Reduced field‐of‐view excitation using second‐order gradients and spatial‐spectral radiofrequency pulses

The performance of multidimensional spatially selective radiofrequency (RF) pulses is often limited by their long duration. In this article, high‐order, nonlinear gradients are exploited to reduce multidimensional RF pulse length. Specifically, by leveraging the multidimensional spatial dependence of second‐order gradients, a two‐dimensional spatial‐spectral RF pulse is designed to achieve three‐dimensional spatial selectivity, i.e., to excite a circular region‐of‐interest in a thin slice for reduced field‐of‐view imaging. Compared to conventional methods that use three‐dimensional RF pulses and linear gradients, the proposed method requires only two‐dimensional RF pulses, and thus can significantly shorten the RF pulses and/or improve excitation accuracy. The proposed method has been validated through Bloch equation simulations and phantom experiments on a commercial 3.0T MRI scanner. Magn Reson Med, 2013. © 2012 Wiley Periodicals, Inc.     

A three‐dimensional variable‐density spiral spatial‐spectral RF pulse with rotated gradients

Three‐dimensional spatial‐spectral radiofrequency pulses using a stack‐of‐spirals trajectory can achieve two‐dimensional spatial localization and one‐dimensional spectral selection simultaneously. These pulses are useful, for example, in reduced field‐of‐view applications that also require frequency specificity such as lipid imaging. A limitation of the pulse design is that the length of the spiral trajectory is fixed by the frequency separation of lipid and water. This restricts the highest possible excitation resolution of the spatial profile over a given field of excitation. In this work, we examine the use of periodically rotated variable‐density spirals to increase the spatial excitation resolution without changing the frequency selectivity. Variable‐density spirals are used to undersample the high spatial frequencies such that higher excitation resolutions can be obtained with a small expense in increased aliasing of the slice profile. The periodic rotation of the spiral trajectories reduces the impact of the undersampling by distributing the aliasing in the frequency domain. The technique is demonstrated with simulations, phantom studies, and imaging human leg muscle at 3 T. It was found in the human study that the spatial excitation resolution could be improved from 6 × 6 to 8 × 8 (matrix size over a fixed field of view) while decreasing aliasing by approximately 40‐60%. Magn Reson Med 63:828–834, 2010. © 2010 Wiley‐Liss, Inc.     

Efficient fat suppression by slice‐selection gradient reversal in twice‐refocused diffusion encoding

Most diffusion imaging sequences rely on single‐shot echo‐planar imaging (EPI) for spatial encoding since it is the fastest acquisition available. However, it is sensitive to chemical‐shift artifacts due to the low bandwidth in the phase‐encoding direction, making fat suppression necessary. Often, spectral‐selective RF pulses followed by gradient spoiling are used to selectively saturate the fat signal. This lengthens the acquisition time and increases the specific absorption rate (SAR). However, in pulse sequences that contain two slice‐selective 180° refocusing pulses, the slice‐selection gradient reversal (SSGR) method of fat suppression can be implemented; i.e., using slice‐selection gradients of opposing polarity for the two refocusing pulses. We combined this method with the twice‐refocused spin‐echo sequence for diffusion encoding and tested its performance in both phantoms and in vivo. Unwanted fat signal was entirely suppressed with this method without affecting the water signal intensity or the slice profile. Magn Reson Med 60:1256–1260, 2008. © 2008 Wiley‐Liss, Inc.     

Dual half‐echo phase correction for implementation of 3D radial SSFP at 3.0 T

Fat/water separation methods such as fluctuating equilibrium magnetic resonance and linear combination steady‐state free precession have not yet been successfully implemented at 3.0 T due to extreme limitations on the time available for spatial encoding with the increase in magnetic field strength. We present a method to utilize a three‐dimensional radial sequence combined with linear combination steady‐state free precession at 3.0 T to take advantage of the increased signal levels over 1.5 T and demonstrate high spatial resolution compared to Cartesian techniques. We exploit information from the two half‐echoes within each pulse repetition time to correct the accumulated phase on a point‐by‐point basis, thereby fully aligning the phase of both half‐echoes. The correction provides reduced sensitivity to static field (B₀) inhomogeneity and robust fat/water separation. Resultant images in the knee joint demonstrate the necessity of such a correction, as well as the increased isotropic spatial resolution attainable at 3.0 T. Results of a clinical study comparing this sequence to conventional joint imaging sequences are included. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.     

