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Biomedical subjects

D Atkinson

Publications and source records attributed to D Atkinson.

At least 19 recordsLinked to original sources
PubMed2003

SMASH navigators.

The additional data acquired when using multiple receiver coils is commonly used to improve SNR or reduce acquisition times. It may also be used to remove image artifacts by selectively replacing corrupt data. In the present study, a correction scheme is presented based on simultaneous acquisition of spatial harmonics (SMASH) that enables detection and correction of motion artifacts caused by 2D translations. Newly measured data is compared with predictions from previously measured data by making negative and positive spatial harmonics. Differences are attributed to motion occurring in the interval between the acquisition of separate phase encode lines and correction parameters are determined. Two types of rigid body motion are considered: 1) object and coil array move, and 2) object only moves, since each causes different phase errors in k-space. Simulation, phantom, and volunteer experiments demonstrate the validity of the technique.

Artifacts↗
PubMed2003

Anisotropic noise propagation in diffusion tensor MRI sampling schemes.

The subject of this study is the controversial choice of directions in diffusion tensor MRI (DT-MRI); specifically, the numerical algebra related to this choice. In DT-MRI, apparent diffusivities are sampled in six or more directions and a least-squares equation is solved to reconstruct the diffusion tensor. Numerical characteristics of the system are considered, in particular the condition number and normal matrix, and are shown to be dependent on the relative orientation of the tensor with respect to the laboratory frame. As a consequence, noise propagation can be anisotropic. However, the class of icosahedral direction schemes is an exception, and icosahedral directions have the same condition number and normal matrix for direction encoding as the ideal scheme with an infinite number of directions. This normal matrix and its condition number are rotationally invariant. Numerical simulations show that for icosahedral schemes with 30 directions the standard deviation of the fractional anisotropy is both low and nearly independent of fiber orientation. The recommended choice of directions for a DT-MRI experiment is therefore the icosahedral set of directions with the highest number of directions achievable in the available time.

Anisotropy↗
PubMed2002

Submicrometer ferromagnetic NOT gate and shift register.

An all-metallic submicrometer device is demonstrated experimentally at room temperature that performs logical NOT operations on magnetic logic signals. When this two-terminal ferromagnetic structure is incorporated into a magnetic feedback loop, the junction performs a frequency division operation on an applied oscillating magnetic field. Up to 11 of these junctions are then directly linked together to create a magnetic shift register.

Journal Article↗
PubMed2001

Respiratory motion compensation for 3-D freehand echocardiography.

Motion of the diaphragm during respiration causes a displacement of the heart relative to the position of a transthoracic ultrasound (US) probe. These respiration-induced shifts of cardiac position can lead to spatial misalignments of data when reconstructed in 3-D. We show how to compensate for this motion using a technique that extends the tracking of the probe to additionally monitor a marker placed on the patient umbilicus. The motion of the umbilicus is calibrated to that of the diaphragm using one additional scan. This calibration is used to correct the 3-D spatial positions of cardiac images acquired from multiple acoustic views. At both systole and diastole, segmentations of the endocardial border visually appear more consistent after our correction than with no correction. Long and short axis segmentations should intersect on the endocardium. After correction, their separation at the closest point is shown to be reduced.

Calibration↗
PubMed2001

The generation and detection of longitudinal guided waves in thin fibers using a conical transformer.

This paper describes a technique to couple ultrasonic energy from a piezoceramic disc transducer into a fiber waveguide to induce longitudinal propagation. A polymer cone is utilized to bond the fiber waveguide onto the surface of the disc and to behave as a mechanical transformer, converting lateral displacements at its base into longitudinal displacements at its apex. Wideband finite element analysis (FEA) results are provided to show that the bond efficiently couples the radial modes of a disc transducer into fiber waveguides for longitudinal mode excitation. Furthermore, narrowband FEA is utilized to investigate how the geometry and material properties of the bond and waveguide influence the coupling efficiency. The technique is then quantified in terms of signal-to-coherent noise ratio (SCNR), reflecting its ability to generate the desired longitudinal waveguide mode and reject erroneous modes. Finally, design parameters are outlined for the successful implementation of this technique.

Journal Article↗
PubMed2000

Probing the lipid-free structure and stability of apolipoprotein A-I by mutation.

