Clinical Physics and Physiological Measurement 

About: Clinical Physics and Physiological Measurement is an academic journal. The journal publishes majorly in the area(s): Electrical impedance tomography & Blood flow. It has an ISSN identifier of 0143-0815. Over the lifetime, 581 publications have been published receiving 12433 citations.

Dielectric properties of body tissues

Abstract: A review is given of the dielectric properties of various mammalian tissues and biological fluids for the frequency range from 1 Hz to 10 GHz The properties considered are the frequency variations of the relative permittivity and electrical conductivity An attempt has been made to present data which can be considered to be the most typical for each material The dielectric properties of aqueous solutions of amino-acids, polypeptides, proteins, and then cells, are first outlined in order to lay the groundwork for the understanding of the properties of tissues The electrical characteristics of various tissues and blood are presented in tabular and graphical form, and the differences between normal and cancerous tissue is also discussed The effects of necrosis and temperature changes are described and the important contribution that water makes to the overall properties is emphasised An insight into some of the dominant physiological and biophysical processes responsible for the dielectric properties of biological materials is also attempted, since this should aid further developments of both the diagnostic and therapeutic applications of radiofrequency and microwave radiation Such information is also relevant to an understanding of the possible biological hazards of such radiation The ways in which dielectric studies can aid an understanding at the molecular level of the basic physiological differences between normal and cancerous tissue, as well as of the physico-chemical state of biological water, are also described

The Sheffield data collection system.

Abstract: Because of the intrinsically low sensitivity of any surface potential measurement to resistivity changes within a volume conductor, any data collection system for impedance imaging must be sensitive to changes in the peripheral potential profile of the order of 0.1%. For example, whilst the resistivity changes associated with lung ventilation and the movement of blood during the cardiac cycle range from 3 to 100% the changes recorded at the surface are very much less than this. The Sheffield data collection system uses 16 electrodes which are addressed through 4 multiplexers. Overall system accuracy is largely determined by the front-end equivalent circuit which is considered in some detail. This equivalent circuit must take into account wiring and multiplexer capacitances. A current drive of 5 mA p-p at 5 kHz is multiplexed to adjacent pairs of electrodes and peripheral potential profiles are recorded by serially stepping around adjacent electrode pairs. The existing Sheffield system collects the 208 data points for one image in 79 ms and offers 10 image data sets per second to the microprocessor. For a homogeneous circular conductor the ratio of the maximum to minimum signals within each peripheral potential profile is 45:1. The temptation to increase the number of electrodes in order to improve resolution is great and an achievable performance for 128 electrodes is given. However, any improvement in spatial resolution can only be made at the expense of speed and sensitivity which may well be the more important factors in determining the clinical utility of APT.

Applied potential tomography: possible clinical applications

Abstract: Applied potential tomography (APT) or electrical impedance imaging has received considerable attention during the past few years and some in vivo images have been produced. This paper reviews the current situation in terms of what in vivo results have been and are likely to be obtained in the near future. Both static and dynamic imaging are possible and these two areas are dealt with separately. Features of the existing in vivo imaging system are good tissue contrast, high-speed data collection, good sensitivity to resistivity changes, low spatial resolution, low cost and no known hazard. It is concluded that the most promising way forward to clinical application in the short term is to use dynamic as opposed to static imaging. An example of lung imaging is shown and the application to measuring regional ventilation and pulmonary oedema is discussed. Use of APT for the detection of intraventricular bleeding in neonates is discussed as is the proven ability to study gastric physiology by imaging resistivity distribution changes following the ingestion of conducting or insulating fluids. Other areas of possible application which are considered are blood flow measurement, cell counting, measurement of lean-fat ratios and the detection of soft tissue lesions.

Non-invasive estimate of the mechanical properties of peripheral arteries from ultrasonic and photoplethysmographic measurements.

Abstract: The non-linear elastic response of arteries implies that their mechanical properties depend strongly on blood pressure Thus, dynamic measurements of both the diameter and pressure curves over the whole cardiac cycle are necessary to characterise properly the elastic behaviour of an artery We propose a novel method of estimating these mechanical properties based on the analysis of the arterial diameter against pressure curves derived from ultrasonic and photoplethysmographic measurements An ultrasonic echo tracking device has been developed that allows continuous recording of the internal diameter of peripheral arteries It measures the diameter 300 times per second with a resolution of 25 microns This system is linked to a commercially available light-plethysmograph which continuously records the finger arterial pressure (025 kPa accuracy) Because of the finite pulse wave velocity, the separation between the diameter and the pressure measurement sites causes a hysteresis to appear in the recorded diameter-pressure curve Using a model based on haemodynamic considerations, the delay between the diameter variations and the finger arterial pressure is first eliminated As the pulse wave velocity depends on the pressure, the delay is determined for each pressure value The relationship between pressure and diameter is then described by a non-linear mathematical expression with three parameters, which best fits the recorded data The dynamic local behaviour of the vessel is fully characterised by these parameters Compliance, distensibility and pulse wave velocity can then be calculated at each pressure level Thus, the mechanical behaviour of peripheral human arteries can now be characterised non-invasively over the pressure range of the whole cardiac cycle The results obtained in vivo on human radial and brachial arteries show that a thorough analysis of the compliance-pressure curves and their modifications (curving, shift) is needed in order to compare two different vessels in a meaningful way

Reference values for resting blood flow to organs of man.

Abstract: The lack of a reliable quantitative description of blood flow in man has hampered the development of accurate biokinetic models of essential elements, drugs, imaging agents, and carcinogens. In this paper we review and analyse data on blood flow and identify representative percentages of cardiac output and absolute blood flow rates to organs and tissues of man for use as reference values for biokinetic models. To keep the review and analysis to a manageable size we have limited attention to the resting state and have suggested reference values for absolute and relative flow rates only for adult males and females.