Fast Acquisition Scheme for Achieving High-ResolutionMRS With J-Scaling Under Inhomogeneous Fields

Xi Chen,1 Meijin Lin,1 Zhong Chen,1* and Jianhui Zhong2*

Intermolecular multiple-quantum coherences (iMQCs) can re-focus the phase dispersion caused by magnetic field inhomo-geneities while preserving the chemical shift, so they have beenapplied to achieve high-resolution MR spectroscopy free of linebroadening caused by susceptibility gradients. However, previ-ous iMQC high-resolution methods all require two-dimensionalspectra sampling of the full range of chemical shifts of soluteevolutions in both F1 and F2 dimensions, resulting in a pro-longed scanning time for data acquisition. In this work, sparsesampling in the t1 dimension and subsequent fold-over correc-tion are used to speed up the intermolecular zero-quantumcoherence spectroscopy by up to 50 times on high-field MRsystems. Furthermore, three types of spectra with homo-de-coupling, original J-coupling constants, and doubled J-cou-pling constants respectively are obtained with manipulation ofthe t1 period. The water suppression is also improved by thecombined use of intermolecular double-quantum filter and ex-citation sculpting. The feasibilities of this group of new se-quences are demonstrated by experiments using an agar gelphantom with an air bubble, in vitro pig brain tissues and anintact postmortem mudskipper. Magn Reson Med 61:775–784,2009. © 2009 Wiley-Liss, Inc.

Key words: high-resolution MRS; intermolecular zero-quantumcoherences; fast acquisition; J-scaling; inhomogeneous broad-ening; water suppression

In vivo magnetic resonance spectroscopy (MRS) offers anoninvasive method to obtain molecular information inbiological tissues. However, broadened spectral lines dueto magnetic field inhomogeneities induced by variations inmagnetic susceptibilities among various tissues and cellu-lar structures can degrade substantially the quality of pro-ton MRS (1). It has been shown that the field gradientscaused by magnetic susceptibility differences at air-tissueboundaries can hardly be homogenized by high-ordershims available on current MR systems (2). Therefore,several effective shimming strategies such as dynamic

shim updating (3) and passive shimming (4,5) were devel-oped. On the other hand, several high-resolution se-quences have been proposed to mitigate the effects ofinhomogeneities in proton MRS (6,7). The signal based onintermolecular multiple-quantum coherences (iMQCs) (8)comes from distant dipolar coupling between spins. It hasbeen shown to be capable of improving spectral resolutionwhen magnetic susceptibilities vary over distances muchlarger than the correlation distance between coupled spins(6). The in vivo intermolecular zero-quantum coherence(iZQC) MRS has been performed on biological samples (9)and rodents (9–11), and the feasibility of this techniquehas been studied numerically (12) and reviewed (13). It isnoted that in all previous iMQC high-resolution studies,the two-dimensional (2D) spectral sampling of the fullrange of chemical shifts (CSs) of solute evolutions in bothF1 and F2 dimensions is required (6,9–11,14–17), so theacquisition time is much longer than its 1D counterpart.Long experimental time degrades the resolutions of NMRspectra when there are unstable magnetic fields or motionsof living objects. A novel fast approach was proposed byWarren and co-workers to speed up 2D acquisition by four-to eightfold (18). It was designed to acquire several t1

points for every single excitation. To achieve this purpose,in a single scan several low flip-angle pulses were used asthe mixing pulse, each transferring only a small fraction ofthe iMQC signal into detectable magnetization. The mag-netization is thus partitioned in small amounts that areeach characterized by a different t1 evolution time. Intensewater signal is also a great obstacle in MRS. Recently, wedesigned an iDQF-HOMOGENIZED (abbreviated as iDH1)sequence including a module of intermolecular double-quantum filter (iDQF) to obtain high-resolution NMR ininhomogeneous fields with good solvent suppression (17).However, the inhomogeneity-independent spectral linesof iDH1 are achieved in the F1 dimension, so a prolongedscanning time is required to obtain sufficient spectral res-olution.

In this work, a group of high-resolution pulse sequences,dubbed iDQF-HOMOGENIZED II (abbreviated as iDH2), isproposed. In iDH2, a substantial reduction in the F1 spec-tral width enables a fast acquisition with an accelerationfactor of several tens. Consequently, more phase cyclingsteps can be used so that two solvent suppression mod-ules, iDQF (17) and excitation sculpting (19), can be jointlyused to improve water suppression efficiency. Further-more, high-resolution spectra with varying degree of J-scaling visualization can be obtained by means of manip-ulation of the t1 period. In addition to the spectrum withthe original coupling constant (dubbed iDH2-1J), the spec-

1Department of Physics and Magnetic Resonance Research Center, StateKey Laboratory of Physical Chemistry of Solid Surface, Xiamen University,Xiamen, China.2Departments of Radiology and Biomedical Engineering, University of Roch-ester, Rochester, New York.Grant sponsor: NNSF of China; Grant number: 20573084; Grant number:10774125; Grant sponsor: Key Project of Health and Science and Technologyof Xiamen; Grant number: 3502Z20051027; Grant sponsor: National Institutesof Health; Grant number: NS41048.*Correspondence to: Zhong Chen, Department of Physics, Xiamen University,Xiamen, 361005, China. E-mail: chenz@xmu.edu.cn or Jianhui Zhong, Uni-versity of Rochester, Rochester, New York, USA. E-mail: jianhui.zhong@rochester.eduReceived 2 April 2008; revised 13 October 2008; accepted 23 October 2008.DOI 10.1002/mrm.21894Published online 2 February 2009 in Wiley InterScience (www.interscience.wiley.com).

