Biomedical subjects

R L Armstrong

Publications and source records attributed to R L Armstrong.

At least 19 recordsLinked to original sources

PubMed1998

Relaxation time mapping of short T*2 nuclei with single-point imaging (SPI) methods.

New techniques for quantitative mapping of T1, T2, and T*2 are proposed, based on the single-point imaging (SPI) method, for materials with short nuclear magnetic resonance relaxation times which cannot be imaged with traditional methods. Relaxation times extracted from two-dimensional images of uniform doped agarose phantoms (T*2 approximately 60-210 microseconds) as well as hardened mortar (T*2 approximately 220 microseconds) and polymers (T*2 approximately 20-100 microseconds), using these techniques, agreed with bulk measurements. The method was then applied to a partially dried cylindrical concrete sample (T*2 approximately 115 microseconds).

Image Enhancement↗

PubMed1998

The influence of shrinkage-cracking on the drying behaviour of White Portland cement using Single-Point Imaging (SPI).

The removal of water from pores in hardened cement paste smaller than 50 nm results in cracking of the cement matrix due to the tensile stresses induced by drying shrinkage. Cracks in the matrix fundamentally alter the permeability of the material, and therefore directly affect the drying behaviour. Using Single-Point Imaging (SPI), we obtain one-dimensional moisture profiles of hydrated White Portland cement cylinders as a function of drying time. The drying behaviour of White Portland cement, is distinctly different from the drying behaviour of related concrete materials containing aggregates.

Construction Materials↗

PubMed1997

Three dimensional imaging of the Belousov-Zhabotinsky reaction using magnetic resonance.

The combination of 3D magnetic resonance imaging data with polygon based computer graphic display software is ideally suited to the study of the Belousov-Zhabotinsky reaction in extended volumes. In this paper we present the first true three dimensional visualization of experimental data from the Belousov-Zhabotinsky reaction. The time evolution of a twisted scroll wave like isoconcentration surface and its organizing filament are demonstrated for the manganese-catalyzed B-Z mixture. These techniques extend the experimental study of the B-Z reaction as a class of pattern-forming systems to the third dimension. The limitations of the technique are discussed.

Computer Simulation↗

PubMed1997

Concrete thawing studied by single-point ramped imaging.

A series of two-dimensional images of proton distribution in a hardened concrete sample has been obtained during the thawing process (from -50 degrees C up to 11 degrees C). The SPRITE sequence is optimal for this study given the characteristic short relaxation times of water in this porous media (T2* < 200 micros and T1 < 3.6 ms). The relaxation parameters of the sample were determined in order to optimize the time efficiency of the sequence, permitting a 4-scan 64 x 64 acquisition in under 3 min. The image acquisition is fast on the time scale of the temperature evolution of the specimen. The frozen water distribution is quantified through a position based study of the image contrast. A multiple point acquisition method is presented and the signal sensitivity improvement is discussed.

Image Processing, Computer-Assisted↗

PubMed1996

Purification and characterization of the hexokinase from Schistosoma mansoni, expressed in Escherichia coli.

