Spectrally selective B1‐insensitive T2 magnetization preparation sequence

A T₂ magnetization‐preparation (T₂ Prep) sequence is proposed that is insensitive to B₁ field variations and simultaneously provides fat suppression without any further increase in specific absorption rate (SAR). Increased B₁ inhomogeneity at higher magnetic field strength (B₀ ≥ 3T) necessitates a preparation sequence that is less sensitive to B₁ variations. For the proposed technique, T₂ weighting in the image is achieved using a segmented B₁‐insensitive rotation (BIR‐4) adiabatic pulse by inserting two equally long delays, one after the initial reverse adiabatic half passage (AHP), and the other before the final AHP segment of a BIR‐4 pulse. This sequence yields T₂ weighting with both B₁ and B₀ insensitivity. To simultaneously suppress fat signal (at the cost of B₀ insensitivity), the second delay is prolonged so that fat accumulates additional phase due to its chemical shift. Numerical simulations as well as phantom and in vivo image acquisitions were performed to show the efficacy of the proposed technique. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.     

Flow‐independent T2‐prepared inversion recovery black‐blood MR imaging

Purpose:

To develop a magnetization preparation method to achieve robust, flow‐independent blood suppression for cardiac and vascular magnetic resonance imaging (MRI).

Materials and Methods:

T₂Prep‐IR sequence consists of a T₂ preparation followed by a nonselective adiabatic inversion pulse. T₂Prep separates the initial longitudinal magnetization of arterial wall from lumen blood. After the inversion recovery pulse the imaging acquisition is then delayed for a period that allows the blood signal to approach the zero‐crossing point. Compared to the conventional double inversion recovery (DIR) preparation, T₂Prep‐IR prepares all the spins regardless of their velocity and direction. T₂Prep‐IR was incorporated into the fast spin echo and fast gradient echo acquisition sequences and images in various planes were acquired in the carotid arteries, thoracic aorta, and heart of normal volunteers. Blood suppression and image quality were compared qualitatively between two different preparations.

Results:

For in‐plane flow carotid images, persistent flow‐related artifacts on the DIR images were removed with T₂Prep‐IR. For cardiac applications, T₂Prep‐IR provided robust blood suppression regardless of the flow direction and velocity, including the cardiac long‐axis views and the aorta that are often problematic with DIR.

Conclusion:

T₂Prep‐IR may overcome the flow dependence of DIR by providing robust flow‐independent black‐blood images. J. Magn. Reson. Imaging 2010;31:248–254. © 2009 Wiley‐Liss, Inc     

Spatially selective implementation of the adiabatic T2prep sequence for magnetic resonance angiography of the coronary arteries

In coronary magnetic resonance angiography, a magnetization‐preparation scheme for T₂‐weighting (T₂Prep) is widely used to enhance contrast between the coronary blood‐pool and the myocardium. This prepulse is commonly applied without spatial selection to minimize flow sensitivity, but the nonselective implementation results in a reduced magnetization of the in‐flowing blood and a related penalty in signal‐to‐noise ratio. It is hypothesized that a spatially selective T₂Prep would leave the magnetization of blood outside the T₂Prep volume unaffected and thereby lower the signal‐to‐noise ratio penalty. To test this hypothesis, a spatially selective T₂Prep was implemented where the user could freely adjust angulation and position of the T₂Prep slab to avoid covering the ventricular blood‐pool and saturating the in‐flowing spins. A time gap of 150 ms was further added between the T₂Prep and other prepulses to allow for in‐flow of a larger volume of unsaturated spins. Consistent with numerical simulation, the spatially selective T₂Prep increased in vivo human coronary artery signal‐to‐noise ratio (42.3 ± 2.9 vs. 31.4 ± 2.2, n = 22, P < 0.0001) and contrast‐to‐noise‐ratio (18.6 ± 1.5 vs. 13.9 ± 1.2, P = 0.009) as compared to those of the nonselective T₂Prep. Additionally, a segmental analysis demonstrated that the spatially selective T₂Prep was most beneficial in proximal and mid segments where the in‐flowing blood volume was largest compared to the distal segments. Magn Reson Med, 2013. © 2012 Wiley Periodicals, Inc.     

