Selection of high-definition 2D virtual profiles with multiple RF pulse excitations along interleaved echo-planar k-space trajectories

MRI exploiting frequency-modulated pulses and their nonlinear phase

Frequency-modulated (FM) pulses can provide several advantages over conventional amplitude-modulated pulses in the field of MRI; however, the manner in which spins are manipulated imprints a quadratic phase on the resulting magnetization. Historically this was considered a hindrance and slowed the widespread adoption of FM pulses. This article seeks to provide a historical perspective of the different techniques that researchers have used to exploit the benefits of FM pulses and to compensate for the nonlinear phase created by this class of pulses in MRI. Expanding on existing techniques, a new method of phase compensation is presented that utilizes nonlinear gradients to mitigate the undesirable phase imparted by this class of pulses.

Accelerated imaging with segmented 2D pulses using parallel imaging and virtual coils

Large magnetic field inhomogeneity can be a significant cause of spatial flip-angle variation when using ordinary, limited-bandwidth RF pulses. Multidimensional RF pulses are particularly sensitive to inhomogeneity due to their extended pulse length, which decreases their bandwidth. Previously, it was shown that, by breaking a 2D pulse into multiple undersampled k-space segments, the excitation bandwidth can be increased at the expense of increased imaging time. The present study shows how this increased imaging time can be offset by undersampling acquisition k-space in a phase-encoded dimension that is in the direction of excitation segmentation. Data from each segment are viewed as originating from "virtual receive coils" rather than multiple physical coils. The undersampled data are reconstructed using parallel imaging techniques (e.g. as in GRAPPA). The method was tested in vivo with brain imaging at both 3 T and 4 T, and used in conjunction with a 32-channel head coil and conventional GRAPPA on the 3 T data. Relationships with existing techniques and future applications are discussed.

Interleaved spatial/spectral encoding in ultrafast 2D NMR spectroscopy

The possibility to record a full 2D spectrum in less than a second using ultrafast 2D NMR (UF2DNMR) is beneficial in many applications. However, the spatial encoding process on which UF2DNMR is based sets specific constraints on the spectral width and resolution of the resulting spectra. To overcome these limitations, a tailored encoding method using spatial/spectral pulses (SPSP) can be employed as an alternative to the traditional linear spatial encoding of interactions. Here we analyze and further develop this alternative spatial encoding strategy. We first carry out numerical simulations to describe the features of bidimensional SPSP pulses. Sidebands are identified along the spectral dimension of the excitation profile. An interleaved excitation scheme is then developed and implemented experimentally to suppress the unwanted signals that arise from these harmonic sidebands. Two examples are shown to illustrate the potential of the proposed approach. An ultrafast selective TOCSY spectrum is recorded to access sub-spectra and fully assign ¹H NMR resonances of individual residues of cyclosporin A. An ultrafast HSQC spectrum of a mixture of metabolites is recorded with an optimized spectral width in the spatially encoded dimension.

Two-dimensional frequency-swept pulse with resilience to both B1 and B0 inhomogeneity

Applications of multidimensional spatially-selective pulses are sometimes limited by their long pulse durations resulting from the need to execute a modulated gradient waveform in concert with RF transmission. Here, we introduce a method to design two-dimensional selective adiabatic pulses using a Cartesian k-space trajectory. The full pulse can be sampled using various undersampled segments to create a multidimensional pulse resilient to large off-resonances. Moreover, the pulse can be designed to be resilient to B1+ inhomogeneity. Experimental demonstrations of fully segmented and single-shot k-space sampling patterns are presented.

A dual K-space UNFOLD method for 3D functional brain imaging: A preliminary study

PURPOSE

To investigate a method of dual k-space unaliasing by Fourier-encoding the overlaps using the temporal dimension (DUNFOLD), a novel technique for high temporal resolution 3D functional brain imaging.

METHODS

Two different methods of unaliasing by Fourier-encoding the overlaps using the temporal dimension (UNFOLD), excitation UNFOLD (XUNFOLD) and acquisition UNFOLD, were merged to obtain a DUNFOLD. The feasibility of the DUNFOLD technique was examined by using a phantom and comparing its result to that of the previous XUNFOLD method. A high temporal resolution 3D functional brain imaging study was also performed, focusing on the microvascular response. Three different temporal resolutions, 20s, 10s and 5s, were tested with a spatial resolution of 0.6(3) mm3 to evaluate the method. The vascular regions of interest were selected for data analysis.

RESULTS

The DUNFOLD method achieved a temporal resolution approximately four times greater than those of the UNFOLD and XUNFOLD methods, without apparent signal degradation. The vascular responses in the visual cortex were obtained with high spatiotemporal resolution by using the DUNFOLD method during visual stimulation. For small vessels, the percentage change in the signal reached 18%.

CONCLUSION

The proposed DUNFOLD method yields a temporal resolution higher than those of the previous UNFOLD and XUNFOLD methods. The conclusions are likely to be important for functional imaging studies, especially those targeting cerebral vascular responsiveness.

