Flow‐sensitive 4D MRI of the thoracic aorta: Comparison of image quality,quantitative flow,and wall parameters at 1.5 T and 3 T

Purpose:

To evaluate the effect of field strength on flow‐sensitive 4D magnetic resonance imaging (MRI) of the thoracic aorta. A volunteer study at 1.5 T and 3 T was conducted to compare phase‐contrast MR angiography (MRA) and 3D flow visualization quality as well as quantification of aortic hemodynamics.

Materials and Methods:

Ten healthy volunteers were examined by flow‐sensitive 4D MRI at both 1.5 T and 3 T MRI with identical imaging parameters (TE/TR = 6/5.1 msec, spatial/temporal resolution ≈2 mm/40.8 msec). Analysis included assessment of image quality of derived aortic 3D phase contrast (PC) angiography and 3D flow visualization (semiquantitative grading on a 0–2 scale, two blinded observers) and quantification of blood flow velocities, net flow per cardiac cycle, wall shear stress (WSS), and velocity noise.

Results:

Quality of 3D blood flow visualization (average grading = 1.8 ± 0.4 at 3 T vs. 1.1 ± 0.7 at 1.5 T) and the depiction of aortic lumen geometry by 3D PC‐MRA (1.7 ± 0.5 vs. 1.2 ± 0.6) were significantly (P < 0.01) improved at 3 T while velocity noise was significantly higher (P < 0.01) at 1.5 T. Velocity quantification resulted in minimally altered (0.05 m/s, 3 mL/cycle and 0.01 N/m²) but not statistically different (P = 0.40, P = 0.39, and P = 0.82) systolic peak velocities, net flow, and WSS for 1.5 T compared to 3 T.

Conclusion:

Flow‐sensitive 4D MRI at 3 T provided improved image quality without additional artifacts related to higher fields. Imaging at 1.5 T MRI, which is more widely available, was also feasible and provided information on aortic 3D hemodynamics of moderate quality with identical performance regarding quantitative analysis. J. Magn. Reson. Imaging 2012;36:1097–1103. © 2012 Wiley Periodicals, Inc.     

Time-resolved three-dimensional phase-contrast MRI

PURPOSE: To demonstrate the feasibility of a four-dimensional phase contrast (PC) technique that permits spatial and temporal coverage of an entire three-dimensional volume, to quantitatively validate its accuracy against an established time resolved two-dimensional PC technique to explore advantages of the approach with regard to the four-dimensional nature of the data. MATERIALS AND METHODS: Time-resolved, three-dimensional anatomical images were generated simultaneously with registered three-directional velocity vector fields. Improvements compared to prior methods include retrospectively gated and respiratory compensated image acquisition, interleaved flow encoding with freely selectable velocity encoding (venc) along each spatial direction, and flexible trade-off between temporal resolution and total acquisition time. RESULTS: The implementation was validated against established two-dimensional PC techniques using a well-defined phantom, and successfully applied in volunteer and patient examinations. Human studies were performed after contrast administration in order to compensate for loss of in-flow enhancement in the four-dimensional approach. CONCLUSION: Advantages of the four-dimensional approach include the complete spatial and temporal coverage of the cardiovascular region of interest and the ability to obtain high spatial resolution in all three dimensions with higher signal-to-noise ratio compared to two-dimensional methods at the same resolution. In addition, the four-dimensional nature of the data offers a variety of image processing options, such as magnitude and velocity multi-planar reformation, three-directional vector field plots, and velocity profiles mapped onto selected planes of interest.     

Variable velocity encoding in a three‐dimensional,three‐directional phase contrast sequence: Evaluation in phantom and volunteers

Purpose:

To evaluate accuracy and noise properties of a novel time‐resolved, three‐dimensional, three‐directional phase contrast sequence with variable velocity encoding (denoted 4D‐vPC) on a 3 Tesla MR system, and to investigate potential benefits and limitations of variable velocity encoding with respect to depicting blood flow patterns.

Materials and Methods:

A 4D PC‐MRI sequence was modified to allow variable velocity encoding (VENC) over the cardiac cycle in all three velocity directions independently. 4D‐PC sequences with constant and variable VENC were compared in a rotating phantom with respect to measured velocities and noise levels. Additionally, comparison of flow patterns in the ascending aorta was performed in six healthy volunteers.

