Magnetization transfer affects the proton creatine/phosphocreatine signal intensity: In vivo demonstration in the rat brain

The effect of off-resonance preirradiation on proton spectra acquired from the healthy rat brain at 4.7 T is examined using a PRESS sequence. The creatin/phosphocreatine (Cr/PCr) signal at 3.0 ppm decreases in signal intensity for offset frequencies between ±10 kHz. This cannot be explained by RF bleedover, but is attributed to magnetization transfer between two pools of Cr/PCr, one with a long T₂ relaxation time giving the observed NMR signal, the other corresponding to a broad resonance line being completely or partially saturated by off-resonance preirradiation. Possible interpretations of these results are discussed.     

Concentrations and magnetization transfer ratios of metabolites in gray and white matter.

The concentrations and magnetization transfer ratios (MTRs) in gray matter (GM) and white matter (WM) of N-acetyl aspartate (NAA), creatine (Cr), choline (Cho), myo-inositol (Ins), and glutamate plus glutamine (Glx) were investigated using magnetic resonance spectroscopic imaging (MRSI). The macromolecule (MM) baseline was studied separately using a metabolite-nulling inversion. Three data sets were collected from a point-resolved spectroscopy (PRESS)-selected volume (TE/TR = 30/3000 ms) of human frontal lobe in vivo: one with MT pulses applied, one with an inversion pulse to null small metabolites, and one with no inversion or MT pulses. The MM signal, which was analyzed by integrating the metabolite-nulled spectrum between 0 and 3 ppm, was estimated to be 38% higher in GM than in WM. MM subtraction decreased the signal-to-noise ratio (SNR) and also decreased the reliability of LCModel quantification of most metabolites, but may have improved the accuracy of quantification of Glx. Glx and Cr were both found to correlate strongly with the GM volume fraction of the voxels. Cr showed the highest MTR, but the other metabolites also showed some attenuation of signal when the MT pulses were applied. The MTRs did not correlate with the GM volume fraction, which implies that the local environment of metabolites does not differ markedly between GM and WM.     

Relative Concentrations of Proton MR Visible Neurochemicals in Gray and White Matter in Human Brain

The relative distributions of N-acetylaspartate (NAA) + N-acetylaspartylglutamate (NAAG), creatine + phosphocreatine (Cr/PCr), and choline (Cho) in the gray and white matter of human brain were determined by utilizing proton magnetic resonance spectroscopic imaging (SI). The SI data was processed using an automated spectroscopic image processing algorithm, and image segmentation was performed using a supervised technique. Linear regression analysis indicated that the NAA + NAAG (2.01 ppm) and Cr/PCr (3.02 ppm) peaks are greater in gray matter compared with white matter. The large intersubject variation observed in the Cho (3.20 ppm) resonance prevented the assessment of its regional distribution with confidence.     

Temporal and regional changes during focal ischemia in rat brain studied by proton spectroscopic imaging and quantitative diffusion NMR imaging

The early development of focal ischemia after permanent occlusion of the right middle cerebral artery (MCA) was studied in six rats using interleaved measurements by diffusion-weighted NMR imaging (DWI) of water and two variants of proton spectroscopic imaging (SI), multiecho SI (TE: 136, 272, 408 ms) and short TE SI (TE: 20 ms). Measurements on a 4.7-T NMR imaging system were performed between the control phase and approximately 6 h postocclusion. In the center of the ischemic lesion of all rats, the apparent diffusion coefficient (ADC) decreased rapidly to 84.4 ± 4.2% (mean ± SD) of the control values approximately 2 min postocclusion. Approximately 6 h postocclusion, the ADC was reduced to 67.1 ± 5.9%. In contrast, large differences between the animals were observed for the temporal increase of lactate (Lac) in the ipsilateral hemisphere. The maximum Lac signal was reached in four rats after 0.5-1.5 h, and in two rats was not reached even after 6 h postocclusion. Six h postocclusion, SI spectra measured at a TE of 136 ms revealed a decrease in the CH³ signal of N-acetylaspartate (NAA) to 67 ± 13% of the control values. Differences were observed between the spatial regions of decreased NAA and increased Lac. In the lesions, a T2 relaxation time of Lac of 292 ± 40 ms, considering a J-cou-pling constant of 6.9 Hz, was measured. Furthermore, a prolongation of the T₂ of the CH₃ signal of creatine/phosphocre-atine (Cr/PCr) was observed in the lesion, from 163 ± 22 ms during control to 211 ± 41 ms approximately 6 h postocclusion. The experiments proved that DWI and proton SI are valuable tools to provide complementary information on processes associated with brain infarcts.     