775© 2009 Wiley-Liss, Inc.

Magnetic Resonance in Medicine 61:775–784 (2009)FULL PAPERS

trum with a J-scaling factor of 2 (dubbed iDH2-2J) is de-signed to better resolve line splittings in weakly coupledspins. On the other hand, because broad lines and over-lapped adjacent multiplets can cause poor signal separa-tion in proton MRS (20), a high-resolution homonucleardecoupled iDH2 version (dubbed iDH2-0J) is also pro-posed to improve signal separation for peak assignmentand quantification.

THEORY

The iDH2 sequences shown in Figure 1 can be best under-stood with the raising and lowing operator formalism:

IzSzO¡

�

2Ix,

�

2Sx

14I�S�O¡

�Sx 14I�S�O¡

�

2Ix

14IzS�O¡

�Ix,�Sx,DISIzSzt2 14S�, [1]

where I spin (corresponding to the solvent) and S spins(corresponding to solutes) are spin-1/2 systems. Signalsfrom higher than two-spin order terms are very weak, sothese terms are ignored in Equation [1]. Theoretically,neither the longitudinal magnetization Iz nor two-spinterms of solvent such as I�I�, IzIz in the iZQC period(between the �/2 excitation and the first � pulse) can betransferred into �2 order by the solute-selective � pulse,so none of them will pass through the iDQF period, whichis between the (�)S and (�/2)I pulses. The excitationsculpting scheme (19) that is used as the water suppres-sion (WS) module right before acquisition (16) can furtherimprove solvent suppression efficiency.

Removing the CS evolutions of solutes in the t1 periodcan reduce greatly the spectral width in the F1 dimensionand acquisition time. In the iDH2-2J sequence (Fig. 1a), theevolution period includes two t1/2 intervals before andafter the second radio frequency (RF) pulse, respectively.Assuming that �m is the frequency offset of spin m (m � I,S) in the rotating frame in the absence of magnetic fieldinhomogeneity, and �� is the deviation of the angularfrequency caused by magnetic field inhomogeneity, thefrequency offset of spin m is then:

�m � �m � ��, �m � I, S. [2]

For a weakly J-coupled AX system for solute (with acoupling constant J), the iDH2-2J sequence leads to thesolvent–solute peaks at

��1,�2 � ��l � �S � �J/2 � ��l � �S � �J/2,�S � �J�

� ��l � �J,�S � �J � ��l � �� � �J,�S � �� � �J,

[3]

where the CS evolutions of solutes no longer exist in the t1

period. To obtain a 1D high-resolution accumulated pro-jection, a shearing process (21) is performed along the F2

dimension by � � �arctan(��/��) � ��/4 (clockwise isdefined as positive for the shearing operation). This oper-ation transforms the matrix (�1, �2) to:

��1,� 2 � ��1,�2 � �1 � ��I � �� � �J,�S � �l � 2�J.

[4]

Equation [4] shows a twofold J scaling-up in the F2

dimension.To obtain spectra with the original J-coupling constants,

the evolution period is placed as one interval t1 betweenthe first and the second RF pulses in the iDH-1J sequence(Fig. 1b). Equation [1] suggests the solvent–solute peaksappear in this case at:

��1,�2 � ��l � �S � �J,�S � �J � ��I � �S � �J,�S

� �� � �J, [5]

where both CSs of solutes �S and scalar-coupling multipletsplittings align along the same direction with a slope �1 inthe 2D frequency domain. Therefore, scalar-coupling con-stants retain the same scaling factor as the CS differencesafter any kind of full-spectrum shearing process, and un-altered J-coupling constants can be obtained. In Equation[5], the �1 domain contains CSs of solutes. It seems impos-sible to reduce the sampling spectral width in the F1 axisto cut down acquisition time because the Nyquist theoremwould be violated. However, because all peaks present ina band near anti-diagonal (see Eq. [5]), a fast acquisitioncan be achieved without information loss by reducing thesampling rate in the t1 dimension with a subsequent fold-over correction (FOC) (21,22), as detailed below.

