Computer simulation of MRS localization techniques: an analysis of ISIS

In vivo absolute quantification of hepatic γ-ATP concentration in mice using 31 P MRS at 11.7 T

Measurement of ATP concentrations and synthesis in humans indicated abnormal hepatic energy metabolism in obesity, non-alcoholic fatty liver disease (NAFLD) and Type 2 diabetes. Further mechanistic studies on energy metabolism require the detailed phenotyping of specific mouse models. Thus, this study aimed to establish and evaluate a robust and fast single voxel ³¹ P MRS method to quantify hepatic γ-ATP concentrations at 11.7 T in three mouse models with different insulin sensitivities and liver fat contents (72-week-old C57BL/6 control mice, 72-week-old insulin resistant sterol regulatory-element binding protein-1c overexpressing (SREBP-1c⁺ ) mice and 10-12-week-old prediabetic non-obese diabetic (NOD) mice). Absolute quantification was performed by employing an external reference and a matching replacement ATP phantom with 3D image selected in vivo spectroscopy ³¹ P MRS. This single voxel ³¹ P MRS method non-invasively quantified hepatic γ-ATP within 17 min and the repeatability tests provided a coefficient of variation of 7.8 ± 1.1%. The mean hepatic γ-ATP concentrations were markedly lower in SREBP-1c⁺ mice (1.14 ± 0.10 mM) than in C57BL/6 mice (2.15 ± 0.13 mM; p < 0.0002) and NOD mice (1.78 ± 0.13 mM; p < 0.006, one-way ANOVA test). In conclusion, this method allows us to rapidly and precisely measure hepatic γ-ATP concentrations, and thereby to non-invasively detect abnormal hepatic energy metabolism in mice with different degrees of insulin resistance and NAFLD. Thus, this ³¹ P MRS will also be useful for future mechanistic as well as therapeutic translational studies in other murine models.

Comparison of four 31P single-voxel MRS sequences in the human brain at 9.4 T

PURPOSE

In this study, different single-voxel localization sequences were implemented and systematically compared for the first time for phosphorous MRS (³¹ P-MRS) in the human brain at 9.4 T.

METHODS

Two multishot sequences, image-selected in vivo spectroscopy (ISIS) and a conventional slice-selective excitation combined with localization by adiabatic selective refocusing (semiLASER) variant of the spin-echo full intensity-acquired localized spectroscopy (SPECIAL-semiLASER), and two single-shot sequences, semiLASER and stimulated echo acquisition mode (STEAM), were implemented and optimized for ³¹ P-MRS in the human brain at 9.4 T. Pulses and coil setup were optimized, localization accuracy was tested in phantom experiments, and absolute SNR of the sequences was compared in vivo. The SNR per unit time (SNR/t) was derived and compared for all four sequences and verified experimentally for ISIS in two different voxel sizes (3 × 3 × 3 cm³ , 5 × 5 × 5 cm³ , 10-minute measurement time). Metabolite signals obtained with ISIS were quantified. The possible spectral quality in vivo acquired in clinically feasible time (3:30 minutes, 3 × 3 × 3 cm³ ) was explored for two different coil setups.

RESULTS

All evaluated sequences performed with good localization accuracy in phantom experiments and provided well-resolved spectra in vivo. However, ISIS has the lowest chemical shift displacement error, the best localization accuracy, the highest SNR/t for most metabolites, provides metabolite concentrations comparable to literature values, and is the only one of the sequences that allows for the detection of the whole ³¹ P spectrum, including β-adenosine triphosphate, with the used setup. The SNR/t of STEAM is comparable to the SNR/t of ISIS. The semiLASER and SPECIAL-semiLASER sequences provide good results for metabolites with long T₂ .

CONCLUSION

At 9.4 T, high-quality single-voxel localized ³¹ P-MRS can be performed in the human brain with different localization methods, each with inherent characteristics suitable for different research issues.

GABA editing with macromolecule suppression using an improved MEGA-SPECIAL sequence

PURPOSE

The most common γ-aminobutyric-acid (GABA) editing approach, MEGA-PRESS, uses J-editing to measure GABA distinct from larger overlapping metabolites, but suffers contamination from coedited macromolecules (MMs) comprising 40 to 60% of the observed signal. MEGA-SPECIAL is an alternative method with better MM suppression, but is not widely used primarily because of its relatively poor spatial localization. Our goal was to develop an improved MM-suppressed GABA editing sequence at 3 Tesla.

METHODS

We modified a single-voxel MEGA-SPECIAL sequence with an oscillating readout gradient for improved spatial localization, and used very selective 30-ms editing pulses for improved suppression of coedited MMs.

RESULTS

Simulation and in vivo experiments confirmed excellent MM suppression, insensitive to the range of B₀ frequency drifts typically encountered in vivo. Both intersubject and intrasubject studies showed that MMs, when suppressed by the improved MEGA-SPECIAL method, contributed approximately 40% to the corresponding MEGA-PRESS measurements. From the intersubject study, the coefficient of variation for GABA+/Cre (MEGA-PRESS) was 11.2% versus 7% for GABA/Cre (improved MEGA-SPECIAL), demonstrating significantly reduced variance (P = 0.005), likely coming from coedited MMs.

CONCLUSIONS

This improved MEGA-SPECIAL sequence provides unbiased GABA measurements with reduced variance as compared with conventional MEGA-PRESS. This approach is also relatively insensitive to the range of B₀ drifts typically observed in in vivo human studies. Magn Reson Med 79:41-47, 2018. © 2017 International Society for Magnetic Resonance in Medicine.

In vivo mouse myocardial (31)P MRS using three-dimensional image-selected in vivo spectroscopy (3D ISIS): technical considerations and biochemical validations

