Spiral out–in trajectories were designed and compared with conventional rewinder-based spiral MRSI and normal phase encoded MRSI. Both trajectories were designed using the analytic spiral design methods first described by Glover (18
). Gradient moments (first and higher moments) were calculated and compared for spiral waveforms with matrix size parameters of 8 × 8, 16 × 16, and 32 × 32. To demonstrate the reduced ghosting artifacts, 16 × 16 voxel spiral out–in trajectory was compared with normal phase encoding readouts. Data were acquired with a PRESS excitation scheme on a moving brain MRS phantom(19
). Therefore, the pulse sequence basically consisted of a 90°–90°–180° excitation scheme followed by the spiral readout. The echo time was 133 ms. Movement was induced by using a motion-generating feature available in our 1.5T scanner (GE Healthcare, Waukesha, WI, USA). The sinusoidal motion that was generated traveled 10 mm with peak velocity of 5 mm/s (frequency 0.25 Hz) similar to respiratory motion. The central 10 cm was prescribed by the PRESS box over a 20-cm FOV. The slice thickness was 1 cm. Data analysis was performed by examining the variation of the water spectra measured from both inside and outside the PRESS selected region.
For comparison with conventional rewinder-based MRSI, 32 × 32 matrix size spiral trajectories were used. First, simulations of the spectral point spread function were performed to estimate the amount of spectral ringing of a moving impulsive water component. By assuming that the impulse was located at δ2 [x, y − x(t); y(t)] during data acquisition, where x(t) and y(t) correspond to the location of the impulse, raw data were simulated and reconstruction was performed on this data set. The impulsive water component was assumed to be centered at the origin moving periodically in the in-plane direction (Y direction) at 1 cm/s during readout. Data from a spectroscopy phantom were also collected with and without motion to evaluate the performance of both trajectories. Periodic motion similar to respiration was induced using the built-in table motion feature of our scanner. For both trajectories, the readout length was set to 0.5 s, with a total scan time of 2 min (echo time[TE]/pulse repetition time [TR] 133/ 1500 ms). The conventional spiral had approximately 350 lobes, and each lobe had length of 1.6 ms, whereas the out–in spiral had approximately 256 lobes and each lobe had length of 2 ms. Data analysis was performed by examining the average SNR of the N-acetyl aspartate (NAA) spectra measured from conventional and out–in spiral acquisitions.
Finally, to illustrate the potential for in vivo use, we applied our sequence to water/fat spectroscopic imaging of the body. In this example, we evaluated multivoxel spectroscopy of the human liver for imaging water and fat components, which can be a useful tool for liver fat assessment. Spiral-based MRSI data sets, both conventional and out–in spirals, were acquired using a phased array abdominal coil on healthy volunteers from an axial slice through the liver (TE/TR 133/1500 ms, 30 cm FOV with 10 cm PRESS box). For both cases, a 32 × 32 voxel over a 32-cm FOV acquisition was used to look at the distribution of water and fat components. The out–in trajectory requires twice the number of readouts compared to conventional spirals due to the factor of two decrease in the effective FOV (). For our experiment, the total imaging time was kept the same for both trajectories by acquiring multiple averages for the conventional spirals. The total scan time was therefore 1.5 min for each data set. No water suppression or spatial saturation pulses were used during the acquisition because the objective of the study was to evaluate the water and fat components explicitly. To the degree possible, volunteers were asked to take short breaths during the dead time of the sequence and to not breathe during pulse excitation and readout period.
All phantom and in vivo data sets were reconstructed with gridding, apodization, and inverse FFT in the kx
, and kf
directions. Prior to gridding, a phase correction scheme was used to further reduce any phase discrepancies (20
). The Institutional Review Board (IRB) approved all in vivo studies.