Folding in the �1 domain of a spectrum occurs if thefrequency offset �1 exceeds the Nyquist frequency �1

N.Folding leads �1 in Equation [5] to an apparent frequency:

�1F � ��1 � �1

Nmod�2�1N� � �1

N, [6]

where mod is the modulo operator for finding the remain-der of division. If the spectral width in �1 is sufficient tocover the width of the frequency band, FOC can untanglethe folded spectrum by shearing the experimental matrix��1

F,�2 into ��1FOC,�2:

�1FOC � ��1

F � �2 � �1Nmod�2�1

N� � �1N. [7]

FIG. 1. a–c: The schematics of the iDH2 pulse sequences: (a)iDH2-2J, (b) iDH2-1J, and (c) iDH2-0J.

776 Chen et al.

The resulted spectrum is equivalent to the fully sampledspectrum after a shearing by ��/4 along the F1 dimension:

��1FOC,�2 � ��1 � �2,�2 � ��I � ��,�S � �� � �J.

[8]

As a result, the anti-diagonal band is tilted to appearhorizontal along the F2 axis, but the line broadening ap-pears parallel to the diagonal. Another shearing along F2

by ��/4 is then performed to obtain a high-resolutionprojection:

��1FOC,� 2 � ��1

FOC,�2 � �1FOC � ��I � ��,�S � �I � �J.

[9]

In the iDH2-0J sequence (Fig. 1c), the constant-timescheme (23) is used with a hard � pulse placed within theiZQC evolution period, which is now made up of an in-terval �� t1/2 (� � t1max/2, where � is a constant) before this� pulse and an interval t1/2 after this � pulse. Therefore,the solvent–solute peaks are located at:

��1,�2 � � � � �l � �S � �J/2 � ��l � �S � �J/2,�S

� �J] � ��l � �S,�S � �J � ��l � �S,�S � �� � �J,

[10]

which is similar to the iDH2-1J (Eq. [5]) except for thehomonuclear decoupling in the indirect dimension.Therefore, the same procedures as iDH2-1J, that is, under-sampling, FOC and further shearing, are applied to theiDH2-0J spectrum:

��1FOC,� 2 � ��1 � �2,�2 � �1

FOC � ��1 � �2, � �1

� ��I � �� � �J,�S � �l. [11]

Hence, the high-resolution spectrum with no J-splittingis obtained from the projection onto the F2 axis.

The iDH2-2J data is rather different to the 1J and 0J datain the time domain: the echo center of every t1 incrementin the iDH2-2J data aligns at diagonal direction, whilethose of the iDH2-1J and 0J align vertically. This is thereason that iDH2-2J does not need FOC. However, more t2

points have to be acquired with the iDH2-2J due to thetime shift of the echo centers, which only brings morenoises without additional information. Therefore, aniDH2-2J sequence with a t1 acquisition delay can be used toshorten the t2 acquisition time and improve the SNR. Thesolvent–solute peaks of the delayed acquired iDH2-2J arelocated at:

��1,�2 � ��l � �S � �J/2 � ��l � �S � �J/2 � �S

� �J,�S � �J� � ��l � �S � 2�J,�S � �J � ��l � �S

� 2�J,�S � �� � �J, [12]

which is similar to the iDH2-1J and 0J (Eq. [5] and [10])except for the 2 times J-scaling in the indirect dimension.Hence, the same procedures as iDH2-1J, that is, undersam-

pling and FOC along F1 are applied to the iDH2-2J spec-trum with delayed acquisition:

��1FOC,�2 � ��1 � �2,�2 � ��l � �J,�S � �J � ��l � ��

� �J,�S � �� � �J, [13]

which is exactly the same as Equation [3]. Then furthershearing along F2 (Eq. [4]) can be applied. The delayed-acquiring version of the iDH2-2J sequence is used in thefollowing experiments and discussions instead of the orig-inal one.

METHODS

All experiments were performed on an 11.74 T VarianNMR System with 54 mm narrow bore, using a 5-mmindirect detection probe at 298 K. An agar gel phantomcontaining 14 mM creatine (Cr) and 4.5 mM lactate (Lac)was used to demonstrate implementation details of theiDH2 sequences. The Z1 shimming coil was deliberatelydetuned to produce a linewidth of 50 Hz to simulate thestatic field inhomogeneity. The excitation sculptingscheme (19) of two frequency selective refocusing pulseswas used as the WS module. The W5 composite pulse (24)was used for the water-exclusive � pulse right before theiDQF period and the two water-exclusive � pulses in theWS module. The parameters (strength � duration) of co-herence selective gradients (CSGs) were G � 0.07 T/m �1.2 ms, G � 0.16 T/m � 1.2 ms, G1 � 0.14 T/m � 1.0 ms,and G2 � 0.24 T/m � 1.0 ms, respectively. An 8-step phasecycling was applied for the iDH2 sequences: � � (x, y, �x,�y, x, y, �x, �y), � � (x, x, x, x, �x, �x, �x, �x) andreceiver � (x, �x, x, �x, x, �x, x, �x), where � is the phaseof the water-exclusive � pulses in the WS module. Therepetition time (TR) and the echo time (TE) were TR/TE �2000/100 ms. This TE value was determined by a series ofvarying TE measurements to seek the maximum signalintensity for the iMQCs. 500 � 25 points were acquiredwith spectral widths of 5000 Hz � 120 Hz in F2 � F1

