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

Imaging the extracellular pH of tumors by MRI after injection of a single cocktail of T1 and T2 contrast agents

The extracellular pH (pH(e) ) of solid tumors is acidic, and there is evidence that an acidic pH(e) is related to invasiveness. Herein, we describe an MRI single-infusion method to measure pH(e) in gliomas using a cocktail of contrast agents (CAs). The cocktail contained gadolinium-1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraaminophosphonate (GdDOTA-4AmP) and dysprosium-1,4,7,10-tetraazacyclododecane-N,N',N'',N'''-tetrakis(methylenephosphonic acid) (DyDOTP), whose effects on relaxation are sensitive and insensitive to pH, respectively. The Gd-CA dominated the spin-lattice relaxivity ΔR(1) , whereas the Dy-CA dominated the spin-spin relaxivity ΔR(2)*. The ΔR(2)* effects were used to determine the pixel-wise concentration of [Dy] which, in turn, was used to calculate a value for [Gd] concentration. This value was used to convert ΔR(1) values to the molar relaxivity Δr(1) and, hence, pH(e) maps. The development of the method involved in vivo calibration and measurements in a rat brain glioma model. The calibration phase consisted of determining a quantitative relationship between ΔR(1) and ΔR(2)* induced by the two pH-independent CAs, gadolinium-diethylenetriaminepentaacetic acid (GdDTPA) and DyDOTP, using echo planar spectroscopic imaging (EPSI) and T(1) -weighted images. The intensities and linewidths of the water peaks in EPSI images were affected by CA and were used to follow the pharmacokinetics. These data showed a linear relationship between inner- and outer-sphere relaxation rate constants that were used for CA concentration determination. Nonlinearity in the slope of the relationship was observed and ascribed to variations in vascular permeability. In the pH(e) measurement phase, GdDOTA-4AmP was infused instead of GdDTPA, and relaxivities were obtained through the combination of interleaved T(1) -weighted images (R(1) ) and EPSI for ΔR(2)*. The resulting r(1) values yielded pH(e) maps with high spatial resolution.

In vivo three-dimensional whole-brain pulsed steady-state chemical exchange saturation transfer at 7 T

Chemical exchange saturation transfer (CEST) is a technique to indirectly detect pools of exchangeable protons through the water signal. To increase its applicability to human studies, it is needed to develop sensitive pulse sequences for rapidly acquiring whole-organ images while adhering to stringent amplifier duty cycle limitations and specific absorption rate restrictions. In addition, the interfering effects of direct water saturation and conventional magnetization transfer contrast complicate CEST quantification and need to be reduced as much as possible. It is shown that for protons exchanging with rates of less than 50-100 Hz, such as imaged in amide proton transfer experiments, these problems can be addressed by using a three-dimensional steady state pulsed acquisition of limited B(1) strength (≈ 1 μT). Such an approach exploits the fact that the direct water saturation width, magnetization transfer contrast magnitude, and specific absorption rate increase strongly with B(1) , while the size of the CEST effect for such protons depends minimally on B(1) . A short repetition time (65 ms) steady-state sequence consisting of a brief saturation pulse (25 ms) and a segmented echo-planar imaging train allowed acquisition of a three-dimensional whole-brain volume in approximately 11 s per saturation frequency, while remaining well within specific absorption rate and duty cycle limits. Magnetization transfer contrast was strongly reduced, but substantial saturation effects were found at frequencies upfield from water, which still confound the use of magnetization transfer asymmetry analysis. Fortunately, the limited width of the direct water saturation signal could be exploited to fit it with a Lorentzian function allowing CEST quantification. Amide proton transfer effects ranged between 1.5% and 2.5% in selected white and grey matter regions. This power and time-efficient 3D pulsed CEST acquisition scheme should aid endogenous CEST quantification at both high and low fields.

Magnetic resonance characterization of ischemic tissue metabolism

Magnetic resonance imaging (MRI) and spectroscopy (MRS) are versatile diagnostic techniques capable of characterizing the complex stroke pathophysiology, and hold great promise for guiding stroke treatment. Particularly, tissue viability and salvageability are closely associated with its metabolic status. Upon ischemia, ischemic tissue metabolism is disrupted including altered metabolism of glucose and oxygen, elevated lactate production/accumulation, tissue acidification and eventually, adenosine triphosphate (ATP) depletion and energy failure. Whereas metabolism impairment during ischemic stroke is complex, it may be monitored non-invasively with magnetic resonance (MR)-based techniques. Our current article provides a concise overview of stroke pathology, conventional and emerging imaging and spectroscopy techniques, and data analysis tools for characterizing ischemic tissue damage.

(1)H-MRS can detect aberrant glycosylation in tumour cells: a study of the HeLa cell line

Glycosylation is the most abundant and diverse form of post-translational modification of proteins. Two types of glycans exist in glycoproteins: N-glycans and O-glycans often coexisting in the same protein. O-glycosylation is frequently found on secreted or membrane-bound mucins whose overexpression and structure alterations are associated with many types of cancer. Mucins have several cancer-associated structures, including high levels of Lewis antigens characterized by the presence of terminal fucose. The present study deals with the identification of MR signals from N-acetylgalactosamine and from fucose in HeLa cells by detecting a low-field signal in one-dimensional (1D) spectra assigned to the NH of N-acetylgalactosamine and some cross peaks assigned to fucose in two-dimensional (2D) spectra. The increase of Golgi pH by treatment with ammonium chloride allowed the N-acetylgalactosamine signal assignment to be confirmed. Behaviour of MR peak during cell growth and comparison with studies from literature taken together made it possible to have more insight into the relationship between aberrantly processed mucin and the presence of non-processed N-acetylgalactosamine residues in HeLa cells. Fucose signals, tentatively ascribed to residues bound to galactose and to N-acetylglucosamine, are visible in both intact cell and perchloric acid spectra. Signals assigned to fucose bound to galactose are more evident in ammonium chloride-treated cells where structural changes of mucin-related Lewis antigens are expected as a result of the higher Golgi pH. A common origin for the N-acetylgalactosamine and fucose resonances attributing them to aberrantly processed mucin can be inferred from the present results.

