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D G Murphy

Publications and source records attributed to D G Murphy.

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

Local histogram correction of MRI spatially dependent image pixel intensity nonuniformity.

We describe a computationally straightforward post-hoc statistical method of correcting spatially dependent image pixel intensity nonuniformity based on differences in local tissue intensity distributions. Pixel intensity domains for the various tissues of the composite image are identified and compared to the distributions of local samples. The nonuniformity correction is calculated as the difference of the local sample median from the composite sample median for the tissue class most represented by the sample. The median was chosen to reduce the effecters on determining the sample statistic and to allow a sample size small enough to accurately estimate the spatial variance of the image intensity nonuniformity. The method was designed for application to two-dimensional images. Simulations were used to estimate optimal conditions of local histogram kernel size and to test the accuracy of the method under known spatially dependent nonuniformities. The method was also applied to correct a phantom image and cerebral MRIs from 15 healthy subjects. Results show that the method accurately models simulated spatially dependent image intensity differences. Further analysis of clinical MR data showed that the variance of pixel intensities within the cerebral MRI slices and the variance of slice volumes within individuals were significantly reduced after nonuniformity correction. Improved brain-cerebrospinal fluid segmentation was also obtained. The method significantly reduced the variance of slice volumes within individuals, whether it was applied to the native images or images edited to remove nonbrain tissues. This statistical method was well behaved under the assumptions and the images tested. The general utility of the method was not determined, but conditions for testing the method under a variety of imaging sequences is discussed. We believe that this algorithm can serve as a method for improving MR image segmentation for clinical and research applications.

Adult↗
PubMedDate not supplied

NIH gerontology research support: policy evaluation of in vitro cell lineage studies.

Populations of cultured human diploid cells, used extensively in research on aging, are not well understood in terms of kinetics and mitotic activity of constituent cells. NICHD has supported, by contract, cell lineage studies to elucidate such cell characteristics contributing to the cell population degeneration and death. The policy of NICHD (NIA) encouragment of such studies has been evaluated. A need for cell lineage and related data from cultured-cell populations is recognized, and research necessary to achieve these data will be encouraged. Grant applications to this end are sought.

Cell Line↗
PubMedDate not supplied

Method for quantification of brain, ventricular, and subarachnoid CSF volumes from MR images.

We describe a simple, rapid, and semiautomated method of MR analysis based on mathematical modeling of MR pixel intensity histograms. The method is shown to be accurate and reliable for regional analysis of brain, central, and subarachnoid CSF volumes. Application of the method to five young and six older subjects revealed significant age-related changes in regional brain volumes whereas no difference was found for traced central CSF volumes or subarachnoid CSF volumes. We conclude that this is a simple method that can be applied to further studies of quantification of brain structure in healthy aging and brain disease.

Adult↗
PubMedDate not supplied

The effect of polishing on surface roughness of tissue conditioners.

PURPOSE: Surface roughness can affect microbial colonization of long-term denture liners, alloys, and denture acrylic. The purpose of the present study was to examine the effect of finishing and polishing procedures on surface roughness of 4 temporary resilient denture liners (tissue conditioners). MATERIALS AND METHODS: Mean surface roughness was measured for 4 materials (Lynal, Visco-gel, Coe-Soft, and Functional Impression Tissue Toner [FITT]) finished in 4 ways: unfinished (control); polished; reduced, unpolished; and reduced, polished. Samples were allowed to polymerize at 37 degrees C for 24 hours, and the surface roughness was measured using a Mitutoyo Surftest 212. RESULTS: Mean surface roughness ranged from 1.8 +/- 0.8 microns for polished Lynal to 7.8 +/- 1.1 microns for reduced, unpolished FITT. All polished samples were smoother than unpolished samples (including controls), whether or not the samples were reduced with a bur. CONCLUSION: Polished samples of tissue conditioning material had lower mean surface roughness measurements than control or reduced, unpolished samples at the 95% level of confidence. There was no difference in mean surface roughness measurements of control samples and unpolished samples reduced with a bur at the 95% level of confidence. Mean surface roughness differed significantly between the materials tested.

Acrylic Resins↗