Fast‐spin‐echo imaging of inner fields‐of‐view with 2D‐selective RF excitations

Purpose:

To demonstrate the feasibility of two‐dimensional selective radio frequency (2DRF) excitations for fast‐spin‐echo imaging of inner fields‐of‐view (FOVs) in order to shorten acquisitions times, decrease RF energy deposition, and reduce image blurring.

Materials and Methods:

Fast‐spin‐echo images (in‐plane resolution 1.0 × 1.0 mm² or 0.5 × 1.0 mm²) of inner FOVs (40 mm, 16 mm oversampling) were obtained in phantoms and healthy volunteers on a 3 T whole‐body MR system using blipped‐planar 2DRF excitations.

Results:

Positioning the unwanted side excitations in the blind spot between the image section and the slice stack to measure yields minimum 2DRF pulse durations (about 6 msec) that are compatible with typical echo spacings of fast‐spin‐echo acquisitions. For the inner FOVs, the number of echoes and refocusing RF pulses is considerably reduced which compared to a full FOV (182 mm) reduces the RF energy deposition by about a factor of three and shortens the acquisition time, e.g., from 39 seconds to 12 seconds for a turbo factor of 15 or from 900 msec to 280 msec for a single‐shot acquisition, respectively. Furthermore, image blurring occurring for high turbo factors as in single‐shot acquisitions is considerably reduced yielding effectively higher in‐plane resolutions.

Conclusion:

Inner‐FOV acquisitions using 2DRF excitations may help to shorten acquisitions times, ameliorate image blurring, and reduce specific absorption rate (SAR) limitations of fast‐spin‐echo (FSE) imaging, in particular at higher static magnetic fields. J. Magn. Reson. Imaging 2010;31:1530–1537. © 2010 Wiley‐Liss, Inc.     

SNR‐optimized phase‐sensitive dual‐acquisition turbo spin echo imaging: A fast alternative to FLAIR

Phase‐sensitive dual‐acquisition single‐slab three‐dimensional turbo spin echo imaging was recently introduced, producing high‐resolution isotropic cerebrospinal fluid attenuated brain images without long inversion recovery preparation. Despite the advantages, the weighted‐averaging‐based technique suffers from noise amplification resulting from different levels of cerebrospinal fluid signal modulations over the two acquisitions. The purpose of this work is to develop a signal‐to‐noise ratio‐optimized version of the phase‐sensitive dual‐acquisition single‐slab three‐dimensional turbo spin echo. Variable refocusing flip angles in the first acquisition are calculated using a three‐step prescribed signal evolution while those in the second acquisition are calculated using a two‐step pseudo‐steady state signal transition with a high flip‐angle pseudo‐steady state at a later portion of the echo train, balancing the levels of cerebrospinal fluid signals in both the acquisitions. Low spatial frequency signals are sampled during the high flip‐angle pseudo‐steady state to further suppress noise. Numerical simulations of the Bloch equations were performed to evaluate signal evolutions of brain tissues along the echo train and optimize imaging parameters. In vivo studies demonstrate that compared with conventional phase‐sensitive dual‐acquisition single‐slab three‐dimensional turbo spin echo, the proposed optimization yields 74% increase in apparent signal‐to‐noise ratio for gray matter and 32% decrease in imaging time. The proposed method can be a potential alternative to conventional fluid‐attenuated imaging. Magn Reson Med, 2013. © 2012 Wiley Periodicals, Inc.     

Half‐fourier‐acquisition single‐shot turbo spin‐echo (HASTE) MRI of the lung at 3 Tesla using parallel imaging with 32‐receiver channel technology

Purpose

To assess the feasibility of half‐Fourier‐acquisition single‐shot turbo spin‐echo (HASTE) of the lung at 3 Tesla (T) using parallel imaging with a prototype of a 32‐channel torso array coil, and to determine the optimum acceleration factor for the delineation of intrapulmonary anatomy.

Materials and Methods

Nine volunteers were examined on a 32‐channel 3T MRI system using a prototype 32‐channel‐torso‐array‐coil. HASTE‐MRI of the lung was acquired at both, end‐inspiratory and end‐expiratory breathhold with parallel imaging (Generalized autocalibrating partially parallel acquisitions = GRAPPA) using acceleration factors ranging between R = 1 (TE = 42 ms) and R = 6 (TE = 16 ms). The image quality of intrapulmonary anatomy and subjectively perceived noise level was analyzed by two radiologists in consensus. In addition quantitative measurements of the signal‐to‐noise ratio (SNR) of HASTE with different acceleration factors were assessed in phantom measurements.

