![]() ![]() The PC-VIPR data were used as a composite image (angiographic constraint) for HYPR-LR reconstruction and for hemodynamic evaluation. Subsequently, velocity encoding was performed by using a high-resolution dual-echo 3D-radial PC acquisition (PC-VIPR). 18 Contrast was injected during the second scan. Two fast low-resolution scans (2 × 2 × 2 mm 3) were used in the time-resolved multiecho 3D radial acquisition (CE-VIPR). Before contrast injection, a fast 2D-PC scan of each MCA was obtained to estimate the velocity to determine optimum VENC to prevent aliasing, followed by a PC HYPRFlow acquisition. Ten healthy volunteers ranging from 19 to 58 years of age were imaged (6 women, 4 men) with a clinical 3T MR imaging system (MR HD 750 GE Healthcare, Milwaukee, Wisconsin) with a 8-channel head coil (Excite HD Brain Coil, GE Healthcare). Volunteer studies were performed in compliance with Health Insurance Portability and Accountability Act regulations and by using a protocol approved by the local institutional review board. With current protocols, PC HYPRFlow provides whole-brain angiograms with excellent spatial resolution (0.68 × 0.68 × 0.68 mm 3) with scanning times of 5–6 minutes. The low-resolution source images are then convolved with the high-resolution PC velocity acquisition by using a technique called HYPR-LR 14 to yield a time series of high-resolution morphologic angiograms with associated velocity information. A series of low-resolution 3D-radial CE source images are acquired (CE-VIPR), followed by a high-resolution velocity-encoded PC acquisition (PC-VIPR 13). Figure 1 shows a flow chart of how images are acquired and reconstructed by using PC HYPRFlow. We have combined radial undersampling with a novel constrained image reconstruction technique to create PC HYPRFlow, 9, 12 a comprehensive MRA technique. For example, azimuthally undersampled radial acquisitions can be used to acquire images in much shorter scanning times than Cartesian acquisitions of a similar resolution with acceptable SNR and image quality, 9 due to the relatively benign nature of streaklike artifacts caused by the undersampling. Because MRA is sparse, consisting of few changing nonzero elements, accelerated acquisitions can often be performed with tolerable artifacts. ![]() Recently, we implemented radial imaging techniques that are particularly well-suited for neurovascular MRA. However, obtaining high-resolution whole-brain angiograms with velocity information within clinically useful imaging times has been challenging. With these methods, in-plane spatial resolutions on the order of 0.6 × 1 mm with scanning times in the 8- to 12-minute range can be achieved. Investigators have used view-sharing, 5 spatial harmonics, 6 and parallel imaging 7 to reduce scanning time. 3, 4 Advances in PC Cartesian acquisition have reduced scanning times and increased resolution to some degree. 2 Previous implementation of whole-brain cardiac-gated PC Cartesian 4D MR imaging has resulted in scanning times too long to be clinically useful. 1 3D and 4D methods have higher SNR, fewer partial volume effects, and improved spatial resolution but have an increased scanning time. 2D-PC MR imaging has been used to obtain hemodynamic data for the past 20 years but has limited coverage and exhibits partial volume effects. However, the sonic properties of the cranial vault prevent measurement of velocity in many intracranial arteries by TCD. TCD has been used clinically for decades and allows the acquisition of velocity measurements in the MCAs and several other vessels through the temporal window. The acquisition of intracranial velocity measurements and velocity derivatives (WSS) is clinically useful for the evaluation of neurovascular disorders such as vasospasm, stenoses, and aneurysms, but measurement of intracranial velocity has proved challenging. Abbreviations CE contrast-enhanced CE-VIPR contrast-enhanced vastly undersampled isotropic projection reconstruction 2D-PC 2D phase-contrast HYPR-LR highly constrained local projection reconstruction MCA middle cerebral artery MRA MR angiography PC phase-contrast PC HYPRFlow time-resolved MRA using highly constrained projection reconstruction and PC-VIPR data for the reconstruction convolution PC-MRA phase-contrast MR angiography PC-VIPR phase-contrast vastly undersampled isotropic projection reconstruction SNR signal intensity–to-noise ratio TCD transcranial Doppler sonography VENC velocity encoding WSS wall shear stress ![]()
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