Magnetic Resonance Velocity Research Articles

Heart motion-adapted MR velocity mapping of blood velocity distribution downstream of aortic valve prostheses: initial experience.

To investigate blood flow velocities and shear rates at two distances downstream of an artificial aortic valve in patients. Blood velocity was quantified downstream of the valve prosthesis (for replacement after aortic valve stenosis or combined stenosis and regurgitation) in 10 patients by using a magnetic resonance (MR) cine velocity mapping method in which the imaging section position is adapted according to the excursion of the valvular plane of the heart. Two acquisitions were performed to display the blood velocity distributions one-fourth valve diameter and one valve diameter downstream of the valve and to quantify blood volumes and shear rates. The velocity profiles measured during flow acceleration one-fourth valve diameter downstream were characterized by a distinct pattern of two lateral jets and one central jet of antegrade flow. High shear rates were found along the leaflet tips. The profiles obtained one valve diameter downstream were skewed, with varying velocity patterns among patients. Peak shear rates were found close to the vessel wall. With correction for through-plane motion of the valve, the mean apparent regurgitant fraction (+/- SD) was 14% +/- 6; the mean regurgitant fraction without correction was 9% +/- 5. The described noninvasive procedure for velocity mapping enables measurements close to the valve and thus evaluation of blood flow patterns with respect to valve design in humans.

Clinical applications of cardiac magnetic resonance imaging after repair of tetralogy of Fallot.

In the past 15 years, cardiovascular magnetic resonance (MR) has evolved into an imaging technique that provides adequate, and in part unique, information on residual problems in the follow-up of patients operated for tetralogy of Fallot. Spin-echo or gradient-echo cine magnetic resonance imaging allow detailed assessment of intracardiac and large vessel anatomy, which is particularly helpful in Fallot patients with residual abnormalities of right ventricular outflow and/or pulmonary artery. Multisection gradient-echo cine MRI can be used to obtain accurate measurements of biventricular size, ejection fraction, and wall mass. This allows serial follow-up of biventricular function. MR velocity mapping is the only imaging technique available that provides practical quantification of pulmonary regurgitation volume. MR velocity mapping can also be used to quantify right ventricular diastolic function in the presence of pulmonary regurgitation.

Portal venous blood flow while breath-holding after inspiration or expiration and during normal respiration in controls and cirrhotics.

In this study, we used magnetic resonance (MR) imaging to measure portal blood flow in 12 healthy controls and 17 cirrhotics while they were breath-holding after inspiration and after expiration. We then compared the results with measurements made during normal respiration in the healthy controls and cirrhotics. Blood flow in the main portal vein under basal fasting conditions was quantitated using the cine phase-contrast MR velocity mapping method. Three measurements were made on one occasion, as follows: (1) throughout the cardiac cycle during normal respiration, (2) with the subject breath-holding after maximal inspiration, and (3) with the subject breath-holding after maximal expiration. During normal respiration, portal blood flow was 1.3 +/- 0.21/min in controls vs 1.0 +/- 0.11/min in cirrhotics (P < 0.0001); while subjects were breath-holding after inspiration, portal blood flow was 1.0 +/- 0.21/min in controls vs 0.9 +/- 0.11/min in cirrhotics; and while subjects were breath-holding after expiration, portal blood flow was 1.5 +/- 0.21/min in controls vs 1.1 +/- 0.21/min in cirrhotics (P < 0.0001). The differences were primarily due to changes in flow velocity. When the magnitude of these hemodynamic changes in the three respiratory conditions was compared in controls and cirrhotics, analysis of variance (ANOVA) showed a significant difference (P < 0.0001). In controls, portal blood flow decreased during maximal inspiration relative to flow during normal respiration (-24.6 +/- 8.3%). Changes in portal blood flow in controls were greater than in cirrhotics (-13.5 +/- 4.5%) (P < 0.0001); however, the difference in blood flow increase associated with maximal expiration between the two groups (+11.8 +/- 9.4% vs +5.9 +/- 11.5%) was not significant. We found that the respiration-induced hemodynamic variation in portal blood flow was less in cirrhotics than in the healthy controls. Portal blood flow measurements made during normal respiration using MR imaging closely reflect nearly physiologic conditions.

