Gradient- And Spin-echo Imaging Research Articles

Use of the NESMA Filter to Improve Myelin Water Fraction Mapping with Brain MRI.

Myelin water fraction (MWF) mapping permits direct visualization of myelination patterns in the developing brain and in pathology. MWF is conventionally measured through multiexponential T2 analysis which is very sensitive to noise, leading to inaccuracies in derived MWF estimates. Although noise reduction filters may be applied during postprocessing, conventional filtering can introduce bias and obscure small structures and edges. Advanced nonblurring filters, while effective, exhibit a high level of complexity and the requirement for supervised implementation for optimal performance. The purpose of this paper is to demonstrate the ability of the recently introduced nonlocal estimation of multispectral magnitudes (NESMA) filter to greatly improve the determination of MWF parameter estimates from gradient and spin echo (GRASE) imaging data. We evaluated the performance of the NESMA filter for MWF mapping from clinical GRASE imaging data of the human brain, and compared the results to those calculated from unfiltered images. Numerical and in vivo analyses of the brains of three subjects, representing different ages, were conducted. Our results demonstrated the potential of the NESMA filter to permit high-quality in vivo MWF mapping. Indeed, NESMA permits substantial reduction of random variation in derived MWF estimates while preserving accuracy and detail. In vivo estimation of MWF in the human brain from GRASE imaging data was markedly improved through use of the NESMA filter. The use of NESMA may contribute to the goal of high-quality MWF mapping in clinically feasible imaging times.

7T MRI‐Histologic Correlation Study of Low Specific Absorption Rate T2‐Weighted GRASE Sequences in the Detection of White Matter Involvement in Multiple Sclerosis

The high value of the specific absorption rate (SAR) of radio-frequency (RF) energy arising from the series of RF refocusing pulses in T2-weighted (T2-w) turbo spin echo (TSE) MRI hampers its clinical application at 7.0 Tesla (7T). T2-w gradient and spin echo (GRASE) uses the speed from gradient refocusing in combination with the chemical-shift/static magnetic field (B0) inhomogeneity insensitivity from spin-echo refocusing to acquire T2-w images with a limited number of refocusing RF pulses, thus reducing SAR. To investigate whether low SAR T2-w GRASE could replace T2-w TSE in detecting white matter (WM) disease in MS patients imaged at 7T. The .7 mm3 isotropic T2-w TSE and T2-w GRASE images with variable echo times (TEs) and echo planar imaging (EPI) factors were obtained on a 7T scanner from postmortem samples of MS brains. These samples were derived from brains of 3 female MS patients. WM lesions (WM-Ls) and normal-appearing WM (NAWM) signal intensity, WM-Ls/NAWM contrast-to-noise ratio (CNR) and MRI/myelin staining sections comparisons were obtained. GRASE sequences with EPI factor/TE = 3/50 and 3/75 ms were comparable to the SE technique for measures of CNR in WM-Ls and NAWM and for detection of WM-Ls. In all sequences, however, identification of areas with remyelination, Wallerian degeneration, and gray matter demyelination, as depicted by myelin staining, was not possible. T2-w GRASE images may replace T2-w TSE for clinical use. However, even at 7T, both sequences fail in detecting and characterizing MS disease beyond visible WM-Ls.

Anatomical brain imaging at 7T using two-dimensional GRASE

Specific absorption rate is a serious problem at high field strengths, especially for sequences involving many high power radiofrequency pulses, such as turbo spin echo (TSE). GRASE (gradient and spin echo) may overcome this problem by omitting a certain number of refocusing pulses of a TSE sequence, and replacing them with segmented echo-planar imaging readouts. GRASE and TSE were compared using similar sequence parameters at a field strength of 7T. The signal-to-noise ratio (SNR) per unit time, contrast, and point spread function (PSF) were determined. High-resolution human brain images were acquired and the implementation of an inversion recovery preparation for T(1) weighting was evaluated. TSE and GRASE images at 7T showed very similar SNR and contrast. The slightly worse PSF for GRASE is balanced by a significant reduction in scan time or increase in spatial coverage compared with TSE. Furthermore, implementing an additional inversion recovery preparation enables the acquisition of T(1)-weighted images with high SNR per unit time. GRASE is highly suitable for structural scanning at ultra-high field strengths and is a valid alternative to the commonly used TSE sequence.

