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How is the Doppler effect and ultrasound used to diagnose illness?

Updated on September 30, 2015
  • In Doppler ultrasound the change in frequency is measured and analysed to give information about the blood flow in the body, particularly the heart. An ultrasound is directed into the body and some of this ultrasound is reflected off blood cells moving in the blood. Due to the movement of the cells the reflected ultrasound received by the transducer will have a changed frequency compared with the incoming signal. The motion of the heart walls was monitored by observing the movement of the heart itself. During the ultrasound, a dark region of bluish-tinge was detected in the arteries that flowed from the heart into the rest of the body->indicating an abnormally slow blood flow.

  • The Doppler effect is the apparent change in frequency observed when there is relative movement between a source of a sound and an observer and is founded by Johann Christian Andreas Doppler. This can be best observed when a car passes you from behind or moves between two observers. A stationary car noise will be heard at the same frequency and time by both observers. However, if the car approaches an observer the frequency will become apparently greater and smaller when the source of car moves away. If a pulse of ultrasound reflects off a stationary boundary it will return with the same frequency and wavelength as emitted. If the boundary is moving away from the transducer there will be a Doppler shift. The waves will undergo a Doppler shift on their outward and reflected journeys, producing a double Doppler shift. Ultrasound used in blood flow measurement is typically in the range 5 to 15 MHz. The ‘moving boundary’ comprises the surfaces of multiple red blood cells, as an individual red blood cell is too small to be a boundary on its own. The Doppler Effect has to be taken into account twice. Consider blood flowing towards the transducer. The blood cells will receive a signal at a higher frequency than that given out by the transducer. These blood cells then act as a source when they reflect the signal. They reflect the higher frequency wave and then move into the wave at the same time, resulting in a further increase in frequency. This higher frequency is then received by the transducer. An experienced practitioner can listen to the frequency change and make judgements about whether the flow is towards or away from the transducer and whether the blood flow rate is normal. Also the signal can be electronically analysed and displayed on a screen for examination. The ultrasound reflected from internal tissues may pose a problem as these waves interfere with the echo ultrasound being analysed. The Doppler Shift is typically a change in frequency up to 3kHz. This value is positive when blood flows towards the probe and negative when the blood flows away. Sophisticated computer software can assign colours to an ultrasound scan on the basis of its Doppler shift. Doppler ultrasound is commonly used to examine liquid flow. In this way flow velocity can be seen. A colour change can indicate increased velocity, indicating a narrowed artery. Colour intensity can indicate flow volume. Mixed colours can indicate flow turbulence due to partial blockage. The wrong colour can indicate a leaking heart valve. Doppler shifts can be in the audible range and so can be heard and an experienced operator can make a diagnosis from this sound.

  • Cardiac problems are able to be detected using ultrasound techniques with the Doppler effect in mind. When the scan is combined with Doppler colour imaging, the valves can be checked to see if they open and shut correctly and if they leak. Echocardiography is the use of ultrasound to diagnose heart problems. A transducer placed on the chest walls emits a short pulse of ultrasound. The reflection is detected from the cardiac structures such as valves and heart muscle walls. Form this, the condition of the heart and its components can be determined. In Doppler echocardiography, ultrasound waves reflect off individual blood cells moving towards or away from the transducer. If the blood flow is parallel to the ultrasound beam, the velocity of blood flow can be measured. The greater the speed of flow, the more the frequency shifts. To improve the information displayed with Doppler ultrasonography, the computer can add ‘false colour’. Commonly blood flow towards the transducer is coloured red and flow away is blue. The speed can be indicated by variations in brightness and colour. Cardiac problems that can be detected: Narrowing of the arteries. The velocity increase where the artery narrows. Leaking valves: Blood flowing ‘the wrong way’ can indicate a faulty heart valve. Blood vessel blockages or the build up of plague. A mix of colours may indicate a range of Doppler shifts resulting from turbulence created by a part blockage or rough walls of the blood vessel. However the drawback of the continuous Doppler signal is that it does not convey clear information about deep blood vessels due to scattering and reflection from soft tissues encountered by the ultrasound as it penetrates the body.

    Hopefully you guys are able to gain certain facts and knowledge about the chemistry of the Doppler effect and how it assists in making our world a better place.


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