Sources of variability in the response of coupled spins to the PRESS sequence and their potential impact on metabolite quantification

Spatial localization in nuclear magnetic resonance spectroscopy

The ability to select a discrete region within the body for signal acquisition is a fundamental requirement of in vivo NMR spectroscopy. Ideally, it should be possible to tailor the selected volume to coincide exactly with the lesion or tissue of interest, without loss of signal from within this volume or contamination with extraneous signals. Many techniques have been developed over the past 25 years employing a combination of RF coil properties, static magnetic field gradients and pulse sequence design in an attempt to meet these goals. This review presents a comprehensive survey of these techniques, their various advantages and disadvantages, and implications for clinical applications. Particular emphasis is placed on the reliability of the techniques in terms of signal loss, contamination and the effect of nuclear relaxation and J-coupling. The survey includes techniques based on RF coil and pulse design alone, those using static magnetic field gradients, and magnetic resonance spectroscopic imaging. Although there is an emphasis on techniques currently in widespread use (PRESS, STEAM, ISIS and MRSI), the review also includes earlier techniques, in order to provide historical context, and techniques that are promising for future use in clinical and biomedical applications.

Optimized detection of lactate at high fields using inner volume saturation

In localized proton MR spectroscopy ((1)H-MRS) in vivo, the detection of lactate (Lac) is affected by modulation of its resonances due to homonuclear scalar couplings (J). A simple and convenient way to distinguish Lac from lipids is to set the TE to 1/J so that the Lac signal is inverted while other resonances (such as lipid) remain in-phase. However, at high field strengths, such as 3 Tesla or above, the modulation of the Lac signal is complicated by chemical shift effects that cause modulation patterns to vary within different subregions of the localized volume. Under some conditions the Lac signal may even disappear completely. In this note we introduce the concept of inner volume saturation (IVS), which makes use of high bandwidth spatial pulses to remove the signal corresponding to the regions of the localized volume that contribute unwanted modulation patterns. The method is described theoretically and demonstrated experimentally at 3 Tesla in a phantom and a patient with acute stroke. The phantom measurements indicate that virtually 100% of the Lac signal can be recovered using this method. The method should be feasible at magnetic fields above 3 Tesla, and may also be applied to other coupled spin systems in which modulation effects are important.

Echo-time independent signal modulations using PRESS sequences: a new approach to spectral editing of strongly coupled AB spin systems

In clinical MR spectroscopy, double spin-echo point resolved spectroscopy (PRESS) sequences are routinely used for volume selection. For strongly coupled AB spin systems under PRESS excitation, the dependence of the signal on the echo time TE has been thoroughly investigated, whereas less attention has been paid to the signal modulation which occurs at constant TE with varying interpulse delays. A substantial TE-independent J modulation is here predicted from analytical solutions of the Liouville equation and density matrix simulations, and verified with experiments on citrate at 1.5 and 3T. It is also shown that this modulation effect could be exploited for editing of strongly coupled AB resonances or for removal of singlets in spectra-by means of difference spectroscopy-just using a standard PRESS sequence. The applicability in vivo of this new spectral editing approach is also demonstrated, with selective detection of citrate resonances in the human prostate. This novel approach has the advantages of being simple, and directly applicable on standard clinical MR scanners, provided that the exact behavior of the resonance is known.

Optimizing PRESS localized citrate detection at 3 Tesla

Analytical methods are used to characterize the response of the strongly coupled two-spin system of citrate to point-resolved spectroscopy (PRESS)-based sequences at 3 T. The signal output is analyzed line by line, as well as in the Cartesian product operator basis. Patterns with a periodicity of 80.9 ms are identified. Furthermore, it is shown that at TE = n . 80.9 ms (n in {0,1,2,...}), the spin evolution can be described without direct reference to strong coupling terms. The theoretical results are found to be in good agreement with in vivo experiments. These results can be used to design protocols for prostate MRS and MRSI at 3 T, and give guidelines for optimizing spin-echo-based acquisition schemes for detecting two-spin systems at arbitrary field strengths.

