Synopsis

This abstract reviews and highlights the various properties and advantages of the LASER pulse sequence

Purpose

Properties of LASER sequence

1. Fully adiabatic single-scan 3D localization

2. No need for outer volume suppression (OVS)

The LASER sequence¹ consists of a nonselective adiabatic half-passage (AHP) pulse followed by three pairs of gradient selective adiabatic full-passage (AFP) pulses of the HSn family (Figure 1). The echo time (T_E) is defined as the total time from the end of the AHP pulse to the start of signal acquisition. At 9.4 T on pre-clinical scanner, a typical LASER sequence using a 4 ms AHP pulse and 1.5 ms AFP pulses (HS1 R25) results in a minimum T_E of 18 ms. At 3 T, on Siemens scanner using gradient-modulated refocusing pulse like GOIA², LASER can be executed with a minimum T_E of 35 ms.

3. Well defined localization: minimal chemical shift displacement and sharp and clean profiles

Due to properties of the AFP pulses, clean profiles with sharp transition are obtained (Figure 2). The chemical shift displacement (CSD) error is minimal in LASER due to the large bandwidth of the AFP pulses which are typically 10 kHz or higher (Table 1). At 9.4 T on pre-clinical scanner, the CSD error is relatively small (2.4%/ppm). On human scanner with 3.2 ms GOIA pulses, the CSD error is negligible. However, off-resonance small smearing artifacts exist.

4. Suppression of J-evolution

The successive application of multiple AFP pulses in LASER acts as a Carr-Purcell pulse train³ such that the J-coupling evolution in J-coupled metabolites is minimized. This occurs when the time evolution due to J-modulation becomes negligible, i.e. when $$τ √δ^2+J^2 << 1$$ where τ is an interpulse delay, J is the coupling constant and δ is the chemical shift difference between coupled spins. This effect is shown Figure 3 where the spectral patterns of coupled metabolites like glutamate, glutamine, myo-inositol are still preserved with LASER at 60 ms compared to STEAM at a similar T_E and are comparable to short-T_E STEAM spectrum4. By adding train of AFP pulses without any delay between the AHP and the first gradient-selective AFP pulse, sequence called T_2p-LASER⁵, J-modulation can be further preserved especially at long T_E. For instance, data from 9.4 T show that the multiplets of taurine and myo-inositol do not undergo any J-evolution when T_E is extended from 32 to 200 ms, although their signal intensities are reduced due to transverse relaxation (Figure 4).

5. Prolonged apparent T₂

At the minimum achievable T_E in LASER, all metabolites undergo T₂ relaxation in the T_2p regime while at long T_E the relaxation is mostly dominated by the diffusion and exchange due to longer τ delay. LASER results in longer apparent T₂ times than with standard sequences such as STEAM or PRESS. For instance, an increase in T₂ of tissue water and singlet resonances (e.g., NAA and tCr) was reported in the human brain at 4 T and 7 T⁶. With T_2p-LASER sequence, the increase in T₂ for strongly coupled metabolites were higher (up to four times) compared to singlet and weakly coupled metabolites⁵. The suppression of J-evolution and prolonged apparent T₂ result in much smaller signal loss for LASER than what is observed for other sequences at the similar T_E.

Acknowledgements

References

1. Garwood & DelaBarre JMR 2001

2.Tannus & Garwood NMR Biomed 1997

3. Carr & Purcell Phys Rev 1954

4. Deelchand et al. MRM 2014

5. Deelchand et al. MRM 2011

6. Michaeli et al. MRM 2002