A portable continuous wave ultrasound system is being developed to observer venous and arterial blood flow. This ultrasound system contains an onboard direct digital synthesis (DDS) microcontroller to produces a frequency variable sinusoidal signal. A 5 MHz signal produce by the DDS is amplified, using high-speed op amps, to excite the piezoelectric ultrasound transducer (PZT).
The transmitted signal is reflected by the red blood cells in the vascular system. The reflected signal excites a secondary piezoelectric material producing a low amplitude waveform (receiver signal). The receive signal contains the transmitted signal along with backscattering signals from the motion of the red blood cells. Using a LM1496 Mixer IC, the backscattered signals (Doppler frequency) is isolated from the receiver signals by mixing the transmitted signal with the receiver signals. The Doppler signal is represented by the velocity of blood, speed of sound through body, angle of transmitting element, and angle of receiver element. The filtered demodulated Doppler signals are typically in the audible range and are sent to a remote computer through soundcard for real-time and post evaluation.
The Doppler signals are sampled by a remote computer sound card at a frequency of 8kHz. Using a Hanning windowsize of 256 a sonogram was constructed using Complex Fast Fourier Transform (CFFT). Currently the system has problems determining the correct Doppler frequency in human testing. However the system is able to characterize flow rate in a blood mimicking phantom. The problem is believed to do with the angle (θ) in which the probe is applied. The current probes rest comfortable when the piezoelectric elements are 90 degrees to the surface of the skin. This angle drives the above equations to instability. The current research objective is to design and fabricate custom probes to meet the following requirements:
- The probe housing shall rest comfortable on the surface of the skin
- The elements should be position such that they are in an angle with blood vessels
- The probes shall be easily to manufacture