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Measurement of chest wall movement; design, performance and clinical use of a four-channel magnetometer instrument.

A four-channel magnetometer instrument is described. It has been developed to measure changes in diameters of the chest and abdomen during respiration. A linearisation correction allows for the magnetic field strength detected by the receiver being proportional to the inverse cube of the diameter being measured. Diameters and changes in diameter are measured to an accuracy of 4% (1 SD) over the working range (15-50 cm). The effects of non-ideal positioning and extraneous movements of the magnetometer coils, cross-channel interference and the influence of nearby metals are described. With minimal care in positioning the coils, these effects are negligible. The instrument has been in use for one year, assessing chest wall movement in patients with abnormal lung function. Recordings are presented illustrating typical normal and abnormal chest wall movements.