Magnetic field gradients in the MRI suite and their effects on aneurysm clips.

We studied magnetic field intensity in the magnetic resonance imaging suite at our hospital and its possible effect on several different types of aneurysm clips, including one Heifetz 17-7 PH, six Heifetz Elgiloy, one Mayfield, six Perneczky, fifteen Sugita, one Sundt-Kees Variangle, four Variangle-McFadden and fifteen Yasargil clips. We carefully observed the clips for any translational or rotational movements along the path from the door towards the magnetic resonance imaging gantry. The magnetic field strength was 0.04 kiloGauss at the entrance of the room, with an acute increase of magnetic strength at 310 cm away from the entrance to the room, 90 cm to the entrance of the gantry. The magnetic strength continued to increase at a rate of 1.0-1.5 kiloGauss for every 20 cm up to the entrance to the gantry. No movement was observed in any of the clips at the entrance to the suite except for the Heifetz 17-7 PH clip, which showed small movement in the longitudinal plane of the clip. At the entrance to the gantry, the Heifetz 17-7 PH, Sundt-Kees Variangle, and Mayfield clips were aligned on the walls of the test container perpendicular to the magnetic bore. The, Heifetz Elgiloy, Perneczky, Sugita, Variangle-McFadden, and Yasargil clips showed no movement throughout the path of the stretcher or near the gantry.

Determination of regional cerebral oxygen consumption in the human: 17O natural abundance cerebral magnetic resonance imaging and spectroscopy in a whole body system.

17O natural abundance imaging in a whole body imager is demonstrated using standard MRI spectrometer and 1H imaging methods. A novel design of a highly sensitive 17O/1H doubly tuned surface head coil is shown. The head probe allows simultaneous acquisition of 17O and 1H images using a single coil. The relatively low 17O signal intensity due to the low natural abundance of 17O (0.037 atom percent) is partially compensated by fast repetition of the pulse sequence, achievable due to the short spin lattice relaxation time, T1. A small number of signal averages (e.g., NEX = 50) is sufficient for obtaining images having signal to noise of about 5:1. Due to the short longitudinal relaxation time of 17O, i.e., 2-5 msec, short TR values can be used. 128 phase encoding steps with TR = 10-25 msec correspond to total acquisition time of 1 to 2.5 min. Due to the small gyromagnetic ratio of 17O and the relatively small gradients in a standard whole body system, i.e. 0.5 G/cm, the image in-plane resolution is about 3 mm and a slice thickness of 15 mm. In vivo 17O MRS and MRI natural abundance spectroscopic signals and images of human brain have been observed. The transverse relaxation time, T2 was found to be 2.00 +/- 0.17 msec at 1.5 T. MRS 17O measurements of signal intensity in the occipital cortex during inhalation of oxygen gas, 21.8% 17O enriched, showed a maximum signal enhancement of 25% within the inhalation period. The rate of the metabolism of oxygen (CMRO2) in the occipital cortex was found to be 1.5 mumole/(g tissue) in good agreement with the value of 1.435 mumole/(g tissue) given in the literature. Current measurements using higher 17O enrichments and larger quantities of 17O enriched oxygen gas will enhance resolution and provide more accurate determination of the rate of oxygen metabolism rate and blood flow. The potential of 17O imaging is thus demonstrated in physiological in vivo studies of cerebral metabolism of oxygen and blood flow.

Determination of the rate of cerebral oxygen consumption and regional cerebral blood flow by non-invasive 17O in vivo NMR spectroscopy and magnetic resonance imaging. Part 2. Determination of CMRO2 for the rat by 17O NMR, and CMRO2, rCBF and the partition coefficient for the cat by 17O MRI.

The rate of oxygen consumption (CMRO2) in intact rat and cat brains was calculated by novel data analysis methods from data obtained in in vivo 17O NMR spectroscopy and imaging inhalation studies. Data analysis methods of 17O inhalation measurements are applied to the calculation of CMRO2, regional cerebral blood flow (rCBF), the reflow (R), the arterial venous difference (AVD) and the partition coefficient (lambda). Several of the applied methods for the determination of CMRO2 do not require measurements of regional cerebral blood flow and H2 17O arterial concentration. The proposed methods have been tested, and the results obtained by the different methods are in very good agreement. It is shown that 17O NMR is unique in providing the rate of blood water flow, where the natural abundance 17O NMR signal consists of an internal reference and that lambda is essentially 1. The average values of CMRO2 for rats and cats were found to be 2.09 +/- 0.35 and 1.18 +/- 0.58 (mumol/O2/g tissue)/min, respectively. The average value for rCBF for the cat was found to be 0.38 +/- 0.12 [(mg/g)/min]. The average value for lambda was found to be 1.00 +/- 0.04. The ratio of AVD due to organs other than the brain to AVD due to the brain is smaller than 1 for the rat and the cat. The time and spatial resolution accuracy of the spectroscopic and imaging methods are compared, discussed and statistically analysed. It is concluded that the accuracy of determination of CMRO2 for the rat is higher than for the cat by a factor of 8.