Consistent fat suppression with compensated spectral-spatial pulses

Reliable fat suppression is especially important with fast imaging techniques such as echo-planar (EPI), spiral, and fast spin-echo (FSE) T₂-weighted imaging. Spectral-spatial excitation has a number of advantages over spectrally selective presaturation techniques, including better resilience to B₀ and B₁, inhomogeneity. In this paper, a FSE sequence using a spectral-spatial excitation pulse for superior fat suppression is presented. Previous problems maintaining the CPMG condition are solved using simple methods to accurately program radio-frequency (RF) phase. Next an analysis shows how B₀ eddy currents can reduce fat suppression effectiveness for spectral-spatial pulses designed for conventional gradient systems. Three methods to compensate for the degradation are provided. Both the causes of the degradation and the compensation techniques apply equally to gradient-recalled applications using these pulses. These problems do not apply to pulses designed for high-speed gradient systems. The spectral-spatial FSE sequence delivers clinically lower fat signal with better uniformity than spectrally selective pre-saturation techniques.     

Improved delayed enhanced myocardial imaging with T2-Prep inversion recovery magnetization preparation

Purpose

To develop a magnetization preparation method that improves the differentiation of enhancing subendocardial infarction (MI) from ventricular blood for myocardial delayed‐enhancement (DE) magnetic resonance imaging (MRI).

Materials and Methods

T₂Prep‐IR is a magnetization preparation pulse that consists of a T₂ preparation (T₂Prep) followed immediately by a nonselective inversion recovery (IR) pulse. The first imaging excitation is then delayed an inversion time (TI) to allow nulling of normal myocardium in DE study. The amount of T₂ contrast is determined by the effective echo time of the T₂Prep pulse, TE_eff. TE_eff is selected to differentiate MI and blood that share similar T₁ values but have different T₂ values. The T₂Prep‐IR preparation was incorporated into a fast gradient echo sequence to produce an image with both T₁ and T₂ weighting. Simulations predict that this method will generate improved contrast between MI and chamber blood compared to conventional IR methods.

Results

Comparisons between images acquired using conventional IR and T₂Prep‐IR in patients with MI indicate that this new approach significantly improves the blood‐MI contrast (122 ± 32% higher than that of IR with P < 0.05).

Conclusion

Our preliminary patient studies confirm that this preparation is helpful for improved delineation of subendocardial infarction. J. Magn. Reson. Imaging 2008;28:1280–1286. © 2008 Wiley‐Liss, Inc.     

Pulsed magnetization transfer contrast for MR imaging with application to breast

The relative populations and transverse relaxation times of the solid-like hydrogen pool (P_B and T_2B) and the magnetization transfer (MT) rates between the solid-like and liquid-like hydrogen pools (K) have been determined for three different agar gel concentrations (2%, 4%, and 8% by weight) as well as excised fibroglandular breast tissue specimens. P_B was determined to be .003(.001), .01(.002), .02(.01), and .06(.01); T_2B was determined to be 13.0(.2), 14.0(.1), 14.5(.1) and 15.2(1.3) μs; and K was determined to be 0.78(.01), 1.15(.02), 2.00(.02), and 3.55(1.5) sec^?1 for the 2%, 4%, and 8% agar gels and the fibroglandular tissue, respectively. The image signal intensities of a pulsed MTC-prepared gradient-echo imaging technique are predicted using these MT parameters and are shown to agree well with experimental data obtained from a clinical MR imaging system. This technique is shown to suppress signal intensity of fibroglandular breast tissue by 40%–50% without exceeding SAR limits (≤ 8W/kg) and is helpful for visualizing lesions and silicone implants.     

