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Biomedical subjects

C A MacDonald

Publications and source records attributed to C A MacDonald.

14 recordsLinked to original sources
PubMed2004

Beam collimation with polycapillary x-ray optics for high contrast high resolution monochromatic imaging.

Monochromatic imaging can provide better contrast and resolution than conventional broadband radiography. In broadband systems, low energy photons do not contribute to the image, but are merely absorbed, while high energy photons produce scattering that degrades the image. By tuning to the optimal energy, one can eliminate undesirable lower and higher energies. Monochromatization is achieved by diffraction from a single crystal. A crystal oriented to diffract at a particular energy, in this case the characteristic line energy, diffracts only those photons within a narrow range of angles. The resultant beam from a divergent source is nearly parallel, but not very intense. To increase the intensity, collimation was performed with polycapillary x-ray optics, which can collect radiation from a divergent source and redirect it into a quasi parallel beam. Contrast and resolution measurements were performed with diffracting crystals with both high and low angular acceptance. Testing was first done at 8 keV with an intense copper rotating anode x-ray source, then 17.5 keV measurements were made with a low power molybdenum source. At 8 keV, subject contrast was a factor of five higher than for the polychromatic case. At 17.5 keV, monochromatic contrast was two times greater than the conventional polychromatic contrast. The subject contrasts measured at both energies were in good agreement with theory. An additional factor of two increase in contrast, for a total gain of four, is expected at 17.5 keV from the removal of scatter. Scatter might be simply removed using an air gap, which does not degrade resolution with a parallel beam.

Algorithms↗
PubMed2002

The dependence of ultrasound contrast agents backscatter on acoustic pressure: theory versus experiment.

Experimental investigations have not fully explored the interaction between ultrasound beams and microbubble contrast agents. Moreover theoretical investigations have not solved the problem of the microbubble oscillation. A simple in-vitro system based on a commercial scanner (ATL UM9) was used to insonate (3 MHz transmission) diluted contrast suspensions of Definity and Quantison at different acoustic pressures (0.27-1.52 MPa). The experimental data were referred to a blood mimicking fluid in order to extract an estimate of their scattering cross-section. The results were compared with the solutions of the three main bubble oscillatidn models, Rayleigh-Plesset, Herring and Gilmore. Non-linear solutions of the above models were produced numerically using the Mathematica Package Software. The experiments showed that both agents provided a linear increase in scattering cross-section with increasing acoustic pressure. The thick shelled Quantison provided an increasing number of scatterers with increasing acoustic pressure, which proved that free bubbles leaked out of the shell. At high acoustic pressures both Quantison and Definity scattering cross-sections were almost identical, and were probably that of a free bubble. The Rayleigh-Plesset model provided a scattering cross-section almost independent of acoustic pressure. On the contrary the scattering cross-sections calculated by the Herring and Gilmore models solutions displayed a definite dependence on acoustic pressure of an order higher than one, which is slightly higher than the order of dependence exhibited by the experimental data. However, the increase of the experimentally measured scattering cross-section with acoustic pressure was sharper than the calculated one by the above two models. This is most probably due to the fact that the models simulated damped and not free bubble oscillations. In conclusion the Rayleigh-Plesset model was inadequate in describing the bubble oscillations even at small diagnostic acoustic pressures. The Herring and Gilmore models could simulate the dependence of the scattering cross-section of encapsulated microbubbles on acoustic pressure. However the contribution of free bubble oscillations has still to be modelled.

Journal Article↗
PubMed2002

X-ray optics for better diagnostic imaging.

