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The principle of ultrasound: Difference between revisions
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Exploring the heart through ultrasound. | |||
A basic knowledge of ultrasound physics is vital to the correct application of ultrasound for diagnostic and therapeutic interventions | |||
Image acquisition is highly operator dependent | |||
A knowledge of the physical attributes of ultrasound waves and image generation is critical to recognition of artefacts and prevention of misdiagnosis | |||
Ultrasound has been used in medicine since the beginning of the 20th century. | |||
Its current importance can be judged by the fact that, of all the various kinds of diagnostic images produced in the world, 1 in 4 is an ultrasound scan. | |||
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). | |||
CHAPTER 1 the general principles of echocardiography | |||
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. | |||
Because of the longitudinal motion of the air particles, there are regions in the air where the air particles are compressed together and other regions where the air particles are spread apart. These regions are known as compressions and rarefactions, respectively. The compressions are regions of high air pressure while the rarefactions are regions of low air pressure. | |||
Since a sound wave consists of a repeating pattern of high-pressure and low-pressure regions moving through a medium, it is sometimes referred to as a pressure wave. | |||
http://www.physicsclassroom.com/class/sound/u11l1c.cfm | |||
Hier moeten we een mooie tekening voor laten maken….. | |||
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 waves are characterized by different generic properties: | |||
▪ Frequency (f = 1/s = s -1 = Hz) | |||
▪ Wavelength (λ = m) | |||
▪ Amplitude (dB) | |||
Frequency (f) = is the number of wavelengths that pass per unit time. It is measured as cycles (or wavelengths) per second and the unit is hertz (Hz). | |||
Wavelength (λ) = the distance between two areas of maximal compression (or rarefaction). The importance of wavelength is that the penetration of the ultrasound wave is proportional to wavelength and image resolution is no more than 1-2 wavelengths. | |||
Propagation velocity = frequency x wavelength | |||
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 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. | |||
In soft tissue propagation velocity is relatively constant at 1540 m/sec and this is the value assumed by ultrasound machines for all human tissue. | |||
Current echocardiography uses intermittent repetitive generation of ultrasound pulses consisting of a few cycles each. | |||
Amplitude and intensity drop as ultrasound travels through tissues. This phenomenon is called attenuation (measured in decibels, dB). Sources of attenuation are: specular reflection, scattering and absorption. Attenuation increases with travelled distance and ultrasound frequency. | |||
The ultrasound waves enter the tissue, are transmitted through the tissues and are reflected back from tissues based on the acoustic impedance of the tissue. Acoustic impedance of a tissue is its density times the velocity at which sound travels through the tissue. The greater the mismatch in acoustic impedance between two adjacent tissues, the greater the amount of ultrasound reflected back to the transducer. Bone/tissue and air/tissue interfaces are highly reflective due to the large mismatch in their acoustic impedances of adjacent tissues. Bone has a very high acoustic impedance and air has a very low acoustic impedance relative to soft tissue. Thus, when the ultrasound beam intersects a bony structure or air-filled interface, the ultrasound beam is reflected back to the transducer, preventing imaging of deeper structures. Therefore, echocardiography must be performed in the intercostal spaces within the cardiac windows (where the heart is against the thorax, without intervening lung) or from subcostal windows. The high mismatch at air/soft tissue interfaces explains the need of using ultrasound gel as a coupling medium during examination. | |||
Encountering an interface, the ultrasound partially returns towards the source and is partially transmitted. At a smooth and large interface the ultrasound obeys rules of specular reflection. The reflected ultrasound returns to
the source in cases of perpendicular incidence, but does not return to
the source in cases of an oblique incidence. Transmission of ultrasound occurs with a change in direction – refraction – in cases of oblique incidence. At a rough interface or when encountering small structures (with dimensions in the range of the wavelength) the ultrasound suffers scattering, returning towards the source and being transmitted in many directions. The proportion of ultrasound returning to the source (backscatter) is independent of insonation angle. Scatter reflections allow generation of an image of examined structures instead of a mirror (specular) image of the transducer. The backscatter is higher with higher ultrasound frequency and depends on scatterer size. A point scatterer sends ultrasound homogenously in all directions. The backscatter from the multitude
of scatterers encountered by the ultrasound wave interfere enhancing (constructive interference) or neutralizing each other (destructive interference). This explains why the image of tissues contains speckles and apparent free spaces instead of having homogeneous appearance. | |||
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. | |||
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. | |||
Ultrasound has been used in medicine for at least 50 years. Its current importance can be judged by the fact that, of all the various kinds of diagnostic images produced in the world, 1 in 4 is an ultrasound scan. The definition of Ultrasound(US)is sound with a frequency > 20kHz. Ultrasound energy is exactly like sound energy, it is a variation in the pressure within a medium. The only difference is that the rate of variation of pressure, the frequency of the wave, is too rapid for humans to hear. Medical ultrasound lies within a frequency range of 30 kHz to 500 MHz. Generally, the lower frequencies (30 kHz to 3 MHz) are for therapeutic purposes, the higher ones (2 to 40 MHz) are for diagnosis (imaging and Doppler), the very highest (50 to 500 MHz) are for microscopic images. For diagnostic purposes two main techniques are employed; the pulse-echo method is used to create images of tissue distribution; the Doppler effect is used to assess tissue movement and blood flow. | Ultrasound has been used in medicine for at least 50 years. Its current importance can be judged by the fact that, of all the various kinds of diagnostic images produced in the world, 1 in 4 is an ultrasound scan. The definition of Ultrasound(US)is sound with a frequency > 20kHz. Ultrasound energy is exactly like sound energy, it is a variation in the pressure within a medium. The only difference is that the rate of variation of pressure, the frequency of the wave, is too rapid for humans to hear. Medical ultrasound lies within a frequency range of 30 kHz to 500 MHz. Generally, the lower frequencies (30 kHz to 3 MHz) are for therapeutic purposes, the higher ones (2 to 40 MHz) are for diagnosis (imaging and Doppler), the very highest (50 to 500 MHz) are for microscopic images. For diagnostic purposes two main techniques are employed; the pulse-echo method is used to create images of tissue distribution; the Doppler effect is used to assess tissue movement and blood flow. | ||