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D N Firmin

Publications and source records attributed to D N Firmin.

At least 91 records · Page 5Linked to original sources
PubMed1984

Measurement of flow with NMR imaging using a gradient pulse and phase difference technique.

A method for determining flow by nuclear magnetic resonance (NMR) imaging is described. A conventional spin-echo imaging sequence is employed with the addition of balanced gradient pulses on either side of the pi radiofrequency pulse. Flow velocities in the direction orthogonal to the image plane are determined by the phase shifts in the NMR image. Experimental validation of the technique in vitro was achieved with a phantom designed to give a continuous flow of water. The flow rate measured by NMR agreed well with the volume flow rate through the phantom. In vivo, NMR flow measurements on the carotid and femoral arteries of two volunteers were compared with Doppler ultrasound results. Velocity measurements were in general agreement. Rapid changes in flow are difficult to follow with the NMR method unless particular care is taken in gradient profile design. The technique can readily be used in existing NMR imaging machines and may have a useful clinical role.

Blood Flow Velocity↗
PubMedDate not supplied

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.

Adult↗
PubMedDate not supplied

Mitral valve flow measured with cine MR velocity mapping in patients with ischemic heart disease: comparison with Doppler echocardiography.

Diastolic function is an important element of overall left ventricular function. The pattern of flow across the mitral valve is commonly used as a measure of diastolic ventricular function. Magnetic resonance (MR) velocity mapping of blood flow across the mitral valve was compared with Doppler echocardiography. Nineteen patients with known coronary artery disease (mean age, 62 years; 11 with previous myocardial infarction) were studied. The mean value of peak early filling velocity (+/- standard deviation) was 60.1 cm/sec +/- 14.3 with the MR method and 59.4 cm/sec +/- 13.7 with echocardiography (P = .732). The mean difference between the two measurements (95% confidence interval) was -0.8 cm/sec (-5.2 cm/sec, +2.2 cm/sec). The mean value of early deceleration was 4.3 m/sec2 +/- 1.5 with the MR method and 4.0 m/sec2 +/- 1.5 with echocardiography (P = .073). The mean difference was -0.4 cm/sec2 (-0.92 cm/sec2, +0.05 cm/sec2). The mean value of peak atriosystolic velocity was 51 cm/sec +/- 14.6 with the MR method and 62 cm/sec +/- 17.2 with echocardiography (P = .002). The mean difference was -11 cm/sec (-18.1 cm/sec, -3.45 cm/sec). Peak atrial filling was consistently lower with the MR method than with echocardiography. Time-averaged measurements of ventricular filling with MR velocity mapping are an accurate measure of early diastolic filling but underestimate the velocity of atriosystolic filling.

Adult↗
PubMedDate not supplied

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.

Adult↗
PubMedDate not supplied

Measurement of regional aortic compliance by MR imaging: a study of reproducibility.

The reproducibility of MR imaging for the measurement of aortic compliance was studied in 47 healthy volunteers. Long and short term reproducibility and intraobserver variability were tested. The method was modified to improve image quality and short term reproducibility and intraobserver variability retested. For comparison, spin echo imaging was compared with cine gradient echo imaging. Initial long term reproducibility showed a mean difference (+/-SE) of 3% (+/- 7%) with 95% confidence interval (CI) for limits of agreement of +/- 69%. Short term reproducibility (7% +/- 6%, 95% CI +/- 46%) and intraobserver variability (1% +/- 2%, 95% CI +/- 31%) were better. After modification of the technique and optimization of image quality, both short term reproducibility and intraobserver variability improved (0% +/- 3%, 95% CI +/- 17% and 5% +/- 2%, 95% CI +/- 16% respectively). Aortic compliance can be measured using spin echo MR imaging with good reproducibility provided care is taken to obtain good quality images with high spatial resolution.

Adult↗
PubMedDate not supplied

Dynamic range extension of cine velocity measurements using motion-registered spatiotemporal phase unwrapping.

A motion-registered spatiotemporal phase-unwrapping method for extending the dynamic range of cine magnetic resonance phase velocity measurements is presented. The interframe cardiac movement is estimated from the magnitude image derived from the velocity encoded raw data, which ensures that the phase signal is unwrapped in the temporal direction with reference to pixels belonging to the same anatomic flow region. An extra step of spatial phase correction is then used to further eliminate any residual errors. The combination of spatial and temporal information for phase unwrapping provides a robust way of extending the dynamic range of cine velocity data in the presence of large phase aliasing errors.

Algorithms↗
PubMedDate not supplied

MR navigator-echo monitoring of temporal changes in diaphragm position: implications for MR coronary angiography.

