MAGNETIC RESONANCE IN MEDICINE 19, 477-482 ( 199 I )

Communications

Longitudinal Spin-Order-Based Pulse Sequence for Lactate Editing

RAVINDER REDDY, V. H. SUBRAMANIAN, €3. J. CLARK,* AND JOHN S. LEIGH

School of Medicine, Department of Biochemistry and Biophysics, Univmity of Pennsylvania, and *Division of Pediatric Cardiology, Children’s Hospital of Philadelphia, Philadelphia, Pennsylvania I9104

Received September 17, 1990; revised November 20, 1990

A new pulse sequence which edits proton spectra of lactate with full signal return and gives good suppression of water and fat signals is described. This sequence exploits lon- gitudinal spin-order from lactate to edit lactate from fat. Experimental results from phantoms and excised pig heart are presented. o 1991 Academic Press. Inc.

INTRODUCTION

The direct observation of in vivo lactic acid by proton NMR spectroscopy is difficult due to interfering lipid resonances and the presence of a highly dominant water signal which causes severe dynamic range problems. Consequently it is essential to employ some sort of spectral editing in conjunction with water suppression to observe in vivo lactic acid. There have been several pulse sequences proposed in the literature for editing proton spectra of lactate ( 1-20). Hetherington et al. ( 6 ) have edited lactate from skeletal muscle and von Kienlin et al. ( 9 ) have edited lactate by saturating the fat spectrum in the lactate methyl group region and used polarization transfer from methine to methyl protons of lactate. In this communication we present a pulse se- quence based on longitudinal spin-order which edits small concentrations of lactate from fat, with full lactate signal return, and gives very good water suppression.

METHOD

The pulse sequence is shown in Fig. 1. The last 7r pulse and associated gradients (shown in dotted lines) are used to refocus magnetic field inhomogeneities. Although we used semiselective ( 1-3-3-1) binomial pulses (22) here the sequence works equally well with other semiselective pulses or with frequency-selective pulses.

Lactate is an A 3 X spin system and its CH3 protons resonate around 1.3 ppm. The (CH,), fragment of lipids which give dominant resonance around 1.4 ppm can be treated as being of the form of an A2B2C2 spin system. The dynamics of the A3X spin system under the influence of this pulse sequence can be easily followed using product operator formalism ( 17, 18, 23). But for lipid ( CH2), all the pulses act as nonselective pulses and it is rather difficult to use the product operator formalism for the analysis of an A2 B2C2 spin system (including strong coupling effects). We, therefore, have evaluated the evolution of the density matrix numerically for analyzing the influence of this sequence on these spin systems.

471 0740-3 19419 1 $3.00 Copyright 0 1991 by Academic Press, Inc. All rights of reproduction in any form reserved.

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FIG. 1. All the 7r/2 pulses are binomial ( 1-3-3-1) pulses and the T pulses are hard pulses. The excitation regions of the binomial pulses are set at 1.3 ppm and the nonexcitation regions are set between 4 and 5 ppm. The gradient (0.6 G/cm) pulse length is 10 ms and Tdelay is about 100 ms. Sequence repetition time is 3 s and 1 / 4 J duration is 34 ms.

For the pulse sequence shown in Fig. 1 if the first and third pulse phases differ by +90°, the density matrix at the end of the first three pulses reveals the following: For lactate the nonzero matrix elements correspond to longitudinal spin order and since the longitudinal spin order is insensitive to magnetic field gradients, a gradient pulse following the third pulse has no effect. The fourth pulse converts the longitudinal spin order into antiphase magnetization which evolves into observable magnetization fol- lowing a I / 2 J period. Since all the binomial pulses are selectively exciting only the

TABLE 1

Phase Cycle Scheme

X X Y Y X

X X Y -Y Y X X -Y -Y -X

X X -Y Y -Y X Y Y -Y X

X Y Y Y Y X Y -Y Y -X

X Y -Y -Y -Y X -X Y Y X

X -X Y -Y Y X -X -Y -Y -X

X -X -Y Y -Y X -Y Y -Y X

X -Y Y Y Y X -Y -Y Y -X

X -Y -Y -Y -Y

Note. x, y , -x, and -y correspond to O", 90", 180", and 270" phases in that order. Receiver phase is constant through- out.

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CH3 resonance of lactate, the observed signal corresponds to the CH3 component of lactate.

In the case of an A2 B2C2 spin system immediately following the third pulse the nonzero matrix elements correspond to zero-quantum coherence (ZQC ), single- quantum coherence ( SQC ), and higher order multiple-quantum coherences (MQCs) . Although the SQCs and MQCs are dephased by the static field gradient pulse, the ZQCs which evolve with chemical-shift differences are not affected by the gradient pulse. ZQCs can be suppressed by adding spectra with different ZQC evolution times ( 2 1 ) . However, experimentally we found that a single experiment with a constant ZQC evolution time of the order of 100 ms yields good suppression of fat. This is due to the fact that in addition to the T2 losses of ZQCs, in lipid (CH2)n fragment there is a distribution of chemical shifts and the overall ZQC evolution is effectively averaged to zero similar to the suppression achieved in the case of a single chemical-shift difference by adding spectra with different ZQC evolution times.