The hexokinase (ATP;D-hexose 6-phosphotransferase, EC 2.7.1.1) of Schistosoma mansoni has been expressed in Escherichia coli as a fusion protein including an N-terminal polyhistidine tag and enterokinase cleavage site. The enzyme was purified by metal chelate chromatography to > 95% homogeneity, based on analysis by SDS-gel electrophoresis and isoelectric focusing. The absorbance at 280 nm was 0.54 for a 1 mg/ml solution (molar extinction coefficient 2.7 x 10(4) cm2 mol). The pI of the S. mansoni hexokinase was 6.0-6.2, slightly more acidic than the rat Type I isozyme (pI 6.35). The S. mansoni enzyme migrated as a single band of activity during nondenaturing cellulose acetate electrophoresis; the mobility was slightly greater than the rat Type I isozyme, consistent with the estimated pI. The Km values for substrates glucose and ATP were 128 +/- 10 and 927 +/- 41 microM, respectively. In accord with a previous report, the S. mansoni hexokinase exhibited moderate sensitivity to inhibition (competitive vs ATP) by the product, glucose 6-phosphate, with a Ki approximately 150 microM; the product analog, 1,5-anhydroglucitol 6-phosphate, was somewhat less effective as an inhibitor, with Ki approximately 500 microM. These kinetic properties were not altered by removal of the N-terminal fusion partner by enterokinase treatment. Immunological crossreactivity between the rat Type I isozyme and the S. mansoni hexokinase was demonstrated by immunoblotting, but this was markedly dependent on the preparation of antiserum used. The activity of the enzyme is apparently highly dependent on maintenance of free sulfhydryl groups. Activity was maintained during storage in the presence of monothioglycerol; activity lost during storage in the absence of monothioglycerol could be partially restored by treatment with this reagent.

Adenosine Triphosphate↗

PubMed1995

Rotating frame RF current density imaging.

RF current density imaging (RF-CDI) is a new MRI technique for imaging the Larmor frequency current density parallel to B0 in electrolytic media. To extend the use of RF-CDI to biological tissue for generating conductivity contrast, the sensitivity must be increased and the data requirements reduced. A rotating frame approach, in which a large B1 field is applied simultaneously as a rotary echo with RF current, is proposed to meet these requirements. Rotating frame magnetic fields are encoded in the phase of an MRI image. Trials have now been performed with this sequence in a three-compartment cylindrical phantom containing doped water or mineral oil for detecting displacement, conduction and fringe field currents. In a postmortem rat study, 85.56 MHz RF currents injected by implanted electrodes created tissue dependent contrast because of the electrical properties of tissue. A sensitivity and artifact analysis was also performed. The sensitivity of this method is determined by the maximum RF pulse duration. SAR limits pose an upper bound on this time and B1, whereas the avoidance of phase artifacts imposes a lower bound on B1.

Algorithms↗

PubMed1993

Effect of fasting, hypocaloric feeding, and refeeding on the energetics of stimulated rat muscle as assessed by nuclear magnetic resonance spectroscopy.

A previous study suggested that muscles from hypocalorically fed rats were limited in their ability to rephosphorylate ADP. During muscle contraction hydrolysis of ATP results in an increase in phosphorus, free ADP, delta GATP, and a reduction in phosphocreatine levels that is reversed during rest by rephosphorylation of ADP to ATP and the resynthesis of phosphocreatine by ATP. We therefore hypothesized that these changes would be restored more slowly during postcontraction rest in hypocalorically fed rats as compared with controls. We compared controls fed ad lib to 2-d fasted and hypocalorically fed rats, losing 20% of their weight. We also compared hypocalorically fed rats that had been refed ad lib for 7 d with age-matched controls fed ad lib. The results showed that ATP, muscle pH, and total muscle creatine levels were not different in all groups. The raised phosphorus and delta GATP levels and lower phosphocreatine/phosphorus ratio at the end of contraction changed more slowly during rest in the hypocaloric rats. These abnormalities were partially corrected by refeeding. The data taken as a whole support the concept of impaired rephosphorylation of ADP in malnourished muscle that is not completely restored by refeeding in stimulated muscle.

Adenosine Diphosphate↗

PubMed1992

RF current density imaging in homogeneous media.

MRI has proven capable of imaging quasistatic volume current densities in electrolytic and biological media. In this paper, the feasibility of extending the method to image RF current density at the Larmor frequency is studied. RF current imaging could be relevant to MR power absorption and safety and to hyperthermia analysis, as well as creating dielectric and conductivity-dependent tissue contrast. The approach is to deliberately induce or inject RF currents in a sample synchronous with an MR pulse sequence and measure the resulting transverse RF magnetic field components. Current density is extracted by computing the curl of the magnetic fields. The preliminary theory has been developed for uniform media where both displacement and conduction currents exist while skin effects or eddy currents are absent. If the derivative in the B0 direction of the RF magnetic field component parallel to B0 is negligible, then sufficient information exists to reconstruct the RF current density component that is parallel to B0 without rotating the sample. The relative phase of the current can also be estimated. The method has been proven feasible by successfully imaging a uniform 85.6-MHz current density in a salt water phantom. The experiment conforms closely to capacitively coupled hyperthermia heating.

