume can rapidly be reconstructed and initially used for contour propagation. Data collection is continued and a second reconstruction is performed with increased spatial resolution (1.5x1.5x4mm). Next, the inherent self-gating is exploited to perform a motion-weighted image reconstruction (soft-gating) [3] to reduce respiratory- induced blurring of the mid-position. After completing the acquisition, a full 4D-MRI with 10 respiratory phases is reconstructed. each instant in time. Material and Methods

Results Fig 2 shows the results of in-silico simulations and an in- vivo abdominal acquisition on the MRL of MR-RIDDLE for a 6 min scan. Top row depicts the reconstructions on three different time points for the phantom that periodically translates in the left-right direction with a sine wave. From left to right the spatial resolution increases and motion artefacts are reduced. Bottom row shows fat- suppressed balanced gradient echo MRL data with corresponding acquisition and reconstruction times. From left to right the vessels in the liver are better defined, contours become sharper and artifacts are reduced. At the end of the acquisition the data are used to perform the slow and computationally expensive 4D reconstruction.

Conclusion MR-RIDDLE is capable of producing a mid-position volume for initialization of contouring or contour propagation in well under a minute after which the image acquisition continues to produce high-resolution updates and eventually a 4D-MRI reconstruction. Moreover, it improves the validity of the observed anatomy and corresponding motion due to the decreased latency with respect to the radiation treatment. PV-0535 Optimizing Acquisition Speed and Contrast of Respiratory Correlated 4D-MRI on a 1.5T MRI-Linac B. Stemkens 1 , T. Bruijnen 1 , C.A.T. Van den Berg 1 , J.J.W. Lagendijk 1 , R.H.N. Tijssen 1 1 UMC Utrecht, Department of Radiation Oncology, Utrecht, The Netherlands Purpose or Objective Over the last few years various 4D-MRI methods have been developed, but mostly on diagnostic 1.5T or 3T scanners. The goal of this work is two-fold: 1) to assess the image quality of respiratory-resolved 4D-MRI acquisition strategies with varying contrast on a 1.5T MRI- Linac system and 2) to optimize image speed for online abdominal motion characterization.