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Biomedical subjects

P N Wells

Publications and source records attributed to P N Wells.

At least 19 recordsLinked to original sources
PubMed1992

Ultrasonics: a window into biomedical science.

Ultrasonics has always been concerned with biomedical research. When the journal started, most applications were in therapy and surgery, but diagnosis soon became dominant. Many key developments received early publication: these included Doppler ultrasound, real-time imaging, tumour neovascularization and, arguably most significant of all, phased array scanning. Now in its thirtieth year of publication, Ultrasonics has contributed greatly to the development of biomedical science and it promises to have an important continuing role.

History, 20th Century↗
PubMed1992

Estimation of total hepatic blood flow by duplex ultrasound.

The volume flow rate of blood in the portal vein and the hepatic artery was measured using a duplex ultrasound system. Two sections of the hepatic artery were studied; the common hepatic artery where measurements were made just after the bifurcation of the coeliac axis to splenic and hepatic arteries and the hepatic artery itself, where measurements were made just proximal to the porta hepatis in a straight stretch of artery overlying the portal vein. Total hepatic blood flow was taken as the sum of hepatic artery and portal vein flows. A group of 10 normal healthy volunteers in the fasting state was studied. The mean (SD) volume blood flow in the vessels was measured to be: hepatic artery 3.5 (45%) ml/min/kg, common hepatic artery 6.9 (30%) ml/min+/kg, portal vein 13.5 (21%) ml/min/kg, total hepatic flow 17.0 (16%) ml/min/kg.

Blood Flow Velocity↗
PubMed1992

The envelope that tissue imposes on achievable ultrasonic imaging.

Ultrasonic images are imperfect because practical imaging systems have limited spatial, contrast, and temporal resolutions. The envelope within which an ultrasonic imaging system operates is imposed by the physical and biological properties of the imaged tissue. The relevant properties are speed, attenuation, inhomogeneity, nonlinearity, scattering, motion, and the induction of biological damage. The system designer begins by choosing the overall dimensions of the tissue structures to be imaged and the imaging rate. Then, optimization of system design allows the imaging performance to coincide with the envelope imposed by the characteristics of the tissue.

Humans↗
PubMed1992

Quality assurance and radiologic audit.

Quality assurance is essential in the cost-effective provision of radiologic services. Technical aspects include the routine measurement of equipment performance and the control of reject and retake rates. The current emphasis is on reducing patient radiation dose while maintaining acceptable image quality. The reduction of unnecessary radiologic investigations is also a priority. Clinical and medical audits, integral parts of the quality assurance process, form part of a continuously repeated cycle designed to raise standards of performance by changing clinical practice on the basis of outcome analysis. Quality assurance in mammography is presented as an example.

Diagnosis, Differential↗
PubMed1991

Ultimate limits in ultrasonic imaging resolution.

According to elementary theory, the resolution of an ultrasonic imaging system increases with the ultrasonic frequency. However, frequency is limited by frequency-dependent attenuation. For imaging at any required depth, resolution improvement beyond the limit imposed by ultrasonic frequency can be obtained by increasing the ultrasonic intensity. This is itself, however, dependent on safety considerations and the effects of nonlinearity. In homogeneous media, image resolution increases with decreasing f-number. Particularly at low f-numbers, however, tissue inhomogeneity leads to a deterioration in image quality. Inhomogeneity may also be considered in terms of phase aberration. It has been found that for a given aperture, image degradation due to phase aberration is worse at higher frequencies. Schemes have been proposed for correction of this problem, but so far model systems do not lend themselves to clinical application. Deconvolution is unsatisfactory, speed correction is impracticable and synthetic aperture scanning and holography are virtually useless in biological tissues. Ultrasound-computed tomography has had only limited success. Speckle reduction can improve target detectability, but at the expense of resolution. Time-frequency control provides a useful partial solution to the problem of resolution reduction resulting from attenuation. It is clear that improved resolution would result in significant clinical benefits. An optimisation system for aperture size and ultrasonic frequency is proposed with signal averaging for resolution enhancement of a defined object area. This would have a compact ultrasonic beam and would allow frame rate to be traded for resolution, by means of signal averaging.

Humans↗
PubMed1991

A new philosophy of medical imaging.

