Search

X‐ray microtomography (XMT), a miniaturised form of computed tomography, has been used to generate 3‐dimensional volume data sets of the X‐ray absorption of human teeth in vitro, with a resolution of ∼40 μm (cubic voxel sidelength). Examples are presented of images relevant to dental morphology, cariology, and cavity preparation and restoration. Applications of XMT imaging to dental education are discussed in the context of new approaches to visual learning through computer‐assisted methods.

X‐ray absorption and backscattered electron (BSE) microscopies are two commonly used techniques for estimating mineral contents in calcified tissues. The resolution in BSE images is usually higher than in x‐ray images, but due to the previous lack of good standards to quantify the grey levels in BSE images of bones and teeth, x‐ray microtomog‐raphy (XMT) images of the same specimens have been used for calibration. However, the physics of these two techniques is different: for a specimen with a given composition, the x‐ray linear attenuation coefficient is proportional to density, but there is no such relation with the BSE coefficient. To understand the reason that this calibration appears to be valid, the behaviour of simulated bone samples was investigated. In this, the bone samples were modelled as having three phases: hydroxyapatite (Ca10(PO4)6(OH)2), protein, and void (either empty or completely filled with polymethylmethacrylate (PMMA), a resin which is usually used for embedding bones and teeth in microscopic studies). The x‐ray linear attenuation coefficients (calculated using published data) and the BSE coefficients (calculated using Monte Carlo simulation) were compared for samples of various phase proportions. It was found that the BSE coefficient correlated only with the x‐ray attenuation coefficient for samples with PMMA infiltration. This was attributed to the properties of PMMA (density and mean atomic number) being very similar to those of the protein; therefore, the sample behaves like a two‐phase system which allows the establishment of a monotonic relation between density and BSE coefficient. With the newly developed standards (brominated and iodinated dimethacrylate esters) for BSE microscopy of bone, grey levels can be converted to absolute BSE coefficients by linear interpolation, from which equivalent densities can be determined.

OBJECTIVES: To examine whether antibiotics are indicated in treating uncomplicated acute sinusitis and, if so, whether newer and more expensive antibiotics with broad spectra of antimicrobial activity are more effective than amoxycillin or folate inhibitors. DESIGN: Meta-analysis of randomised trials. SETTING: Outpatient clinics. SUBJECTS: 2717 patients with acute sinusitis or acute exacerbation of chronic sinusitis from 27 trials. INTERVENTIONS: Any antibiotic versus placebo; amoxycillin or folate inhibitors versus newer, more expensive antibiotics. MAIN OUTCOME MEASUREMENTS: Clinical failures and cures. RESULTS: Compared with placebo, antibiotics decreased the incidence of clinical failures by half (risk ratio 0.54 (95% confidence interval 0.37 to 0.79)). Risk of clinical failure among 1553 randomised patients was not meaningfully decreased with more expensive antibiotics as compared with amoxycillin (risk ratio 0.86 (0.62 to 1.19); risk difference 0.9 fewer failures per 100 patients (1.4 more failures to 3.1 fewer failures per 100 patients)). The results were similar for other antibiotics versus folate inhibitors (risk ratio 1.01 (0.52 to 1.97)), but data were sparse (n=410) and of low quality. CONCLUSIONS: Amoxycillin and folate inhibitors are essentially as effective as more expensive antibiotics for the initial treatment of uncomplicated acute sinusitis. Small differences in efficacy may exist, but are unlikely to be clinically important.

A novel technique for forming superconducting Y1Ba2Cu3O7-y, thick films by screen printing pastes made from spray-dried nitrate powders on alumina and magnesia is described. Manufacture and characterization of nitrate powders are discussed. A model based on dissolution of yttrium nitrate is proposed to explain the rheological and film-forming properties of the nitrate pastes. Low-viscosity pastes are found to give better performance in terms of printability and quality of prints. Barium carbonate formation can be avoided by use of the nitrate pastes and by firing in an oxygen atmosphere. Superconducting thick films were achieved on single-crystal (1 0 0) MgO (T0=77 K) and on polycrystalline alumina with a prereacted barrier layer (T0=55 K).