shows the image of a single slice through the brain of a normal volunteer scanned at 7T. 1
H MRSI data were obtained with a conventional PRESS sequence and the interleaved narrow-band PRESS sequence with adiabatic SPSP refocusing pulses. The PRESS box and spectral grid location for both 1
H MRSI experiments are shown on the image in . The measured B1
map for the same slice, acquired using the double-angle method [22
], can be seen in . An approximate 40% reduction in B1
from the center to the periphery of the brain is evident. The B0
map in was obtained after first and second order shimming was used to optimize the B0
homogeneity (17.75 Hz RMS, 180 Hz peak-to-peak). Changes in B0
over the region of interest stay well below the width of the spectral band of the SPSP pulses used in our sequence.
Figure 6 (A) Water image, (B) B1 map and (C) B0 map of a 1.5 mm slice of a normal human brain for which 1H MRSI data was obtained at 7T. The 5×5 spectral grid within the prescribed PRESS box is shown in (A). The location of the spectral grid is also overlaid (more ...)
The data obtained for the spectral grid location shown in , using a conventional PRESS sequence, can be seen in . When the same region is excited using the interleaved narrow-band PRESS sequence with adiabatic SPSP refocusing pulses, the spectral grids for Cho,Cre and NAA shown in are obtained. The scale was adjusted such that the noise, as determined from the spectral region without peaks, was equivalent for both data sets. Increased spatial coverage is clearly visualized.
In the spectra obtained using the standard PRESS sequence (), non-central voxels, especially those in the anterior portion of the grid, have reduced overall signal due to severe B1 drop-off. This reduction in B1 is visible in the 2D B1 profile shown in . The interleaved narrow-band sequence provides much more signal in these areas (). The SPSP 90° pulse is still not adiabatic and shading due to the B1 receive profile still exists, so some signal loss due to B1 inhomogeneity is to be expected. For the standard PRESS sequence (), the column of voxels along the left (i.e. patient’s left, reader’s right) edge of the PRESS box contain almost no NAA signal due to chemical shift localization error. This is considerably improved in the interleaved narrow-band acquisition (). See for the expected shift of the selected NAA volume for the two sequences. Signal variation may also be attributed to biological variation due to tissue type. When all voxels are averaged, the overall signal increase obtained with our sequence, relative to standard PRESS, is approximately 70% for Cho+Cre and 110% for NAA.
It is important to note that to remain under peak RF amplifier limits and within SAR constraints, the 180° refocusing pulses in the conventional PRESS sequence are replaced by 137° pulses. This is the standard GE Healthcare implementation for PRESS sequences at 3T and above, and involves a trade-off between signal amplitude and pulse bandwidth. High bandwidths are needed to reduce chemical shift misregistration errors. The adiabatic SPSP pulses used in the interleaved narrow-band sequence provide a 180° flip angle while remaining below RF peak amplifier and SAR limits, even when overdriven. As seen in , chemical shift localization error is negligible, regardless of the flip angle. Our sequence results in approximately 55% more signal than conventional PRESS at the central voxels due to this difference in flip angle as well as some B1 inhomogeneity.