Ultrasound Flashcards
How is a sound wave different to an EM wave?
Unlike electromagnetic waves, sound waves cannot be transmitted through a vacuum; they require a medium in the form of a gas (such as air), a liquid (such as water or blood) or a solid (such as soft tissue, bone or metal).
An electromagnetic wave is transverse but a sound wave is longitudinal i.e. the pressure variation occurs in the direction of travel. Like all other sinusoidal waves, a sound wave is characterised by frequency, wavelength and speed (or velocity).
What is the pulse-echo principle?
The pulse-echo principle states that the distance of a reflecting object can be established by timing the go-and-return of a short pulse if the speed of the pulse is known. The returning pulse is known as the echo.
What is the rough speed of sound in tissue?
around 1540 m/s
What is ultrasound?
The answer is that it is simply sound at a frequency (pitch) which is too high for us to hear. In all other respects it is the same as normal sound.
Why are echoes smaller for deeper tissues?
some of the signal is lost (attenuated) on the way and hence there is a general tendency for echoes to get smaller as the depth increases.
What is TGC?
control which allows the operator to adjust the amount of gain (amplification) which is applied to echoes from different depths. This is called time gain compensation, and is an important control for the operator to identify and master. It normally needs to be adjusted during a scan and certainly between patients.
What is A-mode scanning?
This way of presenting information about the targets along a single direction is called the A-scan. it does not resemble a 2D cross-section image but may be regarded as a 1D image line
What is b-mode scanning?
The assembly of many A-scan lines forming a composite image in this way is called a B-scan. In a modern B-scan, the dots do not have the same brightness. Dot brightness depends on echo amplitude and is displayed on a greyscale with white representing very large amplitude echoes and black being used when no echoes are detected (Fig 2). Furthermore, each B-scan is acquired so rapidly that the image is obtained in ‘real time’.
Concerning the use of ultrasound in a medical setting:
A. Time gain compensation is used to correct for variations in the speed of sound between tissues
B. Medical ultrasound imaging typically uses frequencies in the range 3-150 kHz
C. Each single transmitted pulse may result in the creation of a large number of echoes
D. Soft tissues typically have sound speeds of the order of 1500 m/s
E. The angle at which the beam meets an organ boundary may have a large impact on the echo generated
F
F
T
T
T
Time gain compensation is not used to correct for variations in the speed of sound between tissues, but for the increase in echo attenuation with tissue depth.
Medical ultrasound imaging typically uses frequencies from around 1 MHz to around 10 MHz.
Which of the answers below is/are a correct description of TGC?
A. It is fitted to virtually all ultrasound scanners
B. It alters the way ultrasound is generated
C. It needs adjustment between patients and between views
D. It can create artefacts if incorrectly applied
E. It influences only the B-scan
Answers A, C and D are correct.
Time gain compensation does not alter the way ultrasound is generated.
Time gain compensation influences the A-scan as well as the B-scan.
Although some scanners are now fitted with automatic TGC, it is always present in some guise. Its effect is confined to processing signals from received echoes. Since the TGC is an attempt to correct for tissue attenuation, it follows that it needs to be altered between patients, but also within a scan on an individual patient when moving from one organ or section to another with different acoustic properties.
What is the piezoelectric effect?
If an electric voltage is applied to specific materials, they deform, and if a mechanical pressure is applied to them, they develop a voltage on their surface.
What materials are used for ultrasound transducers?
Some naturally-occurring materials (such as quartz crystals) have piezoelectric properties and these were originally used as ultrasound transducers. However, modern imaging transducers are made of synthetic crystalline ceramic materials such as lead zirconate titanate (PZT).
What is the curie temperature?
Immediately after manufacture, this material is not piezoelectric. However, heating to raise its temperature beyond a certain value (the Curie temperature) while applying an external voltage causes electric dipoles within the crystal to align and give the material piezoelectric properties. If the voltage is maintained as the material cools below the Curie temperature the dipole alignment and hence the piezoelectricity is preserved. The material is then cut into a suitable size and shape for use as an ultrasound transducer. Note that further heating above the Curie temperature destroys the piezoelectric properties.
