Spin‐echo micro‐MRI of trabecular bone using improved 3D fast large‐angle spin‐echo (FLASE)

Fast large‐angle spin echo (FLASE) is a common pulse sequence designed for quantitative imaging of trabecular bone (TB) microarchitecture. However, imperfections in the nonselective phase‐reversal pulse render it prone to stimulated echo artifacts. The problem is further exacerbated at isotropic resolution. Here, a substantially improved RF‐spoiled FLASE sequence (sp‐FLASE) is described and its performance is illustrated with data at 1.5T and 3T. Additional enhancements include navigator echoes for translational motion sensing applied in a slice parallel to the imaging slab. Whereas recent work suggests the use of fully‐balanced FLASE (b‐FLASE) to be advantageous from a signal‐to‐noise ratio (SNR) point of view, evidence is provided here that the greater robustness of sp‐FLASE may outweigh the benefits of the minor SNR gain of b‐FLASE for the target application of TB imaging in the distal extremities, sites of exclusively fatty marrow. Results are supported by a theoretical Bloch equation analysis and the pulse sequence dependence of the effective T₂ of triglyceride protons. Last, sp‐FLASE images are shown to provide detailed and reproducible visual depiction of trabecular networks in three dimensions at both anisotropic (137 × 137 × 410 μm³) and isotropic (160 × 160 × 160 μm³) resolutions in the human distal tibia in vivo. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.     

Contrast‐enhanced,three‐dimensional,whole‐brain,black‐blood imaging: Application to small brain metastases

Contrast‐enhanced three‐dimensional T₁‐weighted imaging based on magnetization‐prepared rapid‐gradient recalled echo is widely used for detecting small brain metastases. However, since contrast materials remain in both blood and the tumor parenchyma and thus increase the signal intensity of both regions, it is often challenging to distinguish brain tumors from blood. In this work, we develop a T₁‐weighted, black‐blood version of single‐slab three‐dimensional turbo/fast spin echo whole‐brain imaging, in which the signal intensity of the brain tumor is selectively enhanced while that of blood is suppressed. For blood suppression, variable refocusing flip angles with flow‐sensitizing gradients are employed. To avoid a signal loss resulting from the flow‐sensitizing scheme, the first refocusing flip angle is forced to 180°. Composite restore pulses at the end of refocusing pulse train are applied to achieve partial inversion recovery for the T₁‐weighted contrast. Simulations and in vivo volunteer and patient experiments are performed, demonstrating that this approach is highly efficient in detecting small brain metastases. Magn Reson Med 63:553–561, 2010. © 2010 Wiley‐Liss, Inc.     

Effects of inflow and radiofrequency spoiling on the arterial input function in dynamic contrast‐enhanced MRI: A combined phantom and simulation study

The arterial input function is crucial in pharmacokinetic analysis of dynamic contrast‐enhanced MRI data. Among other artifacts in arterial input function quantification, the blood inflow effect and nonideal radiofrequency spoiling can induce large measurement errors with subsequent reduction of accuracy in the pharmacokinetic parameters. These errors were investigated for a 3D spoiled gradient‐echo sequence using a pulsatile flow phantom and a total of 144 typical imaging settings. In the presence of large inflow effects, results showed poor average accuracy and large spread between imaging settings, when the standard spoiled gradient‐echo signal equation was used in the analysis. For example, one of the investigated inflow conditions resulted in a mean error of about 40% and a spread, given by the coefficient of variation, of 20% for K^trans. Minimizing inflow effects by appropriate slice placement, combined with compensation for nonideal radiofrequency spoiling, significantly improved the results, but they remained poorer than without flow (e.g., 3–4 times larger coefficient of variation for K^trans). It was concluded that the 3D spoiled gradient‐echo sequence is not optimal for accurate arterial input function quantification and that correction for nonideal radiofrequency spoiling in combination with inflow minimizing slice placement should be used to reduce the errors. Magn Reson Med, 2011. © 2011 Wiley‐Liss, Inc.     

