Biomedical subjects

G Navon

Publications and source records attributed to G Navon.

At least 19 recordsLinked to original sources

PubMed2005

Self-diffusion anisotropy of water in sheep Achilles tendon.

The principal values of the diffusion tensor of free water in the pores of sheep Achilles tendon were determined. For this purpose, the azimuthally angular dependence of the self-diffusion coefficient was measured using a radiofrequency tilt coil and pulsed-field-gradient stimulated-echo (PFGSE) NMR. Combining the PFGSE with multiple acquisitions of Hahn echoes using the Carr-Purcell-Meiboom-Gill pulse sequence reduced the measuring time. The diffusion measurements revealed two diffusion process characterized by a fast and a slow effective diffusion coefficient. A model which describes the stimulated-echo amplitude, encoded by the water diffusion and magnetization transfer, was used for evaluation of the fast diffusion coefficients. The fast diffusion process characterizes the water molecules in pores surrounding the collagen fibrils. The diffusion coefficients characterizing the fast process show a well-defined anisotropy. The principal values of the diffusion tensors were determined assuming the elongated pores to be oriented parallel to the tendon fibrils and thus the orientation distribution function of the pores followed that of the collagen fibrils. The average aspect ratio of pores was estimated from the principal values of the water diffusion tensor and is suitable to characterize quantitatively the changes in tendon morphology due to healing or aging. The methods in this investigation can also be applied to measurements of the diffusion anisotropy using ex situ NMR sensors.

Achilles Tendon↗

PubMed2003

Anisotropy of collagen fiber orientation in sheep tendon by 1H double-quantum-filtered NMR signals.

The anisotropy of the angular distribution of collagen fibrils in a sheep tendon was investigated by 1H double-quantum (DQ) filtered NMR signals. Double-quantum build-up curves generated by the five-pulse sequence were measured for different angles between the direction of the static magnetic field and the axis of the tendon plug. Proton residual dipolar couplings determined from the DQ build-up curves in the initial excitation/reconversion time regime which mainly represent the bound water are interpreted in terms of a model of spin-1/2 pairs with their internuclear axes oriented on average along the fibril direction in the presence of proton exchange. The angular distribution of collagen fibrils around the symmetry axis of the tendon measured by the anisotropy of the residual dipolar couplings was described by a Gaussian function with a standard deviation of 12 degrees +/-1 degrees and with the center of the distribution at 4 degrees +/-1 degrees. The existence of this distribution is directly reflected in the finite value of the residual dipolar couplings at the magic angle, the value of the angular contrast, and the oscillatory behavior of the DQ build-up curves. The 1H residual dipolar couplings were also measured from the doublets recorded by the DQ-filtered signals. From the angular dependence of the normalized splitting the angular distribution of the collagen fibrils was evaluated using a Gaussian function with a standard deviation of 19 degrees +/-1 degrees and with the center of distribution at 2 degrees +/-1 degrees. The advantages and disadvantages of these approaches are discussed.

Achilles Tendon↗

PubMed2001

Multiquantum filters and order in tissues.