Subject‐specific water‐selective imaging using parallel transmission

Spectral‐spatial excitation pulses are an efficient means of achieving water‐ or fat‐only imaging and can be used in conjunction with a variety of pulse sequences. However, the approach lacks reliability since its performance is dependent on the homogeneity of the static magnetic field. Sensitivity to static magnetic field variation can be reduced by designing pulses with wider frequency stop bands, but these require longer pulse durations. In the proposed method, spectral‐spatial pulses are optimized on a subject‐dependent basis to take into account measured subject‐specific static magnetic field variation. Extra control of the radiofrequency (RF) field from multichannel transmission is used to achieve this without increasing the length of the pulses. The method characterizes RF pulses using relatively few parameters and has been applied to abdominal imaging at 3 T with an eight‐channel system. In a comparison of standard and subject‐specific pulses on five healthy volunteers, the latter improved fat suppression in all subjects, with a reduction in RF power of 13% ± 6%. A forward model suggests that the mean flip angle in fat was reduced from 0.72° ± 0.55° to 0.12° ± 0.04° for a 20° excitation; uniformity of water excitation also improved, with the standard deviation divided by mean reduced from 0.26 ± 0.05 to 0.16 ± 0.05. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.     

Inner‐volume imaging in vivo using three‐dimensional parallel spatially selective excitation

This work describes the first experimental realization of three‐dimensional spatially selective excitation using parallel transmission in vivo. For the design of three‐dimensional parallel excitation pulses with short durations and high excitation accuracy, the choice of a suitable transmit k‐space trajectory is crucial. For this reason, the characteristics of a stack‐of‐spirals trajectory and of a concentric‐shells trajectory were examined in an initial simulation study. It showed that, especially when undersampling the trajectories in combination with parallel transmission, experimental parameters such as transmit‐coil geometry and off‐resonance conditions have an essential impact on the suitability of the selected trajectory and undersampling scheme. Both trajectories were applied in MR inner‐volume imaging experiments which demonstrate that acceptably short and robust three‐dimensional selective pulses can be achieved if the trajectory is temporally optimized and its actual path is measured and considered during pulse calculation. Pulse durations as short as 3.2 ms were realized and such pulses were appropriate to accurately excite arbitrarily shaped volumes in a corn cob and in a rat in vivo. Reduced field‐of‐view imaging of these selectively excited targets allowed high spatial resolution and significantly reduced measurement times and furthermore demonstrates the feasibility of three‐dimensional parallel excitation in realistic MRI applications in vivo. Magn Reson Med, 2013. © 2012 Wiley Periodicals, Inc.     

A multislice gradient echo pulse sequence for CEST imaging

Chemical exchange–dependent saturation transfer and paramagnetic chemical exchange–dependent saturation transfer are agent‐mediated contrast mechanisms that depend on saturating spins at the resonant frequency of the exchangeable protons on the agent, thereby indirectly saturating the bulk water. In general, longer saturating pulses produce stronger chemical and paramagnetic exchange–dependent saturation transfer effects, with returns diminishing for pulses longer than T₁. This could make imaging slow, so one approach to chemical exchange–dependent saturation transfer imaging has been to follow a long, frequency‐selective saturation period by a fast imaging method. A new approach is to insert a short frequency‐selective saturation pulse before each spatially selective observation pulse in a standard, two‐dimensional, gradient‐echo pulse sequence. Being much less than T₁ apart, the saturation pulses have a cumulative effect. Interleaved, multislice imaging is straightforward. Observation pulses directed at one slice did not produce observable, unintended chemical exchange–dependent saturation transfer effects in another slice. Pulse repetition time and signal‐to noise ratio increase in the normal way as more slices are imaged simultaneously. Magn Reson Med, 2010. © 2009 Wiley‐Liss, Inc.     

Magnetic resonance separation imaging using a divided inversion recovery technique (DIRT)

The divided inversion recovery technique is an MRI separation method based on tissue T₁ relaxation differences. When tissue T₁ relaxation times are longer than the time between inversion pulses in a segmented inversion recovery pulse sequence, longitudinal magnetization does not pass through the null point. Prior to additional inversion pulses, longitudinal magnetization may have an opposite polarity. Spatial displacement of tissues in inversion recovery balanced steady‐state free‐precession imaging has been shown to be due to this magnetization phase change resulting from incomplete magnetization recovery. In this paper, it is shown how this phase change can be used to provide image separation. A pulse sequence parameter, the time between inversion pulses (T180), can be adjusted to provide water‐fat or fluid separation. Example water‐fat and fluid separation images of the head, heart, and abdomen are presented. The water‐fat separation performance was investigated by comparing image intensities in short‐axis divided inversion recovery technique images of the heart. Fat, blood, and fluid signal was suppressed to the background noise level. Additionally, the separation performance was not affected by main magnetic field inhomogeneities. Magn Reson Med 63:1007–1014, 2010. © 2010 Wiley‐Liss, Inc.     

