BURST excitation pulses

A theory of optimum burst excitation is developed in the frame-work of the Shinnar-LeRoux spinor formalism. An N pulse RF train of constant or linear phase cannot improve on an average echo strength of M₀/N, and a phase modulation of the pulse train is necessary to improve the signal yield to the theoretical maximum value M₀√;N. Several methods are presented yielding pulse trains of nearly optimum average amplitude for arbitrary N. It is shown that RF phase spoiling can be analyzed with the same framework. The presented pulse trains may also be useful when ultrawide spectrum hard pulses are required, but only limited RF power is available, e.g., for NMR experiments in extremely inhomogeneous B₀ fields.     

Double half RF pulses for reduced sensitivity to eddy currents in UTE imaging

Ultrashort echo time imaging with half RF pulse excitation is challenging as eddy currents induced by the slice‐select gradient distort the half pulse slice profile. This work presents two pulses with T₂‐dependent slice profiles that are less sensitive to eddy currents. The double half pulse improves the slice selectivity for long T₂ components, while the inverted double half pulse suppresses the unwanted long T₂ signal. Thus, both approaches prevent imperfect cancellation of out‐of‐slice signal from contaminating the desired slice. Experimental results demonstrate substantially improved slice selectivity and R₂* quantitation accuracy with these pulses. These pulses are effective in making short T₂ imaging and quantitation less sensitive to eddy currents and provide an alternative to time‐consuming gradient characterization. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.     

Reduced field‐of‐view single‐shot fast spin echo imaging using two‐dimensional spatially selective radiofrequency pulses

Purpose:

To demonstrate reduced field‐of‐view (RFOV) single‐shot fast spin echo (SS‐FSE) imaging based on the use of two‐dimensional spatially selective radiofrequency (2DRF) pulses.

Materials and Methods:

The 2DRF pulses were incorporated into an SS‐FSE sequence for RFOV imaging in both phantoms and the human brain on a 1.5 Tesla (T) whole‐body MR system with the aim of demonstrating improvements in terms of shorter scan time, reduced blurring, and higher spatial resolution compared with full FOV imaging.

Results:

For phantom studies, scan time gains of up to 4.2‐fold were achieved as compared to the full FOV imaging. For human studies, the spatial resolution was increased by a factor of 2.5 (from 1.7 mm/pixel to 0.69 mm/pixel) for RFOV imaging within a scan time (0.7 s) similar to full FOV imaging. A 2.2‐fold shorter scan time along with a significant reduction of blurring was demonstrated in RFOV images compared with full FOV images for a target spatial resolution of 0.69 mm/pixel.

Conclusion:

RFOV SS‐FSE imaging using a 2DRF pulse shows advantages in scan time, blurring, and specific absorption rate reduction along with true spatial resolution increase compared with full FOV imaging. This approach is promising to benefit fast imaging applications such as image guided therapy. J. Magn. Reson. Imaging 2010;32:242–248. © 2010 Wiley‐Liss, Inc.     

Design of symmetric-sweep spectral-spatial RF pulses for spectral editing.

Spectral-spatial RF (SSRF) pulses allow simultaneous selection in both frequency and spatial domains. These pulses are particularly important for clinical and research MR spectroscopy (MRS) applications for suppression of large water and lipid resonances. Also, the high bandwidth of the subpulses (5-10 kHz) greatly reduces the spatial-shift errors associated with different chemical shifts. However, the use of high-bandwidth subpulses along with enough spectral bandwidth to measure a typical range of metabolite frequencies (e.g., 300 Hz at 3 T) can require RF amplitudes beyond the limits of the RF amplifier of a typical scanner. In this article, a new method is described for designing nonlinear-phase 180 degrees SSRF pulses that can be used for spectral editing. The novel feature of the pulses is that the spectral profile develops as a symmetric sweep, from the outside edges of the spectral window towards the middle, so that coupled components are tipped simultaneously and over a short interval. Pulses were designed for lactate editing at 1.5 T and 3 T. The spectral and spatial spin-echo profiles of the new pulses were measured experimentally. Spectra acquired in phantom experiments showed a well-resolved, edited lactate doublet, with 91% to 93% editing efficiency.     

