G.R. Davis

X-ray microtomography: nondestructive three-dimensional imaging for in vitro endodontic studies.

Article shows the application of a laboratory x-ray microtomography system, a miniaturized form of conventional computerized axial tomography, to the study of root canal morphologic characteristics and changes in the course of root canal treatment in extracted teeth. After reconstruction of the three-dimensional images, the IDL software package (Research Systems, Inc., Colorado) was used to obtain cross-sectional slices of the tooth and three-dimensional views of rendered surfaces of constant mineral density. The root canal systems and changes in these were imaged at a resolution (cubic voxel side-length) of approximately 40 microns.

3D X-ray microscopic study of the extent of variations in enamel density in first permanent molars with idiopathic enamel hypomineralisation.

Objective To measure mineral concentration distributions within teeth with idiopathic enamel hypomineralisation, a condition in which developmental defects are seen in first permanent molars, and/or incisors.Design X-ray microtomographic and 3D x-ray microscopy.Setting UK University, 2001.Materials and methods X-ray microtomographic measurements of the extent of hypomineralisation in two affected molars and two contralateral controls extracted from the same patient.Results The control molars were visibly normal. The affected molars showed hypomineralised yellow opaque enamel with regions of breakdown. X-ray microtomographic images showed; a 20% reduction in mineral concentration of affected enamel (most cases involved full enamel thickness); hypomineralised enamel had a mineral concentration gradient opposite to that of normal enamel; regions of hypomineralisation distributed randomly throughout affected teeth, (apart from cervical region which was less severely affected).Conclusions The pattern of mineral concentration suggests a disturbance during the maturation process. Differences in susceptibility of the ameloblasts during different stages of dental development may explain the asymmetric distribution of the defects. Topical fluoride applications may help promote post eruption maturation of the surface layer in these teeth. The use of fissure sealants and adhesive materials appears to prevent further breakdown.

X-ray microtomography scanner using time-delay integration for elimination of ring artefacts in the reconstructed image

Abstract Most X-ray microtomography scanners work on the same principle as third-generation medical CT scanners, that is, the same point in each projection is measured by the same detector element. This leads to ring artefacts in the reconstructed image if the X-ray sensitivities of the individual detector elements, after any analytical correction, are not all identical. We have developed an X-ray microtomography scanner which uses the time-delay integration method of imaging with a CCD detector to average the characteristics of all the detector elements in each linear projection together. This has the added advantage of allowing specimens which are larger than the detector and X-ray field to be scanned. The device also uses a novel mechanical stage to “average out” inhomogeneities in the X-ray field. The results show that ring artefacts in microtomographic images are eliminated using this technique.

Synchrotron X-Ray Microtomographic Investigation of Mineral Concentrations at Micrometre Scale in Sound and Carious Enamel

Synchrotron X-ray microtomography (XMT) was used to measure the linear attenuation coefficient (LAC) for 1.9-µm sidelength voxels within approximal brown spot lesions and sound human enamel. XMT demonstrated three-dimensional features, notably sheets with ∼30 µm periodicity having low LAC, identified as regions of demineralization corresponding to Retzius lines. Quantitative three-dimensional measurements of mineral concentration, derived from LAC with assumption of a single model composition, were consistent with previous measurements of sound and carious enamel from microradiographic projections. The uncertainty in measurements of mineral concentration and mineral fraction volume was investigated by modelling enamel with a range of composition and component densities. This analysis showed that, although mineral concentration can be determined from LAC with an error of <0.2 g cm–3, the variation in pore fraction volume within caries lesions cannot be reliably determined from X-ray attenuation measurements alone.

Longitudinal Study of the Three-Dimensional Development of Subsurface Enamel Lesions during in vitro Demineralisation

A longitudinal study was made of the 3D development of subsurface enamel lesions in whole human molars. X-ray microtomography (XMT) was used to measure the 3D distribution of linear attenuation coefficients in the tissue at 8–15 stages during cumulative times of 36–107 days demineralisation through ∼1-mm-wide windows. Although lesion morphology was consistent with preferential anisotropic dissolution parallel to enamel prisms at the advancing front, detail (at a scale of ∼100 µm) varied in relation to exposed surface sites separated by <1 mm. The distribution of mineral in the most superficial region varied across the exposed face of each lesion. Within lesions, localised foci of low mineral concentration (at a scale of ∼200 µm) retained their general form through successive stages of demineralisation before coalescing. The most advanced regions within a lesion seemed to correspond with surface regions with lowest mineral concentration. These findings indicate that local variations in fractional pore volume of partially demineralised enamel influence the subsequent spatial development of lesions.

Artefacts in X-ray microtomography of materials

Abstract X-ray microtomography is becoming an increasingly popular tool in the study of microstructure and failure mechanisms in biological and engineering materials, producing three-dimensional (3D) maps of the X-ray linear attenuation coefficient. Limitations of the technique are due to, for example, limited X-ray flux, use of polychromatic radiation (in laboratory systems), finite resolution, discrete sampling and X-ray scatter. These give rise to artefacts in the reconstructed image. Knowledge of these artefacts helps to distinguish them from real features and is important for optimising experimental design so as to minimise their effect on the results. To aid identification, artefacts were simulated computationally using an analytical phantom projection generator. Streak, ring, motion and beam hardening artefacts are considered as well as the results of errors in the centre of rotation and missing information in cone beam geometry. Examples of tomographic images of real materials specimens with motion and beam hardening artefacts as well as centring errors are also shown.