In general, the traditional approach to medical imaging is based on the solution of the inverse problem of deducing the characteristics of tissues within the body from the received field resulting from probing radiation. Ambiguities and lack of complete data, and physical limitations such as diffraction, field non-uniformity and so on, prevent the image from being an exact representation of what would be seen if the imaged part of the patient were to be exposed to direct vision or drawn by an artist. Much more exact representation could be produced, however, if the philosophy of imaging were to be changed to involve the solution of the forward problem in which the received field is iteratively compared with that calculated to be produced by a computer-simulated model of the object from a knowledge base of anatomy, pathology, histology, physical properties of tissues etc. As a result of this converging process of comparison and iterative minimisation of the difference signal, a model of the object would be simulated consistent with the received field resulting from probing radiation as well as with all the relevant information form a knowledge base containing general knowledge and also including the personal records of the particular patient. Using such a model, any required cross-section or three-dimensional pictorial representation of the region of interest in the investigated object could be displayed in the form of a pictorial image. Thus the proposed philosophy would make medical imaging much less subjective and increase its information value, because inherently it includes all relevant biomedical data in the displayed image.

Diagnostic Imaging↗
PubMed1991

Splanchnic haemodynamic changes during acute hypoglycaemia in man.

1. Splanchnic haemodynamic changes were studied in seven healthy subjects during hypoglycaemia induced by the intravenous infusion of insulin. Superior mesenteric artery blood flow and cardiac output were examined noninvasively by a Doppler ultrasound technique. 2. Blood glucose concentration fell from 4.5 (0.14) mmol/l basally to 1.5 (0.09) mmol/l [mean (SEM), P less than 0.003] at the hypoglycaemic reaction ('R') and recovered to baseline by 'R' + 60 min. There was an associated rise in plasma glucagon, adrenaline and noradrenaline levels. 3. Superior mesenteric artery blood flow rose at 'R' from a basal value of 532 (38) ml/min to a peak of 803 (73) ml/min at 'R' + 10 min [mean (SEM), P less than 0.005] and remained significantly elevated until 'R' + 40 min. Resistance in this vessel fell by 33% at 'R' + 10 min (P less than 0.005) and remained significantly low until 'R' + 40 min. 4. Cardiac output rose by 33% at 'R' (P less than 0.004) and returned to normal by 'R' + 20 min. This was associated with a 24% rise in pulse rate (P less than 0.03), but no change in stroke volume or mean arterial pressure. Total peripheral resistance fell by 21% at 'R' (P less than 0.005) and had returned to normal by 'R' + 20 min. 5. The sustained rise in splanchnic blood flow during hypoglycaemic recovery may be of homoeostatic importance by providing metabolic fuel to the liver for gluconeogenesis.

Adult↗
PubMed1991

Optical attenuation characteristics of breast tissues at visible and near-infrared wavelengths.

Optical experiments are described for measuring the attenuation characteristics of breast tissues at visible and near-infrared wavelengths. Total attenuation coefficients post mortem were measured directly in thin tissue sections. They are usually within the range from 10 to 30 mm-1, are rather higher in fat than in fibroglandular specimens and decrease with increasing wavelength. The scattering phase function is strongly forward-peaked with the mean cosine of scattering in the range from 0.85 to 0.97 and appearing more forward-peaked in fat than in fibroglandular tissue. The reduced scattering coefficient is of the order of 1 mm-1 in all tissues. Absorption coefficients were measured indirectly in optically thick sections. They are typically between 0.1 and 0.5 mm-1 at wavelengths around 580 nm and an order of magnitude lower at 850 nm. At 580 nm and shorter wavelengths the absorption in carcinoma is significantly higher than in adjacent uninvolved tissue. Significant differences were observed in the first-order derivatives of the transmission spectra of carcinoma and surrounding tissues at certain infrared wavelengths. Transmission spectra measured in vivo across the wavelength range from 500 to 860 nm have a similar form to the spectra of excised samples. Linear absorption coefficients are generally of the same order of magnitude as those found in vitro although they are lower at green wavelengths.

Adenocarcinoma, Scirrhous↗
PubMed1991

Monte Carlo modelling of light propagation in breast tissue.

Light transport in three-dimensional plane-parallel tissue slabs has been modelled by Monte Carlo analogue simulation. The model design has allowed the study of transmission properties that are pertinent to imaging systems for the detection of breast cancer. An important aspect of the investigations is that they make use of data obtained from quantitative measurements of light scattering and absorption in normal and pathological breast tissues. It is shown that an imaging technique which used a raster scanning laser and detector arrangement and plane-parallel compression of the breast could have considerable advantages in terms of improved transmittance, spatial unsharpness and contrast. Time-of-flight gating of images is also found to be beneficial provided that the light intensities after temporal filtering remain adequate.

Breast↗
PubMed1991

Assessment criteria for diagnostic imaging technologies.

