Time is the key factor to keep in mind. A conventional two-dimensional ultrasound imaging system is already working as hard as it can. Pulses must be transmitted along a given line, reflected from the heart valves and walls, then received. The process is repeated, line by line, through the entire sector arc that comprises several hundred lines. This completes one frame of information, usually in one-thirtieth of a second. In order to have the image appear as though it is continuously moving, the entire image must be updated 30 times in a second (30 frames/second). This results in relatively long waiting periods for the transmit-receive sequence to be completed. It also means that considerable amounts of information need to be quickly processed and presented in the image.
A problem, therefore, results. If all this time is taken to simply create the image, where is there time to sample rapidly with the Doppler, left and right, in all portions of the image field? From earlier units in this series, we have already learned that high quality imaging and high quality pulsed Doppler cannot really be conducted simultaneously.
2004