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R S Balaban

Publications and source records attributed to R S Balaban.

At least 55 records · Page 3Linked to original sources
PubMed1997

High-energy phosphates and tension production in rabbit tibialis anterior/extensor digitorum longus muscles.

The effects of repetitive muscle contraction on energy state and tension production were studied in rabbit tibialis anterior/extensor digitorum longus muscles that had been subjected to 90 days of continuous indirect electrical stimulation at 10 Hz. Anesthetized chronically stimulated and control rabbits were challenged with 15 min of stimulation at 4 and 15 tetani/min. Pi-to-phosphocreatine (PCr) ratio (Pi/PCr) was measured in vivo before, during, and after acute stimulation by 31P-magnetic resonance spectroscopy, and tension was recorded at the same time. Although Pi/PCr was low at rest, it was significantly higher in chronically stimulated muscle than in control muscle (0.20 +/- 0.02 vs. 0.05 +/- 0.01, P < 0.05). Stimulation of control muscle for 15 min at both 4 and 15 tetani/min induced a significant rise in Pi/PCr, whereas the same conditions in chronically stimulated muscle did not produce any significant departure from initial levels. The tension produced by control muscle fell to 93 +/- 3% of its initial value during stimulation at 4 tetani/min and to 61 +/- 7% at 15 tetani/min, respectively. In chronically stimulated muscle, on the other hand, tension was potentiated above its initial level at both stimulation rates (135 +/- 15 and 138 +/- 11%, respectively) and remained significantly elevated throughout each trial. The ability of chronically stimulated muscle to sustain high levels of activity with minimal perturbations in Pi/PCr or decrement in tension is attributable to cellular adaptations that include a well-documented increase in oxidative capacity.

Animals↗
PubMed1997

Efficiency of human skeletal muscle in vivo: comparison of isometric, concentric, and eccentric muscle action.

The purpose of this study was to estimate the efficiency of ATP utilization for concentric, eccentric, and isometric muscle action in the human tibialis anterior and extensor digitorum longus in vivo. A dynamometer was used to quantitate muscle work, or tension, while simultaneous 31P-nuclear magnetic resonance data were collected to monitor ATP, phosphocreatine, inorganic phosphate, and pH. The relative efficiency of the actions was estimated in two ways: steady-state effects on high-energy phosphates and a direct comparison of ATP synthesis rates with work. In the steady state, the cytosolic free energy dropped to the lowest value with concentric activity, followed by eccentric and isometric action for comparative muscle tensions. Estimates of ATP synthesis rates revealed a mechanochemical efficiency [i.e., ATP production rate/work (both in J/s)] of 15.0 +/- 1.3% in concentric and 34.7 +/- 6.1% in eccentric activity. The estimated maximum ATP production rate was highest in concentric action, suggesting an activation of energy metabolism under these conditions. By using direct measures of metabolic strain and ATP turnover, these data demonstrate a decreasing metabolic efficiency in human muscle action from isometric, to eccentric, to concentric action.

Adenosine Diphosphate↗
PubMed1997

Normalized metabolic stress for 31P-MR spectroscopy studies of human skeletal muscle: MVC vs. muscle volume.

A critical requirement of submaximal exercise tests is the comparability of workload and associated metabolic stress between subjects. In this study, 31P-magnetic resonance spectroscopy was used to estimate metabolic strain in the soleus muscle during dynamic, submaximal plantar flexion in which target torque was 10 and 15% of a maximal voluntary contraction (MVC). In 10 healthy, normally active adults, (PCr + Pi)/PCr, where PCr is phosphocreatine, was highly correlated with power output normalized to the volume of muscle in the plantar flexor compartment (r = 0.89, P < 0.001). The same variable was also correlated, although less strongly (r = 0.78, P < 0.001), with power normalized to plantar flexor cross-sectional area. These findings suggest that comparable levels of metabolic strain can be obtained in subjects of different size when the power output, or stress, for dynamic plantar flexion is selected as a function of plantar flexor muscle volume. In contrast, selecting power output as a function of MVC resulted in a positive linear relationship between (PCr + Pi)/PCr and the torque produced, indicating that metabolic strain was increasing rather than achieving constancy as a function of MVC. These findings provide new insight into the design of dynamic muscle contraction protocols aimed at detecting metabolic differences between subjects of different body size but having similar blood flow capacity and mitochondrial volume per unit of muscle.