Expensive diagnostic imaging technologies need to be assessed. Assessment is most difficult with new technologies. Rapid technical change creates pressure for widespread provision because of apparently self-evident advantages. For the health care provider, technology assessment is of central importance. Timing is crucial; economic and clinical studies have to be carried out simultaneously; and studies must be broadly based. Assessment should be carried out only on properly selected technologies. Apart from picture archiving and communication systems, existing expensive diagnostic imaging technologies are now so mature that assessment is concerned mainly with costs and benefits. To be provided, a technology needs to be effective, economic, appropriate and needed. The history of computed tomography illustrates technology assessment from innovation to routine application. Technology assessment also provides essential guidance for equipment selection. Different regulations and controls, explicit or implicit, exist in different countries. There are special requirements for technology assessment in each country according to overall health requirements and policies. The main contemporary challenges are the correct selection of equipment for assessment, and the development of predictive methods of assessment.

Diagnostic Imaging↗
PubMed1990

Blood flow: insights from ultrasound.

Ultrasonic pulse-echo systems can provide range-finding, time-position and real-time two-dimensional images of soft-tissue structures within the body. The Doppler effect can be used to study motion and blood flow. Continuous wave Doppler instruments provide information about velocity and direction of flow; depth discrimination can be obtained by pulsing the ultrasound. Two-dimensional Doppler flow imaging can be achieved by manual scanning of a probe over the skin surface. The combination of real-time pulse-echo imaging with pulsed Doppler blood flow detection in the duplex scanner makes it possible to localize the anatomical position of the Doppler sample volume. Real-time Doppler colour flow imaging combines traditional ultrasonic scanning with a two-dimensional flow map. Using appropriate ultrasonic instruments, blood flow volume rates, blood flow velocity profiles, pressure gradients, orifice areas, flow disturbances, jets, characteristics of blood vessels and the circulatory system, and tissue perfusion can all be investigated. These investigations have clinical applications in the study of cardiac, cerebral and peripheral blood flow, blood flow in the female pelvis, the fetus, the abdomen, the neonate, and in malignant tumours. Contemporary ultrasonic diagnosis employs exposure levels that are apparently free from biological risk, but other factors need to be taken into account in considering the prudent use of ultrasonic methods. Promising research is being carried out into the mechanism of ultrasonic scattering by blood, Doppler speckle, time-domain processing for blood flow imaging, methods for increasing the scanning speed, Doppler flow microscopy and contrast agents. The new technology that will result from this research should lead to further substantial progress in ultrasonic blood flow studies.

Blood Circulation↗
PubMed1989

A new approach to the noninvasive measurement of cardiac output using an annular array Doppler technique--I. Theoretical considerations and ultrasonic fields.

This paper describes the development of a Doppler flowmeter capable of measuring blood volume flow rate without the need to measure the vessel lumen area or beam-vessel angle. It requires the production of a uniform wide ultrasound beam to encompass the whole vessel and thus to produce a Doppler spectrum which corresponds to all the flowing blood, and a narrow reference beam placed within the lumen to compensate for various unknown quantities, such as tissue attenuation. The general definition of volume flow rate is described and applied to a new flowmeter, which allows an absolute value of volume flow rate to be measured independently of vessel size, beam-vessel angle, and tissue attenuation. By electronically apodising an annular array transducer in transmission and reception, a uniform wide beam and a narrow reference reception beam are produced. Theory to predict these beam patterns is developed and a computer simulation is made. The ultrasonic fields obtained from an annular array transducer in water are compared with the theoretical fields.

Adult↗
PubMed1989

A new approach to the noninvasive measurement of cardiac output using an annular array Doppler technique--II. Practical implementation and results.

An experimental Doppler flowmeter system has been developed which can noninvasively measure blood flow volume rate in a vessel. It is based on the attenuation compensated technique and does not require knowledge of the vessel size or beam-vessel angle. In vitro results have shown that the measurement of volume flow rate is independent of vessel angle to within +/- 4%, and independent of vessel diameter to within +/- 5%. Flow rate linearity is better than +/- 3%. A good comparison has been obtained, in vivo, of aortic diameters measured by an imaging system and with this flowmeter; the r value was 0.98. The noninvasive measurement of cardiac output using this flowmeter has been compared with conventional dilution techniques in 54 patients, with a resulting correlation coefficient of r = 0.96.

Aorta↗
PubMed1989

Tissue characterisation.

Many attempts have been made in the past 20 years to derive more quantitative information relating to tissue structure from the returned ultrasound beam. The qualitative method of characterising tissue from visual inspection of the image with interpretation of artifacts remains the most successful one. Potential quantitative methods include estimation of: attenuation, speed of sound, acoustic impedance, scattering characteristics, non-linearity of propagation, tissue motion and the specific perfusion patterns detected by the Doppler shift. The authors conclude that only image inspection and Doppler techniques have shown progress toward the concept of telehistology expressed in the 1970s. Further advances in quantitation of the Doppler signals and development of contrast agents appear promising for the future.

Humans↗