Reactivity of larger intracranial arteries using 7 T MRI in young adults

Abstract

The larger intracranial conduit vessels contribute to the total cerebral vascular resistance, and understanding their vasoreactivity to physiological stimuli is required when attempting to understand regional brain perfusion. Reactivity of the larger cerebral conduit arteries remains understudied due to a need for improved imaging methods to simultaneously assess these vessels in a single stimulus. We characterized reactivity of basal intracranial conduit arteries (basilar, right and left posterior, middle and anterior cerebral arteries) and the right and left internal carotid arteries, to manipulations in end-tidal CO₂ (PetCO₂). Cross-sectional area changes (%CSA) were evaluated from high-resolution (0.5 mm isotropic) images collected at 7 T using a T1-weighted 3D SPACE pulse sequence, providing high contrast between vessel lumen and surrounding tissue. Cerebrovascular reactivity was calculated as %CSA/ΔPetCO₂ in eight healthy individuals (18-23 years) during normocapnia (41 ± 4 mmHg), hypercapnia (48 ± 4 mmHg; breathing 5% CO₂, balance oxygen), and hypocapnia (31 ± 8 mmHg; via hyperventilation). Reactivity to hypercapnia ranged from 0.8%/mmHg in the right internal carotid artery to 2.7%/mmHg in the left anterior cerebral artery. During hypocapnia, vasoconstriction ranged from 0.9%/mmHg in the basilar artery to 2.6%/mmHg in the right posterior cerebral artery. Heterogeneous cerebrovascular reactivity to hypercapnia and hypocapnia was characterized across basal intracranial conduit and internal carotid arteries.

Keywords: Cerebral blood flow; hemodynamics; imaging; magnetic resonance imaging; ultrasound.

Figure 1.

Representative schematic of the cross-sectional area for the nine arteries during normocapnia. Images were acquired using the T1-weighted sequence and were cropped for this schematic. White arrow points to indicated artery, and scale only applies to cropped MRI images.

Figure 2.

Cerebrovascular cross-sectional area reactivity (%CSA/mmHg of PetCO₂) to hypocapnia and hypercapnia across nine arteries. LPCA and RPCA: left and right posterior cerebral artery; LMCA and RMCA: left and right middle cerebral artery; LICA and RICA: left and right internal carotid artery; LACA and RACA: left and right anterior cerebral artery (n = 8 participants). Data presented as mean ± S.E.M. There was a difference between hypercapnia CVR and hypocapnia CVR at each vessel; paired t-test (P < 0.05).

Figure 3.

Schematic of measurements taken from the laboratory and MRI sessions. Representative MRI image highlighting a transverse cross-section in the sagittal plane of the right middle cerebral artery during hypocapnia, normocapnia and hypercapnia breathing conditions, and the corresponding condition’s PetCO₂ levels and right MCA velocity measures during the laboratory session.

Figure 4.

Right middle cerebral artery (MCA) velocity, cross-sectional area (CSA), calculated flow, and conductance during hypocapnia, normocapnia and hypercapnia for each participant. Each participant’s data set is indicated by an individual line (n = 8). Panel A: MCA velocity increased from the hypocapnic to the hypercapnic states, as highlighted by positive slopes with increasing PetCO₂ levels. Panel B: MCA CSAs increased from the hypocapnic to the hypercapnic states, as highlighted by positive slopes with increasing PetCO₂ levels. Panel C: MCA flow (ml/min) increased from the hypocapnic to the hypercapnic states, as highlighted by positive slopes with increasing PetCO₂ levels. Panel D: MCA conductance increased from the hypocapnic to the hypercapnic states, as highlighted by positive slopes with increasing PetCO₂ levels.