Results

Using an acceleration factor of R = 4 image blurring was substantially reduced compared with lower acceleration factors resulting in sharp delineation of intrapulmonary structures in expiratory scans. For inspiratory scans an acceleration factor of 2 provided the best image quality. Expiratory scans had a higher subjectively perceived SNR than inspiratory scans.

Conclusion

Using optimized multi‐element coil geometry HASTE‐MRI of the lung is feasible at 3T with acceleration factors up to 4. Compared with nonaccelerated acquisitions, shorter echo times and reduced image blurring are achieved. Expiratory scanning may be favorable to compensate for susceptibility associated signal loss at 3T. J. Magn. Reson. Imaging 2009;30:541–546. © 2009 Wiley‐Liss, Inc.     

Functional perfusion imaging using pseudocontinuous arterial spin labeling with low‐flip‐angle segmented 3D spiral readouts

Arterial spin labeling (ASL) provides quantitative and reproducible measurements of regional cerebral blood flow, and is therefore an attractive method for functional MRI. However, most existing ASL functional MRI protocols are based on either two‐dimensional (2D) multislice or 3D spin‐echo and suffer from very low image signal‐to‐noise ratio or through‐plane blurring. 3D ASL with multishot (segmented) readouts can improve the signal‐to‐noise ratio efficiency relative to 2D multislice and does not suffer from T₂‐blurring. However, segmented readouts require lower imaging flip‐angles and may increase the susceptibility to temporal signal fluctuations (e.g., due to physiology) relative to 2D multislice. In this article, we characterize the temporal signal‐to‐noise ratio of a segmented 3D spiral ASL sequence, and investigate the effects of radiofrequency phase cycling scheme and flip‐angle schedule on image properties. We show that radiofrequency‐spoiling is essential in segmented 3D spiral ASL, and that 3D ASL can improve temporal signal‐to‐noise ratio 2‐fold relative to 2D multislice when using a simple polynomial (cubic) flip‐angle schedule. Functional MRI results using the proposed optimized segmented 3D spiral ASL protocol show excellent activation in the visual cortex. Magn Reson Med, 2013. © 2012 Wiley Periodicals, Inc.     

Reduction of B1 sensitivity in selective single‐slab 3d turbo spin echo imaging with very long echo trains

Single‐slab 3D turbo/fast spin echo (SE) imaging with very long echo trains was recently introduced with slab selection using a highly selective excitation pulse and short, nonselective refocusing pulses with variable flip angles for high imaging efficiency. This technique, however, is vulnerable to image degradation in the presence of spatially varying B₁ amplitudes. In this work we develop a B₁ inhomogeneity‐reduced version of single‐slab 3D turbo/fast SE imaging based on the hypothesis that it is critical to achieve spatially uniform excitation. Slab selection was performed using composite adiabatic selective excitation wherein magnetization is tipped into the transverse plane by a nonselective adiabatic‐half‐passage pulse and then slab is selected by a pair of selective adiabatic‐full‐passage pulses. Simulations and experiments were performed to evaluate the proposed technique and demonstrated that this approach is a simple and efficient way to reduce B₁ sensitivity in single‐slab 3D turbo/fast SE imaging with very long echo trains. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.     

Compensation of slice profile mismatch in combined spin‐ and gradient‐echo echo‐planar imaging pulse sequences

Combined acquisition of gradient‐echo and spin‐echo signals in MRI time series reveals additional information for perfusion‐weighted imaging and functional MRI because of differences in the sensitivity of gradient‐echo and spin‐echo measurements to the properties of the underlying vascular architecture. The acquisition of multiple echo trains within one time frame facilitates the simultaneous estimation of the transversal relaxation parameters R₂ and R. However, the simultaneous estimation of these parameters tends to be incorrect in the presence of slice profile mismatches between signal excitation and subsequent refocusing pulses. It is shown here that improvements in pulse design reduced R₂ and R estimation errors. Further improvements were achieved by augmented parameter estimation through the introduction of an additional parameter δ to correct for discordances in slice profiles to facilitate more quantitative measurements. Moreover, the analysis of time‐resolved acquisitions revealed that the temporal stability of R₂ estimates could be increased with improved pulse design, counteracting low contrast‐to‐noise ratios in spin‐echo‐based perfusion and functional MRI. Magn Reson Med, 2012. © 2011 Wiley Periodicals, Inc.     