To probe the secondary structure of the C-terminus (residues 165-243) of lipid-free human apolipoprotein A-I (apoA-I) and its role in protein stability, recombinant wild-type and seven site-specific mutants have been produced in C127 cells, purified, and studied by circular dichroism and fluorescence spectroscopy. A double substitution (G185P, G186P) increases the protein stability without altering the secondary structure, suggesting that G185 and G186 are located in a loop/disordered region. A triple substitution (L222K, F225K, F229K) leads to a small increase in the alpha-helical content and stability, indicating that L222, F225, and F229 are not involved in stabilizing hydrophobic core contacts. The C-terminal truncation Delta(209-243) does not change the alpha-helical content but reduces the protein stability. Truncation of a larger segment, Delta(185-243), does not affect the secondary structure or stability. In contrast, an intermediate truncation, Delta(198-243), leads to a significant reduction in the alpha-helical content, stability, and unfolding cooperativity. The internal 11-mer deletion Delta(187-197) has no significant effect on the conformation or stability, whereas another internal 11-mer deletion, Delta(165-175), dramatically disrupts and destabilizes the protein conformation, suggesting that the presence of residues 165-175 is crucial for proper apoA-I folding. Overall, the findings suggest the presence of stable helical structure in the C-terminal region 165-243 of lipid-free apoA-I and the involvement of segment 209-243 in stabilizing interactions in the molecule. The effect of the substitution (G185P, G186P) on the exposure of tryptophans located in the N-terminal half suggests an apoA-I tertiary conformation with the C-terminus located close to the N-terminus.

Amino Acid Substitution↗
PubMed2000

Sampling and reconstruction effects due to motion in diffusion-weighted interleaved echo planar imaging.

Subject motion during diffusion-weighted interleaved echo-planar imaging causes k-space offsets which lead to irregular sampling in the phase-encode direction. For each image, the k-space shifts are monitored using 2D navigator echoes, and are shown to lead to a frequent violation of the Nyquist condition when an ungated sequence is used on seven subjects. Combining data from four repeat acquisitions allows the Nyquist condition to be satisfied in all but 1% of images. Reconstruction of the irregularly-sampled data can be performed using a matrix inversion technique. The repeated acquisitions make the inversion more stable and additionally improve the signal-to-noise ratio. The resultant isotropic diffusion-weighted images and average apparent diffusion coefficient (ADC) maps show high resolution and enable clear localization of a stroke lesion. Residual ADC artifacts with a slow spatial variation are observed and assumed to originate from non-rigid pulsatile brain motion. Magn Reson Med 44:101-109, 2000.

Artifacts↗
PubMed2000

Rhys-Davies exsanguinator: pressure characteristics and technique for improving performance.

This paper reports the results of an investigation into the pressure exerted by the Rhys-Davies exsanguinator on the palm and dorsum of the hand. We hypothesised that, due to the shape of the hand, the palm is shielded from the full effect of the exsanguinator, but our study showed that it is compressed as well as the dorsum. However, placing a bag of fluid in the patient's palm significantly increased the pressure applied to the palm.

Adult↗
PubMed2000

Beating the grog: an evaluation of the Tennant Creek liquor licensing restrictions.

OBJECTIVE: To review the effectiveness of, and community attitudes towards, increased restrictions on the availability of alcohol in Tennant Creek. METHOD: Estimates of quarterly per capita consumption of pure alcohol by persons aged > or = 15 years, admissions data from the local hospital, women's refuge and sobering up shelter, and police data on detentions in custody and common offences were compared for the 12 months prior and 24 months subsequent to the introduction of the restrictions. A random sample survey of residents aged > or = 18 years was conducted to ascertain attitudes towards the restrictions. RESULTS: Over the two years following the introduction of the restrictions, there was a reduction of 19.4% in annual per capita consumption of pure alcohol. This was accompanied by declines in: hospital admissions for acute alcohol-related diagnostic related groups; and persons taken into police custody and the proportions of offences reported on Thursdays. A majority of survey respondents was in favour of retaining or strengthening the existing restrictions. CONCLUSIONS: The restrictions were effective in reducing alcohol consumption and acute related harm and had the support of the majority of people in Tennant Creek. On the basis of this evidence, the NT Liquor Commission made a decision to retain them. Restrictions do not provide a simple answer to the problems associated with excessive alcohol consumption. However, they can be an effective part of a broad public health strategy to deal with such problems.