Spin-echo MRS in humans at high field: LASER localisation using FOCI pulses

Significant improvements in spin-echo MRS are possible when voxel localisation is performed using high bandwidth frequency offset corrected inversion (FOCI) pulses as opposed to more conventional lower bandwidth pulses. The reduced chemical shift displacement errors result in a spectrum that more accurately reflects the actual metabolite distribution within any region of interest that is selected graphically on a series of scout images, and can lead to improved metabolite detection in the case of homonuclear J-coupled spins. At 4.7T, FOCI pulses with a 20 kHz bandwidth result in extremely sharp and uniform selection profiles, and negligible contamination from outside of the voxel of interest, for all signals in the 1H spectral range that is normally studied. A 'FOCI' adiabatic half-passage is observed to provide good excitation over the 1H spectral range. Single shot performance with echo-time (TE)48 ms is reported using a four-port drive birdcage head coil. GAMMA simulations show that, for many detectable metabolites at 4.7 T, LASER localisation using FOCI pulses with TE=48 ms results in 1H anti-phase spectral components that are the same order as would be obtained from a symmetric PRESS sequence with TE=32 ms. Timing schemes are proposed to enable good measurement of lactate with very little signal loss arising from chemical shift displacement errors at TE=144 and 288 ms.

Numerical simulation of PRESS localized MR spectroscopy

Numerical simulations of NMR spectra can provide a rapid and convenient method for optimizing acquisition sequence parameters and generating prior spectral information required for parametric spectral analysis. For spatially resolved spectroscopy, spatially dependent variables affect the resultant spectral amplitudes and phases, which must therefore be taken into account in any spectral simulation model. In this study, methods for numerical simulation of spectra obtained using the PRESS localization pulse sequence are examined. A comparison is made between three different simulation models that include different levels of detail regarding the spatial distributions of the excitation functions, and spin evolution during application of the pulses. These methods were evaluated for measurement of spectra from J-coupled spin systems that are of interest for in vivo proton spectroscopy and results compared with experimental data. It is demonstrated that for optimized refocusing pulses it is sufficient to account for chemical shift effects only, although there is some advantage to implementing a more general numerical simulation approach that includes information on RF pulse excitation profiles, which provides sufficient accuracy while maintaining moderate computational requirements and flexibility to handle different spin systems.

Variability of metabolite yield using STEAM or PRESS sequences in vivo at 3.0 T, illustrated with myo-inositol

Using as an example the myo-inositol (mI) band at approximately 3.6 ppm in the proton spectrum from brain, an evaluation is presented that highlights the difficulties of quantifying metabolites with strongly coupled spins with either STEAM or PRESS and demonstrates some advantages of prospective sequence analysis when measuring their concentrations. The analysis emphasizes the variation in coupled-spin signal yield and lineshape, compared with that of uncoupled singlets such as N-acetylaspartate, a variation that differs from one metabolite spin system to another. This difference in variation between a target metabolite (e.g., mI) and its contaminating background metabolites (e.g., glutamate and taurine, etc.) is shown to provide in certain circumstances a substantial reduction in background contamination (both metabolite and macromolecule) while maintaining sufficient signal-to-noise ratio for precise quantification. For example, sequence times are demonstrated, both for STEAM and for PRESS, that, relative to the short echo-time sequences typical in the literature, enhance the signal to metabolite background of the 3.6-ppm band of mI by factors of 1.7 and 1.3, respectively, essentially eliminate the macromolecular baseline, and yet in vivo retain an S/N approximately 10 in both cases.

Effect of strong homonuclear proton coupling on localized13C detection using PRESS

The effect of strong homonuclear proton coupling on (13)C incorporation measurements by either indirect or direct means was investigated (and illustrated with glutamate) both numerically and experimentally at 3.0 T. In particular, two sequences were considered, each using a proton PRESS sequence for localization. The indirect (13)C detection method incorporated the POCE (proton observe carbon edited) technique onto PRESS, and for direct (13)C detection a DEPT (distortionless enhancement by polarization transfer) sequence was appended to the PRESS localization. Both analysis and experiment demonstrate that when strong homonuclear coupling of protons is additional to heteronuclear coupling with (13)C spins, the (13)C measures derived from either the indirect PRESS-POCE sequence or the direct-but-enhanced PRESS-DEPT sequence are significantly modified. Specifically, the MR lineshapes of both (13)C-bonded and nonbonded protons are changed during (13)C incorporation, giving rise, for example, to a potential cross-contamination of < or =30% between glutamate (13)C(3) and (13)C(4) measures from the PRESS-POCE indirect method. During direct-but-enhanced detection, the DEPT enhancement is reduced for glutamate (13)C(2), (13)C(3), and (13)C(4) but not equally, and the reduction is further exacerbated by proton PRESS localization, which gives rise to enhancements that are strong functions of PRESS TE(1) and TE(2).

