Pulse sequence design for MR velocity mapping of complex flow: Notes on the necessity of low echo times

Lowering of the echo time (TE) has been proposed as a way to reduce effects of phase dispersion in MR velocity mapping, because a low TE reduces sensitivity to higher-order motion terms while first-order velocity sensitivity is maintained. Methods of lowering TE involves the use of extreme gradient ramp times and gradient strengths as well as reduction of the duration of transmit/receive windows, the latter method causing decrements in image resolution. When reducing higher-order sensitivity, however, it is not the overall TE that is the critical parameter, but rather the time pattern of the gradients used in the experiment. Hence, changes in TE without subsequent variations in gradient pattern would, according to theory, not affect quantitative measurements of complex flow and vice versa. In this study, we experimentally demonstrate this relation and utilize the experience to create a sequence robust towards complex flow without sacrifices in image resolution. Our experimental observations show that variations in TE alone while maintaining the time course of the velocity-encoding gradient does not significantly affect measurements of through-plane average complex flow in the studied velocity range. A parameter that cannot be measured as accurately if TE is increased is the peak flow. A phase mapping sequence with prolonged TE from 3 ms to 5 ms but with short duration of the velocity-encoding (section-selective) gradient and improved in-plane resolution was demonstrated in vivo.     

Recovery of signal loss due to an in-plane susceptibility gradient in the gradient echo EPI through acquisition of extended phase-encoding lines

In gradient echo imaging the in-plane susceptibility gradient causes an echo shift which results in signal loss. The loss of signal becomes more severe in gradient echo EPI, due to the low amplitude of the gradient which is applied in the phase-encoding direction during a long echo train. As the readout gradient amplitude is set to be very high in gradient echo EPI, the echo shift in the readout direction is negligible compared to that in the phase-encoding direction. Traditionally, a z-shimming technique has been applied to the phase-encoding direction of gradient echo EPI to restore the lost signal. This technique, however, requires a significant increase of scan time, as is also the case with the through-plane z-shimming technique. A new approach that allows one to restore the lost signal is to acquire additional phase-encoding lines beyond the regular phase-encoding range. The extension of the phase-encoding lines prior to the regular phase-encoding range exploits the delay time for optimum echo time of the BOLD sensitivity. Therefore, scan time is increased only for the extended phase-encoding lines posterior to the regular phase-encoding range. This technique has been confirmed experimentally by imaging human subject's heads at 3T.     

Determination of Velocity Autocorrelation Functions by Multiple Data Acquisition in NMR Pulsed-Field Gradient Experiments

An extension of NMR pulsed-field gradient experiments toward the generation, acquisition, and analysis of multiple echoes is presented. In contrast to currently used measurements where a single or double encoding of displacements by gradient pulses is followed by an acquisition of the echo signal at the end of the sequence, sampling and analyzing the intermediately occurring echoes allows a direct distinction between coherent and dispersive contributions to fluid motion without additional referencing measurements. It is shown that a series of gradient pulse pairs, leading to a train of echoes, can be employed to map the time-dependence of the velocity autocorrelation function between displacements within a single experiment for a system undergoing flow or motion.     

A method for MR quantification of flow velocities in blood and CSF using interleaved gradient-echo pulse sequences

The aim of this study was to establish a rapid method for in vivo quantification of a large range of flow velocities using phase information. A basic gradient-echo sequence was constructed, in which flow was encoded along the slice selection direction by variation of the amplitude of a bipolar gradient without changes in sequence timings. The influence of field inhomogeneities and eddy currents was studied in a 1.5 T scanner. From the basic sequence, interleaved sequences for calibration and in vivo flow determination were constructed, and flow information was obtained by pairwise subtraction of velocity-encoded from velocity non-encoded phase images. Calibration was performed in a nongated mode using flow phantoms, and the results were compared with theoretically calculated encoding efficiencies. In vivo flow was studied in healthy volunteers in three different areas using cardiac gating; central blood flow in the great thoracic vessels, peripheral blood flow in the popliteal vessels, and flow of cerebrospinal fluid (CSF) in the cerebral aqueduct. The results show good agreement with results obtained with other techniques. The proposed method for flow determination was shown to be rapid and flexible, and we thus conclude that it seems well suited for routine clinical MR examinations.     

