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Purpose: To develop a method for targeted volumetric, three directional cine phase contrast (PC) imaging with high spatial resolution in clinically feasible scan times., Materials and Methods: A hybrid radial-Cartesian k-space trajectory is used for cardiac gated, volumetric imaging with three directional velocity encoding. Imaging times are reduced by radial undersampling and temporal viewsharing. Phase contrast angiograms are displayed in a new approach that addresses the concern of signal drop out in regions of slow flow. The feasibility of the PC stack of stars (SOS) trajectory was demonstrated with an in vivo study capturing 14 small intracranial aneurysms (2-10 mm). Aneurysm measures from six aneurysms also imaged with digital subtraction angiography (DSA) were compared with linear regression with those from the PC SOS images., Results: All aneurysms were identified on the phase contrast angiograms. The geometric measures from PC SOS and DSA were in good agreement (linear regression: slope = 0.89, intercept = 0.35, R∧2 = 0.88)., Conclusion: PC SOS is a promising method for obtaining volumetric angiograms and cine phase contrast velocity measurements in three dimensions. Acquired spatial resolutions of 0.4 × 0.4 × (0.7-1.0) mm make this method especially promising for studying flow in small intracranial aneurysms., (Copyright © 2011 Wiley-Liss, Inc.)

Purpose: To investigate the feasibility of using time-of-flight (TOF) images as a constraint in the reconstruction of a series of highly undersampled time-resolved contrast-enhanced MR images (HYPR TOF), to allow simultaneously high temporal and spatial resolution and increased SNR., Materials and Methods: Ten healthy volunteers and three patients with aneurysms underwent a HYPR TOF study, which includes a clinical routine TOF scan followed by a first pass time-resolved contrast-enhanced exam using an undersampled three-dimensional (3D) projection trajectory (VIPR). Image quality, waveform fidelity and signal to background variation ratio measurements were compared between HYPR TOF images and VIPR images without HYPR reconstruction., Results: Volunteer results demonstrated the feasibility of using the clinical routine TOF as the spatial constraint to reconstruct the first pass time-resolved contrast-enhanced MRA acquired using highly undersampled 3D projection trajectory (VIPR). All the HYPR TOF images are superior to the corresponding VIPR images with the same temporal reconstruction window on both spatial resolution and SNR., Conclusion: HYPR TOF improves the spatial resolution and SNR of the rapidly acquired dynamic images without losing the temporal information., (Copyright © 2011 Wiley-Liss, Inc.)

Purpose: To evaluate a novel time-resolved contrast-enhanced (CE) projection reconstruction (PR) magnetic resonance angiography (MRA) method for identifying potential bypass graft target vessels in patients with Class II-IV peripheral vascular disease., Materials and Methods: Twenty patients (M:F = 15:5, mean age = 58 years, range = 48-83 years), were recruited from routine MRA referrals. All imaging was performed on a 1.5 T MRI system with fast gradients (Signa LX; GE Healthcare, Waukesha, WI). Images were acquired with a novel technique that combined undersampled PR with a time-resolved acquisition to yield an MRA method with high temporal and spatial resolution. The method is called PR hyper time-resolved imaging of contrast kinetics (PR-hyperTRICKS). Quantitative and qualitative analyses were used to compare two-dimensional (2D) time-of-flight (TOF) and PR-hyperTRICKS in 13 arterial segments per lower extremity. Statistical analysis was performed with the Wilcoxon signed-rank test., Results: Fifteen percent (77/517) of the vessels were scored as missing or nondiagnostic with 2D TOF, but were scored as diagnostic with PR-hyperTRICKS. Image quality was superior with PR-hyperTRICKS vs. 2D TOF (on a four-point scale, mean rank = 3.3 +/- 1.2 vs. 2.9 +/- 1.2, P < 0.0001). PR-hyperTRICKS produced images with high contrast-to-noise ratios (CNR) and high spatial and temporal resolution. 2D TOF images were of inferior quality due to moderate spatial resolution, inferior CNR, greater flow-related artifacts, and absence of temporal resolution., Conclusion: PR-hyperTRICKS provides superior preoperative assessment of lower limb ischemia compared to 2D TOF.

