Jackie Leung1, Datta Goolaub1, Christopher Macgowan1, Lindsay Cahill1, John Sled1,2, and Andrea Kassner1,2
1SickKids Hospital, Toronto, ON, Canada, 2University of Toronto, Toronto, ON, Canada
Synopsis
The mechanical and morphological properties
of the cerebral vasculature can be studied by measuring blood flow dynamics of the
feeding arteries. In this study, we used high resolution phase contrast MRI to
measure the blood flow and vessel area of the common carotid arteries of
children with and without sickle cell disease (SCD). Preliminary results show
that pulse wave velocity and wave reflection is significantly higher in
patients with SCD, suggesting increased vessel stiffness and downstream
abnormalities in the vasculature. This method is clinically feasible and may
provide useful insight into the vascular properties of children with
cerebrovascular disease.
Introduction
The cerebral microvasculature has the vital
role of changing cerebral blood flow (CBF) in response to alterations in
metabolic demand. In cerebrovascular diseases such as sickle cell disease (SCD),
this function is impaired, resulting in an increased risk of hypoxic injury in
the brain [1]. A potentially valuable approach to assess the microvasculature
involves measuring the flow properties within larger feeding arteries. By
quantifying the time-varying blood flow and cross-sectional area of the common
carotid artery (CCA) across a cardiac cycle, parameters such as pulse-wave
velocity (PWV) and wave reflection coefficient (Γ) can be derived [2,3]. PWV describes the propagation speed of
pressure waves and is related to the mechanical characteristics of the vessel
walls. When these waves encounter a downstream change in vessel morphology, a
partial reflection is generated, which can be characterized as Γ. In this study, we used cine MRI to characterize the vessel and flow
properties of the CCA in children with SCD and healthy controls. We hypothesize
that the PWV and Γ in patients with SCD
will be higher than in controls due to reduced vessel compliance and
abnormalities in the microvasculature.
Methods
Six pediatric patients with SCD (14.5±2.3 years; 4 females, 2 males) and 3 age-matched healthy volunteers underwent
imaging on a 3T clinical MRI system (Siemens Healthcare, Germany). The protocol
included a high resolution phase contrast MRI (PCMRI) sequence to quantify
intravascular flow and vessel size, in addition to standard structural imaging
(T1, T2, diffusion). The PCMRI scan consisted of a single slice radial
acquisition perpendicular to the CCA with the following parameters: TR/TE =
24.3/8.1 ms, voxel = 0.29×0.29 mm, FOV
= 130×130 mm, slice = 5 mm, FA = 30°,
phases = 20, Venc = 100cm/s, spokes=16000, time = 6.5 min. The acquisition
strategy was adapted from real-time cardiac imaging to obtain high spatial and
temporal resolution images with an ECG-free retrospective gating approach
[4,5]. The raw data was transferred for offline reconstruction using in-house
Matlab scripts.
Analysis
of the PCMRI data was performed using a freely available Matlab software
package (Segment v2.0 R5557; http://segment.heiberg.se). A region of interest
(ROI) of the cross section for both the left and right CCA was defined on the first
phase of the magnitude image using a region growing tool. The ROIs were then
automatically propagated across the remaining phases, accounting for the change
in vessel caliber (see Figure 1), and associated pulsatile flow volumes
computed from the phase data. PWV was calculated as the slope between area vs.
flow during the systolic phase, and Γ was calculated as the ratio of backward
to forward wave amplitude at the first harmonic (Figure 2) [2]. Group averages of PWV and Γ were compared using Student's
t-test, with statistical significance defined as p<0.05.
Results
Mean PWV was found to be significantly
greater in the SCD group compared to controls (348±47cm/s vs. 272±73cm/s; p=0.017). Furthermore, mean Γ was also shown to be higher in SCD patients (0.52±0.14cm/s vs. 0.37±0.06cm/s; p=0.027). The results are graphed in
Figure 3.Discussion
Using PCMRI, we could characterize the flow
and reflection properties within the CCA and show significant differences
between pediatric SCD patients and healthy controls. Increased PWV and Γ
in SCD suggests that the CCAs in SCD are less compliant and blood delivery into
the microvasculature is less efficient. Future work will involve incorporating
compressed sensing reconstruction to further increase temporal resolution, and
correlating these measures with clinical outcomes.Conclusion
The parameters derived from this scan can
provide useful information on the mechanical properties and vascular morphology
of the cerebrovasculature in SCD patients that could
potentially aid in the clinical evaluation of children with cerebrovascular
disorders.Acknowledgements
No acknowledgement found.References
1. Kosinski PD, et al. The
severity of anaemia depletes cerebrovascular dilatory reserve in children with
sickle cell disease: a quantitative magnetic resonance imaging study. Br J
Haematol. 2016; 176(2):280-7.
2.
Macgowan CK, et al. Evaluation
of cereborvascular impedance and wave reflection in mouse by ultrasound. J
Cereb Blood Flow & Metab. 2015; 35:521-6.
3.
Vulliemoz S, et al. Estimation
of local aortic elastic properties with MRI. Magn Reson Med. 2002; 47(4):649-54.
4.
Roy CW, et al. Motion
compensated cine CMR of the fetal heart using radial undersampling and
compressed sensing. J Cardiovasc Magn Reson. 2017; 19(1):29.
5.
Roy CW, et al. Accelerated MRI
of the fetal heart using compressed sensing and metric optimized gating. Magn
Reson Med. 2016; 0:1-11.