In ordered systems, where the molecular motion is anisotropic, quadrupolar and dipolar interactions are not averaged to zero. In such cases, double quantum (DQ) coherences can be formed. This review deals mainly with the effect of anisotropic motion of water molecules and sodium ions in intact biological tissues on (2)H, (1)H and (23)Na NMR spectroscopy and its application to NMR imaging (MRI). Double quantum filtered (DQF) spectra of water molecules and sodium ions were detected in a variety of ordered biological tissues. In collagen-containing tissues such as ligaments, tendons, cartilage, skin, blood vessels and nerves, the DQ coherences are formed as a result of the interaction with the collagen fibers. In red blood cells and presumably also in nerve axons it stems from the interaction with the cytoskeleton. For (23)Na, an I = 3/2 nucleus, the DQ coherences can also be formed in isotropic media. By a judicial choice of the pulse angle in the DQ pulse sequence only the DQ coherences arising from anisotropic motion are detected. For I = 1 nuclei such as 2H, DQF spectra can be observed only in ordered structures. Thus, the observation of 2H DQF spectra is an indication of order. The same is true for pairs of equivalent 1H nuclei. The dependence of the DQF signal on the creation time of the double quantum coherences is characteristic to each tissue and allows signals to be resolved from different tissues by performing the measurements at different creation times. In this way, the 2H DQF signals of the different compartments of sciatic nerve were resolved and water diffusion in each compartment was studied independently. In the axon, the diffusion was heavily restricted perpendicular to the axon's long axis, a result from which the axon diameter could be deduced. In blood vessel walls, this characteristic enabled the different layers of the vessel to be viewed and studied under strain. For 2H, a DQF spectroscopic imaging sequence was used to study the orientation of the collagen fibers in the different zones of articular cartilage and bone plug. The effect of pressure on the fibers and their return to equilibrium was studied as well. In blood vessels, a DQF image was obtained and strain maps of the different layers were calculated. The efficiency of the 1H DQF imaging technique was demonstrated on a phantom of rat tail where only the four tendons were detected at short creation times. 1H DQF imaging and spectroscopy followed the healing of a rabbit's ruptured Achilles tendon and the results were far more sensitive to the process than conventional imaging. Finally, the method was implemented on a commercial whole body MRI spectrometer. Images of human wrist and ankle showed a positive contrast for the tendons and ligaments, indicating the potential of the method for clinical imaging. (c) 2001 John Wiley & Sons, Ltd.

Animals↗

PubMed2001

NMR spectroscopic characterization of sarcolemmal permeability during myocardial ischemia and reperfusion.

This study aims to characterize the pattern of membrane disintegration during myocardial ischemia and reperfusion. Intracellular volumes were measured by 1H and 59Co NMR in isolated rat hearts during 10, 30 and 60 min of total ischemia and 30 min of reperfusion at normothermia. Perfusion with hypo-osmotic medium (210 mosm/l) increased intracellular water from 2.50+/-0.06 to 3.07+/-0.07 ml/g dry weight (P<0.001) during pre-ischemia. Hypo-osmotic swelling decreased by 16+/-3, 32+/-6 and 44+/-11% of the pre-ischemic value after 10, 30 and 60 min of ischemia (n.s., P<0.005, P<0.001) respectively, indicating that membrane permeabilization facilitated efflux of osmolytes and counterbalanced the osmotic driving force for water influx. Hypo-osmotic swelling decreased during 30 min of reperfusion by 18+/-5% in all groups (P<0.0.005 v post-ischemia). The volume of distribution of the extracellular marker cobalticyanide increased by more than 3.2+/-0.4 and 5.8+/-0.5% of the intracellular space after 30 and 60 min of ischemia respectively (P<0.001), and by an additional 2% after reperfusion. During 30 min of reperfusion, hearts released 1.6+/-0.2 and 3.2+/-0.4% of the intracellular creatine kinase contents after 30 and 60 min of ischemia, respectively (P<0.001). In addition to the correlation between ischemia duration and membrane permeability, evident from the analysis of each probe, the data showed a progressive increase in severity of membrane injury over time and permeabilization to larger molecules. 23Na NMR spectroscopy in conjunction with an extracellular shift reagent (SR) showed formation of a resonance at an intermediate chemical shift in between the intra and extracellular Na+ peaks, suggesting penetration of SR into cells with disrupted membranes. The constant chemical shift and narrow line shape of this resonance, characteristic of a homogeneous chemical environment, suggested that the distribution of SR was contained within the cytosol of cardiomyocytes. We propose that sarcolemmal membranes are gradually permeabilized to larger molecules by ischemia, and the evolving chemical instability is spatially contained within the myocyte.

Animals↗

PubMed2001

Excretion of a phosphorus-containing carbohydrate by Streptomyces sp. A50.

A new phosphorus-containing compound (1) was detected by (31)P NMR spectroscopy in Streptomyces sp. A50. Compound 1, 1(alpha)-O-methyl-2-(N-acetyl)glucoseamine-6-O-phosphate-1(alpha)-2-(N-acetyl)glucosamine, exhibited a pK(a) value around zero. The compound was found both in the extracellular culture broth and in the cells. While very low concentrations of 1 were found in the culture broth of other species of Streptomyces, its presence in high concentrations was specific to Streptomyces sp. A50. The highly acidic compound was isolated from the broth, and its structure was elucidated by a combination of 1D-, 2D-homonuclear, and inverse heteronuclear NMR techniques and mass spectroscopy.