Adenosine Diphosphate↗
PubMed1996

Spectroscopic determination of cytochrome c oxidase content in tissues containing myoglobin or hemoglobin.

A simple spectroscopic method for determining the cytochrome c oxidase, cytochrome a, a3, content in tissue and mitochondria samples independent of myoglobin or blood contamination is described. Using tissue homogenates solubilized in Triton X-100, this assay relies on the selective reduction of mitochondrial cytochromes by the action of potassium cyanide. Monitoring the optical absorbance of these samples at 605 nm provided a quantitative determination of cytochrome c oxidase content in the presence of myoglobin or blood. The cytochrome c oxidase content of porcine heart mitochondria and whole tissue was determined to be 0.85 nmol/mg protein and 30.5 nmol/g wet wt, respectively.

Animals↗
PubMed1996

Neutral carrier-based "Ca(2+)-selective" microelectrodes for the measurement of tetraphenylphosphonium.

Ca(2+)-selective microelectrodes with Simon's neutral carrier ETH 1001 are commercially available and have been widely used for the measurement of both extra- and intracellular calcium. The electrodes demonstrate high selectivity against other cations such as magnesium, sodium, and potassium. We report, however, that the ETH 1001-based microelectrode is a superior tetraphenylphosphonium (TPP+)-sensitive electrode. The electrode exhibits a Nernstian response for [Ca2+] > 10(-5) M but for [TTP+] > 10(-7) M. Using two different methods, we found that log kTTPCa (selectivity coefficient for TPP+ with respect to Ca2+) is in the range of -3.0 to -5.3. We argue that the ETH 1001 microelectrode can be used as a commercially available TPP+ electrode. We illustrate this application by making membrane potential recordings in respiring mitochondria. The results are identical to those obtained using conventional ion-exchange TPP+ electrodes.

Animals↗
PubMed1996

Evaluation of methemoglobin as an autologous intravascular MRI contrast agent.

Methemoglobin (MetHb) was evaluated as an intravascular paramagnetic contrast agent. Methemoglobin formation was induced by 4-dimethylaminophenol (4-DMAP), causing a reduction in blood T2* in vitro. The 4-DMAP generated metHb with a time constant of 62 s. A 4-DMAP bolus did not decrease measurably the signal intensity in the in vivo rabbit kidney in the first pass. At steady state, a MetHb concentration of 24.8 +/- 2.3% resulted in a signal decrease of 9.2 +/- 2.6% in the kidney. Methemoglobin is an effective vascular T2* relaxation agent, but the formation of MetHb by 4-DMAP is too slow for first-pass imaging. A more effective conversion agent resulting in a bolus of at least 25% MetHb within 5 s would result in a detectable first-pass signal and a viable contrast technique.

4-Aminopyridine↗
PubMed1996

A method to improve the B0 homogeneity of the heart in vivo.

A homogeneous static (B0) magnetic field is required for many NMR experiments such as echo planar imaging, localized spectroscopy, and spiral scan imaging. Although semi-automated techniques have been described to improve the B0 field homogeneity, none has been applied to the in vivo heart. The acquisition of cardiac field maps is complicated by motion, blood flow, and chemical shift artifact from epicardial fat. To overcome these problems, an ungated three-dimensional (3D) chemical shift image (CSI) was collected to generate a time and motion-averaged B0 field map. B0 heterogeneity in the heart was minimized by using a previous algorithm that solves for the optimal shim coil currents for an input field map, using up to third-order current-bounded shims (1). The method improved the B0 homogenelty of the heart in all 11 normal volunteers studied. After application of the algorithm to the unshimmed cardiac field maps, the standard deviation of proton frequency decreased by 43%, the magnitude 1H spectral linewidth decreased by 24%, and the peak-peak gradient decreased by 35%. Simulations of the high-order (second- and third-order) shims in B0 field correction of the heart show that high order shims are important, resulting for nearly half of the improvement in homogeneity for several subjects. The T2* of the left ventricular anterior wall before and after field correction was determined at 4.0 Tesis. Finally, results show that cardiac shimming is of benefit in cardiac 31P NMR spectroscopy and cardiac echo planar imaging.

