Analysis of systematic and random error in MR volumetric flow measurements

Effect of MRI acquisition parameters on accuracy and precision of phase-contrast measurements in a small-lumen vessel phantom

BACKGROUND

Phase-contrast magnetic resonance imaging (PC-MRI) quantifies blood flow and velocity noninvasively. Challenges arise in neurovascular disorders due to small vessels. We evaluated the impact of voxel size, number of signal averages (NSA), and velocity encoding (VENC) on PC-MRI measurement accuracy and precision in a small-lumen vessel phantom.

METHODS

We constructed an in vitro model with a constant flow rate using a 2.2-mm inner diameter plastic tube. A reservoir with a weight scale and timer was used as standard reference. Gradient-echo T1 weighted PC-MRI sequence was performed on a 3-T scanner with varying voxel size (2.5, 5.0, 7.5 mm3), NSA (1, 2, 3), and VENC (200, 300, 400 cm/s). We repeated measurements nine times per setting, calculating mean flow rate, maximum velocity, and least detectable difference (LDD).

RESULTS

PC-MRI flow measurements were higher than standard reference values (mean ranging from 7.3 to 9.5 mL/s compared with 6.6 mL/s). Decreased voxel size improved accuracy, reducing flow rate measurements from 9.5 to 7.3 mL/s. The LDD for flow rate and velocity varied between 1 and 5%. The LDD for flow rate decreased with increased voxel size and NSA (p = 0.033 and 0.042). The LDD for velocity decreased with increased voxel size (p < 10-16). No change was observed when VENC varied.

CONCLUSIONS

PC-MRI overestimated flow. However, it has high precision in a small-vessel phantom with constant flow rate. Improved accuracy was obtained with increasing spatial resolution (smaller voxels). Improved precision was obtained with increasing signal-to-noise ratio (larger voxels and/or higher NSA).

RELEVANCE STATEMENT

Phase-contrast MRI is clinically used in large vessels. To further investigate the possibility of using phase-contrast MRI for smaller intracranial vessels in neurovascular disorders, we need to understand how acquisition parameters affect phase-contrast MRI-measured flow rate and velocity in small vessels.

KEY POINTS

• PC-MRI measures flow and velocity in a small lumen phantom with high precision but overestimates flow rate. • The precision of PC-MRI measurements matches the precision of standard reference for flow rate measurements. • Optimizing PC-MRI settings can enhance accuracy and precision in flow rate and velocity measurements.

Automatic 4D Flow MRI Segmentation Using the Standardized Difference of Means Velocity

We present a method to automatically segment 4D flow magnetic resonance imaging (MRI) by identifying net flow effects using the standardized difference of means (SDM) velocity. The SDM velocity quantifies the ratio between the net flow and observed flow pulsatility in each voxel. Vessel segmentation is performed using an F-test, identifying voxels with significantly higher SDM velocity values than background voxels. We compare the SDM segmentation algorithm against pseudo-complex difference (PCD) intensity segmentation of 4D flow measurements in in vitro cerebral aneurysm models and 10 in vitro Circle of Willis (CoW) datasets. We also compared the SDM algorithm to convolutional neural network (CNN) segmentation in 5 thoracic vasculature datasets. The in vitro flow phantom geometry is known, while the ground truth geometries for the CoW and thoracic aortas are derived from high-resolution time-of-flight (TOF) magnetic resonance angiography and manual segmentation, respectively. The SDM algorithm demonstrates greater robustness than PCD and CNN approaches and can be applied to 4D flow data from other vascular territories. The SDM to PCD comparison demonstrated an approximate 48% increase in sensitivity in vitro and 70% increase in the CoW, respectively; the SDM and CNN sensitivities were similar. The vessel surface derived from the SDM method was 46% closer to the in vitro surfaces and 72% closer to the in vitro TOF surfaces than the PCD approach. The SDM and CNN approaches both accurately identify vessel surfaces. The SDM algorithm is a repeatable segmentation method, enabling reliable computation of hemodynamic metrics associated with cardiovascular disease.

Comparison of 2D and 4D Flow MRI in Neonates Without General Anesthesia

BACKGROUND

Neonates with critical congenital heart disease require early intervention. Four-dimensional (4D) flow may facilitate surgical planning and improve outcome, but accuracy and precision in neonates are unknown.

PURPOSE

To 1) validate two-dimensional (2D) and 4D flow MRI in a phantom and investigate the effect of spatial and temporal resolution; 2) investigate accuracy and precision of 4D flow and internal consistency of 2D and 4D flow in neonates; and 3) compare scan time of 4D flow to multiple 2D flows.

STUDY TYPE

Phantom and prospective patients.

POPULATION

A total of 17 neonates with surgically corrected aortic coarctation (age 18 days [IQR 11-20]) and a three-dimensional printed neonatal aorta phantom.

FIELD STRENGTH/SEQUENCE

1.5T, 2D flow and 4D flow.

ASSESSMENT

In the phantom, 2D and 4D flow volumes (ascending and descending aorta, and aortic arch vessels) with different resolutions were compared to high-resolution reference 2D flow. In neonates, 4D flow was compared to 2D flow volumes at each vessel. Internal consistency was computed as the flow volume in the ascending aorta minus the sum of flow volumes in the aortic arch vessels and descending aorta, divided by ascending aortic flow.

STATISTICAL TESTS

Bland-Altman plots, Pearson correlation coefficient (r), and Student's t-tests.

RESULTS

In the phantom, 2D flow differed by 0.01 ± 0.02 liter/min with 1.5 mm spatial resolution and -0.01 ± 0.02 liter/min with 0.8 mm resolution; 4D flow differed by -0.05 ± 0.02 liter/min with 2.4 mm spatial and 42 msec temporal resolution, -0.01 ± 0.02 liter/min with 1.5 mm, 42 msec resolution and -0.01 ± 0.02 liter/min with 1.5 mm, 21 msec resolution. In patients, 4D flow and 2D flow differed by -0.06 ± 0.08 liter/min. Internal consistency in patients was -11% ± 17% for 2D flow and 5% ± 13% for 4D flow. Scan time was 17.1 minutes [IQR 15.5-18.5] for 2D flow and 6.2 minutes [IQR 5.3-6.9] for 4D flow, P < 0.0001.

DATA CONCLUSION

Neonatal 4D flow MRI is time efficient and can be acquired with good internal consistency without contrast agents or general anesthesia, thus potentially expanding 4D flow use to the youngest and smallest patients.

