Patent GB-1322248-A (GB)
Title: Diagnostic apparatus
IPC: A61B8/02
Filed: 1970-10-20
Abstract: 1322248 Ultrasonic diagnostic apparatus; pulse rate discriminators; ramp generators; frequency multipliers SONICAID Ltd 7 Oct 1971 [20 Oct 1970 (3)] 49650/70 49701/70 and 49702/70 Headings H3A, H3D and H4T Diagnostic apparatus, e.g. for continuous monitoring of foetal heart rate, derives a Doppler signal by mixing an ultrasonic signal as transmitted with the ultrasonic signal received by reflection from a moving surface, derives a first pulse train from at least one selected Doppler frequency band, produces a voltage proportional to the pulse repetition period of the first pulse train, and produces in response to the voltage a second pulse train the pulse repetition frequency of which is greater than the repetition frequency of the first pulse train and is inversely proportional to the voltage. Heart rate is thus detected substantially instantaneously, by effectively measuring the interval between individual beats. In the embodiment shown, a pulse train comprising one pulse per heart beat is derived from a frequency-filtered and limited Doppler signal. Each pulse triggers a first monostable circuit, providing a brief output pulse which immediately triggers a second monostable circuit providing a further brief output pulse. This further pulse, applied at 64 (Fig. 2), re-sets to zero an integrator comprising op-amp 57, capacitor 60, and constant current source 51, by turning on an F.E.T., 61 briefly. The integrator then generates a ramp voltage until it is reset again. To compensate for the ramp's not starting until the reset pulse has ended, capacitor 65 is provided: this charges during the reset pulse and injects a boost charge into the integrator when the reset pulse ends. The peak voltage of the ramp is proportional to the interval between consecutive heart beats and is sampled by applying a pulse from the first monostable circuit to terminal 68 to turn on F.E.T. switch 66 and so store the ramp peak on capacitor 69. A square wave of period proportional to this voltage is obtained at the output of a voltage comparator 70 as follows. With F.E.T. 74 and constant current source 81 switched on, capacitor 80 charges positively until it reaches the capacitor 69 voltage, whereupon the comparator output changes state, thereby turning F.E.T. 74 and current source 81 off and turning F.E.T. 75 and constant-current source 82 on. The capacitor 80 then discharges to zero volts, whereupon the comparator output again changes state and the cycle is repeated. The comparator output frequency is a measure of heart rate and drives a digital or analogue display. For an analogue display the frequency is converted by transistor pump circuit 111 (Fig. 3) to a voltage which is stored on capacitor 147 and drives a recorder. To prevent spurious perturbations of the stored voltage from being recorded, a second transistor pump circuit, 101, of much longer time constant than 111 derives a voltage representing average frequency, and if this voltage differs from the capacitor 147 voltage by more than some threshold value (as determined by a differential amplifier 122 driving threshold switches 131, 132) then a switching signal is applied to turn off F.E.T. 145 to prevent capacitor 147 voltage from changing. F.E.T. 145 is also switched off in the interval between heart beats by a blanking signal applied at 150. A track/ store amplifier may be used instead of capacitor 147. For transmitting and receiving the ultrasonic signals a tuned, 2MHz, divergent widebeam transmitting transducer surrounded by a group of receiving transducers may be used.