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A W Anderson

Publications and source records attributed to A W Anderson.

78 records · Page 5Linked to original sources
PubMedDate not supplied

Diffusion-weighted imaging in tissues: theoretical models.

Typical diffusion measurements use Stejskal-Tanner pulsed gradient spin echo sequences to provide information about the average diffusion and displacement profiles of particles in a sample. To derive structural information, a measured displacement profile has to be related by means of a model to the physical and geometrical properties of the tissue, such as diffusion coefficients and shapes of semi-permeable membranes of compartments in the system. The behavior of the NMR signal and the measured ADC are greatly affected by the cellular architecture of a tissue, mainly because cellular membranes are relatively impermeable to water. For long diffusion times, and small signal attenuations, ADC is relatively insensitive to how it is measured. In general, however, ADC values are not readily interpreted unless the measuring conditions are specified in detail. For given measuring conditions, ADC depends on intra- and extracellular diffusion coefficients, membrane permeabilities, cell sizes and the cellular volume fraction. If intra- and extracellular T2 relaxation rates are different enough, ADC may also depend on the relaxation properties of the system and the echo time. An improved understanding of the precise influence of these factors has been obtained by detailed consideration of theoretical and computer models that can be related to experimental data in simple systems. Further refinements of such models should advance our understanding of water diffusion in tissues.

Body Water↗
PubMedDate not supplied

The relationship of problems in biomedical MRI to the study of porous media.

The NMR methods that are used to characterize inanimate porous media measure relaxation times and related phenomena and material transport, fluid displacement and flow. Biological tissues are comprised of multiple small, fluid-filled compartments, such as cells, that restrict the movement of the bulk solvent water and whose constituents influence water proton relaxation times via numerous interactions with macromolecular surfaces. Several of the methods and concepts that have been developed in one field of application are also of great value in the other, and it may be expected that technical developments that have been spurred by biomedical applications of MR imaging will be used in the continuing study of porous media. Some recent specific studies from our laboratory include the development of multiple quantum coherence methods for studies of ordered water in anisotropic macromolecular assemblies, studies of the degree of restriction of water diffusion in cellular systems, multiple selective inversion imaging to depict the ratios of proton pool sizes and rates of magnetization transfer between proton populations, and diffusion tensor imaging to depict tissue anisotropies. These illustrate how approaches to obtain structural information from biological media are also relevant to porous media. For example, the recent development of oscillating gradient spin echo techniques (OGSE), an approach that extends our ability to resolve apparent diffusion changes over different time scales in tissues, has also been used to compute surface to volume measurements in assemblies of pores. Each of the new methods can be adapted to provide spatially resolved quantitative measurements of properties of interest, and these can be efficiently acquired with good accuracy using fast imaging methods such as echo planar imaging. The community of NMR scientists focused on applications to porous media should remain in close communication with those who use MRI to study problems in biomedicine, to their mutual benefits.

Animals↗
PubMedDate not supplied

The absence of rhamnose in human urine following the ingestion of gum karaya (Sterculia).

Urinary rhamnose estimations following ingestion of gum karaya were requested by the Scientific Committee for Food (EEC) in July 1983. Five male volunteers have therefore made 24-h urine collections prior to, and following, the ingestion of 10 g gum karaya for 15 days, an intake ten-fold greater than that approved in terms of the present temporary ADI (0-12 X 5 mg/kg b.w.). Paper chromatographic separations, with two solvent systems, were made on the fresh urine specimens and also after ten-fold enrichments of all urinary constituents. Standard aqueous solutions of rhamnose, and urine to which rhamnose had been added, showed the detection limit to be 0.2 microgram rhamnose. Independent examinations in two laboratories failed to detect rhamnose at this level in any of the urine specimens, Had 1% of the rhamnose present in 10 g gum karaya appeared in the 24-h urine specimens, it would have been detected. This confirms previous evidence that dietary gum karaya is neither digested nor degraded by enteric bacteria and is not absorbed to any significant extent in Man.

Adult↗
PubMedDate not supplied

A comparison of the pulpal response to freeze-dried bone, calcium hydroxide, and zinc oxide-eugenol in primary teeth in two cynomolgus monkeys.

This study assessed the pulp healing response to human freeze-dried bone (FDB) in two cynomolgus monkeys using 36 noncarious primary teeth pulpotomized and randomly assigned to three medicaments. FDB was applied on the pulp stumps and covered with sterile tin foil as experimental group. The two other groups received either calcium hydroxide [Ca(OH)2], or IRM (reinforced ZOE). All teeth were restored with amalgam. One animal was sacrificed at 6 weeks and the other 6 months after treatment. Teeth were extracted and placed in 10% formalin. Histological evaluation indicated that 100% of teeth treated with FDB had vital pulps compared with 75% of the Ca(OH)2 group after 6 weeks. Dentin bridges were present in 87.5% of FDB versus 75% of Ca(OH)2 group. Inflammatory cells were absent or mild in 100% of FDB-treated versus 75% of the Ca(OH)2 group. After 6 months, 83.3% of FDB-treated teeth had vital pulps compared with the Ca(OH)2 group, which showed 100% pulpal necrosis. In FDB-treated pulps, 100% of teeth showed dentin bridges versus 50% of teeth treated with Ca(OH)2. Inflammatory cells were absent or mildly present in 83.3% of FDB-treated teeth while 100% of Ca(OH)2 showed moderate to severe inflammation. IRM-treated teeth all showed pulpal necrosis after 6 months. We concluded that FDB was superior to calcium hydroxide in treating primary pulp dentition in cynomolgus monkeys.

Animals↗