T2‐weighted 3D fast spin echo imaging with water–fat separation in a single acquisition

Purpose:

To develop a robust 3D fast spin echo (FSE) T₂‐weighted imaging method with uniform water and fat separation in a single acquisition, amenable to high‐quality multiplanar reformations.

Materials and Methods:

The Iterative Decomposition of water and fat with Echo Asymmetry and Least squares estimation (IDEAL) method was integrated with modulated refocusing flip angle 3D‐FSE. Echoes required for IDEAL processing were acquired by shifting the readout gradient with respect to the Carr‐Purcell‐Meiboom‐Gill echo. To reduce the scan time, an alternative data acquisition using two gradient echoes per repetition was implemented. Using the latter approach, a total of four gradient echoes were acquired in two repetitions and used in the modified IDEAL reconstruction.

Results:

3D‐FSE T₂‐weighted images with uniform water–fat separation were successfully acquired in various anatomies including breast, abdomen, knee, and ankle in clinically feasible scan times, ranging from 5:30–8:30 minutes. Using water‐only and fat‐only images, in‐phase and out‐of‐phase images were reconstructed.

Conclusion:

3D‐FSE‐IDEAL provides volumetric T₂‐weighted images with uniform water and fat separation in a single acquisition. High‐resolution images with multiple contrasts can be reformatted to any orientation from a single acquisition. This could potentially replace 2D‐FSE acquisitions with and without fat suppression and in multiple planes, thus improving overall imaging efficiency. J. Magn. Reson. Imaging 2010;32:745–751. © 2010 Wiley‐Liss, Inc.     

High‐resolution functional MRI at 3 T: 3D/2D echo‐planar imaging with optimized physiological noise correction

High‐resolution functional MRI (fMRI) offers unique possibilities for studying human functional neuroanatomy. Although high‐resolution fMRI has proven its potential at 7 T, most fMRI studies are still performed at rather low spatial resolution at 3 T. We optimized and compared single‐shot two‐dimensional echo‐planar imaging (EPI) and multishot three‐dimensional EPI high‐resolution fMRI protocols. We extended image‐based physiological noise correction from two‐dimensional EPI to multishot three‐dimensional EPI. The functional sensitivity of both acquisition schemes was assessed in a visual fMRI experiment. The physiological noise correction increased the sensitivity significantly, can be easily applied, and requires simple recordings of pulse and respiration only. The combination of three‐dimensional EPI with physiological noise correction provides exceptional sensitivity for 1.5 mm high‐resolution fMRI at 3 T, increasing the temporal signal‐to‐noise ratio by more than 25% compared to two‐dimensional EPI. Magn Reson Med, 2013. © 2012 The Authors. Magnetic Resonance in Medicine Published by Wiley Periodicals, Inc. on behalf of International Society of Medicine in Resonance.     

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.     

Combined use of T2‐weighted MRI and T1‐weighted dynamic contrast–enhanced MRI in the automated analysis of breast lesions

A multiparametric computer‐aided diagnosis scheme that combines information from T₁‐weighted dynamic contrast–enhanced (DCE)‐MRI and T₂‐weighted MRI was investigated using a database of 110 malignant and 86 benign breast lesions. Automatic lesion segmentation was performed, and three categories of lesion features (geometric, T₁‐weighted DCE, and T₂‐weighted) were automatically extracted. Stepwise feature selection was performed considering only geometric features, only T₁‐weighted DCE features, only T₂‐weighted features, and all features. Features were merged with Bayesian artificial neural networks, and diagnostic performance was evaluated by ROC analysis. With leave‐one‐lesion‐out cross‐validation, an area under the ROC curve value of 0.77 ± 0.03 was achieved with T₂‐weighted‐only features, indicating high diagnostic value of information in T₂‐weighted images. Area under the ROC curve values of 0.79 ± 0.03 and 0.80 ± 0.03 were obtained for geometric‐only features and T₁‐weighted DCE‐only features, respectively. When all features were considered, an area under the ROC curve value of 0.85 ± 0.03 was achieved. We observed P values of 0.006, 0.023, and 0.0014 between the geometric‐only, T₁‐weighted DCE‐only, and T₂‐weighted‐only features and all features conditions, respectively. When ranked, the P values satisfied the Holm–Bonferroni multiple‐comparison test; thus, the improvement of multiparametric computer‐aided diagnosis was statistically significant. A computer‐aided diagnosis scheme that combines information from T₁‐weighted DCE and T₂‐weighted MRI may be advantageous over conventional T₁‐weighted DCE‐MRI computer‐aided diagnosis. Magn Reson Med, 2011. © 2011 Wiley‐Liss, 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.     

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.     

Improved T2 mapping accuracy with dual‐echo turbo spin echo: Effect of phase encoding profile orders

Turbo spin echo (TSE) pulse sequences have been applied to estimate T₂ relaxation times in clinically feasible scan times. However, T₂ estimations using TSE pulse sequences has been shown to differ considerable from reference standard sequences due to several sources of error. The purpose of this work was to apply voxel‐sensitivity formalism to correct for one such source of error introduced by differing phase encoding profile orders with dual‐echo TSE pulse sequences. The American College of Radiology phantom and the brains of two healthy volunteers were imaged using dual‐echo TSE as well as 32‐echo spin‐echo acquisitions and T₂ estimations from uncorrected and voxel‐sensitivity formalism‐corrected dual‐echo TSE and 32‐echo acquisitions were compared. In all regions of the brain and the majority of the analyses of the American College of Radiology phantom, voxel‐sensitivity formalism correction resulted in considerable improvements in dual‐echo TSE T₂ estimation compared with the 32‐echo acquisition, with improvements in T₂ value accuracy ranging from 5.2% to 18.6%. Magn Reson Med, 2013. © 2012 Wiley Periodicals, Inc.     

A technique for rapid single‐echo spin‐echo T2 mapping

A rapid technique for mapping of T₂ relaxation times is presented. The method is based on the conventional single‐echo spin echo approach but uses a much shorter pulse repetition time to accelerate data acquisition. The premise of the new method is the use of a constant difference between the echo time and pulse repetition time, which removes the conventional and restrictive requirement of pulse repetition time ? T₁. Theoretical and simulation investigations were performed to evaluate the criteria for accurate T₂ measurements. Measured T₂s were shown to be within 1% error as long as the key criterion of pulse repetition time/T₂ ≥3 is met. Strictly, a second condition of echo time/T₁ ? 1 is also required. However, violations of this condition were found to have minimal impact in most clinical scenarios. Validation was conducted in phantoms and in vivo T₂ mapping of healthy cartilage and brain. The proposed method offers all the advantages of single‐echo spin echo imaging (e.g., immunity to stimulated echo effects, robustness to static field inhomogeneity, flexibility in the number and choice of echo times) in a considerably reduced amount of time and is readily implemented on any clinical scanner. Magn Reson Med, 2010. © 2010 Wiley‐Liss, 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.     

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.