High Field MRI: Developments for carotid artery and brain perfusion imaging

Abstract

In this research some innovative approaches to image the carotid arteries with 7T MRI are introduced. It shows the potential of ultra-high field MRI for carotid artery imaging, and addresses some major technical hurdles related to the use of high frequency RF pulses as well. A so-called leaky waveguide transmitter was developed which was used in combination with a high density receive array. A group of healthy subjects was scanned with the developed setup at 7T, as well as on a state-of-the-art setup at 3T. At an average depth of the carotids, a considerable gain in SNR of a factor 2.0 was measured. The setup benefits from the high density of receivers by increased acceleration capacity. T1 and T2 in the healthy vessel wall was measured and reported both for 3T and 7T. T2 was found to be decreased at 7T, whereas the an increase of T1 was reported. These values were unknown at 7T and can be used for sequence optimization. The leaky waveguide transmitter that was developed for RF excitation in the neck, could also be used to extend the reach of the standard 7T head coil for brain imaging. In this combination the waveguide transmitter was used for the magnetic labelling of the blood passing through the carotids. Arterial spin labelling (ASL) sequences were applied to quantify perfusion in the brain. Whole brain ASL has shown to be challenging at 7T, due to the effects of tissue heating and limited power efficiency. The combined setup of the head coil together with external label coil allowed for efficient labelling, far enough below the brain to allow for whole brain ASL. B1 performance was quantified, and possibilities were explored. RF shimming for territorial selection and increased labelling delays up to 4000ms were performed. Essential blood suppression pre-pulses that are used at 3T cannot be used at 7T. The culprit of this effect is the local character of transmit coils at 7T. An evaluation of alternative blood suppression techniques was done, that may require no or less B1+ coverage. The results can serve as a basis for further developments for in vivo high field blood suppression techniques. A good blood suppression technique would facilitate more freedom in sequence design for carotid plaque imaging.

A group of patients with a higher than 70% stenosed carotid artery was scanned with the developed setup at 7T MRI and compared with histopathology. At this stage, full plaque component analysis was not possible yet due to the limited set of sequences. However, we showed carotid plaque imaging at 7T is feasible, even though sequence development could improve the outcome, especially with respect to the T1W scan. This is mainly due to blood suppression complications, which are not available at 7T at the moment of this study.