Amide proton transfer MR imaging of prostate cancer: a preliminary study

PURPOSE

To evaluate the capability of amide proton transfer (APT) MR imaging for detection of prostate cancer that typically shows a higher tumor cell proliferation rate and cellular density leading to an MRI-detectable overall elevated mobile protein level in higher grade tumors.

MATERIALS AND METHODS

Twelve patients with biopsy-proven prostate cancer were imaged on a 3 Tesla MR imaging system before prostatectomy. APT-MR images were acquired by means of a single-slice single-shot turbo spin echo sequence with a saturation prepulse preparation using 33 different frequency offsets (-8 to 8 ppm, interval 0.5 ppm). For quantification we used the APT ratio (APTR) based on the asymmetry of the magnetization transfer ratio at 3.5 ppm in respect to the water signal. Tumor and peripheral zone benign regions of interest (ROIs) were delineated based on whole mount pathology slides after prostatectomy.

RESULTS

APTR in prostate cancer ROIs was 5.8% ± 3.2%, significantly higher than that in the peripheral zone benign regions (0.3% ± 3.2%, P = 0.002).

CONCLUSION

APT-MR imaging is feasible in prostate cancer detection and has the potential to discriminate between cancer and noncancer tissues.

Amide proton transfer imaging with improved robustness to magnetic field inhomogeneity and magnetization transfer asymmetry using saturation with frequency alternating RF irradiation

Amide proton transfer (APT) imaging has shown promise as an indicator of tissue pH and as a marker for brain tumors. Sources of error in APT measurements include direct water saturation, and magnetization transfer (MT) from membranes and macromolecules. These are typically suppressed by postprocessing asymmetry analysis. However, this approach is strongly dependent on B(0) homogeneity and can introduce additional errors due to intrinsic MT asymmetry, aliphatic proton features opposite the amide peak and radiation damping-induced asymmetry. Although several methods exist to correct for B(0) inhomogeneity, they tremendously increase scan times and do not address errors induced by asymmetry of the z-spectrum. In this article, a novel saturation scheme-saturation with frequency alternating RF irradiation (SAFARI)-is proposed in combination with a new magnetization transfer ratio (MTR) parameter designed to generate APT images insensitive to direct water saturation and MT, even in the presence of B(0) inhomogeneity. The feasibility of the SAFARI technique is demonstrated in phantoms and in the human brain. Experimental results show that SAFARI successfully removes direct water saturation and MT contamination from APT images. It is insensitive to B(0) offsets up to 180 Hz without using additional B(0) correction, thereby dramatically reducing scanning time.

Chemical exchange saturation transfer (CEST): what is in a name and what isn't?

Chemical exchange saturation transfer (CEST) imaging is a relatively new magnetic resonance imaging contrast approach in which exogenous or endogenous compounds containing either exchangeable protons or exchangeable molecules are selectively saturated and after transfer of this saturation, detected indirectly through the water signal with enhanced sensitivity. The focus of this review is on basic magnetic resonance principles underlying CEST and similarities to and differences with conventional magnetization transfer contrast. In CEST magnetic resonance imaging, transfer of magnetization is studied in mobile compounds instead of semisolids. Similar to magnetization transfer contrast, CEST has contributions of both chemical exchange and dipolar cross-relaxation, but the latter can often be neglected if exchange is fast. Contrary to magnetization transfer contrast, CEST imaging requires sufficiently slow exchange on the magnetic resonance time scale to allow selective irradiation of the protons of interest. As a consequence, magnetic labeling is not limited to radio-frequency saturation but can be expanded with slower frequency-selective approaches such as inversion, gradient dephasing and frequency labeling. The basic theory, design criteria, and experimental issues for exchange transfer imaging are discussed. A new classification for CEST agents based on exchange type is proposed. The potential of this young field is discussed, especially with respect to in vivo application and translation to humans.

Magnetization exchange with water and T1 relaxation of the downfield resonances in human brain spectra at 3.0 T

The use of water suppression for in vivo proton MR spectroscopy diminishes the signal intensities from resonances that undergo magnetization exchange with water, particularly those downfield of water. To investigate these exchangeable resonances, an inversion transfer experiment was performed using the metabolite cycling technique for non-water-suppressed MR spectroscopy from a large brain voxel in 11 healthy volunteers at 3.0 T. The exchange rates of the most prominent peaks downfield of water were found to range from 0.5 to 8.9 s(-1), while the T(1) relaxation times in absence of exchange were found to range from 175 to 525 ms. These findings may help toward the assignments of the downfield resonances and a better understanding of the sources of contrast in chemical exchange saturation transfer imaging.

Saturation power dependence of amide proton transfer image contrasts in human brain tumors and strokes at 3 T

Amide proton transfer (APT) imaging is capable of detecting mobile cellular proteins and peptides in tumor and monitoring pH effects in stroke, through the saturation transfer between irradiated amide protons and water protons. In this work, four healthy subjects, eight brain tumor patients (four with high-grade glioma, one with lung cancer metastasis, and three with meningioma), and four stroke patients (average 4.3 ± 2.5 days after the onset of the stroke) were scanned at 3 T, using different radiofrequency saturation powers. The APT effect was quantified using the magnetization transfer ratio (MTR) asymmetry at 3.5 ppm with respect to the water resonance. At a saturation power of 2 μT, the measured APT-MRI signal of the normal brain tissue was almost zero, due to the contamination of the negative conventional magnetization transfer ratio asymmetry. This irradiation power caused an optimal hyperintense APT-MRI signal in the tumor and an optimal hypointense signal in the stroke, compared to the normal brain tissue. The results suggest that the saturation power of 2 μT is ideal for APT imaging of these two pathologies at 3 T with the existing clinical hardware.