Although the above sequence edits the spectrum in a single scan the suppression of unwanted signals can be improved by phase cycling to compensate for pulse imper- fections. On the basis of theoretical analysis we have designed a 64-step phase cycle which suppresses the residual fat and water signals. A part of the phase cycle is given in Table 1 and the other phase combinations can be generated from this by changing the phases of the first and third pulses in steps of 90" in such a way that the difference between them is always +90". It should be mentioned here that the phase cycle is not the major means of fat suppression.

I I I I I 4 41 3 29 2 16 I 04 -0 09

PPM

FIG. 2. ( a ) Single (1-9-3- i ) pulse spectrum of 100 m M lithium lactate (50 ml). (b) Lactate-edited spectrum of lactate solution in (a ) without the ?r pulse and gradients in dotted lines. (c) Lactate-edited spectrum of sample in ( a ) with complete sequence.

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RESULTS AND DISCUSSION

All the experiments on phantoms have been performed on a homebuilt 1.8-T whole- body NMR imaging spectrometer. We used a 10-cm-diameter cosine coil ( 2 4 ) for editing lactate from oil. The hard a pulse length was 0.320 ms and the 90" 1-3-3-1 pulse length was set according to the hard a / 2 pulse width. The overall length of a 1-3-3-1 sequence was about 5.5 ms. In Fig. 2a a single 1-3-3-1 pulse spectrum of 100 m M (50 ml) lithium lactate is given. Figure 2c shows a lactate-edited spectrum of the above sample. For comparison in Fig. 2b we show a lactate-edited spectrum of the sample in Fig. 2a obtained with our sequence but without the 7r pulse and the gradients in dotted lines. From Figs. 2a and 2b, it is evident that there is about 10 times signal loss due to the field inhomogeneities. However, if the magnetic field is shimmed so that the lactate doublet is clearly resolved one can use the sequence without the a pulse and the gradients in dotted lines but the spectra must be added and subtracted on alternate scans. In Fig. 3a a single-scan spectrum of a phantom made of 10 ml of lactate (20 mM) and 2 ml of olive oil obtained with a 1-3-3-1 sequence is presented. Figure 3b shows the lactate-edited spectrum of the above phan- tom. In Fig. 4a a single-scan spectrum of an excised heart of a pig obtained with a I- 5-3-1 sequence is given and Fig. 4b shows lactate-edited spectrum of the heart. Spectra on excised heart are obtained with a 4.5-cm-diameter cosine coil on a 2.7-T NMR spectrometer dedicated to animal studies. In this case the hard a / 2 pulse width was 0.150 ms and overall length of the 1-3-3-i sequence was about 3.75 ms. In lactate

Y ' I I I I

4 2 0 0 PPM B 6 ppm 4.41 3.29 2.16 1.04 -0.09

FIG. 3. ( a ) Single ( 1-3-3-7) pulse spectrum of a phantom consisting of 10 ml lithium lactate (20 ml)

FIG. 4. (a) Single ( 1-3-3- 1) spectrum of an excised heart of a baby pig. (b ) Lactate-edited spectrum of and 2 ml olive oil. ( b ) Lactate-edited spectrum of the phantom in ( a ) with complete pulse sequence.

the heart.

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editing short T2 of fat also contributes to fat suppression. By comparing a single-pulse spectrum of a lactate and oil phantom with a spectrum obtained with a spin-echo sequence having the same length as the editing sequence in Fig. 1 we have estimated the fat suppression due to only T2 losses to be approximately a factor of 15.

In vivo application of any metabolite editing method is plagued by potential problems such as magnetic and rf field inhomogeneities and subject motion. The other factors which might affect an editing pulse sequence are the short relaxation times of metab- olites of interest. The present sequence works well in the presence of magnetic field inhomogeneities and this is evident from Fig. 4 where the linewidth at half-height was about 0.4 ppm. With the phase cycle the sequence has a flip angle dependence of sin "0 on the lactate doublet signal whereas the spin-echo-based editing sequences have a flip angle dependence of sin%. Similar to the spin-echo-based editing sequences (6, 7) in this sequence the edited signal depends on its transverse relaxation time T2 during two 1 / 2 J evolution periods. Literature studies of lactate editing (see Refs. (6- 9)) from heart, brain, and skeletal muscle indicate that lactate T2 in these tissues is expected to be greater than or equal to 1 / J period. In this sequence the edited signal has some T I losses during the 1 00-ms delay. However, the disadvantage of this sequence is that the very phase cycles which compensate for pulse imperfections make the editing susceptible to subject motion.