What does the resonant frequency depend on?
the thickness of the transducer and so the same voltage spike will create different damped responses in different transducers.
At the resonant frequency, the wavelength of an ultrasound wave in the transducer is twice the transducer thickness. Therefore, for a given transducer material, resonant frequency and transducer thickness are inversely proportional to each other; a thick crystal gives a low resonant frequency and vice versa
What is the spatial resolution of ultrasound?
The spatial resolution of an imaging system may be thought of as the extent to which the system blurs the image of an object. Alternatively, it can be regarded as the ability of the system to distinguish two separate small objects lying close together. As the distance between the objects is reduced, there will come a point at which the system merges the two and they blur into a single image
What is axial resolution?
Axial resolution refers to a situation in which two objects lie along a single scan line i.e. along the axis of the ultrasound beam; it is sometimes called the longitudinal resolution.
when the object separation is half the length of the ultrasound pulse i.e. half the spatial pulse length the objects will not be resolvable. This distance is a measure of the axial resolution.
How does axial resolution change with frequency?
speed = frequency x wavelength; if ultrasound frequency increases, wavelength must decrease to compensate since speed is more or less constant (assumed to be 1540 m/s in soft tissue). Therefore, higher frequencies are associated with shorter wavelengths and so 2.5 cycles at a higher frequency will occupy a shorter distance (spatial pulse length).
What is lateral resolution?
resolving objects in the lateral (aka azimuthal plane)
The task of resolving these objects is a different one. Note firstly that it requires more than one beam. In fact we need a minimum of three:
The first beam must hit one object but not the other
The second beam must go through the gap between them
The third must hit the second object only
The beam width is a measure of the lateral or azimuthal resolution.
What is slice thickness resolution?
out-of-plane resolution or elevational resolution. Slice thickness is a measure of elevational resolution and it depends on the dimension of the transducer in this direction
What crtieria does the matching layer of the probe have to meet?
In front of the transducer is a matching layer. It serves as an interface between the PZT transducer element and the patient’s skin. Specifically, it reduces the effect of differences in a property called acoustic impedance, whose value is much greater for PZT than for soft tissue. For it to be effective, the matching layer has a thickness equal to a quarter of the ultrasound wavelength in the material of the layer and acoustic impedance equal to the geometric mean of the values for PZT and tissue. In the absence of a matching layer, much of the ultrasound would be reflected back towards the transducer at its interface with the skin.
What lies behind the PZT crystal in the transducer?
Behind the transducer there is a backing block and an acoustic absorber. The backing block is made of a material that is highly attenuating and has acoustic impedance similar to that of PZT, such as epoxy resin loaded with tungsten. This means that ultrasound energy emitted by the back face of the transducer is transmitted into the block and attenuated by it. This dampens the vibration of the transducer producing pulses of short spatial length and so improving axial resolution. The acoustic absorber attenuates stray ultrasound energy.
What is SPL determined by?
In imaging with pulsed ultrasound, a short spatial pulse length (SPL) improves axial resolution. The SPL is determined by the degree of transducer damping.
What is the Q factor of an ultrasound pulse?
The range of frequencies in the spectrum is known as the bandwidth of the pulse and the Q factor is defined as fo/bandwidth.
What properties does a lightly damped US beam have?
A lightly damped transducer is associated with long SPL, narrow bandwidth and high Q
What properties does a heavily damped US beam have?
a heavily damped transducer is associated with short SPL, broad bandwidth and low Q
What sort of imaging is a high Q pulse good for?
higher Q is more appropriate for Doppler flow measurement.
What sort of imaging is a low Q pulse good for?
A low Q transducer is best suited for B-mode imaging
What happens to beams from narrow transducers?
beams from smaller sources begin to diverge at shorter distances from the source surface.
What is the fresnel zone?