MR spectroscopy and spectroscopic imaging of the brain

Magnetic resonance spectroscopy (MRS) and the related technique of magnetic resonance spectroscopic imaging (MRSI) are widely used in both clinical and preclinical research for the non-invasive evaluation of brain metabolism. They are also used in medical practice, although their ultimate clinical value continues to be a source of discussion. This chapter reviews the general information content of brain spectra and commonly used protocols for both MRS and MRSI and also touches on data analysis methods and quantitation. The main focus is on proton MRS for application in humans, but many of the methods are also applicable to other nuclei and studies of animal models as well.

Detection of Amide and Aromatic Proton Resonances of Human Brain Metabolites Using Localized Correlated Spectroscopy Combined with Two Different Water Suppression Schemes

The purpose of the study was to demonstrate the J-coupling connectivity network between the amide, aliphatic, and aromatic proton resonances of metabolites in human brain using two-dimensional (2D) localized correlated spectroscopy (L-COSY). Two different global water suppression techniques were combined with L-COSY, one before and another after localizing the volume of interest (VOI). Phantom solutions containing several cerebral metabolites at physiological concentrations were evaluated initially for sequence optimization. Nine healthy volunteers were scanned using a 3T whole body MRI scanner. The VOI for 2D L-COSY was placed in the right occipital white/gray matter region. The 2D cross and diagonal peak volumes were measured for several metabolites such as N-acetyl aspartate (NAA), creatine (Cr), free choline (Ch), glutamate/glutamine (Glx), aspartate (Asp), myo-inositol (mI), GABA, glutathione (GSH), phosphocholine (PCh), phosphoethanolamine (PE), tyrosine (Tyr), lactate (Lac), macromolecules (MM) and homocarnosine (Car). Using the pre-water suppression technique with L-COSY, the above mentioned metabolites were clearly identifiable and the relative ratios of metabolites were calculated. In addition to detecting multitude of aliphatic resonances in the high field region, we have demonstrated that the amide and aromatic resonances can also be detected using 2D L-COSY by pre water suppression more reliably than the post-water suppression.

Changes in Proton Dynamics in Articular Cartilage Caused by Phosphate Salts and Fixation Solutions

The objective was to study the effect of phosphate salts and fixation solutions on the proton dynamics in articular cartilage in vitro. Microscopic magnetic resonance imaging (μMRI) T(2) anisotropy and nuclear magnetic resonance (NMR) double quantum-filtered (DQF) spectroscopy were used to study the full-thickness articular cartilage from several canine humeral heads. The in-plane pixel size across the depth of the cartilage tissue was 13 μm. The acid phosphate salt was an effective exchange catalyst for proton exchange in the cartilage with an organized structure of collagen fibrils, while the alkaline phosphate salt was not. For cartilage tissue containing less organized collagen fibrils, both acid and alkaline phosphate salts have no significant effect on the T(2) value at low concentration but decrease the T(2) value at high concentration. The solutions of NaCl, KCl, CaCl(2), and D-PBS were found to have no significant effect on T(2) and DQF in cartilage. This study demonstrates the ability to modify the proton exchange in articular cartilage using the solutions of phosphate salts. The ability to modify the proton exchange in articular cartilage can be used to modulate the laminar appearance of articular cartilage in MRI.

Identification of amide protons of glutathione in MR spectra of tumour cells

Signals attributable to amide protons and used in previous studies to measure intracellular pH were observed in the low-field region of the (1)H-MR spectra of four tumour cell lines: T98G, MCF-7, A172 and HeLa. The signals were more intense in the spectra of the two cell lines (T98G and MCF-7) characterised by higher concentrations of glutathione (GSH). After comparison with (1)H-MR spectra of GSH in solution at different pH values, the peaks were attributed to NHs of the Cys and Gly residues of GSH. Modification of the intracellular concentration of GSH by treatment with buthionine sulfoximine produced comparable decreases in the intensity of aliphatic signals of GSH and NHs under examination. The assignment was therefore confirmed.

Amide proton transfer imaging of 9L gliosarcoma and human glioblastoma xenografts

Amide proton transfer (APT) imaging is a variant of magnetization transfer (MT) imaging, in which the contrast is determined by a change in water intensity due to chemical exchange with saturated amide protons of endogenous mobile proteins and peptides. In this study, eight Fisher 344 rats implanted with 9L gliosarcoma cells and six nude rats implanted with human glioblastoma cells were imaged at 4.7 T. There were increased signal intensities in tumors in the APT-weighted images. The contrast of APT imaging between the tumor and contralateral brain tissue was about 3.9% in water intensity (1.49 +/- 0.66% vs -2.36 +/- 0.19%) for the more uniformly hypercellular 9L brain tumors, and it was reduced to 1.6% (-1.18 +/- 0.60% vs -2.77 +/- 0.42%) for the human glioblastoma xenografts that contained hypocellular zones of necrosis. The preliminary results show that the APT technique at the protein level may provide a unique MRI contrast for the characterization of brain tumors.

