Transcript Hemodynamics at the Carotid Terminus and Surrounding Segments

Hemodynamics at the Carotid Terminus and
Surrounding Segments Assessed Using
Highly Accelerated High-Resolution Phase
Contrast MR Velocimetry and Automated
Spline Interpolation
W. Chang, MBA, A. Frydrychowicz, MD, B. Landgraf, BS, S.
Kecskemeti, MS, K. Johnson, PhD, Y. Wu, PhD, J. Velikina,
PhD, O. Wieben, PhD, C. Mistretta, PhD, and P. Turski, MD
3D-Radial vs Cartesian Acquisitions
• The Nyquist conditions regulate how
many projections are needed to avoid
aliasing in MRA and other modalities.
With a Cartesian readout, violating the
Nyquist conditions may result in streak
artifacts, compromising image quality.
• In radial undersampling, violating the
Nyquist conditions will result in artifacts
localized to the periphery away from
vessels, and are better tolerated,
resembling normal noise.1,2
•This effect is enhanced when the
Schematic of 3D PC-VIPR acquisition.
region of interest is sparse, allowing
increased radial undersampling without
excessive aliasing, allowing higher
resolution and lower scan times.
1) Peters et al, MRM, 2000 2) Mistretta, JMRI, 2009
PC-VIPR, a fast 3D-radial PC-MRA technique
• PC-VIPR is a phase-contrast 3D radial technique with high spatial
resolution (0.68x0.68x0.68mm) capable of acquiring whole-brain
angiograms with velocity information with scan times of 5 minutes.3,4
3) Wu et al, AJNR, 2011 4)Nett, et al, JCMR, 2009
Wall Shear Stress
• Wall Shear Stress (WSS) is defined
as the derivative of velocity with
respect to the distance from the wall,
multiplied by the viscosity. It
represents the drag that parallel
flowing fluid imposes on the wall.
• Phase Contrast techniques such as
PC-VIPR have velocity information
that can be used to estimate WSS
values. High spatial resolution is
necessary to accurately acquire WSS
at the boundary zone of the vessel.
• WSS can’t be acquired in-vivo
therefore measurements represent an
estimate of the true WSS. However,
they can differentiate normal and
pathologic flow conditions.
Relationship between velocity and WSS.
Relationship between Wall Shear Stress and Atherosclerosis
Abnormal wall shear stress
(WSS) is prevalent at areas
prone to atherosclerosis and
promotes an atherogenic
phenotype.5
Non-laminar flow, leading to
abnormal WSS, changes
endothelial gene expression
and affects the boundary
zone, leading to plaque
formation and progression.6
Both high and low WSS has
also been associated with
the formation and
progression of saccular
cerebral aneurysms.7
Atherosclerotic stenosis in the left MCA showing low WSS
distal to the plaque.
5) Malek et al, JAMA,1998 6) Cunningham and Gottlieb, Lab Inv, 2005 7) Hoi et al, JNS, 2004
Materials and Methods
• 10 healthy volunteers (6 female, 4 male) were scanned using a GE
Health Discovery 750 3.0T MR Scanner (GE Healthcare, Waukesha, WI)
with an 8 channel head coil (Excite HD, GE Healthcare, Waukesha, WI)
using PC-VIPR.8 A low resolution scan was followed by contrast injection
and PC-VIPR acquisition to acquire a time series of high-res images.
PC VIPR Scan Parameters were:
• TR/TE: 8.2/2.8
• α = 20o
• Bandwidth = 83.3 KHz
• Scan time = 300 sec
• Velocity encoding = 80 cm/s
• Field of View = 220x220x220 mm3
• Resolution = 0.68 x 0.68 x 0.68 mm3
8) Johnson et al, MRM, 2008
Acquisition of WSS with PC-VIPR
• Images were reconstructed using HYPR-LR
reconstruction9 and imported into Ensight where cutplanes
were made axial to vessels of interest (MCA, ICA, ACA).
Cutplane being made in the left MCA using Ensight.
9) Mistretta et al, MRM, 2006
Calculation of WSS using spline interpolation
• The cutplanes were then imported into a MATLAB runtime
environment developed in conjunction with the University of Freiburg,
Germany.10
• Points were selected
around the border of the
vessel and the program
automatically calculated
WSS using B-spline
interpolation and Green’s
theorem. Total
processing time per
patient was less than 30
minutes. PC-VIPR
images allow whole-brain
WSS mapping.
10) Stalder, et al, 2008, MRM
Results
Vessel
Wall Shear
Stress
Standard
Deviation
Middle Cerebral Arteries
0.95 Pa
0.32 Pa
Anterior Cerebral Arteries
0.89 Pa
0.59 Pa
Terminal Internal Carotid
Arteries
1.17 Pa
0.60 Pa
• These values were consistent with values found in the literature for
WSS acquired using PC-MRA.11,12
11) Oyre, et al, JACC, 1998 12) Chang et al, AJNR 2011
Complex flow in normal volunteers
13) Chang et al, NRJ, 2011
Complex flow in normal volunteers (cont)
Clinical Applications
High resolution PC-MRA imaging can be
used to evaluate stenoses and aneurysms.
This patient has
bilateral vertebral
artery stenoses and
basilar and vertebral
artery aneurysms that
are visualized using
high-resolution PCMRA.14
Increased velocity is
seen in the stenoses
and a helical flow
pattern can be seen in
the aneurysms.
14) Chang et al, RSNA, 2010
Clinical Applications (cont)
Comparison of WSS map of the aneurysm with streamline visualization.
Clinical Applications (cont)
14) Chang et al, RSNA, 2010
Clinical Applications (cont)
Clinical Applications (cont)
Clinical Applications (cont)
13) Chang et al, NRJ, 2011
Conclusion/Future Work
• PC VIPR is a radial undersampled PC-MRA technique capable of
acquiring whole-brain images with velocity information. Using highresolution velocity information from PC-VIPR, spline interpolation can be
used to localize the boundary zone and calculate axial WSS.
•WSS has prognostic value in determining areas vulnerable to
atherosclerotic plaque deposition and aneurysm development.
•We are currently using a combination of velocity, its derivatives such as
streamlines and WSS, and morphology to evaluate patients with
stenoses, aneurysms, arteriovenous malformations, and vasospasm.
This workup can identify and characterize pathologic flow conditions.
•Radial PC-MRA may offer clinicians a non-invasive imaging solution
that does not expose patients to invasive techniques or radiation.