W. Chang, MBA, A. 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 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 region of interest is sparse, allowing increased radial undersampling without excessive aliasing, allowing higher resolution and lower scan times. 1) Peters et al, MRM, ) Mistretta, JMRI, 2009 Schematic of 3D PC-VIPR acquisition.

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, )Nett, et al, JCMR, 2009

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. Wall Shear Stress Relationship between velocity and WSS.

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 Relationship between Wall Shear Stress and Atherosclerosis 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, ) Hoi et al, JNS, 2004

Materials and Methods 10 healthy volunteers (6 female, 4 male) were scanned using a GE Health Discovery T 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 = 20 o Bandwidth = 83.3 KHz Scan time = 300 sec Velocity encoding = 80 cm/s Field of View = 220x220x220 mm 3 Resolution = 0.68 x 0.68 x 0.68 mm 3 8) Johnson et al, MRM, 2008

Acquisition of WSS with PC-VIPR Images were reconstructed using HYPR-LR reconstruction 9 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 These values were consistent with values found in the literature for WSS acquired using PC-MRA. 11,12 11) Oyre, et al, JACC, ) Chang et al, AJNR 2011 VesselWall Shear Stress Standard Deviation Middle Cerebral Arteries0.95 Pa0.32 Pa Anterior Cerebral Arteries0.89 Pa0.59 Pa Terminal Internal Carotid Arteries 1.17 Pa0.60 Pa

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 PC- MRA. 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)

13) Chang et al, NRJ, 2011 Clinical Applications (cont)

PC VIPR is a radial undersampled PC-MRA technique capable of acquiring whole-brain images with velocity information. Using high- resolution 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. Conclusion/Future Work