Echo-Planar Imaging↗
PubMed1996

The evaluation of dielectric resonators containing H2O or D2O as RF coils for high-field MR imaging and spectroscopy.

Electromagnetic resonators consisting of low-loss dielectric material and/or metallic boundaries are widely used in microwave technologies. These dielectric resonators usually have high Q factors and well-defined field distributions. Magnetic resonance imaging was shown as a way of visualizing the magnetic field distribution of the resonant modes of these resonators, if the dielectric body contains NMR sensitive nuclei. Dielectric resonators have also been proposed as RF coils for magnetic resonance experiments. The feasibility of this idea in high-field MR is discussed here. Specifically, the dielectric resonances of cylindrical water columns were characterized at 170.7 MHz (4 T 1H Larmor frequency), and evaluated as NMR transmit and receive coils. The dielectric resonance of a cylindrical volume of D2O was used to image a hand at 170.7 MHz. This study demonstrated that MRI is an effective way of visualizing the magnetic field in dielectric structures such as a water cylinder, and can potentially be generalized to solid-state dielectric devices. The possible applications of dielectric resonators other than simple cylindrical volumes in MRI and MR solution spectroscopy at high field strengths are also discussed.

Deuterium↗
PubMed1996

Relationship between skeletal muscle intracellular ionized magnesium and measurements of blood magnesium.

The current laboratory approach to assessing magnesium status is based on determining the concentration of total Mg ((Mg)) in serum or plasma. This strategy is problematic in that the amount of Mg in blood is less than 1% of total body Mg and does not accurately reflect (Mg) in other tissues. Furthermore, the (Mg) of blood does not distinguish biologically active, ionized Mg from the bound fraction. The goal of this study was to determine intracellular ionized Mg ((Mg++)i) of skeletal muscle in vivo and to compare results with the (Mg) of blood constituents. (Mg++)i was determined in resting skeletal muscle by using phosphorus 31 magnetic resonance (31P-MR) spectroscopy. (Mg) was measured in serum (S(Mg)), serum ultrafiltrate (UF(Mg)), mononuclear blood cells (MBC(Mg)), and red blood cells (RBC(Mg)) by using atomic absorption spectroscopy or a colorimetric assay. In a sample of 60 healthy adult subjects, skeletal muscle (Mg++)i = 557 +/- 97 mumol/L (mean +/- SD); S(Mg) = 0.78 +/- 0.09 mmol/L; UF(Mg) = 0.60 +/- 0.12 mmol/L; MBC(Mg) = 13.8 +/- 2.3 mmol/L; and, RBC(Mg) = 1.92 +/- 0.33 mmol/L. A significant negative correlation was found between (Mg++)i and S(Mg) (r = -0.43, p < 0.05). S(Mg) was significantly lower (p < 0.05) and (Mg++)i significantly higher (p < 0.05) in women than in men, but neither was related to age. These findings provide new insight into the relationship between blood Mg measures and (Mg++)i of the largest soft tissue mass of the human body.

Adult↗
PubMed1996

MR relaxation times in human brain: measurement at 4 T.

PURPOSE: To determine the values for relaxation times in human brain for magnetic resonance (MR) imaging at 4 T. MATERIALS AND METHODS: T1 measurements were made with a progressive saturation sequence, an implementation of the Look-Locker sequence, and an inversion-recovery (IR) interleaved echo-planar imaging (IEPI) sequence. T2 measurements were made with a standard spin-echo (SE) sequence and an SE IEPI sequence. RESULTS: The T1 measurements yielded values of 1,724 msec +/- 51 for gray matter, 1,043 msec +/- 27 for white matter, and 4,550 msec +/- 800 msec for cerebrospinal fluid. The deep gray matter regions had T1 values of 1,458 +/- 38 (caudate nucleus) and 1,372 +/- 60 (putamen). The T2 measurements yielded results of 63 msec +/- 6.2 for gray matter and 49.8 msec +/- 2.2 for white matter. CONCLUSION: The T1 values measured at 4 T show a higher value than predicted from extrapolation at lower field strengths. The T2 measurements showed a slight decrease in values over those measured at lower-field strength. The gain in signal-to-noise ratio from the higher field strength may be substantially offset by these altered relaxation time values to a degree that is sequence dependent.

Adult↗
PubMed1996

MR imaging contrast in human brain tissue: assessment and optimization at 4 T.

