Continuous arterial spin labeling perfusion measurements using single shot 3D GRASE at 3 T.

Single shot 3D GRASE is less sensitive to field inhomogeneity and susceptibility effects than gradient echo based fast imaging sequences while preserving the acquisition speed. In this study, a continuous arterial spin labeling (CASL) pulse was added prior to the single shot 3D GRASE readout and quantitative perfusion measurements were carried out at 3 T, at rest and during functional activation. The sequence performance was evaluated by comparison with a CASL sequence with EPI readout. It is shown that perfusion measurements using CASL GRASE can be performed safely on humans at 3 T without exceeding the current RF power deposition limits. The maps of resting cerebral blood flow generated from the GRASE images are comparable to those obtained with the 2D EPI readout, albeit with better coverage in the orbitofrontal cortex. The sequence proved effective for functional imaging, yielding time series of images with improved temporal SNR with respect to EPI and group activation maps with increased significance levels. The method was further improved using parallel imaging techniques to provide increased spatial resolution and better separation of the gray-white matter cerebral blood flow maps.     

GRASE (gradient- and spin-echo) MR imaging: a new fast clinical imaging technique

A novel technique of magnetic resonance (MR) imaging, which combines gradient-echo and spin-echo (GRASE) technique, accomplishes T2-weighted multisection imaging in drastically reduced imaging time, currently 24 times faster than spin-echo imaging. The GRASE technique maintains contrast mechanisms, high spatial resolution, and image quality of spin-echo imaging and is compatible with clinical whole-body MR systems without modification of gradient hardware. Image acquisition time is 18 seconds for 11 multisection body images (2,000/80 [repetition time msec/echo time msec]) and 36 seconds for 22 brain images (4,000/104). With a combination of multiple Hahn spin echoes and short gradient-echo trains, the GRASE technique overcomes several potential problems of echo-planar imaging, including large chemical shift, image distortions, and signal loss from field inhomogeneity. Advantages of GRASE over the RARE (rapid acquisition with relaxation enhancement) technique include faster acquisition times and lower deposition of radio-frequency power in the body. Breath holding during 18-second GRASE imaging of the upper abdomen eliminates respiratory-motion artifacts in T2-weighted images. A major improvement in T2-weighted abdominal imaging is suggested.     

3D GRASE PROPELLER: Improved image acquisition technique for arterial spin labeling perfusion imaging

Arterial spin labeling is a noninvasive technique that can quantitatively measure cerebral blood flow. While traditionally arterial spin labeling employs 2D echo planar imaging or spiral acquisition trajectories, single‐shot 3D gradient echo and spin echo (GRASE) is gaining popularity in arterial spin labeling due to inherent signal‐to‐noise ratio advantage and spatial coverage. However, a major limitation of 3D GRASE is through‐plane blurring caused by T₂ decay. A novel technique combining 3D GRASE and a periodically rotated overlapping parallel lines with enhanced reconstruction trajectory (PROPELLER) is presented to minimize through‐plane blurring without sacrificing perfusion sensitivity or increasing total scan time. Full brain perfusion images were acquired at a 3 × 3 × 5 mm³ nominal voxel size with pulsed arterial spin labeling preparation sequence. Data from five healthy subjects was acquired on a GE 1.5T scanner in less than 4 minutes per subject. While showing good agreement in cerebral blood flow quantification with 3D gradient echo and spin echo, 3D GRASE PROPELLER demonstrated reduced through‐plane blurring, improved anatomical details, high repeatability and robustness against motion, making it suitable for routine clinical use. Magn Reson Med, 2011. © 2011 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.     

3D steady‐state diffusion‐weighted imaging with trajectory using radially batched internal navigator echoes (TURBINE)

