47 Automated respiratory cycle binning for liver 4D-MR imaging

Abstract

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. Download : Download high-res image (278KB) Download : Download full-size image 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.