Correction for artifacts induced by B(0) and B(1) field inhomogeneities in pH-sensitive chemical exchange saturation transfer (CEST) imaging

Chemical exchange saturation transfer (CEST) imaging provides an indirect detection mechanism that allows quantification of certain labile groups unobservable using conventional MRI. Recently, amide proton transfer (APT) imaging, a variant form of CEST imaging, has been shown capable of detecting lactic acidosis during acute ischemia, providing information complementary to that of perfusion and diffusion MRI. However, CEST contrast is usually small, and therefore, it is important to optimize experimental conditions for reliable and quantitative CEST imaging. In particular, CEST imaging is sensitive to B(0) and B(1) field, while on the other hand; field inhomogeneities persist despite recent advances in magnet technologies, especially for in vivo imaging at high fields. Consequently, correction algorithms that can compensate for field inhomogeneity-induced measurement errors in CEST imaging might be very useful. In this study, the dependence of CEST contrast on field distribution was solved and a correction algorithm was developed to compensate for field inhomogeneity-induced CEST imaging artifacts. In addition, the proposed algorithm was verified with both numerical simulation and experimental measurements, and showed nearly complete correction of CEST imaging measurement errors caused by moderate field inhomogeneity.

High-resolution MAS NMR spectroscopy detection of the spin magnetization exchange by cross-relaxation and chemical exchange in intact cell lines and human tissue specimens

High-resolution magic-angle-spinning (HR-MAS) NMR spectroscopy detects resolved signals from membrane phospholipids and proteins in intact cell and tissue samples. MAS has the additional advantage of quenching spin-diffusion through a mutual "flip-flop" of neighbor spins by time-independent dipolar coupling as long as the dipolar coupling is "inhomogeneous." Under MAS, significant magnetization transfer (MT) was observed between water and each proton site in membrane phospholipid and between water and the NMR-observable protein proton signals. The MT rates between water and membrane phospholipids are lower than those between water and protein proton signals. The interaction of water to other small molecules is selective with the observation of MT from water to creatine, lactate, taurine, and glycine, but not to triglyceride, phosphocholine, choline, or myo-inositol. HR-MAS NMR allows the detection of a complete MT network between water and each proton group of creatine. Two creatine pools (one motion-restricted and one motion-free) were identified in skeletal muscle.

Amide proton transfer imaging of human brain tumors at 3T

Amide proton transfer (APT) imaging is a technique in which the nuclear magnetization of water-exchangeable amide protons of endogenous mobile proteins and peptides in tissue is saturated, resulting in a signal intensity decrease of the free water. In this work, the first human APT data were acquired from 10 patients with brain tumors on a 3T whole-body clinical scanner and compared with T1- (T1w) and T2-weighted (T2w), fluid-attenuated inversion recovery (FLAIR), and diffusion images (fractional anisotropy (FA) and apparent diffusion coefficient (ADC)). The APT-weighted images provided good contrast between tumor and edema. The effect of APT was enhanced by an approximate 4% change in the water signal intensity in tumor regions compared to edema and normal-appearing white matter (NAWM). These preliminary data from patients with brain tumors show that the APT is a unique contrast that can provide complementary information to standard clinical MRI measures.

Quantifying exchange rates in chemical exchange saturation transfer agents using the saturation time and saturation power dependencies of the magnetization transfer effect on the magnetic resonance imaging signal (QUEST and QUESP): Ph calibration for poly-L-lysine and a starburst dendrimer

The ability to measure proton exchange rates in tissue using MRI would be very useful for quantitative assessment of magnetization transfer properties, both in conventional MT imaging and in the more recent chemical exchange saturation transfer (CEST) approach. CEST is a new MR contrast mechanism that depends on several factors, including the exchange rate of labile protons in the agent in a pH-dependent manner. Two new methods to monitor local exchange rate based on CEST are introduced. The two MRI-compatible approaches to measure exchange are quantifying exchange using saturation time (QUEST) dependence and quantifying exchange using saturation power (QUESP) dependence. These techniques were applied to poly-L-lysine (PLL) and a generation-5 polyamidoamine dendrimer (SPD-5) to measure the pH dependence of amide proton exchange rates in the physiologic range. Data were fit both to an analytical expression and to numerical solutions to the Bloch equations. Results were validated by comparison with exchange rates determined by two established spectroscopic methods. The exchange rates determined using the four methods were pooled for the pH-calibration curve of the agents consisting of contributions from spontaneous (k0) acid catalyzed (ka), and base catalyzed (kb) exchange rate constants. These constants were k0 = 68.9 Hz, ka = 1.21 Hz, kb = 1.92 x 10(9) Hz, and k0 = 106.4 Hz, ka = 25.8 Hz, kb = 5.45 x 10(8) Hz for PLL and SPD-5, respectively, showing the expected predominance of base-catalyzed exchange for these amide protons.

Functional MRI of the kidney: tools for translational studies of pathophysiology of renal disease

Magnetic resonance imaging (MRI) provides exquisite anatomic detail of various organs and is capable of providing additional functional information. This combination allows for comprehensive diagnostic evaluation of pathologies such as ischemic renal disease. Noninvasive MRI techniques could facilitate translation of many studies performed in controlled animal models using technologies that are invasive to humans. Such a translation is being recognized as essential because many proposed interventions and drugs that prove efficacious in animal models fail to do so in humans. In this article, we review the state-of-the-art functional MRI technique as applied to the kidneys.

Microscopic images of intraspheroidal pH by 1H magnetic resonance chemical shift imaging of pH sensitive indicators

OBJECTIVE

We investigate microscopic pH heterogeneity within tumor spheroids using a novel 1H NMR methodology that provides high resolution measurements of intraspheroidal pH.