Time‐efficient fast spin echo imaging at 4.7 T with low refocusing angles

An implementation of fast spin echo at 4.7 T designed for versatile and time‐efficient T₂‐weighted imaging of the human brain is presented. Reduced refocusing angles (α < 180°) were employed to overcome specific absorption rate (SAR) constraints and their effects on image quality assessed. Image intensity and tissue contrast variations from heterogeneous RF transmit fields and incidental magnetization transfer effects were investigated at reduced refocusing angles. We found that intraslice signal variations are minimized with refocusing angles near 180°, but apparent gray/white matter contrast is independent of refocusing angle. Incidental magnetization transfer effects from multislice acquisitions were shown to attenuate white matter intensity by 25% and gray matter intensity by 15% at 180°; less than 5% attenuation was seen in all tissues at flip angles below 60°. We present multislice images acquired without excess delay time for SAR mitigation using a variety of protocols. Subsecond half Fourier acquisition single‐shot turbo spin echo (HASTE) images were obtained with a novel variable refocusing angle echo train (20° < α < 58°) and high‐resolution scans with a voxel volume of 0.18 mm³ were acquired in 6.5 min with refocusing angles of 100°. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.     

Turboprop+: Enhanced turboprop diffusion‐weighted imaging with a new phase correction

Faster periodically rotated overlapping parallel lines with enhanced reconstruction (PROPELLER) diffusion‐weighted imaging acquisitions, such as Turboprop and X‐prop, remain subject to phase errors inherent to a gradient echo readout, which ultimately limits the applied turbo factor (number of gradient echoes between each pair of radiofrequency refocusing pulses) and, thus, scan time reductions. This study introduces a new phase correction to Turboprop, called Turboprop+. This technique employs calibration blades, which generate 2‐D phase error maps and are rotated in accordance with the data blades, to correct phase errors arising from off‐resonance and system imperfections. The results demonstrate that with a small increase in scan time for collecting calibration blades, Turboprop+ had a superior immunity to the off‐resonance‐related artifacts when compared to standard Turboprop and recently proposed X‐prop with the high turbo factor (turbo factor = 7). Thus, low specific absorption rate and short scan time can be achieved in Turboprop+ using a high turbo factor, whereas off‐resonance related artifacts are minimized. Magn Reson Med 70:497–503, 2013. © 2012 Wiley Periodicals, Inc.     

Rigid‐body motion correction with self‐navigation MRI

The use of phase correlation to detect rigid‐body translational motion is reviewed and applied to individual echotrains in turbo‐spin‐echo data acquisition. It is shown that when the same echotrain is acquired twice, the subsampled correlation provides an array of delta‐functions, from which the motion that occurred between the acquisitions of the two echotrains can be measured. It is shown further that a similar correlation can be found between two sets of equally spaced measurements that are adjacent in k‐space. By measuring the motion between all adjacent pairs of k‐space subgroups, the complete motion history of a subject can be determined and the motion artifacts in the image can be corrected. Some of the limiting factors in using this technique are investigated with turbo‐spin‐echo head and hand images. Magn Reson Med, 2009. © 2008 Wiley‐Liss, Inc.     

Spatially resolved extended phase graphs: Modeling and design of multipulse sequences with parallel transmission

A spatially resolved extended phase graph (SR‐EPG) framework is proposed for prediction of echo amplitudes in the presence of spatially variable radio frequency (RF) fields. The method may be used to examine any regularly repeating pulse sequence and provides a design framework for parallel transmission (PTx) systems; in this work signal homogeneity in static pseudo‐steady state (SPSS) turbo spin echo (TSE) imaging was investigated. Building on SR‐EPG calculations with PTx, a dynamic RF‐shimming approach is proposed in which, RF pulse amplitudes and phases are optimized on a per channel and per pulse basis to yield the desired signal response for all echoes. Results show significant improvements over “static” RF shimming (in which the relative amplitude/phase of the PTx channels are fixed for all pulses). SPSS‐TSE imaging using dynamic RF shimming resulted in excellent image quality, both in phantoms and in vivo, and confirmed SR‐EPG predictions. Magn Reson Med, 2012. © 2012 Wiley Periodicals, Inc.     

