Variablemangle uniform signal excitation (vuse) for threel dimensional time-of-flight MR angiography

A spatially asymmetric RF pulse that improves the uniformity of blood signal intensity and vascular contrast in three-dimensional (3D) MR angiorpgphy [MRA] is presented. The pulse, called variable-angle uniform signal excitation (VUSE), was designed to provide uniform signal response and improved contrast for blood flowing through a 3D imagine volume during a FLASH sequence. The VUSE excitation proffle was optimized on the bash of the number of pulses seen by the blood, which varied with the velocity of through-plane flow, repetition time, and slab thickness with the maximum flip angle at the flow udt constrained at 90°. The theoretical results show that the optimal RF pulse gives more uniformity for flow signal than does a linear ramp excitation proffle or a Gaussian pulse combined with a presaturation pulse. After truncation and ffltering of the VUSE pulse in the time domain, the general shape of the VUSE RF excitation profile is maintained. but the maximum flip angle is reduced. The arteries of the neck in a healthy volunteer were imaged with the VUSE pulse, a constant flip angle (flat) pulse, and a linear ramp pulse in flow-compensated 3D MRA requences. The WSE pulse produced the most uniform signal as evidenced by the lowest relative dispersion of signal along the left vertebral artery (18.0 versus 26.4 to 23.6 for the other studies). F-distribution tests also showed that the signal distribution obtained with the VUSE pulse in a 3D FLASH sequence was statistically different from that for the flat and the linear ramp pulses.     

The effects of linearly increasing flip angles on 3D inflow MR angiography

As recently demonstrated, spin saturation effects in 3D time-of-flight (TOF) MR angiography (MRA) can be reduced by using RF pulses with linearly increasing flip angles (ramp pulses) in the main direction of flow. We developed a model for calculating the signal distribution of proton flow within the excitation volume (slab) for different ramp slopes and compared the results with the measured distribution for the lower-leg arteries. The ramp pulses were generated using the Fourier transformation of the desired excitation profiles. With a bandwidth of 6 kHz and a pulse length of 2.56 ms satisfactory ramps with variable slopes were generated and applied in a standard flow-compensated 3D FISP sequence. The effects on the signal distribution in the resulting angiograms of the lower limbs revealed a considerable reduction of saturation losses in agreement with the calculations. Calculated optimal ramp slopes are provided for flow velocities ranging from 5 to 50 cm/s and excitation volumes ranging from 5 to 25 cm.     

Evaluation of radiofrequency pulses and contrast agent doses for use in 3D pulmonary magnetic resonance angiography

Ten healthy volunteers were imaged with breath-hold, three-dimensional (3D) time-of-flight (TOF) magnetic resonance angiography (MRA) using single-variable-angle uniform signal excitation (VUSE), double-VUSE, and flat radiofrequency (RF) pulses with various doses of contrast agent. The ability of each technique to display pulmonary vasculature was evaluated. Images were segmented to isolate lungs, and maximum intensity projections (MIPs) were computed. All MIPs were assigned an image quality (IQ) rating, and signal-to-noise ratios (SNRs) were measured in pulmonary vessels. Without contrast agent, subsegmental vessels were displayed in single- and double-VUSE images while no vessels were visible in flat images. With equal doses of contrast agent, SNRs and IQ ratings were comparable for images obtained with VUSE and flat pulses. In addition, single-VUSE pulses produced more uniform signal from vessels than flat pulses in contrast-enhanced images. The results indicate that non-contrast-enhanced 3D TOF pulmonary MRA with VUSE RF pulses may be a useful screening tool. In addition, contrast-enhanced 3D TOF MRA with VUSE pulses may be useful as a stand-alone technique for assessing the pulmonary vasculature or as an adjunct to contrast-enhanced 3D TOF MRA with flat pulses. J. Magn. Reson. Imaging 10:929-938, 1999.     