EVIDENCE LEVEL

1 TECHNICAL EFFICACY: Stage 2.

Wall Shear Stress Estimation for 4D Flow MRI Using Navier-Stokes Equation Correction

This study introduces a novel wall shear stress (WSS) estimation method for 4D flow MRI. The method improves the WSS accuracy by using the reconstructed pressure gradient and the flow-physics constraints to correct velocity gradient estimation. The method was tested on synthetic 4D flow data of analytical Womersley flow and flow in cerebral aneurysms and applied to in vivo 4D flow data acquired in cerebral aneurysms and aortas. The proposed method's performance was compared to the state-of-the-art method based on smooth-spline fitting of velocity profile and the WSS calculated from uncorrected velocity gradient. The proposed method improved the WSS accuracy by as much as 100% for the Womersley flow and reduced the underestimation of mean WSS by 39 to 50% for the synthetic aneurysmal flow. The predicted mean WSS from the in vivo aneurysmal data using the proposed method was 31 to 50% higher than the other methods. The predicted aortic WSS using the proposed method was 3 to 6 times higher than the other methods and was consistent with previous CFD studies and the results from recently developed methods that take into account the limited spatial resolution of 4D flow MRI. The proposed method improves the accuracy of WSS estimation from 4D flow MRI, which can help predict blood vessel remodeling and progression of cardiovascular diseases.

Modeling Bias Error in 4D Flow MRI Velocity Measurements

We present a model to estimate the bias error of 4D flow magnetic resonance imaging (MRI) velocity measurements. The local instantaneous bias error is defined as the difference between the expectation of the voxel's measured velocity and actual velocity at the voxel center. The model accounts for bias error introduced by the intra-voxel velocity distribution and partial volume (PV) effects. We assess the intra-voxel velocity distribution using a 3D Taylor Series expansion. PV effects and numerical errors are considered using a Richardson extrapolation. The model is applied to synthetic Womersley flow and in vitro and in vivo 4D flow MRI measurements in a cerebral aneurysm. The bias error model is valid for measurements with at least 3.75 voxels across the vessel diameter and signal-to-noise ratio greater than 5. All test cases exceeded this diameter to voxel size ratio with diameters, isotropic voxel sizes, and velocity ranging from 3-15mm, 0.5-1mm, and 0-60cm/s, respectively. The model accurately estimates the bias error in voxels not affected by PV effects. In PV voxels, the bias error is an order of magnitude higher, and the accuracy of the bias error estimation in PV voxels ranges from 67.3% to 108% relative to the actual bias error. The bias error estimated for in vivo measurements increased two-fold at systole compared to diastole in partial volume and non-partial volume voxels, suggesting the bias error varies over the cardiac cycle. This bias error model quantifies 4D flow MRI measurement accuracy and can help plan 4D flow MRI scans.

Real‐time imaging of respiratory effects on cerebrospinal fluid flow in small diameter passageways

Purpose

Respiration‐related CSF flow through the cerebral aqueduct may be useful for elucidating physiology and pathophysiology of the glymphatic system, which has been proposed as a mechanism of brain waste clearance. Therefore, we aimed to (1) develop a real‐time (CSF) flow imaging method with high spatial and sufficient temporal resolution to capture respiratory effects, (2) validate the method in a phantom setup and numerical simulations, and (3) apply the method in vivo and quantify its repeatability and correlation with different respiratory conditions.

Methods

A golden‐angle radial flow sequence (reconstructed temporal resolution 168 ms, spatial resolution 0.6 mm) was implemented on a 7T MRI scanner and reconstructed using compressed sensing. A phantom setup mimicked simultaneous cardiac and respiratory flow oscillations. The effect of temporal resolution and vessel diameter was investigated numerically. Healthy volunteers (n = 10) were scanned at four different respiratory conditions, including repeat scans.

Results

Phantom data show that the developed sequence accurately quantifies respiratory oscillations (ratio real‐time/reference Q_R = 0.96 ± 0.02), but underestimates the rapid cardiac oscillations (ratio Q_C = 0.46 ± 0.14). Simulations suggest that Q_C can be improved by increasing temporal resolution. In vivo repeatability was moderate to very strong for cranial and caudal flow (intraclass correlation coefficient range: 0.55–0.99) and weak to strong for net flow (intraclass correlation coefficient range: 0.48–0.90). Net flow was influenced by respiratory condition (p < 0.01).

Conclusions

The presented real‐time flow MRI method can quantify respiratory‐related variations of CSF flow in the cerebral aqueduct, but it underestimates rapid cardiac oscillations. In vivo, the method showed good repeatability and a relationship between flow and respiration.

Influence of Spatial Resolution and Compressed SENSE Acceleration Factor on Flow Quantification with 4D Flow MRI at 3 Tesla

Four-dimensional (4D) flow MRI allows quantifying flow in blood vessels–non invasively and in vivo. The clinical use of 4D flow MRI in small vessels, however, is hampered by long examination times and limited spatial resolution. Compressed SENSE (CS-SENSE) is a technique that can accelerate 4D flow dramatically. Here, we investigated the effect of spatial resolution and CS acceleration on flow measurements by using 4D flow MRI in small vessels in vitro at 3 T. We compared the flow in silicon tubes (inner diameters of 2, 3, 4, and 5 mm) measured with 4D flow MRI, accelerated with four CS factors (CS = 2.5, 4.5, 6.5, and 13) and three voxel sizes (0.5, 1, and 1.5 mm³) to 2D flow MRI and a flow sensor. Additionally, the velocity field in an aneurysm model acquired with 4D flow MRI was compared to the one simulated with computational fluid dynamics (CFD). A strong correlation was observed between flow sensor, 2D flow MRI, and 4D flow MRI (rho > 0.94). The use of fewer than seven voxels per vessel diameter (nROI) resulted in an overestimation of flow in more than 5% of flow measured with 2D flow MRI. A negative correlation (rho = −0.81) between flow error and nROI were found for CS = 2.5 and 4.5. No statistically significant impact of CS factor on differences in flow rates was observed. However, a trend of increased flow error with increased CS factor was observed. In an aneurysm model, the peak velocity and stagnation zone were detected by CFD and all 4D flow MRI variants. The velocity difference error in the aneurysm sac did not exceed 11% for CS = 4.5 in comparison to CS = 2.5 for all spatial resolutions. Therefore, CS factors from 2.5–4.5 can appear suitable to improve spatial or temporal resolution for accurate quantification of flow rate and velocity. We encourage reporting the number of voxels per vessel diameter to standardize 4D flow MRI protocols.

Impact of turbulence-induced asymmetric propagators on the accuracy of phase-contrast velocimetry

Phase-contrast magnetic resonance velocimetry (PC-MRI) has been widely used to investigate flow properties in numerous systems. In a horizontal cylindrical pipe (3 mm diameter), we investigated the accuracy of PC-MRI as the flow transitioned from laminar to turbulent flow (Reynolds number 352-2708). We focus primarily on velocimetry errors introduced by skewed intra-voxel displacement distributions, a consequence of PC-MRI theory assuming symmetric distributions. We demonstrated how rapid fluctuations in the velocity field, can produce broad asymmetric intravoxel displacement distributions near the wall. Depending on the shape of the distribution, this resulted in PC-MRI measurements under-estimating (positive skewness) or over-estimating (negative skewness) the true mean intravoxel velocity, which could have particular importance to clinical wall shear stress measurements. The magnitude of these velocity errors was shown to increase with the variance and decrease with the kurtosis of the intravoxel displacement distribution. These experimental results confirm our previous theoretical analysis, which gives a relationship for PC-MRI velocimetry errors, as a function of the higher moments of the intravoxel displacement distribution (skewness, variance, and kurtosis) and the experimental parameters q and Δ. This suggests that PC-MRI errors in such unsteady/turbulent flow conditions can potentially be reduced by employing lower q values or shorter observation times Δ.