Phase‐sensitive,dual‐acquisition,single‐slab, 3D,turbo‐spin‐echo pulse sequence for simultaneous T2‐weighted and fluid‐attenuated whole‐brain imaging

Conventional T₂‐weighted turbo/fast spin echo imaging is clinically accepted as the most sensitive method to detect brain lesions but generates a high signal intensity of cerebrospinal fluid (CSF), yielding diagnostic ambiguity for lesions close to CSF. Fluid‐attenuated inversion recovery can be an alternative, selectively eliminating CSF signals. However, a long time of inversion, which is required for CSF suppression, increases imaging time substantially and thereby limits spatial resolution. The purpose of this work is to develop a phase‐sensitive, dual‐acquisition, single‐slab, three‐dimensional, turbo/fast spin echo imaging, simultaneously achieving both conventional T₂‐weighted and fluid‐attenuated inversion recovery–like high‐resolution whole‐brain images in a single pulse sequence, without an apparent increase of imaging time. Dual acquisition in each time of repetition is performed, wherein an in phase between CSF and brain tissues is achieved in the first acquisition, while an opposed phase, which is established by a sequence of a long refocusing pulse train with variable flip angles, a composite flip‐down restore pulse train, and a short time of delay, is attained in the second acquisition. A CSF‐suppressed image is then reconstructed by weighted averaging the in‐ and opposed‐phase images. Numerical simulations and in vivo experiments are performed, demonstrating that this single pulse sequence may replace both conventional T₂‐weighted imaging and fluid‐attenuated inversion recovery. Magn Reson Med 63:1422–1430, 2010. © 2010 Wiley‐Liss, Inc.     

Spin‐echo MRI using π/2 and π hyperbolic secant pulses

Frequency‐modulated (FM) pulses have practical advantages for spin‐echo experiments, such as the ability to produce a broadband π rotation, with an inhomogeneous radiofrequency (RF) field. However, such use leads to a nonlinear phase of the transverse magnetization, which is why FM pulses like the hyperbolic secant (HS) pulse are not commonly used for multislice spin‐echo magnetic resonance imaging (MRI). Here, a general theory and methods are described for conventional spin‐echo imaging using a π HS pulse for refocusing. Phase profiles produced by the HS pulse are analytically described. The analysis is extended to yield the specific relationships between pulse parameters and gradients, which must be satisfied to compensate the nonlinear phase variation produced with a spin‐echo sequence composed of π/2 and π HS pulses (the π/2 HS ? π HS sequence). The latter offers advantages for multislice spin‐echo MRI, including excellent slice‐selection and partial compensation for RF inhomogeneity. Furthermore, it can be implemented with a shorter echo time and lower power deposition than a previously described method using a pair of π HS pulses. Magn Reson Med 61:175–187, 2009. © 2008 Wiley‐Liss, Inc.     

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.     

MR imaging near metal with undersampled 3D radial UTE‐MAVRIC sequences

Recently developed techniques such as the multiple acquisition with variable resonance image combination and slice encoding for metal artifact correction techniques have improved the ability of clinical magnetic resonance scanners to image near metal implants. These sequences are based on fast spin echo sequences which preclude detection of short T₂ tissues such as tendons, ligaments, and cortical bone. Ultrashort echo time sequences have the potential to detect signals from these tissues. In this study, we investigate the potential of combining ultrashort echo time with multiple acquisition with variable resonance image combination to image short T₂ musculoskeletal tissues adjacent to metallic implants. Different radio frequency excitation pulse types and spectral binning strategies were studied. We found that ultrashort echo time‐multiple acquisition with variable resonance image combination sequences were able to significantly reduce typical artifacts near metal, as well as detect very short T₂ signals that are usually not visualized using clinical pulse sequences. Magn Reson Med, 2013. © 2012 Wiley Periodicals, Inc.     

A new paradigm for high‐sensitivity 19F magnetic resonance imaging of perfluorooctylbromide

In the present work, the NMR properties of perfluorooctylbromide are revisited to derive a high‐sensitivity fluorine MRI strategy. It is shown that the harmful effects of J‐coupling can be eliminated by carefully choosing the bandwidth of the 180° pulses in a spin‐echo sequence. The T₂ of the CF₃ resonance of the molecule is measured using a multispin‐echo sequence and shown to dramatically depend on the interpulse delay. Following these observations, an optimized multispin‐echo imaging sequence is derived and compared with short TE/pulse repetition time gradient echo and chemical shift imaging sequences. The unparalleled sensitivity yielded by the multispin‐echo sequence is promising for future applications, in particular for targeted contrast agents such as perfluorooctylbromide nanoparticles. Magn Reson Med 63:1119–1124, 2010. © 2010 Wiley‐Liss, Inc.     

Whole heart magnetization‐prepared steady‐state free precession coronary vein MRI

Purpose

To compare two coronary vein imaging techniques using whole‐heart balanced steady‐state free precession (SSFP) and a targeted double‐oblique spoiled gradient‐echo (GRE) sequences in combination with magnetization transfer (MT) preparation sequence for tissue contrast improvement.

Materials and Methods

Nine healthy subjects were imaged with the proposed technique. The results are compared with optimized targeted MT prepared GRE acquisitions. Both quantitative and qualitative analyses were performed to evaluate each imaging method.