Acetylglucosamine↗

PubMed2000

Slice-selective proton double quantum filtered MRI of joint connective tissues.

1H double quantum filtered (DQF) imaging has been shown to highlight tendons. In this work the DQF magnetic resonance imaging pulse sequence is extended to include slice selection. The short transverse relaxation time of the 1H nuclear magnetic resonance in connective tissues, presents a stringent demand on the application of gradients and soft radiofrequency pulse lengths needed for slice selection. In the present work a slice selection pulse sequence is implemented by postponing the application of the slice refocusing gradient to the period after the last pulse just before the acquisition. Slice-selective DQF images of rat lower leg and knee are given to demonstrate the efficacy of the technique.

Animals↗

PubMed2000

In vivo (1)H double quantum filtered MRI of the human wrist and ankle.

Proton double quantum filtered (DQF) NMR imaging was applied in vivo to the human wrist and ankle with a clinical 1.5 T MR scanner. Water molecules having anisotropic motion were detected from tendons and ligaments. Images of Achilles tendon were obtained for a voxel size of 1.25 x 1.25 x 20 mm with three values of TR = 1.0, 0.5, and 0.2 sec, resulting in total acquisitions time of 17, 8.5, and 3.4 mins, respectively. Some degradation of the signal-to-noise ratio was observed at the shortest TR value and the contrast was significantly reduced due to SQ coherence leakage. The in vivo DQF images showed structure within the tendon that is otherwise not visible by conventional gradient-recalled echo (GRE) methods. Published 2000 Wiley-Liss, Inc.

Achilles Tendon↗

PubMed1999

The source of heterogeneity in the heme vicinity of ferricytochrome c.

Heterogeneity in the heme vicinity of ferricytochrome c was reported to be detectable by a split of the NMR signal of the heme methyl 3 group [P.D. Burns and G.N. La Mar, J. Am. Chem. Soc. 101 (1979) 5844]. Using cytochrome c mutants and computer simulations of the native and mutated cytochromes, the source of this heterogeneity is found to originate from the His-33 residue motions. The H33F mutation abolished the NMR split and computer simulations of the H33F mutant revealed a narrower distribution of fluctuations of the radius of gyration, suggesting a more rigid structure due to the mutation. The stabilization of the mutant was further demonstrated by a reduction in the H33F mutant of 4 Kcal/mol in the calculated interaction energy between residue 33 and the rest of the cytochrome, in keeping with known experimental results.

Animals↗

PubMed1999

Anisotropic and restricted diffusion of water in the sciatic nerve: A (2)H double-quantum-filtered NMR study.

The signals of water in the different compartments of rat sciatic nerve are resolved in the (2)H double-quantum-filtered NMR spectrum, due to their different quadrupolar splittings and relaxation rates. This resolution allowed the independent measurement of the water diffusion coefficients in the different compartments. The water diffusion in all three compartments, the endoneurium, the epineurium and the axon was found to be anisotropic. Parallel to the nerve fiber the average intraxonal water diffusion coefficient was 1.11 x 10(-5) cm(2)/sec, while in the perpendicular direction the diffusion is heavily restricted. The average perpendicular diffusion coefficient ranged from 0.29 x 10(-5) cm(2)/sec to 0.05 x 10(-5) cm(2)/sec for diffusion times of 7 msec and 50 msec, respectively. Assuming restricted diffusion in nonpermeable cylinders, intra-axonal mean diameters of 6.0, 7.4 and 9.0 microm were obtained for nerves taken from three different rats. Magn Reson Med 42:461-466, 1999.

Animals↗

PubMed1999

1H double-quantum-filtered MR imaging as a new tool for assessment of healing of the ruptured Achilles tendon.

1H double-quantum-filtered magnetic resonance imaging (DQF MRI) was applied to monitor the healing process of the Achilles tendons in rabbits after tenotomy. DQF MRI provides a new contrast, which is based on the non-zero average of the dipolar interaction caused by anisotropic motion of water molecules, determined mainly by their interaction with the ordered collagen fibers. Tissues are characterized by the dependence of their DQF signal on the DQ creation time, tau. With the use of DQF MRI, higher tissue contrast is obtained between tendon, bone, skin, and muscle. The tendons, which give weak signals in standard MRI techniques, are highlighted in the (1)H DQF image. The image changed dramatically during the healing process of the injured Achilles tendon. These changes matched the phases of the healing process. By using a tau-weighted contrast, the DQF images indicate the part of tendon that has not completely healed, even after the conventional MRI appeared normal. Magn Reson Med 42:884-889, 1999.

