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.     

Mitral and aortic valvular flow: quantification with MR phase mapping.

When magnetic resonance phase mapping is used to quantitate valvular blood flow, the presence of higher-order-motion terms may cause a loss of phase information. To overcome this problem, a sequence with reduced encoding for higher-order motion was used, achieved by decreasing the duration of the flow-encoding gradient to 2.2 msec. Tested on a flow phantom simulating a severe valvular stenosis, the sequence was found to be robust for higher-order motion within the clinical velocity range. In eight healthy volunteers, mitral and aortic volume flow rates and peak velocities were quantified by means of phase mapping and compared with results of the indicator-dilution technique and Doppler echocardiography, respectively. Statistically significant correlations were found between phase mapping and the other two techniques. Similar studies in patients with valvular disease indicate that phase mapping is also valid for pathologic conditions. Phase mapping may be used as a noninvasive clinical tool for flow quantification in heart valve disease.     

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. Reproduclbility 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 mllliliters 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 < 0.001), 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 < 0.001). 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.     

Validation of cine phase-contrast MR imaging for motion analysis

The accuracy of cine phase-contrast magnetic resonance (MR) imaging for motion analysis was evaluated. By using a rotating phantom and postprocessing algorithm for phase tracking, errors arising during data acquisition were identified and compensation methods were developed. A spatially varying background phase offset in the velocity images was found to be due to eddy current-induced fields. The magnitude of the offset was in the range of 0–20 cm/sec, which is of the same order of magnitude as cardiac contractile velocities. Background offset is thus an important source of error in tracking cardiac motion. Study of different tracking algorithms revealed the need for an integration scheme using motion terms higher than velocity. Also, considerable improvement in the accuracy and stability of the predicted trajectories was obtained by averaging the trajectories proceeding both forward and backward in time from the starting point. With the algorithm developed, the motion of the phantom was tracked through a complete rotation of the phantom to an accuracy of 2 pixels.     

Semiautomated method for noise reduction and background phase error correction in MR phase velocity data

Background phase distortion and random noise can adversely affect the quality of magnetic resonance (MR) phase velocity measurements. A semiauto-mated method has been developed that substantially reduces both effects. To remove the background phase distortion, the following steps were taken: The time standard deviations of the phase velocity images over a cardiac cycle were calculated. Static regions were identified as those in which the standard deviation was low. A flat surface representing an approximation to the background distortion was fitted to the static regions and subtracted from the phase velocity images to give corrected phase images. Random noise was removed by setting to zero those regions in which the standard deviation was high. The technique is demonstrated with a sample set of data in which the in-plane velocities have been measured in an imaging section showing the left ventricular outflow tract of a human left ventricle. The results are presented in vector and contour form, superimposed on the conventional MR angiographic images.     

Carotid and vertebral artery blood flow in left- and right-handed healthy subjects measured with MR velocity mapping

The goal of the study was to establish normal carotid artery flow rates in left-handed and right-handed individuals as a standard against which patients with carotid artery disease could be compared. Antegrade and retrograde flow were measured in the ascending aorta, in the right and left common, internal, and external carotid arteries, and in the vertebral arteries of 12 healthy subjects. Five subjects were right-handed, five left-handed, and two ambidextrous. Measured flow rates were as follows: common carotid arteries, 360–557 mL/min (mean [± standard deviation], 465 mL/min ± 52); internal carotid arteries, 132–367 mL/min (mean, 265 mL/min ± 60); external carotid arteries, 113–309 mL/min (mean, 186 mL/min ± 51); vertebral arteries from 133–308 mL/min (mean, 244 mL/min ± 43); and cerebral circulation, 546–931 mL/min (mean, 774 mL/min ± 134). All right-handed subjects had higher flow rates in the left internal carotid artery than in the right, and all left-handed subjects had higher flow rates in the right internal carotid artery (P =.007). There were no significant differences in left and right common carotid artery flow rates between left- and right-handed subjects. The standard deviation of a single measurement was 5%. The flow rates were similar to those obtained previously with other techniques and could be used as a normal standard.     

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.     

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.     

In vivo comparison of two through-plane MR velocity mapping methods: Fast fourier encoding and phase mapping

Magnetic resonance imaging maps of velocity were acquired with a 1.5-T system in 10 subjects in a plane perpendicular to the main pulmonary artery. Velocity images were successively acquired with a method developed from Fourier-encoding velocity imaging (FEVI) principles with eight gradient steps and one excitation, and with two-point phase-subtraction mapping. Reconstruction in FEVI was implemented by zero-filling interpolation around the eight gradient steps and then around the four central steps. The methods were compared by using estimates of noise in velocity measurements based on the difference between the experimental map and a smooth fitted map. For the same acquisition time, FEVI with four encoding steps was more precise in velocity measurements than phase mapping. Precision was further increased by the use of eight encoding steps, but acquisition time was doubled.     