Magnetization transfer effect on human brain metabolites and macromolecules.

A pulse sequence was implemented to observe the magnetization transfer (MT) effect on metabolites, water, and macromolecules in human frontal lobes in vivo at 1.5 Tesla. Signals were compared following the application of three hard pulses of 0.745 muT amplitude, applied at frequency offsets of either 2500 Hz or 30 kHz, preceding a conventional point-resolved spectroscopy (PRESS)-localized acquisition with an echo time (TE) of 30 ms and repetition time (TR) of 3 s. This gave an MT effect on water in vivo of 46%, while direct saturation by the MT pulses at 2.5 kHz offset was confirmed to be under 4% for all metabolites. We observed significant MT saturation in vivo for N-acetylated compounds, choline (Cho), myo-inositol, and lactate (Lac); a trend of an effect on glutamate + glutamine (Glx); and the typically observed effect on creatine (Cr). No significant MT effect was seen on the macromolecule signal, which was observed using metabolite nulling.     

Biexponential transverse relaxation (T(2)) of the proton MRS creatine resonance in human brain.

Differences in proton MRS T(2) values for phosphocreatine (PCr) and creatine (Cr) methyl groups (3.0 ppm) were investigated in studies of phantoms and human brain. Results from phantom studies revealed that T(2) of PCr in solution is significantly shorter than T(2) of Cr. Curve-fitting results indicated that the amplitude-TE curves of the total Cr resonance at 3.0 ppm in human brain (N = 26) fit a biexponential decay model significantly better than a monoexponential decay model (P < 0.006), yielding mean T(2) values of 117 +/- 21 ms and 309 +/- 21 ms. Using a localized, long-TE (272 ms) point-resolved spectroscopy (PRESS) proton MRS during 2 min of photic stimulation (PS), an increase of 12.1% +/- 3.5% in the mean intensity of the total Cr resonance in primary visual cortex (VI) was observed at the end of stimulation (P < 0.021). This increase is consistent with the conversion of 26% of PCr in VI to Cr, which is concordant with (31)P MRS findings reported by other investigators. These results suggest a significantly shorter T(2) for PCr than for Cr in vivo. This difference possibly could be exploited to quantify regional activation in functional spectroscopy studies, and could also lead to inaccuracies in some circumstances when the Cr resonance is used as an internal standard for (1)H MRS studies in vivo.     

Quantitative proton spectroscopy of canine brain: in Vivo and in Vitro correlations

Quantitative, single-voxel proton NMR spectroscopy of normal brain was performed in five adult beagle dogs using the cerebral water signal as an internal intensity reference. The same brain regions were then rapidly isolated and frozen using a pneumatic biopsy drill, perchloric acid extracted, and analyzed by biochemical assay and high-resolution NMR spectroscopy. The concentrations of the major resonances in the in vivo and in vitro spectra were compared, and good agreement was found between the different measurements. The in vivo spectra contained three peaks at 3.21, 3.04, and 2.02 ppm, which are usually assigned to trimethylamines (TMA), creatines, and N-acetyl derivatives (NAc), which corresponded to be the following metabolite concentration values: 1.7 ± 0.6, 7.7 ± 2.1, and 10.9 ± 2.7 μmol/g wet weight respectively. In vitro, the following metabolite concentrations were measured: glycerophosphocholine (GPC) 1.3 ± 0.2, phosphocholine (PC) 0.5 ± 0.1, phosphocreatine (PCr) 2.6 ± 0.4, creatine (Cr) 5.9 ± 1.4, and N-Acetyl aspartate (NAA) 8.9 ± 1.8 μmol/g wet weight. Therefore, the 3.21 ppm resonance observed in the in vivo spectrum is predominantly GPC and PC in a ratio of 2.6:1, the 3.04 ppm resonance is Cr and PCr in a ratio of 2.3:1, and the 2.02 ppm resonance is predominantly (≈?80%) NAA with small contributions from N-acetylaspartyl-glutamate (NAAG) and glutamate. The data presented here validate the technique of water referencing as a simple and convenient means of quantitating single-voxel in vivo proton NMR spectra of the brain.     