Biventricular systolic function and mass studied with MR imaging in children with pulmonary regurgitation after repair for tetralogy of Fallot.

To study biventricular systolic function and mass of the heart in young patients with residual pulmonary regurgitation who had undergone surgical correction of tetralogy of Fallot. Transverse gradient-echo magnetic resonance (MR) images covering both ventricles were obtained in 19 patients who had undergone corrective surgery for tetralogy of Fallot at the age of 1.5 years +/- 1 and in 12 age-matched control subjects. In addition, MR velocity maps of the pulmonary artery were obtained. Biventricular volumes, ejection fraction and myocardial mass, and pulmonary flow volumes were measured. Exercise tests were performed in 17 patients. The right ventricular ejection fraction was lower (P < .001) and the right ventricular mass was higher (P < .0005) in patients than in control subjects; the left ventricular ejection fraction was lower (P < .0005) in patients and correlated statistically significantly with pulmonary regurgitation (r = -.68; P < .005). Exercise performance inversely correlated with pulmonary regurgitation (tau = -0.5; P = .01). In children who undergo early surgical repair of tetralogy of Fallot, residual pulmonary regurgitation correlates with biventricular systolic dysfunction and diminished exercise capacity. Despite successful surgical correction, right ventricular hypertrophy may persist.

Quantification of pulmonary and systemic blood flow by magnetic resonance velocity mapping in the assessment of atrial-level shunts.

The objective of this study was to asses the feasibility and accuracy of magnetic resonance (MR) velocity mapping to calculate pulmonary-to-systemic flow ratio (Qp:Qs) in patients with a suspected or diagnosed atrial-level shunt. During a one-year period, all patients referred to our department for further evaluation of an atrial-level shunt underwent the same imaging protocol. Multiphase-multisection gradient-echo MR image sets of the heart were acquired to measure left and right ventricular stroke volumes for validation. Ascending aorta and main pulmonary artery volume flow were measured with MR velocity mapping. Qp:Qs ratios were calculated from both stroke volume data and flow data. Twelve patients, including 6 children, were studied. Six patients had an established diagnosis of atrial septal defect, and the other 6 patients were suspected to have an atrial-level shunt. Measurements of left and right ventricular stroke corresponded closely with those of aortic (r = 0.98) and pulmonary flow (r = 0.99) respectively, and Qp:Qs flow ratios agreed with stroke volume ratios (r = 0.92). In 5 patients with a suspected shunt, the diagnosis could be rejected. Shunts were demonstrated in the other 7 patients. M(r) velocity mapping offers an accurate method to measure aortic and pulmonary artery volume flow that can be useful in the evaluation of atrial-level shunts, in order to establish a definite diagnosis and/or to quantify the Qp:Qs ratio.

Tracking myocardial deformation using phase contrast MR velocity fields: a stochastic approach

The authors propose a new approach for tracking the deformation of the left-ventricular (LV) myocardium from two-dimensional (2-D) magnetic resonance (MR) phase contrast velocity fields. The use of phase contrast MR velocity data in cardiac motion problems has been introduced by others (N.J. Pelc et al., 1991) and shown to be potentially useful for tracking discrete tissue elements, and therefore, characterizing LV motion. However, the authors show here that these velocity data: 1) are extremely noisy near the LV borders; and 2) cannot alone be used to estimate the motion and the deformation of the entire myocardium due to noise in the velocity fields. In this new approach, the authors use the natural spatial constraints of the endocardial and epicardial contours, detected semiautomatically in each image frame, to help remove noisy velocity vectors at the LV contours. The information from both the boundaries and the phase contrast velocity data is then integrated into a deforming mesh that is placed over the myocardium at one time frame and then tracked over the entire cardiac cycle. The deformation is guided by a Kalman filter that provides a compromise between 1) believing the dense field velocity and the contour data when it is crisp and coherent in a local spatial and temporal sense and 2) employing a temporally smooth cyclic model of cardiac motion when contour and velocity data are not trustworthy. The Kalman filter is particularly well suited to this task as it produces an optimal estimate of the left ventricle's kinematics (in the sense that the error is statistically minimized) given incomplete and noise corrupted data, and given a basic dynamical model of the left ventricle. The method has been evaluated with simulated data; the average error between tracked nodes and theoretical position was 1.8% of the total path length. The algorithm has also been evaluated with phantom data; the average error was 4.4% of the total path length. The authors show that in their initial tests with phantoms that the new approach shows small, but concrete improvements over previous techniques that used primarily phase contrast velocity data alone. They feel that these improvements will be amplified greatly as they move to direct comparisons in in vivo and three-dimensional (3-D) datasets.