dimensions in circa 16 min. For the iDH2-1J spectrumwith full-sampling in the indirect dimension, 500 � 1000points were acquired with spectral widths of 5000 Hz �5000 Hz in F2 � F1 dimensions in 158 min. The number ofaverages was 4 and the phase cycling used the first 4 stepsof the above 8-step one. For the signal to noise ratio (SNR)evaluation, the signal intensity was calculated using theintegral value instead of the peak height for a better com-parison of spectra with different linewidths. The noise wasestimated by the root-mean-square noise level between9 ppm and 7 ppm, where no signals were expected. The Cr(3.0 ppm) resonance was used for the SNR and the line-width measurements. The sensitivity, which is defined asSNR/�acquisition time (25), was calculated to estimate thetime efficiency of SNR. For the localized spectral studies,localization was only applied along the z direction for thesample inside an NMR tube of 5 mm outer diameter (o.d.)and 4.24 mm inner diameter (i.d.). Double spin echoesusing sinc-shape selective RF pulses were used as thespatial localization module. It was applied right before theWS module as proposed in Reference (10). The length ofthe sinc pulses was 2 ms. Localization lengths of 12, 6 and

A Fast Method for High-Resolution MRS With J-Scaling 777

3 mm were applied in the z direction, resulting in voxelvolumes of 169, 85, and 42 �L, respectively. For the iDH2spectra, the 500 � 21 points were acquired with spectralwidths of 5000 Hz � 120 Hz in 24 min. The TR/TE were2000/100 ms and the average number was 32. The conven-tional MRS was also performed using a nonselective spinecho sequence followed by the localization and the WSmodules. For the spectra of conventional SQCs, the TR/TEwere 2000/100 ms and the average number was 16. 500points were acquired with the spectral width of 5000 Hz in38 s.

To simulate susceptibility gradients present in tissues ofliving organisms, a 5 mm NMR tube filled with agar gelcontaining an air bubble was used as a phantom. 12 mM Crand 4.5 mM Lac were dissolved in the phantom. For thespin echo images, TR/TE � 2000/20 ms, and for the gra-dient echo images, TR/TE � 200/6 ms were used. Forspectral studies of the bubble phantom, localization wasonly applied along the z direction using double sinc-shape� pulses. Two localization lengths of 10 and 5 mm wereapplied in the z direction, resulting in voxel volumes of141 and 71 �L, respectively. The FASTMAP method (26)was used to adjust the magnetic field homogeneity over thevoxels. For the iDH2 spectra of 10-mm localization length,400�19 points were acquired with spectral widths of5000 Hz � 120 Hz in circa 23 min. The TR/TE were2000/100 ms, and the average number was 32. For theiDH2 spectra of 5-mm localization length, 100 � 17 pointswere acquired with spectral widths of 5000 Hz � 200 Hz incirca 200 min. The TR/TE were 4000/100 ms, and theaverage number was 160. For the conventional localizedspectra of two different voxel sizes, the TR/TE were 2000/100 ms and the average number was 16. Four hundredpoints were acquired with the spectral width of 5000 Hz in72 s. Conventional localized spectra with TE � 20 ms werealso acquired for comparison.

Nonlocalized measurements were also carried out on asample of intact pig brain tissues fitted in a 5-mm NMRtube. 256 � 12 points were acquired with spectral widthsof 5000 Hz � 100 Hz in F2 � F1 dimensions in 7 min. TheTR/TE were 4000/100 ms and the average number was 8.The short T2 of tissue water at 11.74 T (27) greatly in-creases the water–metabolite iMQC transverse relaxationand thus causes extra homogeneous broadenings. There-fore, an exponential line narrowing function of 6 Hz wasapplied in the indirect dimension for the iDH2-2J and -1Jspectra to compensate for the effect. A conventional water-presaturated J-resolved spectroscopy was performed with512 � 40 matrix and spectral widths of 5000 Hz � 50 Hz.The repetition time was TR � 4 s and the number ofaverages was 4. A conventional 1D water-presaturatedspin echo spectrum with the same TE as iDH2 was alsoobtained. For comparison, a conventional 1D water-pre-saturation experiment of the extract from the same brainwas performed as well. The repetition time was TR � 4 sand the number of averages was 128 for these two 1Dexperiments.

