Vascular reactivity in small cerebral perforating arteries with 7 T phase contrast MRI - A proof of concept study

Abstract

Existing cerebrovascular reactivity (CVR) techniques assess flow reactivity in either the largest cerebral vessels or at the level of the parenchyma. We examined the ability of 2D phase contrast MRI at 7 T to measure CVR in small cerebral perforating arteries. Blood flow velocity in perforators was measured in 10 healthy volunteers (mean age 26 years) using a 7 T MR scanner, using phase contrast acquisitions in the semioval center (CSO), the basal ganglia (BG) and the middle cerebral artery (MCA). Changes in flow velocity in response to a hypercapnic breathing challenge were assessed, and expressed as the percentual increase of flow velocity as a function of the increase in end tidal partial pressure of CO₂. The hypercapnic challenge increased (fit ± standard error) flow velocity by 0.7 ± 0.3%/mmHg in the CSO (P < 0.01). Moreover, the number of detected perforators (mean [range]) increased from 63 [27-88] to 108 [61-178] (P < 0.001). In the BG, the hypercapnic challenge increased flow velocity by 1.6 ± 0.5%/mmHg (P < 0.001), and the number of detected perforators increased from 48 [24-66] to 63 [32-91] (P < 0.01). The flow in the MCA increased by 5.2 ± 1.4%/mmHg (P < 0.01). Small vessel specific reactivity can now be measured in perforators of the CSO and BG, using 2D phase contrast at 7 T.

Keywords: Blood flow velocity; Cerebral perforating artery; Cerebrovascular reactivity; Hypercapnia; Phase contrast MRI; White matter.

Graphical abstract

Fig. 1

Slice planning for the 2D phase contrast sequences. The left image shows the planning of the BG slice (in red) and the CSO slice (in green) on a sagittal T1 weighted image. The BG slice touches the underside of the corpus callosum, indicated by the dashed circles. The CSO slice is planned parallel to the BG slice and positioned 15 mm above the corpus callosum (Geurts et al., 2018). The center and right images show the planning of the MCA (in blue) on transverse and coronal T1 weighted images, respectively.

Fig. 2

Breathing protocol. The graph shows the PetCO₂ trace for subject 5, with color coded lines representing each acquisition. The images on top are the actual mean magnitude images of each phase contrast measurement for this subject, in the order in which they were acquired. The color coded lines schematically indicate at which position on the trace they were acquired. The solid lines indicate baseline acquisitions and the dashed lines indicate challenge acquisitions. The phase contrast measurements were alternated with T1w acquisitions, which are indicated with a solid grey line.

Fig. 3

Manual region of interest adjustments. These images show the mean magnitude (M, top row) and velocity (V, bottom row) data of 2D phase contrast acquisitions of mediocre quality, in which pulsation artefacts had to be excluded. The pulsation artefacts show up as vertical lines (in the phase encoding direction) of repetitions of the originating vessel. At these locations the data is corrupted and has to be excluded. The left image shows a CSO slice in which the automatically segmented ROI contained pulsation artefacts from surrounding vessels (red arrows). The red lines show ROI portions that were manually removed, the green lines show the remaining ROI that was included. The right image shows a BG slice in which pulsation artefacts from surrounding vessels (red arrows) had to be avoided during manual segmentation (green lines).

Fig. 4

Measured velocities and fitted reactivity. These plots show a colored boxplot for the vessels that could be matched between baseline and challenge measurements, for each scan of each subject. The boxplots corresponding to the same volunteer are connected with a colored line, representing the individual fit. The fitted reactivity for all subjects is shown in black, with the 95% CI in grey. The horizontal axes show PetCO₂ and the vertical axes show the measured quantity, which is mean flow (F_mean) for the MCA and mean velocity (V_mean) for the BG and the CSO. The left, middle and right graphs show the results for the CSO (green), BG (red) and MCA (blue) measurements respectively. Note that attained PetCO₂ values are nearly identical between experiments, both for baseline and challenge measurements.

Fig. 5

Mean magnitude at baseline and during challenge. The left image shows the mean CSO magnitude of the right hemisphere of subject 5 at baseline and the right image shows the same during challenge. Note the higher blood signal on the right image during the hypercapnic breathing challenge, which caused a larger number of perforators to be detected during the challenge acquisitions. The increase in blood signal is caused by an increase in flow and the T1-inflow effect. The green circles show one example of an included perforator, which was located in the white matter, was not corrupted by ghosting during challenge, and was detected in both acquisitions. The blue arrows points at a curved sulcus with intra-sulcal vessels running through. These sulci and surrounding gray matter were automatically excluded. The red arrows show an area with a ghosting artefact of an intra-sulcal vessel during the challenge acquisition. These areas with ghosting artefacts during challenge were manually annotated and excluded from both acquisitions. The images are masked and scaled identically.

Fig. 6

Number of detected perforators. The markers show the number of detected perforators at baseline (B) and the round markers show those during challenge (C). Corresponding subjects are denoted with solid lines. The left graph shows the results for the CSO measurements (green), the right graph shows the results for the BG measurements (red).