Search PubMed⌕ Search

Biomedical subjects

R Speller

Publications and source records attributed to R Speller.

11 recordsLinked to original sources
PubMed2006

Experimental validation of a simple model capable of predicting the phase contrast imaging capabilities of any x-ray imaging system.

Phase contrast (PC) imaging is one of the most exciting emerging x-ray imaging techniques, with the potential of removing some of the main limitations of conventional radiology. After extensive experimentation carried out particularly at synchrotron radiation (SR) facilities, the scientific community agrees that it is now time to translate these ideas towards the first clinical implementations. In this framework, a complete model, based on Fresnel/Kirchoff diffraction integrals, was devised. This model accounts for source dimensions, beam spectrum and divergence and detector point spread function (PSF), and can thus be applied to any x-ray imaging system. In particular, by accepting in input the above parameters along with the ones describing the sample, the model can be used to optimize the geometry of the set-up, i.e. to assess the source-to-sample and sample-to-detector distances which maximize feature detection. The model was evaluated by acquiring a range of images of different samples with a laboratory source, and a good agreement was found between simulated and experimental data in all cases. In order to maximize the generality of the results, all acquisitions were carried out using a polychromatic source and an energy-resolving detector; in this way, a range of monochromatic images could be obtained as well as polychromatic images, which can be created by integrating different parts of the acquired spectra. One of the most notable results obtained is that in many practical cases polychromatic PC imaging can provide the same image quality as its monochromatic counterpart. This is an important step in the wider application of PC using conventional sources.

Algorithms↗
PubMed2006

Preliminary estimates of the calcium/phosphorus ratio at different cortical bone sites using synchrotron microCT.

The Ca/P ratio was measured in cortical bone samples from the femoral neck, front and rear tibia of female rats (1.5 years of age), using synchrotron radiation microtomography. The use of a monoenergetic x-ray beam, as provided by the synchrotron facility, generates accurate 3D maps of the linear attenuation coefficient within the sample and hence gives the ability to map different chemical components. Data sets were taken at 20 keV for each bone sample and calibration phantoms. From the 3D data sets, multiple 2D slices were reconstructed with a slice thickness of approximately 28 microm and converted to Ca/P ratios using the calibration phantom results. Mean values (M +/- SD) for cortical femoral, front and rear tibias are 2.12 +/- 0.08, 1.75 +/- 0.06 and 1.94 +/- 0.07 respectively. These values were compared with those derived from different animals. Differences between the same bone sites from different animals are not significant (0.1 < p < or = 0.9) while those between different bone sites are highly significant (p < 10(-3)) demonstrating a dependence upon life style and bone use.

Animals↗
PubMed2006

End-user survey for digital sensor characteristics: a pilot questionnaire study.

OBJECTIVE: To survey end-user opinions on dental digital sensor characteristics for the design of a new X-ray imaging sensor. MATERIALS AND METHODS: 100 questionnaires were sent out to dentists and dental radiologists. The questionnaire consisted of six parts related to dental sensors. A: Details about the respondent; B: Prioritization of most important aspects of digital sensors; C: Rating advantages and D: disadvantages of digital sensors; F: Dental features that need to be enhanced by digital sensors; G: End-user comments. RESULTS: Fifty-six questionnaires were returned. Contrast resolution and imaging time were assessed as the most and the least important aspects, respectively. Aspects considered as advantages by approximately 80% of respondents were: optimal contrast resolution, increased specificity, increased discrimination between diseased and healthy tissue, increased sensitivity, lower dose to the patient by more than 20%; as a disadvantage, increased patient dose was given. Dental features considered important by over 80% of the respondents were for intraoral radiographs: the visibility of caries, periapical, periodontal, and bone lesions; for panoramic radiographs: reduction of ghost images, image sharpness and bone. Bone and soft tissues on cephalograms and bone on temporomandibular joint (TMJ) tomograms were mentioned by over 70%. CONCLUSION: The most desired characteristic for a new sensor is contrast resolution. Dose saving is not considered as important unless it is more than 20%. Examination time is not a major issue. Caries, periapical and periodontal, and bone lesions for intraoral radiographs, reduction of ghost images, image sharpness and bone for panoramic radiographs were the most frequently mentioned dental features that should be enhanced by digital sensors.

