Simultaneous acquisition of image and navigator slices using CAIPIRINHA

Introduction Respiratory organ motion is a complicating factor in many treatments. Especially for dynamic dose delivery such as in tumor therapy, exact knowledge of organ motion and its influence on the dose distribution is crucial. Siebenthal et al [1] recently purposed a novel respiratory correlated MR imaging method that acquires a full 2D sagittal navigator image instead of the commonly used one-dimensional diaphragm navigator. This method allows for tracking vascular structures during complete breathing cycles with minimal out-of-plane motion. The 4D-MRI (three dimensional and one temporal dimension) model of organ motion requires rapid acquisition of 2D images. It also requires high contrast between liver tissue and vessels, in contrast to conventional (and less precise) methods that only detect the diaphragm position. High vessel-tissue contrast is generated with balanced Steady State Free Precession (bSSFP) sequences giving bright vessel signal enhancement. In the original method, image and navigator slices were acquired interleaved. Since the navigator image is used to estimate the exact position and shape of organ, a time lag between image and navigator could lead to discrepancy in images and even to building of incorrect models. In this paper, we describe a new approach to image and navigator acquisition based on multislice Controlled Aliasing in Parallel Imaging Results in Higher Acceleration (CAIPIRINHA) [2], which allows a true simultaneous acquisition with consequent decrease of lag and increase of temporal resolution.