3D Phase-contrast Magnetic Resonance Research Articles

MR angiography of the intracranial venous system.

To compare the effectiveness of different imaging planes at time-of-flight (TOF) magnetic resonance (MR) angiography and phase-contrast MR angiography in the visualization of the normal intracranial venous system. In 12 healthy volunteers, two-dimensional (2D) TOF MR angiography and three-dimensional (3D) phase-contrast MR angiography were performed in transverse, sagittal, and coronal planes. All data were displayed as maximum intensity projection (MIP) images. Four neuroradiologists assessed the visibility of 28 intracranial venous structures on the MIP images. Statistical analysis was performed by using the Friedman two-way analysis of variance and the Cochran Q test. Visualization of the normal intracranial venous system was better with 3D phase-contrast and coronal 2D TOF MR angiography than with transverse or sagittal 2D TOF MR angiography (P < .05, Friedman test) for each observer and the group of observers. Differences were found between each of the 2D TOF and 3D phase-contrast MR angiographic sequences in the visualization of individual venous structures (Cochran Q test). The kappa values ranged from 0.36 to 0.71, which indicated a moderate to good agreement between observers. The normal intracranial venous system is adequately visualized with 3D phase-contrast and coronal 2D TOF MR angiography.

MR angiography after renal revascularization: spectrum of expected anatomic results and postintervention complications.

The use of magnetic resonance (MR) angiography in screening for renal artery stenosis has been extensively evaluated. However, the MR angiographic findings after renal artery revascularization are not as well characterized. The renal artery and parenchyma can be evaluated after revascularization with a comprehensive MR imaging protocol that includes T1- and T2-weighted spin-echo sequences, three-dimensional (3D) gadolinium-enhanced MR angiography, and 3D phase-contrast MR angiography. Because surgical techniques for revascularization vary, knowledge of the surgical procedure is necessary to ensure inclusion of the pertinent anatomy at 3D gadolinium-enhanced MR angiography and to define appropriate 3D phase-contrast MR angiography volumes. The 3D gadolinium-enhanced MR angiography volume can be manipulated to view relevant vascular anatomy at the optimal obliquity and section thickness. This protocol allows robust, noninvasive evaluation of the expected arterial anatomy after revascularization, including renal artery endarterectomy, aortorenal bypass grafts, and extraanatomic reconstructions. In cases of suspected postrevascularization complications, gadolinium-enhanced MR angiography is useful because of its lack of nephrotoxicity and radiation exposure. Immediate complications of renal revascularization include renal artery thrombosis, renal infarction, and perinephric hemorrhage. Long-term complications include aneurysms of bypass grafts and recurrent stenosis of the renal artery.

GD-enhanced 3D phase-contrast MR angiography and dynamic perfusion imaging in the diagnosis of renal artery stenosis

The objective of this study was to investigate the role of contrast enhancement using a three-dimensional (3D) phase-contrast (PC) magnetic resonance (MR) sequence (3D PC-MRA) and to assess the value of a dynamic MR perfusion study of the kidneys to determine the hemodynamic relevance of unilateral renal artery stenosis (RAS). Seventeen patients with unilateral RAS were examined on a standard 1.0 T imaging system using a phase shift and magnitude sensitive 3D PC sequence (TR = 160 ms, TE = 9 ms, venc. 30 cm/s). Following the initial pre-contrast 3D PC-MRA a dynamic first pass perfusion study was performed using a Turbo-FLASH 2D sequence (TR = 4.5 ms, TE = 2.2 ms, TI = 400 ms) after bolus injection of 0.15 mmol gadolinium-diethylenetriamine pentaacetic acid (Gd-DTPA)/kg body weight. The 3D PC-MRA was then repeated during infusion of 0.15 mmol Gd-DTPA/kg body weight. Evaluation by three independent readers was based on maximum intensity projection images. Source images were rendered on request. Signal intensity (SI) over time curves of the renal cortex were obtained from the dynamic perfusion study and analyzed for maximum signal enhancement as well as temporal relationship to the aortic SI curve. Results from 3D PC-MRA revealed a sensitivity (pre-/post-contrast) of 100%/89%, specificity of 76%/63%, positive predictive value of 80%/69%, negative predictive value of 90%/78%, and accuracy of 85%/75% ( p = 0.07). Interobserver agreement was κ = 0.61/κ = 0.47 (pre/post Gd-DTPA), respectively. Increased signal-to-noise was present in all segments of the renal arteries post contrast ( p = 0.0003). This came along with image degradation due to aliasing and elevated SI of venous flow that partially obscured the renal arteries. Dynamic SI curves showed a significantly decreased maximum SI in RAS ( p = 0.01–0.001). A temporal delay of cortical signal intensity enhancement could not be confirmed in this setting. Gd-enhanced 3D PC-MRA did not yield a superior diagnostic value in the diagnosis of RAS compared to pre-contrast measurements. Dynamic perfusion imaging of the kidneys, in combination with 3D PC-MRA, can contribute additional information in suspected unilateral RAS.