Achilles Tendon↗

PubMed1999

Efficient limitation of intracellular edema and sodium accumulation by cardioplegia is dissociated from recovery of rat hearts from cold ischemic storage.

Energy deficiency and disturbances of sodium and water homeostasis are considered as mechanisms of injury during hypothermic preservation of cardiac muscle. The present study attempts to characterize the effect of potassium (K+) and magnesium (Mg2+) cardioplegia on these mechanisms. Cellular parameters were measured by multinuclear NMR spectroscopy in isolated rat hearts during 12 h of ischemia at 4 degrees C and 2 h of normothermic reperfusion with an isoosmotic Krebs-Henseleit (KH) solution. Potassium and magnesium cardioplegia (a) reduced the rate of ATP hydrolysis and cellular acidification during early stages of ischemia; (b) caused an early cessation of the phase of fast sodium influx after 40 min (P<0.001 vs 120 min with KH); (c) reduced intracellular sodium accumulation to 148-165 micromol/gdw after 12 h (P<0.01 vs 268+/-15 micromol/gdw with KH); (d) decreased ischemic volumes to 2.7+/-0.1 and 2.8+/-0.1 ml/gdw after 8 and 12 h of storage, respectively (P<0.005 v 3.0 and 3.3 ml/gdw with KH). Quantitative analysis of these parameters showed that both hypothermia and cardioplegia increased the relative contribution of sodium to intracellular water accumulation by a factor of 2-2.5. In view of the marked reduction in absolute sodium and water contents, the data indicate that cold cardioplegia limits the increase in intracellular osmolarity. Myocardial mechanical and metabolic recoveries, and cellular viability deteriorated during prolongation of the ischemic period from 8 to 12 h in all experimental groups (P<0.005). Reperfusion was efficient in reversing intracellular sodium and water accumulation in hearts stored with cardioplegia, in contrast to hearts stored in KH. Magnesium, but not potassium cardioplegia, lowered interstitial water contents (P<0.01 v KH), increased intracellular magnesium concentrations (P<0.001), improved mechanical and metabolic recoveries (P<0.01) and cellular viability (P<0.001). These results indicate (a) cardioplegia reduces intracellular sodium (by approximately 46%) and water accumulation (by 66%) during cold ischemia; (b) both hypothermia and cardioplegia limit the rise in intracellular osmolarity and increase the contribution of sodium to cellular swelling; (c) intracellular sodium and water contents were dissociated from myocardial viability and recovery from cold ischemia in potassium and magnesium cardioplegic solutions. It is concluded that intracellular sodium and water accumulation are not dominant factors in determination of cardiac outcome from ischemia.

Animals↗

PubMed1999

A study of dipolar interactions and dynamic processes of water molecules in tendon by 1H and 2H homonuclear and heteronuclear multiple-quantum-filtered NMR spectroscopy.

The effect of proton exchange on the measurement of 1H-1H, 1H-2H, and 2H-2H residual dipolar interactions in water molecules in bovine Achilles tendons was investigated using double-quantum-filtered (DQF) NMR and new pulse sequences based on heteronuclear and homonuclear multiple-quantum filtering (MQF). Derivation of theoretical expressions for these techniques allowed evaluation of the 1H-1H and 1H-2H residual dipolar interactions and the proton exchange rate at a temperature of 24 degrees C and above, where no dipolar splitting is evident. The values obtained for these parameters at 24 degrees C were 300 and 50 Hz and 3000 s-1, respectively. The results for the residual dipolar interactions were verified by repeating the above measurements at a temperature of 1.5 degrees C, where the spectra of the H2O molecules were well resolved, so that the 1H-1H dipolar interaction could be determined directly from the observed splitting. Analysis of the MQF experiments at 1.5 degrees C, where the proton exchange was in the intermediate regime for the 1H-2H dipolar interaction, confirmed the result obtained at 24 degrees C for this interaction. A strong dependence of the intensities of the MQF signals on the proton exchange rate, in the intermediate and the fast exchange regimes, was observed and theoretically interpreted. This leads to the conclusion that the MQF techniques are mostly useful for tissues where the residual dipolar interaction is not significantly smaller than the proton exchange rate. Dependence of the relaxation times and signal intensities of the MQF experiments on the orientation of the tendon with respect to the magnetic field was observed and analyzed. One of the results of the theoretical analysis is that, in the fast exchange regime, the signal decay rates in the MQF experiments as well as in the spin echo or CPMG pulse sequences (T2) depend on the orientation as the square of the second-rank Legendre polynomial.

