d) Doppler scan

d) Doppler effect in ultrasound; speed of blood in the patient; Δf/f = (2vcosθ)/c for determining the speed v of blood

scanning process

• transducer is placed over the blood vessel

• sends ultrasound pulses

  • reflected off tissues → received with the same frequency

  • reflected off blood moving away from the transducer → received with decreased frequency

  • reflected off blood moving towards the transducer → received with increased frequency

• computer displays a colour-coded image depending on the direction of blood

Δf = (2fvcosθ)/c

• change in ultrasound frequency (Hz) = (2 × initial ultrasound frequency (Hz) × cosine(angle between the pulse and the skin)) / speed of ultrasound in blood (ms^-1)