B1-insensitive fast spin echo using adiabatic square wave enabling of the echo train (SWEET) excitation

Abdominal images at 3T acquired with fast spin echo (FSE) sequences often exhibit signal voids due to RF transmit field inhomogeneities. Theory suggests, however, that the repeated refocusing pulses of FSE are capable of maintaining signal even at reduced RF amplitudes if the magnetization is suitably prepared. Here we propose a modified excitation strategy for FSE that is more robust to transmit field inhomogeneities than conventional FSE. The new excitation approach replaces the standard 90 degrees excitation pulse with a discretely sampled hyperbolic secant pulse that creates a square wave longitudinal magnetization as a function of gradient and off-resonance induced phase shifts between the subsequent echoes of the FSE sequence. This pulse is followed by the conventional train of refocusing pulses except that the first few pulses increase from near zero to the desired refocusing amplitude. Simulations and in vivo results at 3T indicate preserved image quality and much greater robustness of this new sequence to nonuniform RF fields. This robustness comes at the cost of 20% reduction in signal when the RF field is uniform and increased motion sensitivity. This RF field-insensitive sequence may overcome challenges of body imaging at high field and in patients with ascites.     

Concomitant magnetic-field-induced artifacts in axial echo planar imaging

When a linear magnetic field gradient is used, spatially higher-order magnetic fields are produced to satisfy the Maxwell equations. It has been observed that the higher-order magnetic field produced by the readout gradient causes axial echo planar images acquired with a horizontal solenoid magnet to shift along the phase-encoding direction and lose image intensities. Both the shift and intensity reduction become increasingly severe as the slice offset from the isocenter increases. These phenomena are quantitatively analyzed, and good correlation between experiments and theory has been established. The analysis also predicts a previously unre-ported Nyquist ghost on images with very large slice offsets. This ghost has been verified with computer simulations. Based on the analysis, several methods have been developed to eliminate the image shift, the intensity reduction, and the ghost. Selected methods have been implemented on a commercial scanner and proved effective in removing these image artifacts.     

Use of fast spin echo for phase shift magnetic resonance thermometry

PURPOSE: To propose a modified fast spin echo (FSE) magnetic resonance imaging sequence for MR thermometry, employing the proton resonance frequency (PRF) shift by means of MR phase maps. Despite their obvious advantages of speed and high signal-to-noise ratio (SNR), FSE sequences have not until now been used for this purpose due to the restraints imposed by the Carr-Purcell-Meiboom-Gill (CPMG) conditions. MATERIALS AND METHODS: The new FSE combines a new phase modulation scheme that maintains magnetization that ordinarily is destroyed under CPMG conditions, while employing conventional FSE gradient waveforms. The echoes are read in a single shot using 128 readouts in 650 msec, with a phase sensitive preparation using an optional time shift tau before the start of the refocusing gradient waveforms. This feature allows the quantification of temperature dependent phase shifts. We tested the sequence by imaging a heated agar gel phantom while cooling, using different values for tau. RESULTS: There was good correlation between FSE and fiberoptic-based temperature measurements in the phantom(r(2) >or= 0.95). Temperature sensitivity could be adjusted by varying the tau value. CONCLUSION: With the proposed non-CPMG FSE sequence it is feasible to quantify temperature changes by means of the PRF shift.     

