Analysis of MR phase-contrast measurements of pulsatile velocity waveforms

Fetal cardiovascular blood flow MRI: techniques and applications

Fetal cardiac MRI is challenging due to fetal and maternal movements as well as the need for a reliable cardiac gating signal and high spatiotemporal resolution. Ongoing research and recent technical developments to address these challenges show the potential of MRI as an adjunct to ultrasound for the assessment of the fetal heart and great vessels. MRI measurements of blood flow have enabled the assessment of normal fetal circulation as well as conditions with disrupted circulations, such as congenital heart disease, along with associated organ underdevelopment and hemodynamic instability. This review provides details of the techniques used in fetal cardiovascular blood flow MRI, including single slice and volumetric imaging sequences, post-processing and analysis, along with a summary of applications in human studies and animal models.

Fetal hemodynamics and cardiac streaming assessed by 4D flow cardiovascular magnetic resonance in fetal sheep

BACKGROUND

To date it has not been possible to obtain a comprehensive 3D assessment of fetal hemodynamics because of the technical challenges inherent in imaging small cardiac structures, movement of the fetus during data acquisition, and the difficulty of fusing data from multiple cardiac cycles when a cardiac gating signal is absent. Here we propose the combination of volumetric velocity-sensitive cardiovascular magnetic resonance imaging ("4D flow" CMR) and a specialized animal preparation (catheters to monitor fetal heart rate, anesthesia to immobilize mother and fetus) to examine fetal sheep cardiac hemodynamics in utero.

METHODS

Ten pregnant Merino sheep underwent surgery to implant arterial catheters in the target fetuses. Anesthetized ewes underwent 4D flow CMR with acquisition at 3 T for fetal whole-heart coverage with 1.2-1.5 mm spatial resolution and 45-62 ms temporal resolution. Flow was measured in the heart and major vessels, and particle traces were used to visualize circulatory patterns in fetal cardiovascular shunts. Conservation of mass was used to test internal 4D flow consistency, and comparison to standard 2D phase contrast (PC) CMR was performed for validation.

RESULTS

Streaming of blood from the ductus venosus through the foramen ovale was visualized. Flow waveforms in the major thoracic vessels and shunts displayed normal arterial and venous patterns. Combined ventricular output (CVO) was 546 mL/min per kg, and the distribution of flows (%CVO) were comparable to values obtained using other methods. Internal 4D flow consistency across 23 measurement locations was established with differences of 14.2 ± 12.1%. Compared with 2D PC CMR, 4D flow showed a strong correlation (R2 = 0.85) but underestimated flow (bias = - 21.88 mL/min per kg, p < 0.05).

CONCLUSIONS

The combination of fetal surgical preparation and 4D flow CMR enables characterization and quantification of complex flow patterns in utero. Visualized streaming of blood through normal physiological shunts confirms the complex mechanism of substrate delivery to the fetal heart and brain. Besides offering insight into normal physiology, this technology has the potential to qualitatively characterize complex flow patterns in congenital heart disease phenotypes in a large animal model, which can support the development of new interventions to improve outcomes in this population.

Quantitative Flow Imaging in Human Umbilical Vessels In Utero Using Nongated 2D Phase Contrast MRI

BACKGROUND

Volumetric assessment of afferent blood flow rate provides a measure of global organ perfusion. Phase-contrast magnetic resonance imaging (PCMRI) is a reliable tool for volumetric flow quantification, but given the challenges with motion and lack of physiologic gating signal, such studies, in vivo on the human placenta, are scant.

PURPOSE

To evaluate and apply a nongated (ng) PCMRI technique for quantifying blood flow rates in utero in umbilical vessels.

STUDY TYPE

Prospective study design.

STUDY POPULATION

Twenty-four pregnant women with median gestational age (GA) 30 4/7 weeks and interquartile range (IQR) 8 1/7 weeks.

FIELD STRENGTH/SEQUENCE

All scans were performed on a 3.0T Siemens Verio system using the ng-PCMRI technique.