Quantification of blood flow in the fetus with cardiovascular magnetic resonance imaging using Doppler ultrasound gating: validation against metric optimized gating

INTRODUCTION

Fetal cardiovascular magnetic resonance (CMR) imaging is used clinically and for research, but has been previously limited due to lack of direct gating methods. A CMR-compatible Doppler ultrasound (DUS) gating device has resolved this. However, the DUS-gating method is not validated against the current reference method for fetal phase-contrast blood flow measurements, metric optimized gating (MOG). Further, we investigated how different methods for vessel delineation affect flow volumes and observer variability in fetal flow acquisitions.

AIMS

To 1) validate DUS gating versus MOG for quantifying fetal blood flow; 2) assess repeatability of DUS gating; 3) assess impact of region of interest (ROI) size on flow volume; and 4) compare time-resolved and static delineations for flow volume and observer variability.

METHODS

Phase-contrast CMR was acquired in the fetal descending aorta (DAo) and umbilical vein by DUS gating and MOG in 22 women with singleton pregnancy in gestational week 360 (265-400) with repeated scans in six fetuses. Impact of ROI size on measured flow was assessed for ROI:s 50-150% of the vessel diameter. Four observers from two centers provided time-resolved and static delineations. Bland-Altman analysis was used to determine agreement between both observers and methods.

RESULTS

DAo flow was 726 (348-1130) ml/min and umbilical vein flow 366 (150-782) ml/min by DUS gating. Bias±SD for DUS-gating versus MOG were - 45 ± 122 ml/min (-6 ± 15%) for DAo and 19 ± 136 ml/min (2 ± 24%) for umbilical vein flow. Repeated flow measurements in the same fetus showed similar volumes (median CoV = 11% (DAo) and 23% (umbilical vein)). Region of interest 50-150% of vessel diameter yielded flow 35-120%. Bias±SD for time-resolved versus static DUS-gated flow was 33 ± 39 ml/min (4 ± 6%) for DAo and 11 ± 84 ml/min (2 ± 15%) for umbilical vein flow.

CONCLUSIONS

Quantification of blood flow in the fetal DAo and umbilical vein using DUS-gated phase-contrast CMR is feasible and agrees with the current reference method. Repeatability was generally high for CMR fetal blood flow assessment. An ROI similar to the vessel area or slightly larger is recommended. A static ROI is sufficient for fetal flow quantification using currently available CMR sequences.

Quantification of pulmonary/systemic shunt ratio by single-acquisition phase-contrast cardiovascular magnetic resonance

PURPOSE

Phase-contrast cardiovascular magnetic resonance (PC-CMR) quantification of intracardiac shunt (measuring the pulmonary to systemic flow ratio, Qp/Qs) is typically determined by measuring flow through planes perpendicular the pulmonary trunk (PA) and ascending aorta (Ao). This method is subject to error from presence of background velocity offsets and requires two scan acquisitions. We evaluated an alternate PC-CMR technique for quantifying Qp/Qs using a single modified plane that encompasses both the PA and Ao.

MATERIAL AND METHODS

In 53 patients evaluated for intracardiac shunting, PC-CMR measurement in the individual Ao and PA planes and also in a single-acquisition plane was obtained and Qp/Qs calculated by each method. Bland-Altman analysis was performed to evaluate the agreement between the two methods.

RESULTS

The 95% confidence limits of agreement ranged from -0.52 to +0.34 indicating good agreement between the two methods. There was excellent agreement on the clinically relevant threshold value of Qp/Qs ratio of 1.5 (representing criteria for surgical correction of shunt).

CONCLUSIONS

Qp/Qs determined from the single-acquisition approach agrees well with that of the individual PA and Ao method and offers potential improved accuracy (due to background velocity offset).

Investigation of cerebrospinal fluid flow in the cerebral aqueduct using high-resolution phase contrast measurements at 7T MRI

Background The cerebral aqueduct is a central conduit for cerebrospinal fluid (CSF), and non-invasive quantification of CSF flow in the aqueduct may be an important tool for diagnosis and follow-up of treatment. Magnetic resonance (MR) methods at clinical field strengths are limited by low spatial resolution. Purpose To investigate the feasibility of high-resolution through-plane MR flow measurements (2D-PC) in the cerebral aqueduct at high field strength (7T). Material and Methods 2D-PC measurements in the aqueduct were performed in nine healthy individuals at 7T. Measurement accuracy was determined using a phantom. Aqueduct area, mean velocity, maximum velocity, minimum velocity, net flow, and mean flow were determined using in-plane resolutions 0.8 × 0.8, 0.5 × 0.5, 0.3 × 0.3, and 0.2 × 0.2 mm². Feasibility criteria were defined based on scan time and spatial and temporal resolution. Results Phantom validation of 2D-PC MR showed good accuracy. In vivo, stroke volume was -8.2 ± 4.4, -4.7 ± 2.8, -6.0 ± 3.8, and -3.7 ± 2.1 µL for 0.8 × 0.8, 0.5 × 0.5, 0.3 × 0.3, and 0.2 × 0.2 mm², respectively. The scan with 0.3 × 0.3 mm² resolution fulfilled the feasibility criteria for a wide range of heart rates and aqueduct diameters. Conclusion 7T MR enables non-invasive quantification of CSF flow and velocity in the cerebral aqueduct with high spatial resolution.

Multiple blood flow measurements before and after carotid artery stenting via phase-contrast magnetic resonance imaging: An observational study

After carotid artery stenting, the procurement of information about blood flow redistribution among brain-feeding arteries and its time trend is essential to understanding a patient’s physiological background and to determine their care regimen. Cerebral blood flow has been measured twice following carotid artery stenting in few previous studies, with some discrepancies in the results. The purpose of this study was to measure cerebral blood flow at multiple time points after carotid artery stenting, and to elucidate the time trend of cerebral blood flow and redistribution among arteries. Blood flow rates in 11 subjects were measured preoperatively, at one day, one week, and about three months, respectively after carotid artery stenting by using phase-contrast magnetic resonance imaging. The target vessels were the bilateral internal carotid arteries, the basilar artery, and the bilateral middle cerebral arteries. Lumen was semi-automatically defined using an algorithm utilizing pulsatility. The results showed that blood flow rates in the stented internal carotid artery and the ipsilateral middle cerebral artery increased following carotid artery stenting. Blood flow rates in the contralateral internal carotid artery and the basilar artery gradually declined, and they were lower than the preoperative values at three months after stenting. The sum of blood flow rates of the bilateral internal carotid arteries and the basilar artery increased after carotid artery stenting, and then decreased over the next three months. There was no significant change in the blood flow rate in the contralateral middle cerebral artery. From these results, it was concluded that redistribution among the bilateral internal carotid arteries and the basilar artery occurs after carotid artery stenting, and that it takes months thereafter to reach another equilibrium.