(31)P MRS provides a unique non-invasive window into myocardial energy homeostasis. Mouse models of cardiac disease are widely used in preclinical studies, but the application of (31)P MRS in the in vivo mouse heart has been limited. The small-sized, fast-beating mouse heart imposes challenges regarding localized signal acquisition devoid of contamination with signal originating from surrounding tissues. Here, we report the implementation and validation of three-dimensional image-selected in vivo spectroscopy (3D ISIS) for localized (31)P MRS of the in vivo mouse heart at 9.4 T. Cardiac (31)P MR spectra were acquired in vivo in healthy mice (n = 9) and in transverse aortic constricted (TAC) mice (n = 8) using respiratory-gated, cardiac-triggered 3D ISIS. Localization and potential signal contamination were assessed with (31)P MRS experiments in the anterior myocardial wall, liver, skeletal muscle and blood. For healthy hearts, results were validated against ex vivo biochemical assays. Effects of isoflurane anesthesia were assessed by measuring in vivo hemodynamics and blood gases. The myocardial energy status, assessed via the phosphocreatine (PCr) to adenosine 5'-triphosphate (ATP) ratio, was approximately 25% lower in TAC mice compared with controls (0.76 ± 0.13 versus 1.00 ± 0.15; P < 0.01). Localization with one-dimensional (1D) ISIS resulted in two-fold higher PCr/ATP ratios than measured with 3D ISIS, because of the high PCr levels of chest skeletal muscle that contaminate the 1D ISIS measurements. Ex vivo determinations of the myocardial PCr/ATP ratio (0.94 ± 0.24; n = 8) confirmed the in vivo observations in control mice. Heart rate (497 ± 76 beats/min), mean arterial pressure (90 ± 3.3 mmHg) and blood oxygen saturation (96.2 ± 0.6%) during the experimental conditions of in vivo (31)P MRS were within the normal physiological range. Our results show that respiratory-gated, cardiac-triggered 3D ISIS allows for non-invasive assessments of in vivo mouse myocardial energy homeostasis with (31)P MRS under physiological conditions.

Small animal cardiovascular MR imaging and spectroscopy

The use of MR imaging and spectroscopy for studying cardiovascular disease processes in small animals has increased tremendously over the past decade. This is the result of the remarkable advances in MR technologies and the increased availability of genetically modified mice. MR techniques provide a window on the entire timeline of cardiovascular disease development, ranging from subtle early changes in myocardial metabolism that often mark disease onset to severe myocardial dysfunction associated with end-stage heart failure. MR imaging and spectroscopy techniques play an important role in basic cardiovascular research and in cardiovascular disease diagnosis and therapy follow-up. This is due to the broad range of functional, structural and metabolic parameters that can be quantified by MR under in vivo conditions non-invasively. This review describes the spectrum of MR techniques that are employed in small animal cardiovascular disease research and how the technological challenges resulting from the small dimensions of heart and blood vessels as well as high heart and respiratory rates, particularly in mice, are tackled.

J-refocused 1H PRESS DEPT for localized 13C MR spectroscopy

Proton point-resolved spectroscopy (PRESS) localization has been combined with distortionless enhanced polarization transfer (DEPT) in multinuclear MRS to overcome the signal contamination problem in image-selected in vivo spectroscopy (ISIS)-combined DEPT, especially for lipid detection. However, homonuclear proton scalar couplings reduce the DEPT enhancement by modifying the spin coherence distribution under J modulation during proton PRESS localization. Herein, a J-refocused proton PRESS-localized DEPT sequence is presented to obtain simultaneously enhanced and localized signals from a large number of metabolites by in vivo (13) C MRS. The suppression of J modulation during PRESS and the substantial recovery of signal enhancement by J-refocused PRESS-localized DEPT were demonstrated theoretically by product operator formalism, numerically by the spin density matrix simulations for different scalar coupling conditions, and experimentally with a glutamate phantom at various TEs, as well as a colza oil phantom. The application of the sequence for localized detection of saturated and unsaturated fatty acids in the calf bone marrow and skeletal muscle of healthy subjects yielded high signal enhancements simultaneously obtained for all components.

In vivo free induction decay based 3D multivoxel longitudinal hadamard spectroscopic imaging in the human brain at 3 T

We propose and demonstrate a full 3D longitudinal Hadamard spectroscopic imaging scheme for obtaining chemical shift maps, using adiabatic inversion pulses to encode the spins' positions. The approach offers several advantages over conventional Fourier-based encoding methods, including a localized point spread function; no aliasing, allowing for volumes of interest smaller than the object being imaged; an option for acquiring noncontiguous voxels; and inherent outer volume rejection. The latter allows for doing away with conventional outer volume suppression schemes, such as point resolved spectroscopy (PRESS) and stimulated echo acquisition mode (STEAM), and acquiring non-spin-echo spectra with short acquisition delay times, limited only by the excitation pulse's duration. This, in turn, minimizes T2 decay, maximizes the signal-to-noise ratio, and reduces J-coupling induced signal decay. Results are presented for both a phantom and an in vivo healthy volunteer at 3 T.

Fully adiabatic 31P 2D-CSI with reduced chemical shift displacement error at 7 T--GOIA-1D-ISIS/2D-CSI

A fully adiabatic phosphorus (31P) two-dimensional (2D) chemical shift spectroscopic imaging sequence with reduced chemical shift displacement error for 7 T, based on 1D-image-selected in vivo spectroscopy, combined with 2D-chemical shift spectroscopic imaging selection, was developed. Slice-selective excitation was achieved by a spatially selective broadband GOIA-W(16,4) inversion pulse with an interleaved subtraction scheme before nonselective adiabatic excitation, and followed by 2D phase encoding. The use of GOIA-W(16,4) pulses (bandwidth 4.3-21.6 kHz for 10-50 mm slices) reduced the chemical shift displacement error in the slice direction ∼1.5-7.7 fold, compared to conventional 2D-chemical shift spectroscopic imaging with Sinc3 selective pulses (2.8 kHz). This reduction was experimentally demonstrated with measurements of an MR spectroscopy localization phantom and with experimental evaluation of pulse profiles. In vivo experiments in clinically acceptable measurement times were demonstrated in the calf muscle (nominal voxel volume, 5.65 ml in 6 min 53 s), brain (10 ml, 6 min 32 s), and liver (8.33 ml, 8 min 14 s) of healthy volunteers at 7 T. High reproducibility was found in the calf muscle at 7 T. In combination with adiabatic excitation, this sequence is insensitive to the B1 inhomogeneities associated with surface coils. This sequence, which is termed GOIA-1D-ISIS/2D-CSI (goISICS), has the potential to be applied in both clinical research and in the clinical routine.

In vivo 31P spectroscopy by fully adiabatic extended image selected in vivo spectroscopy: a comparison between 3 T and 7 T

An improved image selected in vivo spectroscopy (ISIS) sequence for localized (31)P magnetic resonance spectroscopy at 7 T was developed. To reduce errors in localization accuracy, adiabatic excitation, gradient offset independent adiabatic inversion pulses, and a special extended ISIS ordering scheme were used. The localization accuracy of extended ISIS was investigated in phantoms. The possible spectral quality and reproducibility in vivo was explored in a volunteer (brain, muscle, and liver). A comparison between 3 T and 7 T was performed in five volunteers. Adiabatic extended ISIS provided high spectral quality and accurate localization. The contamination in phantom experiments was only ∼5%, even if a pulse repetition time ∼ 1.2·T(1) was chosen to maximize the signal-to-noise ratio per unit time. High reproducibility was found in the calf muscle for 2.5 cm isotropic voxels at 7 T. When compared with 3 T, localized (31)P magnetic resonance spectroscopy in the human calf muscle at 7 T provided ∼3.2 times higher signal-to-noise ratio (as judged from phosphocreatine peak amplitude in frequency domain after matched filtering). At 7 T, extended ISIS allowed the performance of high-quality localized (31)P magnetic resonance spectroscopy in a short measurement time (∼3 to 4 min) and isotropic voxel sizes of ∼2.5 to 3 cm. With such short measurement times, localized (31)P magnetic resonance spectroscopy has the potential to be applied not only for clinical research but also for routine clinical practice.

