Biomedical subjects
Publications and source records attributed to A J Lucas.
Studies of echo attenuation at long diffusion times in pulsed field gradient NMR experiments on a variety of rock core samples are interpreted in the light of recent theoretical analysis of the effect of pore geometry and surface relaxation. This study is motivated by the need to test the applicability of that theory to real rock systems.
A data analysis methodology is used to process 3D NMR image data acquired for porous systems. The method extracts the mean size of those repeating elements in the image data which are largely compared with the image voxel dimensions. In this work we extend the two-dimensional (2D) image analysis method described by others to three spatial dimensions (3D). 3D image data were acquired at a magnetic field strength of 7 T using NMR microscopy hardware. The 3D autocorrelation function obtained from the data reveals a characteristic pore size in each dimension.
The simple pulse-acquire experiment has been used to evaluate the level of accuracy and precision achievable in NMR fluid saturation measurements for a range of rock core samples saturated with either brine or hydrocarbons. For a set of more than 70 cores measured at 0.66 T the mean error in the NMR measurement is only 0.35% porosity when the sample linewidths are less than 50 ppm. However, for a significant portion of cores, those with very broad NMR linewidths (> 50 ppm), difficulties associated with nonuniform excitation are encountered. The magnetic susceptibility difference between pore fluid and rock matrix translates into relatively broad NMR linewidths, and this feature of petrophysical samples is the major difficulty in performing quantitative NMR experiments. Numerical simulations are used to complement the experimental results in order to develop strategies for obtaining accurate NMR results with these difficult samples.
A technique for obtaining quantitative longitudinal saturation/porosity profiles of rock cores which are longer than the NMR coil has been developed. The slice-selected experiment uses a prefocused pulse in conjunction with a magnetic field gradient for the localization and a CPMG sequence to sample the data. A variety of rock core samples has been studied ranging from limestones to shaly sandstones. Comparison of the relaxation decay curves obtained from these experiments and the bulk experiments show that reliable localized relaxation data are obtained.
Human fetal breathing movements (FBM) differ from neonatal breathing in that they are predominantly paradoxical and are not present all the time. Respiratory pauses are common in the fetus while prolonged apneas are life-threatening in the neonate. In the latter they occur during state 2 (active sleep) when paradoxical breathing is more frequent. Then the inward movement of the thoracic wall causes spreading of the caudal ribs which may elicit an intercostal-to-phrenic inhibitory reflex (IPIR) leading to apnea. This is not the case in state 1 (quiet sleep). In the fetus similar sleep states, state 2F and 1F respectively, have been identified and we investigated if the IPIR is already present before birth and if so, if the response is different between 1F and 2F. In 21 healthy fetuses between 37 and 40 weeks, two real time scanners and a cardiotocograph were used to asses the fetal behavioural state. If FBM were present, the caudo-lateral part of the fetal thoracic wall was shortly compressed manually and the period that FBM ceased measured. During the same epoch moments of sham-stimulation, i.e. control periods without actual compression were also carried out. In 15 out of 16 fetuses with FBM in 1F, and in 19 out of 19 fetuses in 2F, compressions were immediately followed by an apnea. In contrast, moments of sham-stimulation never induced apnea, indicating that the IPIR is indeed present in both behavioural states.(ABSTRACT TRUNCATED AT 250 WORDS)
A system for computer-assisted instruction in performing ophthalmic visual field examinations is presented. The system is implemented on a microcomputer making use of specially developed algorithms to summarize the information contained in a visual field in a very compact, efficient manner and to determine cursor position relative to stored visual field landmarks. The computer program presents a challenging testing situation by using visual fields, based on those of actual patients, of differing degrees of complexity. It is particularly well suited for trainees as well as for the physicians and ophthalmic technicians who frequently perform this examination.
Quality control of manual visual field examination is impaired because the process is difficult to monitor, and because examiners have minimal formal training and supervision, and are subject to rapid turnover. Computer simulation can be used both to teach perimetry and to provide a reference standard against which performance may be measured.
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Altogether 2,798 cytopipette smears have been taken by patients entering the general medical and surgical wards of a hospital. Twelve unsuspected positive smears were found, a rate of 4 per 1,000. Biopsy of these patients has shown five pre-malignant lesions, one invasive carcinoma of cervix, and one carcinoma of corpus uteri. Five patients await biopsy.The cytopipette is both acceptable to the patient (62% acceptance) and a feasible laboratory technique and can produce good cytological smears. Only 30% of patients entering the hospital had already had the opportunity to obtain a smear before, and most of them welcomed the opportunity for the test.Though the self-pipette may be criticized for its inaccuracy we feel that any test, if it can detect cervical abnormality at the rate of 4 women per 1,000, is better than no test at all. It should have a place in routine cytological testing of women second only to the use of Ayre's spatula.
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