Traveling Wave MRI for the Acquisition of Reference Images for Parallel Imaging at the Carotid Artery at 7T - Proof of Concept

Introduction: MRI of the carotid artery and specifically of plaque formation near its bifurcation, is important to asses plaque composition and the associated risk of stroke [1]. Imaging these plaques at 7T may result in higher resolution and in better qualification and risk assessment of the different plaque components, compared to 1.5T and 3T. Dedicated surface coils that can be positioned close to the carotids have been used in phased array for reception of RF signal with maximal efficiency. Undersampling of k-space can then be applied using techniques based on k-space reconstruction (SMASH, GRAPPA) or on image-based reconstruction (SENSE). For most of these methods good sensitivity profiles of the individual coils are needed, which can be obtained by dividing each individual coil image by an additional reference image acquired with uniform spatial sensitivity either using RF body coils or using a virtual body coil by a fixed combination of the images obtained with the receiver coils. In the absence of a uniform B1-field (i.e. at high field strength like 7T), transceivers are applied that can be used for excitation and optimal reception. However, optimal reception requires closely positioned RF coils at the expense of uniformity. Specifically at body parts that vary substantially between subjects, like the human neck, uniformity may be substantially compromised because of the use of small surface coils, which eliminates the possibility of using such setup for reference images. Therefore, we propose to use travelling wave MRI [2] for the acquisition of uniform reference images. As such antennas have a low efficiency, the carotid array was used for transmitting. This reference scan can also be used to correct images for image inhomongeneity by dividing them by this reference image (CLEAR). This abstract shows a proof of concept by means of experimental results on a volunteer subject.