Design of self-refocused pulses under short relaxation times

The effect of using self-refocused RF pulses of comparable duration to relaxation times is studied in detail using numerical simulation. Transverse magnetization decay caused by short T2 and longitudinal component distortion due to short T1 are consistent with other studies. In order to design new pulses to combat short T1 and T2 the relaxation terms are directly inserted into the Bloch equations. These equations are inverted by searching the RF solution space using simulated annealing global optimization technique. A new T2-decay efficient excitation pulse is created (SDETR: single delayed excursion T2 resistive) which is also energy efficient. Inversion pulses which improve the inverted magnetization profile and achieve better suppression of the remaining transverse magnetization are also created even when both T1 and T2 are short. This is achieved, however, on the expense of a more complex B1 shape of larger energy content.

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.

Cerebral energy metabolism in phenylketonuria: findings by quantitative In vivo 31P MR spectroscopy

Both severe impairments of brain development in untreated infants and acute reversible neurotoxic effects on brain function are clinical features of phenylketonuria (PKU). For determining whether impairments of cerebral energy metabolism play a role in the pathophysiology of PKU, quantitative in vivo 31P magnetic resonance spectroscopy (MRS) was performed in a supratentorial voxel of 11 adult PKU patients and controls. Peak areas of inorganic phosphate; phosphocreatine; alpha-, beta-, and gamma-ATP; NAD; phosphomonoesters; phosphodiesters; and a broad phospholipid signal were converted to millimolar concentrations. Mg2+, pH, ADP, the phosphorylation potential, and the relative velocity of oxidative metabolism V/Vmax were derived. Clinical evaluation included mutation analysis, neurologic investigation, intelligence testing, magnetic resonance imaging, and concurrent plasma and brain phenylalanine (Phe), the last by 1H-MRS. Phe loading was performed in five patients with an oral dose of 100 mg/kg body wt L-Phe monitored by spectral EEG analysis. Under steady-state conditions, 31P-MRS revealed normal values for ATP, phosphocreatine, NAD, phosphomonoesters, phosphodiesters, Mg2+, and pH in PKU. ADP (+11%) and the phosphorylation potential (+22%) were increased. Peak areas of inorganic phosphate (-22%) and phospholipid (-8%) were decreased. ADP correlated with concurrent plasma (r = 0.65) and brain (r = 0.55) Phe. During the Phe load, blood Phe levels increased steeply. EEG revealed slowing of background activity. The phosphorylation potential decreased, whereas ADP and V/Vmax increased. In vivo 31P-MRS demonstrated subtle abnormalities of cerebral energy metabolism in PKU in steady-state conditions that were accentuated by a Phe load, indicating a link between Phe neurotoxicity and imbalances of cerebral energy metabolism.

The magnitude of signal errors introduced by ISIS in quantitative 31P MRS

It is well known that the quality of a quantitative 31P MRS measurement relies largely on the performance of the volume selection method, and that image selected in vivo spectroscopy (ISIS) suffers from contaminating signal caused mostly by T1 smearing. However, these signal errors and their magnitude are seldom addressed in clinical studies. The aim of this study was therefore to investigate the magnitude of signal errors in 31P MRS when using ISIS. The results from the measurements with a homogeneous head phantom are as follows: at low TR/T1 ratios the contamination increases rapidly, especially for small (<27 cm3) VOI sizes; at TR/T1=1, the signal from a 27 cm3 VOI was 20% too high, and from an 8 cm3 VOI 150% too high. The signal obtained from different VOI positions varied between 80 and 127%. The signal varied linearly with the 31P concentration in the object. However, a too high signal was obtained when the concentration was lower in the region of interest (inner container) than in the rest of the phantom. The agreement between the simulations and measurements shows that the results of this study are generally applicable to the measurement geometry and the ISIS experiment order rather than being specific for the MR system studied. The errors obtained both experimentally and in computer simulations are too large to be ignored in clinical studies using the ISIS pulse sequence.

The performance of volume selection sequences for in vivo NMR spectroscopy: implications for quantitative MRS

Previous work has demonstrated that deficiencies in volume selection sequences used in magnetic resonance spectroscopy may compromise the quality of the spectra obtained. In this paper, further studies on the ISIS and PRESS sequences are presented. Under conditions of partial saturation, ISIS can exhibit serious contamination with extraneous signal, particularly when a small volume of interest (VOI) is selected. ISIS protocols should therefore use VOIs that are large relative to the target volume, and repetition times that are as long as practicable. In PRESS, contamination is found to be minimised by using a VOI that is small relative to the target volume, and to be independent of repetition time. PRESS performance is also independent of echo time, except when very short echo times are used. These results are consistent with previously published work on ISIS and PRESS, and it is now possible to establish generic features of these sequences and to understand the implications for quantitative spectroscopy. T(1)-weighting of contamination in ISIS can compromise both relative and absolute quantification techniques in several respects. Contamination in PRESS is largely independent of relaxation times and would be easier to model and correct for in the context of quantitative spectroscopy.

Quantitation of localized (31)P magnetic resonance spectra based on the reciprocity principle

There is a need for absolute quantitation methods in (31)P magnetic resonance spectroscopy, because none of the phosphorous-containing metabolites is necessarily constant in pathology. Here, a method for absolute quantitation of in vivo (31)P MR spectra that provides reproducible metabolite contents in institutional or standard units is described. It relies on the reciprocity principle, i.e., the proportionality between the B(1) field map and the map of reception strength for a coil with identical relative current distributions in receive and transmit mode. Cerebral tissue contents of (31)P metabolites were determined in a predominantly white matter-containing location in healthy subjects. The results are in good agreement with the literature and the interexamination coefficient of variance is better than that in most previous studies. A gender difference found for some of the (31)P metabolites may be explained by different voxel composition.

