Interleaved narrow-band PRESS sequence with adiabatic spatial-spectral refocusing pulses for 1H MRSI at 7T

Abstract

Proton magnetic resonance spectroscopic imaging ((1)H MRSI) is a useful technique for measuring metabolite levels in vivo, with Choline (Cho), Creatine (Cre), and N-Acetyl-Aspartate (NAA) being the most prominent MRS-detectable brain biochemicals. (1)H MRSI at very high fields, such as 7T, offers the advantages of higher SNR and improved spectral resolution. However, major technical challenges associated with high-field systems, such as increased B(1) and B(0) inhomogeneity as well as chemical shift localization (CSL) error, degrade the performance of conventional (1)H MRSI sequences. To address these problems, we have developed a Position Resolved Spectroscopy (PRESS) sequence with adiabatic spatial-spectral (SPSP) refocusing pulses, to acquire multiple narrow spectral bands in an interleaved fashion. The adiabatic SPSP pulses provide magnetization profiles that are largely invariant over the 40% B(1) variation measured across the brain at 7T. Additionally, there is negligible CSL error since the transmit frequency is separately adjusted for each spectral band. in vivo (1)H MRSI data were obtained from the brain of a normal volunteer using a standard PRESS sequence and the interleaved narrow-band PRESS sequence with adiabatic refocusing pulses. In comparison with conventional PRESS, this new approach generated high-quality spectra from an appreciably larger region of interest and achieved higher overall SNR.

Figure 1

(A) Interleaved narrow-band SPSP pulse sequence with first interleaf exciting spectral band 1 (centered between Cho & Cre resonances) and the second exciting spectral band 2 (centered on the NAA resonance). Very Selective Saturation (VSS) pulses are used to spatially saturate signal from subcutaneous fat. Timing parameters for the sequence are: readout duration (TAD) = 260 ms, TE = 90 ms and TR = 3 s. The long TR’s required at 7T leave room for the insertion of a third interleaf. Excitation RF and gradient waveforms for each interleaf are shown in (B).

Figure 2

(A) Magnitude and (B) phase of the 24 ms adiabatic SPSP 180° pulse used for refocusing. The peak B₁ value of the pulse is well below the limit of our 7T RF amplifier, which is 17 μT.

Figure 3

Simulated magnetization profiles for the adiabatic SPSP 180° pulse. (A) Spatial profile and (B) central passband of the spectral profile versus B₁ overdrive factor. Pulses can be overdriven by 60% before reaching the peak B₁ limit of the RF amplifier (17 μT) on our 7T magnet; however, the pulses maintain adiabaticity well beyond that limit.

Figure 4

Chemical shift localization error at 7T between selected volumes for NAA and Cho when using (A) a conventional PRESS sequence, (B) a PRESS sequence with SPSP 180° pulses and (C) our interleaved narrow-band PRESS sequence with adiabatic SPSP 180° pulses. CSL error is greatly reduced with the use of SPSP pulses and practically eliminated when an interleaved approach is used with adiabatic SPSP pulses for refocusing.

Figure 5

Simulated magnetization profiles for the final spin echo after the last 180° pulse. (A)Spatial profile, (B) spectral profile showing the main passband as well as the location of the sidebands (±1.9 kHz). All pulses were designed such that the frequency separation between the main passband and sidebands was large enough not to erroneously excite metabolites designated for the next interleaf. Dashed lines show resonant frequencies for metabolites when the main spectral passband is centered on Cho & Cre. (C) 2D spatial-spectral magnitude profile for pulse, showing selectivity in both space and frequency. The opposed sidebands visible at ±950 Hz are also located such that there is no spectral interference between interleaves. (D) 2D spatial-spectral phase profile demonstrating that flat phase for the main passband is achieved.

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 on the maps in (B) and (C) to show the expected B₁ and B₀ changes between voxels. The obtained ¹H MRSI data is shown for (D) conventional PRESS and (E) our interleaved narrow-band sequence with adiabatic SPSP refocusing pulses. Greater spatial coverage and overall SNR is achieved with our sequence in comparison to conventional PRESS. Acquisition parameters are: single 1.5 cm slice, 18×18 cm FOV, 12×12 matrix (only central 5×5 portion within PRESS box is shown), 3.4 cc voxels, TE/TR: 90/3000 ms, 1 NEX, 7:10 min scan time.