ASSESSMENT

The GA-dependent increase in umbilical vein (UV) and arterial (UA) flow was compared to previously published values. Systematic error to be expected from ng-PCMRI, in the context of pulsatile UA flow and partial voluming, was studied through Monte-Carlo simulations, as a function of resolution and number of averages.

STATISTICAL TESTS

Correlation between the UA and UV was evaluated using a generalized linear model.

RESULTS

Simulations showed that ng-PCMRI measurement variance reduced by increasing the number of averages. For vessels on the order of 2 voxels in radius, partial voluming led to 10% underestimation in the flow. In fetuses, the average flow rates in UAs and UV were measured to be 203 ± 80 ml/min and 232 ± 92 ml/min and the normalized average flow rates were 140 ± 59 ml/min/kg and 155 ± 57 ml/min/kg, respectively. Excellent correlation was found between the total arterial flow vs. corresponding venous flow, with a slope of 1.08 (P = 0.036).

DATA CONCLUSION

Ng-PCMRI can provide accurate volumetric flow measurements in utero in the human umbilical vessels. Care needs to be taken to ensure sufficiently high-resolution data are acquired to minimize partial voluming-related errors.

LEVEL OF EVIDENCE

2 Technical Efficacy Stage 1 J. Magn. Reson. Imaging 2017.

Comparison of global cerebral blood flow measured by phase-contrast mapping MRI with 15 O-H2 O positron emission tomography

Purpose

To compare mean global cerebral blood flow (CBF) measured by phase‐contrast mapping magnetic resonance imaging (PCM MRI) and by ¹⁵O‐H₂O positron emission tomography (PET) in healthy subjects. PCM MRI is increasingly being used to measure mean global CBF, but has not been validated in vivo against an accepted reference technique.

Materials and Methods

Same‐day measurements of CBF by ¹⁵O‐H₂O PET and subsequently by PCM MRI were performed on 22 healthy young male volunteers. Global CBF by PET was determined by applying a one‐tissue compartment model with measurement of the arterial input function. Flow was measured in the internal carotid and vertebral arteries by a noncardiac triggered PCM MRI sequence at 3T. The measured flow was normalized to total brain weight determined from a volume‐segmented 3D T₁‐weighted anatomical MR‐scan.

Results

Mean CBF was 34.9 ± 3.4 mL/100 g/min measured by ¹⁵O‐H₂O PET and 57.0 ± 6.8 mL/100 g/min measured by PCM MRI. The measurements were highly correlated (P = 0.0008, R² = 0.44), although values obtained by PCM MRI were higher compared to ¹⁵O‐H₂O PET (absolute and relative differences were 22.0 ± 5.2 mL/100 g/min and 63.4 ± 14.8%, respectively).

Conclusion

This study confirms the use of PCM MRI for quantification of global CBF, but also that PCM MRI systematically yields higher values relative to ¹⁵O‐H₂O PET, probably related to methodological bias.

Level of Evidence: 3

J. Magn. Reson. Imaging 2017;45:692–699.

Development of a consensus document to improve multireader concordance and accuracy of aortic regurgitation severity grading by echocardiography versus cardiac magnetic resonance imaging

Current guidelines recommend a multiparametric echocardiographic assessment of aortic regurgitation (AR). However, the absence of a hierarchical weighting of discordant parameters could cause interobserver variability. In the present study, we sought to define and improve the interobserver variability of AR assessment. Seventeen level 3 readers graded 20 randomly selected patients with AR. The readers also provided a usefulness score for each parameter, depending on its influence on their decision of the AR severity grade. A consensus strategy was subsequently formulated and validated against cardiac magnetic resonance imaging in a separate group of 80 patients. The readers were updated with the consensus document and recalibrated using the same cases. Agreement was statistically assessed using Randolph's free-marginal multirater kappa. At baseline, no uniform approach was used to combine the individual parameters, contributing to the interobserver variability (overall kappa 0.5). A consensus strategy to categorize AR severity was developed in which the left ventricular volume took precedence over the other parameters and was used to differentiate chronic severe AR from less severe categories. Recalibration of the readers using this consensus strategy improved concordance (kappa increased to 0.7). The new strategy also improved the accuracy relative to cardiac magnetic resonance imaging, as evidenced by full agreement on severe AR between the consensus document-based grading and AR severity defined by cardiac magnetic resonance imaging in the separate validation group of 80 patients. In conclusion, grading of chronic AR using a multiparametric approach has suboptimal consistency between readers and a left ventricular volume-based consensus document improved concordance and accuracy.

