The principle of ultrasound

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Author	I.A.C. van der Bilt
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Exploring the heart through ultrasound

Basic knowledge of ultrasound physics is vital to the correct application of ultrasound for diagnostic and therapeutic interventions

Image acquisition is highly operator dependent

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…..

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.

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.