Image Processing Reference
In-Depth Information
11
Complex Digital Filter Designs for Audio
Processing in Doppler Ultrasound System
Baba Tatsuro
Toshiba Medical Systems Corporation
Japan
1. Introduction
A medical Doppler ultrasound system has a spectrum display that indicates the blood flow
direction, whether the blood flows forward or away from a probe. It also has Doppler audio
outputs. In particular, the latter is a special process peculiar to the Doppler ultrasound
system and separates the blood flow direction and outputs from the left and right speakers.
Owing to this function, the existence of a blood flow is quickly detectable. When changing
conventional analog signal-processing into digital signal-processing, we researched many
processing systems of Doppler audio. First, target performances, such as a response time
and direction separation, were set up, and six kinds of digital signal-processing systems
were examined. Further, we investigated some new anti-aliasing processing systems unique
to Doppler ultrasound system. We compared three kinds of anti-aliasing processing
systems. Consequently, we clarified that a complex IIR (infinite impulse response) filter
system has an excellent response and a low calculation load.
2. Outline of Doppler ultrasound system and conventional analog signal-
processing
Recently, the diagnostic ultrasound system has been popular in many diagnostic fields, such
as cardiac, abdomen, and so on. In Section 2.1, an example of diagnostic image and its
principle are introduced. In Section 2.2, the phase shift system that is an example of
representation of conventional analog signal-processing is introduced.
2.1 Outline of Doppler ultrasound system
An example of diagnostic image of a carotid artery is shown in Fig. 1. The upper is a
tomogram image and bottom is a spectrum Doppler image. This image expresses the time
change of the flow velocity in the PWD (Pulse Wave Doppler) range gate set up in the
central of a blood vessel in a tomogram. A horizontal axis and a vertical axis are the flow
velocities corresponding to Doppler shift frequency and time, respectively.
Signal processing of the ultrasound echo signal is shown in Fig. 2. An ultrasonic wave is
transmitted for every cycle of PRF (pulse repetition frequency: fs ) in the transceiver
processing part of Fig. 2(a), and a reflective echo is received. An ultrasonic beam is scanned
in the transverse direction, and envelope detection of the received signal is carried out in the
range direction. This scanning constitutes the tomogram image.
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