Determination of the rate of cerebral oxygen consumption and regional cerebral blood flow by non-invasive 17O in vivo NMR spectroscopy and magnetic resonance imaging: Part 1. Theory and data analysis methods.

Theory and novel data analysis methods of 17O inhalation measurements are presented for the calculation of CMRO2, regional cerebral blood flow (rCBF), the reflow (R), the arterial venous difference (AVD) and the partition coefficient (lambda). Several of the methods proposed for the determination of CMRO2 do not require measurements of regional cerebral blood flow and H2(17)O arterial concentration. All methods of analysis are based on the Kety-Schmidt approach.

In vivo 17O NMR study of rat brain during 17O2 inhalation.

In vivo 17O NMR has been used to monitor the H2(17)O concentration in rat brain during inhalation of 17O2. The results are discussed in terms of oxygen consumption in the brain and recirculation into the brain of H2(17)O produced in other organs.

In vivo measurement of cerebral oxygen consumption and blood flow using 17O magnetic resonance imaging.

We used 17O NMR imaging techniques to measure the H2(17)O concentration in a 0.8-ml voxel in the cat brain following injection of an arterial bolus of enriched H2(17)O and during inhalation of enriched 17O2. We also measured the H2(17)O concentration in arterial blood during 17O2 inhalation. The data from the first measurement were used to calculate the blood flow in the voxel. The data from all three measurements were combined to calculate the oxygen consumption in the voxel. The values of cerebral blood flow and oxygen consumption calculated with 17O NMR techniques agree reasonably well with values calculated for a similar region of the cat brain using autoradiographic techniques.

17O, 14N and 15N n.m.r. studies of the Co2+ complexes of cyclo(Pro17O-Gly15N) and cyclo(Gly17O-Pro) in aqueous solution.

The complexation of cyclo(Pro17O-Gly15N) and cyclo(Gly17O-Pro) with Co2+ ions has been studied by 17O, 14N and 15N n.m.r. spectroscopy in aqueous solution. 17O, 14N and 15N transverse relaxation times and chemical shifts were measured as a function of temperature. The 17O n.m.r. studies unequivocally demonstrate that the cobaltous ion binds to the peptide oxygen of both compounds. The hyperfine coupling constant and the peptide residence times were found to be A = -0.165 MHz and -0.145 MHz, tau m = 16, and 92 microseconds for cyclo(Pro17O-Gly15N) and cyclo(Gly17O-Pro), respectively. The 14N and 15N studies of labeled cyclo(Pro17O-Gly15N) do not indicate binding at either the Gly15N or the Pro14N site.

Synthesis of 17O isotope labeled Leu-enkephalin and 17O n.m.r.

Oxygen-17 isotope was introduced into the alpha-carboxyl group of glycine, 1-phenylalanine, 1-leucine and 1-tyrosine by acid catalyzed exchange of 17O from H2O(17) or by acid hydrolysis of respective amino acid methyl esters in H2O(17). Quantitative enrichment of glycine was achieved by acid hydrolysis of amino acetonitrile in H2O(17). For alpha-amino protection in amino acids t-butoxycarbonyl (Boc) group was employed for 17O labeled enkephalin synthesis. Five analogues of Leu-enkephalins (I-V) labeled with 17O at different amino acid residues were synthesized by solid phase method. 17O n.m.r. spectra were measured at 24.4 and 67.8 MHz for Leu-enkephalins 17O labeled at Gly2 and Phe4 positions. A downfield shift was observed for 17O labeled Gly2 Leu-enkephalin upon heating. This shift is indicative of the rupture of intramolecular hydrogen bonds. The preliminary results confirm the hypothesis that an intramolecular hydrogen bond exists between the carbonyl group of Gly2 and NH group of Leu5.