Signal scaling improves the signal-to-noise ratio of measurements with segmented 2D-selective radiofrequency excitations

PURPOSE

Segmented 2D-selective radiofrequency excitations can be used to acquire irregularly shaped target regions, e.g., in single-voxel MR spectroscopy, without involving excessive radiofrequency pulse durations. However, segments covering only outer k-space regions nominally use reduced B1 amplitudes (i.e., smaller flip angles) and yield lower signal contributions, which decreases the efficiency of the measurement. The purpose of this study was to show that applying the full flip angle for all segments and scaling down the acquired signal appropriately (signal scaling) retains the desired signal amplitude but reduces the noise level accordingly and, thus, increases the signal-to-noise ratio.

METHODS

The principles and improvements of signal scaling were demonstrated with MR imaging and spectroscopy experiments at 3 T for a single-line segmentation of a blipped-planar trajectory.

RESULTS

The observed signal-to-noise ration gain depended on the 2D-selective radiofrequency excitation's resolution, field-of-excitation, and its excitation profile and was between 40 and 500% for typical acquisition parameters.

CONCLUSION

Signal scaling can further improve the performance of measurements with segmented 2D-selective radiofrequency excitations, e.g., for MR spectroscopy of anatomically defined voxels.

Small field of view imaging using wavelet encoding with 2 dimensional RF pulses and gradient echo: phantom results

OBJECT

The objective of this work is to propose an imaging sequence based upon the wavelet encoding approach to provide MRI images free from folding artifacts, in the small field of view (FOV) regime, such as dynamic magnetic resonance imaging (MRI) studies.

MATERIALS AND METHODS

The method consists of using a 2D spatially selective RF excitation pulse inserted into a gradient- echo pulse sequence to excite spins within a determined plane where wavelet encoding is achieved in one direction and slice selection is performed in the second direction. Wavelet encoding allows for spatially localized excitation and consequently restricts the spins excited within a reduced FOV. It consists of varying, according to a predetermined scheme, the width and position of the profile of the so-called fast RF pulse of the 2D RF excitation pulse, to obey wavelet encoding translation and dilation conditions. This sequence is implemented on a 3 Tesla whole body Siemens scanner.

RESULTS

Compared to Fourier encoding, the proposed technique tested on phantoms with different shapes and structures, is able to provide gradient-echo reduced FOV images free from aliased signals.

CONCLUSION

Wavelet encoding is suitable for small FOV imaging in dynamic MRI studies.

Calculation of RF Peak Power for 2D Arbitrary Shape Excitation in MRI

An exact formula has been derived to calculate the required RF (B1) peak power for 2D arbitrary shape excitation in MRI with rectilinear K-space trajectory. An approximate formula has also been derived for spiral trajectory.

Basis function cross-correlations for Robustk-space sample density compensation, with application to the design of radiofrequency excitations

The problem of k-space sample density compensation is restated as the normalization of the independent information that can be expressed by the ensemble of Fourier basis functions corresponding to the trajectory. Specifically, multiple samples (complex exponential functions) may be contributing to each independent information element (independent basis function). Normalization can be accomplished by solving a linear system based on the cross-correlation matrix of the underlying Fourier basis functions. The solution to this system is straightforward and can be obtained without resorting to discretization since the cross-correlations of Fourier basis functions are analytically known. Furthermore, no restrictions are placed on the k-space trajectory and its point-spread function. Additionally, the linear system can be used to elucidate key trade-offs involved in k-space trajectory design. The approach can be used to compensate samples acquired for image reconstruction or designed for low flip angle radiofrequency (RF) excitation. Here it is demonstrated for the latter application, using reversed spiral trajectories. In this case the linear system approach enables one to easily incorporate additional constraints such as smoothness to the solution. For typical RF excitation durations (<20 ms) it is shown that density compensation can even be achieved without numerical iteration.

2D arbitrary shape-selective excitation summed spectroscopy (ASSESS)

Conventional single-voxel localization for MR spectroscopy (MRS) is restricted to selecting only rectangular-shaped regions of interest (ROIs). The complexity of tissue shapes of interest and the desire to maximize the signal-to-noise ratio (SNR) while minimizing partial-volume effects require more sophisticated localization techniques. A group of spatially selective RF pulses are proposed in this work for the measurement of spectra from regions of arbitrary shape based on using a radial trajectory in k-space. Utilizing a single k-line per excitation results in a broad spectroscopic bandwidth. However, spatial localization accuracy is compromised for nutation angles > 10 degrees because of the small-tip-angle approximation of the Bloch equations. By interleaving multiple radial k-lines per excitation with nonselective refocusing pulses, one can achieve accurate localization for nutation angles up to 90 degrees while simultaneously maintaining the spectral bandwidth. The technique is described and compared with existing localization methods, and in vivo results are demonstrated.