Conventional and fast spin-echo MR imaging: Minimizing echo time

Magnetic resonance imaging is frequently complicated by the presence of motion and susceptibility gradients. Also, some biologic tissues have short T2s. These problems are particularly troublesome in fast spin-echo (FSE) imaging, in which T2 decay and motion between echoes result in image blurring and ghost artifacts. The authors reduced TE in conventional spin-echo (SE) imaging to 5 msec and echo spacing (E-space) in FSE imaging to 6 msec. All magnetic gradients (except readout) were kept at a maximum, with data sampling as fast as 125 kHz and only ramp waveforms used. Truncated sine radio-frequency pulses and asymmetric echo sampling were also used in SE imaging. Short TE (5.8 msec) SE images of the upper abdomen were compared with conventional SE images (TE =11 msec). Also, FSE images with short E-space were compared with conventional FSE images in multiple body sites. Short TE significantly improved the liver-spleen contrast-to-total noise ratio (C/N) (7.9 vs 4.1, n = 9, P <.01) on T1-weighted SE images, reduced the intensity of ghost artifacts (by 34%, P <.02), and increased the number of available imaging planes by 30%. It also improved delineation of cranial nerves and reduced susceptibility artifacts. On short E-space FSE images, spine, lung, upper abdomen, and musculoskeletal tissues appeared crisper and measured spleen-liver C/N increased significantly (6.9 vs 4.0, n = 12, P <.01). The delineation of tissues with short T2 (eg, cartilage) and motion artifact suppression were also improved. Short TE methods can improve image quality in both SE and FSE imaging and merit further clinical evaluation.     

Single‐shot 3D GRASE with cylindrical k‐space trajectories

Development of GRASE (gradient‐ and spin‐echo) pulse sequences for single‐shot 3D imaging has been motivated by physiologic studies of the brain. The duration of echo‐planar imaging (EPI) subsequences between RF refocusing pulses in the GRASE sequence is determinant of image distortions and susceptibility artifacts. To reduce these artifacts the regular Cartesian trajectory is modified to a circular trajectory in 2D and a cylindrical trajectory in 3D for reduced echo train time. Incorporation of “fly‐back” trajectories lengthened the time of the subsequences and proportionally increased susceptibility artifact but the unipolar readout gradients eliminate all ghost artifacts. The modified cylindrical trajectory reduced susceptibility artifact and distortion artifact while raising the signal‐to‐noise ratio in both phantom and human brain images. Magn Reson Med 60:976–980, 2008. © 2008 Wiley‐Liss, Inc.     

Direct FLASH MR imaging of magnetic field inhomogeneities by gradient compensation

MR images based on gradient echoes are sensitive to artifacts caused by inhomogeneities of the static magnetic field. This paper describes the effects of local gradients in rapid FLASH MR images and presents a way of directly imaging affected areas. The idea is to compensate for signal losses due to mutual cancellation of dephased magnetizations by deliberate "misadjustments" of the refocusing part of the slice selection gradient. In contrast to conventional field imaging techniques no three-dimensional data acquisition or subsequent Fourier analysis is required to obtain images at a particular gradient strength. Conventional as well as inhomogeneity compensated FLASH images have been obtained on phantoms and human heads using a 2.35-T 40-cm magnet and a 1.5-T whole-body system, respectively.     

Respiratory effects in two-dimensional Fourier transform MR imaging

Respiratory and other regular motions during two-dimensional Fourier transform magnetic resonance imaging produce image artifacts consisting of local blurring and more or less regularly spaced "ghost" images propagating along the direction of the phase-encoding magnetic field gradient. The patterns of these ghost artifacts can be understood in terms of the technique of image production and basic properties of the discrete Fourier transform. This understanding permits, without respiratory gating, production of images of improved quality in body regions in which there is significant respiratory motion. In particular, the ghosts can be maximally separated from the primary image by choosing intervals between phase-encoding gradient pulse increments that are equal to one-half the respiratory period; they can be minimally separated by choosing an interval equal to the respiratory period. Increasing the number of signal averages between each phase-encoding increment decreases the intensity of the ghosts.     

Reduction of phase error ghosting artifacts in thin slice fast spin-echo imaging

Fast spin-echo (FSE) imaging techniques are very sensitive to the relative phase between the 90° (excitation) RF pulse and the 180° (refocusing) RF pulses. In this paper, it is demonstrated that a phase shift can be created between the excitation and refocusing pulses in such a manner that the received signal is divided into two components of distinctly different phase shifts. The nature of these two components is reviewed. It is demonstrated that ghosting artifacts will occur when images are reconstructed from this received signal. The ghosting is shown to be object dependent. A correction technique is presented which calculates the phase errors among different echoes based on measurements from a single echo train acquired without phase encoding gradients. The results in both phantom and human studies show that this method is capable of reducing the ghosting artifact in thin slice FSE images.     