Extended ISIS sequences insensitive to T(1) smearing

Image selected in vivo spectroscopy (ISIS) is a volume selection method often used for in vivo (31)P MRS, since it is suitable for measurements of substances with short T(2). However, ISIS can suffer from significant signal contributions caused by T(1) smearing from regions outside the VOI. A computer model was developed to simulate this contamination. The simulation results for the ISIS experiment order implemented in our MR system (ISIS-0) were in agreement with results obtained from phantom measurements. A new extended ISIS experiment order (E-ISIS) was developed, consisting of four "optimal" ISIS experiment orders (ISIS-1 to ISIS-4) performed consecutively with dummy ISIS experiments in between. The simulation results show that contamination due to T(1) smearing is, effectively, eliminated with E-ISIS and is significantly lower than for ISIS-0 and ISIS-1. E-ISIS offers increased accuracy for quantitative and qualitative determination of substances studied using in vivo MRS. Hence, E-ISIS can be valuable for both clinical and research applications.

Single-shot, three-dimensional "non-echo" localization method for in vivo NMR spectroscopy

Metabolite signals with short T(1) or T(2) are difficult to localize with full sensitivity. This limitation was overcome with the development and implementation of a single-shot, complete three-dimensional "non-echo" localization method with reduced sensitivity to spatial B(1) variation, which is suitable for measuring signals with very short T(1) or T(2), e.g., the (13)C NMR signals of glycogen. The proposed method is based on a T(1)-optimized outer volume suppression scheme using pulses of the hyperbolic secant type applied at different power levels, which is robust over a fivefold range of T(1). Strong lipid, muscle glycogen, and glucose signals originating outside the rat brain were suppressed. Signals of glycogen, aspartate, glutathione, GABA C4, N-acetyl aspartate as well as the C3 and C4 signals of glutamate and glutamine with resolved homonuclear (13)C-(13)C coupling were fully resolved in vivo at 9.4 Tesla using higher-order shimming. The method can be extended to other nuclei and to localized MRS of humans.

Quality assessment of localization technique performance in small volume in vivo 1H MR spectroscopy

A new phantom and evaluation method for experimental evaluation of 1H-magnetic resonance spectroscopy single volume localization techniques regarding signal contamination (C), defined as the part of the signal originating outside the volume of interest, is presented. The quality assessment method is based on a spherical phantom with an oil/water interface in order to reduce susceptibility effects, and applied for stimulated-echo acquisition method (STEAM) and spin-echo (SE) sequences, echo times of 270, 135, and 10 ms, and cubic volumes of interest (VOI) of 1(3), 1.5(3), 2(3), 2.5(3), and 3(3) cm3. To be able to mimic measurements of the contamination in three dimensions the physical gradients representing the three orthogonal directions for slice selection were shifted in the pulse sequences. Contamination values in one dimension differed between 6.5% and 8.4% in SE sequences, and between 0.7% and 13.8% in STEAM sequences. In STEAM sequences a decrease of C with increasing VOI size was observed while SE sequences showed comparable C values for the different VOI sizes tested. The total contamination in three dimensions were 19% and 18% in SE and STEAM sequences with a TE of 270 ms, and 7% in a STEAM sequence with a TE of 10 ms, respectively. The presented evaluation method is easily applied to the new phantom and showed high reproducibility.

MR spectroscopy using multi-ring surface coils

A spatially uniform B(1)-field is preferred for MR imaging and spectroscopy. Unfortunately, volume coils are sometimes unavailable, or do not provide adequate RF power or SNR for some applications. In quantitative MRS, mean metabolite concentration cannot be evaluated when the coil response is nonuniform, unless an assumption is made concerning the metabolite spatial distribution. It is well known that standard single-loop surface coils, although offering high SNR characteristics, have poor B(1) homogeneity. New multi-ring surface coils are proposed which produce a locally uniform B(1) field, with sensitivity and power requirements comparable to those of standard surface coils. MR spectroscopy using two and three-ring versions of this "local volume coil" result in spatial localization essentially identical to that obtained with a volume coil but with much improved RF power and SNR characteristics. When compared to standard surface coils, the multi-ring coil offers much improved water suppression and localization, as well as reduced outer voxel contamination, with only a small loss in SNR and moderate increase in SAR. In summary, the multi-ring coil operates midway between the volume coil and the standard surface coil, retaining the most advantageous properties of both. Magn Reson Med 42:655-664, 1999. Published 1999 Wiley-Liss, Inc.

In vivo detection of (15)N-coupled protons in rat brain by ISIS localization and multiple-quantum editing

Three-dimensional image-selected in vivo spectroscopy (ISIS) was combined with phase-cycled (1)H-(15)N heteronuclear multiple-quantum coherence (HMQC) transfer NMR for localized selective observation of protons J-coupled to (15)N in phantoms and in vivo. The ISIS-HMQC sequence, supplemented by jump-return water suppression, permitted localized selective observation of 2-5 micromol of [(15)N(indole)]tryptophan, a precursor of the neurotransmitter serotonin, through the (15)N-coupled proton in 20-40 min of acquisition in vitro at 4.7 T. In vivo, the amide proton of [5-(15)N]glutamine was selectively observed in the brain of spontaneously breathing (15)NH(4)(+)-infused rats, using a volume probe with homogeneous (1)H and (15)N fields. Signal recovery after three-dimensional localization was 72-82% in phantoms and 59 +/- 4% in vivo. The result demonstrates that localized selective observation of (15)N-coupled protons, with complete cancellation of all other protons except water, can be achieved in spontaneously breathing animals by the ISIS-HMQC sequence. This sequence performs both volume selection and heteronuclear editing through an addition/subtraction scheme and predicts the highest intrinsic sensitivity for detection of (15)N-coupled protons in the selected volume. The advantages and limitations of this method for in vivo application are compared to those of other localized editing techniques currently in use for non-exchanging protons.

Localized 15N NMR spectroscopy of rat brain by ISIS

Three-dimensional image-selected in vivo spectroscopy (ISIS), combined with proton-decoupled nuclear-Overhauser-enhanced 15N nuclear magnetic resonance (NMR), was used to localize [15N]metabolites, observed by a head coil, to the brain in rats. In spontaneously breathing anesthetized rats given intravenous [15N]ammonium acetate infusion, brain [5-15N]glutamine was observed in the localized spectrum with a v1/2 of 5 Hz in 19-28 min at 4.7 T, while the signal from blood [15N]urea was eliminated by the localization process. In rats given [15N]leucine infusion, the peak representing predominantly (89%) brain [15N]glutamate was observed, with elimination of the signal from muscle [15N]alanine. In vivo peak areas of the brain [15N]metabolites in the localized spectra were proportional to their concentrations. The advantages and limitations of localization by ISIS using a volume coil with a homogeneous B1 field are compared with those of localization by a surface coil for in vivo 15N NMR study of neurotransmitters in the brain.