Excitation UNFOLD (XUNFOLD) to improve the temporal resolution of multishot tailored RF pulses

An extension of the "UNaliasing by Fourier encoding the Overlaps using the temporaL Dimension" (UNFOLD) method to the excitation domain (XUNFOLD) is presented to improve the temporal resolution of multishot tailored RF (TRF) pulses. Multishot three-dimensional TRF pulses were designed to produce a time series of images with periodically aliased excitation profiles. The XUNFOLD method is shown to remove the excitation profile aliasing from the dynamic imaging data by filtering in the temporal frequency dimension. The technique is demonstrated to improve the temporal resolution of simulated functional MRI (fMRI) activation in a time series of brain images.

PSF-choice: a novel MRI method for shaping point-spread functions in phase-encoding dimensions

An imaging method to obtain arbitrary point-spread functions (PSFs) in phase-encoding dimensions is described. This method, called PSF-Choice, is particularly relevant for applications, such as spectroscopic imaging, in which only a very few phase encodes are acquired and ringing artifact can be a serious problem. PSF-Choice uses partial 2D RF excitations to produce aliased excitations that are encoded using standard phase-encoding gradients. Theoretically, the PSF of the reconstructed result depends only on the RF excitation profile. Simulations demonstrate that a Gaussian-like PSF can be achieved, eliminating the side lobes that are associated with ringing artifact. It is further shown that neither the spatial resolution (as represented by the width of the PSF) nor the signal-to-noise ratio (SNR) of the method is adversely affected when compared to standard phase encoding. In the sense that the same number of encodes are required as with standard phase encoding, temporal resolution is also maintained. Phantom experiments demonstrate the initial feasibility of the method to eliminate ringing artifact.

High-resolution fast spin echo imaging of the human brain at 4.7 T: implementation and sequence characteristics

In this work, a number of important issues associated with fast spin echo (FSE) imaging of the human brain at 4.7 T are addressed. It is shown that FSE enables the acquisition of images with high resolution and good tissue contrast throughout the brain at high field strength. By employing an echo spacing (ES) of 22 ms, one can use large flip angle refocusing pulses (162 degrees ) and a low acquisition bandwidth (50 kHz) to maximize the signal-to-noise ratio (SNR). A new method of phase encode (PE) ordering (called "feathering") designed to reduce image artifacts is described, and the contributions of RF (B(1)) inhomogeneity, different echo coherence pathways, and magnetization transfer (MT) to FSE signal intensity and contrast are investigated. B(1) inhomogeneity is measured and its effect is shown to be relatively minor for high-field FSE, due to the self-compensating characteristics of the sequence. Thirty-four slice data sets (slice thickness = 2 mm; in-plane resolution = 0.469 mm; acquisition time = 11 min 20 s) from normal volunteers are presented, which allow visualization of brain anatomy in fine detail. This study demonstrates that high-field FSE produces images of the human brain with high spatial resolution, SNR, and tissue contrast, within currently prescribed power deposition guidelines.

Multishot 3D slice-select tailored RF pulses for MRI

A multishot 3D slice-select tailored RF pulse method is presented for the excitation of slice profiles with arbitrary resolution. This method is derived from the linearity of the small tip angle approximation, allowing for the decomposition of small tip angle tailored RF pulses into separate excitations. The final image is created by complex summation of the images acquired from the individual excitations. This technique overcomes the limitation of requiring a long pulse to excite thin slices with adequate resolution. This has implications in applications including T*(2)-weighted functional MRI in brain regions corrupted by intravoxel dephasing artifacts due to susceptibility variations. Simulations, phantom experiments, and human brain images are presented. It is demonstrated that at most four shots of 40 ms pulse length are needed to excite a 5 mm-thick slice in the brain with reduced susceptibility artifacts at 3T.

Slab scan diffusion imaging

For maximum robustness of a diffusion-weighted MR imaging sequence, it is desirable to use a single-shot imaging method. This article introduces a new single-shot imaging approach that combines the advantages of multiple spin-echoes with the technique of line scan diffusion imaging. A slab volume, which can be spatially encoded with fewer phase encodes than a regular field of view, is selected with 2D selective pulses. With the shorter echo train, the sensitivity to field inhomogeneities and chemical shift is thus greatly diminished. Further reduction is achieved by interleaving short gradient echo trains with refocusing spin-echo pulses. Optimized slice-selective RF pulses that produce flip angles close to 180 degrees are used to minimize the stimulated echo component. Motion-related phase shifts, which change polarity with each spin-echo excitation, will give rise to artifacts that are avoidable by processing even and odd spin-echoes separately. As with line scan diffusion imaging, the complete field of view is acquired by sequential scanning. Since with each shot several lines of data are collected, a considerable improvement over line scan diffusion imaging in terms of scanning speed is achieved. Diffusion data obtained in phantoms and normal subjects demonstrate the feasibility of this novel approach.