Application of linear optimization techniques to MRI phase contrast blood flow measurements

The goal of this study was to use linear optimization techniques as a systematic method of cine phase contrast pulse sequence design and to apply this technique to the measurement of blood flow in vivo. The optimized waveforms were validated in a constant flow phantom with average velocities ranging from 5 to 50 cm/s. The same optimized sequence was also run in a segmented k-space variation with five phase encoding lines per segment. The magnetic resonance (MR) derived velocity measurements were accurate over the entire range of velocities tested (p < .05) in both cases. The same optimized pulse sequence was applied to the measurement of flow in the main pulmonary artery of five normal volunteers and compared with stroke volumes and cardiac outputs calculated from right ventricular volume measurements. These measurements showed a mean difference between the MR phase contrast calculated stroke volume and the volumetric stroke volume measurement of 9.8 ± 11.6%. The mean difference between the calculated phase contrast cardiac output and the volumetric cardiac output was 4.4 ± 10%. These results imply that optimization techniques are an efficient method for designing cine phase contrast pulse sequences.     

Cardiac T2 measurements in patients with iron overload: a comparison of imaging parameters and analysis techniques

Measurement of cardiac T2 has emerged as an important tool to noninvasively quantify cardiac iron concentration in order to detect preclinical evidence of toxic levels and titrate chelation therapy. However, there exists variation among practitioners in cardiac T2 measurement methods. This study examines the impact of different imaging parameters and data analysis techniques on the calculated cardiac R2 (1/T2) in patients at risk for cardiac siderosis. The study group consisted of 36 patients with thalassemia syndromes who had undergone clinical magnetic resonance imaging assessment of cardiac siderosis using a standardized protocol and who were selected to yield a broad range of cardiac R2 values. Cardiac R2 measurements were performed on a 1.5-T scanner using an electrocardiogram-gated, segmented, multiecho gradient echo sequence obtained in a single breath-hold. R2 was calculated from the signal intensity versus echo time data in the ventricular septum on a single midventricular short-axis slice. There was good agreement between R2 measured with a blood suppression prepulse (black blood technique) and without (mean difference 6.0 ± 10.7 Hz). The black blood technique had superior within-study reproducibility (R2 mean difference 1.6 ± 8.6 Hz versus 2.7 ± 14.6 Hz) and better interobserver agreement (R2 mean difference 3.4 ± 8.2 Hz versus 8.3 ± 16.5 Hz). With the same minimum echo time, the use of small (1.0 ms) versus large (2.2 ms) echo spacing had minimal impact on cardiac R2 (mean difference 0.3 ± 8.7 Hz). The application of a region-of-interest-based versus a pixel-based data analysis also had little effect on cardiac R2 calculation (mean difference 8.4 ± 6.9 Hz). With black blood images, fitting the signal curve to a monoexponential decay or to a monoexponential decay with a constant offset yielded similar R2 values (mean difference 3.4 ± 8.1 Hz). In conclusion, the addition of a blood suppression prepulse for cardiac R2 measurement yields similar R2 values and improves reproducibility and interobserver agreement. The findings regarding other variations may be helpful in establishing a broadly accepted imaging and analysis technique for cardiac R2 calculation.     

NMR velocimetry with 13-interval stimulated echo multi-slice imaging in natural porous media under low flow rates

Characterization and quantification of root water uptake processes play a key role in understanding and managing the effects of global climate change on agricultural production and ecosystem dynamics. Part of this understanding is related to the flow of water towards plant roots in soils. In this study we demonstrate for the first time, to our knowledge, that fluid flow in the voids of the pore space of a model soil system (natural sand) can be detected and mapped to an NMR image for mean flows as low as 0.06 mm/s even under the influence of internal magnetic field gradients. To accomplish this we combined multi-slice imaging with a 13-interval pulse sequence to the NMR pulse sequence 13-interval stimulated echo multi-slice imaging (13-interval STEMSI). The result is a largely reduced influence of the internal magnetic field gradients, leading to an improved signal-to-noise ratio which in turn enables one to acquire velocity maps where conventional stimulated echo methods fail.     

