Synopsis

Keywords: Pulse Sequence Design, Spectroscopy

In this abstract, a spectral editing method using MEGA-PRESS with conditional frequency-selective saturation pulses is proposed. The proposed method used additional frequency-selective saturation blocks before the excitation block. The saturation pulses were activated only when the MEGA pulses were inactive. When saturation pulses were activated, the sequence acted as the conventional MEGA-PRESS whose editing pulses were inactive except that the passband of the saturation pulse was not excited. Spectrum reconstruction for the proposed method was same as that for the conventional MEGA-PRESS. Experimental results showed that the proposed method selectively extracted J-coupled signals with all frequencies.

Introduction

For edited MR spectroscopy¹, sequences with frequency-editing refocusing pulses such as band selective inversion with gradient dephasing (BASING)² and Mescher-Garwood (MEGA)³ pulses are used commonly for analyzing J-coupled evolution. These sequences acquire differences between signals with editing pulses and signals without the editing pulses. For non-editing frequency bands, J-coupled evolution can be selectively observed. However, in the editing frequency band, J-coupled evolution cannot be analyzed easily since non-J-coupled signals such as singlets are overlapped with J-coupled signals. Such signals are not observable when other editing techniques such as a double quantum filter⁴ are used.
In this abstract, a spectral editing method using 1H MEGA- Point RESolved Spectroscopy (PRESS) with conditional frequency-selective saturation pulses is proposed. Similar to other editing techniques, the proposed method selectively extracted J-coupled signals with all frequencies.

Methods

Pulse sequence
As shown in Figure 1, the proposed method used additional frequency-selective saturation blocks before the excitation block of PRESS. The saturation pulses were centered with the editing frequency (referred to as active) only when the MEGA pulses were centered with a non-editing frequency (referred to as inactive). When saturation pulses were activated (named Sat), the sequence acted as MEGA-PRESS whose editing pulses were inactive except that the passband of the saturation pulse was not excited. When the MEGA pulses were activated (named Nosat), the sequence acted as MEGA-PRESS whose editing blocks were used. For both the saturation and the MEGA pulses, a Gaussian pulse with duration of 21.2 milliseconds was used. A block of VAriable Power and Optimized Relaxations delays (VAPOR)⁵ was used before the excitation block for water suppression.
To extract J-coupled signals selectively, the signals of Nosat were subtracted from the signals of Sat. The evolution of unsaturated spins and J-coupling was active in the Sat cases. In the Nosat cases, the evolution of all spins and J-coupling was active and the evolution of J-coupling in the passband of the MEGA pulses was spoiled in the MEGA blocks.
In the following evaluations, the editing frequency was adjusted to approximately 1.9 ppm using estimated frequency of water. The editing frequency for inactive cases was determined for simultaneous water suppression of MEGA-PRESS.

Evaluations using phantoms
Two phantoms named Phantom #1 and Phantom #2 were scanned using a 3-T scanner and a 32-channel receiver head coil with NEX of $$$64 \times 2$$$. Other scan parameters were TE of 68 milliseconds, TR of 2000 milliseconds, dwell time of 384 microseconds, samples per readout of 1024, and volume-of-interest (VOI) of $$$3 \times 3 \times 3$$$ cm³. In addition to the subtraction of the Nosat spectrum from the Sat spectrum, individual spectra were also reconstructed for Phantom #1.
The concentrations of the metabolites used in these phantoms are shown in Figure 2. Phantom #1 simulated the spectra of a normal brain⁶ with an inhibitory neurotransmitter gamma-aminobutyric acid (GABA). Phantom #2 simulated isocitrate dehydrogenase (IDH)-mutant brain tumor tissue⁷ with oncometabolite 2-hydroxyglutarate (2-HG). Spectra of GABA and 2-HG have measurable spins with J-couplings at approximately 3.0 ppm and 4.0 ppm, respectively, when editing pulses are applied to 1.9 ppm ¹. These spectra also have measurable spins in 2.2 - 2.3 ppm for the same editing pulses. In addition, N-acetylaspartate (NAA) has a singlet at 2.0 ppm near editing frequency. These phantoms also contained several other metabolites including NAA as shown in Figure 2.

Evaluation using volunteer data
A healthy volunteer was scanned using another 3-T scanner and a 12-channel receiver head neck coil with NEX of $$$128 \times 2$$$. Other scan parameters were same as those for the phantoms. The subtracted spectra were reconstructed for the volunteer data.

Results

Individual spectra of Phantom #1 are shown in Figure 3. It could be confirmed that the unsuppressed spectrum of MEGA off was suppressed in the spectrum of Sat near the editing frequency at 1.9 ppm.
The subtracted spectra of Phantom #1 and Phantom #2 are shown in Figure 4. The editing-related spectra were found at 3.0 ppm in Phantom #1 and at 4.0 ppm in Phantom #2. In addition, unlike the spectrum of MEGA, the spectrum of the proposed method clearly suppressed the singlet of NAA at 2.0 ppm.
The spectra of the volunteer are shown in Figure 5. The editing-related spectra were detectable at 3.0 ppm. Similar to the phantom cases, the spectrum of the proposed method revealed J-coupled signals near the editing frequency at 1.9 ppm.

Discussion

The results showed that the singlet of NAA at 2.0 ppm was selectively suppressed in the spectra of the proposed method. These results implied that the proposed method could reveal J-coupled signals around 1.9 ppm. While it could not be confirmed clearly, spectra come from macromolecule (MM)⁸ seemed to be suppressed except but signals coupled within the editing frequency band.

Conclusion

A spectral editing method using MEGA-PRESS with conditional frequency-selective saturation pulses has been proposed. The proposed method used additional frequency-selective saturation blocks before the excitation block of PRESS. The obtained spectra implied that the information of J-coupled signals could be used as additional information for analyzing spectra.

Acknowledgements

No acknowledgement found.

References

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