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.     

Diffusion sensitivity of turbo spin echo sequences

This article introduces an effective b-factor b(TSE) for turbo spin echo (TSE) sequences to quantify their inherent diffusion sensitivity. b(TSE) is investigated for a broad variety of two-dimensional- and three-dimensional-TSE sequences using constant and varying flip angles (transitions between pseudo steady states, SPACE, VISTA, Cube, etc.). The inherent TSE diffusion sensitivity becomes important for high-resolution protocols, which can lead to subtle contrast modifications or even fluid suppressions in a clinical setting or animal imaging regime. The b(TSE) values obtained considerably depend on the relaxation times and diffusion coefficient and, thus, on the tissue under observation. The fractional b(TSE) contributions per TSE imaging encoding axis are highly anisotropic. Further noteworthy effects such as decreasing b-factors along a TSE train are pointed out and explained. The results are also discussed in combination with recent findings regarding contrast properties and possible diffusion sensitivity of TSE sequences. Identical but well more pronounced b(TSE) effects are observed in the animal imaging regime due to smaller field of view and higher resolutions.     

Density weighted turbo spin echo imaging

Purpose

To optimize the spatial response function (SRF) while maintaining optimal signal to noise ratio (SNR) in T₂ weighted turbo spin echo (TSE) imaging by prospective density weighting.

Materials and Methods

Density weighting optimizes the SRF by sampling the k‐space with variable density without the need of retrospective filtering, which would typically result in nonoptimal SNR. For TSE, the T₂ decay needs to be considered when calculating an optimized sampling pattern. Simulations were carried out and T₂ weighted in vivo TSE measurements were performed on a 3 Tesla MRI system. To evaluate the SNR, reversed centric density weighted and retrospectively filtered Cartesian acquisitions with identical measurement parameters and SRFs were compared with TE_eff = 90 ms and a density weighted k‐space sampling optimized to yield a Kaiser function for SRF side lobe suppression for white matter.

Results

Density weighting of a reversed centric reordering scheme resulted in an SNR increase of (43 ± 13)% compared with the Cartesian acquisition with retrospective filtering while maintaining comparable contrast behavior.

Conclusion

Density weighting is applicable to TSE imaging and results in significantly increased SNR. The gain can be used to shorten the measurement time, which suggests applying density weighting in both time and SNR constrained MRI. J. Magn. Reson. Imaging 2013;37:965–973. © 2013 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.     

Axial black blood turbo spin echo imaging of the right ventricle

Black blood turbo spin echo (TSE) imaging of the right ventricle (RV) free wall is highly sensitive to cardiac motion, frequently resulting in nondiagnostic images. Temporal and spatial parameters of a black blood TSE pulse sequence were evaluated for visualization of the RV free wall. Seventy‐four patient studies were retrospectively evaluated for the effects of acquisition timing on image quality. Axial black blood TSE images were acquired on 10 healthy volunteers to assess the role of spatial misregistration on right ventricle visualization; increasing the double inversion recovery (DIR) slice thickness beyond 300% had no effect on image quality (P = 0.2). Thirty‐five patient studies were prospectively evaluated with inversion times (TIs) corresponding to the mid‐diastolic rest period and end‐systole based on visual analysis of a four chamber cine. When TIs were chosen to be within the patients' RV rest period, mean image quality score was significantly improved (2.3 vs 1.86; P < 0.001) and the number of clinically diagnostic images increased from 32% to 46%. Black blood TSE imaging of the RV free wall is highly sensitive to cardiac motion. Image quality can be improved by choosing TIs concordant with the rest period of the patient's RV that may occur at mid‐diastole or end‐systole. Magn Reson Med 61:307–314, 2009. © 2009 Wiley‐Liss, Inc.     

T1 maps from shifted spin echoes and stimulated echoes.

A new pulse sequence for fast multislice T1 mapping is presented. This method is based on calculating T1 from spin echo (SE) and stimulated echo (STE) images obtained with different degrees of T1 weighting, and uses the interleaved acquisition scheme of the fast phase acquisition of composite echoes (FastPACE) technique. In contrast to the FastPACE technique, the two echoes are sampled separately. Experimental comparisons confirm that the new sequence layout overcomes most of the FastPACE restrictions, such as its motion sensitivity and the need for a fully complex data set. Moreover, this method offers a higher precision for long T1 values and a further reduction of acquisition time.     

Full-brain T1 mapping through inversion recovery fast spin echo imaging with time-efficient slice ordering.

Brain T1 mapping has important clinical applications in detecting brain disorders. Conventional T1 mapping techniques are usually based on inversion recovery spin echo (IRSE) imaging or its more time-efficient counterpart inversion recovery fast spin echo (IRFSE) imaging because they can deliver good image quality. Multiple inversion times are required to accurately estimate T1 over a wide range of values. Without acquisition optimization, both the IRSE and the IRFSE T1 mapping techniques require long scan times to image the whole brain. To reduce the scan time and maintain the quality of the T1 maps, we propose a new full-brain T1 mapping pulse sequence based on a multislice inversion recovery fast spin echo imaging using a time-efficient slice ordering technique.     

Intuitive design guidelines for fast spin echo imaging with variable flip angle echo trains.

We present a simple and intuitive means for determining the flip angles (FAs) required for smooth transitions between static pseudo steady states (SPSSs) in fast spin echo (FSE) imaging with variable FA (VFA) echo trains. We demonstrate the effectiveness of single and multiple transition pulses to successfully vary refocusing FAs while retaining high signal levels. The graphical interpretation presented here is consistent with previous analytical techniques and permits accurate signal-intensity predictions along the echo train.     