Rapid imaging of fluid flow patterns in a narrow packed bed using MRI

A gradient echo rapid velocity and acceleration imaging sequence (GERVAIS) has been developed and implemented to image liquid flow within a narrow packed bed. Two-dimensional velocity images have been acquired with an in-plane pixel size of 781 microm x 781 microm, with a data acquisition time of 20 ms for a single velocity component. Images of the x, y and z velocity vectors are reported. Data are reported for Reynolds numbers (based on particle diameter) of 200 and 300. In each case, GERVAIS images are compared with the results of a standard spin-echo phase-encoding velocity measurement. At Re = 200, steady-state flow is expected and the velocity images acquired using both techniques are consistent. At Re = 300, the GERVAIS sequence is able to image the unsteady-state flow field within this system. In contrast, the standard phase-encoding velocity measurement contains significant artefacts.     

Pulse sequence optimization for T2-weighted MR imaging of the brain

The authors implemented bipolar velocity compensated pulse techniques for T2-weighted MR imaging of the brain. Signal-to-noise (S/N) and image quality was compared for pulse sequences with standard and optimized RF pulses, low and regular bandwidth versions and cardiac triggering. Images from bipolar velocity compensated sequences allowed better visualization of vessels and basilar cisterns and improved image quality relative to standard sequences without velocity compensation. The implementation of optimized RF pulses with bipolar sequences resulted in further improvement in image quality. Single echo sequences consistently had improved image quality and signal-to-noise relative to the second echo of a double echo sequence. Low bandwidth bipolar sequences with extended sampling period had 30% higher S/N, but at the cost of slight loss in edge definition. The highest image quality was obtained with the bipolar, optimized RF, single echo sequence. Using this technique contiguous high quality image slices could be obtained with velocity compensation. The addition of cardiac triggering to bipolar sequences resulted in slight improvement in image quality, but this difference was marginal and probably rarely necessary for MR imaging of the brain.     

Evaluation of steady and pulsatile flow with dynamic MRI using limited flip angles and gradient refocused echoes

Magnetic resonance imaging sequences utilizing limited flip angles and gradient echoes yield rapid (less than 2 min) dynamic images of the cardiovascular system. These images contain both accurate anatomical and functional information. Using a gradient refocused acquisition in the steady state (GRASS) in the CINE mode, we studied the relationship between gradient echo signal intensity and velocity of steady and pulsatile flow in a phantom simulating medium to large vessels. Images were acquired on a 1.5 Tesla system (repetition TIME = 21 ms, echo TIME = 12 ms, flip ANGLE = 30 degrees). Data from each pulse interval were sorted in 16 images. Signal intensities from flow tube lumina and surrounding stationary water jacket were used to calculate contrast ratios which were compared to velocity measurements made with electromagnetic (EM) flow probes outside the magnet room. During steady flow, signal intensity contrast ratios increased with increasing flow and in a 10 mm thick slice, reached a peak at 48 cm/s, and declined for velocities up to 90 cm/s. Changes in instantaneous velocity during pulsatile flow correlated well (r > .88) with signal intensity changes up to a maximum mean velocity of 17 cm/s. Total signal intensity from the lumen for an “R to R” interval correlated extremely well (r > .97) with mean pulsatile flow velocities up to 30 cm/s. The excellent correlation between gradient echo signal intensity and actual flow velocities suggests that this imaging sequence might be useful for evaluating normal and pathologic flow phenomena.     

Quantitative comparison between a multiecho sequence and a single-echo sequence for susceptibility-weighted phase imaging

The aim of this study was to investigate the benefits arising from the use of a multiecho sequence for susceptibility-weighted phase imaging using a quantitative comparison with a standard single-echo acquisition. Four healthy adult volunteers were imaged on a clinical 3-T system using a protocol comprising two different three-dimensional susceptibility-weighted gradient-echo sequences: a standard single-echo sequence and a multiecho sequence. Both sequences were repeated twice in order to evaluate the local noise contribution by a subtraction of the two acquisitions. For the multiecho sequence, the phase information from each echo was independently unwrapped, and the background field contribution was removed using either homodyne filtering or the projection onto dipole fields method. The phase information from all echoes was then combined using a weighted linear regression. R2 maps were also calculated from the multiecho acquisitions. The noise standard deviation in the reconstructed phase images was evaluated for six manually segmented regions of interest (frontal white matter, posterior white matter, globus pallidus, putamen, caudate nucleus and lateral ventricle). The use of the multiecho sequence for susceptibility-weighted phase imaging led to a reduction of the noise standard deviation for all subjects and all regions of interest investigated in comparison to the reference single-echo acquisition. On average, the noise reduction ranged from 18.4% for the globus pallidus to 47.9% for the lateral ventricle. In addition, the amount of noise reduction was found to be strongly inversely correlated to the estimated R2 value (R=-0.92). In conclusion, the use of a multiecho sequence is an effective way to decrease the noise contribution in susceptibility-weighted phase images, while preserving both contrast and acquisition time. The proposed approach additionally permits the calculation of R2 maps.     

