In vitro methods for studying the accuracy of velocity determination and spatial resolution of a color Doppler flow mapping system

We tested a color Doppler flow mapping system (Aloka 880, 3.5 mHz) to image the steady flow of a suspension of cornstarch (0.1% to 1.75%) in water, flowing at known velocities through a series of Tygon tubing with internal diameters of 1.6 to 4.8 mm and latex rubber tubing (7.9 to 15.9 mm) in a water tank. Flow through the tubes was imaged at 2 to 12 cm and 2 to 18 cm distances from the transducer, by the use of the near and far focus settings of the instrument. First, the color flow diameters were compared to the known internal diameters of the tubing oriented at a 30- to 70- degree flow angle to the direction of interrogation to test axial resolution as well as laterel resolution. For the near transmit focus setting, the color diameters increasingly overestimated the actual flow diameters as the distance from the transducer increased progressively beyond 6 cm (i.e., overestimation of up to 25 mm or 500% for the 4.8 mm tubing and 250% for the 9.5 mm tubing at 16 cm from the transducer). For the far transmit focus setting and for distances within 10 cm, the color diameters did not differ significantly (i.e., more than 1 mm) from the actual diameter. The lowest velocity color encoded by the instrument was 3 cm/sec for a pulse repetition frequency of 4 kHz. At a flow velocity of about 50 cm/sec, 6 cm distance from the transducer and mid gain settings, flow through the 1.6 mm tube barely color coded, while flow thorugh 2.4, 3.2, and 4.8 mm tubes was easily imaged. These results have direct implications in clinical studies for imaging small septal defects, small orifices, or when measuring color flow Doppler diameters of vessels or valves for volume flow calculations. They also established a method for objectively evaluating the evolving capabilities of this new imaging technology.