Achilles Tendon↗

PubMed1999

Detection and characterization of boric acid and borate ion binding to cytochrome c using multiple quantum filtered NMR.

The application of multiple quantum filtered (MQF) NMR to the identification and characterization of the binding of ligands containing quadrupolar nuclei to proteins is demonstrated. Using relaxation times measured by MQF NMR multiple binding of boric acid and borate ion to ferri and ferrocytochrome c was detected. Borate ion was found to have two different binding sites. One of them was in slow exchange, k(diss) = 20 +/- 3 s(-1) at 5 degrees C and D(2)O solution, in agreement with previous findings by (1)H NMR (G. Taler et al., 1998, Inorg. Chim. Acta 273, 388-392). The triple quantum relaxation of the borate in this site was found to be governed by dipolar interaction corresponding to an average B-H distance of 2.06 +/- 0.07 A. Other, fast exchanging sites for borate and boric acid could be detected only by MQF NMR. The binding equilibrium constants at these sites at pH 9.7 were found to be 1800 +/- 200 M(-1) and 2.6 +/- 1.5 M(-1) for the borate ion and boric acid, respectively. Thus, detection of binding by MQF NMR proved to be sensitive to fast exchanging ligands as well as to very weak binding that could not be detected using conventional methods.

Animals↗

PubMed1998

Imaging of H217O distribution in the brain of a live rat by using proton-detected 17O MRI.

Imaging of H217O has a number of important applications. Mapping the distribution of H217O produced by oxidative metabolism of 17O-enriched oxygen gas may lead to a new method of metabolic functional imaging; regional cerebral blood flow also can be measured by measuring the H217O distribution after the injection of 17O-enriched physiological saline solution. Previous studies have proposed a method for indirect detection of 17O. The method is based on the shortening of the proton T2 in H217O solutions, caused by the residual 17O-1H scalar coupling and transferred to the bulk water via fast chemical exchange. It has been shown that the proton T2 of H217O solutions can be restored to that of H216O by irradiating the resonance frequency of the 17O nucleus. The indirect 17O image thus is obtained by taking the difference between two T2-weighted spin-echo images: one acquired after irradiation of the 17O resonance and one acquired without irradiation. It also has been established that, at relatively low concentrations of H217O, the indirect method yields an image that quantitatively reflects the H217O distribution in the sample. The method is referred to as PRIMO (proton imaging of oxygen). In this work, we show in vivo proton images of the H217O distribution in a rat brain after an i.v. injection of H217O-enriched physiological saline solution. Implementing the indirect detection method in an echo-planar imaging sequence enabled obtaining H217O images with good spatial and temporal resolution of few seconds.

Animals↗

PubMed1998

Mapping strain exerted on blood vessel walls using deuterium double-quantum-filtered MRI.

A technique is described for displaying distinct tissue layers of large blood vessel walls as well as measuring their mechanical strain. The technique is based on deuterium double-quantum-filtered (DQF) spectroscopic imaging. The effectiveness of the double-quantum filtration in suppressing the signal of bulk water is demonstrated on a phantom consisting of rat tail tendon fibers. Only intrafibrillar water is displayed, excluding all other signals of water molecules that reorient isotropically. One- and two-dimensional spectroscopic imaging of bovine aorta and coronary arteries show the characteristic DQF spectrum of each of the tissue layers. This property is used to obtain separate images of the outer layer, the tunica adventitia, or the intermediate layer, the tunica media, or both. To visualize the effect of elongation, the average residual quadrupole splitting is calculated for each pixel. Two-dimensional deuterium quadrupolar splitting images are obtained for a fully relaxed and a 55% elongated sample of bovine coronary artery. These images indicate that the strong effect of strain is associated with water molecules in the tunica adventitia whereas the DQF NMR signal of water in the tunica media is apparently strain-insensitive. After appropriate calibration, these average quadrupolar splitting images can be interpreted as strain maps.

