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Biomedical subjects

J M Pope

Publications and source records attributed to J M Pope.

At least 37 records · Page 2Linked to original sources
PubMed1995

The effect of sacrifice on image signal, T1, T2, and T*2 in liver, kidney, and brain of the Wistar rat.

Signal intensity in SE240/25 and GE240/25 images, T*2, T2, and T1 were measured prior to and after sacrifice at 1.5 T in liver, kidney, and brain of Wistar rats. Presacrifice, animals were maintained on 2-3% halothane, ensuring a high saturation of venous haemoglobin with oxygen, which emphasised blood oxygenation level dependent (BOLD) effects upon deoxygenation of haemoglobin postsacrifice. T*2 values (in ms) changed on sacrifice from 27.8 +/- 4.1 to 15.0 +/- 1.1 in liver (p << .001), from 57.4 +/- 6.5 to 52.8 +/- 5.7 in brain (p < .02), and from 52.3 +/- 10.3 to 37.6 +/- 11.9 in kidney cortex (p < .01). T*2 was also reduced in kidney medulla and papilla. T2 reduced on sacrifice (by a lesser amount than T*2), except in brain, which showed a small increase. T1 (ms) changed only in liver, showing a significant increase from 670 +/- 31 to 834 +/- 69 (p << .001). The ratio of signal postsacrifice to presacrifice was lowest in the SE240/25 and GE240/25 images of liver (0.73 +/- 0.03 and 0.52 +/- 0.05, respectively), but increased T1 may account for some of these changes. Expected signal ratios at sacrifice calculated from measured relaxation times agreed well with those observed experimentally. Relaxation changes observed on sacrifice were consistent with deoxygenation of haemoglobin. These results give an estimate of the maximum effects which changes of blood oxygen level might produce at 1.5 T and suggest that contrast due to variation of endogenous deoxyhaemoglobin may be exploitable in the liver.

Animals↗
PubMed1995

Quantitative magnetic resonance flow and diffusion imaging in porous media.

Quantitative flow and diffusion measurements have been made for water in model porous media, using magnetic resonance micro-imaging methods. The samples consisted of compacted glass beads of various sizes down to 1 mm diameter. Typical flow and diffusion images exhibited a spatial resolution of 117 microns x 117 microns and velocities in the range 1-2 mm/s. Comparison of volume flow rates calculated from the flow velocity maps with values measured directly yielded good agreement in all cases. There was also good agreement between the mean diffusion coefficient of water calculated from the diffusion maps and the bulk diffusion coefficient for pure water at the same temperature. In addition, the mean diffusion coefficient did not depend on the pore sizes in the bead diameter range of 1-3 mm. Our results also show that partial volume effects can be compensated by appropriate thresholding of the images prior to the final Fourier transformation in the flow-encoding dimension.

Diffusion↗
PubMed1995

Choice of soft pulse shapes for signal excitation in chemical shift selective imaging.

The choice of soft pulse shapes for chemical shift selective excitation in chemical shift imaging is discussed. In the presence of inhomogeneities in the static magnetic field resulting from susceptibility anomalies, it is important to optimise pulse bandshape and frequency offset as well as bandwidth, in order to minimize artefacts arising from excitation of unwanted resonances. A comparison of the use of Gaussian and sinc shaped excitation pulses in the chemical shift micro-imaging of grapes serves to illustrate some of the effects that may be observed.

Magnetic Resonance Spectroscopy↗
PubMed1994

Hydration forces and membrane stresses: cryobiological implications and a new technique for measurement.

Very large, repulsive forces are measured between various surfaces in water at separations of about a nanometer or less. These forces are important in cryobiology because extracellular freezing usually causes extreme osmotic dehydration of cells. This brings membranes and macromolecules into close approach, and imposes large, anisotropic stresses on them. It is therefore important to study these forces at freezing temperatures. We have studied the freezing and thawing behaviour of lamellar phases of egg yolk lecithin and D2O. Force-hydration and force-separation relations are obtained from the deuterium nuclear magnetic resonance signal as a function of temperature. From these measurements we estimate the magnitude of freezing-induced membrane stresses and discuss their effect on the response of cells and organelles to freezing and thawing.