Comparison of MR flow quantification in peripheral and main pulmonary arteries in patients after right ventricular outflow tract surgery: A retrospective study

PURPOSE

To compare the quantification of pulmonary stroke volume (SV) by phase contrast magnetic resonance (PC-MR) in the main pulmonary artery (MPA) to the sum of SVs in both peripheral pulmonary arteries (PPA) in different right ventricular (RV) outflow pathologies.

MATERIALS AND METHODS

Pulmonary SV was determined by PC-MR in the MPA and the PPA in healthy individuals (H, n = 54), patients after correction for tetralogy of Fallot with significant pulmonary regurgitation and without pulmonary or RV outflow tract stenosis (PR, n = 50), and in patients with RV outflow tract or pulmonary valve stenosis (PS, n = 50). Resulting SVs were compared to aortic SV in the ascending aorta.

RESULTS

Mean age was similar between the groups: H 28 ± 17 vs. PR 24 ± 11 vs. PS 22 ± 10 years. Bland-Altman analyses revealed in all groups a relatively small systemic (bias) but large random error (limits of agreement) for pulmonary SV determined in the MPA as compared to summed SVs in the PPA. The largest limits of agreement were present in PS patients: H: MPA 3.9% (-11, + 19) vs. PPA 0.4% (-15, + 15); PR: MPA 5.2% (-25, + 36) vs. PPA 0.6% (-24, + 26); PS: MPA 5% (-36; + 46), PPA -0.03% (-34, + 35).

CONCLUSION

The accuracy of PC-MR in the MPA is reasonable; however, a large random error (precision) is observed that is most pronounced in PS patients. This potential error should be taken into consideration when interpreting MPA flow measurements.

LEVEL OF EVIDENCE

3 Technical Efficacy: Stage 2 J. Magn. Reson. Imaging 2017;46:1839-1845.

Cardiovascular magnetic resonance phase contrast imaging

Cardiovascular magnetic resonance (CMR) phase contrast imaging has undergone a wide range of changes with the development and availability of improved calibration procedures, visualization tools, and analysis methods. This article provides a comprehensive review of the current state-of-the-art in CMR phase contrast imaging methodology, clinical applications including summaries of past clinical performance, and emerging research and clinical applications that utilize today's latest technology.

Jugular Venous Flow Abnormalities in Multiple Sclerosis Patients Compared to Normal Controls

BACKGROUND

To determine if extracranial venous structural and flow abnormalities exist in patients with multiple sclerosis (MS).

METHODS

Magnetic resonance imaging was used to assess the anatomy and function of major veins in the neck in 138 MS patients and 67 healthy controls (HC). Time-of-flight MR angiography (MRA) was used to assess stenosis while 2-dimensional phase-contrast flow quantification was used to assess flow at the C2/C3 and C5/C6 levels. Venous flow was normalized to the total arterial flow. The MS patients were divided into stenotic (ST) and nonstenotic (NST) groups based on MRA assessment, and each group was compared to the HC group in anatomy and flow.

RESULTS

The MS group showed lower normalized internal jugular vein (IJV) blood flow (tIJV/tA) than the HC group (P < .001). In the MS group, 72 (52%) were classified as ST while 66 (48%) were NST. In the HC group, 11 (23%) were ST while 37 (77%) were NST. The ST-MS group had lower IJV flow than both HC and NST-MS groups.

CONCLUSION

After categorizing the MS population into two groups based upon anatomical stenosis, as determined from an absolute quantification of IJV cross section, clear differences in IJV flow between the ST-MS and HC samples became evident. Despite the unknown etiology of MS, abnormal venous flow was noted in a distinct group of MS patients compared to HC.

Use of multi-velocity encoding 4D flow MRI to improve quantification of flow patterns in the aorta

PURPOSE

To show that the use of a multi-velocity encoding (VENC) 4D-flow approach offers significant improvements in the characterization of complex flow in the aorta. Four-dimensional flow magnetic resonance imaging (MRI) (4D-flow) can be used to measure complex flow patterns and dynamics in the heart and major vessels. The quality of the information derived from these measures is dependent on the accuracy of the vector field, which is limited by the vector-to-noise ratio.

MATERIALS AND METHODS

A 4D-flow protocol involving three different VENC values of 150, 60, and 20 cm/s was performed on six control subjects and nine patients with type-B chronic aortic dissection at 3T MRI. Data were processed using a single VENC value (150 cm/s) or using a fused dataset that selected the lowest appropriate VENC for each voxel. Performance was analyzed by measuring spatial vector angular correlation, magnitude correlation, temporal vector conservation, and "real-world" streamline tracing performance.

RESULTS

The multi-VENC approach provided a 31% improvement in spatial and 53% improvement in temporal precision of velocity vector measurements during the mid-late diastolic period, where 99% of the flow vectors in the normal aorta are below 20 cm/s. In low-flow conditions this resulted in practical improvements of greater than 50% in pathline tracking and streamline tracing quantified by streamline curvature measurements.

CONCLUSION

A multi-VENC 4D-flow approach provides accurate vector data across normal physiological velocities observed in the aorta, dramatically improving outputs such as pathline tracking, streamline estimation, and further advanced analyses.

Characterization and quantification of dynamic eccentric regurgitation of the left atrioventricular valve after atrioventricular septal defect correction with 4D Flow cardiovascular magnetic resonance and retrospective valve tracking

BACKGROUND

To characterize and directly quantify regurgitant jets of left atrioventricular valve (LAVV) in patients with corrected atrioventricular septal defect (AVSD) by four-dimensional (4D)Flow Cardiovascular Magnetic Resonance (CMR), streamline visualization and retrospective valve tracking.

METHODS

Medical ethical committee approval and informed consent from all patients or their parents were obtained. In 32 corrected AVSD patients (age 26 ± 12 years), echocardiography and whole-heart 4DFlow CMR were performed. Using streamline visualization on 2- and 4-chamber views, the angle between regurgitation and annulus was followed throughout systole. On through-plane velocity-encoded images reformatted perpendicular to the regurgitation jet the cross-sectional jet circularity index was assessed and regurgitant volume and fraction were calculated. Correlation and agreement between different techniques was performed with Pearson's r and Spearman's rho correlation and Bland-Altman analysis.