Magnetic resonance imaging of the cervical spine: comparison of 2D T2-weighted turbo spin echo, 2D T2*weighted gradient-recalled echo and 3D T2-weighted variable flip-angle turbo spin echo sequences

To compare an isotropic three-dimensional (3D) high-resolution T2-weighted (w) MR sequence and its reformations with conventional sequences for imaging of the cervical spine. Fifteen volunteers were examined at 1.5 T using sagittal and axial 3D T2-w, sagittal and axial 2D T2w, and axial 2D T2*w MR sequences. Axial reformations of the sagittal 3D dataset were generated (3D MPR T2w). Signal-to-noise and image homogeneity were evaluated in a phantom and in vivo. Visibility of ten anatomical structures of the cervical spine was evaluated. Artifacts were assessed. For statistical analysis, Cohen’s kappa, Wilcoxon matched pairs, and t-testing were utilized. There were no significant differences in homogeneity between the sequences. Sagittal 3D T2w enabled better delineation of nerve roots, neural foramina, and intraforaminal structures compared to sagittal 2D T2w. Axial 3D T2w and axial 3D MPR T2w resulted in superior visibility of most anatomical structures compared to axial 2D T2w and comparable results to 2D T2*w concerning the spinal cord, nerve roots, intraforaminal structures, and fat. Artifacts were most pronounced in axial 2D T2w and axial 3D T2w. Acquisition of a 3D T2w data set is feasible in the cervical spine with superior delineation of anatomical structures compared to 2D sequences.     

Bone marrow lesions: evaluation with fat-suppression turbo spin echo MR imaging at 0.5 T

The purpose of this study was the assessment of the diagnostic value of fat-suppression T2-weighted images for a variety of bone marrow lesions. We performed 40 studies of the axial or appendicular skeleton in 33 patients (age range 4–80 years) with neoplastic, inflammatory or traumatic lesions with a 0.5 T system (Glyroscan T5, Philips Medical Systems, Best, The Netherlands). Fat-suppression T2-weighted images [turbo spin echo (TSE) with spectral presaturation with inversion recovery (SPIR)] were obtained in addition to the routine T1-weighted SE and T2-weighted TSE sequences. Fat-suppression TSE T2-weighted images were better than standard TSE T2-weighted images in 25 studies. In 11 of them demonstration and characterization of the lesions (known from T1-weighted images) was possible only after fat suppression In the other 14 patients demonstration of the full extent of the lesion especially to the nearby soft tissues was possible only after fat suppression. In 13 studies no advantage was conferred by SPIR, whereas in two instances T2-weighted images were better. Fat-suppression T2-weighted images are diagnostically usefull in a variety of lesions of the musculoskeletal system, but their limitations should be known.Correspondence to: H. Chrysikopoulos     

Evaluation of turbo spin echo sequences for MRI of focal liver lesions at 0.5 T

To determine whether turbo spin echo (TSE) sequences can replace conventional T2-weighted spin echo (SE) sequences in MRI of the liver, 40 patients with focal liver lesions were imaged at 0.5 T. A T2-weighted SE sequences (TR/TE 1800/90 ms, number of signals averaged [NEX]=2, scan time=7:16 min), a TSE sequence (TR/TE 1800/90 ms, NEX=4, number of echos per excitation=13, echo spacing=12.9 ms, scan time=4:16 min) and a T1-weighted SE sequence (TR/TE 350/15 ms, NEX=2, scan time=4:21 min) were obtained and image quality, lesion detectability and lesion differentiation were evaluated qualitatively by subjective assessment using scores and quantitatively by lesion-liver contrast-to-noise (CNR) and tumour/liver signal intensity (SI) ratios. The image quality of the TSE sequence was substantially better compared with the T2-weighted SE sequence due to a reduction in motion artefacts and better delineation of anatomical details. Of a total of 158 visible lesions the T1-weighted SE, TSE, and T2-weighted SE sequences showed 91%, 81% and 65% of the lesions, respectively. Thus the TSE sequence depicted 24% (P< 0.001) more lesions than the T2-weighted SE sequence. In all types of pathology the lesion-liver CNR of the TSE sequence was significantly (P< 0.001) higher compared to the CNR of the T2-weighted SE sequence (+ 55–65%), indicating superior lesion conspicuity. Lesion characterization was equally good on the two T2-weighted sequences with no difference in the tumour/liver SI ratio. Using a criterion of tumour/liver SI ratio equal to or higher than 2, haemangiomas larger than 1 cm in diameter could be differentiated from other lesions with a sensitivity and specificity of 95% and 96%, respectively. Our results indicate that the TSE sequence is suitable for replacing the conventional T2-weighted SE sequence in MRI of focal liver lesions.This paper was presented at ECR 1993 Correspondence to: B. Kreft     

Bayesian algorithm using spatial priors for multiexponential T(2) relaxometry from multiecho spin echo MRI

Multiexponential T₂ relaxometry is a powerful research tool for detecting brain structural changes due to demyelinating diseases such as multiple sclerosis. However, because of unusually high signal‐to‐noise ratio requirement compared with other MR modalities and ill‐posedness of the underlying inverse problem, the T₂ distributions obtained with conventional approaches are frequently prone to noise effects. In this article, a novel multivoxel Bayesian algorithm using spatial prior information is proposed. This prior takes into account the expectation that volume fractions and T₂ relaxation times of tissue compartments change smoothly within coherent brain regions. Three‐dimensional multiecho spin echo MRI data were collected from five healthy volunteers at 1.5 T and myelin water fraction maps were obtained using the conventional and proposed algorithms. Compared with the conventional method, the proposed method provides myelin water fraction maps with improved depiction of brain structures and significantly lower coefficients of variance in white matter. Magn Reson Med, 2012. © 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.