Improvement of vessel visibility in time-of-flight MR angiography of the brain

PURPOSE: To improve vessel visibility in time-of-flight MR angiography (TOF-MRA) by careful consideration of coil choice, coil position, and frequency offset and profile of the nonspatially selective chemical shift selective (CHESS) presaturation pulse. MATERIALS AND METHODS: The effects of both the CHESS and the excitation radiofrequency (RF) pulses on flow signal and signals from stationary substances were evaluated by changing the spatial area where RF pulses were applied to upstream flow in a flow phantom and in human subjects. The difference between the eight-channel phased-array receive-only coil and the transmit-receive coil was evaluated. RESULTS: The CHESS pulse suppresses the flow signal over a wider frequency range than the signals from stationary substances, especially when using the body coil for transmission. Even without presaturation pulse, the excitation pulse slightly suppressed the flow signal. Adjusting the position of the transmit-receive coil relative to the head improved these TOF-MRA images. The results were better than those obtained with the eight-channel coil. CONCLUSION: The excitation and the nonspatially selective CHESS pulses degraded the flow signal. Our results suggest that reduced spatial extent of RF pulse application to upstream flow can improve image quality of TOF-MRA. This result can be implemented on conventional scanners.     

Superbalanced steady state free precession

Steady state free precession (SSFP) signal theory is commonly derived in the limit of quasi-instantaneous radiofrequency (RF) excitation. SSFP imaging protocols, however, are frequently set up with minimal pulse repetition times and RF pulses can thus constitute a considerable amount to the actual pulse repetition time. As a result, finite RF pulse effects can lead to 10-20% signal deviation from common SSFP theory in the transient and in the steady state which may impair the accuracy of SSFP-based quantitative imaging techniques. In this article, a new and generic approach for intrinsic compensation of finite RF pulse effects is introduced. Compensation is based on balancing relaxation effects during finite RF excitation, similar to flow or motion compensation of gradient moments. RF pulse balancing, in addition to the refocusing of gradient moments with balanced SSFP, results in a superbalanced SSFP sequence free of finite RF pulse effects in the transient and in the steady state; irrespective of the RF pulse duration, flip angles, relaxation times, or off-resonances. Superbalancing of SSFP sequences can be used with all quantitative SSFP techniques where finite RF pulse effects are expected or where elongated RF pulses are used.     

B1-insensitive fast spin echo using adiabatic square wave enabling of the echo train (SWEET) excitation

Abdominal images at 3T acquired with fast spin echo (FSE) sequences often exhibit signal voids due to RF transmit field inhomogeneities. Theory suggests, however, that the repeated refocusing pulses of FSE are capable of maintaining signal even at reduced RF amplitudes if the magnetization is suitably prepared. Here we propose a modified excitation strategy for FSE that is more robust to transmit field inhomogeneities than conventional FSE. The new excitation approach replaces the standard 90 degrees excitation pulse with a discretely sampled hyperbolic secant pulse that creates a square wave longitudinal magnetization as a function of gradient and off-resonance induced phase shifts between the subsequent echoes of the FSE sequence. This pulse is followed by the conventional train of refocusing pulses except that the first few pulses increase from near zero to the desired refocusing amplitude. Simulations and in vivo results at 3T indicate preserved image quality and much greater robustness of this new sequence to nonuniform RF fields. This robustness comes at the cost of 20% reduction in signal when the RF field is uniform and increased motion sensitivity. This RF field-insensitive sequence may overcome challenges of body imaging at high field and in patients with ascites.     

MR angiography with adiabatic flow excitation.

A new method of magnetic resonance (MR) angiography is presented that produces signal from flowing spins and suppresses that from stationary spins by means of a flow excitation pulse sequence consisting of adiabatic 90 degrees and 180 degrees radio-frequency (RF) pulses interleaved with flow-dephasing gradient lobes. Stationary spins are refocused along the z axis, while flowing spins are dephased by the gradient lobes and generate a transverse component that can be measured directly to produce the angiogram. Adiabatic RF pulses and unipolar gradient lobes give the pulse sequence a high degree of immunity to RF and magnetic field inhomogeneity. The pulse sequence can be successfully applied with a transmit/receive surface coil. The disadvantage of adiabatic RF pulses is that their long duration makes it difficult to suppress the signal of stationary spins with short T2.     

Combining spin echoes with gradient echoes in the context of the global coherent free precession pulse sequence.