While most diffusion‐weighted imaging (DWI) is acquired using single‐shot diffusion‐weighted spin‐echo echo‐planar imaging, steady‐state DWI is an alternative method with the potential to achieve higher‐resolution images with less distortion. Steady‐state DWI is, however, best suited to a segmented three‐dimensional acquisition and thus requires three‐dimensional navigation to fully correct for motion artifacts. In this paper, a method for three‐dimensional motion‐corrected steady‐state DWI is presented. The method uses a unique acquisition and reconstruction scheme named trajectory using radially batched internal navigator echoes (TURBINE). Steady‐state DWI with TURBINE uses slab‐selection and a short echo‐planar imaging (EPI) readout each pulse repetition time. Successive EPI readouts are rotated about the phase‐encode axis. For image reconstruction, batches of cardiac‐synchronized readouts are used to form three‐dimensional navigators from a fully sampled central k‐space cylinder. In vivo steady‐state DWI with TURBINE is demonstrated in human brain. Motion artifacts are corrected using refocusing reconstruction and TURBINE images prove less distorted compared to two‐dimensional single‐shot diffusion‐weighted‐spin‐EPI. Magn Reson Med, 2010. © 2009 Wiley‐Liss, Inc.     

A comparison of phase encoding ordering schemes in T2-weighted GRASE imaging

Gradient and spin echo (GRASE) imaging is an echo train imaging sequence that combines gradient and RF refocusing. This combination introduces phase modulations into the echo train. If the phase encoding order is linear with echo time, these modulations cause severe ghosting artifacts. Changing the order of phase encoding can greatly reduce these artifacts. Several phase encoding orders for T₂-weighted sequences are compared in this paper: linear, partially randomized, standard GRASE ordering, and k-banded (kb) GRASE ordering. Different possible implementations of GRASE and kbGRASE are also considered. Computer simulation is used to compare resolution and artifact levels. Phantom and volunteer images are presented. The linear order is most sensitive to ghosting artifacts associated with chemical shift, susceptibility differences and static field inhomogeneities. The standard GRASE order is least sensitive to these but most vulnerable to artifacts associated with short T₂ signals. kbGRASE is a good intermediate between linear and standard GRASE and generally shows the lowest artifact levels. The partially randomized order gives the most diffuse artifacts. Computer simulations show that spatial resolution and contrast with all phase encoding orders are similar.     

A new method for fast proton spectroscopic imaging: spectroscopic GRASE.

A new fast spectroscopic imaging method is presented which allows both a very short minimum total measurement time and effective homonuclear decoupling. After each excitation, all data points from N(GE) k(x)-k(y)-slices at different k(omega)-values are acquired by using a gradient and spin echo (GRASE) imaging sequence. The delay between consecutive gradient echoes, which are measured with uniform phase encoding between consecutive refocusing alpha-pulses, is the inverse of the spectral width (SW). A refocusing 180 degrees pulse, which is applied within a constant delay between excitation and the GRASE sequence, is shifted in a series of measurements by an increment N(GE)/(2 * SW) to cover the whole k(omega)-k(x)-k(y)-space. Spectroscopic GRASE was implemented on a 4.7 T imaging system and tested on phantoms and normal rat brain in vivo. Measurements were performed with a nominal voxel size of 1.5 x 1.5 x 3 mm(3) and a spatial 64 x 64 matrix. The total measurement time was 2 or 4 min using a repetition time of 1.9 sec, 96 chemical shift encoding steps, SW = 800 Hz, N(GE) = 3, and 2 or 4 accumulations.     

Improved echo volumar imaging (EVI) for functional MRI

Echo volumar imaging (EVI) is a 3D modification of echo‐planar imaging (EPI) that allows data from an entire volume to be acquired following a single RF excitation. EVI provides a high rate of volumar data acquisition, which is advantageous for functional MRI (fMRI). However, few studies to date have applied EVI to fMRI, since because of gradient hardware limitations EVI generally has to be used with long sampling times, resulting in high sensitivity to susceptibility‐induced distortions. In this study we modified the EVI sequence to improve its suitability for fMRI. The sampling time is reduced by the use of a high gradient‐switching frequency, a small number of echoes, and outer volume suppression (OVS); rewind gradients ameliorate Nyquist ghosting; and phase correction via a calibration scan reduces ghosting and distortion. It is shown that the modified EVI sequence allows fMRI data to be acquired with a temporal resolution of 167 ms. Magn Reson Med, 2006. © 2006 Wiley‐Liss, Inc.     