Polycapillary x-ray optics provide an innovative new way to control x-ray beams. Placing these optics after the object to be imaged provides very efficient rejection of Compton scatter, while allowing image magnification without loss of resolution, image demagnification, or image shaping to match with digital detectors. Measured scatter rejection optics had primary transmissions greater than 50% and scatter transmission of less than 1%. For a 5-cm thick Lucite phantom, this resulted in a contrast enhancement of nearly a factor of two at 20 keV and three at 40 keV. The magnification from the tapered capillary optics improved the MTF at all frequencies out to 1.8 times the original system resolution. Increases below the system resolution are most important because clinically relevant structures generally occupy lower spatial frequencies. Alternatively, placing a collimating optic and diffracting crystal before the patient provides sufficient monochromatic beam intensity for medical imaging. Contrast, resolution, and intensity measurements were performed with both high and low angular acceptance crystals. At 8 keV, contrast enhancement was a factor of 5 relative to the polychromatic case, in good agreement with theoretical values. At 17.5 keV, monochromatic subject contrast was more than a factor of 2 times greater than the conventional polychromatic contrast. An additional factor of two increase in contrast, for a total factor of four, is expected from the removal of scatter in a large beam clinical system. The measured angular resolution after the crystal was 0.4 mrad for a silicon crystal.

Humans↗
PubMed2001

The Newfoundland and Labrador Heart Health Program dissemination story: the formation and functioning of effective coalitions.

Newfoundland and Labrador is the most easterly province of Canada (O'Loughlin et al., Figure 1) with a population of 537,000. Rural in nature, 50% of the population resides in widely-dispersed communities of less than 2500 people. The economy traditionally relied on the fishing industry, but with the closure of the once lucrative cod fishery in 1991, the poorest province in Canada faced a difficult economic climate with up to 20% unemployment rates. With little funding available to supplement or sustain expensive initiatives, the Demonstration Phase of the Newfoundland and Labrador Heart Health Program (1990-1996) focused on how community-based programs are developed and sustained, with a view to diffusion throughout the province. The whole province was defined as the demonstration site for the project, and a community mobilization strategy was used with extensive reliance on community health professionals and volunteer contributions.

Cardiovascular Diseases↗
PubMed1996

Imaging characteristics of x-ray capillary optics in digital mammography.

Computed radiography (CR) has shown promise in digital mammographic screening due to its good low spatial frequency MTF and its relatively wide exposure latitude. The CR image format has not gained acceptance clinically because of reduced high spatial frequency resolution as compared to film-screen images. X-ray capillary optics, aligned between the breast and CR phosphor imaging plate, will capture primary x-ray photons almost exclusively. Due to the very small angle of acceptance, scattered photons angled more than about 1.6 x 10(-3) radians from primary trajectory will not be accepted at the capillary optic entrance. The virtual elimination of detected scatter means almost 100% of the possible primary contrast should be visible in the image. In addition, the image can be magnified without focal spot blurring. Effective resolution of CR images can be increased by a factor equal to that magnification. Clinical implementation of future capillary optics are expected to be either in the form of a large, stationary, post-patient optic that accepts primary from the entire breast or a fan-shaped optic that is scanned across the breast. Measurements of a test capillary optic showed a reduction of scatter fraction to 0.018. Images of a lucite contrast detail phantom revealed a corresponding increase in image contrast when compared to anti-scatter grid and no grid methods. Spectral transmission measurements using a high-purity germanium detector showed good primary transmission (45%-50%) in the mammographic energy range. The MTF measurements of both stationary and scanned capillary optics showed improvement at the 5% MTF level to 8.4 mm-1 for scanned optics and 9.2 mm-1 for stationary optics representing a 68% and 84% respective increase over the CR MTF without magnification or capillary optics.

Capillary Action↗
PubMed1995

Measurements of capillary x-ray optics with potential for use in mammographic imaging.

Capillary optic arrays are bundles of hollow glass capillaries which guide x rays in a manner similar to the way fiber optics guide light. Focused postpatient capillary optic arrays have the potential to significantly improve both contrast and resolution of mammographic images compared to conventional antiscatter grids. Contrast can be improved by the nearly total scatter rejection of the optic. Effective resolution can be improved by geometric magnification without increased focal spot blurring. The best results were found for borosilicate glasses, with transmissions in excess of 60% for 22-cm-long fibers. To evaluate the scatter rejection properties, the transmission of off-axis radiation was measured. Transmission drops to < 1% at an angular displacement of 2.7 mrad. Transmission of a bulk capillary array dropped to near zero if the source was at an angle of 2.5 mrad. This implies excellent scatter rejection capabilities. To evaluate whether unchanneled photons might still reach the detector, absorption measurements were also performed on fibers and arrays. Absorption was found to be adequate for scatter rejection. All of the data agreed well with numerical simulations. Performance calculations for two potential optics geometries gave promising results.

