ESTRO 2020 Abstract book

S1014 ESTRO 2020

Conclusion Extended FOV reconstructions can affect dose distributions and the patient volume outside the sFOV and this volume should be as low as reasonably possible. There is no alternative to the use of truncated data to reconstruct images outside the sFOV. Dose differences can be mitigated with multiple photon beams from different directions (e.g. VMAT treatments). VB10 shows an improved performance compared to VA62 for phantoms, but there is no ground truth for patients. Current accuracy is probably enough for photon treatments with multiple beams, but caution needs to be taken for single field photon treatment or particle therapy. PO-1735 Superparamagnetic nickel ferrite nanoparticles as contrast agents in Magnetic Resonance Imaging C. Ferrer 1 , C. Huertas 1 , R. Plaza 1 , E. Corredoira 1 1 Hospital universitaria La Paz, Radiofísica y Radioprotección, Madrid, Spain Purpose or Objective This work studies the possibility of using NiFe 2 O 4 superparamagnetic nickel ferrite nanoparticles (SPION) as contrast agent in Magnetic Resonance Imaging (MRI). Specifically, its use is tested in a Brachytherapy application, to make visible the source path inside a catheter and the interstitial needle end. Material and Methods The SPIONs used present a medium size of 20 nm and, due the nickel toxicity, are coated with silica. The high magnetic moment of the SPIONs produces inhomogeneities in the local magnetic field. SPIONs create microscopic field gradients in the presence of a strong magnetic field, which causes the reduction and the relationship with two phases of longitudinal relaxation time (T1) or transverse time (T2 and T2 *) in the nearby nuclei. This induces areas in the image by hyper- (for T1) or hypo-intense MRI (for T2W and T2W *), highlighting the areas where NPs are concentrated. SPIONs suspended in agarose gel have been injected into a flexible 6F catheter inserted into the ovoid of a Fletcher- type applicator supplied by Elekta, into an agar dummy. A 3 T MRI acquisition was performed using an axial and 3D T2W sequence, with 200 ms repetition time and 90 ms echo time. In addition, a plastic needle has been filled with the same SPION suspension. Results The silica coated SPIONs, have shown to be effective in their function as a contrast agent in the ovoid's MR image, correctly assessing its path, using the same brachytherapy MRI sequence than with patients. In addition, the needle tip end is visible, producing a smaller artifact that the one achieved with titanium. Measured distances in CT imaging and MRI showed to be coincident. SPIONs are also visible in the CT image, so they could be use equally as CT contrast agents. As SPIONs are within the catheter, there are no problems due to possible toxicity, nor by phagocytosis by the endoplasmic reticulum.

Conclusion The use of SPION improves the sensitivity in the MR image and allows the source path visualization inside the catheter for its subsequent reconstruction, as well as the interstitial needle tip end. PO-1736 High frame rate CAIPIRINHA-TWIST-VIBE MRI for volumetric motion monitoring in MRgRT: a pilot study J. Yuan 1 , O.L. Wong 1 , R.Y.W. Ho 1 , Y. Zhou 1 , K.Y. Cheung 1 , S.K. Yu 1 1 Hong Kong Sanatorium & Hospital, Medical Physics and Research Department, Happy Valley, Hong Kong SAR China Purpose or Objective Orthogonal cine MRI has been clinically used for motion monitoring/gating in MR-guided-radiotherapy (MRgRT), but might misestimate motion due to the lack of volumetric motion information. We aim to develop a fast 3D volumetric dynamic MRI for potential respiratory motion monitoring/gating in MRgRT. Material and Methods 7 healthy volunteers underwent free-breathing abdominal scans on a 1.5T MR-sim in RT position with an 18-channel body matrix coil and a spine coil. 3D gradient-echo sequence VIBE (Volumetric Interpolated Breath-hold Examination) was accelerated by 4-fold CAIPIRINHA (Controlled aliasing in parallel imaging results in higher acceleration). View sharing technique TWIST was used (20% and 25% undersampling rate for central and peripheral K-space) to further accelerate acquisition. This imaging protocol (Axial, TE/TR=0.6/1.5ms, flip angle=5 o , partial-Fourier factor=0.75, 56 slices/volume, voxel size=3x3x4.5mm 3 , temporal resolution= ~0.33s) yielded a frame rate of ~3 frame-per-second (fps) to cover the entire abdominal motion. 144 time frames (8064 slice images) were acquired with a scan duration of ~49s. Dynamic images were retrospectively reconstructed and reformatted. Abdominal organ motions with respiration in 3D were demonstrated by rigid and deformable registration using 3D slicer and Elastix. Results All scans were successful. The high 3fps frame rate was fast enough to divide a normal respiratory cycle of ~3-4s into ten phases directly on volumetric images, without relying on any external/internal respiration surrogate. It completely avoided the volume discontinuity (missing, overlap, zig-zag boundary etc) problems associated with slice-by-slice MRI acquisition and complicated respiration phase or magnitude sorting algorithms. The reformatted CAIPIRINHA-TWIST-VIBE images of a volunteer were illustrated (Fig. 1), along with the voxel-wise dynamic displacement vector field (DVF) calculated by deformable registration. The respiration induced blurring was minor at diaphragm in SI direction. The cardiac motion effect on image quality was unnoticeable. Slight ghosting artifact along AP was observed due to the highly undersampled K- space acquisition, which might not much affect image segmentation and registration for clinical purpose, but need to be further validated. Other limitations in image