Rapid whole cerebrum myelin water imaging using a 3D GRASE sequence

Myelin water imaging, a magnetic resonance imaging technique capable of resolving the fraction of water molecules which are located between the layers of myelin, is a valuable tool for investigating both normal and pathological brain structure in vivo. There is a strong need for pulse sequences which improve the quality and applicability of myelin water imaging in a clinical setting. In this study, we validated the use of a fast multi echo T(2) relaxation sequence for myelin water imaging. Using a multiple combined gradient and spin echo (GRASE) technique, we attain whole cerebrum myelin water images in under 15 minutes. Region of interest analysis indicates that this fast GRASE imaging sequence produces results which are in good agreement with pure spin echo measurements (R(2)=0.95, p<0.0001). This drastic improvement in speed and brain coverage compared to current spin echo standards will allow increased inclusion of myelin water imaging in neurological research protocols and opens up the possibility of applications in a clinical setting.

Gradient‐ and spin‐echo T2‐weighted imaging for SPIO‐enhanced detection and characterization of focal liver lesions

To evaluate superparamagnetic iron oxide (SPIO)-enhanced breathhold T2-weighted GRASE imaging in detection and characterization of focal liver lesions. In 30 patients (including 20 with cirrhosis) with 39 malignant and 25 benign lesions, gradient- and spin-echo (GRASE) images with two echo times (75 and 90 msec; GRASE75 and GRASE90) were obtained prior to and following administration of SPIO, and compared with respiratory-triggered and breathhold fast spin-echo (RT-FSE and BH-FSE) images. Two readers evaluated image quality and reviewed 240 liver segments for sensitivity and specificity. Signal-to-noise ratio (SNR), and its reduction in liver and spleen after administration of SPIO, and lesion-to-liver contrast-to-noise ratio (CNR) were measured. Compared with RT-FSE and BH-FSE, GRASE reduced scan time by 77% to 82% and 21% to 27%, respectively. The image qualities with BH-FSE and GRASE75 were higher than with BH-FSE and GRASE90. BH-FSE showed higher specificity than RT-FSE and GRASE90, but otherwise there were no significant differences between pulse sequences for sensitivity or specificity. The mean SNR and CNR of the lesions with RT-FSE were significantly higher than with the other methods. SPIO-induced signal reduction of liver SNR was smallest with BH-FSE. GRASE is faster and more sensitive to SPIO than FSE, but its sensitivity and specificity were slightly inferior to those of BH-FSE. Image quality is a current limitation.

Focal liver lesions: Breathhold gradient‐ and spin‐echo T2‐weighted imaging for detection and characterization

To evaluate breathhold gradient- and spin-echo (GRASE) T2-weighted imaging for the detection and characterization of focal liver lesions. Two GRASE sequences with different echo times (75 and 90 msec, GRASE75 and GRASE90) were compared with respiratory-triggered fast spin-echo (SE) and breathhold fast SE in 64 patients with 103 malignant and 51 benign lesions. Compared with respiratory-triggered and breathhold fast SE, GRASE reduced scan time by 77% to 82% and 21% to 27%, respectively. Two independent readers evaluated image quality and reviewed 504 liver segments on a segment-by-segment basis. Observer performance was evaluated with receiver operating characteristic (ROC) curve analysis. The signal-to-noise ratio (SNR) of liver and spleen, and lesion-to-liver contrast-to-noise ratio (CNR) were also measured. The overall quality of the GRASE images was higher than that of the respiratory-triggered and breathhold fast SE images, although signal inhomogeneities were more frequently observed with GRASE. No significant difference in the values of the area under the ROC curve (Az) for malignant lesion detection was found. The mean SNR and CNR were highest for respiratory-triggered fast SE. T2-weighted breathhold GRASE has the potential to provide faster liver imaging.

Functional MRI of the human brain with GRASE-based BOLD contrast.