Renal artery stenosis: value of screening with three-dimensional phase-contrast MR angiography with a phased-array multicoil.

To evaluate screening with three-dimensional (3D) phase-contrast magnetic resonance (MR) angiography with a phased-array multicoil to detect renal artery stenosis. Fifty consecutive patients suspected of having renovascular disease were prospectively examined with 3D phase-contrast MR imaging with a phased-array multicoil. Findings were correlated with those at intra-arterial digital subtraction angiography (DSA) as the standard of reference for grade of stenosis. MR angiography depicted 101 of 103 renal arteries depicted at intraarterial DSA; the two missed arteries were an accessory artery outside the imaging volume and an artery with a stent. At intraarterial DSA, a stenosis was found in 31 of 101 arteries. On the basis of findings at 3D phase-contrast MR angiography, the presence of any degree of stenosis was correctly depicted in 29 of 31 cases and the absence of stenosis was correctly depicted in 66 of 70 cases (accuracy, 94%; sensitivity, 94%; negative predictive value, 97%). Overall accuracy was 97% for correct depiction of severe renal artery stenosis (> 50%). 3D phase-contrast MR angiography with a phased-array multicoil was an accurate noninvasive screening technique in patients with suspected renal artery stenosis.

Role of 3D Display MR Angiography in the Study of Intracranial Aneurysms

This paper determines the usefulness of threedimensional (3D) phase-contrast (PC) magnetic resonance (MR) angiography with 3D display in characterizing large intracranial saccular aneurysms. Eight patients referred for non invasive confirmation of unruptured aneurysm (n=9) detected by conventional CT and/or MR imaging were studied by 3D PC (VENC of 20 cm/s, 3 axes) in axial acquisition localised on the circle of Willis using a 0.5 T unit (Vectra, GE). We used 3D display images to assess the feasibility of selective treatment. Results were correlated with catheter angiograms in 3 patients. In two thirds of the cases, location and size of the orifice, size of the sac, and clarification of parent artery were clearly provided allowing prediction of therapeutic management. In the remaining cases results remained doubful because of lack of spatial resolution. The benefit of the method was coupled with the clinical status of the patients to influence the final decision. 3D PC MR angiography with 3D display has emerged as an improvement in predicting therapeutic management, and is especially useful in large unruptured aneurysms in elderly or ill patients.

Noise reduction in three-dimensional phase-contrast MR velocity measurements.

The authors have developed a method to reduce noise in three-dimensional (3D) phase-contrast magnetic resonance (MR) velocity measurements by exploiting the property that blood is incompressible and, therefore, the velocity field describing its flow must be divergence-free. The divergence-free condition is incorporated by a projection operation in Hilbert space. The velocity field obtained with 3D phase-contrast MR imaging is projected onto the space of divergence-free velocity fields. The reduction of noise is achieved because the projection operation eliminates the noise component that is not divergence-free. Signal-to-noise ratio (S/N) gains on the order of 15%-25% were observed. The immediate effect of this noise reduction manifests itself in higher-quality phase-contrast MR angiograms. Alternatively, the S/N gain can be traded for a reduction in imaging time and/or improved spatial resolution.

Visualizing three-dimensional flow with simulated streamlines and three-dimensional phase-contrast MR imaging.

Three-dimensional (3D) velocity maps acquired with 3D phase-contrast magnetic resonance (MR) imaging contain information regarding complex motions that occur during imaging. A technique called simulated streamlines, which facilitates the display and comprehension of these velocity data, is presented. Single or multiple seed points may be identified within blood vessels of interest and tracked through the velocity field. The resulting trajectories are combined with a 3D MR angiogram and displayed with 3D volume visualization software. Mathematical analysis highlights potential applications and pitfalls of the technique, which was implemented both in phantoms and in vivo with excellent results. For example, single streamlines reveal helical flow patterns in aneurysms, and multiple streamlines seeded in the common carotid artery reveal branch filling-time relationships and slow filling of the carotid bulb. The technique is helpful in understanding these complex flow patterns.