Cell Membrane↗
PubMed1994

Dose distribution measurements in superficial x-ray beams using NMR dosimetry.

The conversion of Fe2+ to Fe3+ in a Fricke solution due to ionizing radiation can be detected using nuclear magnetic resonance (NMR) imaging. The aim of the present study was to develop a suitable system for the study of dose distributions in superficial radiation beams making use of the water equivalence of the dosimetric gel system. Agarose gels (1.5%) doped with 0.5 mM ammonium ferrous sulphate and 125 mM sulphuric acid were exposed to x-rays from a superficial radiotherapy treatment unit (HVT 1.4, 2.4 and 7.25 mm A1). In the experiments doses between 10 Gy and 20 Gy were given, and the gel surface was in direct contact with the lead glass cone applicators (diameters 1 cm and 2.5 cm) at an FSD of 10 cm. Images were obtained within 4 h after irradiation in the head coil of a 1.5 T clinical MR scanner. Using spin-echo sequences with seven different repetition times between 120 ms and 4 s, the spin-lattice relaxation time (T1) was calculated for specified regions of interest with about 1 mm spacing. The inverse of T1 was shown to be proportional to the given dose and 1/T1 maps were obtained for all three superficial radiation qualities. The depth-dose curves determined with NMR dosimetry compare well with those obtained with a thin-window parallel plate ion chamber and thermoluminescence dosimetry in the same radiation beams.

Dose-Response Relationship, Radiation↗
PubMed1993

Ni-DTPA doped agarose gel--a phantom material for Gd-DTPA enhancement measurements.

In order to study the relationship between the concentration of Gd-DTPA in tissue, and the resulting changes in relaxation times and signal intensity, a phantom material that has similar relaxation times to tissue and that can be doped with Gd-DTPA is required. The "tissue-equivalent" material should not contain Gd; nor should it alter the relaxivities of Gd-DTPA from their values in aqueous solution (R1 = 4.5 sec-1 mM-1; R2 = 5.5 sec-1 mM-1 at 1.5 T). Conventional materials, based on CuSO4-, MnCl2-, or GdCl3/LaCl3-agarose mixtures, are not suitable, since Gd is displaced from the Gd-DTPA chelate. The new material, consisting of Ni-DTPA dissolved in agarose, is easy to prepare and does not interact with Gd-DTPA. Its relaxation times are stable; relaxivity R1 was within 4% of its aqueous value over 109 days. T1s have low dependence on temperature (0.2-1.0%/degrees C at 21 degrees C) and on field strength, allowing the material to be used as a relaxation time standard for quality assurance. Equations giving the concentration of Ni-DTPA and agarose to produce a required T1 and T2 are provided.

Contrast Media↗
PubMed1993

Flow-selective pulse sequences.

A series of pulse sequences is described which selectively excite signals only from flowing spins. The method is based on the binomial selective excitation sequences employed for solvent suppression in NMR spectroscopy, combined with 180 degrees refocusing of chemical shift and static field inhomogeneity effects and the application of bipolar gradients to distinguish stationary from flowing spins. The effectiveness of the method is demonstrated by experiments on a simple flow phantom.

Humans↗
PubMed1993

Investigation of the tissue equivalence of gels used for NMR dosimetry.

