Structure and performances of mono- and bidimensional pulsed Doppler systems
Pulsatility of cardiac events limits the time allowed for acquiring informations. Biological tissue, ultrasonic probe and electronical circuits produce noise which is added to the signal. The theoretical accuracy of any velocity measurement can be expressed using the Woodward’s relation: sv = k/(Tm × R1/2), where sv is the variance of the velocity estimation, R the signal-to-noise ratio and Tm the measurement duration. Technical problems are further complicated by the presence of wall motion producing strong low frequency Doppler signals.
In pulsed Doppler systems, transmitted bursts generate backscattered echos. Each of them is processed in order to get I and Q signals, thus defining a point. Successive bursts produce successive points, the rotation of them defining the velocity and the orientation of the target motion.
For a given depth, when a single line is studied, the number of bursts corresponding to the pulse repetition rate is used both for filtering and velocity estimation. At the opposite, when considering 2D Doppler imaging, the same number of bursts is shared between the lines required for building the image. Therefore, the number of informations available for filtering and velocity estimation is far smaller, leading to a poor accuracy.
KeywordsWall Motion Pulse Repetition Rate Doppler Frequency Doppler Signal Velocity Estimation
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