Adolescent↗
PubMed2000

Determinants of use rate of oral rehydration therapy for management of childhood diarrhoea in rural Bangladesh.

In rural Bangladesh, mothers were interviewed to identify factors that determine the use of oral rehydration therapy (ORT) for management of diarrhoea in children aged less than 5 years. The point prevalence of diarrhoea among 1,600 children was 11.6%, with 46% having acute watery diarrhoea. The overall ORT-use rate was 29%; only 17% of the cases used it adequately. Common reasons for not using ORS included misperception about diarrhoea and age of patients. Other reasons included incorrect assessments, severity, and difficulties with the administration of oral rehydration solutions. Promotion of ORT can be effected by improving the level of understanding of mothers with regard to assessment of severity, early initiation of treatment regardless of age, sex, type of diarrhoea, breast-feeding, and nutrition status.

Bangladesh↗
PubMed1999

Automatic compensation of motion artifacts in MRI.

Patient motion during the acquisition of a magnetic resonance image can cause blurring and ghosting artifacts in the image. This paper presents a new post-processing strategy that can reduce artifacts due to in-plane, rigid-body motion in times comparable to that required to re-scan a patient. The algorithm iteratively determines unknown patient motion such that corrections for this motion provide the best image quality, as measured by an entropy-related focus criterion. The new optimization strategy features a multi-resolution approach in the phase-encode direction, separate successive one-dimensional searches for rotations and translations, and a novel method requiring only one re-gridding calculation for each rotation angle considered. Applicability to general rigid-body in-plane rotational and translational motion and to a range of differently weighted images and k-space trajectories is demonstrated. Motion artifact reduction is observed for data from a phantom, volunteers, and patients.

Algorithms↗
PubMed1999

Water temperature influences the shoaling decisions of guppies, Poecilia reticulata, under predation threat.

In previous experiments, we found antipredator behaviours in female Trinidadian guppies to be influenced by water temperature. To distinguish between hypotheses suggested to explain these observations, we examined the effect of an increase in water temperature on the choice of shoal size of individual female guppies, in the absence and presence of predation threat from a confined cichlid. At 22 degrees C, guppies showed no significant preference for either of two shoal sizes of female guppies, although other responses indicated a reaction to the threat. At 26 degrees C, females chose to shoal with the smaller group when the predator was absent, but, once threatened, switched to a strong preference for the larger group. Variation in preference between individuals was quite high in all treatments, and was somewhat lower for guppies under threat in warmer water, although not significantly so. Our evidence indicates that guppies in warmer water behave as if they are at greater risk of predation, but only when under threat. Temperature-dependent increases in predation risk from ectothermic piscivores could result from less efficient escape responses and from greater vulnerability because of increased foraging activity. We suggest that selection favouring guppies displaying an optimal balance between foraging and antipredator behaviours should act more strongly at higher temperature. Copyright 1999 The Association for the Study of Animal Behaviour.

Journal Article↗
PubMed1999

Effects of standing on cerebrovascular resistance in patients with idiopathic orthostatic intolerance.

PURPOSE: Patients with idiopathic orthostatic intolerance often have debilitating symptoms on standing that are suggestive of cerebral hypoperfusion despite the absence of orthostatic hypotension. SUBJECTS AND METHODS: We evaluated the effects of graded head-up tilt on cerebral blood flow as determined by transcranial Doppler measurements in 10 patients with idiopathic orthostatic intolerance (nine women, one man, 22 to 47 years) and nine age- and sex-matched control subjects. RESULTS: In patients, mean (+/- SD) arterial pressure at 0 degrees head-up tilt was 90 +/- 11 mm Hg and was well maintained at all tilt angles (90 +/- 11 mm Hg at 75 degrees). In controls, mean arterial pressure was 85 +/- 7 mm Hg at 0 degrees and 82 +/- 11 mm Hg at 75 degrees head-up tilt. There was a substantial decrease in peak velocity with increasing tilt angle in patients (28% +/- 10%) but not in controls (10% +/- 10% at 75 degrees, P <0.001). Similarly, mean velocity decreased 26% +/- 13% in patients and 12% +/- 11% in controls (P = 0.01). With increasing head-up tilt, patients had a significantly greater increase in regional cerebrovascular resistance than controls. CONCLUSIONS: In patients with idiopathic orthostatic intolerance, peak and mean middle cerebral artery blood flow velocity decreased in response to head-up tilt despite well sustained arterial blood pressure. These observations indicate that in this group of patients, regulation of cerebrovascular tone may be impaired and might therefore be a target for therapeutic interventions.