Strategy for the spectral filtering of myo-inositol and other strongly coupled spins

A multiple quantum filter strategy is presented for spectrally discriminating metabolites with strongly coupled spins from those whose spins are either uncoupled or weakly coupled. The strategy also includes a means for selectively suppressing the background multiplets of metabolites that also have strongly coupled spins. As a demonstration of its efficacy at 3.0 T, the strategy is shown to enhance by a factor of approximately 5 the signal-to-background ratio of the myo-inositol band at 3.6 ppm relative to that in response to a PRESS sequence with the same sequence timings. This is done by eliminating the uncoupled resonance of glycine and the weakly coupled multiplets of glutamate and glutamine, and by selectively suppressing the strongly coupled taurine multiplet 3-fold. The macromolecular background was effectively removed through its transverse decay over 105 ms. The associated cost of gaining the signal to background enhancement is a drop in signal yield by a factor of 0.75 relative to PRESS at the same timings. The myo-inositol signal to noise ratio was nevertheless maintained by the filter at approximately 12.

In vivo 1H NMR spectroscopy of individual human brain metabolites at moderate field strengths

This article reviews spectral editing techniques for in vivo 1H NMR spectroscopy of human brain tissue at moderate field strengths of 1.5-3 Tesla. Various aspects of 1H NMR spectroscopy are discussed with regard to in vivo applications. The parameter set [delta, J, n] (delta being the relative chemical shift, J the scalar coupling constant and n the number of coupled spins) is used to characterize the spin systems under investigation and to classify the editing techniques that are used in in vivo 1H NMR spectroscopy.

Changes in the proton T2 relaxation times of cerebral water and metabolites during forebrain ischemia in rat at 9.4 T

Proton T(2) relaxation times of cerebral water and metabolites were measured before, during, and after transient forebrain ischemia in rat at 9.4 T using localized proton magnetic resonance spectroscopy ((1)H-MRS) with Hahn echoes formed at different echo times (TEs). It was found that the T(2) values of water and N-acetyl aspartate (NAA) methyl, but not total creatine (tCr) methyl, decrease significantly (approximately 10%) during ischemia, and this T(2) reduction is reversed by reperfusion. The T(2) reduction observed for NAA was most likely caused by the extravascular component of the blood oxygenation level-dependent (BOLD) effect induced by a drastically increased deoxyhemoglobin content during ischemia. The absence of T(2) changes for tCr can probably be explained by the fact that the BOLD-related T(2) decrease was counterbalanced by the conversion of phosphocreatine (PCr) to creatine (Cr), which has a longer T(2) than PCr, during ischemia. The changes in T(2) should be taken into account for the quantification of metabolite concentrations during ischemia.

1H MRS of a boron neutron capture therapy 10B-carrier, L-p-boronophenylalanine-fructose complex, BPA-F: phantom studies at 1.5 and 3.0 T

The quantification of a BNCT 10B-carrier, L-p-boronophenylalanine-fructose complex (BPA-F), was evaluated using 1H magnetic resonance spectroscopy (1H MRS) with phantoms at 1.5 and 3.0 T. For proper quantification, relaxation times T1 and T2 are needed. While T1 is relatively easy to determine, the determination of T2 of a coupled spin system of aromatic protons of BPA is not straightforward with standard MRS sequences. In addition, an uncoupled concentration reference for aromatic protons of BPA must be used with caution. In order to determine T2, the response of an aromatic proton spin system to the MRS sequence PRESS with various echo times was calculated and the product of the response curve with exponential decay was fitted to the measured intensities. Furthermore, the response curve can be used to correct the intensities, when an uncoupled resonance is used as a concentration reference. BPA was quantified using both phantom replacement and internal water referencing methods with accuracies of +/- 5% and +/- 15%. Our phantom results suggest that in vivo studies on BPA concentration determination will be feasible.

