Radiation doses from common radiographic procedures: a ten year perspective.

Using a semi-empirical model patient doses for a number of plain film radiographic procedures following the implementation of Computed Radiography (CR) technology in our Hospital have been evaluated. The results are presented in terms of the entrance surface dose (ESD) and the effective dose. A comparison of these results, with those reported previously for 1988, suggests that with the exception of chest radiography, patient doses have decreased although in many instances the decreases are not statistically significant. The finding for chest examinations stands apart from all others in that the introduction of CR technology has resulted in a substantial increase in patient dose for the PA view by at least 18%. The major reason for these apparently contradictory findings has its roots in the effectively variable speed of CR systems and the willingness of radiologists to accept more noise in some CR images.

An investigation into the impact of anatomical variation upon mean glandular dose produced within a standard breast.

By using the EGS4 Monte Carlo computer program the normalised mean glandular dose (MGD) was calculated for a breast model that is intended to reflect the composition of an "average" breast. The reliability of the calculation was established by comparing the predictions with previously published values for a breast model. The breast model used was then altered in order to reflect the possible extremes in glandular distribution that could occur within a compressed breast. These results show that the MGD could vary by up to a factor of four depending upon where the majority of glandular tissue is located within the breast. The impact of this variation in MGD upon risk assessment within mammography is then discussed.

Determination of correct AEC function with computed radiography cassettes.

It has long been recognized that the assessment of Automatic Exposure Control (AEC) function is an important criterion in the assessment of any general radiographic x-ray installation. Of the tests commonly performed on an AEC, only Beam Quality Dependence relates intimately with the optical density produced on the resultant radiograph. In this test the AEC function is ascertained as being satisfactory or otherwise, on the basis of whether the optical density on the radiograph is controlled within well defined limits. With computed radiography (CR), where the conventional screen-film combination is replaced by a storage phosphor screen, the process by which the digital image is presented on film restricts the resultant optical densities to an extremely tight range, independent of the air kerma incident upon the CR phosphor plate. The implication of this is that the measurement of optical density can not be used as a direct measure of AEC function, and a test such as Beam Quality Dependence cannot be performed in the conventional manner. To overcome this, the relationship between incident air kerma and a computer generated index supplied with the CR system was investigated for two commercially available systems, one supplied by Fuji Medical Systems and the other by Kodak. The relationship between the index and incident air kerma could then be used as a substitute for optical density in monitoring AEC performance with respect to beam quality. It has been determined that an inverse linear relationship exists between the incident air kerma and the Sensitivity Number for the Fuji CR system and that a logarithmic relationship exists between the Exposure Index and incident air kerma for the Kodak CR system. In order to quantify these relationships further, the dependence of Sensitivity Number and Exposure Index on beam quality and incident kVp have been investigated. Using the results from the above investigations, acceptable limits for both Sensitivity Number and Exposure Index, can be established to test for correct AEC function, with respect to Beam Quality Dependence.

Acute inflammatory changes in subcutaneous microtumors in the ears of mice induced by intravenous CM101 (GBS toxin).

CM101, a bacterial polysaccharide derived from group B streptococcus, induces pronounced inflammatory changes in and around tumor blood vessels 60 min after i.v. injection. A technique has been developed for implanting small numbers of tumor cells in the ear skin of mice. This allows macroscopic examination of the tumor and its supporting blood vessels as it reaches the 10000 cell size and greater. Treatments can be monitored in this model for effects on small "metastatic-like" tumor nodules by direct observation and by histological examination. Inflammatory changes were indicated by increased numbers of polymorphonuclear leukocytes (PMN) adjacent to and marginating within thin-walled blood vessels and within the tumor tissue. PMN were seen in the process of migrating through venules and enlarged capillaries, each with prominent endothelial cells. Tumor morphology was variable with evidence of occasional single necrotic cells. This contrasted with tumors in ears of dextran-treated or untreated mice, which had uniform tumor morphology, and acute inflammatory cells were rarely present.

Evidence for two conductance/exchange pathways for chloride in rat parotid secretory granules.

Electron probe x-ray microanalysis was used to determine that bromide is localized to rat parotid secretory granules at early stages of an in situ Cl/Br washout experiment. Chloride efflux and bromide influx across the secretory granule membrane occurred with a time order of minutes. Since the Cl washout data indicated minimal Cl binding within the granule, and therefore minimal Br binding, the Br localization results suggest the presence of two or more anion conductance/exchange pathways in the granule membrane for the Cl (Br) ion.

Beam induced mass loss in high resolution biological microanalysis.