Adult↗
PubMed1999

Postpartum health in a Dhaka slum.

This study examines the health, nutritional status, and health care seeking behaviour of a community based sample of 122 postpartum women from an urban slum in Dhaka, Bangladesh. It describes a physically impoverished environment in which malnutrition is serious, and non-trivial morbid episodes as a consequence of childbirth are very common. Malnutrition was found to be widespread: about one-quarter of the study mothers were short in stature, measuring 145cm or less in height; over two-thirds of the women weighed <45 kg; and a similar proportion had a BMI of <20. Based on mid-upper arm circumference, an overwhelming majority (96%) suffered from some degree of malnutrition. During the first 6 weeks postpartum over three-quarters of the women reported a non-trivial illness. The frequency of reported illnesses was significantly associated with both increasing age and parity. Despite severe poverty, most of the women reporting illnesses (71%) received some form of health care from a wide range of western and traditional health care providers, with Traditional Birth Attendants (TBAs) and unqualified western care providers being the most frequently utilised. This study highlights the plight of these women in a precarious environment and shows how their health is compromised by cultural and political constraints. We conclude that while the burden of postpartum morbidity is very high, the incorporation of traditional practitioners and unqualified western care providers into maternal health training programs, together with efforts to empower women, could be effective in improving the health status of mothers in this marginalised and fragmented community. To achieve this outcome, a clearly articulated and integrated approach to development in slum communities is required.

Adolescent↗
PubMed1999

Bubble contrast echocardiography in detecting pulmonary arteriovenous shunting in children with univentricular heart after cavopulmonary anastomosis.

OBJECTIVES: We sought to compare bubble contrast echocardiography and pulmonary angiography in detecting pulmonary arteriovenous malformation (PAVM) in children with cavopulmonary anastomosis (CPA), and to examine anatomic and physiologic variables associated with the development of PAVM. BACKGROUND: Development of PAVM in patients with CPA may cause profound cyanosis. Pulmonary arteriovenous malformation has been traditionally diagnosed by pulmonary angiography with reported incidence of 20% to 25% in patients with CPA. METHODS: Fourteen patients (age 1.1 to 12.6 years) with any forms of CPA and normal pulmonary venous drainage formed the study population. All patients underwent cardiac catheterization and pulmonary angiography. Bubble contrast echocardiographic studies were performed with injection of 10 ml of agitated saline solution into branch pulmonary arteries. Transthoracic echocardiograms using an apical view were performed to assess the appearance of bubble contrast in the systemic ventricles. We compared the results of pulmonary angiograms and contrast echocardiograms, and findings of contrast echocardiograms between lungs with hepatic venous blood flow and lungs without hepatic venous blood. RESULTS: Ten of the 14 patients (71%) had positive contrast echocardiographic studies, compared with three (21%) detected by pulmonary angiograms (p = 0.01). No difference was found in pulmonary artery pressure, transpulmonary gradient or presence of heterotaxy syndrome between patients with positive contrast echocardiographic studies and patients with negative studies. However, patients with positive contrast echocardiograms tended to have lower oxygen saturation (81%) and higher hemoglobin (16.4 g/dl) compared with patients with negative studies (88% and 14.7 g/dl, p = 0.10 and p = 0.18 respectively). Patients with Glenn shunt or unidirectional Fontan had higher incidence of PAVM (10/11) compared with patients with classic or lateral tunnel Fontan (0/3, p = 0.01). All 12 lungs with no perfusion of hepatic venous blood had positive contrast echocardiographic studies. Lungs with no hepatic venous blood flow were more likely to develop PAVM compared with lungs with hepatic venous blood flow (12/12 and 3/16 respectively, p < 0.01). CONCLUSIONS: Bubble contrast echocardiography is more sensitive in detecting PAVM compared with pulmonary angiography. The prevalence of PAVM in patients with CPA may be much higher than what had been reported previously. Lungs with no hepatic venous blood flow are more likely to develop PAVM than lungs with hepatic venous blood flow.