Cine gradient-echo MR imaging and MR velocity mapping in the evaluation of congenital heart disease.

Cine gradient-echo (GRE) magnetic resonance (MR) imaging and MR velocity mapping can be applied to assess cardiovascular function and to measure volumetric flow in patients with repaired and untreated congenital heart disease. GRE images, particularly when displayed in a cine loop, are very useful for visualizing surgical anastomoses and conduits and for detecting stenosis, regurgitation, and left-to-right shunts. Multisection cine GRE imaging is a suitable method for measuring global right ventricular function, an important issue in congenital heart disease. MR velocity mapping is used for quantitative assessment of hemodynamics. Pressure gradients can be estimated from peak blood flow velocities in stenotic pulmonary arteries or surgical conduits. Because flow volume can be measured with MR velocity mapping, main-, left-, and right pulmonary artery flow, valvular regurgitation, left-to-right shunts, and ventricular filling can all be quantified. The abilities to elucidate complex cardiac anatomy and to measure cardiac function and flow in one examination make MR imaging a useful comprehensive tool for follow-up of congenital heart disease.

Three-dimensional reconstruction of the flow in a human left heart by using magnetic resonance phase velocity encoding.

Intraventricular flows have been correlated with disease and are of interest to cardiologists as a possible means of diagnosis. This study extends a method that use magnetic resonance (MR) to measure the three-dimensional nature of these flows. Four coplanar, sagittal MR slices were located that spanned the left ventricle of a healthy human. All three velocity components were measured in each slice and 18 phases were obtained per beat. With use of the MR magnitude images, masks were created to isolate the velocity data within the heart. These data were read into the software package, Data Visualizer, and the data from the four slices were aligned so as to reconstruct the three-dimensional volume of the left ventricle and atrium. By representing the velocity in vectorial form, the three-dimensional intraventricular flow field was visualized. This revealed the presence of one large line vortex in the ventricle during late diastole but a more ordered flow during early diastole and systole. In conclusion, the use of MR velocity acquisition is a suitable method to obtain the complex intraventricular flow fields in humans and may lead to a better understanding of the importance of these flows.

Tracking of cyclic motion with phase-contrast cine MR velocity data.

A method of computing trajectories of objects by using velocity data, particularly as acquired with phase-contrast magnetic resonance (MR) imaging, is presented. Starting from a specified location at one time point, the method recursively estimates the trajectory. The effects of measurement noise and eddy current-induced velocity offsets are analyzed. When the motion is periodic, trajectories can be computed by integrating in both the forward and backward temporal directions, and a linear combination of these trajectories minimizes the effect of velocity offsets and maximizes the precision of the combined trajectory. For representative acquisition parameters and signal-to-noise ratios, the limitations due to measurement noise are acceptable. In a phantom with reciprocal rotation, the measured and true trajectories agreed to within 3.3%. Sample trajectory estimates of human myocardial regions are encouraging.

Magnetic resonance imaging kappa-space segmentation using phase-encoding groups the accuracy of quantitative measurements of pulsatile flow.