MATERIAL AND METHODS

High resolution microscopic images of intraspheroidal pH were obtained by 1H NMR using chemical shift selective excitation of the H2 resonance of imidazole added to the incubation medium. Imidazole accumulated in the intraspheroidal space in a pH dependent manner. Maps of intraspheroidal pH could be obtained by transforming pixel by pixel (32 x 32 micro) the regional variation of imidazole H2 intensity into a relative pH scale.

RESULTS

Our analysis revealed drastic intraspheroidal pH alterations depending on the size of the spheroid, ca. 0.6 pH units more acidic in the necrotic core than in the periphery, for spheroids of 600 mum diameter. The presence of concentric regions having similar intraspheroidal pH was consistently observed. The thickness of these regions decreased from pH 7.2 to pH 6.8 and increased below the latter pH value.

CONCLUSION

Our observations are compatible with the general model of spheroid growth where the more external layers of cells are in active growth and depict more alkaline pH values while the inner layers remain quiescent or evolve to a necrotic core, depicting more acidic pH values.

Suppression of lipid artifacts in amide proton transfer imaging

Amide proton transfer (APT) imaging is a type of chemical exchange saturation transfer imaging in which the amide protons of cellular proteins and peptides are saturated and detected via the water resonance. To study this effect, conventional magnetization transfer and direct saturation effects in the frequency-dependent water saturation spectrum (z-spectrum) need to be removed by asymmetry analysis with respect to the water frequency offset. When using echo planar imaging, it was found that unequal pericranial fat saturation at equidistant higher and lower frequencies with respect to water leads to a lipid artifact in APT asymmetry images. It is demonstrated that a chemical-shift-selective refocusing pulse in combination with crusher gradients can suppress this artifact and provide high-quality images.

pH imaging

Acid-base balance is altered in a variety of common pathologies, including COPD, ischemia, renal failure, and cancer. Because of robust cellular pH homeostatic mechanisms, most of the pathological alterations in pH are expressed as changes in the extracellular, systemic pH. There are data to indicate that altered pH is not simply an epiphenomenon of metabolic or physiologic imbalance but that chronic pH alterations can have important sequelae. MRSI and MRI measurements indicate that pH gradients of up to 1.0 pH unit can exit within 1-cm distance. Although measurement of blood pH can indicate systemic problems, it cannot pinpoint the lesion or quantitatively assess the magnitude of excursion from normal pHe. Hence, there is a need to develop pHe measurement methods with high spatiotemporal resolution. The two major approaches being investigated include magnetization transfer methods and relaxation methods. pH-dependent MT effects can observed with endogenous signals or exogenously applied CEST agents. While endogenous signals have the advantage of being fully noninvasive and relatively straightforward to apply, they lack a full biophysical characterization and dynamic range that might be afforded by future CEST agents. pH-dependent relaxivity also requires the injection or infusion of exogenous contrast reagents. In both MT and relaxographic approaches, the magnitude of the effect, and, thus, the ability to quantify pHe, depends on a spatially and temporally varying concentration of the CR. A number of approaches have been proposed to solve this problem and, once it is solved, pH imaging methods will be applicable to human clinical pathologies.

Quantitative description of proton exchange processes between water and endogenous and exogenous agents for WEX, CEST, and APT experiments

The proton exchange processes between water and solutes containing exchangeable protons have recently become of interest for monitoring pH effects, detecting cellular mobile proteins and peptides, and enhancing the detection sensitivity of various low-concentration endogenous and exogenous species. In this work, the analytic expressions for water exchange (WEX) filter spectroscopy, chemical exchange-dependent saturation transfer (CEST), and amide proton transfer (APT) experiments are derived by the use of Bloch equations with exchange terms. The effects of the initial states for the system, the difference between a steady state and a saturation state, and the relative contributions of the forward and backward exchange processes are discussed. The theory, in combination with numerical calculations, provides a useful tool for designing experimental schemes and assessing magnetization transfer (MT) processes between water protons and solvent-exchangeable protons. As an example, the case of endogenous amide proton exchange in the rat brain at 4.7 T is analyzed in detail.

Amide proton transfer (APT) contrast for imaging of brain tumors

In this work we demonstrate that specific MR image contrast can be produced in the water signal that reflects endogenous cellular protein and peptide content in intracranial rat 9L gliosarcomas. Although the concentration of these mobile proteins and peptides is only in the millimolar range, a detection sensitivity of several percent on the water signal (molar concentration) was achieved. This was accomplished with detection sensitivity enhancement by selective radiofrequency (RF) labeling of the amide protons, and by utilizing the effective transfer of this label to water via hydrogen exchange. Brain tumors were also assessed by conventional T(1)-weighted, T(2)-weighted, and diffusion-weighted imaging. Whereas these commonly-used approaches yielded heterogeneous images, the new amide proton transfer (APT) technique showed a single well-defined region of hyperintensity that was assigned to brain tumor tissue.

Oral choline decreases brain purine levels in lithium-treated subjects with rapid-cycling bipolar disorder: a double-blind trial using proton and lithium magnetic resonance spectroscopy

OBJECTIVES

Oral choline administration has been reported to increase brain phosphatidylcholine levels. As phospholipid synthesis for maintaining membrane integrity in mammalian brain cells consumes approximately 10-15% of the total adenosine triphosphate (ATP) pool, an increased availability of brain choline may lead to an increase in ATP consumption. Given reports of genetic studies, which suggest mitochondrial dysfunction, and phosphorus (31P) magnetic resonance spectroscopy (MRS) studies, which report dysfunction in high-energy phosphate metabolism in patients with bipolar disorder, the current study is designed to evaluate the role of oral choline supplementation in modifying high-energy phosphate metabolism in subjects with bipolar disorder.