Artifacts↗
PubMed2005

High resolution Ca/P maps of bone architecture in 3D synchrotron radiation microtomographic images.

The Ca/P ratio was measured in cortical bone samples from the femoral neck and tibia of different animal species, using synchrotron radiation microtomography. Use of a monoenergetic X-ray beam, as provided by the synchrotron facility, generates accurate 3D maps of the linear attenuation coefficient within the sample and hence gives the ability to map different chemical components. Also, by comparing normal and abnormal bones, i.e. osteoporotic (induced by inflammation), changes in the Ca/P ratio brought about by bone diseases can be detected. MicroCT data sets were collected at 20 and 28 keV for each bone sample and two calibration phantoms. From the 3D data sets, multiple 2D slices were reconstructed with a slice thickness of approximately 30 microm. Regions of interest were defined around suitable sites and were converted to Ca/P ratios using the data collected from the test phantoms. A significant difference (p<0.001) between osteoporotics and age-matched normals at both energies was detected. Differences between different bone sites from the same animal are not significant (p>0.5) while those between the same bone sites from different animals are highly significant (p<0.001). Differences between estimates made at 20 and 28 keV are not significant (p>0.5). An important aspect is the ability to map the spatial distribution of the Ca/P ratio.

Animals↗
PubMed1999

In vivo MRI measurements of bone quality in the calcaneus: a comparison with DXA and ultrasound.

Magnetic resonance imaging (MRI) has shown promise in the assessment of bone architecture. The precision and feasibility of MRI measurements in osteoporosis in vivo have been assessed in this study. T2' was calculated from measurements of T2 and T2* in the calcaneus of 32 postmenopausal women using a gradient-echo sequence PRIME (Partially Refocused Interleaved Multiple Echo). This sequence allows the measurement of T2 and T2* in one acquisition. In vivo measurements of bone mineral density (BMD) by dual-energy X-ray absorptiometry (DXA) were made in the calcaneus, spine and femoral neck. The ultrasound parameters broadband ultrasound attenuation (BUA) and speed of sound (SOS) were also measured in the calcaneus. These three techniques have not previously been compared in the same study population. The precision of the MRI technique was poor relative to the DXA and ultrasound techniques, with a CV of 6.9% +/- 4.4% for T2' and 5.5% +/- 3.6% for T2*. Approximately 4% of this is due to system error as determined by phantom measurements. The postmenopausal women were classified as having low BMD if they had a lumbar spine (L2-4) BMD of less than 0.96 g/cm2 (more than 2 standard deviations below normal peak bone mass). Calcaneal T2' was significantly correlated with calcaneal BMD (r = -0.79, p < 0.0001), BUA (r = -0.59, p = 0.0004) and SOS (r = -0.58, p = 0.0006). T2' was significantly different in postmenopausal women with normal BMD and those with low BMD (p < 0.01). However, the difference was of only borderline significance (p < 0.06) after adjustment for age and years since menopause.

Absorptiometry, Photon↗
PubMed1999

R'2 measured in trabecular bone in vitro: relationship to trabecular separation.