Absolute metabolite quantification by in vivo NMR spectroscopy: IV. Multicentre trial on MRSI localisation tests

The difference between the experimental and theoretical spatial response function (SRF) of a narrow tube with water is used for a localization test for magnetic resonance spectroscopic imaging (MRSI). From this difference a quantitative performance parameter is derived for the relative amount of signal within a limited region in the field of view. The total signal loss by the MRSI experiment and eddy currents is described by a parameter SL derived from the signal intensities of two echoes. Results of a European multi-centre trial show that this approach is suited for assessment of MRSI localization performance.

Signal profile measurements of single- and double-volume acquisitions with image-selected in vivo spectroscopy for 31P magnetic resonance spectroscopy

The volume-selection performance was studied for single- and double-volume-of-interest (VOI) acquisition with the volume-selection method image-selected in vivo spectroscopy for 31P magnetic resonance spectroscopy. High-resolution signal profiles were measured using a phantom simulating a brain. Inside the phantom there was a small, remotely controlled, movable signal source filled with ortho-phosphoric acid. Signal profiles of the VOI were measured in three perpendicular directions for 1VOI (single VOI) and 2VOI (double VOI) acquisition. The measured signal profiles for both acquisitions were very similar, but they showed a discrepancy with regard to the intended VOI (iVOI). The transition regions were on average 3.8 mm and the average full width at half maximum of the signal profile was 30 mm for an iVOI size of 30*30*30 (mm3). No displacement was observed in the signal profiles. To avoid overlapping signal profiles, the minimum separation between two iVOIs was found to be 10 mm in our magnetic resonance (MR) system. A substantial negative signal contribution from regions outside the iVOI was measured in the y-direction for 1VOI acquisition and one of the two VOIs in 2VOI acquisition. The other VOI in 2VOI acquisition exhibited only minor contamination. The measurements presented underline the importance of detailed knowledge on the volume selection performance in in vivo magnetic resonance spectroscopy.

A two-compartment phantom for VOI profile measurements in small-bore 31P MR spectroscopy

A two-compartment gel phantom for VOI profile measurements in volume-selective 31P spectroscopy in small-bore units is presented. The phantom is cylindrical with two compartments divided by a very thin (30 microm) polyethene film. This thin film permits measurements with a minimum of susceptibility influences from the partition wall. The phantom was used for evaluation of the volume selection method ISIS (image-selected in vivo spectroscopy). The position of the phantom was fixed in the magnet during the measurements, while the volume of interest (VOI) was moved stepwise over the border. The signal from the two compartments was measured for each position and the data were evaluated following differentiation. We have found this phantom suitable for VOI profile measurements of ISIS in small-bore systems. The phantom forms a useful complement to recommended phantoms for small bore-spectroscopy.

Reproducibility of human cardiac 31P-NMR spectroscopy

The reproducibility of the phosphocreatine to adenosine triphosphate ratio (PCr/ATP) was assessed from cardiac phosphorus-31 (31P) NMR spectra of the human left ventricle acquired with three different localization techniques. Cardiac 31P-NMR spectra (n = 68) were obtained at rest from 16 healthy subjects with three-dimensional (3D) image selected in vivo spectroscopy (ISIS), 1D spectroscopic imaging (SI), or with a combination of 2D ISIS and the 1D SI technique (ISIS + SI). The average PCr/ATP ratios were 1.41 +/- 0.20 for ISIS + SI and 1.31 +/- 0.19 for ISIS and were in the lower range of values obtained in previous studies, mainly because of a lower saturation correction factor for the cardiac PCr/ATP ratio. The SI experiment yielded an average PCr/ATP value of 0.98 +/- 0.20, significantly lower as compared to the correct values obtained with ISIS + SI and ISIS (p < 0.001), underscoring the need for 3D localization to avoid contamination of the NMR signal by liver tissue. Intersubject standard deviations of the PCr/ATP ratio were comparable to values reported previously. For all three localization techniques the absolute intra-examination differences in PCr/ATP (0.06 for ISIS to 0.15 for ISIS + SI) were significantly smaller (p approximately 0.03) than inter-examination differences (0.24 for ISIS to 0.29 for ISIS + SI). Therefore, consecutive acquisition of cardiac 31P-NMR spectra from the same patient during a single examination, e.g. under various cardiac loading conditions, appears to be a reliable approach for metabolic evaluation of heart disease.

Endurance-trained and untrained skeletal muscle bioenergetics observed with magnetic resonance spectroscopy

Resting and submaximal isometric exercise 31P magnetic resonance spectroscopy (MRS) was carried out on 7 endurance-trained males (26.0 +/- 3 yrs) and 7 sedentary males (27.0 +/- 4 yrs). Spectral analysis provided peak areas of phosphocreatine (PCr), inorganic phosphate (Pi), adenosine triphosphate (ATP), and the chemical shift of Pi relative to PCr. The ratio of PCr/Pi was moderately lower during rest (preexercise p = .13, postexercise p = .18), and significantly higher during exercise (p < .05) in the trained subjects. Intracellular pH patterns were the same for both groups; a transient alkalosis was observed at the onset of exercise with a return to resting levels after 2 min. Differences suggest improved ATP resynthesis rate in the trained subjects during exercise. Intracellular pH changes can be attributed to the utilization of hydrogen ions that accompany PCr hydrolysis during work. The findings are congruent with previous reports indicating a superior oxidative capacity in trained skeletal muscle.

Signal profile measurements for evaluation of the volume-selection performance of ISIS

High-resolution signal profiles obtained with a test phantom were used in this study to evaluate the volume-selection performance of an implementation of ISIS (Image Selected In vivo Spectroscopy). The phantom simulated the brain with regard to volume and loading of coil. A remotely controlled, movable signal source inside the phantom was filled with orthophosphoric acid. Signal profiles of the volume of interest (VOI) were measured in three perpendicular directions. Special interest was focused on the transition zones, the position of the profiles, and the effects of off-resonance and T1 smearing. The transition zones were on average 5.6 mm wide and the full width at half maximum (FWHM) was 35 mm for a VOI of 40 x 40 x 40 mm3. The positions of the centre of the signal profiles were x = 3.2, y = -0.7 and z = 3.3 mm off-centre. The deviation of the volume position could be explained by off-resonance effects during imaging and spectroscopy. These data illustrate the importance of detailed knowledge of the volume-selection performance when attempting precision measurements using image-guided in vivo MRS.