Fat and water magnetic resonance imaging

A wide variety of fat suppression and water–fat separation methods are used to suppress fat signal and improve visualization of abnormalities. This article reviews the most commonly used techniques for fat suppression and fat–water imaging including 1) chemically selective fat suppression pulses “FAT‐SAT”; 2) spatial‐spectral pulses (water excitation); 3) short inversion time (TI) inversion recovery (STIR) imaging; 4) chemical shift based water–fat separation methods; and finally 5) fat suppression and balanced steady‐state free precession (SSFP) sequences. The basic physical background of these techniques including their specific advantages and disadvantages is given and related to clinical applications. This enables the reader to understand the reasons why some fat suppression methods work better than others in specific clinical settings. J. Magn. Reson. Imaging 2010;31:4–18. © 2009 Wiley‐Liss, Inc.     

Dual inversion recovery,ultrashort echo time (DIR UTE) imaging: Creating high contrast for short‐T2 species

Imaging of short‐T₂ species requires not only a short echo time but also efficient suppression of long‐T₂ species in order to maximize the short‐T₂ contrast and dynamic range. This paper introduces a method of long‐T₂ suppression using two long adiabatic inversion pulses. The first adiabatic inversion pulse inverts the magnetization of long‐T₂ water and the second one inverts that of fat. Short‐T₂ species experience a significant transverse relaxation during the long adiabatic inversion process and are minimally affected by the inversion pulses. Data acquisition with a short echo time of 8 μs starts following a time delay of inversion time (TI1) for the inverted water magnetization to reach a null point and a time delay of TI2 for the inverted fat magnetization to reach a null point. The suppression of long‐T₂ species depends on proper combination of TI1, TI2, and pulse repetition time. It is insensitive to radiofrequency inhomogeneities because of the adiabatic inversion pulses. The feasibility of this dual inversion recovery ultrashort echo time technique was demonstrated on phantoms, cadaveric specimens, and healthy volunteers, using a clinical 3‐T scanner. High image contrast was achieved for the deep radial and calcified layers of articular cartilage, cortical bone, and the Achilles tendon. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.     

A simple low‐SAR technique for chemical‐shift selection with high‐field spin‐echo imaging

We have discovered a simple and highly robust method for removal of chemical shift artifact in spin‐echo MR images, which simultaneously decreases the radiofrequency power deposition (specific absorption rate). The method is demonstrated in spin‐echo echo‐planar imaging brain images acquired at 7 T, with complete suppression of scalp fat signal. When excitation and refocusing pulses are sufficiently different in duration, and thus also different in the amplitude of their slice‐select gradients, a spatial mismatch is produced between the fat slices excited and refocused, with no overlap. Because no additional radiofrequency pulse is used to suppress fat, the specific absorption rate is significantly reduced compared with conventional approaches. This enables greater volume coverage per unit time, well suited for functional and diffusion studies using spin‐echo echo‐planar imaging. Moreover, the method can be generally applied to any sequence involving slice‐selective excitation and at least one slice‐selective refocusing pulse at high magnetic field strengths. The method is more efficient than gradient reversal methods and more robust against inhomogeneities of the static (polarizing) field (B₀). Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.     

Parallel excitation in the human brain at 9.4 T counteracting k‐space errors with RF pulse design

Multidimensional spatially selective radiofrequency (RF) pulses have been proposed as a method to mitigate transmit B1 inhomogeneity in MR experiments. These RF pulses, however, have been considered impractical for many years because they typically require very long RF pulse durations. The recent development of parallel excitation techniques makes it possible to design multidimensional RF pulses that are short enough for use in actual experiments. However, hardware and experimental imperfections can still severely alter the excitation patterns obtained with these accelerated pulses. In this note, we report at 9.4 T on a human eight‐channel transmit system, substantial improvements in two‐dimensional excitation pattern accuracy obtained when measuring k‐space trajectories prior to parallel transmit RF pulse design (acceleration ×4). Excitation patterns based on numerical simulations closely reproducing the experimental conditions were in good agreement with the experimental results. Magn Reson Med, 2010. © 2009 Wiley‐Liss, Inc.     

Design of improved spectral-spatial pulses for routine clinical use.

