· Spectral editing enables improved quantification of metabolites that are otherwise difficult to measure.

· Two key spectral editing techniques are J-difference editing and multiple quantum filtering.

· The lecture covers basic principles and sequences of difference and multiple quantum editing.

Students and researchers working in the field of in-vivo magnetic resonance spectroscopy will benefit from the lecture. At the end of the lecture the audience will have an understanding of how spectral editing can improve the quantification of some difficult to measure metabolites. In addition, the audience will understand the physical principles behind difference and multiple quantum filtering pulse sequences.In-vivo proton magnetic resonance spectra exhibit poor spectral resolution, particularly at clinical field strengths, due to the overlap of peaks with similar chemical shifts and peak splitting due to homonuclear scalar coupling interactions. Spectral editing techniques have been designed and implemented to enable retention of peaks from metabolites of interest while suppressing background contaminating peaks. The techniques of difference spectral editing (1) and multiple quantum filtering (2) are presented and discussed in this lecture.

Difference Spectral Editing:

Basic J-Difference Editing:

Figure 1 displays a basic difference editing spin echo pulse sequence. The sequence is a variation of that employed by Rothman et al. in the first in-vivo application of J-difference editing (1). Consider a weakly-coupled AX spin system (scalar coupling constant = J_AX) and assume that the signal of interest is from spin A and that it is overlapped by undesired signal from uncoupled spin I. On the first application of the sequence, the frequency selective pulses are off and spin A evolves according to A_Ycos(πJ_AXTE) - 2A_XX_Zsin(πJ_AXTE) under J-coupling evolution. Setting the echo time, TE, to 1/J_AX reduces the expression to - A_Y resulting in an inverted doublet for spin A. Therefore, ignoring signal from spin X, the output of the first scan is I_Y - A_Y . Turning on the 180° X frequency selective pulses rewinds the J-coupling evolution of spin A resulting in I_Y + A_Y. Subtracting the first scan from the second yields 2A_Y and the unwanted singlet signal from spin I is removed and the result is an upright doublet for spin A. For the observation of quartets (AX₃ spin system) TE is also set to 1/J_AX; however, for triplets (AX₂ spin system) TE is set to 1/(2J_AX). For the latter case, only the two outer peaks are retained resulting in a 50 % signal loss.

MEGA-PRESS:

The MEscher-GArwood (MEGA) sequence was developed for solvent suppression by exploiting asymmetric gradient pulses to dephase spins affected by frequency selective pulses (3). The MEGA sequence has been incorporated into the commonly employed in-vivo magnetic resonance spectroscopy (MRS) pulse sequence Point RESolved Spectroscopy (PRESS) resulting in the MEGA-PRESS sequence (4), shown in Figure 2. The PRESS (5,6) sequence’s versatile nature renders it suitable for the incorporation of a number of editing techniques (7). In MEGA-PRESS, the frequency selective pulses of MEGA can be exploited for J-difference editing while PRESS enables three-dimensional spatial localization. MEGA-PRESS has been employed to observe a number of metabolites in human brain by applying the frequency selective pulses on alternate scans, which are subtracted. The echo time is optimized for the spins to be observed and the frequency selective pulses target the spins weakly-coupled to them. MEGA-PRESS has been employed to resolve the 3 ppm GABA (gamma aminobutyric acid) resonance from overlapping Cr (creatine) signal at 4 T (4). In addition, MEGA-PRESS has been successfully applied to resolve the 3 ppm GSH (glutathione) resonance from overlapping GABA and Cr peaks (8) at 4 T. Furthermore, it has been used to measure Asc (ascorbate) at 4 T (9) and to distinguish NAA (N-acetyl aspartate) from NAAG (N-acetyl aspartyl glutamate) at 3 T (10).

Multiple Quantum Filtering:

Basic Multiple Quantum Filter:

The order of a multiple quantum coherence (MQC) depends on the difference between the quantum numbers related to the energy levels involved in an energy state transition. Single quantum coherences (SQCs) have an order of ±1 and are the only directly observable coherences. For a coupled two-spin system, zero quantum coherences (ZQCs) and double quantum coherences (DQCs) can also be created. Multiple quantum filters (MQFs) pass through spin coherences of a certain order. Piantini et al. demonstrated how a sequence of pulses with an appropriate phase cycling scheme can selectively refocus a coherence pathway of interest (2). However, the more common single-shot MQF approach in in-vivo MRS is to employ gradient pulses of appropriate relative areas to select the desired pathway. Figure 3 shows a basic MQF pulse sequence. Consider a weakly-coupled AX spin system and let us examine the response of the A proton. Setting the delay TE₁ to 1/(2J_AX) maximizes the creation of antiphase magnetization and the second 90° pulse transforms the coherence to , a MQC which is a combination of ZQCs (A⁺X^- and A^-X⁺) and DQCs (A⁺X⁺ and A^-X^-). The former are not affected by gradient G₁; however, the DQCs accumulate a phase shift that is twice that accumulated by the SQCs, namely 2Ø, resulting in the states A⁺X⁺e^-i2Ø and A^-X^-e^i2Ø. The third 90° pulse, termed the read pulse, transforms the MQCs back to antiphase SQCs which get converted to in-phase magnetization by the gradient G₂ and a TE₂ delay of 1/(2J_AX)­. Setting the area of G₂ to zero will refocus the ZQC pathway and will also pass through signal from uncoupled spins. If the area of G₂ is set to twice, or negative twice, that of G₁­ one of the DQC pathways will be selected. The signal yield for a zero quantum filter (ZQF) is 50 % while that of a double quantum filter (DQF) is only 25 %. However, it has been shown that the yield of a DQF can be increased by employing a frequency selective pulse for the read pulse that only targets the X protons (11,12).

MQFs with Spatial Localization

The MQF sequence can be incorporated into PRESS to enable three-dimensional spatial localization as shown in Figure 4. For spin systems that are more complex than a simple AX spin system, particularly those involving strong coupling, the optimal timings need to be determined for the specific protons being targeted. For example, it was determined that a PRESS-DQF withTE₁ = TE₂ = 37.5 ms was optimal for observing the AB protons (≈ 2.95 ppm) of the ABX spin system of the cysteinyl group of GSH at 1.5 T (13). The DQF effectively suppressed overlapping Cr signal. PRESS-DQF parameters have also been optimized to enable the detection of strongly-coupled Glu (glutamate) protons at ≈ 2.3 ppm with minimal contamination from Gln (glutamine) at 3 T (14) and for the in-vivo detection of the 3 ppm GABA resonances at 1.5 T (15,16) and at 4.1 T (17). A modified PRESS-ZQF was employed to detect the strongly-coupled protons of mI (myo-inositol) resonating in the vicinity of 3.6 ppm while minimizing signal contamination from background Glu, Gln, Gly (glycine) and Tau (taurine) signal at 3 T (18). Setting the phase of the second 90° pulse so that it is orthogonal to that of the first (19) suppresses signal from weakly-coupled Glu and Gln protons and from uncoupled Gly protons, whereas optimal timings were determined to suppress Tau signal (18).

Difference editing and multiple quantum filter pulse sequences have been tailored and applied in vivo to facilitate the measurement of a number of relevant metabolites such as GABA, Glu, mI and GSH. The pulse sequences can be optimized for other metabolites and employed in research studies where specific metabolites are of interest.