Clinical Physics and Physiological Measurement

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 are 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 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. The authors 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 they have limited attention to the resting state and have suggested reference values for absolute and relative flow rates only for adult males and females.

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.

Six hundred 'normal' volunteers have been examined, ranging in age from neonates of three days to adults of 65 years. Average compliance was measured for the aortic, iliac, and leg arterial pathways using a non-invasive atraumatic Doppler ultrasound method in non-sedated subjects. No variation of compliance with mean blood pressure was found but there was a strong dependence on age and sex. At birth, leg arteries appear to be markedly more compliant than the aorta. Relative values are reversed in the first five years, after which the aorta remains more compliant, reaching a maximum value around 10 years and then decreasing sharply towards leg values until 20 years. Beyond this age aortic compliance decreases very slowly, and aorta, iliac, and leg arteries appear to tend towards equal compliance in the sixth decade. An explanation for the significant differences between aortic compliance in males and females from menarchy to menopause is offered and implications of these compliance variations in atherogenesis are discussed.

The design and implementation of high performance differential voltmeters for semi-parallel data acquisition are described. The general requirements and specific conditions encountered in eletrical impedance tomography (EIR) for accurate measurements are analysed. The major parameters are common-mode rejection and bandwidth. A specific implementation of the voltmeters, with separate DC supplies and independent signal references, is described. This arrangement, in which each voltmeter follows the input signal, automatically cancels any common-mode voltage at the input. The signal is fed to the remainder of the instrumentation through a transformer. The use of a reduced number of components contributes to the minimisation of the inter-channel variations. Furthermore, the geometrical distribution of the voltmeters around the object minimises the length of electrode wires, also reducing the input capacitance. The number of modular voltmeters and DC/DC converters is 32. The common-mode rejection of these voltmeters is greater than 72 dB in the frequency range 3.6–560 kHz. In conclusion, the proposed solution ensures a minimisation of common-mode errors and enables the use of a 250 kHz frequency.

When a current is injected into a body, in addition to the voltage profile developed on the surface, a common-mode voltage (CMV) which produces errors in the measurement also appears. The great accuracy needed to reconstruct images in electrical impedance tomography (EIT) requires the use of differential amplifiers with a high common-mode rejection ratio (CMRR) to avoid this error. Nevertheless, the effective CMRR is lower than the differential amplifier ratio due to mismatches in the electrode impedances and other circuits in the measurement channel. The use of common-mode feedback (CMFB) is an alternative to reducing the error pmduced by the CMV. The stability of the feedback loop is analysed for a broadband system. Simulation and experimental results show that it is possible to obtain an improvement of 40 dB in the measurements at frequencies of up to 10 kHz.

The objective of this study is to improve the quality of the hardware of the existing Lund impedance tomography system. Improvement in the current generator and different isolation proposals are presented.

The electrode–electrolyte interface impedance may be simplistically modelled by an equivalent circuit comprising a resistance, R_TOTAL, in series with an empirical, constant phase angle impedance, Z_CPA. This pseudo-capacitance can be thought of as representing empirically the non-faradaic, double layer capacitance in the presence of specific adsorption and surface roughness effects. R_TOTAL is the sum of the lead and electrolyte resistance. Depositing a thin layer of silver chloride on silver electrodes can yield improved electrical performance characteristics (potential and impedance) when used in conjunction with a chloride gel. An electrolytically deposited AgCl layer tends to have a rough surface profile thus leading to an increase in the effective interface area. This gives rise to a decrease in R_TOTAL and Z_CPA, both of which are desirable. Unfortunately AgCl is a relatively poor conductor. Increasing layer thickness causes R_TOTAL to increase, thus adversely affecting the inter-electrode impedance at high frequences. Electrode systems for use in electrical impedance tomography therefore require only relatively thin layers of AgCl.

Multiple applied current impedance measurement systems require numbers of current sources which operate simultaneously at the same frequency and within the same phase but at variable amplitudes. Investigations into the performance of some integrated operational transconductance amplifiers as variable current sources are described. Measurements of breakthrough, non-linearity and common-mode output levels for LM13600, NE5517 and CA3280 were carried out. The effects of such errors on the overall performance and stability of multiple current systems when driving floating loads are considered.

A variety of nuclear-based techniques for the in vivo study of human body decomposition is now available for clinical diagnosis and research, and the number of centres where such work is performed is likely to grow substantially in the next few years. Their most important applications at present are in the measurement of bone mineral mass (calcium), body protein (nitrogen) and organ contents of cadmium and lead. Although still mainly used as research tools, they are already at a stage where routine applications can be justified in clinical diagnosis and occupational health monitoring. The radiation doses delivered to the subject vary according to the sensitivity and precision required, but are generally low compared with many common radiological examinations and diagnostic tests.

This article reviews present techniques of body temperature measurement. The extent to which currently available thermometers satisfy the various requirements of medical thermometry are discussed.

Mass spectrometry has developed rapidly from being a research tool used primarily in physical chemistry to the point where biochemical and medical research applications not only account for a high proportion of its usage but also direct much of the effort concerned with technique development. Two major recent developments have increased and continue to expand the scope of application in medical research and other areas. These are (i) the effective use of high mass spectrometric sensitivity and (ii) the development of techniques for the analysis of compounds of high mass and low volatility which were previously impossible to analyse directly. The author considers the application of mass spectrometry in medical research and attempts to highlight and exemplify recent trends of particular importance. Special emphasis is given to the consideration of the unique advantages of mass spectrometry as an analytical method.

This review deals with the following topics: why use lasers?; what is a laser?; containing and controlling laser beams; types of medical lasers; laser beam delivery systems; evaluation and safety of medical lasers; buying a laser; laser tubes; general maintenance; staff training and safety codes; future outlook.

The main impression received by the authors whilst writing this review is the scarcity of technical data in the clinical studies and the total absence of controlled trials. The spatial patterns of the stimulus have not been measured, and no experiments have proved that there is an effect from the magnetic field itself. Control experiments using dummy stimulators must be done, since the orthopaedic management of the stimulated patients is different from conventional management and this may have significant and beneficial clinical effects. There is no clinical study at present which shows a direct therapeutic benefit due solely to the application of the magnetic field component of the overall treatment regime. The in vivo animal experiments suggest that there may be effects due to the magnetic fields used but results are very scarce compared with the accumulated data from direct current stimulation. In vitro studies are far removed from the clinical situation, but could nonetheless prove useful if the opportunity of controlling the stimulus can be tanks. In conclusion, the authors believe that, unlike steady current work, the pulsed magnetic field treatment of fractures has not been sufficiently well investigated and, although some of the animal experiments suggest significant effects the benefit of using magnetic fields in the clinical management of non-union and delayed union has still to be proven.