To extend the signal longevity of magnetically excited spins in flowing fluids while in a state of global coherent free precession (GCFP), a refocusing radiofrequency (RF) pulse and bipolar gradient waveforms were combined with the GCFP sequence. The data demonstrate that RF refocusing in the presence of flowing blood is possible, but the improvement in signal amplitude depends on the static magnetic field homogeneity along the direction of motion and the displacement of the spins between the excitation and the RF refocusing pulse, as well as displacement during subsequent RF refocusing pulses. The least amount of phase dispersion and thus the longest lasting signal is obtained with the shortest echo spacing where only one line of data is recorded between two RF refocusing pulses. This approach was successfully used in a phantom and in vivo to image fast and slow blood flow. Depending on the experimental conditions, signal persistence is improved significantly compared to playing the same sequence without RF refocusing, but the improvement is limited by the product of blood flow velocity and the time between RF refocusing pulses.     

Parallel excitation with an array of transmit coils.

Theoretical and experimental results are presented that establish the value of parallel excitation with a transmit coil array in accelerating excitation and managing RF power deposition. While a 2D or 3D excitation pulse can be used to induce a multidimensional transverse magnetization pattern for a variety of applications (e.g., a 2D localized pattern for accelerating spatial encoding during signal acquisition), it often involves the use of prolonged RF and gradient pulses. Given a parallel system that is composed of multiple transmit coils with corresponding RF pulse synthesizers and amplifiers, the results suggest that by exploiting the localization characteristics of the coils, an orchestrated play of shorter RF pulses can achieve desired excitation profiles faster without adding strains to gradients. A closed-form design for accelerated multidimensional excitations is described for the small-tip-angle regime, and its suppression of interfering aliasing lobes from coarse excitation k-space sampling is interpreted based on an analogy to sensitivity encoding (SENSE). With or without acceleration, the results also suggest that by taking advantage of the extra degrees of freedom inherent in a parallel system, parallel excitation provides better management of RF power deposition while facilitating the faithful production of desired excitation profiles. Sample accelerated and specific absorption rate (SAR)-reduced excitation pulses were designed in this study, and evaluated in experiments.     

Black-blood MR angiography with grase: Measurement of flow-induced signal attenuation

We investigated the feasibility of performing black-blood MR angiography (MRA) with the gradient and spin-echo (GRASE) pulse sequence. Phantom experiments and human testing were conducted, and the results were compared with those of turbo spin-echo (TSE). We demonstrated that both techniques are able to produce signal suppression of flowing fluid to background level. With fewer radiofrequency (RF)-refocusing pulses, GRASE pulse sequences could serve as an alternative black-blood technique of reduced RF power exposure and shorter scan time. These relative advantages of GRASE may become useful when high-resolution images are taken.     

Selective maximization of (31)P MR spectroscopic signals of in vivo human brain metabolites at 3T

PURPOSE: To develop a short TR, short TE, large flip angle (LFA), in vivo (31)P MR spectroscopy (MRS) technique at 3T that selectively maximizes the signal-to-noise ratio (SNR) of long T(1) human brain metabolites implicated in bipolar disorder. MATERIALS AND METHODS: Two pulse sequences were evaluated for efficiency. Slice profiles acquired with the scaled, sinc-shaped, radiofrequency (RF) LFA pulses were compared to those acquired with Shinnar-Le Roux (SLR) RF LFA pulses. The SLR-based LFA pulse sequence was used to maximize the inorganic phosphate signal in a phantom, after which volunteer metabolite signals were selectively maximized and compared to their correlates acquired with conventional spin-echo methods. RESULTS: The comparison of slice profiles acquired with sinc-shaped RF LFA pulses vs. SLR RF LFA pulses showed that SLR-based pulse sequences, with their improved excitation and slice profiles, yield significantly better results. In vivo LFA spin-echo MRS implemented with SLR pulses selectively increased the (31)P MRS signal, by as much as 93%, of human brain metabolites that have T(1) times longer than the TR of the acquisition. CONCLUSION: The data show that the LFA technique can be employed in vivo to maximize the signal of long T(1) (31)P brain metabolites at a given TE and TR. LFAs ranging between 120 degrees and 150 degrees are shown to maximize the (31)P signal of human brain metabolites at 3T.     