In vivo detection of serine in the human brain by proton magnetic resonance spectroscopy (1H‐MRS) at 7 Tesla

A single‐voxel proton magnetic resonance spectroscopy (¹H‐MRS) filtering strategy for in vivo detection of serine (Ser) in human brain at 7T is proposed. Spectral difference of coupled resonances arising from different subecho times of triple refocusing at a constant total echo time (TE) was utilized to detect the Ser multiplet and cancel the overlapping creatine (Cr) 3.92‐ppm singlet via difference editing. Dependence of the Ser signal on subecho times was investigated using density‐matrix simulation incorporating the slice‐selective radio frequency (RF) pulses. The simulation indicated that the difference‐edited Ser CH₂ multiplet at ～3.96 ppm is maximized with (TE₁, TE₂, TE₃) = (54, 78, 78) and (36, 152, 22) ms. The edited Ser peak amplitude was estimated, with both numerical and phantom analyses of the performance, as 83% with respect to 90° acquisition for a localized volume, ignoring relaxation effects. From the area ratio of the edited Ser and unedited Cr 3.03‐ppm peaks, assuming identical T₁ and T₂ between Ser and Cr, the Ser‐to‐Cr concentration ratio for the frontal cortex of healthy adults was estimated to be 0.8 ± 0.2 (mean ± SD; N = 6). Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.     

Serial proton magnetic resonance spectroscopy of ischemic brain injury in humans

Four patients were observed serially with 1H magnetic resonance spectroscopy (MRS) at times ranging from 3 days to 10 weeks after a documented ischemic event. Spectra were obtained from 8 cc volumes in infarcted regions and contralateral matched normal regions. Reproducible variations in n-acetyl aspartate (NAA), creatine + phosphocreatine (Cr + PCr), and the sequential changes in lactate and lipid resonances are related to the pathophysiology of stroke. A conspicuous lack of significant change in the choline (Cho) resonance with concomitant decrease in NAA and Cr + PCr is reported as a possible marker of ischemic injury.     

Diagnostic value of proton MR spectroscopy and diffusion-weighted MR imaging in childhood inherited neurometabolic brain diseases and review of the literature

The purpose of this study is to evaluate parenchymal diffusion properties and metabolite ratios in affected brain tissues of inherited neurometabolic brain diseases with an overview of the current literature about the diagnostic data of both techniques in childhood inherited metabolic brain diseases. The study group was consisting, 19 patients (15 males, 4 females; mean age, 54 months (4.5 years); age range, 1-171 months (14.25 years)) diagnosed with inherited neurometabolic brain disease. Single- and multivoxel proton MRS was carried out and NAA/Cr, Cho/Cr, mI/Cr, Glx/Cr ratios were calculated. Presence of lactate peak and abnormal different peaks were noted. ADC values were calculated from brain lesions. Results are compared with age and sex matched normal subjects. Elevated NAA/Cr ratio (Canavan disease), galactitol peak (galactosemia) at 3.7 ppm, branched chain amino acids (Maple syrup urine disease—MSUD) at 0.9 ppm were seen on different diseases. In Leigh disease and MSUD restricted diffusion was detected. Different diffusion properties were seen only in one Glutaric aciduria lesions. NAA/Cr ratios and calculated ADC values were significantly different from normal subjects (p < 0.05). DWI combined with MRS are complementary methods to routine cranial MRI for evaluating neurometabolic diseases which can give detailed information about neurochemistry of affected brain areas.     