Accuracy and precision of MR velocity mapping in measurement of stenotic cross-sectional area,flow rate,and pressure gradient

Reliability of magnetic resonance (MR) velocity mapping to assess severity of stenosis was assessed in vitro. Steady flow at different flow rates through five stenoses with a central orifice area ranging from 17 to 176 mm² was measured with velocity mapping performed perpendicular to the stenotic jet. Besides determination of the stenotic cross-sectional area and flow rate, the pressure gradient was calculated with the modified Bernoulli equation and compared with manometer measurements. Cross-sectional areas were measured with an accuracy of ?76%, a precision of ?91%, and an error of ?19 mm². Flow rates had an accuracy of ?72%, a precision of ?94%, and an error of ?1.4 L/min. The modification of the Bernoulli equation limited its reliability to stenoses with areas of 35-113 mm². Pressure gradients were calculated with an accuracy of ?80%, a precision of ?88%, and an error of ?15 mm Hg. The method was applied in a single patient with aortic stenosis and gave estimates that agreed with those obtained by heart catheterization.     

Quantitative MR signal behavior in cardiac cine loops as a function of heart rate.

To evaluate the heart cycle-dependent signal intensity changes in the cardiac chambers, the aorta, and the pulmonary artery, five healthy volunteers were studied with gradient-echo magnetic resonance cine loops at different heart rates. Quantitative evaluation of signal intensity on each side of the cardiac valves showed that there were changes in signal intensity due to section-entry and spin-phase phenomena but none due to the increase in heart rate. The authors conclude that there is no heart rate-dependent signal loss in healthy persons that simulates valvular dysfunction, thus suggesting that signal intensity change can be used as an indicator for this disease, independent of heart rate.     

Validation of volume flow measurements with cine phase-contrast MR imaging for peripheral arterial waveforms

A flow phantom was used to study MR volume flow measurements for monophasic and triphasic waveforms over the flow range expected in peripheral arteries at rest and with exercise (2–24 mL/sec, n = 50). The improvement in accuracy with phase-correction image processing to eliminate errors caused by eddy currents was measured. Volume flow estimates with Doppler sonography were also measured. MR volume flow measurements correlated with volume collection with r = 0.996 and mean error = 4.6%. Phase–correction processing decreased mean error from 12.6% to 4.6% (P <.001, paired t-test). Doppler sonography had a higher mean error of 10.3% (P <.001, unpaired t-test). Cine phase-contrast MR imaging provides accurate estimates of volume blood flow for waveforms and flow ranges expected in peripheral arteries.     

Renal artery velocity mapping with MR imaging

An MR phase imaging sequence with a very short echo time was used to assess blood velocity and flow at the renal artery bifurcation. Cardiac-gated MR imaging data were obtained in six healthy subjects in sagittal planes adjacent to the abdominal aorta and transverse planes above and below the renal artery bifurcation. Average renal artery flow rate was 23.8 ±9 mL/sec. A strong individual variability was found for the velocity profiles in the abdominal aorta during end-systolic regurgitation. Flow rate was also determined in three patients with reduced renal artery blood flow. Two patients received therapy with percutaneous transluminal angioplasty. The successful outcome was documented with MR imaging. A reliable assessment of renal artery flow with MR phase imaging is feasible. Measurement of the velocity profiles yields valuable insights in the complicated flow regime at the renal artery bifurcation.     

Single-shot versus interleaved echo-planar MR -imaging: Application to visualization of cardiac valve leaflets

Visualization of the cardiac valves with standard magnetic resonance (MR) imaging is not adequate because of long acquisition times. Echo-planar imaging (PI) can, however, be performed with a temporal resolution (30–50 msec) comparable to that of echocardiography. The authors evaluated the feasibility of real-time imaging of cardiac valve motion with ultrafast MR techniques. Eight healthy volunteers and three patients with mitral stenosis and re- gurgitation were studied with a 1.5-T whole-body im- ager. Two different EPI sequences were assessed: a standard single-shot gradient-echo EPI (GEPI) SCquence and a fast imaging technique based on multiple-shot EPI with interleaved k-space acquisition (IGEPI). Fat-suppressed images with an in-plane resolution of 3.7 × 3.7 mm were obtained equally spaced through the cardiac cycle. Half-k-space acquisition was used. Morphologic evaluation was superior with IGEPI, owing to the better intracavitary signal homogeneity (P ≤ 0.01). and the mitral valve leaflets were easier to identify on systolic images. IGEPI provided adequate valve visibility in all three patients.     