Results

Whole‐heart images were successfully acquired with no visible image artifact in the vicinity of the coronary veins. The anatomical features and visual grading of both techniques were comparable. However, the targeted small slab acquisition of the left ventricular lateral wall was superior to whole‐heart acquisition for visualization of relevant information for cardiac resynchronization therapy (CRT) lead implantation.

Conclusion

We demonstrated the feasibility of whole‐heart coronary vein MRI using a 3D MT‐SSFP imaging sequence. A targeted acquisition along the lateral left ventricular wall is preferred for visualization of branches commonly used in CRT lead implantation. J. Magn. Reson. Imaging 2009;29:1293–1299. © 2009 Wiley‐Liss, Inc.     

Fast CPMG‐based Bloch‐Siegert B1+ mapping

A novel method for B mapping based on the Bloch‐Siegert (BS) shift was recently presented. This method applies off‐resonant pulses before signal acquisition to encode B₁ information into the signal phase. BS‐based methods possess significant advantages in measurement time and accuracy compared to magnitude‐based B methods. This study extends the idea of BS B mapping to Carr, Purcell, Meiboom, Gill (CPMG)‐based multi‐spin‐echo (BS‐CPMG‐MSE) and turbo‐spin‐echo (BS‐CPMG‐TSE) imaging. Compared to BS‐based spin echo imaging (BS‐SE), faster acquisition of the B information was possible using the BS‐CPMG‐TSE sequence. Furthermore, signal loss by T₂* effects could be minimized using these spin echo‐based techniques. These effects are critical for gradient echo‐based BS methods at high field strengths. However, multi‐spin‐echo‐based BS B₁ methods inherently possess high specific absorption rates. Thus, the relative specific absorption rate of BS‐CPMG‐TSE sequences was estimated and compared with the specific absorption rate produced by BS‐SE sequences. Magn Reson Med, 2012. © 2011 Wiley Periodicals, Inc.     

Self-calibrated multiple-echo acquisition with radial trajectories using the conjugate gradient method (SMART-CG)

Purpose:

To remove phase inconsistencies between multiple echoes, an algorithm using a radial acquisition to provide inherent phase and magnitude information for self correction was developed. The information also allows simultaneous support for parallel imaging for multiple coil acquisitions.

Materials and Methods:

Without a separate field map acquisition, a phase estimate from each echo in multiple echo train was generated. When using a multiple channel coil, magnitude and phase estimates from each echo provide in vivo coil sensitivities. An algorithm based on the conjugate gradient method uses these estimates to simultaneously remove phase inconsistencies between echoes, and in the case of multiple coil acquisition, simultaneously provides parallel imaging benefits. The algorithm is demonstrated on single channel, multiple channel, and undersampled data.

Results:

Substantial image quality improvements were demonstrated. Signal dropouts were completely removed and undersampling artifacts were well suppressed.

Conclusion:

The suggested algorithm is able to remove phase cancellation and undersampling artifacts simultaneously and to improve image quality of multiecho radial imaging, the important technique for fast three‐dimensional MRI data acquisition. J. Magn. Reson. Imaging 2011;33:980–987. © 2011 Wiley‐Liss, Inc.     

Optimized T1-weighted contrast for single-slab 3D turbo spin-echo imaging with long echo trains: application to whole-brain imaging.

T(1)-weighted contrast is conventionally obtained using multislice two-dimensional (2D) spin-echo (SE) imaging. Achieving isotropic, high spatial resolution is problematic with conventional methods due to a long acquisition time, imperfect slice profiles, or high-energy deposition. Single-slab 3D SE imaging was recently developed employing long echo trains with variable low flip angles to address these problems. However, long echo trains may yield suboptimal T(1)-weighted contrast, since T(2) weighting of the signals tends to develop along the echo train. Image blurring may also occur if high spatial frequency signals are acquired with low signal intensity. The purpose of this work was to develop an optimized T(1)-weighted version of single-slab 3D SE imaging with long echo trains. Refocusing flip angles were calculated based on a tissue-specific prescribed signal evolution. Spatially nonselective excitation was used, followed by half-Fourier acquisition in the in-plane phase encoding (PE) direction. Restore radio frequency (RF) pulses were applied at the end of the echo train to optimize T(1)-weighted contrast. Imaging parameters were optimized by using Bloch equation simulation, and imaging studies of healthy subjects were performed to investigate the feasibility of whole-brain imaging with isotropic, high spatial resolution. The proposed technique permitted highly-efficient T(1)-weighted 3D SE imaging of the brain.     

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.