Neurometabolism of active neuropsychiatric lupus determined with proton MR spectroscopy.

PURPOSETo determine the neurometabolism of patients with active neuropsychiatric systemic lupus erythematosus (NPSLE) by using proton MR spectroscopy.METHODSThirty-six patients with SLE and eight control subjects were studied with proton MR spectroscopy to measure brain metabolites. Peaks from N-acetylaspartate (NAA), creatine (Cr), choline (Cho), and at 1.3 parts per million (ppm) lipid, macromolecules, and lactate were measured. Patients were classified as having major NPSLE (seizures, psychosis, major cognitive dysfunction, delirium, stroke, or coma) (n = 15) or minor NPSLE (headache, minor affective disorder, or minor cognitive disorder) (n = 21). Patients with major NPSLE were severely ill and hospitalized.RESULTSSLE patients had lower NAA and increased metabolites at 1.3 ppm than did control subjects (NAA/Cr(SLE) = 1.90 +/- 0.35, NAA/Cr(Control) = 2.16 +/- 0.26; 1.3 ppm/Cr(SLE) = 0.49 +/- 0.41, 1.3 ppm/Cr(Control) = 0.27 +/- 0.05). NAA/Cr in patients with current or prior major NPSLE was lower than in patients without major NPSLE. Increased peaks at 1.3 ppm were present in all SLE subgroups, but particularly in patients with major NPSLE. These resonances were not evident at an echo time of 136, indicating that these signals were not lactate.CONCLUSIONMajor NPSLE, past or present, is associated with decreased levels of NAA. Elevated peaks around 1.3 ppm do not represent lactate even in severely ill patients, indicating that global ischemia is not characteristic of NPSLE. Neurochemical markers determined by MR spectroscopy may be useful for determining activity and degree of brain injury in NPSLE.     

Proton NMR spectroscopy of solvent-saturable resonances: A new approach to study pH effects in Situ

It is shown that the effect of pH changes can be measured in proton NMR spectra through the pH sensitivity of the signal intensities of metabolite protons exchanging with water. To observe this phenomenon, pulse sequences must be used that can sensitively observe these exchangeable protons under physiological conditions, which is achieved by avoiding magnetization transfer signal losses due to water saturation for solvent suppression purposes. These methods provide an order-of-magnitude enhancement of many signals between 5 and 10 ppm, containing both N-bound protons as well as aromatic C-H protons coupled to them, the intensity of which is influenced by exchange-relayed saturation. As a first application, the effects of pH change on these resonances are studied ex vivo (perfused cells) and in vivo (cat brain).     

Separation of lipids from lactate in experimental allergic encephalomyelitis by zero quantum NMR techniques

Using localized proton spectroscopy, the author and associates previously have observed an increase in the resonance at 1.2 +/- 0.2 ppm in the brain of monkeys with experimental allergic encephalomyelitis (EAE). In proton nuclear magnetic resonance (NMR) spectroscopy, the lactate methyl protons resonate at dose to the same chemical shift frequency as the lipid methylene protons (1.2 +/- 0.2 ppm). Noninvasive zero quantum NMR techniques were used to separate lipids from lactate in the brain during the course of EAE development. Out of 53 zero quantum NMR measurements (from three animals), 16 measurements were made at a time when a resonance at 1.2 ppm appeared (after the animals developed EAE). In all these cases, lactate was not detectable. The fact that the resonance (identified as lipids with zero quantum techniques) correlated with histochemical Oil red O staining suggests that these mobile NMR signals are associated with demyelination.     