To further test the feasibilities of the proposed se-quences, postmortem studies were also performed on anintact mudskipper (Periophthalmus modestus). The sam-ple was purchased from a local retailer and was killedright before being fitted into the NMR tube. The spin echo

images of sagittal and axial planes were acquired withTR/TE � 2000/20 ms. For the spectral studies, three-di-mension localization was applied using double spin ech-oes (two sinc-shape � pulses) for each dimension. Thevoxel was 3 � 3 � 10 mm3. For the iDH2 spectra, 400 � 13points were acquired with spectral widths of 5000 Hz �120 Hz in circa 31 min. The TR/TE were 2000/100 ms andthe average number was 64. An exponential line narrow-ing function of 6 Hz was applied in the indirect dimensionfor the iDH2-2J and -1J spectra to compensate for thetransverse relaxation of the tissue water. A point-resolvedspectroscopy (PRESS) spectrum of the same voxel was alsoacquired with the same TE. The variable power and opti-mized relaxation delays (VAPOR) module was used forwater suppression. The TR/TE were 2000/100 ms and theaverage number was 64. Eight hundred points were ac-quired with the spectral width of 5000 Hz in 132 s. Forcomparison, a high-resolution magic angle spinning (HR-MAS) experiment was also performed on a piece of muscletissue of the same mudskipper sample. The sample wasspun at 2 kHz about the magic angle in a Varian Nano-NMR probe. The NOESYPRESAT pulse sequence (28) us-ing the CPMG module in the spin-echo period was used toobtain the spectrum. The TR/TE were 5000/100 ms and theaverage number was 128. 1500 points were acquired withthe spectral width of 5000 Hz in 12 min.

RESULTS AND DISCUSSION

The major steps in postprocessing of the iDH2 spectra aredemonstrated in Figure 2 using data from the agar phan-tom containing 14 mM Cr and 4.5 mM Lac. The four rowsare spectra of iDH2-1J with full-sampling in indirect di-mension, iDH2-1J, iDH2-2J, and iDH2-0J, respectively. Thefirst column is the acquired spectra before any shearingprocess; the second column is the spectra after the shear-ing (with FOC for iDH2 spectra) along F1; the third columnis the spectra after the shearing along F2. The counter-diagonal of the fully sampled spectrum (Fig. 2a) before anyshearing process was plotted with the dashed line. In thesparsely sampled spectra of the same sequence (Fig. 2d),the counter-diagonals were broken and the resonances onthem were subject to fold-over. After the F1 shearing withFOC, it can be seen that the broken counter-diagonals werere-joined, resulting in a same spectrum (Fig. 2e) with thedesired region of the fully sampled spectrum (Fig. 2b). TheFOC processes for the spectra of 2J and 0J (Row 3 and 4)were the same. The doublet of Lac is expanded in the inset,with apparent J constants marked in two dimensions todemonstrate the J-scaling process, which has been ana-lyzed in the Theory section. The accumulated projection isalso presented for each spectrum (the projections of fullysampled spectra were calculated for the marked regionwith a 120 Hz F1 frequency width, which is the same as theF1 spectral width of the undersampled spectra). It shouldbe noted that the shearing process applied along the F1

dimension, which transforms the data matrix from ��1F,�2

to ��1FOC,�2, only affects the projection onto the F1 axis,

but leaves the F2 projection unchanged. As a result, theprojection spectra of Column 1 and 2 in a same row areexactly the same. The sensitivity, which is defined as

778 Chen et al.

SNR/�acquisition time(25), is also presented for eachspectrum. It can be observed that: (1) the fully and sparselysampled data achieve the same sensitivity if they have thesame t1max; (2) there is no signal or sensitivity loss duringthe FOC and shearing process. The sensitivity of theiDH2-0J is lower than other two iDH2 sequences becausethe transverse relaxation during the constant-time iZQC

period, which should be longer than t1max/2, leads to signalloss (23). On the other hand, the linewidth of the homo-decoupled spectrum (iDH2-0J) is smaller than those of the2J and 1J spectra. Because the iZQCs are insensitive toinhomogeneity, the position of the � pulse in the constant-time iZQC period has minor influence on the signal inten-sity. Therefore, the signal intensity of the iDH2-0J is hardly

FIG. 2. The shearing process for the fully and sparsely sampled iDH2 spectra of the agar gel phantom containing 14 mM Cr and 4.5 mMLac. The four rows are spectra of iDH2-1J with full-sampling of indirect dimension, iDH2-1J, iDH2-2J, and iDH2-0J, respectively. Thepreshearing spectra, the spectra after the shearing (with FOC for sparsely sampled iDH2 spectra) along F1, and the spectra after theshearing along F2 are presented in the first, second and third columns, respectively. The doublet of Lac is expanded in the inset.

A Fast Method for High-Resolution MRS With J-Scaling 779

subject to attenuation as t1 increases. The residual line-width of the iDH2-0J is mainly caused by the short t1max

(200 ms), which is limited by the large water-metaboliteiZQC relaxation rate (17). Measurements with localizationon the same agar phantom were also performed to examthe voxel size dependence of the signal properties. Theone-dimensional localization was applied along the z di-rection, and the voxel positions were calculated and ac-quired relative to the Cr resonance at 3.0 ppm for thechemical shift displacement consideration (2). The resultsof the signal sensitivities and the linewidths were listed inTable 1. It can be seen that the sensitivities of both iZQCand conventional signals are linearly depend on the voxelsize and the iZQC signal is one order of magnitude weakerthan the conventional signal. The linewidth (Cr at3.0 ppm) of conventional SQCs is broadened as the voxelsize increases because the background gradients causinginhomogeneous broadenings are of the same direction aslocalization, while the linewidth of iZQCs is independentof the voxel size.