Quantification of signal selection efficiency, extra volume suppression and contamination for ISIS, STEAM and PRESS localized 1H NMR spectroscopy using an EEC localization test object

The three most widely used single-volume NMR localization techniques (ISIS, STEAM and PRESS) are assessed quantitatively for 1H spectroscopy using an EEC localization test object. Signal selection efficiency, suppression of outer volume signals and contamination are measured on a 1.5 T whole-body Siemens GBS1 system. The ISIS signal selection efficiency (volume of interest (VOI), 1-125 cm3) ranged from 90% to 95%, with T1 relaxation during the sequence shown to account for the observed 5-10% signal loss. Contamination for ISIS was found to be higher for smaller VOIS and ranged from approximately 45% (VOI = 1 cm3) to approximately 9% (VOI = 125 cm3). For PRESS, contamination ranged from 7% to 12% and it was between 3% and 8% for STEAM. However, the maximum signal selection efficiency for the latter two techniques (echo time, 270 ms) was relatively low (10-17%), and limited by T2 losses and the non-rectangular slice profiles of sinc pulses.

Evaluation of volume selection methods in in vivo MRS. Design of a new test phantom

In vivo MR spectroscopy (MRS) requires some kind of volume selection method to be able to measure the signal from a selected part of the body. To be able to interpret the spectra correctly, the quality of the volume selection must be investigated for each new MRS application using phantom measurements. A new phantom, especially suitable for precision measurements of the volume selection performance, is presented. It contains a small, remotely controlled signal source placed inside a larger vessel. This principle can be applied to various body regions, coil types and nuclei. The measurement conditions are close to the clinical situation. The phantom does not have to be repositioned during a signal profile measurement and the signal contribution from each point along the profile is determined regarding sign and amplitude.

Quality assessment in in vivo NMR spectroscopy: IV. A multicentre trial of test objects and protocols for performance assessment in clinical NMR spectroscopy

A multicentre trial of test objects and protocols for performance assessment in single volume and slice selective magnetic resonance spectroscopy (MRS) was conducted by the European Community Concerted Action on MRI and MRS. The trial assessed phosphorus and proton localisation techniques implemented on commercially available MR systems at ten sites in Europe. At each site, a number of parameters devised by the Concerted Action were measured using prototype test objects. Some of these parameters related to the quality of localisation and others to the overall performance of the spectrometer. Results were obtained for the ISIS, DRESS, STEAM, and PRESS sequences with a range of acquisition parameters, allowing evaluation of the assessment methodology and comparison of the efficacy of various implementations of these localisation techniques. The results of this trial have been important in the development of the Concerted Action's final recommendations for MRS performance assessment, and demonstrate that such assessment provides valuable information in the comparison of spectroscopy data from different sites and in the development of new localisation sequences, and provides a means of quality assurance in MRS.

Calculation of sensitivity correction factors for surface coil MRS

Quantification of MRS signals obtained with surface coils is difficult due to the inhomogeneous response of these coils. This inhomogeneity results in the measured signal from a defined volume of interest (VOI) being spatially dependent. To account for the sensitivity variation with position from the surface coil, we have developed a method of calculating correction factors for defined VOIs based on an experimentally obtained 3D sensitivity coil map. These factors may then be applied to spectra obtained from these VOIs to accurately take into consideration the varying coil sensitivity resulting in a reduction of measured signal. This method is demonstrated here to be able to correct for the inhomogeneity of surface coils over a range of two coil radii to within 4% accuracy.

Phosphorus magnetic resonance spectroscopy of human masseter muscle

Masseter muscle metabolism is poorly understood. 31P Magnetic Resonance Spectroscopy (MRS) provides an opportunity for non-invasive study of muscle metabolism during rest, exercise, and recovery. The aim of this study was to investigate the changes in high-energy phosphates and pH in human masseter muscle associated with exertional pain. Phosphates and pH were measured with 31P Magnetic Resonance at 2.0 Tesla. The bite force was simultaneously measured with a force transducer. Continuous biting at maximum voluntary bite force (MVBF) and two intermittent biting exercises with different duty cycles were performed to pain intolerance. The light intermittent exercise did not produce pain. Brief MVBF requested at the beginning, during, and end of each exercise showed no decay. Qualitatively, changes in phosphates were similar to those reported from comparable limb muscle exercises: increased inorganic phosphate (Pi), decreased phosphocreatine (PCr), and no changes in ATP level. Quantitatively, however, the Pi/PCr ratio did not reach the levels reported in limb muscles during similar exercises. Also, the pH changed very little. Thus, the lack of fatigue was no surprise, since the level of changes in Pi/PCr and pH, reported to be associated with fatigue in limb muscles, was far less in the masseter. Pain development toward the end of the heavy exercises prevented further depletion of metabolites. Thus, the lack of fatigue generally postulated for the masseter muscle may not be due to resistance to fatigue of these fibers, but rather to the presence of pain preventing the fatigue. However, no specific metabolic changes associated with exertional pain were found.

An integrated program for amplitude-modulated RF pulse generation and re-mapping with shaped gradients

Efficient generation of amplitude modulated, frequency selective RF pulses has been demonstrated by the Shinnar-Le Roux (SLR) algorithm. In the present article, we provide an overview of a relatively comprehensive computer program that includes a version of the SLR algorithm and also incorporates an algorithm for re-mapping a selective RF pulse onto a new dwell time with modulated gradients. The re-mapping may be used to reduce SAR, or to shorten the RF pulse time by increasing the gradient and RF strength in regions where the original RF pulse amplitude was low. The program includes additional useful features including a Bloch equations algorithm, and pulse scaling, to enable examination of pulse profiles under a variety of conditions such as RF inhomogeneity and even nuclear relaxation. The program, MATPULSE, was developed with the MATLAB for Windows programming language and makes extensive use of the MATLAB graphical user interface (GUI) features to generate a user-friendly interface. A number of examples are provided to illustrate the capabilities of the MATPULSE program.

Magnetic resonance imaging and spectroscopy of the human heart

Magnetic resonance imaging and spectroscopy have a great potential both for clinical cardiac diagnostics and for research in cardiac physiology, metabolism and disease. At the present time, cardiac MRI already is the method of choice in several clinical conditions, especially in imaging central vasculature and intra- and paracardiac masses. With the recent development of contrast agents and ability to measure both flow velocities and flow volume, the cardiac MRI is likely to have a profound role in evaluating coronary arterial disease as well as valvular heart disease. The limitations due to long imaging times of cardiac MRI-studies are likely to be overcome with the development of ultrafast imaging techniques in the near future. On the other hand, cardiac MRS is still a research tool, which needs technical improvements before it can be widely utilized in clinical work. However, attempts to this aim are highly justified, when the possibility that MRS will provide metabolic information of the heart is considered and bearing in mind, that MR-magnets with sufficient field strength for MRS are increasingly in use in most modern hospitals. The role of magnetic resonance imaging (MRI) and spectroscopy (MRS) in the evaluation of heart diseases is still evolving. Some clear indications for clinical use of cardiac MRI have already become apparent, whereas cardiac MRS is still confined to research applications. The current paper consists of a review of the role of MRI for cardiovascular diagnosis together with a review of the currents status of cardiac MRS.