Spectral-spatial pulses (spsp pulses) selectively excite spins at spatial location z and spectral frequency (due to chemical shift and/or field inhomogeneity) v. In this work we discuss the design of improved spsp pulses for fat signal suppression. Optimal pulses are designed as optimal constant ripple FIR filters using the inverse SLR transform. Spsp pulses with thin slices are obtained by modifying the phases between subpulses, thereby eliminating unwanted magnetization lobes. Robust spsp pulses at off-center slices are obtained with a prescan calibration. These pulses are used either for selective fat saturation or for selective water excitation. It is shown that spsp pulses suppress fat signal better than conventional fat saturation pulses. Using the techniques presented in this article, we replaced all the fat saturation pulses on our systems with spsp pulses and obtained a significant improvement in image quality.     

Reproducibility study of whole‐brain 1H spectroscopic imaging with automated quantification

A reproducibility study of proton MR spectroscopic imaging (¹H‐MRSI) of the human brain was conducted to evaluate the reliability of an automated 3D in vivo spectroscopic imaging acquisition and associated quantification algorithm. A PRESS‐based pulse sequence was implemented using dualband spectral‐spatial RF pulses designed to fully excite the singlet resonances of choline (Cho), creatine (Cre), and N‐acetyl aspartate (NAA) while simultaneously suppressing water and lipids; 1% of the water signal was left to be used as a reference signal for robust data processing, and additional lipid suppression was obtained using adiabatic inversion recovery. Spiral k‐space trajectories were used for fast spectral and spatial encoding yielding high‐quality spectra from 1 cc voxels throughout the brain with a 13‐min acquisition time. Data were acquired with an 8‐channel phased‐array coil and optimal signal‐to‐noise ratio (SNR) for the combined signals was achieved using a weighting based on the residual water signal. Automated quantification of the spectrum of each voxel was performed using LCModel. The complete study consisted of eight healthy adult subjects to assess intersubject variations and two subjects scanned six times each to assess intrasubject variations. The results demonstrate that reproducible whole‐brain ¹H‐MRSI data can be robustly obtained with the proposed methods. Magn Reson Med 60:542–547, 2008. © 2008 Wiley‐Liss, Inc.     

Reduced field‐of‐view single‐shot fast spin echo imaging using two‐dimensional spatially selective radiofrequency pulses

Purpose:

To demonstrate reduced field‐of‐view (RFOV) single‐shot fast spin echo (SS‐FSE) imaging based on the use of two‐dimensional spatially selective radiofrequency (2DRF) pulses.

Materials and Methods:

The 2DRF pulses were incorporated into an SS‐FSE sequence for RFOV imaging in both phantoms and the human brain on a 1.5 Tesla (T) whole‐body MR system with the aim of demonstrating improvements in terms of shorter scan time, reduced blurring, and higher spatial resolution compared with full FOV imaging.

Results:

For phantom studies, scan time gains of up to 4.2‐fold were achieved as compared to the full FOV imaging. For human studies, the spatial resolution was increased by a factor of 2.5 (from 1.7 mm/pixel to 0.69 mm/pixel) for RFOV imaging within a scan time (0.7 s) similar to full FOV imaging. A 2.2‐fold shorter scan time along with a significant reduction of blurring was demonstrated in RFOV images compared with full FOV images for a target spatial resolution of 0.69 mm/pixel.

Conclusion:

RFOV SS‐FSE imaging using a 2DRF pulse shows advantages in scan time, blurring, and specific absorption rate reduction along with true spatial resolution increase compared with full FOV imaging. This approach is promising to benefit fast imaging applications such as image guided therapy. J. Magn. Reson. Imaging 2010;32:242–248. © 2010 Wiley‐Liss, Inc.     

Improved 3‐Tesla cardiac cine imaging using wideband

Cine balanced steady‐state free precession (SSFP) is the most widely used sequence for assessing cardiac ventricular function at 1.5 T because it provides high signal‐to‐noise ratio efficiency and strong contrast between myocardium and blood. At 3 T, the use of SSFP is limited by susceptibility‐induced off‐resonance, resulting in either banding artifacts or the need to use a short‐sequence pulse repetition time that limits the readout duration and hence the achievable spatial resolution. In this work, we apply wideband SSFP, a variant of SSFP that uses two alternating pulse repetition times to establish a steady state with wider band spacing in its frequency response and overcome the key limitations of SSFP. Prospectively gated cine two‐dimensional imaging with wideband SSFP is evaluated in healthy volunteers and compared to conventional balanced SSFP, using quantitative metrics and qualitative interpretation by experienced clinicians. We demonstrate that by trading off temporal resolution and signal‐to‐noise ratio efficiency, wideband SSFP mitigates banding artifacts and enables imaging with approximately 30% higher spatial resolution compared to conventional SSFP with the same effective band spacing. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.     