The use of phase-encode grouping (PEG) allows acquisition of a complete cardiac cine in a single breath hold, eliminating respiratory artifacts and improving edge definition. One approach to quantitative magnetic resonance (MR) flow measurements in pulmonary, coronary, and renal arteries uses phase velocity mapping with a PEG, single breath-hold, cine technique. Phantom studies were designed to evaluate the accuracy of MR phase velocity mapping (MRV) with PEG in continuous and pulsatile flow measurements in small vessels. Experiments were performed on a 1.5-T MRI system and results were compared with those obtained by timed volume measurements and a transit-time ultrasound flowmeter. Flow velocity, pulsatile flow wave form, pulsing rate, and PEG size were varied. For continuous flow, MRV measured velocities correlated well with those calculated from timed volume measurements (r > 0.998) under mean spatial velocities ranging from 20-100 cm s-1. The temporal mean velocity difference between MRV and ultrasound flowmeter measurements was less than +/- 2.9 cms-1 under pulsatile flow conditions with PEG sizes of 5 or less, however, the mean velocity difference was > 7 cm s-1 for PEG sizes of 8 or more. Accurate MR velocity measurement with PEG in small vessels under conditions of pulsatile flow is possible if an appropriate PEG size is selected.

Exercise-related changes in aortic flow measured with spiral echo-planar MR velocity mapping.

Spiral echo-planar magnetic resonance (MR) velocity mapping was used to measure exercise-related changes in flow in the descending thoracic aorta in 10 healthy volunteers. Flow was measured at rest and immediately after dynamic exercise, with a 0.5-T imager with a surface receiving coil and electrocardiographic triggering. Supine exercise was performed with a home-built pedaling apparatus. Spiral velocity mapping was performed in a transverse plane through the descending thoracic aorta with the subject at rest. The subject was then asked to perform maximum exercise, stop, and hold his breath during a four-heartbeat acquisition time. Eight cine frames with a temporal resolution of 50 msec were acquired through systole. Each image was acquired in 40 msec during spiral acquisition of k-space data, starting at the center, 6 msec after the excitation pulse. Reproducibility of the technique was established by repeating the flow measurement in four consecutive heartbeats. At rest, the heart rate (mean +/- standard deviation), mean aortic flow, peak aortic flow, and time to peak flow were 68 beats per minute +/- 6, 41 milliliters per beat +/- 8, 107 mL/sec +/- 20, and 175 msec +/- 25, respectively. After exercise, the heart rate and mean and peak aortic flow were significantly increased (P < .001), measuring 101 beats per minute +/- 12, 57 milliliters per beat +/- 11, and 158 mL/sec +/- 29, respectively, while the time to peak flow (115 msec +/- 32) was significantly reduced (P < .001). The four sets of values obtained for the first four consecutive heartbeats measured at rest were similar, as were those obtained for the first four heartbeats after exercise.

MR velocity mapping of tricuspid flow in healthy children and in patients who have undergone Mustard or Senning repair.

To determine the feasibility and accuracy of measuring tricuspid volume flow with magnetic resonance (MR) velocity mapping in healthy children and in patients after a Mustard or Senning repair. MR studies were performed in 14 healthy children (mean age, 12 years +/- 3) and in 12 patients (mean age, 17 years +/- 5) late after a Mustard or Senning repair. MR measurements of tricuspid volume flow were validated against right ventricular stroke volumes measured tomographically. Diastolic filling parameters were derived from the flow measurements. Tricuspid volume flow and right ventricular stroke volume showed close agreement in the healthy children (r = .98) and in the patients (r = .94). Children after Senning repair, compared with healthy children, showed a delayed and higher peak tricuspid flow rate during early filling and a lower peak flow rate during atrial contraction (P < .05). MR measurement of tricuspid flow is feasible and accurate in healthy children and in patients after a Mustard or Senning operation, who often demonstrate abnormal tricuspid flow patterns.

Skeletal muscle contraction: analysis with use of velocity distributions from phase-contrast MR imaging.