METHODS

Eight lithium-treated patients with DSM-IV bipolar disorder, rapid cycling type were randomly assigned to 50 mg/kg/day of choline bitartrate or placebo for 12 weeks. Brain purine, choline and lithium levels were assessed using 1H- and 7Li-MRS. Patients received four to six MRS scans, at baseline and weeks 2, 3, 5, 8, 10 and 12 of treatment (n = 40 scans). Patients were assessed using the Clinical Global Impression Scale (CGIS), the Young Mania Rating Scale (YRMS) and the Hamilton Depression Rating Scale (HDRS) at each MRS scan.

RESULTS

There were no significant differences in change-from-baseline measures of CGIS, YMRS, and HDRS, brain choline/creatine ratios, and brain lithium levels over a 12-week assessment period between the choline and placebo groups or within each group. However, the choline treatment group showed a significant decrease in purine metabolite ratios from baseline (purine/n-acetyl aspartate: coef = -0.08, z = -2.17, df = 22, p = 0.030; purine/choline: coef = -0.12, z = -1.97, df = 22, p = 0.049) compared to the placebo group, controlling for brain lithium level changes. Brain lithium level change was not a significant predictor of purine ratios.

CONCLUSIONS

The current study reports that oral choline supplementation resulted in a significant decrease in brain purine levels over a 12-week treatment period in lithium-treated patients with DSM-IV bipolar disorder, rapid-cycling type, which may be related to the anti-manic effects of adjuvant choline. This result is consistent with mitochondrial dysfunction in bipolar disorder inadequately meeting the demand for increased ATP production as exogenous oral choline administration increases membrane phospholipid synthesis.

Using the amide proton signals of intracellular proteins and peptides to detect pH effects in MRI

In the past decade, it has become possible to use the nuclear (proton, 1H) signal of the hydrogen atoms in water for noninvasive assessment of functional and physiological parameters with magnetic resonance imaging (MRI). Here we show that it is possible to produce pH-sensitive MRI contrast by exploiting the exchange between the hydrogen atoms of water and the amide hydrogen atoms of endogenous mobile cellular proteins and peptides. Although amide proton concentrations are in the millimolar range, we achieved a detection sensitivity of several percent on the water signal (molar concentration). The pH dependence of the signal was calibrated in situ, using phosphorus spectroscopy to determine pH, and proton exchange spectroscopy to measure the amide proton transfer rate. To show the potential of amide proton transfer (APT) contrast for detecting acute stroke, pH effects were noninvasively imaged in ischemic rat brain. This observation opens the possibility of using intrinsic pH contrast, as well as protein- and/or peptide-content contrast, as diagnostic tools in clinical imaging.

Sensitive CEST agents based on nucleic acid imino proton exchange: detection of poly(rU) and of a dendrimer-poly(rU) model for nucleic acid delivery and pharmacology

It is shown that the exchange properties of the imino and hydroxyl protons of polyuridilic acid (poly(rU)) allow use of this compound as a chemical-exchange saturation transfer (CEST) contrast agent. A proton/proton sensitivity enhancement factor of over 5000 per imino proton allowed the detection of a few micromolar of polymer (2000 uridine units; 644 kD) with a 50% change in the water signal. The enhancement factor would increase further at even lower concentrations, opening up the submicromolar range. When poly(rU) was complexed to a dendrimer carrying 250 positive charges, the stoichiometry was approximately one RNA for 10 dendrimers. The sensitivity enhancement was reduced but remained large (2300/imino proton), bringing enhanced CEST visibility to the dendrimers. The net charge of the complex was positive, suggesting that the complexed dendrimers would still interact with cell membranes, and that the RNA-dendrimer complex could provide a model for a gene delivery system with good CEST visibility.

Mechanism of magnetization transfer during on-resonance water saturation. A new approach to detect mobile proteins, peptides, and lipids

The mechanism of magnetization transfer (MT) between water and components of the proton spectrum was studied ex vivo in a perfused cell system and in vivo in the rat brain (n = 5). Water was selectively labeled and spectral buildup consequential to transfer of longitudinal magnetization was followed as a function of time. At short mixing time (T(m)), nitrogen-bound solvent-exchangeable protons were observed, predominantly assigned to amide groups of proteins and peptides. At longer T(m), intramolecular nuclear Overhauser enhancement (NOE) was observed in the aliphatic proton region, leading to a mobile-macromolecule-weighted spectrum that resembles typical protein spectra described in the literature. This effect on the proton spectrum is distinct from that of classical off-resonance MT, which has been shown to be due to the immobile solid-like proton pool. When studying a solution of major brain metabolites under physiological concentrations and conditions (pH), no transfer effects were observed, in line with expectations based on reduced NOE effects in rapidly tumbling molecules and the fast proton exchange rates of amino, amine, SH, and OH groups. The spectral intensities of the amide protons may serve as indicators for pH and cellular levels of mobile proteins and peptides, while the aliphatic components are representative of several types of mobile macromolecules, including proteins, peptides, and lipids.

Renal and systemic pH imaging by contrast-enhanced MRI

Perturbations of renal and systemic pH accompany diseases of the kidney, such as renal tubular acidosis, and the ability to image tissue pH would be helpful to assess the extent and severity of such conditions. A dual-contrast-agent strategy using two gadolinium agents, the pH-insensitive GdDOTP(5-) and the pH-sensitive GdDOTA-4AmP(5-), has been developed to generate pH maps by MRI. The renal pharmacokinetics of the structurally dissimilar pH-insensitive contrast agents GdDTPA(2-) and GdDOTP(5-) were found to be similar. On that basis, and on the basis of similarity of structure and charge, the renal pharmacokinetics of GdDOTP(5-) and GdDOTA-4AmP(5-) were assumed to be identical. Dynamic T(1)-weighted images of mice were acquired for 1 hr each following boluses of GdDOTP(5-) and GdDOTA-4AmP(5-). The time-varying apparent concentration of GdDOTP(5-) and the time-varying enhancement in longitudinal relaxation rate following GdDOTA-4AmP(5-) were calculated for each pixel and used to compute pH images of the kidneys and surrounding tissues. MRI pH maps of control mice show acidic regions corresponding to the renal papilla, calyx, and ureter. Pretreatment of mice with the carbonic anhydrase inhibitor acetazolamide resulted in systemic metabolic acidosis and accompanying urine alkalinization that was readily detected by this dual-contrast-agent approach.