Small‐tip fast recovery imaging using non‐slice‐selective tailored tip‐up pulses and radiofrequency‐spoiling

Small‐tip fast recovery (STFR) imaging is a new steady‐state imaging sequence that is a potential alternative to balanced steady‐state free precession. Under ideal imaging conditions, STFR may provide comparable signal‐to‐noise ratio and image contrast as balanced steady‐state free precession, but without signal variations due to resonance offset. STFR relies on a tailored “tip‐up,” or “fast recovery,” radiofrequency pulse to align the spins with the longitudinal axis after each data readout segment. The design of the tip‐up pulse is based on the acquisition of a separate off‐resonance (B0) map. Unfortunately, the design of fast (a few ms) slice‐ or slab‐selective radiofrequency pulses that accurately tailor the excitation pattern to the local B0 inhomogeneity over the entire imaging volume remains a challenging and unsolved problem. We introduce a novel implementation of STFR imaging based on “non‐slice‐selective” tip‐up pulses, which simplifies the radiofrequency pulse design problem significantly. Out‐of‐slice magnetization pathways are suppressed using radiofrequency‐spoiling. Brain images obtained with this technique show excellent gray/white matter contrast, and point to the possibility of rapid steady‐state T₂/T₁‐weighted imaging with intrinsic suppression of cerebrospinal fluid, through‐plane vessel signal, and off‐resonance artifacts. In the future, we expect STFR imaging to benefit significantly from parallel excitation hardware and high‐order gradient shim systems. Magn Reson Med, 2013. © 2012 Wiley Periodicals, Inc.     

Signal fluctuations induced by non‐T1‐related confounds in variable TR fMRI experiments

Purpose

To assess and model signal fluctuations induced by non‐T₁‐related confounds in variable repetition time (TR) functional magnetic resonance imaging (fMRI) and to develop a compensation procedure to correct for the non‐T₁‐related artifacts.

Materials and Methods

Radiofrequency disabled volume gradient sequences were effected at variable offsets between actual image acquisitions, enabling perturbation of the measurement system without perturbing longitudinal magnetization, allowing the study of non‐T₁‐related confounds that may arise in variable TR experiments. Three imaging sessions utilizing a daily quality assurance (DQA) phantom were conducted to assess the signal fluctuations, which were then modeled as a second‐order system. A modified projection procedure was implemented to correct for signal fluctuations arising from non‐T₁‐related confounds, and statistical analysis was performed to assess the significance of the artifacts with and without compensation.

Results

Assessment using phantom data reveals that the signal fluctuations induced by non‐T₁‐related confounds was consistent in shape across the phantom and well‐modeled by a second‐order system. The phantom exhibited significant spurious detections (at P < 0.01) almost uniformly across the central slices of the phantom.

Conclusion

Second‐order system modeling and compensation of non‐T₁‐related confounds achieves significant reduction of spurious detection of fMRI activity in a phantom. J. Magn. Reson. Imaging 2009;29:1234–1239. © 2009 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.     

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     

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.     

Determination of the optimal first‐order gradient moment for flow‐sensitive dephasing magnetization‐prepared 3D noncontrast MR angiography

Flow‐sensitive dephasing (FSD) magnetization preparation has been developed for black‐blood vessel wall MRI and noncontrast MR angiography. The first‐order gradient moment, m₁, is a measure of the flow‐sensitization imparted by an FSD preparative module. Determination of the optimal m₁ for each individual is highly desirable for FSD‐prepared MR angiography. This work developed a 2D m₁‐scouting method that evaluates a range of m₁ values for their effectiveness in blood signal suppression in a single scan. The feasibility of using the 2D method to predict blood signal suppression in 3D FSD‐prepared imaging was validated on a flow phantom and the popliteal arteries of 5 healthy volunteers. Excellent correlation of the blood signal measurements between the 2D scouting and 3D FSD imaging was obtained. Therefore, the optimal m₁ determined from the 2D m₁‐scouting scan may be directly translated to 3D FSD‐prepared imaging. In vivo studies of additional 10 healthy volunteers and 2 patients have demonstrated the proposed method can help significantly improve the signal performance of FSD MR angiography, indicating its potential to enhance diagnostic confidence. Further systematic studies in patients are warranted to evaluate its clinical value. Magn Reson Med, 2011. © 2011 Wiley‐Liss, Inc.     