The application of T2*-weighted gradient and spin-echo (GRASE) imaging was investigated as a method for blood oxygenation level-dependent (BOLD)-based functional magnetic resonance imaging (fMRI). The displaced-echo method was implemented to produce single-shot T2*-weighted GRASE images. This technique removes the requirement that the Carr-Purcell Meiboom-Gill (CPMG) condition be fulfilled. T2*-weighted GRASE images that are free from interference artifacts can thus be obtained, hence allowing the possibility of using single-shot GRASE for BOLD-based functional imaging. The method was demonstrated at 3 T and gave robust and reproducible activation-induced signal changes.

Improved three-dimensional GRASE imaging with the SORT phase-encoding strategy

The phase-encoding strategy plays a critical role in determining the quality of gradient- and spin-echo (GRASE) images. Phase-encoding methods developed for two-dimensional GRASE imaging strive to achieve a balance between artifacts from T2-dependent signal amplitude modulations and off-resonance-dependent signal phase shifts, although no current method provides smooth and nonperiodic evolutions for both of these signal changes. In three-dimensional GRASE imaging, the use of two phase-encoding directions presents the opportunity for improved phase-encoding strategies. In this report a phase-encoding strategy for three-dimensional GRASE, termed SORT, is described; this strategy separates off-resonance and T2 effects, mapping one along each of the two phase-encoding directions. Thus, off-resonance-induced artifacts can be minimized while eliminating T2-dependent periodic signal modulations and allowing complete flexibility in the selection of echo time. The performance of the SORT phase-encoding method for T2-weighted GRASE imaging was compared with that of existing methods based on calculated point spread functions and simulated images. The predicted performance of SORT phase encoding was verified experimentally using T2-weighted three-dimensional GRASE imaging of the brain. Generally artifact-free images were obtained even in the presence of fat, susceptibility interfaces, and a wide range of T2 values.

GRASE imaging at 3 Tesla with template interactive phase-encoding.

A new method for ordering the phase-encoding gradient is proposed, and an application for short effective TE gradient- and spin-echo (GRASE) imaging is demonstrated. The proposed method calculates the phase-encoding order from the signal decay of a template scan (hence "template interactive phase-encoding" or TIPE). Computer simulations are used to compare the point spread functions of different phase-encoding orders giving short effective echo times (kb centric GRASE, centric GRASE, centric TIPE). The conventional centric phase-encoding order is also considered for GRASE. The conventional centric method is sensitive to both amplitude and phase modulation of the signal in k-space. The centric TIPE method gives the least amplitude modulation artifacts but is vulnerable to phase artifacts. The TIPE experiment was implemented on a 3 Tesla system. To the best of our knowledge, we present the first in vivo GRASE images at this field strength.

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.

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, kb-GRASE 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.

Increased flexibility in GRASE imaging by k space-banded phase encoding.

GRASE (GRadient and spin Echo) is an echo train imaging technique that combines gradient and RF refocusing. Although overall signal decay is with T2 and field inhomogeneity phase errors do not accumulate, the small residual phase errors are periodic with echo number. The echo order described previously eliminates the phase error periodicity in k space but instead creates periodicity in the T2 modulation function that can also cause artifacts. In addition, with this order, the effective TE must be half the echo train time, and asymmetric Fourier sampling is difficult to implement. A new method is described that greatly reduces artifacts due to T2 decay, permits greater control of T2 contrast, and lends itself to asymmetric Fourier sampling. Different time segments of the echo train are encoded with different bands of spatial frequency in k space (hence "k banding"). Both computer simulations and experimental results demonstrate improvements in GRASE images acquired by this method.

GRASE (Gradient‐and Spin‐Echo) imaging: A novel fast MRI technique

A fast multi-section MR imaging technique is described. Gradient- and spin-echo (GRASE) imaging utilizes the speed advantages of gradient refocusing while overcoming the image artifacts arising from static field inhomogeneity and chemical shift. Image contrast is determined by the T2 contrast in the Hahn spin echoes. A novel k-space trajectory temporally modulates signals and demodulates artifacts.