Spatially localized 31P NMR measurements of longitudinal relaxation rates in the canine myocardium

FLAX-ISIS spatial localization was combined with inversion recovery to enable the measurement of spatially localized T1 values. This approach was applied to the transmural determination of creatine phosphate longitudinal relaxation times in the canine myocardium. By examining five voxels spanning the left myocardial wall, we observed that transmural T1 values for creatine phosphate ranged from 3.61 +/- 0.20 in the endocardium to 4.00 +/- 0.20 in the epicardium at 4.7 Tesla. As such, the canine myocardium exhibits no transmural variation in the T1 values of creatine phosphate. This simple approach can be extended to enable the in vivo measurement of transmural enzymatic rates.

Use of computer simulations for quantitation of 31P ISIS MRS results

The difficulties in quantitation of in vivo 31P spectra are exacerbated by the fact that, in general, coils with inhomogeneous B1 fields are used with in vivo samples. A general method for quantitation of in vivo 31P MRS results obtained with the ISIS localization method was developed using computer simulations. The simulation calculates the preparation of the sample magnetization throughout the sample by the ISIS pulse sequence, as well as the sensitivity of signal reception. The calculation accounts for both the B1 field and the B0 gradients applied to the sample. The sensitivity of the experiment is expressed by integration of the simulated signal over the sample, assuming a homogeneous sample. The primary advantage of this approach is that a separate localization experiment on a phantom of known concentration is not required each time parameters of the localization experiment, such as dimensions or location of the localized volume, are altered. In addition, the simulations indicate the degree of contamination (signal from outside of the localized volume) that occurs, and provide a means of comparing different executions of the ISIS experiment. Experiments were performed on phantoms to verify the simulations, and experimental results on human brain and liver are reproduced to show that this approach provides reasonable estimates of metabolite levels in terms of molar concentrations.

Physiological concentrations of inorganic phosphate affect MgATP-dependent Ca2+ storage and inositol trisphosphate-induced Ca2+ efflux in microsomal vesicles from non-hepatic cells

1. MgATP-dependent 45Ca2+ uptake by microsomes obtained from various non-hepatic tissues, namely rat brain, rat solid Morris hepatoma 3924A and human platelets, was measured in the presence of P(i) at low, cytosol-like, concentrations. 2. Increasing P(i) concentrations (0.5-3 mM) caused a progressive enlargement of the 45Ca(2+)-storage capacity of all the microsomal fractions. 3. As a result of P(i) stimulation of Ca2+ uptake, 45Ca2+ and [32P]P(i) were co-accumulated by the three microsomal fractions. 4. The time course for 45Ca2+ and [32P]P(i) accumulation in brain microsomes revealed a biphasic 45Ca2+ uptake: a rapid phase was followed by a second, slower, phase, which depended on the presence of P(i). During the P(i)-dependent phase, the uptake of 45Ca2+ was paralleled by the uptake of [32P]Pi. 5. The passive efflux of Ca2+ was paralleled by the efflux of P(i) and vice versa. In fact, the inhibition of active Ca2+ uptake by excess EGTA, or lowering the P(i) concentration of the incubation system by dilution, caused the release of 45Ca2+ and [32P]P(i) from 45Ca2+ or [32P]P(i) pre-loaded brain microsomes. The Ca2+ ionophore A23187 also released 45Ca2+ and [32P]P(i). 6. Ca2+ efflux by A23187 was rapid (t 1/2 approx. 2 s) and independent of the extent of intravesicular Ca2+ loading, which indicates that Ca2+ and P(i) do not form intravesicular insoluble complexes. 7. The progressive increase in Ca2+ accumulation, depending on P(i) stimulation, resulted in a proportional increase in the amount of Ca2+ releasable by InsP3 in the three non-hepatic microsomal fractions and in digitonin-permeabilized platelets. 8. Concomitantly to Ca2+, microsomal P(i) was also released by InsP3.

31P magnetic resonance spectroscopy in dilated cardiomyopathy and coronary artery disease. Altered cardiac high-energy phosphate metabolism in heart failure

BACKGROUND

The purpose of this work was to further define the value of cardiac 31P magnetic resonance (MR) spectroscopy for patients with coronary artery disease and dilated cardiomyopathy.

METHODS AND RESULTS

Blood-corrected and T1-corrected 31P MR spectra of anteroseptal myocardium were obtained at rest using image-selected in vivo spectroscopy localization, a selected volume of 85 +/- 12 cm3, and a field strength of 1.5 T. Nineteen volunteers had a creatine phosphate (CP)/ATP ratio of 1.95 +/- 0.45 (mean +/- SD) and a PDE/ATP ratio of 1.06 +/- 0.53; in four patients with left anterior descending coronary artery (LAD) stenosis, six patients with chronic anterior wall infarction, and four patients with chronic posterior wall infarction, CP/ATP and phosphodiester (PDE)/ATP ratios did not differ from those in volunteers. Twenty-five measurements of 19 patients with dilated cardiomyopathy yielded a CP/ATP of 1.78 +/- 0.51 and a PDE/ATP of 0.98 +/- 0.56 (p = NS versus volunteers). When these patients were grouped according to the severity of heart failure, however, CP/ATP was 1.94 +/- 0.43 in mild (p = NS versus volunteers) and 1.44 +/- 0.52 in severe DCM (p < 0.05), respectively. No correlation was found between CP/ATP and left ventricular ejection fraction or fractional shortening, but correlation of CP/ATP with the New York Heart Association (NYHA) class was significant (r = 0.60, p < 0.005). Six patients with dilated cardiomyopathy were studied repeatedly before and after 12 +/- 6 weeks of drug treatment leading to clinical recompensation with improvement of the NYHA status by 0.8 +/- 0.3 classes. Concomitantly, CP/ATP increased from 1.51 +/- 0.32 to 2.15 +/- 0.27 (p < 0.01), whereas PDE/ATP did not change significantly.

CONCLUSIONS

Cardiac high-energy phosphate metabolism at rest is normal in LAD stenosis and chronic myocardial infarction in the absence of heart failure. The CP/ATP ratio has low specificity for the diagnosis of dilated cardiomyopathy. However, CP/ATP correlated with the clinical severity of heart failure and may improve during clinical recompensation.