Reducing contamination while closing the gap: BASSI RF pulses in PASL.

Bandwidth-modulated selective saturation and inversion (BASSI) pulses are a class of frequency- and gradient-modulated radiofrequency (RF) pulses, derived from the hyperbolic secant pulse by temporal variation of the bandwidth parameter. These pulses afford optimal amplitude modulation, achieving uniform and highly selective profiles at any effective flip angle. In this paper, BASSI pulses are parameterized to obtain low RF energy pulsed arterial spin labeling (PASL) label pulses with minimal contamination of static spins outside the label region and highly selective PICORE/QUIPSS II saturation pulses allowing for small label gaps. They are compared to frequency offset corrected inversion (FOCI) label pulses and sinc saturation pulses in simulations and a phantom experiment. Drawing on the outstanding selectivity of bandwidth-modulated saturation pulses, a new noninvasive method to measure in vivo the contamination effects due to direct and indirect saturation of static spins by the label pulse is presented. In an in vivo study on four subjects, contamination effects in a QUIPSS II PASL implementation based on BASSI pulses are compared to those present in a state-of-the-art Q2TIPS sequence employing a FOCI label pulse. Residual contamination in the QUIPSS II/BASSI sequence is shown to be reduced by a factor of 3, compared to the Q2TIPS/FOCI sequence. In vivo human perfusion images obtained with a label gap of only 2 mm are presented.     

Steady state of echo-shifted sequences with radiofrequency phase cycling.

Due to a delayed echo-formation, echo-shifted gradient echo sequences (ES-GRE) allow for an enhanced T(2)*-weighting at short repetition times. While they are in use with and without RF spoiling, analytical solutions are only known for the latter. The signal formation in the former could only be assessed in approximative form, so far. In this article an exact analytical solution is presented for TR-periodic ES-GRE sequences with RF phase cycling. Besides providing a better numerical performance, it should be useful for systematic sequence development. The relation to recent approximative solutions is discussed.     

Variable-averaging RARE

The RARE method and its variants have become popular, rapid-imaging alternatives to conventional spin-echo imaging, particularly for long repetition time proton density and T²-weighted imaging. One variant is to generate both early and late echo images using the same pulse sequence, which has the added benefit of reduced edge artifacts and blurring. Described in this paper is variable-averaging RARE (VARARE), a method by which independent amounts of averaging can be set for the early and late echo images generated by a single scanning sequence. Through the use of this method, the signal-to-noise ratio (SNR) of late echo images can be improved without unnecessarily increasing the number of averages for the early echo image, thus saving scanning time. Comparisons to various alternatives are made with respect to scanning time and image quality. Phantom measurements and in vivo images are given to demonstrate the effectiveness of VA-RARE as an efficient method for improving SNR of late echo time images in RARE imaging.     

Two-dimensional spatially-selective RF excitation pulses in echo-planar imaging.

Two-dimensional spatially-selective RF (2DRF) excitation pulses were developed for single-shot echo-planar imaging (EPI) with reduced field of view (FOV) in the phase-encoding direction. The decreased number of k-space lines significantly shortens the length of the EPI echo train. Thus, both gradient-echo and spin-echo 2DRF-EPI images of the human brain at 2.0 T exhibit markedly reduced susceptibility artifacts in regions close to major air cavities. Based on a blipped-planar trajectory, implementation of a typical 2DRF pulse resulted in a 26-ms pulse duration, a 5-mm section thickness, a 40-mm FOV along the phase-encoding direction, and a 200-mm distance of the unavoidable side excitations from the center of the FOV. For the above conditions and at 2 x 2 mm(2) resolution, 2DRF-EPI yielded an echo train length of only 21 ms, as opposed to 102 ms for conventional EPI. This gain in time may be used to achieve higher spatial resolution. For example, spin-echo 2DRF-EPI of a 40-mm FOV at 1 x 1 mm(2) resolution led to an echo train of 66 ms. Although the current implementation still lacks user-friendliness, 2DRF pulses are likely to become a useful addition to the arsenal of advanced MRI tools. .     