Phenylalanine can be detected in brain tissue of healthy subjects by 1H magnetic resonance spectroscopy

Transport of phenylalanine (Phe) and the other large neutral amino acids across the blood-brain barrier plays a crucial role in the pathogenesis of phenylketonuria (PKU). Thus, investigation of Phe transport kinetics by means of proton magnetic resonance spectroscopy (1H MRS) became an important research area in the mid 1990s. As 1H MRS measurements of brain phenylalanine are restricted to tissue concentrations above 100-150 micromol/kg wet weight, this approach was possible only in PKU patients, and comparison with healthy controls was not achieved. Using standardized single-dose oral Phe loading in three healthy subjects, it was shown that Phe values increase steeply, peak at about 1 h post load, and decrease thereafter. In a single case study, repetitive Phe loading was then performed to achieve a plateau of high blood Phe concentrations for several hours. It was demonstrated that detection and monitoring of brain Phe concentrations is feasible by means of 1H MRS. This approach constitutes a prerequisite for describing carrier kinetics in health.

MRI of the tumor microenvironment

The microenvironment within tumors is significantly different from that in normal tissues. A major difference is seen in the chaotic vasculature of tumors, which results in unbalanced blood supply and significant perfusion heterogeneities. As a consequence, many regions within tumors are transiently or chronically hypoxic. This exacerbates tumor cells' natural tendency to overproduce acids, resulting in very acidic pH values. The hypoxia and acidity of tumors have important consequences for antitumor therapy and can contribute to the progression of tumors to a more aggressive metastatic phenotype. Over the past decade, techniques have emerged that allow the interrogation of the tumor microenvironment with high resolution and molecularly specific probes. Techniques are available to interrogate perfusion, vascular distribution, pH, and pO(2) nondestructively in living tissues with relatively high precision. Studies employing these methods have provided new insights into the causes and consequences of the hostile tumor microenvironment. Furthermore, it is quite exciting that there are emerging techniques that generate tumor image contrast via ill-defined mechanisms. Elucidation of these mechanisms will yield further insights into the tumor microenvironment. This review attempts to identify techniques and their application to tumor biology, with an emphasis on nuclear magnetic resonance (NMR) approaches. Examples are also discussed using electron MR, optical, and radionuclear imaging techniques.

Effects of intracellular pH, blood, and tissue oxygen tension on T1rho relaxation in rat brain

The effects of intracellular pH (pH(i)), paramagnetic macroscopic, and microscopic susceptibility on T(1) in the rotating frame (T(1rho)) were studied in rat brain. Intracellular acidosis was induced by hypercapnia and pH(i), T(1rho), T(2), diffusion, and cerebral blood volume (CBV) were quantified. Taking into account the CBV contribution, a prolongation of parenchymal T(1rho) by 4.5% was ascribed to a change in tissue water relaxation caused by a one unit drop in pH(i). Blood T(1rho) was found to prolong linearly with blood oxygenation saturation (Y). The macroscopic susceptibility contribution to parenchymal T(1rho) was assessed both through BOLD and an iron oxide contrast agent, AMI-227. The T(1rho) data from these experiments could be described by intravascular effects with insignificant effects of susceptibility gradients on tissue water. Tissue oxygen tension (PtO(2)) was manipulated and monitored with microelectrodes to assess its plausible contribution to microscopic susceptibility and relaxation. Parenchymal T(1rho) was virtually unaffected by variations in the PtO(2), but T(1) was shortened in hyperoxia and T(2) showed a negative BOLD effect in hypoxia. It is demonstrated that pH(i) directly modulates tissue T(1rho), possibly through its effect on proton exchange; however, neither BOLD nor PtO(2) directly influence tissue T(1rho). The observations are discussed in the light of physicochemical mechanisms contributing to the ischemic T(1rho) changes.

Localized proton spectroscopy without water suppression: removal of gradient induced frequency modulations by modulus signal selection

Most Magnetic Resonance Spectroscopy (MRS) localization methods can generate gradient vibrations at acoustic frequencies and/or magnetic field oscillation, which can cause a time-varying magnetic field superimposed onto the static one. This effect can produce frequency modulations of the spectral resonances. When localized MRS data are acquired without water suppression, the associated frequency modulations are manifested as a manifold of spurious peaks, called sidebands, which occur symmetrically around the water resonance. These sidebands can be larger than the small metabolite resonances and can present a problem for the quantitation of the spectra, especially at short echo times. Furthermore, the resonance lineshapes may be distorted if any low frequency modulations are present. A simple solution is presented which consists of selecting the modulus of the acquired Free Induction Decay (FID) signal. Since the frequency modulations affect only the phase of the FID signal, the obtained real spectrum of the modulus is free from the spurious peaks where quantitative results may be directly obtained. Using this method, the distortions caused by the sidebands are removed. This is demonstrated by processing proton MRS spectra acquired without water suppression collected from a phantom containing metabolites at concentrations comparable to those in human brain and from a human subject using two different localization methods (PRESS and Chemical Shift Imaging PRESS-(CSI)). The results obtained illustrate the ability of this approach to remove the spurious peaks. The corrected spectra can then be fit accurately. This is confirmed by the results obtained from both the relative and the absolute metabolites concentrations in phantoms and in vivo.