17O n.m.r. studies of amino acids in the solid state, in single- and polycrystalline forms.

A single crystal of 17O enriched alpha-glycine was grown by slow evaporation of aqueous solution. 17O n.m.r. studies of the alpha-glycine molecule in single crystalline form revealed five 17O transitions each consisting of two lines due to inequivalency of the oxygen atoms in the unit cell, with each of these lines revealing a dipolar interaction between the 17O and the nearest hydrogen atom. The spectral width was found to be of the order of magnitude of MHz and the linewidths of the order of magnitude of 2 kHz.

17O and 14N n.m.r. studies of the Co (II) interaction with cyclo(Ala*-Ala) in aqueous solution.

The complexation of cyclo(Ala*-Ala) with the cobaltous ions in aqueous solution was investigated by 17O and 14N n.m.r. spectroscopy. The 17O and 14N transverse relaxation time (T2p) and chemical shift (delta omega a) of cyclo(Ala*-Ala) were measured as a function of the temperature at pH = 7.03 +/- 0.02, and pH = 6.45 +/- 0.02, and as a function of pH at room temperature. No effects of pH on the transverse relaxation time and chemical shift were observed. Complementary 17O studies of the solvent water molecules were also carried out. The hyperfine coupling constant and the entropy and enthalpy of activation for the exchange of cyclo(Ala*-Ala) and water molecules between the coordinated and noncoordinated states were determined by least-square fit of theoretical equation for the chemical shift delta omega a to experimental data. The hyperfine coupling constant of the peptide bound oxygen was determined to be (-1.6 +/- 0.1) X 10(5) Hz and the entropy and enthalpy (32.0 +/- 3.0) kJ/mol and (-12.0 +/- 1.0) e.u, respectively. Information obtained from 17O n.m.r. study allows some inferences concerning the probable coordination sphere of the cobaltous ion. There are three types of complexes: Co(H2O)6(2+), CoL X 5H2O and CoL2 X 4H2O, with relative concentrations 19.9%, 2.9%, and 77.2%, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Oxygen-18 isotope labeling and its effect on carbon-13 chemical shifts of the peptide bond.

The effects of 18O isotopes on 13C NMR chemical shifts of peptide carbonyl carbons were found to be 0.028 ppm for glyclyglycine and 0.029 ppm for glycylproline at 50.31 MHz.

Isotopic labeling of tyrosine, followed by 17O n.m.r.

A simple chemical procedure has been developed in order to introduce oxygen-17 labels into the carboxylic and phenolic sites of L-tyrosine. Detailed studies of the 17O n.m.r. chemical shift as a function of pH reveal an unusually large titration shift upon the deprotonation of the phenol group. This result suggests that 17O n.m.r. may contribute useful information about side chain properties in peptides and proteins.

17O NMR investigation of the hydration of L-alanine and L-proline in water/Me2SO mixtures.

The hydration state of L-Alanine and L-Proline has been assessed via 17O NMR. At neutral and basic pH, two water molecules are hydrogen bonded at the carboxylate group, one to each oxygen, whereas a third water molecule is hydrogen bonded to the protonated COOH group at acidic pH, via the hydroxyl hydrogen. The possible formation of dimers and/or higher complexes in DMSO is indicated not only from the chemical shift but also from the linewidth of the amino acids.

Oxygen-17 n.m.r. of peptides.

Peptide-17O chemical shifts of linear dipeptides with and without protecting groups in H2O, CH3OH, CH2Cl2, CHCl3, CCl4, CH3CN and DMSO were between 256-350 ppm downfield from external water. Increasing solvent H-bond donating ability correlated with shifts to higher field. The 17O resonance of several cyclic dipeptides appeared at higher field relative to comparable linear dipeptides (303-317 p.p.m. vs. 327-337 p.p.m.). Separate signals were simultaneously observed by 13C and 17O n.m.r. for cis and trans N-tert.-butyl-formamide in binary mixtures with H2O, (CH3)2CO, and CCl4. The differences in the 17O nuclear screening of the amide isomers and most probably for cis and trans peptides were independent of contributions from H-bonding at the amide or peptide linkage, apparently reflecting differences between geometric isomers in electron distribution and through space effects. Peptide-17O of Gly-Ala, Gly-Leu and Gly-Glu in aqueous solution experienced upfield shifts of 6-12 p.p.m. and 12-16 p.p.m. upon deprotonation of the C-terminal COOH and of the N-terminal NH3+ groups respectively. These observations were rationalized in terms of the attendant changes in substituent effects, especially on the pi electron donating ability of the N atom at the peptide linkage and increased partial negative charge on the peptide oxygen. Temperature studies of peptide-17O of Gly-Ala between pH 1.5-9.0 revealed a chemical shift coefficient of 0.08 p.p.m./degree K and similar behavior of T1 and T2 relaxation times. Ea for molecular rotation was 5 kcal/mol between 301-331 degrees K. Rotational correlation times, tau c, were within the range expected from the Stokes-Einstein relation.