Results:

Phantom measurements showed a linear relationship between velocity noise and velocity encoding. 4D‐vPC MRI presented lower noise levels than 4D‐PC both in phantom and in volunteer measurements, in agreement with theory. Volunteer comparisons revealed more consistent and detailed flow patterns in early diastole for the variable VENC sequences.

Conclusion:

Variable velocity encoding offers reduced noise levels compared with sequences with constant velocity encoding by optimizing the velocity‐to‐noise ratio (VNR) to the hemodynamic properties of the imaged area. Increased VNR ratios could be beneficial for blood flow visualizations of pathology in the cardiac cycle. J. Magn. Reson. Imaging 2012; 36:1450–1459. © 2012 Wiley Periodicals, Inc.     

Gradient field switching as a source for artifacts in MR imaging of metallic stents.

Metallic implants, such as stents, have long been a concern in magnetic resonance imaging (MRI). In addition to safety issues, they are commonly associated with image artifacts. The mechanisms of radiofrequency- (RF) and susceptibility-induced artifacts have been thoroughly investigated. However, gradient-switching-induced artifacts have not been analyzed. In this study it was demonstrated that gradient switching may be a source of artifacts in metallic stent MR imaging. These artifacts differ from those caused by the RF pulse. A theoretical explanation is provided as well.     

Arterial input functions from MR phase imaging

A method is presented for obtaining high-sensitivity arterial input functions following bolus intravenous contrast agent administration. Arterial contrast agent is monitored by phase reconstruction of single-shot echo-planar images. During bolus injections of a gadolinium (Gd) agent in a baboon, data were acquired at the mid-abdominal aorta, and magnitude and phase-shift images were reconstructed. Pair-wise image subtraction was used to minimize phase aliasing. The phase-based method is shown to have a significant potential improvement in sensitivity compared to the magnitude approach. The phase method also has a general linear response to concentration. This method may have potential utility in quantitative imaging of blood flow and contrast agent kinetics.     

Analysis of pulse wave velocity in the thoracic aorta by flow-sensitive four-dimensional MRI: reproducibility and correlation with characteristics in patients with aortic atherosclerosis

Purpose:

To measure aortic pulse wave velocity (PWV) using flow‐sensitive four‐dimensional (4D) MRI and to evaluate test–retest reliability, inter‐ and intra‐observer variability in volunteers and correlation with characteristics in patients with aortic atherosclerosis.

Materials and Methods:

Flow‐sensitive 4D MRI was performed in 12 volunteers (24 ± 3 years) and 86 acute stroke patients (68 ± 9 years) with aortic atherosclerosis. Retrospectively positioned 28 ± 4 analysis planes along the entire aorta (inter‐slice‐distance = 10 mm) and frame wise lumen segmentation yielded flow‐time‐curves for each plane. Global aortic PWV was calculated from time‐shifts and distances between the upslope portions of all available flow‐time curves.

Results:

Inter‐ and intra‐observer variability of PWV measurements in volunteers (7% and 8%) was low while test–retest reliability (22%) was moderate. PWV in patients was significantly higher compared with volunteers (5.8 ± 2.9 versus 3.8 ± 0.8 m/s; P = 0.02). Among 17 patient characteristics considered, statistical analysis revealed significant (P < 0.05) but low correlation of PWV with age (r = 0.25), aortic valve insufficiency (r = 0.29), and pulse pressure (r = 0.28). Multivariate modeling indicated that aortic valve insufficiency and elevated pulse pressure were significantly associated with higher PWV (adjusted R² = 0.13).

Conclusion:

Flow‐sensitive 4D MRI allows for estimating aortic PWV with low observer dependence and moderate test–retest reliability. PWV in patients correlated with age, aortic valve insufficiency, and pulse pressure. J. Magn. Reson. Imaging 2012;35:1162‐1168. © 2012 Wiley Periodicals, Inc.     

Reproducibility of flow and wall shear stress analysis using flow-sensitive four-dimensional MRI

Purpose:

To systematically investigate the scan–rescan reproducibility and observer variability of flow‐sensitive four‐dimensional (4D) MRI in the aorta for the assessment of blood flow and global and segmental wall shear stress.