Spot-scanning beam proton therapy vs intensity-modulated radiation therapy for ipsilateral head and neck malignancies: A treatment planning comparison

Radiation therapy for head and neck malignancies can have side effects that impede quality of life. Theoretically, proton therapy can reduce treatment-related morbidity by minimizing the dose to critical normal tissues. We evaluated the feasibility of spot-scanning proton therapy for head and neck malignancies and compared dosimetry between those plans and intensity-modulated radiation therapy (IMRT) plans. Plans from 5 patients who had undergone IMRT for primary tumors of the head and neck were used for planning proton therapy. Both sets of plans were prepared using computed tomography (CT) scans with the goals of achieving 100% of the prescribed dose to the clinical target volume (CTV) and 95% to the planning TV (PTV) while maximizing conformity to the PTV. Dose-volume histograms were generated and compared, as were conformity indexes (CIs) to the PTVs and mean doses to the organs at risk (OARs). Both modalities in all cases achieved 100% of the dose to the CTV and 95% to the PTV. Mean PTV CIs were comparable (0.371 IMRT, 0.374 protons, p = 0.953). Mean doses were significantly lower in the proton plans to the contralateral submandibular (638.7 cGy IMRT, 4.3 cGy protons, p = 0.002) and parotid (533.3 cGy IMRT, 48.5 cGy protons, p = 0.003) glands; oral cavity (1760.4 cGy IMRT, 458.9 cGy protons, p = 0.003); spinal cord (2112.4 cGy IMRT, 249.2 cGy protons, p = 0.002); and brainstem (1553.52 cGy IMRT, 166.2 cGy protons, p = 0.005). Proton plans also produced lower maximum doses to the spinal cord (3692.1 cGy IMRT, 2014.8 cGy protons, p = 0.034) and brainstem (3412.1 cGy IMRT, 1387.6 cGy protons, p = 0.005). Normal tissue V₁₀, V₃₀, and V₅₀ values were also significantly lower in the proton plans. We conclude that spot-scanning proton therapy can significantly reduce the integral dose to head and neck critical structures. Prospective studies are underway to determine if this reduced dose translates to improved quality of life.     

Human brain tumors: spectral patterns detected with localized H-1 MR spectroscopy.

Image-guided localized proton magnetic resonance (MR) spectroscopy of intracranial tumors was performed to correlate spectral patterns and histologic findings. Thirty-six patients were examined prior to any specific treatment. Evaluation based on signal intensity ratios showed that all tumor spectra differed from spectra of healthy brain tissue. Ratios of creatine to choline-containing compounds (Cr/Cho) and nitrogen acetyl-aspartate to Cho (NAA/Cho) were reduced significantly in all tumor spectra compared with spectra of normal tissue in contralateral brain hemispheres (P less than .005). Noncerebral tumors typically showed a vanishing or missing NAA signal, strongly reduced Cr signal, and additional signals, assigned to alanine in meningiomas and lipids in metastases. In contrast, 11 gliomas of grades 2 and 3 exhibited NAA/Cho ratios and Cr/Cho ratios that were less than normal but that were significantly larger (P less than .01) than corresponding values in eight meningiomas. Ten glioblastomas displayed spectra with various signal ratios, so no significant differences between them and other tumor types could be established. In nine gliomas a clearly detectable lactate signal was present. However, no direct correlation between lactate level and histologic tumor grading was found.     

Magnetic coupling of creatine/phosphocreatine protons in rat skeletal muscle, as studied by (1)H-magnetization transfer MRS.

Off-resonance saturation caused a reduction of the 3.04 ppm NMR signal from the methyl protons of creatine in rat hindleg skeletal muscle. (1)H-NMR spectra were recorded over a 200 kHz range of off-resonance saturation frequencies. The span of frequencies over which the creatine signal was reduced greatly exceeded that expected for direct saturation by the off-resonance RF-field. This suggests that there is a motionally restricted proton pool which exchanges magnetization with the free creatine pool. The experimental data were fitted to characterize the immobilized proton pool and the exchange kinetics, using a two-pool exchange model. The immobile pool was estimated to amount to ca. 2.5% of the mobile pool of free creatine, while the rate of exchange between the mobile and immobile configurations is ca. 2.3 sec(-1). After depletion of phosphocreatine by termination of the animal, the MT effect on the creatine methyl protons remained unchanged. This indicates that phosphocreatine and creatine both contribute to the MT phenomenon. Selective saturation of the mobile water pool also led to a reduction in the intensity of the total creatine methyl signal, suggesting that water and creatine are magnetically coupled via a macromolecular interface. The precise mechanism responsible for and the biological significance of the pronounced creatine magnetization transfer effect in rat skeletal muscle remains to be established. Magn Reson Med 42:665-672, 1999.     