Cardiovascular magnetic resonance in the evaluation of heart failure: a luxury or a need?

Heart failure is a common syndrome with multiple causes. Cardiovascular magnetic resonance (CMR), using the available range of technique, is establishing itself as the gold standard noninvasive test for determining the underlying causes, and adding prognostic value, guiding therapy. Progress is continuing and rapid with promising new techniques such as diffuse fibrosis assessment. This article discusses the diverse roles of CMR in heart failure.

Womersley number-based estimates of blood flow rate in Doppler analysis: in vivo validation by means of phase-contrast MRI

A common clinical practice during single-point Doppler analysis is to measure the centerline maximum velocity and to recover the time-averaged flow rate by exploiting an assumption on the shape of velocity profile (a priori formula), either a parabolic or a flat one. In a previous study, we proposed a new formula valid for the peak instant linking the maximum velocity and the flow rate by including a well-established dimensionless fluid-dynamics parameter (the Womersley number), in order to account for the hemodynamics conditions (Womersley number-based formula). Several in silico tests confirmed the reliability of the new formula. Nevertheless, an in vivo confirmation is missing limiting the clinical applicability of the formula. An experimental in vivo protocol using cine phase-contrast MRI (2-D PCMRI) technique has been designed and applied to ten healthy young volunteers in three different arterial districts: the abdominal aorta, the common carotid artery, and the brachial artery. Each PCMRI dataset has been used twice: 1) to compute the value of the blood flow rate used as a gold standard and 2) to estimate the flow rate by measuring directly the maximum velocity and the diameter (i.e., emulating the intravascular Doppler data acquisition) and by applying to these data the a priori and the Womersley number-based formulae. All the in vivo results have confirmed that the Womersley number-based formula provides better estimates of the flow rate at the peak instant with respect to the a priori formula. More precisely, mean performances of the Womersley number-based formula are about three times better than the a priori results in the abdominal aorta, five times better in the common carotid artery, and two times better in the brachial 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.

Assessment of blood flow and valvular heart disease using phase-contrast cardiovascular magnetic resonance

Measurement of blood flow is important for assessing the severity of disease processes involving the cardiovascular system. Phase-contrast cardiovascular magnetic resonance (PC-CMR) can be used to measure blood flow noninvasively without ionizing radiation or limitations imposed by body habitus. This review describes the performance of PC-CMR and its clinical utility in assessing patients with cardiovascular or valvular heart disease.

Time-Resolved 3-Dimensional Velocity Mapping in the Thoracic Aorta

OBJECTIVE

An analysis of thoracic aortic blood flow in normal subjects and patients with aortic pathologic findings is presented. Various visualization tools were used to analyze blood flow patterns within a single 3-component velocity volumetric acquisition of the entire thoracic aorta

METHODS

Time-resolved, 3-dimensional phase-contrast magnetic resonance imaging (3D CINE PC MRI) was employed to obtain complete spatial and temporal coverage of the entire thoracic aorta combined with spatially registered 3-directional pulsatile blood flow velocities. Three-dimensional visualization tools, including time-resolved velocity vector fields reformatted to arbitrary 2-dimensional cut planes, 3D streamlines, and time-resolved 3D particle traces, were applied in a study with 10 normal volunteers. Results from 4 patient examinations with similar scan prescriptions to those of the volunteer scans are presented to illustrate flow features associated with common pathologic findings in the thoracic aorta.

RESULTS

Previously reported blood flow patterns in the thoracic aorta, including right-handed helical outflow, late systolic retrograde flow, and accelerated passage through the aortic valve plane, were visualized in all volunteers. The effects of thoracic aortic disease on spatial and temporal blood flow patterns are illustrated in clinical cases, including ascending aortic aneurysms, aortic regurgitation, and aortic dissection.