Measurement of key parameters of the microstructure of trabecular bone is critical to the study of osteoporosis and bone strength. Density based methods cannot provide this information, and give only the total amount of bone present, and not its arrangement. Magnetic resonance imaging has shown the potential to provide information related to the microarchitecture of the trabecular bone matrix. Twelve samples (8 x 8 x 8 mm3 bone cubes) were cut from sheep vertebrae such that the trabeculae ran either parallel or perpendicular to each face. Detailed measurements of the structure of these bone cubes were made by histomorphometry, and compared to R'2 and R*2 measured with a spin and gradient-echo sequence, Partially Refocused Interleaved Multiple Echo, at 1.5 Tesla. The precision of the R'2 measurement (% coefficient of variation) was 8.7+/-5.1, and 7.7+/-4.3 for R*2. Uncorrected values of R'2 and R*2 were significantly correlated to density measured by quantitative computed tomography (r = 0.87, p = 0.0005, and r = 0.90, p = 0.0002, respectively), and trabecular bone area measured by histomorphometry (r = 0.80, p = 0.002, and r = 0.83, p = 0.0008, respectively). Density correction was effected by imaging the same slice of bone in two orientations (90 degrees and 0 degrees ) to the main magnetic field. For both R'2 and R*2 there was a significant difference between measurements in the 90 degrees and 0 degrees orientations (p < 0.01). The difference between the two values was used, and termed R'2net or R*2net. The net parameters were independent of bone mass. R'2net and R*2net were significantly correlated to trabecular separation (p < 0.05) with r = -0.58 and r = -0.62, respectively. These results demonstrate the ability of magnetic resonance imaging to characterize a key measure of the trabecular microstucture. An increase in trabecular separation has important biomechanical consequences in osteoporosis. This result also strengthens the hypothesis that the sensitivity of R'2 to osteoporosis-related bone changes is due to magnetic susceptibility effects in which rapid transitions between bone and marrow create local magnetic field inhomogeneities that result in an increase in R'2 values.

Absorptiometry, Photon↗
PubMed1999

The effect of region of interest selection on dual energy X-ray absorptiometry measurements of the calcaneus in 55 post-menopausal women.

Bone mineral density (BMD) of the calcaneus was assessed by dual energy X-ray absorptiometry (DXA) in four different regions of the calcaneus in 20 pre-menopausal and 55 post-menopausal women, none of whom were on treatment or suffering from conditions affecting bone. The total body option in the small animal software package of a Lunar DPX-L bone densitometer was used. The precision of the technique (%CV) varied from 0.7 to 2.2% depending on the region scanned. For post-menopausal women, BMD results in the mid and posterior parts of the calcaneus varied by < 7% while BMD in the anterior region was about 20% lower. DXA of the calcaneus was compared with measurements at the spine (L2-L4) and hip (femoral neck) and changes with age were estimated from cross-sectional data. BMD of the calcaneus was significantly reduced in 28 post-menopausal women with low lumbar spine BMD (-2SD) compared with women with normal spine BMD. Calcaneal BMD was significantly correlated to axial BMD (r = 0.45-0.77) and to age (r = 0.45 to -0.63). For a subgroup of 33 post-menopausal women measured twice after approximately 1 year, calcaneus BMD decreased by between 1.2% and 2.5% while axial BMD showed no significant change. Unlike spine or femoral neck BMD, the decrease in calcaneus BMD was significantly greater in women with low spine BMD than in normal women, possibly indicating improved detection of skeletal changes. The optimum measurement sites for BMD in the calcaneus were within the mid or posterior part of the calcaneus or enclosing the whole posterior calcaneus. The calcaneus was shown to be a precise, sensitive and simple measurement site suitable for the assessment of osteoporosis, especially in the elderly where degenerative changes in the spine and hip can complicate BMD assessment.

Absorptiometry, Photon↗
PubMed1998

Comparison of ultrasound and dual energy X-ray absorptiometry measurements in the calcaneus.