Biophysical Phenomena↗
PubMed1994

Use of capillary optics as a beam intensifier for a Compton x-ray source.

The use of Kumakhov capillary optics will significantly enhance the performance of near-monochromatic, Compton backscattered x-ray programs. The Vanderbilt University Medical Free-Electron Laser Center is developing the capability to create these tunable x rays for medical imaging. The present transport has only reflection optics, and the beam is quite large in diameter at the laboratory. Low loss collimation of this beam would allow higher x-ray intensities after transport. This article describes experimental and computer simulation results which predict the expected performance for a multifiber Kumakhov collimator for use in the x-ray beam transport. Estimates from our research are that a multifiber optic formed of individual polycapillary fibers could be used to capture the full 7 mrad of the Vanderbilt x-ray beam and collimate it to a 1-2 mrad divergence with approximately 40%-50% transmission efficiency. This optic should increase the x-ray intensity at the laboratory level by a factor of > or = 5 by decreasing the beam divergence and subsequent spot size. Additionally, analysis of monolithic optics of fused multicapillary fibers predicts an increase in the intensity of the x rays at the laboratory by a factor of 55. These optics can have tapered channels that greatly decrease their exit divergence. This will greatly enhance the capabilities of this unique x-ray source. This article reports the initial results from a collaboration between Vanderbilt, The Center for X-Ray Optics at University at Albany, SUNY, and X-Ray Optical Systems in Albany, NY.

Biophysical Phenomena↗
PubMed1993

Evaluation of Questor urine screening system for bacteriuria and pyuria.

AIMS: To evaluate the Questor automated bacteriuria and pyuria screening system; to compare its performance with that of a reference method; and to assess its usefulness in a routine clinical laboratory. METHODS: The Questor urine screening system was compared with a comprehensive regimen to detect urinary tract infection, using pour-plate viable counts to determine the numbers of bacteria present in urine samples, a wide range of other cultural methods, microscopic findings and clinical information. RESULTS: The optimal performance in detecting significant growths was a sensitivity of 93%, a specificity of 74%, a positive predictive value of 43% and a negative predictive value of 98%. The list price per test is 0.17 pounds and the capital cost of the system is 39,950 pounds. Questor can test 50 samples an hour and can be operated by one member of the laboratory staff, who is not required to make interpretative judgments--for example, a medical laboratory assistant. CONCLUSIONS: The sensitivity and specificity of the Questor was better than that obtained from other screening systems using the same protocol. The system was easy to use and is a useful addition to the methods available for screening for bacteriuria.

Bacteriuria↗
PubMed1991

Biliary atresia.

Care of the child with biliary atresia is a challenge for pediatric nurses. The cause of the disease is not known, and new diagnostic tests and treatment options are continually being developed. This article addresses fundamental issues specific to the care of children with biliary atresia. In order to understand the clinical manifestations and the treatment rationale, pathophysiology and theories of etiology are discussed. Nursing care of the child and issues regarding the teaching and support of the family system are addressed, with suggestions for independent reading.

Biliary Atresia↗
PubMed1986

Synthesis of mutant parathyroid hormone genes via site-specific recombination directed by crossover linkers.

A synthetic 'crossover linker' technique has been designed for gene modification. The linker has a restriction end for an initial 'cohesive end' ligation with one terminus of a linearized plasmid, a middle section carrying modified sequence information, and an 'homology-searching' sequence of 20 bp at its other end, that is homologous to a specific region in the opposite terminus of the plasmid. Inside the Escherichia coli transformation host, intramolecular recombination between the homologous ends of the resultant plasmid intermediate completes the integration of the linker. Using different crossover linkers, a human parathyroid hormone gene which had previously been cloned into plasmid pUC8 was converted to mutant coding sequences via specific base substitution, sequence deletion and sequence insertion.

Base Sequence↗