Doppler sonography/physical principle

'''

Physical principles of Doppler-Sonography
'''

Introduction
Sonongraphy, also sometimes called ultrasound scanning is a safe, painless and non-invasive imaging technique which involves the use of high frequency sound waves to produce real time images of internal organs, tissues and bloodflow in the body. During this process, high frequency sound waves of between 1-5 MHz are transmitted into the body using a transducer, the sound waves are reflected by the tissues being examined back to the same transducer, which then relays the message to the ultrasound machine. The machine is able to interprete this data by calculating the distance between the transducer and the tissues and using the speed of sound in tissue (5,140 m/s) and the time taken for the echo to return, and this enables a two-dimensional image to be formed.

The Doppler-Sonography is a special type of Sonography that enables the physician to display the the bloodflow in blood vessels and the heart in acoustic and visual qualities using the Dopplereffect of soundwaves. In Doppler Sonography it is still necessary to use the method of usual Sonography to find the target structure. The target structures of Doppler Sonography need to be vessels or the heart since a moving object or fluid (in this case blood) is required to create the Doppler effect. Common examined organs or tissues are for instance the common carotid artery, the heart and also the vessles of the brain. Nowadays, modern medical ultrasound device can use Doppler Sonography in order to evaluate the bloodflow of the visualized structures. Because Doppler-Sonography is fast, relativley cheap, and non-invasive it has become essential in the diagnostic of vessles in various areas of medicine such as internal medicine, gynaecology, angiology and cardiology. In this part of the article we are going to discuss in detail how the Doppler effect is applied to the medical ultrasound device.

Applied Doppler effect in Sonography


In Sonography the Dopplereffect works slightly different compared to the examples provided above as the blood does not emmit any sound but rather reflects the soundwaves produced by the probe. Through the reflection of the soundwaves by the erythroctes and their movement due to bloodflow the blood behaves like a moving source of sound. Therefore the soundwaves reflected by the erythrocytes will undergo the Dopplereffect and there will be a change of frequency in comparsim with the emmited frequency by the transducer. Therefore the transducer has to be again, like in sonography, the producer of sound and at the same time the receiver of the reflected waves.

In Doppler sonography, a series of pulses is transmitted using a transducer to detect movement of blood. Echoes from stationary tissues are the same from pulse to pulse, however echoes from moving ‘scatterers’ show slight differences in the time taken for the pulse to be returned to the receiver. These differences are measured in terms of a phase shift, and it is thus possible to obtain a Doppler frequency. These can then be further processed to produce a colour flow display or spectrum.

Depandence of Doppler Shift on Doppler angle


When a physician is trying to examine a vessel via Doppler Sonography it is really important to take the angle of incidence into consideration as the Dopplereffect is dependant on the position of the reseiver to the source of sound. As shown in the picture with the ambulance we obtain either a much higher frequency when movement of the source of sound is towars the receiver or a much lower frequency when the movement of the source of sound is away from the receiver. Therefore the optimal value for the Doppler effect would be at 180° or 0° to the source of sound. Now imagine the observer of the ambulance is standing at 90° to the driving ambulance. The waves the reseiver is hearing now are neither of a higher nor a lower frequency which means there is no doppler effect. This phenomena can be seen even clearer when measureing the distances between the waves at 90° and comparing them to the the distance between the waves when the ambulance is not moving. You should obtain approximatley the same frequency. Therefore the sound the person will hear at 90° to the moving ambulance will be the same sound as if the ambulance was not in movement. This relation is expressed in the formula through cos θ. The cosine of 0° and 180° is 1 and -1. The cosine of 90° is 0. That is why the optimal angle of insidence or beam-flow angle is fixed between 0°- 60° or 120°-180°. In both cases there is a sufficient Doppler effect to measure a accurate value of the velocity of blood. Trough the angle of inscidence the device is able to display different directions of blood-flow as opposite directions of flow will have the same magnitude but either a positive (0°-60°) or a negative (120°-180°) value as the angle is inverted with changing direction. Variying among ultrsound devices blood flowing towards the probe it coloured differently to blood flowing away from the probe. That is an addiational method to differentiate between arterys and veins.

As shown in the images, there has to be motion in the direction of the beam, so the transducer must be inclined at a certain angle to the vessel being examined. If the flow is perpendicular to the beam, there is no relative motion from pulse to pulse. The size of the Doppler signal is dependent on: (1)	Blood velocity: as velocity increases, so does the Doppler frequency; (2) Ultrasound frequency: higher ultrasound frequencies give increased Doppler frequency. As in B-mode, lower ultrasound frequencies have better penetration. (3) The choice of frequency is a compromise between better sensitivity to flow or better penetration; (4 The angle of insonation: the Doppler frequency increases as the Doppler ultrasound beam becomes more aligned to the flow direction (the angle q between the beam and the direction of flow becomes smaller). This is of the utmost importance in the use of Doppler ultrasound. The implications are illustrated schematically in Figure 3. https://sonoworld.com/Client/Fetus/html/doppler/capitulos-html/chapter_01.htm