Materials and Methods:

ECG and respiration‐synchronized flow‐sensitive 4D MRI data (spatio‐temporal resolution = 1.7 × 2.0 × 2.2 mm³/40.8 ms) were acquired in 12 healthy volunteers. To analyze scan–rescan variability, flow‐sensitive 4D MRI was repeated in 10 volunteers during a second visit. Data analysis included calculation of time‐resolved and total flow, peak systolic velocity, and regional and global wall shear stress (WSS) in up to 24 analysis planes distributed along the aorta.

Results:

Scan–rescan, inter‐observer, and intra‐observer agreement was excellent for the calculation of total flow and peak systolic velocity (mean differences <5% of the average flow parameter). Global WSS demonstrated moderate agreement and increased variability regarding wall shear stress (scan‐rescan, inter‐observer, and intra‐observer agreement; mean differences <10% of the average WSS parameters). The segmental distribution of wall shear stress in the thoracic aorta could reliably be reproduced (r > 0.87; P < 0.001) for different observers and examinations.

Conclusion:

Flow‐sensitive 4D MRI‐based analysis of aortic blood flow can be performed with good reproducibility. Robustness of global and regional WSS quantification was limited, but spatio‐temporal WSS distributions could reliably be replicated. J. Magn. Reson. Imaging 2011;33:988–994. © 2011 Wiley‐Liss, Inc.     

3D blood flow characteristics in the carotid artery bifurcation assessed by flow‐sensitive 4D MRI at 3T

To determine three‐dimensional (3D) blood flow patterns in the carotid bifurcation, 10 healthy volunteers and nine patients with internal carotid artery (ICA) stenosis ≥50% were examined by flow‐sensitive 4D MRI at 3T. Absolute and mean blood velocities, pulsatility index (PI), and resistance index (RI) were measured in the common carotid arteries (CCAs) by duplex sonography (DS) and compared with flow‐sensitive 4D MRI. Furthermore, 3D MRI blood flow patterns in the carotid bifurcation of volunteers and patients before and after recanalization were graded by two independent readers. Blood flow velocities measured by MRI were 31–39% lower than in DS. However, PI and RI differed by only 13–16%. Rating of 3D flow characteristics in the ICA revealed consistent patterns for filling and helical flow in volunteers. In patients with ICA stenosis, 3D blood flow visualization was successfully employed to detect markedly altered filling and helical flow patterns (forward‐moving spiral flow) in the ICA bulb and to evaluate the effect of revascularization, which restored filling and helical flow. Our results demonstrate the feasibility of flow‐sensitive 4D MRI for the quantification and 3D visualization of physiological and pathological flow patterns in the carotid artery bifurcation. Magn Reson Med 61:65–74, 2009. © 2008 Wiley‐Liss, Inc.     

Faster flow quantification using sensitivity encoding for velocity-encoded cine magnetic resonance imaging: in vitro and in vivo validation

PURPOSE: To test the agreement between conventional and sensitivity-encoded (SENSE) velocity encoded cine (VEC) MRI in a flow phantom and in subjects with congenital and acquired heart disease. MATERIALS AND METHODS: Flow measurements were performed in a 1.5 T scanner using a segmented k-space VEC MRI sequence and then repeated with a SENSE factor of 2. The flow phantom used a piston pump to generate physiologic arterial waveforms (0.5-4.9 L/min). In the subjects, flow measurements were performed in the ascending aorta (N = 33) and/or the main pulmonary artery (N = 24). RESULTS: Utilization of SENSE reduced the scan time by 50%. In the phantom, measurements without and with SENSE agreed closely with a mean difference of 0.01 +/- 0.08 L/min or 0.12% +/- 3.8% (P = 0.68). In the subjects, measurements without and with SENSE also agreed closely with a mean difference of 0.08 +/- 0.36 L/min or 1.3% +/- 7.2% (P = 0.08). Compared with standard imaging, the use of SENSE reduced the signal-to-noise ratio (SNR) by 28% in the phantom (N = 10) and 27% in vivo (N = 22). CONCLUSION: VEC MRI flow measurements with a SENSE factor of 2 were twice as fast and agreed closely with the conventional technique in vitro and in vivo. VEC MRI with SENSE can be used for rapid and reliable quantification of blood flow.     