Low-field magnetic resonance imaging in the evaluation of mechanical and biological heart valve function

Magnetic resonance imaging (MRI) has been frequently considered unsafe for patients with ferromagnetic implants: risks to be considered include induction of electric current, heating and dislocation of the prosthesis. Previous in vitro and in vivo studies have indicated the possibility of performing MRI examinations on patients with prosthetic heart valves. The aim of our study was to verify the presence of artifacts at the level of the prosthetic heart valve in vivo using a low-field MR unit (0.2 T) and to define the possibility of a functional analysis of the valve in patients with biomedical or mechanical prostheses. We evaluated 14 patients surgically treated for implantation of nine biological and seven mechanical aortic and mitral valves. A low-field MR unit (0.2 T) was employed using cine-MR technique on long- and short-axis view. The images were acquired on planes parallel and perpendicular to the valvular plane. Semiquantitative analysis with double-blind evaluation for definition of the extent of the artifact was performed. Three classes of artifacts were distinguished from minimal to significant. The examinations showed the presence of minimal artifacts in all biological heart valves and moderate artifacts in mechanical valves giving good qualitative data on blood flow near the valve. Analysis of the flow behind the valve showed signs of normal function in 13 prostheses and pathological findings in the remaining three. In these latter cases, MRI was able to define the presence of a pathologic aortic pressure gradient, mitral insufficiency and malpositioning of the mitral valve causing subvalvular turbulence. Nevertheless, we believe that the application of velocity-encoding cine-MR is more promising than semiquantitative analysis of artifacts.     

Ischemic heart disease: Assessment with gadolinium-enhanced ultrafast MR imaging and dipyridamole stress

The potential of magnetic resonance (MR) imaging for the detection of myocardial perfusion abnormalities in patients with coronary artery disease has not been fully explored. A feasibility study was conducted in 10 patients with a novel approach to determine whether myocardial ischemia can be assessed with MR imaging and dynamic first-pass bolus tracking enhanced with gadolinium tetraazacyclododecanetetraacetic acid (DOTA). Three tomographic planes were acquired before and after pharmacologic stress with dipyridamole, with use of the bolus-tracking series at rest as a reference. The change in myocardial rate of enhancement was compared with the results obtained by means of the established methods, exercise thallium scintigraphy and coronary angiography. Detection of ischemic regions with MR imaging showed a sensitivity, specificity, and diagnostic accuracy of 65%, 76%, and 74%, respectively. Ultrafast MR imaging can be used to detect regions of myocardial ischemia.     

Blood flow patterns in the thoracic aorta studied with three-directional MR velocity mapping: The effects of age and coronary artery disease

The objective was to investigate how the blood flows in the thoracic aorta, with special emphasis on flow reversal and flow into the coronary arteries. Three-directional MR velocity mapping was used to map multidirectional flow velocities in the aorta in 14 normal subjects and 14 patients with coronary artery disease. Dynamic flow vector maps and through-plane velocity maps were used. The flow reversed in all subjects in the upper ascending aorta and usually also in the distal aortic arch. Retrograde flow became antegrade again at various levels in the ascending aorta and in the coronary sinuses. Seven flow characteristics were investigated that, lumped together, were significantly different (P = .0005) in normal subjects compared with patients and in normal subjects 70 years of age and older compared with those younger than 70 years of age.     

High-precision MR velocity mapping by 3D-fourier phase encoding with a small number of encoding steps

The final result of Fourier velocity mapping is a set of images, each representing the spatial distribution of spins at a given velocity. To acquire data in a short time, the number of encoding gradient steps must be as small as possible, but this can mean sacrificing velocity resolution. We used interpolation methods to obtain high velocity resolution with a small number of encoding steps involving linear interpolation from 16 encoding steps or more and zero-filling interpolation from two to eight encoding steps. Velocity measured by interpolated Fourier-flow encoding agreed well with values obtained using a calibrated phantom. A simulation of noise on the images of the phantom showed that, for a given acquisition time, increasing number of encoding steps in the Fourier flow encoding gave better precision for velocity measurement than did averaging identical signals in phase-mapping methods.     

First pass of an MR susceptibility contrast agent through normal and ischemic heart: Gradient-recalled echo-planar imaging

Gradient-recalled echo-planar magnetic resonance (MR) imaging was used to monitor the first pass of a magnetic susceptibility contrast agent through the heart of normal rats and rats subjected to 60-minute occlusion of the anterior branch of the left main coronary artery. Each animal (six normal and six ischemic) received four doses (0.05, 0.1, 0.15, and 0.2 mmol/kg) of Dy-DTPA-BMA [diethylenetriaminepentaacetic acid–bis(methylamide)] administered as a bolus volume of 1.0 mL/kg. In both normal and ischemic rats, signal intensity in nonischemic myocardium was reduced by the contrast agent in a dose-dependent manner. Signal intensity in the ischemic zone was reduced much less, so that at a contrast agent dose of 0.1 mmol/kg or greater the ischemic zone was clearly defined as a high-intensity zone on echo-planar images. Plots of the change in the apparent T2* relaxation rate (ΔR2*) during the peak bolus effect versus injected dose were well fit by straight lines for normal, nonischemic, and ischemic myocardium but not for blood in the left ventricle. No difference was seen between myocardial response in normal animals and in nonischemic regions in animals with coronary artery occlusion. These findings suggest that the contrast agent–induced changes in tissue T2* are monoexponential and support the idea that data derived from contrast transit studies may be useful for calculation of myocardial blood flow.