Velocity gradient data from phase-contrast magnetic resonance (MR) imaging were tested for the ability to calculate tensile strain and shear strain (deformation) during cyclical motion of skeletal muscle. Strain data were derived from in vitro and in vivo phase-contrast MR velocity maps. A motion phantom designed to cyclically compress and expand a specimen of skeletal muscle provided a standard of reference to validate deformation, translation, and rotation measurements. The authors studied anterior and posterior muscle compartments of the lower extremity in three healthy volunteers during ankle dorsiflexion and plantar flexion against various resistances and the forearms of five healthy volunteers during flexion and extension of the fingers. The mean in vitro tracking error was 0.5 mm. The gastrocnemius muscle area in vivo changed 20% for both the minimum and maximum force conditions and therefore did not appear to be a good predictor of force. Phase-contrast MR imaging provides quantitative data on muscle contraction and demonstrates that shear and tensile strain can be measured and separated from translation and rotation of muscle.

Magnetic-resonance velocity mapping of the central circulation.

In magnetic-resonance (MR) velocity mapping there exists a linear relationship between the velocity and signal in each element of a tomographic image. The technique can be used for quantitative measurements of linear velocities (m s-1) and flow rates (1 min-1). By using cinematographic images the flow profile during the cardiac cycle can be determined. This allows quantification of forward flow, regurgitant volume and regurgitant fraction in cases of heart-valve insufficiency. In valvular stenosis the transvalvular pressure gradient and valve area can be determined. Magnetic-resonance velocity mapping may also provide information about diastolic function of left ventricular function. Together with other MR imaging techniques, velocity mapping gives an accurate assessment of the severity of aortic dissection. Recent studies indicate that MR velocity mapping provides quantification of renal blood flow, and that MR imaging may be used even for coronary angiography and measurements of coronary blood flow. Therefore MR velocity mapping has the potential to become an important clinical tool for examination of the cardiovascular system providing high accuracy and quantitative measurements.

Blood flow measurements in the aorta and major arteries with MR velocity mapping.

The purpose of this study was to measure antegrade and retrograde flow in the aorta and the major arterial pathways in the body noninvasively with cine magnetic resonance (MR) velocity mapping, to determine the hemodynamic significance of retrograde flow in arteries. Two hundred forty cine velocity maps for blood flow measurements were obtained at 29 sites in the aorta and the major arteries in 31 healthy human subjects of varying age at rest. Synchronous or isolated antegrade and retrograde flow was found in the entire aorta and in arteries supplying muscles. No retrograde flow was found in arteries supplying internal organs, such as the internal carotid or splanchnic arteries. The retrograde flow in the aorta and the extremity arteries contributes substantially to supplying diastolic perfusion of internal organs such as the heart, brain, and kidneys. Antegrade flow tends to be helical in the thoracic aorta.

Assessment of regional left ventricular long-axis motion with MR velocity mapping in healthy subjects.

The pattern of left ventricular long-axis motion during early diastole was assessed with magnetic resonance (MR) velocity mapping in 31 healthy volunteers. Regional long-axis velocity varied with time and position around the ventricle. During systole, the base descended toward the apex. The greatest magnitude of long-axis velocity occurred during early diastole. The lateral wall had the highest velocity (140 mm/sec +/- 40 [mean +/- standard deviation]); the anterior and inferior walls had lower velocities (96 mm/sec +/- 27 and 92 mm/sec +/- 34, respectively). The inferoseptal area consistently had the lowest velocities (87 mm/sec +/- 40). Absolute values of peak early-diastolic velocity declined with age (r = -.64, P < .001). Peak early-diastolic velocity was not dependent on heart rate (r = .014, P = .94). Regional variations in left ventricular wall motion were seen. MR velocity mapping is a useful technique for assessing regional left ventricular long-axis heart function.

