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The principle of ultrasound: Difference between revisions
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'''Period''' of an ultrasound wave is the time that is required to capture one cycle, i.e., the time from the beginning of one cycle till the beginning of the next cycle. The units of period is time and typical values in echo is 0.1 to 0.5 microsecond. Period of ultrasound is determined by the source and cannot be changed by the sonographer. | '''Period''' of an ultrasound wave is the time that is required to capture one cycle, i.e., the time from the beginning of one cycle till the beginning of the next cycle. The units of period is time and typical values in echo is 0.1 to 0.5 microsecond. Period of ultrasound is determined by the source and cannot be changed by the sonographer. | ||
[[File:PhysicsUltrasound_Fig2.svg|left| Fig. 2]] | [[File:PhysicsUltrasound_Fig2.svg|thumb|left| Fig. 2]] | ||
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Frequency is the inverse of the period and is defined by a number of events that occur per unit time. The units of frequency is 1/sec or Hertz (Hz). Since f = 1/P, it is also determined by the source and cannot be changed. | Frequency is the inverse of the period and is defined by a number of events that occur per unit time. The units of frequency is 1/sec or Hertz (Hz). Since f = 1/P, it is also determined by the source and cannot be changed. | ||
[[File:PhysicsUltrasound_Fig3.svg|left| Fig. 3]] | [[File:PhysicsUltrasound_Fig3.svg|thumb|left| Fig. 3]] | ||
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'''Amplitude''' is an important parameter and is concerned with the strength of the ultrasound beam. It is defined as the difference between the peak value and the average value of the waveform. It is expressed in decibels or dB, which is a logarithmic scale. It can be changed by a sonographer. Amplitude decreases as the ultrasound moves through tissue, this is called attenuation. Amplitude decreases usually by 1 dB per 1 MHz per 1 centimeter traveled. For example, if we have a 5 MHz probe and the target is located at 12 cm (24 cm total distance), then the amplitude attenuation will be 1 dB x 5 MHz x 24 cm = 120 dB which nearly 6000 fold decrease. | '''Amplitude''' is an important parameter and is concerned with the strength of the ultrasound beam. It is defined as the difference between the peak value and the average value of the waveform. It is expressed in decibels or dB, which is a logarithmic scale. It can be changed by a sonographer. Amplitude decreases as the ultrasound moves through tissue, this is called attenuation. Amplitude decreases usually by 1 dB per 1 MHz per 1 centimeter traveled. For example, if we have a 5 MHz probe and the target is located at 12 cm (24 cm total distance), then the amplitude attenuation will be 1 dB x 5 MHz x 24 cm = 120 dB which nearly 6000 fold decrease. | ||
[[File:PhysicsUltrasound_Fig4.svg|left|500px| Fig. 4]] | [[File:PhysicsUltrasound_Fig4.svg|thumb|left|500px| Fig. 4]] | ||
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'''Power''' of ultrasound is defined as the rate of energy transfer and is measured in Watts. It is determined by the sound source and it decreases as the beam propagated through the body. | '''Power''' of ultrasound is defined as the rate of energy transfer and is measured in Watts. It is determined by the sound source and it decreases as the beam propagated through the body. | ||
Intensity of the ultrasound beam is defined as the concentration of energy in the beam. Intensity = Power / beam area = (amplitude)^2 / beam area, thus it is measured in Watts per cm^2. It is the key variable in ultrasound safety. Intensity also decreases as the ultrasound propagates through tissue. | Intensity of the ultrasound beam is defined as the concentration of energy in the beam. Intensity = Power / beam area = (amplitude)^2 / beam area, thus it is measured in Watts per cm^2. It is the key variable in ultrasound safety. Intensity also decreases as the ultrasound propagates through tissue. | ||
[[File:PhysicsUltrasound_Fig5.svg|left|500px| Fig. 5]] | [[File:PhysicsUltrasound_Fig5.svg|thumb|left|500px| Fig. 5]] | ||
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'''Wavelength''' is defined as the length of a single cycle. It is measured in the units of length. It is determined by both the source and the medium. Wavelength cannot be changed by the sonographer. It influences the longitudinal image resolution and thus effect image quality. Typical values of wavelength are 0.1 – 0.8 mm. Wavelength (mm) = Propagation speed in tissue (mm/microsecond) / frequency (MHz). High frequency means short wavelength and vice versa. | '''Wavelength''' is defined as the length of a single cycle. It is measured in the units of length. It is determined by both the source and the medium. Wavelength cannot be changed by the sonographer. It influences the longitudinal image resolution and thus effect image quality. Typical values of wavelength are 0.1 – 0.8 mm. Wavelength (mm) = Propagation speed in tissue (mm/microsecond) / frequency (MHz). High frequency means short wavelength and vice versa. | ||
[[File:PhysicsUltrasound_Fig6.svg|left|400px| Fig. 6]] | [[File:PhysicsUltrasound_Fig6.svg|thumb|left|400px| Fig. 6]] | ||
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