PURPOSE: To evaluate multiple magnetic resonance (MR) imaging sequences for their ability to provide T1 and T2-weighted images at a field strength of 4 T, and to validate previously obtained relaxation time measurements. MATERIALS AND METHODS: Different spin-echo, inversion-recovery (IR), gradient-recalled acquisition in the steady state (GRASS), and magnetization transfer contrast-enhanced GRASS sequences were evaluated in a single section, each in at least four volunteers. Also, interleaved echo-planar imaging (IEPI) and interleaved gradient-recalled echo (IGRE) sequences were analyzed and compared with standard sequences. RESULTS: Predicted contrast behavior, according to MR relaxation time measurements, was found to agree well with that of the validation experiments. Fair T1 contrast can be achieved on MR images at 4 T, contrary to early predictions. Under other conditions, however, such as partially spin-density-weighted parameters, lower contrast is observed at the high field strength when compared with conventional field strengths. CONCLUSION: The longer T1 values at higher field strengths have a substantial effect on image contrast. Depending on the sequence and parameters chosen, good image contrast can be realized at 4 T. IEPI and hybrid IR IGRE sequences are useful in shortening the prolonged examination times owing to the longer T1 values at higher field strengths.

Artifacts↗
PubMed1996

Quantitation of galactosemic cataracts in dogs using magnetization transfer contrast-enhanced magnetic resonance imaging.

PURPOSE: Magnetic resonance imaging (MRI) is becoming increasingly important for the diagnosis and characterization of ocular pathologies. A drawback to this technique is that image contrast between different regions of tissue can be obscured because of the similarity of their nuclear magnetic resonance relaxation parameters. This problem is addressed by magnetization transfer contrast (MTC) enhancement, a MRI technique that generates high-contrast images based on characteristic tissue differences resulting from the interaction of water and macromolecules. The purpose of this study was to investigate the feasibility of using MTC-enhanced imaging to monitor quantitatively the lens changes associated with sugar cataract formation in galactose-fed dogs. METHODS: Male beagles fed a diet containing 30% galactose were periodically examined by MRI for changes in tissue character. Each examination included a gradient recalled echo image (M0), an MTC-enhanced gradient recalled echo image (Ms), a T1 image determined from a one-shot T1 imaging sequence, and a T1-weighted image taken from the raw T1 data. Average values were obtained for several regions of interest and tabulated. These were correlated with cataractous stages visually observed by slit lamp biomicroscopy and retroillumination photography. RESULTS: Enhanced image details of the lens and anterior segment that documented osmotic changes from initial cortical vacuole formation to cortical and nuclear changes associated with advanced sugar cataracts were characterized from measurements of parameters obtained from M0, Ms, T1-weighted, and T1 images. Changes in the cross-sectional areas of lenses during sugar cataract formation also were documented. The magnetic resonance images showed visible changes from the onset of cortical vacuole formation. Region of interest (ROI) analysis of the images showed tissue changes occurring throughout the cataract progression. CONCLUSIONS: The MTC-enhanced MRI technique is well suited to detecting lens changes associated with cataractogenesis. All but the earliest changes were readily apparent from the images with no further analysis. Graphic ROI analysis was able to detect regional changes associated the cataract progression for all degrees of severity. Furthermore, the images demonstrated changes in size and shape that would not be detectable by visual inspection.

Animals↗
PubMed1995

Water-macromolecular proton magnetization transfer in infarcted myocardium: a method to enhance magnetic resonance image contrast.

Water proton nuclear magnetic resonance relaxation times and magnetization transfer (MT) parameters of rat hearts were studied 24 h or 4 weeks after ligation of the left coronary artery or sham operation. Compared with sham-operated controls, measured relaxation times (T1sat and T2) of both acute and chronic myocardial infarction increased. The MT effect significantly decreased in the infarcted myocardium. The changes in relaxation times and MT effect were significantly greater in chronic infarcts compared with acute infarcts. Improvements in calculated image contrast between normal and infarcted tissue were supported by images of ex vivo hearts with chronic infarction. Image contrast was increased at short echo times in the presence of macromolecular proton pool irradiation. Exploiting changes in tissue MT following myocardial infarction to enhance contrast between normal and infarcted tissue should allow improved identification and characterization of infarcted myocardium.