Localized studies were also performed on an agar gelphantom with an air bubble to simulate magnetic fielddistortions caused by susceptibility gradients. The imagesof conventional signals are presented in Figure 3. The spinecho images of three planes (the first row: Fig. 3a–c) showthe actual size and position of the air bubble. The suscep-tibility difference between air and tissue leads to pertur-bation of the magnetic field (1,2). Therefore, the gradientecho images (the second row: Fig. 3d–f) demonstrate theinfluence of field distortions caused by the air bubble.Spectral studies were performed with localization alongthe z direction of 5 mm and 10 mm, respectively. Thevoxels were marked on the images in Figure 3. Both con-ventional 1D spectra and three iDH2 spectra were ac-quired. Spectra acquired with localization of 10-mmlength are presented in Figure 4. The signal sensitivitiesand linewidths of conventional and iDH2-1J spectra fordifferent voxel sizes are presented in Table 2. In the spec-trum of conventional SQCs (Fig. 4d,e), the Cr (3.0 ppm) issubject to more severe inhomogeneous broadening thanother resonances because the voxel position were calcu-lated relative to Cr (3.0 ppm). The chemical shift displace-ment effect is rather severe at a magnetic field as high as11.74 T so that the Cr (4.0 ppm) and Lac (1.3 ppm) have20% and 38% displacement, respectively (2). As a result,the voxel for Cr (3.0 ppm) contains the full volume of fielddistortions while the signals of other resonances originate

from voxels dislocated by chemical shift, which includeonly part of the field-distorted volume. The iDH2 spectra(Fig. 4a–c), on the other hand, are narrowed in the line-width for all resonances to a certain degree despite thechemical shift displacement artifact. It should be notedthat differences in voxel positions for different resonancepeaks with respect to the air bubble still cause alteredsignal intensity ratio in the iDH2 spectra, which could leadto misinterpretations of absolute concentrations (2). Tech-niques such as frequency offset corrected inversion (FOCI)pulses (29) can be used to reduce chemical shift displace-ment. The residual water signals in the conventional MRS,which is mainly due to the high-order inhomogeneitiescaused by the air bubble, are better suppressed in the iDH2spectra. Both the signal sensitivities of SQCs and iZQCsdramatically decreases when the voxel size is reducedfrom 10 mm to 5 mm. This phenomenon is consistent withthe numerical studies presented in Reference (12). Thereason may be that, in the latter case, the field-distortedarea occupies most volume of the voxel. The iZQC spec-

FIG. 3. a–f: Images of the agar gel bubble phantom: (a–c) spin echoimages of three orthogonal planes, and (d–f) gradient echo imagesof three orthogonal planes. The 10- and 5-mm lengths of localiza-tion along the z direction are marked with blue and red lines,respectively.

Table 1Signal Sensitivity and the Peak Linewidth at Different Voxel Sizesfor the Agar Gel Phantom in a Magnetic Field with LinearInhomogeneity in the Z Direction*

Experiment Voxel (�L) Sensitivity FWHM (Hz)

iDH2 42 0.43 885 0.81 8

169 1.75 8SQC 42 3.16 10

85 6.48 19169 15.84 27

*The Sensitivity is defined as SNR/�acquisition time(25). For peaklinewidth, the FWHM of Cr (3.0 ppm) is measured.

780 Chen et al.

trum also achieves resolution improvement though it tooka rather long time (circa 200 min) to acquire because of thelow signal sensitivity.

The spectra of the brain tissue sample are presented inFigure 5. Multiplet patterns of metabolites such as those ofLac (1.31 ppm), alanine (Ala, 1.47 ppm), and �-aminobu-tyric acid (GABA, 2.28 ppm) can be resolved in theiDH2-2J spectrum (Fig. 5d). However, the residual line-width (�13 Hz, the singlet of N-acetyl aspartate [NAA]),mainly due to the short t1max (120 ms), hinders resolutionof weaker scalar couplings or more complicated multipletpatterns. Rather than to recognize unidentified compo-nents, the ultimate goal for most studies of proton MRS isto obtain absolute concentrations of the metabolites ob-served (30). Therefore, the homonuclear decoupled iDH2spectroscopy was designed to obtain collapsed resonancepeaks of coupled spins for higher peak intensities as wellas better signal separation, both of which benefit the quan-tification. Several resonances, such as those of glutamate(Glu, 2.34 ppm), asparate (Asp, 2.65 and 2.80 ppm), myo-inositol (m-Ins, 4.05 ppm) and Lac (4.09 ppm), which areconcealed in noise or overlapping in the iDH2-2J and -1J,can now be resolved in the iDH2-0J spectrum (Fig. 5f).Compared with in vivo homo-decoupling PRESS usingconventional spin echoes, such as J-resolved PRESS (J-PRESS) (31) and constant-time PRESS (CT-PRESS) (20),iDH2-0J can suppress the inhomogeneous broadening inits homo-decoupled projection, while the conventionalhomo-decoupled spectroscopy (Fig. 5c) cannot. The differ-ence of relative signal intensities between the iDH2 spectra

(Fig. 5d–f) and the spectrum of extract (Fig. 5a) is causedby T2 weighting and the J-modulation of multiplet inten-sities (30) during the echo time of iDH2. The LCmodelalgorithm (32), which is able to compensate the J-evolutioneffect, can be potentially implemented for the metabolitequantification in iMQC MRS.