Tooth size patterns in patients with hypodontia and supernumerary teeth

AIMS Anomalies of tooth number may not be isolated conditions but may have wider associations in the development of the dentition including tooth size. This study aimed to examine links between hypodontia, supernumerary teeth and crown size, considering the effect on the development of the whole dentition and so increase understanding of the aetiology of these conditions. METHODS AND RESULTS The patients, who were all of European ancestry, were 60 young adults (30 males and 30 females) with hypodontia and 60 age and sex matched controls together with 60 young adults (39 males and 21 females) with supernumerary teeth and 60 age and sex matched controls. Hand measurements of mesiodistal and buccolingual dimensions were made of the teeth on dental study models using Mitutoyo electric callipers. The mean value of two measurements was used and intra-operator and inter-operator reliability determined. Patients with hypodontia had smaller teeth than the control group and this difference was statistically significant (p<0.05) for all teeth except the MD dimensions of 13, 23, 24 and 44. The difference in size was greatest for the BL dimensions in hypodontia patients. Further, the greater the number of missing teeth the smaller the tooth size. The hypodontia patients also showed higher variability in tooth dimensions than the control group. Patients with supernumerary teeth had larger teeth than the controls, with the greatest differences in the MD dimensions. In both hypodontia and supernumerary patients the differences in tooth size were generalised throughout the dentition. CONCLUSIONS In anomalies of tooth number the size of teeth is also involved. In patients with hypodontia and supernumerary teeth the crown size of the whole dentition is affected. These findings are compatible with a multifactorial aetiology of these conditions.

DETERMINATION OF MINERAL CONCENTRATION IN DENTAL ENAMEL FROM X-RAY ATTENUATION MEASUREMENTS

The mineral content of dental enamel is commonly measured by X-ray attenuation experiments. Most studies have used contact microradiography in which intensities are measured with photographic film which is convenient and gives high spatial resolution. However photon counting intensity measurements are to be preferred in many experiments (longitudinal and scanning microradiography, and microtomography), as illustrated here, because they have a larger dynamic range and greater sensitivity to small intensity changes. Additionally, the detector and specimen are well separated which allows the pseudo-continuous study of de- and remineralization. The mineral content is often quoted as 95 wt% or 87 vol% hydroxyapatite for permanent human enamel. This determination from attenuation experiments requires accurate values of elemental mass attenuation coefficients and a number of assumptions. The effects of possible choices of these are considered and it is shown that the most important is the density of enamel mineral used in conversion of wt% to vol%. If the density is taken as 2.99 g cm(-3), as recently suggested (J.C. Elliott, Dental Enamel, Ciba Foundation Symposium 205, Wiley, Chichester, pp. 54-72, 1997), instead of 3.15 g cm(-3) as for hydroxyapatite, the calculated vol% is approximately 93 instead of approximately 87.

Motor nerve conduction velocity distributions in man: results of a new computer-based collision technique

A new computer-based collision technique for direct measurement of the human motor nerve conduction velocity distribution is described. In contrast to previous collision techniques, the test muscle response is progressively cancelled to a null using an arrangement of proximal and distal stimuli which eliminates distortion of the test response caused by transient changes in nerve and muscle fibre conduction. The increased sensitivity of this new technique permits accurate measurement of the slowest 1% of alpha motor nerve fibres. We have used our modified collision technique to determine motor nerve conduction velocity distributions for the median nerve in 20 normal subjects aged between 19 and 59 (mean 35) years. 150% maximal stimulus intensities were used, with a controlled limb temperature of 35 degrees C. Group mean velocities (+/- S.D.) for the fastest (95%), mean (50%) and slowest (5% and 1%) motor fibres were 59.1 +/- 3.0, 56.9 +/- 2.9, 52.7 +/- 3.1 and 51.2 +/- 3.7 m/sec respectively. Data are also presented for the ulnar and peroneal nerves.

Application of scanning microradiography and x-ray microtomography to studies of bones and teeth.

In scanning microradiography (SMR), a thin section is stepped across a 15-μm diameter X-ray beam and the transmitted intensity measured at each point. This technique has permitted more accurate measurements of the spatial variation of the mineral concentration in sections of dentin and enamel than conventional photographic microradiography. Moreover, because the section is not in close contact with an emulsion, SMR allows continuous study while the specimen is bathed in a reaction solution. The present studies have been particularly directed to gaining an understanding of the formation and repair of carious lesions in teeth: one particular puzzle is subsurface demineralization, in which the initial loss of mineral appears to take place some 20 to 50 μm below the tooth surface. SMR studies are reported here on the demineralization in dilute acids and the subsequent partial remineralization in supersaturated calcium phosphate solutions in model systems for dental caries. In order to develop a theoretical model for de- and remineralization of carious lesions, it is necessary to quantify transport processes within the tooth. To this end, we are developing a method of measuring effective diffusion coefficients of strongly X-ray-absorbing ions in water within permeable solids in which the diffusion coefficient varies with position. The method uses sequential concentration/distance profiles determined by SMR. As a test, diffusion coefficients of potassium iodide in water within a permeable glass frit have been measured. X-ray microtomography (XMT) can be carried out by adding an axis of rotation to the SMR apparatus. Using this method, linear absorption coefficients, and hence mineral concentrations, can be measured in 15 X 15 X 15-μm3 voxels. This has advantages over SMR in that superposition within the depth of the section and errors in determining its thickness are avoided. XMT studies of de- and remineralization similar to those described above for SMR, and also XMT studies of the variation in mineral concentration in the cortical bone of a rat femur along its length, are reported.