Correction of main and transmit magnetic field (B0 and B1) inhomogeneity effects in multicomponent‐driven equilibrium single‐pulse observation of T1 and T2

Multicomponent‐driven equilibrium single‐component observation of T₁ and T₂ offers a new approach to multiple component relaxation time and myelin water analysis. The method derives two‐component relaxation information from spoiled and fully balanced steady‐state (SPGR and bSSFP) imaging data acquired over multiple flip angles. Although these steady‐state imaging techniques afford rapid acquisition times and high signal‐to‐noise ratio efficiency, they are also sensitive to main (B₀) and transmit (B₁) magnetic field inhomogeneities. These effects alter the measured signal from their theoretical values and lead to substantive errors in the derived myelin volume fraction estimates. Here, we incorporate correction techniques to mitigate these effects. DESPOT1‐HIFI is used to first calibrate the transmitted flip angles; and B₀ affects are removed through the inclusion of an additional parameter in the multicomponent‐driven equilibrium single‐component observation of T₁ and T₂ fitting, coupled with the acquisition of multiple phase‐cycled bSSFP data. The performance of these correction techniques was evaluated using numerical simulations, demonstrating effective removal of B₀ and B₁‐induced errors in the derived myelin fraction relaxation parameters. The approach was also successfully demonstrated in vivo, with near artifact‐free whole‐brain, high spatial resolution (1.7 mm × 1.7 mm × 1.7 mm isotropic voxels) myelin water fraction maps acquired in a clinically feasible 16 min. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.     

Motion and flow insensitive adiabatic T2‐preparation module for cardiac MR imaging at 3 tesla

A versatile method for generating T₂‐weighting is a T₂‐preparation module, which has been used successfully for cardiac imaging at 1.5T. Although it has been applied at 3T, higher fields (B₀ ≥ 3T) can degrade B₀ and B₁ homogeneity and result in nonuniform magnetization preparation. For cardiac imaging, blood flow and cardiac motion may further impair magnetization preparation. In this study, a novel T₂‐preparation module containing multiple adiabatic B₁‐insensitive refocusing pulses is introduced and compared with three previously described modules [(a) composite MLEV4, (b) modified BIR‐4 (mBIR‐4), and (c) Silver‐Hoult–pair]. In the static phantom, the proposed module provided similar or better B₀ and B₁ insensitivity than the other modules. In human subjects (n = 21), quantitative measurement of image signal coefficient of variation, reflecting overall image inhomogeneity, was lower for the proposed module (0.10) than for MLEV4 (0.15, P < 0.0001), mBIR‐4 (0.27, P < 0.0001), and Silver‐Hoult–pair (0.14, P = 0.001) modules. Similarly, qualitative analysis revealed that the proposed module had the best image quality scores and ranking (both, P < 0.0001). In conclusion, we present a new T₂‐preparation module, which is shown to be robust for cardiac imaging at 3T in comparison with existing methods. Magn Reson Med 70:1360–1368, 2013. © 2012 Wiley Periodicals, Inc.     

T1 corrected B1 mapping using multi‐TR gradient echo sequences

This work presents a new approach toward a fast, simultaneous amplitude of radiofrequency field (B₁) and T₁ mapping technique. The new method is based on the “actual flip angle imaging” (AFI) sequence. However, the single pulse repetition time (TR) pair used in the standard AFI sequence is replaced by multiple pulse repetition time sets. The resulting method was called “multiple TR B₁/T₁ mapping” (MTM). In this study, MTM was investigated and compared to standard AFI in simulations and experiments. Feasibility and reliability of MTM were proven in phantom and in vivo experiments. Error propagation theory was applied to identify optimal sequence parameters and to facilitate a systematic noise comparison to standard AFI. In terms of accuracy and signal‐to‐noise ratio, the presented method outperforms standard AFI B₁ mapping over a wide range of T₁. Finally, the capability of MTM to determine T₁ was analyzed qualitatively and quantitatively, yielding good agreement with reference measurements. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.     