Array-optimized composite pulse for excellent whole-brain homogeneity in high-field MRI.

A number of methods to improve excitation homogeneity in high-field MRI have been proposed, and some of these methods rely on separate control of radiofrequency (RF) coils in a transmit array. In this work we combine accurate RF field calculations and the Bloch equation to demonstrate that by using a sequence of pulses with individually optimized current distributions (i.e., an array-optimized composite pulse), one can achieve remarkably homogeneous distributions of available signal intensity over the entire brain volume. This homogeneity is greater than that achievable using the same transmit array to produce either a single optimized (or RF shimmed) pulse or a single RF shimmed field distribution in a standard 90x-90y composite pulse arrangement. Simulations indicate that with a very simple array-optimized composite pulse, excellent whole-brain excitation homogeneity can be achieved at up to 600 MHz.     

3D MDEFT imaging of the human brain at 4.7 T with reduced sensitivity to radiofrequency inhomogeneity.

A modification to the 3D modified driven equilibrium Fourier transform (MDEFT) imaging technique is proposed that reduces its sensitivity to RF inhomogeneity. This is especially important at high field strengths where RF focusing effects exacerbate B(1) inhomogeneity, causing significant signal nonuniformity in the images. The adiabatic inversion pulse used during the preparation period of the MDEFT sequence is replaced by a hard (nonadiabatic) pulse with a nominal flip angle of 130 degrees. The spatial inhomogeneity of the hard pulse preparation compensates for the inhomogeneity of the excitation pulses. Uniform signal intensity is obtained for a wide range of B(1) amplitudes and the high CNR characteristic of MDEFT is retained. The new approach was validated by numerical simulations and successfully applied to human brain imaging at 4.7 T, resulting in high-quality T(1)-weighted images of the whole human brain at high field strength with uniform signal intensity and contrast, despite the presence of significant RF inhomogeneity.     

Parallel excitation in the human brain at 9.4 T counteracting k‐space errors with RF pulse design

Multidimensional spatially selective radiofrequency (RF) pulses have been proposed as a method to mitigate transmit B1 inhomogeneity in MR experiments. These RF pulses, however, have been considered impractical for many years because they typically require very long RF pulse durations. The recent development of parallel excitation techniques makes it possible to design multidimensional RF pulses that are short enough for use in actual experiments. However, hardware and experimental imperfections can still severely alter the excitation patterns obtained with these accelerated pulses. In this note, we report at 9.4 T on a human eight‐channel transmit system, substantial improvements in two‐dimensional excitation pattern accuracy obtained when measuring k‐space trajectories prior to parallel transmit RF pulse design (acceleration ×4). Excitation patterns based on numerical simulations closely reproducing the experimental conditions were in good agreement with the experimental results. Magn Reson Med, 2010. © 2009 Wiley‐Liss, Inc.     

Fat suppressed MRI of articular cartilage with a spatial-spectral excitation pulse

We developed a three-dimensional, gradient-recalled-echo imaging technique that incorporates a short-duration spatial-spectral excitation pulse from the family of binomial pulses. Binomial pulses of different orders were tested on phantoms and on normal volunteers to find the composite pulse that produced in the shortest duration the most reliable fat suppression. Composite pulses employing unipolar slice-selective gradients with explicit rewinder gradients between each radiofrequency (RF) pulse were compared with composite RF pulses employing alternating-polarity, slice-select gradients. The advantage of the sequences using the unipolar gradients is improved fat suppression. Images of the knees of volunteers produced with the composite RF pulse have contrast between fat and articular cartilage equivalent to that on images created by the gradient-recalled-echo imaging technique employing a conventional chemsat pulse. The optimum RF pulse consisted of three amplitude- and phase-modulated pulses combined with unipolar slice-select gradients.     