Simultaneous imaging of myocardial motion and chamber blood flow with SPAMM n' EGGS (Spatial Modulation of Magnetization With Encoded Gradients for Gauging Speed)

PURPOSE: To provide simultaneous measurements of one-dimensional (1-D) myocardial displacement and 1-D chamber blood flow in a single breath-held acquisition using an MR imaging technique, SPAMM n' EGGS (Spatial Modulation of Magnetization With Encoded Gradients for Gauging Speed). MATERIALS AND METHODS: Velocity encoding bipolar gradients sensitive to chamber blood flow were played out before the readout gradient in a 1-1 SPAMM-tagged MR imaging pulse sequence. For any given motion-flow encoded direction, the acquired image sequence was later postprocessed to separate the tag motion and blood flow terms. Experiments were performed on seven normal volunteers, and two pigs with moderate ischemic mitral regurgitation. Left-ventricular motion and trans-valvular flow obtained using the SPAMM n' EGGS pulse sequence was compared against measurements obtained using standard tagging and phase-contrast pulse sequences, respectively. RESULTS: Results in normal volunteers and diseased pigs demonstrate multiphase correlated measurements of myocardial motion and chamber blood flow using SPAMM n' EGGS. A close correspondence in these measurements to conventional tagging and phase-contrast sequences is confirmed. CONCLUSION: We have demonstrated that simultaneous acquisition of myocardial motion and chamber blood flow is possible within a single breath-hold. The data obtained using the SPAMM n' EGGS pulse sequence may be useful in the planning and evaluation of mitral-valve repair procedures.     

Quantifying in vivo hemodynamic response to exercise in patients with intermittent claudication and abdominal aortic aneurysms using cine phase‐contrast MRI

Purpose:

To evaluate rest and exercise hemodynamics in patients with abdominal aortic aneurysms (AAA) and peripheral occlusive disease (claudicants) using phase‐contrast MRI.

Materials and Methods:

Blood velocities were acquired by means of cardiac‐gated cine phase‐contrast in a 0.5 Tesla (T) open MRI. Volumetric flow was calculated at the supraceliac (SC), infrarenal (IR), and mid‐aneurysm (MA) levels during rest and upright cycling exercise using an MR‐compatible exercise cycle.

Results:

Mean blood flow increased during exercise (AAA: 130%, Claudicants: 136% of resting heart rate) at the SC and IR levels for AAA participants (2.6 ± 0.6 versus 5.8 ± 1.6 L/min, P < 0.001 and 0.8 ± 0.4 versus 5.1 ± 1.7 L/min, P < 0.001) and claudicants (2.3 ± 0.5 versus 4.5 ± 0.9 L/min, P < 0.005 and 0.8 ± 0.2 versus 3.3 ± 0.9 L/min, P < 0.005). AAA participants had a significant decrease in renal and digestive blood flow from rest to exercise (1.8 ± 0.7 to 0.7 ± 0.6 L/min, P < 0.01). The decrease in renal and digestive blood flow during exercise correlated with daily activity level for claudicants (R = 0.81).

Conclusion:

Abdominal aortic hemodynamic changes due to lower extremity exercise can be quantified in patients with AAA and claudication using PC‐MRI. The redistribution of blood flow during exercise was significant and different between the two disease states. J. Magn. Reson. Imaging 2010; 31: 425–429. © 2010 Wiley‐Liss, Inc.

     

Plaques in the descending aorta: A new risk factor for stroke? Visualization of potential embolization pathways by 4D MRI

The combination of morphologic and hemodynamic information can help in assessing the risk of embolic stroke associated with thrombi and plaques in the descending aorta. For two acute stroke patients, the determination of individual embolic pathways using flow-sensitive four-dimensional (4D) MRI are reported. 3D visualization of local flow patterns, i.e., retrograde flow channels originating at the site of the atheroma, in conjunction with exact plaque localization, suggested potential embolization of high-risk plaques in the descending aorta although they are located downstream from the supraaortic arteries. Our findings indicate that taking plaques of the descending aorta into consideration may help improve the spectrum of pathologies considered as high-risk sources for brain ischemia.