Velocity Pulsatility and Arterial Distensibility Along the Internal Carotid Artery

Abstract

Background Attenuation of velocity pulsatility along the internal carotid artery (ICA) is deemed necessary to protect the microvasculature of the brain. The role of the carotid siphon within the whole ICA trajectory in pulsatility attenuation is still poorly understood. This study aims to assess arterial variances in velocity pulsatility and distensibility over the whole ICA trajectory, including effects of age and sex. Methods and Results We assessed arterial velocity pulsatility and distensibility using flow-sensitized 2-dimensional phase-contrast 3.0 Tesla magnetic resonance imaging in 118 healthy participants. Velocity pulsatility index (vPI=(V_max-V_min)/V_mean) and arterial distensibility defined as area pulsatility index (A_max-A_min)/A_mean) were calculated at C1, C3, and C7 segments of the ICA. vPI increased between C1 and C3 (0.85±0.13 versus 0.93±0.13, P<0.001 for averaged right+left ICA) and decreased between C3 and C7 (0.93±0.13 versus 0.84±0.13, P<0.001) with overall no effect (C1-C7). Conversely, the area pulsatility index decreased between C1 and C3 (0.18±0.06 versus 0.14±0.04, P<0.001) and increased between C3 and C7 (0.14±0.04 versus 0.31±0.09, P<0.001). vPI in men is higher than in women and increases with age (P<0.015). vPI over the carotid siphon declined with age but remained stable over the whole ICA trajectory. Conclusions Along the whole ICA trajectory, vPI increased from extracranial C1 up to the carotid siphon C3 with overall no effect on vPI between extracranial C1 and intracranial C7 segments. This suggests that the bony carotid canal locally limits the arterial distensibility of the ICA, increasing the vPI at C3 which is consequently decreased again over the carotid siphon. In addition, vPI in men is higher and increases with age.

Keywords: MRI angiography; cerebral hemodynamics; distensibility; internal carotid artery; velocity pulsatility index.

Figure 1. Planning of the 2D phase contrast velocity measurements on different segments of the internal carotid artery.

A and B, Coronal and sagittal oriented phase contrast angiography surveys used to plan the 2D phase contrast velocity measurements at C1 (blue line) and C3 (red) segments. The planning of the C7 (yellow) segment is also illustrated in the phase contrast angiography surveys. Planning of the C7 segment was fine‐tuned using a maximum intensity projection of the 3D time‐of‐flight angiogram (C and D). C, Until February 2019, a single 2D phase‐contrast measurement was used for simultaneous assessment of both right and left internal carotid artery. D, From February 2019 until October 2019, the phase‐contrast velocity measurements of the right and left internal carotid artery at C7 segments were planned separately to compensate for the slightly diverging angle at C7 level.

Figure 2. Magnitude and phase contrast images shown for 3 different segments for the first heart phase within 1 representative subject.

For every segment the contour (region of interest) was automatically drawn and propagated. All regions of interest were included for this representative subject. A, segment C1. B, segment C3. C, segment C7 right. D, segment C7 left. E, All mean velocity curves (in cm/s) of this subject are combined and shown over trigger delay (in milliseconds) for the 3 segments, C1 (blue), C3 (red) and C7 (yellow) of the internal carotid artery.

Figure 3. Velocity pulsatility index (left) and area pulsatility index (right) per vessel for each segment showing inverse trends across segments for the velocity pulsatility and area pulsatility indices.

The area pulsatility decreases between C1 and C3, and subsequently increases again to C7, the velocity pulsatility increases between C1 and C3 and decreases again at C7. aPI indicates area pulsatility index; BA, basilar artery; ICA, internal carotid artery; and vPI, velocity pulsatility index.

Figure 4. The effect of age on both velocity pulsatility index (upper) and area pulsatility index (lower) across segments are illustrated using trend lines.

Velocity pulsatility index C1: y=0.0028x+0.7167, velocity pulsatility index C3: y=0.0033x+0.7824 and velocity pulsatility index C7=0.0036x+0.6671. For area pulsatility index C1: y=0.0004x+0.1623, area pulsatility index C3: y=0.0003x+0.0979 and area pulsatility index C7: y=−0.0003x+0.3218. aPI indicates area pulsatility index; and vPI, velocity pulsatility index.