Reduction of phase error ghosting artifacts in thin slice fast spin-echo imaging

Fast spin-echo (FSE) imaging techniques are very sensitive to the relative phase between the 90° (excitation) RF pulse and the 180° (refocusing) RF pulses. In this paper, it is demonstrated that a phase shift can be created between the excitation and refocusing pulses in such a manner that the received signal is divided into two components of distinctly different phase shifts. The nature of these two components is reviewed. It is demonstrated that ghosting artifacts will occur when images are reconstructed from this received signal. The ghosting is shown to be object dependent. A correction technique is presented which calculates the phase errors among different echoes based on measurements from a single echo train acquired without phase encoding gradients. The results in both phantom and human studies show that this method is capable of reducing the ghosting artifact in thin slice FSE images.     

Angiographic Imaging with 2D RF Pulses

Magnetic resonance angiography (MRA) was performed by using RF pulses designed to excite a limited spatial extent in two orthogonal directions. The restriction in the second spatial dimension can be used to increase inflow enhancement and to improve small field-of-view imaging. A rectangular excitation was produced with an “echo-planar” k-space trajectory and a sine-modulated RF waveform. In vivo images have demonstrated that vessels are more clearly delineated with the two-dimensional excitation. Aliasing artifacts in small field-of-view imaging are significantly reduced, although in some cases complete elimination is not possible due to the nature of the gradient trajectory.     

VERSE-guided numerical RF pulse design: a fast method for peak RF power control

In parallel excitation, the computational speed of numerical radiofrequency (RF) pulse design methods is critical when subject dependencies and system nonidealities need to be incorporated on-the-fly. One important concern with optimization-based methods is high peak RF power exceeding hardware or safety limits. Hence, online controllability of the peak RF power is essential. Variable-rate selective excitation pulse reshaping is ideally suited to this problem due to its simplicity and low computational cost. In this work, we first improve the fidelity of variable-rate selective excitation implementation for discrete-time waveforms through waveform oversampling such that variable-rate selective excitation can be robustly applied to numerically designed RF pulses. Then, a variable-rate selective excitation-guided numerical RF pulse design is suggested as an online RF pulse design framework, aiming to simultaneously control peak RF power and compensate for off-resonance.     

New FOCI pulses with reduced radiofrequency power requirements

PURPOSE: To propose new frequency offset corrected inversion (FOCI) pulses with significantly reduced radiofrequency (RF) power deposition for spin echo imaging by incorporating the variable-rate selective excitation (VERSE) schemes into the pulse design. MATERIALS AND METHODS: Two schemes are proposed to design the new FOCI pulses with dramatically reduced peak RF power requirements. In scheme A, the time-dilation function is derived from a predefined adiabaticity factor modulation function. In scheme B, the time-dilation function is predefined, while the adiabaticity factor is conserved. RESULTS: The new FOCI pulses are shown to be able to operate at reduced specific absorption rate (SAR), specifically at the same peak RF power as that of a five- or seven-lobe sinc inversion pulse of the same duration. Using the new FOCI pulse, significant gain in sensitivity was observed in in vivo spin-echo echo-planar imaging, which was attributed to the improved refocusing slice profile. CONCLUSION: The new FOCI pulses can replace the 180 degrees five- or seven-lobe sinc pulses in spin-echo imaging with the same peak RF power requirement and significantly improved slice profile.     

Advanced three-dimensional tailored RF pulse for signal recovery in T2*-weighted functional magnetic resonance imaging.

T(2) (*)-weighted functional MR images are plagued by signal loss artifacts caused by susceptibility-induced through-plane dephasing. We present major advances to the original three-dimensional tailored RF (3DTRF) pulse method that pre-compensates the dephasing using three-dimensional selective excitation. The proposed 3DTRF pulses are designed iteratively with off-resonance incorporation and with a novel echo-volumar trajectory that frequency-encodes in z and phase-encodes in x,y. We also propose a computational scheme to accelerate the pulse design process. We demonstrate effective signal recovery in a 5-mm slice in both phantom and inferior brain, using 3DTRF pulses that are only 15.4 ms long. Compared to the original method, the new approach leads to significantly reduced pulse length and enhancement in slice selectivity. 3D images of the slice volume confirm fidelity of the excited phase pattern and slice profile.