A new T2 preparation technique for ultrafast gradient-echo sequence

The T₂, contrast in images obtained with driven equilibrium (90°_x?180°_x?90°_x) prepared ultrafast gradient-echo sequences is compromised by the longitudinal magnetization build-up after the second 90°_x pulse, which does not carry T₂ information. This paper describes a new T₂ contrast preparation technique for ultrafast gradient-echo sequence that suppresses the signal arising from the build-up. By dephasing in the preparation and rephasing in the acquisition of the gradient echoes, the new technique eliminates signals that are not dictated by the T₂ contrast in a driven-equilibrium approach. Consequently, it generates an image that is essentially T₂-weighted. Phantom and in vivo experiments were conducted to validate the technique and to demonstrate its clinical utility. These studies indicate that the technique works properly and can be used for in vivo studies.     

Design of practical T2-selective RF excitation (TELEX) pulses

Traditional T₂-based imaging techniques are geared toward imaging long-T₂, species. Traditional techniques are, therefore, not optimal in clinical situations where the information of interest lies in the short-T₂ species. T₂-selective RF excitation (TELEX) is a technique for obtaining a T₂-based contrast that highlights short-T₂ values while suppressing long-T₂ values-opposite to traditional T₂ contrast. Previously, TELEX has been demonstrated qualitatively to highlight only very short-T₂ values (T₂ = 0.001 s). When applied to longer T₂ values (T₂ ≈? 0.01 s), TELEX becomes sensitive to ΔB_o non-uniformities. This restricts its application to problems in which the T₂ of interest is very short. In this study, TELEX is characterized quantitatively. Furthermore, a bandwidth broadening scheme is developed that reduces the ΔB_o sensitivity of TELEX. This permits the technique to be applied to longer T₂ values. The capabilities and limitations of a practical implementation of TELEX are discussed.     

A method to improve the BO homogeneity of the heart in vivo

A homogeneous static (B_o) magnetic field is required for many NMR experiments such as echo planar imaging, localized spectroscopy, and spiral scan imaging. Although semi-automated techniques have been described to improve the B_o field homogeneity, none has been applied to the in vivo heart. The acquisition of cardiac field maps is complicated by motion, blood flow, and chemical shift artifact from epicardial fat. To overcome these problems, an ungated three-dimensional (3D) chemical shift image (CSI) was collected to generate a time and motion-averaged B_o field map. B_o heterogeneity in the heart was minimized by using a previous algorithm that solves for the optimal shim coil currents for an input field map, using up to third-order current-bounded shims (1). The method improved the B_o homogeneity of the heart in all 11 normal volunteers studied. After application of the algorithm to the unshimmed cardiac field maps, the standard deviation of proton frequency decreased by 43%, the magnitude ¹H spectral linewidth decreased by 24%, and the peak-peak gradient decreased by 35%. Simulations of the high-order (second- and third-order) shims in B_o field correction of the heart show that high order shims are important, resulting for nearly half of the improvement in homogeneity for several subjects. The T₂* of the left ventricular anterior wall before and after field correction was determined at 4.0 Tesla. Finally, results show that cardiac shimming is of benefit in cardiac ³¹P NMR spectroscopy and cardiac echo planar imaging.     

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.     

Optimal radiofrequency and gradient spoiling for improved accuracy of T1 and B1 measurements using fast steady‐state techniques

