Optical fibre plethysmograph.

We describe a method for obtaining blood volume information from the external jugular vein or carotid artery by means of an optical fibre transducer. The device may be used to obtain accurate predictions of the state of the cardiac cycle, thereby enabling a range of physiological measurements to be made. The transducer is entirely non-invasive and non-magnetic; it is extremely robust and is unaffected by the electrically and magnetically hostile environment created by an NMR scanner.