Combining spin echoes with gradient echoes in the context of the global coherent free precession pulse sequence.

To extend the signal longevity of magnetically excited spins in flowing fluids while in a state of global coherent free precession (GCFP), a refocusing radiofrequency (RF) pulse and bipolar gradient waveforms were combined with the GCFP sequence. The data demonstrate that RF refocusing in the presence of flowing blood is possible, but the improvement in signal amplitude depends on the static magnetic field homogeneity along the direction of motion and the displacement of the spins between the excitation and the RF refocusing pulse, as well as displacement during subsequent RF refocusing pulses. The least amount of phase dispersion and thus the longest lasting signal is obtained with the shortest echo spacing where only one line of data is recorded between two RF refocusing pulses. This approach was successfully used in a phantom and in vivo to image fast and slow blood flow. Depending on the experimental conditions, signal persistence is improved significantly compared to playing the same sequence without RF refocusing, but the improvement is limited by the product of blood flow velocity and the time between RF refocusing pulses.     

Multishot diffusion-weighted FSE using PROPELLER MRI.

A method for obtaining diffusion-weighted images that are free from the artifacts associated with echo-planar acquisitions, such as signal pile-up and geometric warping, is introduced. It uses an ungated, multishot fast spin-echo (FSE) acquisition that is self-navigated. The phase of the refocusing pulses is alternated to minimize non-Carr-Purcell-Meiboom-Gill (CPMG) artifacts. Several reconstruction methods are combined to make this method robust against motion artifacts. Examples are shown of clinical diffusion-weighted imaging and high-resolution diffusion tensor imaging.     

Nonlinear phase correction for navigated diffusion imaging.

Motion during diffusion-weighted imaging (DWI) introduces phase errors that can cause significant artifacts in brain images. One method of correcting these errors uses additional navigator data to measure the phase corruptions. Standard navigator methods correct for rigid-body motion but cannot correct for nonrigid deformations of the brain related to the cardiac cycle. This work derives a generalized reconstruction that corrects for nonrigid motion based on a least-squares formulation. Since this reconstruction has the disadvantage of being computationally expensive, an approximation is presented, called a refocusing reconstruction. The refocusing reconstruction is both efficient and straightforward. Each readout is multiplied in image space by the phase conjugate of the navigator image, and these rephased readouts are then summed. The conditions under which the refocusing reconstruction is sufficient are considered and methods to improve the quality of refocused images are discussed. In particular, synchronization of the acquisition to the cardiac cycle can provide data that is well-conditioned to the refocusing reconstruction without incurring the large time penalty traditionally associated with cardiac gating. These methods are applied to steady-state DWI, a promising pulse sequence that is particularly sensitive to motion-induced phase artifacts. The refocusing reconstruction is shown to significantly improve SS-DWI over standard rigid-body corrections.     

Concomitant gradient terms in phase contrast MR: Analysis and correction

Whenever a linear gradient is activated, concomitant magnetic fields with non-linear spatial dependence result. This is a consequence of Maxwell's equations, i.e., within the imaging volume the magnetic field must have zero divergence, and has negligible curl. The concomitant, or Maxwell field has been described in the MRI literature for over 10 years. In this paper, we theoretically and experimentally show the existence of two additional lowest-order terms in the concomitant field, which we call cross-terms. The concomitant gradient cross-terms only arise when the longitudinal gradient G_z is simultaneously active with a transverse gradient (G_x or G_y). The effect of all of the concomitant gradient terms on phase contrast imaging is examined in detail. Several methods for reducing or eliminating phase errors arising from the concomitant magnetic field are described. The feasibility of a joint pulse sequence-reconstruction method, which requires no increase in minimum TE, is demonstrated. Since the lowest-order terms of the concomitant field are proportional to G²/B₀, the importance of concomitant gradient terms is expected to increase given the current interest in systems with stronger gradients and/or weaker main magnetic fields.     