RESULTS

In 8 patients, multiple regurgitant jets over the LAVV were identified. Median variation in regurgitant jet angle within patients was 36°(IQR 18-64°) on the 2-chamber and 30°(IQR 20-40°) on the 4-chamber. Regurgitant jets had a circularity index of 0.61 ± 0.16. Quantification of the regurgitation volume was feasible with 4DFlow CMR with excellent correlation between LAVV effective forward flow and aortic flow (r = 0.97, p < 0.001) for internal validation and moderate correlation with planimetry derived regurgitant volume (r = 0.65, p < 0.001) and echocardiographic grading (rho = 0.51, p = 0.003).

CONCLUSIONS

4DFlow CMR with streamline visualization revealed multiple, dynamic and eccentric regurgitant jets with non-circular cross-sectional shape in patients after AVSD correction. 4DFlow with retrospective valve tracking allows direct and accurate quantification of the regurgitation of these complex jets.

Inter-study reproducibility of interleaved spiral phase velocity mapping of renal artery haemodynamics

BACKGROUND

Qualitative and quantitative assessment of renal blood flow is valuable in the evaluation of patients with renal and renovascular diseases as well as in patients with heart failure. The temporal pattern of renal flow velocity through the cardiac cycle provides important information about renal haemodynamics. High temporal resolution interleaved spiral phase velocity mapping could potentially be used to study temporal patterns of flow and measure resistive and pulsatility indices which are measures of downstream resistance.

METHODS

A retrospectively gated breath-hold spiral phase velocity mapping sequence (TR 19 ms) was developed at 3 Tesla. Phase velocity maps were acquired in the proximal right and left arteries of 10 healthy subjects in each of two separate scanning sessions. Each acquisition was analysed by two independent observers who calculated the resistive index (RI), the pulsatility index (PI), the mean flow velocity and the renal artery blood flow (RABF). Inter-study and inter-observer reproducibility of each variable was determined as the mean +/- standard deviation of the differences between paired values. The effect of background phase errors on each parameter was investigated.

RESULTS

RI, PI, mean velocity and RABF per kidney were 0.71+/- 0.06, 1.47 +/- 0.29, 253.5 +/- 65.2 mm/s and 413 +/- 122 ml/min respectively. The inter-study reproducibilities were: RI -0.00 +/- 0.04 , PI -0.03 +/- 0.17, mean velocity -6.7 +/- 31.1 mm/s and RABF per kidney 17.9 +/- 44.8 ml/min. The effect of background phase errors was negligible (<2% for each parameter).

CONCLUSIONS

High temporal resolution breath-hold spiral phase velocity mapping allows reproducible assessment of renal pulsatility indices and RABF.

Quantifying errors in flow measurement using phase contrast magnetic resonance imaging: comparison of several boundary detection methods

Quantifying flow from phase-contrast MRI (PC-MRI) data requires that the vessels of interest be segmented. The estimate of the vessel area will dictate the type and magnitude of the error sources that affect the flow measurement. These sources of errors are well understood, and mathematical expressions have been derived for them in previous work. However, these expressions contain many parameters that render them difficult to use for making practical error estimates. In this work, some realistic assumptions were made that allow for the simplification of such expressions in order to make them more useful. These simplified expressions were then used to numerically simulate the effect of segmentation accuracy and provide some criteria that if met, would keep errors in flow quantification below 10% or 5%. Four different segmentation methods were used on simulated and phantom MRA data to verify the theoretical results. Numerical simulations showed that including partial volumed edge pixels in vessel segmentation provides less error than missing them. This was verified with MRA simulations, as the best performing segmentation method generally included such pixels. Further, it was found that to obtain a flow error of less than 10% (5%), the vessel should be at least 4 (5) pixels in diameter, have an SNR of at least 10:1 and have a peak velocity to saturation cut-off velocity ratio of at least 5:3.

Peak velocity measurements in tortuous arteries with phase contrast magnetic resonance imaging: the effect of multidirectional velocity encoding

OBJECTIVES

Blood flow velocity measurement with phase contrast magnetic resonance imaging (PC-MRI) is widely applied in clinical routine imaging. Usually, velocity and volumetric flow measurements are performed using unidirectional encoding of the through-plane velocity with a 2-dimensional (2D) acquisition. Single-slice acquisitions and measurements with unidirectional encoding, however, may lead to significant errors, especially in tortuous vessels, but might benefit from higher signal-to-noise ratios (SNRs). To evaluate the impact of volumetric acquisition and multidirectional velocity encoding, blood velocity measurements were performed at 3 locations in the distal internal carotid artery with a 3-dimensional, 3-directional time-resolved phase contrast (PC) sequence (4-dimensional [4D]) and a 2D acquisition with 3-directional (2D-3dir) and through-plane velocity encoding (2D-tp) derived from the same sequence.

MATERIALS AND METHODS

Twenty carotid arteries of 10 healthy volunteers (24-37 years) were evaluated. For each volunteer, 1 4D acquisition and 3 2D 3-directional PC measurements were placed according to a time-of-flight angiography. Unidirectionally encoded through-plane velocities were derived from the multidirectionally encoded 2D scan by discarding the in-plane components. Regions of interest were identified on the slab after postprocessing and visualization for the 4D data set as well as directly on the digital imaging and communications in medicine images for the 2D measurement. Blood flow velocity, volumetric flow, and SNRs were measured at carotid segments C4, C5, and C7 on both sides obtaining 20 values per vessel location. The quantities were tested for significant differences between each modality at all 3 locations with paired t tests.

RESULTS

At the segments C5 and C7, the highest peak velocities (PVs) were measured with the 4D sequence, followed by 2D-3dir and 2D tp. The PV differences between the sequences were significant (P < 0.01) at both locations. At the proximal segment of the carotid siphon (C4), the PV values of the 2D-3dir sequence were significantly higher than the ones measured with 2D-tp. The mean PV value of the 4D sequence was located in between 2D-3dir and 2D-tp without significant differences to either of the 2D sequences. Volumetric flow measurements were also significantly different between 2D and 4D acquisitions, but without a discernible trend. The SNR analysis clearly favored 2D over 4D acquisitions because of higher inflow enhancement.

CONCLUSIONS

The results of the current study show that velocity measurements with a unidirectional encoded through-plane PC sequence lead to a significant underestimation of velocity values in tortuous vessels. In all 3 evaluated segments of the distal internal carotid artery, multidirectional velocity encoding revealed significantly higher PV values than those of unidirectional velocity encoding. These results indicate that multidirectional encoding should be preferred to unidirectional encoding for velocity measurements in tortuous vessels. Furthermore, 4D PC-MRI is superior to 2D-3dir in 2 of 3 locations. However, single-slice measurements with multidirectional velocity encoding have higher SNR and may be an alternative to 4D PC-MRI with a scan time of only approximately 90 seconds per slice.

Assessment of altered three-dimensional blood characteristics in aortic disease by velocity distribution analysis

PURPOSE

To test the feasibility of velocity distribution analysis for identifying altered three-dimensional (3D) flow characteristics in patients with aortic disease based on 4D flow MRI volumetric analysis.