CONCLUSION

Time-resolved 3D velocity mapping was successfully applied in a study of 10 healthy volunteers and 4 patients with documented aortic pathologic findings and has proven to be a reliable tool for analysis and visualization of normal characteristic as well as pathologic flow features within the entire thoracic aorta.

Rapid measurement of time-averaged blood flow using ungated spiral phase-contrast

A novel ungated spiral phase-contrast (USPC) imaging method was developed for rapid measurement of time-averaged blood-flow rates in the presence of pulsatility. The spatial point-spread function was analyzed to provide an intuitive understanding of how spiral trajectories, which sample the k-space origin at every excitation, can mitigate the effects of pulsatility. Pulsatile flow phantom experiments were performed to validate the accuracy and repeatability of the USPC method. The measurement of flow in the renal and femoral arteries of normal volunteers were also performed. The phantom results (error < or = +9%, SD(phantom) < or = 2%, time-averaged pulsatile-flow rates = 3-15 ml/s) and in vivo results (SD(renal) < or = 8%, SD(femoral) < or = 14%) demonstrate the potential of the USPC method for rapidly and repeatedly measuring accurate time-averaged blood flow even in relatively small arteries and in the presence of strong pulsatility.

Characterization of common carotid artery blood-flow waveforms in normal human subjects

Knowledge of human blood-flow waveforms is required for in vitro investigations and numerical modelling. Parameters of interest include: velocity and flow waveform shapes, inter- and intra-subject variability and frequency content. We characterized the blood-velocity waveforms in the left and right common carotid arteries (CCAs) of 17 normal volunteers (24 to 34 years), analysing 3560 cardiac cycles in total. Instantaneous peak-velocity (Vpeak) measurements were obtained using pulsed-Doppler ultrasound with simultaneous collection of ECG data. An archetypal Vpeak waveform was created using velocity and timing parameters at waveform feature points. We report the following timing (post-R-wave) and peak-velocity parameters: cardiac interbeat interval (T(RR)) = 0.917 s (intra-subject standard deviation = +/- 0.045 s); cycle-averaged peak-velocity (V(CYC)) = 38.8 cm s(-1) (+/-1.5 cm s(-1)); maximum systolic Vpeak = 108.2 cm s(-1) (+/-3.8 cm s(-1)) at 0.152 s (+/-0.008 s); dicrotic notch Vpeak = 19.4 cm s(-1) (+/-2.9 cm s(-1)) at 0.398 s (+/-0.007 s). Frequency components below 12 Hz constituted 95% of the amplitude spectrum. Flow waveforms were computed from Vpeak by analytical solution of Womersley flow conditions (derived mean flow = 6.0 ml s(-1)). We propose that realistic, pseudo-random flow waveform sequences can be generated for experimental studies by varying, from cycle to cycle, only T(RR) and V(CYC) of a single archetypal waveform.

MR phase-contrast flow measurement with limited spatial resolution in small vessels: value of model-based image analysis

Magnetic resonance phase-contrast volume flow rate (VFR) measurement with limited resolution in small vessels is subject to two major sources of error: a) partial volume artifacts, causing systematic overestimation of the VFR, and b) errors related to the selection of vessel pixels [region of interest (ROI)], causing large inter-observer and intra-observer variability. Additionally, limited resolution results in Gibbs-ringing around vessels, which adversely affects VFR determination. In this paper, a semi-automatic model-based method is presented that effectively eliminates errors due to both partial volume effect and Gibbs-ringing and also minimizes errors from variability in the ROI selection. The model assumes a parabolic flow profile and cylindrical vessel geometry, incorporates inflow effects, and takes into account the point-spread function of the acquisition. The method automatically estimates maximum velocity, vessel radius, and VFR. The method is validated in phantoms under various conditions and evaluated in vivo. For small vessels with moderately pulsatile flow, it is demonstrated that accurate VFRs and diameter estimates are obtained, virtually independent of the ROI selection, even in vessels covered by just a few pixels. Compared with conventional VFR analysis, both accuracy and reproducibility improve significantly.

