Effect of flip angle on volume flow measurement with nontriggered phase‐contrast MR: In vivo evaluation in carotid and basilar arteries

Purpose

To evaluate the effect of flip angle on volume flow rate measurements obtained with nontriggered phase‐contrast magnetic resonance imaging (MRI) in vivo.

Materials and Methods

We prospectively measured volume flow rates of the bilateral internal carotid artery and the basilar artery with cine and nontriggered phase‐contrast MRI. For nontriggered phase‐contrast imaging, flip angles of 4, 15, 60, and 90° were used for 40 volunteers and of 8, 15, and 30° for 54 volunteers. Lumen boundaries were semiautomatically determined by pulsatility‐based segmentation using cine phase‐contrast MRI. Identical lumen boundaries were used for nontriggered phase‐contrast imaging.

Results

The ratio of volume flow rate obtained with nontriggered phase‐contrast imaging to that obtained with cine phase‐contrast imaging significantly increases with an increase in the flip angle. The mean ratios lie within a relatively narrow range of ±15% with a wide range of flip angles of 8–90°. As the flip angle increases, ghost artifacts become prominent and signal‐to‐noise and contrast‐to‐noise ratios increase.

Conclusion

Flip angles between 8 and 60° are most appropriate for nontriggered phase‐contrast MR measurements in the internal carotid and the basilar artery. J. Magn. Reson. Imaging 2009;29:1218–1223. © 2009 Wiley‐Liss, Inc.     

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.     

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.     

Quantification and visualization of flow in the Circle of Willis: Time‐resolved three‐dimensional phase contrast MRI at 7 T compared with 3 T

The assessment of both geometry and hemodynamics of the intracranial arteries has important diagnostic value in internal carotid occlusion, sickle cell disease, and aneurysm development. Provided that signal to noise ratio (SNR) and resolution are high, these factors can be measured with time‐resolved three‐dimensional phase contrast MRI. However, within a given scan time duration, an increase in resolution causes a decrease in SNR and vice versa, hampering flow quantification and visualization. To study the benefits of higher SNR at 7 T, three‐dimensional phase contrast MRI in the Circle of Willis was performed at 3 T and 7 T in five volunteers. Results showed that the SNR at 7 T was roughly 2.6 times higher than at 3 T. Therefore, segmentation of small vessels such as the anterior and posterior communicating arteries succeeded more frequently at 7 T. Direction of flow and smoothness of streamlines in the anterior and posterior communicating arteries were more pronounced at 7 T. Mean velocity magnitude values in the vessels of the Circle of Willis were higher at 3 T due to noise compared to 7 T. Likewise, areas of the vessels were lower at 3 T. In conclusion, the gain in SNR at 7 T compared to 3 T allows for improved flow visualization and quantification in intracranial arteries. Magn Reson Med, 2013. © 2012 Wiley Periodicals, Inc.     

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.     

4D phase contrast MRI at 3 T: Effect of standard and blood‐pool contrast agents on SNR,PC‐MRA,and blood flow visualization

Time‐resolved phase contrast (PC) MRI with velocity encoding in three directions (flow‐sensitive four‐dimensional MRI) can be employed to assess three‐dimensional blood flow in the entire aortic lumen within a single measurement. These data can be used not only for the visualization of blood flow but also to derive additional information on vascular geometry with three‐dimensional PC MR angiography (MRA). As PC‐MRA is sensitive to available signal‐to‐noise ratio, standard and novel blood pool contrast agents may help to enhance PC‐MRA image quality. In a group of 30 healthy volunteers, the influence of different contrast agents on vascular signal‐to‐noise ratio, PC‐MRA quality, and subsequent three‐dimensional stream‐line visualization in the thoracic aorta was determined. Flow‐sensitive four‐dimensional MRI data acquired with contrast agent provided significantly improved signal‐to‐noise ratio in magnitude data and noise reduction in velocity data compared to measurements without contrast media. The agreement of three‐dimensional PC‐MRA with reference standard contrast‐enhanced MRA was good for both contrast agents, with improved PC‐MRA performance for blood pool contrast agent, particularly for the smaller supra‐aortic branches. For three‐dimensional flow visualization, a trend toward improved results for the data with contrast agent was observed. Magn Reson Med, 2010. © 2009 Wiley‐Liss, Inc.     

Quantification of myocardial blood flow using model based analysis of first‐pass perfusion MRI: Extraction fraction of Gd‐DTPA varies with myocardial blood flow in human myocardium

For the absolute quantification of myocardial blood flow (MBF), Patlak plot‐derived K1 need to be converted to MBF by using the relation between the extraction fraction of gadolinium contrast agent and MBF. This study was conducted to determine the relation between extraction fraction of Gd‐DTPA and MBF in human heart at rest and during stress. Thirty‐four patients (19 men, mean age of 66.5 ± 11.0 years) with normal coronary arteries and no myocardial infarction were retrospectively evaluated. First‐pass myocardial perfusion MRI during adenosine triphosphate stress and at rest was performed using a dual bolus approach to correct for saturation of the blood signal. Myocardial K1 was quantified by Patlak plot method. Mean MBF was determined from coronary sinus flow measured by phase contrast cine MRI and left ventricle mass measured by cine MRI. The extraction fraction of Gd‐DTPA was calculated as the K1 divided by the mean MBF. The extraction fraction of Gd‐DTPA was 0.46 ± 0.22 at rest and 0.32 ± 0.13 during stress (P < 0.001). The relationship between extraction fraction (E) and MBF in human myocardium can be approximated as E = 1 ? exp(?(0.14 × MBF + 0.56)/MBF). The current results indicate that MBF can be accurately quantified by Patlak plot method of first‐pass myocardial perfusion MRI by performing a correction of extraction fraction. Magn Reson Med, 2011. © 2011 Wiley Periodicals, Inc.     

Regional pulmonary blood flow: Comparison of dynamic contrast‐enhanced MR perfusion and phase‐contrast MR

Regional pulmonary blood flow can be assessed using both dynamic contrast‐enhanced (DCE) MR and phase‐contrast (PC) MR. These methods provide somewhat complementary information: DCE MR can assess flow over the entire lung while PC MR can detect rapid changes in flow to a targeted region. Although both methods are considered accurate, one may be more feasible than the other depending on pathology, patient condition, and availability of an intravenous route. The objective of this study was to establish a consensus between the two methods by comparing paired DCE MR and PC MR measurements of relative blood flow in Yorkshire piglets (N = 5, age = 7 days, weight = 3.3 ± 0.6 kg) under various physiological states including regional lung collapse. A strong correlation (R² = 0.71, P < 0.01) was observed between the methods. In conclusion, DCE MR and PC MR provide a consistent measure of changes in regional pulmonary blood flow. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.     

Absolute quantification of cerebral blood flow in neurologically normal volunteers: Dynamic‐susceptibility contrast MRI‐perfusion compared with computed tomography (CT)‐perfusion

To improve the reproducibility of arterial input function (AIF) registration and absolute cerebral blood flow (CBF) quantification in dynamic‐susceptibility MRI‐perfusion (MRP) at 1.5T, we rescaled the AIF by use of a venous output function (VOF). We compared CBF estimates of 20 healthy, elderly volunteers, obtained by computed tomography (CT)‐perfusion (CTP) and MRP on two consecutive days. MRP, calculated without the AIF correction, did not result in any significant correlation with CTP. The rescaled MRP showed fair to moderate correlation with CTP for the central gray matter (GM) and the whole brain. Our results indicate that the method used for correction of partial volume effects (PVEs) improves MRP experiments by reducing AIF‐introduced variance at 1.5T. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.