Temporal changes in respiration could influence navigator-echo (NE)-gated MR coronary angiography (MRCA), but systematic investigation of the effects of such variations and how to limit them has not been performed. We addressed these issues by studying the influence of time in the magnet on diaphragm position and respiratory patterns using NE diaphragm monitoring in volunteers and a phantom model. NE diaphragm monitoring was performed at .5 T in 10 subjects over a total period of 35 minutes. The end-expiratory position was sustained for longer (1.1 vs .4 seconds, P < .001) and with greater position stability (SD 1.9 vs 5.9 mm, P = .01) than the end-inspiratory position. Drift of the end-expiratory position occurred over time, causing a fall in scan efficiency (44-28%, P = .01). Up-drift of the end-expiratory position was most common. Loss of scan efficiency was worse with up-drift because of loss of the end-expiratory pause from the NE window (up-drift 10% mm-1, down-drift 7% mm-1, both P = .03). Scan efficiency also was reduced during sleep (to a nadir of 0%), secondary to loss of the end-expiratory pause, periodic breathing with oscillating end-expiratory position, and periods of apnea. The phantom model used actual diaphragm traces to evaluate the artifact resulting from diaphragm motion during acquisition. Artifact was considerably reduced by NE adaptive motion correction compared with NE gating alone (ghosting ratio 2.0 vs 2.8, P < .01). Artifact also was significantly reduced with up-drift if scan efficiency was maintained above 35% (P = .05). For optimal NE-gated MRCA, the following features are important: the NE window should be placed around the end-expiratory position; subjects should not sleep; scan efficiency should be monitored and the NE window should be repositioned if scan efficiency falls below 35%; and adaptive motion correction should be used.

Adult↗
PubMedDate not supplied

Cine MR Fourier velocimetry of blood flow through cardiac valves: comparison with Doppler echocardiography.

Noninvasive measurement of blood flow velocity through the cardiac valves has important clinical applications. A wide variety of MR methods are available for flow measurement. The aim of this study was to investigate the ability of cine MR Fourier velocimetry to measure flow through healthy cardiac valves and to compare MR and Doppler peak velocity measurements. Ten healthy volunteers (age mean +/- SD, 24 +/- 4 years) without history of valvular disease were studied. Four of the subjects were females. In each subject, aortic, pulmonary, mitral, and tricuspid valves were evaluated with MR and Doppler imaging. A whole-body mobile MR machine was used, operating at .5-T with actively shielded magnetic field gradient coils on all three axes capable of 20 mT/m at a slew rate of 60 mT/ m/msec. The heart rate during MR and Doppler studies was not significantly different. The mean difference between the two studies was 2 beats/min, with a 95% confidence interval of -22 beats/min, +25 beats/ min. Peak systolic flow velocity in the aortic and pulmonary valves and peak diastolic flow velocity in the mitral and tricuspid valves measured with MRI and Doppler echocardiography correlated well. The mean difference between the two measurements (MR-Doppler) was 63 mm/sec, with a 95% confidence interval of -180 mm/sec, +310 mm/sec. The agreement between two observers interpreting the same MR velocity maps was close. The mean difference between their two measurements was 23 mm/sec, with a 95% confidence interval of -20 mm/sec, +60 mm/sec. There was no significant difference between MR and Doppler imaging or between the two MR observers. MR Fourier velocimetry has the necessary ease, reliability, and speed to measure blood flow through the cardiac valves, although measurement of late diastolic flow in the atrioventricular valves is limited. Measurement of peak blood velocity through the cardiac valves by this method showed satisfactory agreement with Doppler, but its clinical application for assessing diseased cardiac valves must be established.

Adult↗
PubMedDate not supplied

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.

Adult↗
PubMedDate not supplied

Hybrid ordered phase encoding (HOPE): an improved approach for respiratory artifact reduction.

Respiration causes continuous change in cardiac position, which leads to image degradation. Phase-encode reordering methods are often used to reduce these artifacts. An improved method for suppressing motion artifacts by reordering the acquisition of k space has been developed that is less sensitive to change of breathing patterns and bulk movement. We describe the theory behind the new approach and compare its results with those of existing methods by use of a phantom with simulated and actual acquired breathing patterns. The comparison was also made in vivo; cardiac scans were performed in 15 subjects with image planes that are known to be particularly susceptible to respiratory artifact. A significant improvement in image quality was achieved compared with conventional nonreordered and existing reordering methods.

Algorithms↗
PubMedDate not supplied

Comparison of fast spiral, echo planar, and fast low-angle shot MRI for cardiac volumetry at .5T.

The application of fast imaging is necessary to reduce the scanning time for cardiac volumetric MRI. Fast spiral, echo planar imaging (EPI), and fast low-angle shot (FLASH) imaging are rapid MRI techniques that allow image acquisition within a fraction of a second. Performed as a multi-shot technique, breath-hold imaging with high temporal and spatial resolution is feasible. This study evaluated the accuracy of interleaved spiral, EPI, and FLASH imaging for measuring ventricular volume and mass at .5T. Breath-hold short-axis cines in parallel planes covering both ventricles were acquired in 16 volunteers with all three fast methods, as well as with conventional gradient-echo imaging for comparison. All fast techniques showed good agreement with conventional imaging. Despite its lower temporal resolution, FLASH imaging yielded higher image quality than EPI and spiral, making FLASH more reliable and suggesting that at .5T, it is the method of choice for rapid cardiac volumetric imaging.