Designing long-T2 suppression pulses for ultrashort echo time imaging.

Ultrashort echo time (UTE) imaging has shown promise as a technique for imaging tissues with T2 values of a few milliseconds or less. These tissues, such as tendons, menisci, and cortical bone, are normally invisible in conventional magnetic resonance imaging techniques but have signal in UTE imaging. They are difficult to visualize because they are often obscured by tissues with longer T2 values. In this article, new long-T2 suppression RF pulses that improve the contrast of short-T2 species are introduced. These pulses are improvements over previous long-T2 suppression pulses that suffered from poor off-resonance characteristics or T1 sensitivity. Short-T2 tissue contrast can also be improved by suppressing fat in some applications. Dual-band long-T2 suppression pulses that additionally suppress fat are also introduced. Simulations, along with phantom and in vivo experiments using 2D and 3D UTE imaging, demonstrate the feasibility, improved contrast, and improved sensitivity of these new long-T2 suppression pulses. The resulting images show predominantly short-T2 species, while most long-T2 species are suppressed.     

Steady state of gradient echo sequences with radiofrequency phase cycling: Analytical solution,contrast enhancement with partial spoiling

Spoiled gradient echo sequences can only reach a homogeneous steady state if sufficiently strong crusher gradients are used in combination with RF phase cycling (RF spoiling). However, the signal depends quite sensitively on the chosen phase increment ? and—lacking analytical solutions—numerical simulations must be used to study the transient and steady‐state magnetization. For the steady state an exact analytical solution is derived, which holds for arbitrary sequence and tissue parameters. Besides a considerably improved computation performance, the analytical approach enables a better understanding of the complicated dependence on ?. For short repetition times (TR) the regime of small ? turns out to be particularly interesting: It is shown that the typical ?_c, where RF spoiling starts to become effective, is essentially inversely proportional to T₂. This tissue dependence implies that contrasts can be considerably larger with partial spoiling (? ≈ ?_c) than with conventional RF spoiling (? ? ?_c). As an example, the uptake of contrast agents in tissues is investigated. For typical parameters a considerably improved contrast enhancement can be obtained, both theoretically and experimentally. Magn Reson Med, 2006. © 2005 Wiley‐Liss, Inc.     

Electrodynamic constraints on homogeneity and radiofrequency power deposition in multiple coil excitations

The promise of increased signal‐to‐noise ratio and spatial/spectral resolution continues to drive MR technology toward higher magnetic field strengths. SAR management and B₁ inhomogeneity correction become critical issues at the high frequencies associated with high field MR. In recent years, multiple coil excitation techniques have been recognized as potentially powerful tools for controlling specific absorption rate (SAR) while simultaneously compensating for B₁ inhomogeneities. This work explores electrodynamic constraints on transmit homogeneity and SAR, for both fully parallel transmission and its time‐independent special case known as radiofrequency shimming. Ultimate intrinsic SAR—the lowest possible SAR consistent with electrodynamics for a particular excitation profile but independent of transmit coil design—is studied for different field strengths, object sizes, and pulse acceleration factors. The approach to the ultimate intrinsic limit with increasing numbers of finite transmit coils is also studied, and the tradeoff between homogeneity and SAR is explored for various excitation strategies. In the case of fully parallel transmission, ultimate intrinsic SAR shows flattening or slight reduction with increasing field strength, in contradiction to the traditionally cited quadratic dependency, but consistent with established electrodynamic principles. Magn Reson Med 61:315–334, 2009. © 2009 Wiley‐Liss, Inc.