Noncontrast‐enhanced renal angiography using multiple inversion recovery and alternating TR balanced steady‐state free precession

Noncontrast‐enhanced renal angiography techniques based on balanced steady‐state free precession avoid external contrast agents, take advantage of high inherent blood signal from the contrast mechanism, and have short steady‐state free precession acquisition times. However, background suppression is limited; inflow times are inflexible; labeling region is difficult to define when tagging arterial flow; and scan times are long. To overcome these limitations, we propose the use of multiple inversion recovery preparatory pulses combined with alternating pulse repetition time balanced steady‐state free precession to produce renal angiograms. Multiple inversion recovery uses selective spatial saturation followed by four nonselective inversion recovery pulses to concurrently null a wide range of background species while allowing for adjustable inflow times; alternating pulse repetition time steady‐state free precession maintains vessel contrast and provides added fat suppression. The high level of suppression enables imaging in three‐dimensional as well as projective two‐dimensional formats, the latter of which has a scan time as short as one heartbeat. In vivo studies at 1.5 T demonstrate the superior vessel contrast of this technique. Magn Reson Med 70:527–536, 2013. © 2012 Wiley Periodicals, Inc.     

Dual‐band water and lipid suppression for MR spectroscopic imaging at 3 Tesla

A dual‐band water and lipid suppression sequence was developed for multislice sensitivity‐encoded proton MR spectroscopic imaging of the human brain. The presaturation scheme consisted of five dual‐band frequency‐modulated radiofrequency pulses based on hypergeometric functions integrated with eight outer volume suppression (OVS) pulses. The flip angles of the dual‐band pulses were optimized through computer simulations to maximize suppression factors over a range of transmitter amplitude of radiofrequency field and water and lipid T₁ values. The resulting hypergeometric dual band with OVS (HGDB + OVS) sequence was implemented at 3 T in a multislice sensitivity‐encoded proton MR spectroscopic imaging experiment and compared to a conventional water suppression scheme (variable pulse power and optimized relaxation delays (VAPOR)) with OVS. The HGDB sequence was significantly shorter than the VAPOR sequence (230 versus 728 msec). Both HGDB + OVS and VAPOR + OVS produced good water suppression, while lipid suppression with the HGDB + OVS sequence was far superior. In sensitivity‐encoded proton MR spectroscopic imaging data, artifacts from extracranial lipid signals were significantly lower with HGDB + OVS. The shorter duration of HGDB compared to VAPOR also allows reduced pulse repetition time values in the multislice acquisition. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.     

DIfferential Subsampling with Cartesian Ordering (DISCO): a high spatio-temporal resolution Dixon imaging sequence for multiphasic contrast enhanced abdominal imaging

Purpose:

To develop and evaluate a multiphasic contrast‐enhanced MRI method called DIfferential Sub‐sampling with Cartesian Ordering (DISCO) for abdominal imaging.

Materials and Methods:

A three‐dimensional, variable density pseudo‐random k‐space segmentation scheme was developed and combined with a Dixon‐based fat‐water separation algorithm to generate high temporal resolution images with robust fat suppression and without compromise in spatial resolution or coverage. With institutional review board approval and informed consent, 11 consecutive patients referred for abdominal MRI at 3 Tesla (T) were imaged with both DISCO and a routine clinical three‐dimensional SPGR‐Dixon (LAVA FLEX) sequence. All images were graded by two radiologists using quality of fat suppression, severity of artifacts, and overall image quality as scoring criteria. For assessment of arterial phase capture efficiency, the number of temporal phases with angiographic phase and hepatic arterial phase was recorded.

Results:

There were no significant differences in quality of fat suppression, artifact severity or overall image quality between DISCO and LAVA FLEX images (P > 0.05, Wilcoxon signed rank test). The angiographic and arterial phases were captured in all 11 patients scanned using the DISCO acquisition (mean number of phases were two and three, respectively).

Conclusion:

DISCO effectively captures the fast dynamics of abdominal pathology such as hyperenhancing hepatic lesions with a high spatio‐temporal resolution. Typically, 1.1 × 1.5 × 3 mm spatial resolution over 60 slices was achieved with a temporal resolution of 4–5 s. J. Magn. Reson. Imaging 2012;35:1484–1492. © 2012 Wiley Periodicals, Inc.