Increasing spoiling efficiency in RF‐spoiled gradient echo sequences by averaging of RF phase‐cycle‐adapted k‐spaces

A method is presented that improves the effectiveness of RF‐spoiling whenever an image consists of the averaging of several identical single acquisitions. Using this method, it becomes possible to reduce the necessary moment of the spoiler gradient by a factor equal to the number of averages. The idea is to perform the collection of k‐space lines such that the averaging takes over a part of the spoiling process. The number of sequence cycles played out between two acquisitions of a certain k‐space line is adjusted based on both the number of total averages of this line and the phase‐cycling pattern of the RF‐pulse train of a RF‐spoiled gradient echo sequence. In this way, ghost artifacts visible in the single images add to zero in the averaged image while the depiction of the object experiences normal averaging with enhanced signal‐to‐noise ratio. Magn Reson Med, 2011. © 2011 Wiley‐Liss, Inc.     

Implementation of vascular‐space‐occupancy MRI at 7T

Vascular‐space‐occupancy (VASO) MRI exploits the difference between blood and tissue T₁ to null blood signal and measure cerebral blood volume changes using the residual tissue signal. VASO imaging is more difficult at higher field because of sensitivity loss due to the convergence of tissue and blood T₁ values and increased contamination from blood‐oxygenation‐level‐dependent (BOLD) effects. In addition, compared to 3T, 7T MRI suffers from increased geometrical distortions, e.g., when using echo‐planar‐imaging, and from increased power deposition, the latter especially problematic for the spin‐echo‐train sequences commonly used for VASO MRI. Third, non‐steady‐state blood spin effects become substantial at 7T when only a head coil is available for radiofrequency transmit. In this study, the magnetization‐transfer‐enhanced‐VASO approach was applied to maximize tissue‐blood signal difference, which boosted signal‐to‐noise ratio by 149% ± 13% (n = 7) compared to VASO. Second, a 3D fast gradient‐echo sequence with low flip‐angle (7°) and short echo‐time (1.8 ms) was used to minimize the BOLD effect and to reduce image distortion and power deposition. Finally, a magnetization‐reset technique was combined with a motion‐sensitized‐driven‐equilibrium approach to suppress three types of non‐steady‐state spins. Our initial functional MRI results in normal human brains at 7T with this optimized VASO sequence showed better signal‐to‐noise ratio than at 3T. Magn Reson Med 69:1003–1013, 2013. © 2012 Wiley Periodicals, Inc.     

Incidental magnetization transfer contrast by fat saturation preparation pulses in multislice look‐locker echo planar imaging

In this study, it is demonstrated that fat saturation (FS) preparation (prep) pulses generate incidental magnetization transfer contrast (MTC) in multislice Look‐Locker (LL) imaging. It is shown that frequency‐selective FS prep pulses can invoke MTC through the exchange between free and motion‐restricted protons. Simulation reveals that the fractional signal loss by these MTC effects is more severe for smaller flip angles (FAs), shorter repetition times (TRs), and greater number of slices (SN). These incidental MTC effects result in a signal attenuation at a steady state (up to 30%) and a T₁ measurement bias (up to 20%) when using inversion recovery (IR) LL echo‐planar imaging (EPI) sequences. Furthermore, it is shown that water‐selective MRI using binomial pulses has the potential to minimize the signal attenuation and provide unbiased T₁ measurement without fat artifacts in MR images. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.     

