4D flow MRI facilitates quantification of cardiac phase-resolved blood velocity vector fields and has successfully been deployed to study cerebrovascular flow. Besides cardiac-induced flow pulsation, respiration is known to modulate arterial and venous blood flow in the brain. Quantification of the respiratory flow modulation (RFM) holds potential to further our insights into vascular coupling and improve our understanding of cerebral circulation in general. A 5D phase-contrast flow MRI framework was developed to volumetrically quantify RFM by resolving velocity vector fields over the cardiac and respiratory cycle, with high spatial (0.82 mm isotropic) and cardiac (55 ms) resolutions, using two respiratory states whilst accounting for variable physiological RFM delays, with a reasonable acquisition time (20 min at 60 bpm). Recent advances in deep learning-based image reconstruction and analysis methods are incorporated to facilitate the approach. The 5D flow MRI framework was validated in 10 healthy volunteers with reference to fully sampled respiratory-resolved 2D flow MRI orthogonal to the internal carotid artery (ICA), yielding Pearson correlation coefficients of 0.97 and 0.90 and biases of and 0.09 % and 1.77 % for RFM of mean velocity magnitude and amplitude, respectively. The value of cerebrovascular 5D flow MRI is demonstrated using full-field spatially resolved RFM quantification of mean velocity and velocity amplitude, revealing a high physiological intra-subject variability. Cerebrovascular 5D flow MRI enables the study of full-field respiratory flow modulation holding potential of furthering our understanding of cerebral circulation.

Magnetic resonance imaging (MRI) is a valuable clinical and research tool for patients managed using deep brain stimulation (DBS). Unfortunately, MRI under these conditions is associated with substantial risks, necessitating stringent regulations that limit its clinical and research utility. In addition, magnetic susceptibility differences between DBS hardware and surrounding tissues significantly compromise spatial encoding mechanisms of conventional MRI sequences, resulting in image signal loss and geometric distortions. The impact on gradient-recalled echo echo-planar imaging sequences commonly utilised for functional MRI in DBS settings is particularly severe. This review presents a range of mitigation strategies aimed at both safety and enhanced image quality, spanning innovations in DBS hardware design to advanced MRI sequences capable of addressing issues inherent to the presence of DBS hardware, such as electrode heating and susceptibility artefacts. Additionally, we highlight approaches incorporating the discussed novel postprocessing techniques and functional MRI acquisition protocols, along with their limitations and associated challenges to enable their wider dissemination, with the overarching objective of improving the quality of life of DBS patients.