Proton exchange as a relaxation mechanism for T1 in the rotating frame in native and immobilized protein solutions

T1 relaxation in the rotating frame (T1rho) is a sensitive magnetic resonance imaging (MRI) contrast for acute brain insults. Biophysical mechanisms affecting T1rho relaxation rate (R1rho) and R1rho dispersion (dependency of R1rho on the spin-lock field) were studied in protein solutions by varying their chemical environment and pH in native, heat-denatured, and glutaraldehyde (GA) cross-linked samples. Low pH strongly reduced R1rho in heat-denatured phantoms displaying proton resonances from a number of side-chain chemical groups in high-resolution 1H NMR spectra. At pH of 5.5, R1rho dispersion was completely absent. In contrast, in the GA-treated phantoms with very few NMR visible side chain groups, acidic pH showed virtually no effect on R1rho. The present data point to a crucial role of proton exchange on R1rho and R1rho dispersion in immobilized protein solution mimicking tissue relaxation properties.

Administration and (1)H MRS detection of histidine in human brain: application to in vivo pH measurement

Measurement of histidine in vivo offers the potential for tissue pH measurement using routinely performed (1)H MR spectroscopy. In the brain, however, histidine concentrations are generally too low for reliable measurement. By using oral loading of histidine, this study demonstrates that brain concentrations can be significantly increased, enabling detection of histidine by localized (1)H MR measurements and making in vivo pH measurement possible. In studies carried out on healthy human subjects at 1.5 T, a consistent spectral quality downfield from water was achieved using a PRESS sequence at short echo times. Measurements at different TE values helped to characterize the downfield spectral region. Histidine loading of 400 mg/kg of body weight increased brain histidine levels by approximately 0.8 mM, with maximum histidine concentration reached 4 to 7 hr after consumption. The pH calculated from histidine resonances was 6.96, and a hyperventilation study demonstrated the potential for measuring altered pH.

Mapping brain metabolites using a double echo-filter metabolite imaging (DEFMI) technique

A double echo-filter metabolite imaging (DEFMI) technique was developed for spatial mapping of low-concentration metabolites in the human brain. This imaging technique simultaneously acquires two images from two individual metabolites, respectively, using conventional imaging acquisition. It provides (i) efficient water and lipid suppressions, (ii) the capability of collecting high-resolution metabolite images within a short time and creating a ratio image from two interesting metabolites within a single experiment, and (iii) flexibility and simplicity for experimental setup and data processing. The technique was examined by both phantom and human brain experiments at 4 Tesla. The results reveal that the DEFMI technique is promising for applications in metabolism studies aimed at investigating physiological and pathological questions.

Molecular aspects of magnetic resonance imaging and spectroscopy

Magnetic resonance imaging (MRI) is a well known diagnostic tool in radiology that produces unsurpassed images of the human body, in particular of soft tissue. However, the medical community is often not aware that MRI is an important yet limited segment of magnetic resonance (MR) or nuclear magnetic resonance (NMR) as this method is called in basic science. The tremendous morphological information of MR images sometimes conceal the fact that MR signals in general contain much more information, especially on processes on the molecular level. NMR is successfully used in physics, chemistry, and biology to explore and characterize chemical reactions, molecular conformations, biochemical pathways, solid state material, and many other applications that elucidate invisible characteristics of matter and tissue. In medical applications, knowledge of the molecular background of MRI and in particular MR spectroscopy (MRS) is an inevitable basis to understand molecular phenomenon leading to macroscopic effects visible in diagnostic images or spectra. This review shall provide the necessary background to comprehend molecular aspects of magnetic resonance applications in medicine. An introduction into the physical basics aims at an understanding of some of the molecular mechanisms without extended mathematical treatment. The MR typical terminology is explained such that reading of original MR publications could be facilitated for non-MR experts. Applications in MRI and MRS are intended to illustrate the consequences of molecular effects on images and spectra.

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.

Proton nuclear magnetic resonance measurement of p-boronophenylalanine (BPA): a therapeutic agent for boron neutron capture therapy

Noninvasive in vivo quantitation of boron is necessary for obtaining pharmacokinetic data on candidate boronated delivery agents developed for boron neutron capture therapy (BNCT). Such data, in turn, would facilitate the optimization of the temporal sequence of boronated drug infusion and neutron irradiation. Current approaches to obtaining such pharmacokinetic data include: positron emission tomography employing F-18 labeled boronated delivery agents (e.g., p-boronophenylalanine), ex vivo neutron activation analysis of blood (and very occasionally tissue) samples, and nuclear magnetic resonance (NMR) techniques. In general, NMR approaches have been hindered by very poor signal to noise achieved due to the large quadrupole moments of B-10 and B-11 and (in the case of B-10) very low gyromagnetic ratio, combined with low physiological concentrations of these isotopes under clinical conditions. This preliminary study examines the feasibility of proton NMR spectroscopy for such applications. We have utilized proton NMR spectroscopy to investigate the detectability of p-boronophenylalanine fructose (BPA-f) at typical physiological concentrations encountered in BNCT. BPA-f is one of the two boron delivery agents currently undergoing clinical phase-I/II trials in the U.S., Japan, and Europe. This study includes high-resolution 1H spectroscopic characterization of BPA-f to identify useful spectral features for purposes of detection and quantification. The study examines potential interferences, demonstrates a linear NMR signal response with concentration, and presents BPA NMR spectra in ex vivo blood samples and in vivo brain tissues.