Angiography↗
PubMed1999

Controlled trial of two formulations of cromolyn sodium in the treatment of asthmatic patients > or = 12 years of age. Intal Study Group.

STUDY OBJECTIVES: This multicenter, randomized, double-blind, placebo-controlled, parallel study compared the efficacy and safety of the hydrofluoroalkane (HFA) formulation of cromolyn sodium metered-dose inhaler (MDI) with the chlorofluorocarbon (CFC) formulation in asthmatic patients > or = 12 years old over a 12-week period. DESIGN: Stable asthmatics using only currently marketed cromolyn sodium and as-needed inhaled beta2-agonists were randomly assigned to treatment with HFA cromolyn sodium, CFC cromolyn sodium, or placebo, administered as two inhalations (2 mg) qid for 12 weeks. Prior to randomization, all patients were required to meet minimum symptom and/or pulmonary function test criteria after discontinuation of cromolyn sodium. Efficacy was assessed by changes in daily symptom scores, albuterol use, peak expiratory flow, pulmonary function measurements, and overall opinions of effectiveness. RESULTS: A total of 280 patients in 29 centers were randomly assigned to treatment with HFA cromolyn sodium (n = 94), CFC cromolyn sodium (n = 91), or placebo (n = 95). Patients treated with the HFA formulation of cromolyn sodium demonstrated a 28 to 33% improvement over placebo for all symptom scores (p < 0.05) and a 35% improvement over placebo in the use of albuterol MDI (p < 0.05). The patients' opinions of overall effectiveness favored HFA cromolyn sodium (p = 0.011) and CFC cromolyn sodium (p = 0.006) over placebo. The investigators' opinions indicated a statistically significant difference favoring CFC cromolyn sodium compared with both placebo (p < 0.001) and HFA cromolyn sodium (p = 0.042). No statistically significant differences existed among groups in the incidence of treatment-related adverse events. CONCLUSION: The HFA formulation of cromolyn sodium MDI is a well-tolerated and effective treatment for asthma patients > or = 12 years old. The safety and efficacy profile of the HFA formulation is comparable to that of the CFC formulation.

Administration, Inhalation↗
PubMed1998

Thermodynamic analysis of human plasma apolipoprotein C-1: high-temperature unfolding and low-temperature oligomer dissociation.

Thermal and chemical unfolding of lipid-free apolipoprotein C-1 (apoC-1), a 6-kDa protein component of very low density and high-density lipoproteins, was analyzed by far-UV CD. In neutral 1 mM Na2HPO4 solutions containing 6-7 micrograms/mL protein, the apoC-1 monomer is approximately 30% alpha-helical at 0-22 degrees C and unfolds reversibly from about 22-80 degrees C with Tm = 51 +/- 3 degrees C and van't Hoff enthalpy delta Hv(Tm) = 19 +/- 3 kcal/mol. The apparent free energy of the monomer stabilization determined from the chemical unfolding at 0 degree C, delta G(0 degree C) = 2.8 +/- 0.8 kcal/mol, decreases by about 1 kcal/mol upon heating to 25 degrees C. A small apparent heat capacity increment suggests the absence of a substantial hydrophobic core for the apoC-1 molecule. At pH 7, increasing apoC-1 concentration above 10 micrograms/mL leads to self-association and formation of additional alpha-helices that unfold upon both heating and cooling from room temperature. The CD data indicate that the high-temperature transition reflects a complete monomer unfolding and the low-temperature transition reflects oligomer dissociation into stable monomers. This suggests the importance of hydrophobic interactions for apoC-1 self-association. Close proximity between the high- and low-temperature transitions and the absence of a plateau in the chemical unfolding curves recorded from oligomeric apoC-1 indicate marginal oligomer stability and suggest that in vivo apoC-1 transfer is mediated via the complexes with other apolipoproteins and/or lipids.

Acetates↗