Variable flip angle T₁ mapping and actual flip‐angle imaging B₁ mapping are widely used quantitative MRI methods employing radiofrequency spoiled gradient‐echo pulse sequences. Incomplete elimination of the transverse magnetization in these sequences has been found to be a critical source of T₁ and B₁ measurement errors. In this study, comprehensive theoretical analysis of spoiling‐related errors in variable flip angle and actual flip‐angle imaging methods was performed using the combined isochromat summation and diffusion propagator model and validated by phantom experiments. The key theoretical conclusion is that correct interpretation of spoiling phenomena in fast gradient‐echo sequences requires accurate consideration of the diffusion effect. A general strategy for improvement of T₁ and B₁ measurement accuracy was proposed based on the strong spoiling regimen, where diffusion‐modulated spatial averaging of isochromats becomes a dominant factor determining magnetization evolution. Practical implementation of strongly spoiled variable flip angle and actual flip‐angle imaging techniques requires sufficiently large spoiling gradient areas (A_G) in combination with optimal radiofrequency phase increments (?₀). Optimal regimens providing <2% relative T₁ and B₁ measurement errors in a variety of tissues were theoretically derived for prospective in vivo variable flip angle (pulse repetition time = 15–20 ms, A_G = 280–450 mT·ms/m, ?₀ = 169°) and actual flip‐angle imaging (pulse repetition time₁/pulse repetition time₂ = 20/100 ms, A_G1/A_G2 = 450/2250 mT·ms/m, ?₀ = 39°) applications based on 25 mT/m maximal available gradient strength. Magn Reson Med 63:1610–1626, 2010. © 2010 Wiley‐Liss, Inc.     

One component? Two components? Three? The effect of including a nonexchanging “free” water component in multicomponent driven equilibrium single pulse observation of T1 and T2

Quantitative myelin content imaging provides novel and pertinent information related to underlying pathogenetic mechanisms of myelin‐related disease or disorders arising from aberrant connectivity. Multicomponent driven equilibrium single pulse observation of T₁ and T₂ is a time‐efficient multicomponent relaxation analysis technique that provides estimates of the myelin water fraction, a surrogate measure of myelin volume. Unfortunately, multicomponent driven equilibrium single pulse observation of T₁ and T₂ relies on a two water‐pool model (myelin‐associated water and intra/extracellular water), which is inadequate within partial volume voxels, i.e., containing brain tissue and ventricle or meninges, resulting in myelin water fraction underestimation. To address this, a third, nonexchanging “free‐water” component was introduced to the multicomponent driven equilibrium single pulse observation of T₁ and T₂ model. Numerical simulations and experimental in vivo data show that the model to perform advantageously within partial volume regions while providing robust and reproducible results. It is concluded that this model is preferable for future studies and analysis. Magn Reson Med, 2013. © 2012 Wiley Periodicals, Inc.     

Transverse relaxation time (T2) mapping in the brain with off‐resonance correction using phase‐cycled steady‐state free precession imaging

Purpose

To investigate a new approach for more completely accounting for off‐resonance affects in the DESPOT2 (driven equilibrium single pulse observation of T₂) mapping technique.

Materials and Methods

The DESPOT2 method derives T₂ information from fully balanced steady‐state free precession (bSSFP) images acquired over multiple flip angles. Off‐resonance affects, which present as bands of altered signal intensity throughout the bSSFP images, results in erroneous T₂ values in the corresponding calculated maps. Radiofrequency (RF) phase‐cycling, in which the phase of the RF pulse is incremented along the pulse train, offers a potential method for eliminating these artifacts. In this work we present a general method, referred to as DESPOT2, with full modeling (DESPOT2‐FM), for deriving T₂, as well as off‐resonance frequency, from dual flip angle bSSFP data acquired with two RF phase increments.

Results

The method is demonstrated in vivo through the acquisition of whole‐brain, 1 mm³ isotropic T₂ maps at 3T and shown to provide near artifact‐free maps, even in areas with steep susceptibility‐induced gradients.

Conclusion

DESPOT2‐FM offers an efficient method for acquiring high spatial resolution, whole‐brain T₂ maps at 3T with high precision and free of artifact. J. Magn. Reson. Imaging 2009;30:411–417. © 2009 Wiley‐Liss, Inc.     