METHODS

Forty patients with aortic (Ao) dilation (mid ascending aortic diameter MAA = 40 ± 7 mm, age = 56 ± 17 years, 11 females) underwent cardiovascular MRI. Four groups were retrospectively defined: mild Ao dilation (n = 10; MAA < 35 mm); moderate Ao dilation (n = 10; 35 < MAA < 45 mm); severe Ao dilation (n = 10; MAA > 45 mm); Ao dilation+aortic stenosis AS (n = 10; MAA > 35 mm and peak velocity > 2.5 m/s). The 3D PC-MR angiograms were computed and used to obtain a 3D segmentation of the aorta which was divided into four segments: root, ascending aorta, arch, descending aorta. Radial chart displays were used to visualize multiple parameters representing segmental changes in the 3D velocity distribution associated with aortic disease.

RESULTS

Changes in the velocity field and geometry between cohorts resulted in distinct hemodynamic patterns for each aortic segment. Disease progression from mild to Ao dilation + AS resulted in significant differences (P < 0.05) in flow parameters across cohorts and increased radial chart size for root and ascending aorta segments by 146% and 99%, respectively.

CONCLUSION

Volumetric 4D velocity distribution analysis has the potential to identify characteristic changes in regional blood flow patterns in patients with aortic disease.

Towards highly accelerated Cartesian time-resolved 3D flow cardiovascular magnetic resonance in the clinical setting

BACKGROUND

The clinical applicability of time-resolved 3D flow cardiovascular magnetic resonance (CMR) remains compromised by the long scan times associated with phase-contrast imaging. The present work demonstrates the applicability of 8-fold acceleration of Cartesian time-resolved 3D flow CMR in 10 volunteers and in 9 patients with different congenital heart diseases (CHD). It is demonstrated that accelerated 3D flow CMR data acquisition and image reconstruction using k-t PCA (principal component analysis) can be implemented into clinical workflow and results are sufficiently accurate relative to conventional 2D flow CMR to permit for comprehensive flow quantification in CHD patients.

METHODS

The fidelity of k-t PCA was first investigated on retrospectively undersampled data for different acceleration factors and compared to k-t SENSE and fully sampled reference data. Subsequently, k-t PCA with 8-fold nominal undersampling was applied on 10 healthy volunteers and 9 CHD patients on a clinical 1.5 T MR scanner. Quantitative flow validation was performed in vessels of interest on the 3D flow datasets and compared to 2D through-plane flow acquisitions. Particle trace analysis was used to qualitatively visualise flow patterns in patients.

RESULTS

Accelerated time-resolved 3D flow data were successfully acquired in all subjects with 8-fold nominal scan acceleration. Nominal scan times excluding navigator efficiency were on the order of 6 min and 7 min in patients and volunteers. Mean differences in stroke volume in selected vessels of interest were 2.5 ± 8.4 ml and 1.63 ± 4.8 ml in volunteers and patients, respectively. Qualitative flow pattern analysis in the time-resolved 3D dataset revealed valuable insights into hemodynamics including circular and helical patterns as well as flow distributions and origin in the Fontan circulation.

CONCLUSION

Highly accelerated time-resolved 3D flow using k-t PCA is readily applicable in clinical routine protocols of CHD patients. Nominal scan times of 6 min are well tolerated and allow for quantitative and qualitative flow assessment in all great vessels.

Mean cerebral blood flow measurements using phase contrast MRI in the first year of life

Alterations in cerebral blood flow (CBF) are believed to be linked to many of the neurological pathologies that affect neonates and small infants. CBF measurements are nonetheless often difficult to perform in this population, as many techniques rely on radioactive tracers or other invasive methods. In this study, mean global CBF was measured in 21 infants under the age of one, using non-invasive MRI techniques adapted to the neonatal population. Mean CBF was computed as the ratio of blood flow delivered to the brain (measured using phase contrast MRI) and brain volume (computed by segmenting anatomical MR images). Tests in adult volunteers and repeated measurements showed the flow measurements using the proposed method to be both accurate and reproducible. It was also found that cardiac gating need not be employed in infants with no known cardiac pathology. The developed technique can easily be appended to a neonatal MRI examination to provide rapid, robust, and non-invasive estimates of mean CBF, thus providing a means to monitor developmental or pathology-related alterations in cerebral perfusion and the impact of different treatment courses. In the imaged cohort, mean CBF and flow to the brain were found to rapidly increase during the first year of life (from approx. 25 to 60 ml blood/100 ml tissue/min), in good agreement with literature from other modalities where available. Mean CBF also showed a significant correlation with arterial oxygen saturation level and heart rate, but no significant correlation was found between CBF and the hematocrit or body temperature.

Phase-contrast MRI and applications in congenital heart disease

A review of phase-contrast magnetic resonance imaging techniques, with specific application to congenital heart disease, is presented. Theory, pitfalls, advantages, and specific examples of multiple, well-described congenital heart disease presentations are discussed.

Phase contrast MRI quantification of pulsatile volumes of brain arteries, veins, and cerebrospinal fluids compartments: repeatability and physiological interactions

PURPOSE

To study measurement repeatability and physiological determinants on measurement stability for phase contrast MRI (PC-MRI) measurements of cyclic volume changes (ΔV) of brain arteries, veins, and cerebrospinal fluid (CSF) compartments.

MATERIALS AND METHODS

Total cerebral blood flow (tCBF), total internal jugular flow (tJBF) and spinal CSF flow at C2-C3 level and CSF in the aqueduct was measured using five repetitions in 20 healthy subjects. After subtracting net flow, waveforms were integrated to calculate ΔV of arterial, venous, and cerebrospinal fluid compartments. The intraclass correlation coefficient (ICC) was used to measure repeatability. Systematic errors were investigated by a series of phantom measurements.

RESULTS

For ΔV calculated from tCBF, tJBF and both CSF waveforms, the ICC was ≥0.85. ΔV from the tCBF waveform decreased linearly between repetitions (P = 0.012). Summed CSF and venous volume being shifted out from the cranium was correlated with ΔV calculated from the tCBF waveform (r = 0.75; P < 0.001). Systematic errors increased at resolutions <4 pixels per diameter.

CONCLUSION

Repeatability of ΔV calculated from tCBF, tJBF, and CSF waveforms allows useful interpretations. The subject's time in the MR system and imaging resolution should be considered when interpreting volume changes. Summed CSF and venous volume changes was associated with arterial volume changes.

Variation in supratentorial cerebrospinal fluid production rate in one day: measurement by nontriggered phase-contrast magnetic resonance imaging

PURPOSE

Measuring the cerebrospinal fluid (CSF) production rate is important for understanding the physiology related to normal conditions and neurological disorders. Triggered phase-contrast magnetic resonance imaging (MRI) has been used to measure CSF production rate, but the use of nontriggered phase-contrast MRI has not been reported. The purposes of this study were to assess the feasibility of using nontriggered phase-contrast MRI to measure CSF flow and to determine whether CSF production exhibits circadian rhythm.