Ultrasound measurements have been proposed as a means of providing structural information on bone but have also been shown to correlate with density. High correlation coefficients have been obtained in vitro, but are lower for in vivo work. The aim of this study was to investigate the relationship of broadband ultrasound attenuation (BUA), speed of sound (SOS) and stiffness, to bone mineral density (BMD) measured in the calcaneus at a closely matched region of interest (ROI). Measurements were made in 55 post-menopausal and 20 young, normal women. Calcaneus BMD measurements were made using an ROI (fixed by reference to external axes) to represent the area and location of the ultrasound transducers and an ROI based on anatomical markers, positioned in the posterior part of the calcaneus. BUA was significantly correlated with calcaneus BMD in the fixed ROI (r = 0.77, p < 0.0001) and the anatomical ROI (r = 0.78, p < 0.0001). Correlation of BUA with axial BMD was lower at r = 0.63, p < 0.0001 for the spine (L2-L4) and r = 0.59, p < 0.0001 for the femoral neck. Results for SOS and stiffness were very similar. From the calcaneus dual energy X-ray absorptiometry (DXA) scan images the region representing the ultrasound measurement area was found to contain cortical bone edges in 60% of cases. In 16% of scans a small part (4.5 +/- 4.0%) of the ROI measurement area fell partially outside the heel. However, there was no obvious discrepancy in the ultrasound results in the individual cases where this occurred. The correlation between calcaneus BMD and BUA was improved from r = 0.72 to r = 0.86 when scans with a cortical edge in the measurement ROI were excluded. The values for SOS were similar. In summary, BMD of the calcaneus, when closely matched to the site of ultrasound measurement shows significant correlation with ultrasound measurements. By excluding scans in which the ultrasound measurement appeared to include bone edges, correlations of approximately r = 0.86 were obtained. However, the BMD result still does not explain all of the variance in the ultrasound results.

Absorptiometry, Photon↗
PubMed1995

A multiparameter optimization of digital mammography.

Multiparameter optimizations have been carried out for a wide range of digital mammography system configurations and requirements, with the aim of optimizing the image quality for a given patient dose. These conditions include a range of slot widths for scanning mammography systems, exposure times from 1 to 10 s, focal spot sizes from 80 to 800 microns, a range of detector resolutions and noise levels, dose restrictions, patient thicknesses and targets, and x-ray tube targets. The influences of these on the optimum system configuration in terms of tube potential, filtration, source to patient distance and target magnification are discussed. It is demonstrated that x-ray tube power constraints can significantly restrict the optimum magnification for slot scanning systems, with the result that poor-resolution detectors are not suited for use in a scanning configuration, and that large-focal-spot-good-detector resolution combinations are more suitable. The use of a detector with increased width, raised tube potential and reduced amount of added filtration is shown to be helpful in reducing x-ray tube power limitations. It is shown that, in many cases, correct optimization can bring the detail SNR for an examination using a given detector-x-ray tube configuration to within 10-15% of the SNR achieved with the optimum combination. This gives the designer some scope to consider other factors such as cost and the implications of image size on storage space.

Biophysical Phenomena↗
PubMed1991

Regulation of androgen receptor mRNA in rat Sertoli and peritubular cells.

Regulation of 9.5-kb androgen receptor mRNA concentrations in Sertoli and peritubular cells from 20-day-old rats was studied by Northern blot analysis. Treatment of cells in vitro for 1-7 days with 300 ng/ml FSH increased androgen receptor mRNA up to 4-fold in Sertoli cells but not in peritubular cells. Testosterone (100 ng/ml) had no effect or slightly decreased androgen receptor mRNA in Sertoli and peritubular cells. Androgen receptor mRNA concentrations in Sertoli and peritubular cells from rats killed 15 days after hypophysectomy were elevated 4-5-fold over those in cells from intact rats. The androgen receptor mRNA concentration was decreased in both Sertoli and peritubular cells isolated from hypophysectomized animals treated with 500 micrograms/day testosterone propionate in vivo and subsequently with 100 ng/ml testosterone in vitro. FSH treatment (100 micrograms/day in vivo, followed by 300 ng/ml in vitro) did not increase androgen receptor mRNA over that in cells from hypophysectomized controls but rather decreased its concentration to varying degrees in Sertoli and peritubular cells. The rise in androgen receptor mRNA in both Sertoli and peritubular cells isolated from hypophysectomized animals is attributable, at least in part, to the absence of the inhibitory influence of testosterone. Other data in the literature suggest positive regulation of Sertoli cell androgen receptor protein by FSH and androgens. Consequently, complex mechanisms involving transcriptional, translational, and post-translational regulation probably control androgen receptor concentrations in the cells of the rat seminiferous tubule.

Animals↗