Effect of vascular crushing on FAIR perfusion kinetics, using a BIR-4 pulse in a magnetization prepared FLASH sequence.

Flow-sensitive alternating inversion recovery (FAIR) perfusion imaging suffers from high vascular signal, resulting in artifacts and overestimation of perfusion. With TurboFLASH acquisition, crushing of vascular signal by bipolar gradients after each excitation is difficult due to the requirement of an ultrashort repetition time. Therefore, insertion of a preparation phase in the FAIR sequence, after labeling and prior to TurboFLASH acquisition, is proposed. A segmented adiabatic BIR-4 pulse, interleaved with crusher gradients, was used for flow crushing. The effect of the crusher preparation is shown as a function of crusher strength for a flow phantom and in rat brain. Influence of crusher strength on the time-dependent FAIR signal from rat brain was also measured. Signal from flowing spins in a flow phantom and from arterial spins in rat brain was significantly suppressed. Image quality was improved and the overestimation of perfusion at short inflow times was eliminated.     

The effect of concomitant gradient fields on diffusion tensor imaging

Concomitant gradient fields are transverse magnetic field components that are necessarily present to satisfy Maxwell's equations when magnetic field gradients are utilized in magnetic resonance imaging. They can have deleterious effects that are more prominent at lower static fields and/or higher gradient strengths. In diffusion tensor imaging schemes that employ large gradients that are not symmetric about a refocusing radiofrequency pulse (unlike Stejskal–Tanner, which is symmetric), concomitant fields may cause phase accrual that could corrupt the diffusion measurement. Theory predicting the error from this dephasing is described and experimentally validated for both Reese twice‐refocused and split gradient single spin‐echo diffusion gradient schemes. Bias in apparent diffusion coefficient values was experimentally found to worsen with distance from isocenter and with increasing duration of gradient asymmetry in both a phantom and in the brain. The amount of error from concomitant gradient fields depends on many variables, including the diffusion gradient pattern, pulse sequence timing, maximum effective gradient amplitude, static magnetic field strength, voxel size, slice distance from isocenter, and partial Fourier fraction. A prospective correction scheme that can reduce concomitant gradient errors is proposed and verified for diffusion imaging. Magn Reson Med, 2012. © 2011 Wiley Periodicals, Inc.     

High-resolution fast spin echo imaging of the human brain at 4.7 T: implementation and sequence characteristics.

In this work, a number of important issues associated with fast spin echo (FSE) imaging of the human brain at 4.7 T are addressed. It is shown that FSE enables the acquisition of images with high resolution and good tissue contrast throughout the brain at high field strength. By employing an echo spacing (ES) of 22 ms, one can use large flip angle refocusing pulses (162 degrees ) and a low acquisition bandwidth (50 kHz) to maximize the signal-to-noise ratio (SNR). A new method of phase encode (PE) ordering (called "feathering") designed to reduce image artifacts is described, and the contributions of RF (B(1)) inhomogeneity, different echo coherence pathways, and magnetization transfer (MT) to FSE signal intensity and contrast are investigated. B(1) inhomogeneity is measured and its effect is shown to be relatively minor for high-field FSE, due to the self-compensating characteristics of the sequence. Thirty-four slice data sets (slice thickness = 2 mm; in-plane resolution = 0.469 mm; acquisition time = 11 min 20 s) from normal volunteers are presented, which allow visualization of brain anatomy in fine detail. This study demonstrates that high-field FSE produces images of the human brain with high spatial resolution, SNR, and tissue contrast, within currently prescribed power deposition guidelines.     

Intensity artifacts in MRI caused by gradient switching in an animal-size NMR magnet.

