METHODS OF MEASURING BLOOD FLOW 



I3 (, 7 



carotid artery and in the jugular vein (62). Its sensi- 

 tivity and frequency response were satisfactory. How- 

 ever, the base line was often shifted by spontaneous 

 changes of the resistances between the electrodes. This 

 was probably due to minute fibrin deposits at the 

 bristle tip (102). 



Further improvement of the electrical transmission 

 was achieved with electromagnetic devices. The bristle 

 or pendulum is made of or mounted with ferromag- 

 netic material which changes, by variation of its posi- 

 tion, the magnetic field of two coils installed on both 

 sides of the pendulum. The coils are fed with alterna- 

 ting current. In 1952, Brecher and Crun [quoted from 

 (9)] arranged two induction coils around a Lucite can- 

 nula in such a way that deflections of ferromagnetic 

 pendulum changed the coil inductances in opposite 

 directions. Although the signal-to-noise ratio was 

 high, temperature changes produced an unfortunate 

 instability of the base line. 



Pieper & Wetterer (102-104) developed several 

 models with electrical transmission based on the prin- 

 ciple of the differential transformer. While in the 

 inductance bridge the amplitude of the resulting 

 signal voltage is the outcome not only of changes in 

 inductance, but also of changes in ohmic resistance, 

 e.g., resulting from temperature changes of the coil 

 wires, the differential transformer separates the ohmic 

 component from the inductive component so that 

 only the latter is measured. The transformer consists 

 of two symmetrical parts, each of which contains a 

 primary and a secondary coil. The primary coils, 

 which are fed with alternating current of 5 to 10 kc per 

 sec, are connected in series so that the directions of 

 their magnetic a-c fields are congruent. The secondary 

 coils are also arranged in series; their winding direc- 

 tions, however, are opposite to each other, and the 

 induced secondary a-c voltage will be proportional 

 to the difference of the mutual inductances present in 

 each of the two parts of the transformer. When the 

 conditions are equal on both sides, the secondary 

 voltage will be null. By shifting the ferromagnetic core, 

 the mutual inductance of one part is augmented, and 

 that of the other part diminished. If the primary 

 alternating current is kept constant, changes in ohmic 

 resistance of both the primary and secondary circuits 

 will have no influence on the secondary voltage. This 

 principle had already proved satisfactory in micro- 

 manometers (49, 135). The first pendulum flowmeter 

 of Pieper and Wetterer consisted of a compact ferro- 

 magnetic cylinder (fig. 1 7/) the deflections of which 

 were detected by differential-transformer coils wound 

 around the tube upstream and downstream from the 



fig. 19. Electromagnetic bristle flowmeter of Pieper and 

 Wetterer. a: Section in the plane of the two axes of the T-can- 

 nula. b: Section at 90 to a. FS, flat spring; TC, transformer 

 case; C, coils of differential transformer; F, ferromagnetic 

 frame of transformer; FC, movable ferromagnetic core; A", 

 needle. Proportions not to scale. Maximal length of side branch 

 about 20 mm. For use, the T-cannula is completely filled with 

 anticoagulant fluid. [From Pieper & Wetterer (104).] 



middle of the cylinder. This model was abandoned in 

 spite of its simplicity because of two disadvantages: 

 The cylinder length is not negligible so that the inertia 

 of the fluid column around the cylinder gives rise to a 

 third term in equation 10 which is proportional to the 

 flow acceleration and may distort the records (see 

 term III in equation 8). In addition, the force exerted 

 on the cylinder by the streaming fluid is due only to 

 the flow near the axis. 



The authors, therefore, built two other models, the 

 second of which corresponds to figure 1 yb and is 

 shown in figure 19. A flat spring is fixed at the end of 

 the vertical limb of the T-cannula and carries a ferro- 

 magnetic core and a needle protruding into the 

 horizontal tube. The differential transformer is in- 

 stalled on both sides of the vertical limb so that deflec- 

 tions of the needle will shift the core toward one or the 

 other transformer part. The device has a high sensi- 

 tivity and satisfactory temperature stability. Its na- 

 tural frequency is about 200 cps. The additional 

 equipment consists of an a-c source of 5 to 10 kc per 

 sec for feeding the primary coils of an amplifier and a 

 rectifier. Attempts to linearize the calibration curve 

 have also been made. A micromanometer was at- 

 tached to the T-cannula (fig. 19) for simultaneous 

 blood-pressure recording. The instrument was used 

 for the registration of pressure and flow in the carotid 

 and femoral arteries of dogs. 



Independently, Scher et al. (118) described a paddle 



