Dissertação de mest., Engenharia Electrónica e Telecomunicações, Faculdade de Ciências e Tecnologia, Universidade do Algarve, 2009Ultrasound has for many years been an important tool in the detection and quantification of various
health problems. In vascular diseases, for example, the ultrasound can be applied with different
techniques such as Transit-Time Flow Measurements (TTFM) [36], Doppler [28][40][41][42] and elastography
[37] [38].
Research has been developed focusing the signal processing of Doppler ultrasound signals. In an
ongoing project, named Desarrollo de Sistemas Ultras´onicos y Computacionales para Diagn´ostico
Cardiovascular (SUCoDiC), Doppler ultrasound signals are processed by an analog signal processing
unit, in order to obtain the inphase (I) and quadrature (Q) components of the Doppler ultrasound
signals, to allow directional blood flow separation.
Problems associated with unbalanced channels’ gain of the employed analog system have been
detected, resulting in an inapropriate directional blood flow separation.
This thesis reports the research performed to eliminate such problems by substituting the analog
system’s demodulator by digital signal processing approaches aiming at the achievement of the same
goals, i.e., obtaining the Doppler ultrasound signal’s inphase (I) and quadrature (Q) components,
for efficient directional blood flow separation.
Five digital quadrature techniques have been studied to achieve such goal. Also, given technical
constraints imposed by the nature of the Doppler ultrasound signals to be used, and limitations of the
sampling rate of the Analog-to-Digital Converter (ADC) used, two strategies to acquire the Doppler
ultrasound signals were studied. Such strategies involved the sampling of a downconversion version
of the Doppler ultrasound signals (by application of the heterodyne function) and direct sampling of
the Doppler ultrasound signals using uniform bandpass sampling.
From the results obtained, three approaches are selected and proposed for real time implementation.
Comparison between both signal sampling strategies employed are also presente
