Abstract
A pressure transducing system utilising light for the modulation of the deflection of a catheter tip pressure sensing diaphragm is investigated. Optical fibres are used for transporting the input light signal to, and the modulated signal output from, the probe tip. The emergent modulated light signal falls on to a photo detector which gives an analogue electrical output. The analysis of the total system is achieved by first considering the effect which the parameters of the optical fibres and their arrangement in the distal tip of the probe have on the fraction flux received to flux emitted as a function of the separation of a 'flat' reflecting diaphragm. This relationship is derived from first principles and considers the emitting fibre ends to be Lambertian sources. This implies that the numerical aperture (N.A.) of the fibre(s) is unity and that the polar distribution is uniform. The resulting expression is termed 'reflection factor', from which a simple method for accounting for fibre systems of other N.A. s is proposed. The conditions for which it is valid are discussed, at which time a critical appraisal is made of the one other original author in this field. The derived expression for reflection factor is modified by introducing a normalised concept for the separation 'd' of the reflecting surface to the distal tip face, and the outside radii 'R' of the receiving system, these being the separation ratio 'B' and the 'A' value respectively. When 'B' = d/r' and 'A = R/r', noting that 'A' signifies the fibre arrangement which, geometrically is that of a central emitter surrounded by an annullar receiver. The introduction of these normalised terms allows for a general description to be defined for the gradient of the reflection factor response curve (slope factor) which is dimensionless. It also allows for the description of what is termed the 'reflection factor total ratio'. This is the ratio of the reflection factor at a separation ratio of 'B' to that when the separation ratio has changed by 'delta B' this being the normalised deflection ratio. Before applying the above mentioned conceptual approach which defines the optical fibre response for different positions for a 'flat' diaphragm, in order to predict the response when a real diaphragm is used, an assessment is made of the errors incurred by considering the real diaphragm to deflect with a flat profile. A separate section has been devoted to considerations of a diaphragm, its deflection characteristics and natural frequency. A graphical presentation by which a choice of diaphragm can be exercised and a factor of safety can be determined has been developed. The light source and photodetector are considered from the point of view of what type and mode of operation are most suitable for application in this system and a relationship for the minimum acceptable signal to noise ratio and the set level of system output is derived for different values of reflection factor total ratio. All the discrete system parameters are combined to form a graphical aid for the specification of system parameters (G.A.S.S.P.), which is used to provide a theoretically proposed system and its response characteristics. It is later applied to a system where system noise and probe reflection factor characteristics have been determined experimentally, to aid the choise as to which diaphragm should be used and at what separation it should be set. The fibre optic probe constructed thereby is experimentally evaluated, the diaphragm being 25 microns thick Berrylium Copper of 1.5 mm radius, for which when set at an initial separation of 35 microns a pressure resolution of 0.5 mm Hg. (sensitivity 0.186 mV / mm Hg.) is achieved.