Animals↗
PubMed1995

Anomalous transverse relaxation in 1H spectroscopy in human brain at 4 Tesla.

Longitudinal (T1) and apparent transverse relaxation times (T2) of choline-containing compounds (Cho), creatine/phosphocreatine (Cr/PCr), and N-acetyl aspartate (NAA) were measured in vivo in human brain at 4 Tesla. Measurements were performed using a water suppressed stimulated echo pulse sequence with complete outside volume presaturation to improve volume localization at short echo times. T1-values of Cho (1.2 +/- 0.1 s), Cr (1.6 +/- 0.3 s), and NAA (1.6 +/- 0.2 s) at 4 Tesla in occipital brain were only slightly larger than those reported in the literature at 1.5 Tesla. Thus, TR will not adversely affect the expected enhancement of signal-to-noise at 4 Tesla. Surprisingly, apparent T2-values of Cho (142 +/- 34 ms), Cr (140 +/- 13 ms), and NAA (185 +/- 24 ms) at 4 Tesla were significantly smaller than those at 1.5 Tesla and further decreased when increasing the mixing interval TM. Potential contributing factors, such as diffusion in local susceptibility related gradients, dipolar relaxation due to intracellular paramagnetic substances and motion effects are discussed. The results suggest that short echo time spectroscopy is advantageous to maintain signal to noise at 4 Tesla.

Artifacts↗
PubMed1995

Correction for geometric distortion in echo planar images from B0 field variations.

A method is described for the correction of geometric distortions occurring in echo planar images. The geometric distortions are caused in large part by static magnetic field inhomogeneities, leading to pixel shifts, particularly in the phase encode direction. By characterizing the field inhomogeneities from a field map, the image can be unwarped so that accurate alignment to conventionally collected images can be made. The algorithm to perform the unwarping is described, and results from echo planar images collected at 1.5 and 4 Tesla are shown.

Algorithms↗
PubMed1995

Skeletal muscle intracellular ionized magnesium measured by 31P-NMR spectroscopy across the menstrual cycle.

OBJECTIVE: The hormonal regulation of magnesium (Mg) metabolism is poorly understood. Preliminary evidence suggests that reproductive hormones may influence Mg concentrations in various tissues. The purpose of this study was to determine if Mg concentrations in blood and muscle are affected by the phase of the menstrual cycle. METHODS: Magnesium measures were obtained from 16 women over the course of one complete menstrual cycle. The principal outcome measure was intracellular ionized Mg ([Mg++]i) in skeletal muscle as measured by 31P nuclear magnetic resonance spectroscopy. Three blood measures (serum, red blood cell, and mononuclear blood cell) of total Mg ([Mg]) were also obtained. RESULTS: Mean Mg concentrations were stable across the menstrual cycle with no evidence of a menstrual cycle phase effect for any of the measures. Furthermore, skeletal muscle [Mg++]i was not correlated with any blood measure of [Mg]. CONCLUSION: These results suggest that physiologic fluctuations in reproductive hormones do not influence either blood [Mg] or skeletal muscle [Mg++]i in healthy, regularly cycling women.

Adult↗
PubMed1995

Musculoskeletal MR imaging at 4 T and at 1.5 T: comparison of relaxation times and image contrast.

PURPOSE: To evaluate the relaxation time-based contrast between the main tissues of the musculoskeletal system as measured in the human knee with magnetic resonance imaging at 4 T and 1.5 T. MATERIALS AND METHODS: Five volunteers underwent 4-T and 1.5-T imaging. Inversion-recovery series were used to measure T1 values, and T2 values were measured with a spin-echo sequence. RESULTS: T1 values increased in all tissues with 4-T imaging. Values increased in muscle from 1 to 1.8 seconds, in fat from 0.3 to 0.4 seconds, and in cartilage from 0.8 to 1.5 seconds. T2 values were 10%-20% shorter in all tissues at 4 T. CONCLUSION: Advantages of 4-T imaging compared with 1.5-T imaging include a higher signal-to-noise ratio and an improved signal difference-to-noise ratio. However, any improvement in signal-to-noise ratio at high field strengths can partially be reduced by the increase in the T1 value. The slightly shorter T2 values at 4 T do not affect image contrast.

Cartilage, Articular↗