Velocity distribution of slow fluid flows in bentheimer sandstone: An NMRI and propagator study

This work addresses the special problems of measuring flow velocity distributions in rock by NMR methods. Specifically, these problems are to measure very slow flows as well as flows in the presence of background magnetic field gradients caused by heterogeneities of the rock. We modify a stimulated echo sequence for use in diffusion measurements, in order to maximize velocity sensitivity and minimize background gradient effects. Accurate velocity images of Soltrol 220 oil in sandstone were made for flow velocities up to around 0.04 mm/s with an imager that does not have echo-planar capability. Accurate velocity distributions by the propagator method can be obtained even with stimulated echo delays of 1.9 T₁ by phase cycling combined with suitable crusher gradients.     

Magnetic resonance visualisation of single- and two-phase flow in porous media.

Three-dimensional MRI and flow visualisation data are presented for single and two-phase flow occurring within packed beds of glass spheres. The initial motivation for this work has been to understand the operation of fixed-bed reactors used in many chemical processing operations; these systems also serve as model porous media in which to investigate the effect of the structure of a pore space on the flow phenomena occurring within it. For the case of single-phase flow, maps of the liquid shear rate components are calculated from which forces on individual spheres within the bed are obtained. The velocity histogram for flow transverse to the direction of superficial flow is exponential in both negative and positive directions. This form of the velocity histogram implies an exponential form for the displacement propagator, in contrast to the Gaussian distribution obtained by pulsed gradient spin echo measurements. This difference arises because the spatially resolved velocity imaging sequence measures only the average velocity within each voxel and is insensitive to the effects of incoherent (diffusive) motion. Visualisations of air-water flow through a sphere pack are also reported and the capability of MRI to yield information on rivulet formation and surface wetting characteristics is illustrated.     

Flow compensation for the fast spin echo triple-echo Dixon sequence

The fast spin echo (FSE) triple-echo Dixon (FTED) sequence uses bipolar triple-echo readout during each echo-spacing period of FSE to collect all the images necessary for Dixon water and fat separation in a single scan. In comparison to other FSE implementations of the Dixon technique, the triple echo readout used in FTED incurs minimal deadtime in the pulse sequence design and thus greatly enhances the overall scan efficiency. A potential drawback of FTED is that the time dependence of the gradient moment along the frequency encode direction becomes more complicated than in FSE and flow compensation based on the gradient moment (GM) nulling is difficult to achieve. In this work, the first order GM along the frequency encode direction of FTED was examined and two different methods to minimize the GM were proposed. The first method nulls the GM at all the locations of the refocusing radiofrequency pulses so that the Carr-Purcell Meiboom-Gill condition is always maintained. The second method minimizes the GM of the spin echo component of the FSE signal at the echo locations. The efficacy of both methods in reducing the first order GM and flow-related artefacts was demonstrated both in phantom and in images in vivo.     

Rapid spectroscopic velocity quantification using periodically oscillating gradients

In this work two spectroscopic methods are described which allow rapid flow velocity quantification in the presence of a parabolic velocity distribution. This method requires only a single excitation and is based on flow encoding by periodically oscillating gradients. In the shown spin echo variant additional refocusing pulses correct for field inhomogeneities. A theoretical model is introduced, which describes the course of the derived spectra even in high flow region, where a significant part of the encoded spins leaves the sensitive area of the coil during data acquisition (outflow-effect). It was demonstrated that both methods can quantify flow velocities within the velocity range of 1mm/s up to 36 cm/s in the presence of a parabolic flow velocity distribution. The maximum velocity of the parabolic distribution is indicated in this method by a peak in the acquired spectrum from which the velocity could be quantified. Flow velocity quantification by periodically oscillating gradients seems a reasonable and fast alternative to established imaging techniques.     

