BASIC PHYSIC OF ULTRASOUND

Basic Physics Of Ultrasound

Definition

Ultrasound is the term used to describe sound of frequencies above 20 000 Hertz (Hz),beyond the range of human hearing. Frequencies of 1–30 megahertz (MHz) are typical for diagnostic ultrasound.Diagnostic ultrasound imaging depends on the computerized analysis ofreflected ultrasound waves, which non-invasively build up fine images of internal body structures. The resolution attainable is higher with shorter wavelengths, with the wavelength being inversely proportional to the frequency. However, the use of high frequencies is limited by their greater attenuation (loss of signal strength) in tissue and thus shorter depth of penetration. For this reason, different ranges of frequency are used for examination of different parts of the body:

 ■ 3–5 MHz for abdominal areas

 ■ 5–10 MHz for small and superficial parts and

 ■ 10–30 MHz for the skin or the eyes.

Generation of ultrasound

Piezoelectric crystals or materials are able to convert mechanical pressure (which causes alterations in their thickness) into electrical voltage on their surface (the piezoelectric effect). Conversely, voltage applied to the opposite sides of a piezoelectric material causes an alteration in its thickness (the indirect or reciprocal piezoelectric effect). If the applied electric voltage is alternating, it induces oscillations which are transmitted as ultrasound waves into the surrounding medium. The piezoelectric crystal, therefore, serves as a transducer, which converts electrical energy into mechanical energy and vice versa. Ultrasound transducers are usually made of thin discs of an artificial ceramic material such as lead zirconate titanate. The thickness (usually 0.1–1 mm) determines the ultrasound frequency. The basic design of a plain transducer is shown in most diagnostic applications, ultrasound is emitted in extremely short pulses as a narrow beam comparable to that of a flashlight. When not emitting a pulse (as much as 99% of the time), the same piezoelectric crystal can act as a receiver.

 Properties of ultrasound

Sound is a vibration transmitted through a solid, liquid or gas as mechanical pressure waves that carry kinetic energy. A medium must therefore be present for the propagation of these waves. The type of waves depends on the medium. Ultrasound propagates in a fluid or gas as longitudinal waves, in which the particles of the medium vibrate to and fro along the direction of propagation, alternately compressing and rarefying the material. In solids such as bone, ultrasound can be transmitted as both longitudinal and transverse waves; in the latter case, the particles move perpendicularly to the direction of propagation. The velocity of sound depends on the density and compressibility of the medium. In pure water, it is 1492 m/s (20 °C), for example. The relationship between

frequency (f ), velocity (c) and wavelength (λ) is given by the relationship: As it does in water, ultrasound propagates in soft tissue as longitudinal waves, with an average velocity of around 1540 m/s (fatty tissue, 1470 m/s; muscle, 1570 m/s). The construction of images with ultrasound is based on the measurement of distances,

which relies on this almost constant propagation velocity. The velocity in bone (ca.3600 m/s) and cartilage is, however, much higher and can create misleading effects in images, referred to as artefacts (see below).

The wavelength of ultrasound influences the resolution of the images that can be obtained; the higher the frequency, the shorter the wavelength and the better the resolution. However, attenuation is also greater at higher frequencies. The kinetic energy of sound waves is transformed into heat (thermal energy) in the medium when sound waves are absorbed. The use of ultrasound for thermotherapy was the first use of ultrasound in medicine.

Energy is lost as the wave overcomes the natural resistance of the particles in the medium to displacement, i.e. the viscosity of the medium. Thus, absorption increases with the viscosity of the medium and contributes to the attenuation of the ultrasound beam. Absorption increases with the frequency of the ultrasound. Bone absorbs ultrasound much more than soft tissue, so that, in general, ultrasound is suitable for examining only the surfaces of bones. Ultrasound energy cannot reach the areas behind bones. Therefore, ultrasound images show a black zone behind bones, called an acoustic shadow, if the frequencies used are not very low  Reflection, scattering, diffraction and refraction (all well-known optical phenomena) are also forms of interaction between ultrasound and the medium. Together with absorption, they cause attenuation of an ultrasound beam on its way through the medium. The total attenuation in a medium is expressed in terms of the distance within the medium at which the intensity of ultrasound is reduced to 50% of its initial level, called the ‘half-value thickness’.

In soft tissue, attenuation by absorption is approximately 0.5 decibels (dB) per centimetre of tissue and per megahertz. Attenuation limits the depth at which examination with ultrasound of a certain frequency is possible; this distance is called the ‘penetration depth’. In this connection, it should be noted that the reflected ultrasound

echoes also have to pass back out through the same tissue to be detected. Energy loss suffered by distant reflected echoes must be compensated for in the processing of the signal by the ultrasound unit using echo gain techniques ((depth gain compensation (DGC) or time gain compensation (TGC)) to construct an image with homogeneous density over the varying depth of penetration Reflection and refraction occur at acoustic boundaries (interfaces), in much the same way as they do in optics. Refraction is the change of direction that a beam

undergoes when it passes from one medium to another. Acoustic interfaces exist between media with different acoustic properties. The acoustic properties of a medium are quantified in terms of its acoustic impedance, which is a measure of the degree to which the medium impedes the motion that constitutes the sound wave. The acoustic impedance (z) depends on the density (d) of the medium and the sound velocity (c) in the medium, as shown in the expression: The difference between the acoustic impedance of different biological tissues and organs is very small. Therefore, only a very small fraction of the ultrasound pulse is reflected, and most of the energy is transmitted.This is a precondition for the construction of ultrasound images by analysing echoes from successive reflectors at different depths. The greater the difference in acoustic impedance between two media, the higher the fraction of the ultrasound energy that is reflected at their interface and the higher the

attenuation of the transmitted part. Reflection at a smooth boundary that has a diameter greater than that of the ultrasound beam is called ‘specular reflection.Air and gas reflect almost the entire energy of an ultrasound pulse arriving through a tissue. Therefore, an acoustic shadow is seen behind gas bubbles. For this reason, ultrasound is not suitable for examining tissues containing air, such as the healthy lungs. For the same reason, a coupling agent is necessary to eliminate air between the transducer and the skin. The boundaries of tissues, including organ surfaces and vessel walls, are not smooth, but are seen as ‘rough’ by the ultrasound beam, i.e. there are irregularities at a scale similar to the wavelength of the ultrasound. These interfaces cause nonspecular reflections,known as back-scattering,over a large angle.Some of these reflections will reach the transducer and contribute to the construction of the image A similar effect is seen with very small reflectors, those whose diameters are similar to that of the wavelength of the ultrasound beam. These reflectors are called scattering’. They reflect (scatter) ultrasound over a wide range of angles,

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