The localized spectra and images from a postmortemmeasurement of an intact mudskipper are shown in Figure6. The sample was fitted in a 5-mm NMR tube. Figure 6a isthe sagittal and axial spin–echo images with TE � 20 ms.The MRS studies were performed with localization onthree dimensions. The voxel, which is marked on the

FIG. 5. a–f: Nonlocalized spectra of pig brain: (a) conventionalwater-presaturated spectrum of the brain extract, (b) 1D water-presaturated spin echo spectrum of intact tissue from the samebrain, (c) water-presaturated J-resolved spectrum and its homo-decoupled projection of the same sample as b, and (d–f) projectionspectra of the same sample as b using the iDH2-2J, -1J, and -0Jsequences, respectively.

FIG. 4. Spectra of the agar gel bubble phantom containing 12 mMCr and 4.5 mM Lac. The localization was applied on the z directionwith a length of 10 mm. a–e: The (a–c) projection spectra of theiDH2-1J, -2J, and -0J, respectively, with TE � 100 ms, (d,e) local-ized spectra of conventional signals with TE � 100 ms and 20 ms,respectively.

A Fast Method for High-Resolution MRS With J-Scaling 781

images, was positioned near the spinal cord with a size of3 � 3 � 10 mm3. The voxel was calculated and acquiredrelative to the Cr resonance at 3.0 ppm. Figure 6b–d areiDH2-1J, 2J, and 0J spectra, respectively. Figure 6e,f are thePRESS spectrum and the in vitro HR-MAS spectrum, re-spectively. The linewidth of Cr (3.0 ppm) was reducedfrom 46 Hz in the PRESS spectrum to 14 Hz in the iDH2-0Jspectrum, which was similar to the linewidth of the HR-MAS spectrum. It can be seen that the apparent J constantwas correctly rescaled on the doublet at 1.3 ppm in theiDH2-1J (Fig. 6b) and the iDH2-2J (Fig. 6c) spectra. On theother hand, higher peak intensities as well as better signalseparations for coupled spins were observed in theiDH2-0J spectrum (Fig. 6d). Resonances of alanine (Ala,1.47 ppm) and taurine (Tau, 3.25 ppm and 3.42 ppm),which can hardly be observed or separated in the iDH2-2Jand -1J, can be resolved in the iDH2-0J spectrum.

From Equations [4], [9], and [11], it can be seen that thefrequency of all resonances in the F1 dimension of the final

iDH2 spectra after all shearing processes depends on �I,�� and J. The natural linewidths of metabolites, which aredetermined by the transverse relaxation constants (27), arenegligible compared with ��. The indirect acquisition fre-quency of the iDH2 sequence group can be reduced to thevalue that is only slightly larger than the maximum F1

frequency to avoid fold-over in final spectra after all shear-ing processes. When the transmitter frequency is set to �I,the range of the F1 frequency is determined by �� iniDH2-1J (Eq. [9]), while it is determined by �� and J iniDH2-2J (Eq. [4]) and iDH-0J (Eq. [11]). Because both thechemical shift frequency difference and the inhomoge-neous broadening increase linearly as B0, the unit “ppm” isused for both the spectral width and the inhomogeneouslinewidth. The F2 spectral width for metabolites is as-sumed to be 10 ppm and the inhomogeneous broadeninglinewidth is 0.1 ppm. The assumed inhomogeneous line-width is thus 50 Hz in a MR system with a high static fieldof 11.7 Tesla (T). Because the linewidth is much largerthan J coupling constants (33) of most metabolites, therange of the F1 resonance frequency of processed iDH2 ismainly determined by the inhomogeneous broadeninglinewidth. If the reduced linewidth of iDH2 group is set to0.2 ppm, which is two times the inhomogeneous line-width, an acceleration factor of 50 can be obtained for dataacquisition. On the other hand, in a clinical 3T MR scan-ner for example, the inhomogeneous broadening reducesto circa 13 Hz so that other factors that influence the F1

frequency range should also be taken into account. In theiDH2-1J spectrum, there is no J splitting in the F1 dimen-sion, so the reduced spectral linewidth can still be set to0.2 ppm and the accelerating factor remains 50 times.However, the J splitting remains in the F1 dimension of theiDH2-2J and -0J to achieve J variations. Because some Jcoupling constants are now comparable to the inhomoge-neous linewidth (circa 13 Hz), the reduced F1 samplingspectral width with 2J and 0J sequences should be ex-tended to a larger value than in 1J to avoid fold-over in thefinal spectra. As a result, the achievable accelerating factorwill reduce in these cases. In examples presented here, thefast acquisition scheme reduced the spectral width in theindirect dimension from 5000 Hz to approximately120 Hz, thus a 40-fold scanning time reduction wasachieved, theoretically. With a 2-step phase cycling, theoriginal iZQC spectroscopy with a full F1 spectral widthtakes more than 1 h if the same t1max and repetition delayare applied, while a single iDH2 experiment takes onlycirca 2 min. The scanning time of the iDH2 can be furthershortened by using the multi-acquisition scheme proposedby the Warren group (18), but the intrinsic low SNR of