pH control in rat skeletal muscle during exercise,recovery from exercise,and acute respiratory acidosis

We used ³¹P magnetic resonance spectroscopy to compare the response of rat skeletal muscle to three kinds of proton load. During exercise (tetanic sciatic nerve stimulation), protons from lactic acid were buffered passively and consumed by net hydrolysis of phosphocreatine (PCr). During recovery from exercise, the pH-dependent efflux of protons produced by PCr resynthesis could be partially inhibited by amiloride or 4,4′-diisothiocyanostilbene-2,2′-disulphonate (DIDS), implicating both sodiudproton and bicarbonatelchloride exchange, but was not inhibited by simultaneous respiratory acidosis. In early recovery, up to 30% of proton efflux was mediated by lactatelproton cotransport. During acute respiratory acidosis at rest, the eventual change in muscle pH was consistent with passive buffering and was unaffected by amiloride or DIDS, implying no significant contribution of proton fluxes.     

Measurement of brain glutamate using TE-averaged PRESS at 3T.

A method is introduced that provides improved in vivo spectroscopic measurements of glutamate (Glu), glutamine (Gln), choline (Cho), creatine (Cre), N-acetyl compounds (NAtot, NAA + NAAG), and the inositols (mI and sI). It was found that at 3T, TE averaging, the f1 = 0 slice of a 2D J-resolved spectrum, yielded unobstructed signals for Glu, Glu + Gln (Glx), mI, NA(tot), Cre, and Cho. The C4 protons of Glu at 2.35 ppm, and the C2 protons of Glx at 3.75 ppm were well resolved and yielded reliable measures of Glu/Gln stasis. Apparent T1/T2 values were obtained from the raw data, and metabolite tissue levels were determined relative to a readily available standard. A repeatibility error of <5%, and a coefficient of variation (CV) of <10% were observed for brain Glu levels in a study of six normal volunteers.     

Detection of glutamate/glutamine resonances by 1H magnetic resonance spectroscopy at 0.5 tesla

Midfield proton magnetic resonance spectroscopy (MRS) provides a noninvasive method to monitor glutamate and glutamine (Glx) levels in vivo. Experiments to detect the γ and β resonances of Glx have been performed by using commercial 0.5 T and 1.5 T MR scanners on seven patients with elevated blood ammonia and eight normal volunteers. Compared with the spectral sensitivity obtained on an otherwise identical system operating at 1.5 T, the singlet resonance of N-acetyl aspartate (NAA) was decreased by a factor of 1.48, which is significantly less than expected using the ratio of Boltzman populations at the two field strengths. However, the resonances of Glx at 0.5 T increased in signal-to-noise ratio (SNR) by a factor of 2. The increased SNR of Glx is principally due to improved B ₀ main-field homogeneity and collapse of the strongly J -coupled Glx resonances. Our preliminary results suggest that midfield proton MRS will provide significant clinical utility in the detection of Glx levels in human brain.     

Proton magnetization transfer effect in rat liver lactate.

Off-resonance lactate magnetization transfer (MT) experiments were performed on the in situ rat liver under perfused and ischemic conditions. A significant MT effect for lactate methyl protons was observed. The effect was larger for the ischemic condition than for the perfused condition, and was largest in the blood-filled ischemic livers. The size of the motionally restricted lactate pool, determined using a two-pool model fit, was estimated to be about 1% in perfused livers and about 1.8-2.5% after more than 1 hr of onset of ischemia, suggesting that lactate in liver is almost fully NMR-visible. The MT data for both the perfused and the ischemic condition appeared to be better approximated when assuming a superLorentzian lineshape for the immobile pool rather than a Gaussian lineshape. Finally, the experiments demonstrated a coupling between the lactate methyl and water protons, which may be mediated by macromolecules.