The transition of Fe2+ to Fe3+ in Fricke solution after irradiation results in a change of NMR proton relaxation times in agarose gels which can be used for the dosimetry of ionizing radiation. The main advantage of this system is the possibility of observing dose distributions in 3 dimensions in a medium which is supposed to be tissue equivalent. The aim of the present study was to quantify parameters which determine the tissue equivalence of NMR dosimetry gels. Electron densities and effective atomic numbers were calculated for gels with varying iron, sulphur and agarose concentration. The Hounsfield CT numbers so derived agree well with the CT numbers measured on a clinical CT scanner (effective photon energy 70.7 keV). The Hounsfield CT number of 7.2 +/- 1.5 (n = 9) measured for a 1.5% agarose gel doped with 0.5 mM ammonium ferrous sulphate and 125 mM sulphuric acid compares well with the calculated one of 8 +/- 5. Relaxation times were measured from a series of MR images obtained on a 1.5 T clinical MR scanner. The observed change in 1/T1 of the gel with dose was found to be linear up to 10 Gy (0.084 s-1 Gy-1). No difference in dose response for 10 Gy delivered by four different superficial radiation qualities (HVT = 1.4-7.5 mm Al) could be observed. These findings and the calculated effective atomic number of 7.46 demonstrate the close tissue equivalence of this agarose gel which makes it an ideal tool for the investigation of low energy therapeutic x-rays.

Ferrous Compounds↗
PubMed1992

Motional narrowing of the 2H NMR spectra near the chain melting transition of phospholipid/D2O mixtures.

The reduction in spectral splitting, or motional narrowing, of the deuterium spectra of D2O/phospholipid mixtures near the main chain melting phase transition was studied for palmitoyloleoylphosphatidylcholine (POPC), palmitoyloleoylphosphatidylethanolamine (POPE) and equimolar mixtures of the two at 10% hydration. For POPC the splitting was about 1700 Hz in both the fluid and gel phases, dropping to zero near the phase transition (as reported previously). For POPE the splitting remained approximately constant above the phase transition. Below the phase transition the spectrum showed a single broad line whose linewidth varied between 100 Hz and 800 Hz. This was interpreted as being due to small domains of water within a weakly hydrated crystal. POPC:POPE (1:1) samples exhibited motional narrowing behaviour similar to that for POPC except that the splitting above the phase transition was approximately twice that below the transition. The relatively broad temperature range (approximately 20 K) of the transition is explained using a simple physical model involving lipid fluctuations near the phase transition.

Deuterium↗
PubMed1992

Artifacts in chemical shift selective imaging.

The role of susceptibility effects in the production of artifacts in chemical shift images generated by the selective excitation technique is discussed. The effects are demonstrated in images of phantom samples of agarose gels containing small air bubbles. The artifacts can lead to erroneous interpretations, in which the resonances to be resolved exhibit a relatively small chemical shift separation, such as that between water and soluble carbohydrates (sugars).

Artifacts↗
PubMed1992

The application of 3D chemical shift microscopy to noninvasive histochemistry.

Three-dimensional chemical shift imaging methods offer significant advantages over two-dimensional (chemical shift selective) techniques for the study of biological samples which exhibit complex spectra. We have applied 3D-CSI methods to the study of a plant system, (dried fennel mericarps) previously studied by 2D-CSI techniques, obtaining images with in-plane resolution down to 30 microns and special resolution of 0.4 ppm. A modification of the 3D-CSI technique which incorporates water suppression by a combination of pre-saturation and selective excitation is also described and its performance evaluated on both a phantom and a fresh fennel sample.

Ferula↗
PubMed1992

Haemolytic disease of the newborn probably due to anti-ELO, an antibody to a low frequency red cell antigen.

Investigation of mild bilirubinaemia and a positive direct antiglobulin test in a 2 day old baby revealed that the mother's serum contained an antibody against the low frequency antigen ELO, which was present on the father's red cells. Family studies showed that the ELO antigen segregates from Rh, Gc, ADA and PGM1. The ELO antigen is enzyme resistant and therefore not likely to be part of the MNS or Duffy systems. No abnormalities were detected in immunoblotting studies. Although insufficient samples were available to attempt elution of the antibody from the cord cells, it is probable that this case represents the first reported haemolytic disease of the newborn due to anti-ELO.