Artifacts↗

PubMed1992

Concentration dependence of fluorine impurity spin-lattice relaxation rate in bone mineral.

The concentration dependence of the fluoride ion spin-lattice relaxation rate has been observed by nuclear magnetic resonance experiments on samples of defatted and dried bone. The 19F spin-lattice relaxation rates increased linearly with the bone fluoride concentration. Different results were obtained from trabecular than from cortical bone. For the same macroscopic fluoride content per gram of bone calcium, the relaxation rate is significantly faster in cortical bone. Relaxation rates in cortical bone samples prepared from both rats and dogs were apparently controlled by the same species-independent processes. For samples from beagle dogs, the bulk fluoride concentrations measured by neutron activation analysis were 3.1 +/- 0.3 times greater in trabecular bone than in the corresponding cortical bone. The beagle spin-lattice relaxation data suggest that the microscopic fluoride concentrations in bone mineral were 1.8 +/- 0.4 times greater in trabecular bone than in cortical bone. It is concluded that the accumulation of fluoride impurities in bone mineral is non-uniform.

Animals↗

PubMed1992

High-resolution magnetic resonance imaging of experimental spinal cord injury in the rat.

The ability of magnetic resonance imaging (MRI) to display the anatomic changes after spinal cord injury in the rat were examined in postmortem specimens. With the clip compression technique, acute spinal cord injuries of three grades of severity were produced in adult male rats. One hour after injury, during which time physiological parameters were measured and maintained within the normal range, the rats were killed by transcardiac perfusion of formalin. The vertebral column containing the cervical and upper thoracic segments was excised and, after further formalin fixation, 20 contiguous MRI scans centered on the injury site were obtained using a spin-echo imaging sequence. The volume of signal acquisition for each image was 15 x 15 x 1.0 mm thick. The spinal cords were then removed from the vertebral column, sectioned, and stained for histological examination. Ten-micrometer serial sections of each cord were examined microscopically. MRI scans and microscopic sections at comparable levels were examined to determine the quality of anatomical detail and spatial resolution of the MRI scans. MRI scans with resolution of about 75 microns per pixel edge were obtained. At the site of injury, there was disruption of the normally well demarcated gray-white interface and variable areas of low signal intensity. Because of the resolution achieved, it was possible to determine that these low signal intensity areas were post-traumatic hemorrhages. Indeed, hemorrhages with dimensions of the order of 100 microns were detected on the MRI scans. Furthermore, by examining the serial sections of the cord, the extent of the injury along the cord could be determined.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1990

The aspirin and heme-binding sites of ovine and murine prostaglandin endoperoxide synthases.

