hide
Free keywords:
-
Abstract:
Purpose/Introduction: Free induction decay (FID) based MRSI has been shown to be highly promising at ultra-high magnetic field [1, 2]. It avoids in-plane chemical shift displacement and allows short echo time. However, the necessity to use fat and water saturation results in an excessive amount of time needed for signal preparation. Thus, our aim was to improve the time efficiency of the MRSI-FID sequence to obtain spectra with high spatial resolution within reasonable time.
Subjects and Methods: Spectra were acquired at a 9.4 T whole body scanner (Siemens, Erlangen,Germany) from the superior part of the brain of a healthy volunteer (male, 34 years old) with approval by the local ethics committee. A 16 channel transmit, 31 channel receive coil [3] was used for signal transmission/reception. B0 field inhomogeneities were minimized by second order image based shimming. The sequence was preceded by a non-localized fat saturation gauss pulse, allowing reduction of the fat contaminations by approximately 40 . The flip angles of three water saturation pulseswere optimized for aT1 range of 800–2800 ms and B1 + inhomogeneities of±50 using aBloch simulation. Exchanging the sinc excitation pulse with an asymmetric pulse shortened the acquisition delay (TE) to 1 ms. The optimized sequence is presented in Fig. 1. The following parameters were used for in vivo data acquisition:
TR = 150 ms, TE = 1 ms, spectral bandwidth = 5000 Hz, acquisition duration = 108 ms, nominal voxel size 6 9 6910 mm (32 9 32 voxels), density weighting with 2 averages and acquisition timeof 2 min 19 s. Coil combinationwas performed using the adaptive combine method [4].
Fig. 1 Time diagram of the optimized MRSI-FID sequence. Abbreviations used: fat saturation pulse, FatSat; water suppression pulse, WET; excitation pulse, Excit
Results: Spectra obtained with the proposed sequence are of reasonable quality (Fig. 2). Although the fat signal is still present, it does not hamper the spectral range of interest (between 4.3 and 1.8 ppm), even in the voxels which are close to scalp (black and green). Differences
in the intensity of the presented spectra could be caused by
B1-field.
Results: Spectra obtained with the proposed sequence are of reasonable quality (Fig. 2). Although the fat signal is still present, it does not hamper the spectral range of interest (between 4.3 and 1.8 ppm), even in the voxels which are close to scalp (black and green). Differences
in the intensity of the presented spectra could be caused by
B1-field inhomogeneities.
Discussion/Conclusion: The presented sequence allows fast and efficient acquisition of MRSI spectra. It allows to acquire a 32 9 32 spectral matrix within less than 2.5 min. Further acceleration could be done using parallel imaging techniques. Relatively large fat residuals indicate the necessity of further optimization of the fat saturation.
Additional effort is also needed for reducing the influence of B0 and B1 +-variations on the measured spectra.
