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The principle of ultrasound: Difference between revisions
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Ultrasound waves are sound waves with a higher than audible frequency. The audible frequency range is 20 hertz (Hz) to 20,000Hz (20kHz). Cardiac imaging applications use an ultrasound frequency range of 1–20MHz (MegaHertz) (1.000.000 – 20.000.000 Hz). | Ultrasound waves are sound waves with a higher than audible frequency. The audible frequency range is 20 hertz (Hz) to 20,000Hz (20kHz). Cardiac imaging applications use an ultrasound frequency range of 1–20MHz (MegaHertz) (1.000.000 – 20.000.000 Hz). | ||
== | ==The general principles of echocardiography== | ||
To understand ultrasound it is important to understand sound (waves). | To understand ultrasound it is important to understand sound (waves). | ||
Sound is a sequence of waves of pressure that propagate through compressible media such as air or water. | Sound is a sequence of waves of pressure that propagate through compressible media such as air or water. | ||
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http://www.physicsclassroom.com/class/sound/u11l1c.cfm Hier moeten we een mooie tekening voor laten maken….. | http://www.physicsclassroom.com/class/sound/u11l1c.cfm Hier moeten we een mooie tekening voor laten maken….. | ||
Sound is transmitted through gases, plasma, and liquids as longitudinal waves, also called compression waves. We won’t go into transverse waves in this chapter about cardiac ultrasound. | Sound is transmitted through gases, plasma, and liquids as longitudinal waves, also called compression waves. We won’t go into transverse waves in this chapter about cardiac ultrasound. | ||
Sound waves are characterized by different generic properties: | Sound waves are characterized by different generic properties: | ||
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Propagation velocity = frequency x wavelength | Propagation velocity = frequency x wavelength | ||
v = f x λ (m/s) | *v = f x λ (m/s) | ||
Propagation velocity is dependent of physical properties of a medium (eg. density, temperature or pressure). Frequency is not dependent of the medium, the wavelength is. | Propagation velocity is dependent of physical properties of a medium (eg. density, temperature or pressure). Frequency is not dependent of the medium, the wavelength is. | ||
Propagation dependens on acoustic impedance of the tissue and the angle of incidence (insonation angle) with the interface. | Propagation dependens on acoustic impedance of the tissue and the angle of incidence (insonation angle) with the interface. | ||
The wavelength determines imaging resolution. In echocardiography, adequate resolution is obtained with wavelengths less than 1mm. A shorter wavelength corresponds to a higher frequency, and vice versa. | The wavelength determines imaging resolution. In echocardiography, adequate resolution is obtained with wavelengths less than 1mm. A shorter wavelength corresponds to a higher frequency, and vice versa. | ||
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Hier moeten we een mooie tekening voor laten maken….. | Hier moeten we een mooie tekening voor laten maken….. | ||
Ultrasound is a form of energy, travelling in a beam. The energy transferred in the unit of time defines the power, measured in milliwatts (mW). The power per unit of beam cross-sectional area represents the average intensity (mW/cm2). Power and intensity are proportional with the square of the wave amplitude. | Ultrasound is a form of energy, travelling in a beam. The energy transferred in the unit of time defines the power, measured in milliwatts (mW). The power per unit of beam cross-sectional area represents the average intensity (mW/cm2). Power and intensity are proportional with the square of the wave amplitude. | ||
The intensity increases with power increase or cross-sectional area decrease by focusing the ultrasound beam. The intensity varies across the beam, being highest in the centre and lower towards the edges. | The intensity increases with power increase or cross-sectional area decrease by focusing the ultrasound beam. The intensity varies across the beam, being highest in the centre and lower towards the edges. | ||
An estimate of peak intensity is given by the mechanical index (MI) calculated from the peak negative pressure (MPa) divided by the square root of transmitted frequency (MHz). The mechanical index (an estimate of the maximum amplitude of the pressure pulse in tissue) can be used as an estimate for the degree of bio-effects a given set of ultrasound parameters will induce. A higher mechanical index means a larger bio-effect. Currently the FDA stipulates that diagnostic ultrasound scanners cannot exceed a mechanical index of 1.9. | An estimate of peak intensity is given by the mechanical index (MI) calculated from the peak negative pressure (MPa) divided by the square root of transmitted frequency (MHz). The mechanical index (an estimate of the maximum amplitude of the pressure pulse in tissue) can be used as an estimate for the degree of bio-effects a given set of ultrasound parameters will induce. A higher mechanical index means a larger bio-effect. Currently the FDA stipulates that diagnostic ultrasound scanners cannot exceed a mechanical index of 1.9. | ||