Gleaning multicomponent T1 and T2 information from steady‐state imaging data

The driven‐equilibrium single‐pulse observation of T₁ (DESPOT1) and T₂ (DESPOT2) are rapid, accurate, and precise methods for voxelwise determination of the longitudinal and transverse relaxation times. A limitation of the methods, however, is the inherent assumption of single‐component relaxation. In a variety of biological tissues, in particular human white matter (WM) and gray matter (GM), the relaxation has been shown to be more completely characterized by a summation of two or more relaxation components, or species, each believed to be associated with unique microanatomical domains or water pools. Unfortunately, characterization of these components on a voxelwise, whole‐brain basis has traditionally been hindered by impractical acquisition times. In this work we extend the conventional DESPOT1 and DESPOT2 approaches to include multicomponent relaxation analysis. Following numerical analysis of the new technique, renamed multicomponent driven equilibrium single pulse observation of T₁/T₂ (mcDESPOT), whole‐brain multicomponent T₁ and T₂ quantification is demonstrated in vivo with clinically realistic times of between 16 and 30 min. Results obtained from four healthy individuals and two primary progressive multiple sclerosis (MS) patients demonstrate the future potential of the approach for identifying and assessing tissue changes associated with several neurodegenerative conditions, in particular those associated with WM. Magn Reson Med 60:1372–1387, 2008. © 2008 Wiley‐Liss, Inc.     

T1ρ of protein solutions at very low fields: Dependence on molecular weight,concentration, and structure

The effect of molecular weight, concentration, and structure on 1/T₁ρ, the rotating frame relaxation rate, was investigated for several proteins using the on-resonance spin-lock technique, for locking fields B₁ < 200 μT. The measured values of 1/T₁ρ, were fitted to a simple theoretical model to obtain the dispersion curves 1/T₁ρ(ω₁) and the relaxation rate at zero B₁ field, 1/T₁ρ,(O). 1/T₁ρ, was highly sensitive to the molecular weight, concentration, and structure of the protein. The amount of intra- and intermolecular hydrogen and disulfide bonds especially contributed to 1/T₁ρ. In all samples, 1/T₁ρ(O) was equal to 1/T₂ρ measured at the main magnetic field B_o = 0.1 T, but at higher locking fields the dispersion curves mono-tonically decreased. The results of this work indicate that a model considering the effective correlation time of molecular motions as the main determinant for 1/T₁ρ relaxation in protein solutions is not valid at very low B₁ fields. The underlying mechanism for the relaxation rate 1/T₁ρ at B₁ fields below 200 μT is discussed.     

An assessment of spin-echo rotating-frame imaging for spatially localized determination of short T2 relaxation times in vivo

The rotating frame method of localized spectroscopy can be augmented by the inclusion of a refocusing pulse to enable the measurement of T₂ relaxation times. This technique is particularly appropriate for determining short relaxation parameters due to the absence of time consuming switched B₀ field gradients. We have evaluated the accuracy of this protocol by measuring localized T₂s in the range of 1 to 20 ms. Preliminary data obtained from muscle and liver of normal and iron overloaded human subjects are also presented.     

Quantitative mapping of T2 using partial spoiling

Fast quantitative MRI has become an important tool for biochemical characterization of tissue beyond conventional T₁, T₂, and T₂*‐weighted imaging. As a result, steady‐state free precession (SSFP) techniques have attracted increased interest, and several methods have been developed for rapid quantification of relaxation times using steady‐state free precession. In this work, a new and fast approach for T₂ mapping is introduced based on partial RF spoiling of nonbalanced steady‐state free precession. The new T₂ mapping technique is evaluated and optimized from simulations, and in vivo results are presented for human brain at 1.5 T and for human articular cartilage at 3.0 T. The range of T₂ for gray and white matter was from 60 msec (for the corpus callosum) to 100 msec (for cortical gray matter). For cartilage, spatial variation in T₂ was observed between deep (34 msec) and superficial (48 msec) layers, as well as between tibial (33 msec), femoral, (54 msec) and patellar (43 msec) cartilage. Excellent correspondence between T₂ values derived from partially spoiled SSFP scans and the ones found with a reference multicontrast spin‐echo technique is observed, corroborating the accuracy of the new method for proper T₂ mapping. Finally, the feasibility of a fast high‐resolution quantitative partially spoiled SSFP T₂ scan is demonstrated at 7.0 T for human patellar cartilage. Magn Reson Med, 2011. © 2011 Wiley‐Liss, Inc.