GABA X2 multiplet measured pre- and post-administration of vigabatrin in human brain

This work demonstrates, in solution and in human brain at 3 tesla, that the X(2)-multiplet of the A(2)M(2)X(2) proton spin system of GABA at 2.315 ppm can be readily resolved from that of the overlapping background, particularly the glutamate multiplet, i.e., the PQ multiplet of the glutamate AMNPQ spin system. Prior to experiment, the values of the stimulated echo acquisition mode (STEAM) sequence parameters TE and TM that maximized the GABA-X(2) discrimination from its background (i.e., 168 ms and 28 ms, respectively) were determined numerically. The determination was made by calculating the spectral response of all contributing metabolites to the STEAM sequence throughout TE/TM space. A baseline GABA concentration (mean +/- standard deviation (SD) of the mean) of 0.78 +/- 0.04 mM was estimated from spectra acquired from a 3 x 3 x 3 cm(3) volume in the parieto-occipital cortex of eight normal control subjects. Five of the eight control subjects were also studied 24 hr post-administration of a single dose of 50 mg.kg(-1) vigabatrin. Four of the five showed increases in GABA in the range of 15-120% of their baseline level.

Proton magnetic resonance spectroscopy in the brain: report of AAPM MR Task Group #9

AAPM Magnetic Resonance Task Group #9 on proton magnetic resonance spectroscopy (MRS) in the brain was formed to provide a reference document for acquiring and processing proton (1H) MRS acquired from brain tissue. MRS is becoming a common adjunct to magnetic resonance imaging (MRI), especially for the differential diagnosis of tumors in the brain. Even though MR imaging is an offshoot of MR spectroscopy, clinical medical physicists familiar with MRI may not be familiar with many of the common practical issues regarding MRS. Numerous research laboratories perform in vivo MRS on other magnetic nuclei, such as 31P, 13C, and 19F. However, most commercial MR scanners are generally only capable of spectroscopy using the signals from protons. Therefore this paper is of limited scope, giving an overview of technical issues that are important to clinical proton MRS, discussing some common clinical MRS problems, and suggesting how they might be resolved. Some fundamental issues covered in this paper are common to many forms of magnetic resonance spectroscopy and are written as an introduction for the reader to these methods. These topics include shimming, eddy currents, spatial localization, solvent saturation, and post-processing methods. The document also provides an extensive review of the literature to guide the practicing medical physicist to resources that may be useful for dealing with issues not covered in the current article.

Quantitative evaluation of the lactate signal loss and its spatial dependence in press localized (1)H NMR spectroscopy

Localized (1)H NMR spectroscopy using the 90 degrees -t(1)-180 degrees -t(1)+t(2)-180 degrees -t(2)-Acq. PRESS sequence can lead to a signal loss for the lactate doublet compared with signals from uncoupled nuclei which is dependent on the choice of t(1) and t(2). The most striking signal loss of up to 78% of the total signal occurs with the symmetrical PRESS sequence (t(1)=t(2)) at an echo time of 2/J (approximately 290 ms). Calculations have shown that this signal loss is related to the pulse angle distributions produced by the two refocusing pulses which leads to the creation of single quantum polarization transfer (PT) as well as to not directly observable states (NDOS) of the lactate AX(3) spin system: zero- and multiple-quantum coherences, and longitudinal spin orders. In addition, the chemical shift dependent voxel displacement (VOD) leads to further signal loss. By calculating the density operator for various of the echo times TE=n/J, n=1, 2, 3,..., we calculated quantitatively the contributions of these effects to the signal loss as well as their spatial distribution. A maximum signal loss of 75% can be expected from theory for the symmetrical PRESS sequence and TE=2/J for Hamming filtered sinc pulses, whereby 47% are due to the creation of NDOS and up to 28% arise from PT. Taking also the VOD effect into account (2 mT/m slice selection gradients, 20-mm slices) leads to 54% signal loss from NDOS and up to 24% from PT, leading to a maximum signal loss of 78%. Using RE-BURP pulses with their more rectangular pulse angle distributions reduces the maximum signal loss to 44%. Experiments at 1.5 T using a lactate solution demonstrated a maximum lactate signal loss for sinc pulses of 82% (52% NDOS, 30% PT) at TE=290 ms using the symmetrical PRESS sequence. The great signal loss and its spatial distribution is of importance for investigations using a symmetrical PRESS sequence at TE=2/J.