Acetylation of Ser-530 of sheep prostaglandin endoperoxide (PGG/H) synthase by aspirin causes irreversible inactivation of the cyclooxygenase activity of the enzyme. To determine the catalytic function of the hydroxyl group of Ser-530, we used site-directed mutagenesis to replace Ser-530 with an alanine. Cos-1 cells transfected with expression vectors containing the native (Ser-530) or mutant (Ala-530) cDNAs for sheep PGG/H synthase expressed comparable cyclooxygenase and hydroperoxidase activities. Km values for arachidonate (8 microM) and ID50 values for reversible inhibition by the cyclooxygenase inhibitors, flurbiprofen (5 microM), flufenamate (20 microM), and aspirin (20 mM), were also the same for both native and mutant PGG/H synthases; however, only the native enzyme was irreversibly inactivated by aspirin. Thus, the "active site" Ser-530 of PGG/H synthase is not essential for catalysis or substrate binding. Apparently, acetylation of native PGG/H synthase by aspirin introduces a bulky sidechain at position 530 which interferes with arachidonate binding. In related studies, a cDNA for mouse PGG/H synthase was cloned and sequenced. A sequence of 35 residues with Ser-530 at the midpoint was identical in the two proteins. Thus, Ser-530 does lie in a highly conserved region, probably involved in cyclooxygenase catalysis. Sequence comparisons of mouse and sheep PGG/H synthase also provided information about the heme-binding site of the enzyme. The sheep HYPR sequence (residues 274-277), which had been proposed to form a portion of the distal heme-binding site, is not conserved in the mouse PGG/H synthase, suggesting that this region is not the distal heme-binding site. One sequence, TIWLREHNRV (residues 303-312 of the sheep enzyme), is very closely related to the sequence TLW(L)LREHNRL common to thyroid peroxidase and myeloperoxidase. The histidine in this latter sequence is the putative axial heme ligand of these peroxidases. We suggest that the histidine (His-309) of sheep PGG/H synthase sequence is the axial heme ligand of this enzyme.

Amino Acid Sequence↗

PubMed1990

In vitro magnetic resonance microimaging of the spinal cord.

A 2.0-T small animal imager equipped with 3 G/cm imaging gradient coils was used to obtain images of en bloc segments of the vertebral column containing the rat spinal cord and of a cadaveric human spinal cord. These images demonstrate the applicability of MR micro-imaging techniques to the study of interesting problems in medicine and biology. Images of transverse sections of the spinal cord with in-plane resolution of better than 0.1 mm were obtained from slices 1 mm thick. The images of normal and injured rat spinal cord vividly show that spinal cord injury in the rat can be visualized using MRI techniques. The MR image of the cadaveric human spinal cord shows subanachroid gray and white matter structures. A comparison with a photomicrograph indicates that it is the superior contrast of the MR image that leads to increased conspicuity.

Animals↗

PubMed1990

Determination of extracellular/intracellular fluid ratios from magnetic resonance images: accuracy, feasibility, and implementation.

This study determines the accuracy and feasibility of using localized spin-lattice (T1) relaxation time measurements from magnetic resonance (MR) images to follow changes in extracellular/intracellular fluid ratios in defined subvolumes of living tissue. A red blood cell suspension was used as a test system and a simple two-compartment model incorporating fast exchange was found to suffice for the conversion of T1 values to volume ratios. The technique requires the addition of gadolinium-DTPA to the model system to selectively enhance relaxation in the extracellular fluid space. No detectable amount of gadolinium-DTPA was found to enter the intracellular fluid space, and all magnetization decay plots obtained from both intracellular constituents and complete RBC suspensions consisted of a single exponential. Both of these results are compatible with assumptions underlying our physical model. The NMR-determined fluid ratio values were compared to those measured via the microhematocrit technique. Partial saturation image-mode determinations are strongly correlated to microhematocrit data (R2 = 0.945) and indicate that localized cell volume changes may be followed with a sensitivity of +/- 2.2%. These values compare favorably with those produced when nonimaging inversion-recovery techniques are used to determine the MR hematocrit (R2 = 0.962, sensitivity = +/- 1.1%). This technique, with modification, should be applicable to the comparison of ratios of extracellular/intracellular fluid volumes in structurally complex tissues where small subvolumes of homogeneous cell structure could be examined.

Body Fluids↗

PubMed1990

Field dependence of 19F NMR in rat bone powders.

