Editorial for “Non‐Invasive Assessment of Damping of Blood Flow Velocity Pulsatility in Cerebral Arteries with 7 T MRI”

Editorial for “Non-Invasive Assessment of Damping of Blood Flow Velocity Pulsatility in Cerebral Arteries with 7 T MRI” The pulsatile blood flows along the cardiovascular tree driven by the aortic pressure. Under normal conditions, the central arteries can dampen the hemodynamic pulsations through Windkessel effect and protect the cerebral microcirculation from excessive pulsatile stress. With aging or diseases, however, the arteries inevitably become stiffer, making the damping less effective. The distal cerebral arteries and microvasculature would then suffer from elevated blood pulsations, resulting in damages to the brain parenchyma, which is thought to contribute to cognitive impairments and neurodegeneration. Therefore, measuring arterial pulsatility is crucial for the prevention and treatment of cerebral diseases. Until now, most research on blood flow pulsatility and damping mainly focused on relatively large arteries such as the aorta, carotid arteries, and the circle of Willis (CoW). Yet, studies on the damping of pressure pulsations distal from the CoW have rarely been reported. A recent study provided a 4D flow magnetic resonance imaging (MRI) approach to automatically assess the global flow pulsatility in distal cerebral vessels, originating from the cerebral cortical arteries. Nevertheless, more distal arteries with diameters smaller than 1 mm cannot be effectively measured by this method due to the insufficient spatial resolution. The recent advancements in ultra-high field systems, such as 7 T MRI, have made it possible to assess blood velocity pulsatility in the small perforating arteries of the brain. For instance, Schnerr et al successfully measured the blood flow waveforms of the small lenticulostriate arteries in a reproducible way using 2D phase contrast (PC)-MRI at 7 T. Based on the calculated pulsatility index and damping factor, their study demonstrated significant differences between young and elderly subjects. In addition, another recent study has managed to assess the blood flow velocity and pulsatility in small cerebral perforating arteries of the basal ganglia (BG) and the semioval center (CSO) in patients with cerebral small vessel disease and healthy subjects and observed increased pulsatility in the patient group. However, the damping effect up to the level of cerebral perforating arteries in the white matter such as CSO was not evaluated. In this issue of JMRI, Arts et al further extended the previous studies and studied damping of pulsations from the first segment (M1) of the middle cerebral artery down to the small perforating arteries of the BG and the CSO in 38 healthy subjects using 2D PC-MRI at 7 T. Specifically, the velocity pulsatilities of the M1 and the small perforating arteries in BG and CSO, and the damping indices between M1 and those perforating arteries using the dampening factor (i.e. dividing the pulsatility of M1 by the pulsatility in BG or CSO) were effectively measured. This well-designed study also compared the proximal measures of pressure and arterial stiffness, including left ventricular stroke volume, mean arterial pressure, pulse pressure (PP), and aortic pulse wave velocity (PWV), with the cerebral velocity pulsatilities and distal damping indices. The results revealed damping between M1 and CSO/BG, as well as variable damping behaviors concerning proximal measures, especially PP and PWV. The current study provides important insights into the protective mechanism in regulating pulsations that distal damping increases with the elevated proximal pressure and arterial stiffness. It further advances our understanding of the pulsatile damping behaviors. The current imaging method is sensitive to motion, and that the pulsatility-related measures of perforating arteries are still noisy even with the high-resolution 2D PC-MRI at 7 T. More robust and accurate measurements of smaller distal cerebral arteries remain a challenge. It might still be worthwhile to further improve the flow imaging methods for small vessels at 7 T in future studies. Moreover, as suggested by the authors, other pulsatility measurements such as near-infrared spectroscopy may also supplement the assessments of perforating arteries. Besides, this work has provided valuable observations for the damping behavior of the more distal and, more importantly, smaller cerebral perforating arteries and its relationship with proximal pressure and arterial stiffness. Further studies of changes in the damping behavior of distal perforating arteries in aging and cerebrovascular diseases are strongly expected.