The principle of ultrasound
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| Author | I.A.C. van der Bilt | |
| Moderator | I.A.C. van der Bilt | |
| Supervisor | ||
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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.
The Four Acoustic Variables: Pressure - the amount of force over a given area. Distance - particle displacement with the wave Temperature - Density
Reflection and Propagation:
Effect of propagation through gaseous zones - poor propagation, inadequate imaging. Effect of propagation through dense zones - nearly all of the US is reflected. Structures below dense zones are poorly imaged. Examples of dense materials - bone, calcium, metal.
Material Speed of Propagation
bone 4080 m/s
blood 1570 m/s
tissue 1540 m/s
fat 1450 m/s
air 330 m/s
Definitions: Cycle - the combination of one rarefaction and one compression equals one cycle. Amplitude - the maximum displacement of a particle or pressure wave. Intensity - the amount of force or energy of sound. Decibel (dB) - a numerical expression of the relative loudness of sound. Wavelength - the distance between the onset of peak compression or cycle to the next. Velocity - the velocity is the speed at which sound waves travel through a particular medium. Velocity is equal to the frequency x wavelength. The velocity of US through human soft tissue is 1540 meters per second.
Frequency - the number of cycles per unit of time. Frequency and wavelength are inversely related. The higher the frequency the smaller the wavelength.
Acoustic Impedance - simply put, acoustic impedance is dependent on the density of the material in which sound is propagated through. The greater the impedance the more dense the material.
Reflection - the portion of a sound that is returned from the boundary of a medium. (echo) The angle of incidence influences the reflected and refracted waves.
Refraction - the change of sound direction on passing from one medium to another.
Acoustic Mismatch - the boundary between two different media where reflection and refraction occurs.
Attenuation - the decrease in amplitude and intensity as a sound wave travels through a medium.
Types of Echoes:
Specular - echoes originating from relatively large, regularly shaped objects with smooth surfaces. These echoes are relatively intense and angle dependent. (i.e. IVS, valves)
Scattered - echoes originating from relatively small, weakly reflective, irregularly shaped objects are less angle dependant and less intense. (ie. blood cells)
Scattering: Reflection and Refraction are affected by the material being imaged.
Frequencies:
Frequencies for adult imaging - 2.0mHz to 3.0mHz.
Frequencies for pediatric imaging - 5.0mHz to 7.5mHz to 12mHz.
Effect of higher frequencies on penetration - the higher the frequency the less penetration, the lower the frequency the greater the penetration.
Artifacts:
Acoustic Shadowing - the loss of information below an object because the greater portion of the sound energy was reflected back by the object. This occurs in objects like prosthetic valves.
Enhancement - the increase in relection amplitude from objects that lie behind a weakly attenuating structure. Enhancement may occur in structures below a cyst.
Reverberation - the unsuitable reflections generated when the sound wave strikes a highly reflective object creating artifacts that degrade the image. The peak of the sector scan window is usually filled with reverberations due to the initial transmission of sound energy reflecting off of the chest wall and being reflected off the transducer face in a repetitious fashion. Reverberations may occur in more internal structures like the diaphragm or from dense objects such as a mechanical valve prothesis. Mirroring may occur as sound energy is reflected off dense structures and displayed on the screen as a double image.
Side-Lobe - produced from the side lobes of the ultrasound beam. This artifact appears as false structures in the scan plane.
DOPPLER PRINCIPLES
Christian Johann Doppler described the effect of motion of sound sources and its effect on the frequency of the sound to the observer. In medical applications we find that the frequency of the reflected signal is modified by the velocity and direction of blood flow. If blood cells are moving towards the transducer, they increase the frequency of the returning signal. As cells move away from the transducer, the frequency of the returning signal decreases.
The mathematical formula is:
The frequency difference is equal to the reflected frequency minus the originating frequency. If the resulting frequency is higher then there is a positive Doppler shift and the object is moving toward the transducer and if the resulting frequency is lower, there is a negative Doppler shift and it is moving away from the transducer.The angle theta, cos D component is the angle of incidence of the beam upon the object. For the most accurate determination of flow, the beam should be parallel to the flow of blood where the angle theta is zero. If the angle of incidence is greater, the results are less reliable. It is generally accepted that results from the Doppler shift where the angle theta is greater than 20 degrees is not used for calculation.
Doppler Instrumentation:
Doppler techniques are dependent on the transducers used. The transducer operating in continuous wave mode utilizes one half of the element(s) and are continuously sending sound energy while the other half is continuously receiving the reflected signals.
If the transducer is being used in a pulsed wave mode, the whole transducer is used to send and then receive the returning signals.
Comparing the two modes of Doppler techniques describes the advantages and disadvantages.
Advantages Disadvantages
Continuous Wave
Accurately measures high velocity flows Lacks range resolution
Pulsed wave Ability to measure velocities at a specific location (range resolution) Aliasing of velocities above the Nyquist limit (inability to measure high velocities accurately)
Pulsed wave techniques have proven to be very valuable in blood flow studies. The technique allows the accurate measurement of blood flow at a specific area in the heart and detection of both velocity and direction. Measurement is performed by timing the reception of the returning signals giving a view of flows at specific depths. The region where flow velocities are measured is called the sample volume. Errors in the accuracy of the information arise if the velocities exceed a certain speed. The highest velocity accurately measured is called the Nyquist limit.
Nyquist Limit - defined as ½ the Pulse Repetition Frequency (the number of pulses per second.) If the velocity of flow exceeds the Nyquist limit, the direction and velocity are inaccurately displayed and, in fact, appear to change direction. Color flow Doppler capitalizes on this effect allowing us to detect flow disturbances from laminar to turbulent flow.
High PRF - a Doppler technique that attempts to overcome the effects of the Nyquist limit. This technique may be seen as a compromise of pulsed wave and continuous wave properties and involves the use of multiple sample volumes thereby increasing the accuracy of velocity measurements at the cost of range ambiguity.
Doppler Displays:
The display of Doppler velocity data is the Doppler frequency shifts versus time. Included in the display are the Doppler settings such as frequency, calibration, range, and timing markers.
Doppler Controls:
Controls used during the Doppler examination are dependent on manufacturers specifications and the modes available. Controls for the cursor, sample volume length and depth, angle correction, gain, filters, and spectral averaging are typically included.
Color Flow Imaging:
Sampling methods - CFI is based on pulsed Doppler technology where multiple sample volumes among multiple planes are detected and displayed utilizing color mapping for direction and velocity flow data. Common mapping formats are BART (Blue Away, Red Towards ) or RABT, and enhanced or variance flow maps where saturations and intensities indicate higher velocities and turbulence or acceleration, respectively. Some maps utilize a third color, green, to indicate accelerating velocities and turbulence.
Artifacts - aliasing of the data displayed in pulsed wave technology is utilized as a benefit in determining transitions from laminar to turbulent flow. Other artifacts associated with CFI and spectral Doppler are artifacts due to gain set too high, "ghosting" from improperly set wall filters (low frequency), mirroring, crying/talking artifacts, and signal loss from data sharing.
Limitations of CFI - CFI is a "qualitative" examination and has not yet been "quantified", that is, results cannot be measured to give discrete numbers for diagnosis. Qualitative assessment gives comment on the overall view or quality of the results as in flow conditions and jet direction, velocity, and pattern. Quantitative results are those measured and given discrete numbers used in calculations. There are some current semi-quantitative results given as ratios of jet length by jet width to determine the degree of regurgitation given as mild, moderate or severe.