This report will focus on the physics, processes and equipment used in Doppler ultrasound. It will also focus on the results, clinical indicators, advantages, limitations and applications of Doppler ultrasound. The Physics of Doppler Ultrasound The basis of Doppler ultrasound is what is known as the Doppler Effect. It is named after an Australian physicist Christian Doppler who theorized it in 1842. It states that the reflected waves from a moving object will undergo a change in frequency. For example when a vehicle with a siren moves toward an observer the frequency received by the observer is higher than that emitted from the siren and this is also the case as the source of the siren moves away.
The magnitude and direction of this change is able to provide information about the moving object. In the medical realm, the movement of blood cells causes changes in the pitch of reflected sound waves from the Doppler ultrasound machine. This hand-held machine is known as a transducer and is lightly applied on the skin over a blood vessel. Information from the reflected sound waves is then processed by a computer into two dimensional color images and graphs as well. Figure 1: Doppler ultrasound measures the movement of scatters in the blood through the beam.
As blood velocity increases so does Doppler frequency There a few basic types of Doppler ultrasound including continuous or pulsed wave Doppler, duplex Doppler and color Doppler ultrasound. Continuous Wave Doppler Also known as bedside Doppler, in this kind of Doppler ultrasound, there is continuous transmission and reception of ultrasound. Deane (2002) states Doppler signals are obtained from all vessels in the path of the ultrasound beam. The change in pitch of the sound waves produced is what is used to provide information about flow of blood in the vessels.
The beam has to become sufficiently attenuated to provide information about the blood vessel. As the name suggest in can be done at the bedside in a hospital with a portable machine. Figure 2: Continuous wave Doppler transducer. Continuous wave Doppler can therefore detect high velocity flows however, is unable to pinpoint where along the line the high velocity is coming from. Pulse wave Doppler can achieve this. Pulse Wave Doppler Pulse wave Doppler allows measurement of velocity along a single point on the line.
The radiographer sends a signal or to a specific depth and then listens for frequency at that depth. Duplex Doppler In duplex ultrasound, a picture of the blood vessels and surrounding organs is produced usually using standard ultrasound methods. It therefore combines conventional imaging information and Doppler flow imaging information to allow the structure of the blood vessels to be seen. During a duplex ultrasound exam the radiographer uses two forms of ultrasound together, conventional ultrasound to show structure of blood vessels and Doppler ultrasound to show flow of blood through the vessels.
Color Doppler In this type of Doppler, a color image of the blood vessel is produced. Doppler sounds are then converted into colors that are overlaid on the blood vessel to come up with information about the speed and direction of the blood flow through the blood vessel. The image viewed in grayscale is difficult to read and color Doppler improves the image. It assigns color values that depend on whether the blood is moving towards or away from the transducer. Figure 3: Color flow Doppler image There are several factors that affect Doppler ultrasound imaging including blood velocity, ultrasound frequency, the choice of frequency and angle of isonation.
Generally as blood velocity increases, Doppler frequency increases as well. Higher ultrasound frequencies yield higher Doppler frequencies. The choice of frequency is a choice between sensitivity to flow or better penetration. The angle of isonation affects in that as the ultrasound beam becomes more aligned to the flow of blood the Doppler frequency increases.
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