Adult↗
PubMed1991

Applications of chemical-shift-selective NMR microscopy to the non-invasive histochemistry of plant materials.

Chemical-shift-selective imaging at microscopic resolution has been applied to the study of various plant materials including orange peel, grape berries and both dried and undried fruits of fennel. It is shown that selective imaging of aromatics and carbohydrates as well as water and oil can be performed with in-plane resolution down to 13 microns and slice thicknesses of 1 mm or less, at a field strength of 4.7 Tesla. The noninvasive nature of the method gives it advantages over established methods of plant histochemistry which involve sectioning and staining to reveal different chemical constituents.

Fruit↗
PubMed1991

Cerebral ischemic events associated with endocarditis, retinal vascular disease, and lupus anticoagulant.

PURPOSE: A group of young patients presenting with cerebral ischemic events, endocardial lesions, and lupus anticoagulant is described in order to highlight the common clinical laboratory features. PATIENTS AND METHODS: Fourteen consecutive patients (10 females, age range 17 to 53 years [mean 38 years]) at onset of symptoms of cerebral ischemia who had evidence of the lupus anticoagulant syndrome and were being followed prospectively are reviewed. All patients had abnormal phospholipid-dependent coagulation test results, and most had anticardiolipin antibody at the time of presentation. Three of 14 had four or more American Rheumatism Association criteria for definite systemic lupus erythematosus and the remaining patients were considered to have primary lupus anticoagulant syndrome. RESULTS: The common features among these patients included at least one cerebral ischemic event at presentation (stroke or transient ischemic attack), or recurrent episodes suggesting cerebral ischemia (amaurosis fugax, recurrent severe migraine headaches), livedo reticularis, endocardial valvular lesions noted on echocardiography (11 mitral, two aortic valve) that were often associated with discrete vegetations, retinal vascular lesions, and computed tomographic/magnetic resonance imaging scanning or angiographic evidence of multiple cerebral infarction. Venous thromboembolic events were uncommon (three of 14). Common laboratory studies included thrombocytopenia (10 of 14), positive direct Coombs' test result (11 of 14), and hypocomplementemia (11 of 14). Follow-up after initial treatment with either salicylates or anticoagulant therapy (warfarin) for up to 10 years indicated that while many patients had recurrent symptoms suggesting cerebral ischemia, major stroke syndromes did not recur nor new episodes emerge. CONCLUSION: The combination of multiple cerebral ischemic lesions and endocardial lesions, including valvular vegetations, suggests that these cerebral ischemic events represent cerebral emboli, and that these cerebral embolic events originate from vegetative lesions on the mitral or, less commonly, aortic valve, in association with lupus anticoagulant.

Adolescent↗
PubMed1989

Chain-length dependence of n-alkane solubility in phosphatidylcholine bilayers: a 2H-NMR study.

Phosphatidylcholine bilayer membranes containing 2H-labelled n-alkanes have been studied by 2H-NMR as a model system for the investigation of molecular theories of general anesthesia. The solubilities of n-alkanes in lipid bilayers have been determined by measurement of the relative intensities of a powder pattern signal arising from orientationally ordered, membrane-soluble alkane and a sharp signal in the 2H-NMR spectrum resulting from isotropically reorienting alkane. The ordering profiles for the ordered n-alkane as determined from the quadrupole splittings for different segments along the chain are similar to those described earlier for n-hexane, n-octane and n-dodecane, suggesting that the restricted motions undergone by the n-alkanes of chain length from 6 to 19 are basically similar. For this homologous series of n-alkanes, it was found that membrane solubility dropped sharply at an alkane chain length which depended on lipid chain length, degree of unsaturation, cholesterol concentration in the bilayer, and temperature. The results show that the incorporation of n-alkanes in lipid bilayers is a complex function of lipid composition. The implications of these results in relationship to the observed 'cut-off' in anesthetic potency in the n-alkane homologous series are discussed.

1,2-Dipalmitoylphosphatidylcholine↗