In contrast-enhanced (CE) magnetic resonance (MR) angiography (MRA), lower injection rates of a fixed contrast agent dose provide longer contrast agent bolus at the expense of lower intravascular signal. This study evaluated the effect of different injection rates in imaging of the vasculature of the lower extremities with time-resolved, CE MRA. In three volunteers, injection rates of 0.5, 1.5 and 3.0 mL/second were administered in a randomized order and imaged in two separate sessions. Contrast agent bolus dynamics measured in volunteers were used in computer simulations to confirm variations in contrast agent concentration as a source of vessel ringing and blurring artifacts. To validate the effect of injection rate in pathologic vessels, 37 patients with peripheral vascular disease were imaged with a time-resolved technique using an injection rate of 0.5 mL/second or 1.5 mL/second and retrospectively divided into two groups. In volunteers, higher injection rates caused a stronger modulation of k-space and resulted in increased ringing artifacts in time-resolved CE MRA. These results were reproduced with computer simulations. In the qualitative patient study, significantly less vessel blurring was observed using a lower injection-rate, without a significant loss of vessel contrast., (Copyright 2001 Wiley-Liss, Inc.)

The purpose of this study was to investigate the effect on three-dimensional (3D) magnetic resonance digital subtraction angiography (MR DSA) images of various injection protocol parameters (ie, injection order, volume, and rate), as well as image masking. The pelves of 10 normal volunteers were scanned using seven different contrast agent volume/injection rate combinations. Subtraction of a precontrast mask image resulted in vascular image contrast improvements of between 4.0 and 7.7 times. Image quality and smaller vessel image contrast in the masked data decreased with increasing injection number. Data acquired with a high (0.150 mmol kg(-1)) volume yielded the highest quality images, although only small nonsignificant differences in image quality and large vessel conspicuity were found between images obtained using the high and medium (0.075 mmol kg(-1)) volumes. Images acquired with a low (0. 038 mmol kg(-1)) volume, while of lower image contrast, were judged to be of reasonable quality, especially when acquired as the first or second injection. Injection rate (1 ml s(-1), 2 ml s(-1), and 4 ml s(-1)) was not found to affect the images significantly, although selection of an injection rate that gave an injection duration of approximately 10 seconds tended to give better vascular image contrast. Based on these data, a series of escalating volumes for multi-injection examination is proposed. J. Magn. Reson. Imaging 2000;12:476-487., (Copyright 2000 Wiley-Liss, Inc.)

With contrast-enhanced MR angiographic techniques, a T1-shortening contrast agent is injected into the blood stream. Imaging during the first pass of the contrast agent permits acquisition of a high-contrast MR angiogram. Scan parameters such as flip angle, repetition time, echo time, and scan duration, and injection parameters, such as dose and rate, must be carefully chosen to achieve maximum contrast between blood vessels and stationary tissues. A critical parameter affecting image quality is the timing of the acquisition relative to the injection. If the collection of the center of k-space does not coincide with peak arterial concentration, artifacts, reduced SNR, and venous enhancement may result. Several techniques have been developed to address the timing issue. Post-processing techniques such as subtracting a pre-contrast image from a post-contrast angiogram can be used to improve image quality. Intravascular contrast agents that may also lead to improved image quality are currently being developed.

