Spatially two-dimensional selective radio frequency (2DRF) excitations are able to excite arbitrarily-shaped profiles in their excitation plane and, hence, can be used to minimize partial volume effects in single-voxel magnetic resonance spectroscopy. In this study, 2DRF excitations based on the PROPELLER trajectory which consists of blades of parallel lines that are rotated against each other, are presented. Because the k-space center is covered with each segment, the trajectory yields a high signal efficiency which, e.g., is considerably improved com-
Conventional localization in single-voxel MR spectroscopy (SVS) is based on cross-sectional RF excitations (1,2) that define a cuboidal measurement volume. However, the target regions of SVS like anatomical structures or focal lesions usually have more complex shapes which results in partial volume effects. The measurement volume can either be chosen to contain the full region-of-interest but then also includes surrounding tissue or only a part of the target region can be covered. These effects hamper the informative value of the metabolite concentrations determined.To minimize these partial volume effects, several approaches to acquire noncuboidal regions-of-interest in MR spectroscopy have been proposed in the past. Some of them introduce additional slice-selective RF excitations for excitation or saturation (3,4) but face limitations when dealing with nonconvex or nonconnected regionsof-interest. More promising is the SLOOP method (5,6), an extension of the SLIM technique (7), that identifies and applies a limited set of phase-encoding steps from which the spectrum of the arbitrary target volume can be determined. An alternative on the excitation side is the application of spatially 2D-selective RF excitations (8-10) that are able to excite an arbitrarily shaped profile within the two-dimensional plane defined by the underlying trajectory. Because of the required two-dimensional sampling of k-space, 2DRF pulses are longer than standard sliceselective RF excitations. To avoid excessive echo times and to minimize chemical-shift related displacement artifacts, the effective pulse duration can be reduced using segmentation (e.g., Refs. 11,12). Thereby, successive acquisitions are performed with different parts of the trajectory and the corresponding RF pulse shapes. The complex average of the acquired signals then yields the desired excitation profile.The first MRS experiments using segmented 2DRF excitations were performed for 31P SVS using pinwheel trajectories (11). More recently, segmented 2DRF excitations based on a radial trajectory were used in an animal study (13) in combination with nonselective refocusing RF excitations (14) to minimize resonance-offset effects. In a different approach applied in the human brain, a blipped-planar trajectory with a single line per segment was used which avoids chemical-shift related displacement artifacts in the blip direction (15). Furthermore, it limits the unwanted side excitations to a single direction which facilitates their elimination, e.g., by a refocussing RF pulse. Thus, pulse durations comparable to that of standard slice-selective RF excitations are achieved, and the short echo times typically used in SVS can be retained. However, the signal-to-noise ratio (SNR) per volume is reduced which is related to the lower signal contributions of outer k-space segments (15).In this study, a weighted averaging approach with flip angle adaptation is presented that can considerably improve the SNR efficiency of segmented 2DRF excitations as is demonstrated for single-line...
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.
Purpose: To improve the efficiency and flexibility of acquisitions of multiple voxels in MR spectroscopy by combining two-dimensional-selective radiofrequency (2DRF) excitations and Hadamard encoding.
Materials and Methods:With 2DRF excitations (PROPEL-LER trajectory, 16 half-Fourier segments, each with five lines) two voxels are defined. By combining the individual 2DRF pulses with Hadamard-like encoded phases, the voxels are acquired simultaneously but the individual contributions can be isolated from the obtained spectra. This is demonstrated on a 3 Tesla whole-body MR system in phantoms and in the human brain in vivo.Results: Compared with sequential single-voxel acquisitions the signal efficiency increases with the number of voxels covered. Furthermore, in comparison to conventional single-voxel MRS based on cross-sectional RF excitations, 2DRF excitations offer a higher flexibility because they allow for arbitrary voxel sizes, orientations, in-plane positions, and shapes.
Conclusion:The presented approach improves the flexibility and efficiency of acquisitions of multiple voxels, i.e., can shorten acquisition times accordingly, and can help to reduce partial volume effects.
Recently, spatially two-dimensional selective radiofrequency excitations based on the PROPELLER trajectory have been presented and were applied to minimize partial volume effects in single-voxel MR spectroscopy. Thereby, residual side excitations appeared due to limitations of the Voronoi diagram that was used to consider the nonconstant sampling density, and trajectory distortions caused by eddy currents varying between the differently rotated blades. In this extension, one of the refocusing radiofrequency pulses of a PRESS-based pulse sequence is applied in the blip direction of each segment to eliminate the side excitations. This corresponds to an infinitely dense sampling of the blade and the required sampling density correction can easily be calculated. Thus, signal contributions from outside the desired region-of-interest are completely avoided. The feasibility of this approach to acquire single-voxel MR spectra of anatomically defined regions-of-interest is demonstrated in the human brain in vivo on a 3T whole-body MR system.
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