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Introduction Stereotactic body radiation therapy (SBRT) has been validated for liver lesions [1]. Because high dose per fraction is delivered on small volume, very accurate lesion segmentation is necessary. MRI is the most relevant imaging for liver lesion contouring but it is prone to motion artifacts if breathing is not managed. Evaluation of liver lesion motion induced by breathing is necessary in radiotherapy to guide patient treatment planning (respiratory gating). A 4D-MRI sequence was previously validated for liver [2]. In this study, an automated method was developed to sort images according to the breathing cycle. Methods The 4D-MRI acquisition is performed with an experimental sequence (bSSFP TrueFISP [3]) on 1.5T Magnetom AeraTM (Siemens). This sequence allows very fast axial and sagittal 2D acquisition during free breathing. Axial slice positions (0.88 s/slice) are incremented to cover the liver volume. Sagittal slices (navigators) are acquired at a fixed position. The navigators are used to assign a phase for each axial slice which depend on an amplitude sequencing of the respiratory cycle (0% = inspiration, 16%, 33%, 50% = expiration, 66%, 83%). Lung volume is estimated on each navigator by automatic segmentation and allows to assign a phase for the coupled axial image in the breathing cycle. As the image number per respiratory cycle is low, the sequence is repeated 20 times in order to achieve satisfactory sampling. The sequence was evaluated on 4 volunteers with an audio coaching (5 and 6 s period) and 2 patients without audio coaching. Results Patient image sorting according to their position in the breathing cycle was realized by automated phase detection (Fig. 1) and images were imported into the treatment planning system Eclipse 13.7TM (Varian) in 6 phases. Audio coaching is not used here. It is required for patients with irregular breathing cycle. From axial images, Eclipse reconstructs slices in sagittal and coronal plans, and allows to display a cine view with the 6 breathing phases in order to analyze the dynamic behavior of the liver. Conclusions 4D-MRI images from volunteers and patients were acquired with bSSFP TrueFISP sequence. After automated sorting with the method developed in this study, 6 breathing cycle phases were imported into Eclipse. Patient 4D-MRI can be registered with 4D-CT to increase the physician's segmentation accuracy. [ABSTRACT FROM AUTHOR]

Introduction Liver imaging is challenging due to its constant movement with breathing. In Magnetic Resonance Imaging (MRI), different strategies have been developed for organ movement consideration [1–3] . These methods are limited by the temporal resolution, as well as image quality. A novel retrospective gating method for dynamic 3D MR imaging during free breathing was developed [1] and further improved by Celicanin et al. [4] . This work aims of assessing the method and the reconstruction technique for liver cases. The final clinical application is to improve liver lesion delineation for Stereotactic Body Radiation Therapy (SBRT). Methods The innovative 4D MRI approach has been evaluated for liver motion estimation during entire respiratory cycle on a healthy volunteer. It is based on simultaneous acquisition of 2D images and a navigator set using acceleration CAIPIRINHA technique [4] providing no temporal divergence between them. During the entire acquisition time, the navigator is fixed in the same position, while the image position is changing, in order to cover the entire organ during free breathing [1] . The entire respiratory cycle is acquired, using a modified balanced Steady State Free Precession sequence (bSSFP) and is reconstructed in a retrospective manner ( Fig. 1 ). Images are acquired with a 1.5T MRI (MAGNETOM Aera; Siemens Medical Solutions, Erlangen, Germany), using an 18-channel design body flex coil. The 3.5 mm 2D axial slices are obtained, using the following sequence parameters: TE = 2.27 ms, TR = 591.97 ms, FA = 70 °, FOV = 37 × 37 cm 2 , matrix = 256 × 246. Total acquisition time is 33.175 s (0.83 s per slice). Results The preliminary results obtained on a healthy volunteer are shown in Fig. 1 . Images are characterized by high resolution and great contrast, making this method promising for liver lesion examination. The reconstruction of 4D MRI and the sequence optimization to cover the entire respiration cycle with the appropriate time sampling is ongoing work. Conclusions The benefit of MRI acquisition in treatment position to improve lesion contouring for liver SBRT has already been demonstrated [5] . The presented 4D MRI method represents a highly time efficient technique for the free breathing liver motion registration, giving the possibility to sort and reconstruct the images according to the respiratory phases. It will allow registration with 4D CT planning, likely to improve delineation accuracy. The same approach can be applied to study moving organs others than liver. [ABSTRACT FROM AUTHOR]