Physical principles

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Diagnostic ultrasonography is a cross-sectional imaging technique based on the physical principles of sound waves. Unlike electromagnetic waves, which can be transmitted within a vacuum, sound waves need a medium (liquid, solid or gas) for propagation. The particles of the medium are oscillated by a sound wave due to alternating pressure deviations from the equilibrium pressure, causing local regions of compression and rarefaction. This chapter explains the production of ultrasound waves and their interaction with tissues to produce the ultrasound image, together with Doppler and harmonic ultrasonography.

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1.1 Sound waves are characterized by wavelength (λ), frequency and amplitude (). Particles of matter at rest. Sound waves cause compression and rarefaction of the particles of matter.
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1.2 Reflection of sound waves is dependent on the difference in impedance of two media. If the incident echo hits the reflecting surface at a 90 degree angle, the reflecting echo goes back to the transducer. If the incident echo does not hit the reflecting surface at a 90 degree angle, the angle of incidence is the same as the angle of reflection (θ= θ). At boundaries between two media with different acoustic velocities, refraction occurs at an angle θ. Diffraction is a change of direction of waves caused by an obstacle or a hole. Scatter causes the beam to diffuse in many directions with only parts of the incident echo returning to the transducer.
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1.4 The transducer sends a pulse and records the time taken for the echo to return. The depth of the reflector recorded on the display is proportional to the distance of the reflector from the transducer. Due to attenuation, returning echoes are weaker with increased depth.
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1.5 Different image display modes (A, B, M) using the heart as an example. LA = left atrium; LV = left ventricle; RV = right ventricle.
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1.6 Reflectors are detected separately if they are separated by a distance of >0.5 spatial pulse lengths. Overlap of the returning echoes occurs if the distance between the two reflectors is <0.5 spatial pulse lengths; the two reflectors cannot be distinguished from one another in the resulting image. Reduction of spatial pulse length results in better axial resolution because reflectors that are closer to each other can be distinguished.
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1.7 Lateral resolution is the ability of the transducer to separate two reflectors perpendicular to the beam direction and is dependent on beam width.
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1.8 Characteristic beam profile.
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1.9 Reverberation artefacts are produced by a pulse bouncing back and forth between two reflecting interfaces. An example of this artefact (arrowed) is gas bubbles in an intestinal loop.
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1.10 Mirror image artefacts are produced by a strong reflective, obliquely oriented surface which reflects the sound beam into an organ instead of returning it to the transducer. An example of this artefact is the appearance of a vessel and liver parenchyma distal to the diaphragm.
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1.11 Complete reflection of the sound beam due to highly reflective surfaces (such as a urolith) within the urinary bladder creates acoustic shadowing, which appears as an anechoic zone deep to the structure preventing that region from being examined.
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1.12 The gallbladder causes a relative lack of sound wave attenuation and distal enhancement. The hypoechoic lines tangential to the gallbladder wall are caused by edge shadowing (white stars).
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1.13 Pseudosludge in the urinary bladder. This artefact is due to the thickness of the ultrasound beam causing the false impression of sludge in the urinary bladder.
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1.14 A non-moving reflector reflects the echo (grey wave) at the same frequency and wavelength as the transmitted echo (black wave). If the reflector moves towards the transducer, the ultrasound wave is reflected with a higher frequency and shorter wavelength (red wave). A lower frequency and longer wavelength (blue wave) results if the reflector moves away from the transducer.
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1.15 B-mode ultrasonograms of the abdominal aorta of a dog and the corresponding pulsed wave Doppler images showing the wave form spectrum and velocity of the aortic blood flow in m/s. As the angle between the transmitted beam (dotted green line and gate) and the moving red blood cells increases, the calculated velocity decreases according to the Doppler formula. Flow direction cannot be detected if the angle is 90 degrees.
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1.17 Colour flow Doppler ultrasonogram of an arteriovenous fistula within the liver of a dog. The mean Doppler shift is encoded to a colour and displayed on top of the B-mode image. The colour map is visible in the upper left corner, and shows the range of velocity which is encoded (–50 to +50 cm/s): flow toward the transducer is displayed in red and flow away from the transducer is displayed in blue. The course of the fistula is like a corkscrew and therefore both flow directions are present within one vessel.
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1.18 PW Doppler spectral tracing of the abdominal aorta of a dog showing aliasing. Aliasing occurs when the blood flow velocity exceeds the rate at which the PW system can record it properly. The aliased portion is cut off the top of the velocity spectrum and wraps around to point up from below the baseline.
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1.19 Colour flow image of the urinary bladder of a dog with a urolith showing twinkling. The rough reflecting irregular surface of the urolith imitates motion and causes a quickly fluctuating mixture of Doppler signals.

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