Study of susceptibility-induced artefacts in GRASE with different echo train length

The aim of this study was to evaluate the sensitivity of gradient-and-spin-echo (GRASE) sequences to susceptibility effects. GRASE sequences with 21 and 33 echoes per echo train were compared with a T2-weighted FSE sequence with an echo train length of 5 by means of MRI in phantoms, volunteers (n = 10), and patients (n = 19) with old hemorrhagic brain lesions. All experiments were performed on a 1.0-T clinical MR system (Impact Expert, Siemens AG, Erlangen, Germany) with constant imaging parameters. Contrast-to-noise ratios (CNRs) of tubes doped with iron oxides at different concentrations, of brain areas with physiological iron deposition (red nucleus, substantia nigra), and of areas of old brain hemorrhage were calculated for FSE and GRASE pulse sequences. Areas of old brain hemorrhage were also qualitatively analyzed for the degree of visible susceptibility effects by blinded reading. The CNR of iron oxide tubes and iron-containing brain areas decreased with increasing echo trains of GRASE sequences. The CNR of GRASE sequences decreased when compared with CNR of their FSE counterparts (GRASE 21 echo trains 23.8 ± 0.8, FSE 5 echo trains 26.7 ± 0.9; p≤ 0.01). Qualitative analysis confirmed these measurements. FSE with an ETL of 5 demonstrated significantly stronger susceptibility effects than their GRASE counterpart with an ETL of 21. The results demonstrate that GRASE sequences do not necessarily compensate for the reduced sensitivity of FSE to susceptibility effects. The complex signal behavior of GRASE makes conventional SE, gradient echo, or FSE sequences containing shorter echo trains preferable when patients with intracranial hemorrhage are clinically evaluated. Received 12 November 1997; Revision received 18 April 1997; Accepted 1 September 1997     

RASER: a new ultrafast magnetic resonance imaging method.

A new MRI method is described to acquire a T(2)-weighted image from a single slice in a single shot. The technique is based on rapid acquisition by sequential excitation and refocusing (RASER). RASER avoids relaxation-related blurring because the magnetization is sequentially refocused in a manner that effectively creates a series of spin echoes with a constant echo time. RASER uses the quadratic phase produced by a frequency-swept chirp pulse to time-encode one dimension of the image. In another implementation the pulse can be used to excite multiple slices with phase-encoding and frequency-encoding in the other two dimensions. The RASER imaging sequence is presented along with single-shot and multislice images, and is compared to conventional spin-echo and echo-planar imaging sequences. A theoretical and empirical analysis of the spatial resolution is presented, and factors in choosing the spatial resolution for different applications are discussed. RASER produces high-quality single-shot images that are expected to be advantageous for a wide range of applications.     

Single-shot GRASE imaging with short effective TEs

A new phase-encoding scheme for gradient- and spin-echo (GRASE) imaging giving a short effective TE is described. Unlike previous orders, phase encoding is centric rather than sequential. The sequence is a development of k-banded GRASE that uses different time segments of the echo train to encode different bands of k space. This phase-encoding order has been implemented in single-shot sequences on an imager with high performance gradients. Approximately 144 phase-encoding lines can be acquired in an echo train time of 390 ms. With centric phase encoding, the effective TE is 8 ms, compared with 75 ms for sequential encoding, and signal-to-noise ratios (SNRs) in brain tissue are 50 to 70% higher. The sequence can be employed in, for example, diffusion and velocity imaging.     

Robust fat suppression at 3T in high‐resolution diffusion‐weighted single‐shot echo‐planar imaging of human brain

Single‐shot echo‐planar imaging is the most common acquisition technique for whole‐brain diffusion tensor imaging (DTI) studies in vivo. Higher field MRI systems are readily available and advantageous for acquiring DTI due to increased signal. One of the practical issues for DTI with single‐shot echo‐planar imaging at high‐field is incomplete fat suppression resulting in a chemically shifted fat artifact within the brain image. Unsuppressed fat is especially detrimental in DTI because the diffusion coefficient of fat is two orders of magnitude lower than that of parenchyma, producing brighter appearing fat artifacts with greater diffusion weighting. In this work, several fat suppression techniques were tested alone and in combination with the goal of finding a method that provides robust fat suppression and can be used in high‐resolution single‐shot echo‐planar imaging DTI studies. Combination of chemical shift saturation with slice‐select gradient reversal within a dual‐spin‐echo diffusion preparation period was found to provide robust fat suppression at 3 T. Magn Reson Med, 2011. © 2011 Wiley Periodicals, Inc.     