The 19F NMR properties of fluoridated rat bone powder samples have been studied in several external magnetic fields. The results show a characteristic field dependence, related to the strength of chemical shift interactions compared to dipole-dipole interactions. While the free induction decay shape is relatively insensitive to the 19F bone mineral concentration, the spin-lattice relaxation time decreases with increasing 19F concentration. Multi-exponential spin-lattice relaxation processes indicate that there are at least two chemically inequivalent incorporation sites for fluorine in bone mineral. Evidence for clustering of 19F fluoride impurities in bone mineral is seen in Hahn echo experiments. Sample preparation and handling methods are shown to affect the values of some of the observed NMR parameters.

Animals↗

PubMed1988

The center-tapped slotted tube autotransformer resonator: a coil for use with a high-resolution small animal imaging system.

A radiofrequency (rf) coil design based on a slotted tube resonator which allows high-resolution imaging of the rodent brain with excellent anatomic definition on a standard G.E. CSI II 2.0-T MR imaging unit with no modification of existing apparatus is described. The coil is easily constructed and permits very rapid tuning to different loading conditions, allowing a high sample throughput.

Animals↗

PubMed1988

Effect of dietary manipulations (fasting, hypocaloric feeding, and subsequent refeeding) on rat muscle energetics as assessed by nuclear magnetic resonance spectroscopy.

31P nuclear magnetic resonance (NMR) spectroscopy in vivo and fluorometry were used to measure muscle ATP, total creatine, pH, and Mg2+ in vivo; and to calculate creatine phosphate (CrP), the ratios of CrP/inorganic phosphate (Pi), CrP/ATP, free ADP levels, and the free-energy change in ATP hydrolysis so nutritional effects could be ascertained. These parameters were determined in vivo in resting control, 2-d-fasted, and hypocalorically fed rats and in animals similarly hypocalorically fed and then refed. The ATP, Pi, and intracellular Mg2+ levels were comparable in the four groups. When the fasted and underfed animals were compared with the control and refed animals, there were falls in the ratios of CrP/Pi and CrP/ATP, in the calculated CrP, and the free-energy change of ATP hydrolysis, but a rise in the calculated free ADP. In the hypocaloric group, intracellular pH fell significantly and a large peak was noted in the phosphodiester region. The data are consistent with the hypothesis that ATP levels are maintained at the cost of CrP, suggesting that ATP production is disturbed by aerobic and anaerobic mechanisms.

Adenosine Diphosphate↗

PubMed1984

Protein synthesis in germinating Saccharomyces cerevisiae ascospores.

The uptake and incorporation of macromolecular precursors in germinating Saccharomyces cerevisiae ascospores were investigated. Addition of cycloheximide at various times during germination revealed that protein synthesis can occur within 20 min after the spores are shifted to glucose-containing media. The time of initiation of uptake and incorporation of several amino acids differed; this can be attributed to differing amino acid pool levels in the spores, as well as differing transport activities. Two-dimensional gel electrophoresis of proteins labeled with [35S]methionine for various 20-min periods after germination began showed at least one protein whose synthesis begins well after the bulk of the proteins.

Amino Acids↗

PubMed1979

Comparison of Dycal and formocresol pulpotomies in young permanent teeth in monkeys.

This study compared Dycal and formocresol pulpotomies on young healthy permanent teeth with respect to continued dentinogenesis and root end development. Pulpotomy was performed on a total of forty permanent teeth with incompletely developed roots in three young stump-tailed monkeys (Macaca speciosa). Twenty teeth were treated with Dycal and twenty with formocresol. At the end of the experimental periods the animals were killed and the specimens were prepared and sectioned en bloc for histologic examination. The interval between treatment and death ranged between 7 and 797 days. Before each experimental procedure a Procion vital dye was administered as a marking agent for continued root development. Twelve of twenty teeth treated with Dycal and seventeen of twenty teeth treated with formocresol were judged to be successful as evidenced by continued root development, absence of periapical pathoses, and the presence of noninflamed or only mildly inflamed pulps. Brown and Brenn staining showed bacteria within the pulps of the teeth that failed. All but one of the teeth in this study showed evidence of continued root development as confirmed by Procion labeling.

Animals↗