MATERIALS AND METHODS

The feasibility of phase-contrast MRI was assessed with a phantom simulated human cerebral aqueduct. CSF flow through the cerebral aqueduct was measured with nontriggered phase-contrast MRI four times during 1 day in 10 normal volunteers.

RESULTS

In the phantom study, linear regression analysis gave the following measured values (ml/h): 0.80 × (value of steady flow) - 10.0 for triggered phase-contrast MRI and 1.27 × (value of steady flow) - 12.2 for nontriggered phase-contrast MRI. One-factor analysis of variance showed no significant effect of the time of the measurements (P = 0.47). The supratentorial CSF production rate was 510 ± 549 ml/day (mean ± SD).

CONCLUSION

Nontriggered phase-contrast MRI provided good estimates of the flow rate in the phantom study. We observed no circadian rhythm in CSF production.

A novel MRI framework for the quantification of any moment of arbitrary velocity distributions

MRI can measure several important hemodynamic parameters but might not yet have reached its full potential. The most common MRI method for the assessment of flow is phase-contrast MRI velocity mapping that estimates the mean velocity of a voxel. This estimation is precise only when the intravoxel velocity distribution is symmetric. The mean velocity corresponds to the first raw moment of the intravoxel velocity distribution. Here, a generalized MRI framework for the quantification of any moment of arbitrary velocity distributions is described. This framework is based on the fact that moments in the function domain (velocity space) correspond to differentials in the Fourier transform domain (kv-space). For proof-of-concept, moments of realistic velocity distributions were estimated using finite difference approximations of the derivatives of the MRI signal. In addition, the framework was applied to investigate the symmetry assumption underlying phase-contrast MRI velocity mapping; we found that this assumption can substantially affect phase-contrast MRI velocity estimates and that its significance can be reduced by increasing the velocity encoding range.

Right coronary artery flow velocity and volume assessment with spiral K-space sampled breathhold velocity-encoded MRI at 3 tesla: accuracy and reproducibility

PURPOSE

To evaluate accuracy and reproducibility of flow velocity and volume measurements in a phantom and in human coronary arteries using breathhold velocity-encoded (VE) MRI with spiral k-space sampling at 3 Tesla.

MATERIALS AND METHODS

Flow velocity assessment was performed using VE MRI with spiral k-space sampling. Accuracy of VE MRI was tested in vitro at five constant flow rates. Reproducibility was investigated in 19 healthy subjects (mean age 25.4 +/- 1.2 years, 11 men) by repeated acquisition in the right coronary artery (RCA).

RESULTS

MRI-measured flow rates correlated strongly with volumetric collection (Pearson correlation r = 0.99; P < 0.01). Due to limited sample resolution, VE MRI overestimated the flow rate by 47% on average when nonconstricted region-of-interest segmentation was used. Using constricted region-of-interest segmentation with lumen size equal to ground-truth luminal size, less than 13% error in flow rate was found. In vivo RCA flow velocity assessment was successful in 82% of the applied studies. High interscan, intra- and inter-observer agreement was found for almost all indices describing coronary flow velocity. Reproducibility for repeated acquisitions varied by less than 16% for peak velocity values and by less than 24% for flow volumes.

CONCLUSION

3T breathhold VE MRI with spiral k-space sampling enables accurate and reproducible assessment of RCA flow velocity.

Renal arterial blood flow measurement by breath-held MRI: Accuracy in phantom scans and reproducibility in healthy subjects

This study evaluates reliability of current technology for measurement of renal arterial blood flow by breath-held velocity-encoded MRI. Overall accuracy was determined by comparing MRI measurements with known flow in controlled-flow-loop phantom studies. Measurements using prospective and retrospective gating methods were compared in phantom studies with pulsatile flow, not revealing significant differences. Phantom study results showed good accuracy, with deviations from true flow consistently below 13% for vessel diameters 3mm and above. Reproducibility in human subjects was evaluated by repeated studies in six healthy control subjects, comparing immediate repetition of the scan, repetition of the scan plane scouting, and week-to-week variation in repeated studies. The standard deviation in the 4-week protocol of repeated in vivo measurements of single-kidney renal flow in normal subjects was 59.7 mL/min, corresponding with an average coefficient of variation of 10.55%. Comparison of renal arterial blood flow reproducibility with and without gadolinium contrast showed no significant differences in mean or standard deviation. A breakdown among error components showed corresponding marginal standard deviations (coefficients of variation) 23.8 mL/min (4.21%) for immediate repetition of the breath-held flow scan, 39.13 mL/min (6.90%) for repeated plane scouting, and 40.76 mL/min (7.20%) for weekly fluctuations in renal blood flow.

Characterization of the time course of the superior mesenteric, abdominal aorta, internal carotid and vertebral arteries blood flow response to the oral glucose challenge test using magnetic resonance imaging

Blood flow to the splanchnic circulation increases postprandially which may cause a reduction in systemic and cerebral perfusion leading to postprandial syncope in the elderly who lack adequate cardiovascular reserve. We used multi-station 2D phase contrast cine magnetic resonance imaging (PC-MRI) with the aim of characterizing the time course of the haemodynamic response to an oral glucose challenge test (OGCT) in the large arteries perfusing the splanchnic, systemic and cerebral circulations (superior mesenteric artery SMA, abdominal aorta AA, internal carotid arteries, ICA and vertebral arteries VA). In this study nine fasted healthy volunteers were studied. Separate cine PC-MRI scans were acquired in the neck and in the abdomen every 88 s, these two measurements being interleaved for ten baseline scans at each station with the scanner automatically moving the subject between the two stations. After ingestion of the OGCT, a further 30 cine PC-MRI scans were acquired at each station. Using this technique we were able to characterize with frequent sampling of volumetric blood flow the time course of blood flow response to the OGCT of the SMA, AA and both VA and ICA. We found a substantial variation between individuals in the amplitude and the time to the peak of the SMA blood flow response to the OGCT which correlated positively with body mass index. MRI provides a robust, non-invasive method of studying normal physiology that could be valuable in studies of diseases such as postprandial hypotension.

Evaluation of hepatic fibrosis with portal pressure gradient in rats

MRI has the potential of providing a noninvasive assessment of liver pathology. This work introduces a portal pressure gradient (PPG) model derived from fluid mechanics, where the PPG is proportional to the average velocity and inversely proportional to the vessel area in the upper part of portal vein. Using a phase-contrast spoiled gradient echo sequence, the PPG model was verified in a phantom study and was tested in an animal study using 35 rats with various degrees of hepatic fibrosis induced by carbon tetrachloride (CCl(4)). Histological examination was conducted to determine the severity of hepatic fibrosis. The fibrosis score monotonically increased with the duration of CCl(4) treatment. The PPG was highly correlated with nonzero fibrosis scores (r(2) = 0.90, P < 0.05). There was a significant difference between control and cirrhosis groups (P < 0.0006, alpha < 0.0018). The difference between control and fibrosis (noncirrhosis) groups (P < 0.002, alpha < 0.006) was also significant. Without the administration of any contrast agent, the MRI-PPG approach shows promise as a noninvasive means of evaluating liver fibrosis.