Construction of a protocol for measuring blood flow by two-dimensional phase-contrast MRA

Our aim is to describe and demonstrate the steps we have found to be useful in the construction and evaluation of protocols for triggered and nontriggered measurement of blood flow by two-dimensional phase-contrast magnetic resonance angiography (MRA). To achieve this goal, we start with a survey of factors governing the accuracy (validity) and precision (repeatability) of MR flow measurements. This knowledge, combined with prior information regarding the diameter of the target vessel and the prevailing flow conditions, is then employed to define a protocol for measuring flow with negligible systematic error. In the absence of a gold standard for in vivo flow measurements, the protocol is subsequently validated for a range of flow conditions by representative phantom experiments. Precision is then calculated from the signal-to-noise ratio (SNR) of blood in the accompanying magnitude images or, less conveniently, estimated from the standard deviation of repeated measurements. The desired precision is finally achieved by adjusting the appropriate SNR parameters. All steps involved in protocol development are demonstrated for both flow-independent and flow-dependent acquisitions.

Morphological and hemodynamic assessments of carotid stenosis using quantitative digital subtraction angiography

BACKGROUND AND PURPOSE

Digital angiography is the best established tool for assessing atheromatous disease of extracranial blood vessels. Advances in computer technology have now made it possible and practicable to extract quantitative information (length, width, cross-sectional area, and flow velocity) from good-quality clinical angiograms, allowing calculation of volume flow and pressure gradient. The technique of quantitative angiography (QA) is used for assessing coronary artery disease, but to date there has been no clinical application in patients with cerebrovascular disease.

SUMMARY OF REPORT

We have developed a computer program for off-line analysis of routine digital subtraction angiographic images. From biplanar images, the program reconstructs the angiogram in three dimensions and performs quantitative analysis of each vessel. From this data, the pressure drop from the aortic arch to the circle of Willis is then calculated. We assessed the clinical applicability of QA in five patients investigated for transient ischemic attack. The carotid artery ipsilateral to the symptomatic hemisphere was occluded in one patient and had minor plaque in another. In the remaining three patients, ipsilateral internal carotid artery stenosis was measured by QA as producing area reductions of 55%, 72%, and 88% (equivalent to diameter reductions of 33%, 48%, and 65%, respectively). In these patients, the quantitative stenosis pressure gradients were calculated as 1.2, 3.0, and 3.5 mm Hg. respectively. Further calculation showed that each stenosis contributed to 18%, 24%, and 60%, respectively, of the total carotid pressure gradient from the aortic arch to the circle of Willis. These carotid arteries carried 47%, 42%, and 26%, respectively, of the total cerebral flow. The results of quantitative analysis were validated by comparing, within each patient, the differences in pressure gradients between right and left carotid systems of between right and left vertebral arteries (overall mean difference in pressure gradient, 0.6 +/- 0.5 mm Hg: P = NS). Finally, comparison was made of pressure gradients across the circle of Willis between the carotid and vertebrobasilar circulations (mean difference in pressure gradient, 4.1 +/- 5.3 mm Hg; P = NS).

CONCLUSIONS

Quantitative angiography allows determination of the hemodynamic parameters of a vessel or stenosis. It has significant potential, both as a research tool and in routine clinical practice, for the investigation of cerebrovascular disease.