An alternative to GRASE: Toward spin-echo-like contrast with independent reconstruction of gradient-echo images

Multiple gradient echoes are generated for each RF echo of a Carr-Purcell-Meiboom-Gill (CPMG) train. Independently, phase-encoded fast spin-echo images are obtained from the different gradient echoes. Presently, three images are formed from three gradient-echoes from each of four RF echoes. The two peripheral gradient echo images are encoded for a late effective TE, then summed after reconstruction: this image has decreased fat intensity and increased susceptibility contrast compared with fast spin echo. The central gradient echoes yielded another image of intermediate contrast useful for neuroimaging. Raw data from the variously timed gradient echoes are not combined as they are in GRASE.     

Selective suppression of artifact‐generating echoes in cine DENSE using through‐plane dephasing

In displacement‐encoded imaging with stimulated echoes (DENSE), tissue displacement is encoded in the phase of the stimulated echo. However, three echoes generally contribute to the acquired signal (the stimulated echo, the complex conjugate of the stimulated echo, and an echo due to T₁ relaxation). It is usually desirable to suppress all except the stimulated echo, since otherwise the additional echoes will cause displacement measurement errors. Ideally, suppression of the artifact‐generating echoes would be independent of time, T₁, and displacement‐encoding frequency, and would not require additional acquisitions. In this study through‐plane gradients were used to selectively dephase artifact‐generating echoes without causing significant signal loss of the stimulated echo. A cine DENSE sequence was modified to include dephasing gradients and perform complementary spatial modulation of magnetization (CSPAMM). For single‐acquisition cine DENSE using dephasing alone, artifact suppression was similar to CSPAMM with two acquisitions. The use of dephasing with CSPAMM required two acquisitions, but demonstrated greater artifact suppression than CSPAMM alone or dephasing alone. Magn Reson Med, 2006. © 2006 Wiley‐Liss, Inc.     

Rapid water and lipid imaging with T2 mapping using a radial IDEAL‐GRASE technique

Three‐point Dixon methods have been investigated as a means to generate water and fat images without the effects of field inhomogeneities. Recently, an iterative algorithm (IDEAL, iterative decomposition of water and fat with echo asymmetry and least squares estimation) was combined with a gradient and spin‐echo acquisition strategy (IDEAL‐GRASE) to provide a time‐efficient method for lipid–water imaging with correction for the effects of field inhomogeneities. The method presented in this work combines IDEAL‐GRASE with radial data acquisition. Radial data sampling offers robustness to motion over Cartesian trajectories as well as the possibility of generating high‐resolution T₂ maps in addition to the water and fat images. The radial IDEAL‐GRASE technique is demonstrated in phantoms and in vivo for various applications including abdominal, pelvic, and cardiac imaging. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.     

Perfusion-based functional magnetic resonance imaging with single-shot RARE and GRASE acquisitions.

For perfusion-based functional magnetic resonance imaging, the previously introduced flow-sensitive alternating inversion recovery (FAIR) technique is combined with single-shot RARE (rapid acquisition with relaxation enhancement) and GRASE (gradient and spin echo) imaging sequences. The advantages of these sequences compared to commonly used echo-planar imaging (EPI) are an increased signal-to-noise ratio and the absence of distortions and artifacts due to magnetic field inhomogeneities. RARE- and GRASE-FAIR are applied to functional brain mapping studies in humans during visual stimulation. Results demonstrate that the presented techniques allow for perfusion maps with higher spatial resolution compared to EPI-FAIR. Relative regional cerebral blood flow change in the occipital cortex during visual stimulation was measured to be 41+/-4% (n = 5). The comparison of FAIR data obtained with RARE and GRASE techniques shows that RARE yields images with the higher signal-to-noise ratio. However, the GRASE technique features a shorter acquisition time and less RF power deposition and is thus better suited for multi-slice acquisitions.     

