4/8/2024 0 Comments Sound waves diffractHowever, no continuous imaging is possible.ĭoppler ultrasonography exploits the well-known Doppler effect. In PW Doppler, which is pulse-based, distance information can be obtained using time-gating. However, no distance information can be measured. It has the advantage that it allows for continuous imaging due to simultaneous emission and detection. In CW Doppler, half of the transducer array emits, and the other half detects pulses. Two modes are frequently used: continuous wave (CW) Doppler and pulsed wave (PW) Doppler. Medical Doppler ultrasonography enables the measuring and visualization of blood flow (blood velocities). With a curved arrangement of transducers, an image detail can be represented as a circle segment.Įlectronic scanners (phased arrays) Every transducer element of an array can be accessed for both sending and receiving with an individual adjustable delay. For scanning, the whole group of elements is shifted. A group of transducers is activated simultaneously. An image consists of a fan of typically 100 lines.Įlectronic scanners (linear/curved arrays) Here, many (60 to 100) and very small (0.5 mm to 1 mm) transducers are used, which are arranged in a row (“array”). The intensity of the echo is transformed into gray scales and is inserted into an image matrix ( B-Mode). Thus, a slice of the human body is represented in the form of a circle segment. Mechanical scanners The transducer librates in front of the patient, without any external movement of the gaging head. To achieve this, two techniques are commonly used: the mechanical and the electronic method. Hence, various rays are sent in different directions. In brief, in order to acquire 2-D images of the inner body, the ultrasound device has to sample not only on a 1-D ray (as in A-mode), but on a 2-D plane in 3-D space. B-mode images are generated by systematically combining a multitude of A-mode (1-D) scans into a single 2-D image, where the intensity of a pixel is defined by the amplitude of the corresponding ultrasonic ray. The repeat rate depends on the penetration depth and therewith on the used frequency.ī-mode is the most common ultrasound mode. The next sonic impulse will be emitted if all echoes of the preliminary sonic impulse are decayed. The signal-to-noise ratio becomes worse with increasing depth. The signal height of an interface reflected signal is independent of the penetration depth from which the echo comes. Signals of high depth (15 cm) are raised up to 120 dB. Later arriving echoes, which are weaker because of the absorption, are more amplified than the signals from the surface. The returning echo is given through a duplexer to a time-dependent amplifier (Time Gain Compensation). From a continuously running high-frequency generator, a wave packet is cut out with a “gate” and is passed to the transducer. The backward scattered ultrasound intensity along a single ray is called A-mode. ![]() However, the major disadvantage is that only very localized information (one single line through the body) is acquired. ![]() Extractable measurements are: frequency, modulated frequency, height of the impulse/amplitude, runtime, wave phase, phase shift, and attenuation. The height of the amplitude of the reflected ultrasound is displayed over the sonic runtime in the sonic ray direction. Diffraction determines the direction in which most sound will be radiated, an important factor for the acoustical engineers who work to make them as quiet as possible.A-mode is the simplest scanning mode. ![]() The white region is a cross-section of the front part of an aircraft engine, the sound wave is produced by the turbofan. The animation below shows another example of diffraction. Thus, this solution for noise reduction is efficient only if the houses are located within the shadow region of the sound barrier. It is characterised by low noise levels due only to the acoustic diffracted wave. A shadow region is observed just behind the barrier (bottom right of the animation). Interference patterns due to the superposition of the incident wave and the diffracted wave are clearly seen just before the barrier (bottom left of the animation). ![]() The animation below illustrates how a travelling wave emitted from the upper left corner by, say, an aeroplane is diffracted by a sound barrier erected to shield homes from the traffic noise. An example of diffraction phenomena is given by the spreading of waves around an obstacle. Diffraction occurs if a wave encounters an object and if the wavelength is of the same size (or greater than) the object size. The spreading of waves when they pass through an opening, or around an obstacle into regions where we would not expect them, is called diffraction.
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