The generating functions formalism for the analysis of spin response to the periodic trains of RF pulses. Echo sequences with arbitrary refocusing angles and resonance offsets

The generating functions (GF) formalism was applied for calculation of spin density matrix evolution under the influence of periodic trains of RF pulses. It was shown that in a general case, closed expression for the generating function can be found that allows in many cases to derive analytical expressions for the generating function of spin density matrix (magnetization, coherences). This approach was shown to be particularly efficient for the analysis of multi-echo sequences, where one has to average over various frequency isochromats. The explicit analytical expressions for the generating function for echo amplitudes in a Carr–Purcell–Meiboom–Gill (CPMG) echo sequence, a multiecho sequence with incremental phase of refocusing pulse, a gradient echo sequence including transient period were obtained for an arbitrary flip angle and an arbitrary resonance offset. Comparison of the theory and the spin-echo experiments was done, demonstrating a good agreement.     

Accurate velocity mapping with FAcE

Spin dislocation between the slice selection, phase encoding, and frequency encoding is a source of image distortions. Two strategies can be pursued to improve the appearance of moving spins in an image. Either the sequence is made equally sensitive to velocity-dependent dislocation artifacts for all spatial directions or the sensitivity is reduced with a shorter echo time. The first approach increases the dislocation for the phase-encoding direction and is therefore not useful if velocity maps with minimal distortion are the goal. FAcE (FID acquired echoes) is a sequence with separate sampling of the left and right k-space half-planes that allows for very short echo times. It was applied for velocity mapping of flow in the slice select direction. Special attention was paid to a compact design of the velocity-encoding select gradient to achieve short echo times even with high velocity sensitivity. Artifacts introduced by in-plane motion were studied for FAcE and conventional gradient-echo sequences, both in phantom experiments and simulation. FAcE allows for very short echo times with inherent motion compensation of the frequency-encoding gradient. Thus, both motion-related dislocation artifacts and signal voids due to coherence loss in regions with irregular flow are minimal.     

On the accuracy of EPI-based phase contrast velocimetry

For over a decade, echo-planar imaging (EPI) has been used in both the medical and applied sciences to capture velocity fields of fluid flows. However, previous studies have not rigorously confirmed the accuracy of the measurements or sought to understand the limitations of the technique. In this study, a bipolar gradient was added to a flow-compensated EPI pulse sequence to obtain rapid phase contrast images of steady and unsteady flows through two step stenoses. For steady Re = 100 and 258 flows, accuracy was measured through systematic comparisons with CFD simulations, mass flow rate measurements, and spin echo phase contrast images. On average, the EPI image data exhibited velocity errors of 5 to 10 percent, while mass was conserved to within 5.6 percent at each axial position. Compared to spin-echo phase contrast images, the EPI images have 50 percent lower signal-to-noise ratio, larger local velocity errors, and similar mass conservation characteristics. An unsteady flow was then examined by starting a pump and allowing it to reach a steady Re = 100 flow. Accuracy in this case was measured by the consistency between mass flow rate measurements at different axial positions. Images taken at 0.3 s intervals captured the velocity field evolution and showed that 50 to 100 percent errors occur when the flow changes on a time scale faster than the image acquisition time.     

Echo-spacing optimization for the simultaneous measurement of reversible (R2') and irreversible (R2) transverse relaxation rates

Accurate measurement of reversible (R2') and irreversible (R2) transverse relaxation rates plays a key role in various magnetic resonance imaging research and applications. Although optimization of echo spacing for a multiecho pulse sequence measuring a single exponential decay has been investigated, optimization in sequences such as Gradient-Echo Sampling of Free Induction Decay and Echo (GESFIDE), in which two echo trains are simultaneously measured to obtain both R2 and R2', has not been reported. In this work, optimum echo spacings for the GESFIDE sequence are determined to improve the accuracy of measured relaxation parameters. Various relaxation rates and the number of acquired echoes are considered, as well as whether the receiver bandwidth is kept fixed or is varied with echo spacing. In the case of constant receiver bandwidth, results show that the echo train length approximately equal to T2* should be used for each echo train in GESFIDE to minimize uncertainty in R2 or R2'. If the receiver bandwidth is allowed to change with echo spacing in order to maximize the image signal-to-noise ratio, the optimum echo train length will vary, generally increasing with the number of echoes.