Electron beam induced loss of mass from the organic matrix and from higher Z constituents of biological samples was measured by monitoring bremsstrahlung and peak changes in EDS spectra. When any effects of contamination, extraneous X-rays, beam current drift, specimen drift, and specimen shrinkage were monitored and corrected for, the three types of samples gave consistent and similar results at 296 K. Bremsstrahlung losses averaged 45%, 46% and 50% respectively for muscle homogenate, salivary gland sections and albumin. Sulphur losses average 74%, 72% and 86% for the same three sample types. No other elements suffered significant losses. D1/e for bremsstrahlung averaged 0.14 C/cm2. Bremsstrahlung loss at 93 K began approximately one order of magnitude higher in dose, and the extent of loss varied. Sulphur losses, however, were greatly reduced at low temperatures.

Analysis of chromatin-associated fiber arrays.

Electron microscopic examination of chromatin from embryonic nuclei of Oncopeltus fasciatus and Drosophila melanogaster reveals arrays of chromatin associated fibers. The lengths and spacings of these fibers were analyzed to provide a basis for defining and interpreting regions of transcriptionally active chromatin. The results of the analysis are consistent with the interpretation of some fibers as nascent RNA with associated protein (RNP). The chromatin segments underlying these fiber arrays were classified as ribosomal or non-ribosomal transcription units according to definitions and criteria described by Foe et al. (1976). Nascent fibers on active ribosomal transcription units were analyzed and compared for Drosophila melanogaster, Triturus viridescens, and Oncopeltus fasciatus. A common feature of the fiber patterns on ribosomal TUs is that origin-distal fibers exhibit greater length variability and a lower slope relative to proximal fibers. The region of increased variability in fiber lengths is correlated with the expected location of 28S ribosomal RNA sequences in the distal half of each ribosomal transcription unit. Because 28S ribosomal RNA appears to contain more extensive regions of base sequence complementarity, we suggest that the length of ribosomal RNP fibers is influenced under our spreading conditions by the secondary structure of the nascent RNA. In order to calculate the RNA content of RNP fibers, chromatin morphology was used to estimate lengths of transcribed DNA. The packing ratio of DNA in chromatin, which we express as the length of B-structure DNA divided by length of chromatin, is 1.1-1.2 and 1.6 for the DNA in active ribosomal and non-ribosomal chromatins, respectively. These DNA packing ratios are used to determine the extent to which nascent RNP fibers are shorter than the transcribed DNA (expressed as DNA/RNP length ratio). For non-ribosomal transcription units and for proximal fibers of ribosomal transcription units. DNA/RNP length ratios are relatively constant within each array. However, considerable variability in this ratio (4-23) is observed for different arrays of fibers. Possible sources of this variability are considered by comparing ratios derived from the presumably identical ribosomal transcription units. Further analysis of the morphology of nascent fibers may elucidate the contributions of proteins and successive RNA sequences to RNP structure.

Comparative organization of active transcription units in Oncopeltus fasciatus.

We have analyzed electron micrographs of chromatin-associated fiber arrays from embryos of the milkweed bug, Oncopeltus fasciatus. The analysis has revealed that the arrays have highly ordered patterns of fiber spacings and lengths. These patterns support the interpretation that the fibers are nascent RNA with associated proteins (RNP fibers) which have resulted from transcription of the DNA in the underlying chromatin segment. In particular, the patterns indicate that the chromatin underlying each array is delimited by specific sites for initiation and termination of transcription. We apply the term transcription unit to a chromatin segment thus bounded. The analysis has further revealed that transcription units can be grouped into two principal classes--ribosomal and nonribosomal. Active transcription units of these two classes differ in DNA content, in their proximity to other active transcription units, and in their chromatin morphology. For certain developmental stages, fiber frequencies (that is, the nubmers of fibers per mum of chromatin) are also useful in distinguishing ribosomal from nonribosomal arrays. The most definitive of the above classification criteria is chromatin morphology as observed under our preparative conditions. We propose that term rho chromatin for the unbeaded or smooth chromatin that underlies nascent ribosomal RNP fibers. DNA in rho chromatin has a calculated packing ratio of approximately 1.2 mum of B structure DNA per mum of chromatin. Nu chromatin is used to designate the beaded chromatin for which we calculate a DNA packing ratio of 1.6-2.3 in our preparations. This calculation for nu chromatin is based on the inference that the beads are nucleosomes (nu bodies, PS particles, unit particles). The beaded morphology is observed between fibers of nonribosomal transcription unit as well as for most fiber-free chromatin. The detection of specific sites of transcriptional initiation and termination and the classification of transcription units can provide a basis for further analysis of transcriptional control.