Super‐resolution for multislice diffusion tensor imaging

Diffusion weighted magnetic resonance images are often acquired with single shot multislice imaging sequences, because of their short scanning times and robustness to motion. To minimize noise and acquisition time, images are generally acquired with either anisotropic or isotropic low resolution voxels, which impedes subsequent posterior image processing and visualization. In this article, we propose a super‐resolution method for diffusion weighted imaging that combines anisotropic multislice images to enhance the spatial resolution of diffusion tensor data. Each diffusion weighted image is reconstructed from a set of arbitrarily oriented images with a low through‐plane resolution. The quality of the reconstructed diffusion weighted images was evaluated by diffusion tensor metrics and tractography. Experiments with simulated data, a hardware DTI phantom, as well as in vivo human brain data were conducted. Our results show a significant increase in spatial resolution of the diffusion tensor data while preserving high signal to noise ratio. Magn Reson Med, 2013. © 2012 Wiley Periodicals, Inc.     

3D single‐shot VASO using a maxwell gradient compensated GRASE sequence

The vascular space occupancy (VASO) method was recently proposed as a functional MRI (fMRI) method that is capable of detecting activation‐related changes in blood volume (CBV), without the need for a blood‐pool contrast agent. In the present work we introduce a new whole‐brain VASO technique that is based on a parallel‐accelerated single‐shot 3D GRASE (gradient and spin echo) readout. The GRASE VASO sequence employs a flow‐compensated correction scheme for concomitant Maxwell gradients which is necessary to avoid smearing artifacts that may occur due to violation of the Carr–Purcell–Meiboom–Gill (CPMG) condition for off‐resonance excitation. Experiments with 6 min of visual‐motor stimulation were performed on eight subjects. At P < 0.01, average percent signal change and t‐score for visual stimulation were ?3.11% and ?8.42, respectively; activation in left and right motor cortices and supplementary motor area was detected with ?2.75% and ?6.70, respectively. Sensitivity and signal changes are comparable to those of echo‐planar imaging (EPI)‐based single‐slice VASO, as indicated by additional visual‐task experiments (?3.39% and ?6.93). The method makes it possible to perform whole‐brain cognitive activation studies based on CBV contrast. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.     

Improvement in signal-to-noise ratio and reduction of chemical shift and motion-induced artifacts by summation of gradient and spin echo data acquisition

Narrow bandwidth magnetic resonance (MR) imaging allows an increase of signal-to-noise ratio (SNR) but causes increased chemical shift and motion-induced artifacts. To obtain MR images with SNR approximately equal to that obtained with narrow bandwidth but with less chemical shift and motion-induced artifact, we introduced triple readout gradient reversal centered around the spin echo. As a result, signals from two gradient echoes and a single spin echo can be collected and summed. Phantom, knee, shoulder, and abdominal MR images were obtained using a 1.5 T GE Signa System at sampling rates ranging from 10 to 60 kHz. Since the bandwidth per pixel was tripled, chemical shift misregistration was reduced by the same factor. The summation image of two gradient echoes and one spin echo had an SNR comparable with that of a single spin echo acquired within the same total sampling interval. Data acquisition at a high sampling ratio also minimizes the dispersion of T2* weighting among three echoes. In addition, summation of the three resulting images decreases motion artifact by effective averaging.     

Susceptibility weighted imaging with multiple echoes

Purpose:

To extend susceptibility weighted imaging (SWI) to multiple echoes with an adapted homodyne filtering of phase images for the computation of venograms with improved signal to noise ratio (SNR) and contrast to noise ratio (CNR) and to produce high resolution maps of R₂* relaxation.

Materials and Methods:

Three‐dimensional multi echo gradient echo data were acquired with five equidistant echoes ranging from 13 to 41 ms. The phase images of each echo were filtered with filter parameters adjusted to the echo time, converted into a phase mask, and combined with the corresponding magnitude images to obtain susceptibility weighted images. The individual images were then averaged. Conventional single echo data were acquired for comparison. Maps of R₂* relaxation rates were computed from the magnitude data. Field maps derived from the phase data were used to correct R₂* for the influences from background inhomogeneities of the static magnetic field.

Results:

Compared with the single echo images, the combined images had an increase in SNR by 46% and an improvement in CNR by 34 to 80%, improved visibility of small venous vessels and reduced blurring along the readout direction. The R₂* values of different tissue types are in good agreement with values from the literature.

Conclusion:

Acquisition of SWI with multiple echoes leads to an increase in SNR and CNR and it allows the computation of high resolution maps of R₂* relaxation. J. Magn. Reson. Imaging 2010;31:185–191. © 2009 Wiley‐Liss, Inc.