The sensitivities of phase-difference (PD) and complex-difference (CD) processing strategies to in-plane motion were examined theoretically and experimentally. Errors in velocity and volume flow rate (VFR) estimates were attributed to (a) motion between different velocity encodings and, in the case of segmented k-space acquisition strategies, (b) motion over the segment duration. PD estimates were found to be insensitive to in-plane motion between velocity encodings, whereas CD VFR estimates were found to be sensitive to this motion. PD estimates, however, were affected by partial volume effects. A corrected CD (CD') scheme was developed that minimizes both partial-volume and in-plane motion effects. Segmented k-space acquisitions with sequential offset and sequential interleaved offset (or centric) phase-encoding schemes were studied. Images obtained using these techniques were found to include both blurring and replication artifacts. The amount of artifact generally increased with the number of views per segment (vps) and the in-plane velocity. PD, CD, and CD' VFR estimates were found to be degraded by these artifacts. The sequential offset phase-encoding scheme generally had acceptable VFR errors (at 4 vps. a CD' VFR error of 7.0%) when averaged over the physiologic range of myocardial motion (> 12 cm second-1); however, larger errors were observed outside this range. VFR estimates obtained using the sequential interleaved phase-encoding scheme at 4 vps were unacceptable. More accurate VFR measurements were obtained using a revised segmented PC strategy, which reversed the order in which the velocity and phase encodings were interleaved. The weighted average CD' VFR error obtained using the revised strategy was 24.5% (for 4 vps). Using displacement information obtained from the two velocity-encoded images, an estimate of the in-plane velocity was obtained and used to correct the acquired data. This decreased the VFR error (weighted average CD' error at 4 vps decreased from 24.5% to -6.3%); however, the implemented correction algorithm could potentially introduced other artifacts in the images.

We have previously reported on a complex-difference (CD) flow measurement technique that produces more accurate results than the phase-difference (PD) flow measurement technique due to the greater immunity of the former method to partial volume effects. We report here on some of the ways in which through-plane myocardial motion affects the accuracy of absolute coronary artery flow measurements obtained using the PD and CD techniques. We also discuss motion correction schemes that can be applied to the PD and CD processing methods to improve their accuracy. Computer simulations have been performed to assess the magnitude of the errors associated with these flow measurement techniques when they are applied to small vessels that are attached to a moving background. Laminar and plug flow, with and without complete background suppression, have been considered. Experiments with a moving vessel phantom have been conducted to test the performance of the PD and CD flow measurement techniques in circumstances similar to those simulated. The simulations and the experiments showed that, after corrections for through-plane motion are made, the CD method generally yields more accurate flow results than the PD method. As shown by the simulations, however, both methods yield compromised results due to subtle saturation effects that occur when the direction of myocardial motion is opposite the direction of blood flow. Unvalidated PD and CD measurements of coronary artery flow waveforms in human volunteers are presented to illustrate the magnitude of the proposed through-plane motion effects in vivo.

Magnetic resonance (MR) phase-difference methods work well for measuring volumetric flow rates when the vessel diameter is large compared with the in-plane voxel dimensions. For small vessels (eg, coronary arteries), partial-volume effects introduce substantial errors in the measured volume flow rate. To correctly measure flow rates through a voxel, both the fraction of the voxel containing moving spins and the phase shift imparted to those spins must be known. The authors propose a flow measurement method that combines information obtained with both the complex-difference and phase-difference processing techniques and thereby provides the fractional volume occupied by the moving spins and the phase of those spins. The complex-difference flow map method proposed results in improved accuracy of MR phase-contrast flow measurements in the presence of partial-volume effects.

This article reviews the general characteristics of several vascular imaging modalities with the purpose of identifying the distinguishing features of magnetic resonance (MR) angiography. Brief discussions of conventional x-ray film angiography, intravenous and intraarterial digital subtraction angiography (DSA), duplex and color Doppler flow ultrasound (US), computed tomographic (CT) angiography, transesophageal and intravascular US, angioscopy, and MR angiography are presented. The advantages and disadvantages of each are discussed. The general attributes and image quality features of MR angiography, intraarterial DSA, CT angiography, and US are compared. It is concluded that no single imaging modality will presently suffice for all purposes. Because of its noninvasiveness, rapidly improving image quality, and ability to directly provide velocity information, MR angiography is likely to play a role in an increasing number of clinical applications.