Hydrogen bonds in the tripeptide Pro-Leu-Gly-NH2. 17O and 1H studies.

17O-NMR measurements of labeled Pro-Leu-Gly-NH2 were carried out at different pH levels and in mixed solvents of water/acetonitrile. Complementary studies of the amide protons were carried out in acetonitrile-d3. Only the prolyl C = 17O group was sensitive to the pH level. Protonation of the amine group resulted in an upfield chemical shift of 18 ppm. The chemical shifts of each of the three oxygen sites was sensitive to the ratio water:acetonitrile. Solvent composition dependence of the chemical shift and linewidth suggests that the prolyl C = 17O is involved in intramolecular hydrogen bond formation when Pro-Leu-Gly-NH2 is dissolved in acetonitrile, while in water there is no intramolecular H bond.

Carbonyl-17O n.m.r. of amino acid and peptide carboxamide and methyl ester derivatives.

Specific carbonyl enrichment with 17O of amino acid OMe esters by up to 10(3) times over natural abundance was affected by treating [17O]-alpha-COOH amino acids with SOCl2 in MeOH. Carbonyl-[17O]-Gly-NH2, Gly-NHCH3 and Gly-N(CH3)2 were obtained from [17O]-Gly-OMe by (methyl)aminolysis with NH3, CH3NH2 and (CH3)2NH gases respectively. Peptide [17O]-carboxamides were prepared by (methyl)aminolysis of Z-Pro-Leu-[17O]-Gly-OMe followed by catalytic hydrogenation to remove the Z group. 17O chemical shifts of amino acid and peptide carboxamides in H2O, MeOH, CH3CN and DMSO were 260-324 p.p.m. downfield relative to H2O, depending on alpha-NH2 ionization, substitution on both amino groups and solvent H bonding, primarily to amide oxygen. Introduction of an amide-N methyl group usually caused upfield shifts (approx. -10 p.p.m.), attributed to elimination of one NH bond, while a second N methylation had the opposite effect. Amino acid and peptide OMe ester carbonyl-[17O] resonances appeared 326-359 p.p.m. downfield relative to H2O reflecting on side chain interactions, state of alpha-NH2 ionization and H-bonding with the solvent. Effective rotational correlation times for glycine and peptide carboxamide-[17O], calculated from T2 relaxation data, were of similar magnitude to values derived from solution properties and depended on the molecular weight and solvent viscosity.

Labeling of amino acids and peptides with isotopic oxygen as followed by 17O-N.M.R.

17O was introduced into the respective alpha- and gamma-COOH groups of Boc-Gly and Boc-Glu by saponification of the corresponding O-methyl esters with 1N NaOH in H2 17O. Other 17O enriched Boc-amino acids were prepared by acid catalyzed exchange into the amino acid alpha-COOH group followed by t-butyloxycarbonylation with t-butyl S-4, 6-dimethylpyrimidin-2-ylthio carbonate. Final enrichment, by approximately three orders of magnitude over natural abundance, was 60-100% of the possible maximum. The synthesis of [17O]-Gly-Ala, [17O]-Gly-Leu and [17O]-Gly-Glu by DCC/HBT mediated coupling of Boc-Gly-[17O]-alpha-COOH with amino acid-O-t-butyl esters followed by deprotection with HCl/EtOAc proceeded without undue loss of the isotope. Boc-[17O]-Pro-Leu-Gly-NH2 was prepared by a similar procedure. [Tyr2-17O]-, [Pro7-17O]- and [Gly4-17O]-oxytocin were synthesized using solid phase support. 17O-chemical shifts of synthetic intermediates and of the final products were as expected for each functional group. Linewidth data correlate with the molecular weights of the compounds prepared.