Measuring blood flow by nontriggered 2D phase-contrast MR angiography

This study was done to assess the validity of nontriggered 2D phase contrast MR angiography for measuring blood flow in human arteries and veins. Volume in the popliteal and internal carotid arteries was measured by nontriggered and triggered 2DPC in 1.3 normal volunteer (mean age 27, range 18-46). Parameter selection was guided by previous phantom experiments. Results were compared by linear regression analysis. Measurement error was determined by one-way analysis of variance of repeated measurements. In the internal carotid arteries, good agreement was found between the volume flow, Q, as determined by a triggered measurement and a nontriggered measurement: Qntr = 0.988 (+/- 0.006) Qtr, r = 0.98, SEE = 0.16 ml/s. The estimated measurement errors of both techniques were of the same order: 0.27 vs. 0.31 ml/s. Substantial deviations between triggered and nontriggered 2DPC were found in the popliteal artery: Qntr = 0.827 (+/- 0.028) Qtr, r = 0.97, SEE = 0.12 ml/s. The estimated measurement error of nontriggered 2DPC turned out to be twice as large as of triggered 2DPC here: 0.22 vs. 0.13 ml/s. We believe that nontriggered 2DPC is a valid technique for measuring blood flow in stationary vessels with weakly pulsatile flow, but merely provides a rough estimation for strongly pulsatile flow. In its current implementation, nontriggered 2DPC provides the data in 40 s, whereas triggered 2DPC requires 3-4 min, and offers additional time savings with regard to patient preparation and data processing.

MR angiography of abdominal and peripheral arteries. Techniques and clinical applications

This review article deals with MR angiography (MRA) of abdominal and peripheral arteries. Pulsatile flow, respiratory motion and peristalsis impose difficulties in imaging the vascular structures in the abdomen and the lower extremities. Development of new techniques, such as segmentation of the data acquisition, using specific acquisition windows in relation to a cardiac trigger, magnetization preparation of the tissue and phase-encoding re-ordering or sorting, have reduced the artifacts associated with abdominal and peripheral MRA. Clinical MR investigations of the arteries branching from the abdominal aorta such as the renal and mesenteric arteries and arteries in the lower extremities have revealed that severe stenoses or occlusions can be diagnosed accurately while the grading of less severe stenosis is more difficult. The phase-contrast method has been used to quantify blood flow and study the hemodynamics in abdominal and peripheral vessels. Quantitative flow information can be used to diagnose vascular disease and provides important physiological information. More prospective clinical studies, in which recently developed MRA techniques are compared with conventional angiography, are necessary before conclusive decisions can be made as to whether MRA may replace these methods.

Estimation of average flow in ungated 3D phase contrast angiograms

Three dimensional (3D) phase contrast angiograms contain velocity data, which is discarded after the reconstruction of the projections. In extension to earlier work on velocity quantification with ungated 2D phase data, this paper shows that a useful estimate of the average velocity and flow rate can be extracted from ungated 3D phase contrast angiograms. Simulations and experiments in a phantom and in vivo were performed. For pulsatile flow and strong spin saturation, an over-estimation of the flow rate at the net in-flow end of the imaging volume and underestimation at the net out-flow end was observed. Imaging at lower RF tip angles yielded flow rates close to the correct value within the entire imaging volume. In contrast to ungated 2D experiments, the flow rates determined by repeated 3D experiments showed no variation.

Error in MR volumetric flow measurements due to ordered phase encoding in the presence of flow varying with respiration

Respiratory ordered phase encoding is often employed in MRI studies to reduce image artifacts due to breathing motion. The purpose of this work was to evaluate error caused by the use of respiratory ordering of phase encoding in MR cine phase-contrast (CPC) volumetric flow measurements when the flow rate is sensitive to respiration. It was hypothesized that this effect is due to the systematic biasing of a respiratory-induced phase modulation function in k-space. A theoretical model for the effects of respiration was developed and then tested in flow phantom studies and in normal volunteer studies. In phantom experiments, the use of respiratory ordering induced an error of as much as 13% in CPC volumetric flow measurements. In preliminary volunteer studies, error was as high as 26% in superior vena cava flow measurements versus less than 1% error in the ascending aorta. It is concluded that a potential for error exists in CPC volumetric flow measurements obtained with the use of respiratory ordering schemes. Volunteer studies with larger numbers are warranted. Clinical applications in which this effect may be important include flow measurements in vessels subject to variations in flow due to respiration, such as the venae cavae, pulmonary vasculature, and portal vein.