Adult↗
PubMedDate not supplied

Combined MRI and CFD analysis of fully developed steady and pulsatile laminar flow through a bend.

A combined MR and computational fluid dynamics (CFD) study is made of flow in a simple phantom laboratory flow rig consisting of a 180 degree bend with straight entry and exit sections. The aim was to investigate the potential of the use of MRI-linked CFD simulations for in vivo use. To this end, the experiment was set up for both steady and pulsatile laminar flow conditions, with Reynolds and Dean numbers and Womersley pulsatility parameter representative of resting flow in the human aorta. The geometrical images of the pipe and the velocity images at entry to the bend were used as boundary conditions for CFD simulations of the flow. The CFD results for both steady and pulsatile cases compared favorably with velocity images obtained at exit from the bend. Additional information such as pressure and wall shear stress, which either could not be measured adequately via MRI, or could not be measured at all, was also extracted from the simulation. Overall, the results were sufficiently promising to justify pursuing subsequent in vivo studies.

Aorta, Thoracic↗
PubMedDate not supplied

Blood flow imaging by cine magnetic resonance.

A technique for measuring blood flow by whole body nuclear magnetic resonance is described. This method uses imaging gradient profiles that combine even echo rephasing with a field echo sequence to overcome the problem of signal loss from flowing blood. The flow velocity component in any desired direction may be measured by appropriate gradient profile modifications, producing velocity dependent phase shifts that can be displayed by phase mapping. The sequence allows for fast repetition so that flow information may be acquired rapidly from many points in the cardiac cycle and has been used in this mode to observe and measure blood flow in the heart chambers and great vessels. Flow measurements in the femoral artery were also carried out using the same technique; these were compared with similar measurements obtained by Doppler ultrasound. The technique can readily be applied using standard imaging equipment and should prove useful in the clinical assessment of many diseases of the cardiovascular system.

Blood Flow Velocity↗
PubMedDate not supplied

In vivo validation of MR velocity imaging.

Calculations of left ventricular stroke volume obtained by summing the areas of multiple contiguous transverse magnetic resonance (MR) slices in systole and diastole using a spin echo sequence have been compared in 10 healthy volunteers with the stroke output derived from velocity maps in the ascending aorta using a field even-echo rephasing sequence. The results gave a correlation coefficient of 0.97 (p less than 0.001) and a standard error of estimate of 3.2 ml. Velocity maps have also been obtained in the pulmonary artery, the descending aorta, and the superior vena cava. The accuracy of this technique and the theoretical limitations of MR measurements have implications for the earlier detection of atheroma in the coronary and other arteries.

Aorta↗
PubMedDate not supplied

Diagnosis of femoral arteriovenous fistula by MR imaging with velocity mapping.

A case of suspected femoral arteriovenous fistula after cardiac catherization remained undiagnosed because of inability to obtain access for arteriography. Magnetic resonance imaging with phase-shift velocity mapping allowed a noninvasive diagnosis and qualitative evaluation of the lesion.

Arteriovenous Fistula↗
PubMedDate not supplied

Automatic enhancement, animation, and segmentation of flow in peripheral arteries from MR phase-shift velocity mapping.

The authors describe a technique for automatic enhancement, segmentation, and analysis of flow from MR phase shift velocity mapping. This method uses a specific flow related enhancement process to extract and localize the flow field within the image, followed by global flow analysis. By animation on low cost equipment (personal computers), this method can provide various ways of visualizing flow in vivo and its variation in different structures and lesions. Our preliminary results using this method for analyzing flow data in the aortic bifurcation of healthy volunteers are encouraging.

Aorta, Abdominal↗
PubMedDate not supplied

Vortical flow feature recognition: a topological study of in vivo flow patterns using MR velocity mapping.

PURPOSE: To describe an automatic method for studying vortical flow features using multidirectional magnetic resonance velocity imaging. METHODS: The technique relied on the use of linear differential equations to provide local approximation to the acquired MR velocity data and to identify vortical flow patterns that were characterised by their circular or swirling motion. Two directional MR velocity images were acquired on a 0.5 T system with a TE of 14 ms, slice thickness of 10 mm, and field of view of 30-40 cm. RESULTS: The method was validated using numerical simulation, and the potential of this technique for studying blood flow features was demonstrated in the ascending aorta of a healthy volunteer and that of a patient with Marfan's syndrome. The difference in vortex flow dynamics of the left ventricle between a normal subject and a patient with dilated ventricle was also compared. CONCLUSION: The approach of detecting critical flow features prior to analysing dynamical indices of the fluid is suited to the topological study of complex flow patterns depicted by MR velocity mapping techniques.

Algorithms↗