Animals↗

PubMed1998

Observation of a 1H double quantum filtered signal of water in biological tissues.

The observation of a 1H double quantum filtered (DQF) NMR signal of water in bovine sciatic nerve, bovine articular cartilage, rat tail tendon, and rat brain is reported. The origin of this signal in rat tail tendon was found to be a result of residual dipolar interaction between water protons and macromolecular protons. The dependence of the width of the 1H DQF spectra on the orientation indicated that in rat tail tendon the effective director of the residual dipolar interaction is parallel to the collagen fibers. 1H DQF NMR may be applied in imaging where the contrast obtained is related to the degree of order in the tissue.

Animals↗

PubMed1998

Proton double-quantum filtered MRI--a new method for imaging ordered tissues.

The imaging of connective tissues such as cartilage and tendons using standard MRI techniques is hampered by their low signal relative to the surrounding tissues. 1H double-quantum filtered (DQF) MRI is an imaging method that detects molecules associated with ordered structures, while the signal from isotropic fluids is filtered out, thus creating a new type of contrast. The technique is demonstrated on an intact rat tail, where the image of the tendons is highlighted. Although the signal-to-noise ratio is inferior to that in gradient-echo MRI, the contrast between the tendons and the surrounding tissues is significantly better in the DQF MRI. It is demonstrated how, by adjusting the parameters of the DQF imaging pulse sequence, one can modify the contrast and enhance the images of specific compartments within an organ. A comparison with 2H DQF imaging of the same tissue is also given.

Animals↗

PubMed1998

Intracellular volumes and membrane permeability in rat hearts during prolonged hypothermic preservation with St Thomas and University of Wisconsin solutions.

The study aims to determine a possible relationship between intracellular water, energy metabolism, functional recovery and membrane permeability, during and after hypothermic cardiac preservation. Isolated rat hearts were stored for 12 h at 4 degrees C with University of Wisconsin (UW), St Thomas Hospital (ST) and Krebs-Henseleit (KH) solutions, and were reperfused for 1 h. Cellular volumes were measured by 1H NMR of water and 59Co NMR of the extracellular marker cobalticyanide, and energetic profiles by 31P NMR spectroscopy. Storage in ST solution reduced ischemic swelling from 2.50 +/- 0.06 to 2.73 +/- 0.09 (P < 0.001 v 3.56 +/- 0.10 ml/g dry weight in KH), while UW solution caused cellular shrinkage to 2.12 +/- 0.08 ml/g dry weight. Intracellular ATP concentrations and pH values were higher in UW as compared to ST solution. At reperfusion, hearts stored in ST shrank while those stored in UW expanded, resulting in similar intracellular volumes. Storage with UW was superior to ST in post-ischemic function 65 +/- 5% (P < 0.01 v 49 +/- 4% with ST) and in recovery of ATP 46 +/- 3% (P < 0.001 v 25 +/- 4% with ST). Storage with both ST and UW solutions did not prevent interstitial edema. Sarcolemmal membrane integrity, as assessed by cellular swelling in response to a hypo-osmotic shock (210 mmol/l), was significantly improved by ST and UW solutions as compared to KH (P < 0.05). Creatine kinase efflux was reduced by ST and UW as compared to KH (P < 0.05), and by UW as compared to ST (P < 0.05). Coronary flow was higher following storage with UW than ST solutions. 66 +/- 6 and 45 +/- 4%, respectively (P < 0.01). According to these data, the beneficial effects of UW and ST solutions on hypothermic ischemic storage of rat hearts included prevention of cellular edema and preservation of sarcolemmal membrane integrity. It is concluded: (a) UW and ST solutions reduce ischemic and reperfusion cellular volumes: (b) both solutions, and UW in particular were efficient in preservation of membrane integrity: (c) prevention of cellular edema is not the single or main mechanism responsible for the improved preservation with UW and ST solutions.

Adenosine↗