Magnetic resonance, a new method for measuring blood flow in hemodialysis fistulae

Blood flow through arteriovenous fistulae in patients undergoing hemodialysis needs to be within a certain range for optimal management. Magnetic resonance (MR) velocity mapping is a new modality that allows the noninvasive measurement of blood flow volume. This technique was applied in 13 patients with either a Brescia-Cimino shunt (N = 6) or a Polytetrafluoroethylene graft (Goretex, N = 7). Fistula flow volume measured by MR velocity mapping was 1386 +/- 411 ml/min (mean +/- SD, range 644 to 1921 ml/min). Interstudy reproducibility of MR velocity mapping was good (r = 0.94). Interstudy reproducibility of MR velocity mapping was good (r = 0.94). MR velocity mapping derived flow volumes showed good correlation with values obtained with an indicator dilution technique using Indocyanine Green (ICG) which was performed simultaneously in eight patients (r = 0.86). We conclude that MR velocity mapping is an accurate noninvasive method to quantify flow volume through arteriovenous fistulae in patients requiring hemodialysis.

Left Ventricular Blood Flow Patterns Assessed by Magnetic Resonance Velocity Mapping in Patients with Ischemic Heart Disease

Myocardial infarction results in wall motion abnormalities potentially associated with abnormal intracavitary blood flow patterns in the left ventricle. Magnetic resonance (MR) technique allows three-dimensional quantitative mapping of blood flow velocities. We performed three-dimensional MR velocity mapping in a modified left ventricular long axis plane in 10 patients with previous myocardial infarction and milder reduction of the left ventricular function. The early diastolic inflow pattern did not differ qualitatively from earlier published findings in normal subjects. At systole, a flow pattern similar to that found in normals was observed in 7 of our patients. In the remaining 3 patients two types of abnormal systolic flow patterns were observed resulting in a paradoxical flow directed towards the apex. The location and timing of the abnormal flow allowed a distinction between paradoxical flow into an aneurism and paradoxical flow towards an akinetic apex. Compared with other techniques currently available, three-dimensional MR velocity mapping permits a more detailed description and understanding of abnormal flow patterns caused by wall motion injury in patients with myocardial infarction. This potential of the method suggests that three-dimensional MR phase velocity mapping may become a useful tool in assessment of significance and treatment of regional left ventricular dysfunction.

Magnetic resonance measurement of velocity and flow: Technique, validation, and cardiovascular applications

With a newly developed magnetic resonance (MR) technique for blood flow measurements, qualitative and quantitative information on both flow volume and flow velocity in the great vessels can be obtained. MR flow quantitation is performed with a gradientecho MR sequence with high temporal resolution enabling measurements at frequent intervals throughout the cardiac cycle. MR flow quantitation uses the phase rather than the amplitude of the MR signal to reconstruct the images. These images, often referred to as MR velocity maps or velocity-encoded cine MR images, are two-dimensional displays of flow velocity. From these velocity maps, velocity and volume flow data can be obtained. Previous validation experiments have demonstrated the accuracy of MR velocity mapping, and this technique is now being applied successfully in several clinical fields. MR velocity mapping may be of considerable value when Doppler echocardiography results are unsatisfactory or equivocal, particularly because MR is suited for the analysis of volumetric flow and complex flow patterns. Among the vastly growing number of clinical cardiovascular applications that have been reported are the great arteries and veins, coronary vessels, valvular disease, and the abdominal and peripheral vessels. These items are reviewed, and some aspects of the technique that need improvement are discussed.

Frequency response to retrospectively gated phase-contrast MR imaging: effect of interpolation.

Retrospectively gated phase-contrast (PC) magnetic resonance velocity and volume flow measurements were evaluated in both in vitro and in vivo experiments. The accuracy of these measurements was found to be affected by the interpolation window width required in the reconstruction of retrospectively gated data. Interpolation modified the frequency content of the series of temporal measurements by decreasing the response at higher frequencies. With a series of sinusoidal flow waveforms, the frequency response of one specific implementation of retrospectively gated PC velocity measurements was experimentally determined. The experimental response agreed with the theoretical response predicted from an analysis of the interpolating function (2.2% root-mean-square difference). In vitro experiments with a simulated carotid flow waveform demonstrated errors in the systolic measurements that were a direct result of the modified frequency response. A volunteer study was also undertaken and confirmed the in vitro findings.