The switching of magnetic field gradients in MRI gives rise to eddy currents in the structural components of superconducting magnet systems. The associated magnetic fields cause intensity artifacts which are particularly severe in some animal-size systems. We treat theoretically three mechanisms which cause intensity artifacts in one-dimensional projection images obtained by a spin-echo technique. The first is an off-resonance effect, caused by applying the refocusing pulse before the read compensation gradient pulse has decayed sufficiently. The other two mechanisms are caused by a spatial dependence of the phase accumulated by the spins at the time of formation of the echo, as a result of the eddy current fields. First, interference causes a loss of transverse magnetization because of a variation in the phase of spins which lie on the same isochromat during the read gradient pulse. Second, a variation of the phase of the spins in a direction orthogonal to the isochromats causes spins throughout the sample to refocus at different times. These two mechanisms are fundamentally different, since interference can occur even if the main magnetic field is homogeneous, whereas improper refocusing does not. It is shown that there is no loss of intensity by the interference mechanism if phase encoding is used to form two-dimensional images. This may well be a major reason why images obtained by 2DFT have been found to be generally superior to those obtained by projection reconstruction. Experimentally, the distribution of intensity in one-dimensional projection images of a square slice phantom is compared with theoretical intensities, estimated using eddy current field reported in the preceding paper.     

High spatial resolution EPI using an odd number of interleaves.

Ghost artifacts in echoplanar imaging (EPI) arise from phase errors caused by differences in eddy currents and gradient ramping during left-to-right traversal of kx(forward echo) versus right-to-left traversal of kx (reverse echo). Reference scans do not always reduce the artifact and may make image quality worse. To eliminate the need for reference scans, a ghost artifact reduction technique based on image phase correction was developed, in which phase errors are directly estimated from images reconstructed separately using only the forward or only the reverse echos. In practice, this technique is applicable only to single-shot EPI that produces only one ghost (shifted 1/2 the field of view from the parent image), because the technique requires that the ghosts do not completely overlap the parent image. For higher spatial resolution, typically an even number of separate k-space traversals (interleaves) are combined to produce one large data set. In this paper, we show that data obtained from an even number of interleaves cannot be combined to produce only one ghost, and image phase correction cannot be applied. We then show that data obtained from an odd number of interleaves can be combined to produce only one ghost, and image phase correction can be applied to reduce ghost intensity significantly. This "odd-number interleaf EPI" provides spatial and temporal resolution tradeoffs that are complementary to, or can replace, those of even-number interleaf EPI. Odd-number interleaf EPI may be particularly useful for MR systems in which reference scans have been unreliable.     

中场强磁共振尿路水成像技术与临床应用初探

目的 评价中场强（０．５Ｔ）磁共振单激发快速自旋回波９ＳＳＦＳＥ０尿路水成像（ＭＲＵ）技术及其临床应用价值。材料与方法 ４２例正常和悄路梗阻患者,在ＧＥ ０．５Ｔ超导ＭＲ成像仪上采用ＳＳＦＳＥ进行ＭＲＵ成像;３８例加作２Ｄ ＦＳＥ重Ｔ２ＷＩ,经最大信号强度投影（ＭＩＰ）重建ＭＲＵ图像。结果 采用ＳＳＦＳＥ所得ＭＲＵ图像可清晰显示正常尿路,效果比２Ｄ ＦＳＥ重Ｔ２ＷＩ经ＭＩＰ重建所得ＭＲＵ钉佳,尿路     

Concomitant gradient field effects in spiral scans.

Maxwell's equations imply that imaging gradients are accompanied by higher order spatially varying fields (concomitant fields) that can cause artifacts in MR imaging. The lowest order concomitant fields depend quadratically on the imaging gradient amplitude and inversely on the static field strength. Time-varying concomitant fields that accompany the readout gradients of spiral scans cause unwanted phase accumulation during the readout, resulting in spatially dependent blurring. Concomitant field phase errors are independent of echo time and, therefore, cannot be detected using Dixon-type field map measurements that are normally used to deblur spiral scan images. Data acquisition methods that reduce concomitant field blurring increase off-resonant spin blurring, and vice versa. Blurring caused by concomitant fields can be removed by variations of image reconstruction methods developed to correct for spatially dependent resonance offsets with nonrectangular k-space trajectories.