The impact of the ISIS experiment order on spatial contamination

When performing volume-localized spectroscopy measurements, the amount of spatial contamination is an important quality criterion. With the ISIS localization technique contamination cannot only arise from the transition regions around the volume of interest, but also from remote regions of the sample. The latter contamination component is a consequence of inhomogeneous excitation pulses, if short repetition times TR are used. Its severity depends both on the order of the eight phase cycling experiments needed for an ISIS measurement, and on the ratio TR/T1. Here it is theoretically discussed from which regions of the sample contamination can arise for a specific phase cycling order. For the worst orders the contaminating regions are almost three times as large as for the optimal orders. The ratio for the effectively measured contamination, however, can be moderated in real experiments, because cancellation effects occur due to the phase distribution of the contaminating signals. 31P phantom experiments clearly demonstrate that contamination is present even if adiabatic excitation pulses are applied and that spatial contamination can be reduced to about a third by an optimal choice of the phase cycling order.

Proton magnetic resonance spectroscopy of human brain: applications to normal white matter, chronic infarction, and MRI white matter signal hyperintensities

A modified ISIS method, for image-selected localized proton magnetic resonance spectroscopy (1H MRS), was used to determine the ratios and T2 relaxation times of proton metabolites in normal subjects and in patients with chronic infarction and MRI white matter signal hyperintensities (WMSH). First, in patients with cerebral infarctions, increased concentrations of lactate were found in the majority of patients, and N-acetyl aspartate (NAA) was reduced to a significantly greater extent than choline (Cho) or creatine (Cre). For TE = 270 ms, the raw ratios of Cho/NAA, Cre/NAA, and Lac/NAA were significantly (P less than 0.05) increased from 0.23 +/- 0.02 (mean +/- SE), 0.20 +/- 0.01, and 0.05 +/- 0.01, respectively in the normal group to 0.39 +/- 0.08, 0.37 +/- 0.05, and 0.48 +/- 0.15 in the stroke group. Also, the T2 relaxation time of creatine was significantly (P = 0.007) increased from 136 ms in normal white matter to 171 ms in cerebral infarcts. Second, in patients with WMSH, no significant change of the proton metabolite concentrations could be detected with the exception of the choline which was significantly (P = 0.003) altered. The Cho/NAA ratio, after T2 and excitation profile correction, increased from 0.47 +/- 0.02 in the normal group to 0.64 +/- 0.05 in the WMSH group. Third, in normal white matter, the concentration of N-acetyl aspartate, choline, and lactate was estimated to 11.5, 2.0, and 0.6 mM, respectively, by assuming a total creatine concentration of 10 mM.

Effect of photic stimulation on human visual cortex lactate and phosphates using 1H and 31P magnetic resonance spectroscopy

Previous animal and human studies showed that photic stimulation (PS) increased cerebral blood flow and glucose uptake much more than oxygen consumption, suggesting selective activation of anaerobic glycolysis. In the present studies, image-guided 1H and 31P magnetic resonance spectroscopy (MRS) was used to monitor the changes in lactate and high-energy phosphate concentrations produced by PS of visual cortex in six normal volunteers. PS initially produced a significant rise (to 250% of control, p less than 0.01) in visual cortex lactate during the first 6.4 min of PS, followed by a significant decline (p = 0.01) as PS continued. The PCr/Pi ratios decreased significantly from control values during the first 12.8 min of PS (p less than 0.05), and the pH was slightly increased. The positive P100 deflection of the visual evoked potential recorded between 100 and 172 ms after the strobe was significantly decreased from control at 12.8 min of PS (p less than 0.05). The finding that PS caused decreased PCr/Pi is consistent with the view that increased brain activity stimulated ATPase, causing a rise in ADP that shifted the creatine kinase reaction in the direction of ATP synthesis. The rise in lactate together with an increase in pH suggest that intracellular alkalosis, caused by the shift of creatine kinase, selectively stimulated glycolysis.

31Phosphorus magnetic resonance spectroscopy of the temporal lobes in schizophrenia

Eleven schizophrenic patients and nine normal controls were studied using in vivo 31Phosphorous magnetic resonance spectroscopy (31P MRS) to test the hypothesis of metabolic asymmetry in the temporal lobes in schizophrenia. The controls did not demonstrate any asymmetry of phosphorous metabolite ratios, percentage of phosphorous metabolites, or pH. In the schizophrenics, however, phosphocreatine/beta-adenosine triphosphate (PCr/beta-ATP) and phosphocreatine/inorganic phosphate (PCr/Pi) effects appeared to primarily reflect higher ratios on the right side, while the percentage of beta-ATP appeared to primarily reflect higher relative concentrations in the left temporal lobe. Moreover, significant negative correlations were noted between total Brief Psychiatric Rating Scale scores and PCr/beta-ATP in both the right and left temporal lobes. These results support the hypothesis of an asymmetric distribution of 31P metabolites in the temporal lobe of schizophrenic patients, and also show an association between temporal lobe phosphorous metabolism and the severity of psychiatric symptomatology.

Excitation characteristics of adiabatic half-passage RF pulses used in surface coil MR spectroscopy. Application to 13C detection of glycogen in the rat liver

Properties of sech/tanh and sin/cos half-passage RF pulses are discussed in view of their use in surface coil MR spectroscopy. We focus on the use of these pulses in a regime which is partially adiabatic, i.e. not strictly adiabatic off-resonance, while on-resonance the adiabaticity condition is fulfilled. It is shown that the frequencies of the singular points of the excitation profiles, as well as their number, depend on the B1 field. This leads to a signal intensity reduction from off-resonance spectral regions over much broader ranges than generally believed. We show in particular that with surface coil, sin/cos RF pulses may perform particularly well, providing optimal excitation on resonance and a desired attenuation over a broad spectral range off-resonance. This feature is applied for the in vivo detection of rat liver glycogen by means of 13C MR spectroscopy. Under suitable RF power conditions, a remarkable attenuation of the signals from the saturated carbons of the subcutaneous fat can be achieved.

Localized 13C NMR spectroscopy of myo-inositol in the human brain in vivo

Natural abundance 13C NMR spectra obtained from 144-cm3 volumes in the human brain contained well-resolved resonances of myo-inositol after 60 min of data accumulation. A mean concentration of 7.2 +/- 0.5 mumol/g (+/- SE, n = 7) was calculated from the comparison with phantoms. 13C NMR spectroscopy thus provides a complementary role in the quantitation of metabolites also observed in the crowded 1H spectrum.