Reduction of artifacts in T2‐weighted PROPELLER in high‐field preclinical imaging

A simple technique is implemented for correction of artifacts arising from nonuniform T₂‐weighting of k‐space data in fast spin echo–based PROPELLER (periodically rotated overlapping parallel lines with enhanced reconstruction). An additional blade with no phase‐encoding gradients is acquired to generate the scaling factor used for the correction. Results from simulations and phantom experiments, as well as in vivo experiments in free‐breathing mice, demonstrate the advantages of the proposed method. This technique is developed specifically for high‐field imaging applications where T₂ decay is rapid. Magn Reson Med, 2011. © 2010 Wiley‐Liss, Inc.     

Rapid T1 mapping of mouse myocardium with saturation recovery look‐locker method

Dynamic contrast‐enhanced MRI using gadolinium or manganese provides unique characterization of myocardium and its pathology. In this study, an electrocardiography (ECG) triggered saturation recovery Look‐Locker method was developed and validated for fast cardiac T₁ mapping in small animal models. By sampling the initial portion of the longitudinal magnetization recovery curve, high temporal resolution (～3 min) can be achieved at a high spatial resolution (195 × 390 μm2) in mouse heart without the aid of parallel imaging or echo‐planar imaging. Validation studies were performed both in vitro on a phantom and in vivo on C57BL/6 mice (n = 6). Our results showed a strong agreement between T₁ measured by saturation recovery Look‐Locker and by the standard saturation recovery method in vitro or inversion recovery Look‐Locker in vivo. The utility of saturation recovery Look‐Locker in dynamic contrast‐enhanced MRI studies was demonstrated in manganese‐enhanced MRI experiments in mice. Our results suggest that saturation recovery Look‐Locker can provide rapid and accurate cardiac T₁ mapping for studies using small animal models. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.     

A novel flow-suppression technique using tailored RF pulses

The pulsatile nature of blood flow makes zipper-like artifacts along the coding direction in the two-dimensional Fourier transform NMR image. So far, spatial presaturation, one of the correction methods, is known to be effective in eliminating flow artifacts when the Fourier spin echo acquisition is employed. However, this method requires an additional RF pulse and a spoiling gradient for presaturation. Described in this paper is a new flow suppression technique, based on spin dephasing, using a set of tailored RF pulses. The proposed method does not require additional saturation RF pulses or spoiling gradient pulses, making it advantageous over other methods. In addition, the method is relatively robust to flow velocity. The proposed technique is equivalent to the existing flow saturation technique except that the elimination of the flow component is achieved by a pair of tailored 90–180° RF pulses in the spin echo sequence. The principle of the proposed method is the creation of a linear phase gradient within the slice along the slice selection direction for the moving material by use of two opposing quadratic phase RF pulses, i.e., 90° and 180° RF pulses with opposing quadratic phase distributions. That is to say, all the spins of the moving materials along the slice selection direction become dephased. Therefore, no observable signal is generated. Computer simulations and experimental results obtained using a 2.0-T whole-body imaging system on both a phantom and a human volunteer are also presented.     

Simplified quantification of labile proton concentration‐weighted chemical exchange rate (kws) with RF saturation time dependent ratiometric analysis (QUESTRA): Normalization of relaxation and RF irradiation spillover effects for improved quantitative chemical exchange saturation transfer (CEST) MRI

Chemical exchange saturation transfer MRI is an emerging imaging technique capable of detecting dilute proteins/peptides and microenvironmental properties, with promising in vivo applications. However, chemical exchange saturation transfer MRI contrast is complex, varying not only with the labile proton concentration and exchange rate, but also with experimental conditions such as field strength and radiofrequency (RF) irradiation scheme. Furthermore, the optimal RF irradiation power depends on the exchange rate, which must be estimated in order to optimize the chemical exchange saturation transfer MRI experiments. Although methods including numerical fitting with modified Bloch‐McConnell equations, quantification of exchange rate with RF saturation time and power (QUEST and QUESP), have been proposed to address this relationship, they require multiple‐parameter non‐linear fitting and accurate relaxation measurement. Our work extended the QUEST algorithm with ratiometric analysis (QUESTRA) that normalizes the magnetization transfer ratio at labile and reference frequencies, which effectively eliminates the confounding relaxation and RF spillover effects. Specifically, the QUESTRA contrast approaches its steady state mono‐exponentially at a rate determined by the reverse exchange rate (k_ws), with little dependence on bulk water T₁, T₂, RF power and chemical shift. The proposed algorithm was confirmed numerically, and validated experimentally using a tissue‐like phantom of serially titrated pH compartments. Magn Reson Med, 2011. © 2011 Wiley‐Liss, Inc.     