Quantifying coronary sinus flow and global LV perfusion at 3T

Background

Despite the large availability of 3T MR scanners and the potential of high field imaging, this technical platform has yet to prove its usefulness in the cardiac MR setting, where 1.5T remains the established standard. Global perfusion of the left ventricle, as well as the coronary flow reserve (CFR), can provide relevant diagnostic information, and MR measurements of these parameters may benefit from increased field strength. Quantitative flow measurements in the coronary sinus (CS) provide one method to investigate these parameters. However, the ability of newly developed faster MR sequences to measure coronary flow during a breath-hold at 3T has not been evaluated.

Methods

The aim of this work was to measure CS flow using segmented phase contrast MR (PC MR) on a clinical 3T MR scanner. Parallel imaging was employed to reduce the total acquisition time. Global LV perfusion was calculated by dividing CS flow with left ventricular (LV) mass. The repeatability of the method was investigated by measuring the flow three times in each of the twelve volunteers. Phantom experiments were performed to investigate potential error sources.

Results

The average CS flow was determined to 88 ± 33 ml/min and the deduced LV perfusion was 0.60 ± 0.22 ml/min·g, in agreement with published values. The repeatability (1-error) of the three repeated measurements in each subject was on average 84%.

Conclusion

This work demonstrates that the combination of high field strength (3T), parallel imaging and segmented gradient echo sequences allow for quantification of the CS flow and global perfusion within a breath-hold.

Assessment of splanchnic blood flow using magnetic resonance imaging

BACKGROUND AND AIMS

The splanchnic circulation has an important function in the body under both physiological and pathophysiological conditions. Despite its importance, no reliable noninvasive procedures for estimating splanchnic circulation have been established. The aim of this study was to evaluate MRI as a tool for assessing intra-abdominal blood flows of the aorta, portal vein (VPO) and the major intestinal and hepatic vessels.

METHODS

In nine healthy volunteers, the proximal aorta (AOP) and distal abdominal aorta (AOD), superior mesenteric artery (SAM), celiac trunk (CTR), hepatic arteries (common and proper hepatic arteries, AHC and AHP, respectively), and VPO were localized on contrast-enhanced magnetic resonance angiography images. Volumetric flow was measured using a two-dimensional cine echocardiogram-gated phase contrast technique. Measurements were taken before and 30 min after continuous intravenous infusion of somatostatin (250 microg/h) and were independently evaluated by two investigators.

RESULTS

Blood flow measured by MRI in the VPO, SAM, AOP, AHP, and CTR significantly decreased after drug infusion. Flows in the AOD and AHC showed a tendency to decrease (P>0.05). Interrater agreement on flows in MRI was very good for large vessels (VPO, AOP, and AOD), with a concordance correlation coefficient of 0.94, as well as for smaller vessels such as the CTR, AHC, AHP, and SAM (concordance correlation coefficient =0.78).

CONCLUSION

Somatostatin-induced blood flow changes in the splanchnic region were reliably detected by MRI. MRI may be useful for the noninvasive assessment of blood flow changes in the splanchnic region.

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 degrees were used for 40 volunteers and of 8, 15, and 30 degrees 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 degrees . 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 degrees are most appropriate for nontriggered phase-contrast MR measurements in the internal carotid and the basilar artery.

The Quantification of Cerebral Blood Flow by Phase Contrast MRA: Basics and Applications

Phase-contrast magnetic resonance (PCMRA) flow quantification can determine vascular velocities and volumetric flow rate (VFR) non-invasively for in vitro and in vivo studies. Recently, the increasing power of MR imaging units and the reduced time for data acquisition and post-processing have led to an increasing number of investigations on the use of phase-contrast flow measurements as an additional source of quantitative functional information in MR imaging. In addition, PCMRA can be added to morphologic MRI sequences, offering the option to correlate flow to morphology based on data generated during one examination. This review discusses the basics of phase-contrast imaging, describing the errors and avoiding methods associated with PCMRA, providing guidelines for flow measurement and data analysis, and presenting the current clinical cerebral applications.

Effect of IV Injection of Radiographic Contrast Media on Human Renal Blood Flow

OBJECTIVE

The purpose of our study was to assess the effect of an i.v. injection of iodinated radiographic contrast medium on human renal blood flow using cine phase-contrast MRI.

SUBJECTS AND METHODS

We examined 12 healthy adult volunteers. Blood flow in one renal artery was measured using cine phase-contrast imaging (1.5-T MR system). Each volunteer received 120 mL of isotonic sodium chloride on study day 1 and 120 mL of a low-osmolar, nonionic, iodinated contrast medium (iomeprol, 400 mg I/mL) on study day 2. Repetitive measurements were performed before (up to five measurements in 5 minutes) and after (up to 13 measurements in 30 minutes) the injection was started.

RESULTS

Mean basal renal artery blood flow was 664 mL/min. In response to the injection of the test substances, we found a significantly larger decrease in average renal blood flow for contrast medium than for sodium chloride (31.9 mL/min vs 18.3 mL/min, p = 0.0481). Furthermore, in analyzing the measurements at early time points, we found a significant decrease (11.4% +/- 4.7% [SD]; Bonferroni-corrected, p < 0.05) in renal blood flow 2 minutes after the injection of the contrast medium was started. Sodium chloride did not produce a significant effect at any time.

CONCLUSION

Cine phase-contrast MRI can measure a decrease in renal blood flow in humans in response to an i.v. injection of iodinated radiographic contrast medium. Therefore, cine phase-contrast MRI can be a helpful and noninvasive tool for further investigations of contrast media-induced changes in human renal blood flow and their possible impact on the development of contrast-induced nephropathy.

Hemodynamics in the mouse aortic arch as assessed by MRI, ultrasound, and numerical modeling

Mice are widely used to study arterial disease in humans, and the pathogenesis of arterial diseases is known to be strongly influenced by hemodynamic factors. It is, therefore, of interest to characterize the hemodynamic environment in the mouse arterial tree. Previous measurements have suggested that many relevant hemodynamic variables are similar between the mouse and the human. Here we use a combination of Doppler ultrasound and MRI measurements, coupled with numerical modeling techniques, to characterize the hemodynamic environment in the mouse aortic arch at high spatial resolution. We find that the hemodynamically induced stresses on arterial endothelial cells are much larger in magnitude and more spatially uniform in the mouse than in the human, an effect that can be explained by fluid mechanical scaling principles. This surprising finding seems to be at variance with currently accepted models of the role of hemodynamics in atherogenesis and the known distribution of atheromatous lesions in mice.