FIG. 6. a–e: Post-mortem studies of an intact mudskipper: (a) sag-ittal and axial spin-echo images; (b–d) localized spectra of theiDH2-1J, -2J, and -0J, respectively; and (e) PRESS spectrum. Thevoxel (3 � 3 � 10 mm3) was marked on the images. f: HR-MASspectrum of muscle tissues of the same mudskipper sample.

Table 2Signal Sensitivity and the Peak Linewidth with Regard to the Voxel Size of the Agar Gel Bubble Phantom

ExperimentVoxel(�L)

Sensitivity

FWHM (Hz)

Cr (3.0 ppm)0% displacement

Cr (3.9 ppm)20% displacement

Lac (1.3 ppm)38% displacement

iDH2 71 0.05 20 19 17141 0.50 14 11 17

SQC 71 0.73 95 N/Aa 89141 8.25 82 51 26

aThe resonance was covered by the strong residual water signal so it could not be observed.

782 Chen et al.

iMQCs may restrict further reduction in acquisition time.On the other hand, if more averages are taken to enhanceSNR, more steps of phase cycling can be used to optimizewater suppression in the iDH2. A suppression efficiency ofmore than 104-fold was achieved in inhomogeneous fieldsin this study.

A weakly coupled system is assumed in the previousdiscussion. In strongly coupled systems, on the otherhand, the � pulse would lead to coherence transfer be-tween various transitions. These “cross peaks” will causeadditional resonances, the so-called “strongly coupled ar-tifacts”, in projections of t1-manipulated or sheared exper-iments, such as J-spectroscopy, sequences using CTscheme (34), or IDEAL (17). The same problem occurs iniDH2-2J and iDH2-0J and it makes the spectral interpreta-tion open to ambiguity. Keeler’s group has proposed sev-eral methods for suppressing these unwanted signals (34).These methods may help to improve the iDH2 sequencesfor its applications in strongly coupled spin systems.

In the spin-echo period of the iMQC sequence, it takessome time for the distant dipole-dipole interaction to con-vert the anti-phase single-quantum terms into observablein-phase terms. This so-called “demagnetizing time” isapproximately 100 ms at 11.7T so that a TE as long as it hasto be used to maximize the iMQC signal intensity. Theinherent low intensity of iMQC signal and the signal losscaused by the water-metabolite iMQC transverse relax-ation in the t1 period is likely to lead to insufficient SNR ofsome resonances for spectral fitting; and the metabolitetransverse relaxation during the long TE also causes T2

weighting of the signal intensities, which may increasequantification errors if the prior knowledge of T2 is notreliable. On the other hand, extra-short echo time (� 2 ms)proton MRS using the regular SQC signal can obtain ex-cellent resolution and sensitivity (32,35), so that concen-tration measurement of metabolites, which are previouslydifficult to quantify in 1H spectra, can be achieved (32).Therefore, due to its disadvantage compared with conven-tional MRS, iMQC cannot replace other routine sequencesfor in vivo proton spectroscopy. Its advantage of beinginsensitive to field inhomogeneity, however, makes theiMQC method a complementary technique to regular SQCMRS for obtaining high-resolution single-voxel protonspectrum in the presence of strong field distortions, espe-cially for high-field in vivo animal studies.

Compared with intramolecular zero-quantum coher-ences (ZQCs), the in vivo intermolecular MQC signalsoriginate from distant dipolar couplings between waterand metabolite spins, instead of scalar couplings in a J-coupling network. In addition to the homogeneous broad-ening caused by short T2 of tissue water, there is alsoresidual inhomogeneous broadening due to the field inho-mogeneity hardly refocused within the correlation dis-tance. The residual linewidth of our experiments(�0.02 ppm) is approximately twice that of intramolecularZQCs (�0.01 ppm) (7) under the same magnetic fieldstrength (11.74T). On the other hand, intermolecularMQCs are not restricted by the scalar coupling; the dipolarfield cast by water affects scalar coupled and noncoupledspins equally. The relative frequencies of metabolites iniMQC MRS are the same as conventional signals, which

makes an easier spectral interpretation than intramolecu-lar MQCs.

In conclusion, a new group of pulse sequences (iDH2) isproposed to achieve fast acquisition of high-resolutionJ-scaled NMR spectra with effective solvent suppression. Itcan be potentially applied to high-field in vivo MRS ofhighly inhomogeneous regions, that is, where susceptibil-ity boundaries between air cavities and tissues generatesignificant field inhomogeneity effects (1).

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