Accuracy and precision of time-averaged flow as measured by nontriggered 2D phase-contrast MR angiography, a phantom evaluation

The purpose of this study was to assess the accuracy and precision of time-averaged flow as measured by nontriggered 2D PC. Mono-, bi-, and triphasic flow patterns, modelling waveforms encountered in the human vascular system, were generated by a computer-controlled flow system. Time-averaged flow velocity was measured by conventional 2D cardiac-triggered cine PC and by nontriggered 2D PC for different settings of the excitation flip angle and the velocity sensitivity. Accuracy and precision were determined by repeating the measurements (N = 6) and comparing the results against precisely known calibration values. Measurements revealed waveform-specific deviations between triggered and nontriggered acquisitions that depended on the velocity sensitivity and, more strongly, on the flip angle of the nontriggered experiment. This confirmed the theoretically predicted predominance of amplitude over phase effects. Systematic errors could be reduced by decreasing the flip angle and the velocity sensitivity, although at the expense of signal-to-noise, so that additional signal averaging was required to maintain a specified precision. The attainable accuracy appeared to be acceptable only for waveforms with a relatively low pulsatility index. The study demonstrates the feasibility of accurate and precise nontriggered velocity measurements for weakly pulsatile flow and indicates a route towards improving the reliability for highly pulsatile flow.

Effects of physiologic waveform variability in triggered MR imaging: theoretical analysis

One of the assumptions inherent in most forms of triggered magnetic resonance (MR) imaging is that the pulsatile waveform (be it cardiac, respiratory, or some other) is purely periodic. In reality, the periodicity condition is rarely met. Physiologic waveform variability may lead to image artifacts and errors in velocity or volume flow rate estimates. The authors analyze the effects of physiologic waveform variability in triggered MR imaging. They propose that this variability be treated as a modulation of the underlying motion waveform. This report concentrates on amplitude modulation of the velocity waveform, which results in amplitude and phase modulation of the transverse magnetization. Established Fourier and modulation theory and the recently described principles of (k,t)-space were used to derive the appearance of physiologic waveform variability artifacts in triggered MR images and to predict errors in time-averaged and instantaneous velocity estimates that may result from such motion effects, including effects such as ghost overlap. Simulations and experimental results are provided to confirm the theory.

Magnetic resonance velocity imaging using a fast spiral phase contrast sequence

Time-resolved velocity imaging using the magnetic resonance phase contrast technique can provide clinically important quantitative flow measurements in vivo but suffers from long scan times when based on conventional spin-warp sequences. This can be particularly problematic when imaging regions of the abdomen and thorax because of respiratory motion. We present a rapid phase contrast sequence based on an interleaved spiral k-space data acquisition that permits time-resolved, three-direction velocity imaging within a breath-hold. Results of steady and pulsatile flow phantom experiments are presented, which indicate excellent agreement between our technique and through plane flow measurements made with an in-line ultrasound probe. Also shown are results of normal volunteer studies of the carotids, renal arteries, and heart.

Experimental study of the effects of "fractional" gating on flow measurements

Velocity encoded phase imaging is subject to errors from phase and amplitude variations of the k-space data caused by beat-to-beat variations of the flow. Fractional cardiac gating is defined as asynchronous gating with each phase encode step occupying a fixed fraction of the RR interval. The gating fraction is the inverse of the number of phase encode steps taken per RR interval. Studies in normal subjects show that deviations and standard errors of ascending and descending aorta flow measurements are significantly greater with decreased gating fraction. Significant errors occur when gating does not separate systolic and diastolic data. The studies establish a graded trade-off between flow measurement accuracy and precision with imaging time, and show that standard nongated phase contrast measurements of strongly pulsatile flow are unreliable.

Time-resolved MR imaging by automatic data segmentation

A method for time-resolved imaging that provides a flexible trade-off between imaging time and temporal resolution is presented. It is based on a view order selection technique that automatically segments the acquired raw data into appropriate temporal frames. When used with cardiac monitoring and phase-contrast imaging, data similar to that obtained with a conventional gated phase-contrast sequence are acquired rapidly. For many applications, the temporal resolution can be reduced enough to permit imaging within a breath-hold interval, while still allowing accurate time-averaged flow quantitation. This is a general technique that can be implemented within a variety of pulse sequences and can resolve other motion cycles, including the respiratory cycle.