Improved half RF slice selectivity in the presence of eddy currents with out‐of‐slice saturation

Ultrashort echo time imaging with half RF pulse excitation is sensitive to eddy currents induced by the slice‐select gradient that distorts the half pulse slice profile. This work demonstrates improvements in the half pulse profile by using spatial saturation on both sides of the imaged slice to suppress the out‐of‐slice magnetization. This effectively improves the selectivity of the half pulse excitation profile. A quadratic phase RF pulse with high bandwidth and selectivity was used to achieve a wide saturation band with sharp edges. Experimental results demonstrate substantially improved slice selectivity and R quantitation accuracy obtained with the out‐of‐slice saturation. This approach is effective in making short T₂ imaging and quantitation with half pulses less sensitive to eddy currents. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.     

Characterization and reduction of saturation banding in multiplanar coherent and incoherent steady‐state imaging

Hypointense band artifacts occur at intersections of nonparallel imaging planes in rapidly acquired MR images; quantitative or numerical analysis of these bands and strategies to mitigate their appearance have largely gone unexplored. The magnetization evolution in the different regions of multiplanar images was simulated for three common rapid steady‐state techniques (spoiled gradient echo, steady state free precession, balanced steady state free precession). Saturation banding was found to be highly dependent on the pulse sequence, acquisition time, and phase‐encoding order. Encoding the center of k‐space at the end of the acquisition of each slice (i.e., reverse centric phase encoding) is demonstrated to be a simple and robust method for significantly reducing the relative saturation in all imaging planes. View ordering and resolution dependence were confirmed in multiplanar abdominal images. The added importance of reducing the artifact in accelerated acquisition techniques (e.g., parallel imaging) is particularly notable in multiplanar balanced steady state free precession images in the brain. Magn Reson Med 63:1415–1421, 2010. © 2010 Wiley‐Liss, Inc.     

Optimizing signal‐to‐noise ratio of high‐resolution parallel single‐shot diffusion‐weighted echo‐planar imaging at ultrahigh field strengths

The potential signal‐to‐noise ratio (SNR) gain at ultrahigh field strengths offers the promise of higher image resolution in single‐shot diffusion‐weighted echo‐planar imaging the challenge being reduced T₂ and T₂* relaxation times and increased B₀ inhomogeneity which lead to geometric distortions and image blurring. These can be addressed using parallel imaging (PI) methods for which a greater range of feasible reduction factors has been predicted at ultrahigh field strengths—the tradeoff being an associated SNR loss. Using comprehensive simulations, the SNR of high‐resolution diffusion‐weighted echo‐planar imaging in combination with spin‐echo and stimulated‐echo acquisition is explored at 7 T and compared to 3 T. To this end, PI performance is simulated for coil arrays with a variable number of circular coil elements. Beyond that, simulations of the point spread function are performed to investigate the actual image resolution. When higher PI reduction factors are applied at 7 T to address increased image distortions, high‐resolution imaging benefits SNR‐wise only at relatively low PI reduction factors. On the contrary, it features generally higher image resolutions than at 3 T due to smaller point spread functions. The SNR simulations are confirmed by phantom experiments. Finally, high‐resolution in vivo images of a healthy volunteer are presented which demonstrate the feasibility of higher PI reduction factors at 7 T in practice. Magn Reson Med, 2012. © 2011 Wiley Periodicals, Inc.