A single spectral editing pulse incorporated into the PRESS sequence produces large Bloch-Siegert shift. The lack of a general method to quantify these shifts for spectral editing experiments has made it necessary to use a second identical editing pulse to cancel the shift. Here we describe a high-speed density matrix simulation method to accurately simulate a PRESS sequence with a single editing pulse for simultaneous detection of glutamate, glutamine, GABA, and glutathione at TE = 56 ms and 7 Tesla. To facilitate in vivo quantification, the frequency dependent Bloch-Siegert shift is accurately calculated and removed from the spectra.
INTRODUCTION
A single spectral editing pulse incorporated into the PRESS sequence can produce large Bloch-Siegert shift (1). The MEGA-PRESS method cancels this shift by adding a second identical editing pulse (2). A recently proposed full density matrix simulation technique (3) converts the time-consuming three-dimensional density matrix simulation of PRESS into three one-dimensional computation problems, shortening the computation time by orders of magnitude. Here, we applied the principle of one-dimensional projection to density matrix simulation of PRESS-based spectral editing using a single editing pulse for simultaneous detection of glutamate, glutamine, γ-aminobutyric acid (GABA), and glutathione at TE = 56 ms and 7 Tesla. The Bloch-Siegert shift as a function of chemical shift was accurately simulated and then removed from the spectra to allow spectral fitting of in vivo data acquired from the frontal cortex of the human brain.METHODS
The density matrix simulation programs were developed using the GAMMA NMR simulation C++ library (4). The simulated pulse sequence for simultaneous spectral editing of glutamate, glutamine, GABA, and glutathione is shown in Fig. 1. To digitize the 2 × 2 × 2 cm3 spectroscopy voxel, we used 200, 2000, and 200 spatial points in the x, y, and z directions, respectively. Based on the one-dimensional projection method (3), the spin density operator before the start of the first crusher gradient flanking the editing pulse was computed by summing up the 200 density matrices in the x direction. To account for frequency drift due to system instability and subject movement during the editing experiment, the rest of the spin density operator evolution was computed for 31 editing pulse frequency drift values ranging from -15 Hz to 15 Hz with one Hz increment. For each frequency drift value, the spin density operator was computed using two one-dimensional summations of the digitized density matrices, first in the y direction (2000 density matrices) and then in the z direction (200 density matrices). The free induction decay (FID) signals were then computed for each of the 31 frequency drift values. The Bloch-Siegert shift caused by the single editing pulse was computed by finding the phase shift values of simulated singlet peaks at 400 different frequencies between 0 – 4 ppm with a 0.01 ppm increment for each of the digitized spatial points in the y direction.DISCUSSION AND CONCLUSION
Keltner et al. (1) pioneered spectral editing of GABA using a PRESS-based sequence. To overcome the large Bloch-Siegert shift, they carefully chose the amplitude and duration of the editing pulse such that the Bloch-Siegert shift at 3.0 ppm became zero. As demonstrated in the current work, full density matrix simulation provides a practical way to accurately account for the Bloch-Siegert shift over the whole spectrum. The use of a single editing pulse can also shorten the echo time and provide additional freedom to design pulse sequences for simultaneously editing multiple metabolite signals.