MR spectroscopy quantitation: a review of time-domain methods

In this article an overview of time-domain quantitation methods is given. Advantages of processing the data in the measurement domain are discussed. The basic underlying principles of the methods are outlined and from them the situations under which these algorithms perform well are derived. Also an overview of methods to preprocess the data is given. In that respect, signal-to-noise and/or resolution enhancement, the removal of unwanted components and corrections for model imperfections are discussed.

Response of metabolites with coupled spins to the STEAM sequence

This article demonstrates that a numerical solution of the full quantum mechanical equations for all metabolites with coupled spins is an efficient and accurate means, first, of predicting the optimum STEAM sequence design for quantifying any target metabolite in brain, and, second, for providing the basis lineshapes and yields of these metabolites to facilitate their accurate quantification. Using as illustrations the weakly coupled AX3 system of lactate, the ABX aspartyl group of N-acetylaspartate, which has only two strongly coupled spins, and the much larger strongly coupled AMNPQ glutamyl group of glutamate, the numerical solutions for the response to STEAM highlight the principal source of response variability, namely, the evolution of and transfer between zero quantum terms during the mixing time, TM. These highlights include the rapid oscillations of zero quantum terms due to the chemical shift difference of the coupled spins, the proliferation of oscillating zero order terms due to strong coupling, and the serendipitous smoothing of the response as the number of strongly coupled spins increases. The numerical solutions also demonstrate that the design of the selective 90 degrees pulses is a far less critical factor in determining the response than was the case for the selective 180 degrees pulses of the PRESS sequence (Thompson and Allen, Magn Reson Med 1999;41:1162-1169). The veracity of the method is demonstrated both in phantom solutions and in the parietal lobe of a normal human brain.

Quantitative analysis of short echo time (1)H-MRSI of cerebral gray and white matter

Quantitative analysis of (1)H-magnetic resonance spectroscopic imaging (MRSI) data was developed using the user-independent spectral analysis routine LCModel. Tissue segmentation was performed using statistical parametric mapping software (SPM 96), and the results were used to correct for cerebrospinal fluid contamination. A correction was developed for the imperfections in the spectroscopic excitation profile in order to improve the uniformity of metabolite images. After validation in phantoms, these techniques were applied to study differences in metabolite concentrations between gray and white matter in normal volunteers (n = 13). A positive correlation was found between concentration and gray matter content for most metabolites studied. The estimated ratios of metabolite concentration in gray vs. white matter were: N-acetyl aspartate + N-acetyl aspartyl glutamate (NAc) = 1.16+/- 0.11; creatine = 1.7+/-0.3; glutamate + glutamine = 2.4+/-0.5; myo-inositol = 1.6+/-0.3; choline = 0.9+/-0.2. The ratio of NAc/Cr was negatively correlated with gray matter content: gray/white = 0.69 +/-0.08. These methods will be useful in the evaluation of metabolite concentrations in MRSI voxels with mixed tissue composition in patient groups.

Off-resonance effects of the radiofrequency pulses used in spectral editing with double-quantum coherence transfer

Spectral editing using gradient selected double-quantum (DQ) coherence transfer is often used for the selective observation of metabolites in vivo. In attempting to optimize the detection sensitivity of a conventional DQ spectral editing sequence, the effects of using radiofrequency (RF) pulses that are not at the resonance frequency of the observed peaks were investigated both theoretically and experimentally. The results show that spectral editing using pulses at the frequency of the observed resonance does not necessarily give the optimal detection sensitivity. At 7 T, the detection sensitivity of lactate observed using a DQ editing method can be increased by up to 30% by setting the RF pulses off resonance at the proper frequency. The results also suggest that slice selective RF pulses used in DQ spectral editing combined with PRESS localization may have slice profiles different from those when the same pulses are used for standard PRESS spatial localization.

Residual dipolar coupling of the Cr/PCr methyl resonance in resting human medial gastrocnemius muscle

This paper presents a detailed analysis of the TE dependence of the creatine methyl proton signal at 3.02 ppm, resulting from a symmetric PRESS sequence applied to the resting human gastrocnemius muscle. The analysis shows that a two-component decay of the central peak of the dipolar-coupled-methyl triplet should be interpreted as the superposition of a rapid ( approximately 34 msec) dipolar dephasing and a less rapid ( approximately 162 msec) transverse relaxation. These data do not support a two-pool hypothesis for TE dependence of this signal. Magn Reson Med 42:421-424, 1999.