We have presented a simple pulse sequence for editing in vivo lactate spectra. It gives both good spectral editing and solvent suppression and the performance of the sequence has been demonstrated by editing small concentrations of lactate from oil. Since this sequence edits lactate with maximum signal return, it facilitates the in vivo detection of lactate. Localized lactate editing can be achieved by adding a localization sequence such as Hadamard spectroscopic imaging ( 2 5 ) or ISIS (26) preceding the editing sequence.

REFERENCES

1. D. L. ROTHMAN, F. ARIAS-MENDOZA, G. 1. SHULMAN, AND R. G. SHULMAN, J. Mugn. Reson. 60,

2. D. L. ROTHMAN, K. L. BEHAR, H. P. HETHERINGTON, AND R. G. SHULMAN, Proc. Natl. Acad. Sci.

3. T. JUE, F. ARIAS-MENDOZA, N. C. GONNELLA, G. I . SHULMAN, AND R. G. SHULMAN, Proc. Natl.

4. C. C. HANSTOCK, M. R. BENDALL, H. P. HETHERINGTON, D. P. BOISVERT, AND P. S. ALLEN, J. Magn.

5. H. P. HETHERINGTON, M. J. AVISON, AND R. G. SHULMAN, Proc. Natl. Acad. Sci. USA 82, 31 15

6. H. P. HETHERINGTON, J. R. HAMM, J. W. PAN, D. L. ROTHMAN, AND R. G. SHULMAN, J. Mugn.

7. S. R. WILLIAMS, D. G. GADIAN, AND E. PROCTOR, J. Mugn. Reson. 66, 562 ( 1986). 8. c. J. HARDY AND c. L. DUMOULIN, Mugn. Reson. Med. 5, 58 ( 1987). 9. M. VON KIENLIN, J. P. ALBKAND, B. AUTHIER, P. BLONDET, S. LOLITO, AND M. DECORPS, J. Magn.

430 (1984).

USA 81,6330 (1984).

Acad. Sci. USA 82, 5246 ( 1985).

Reson. 71, 349 (1987).

(1985).

Reson. 82,86 (1989).

Reson. 75,371 (1987). 10. C. H. SOTAK AND D. M. FREEMAN, J. Mugn. Reson. 77, 382 (1988). 11. C. L. DUMOULIN AND D. VATIS, Mugn. Reson. Med. 3,282 (1986). 12. W. NOSEL, L. A. TRIMBLE, J. F. SHEN, AND P. S. ALLEN, Magn. Reson. Med. 11, 389 (1989). 13. C. H. SOTAK, Mugn. Reson. Med. 7,364 ( 1988).

482 COMMUNICATIONS

14. D. M. DODDRELL, 1. M. BRERETON, L. N. MOXON, AND G. J. GALLOWAY, Mugn. Reson. Med. 9,132

15. C. H. SOTAK, D. M. FREEMAN, AND R. E. HURD, J. Mugn. Reson. 78, 355 (1988). 16. L. A. TRIMBLE, J. F. SHEN, A. H. WILMAN, AND P. S. ALLEN, J. Mugn. Reson. 86, 191 ( 1990). 17. 1. M. BRERETON, G. J. GALLOWAY, S. T. ROSE, AND D. M. DODDRELL, J. Mugn. Reson. 83, 190

18. G. C. M C ~ N N O N AND P. BOESIGER, Mugn. Reson. Med. 8, 355 (1988). 19. A. KNUTTEL AND R. KIMMICH, J. Mugn. Reson. 86,253 ( 1990). 20. D. STRYJEWSKI, H. OSCHKINAT, AND D. LEIBFRITZ, Mogn. Reson. Med. 13, 158 ( 1990). 21. R. R. ERNST, G. BODENHAUSEN, AND A. WOKAUN, “Principles of Nuclear Magnetic Resonance in

22. D. L. TURNER, J. Mugn. Reson. 54, 146 (1983). 23. 0. W. SORENSEN, G. W. EICH, M. H. LEVITT, G. BODENHAUSEN, AND R. R. ERNST. Prog. NMR

24. L. BOLINGER, M. G. PRAMMER, AND J. S. LEIGH, J. Mugn. Reson. 81, 162 (1988). 25. L. BOLINGER AND J. S. LEIGH, J. Mugn. Reson. 80, 162 ( 1988). 26. R. J. ORDIDGE, A. CONNELLY, AND J. A. B. LOHMAN, J. Mugn. Reson. 66,283 ( 1986).

(1989).

(1989).

One and Two Dimensions,” Clarendon Press, Oxford, 1987.

Spectrosc. 16, 163 (1983).