Spin-echo magnetic resonance spectroscopic imaging at 7 T with frequency-modulated refocusing pulses

Abstract

Two approaches to high-resolution SENSE-encoded magnetic resonance spectroscopic imaging (MRSI) of the human brain at 7 Tesla (T) with whole-slice coverage are described. Both sequences use high-bandwidth radiofrequency pulses to reduce chemical shift displacement artifacts, SENSE-encoding to reduce scan time, and dual-band water and lipid suppression optimized for 7 T. Simultaneous B0 and transmit B1 mapping was also used for both sequences to optimize field homogeneity using high-order shimming and determine optimum radiofrequency transmit level, respectively. One sequence ("Hahn-MRSI") used reduced flip angle (90°) refocusing pulses for lower radiofrequency power deposition, while the other sequence used adiabatic fast passage refocusing pulses for improved sensitivity and reduced signal dependence on the transmit-B1 level. In four normal subjects, adiabatic fast passage-MRSI showed a signal-to-noise ratio improvement of 3.2±0.5 compared to Hahn-MRSI at the same spatial resolution, pulse repetition time, echo time, and SENSE-acceleration factor. An interleaved two-slice Hahn-MRSI sequence is also demonstrated to be experimentally feasible.

Figure 1

Pulse sequences developed for slice-selective MRSI at 7T. (A) Hahn-MRSI (90°-90°), (B) AFP-MRSI (90°-180°-180°) sequences. In both cases, signal excitation is preceded by (C) a dual-band water and lipid suppression with integrated outer-volume suppression (OVS).

Figure 2

Theoretical signal dependence of the 90°-90° (B₁×sin(α)×sin²(α/2)) and 90°-AFP (B₁×sin(α)) sequences as a function of the transmit B₁ field relative to a nominal B₁ field corresponding to a 90° rotation. Off-resonance effects and the effects of relaxation are ignored in these simulations. For comparison, the B₁ dependence of a conventional 90°-180° spin echo sequence (B₁×sin³(α)) is also shown.

Figure 3

Metabolic images recorded using the AFP-MRSI sequence in one subject. In addition to the sagittal (showing MRSI slice location) and axial anatomical MR images, the transmit B₁ map, and reconstructed metabolic images of images of glutamate and glutamine (Glx), myo-inositol (mI), choline (Cho), creatine (Cr) and N-acetyl aspartate (NAA) normalized to the H₂O-MRSI are shown. Histograms of the transmit B₁ distribution and the frequency (B₀) distribution after high order shimming in this slice location are also shown. 59% of MRSI voxels fell within the range +/− 10 Hz.

Figure 4

AFP-MRSI spectra from the whole slice of the dataset as shown in Figure 3.

Figure 5

Comparison of spectra using the Hahn-MRSI and AFP-MRSI pulse sequences in the same voxel (left parietal gray matter) from one subject; top trace is the original spectra and baseline fit, middle trace the baseline-subtracted experimental data overlaid with the results of the fitting routine, and the bottom trace the residual. The approximately 3-fold greater SNR of the AFP-MRSI sequence under the same acquisition conditions is apparent. However, the SAR of the AFP-MRSI sequence was more than double that of the Hahn-MRSI sequence (53% vs. 24%, respectively). Mild residual lipid (Lip) contamination is present.

Figure 6

Hahn-MRSI spectra from one slice of the 2-slice dataset recorded in one subject. In these experiments, note the incomplete lipid unfolding upfield from the NAA peak in central voxels, due to errors in the SENSE-reconstruction algorithm used.