The region of the beam close to the transducer where it maintains its original width (the width of the transducer element) is called the near field or Fresnel zone.
What is the Fraunhofer zone?
Further from the transducer is a region in which the beam diverges; this is known as the far field or Fraunhofer zone.
What does the length of the near field depend on?
diameter of the source, wavelength
What is Huygens principle?
the surface of a transducer can be regarded as an infinite number of very small sources each of which emits spherical sound waves (wavelets).
What is the principle of superposition?
when two or more waves coincide, the net amplitude at a point is the sum of the individual wave amplitudes at that point
When does constructive interference happen?
Constructive interference occurs when the waves are in phase (0° phase difference) and destructive interference when they are out of phase (180° phase difference).
How does a change in source diameter effect near field?
a modest decrease in transducer diameter (from 2 cm ro about 1.5 cm) has reduced the near field by a factor of almost two (from 19 cm to 10 cm). The decrease in diameter also means better lateral resolution in the near field.
How does changing requency effect legth of near field?
For a fixed transducer radius, we can reduce the length of the near field by choosing a lower frequency transducer
For a fixed frequency, how can we reduce the length of the near field?
For a fixed frequency, we can reduce the length of the near field by choosing a transducer with reduced diameter - this also reduces overall beam width and so improves lateral resolution
For a fixed near field length, how can we reduce overall beam width and improve lateral resolution?
For a fixed near field length, we can reduce overall beam width and improve lateral resolution by choosing a smaller diameter transducer but the transducer also has to be of higher frequency - thus good lateral resolution is usually associated with higher ultrasound frequency
Where is lateral resolution of a beam best?
ultrasound beam has a degree of natural focusing. It is at its narrowest towards the end of the near field and so this is the point at which lateral resolution is at its best.
What is beamforming?
using electronic lenses. the sophistication of the various focusing techniques is now such that the term ‘beamforming’ is often used instead of beam focusing
At what frame rate can the eye not really distinguish?
Typically, if images are presented at a rate of more than about 20 frames per second, the eye and brain together interpret this as a continuously moving image.
What can limit the frame rate possible?
hen a pulse is sent out from one element, it is necessary to wait for all the echoes to come back from the region in front of that element before sending out another pulse from a nearby element. Failure to do this runs the risk of ambiguity.
If a pulse is sent out from a nearby element before the normal waiting time has elapsed, it is possible for returning echoes to be picked up by the wrong element and therefore misinterpreted. To avoid this, it is normal to wait between pulse firings. This has the effect of slowing down the imaging process and limiting the frame rate.
What is a ‘synthetic aperture’?
firing of elements in groups to avoid issues of very short near field and wide divergence and hence very poor lateral resolution due to <1 mm wide elements.
Why are electronic dealys used in the transducer?
simultaneous firing (excitation) of transducer elements generates an ultrasound beam whose axis corresponds to the position of the central element. This beam has a useful near field length but its overall width limits lateral resolution.
electronic delaying allows ‘electronic focusing of the beam. This symmetrical arrangement of delays produces wavefronts that are concave in the direction of travel and converging on the focal point.
It is at the focus that the beam is at its narrowest and lateral resolution at its best
How are electronic delats used in signal reception?
echoes that start from T in phase are out of phase when they reach the individual transducer elements and the signals they produce have small amplitude when summed by the scanner electronics.
However, the signals on the return journey can be brought into phase by applying exactly the same electronic delays as were applied to the excitation pulses on the outward journey (because the distances to and from T to individual transducer elements are the same in both cases). Addition of the individual delayed signals now gives a strong total signal
What is dynamic focusing?
the receiving electronic focus to change to a greater depth and repeat this process to steadily move out the focal point so that it is always in the right place for the echoes arriving at the instant. In fact, electronically it is possible to make a continuous smooth sweep of the delays so that the focal length of the receiving electronic lens is always correct
What is multiple zone focusing?
The transmitted pulse cannot be heavily focused if it is required to generate echoes over a wide range of tissue depths as this would cause image degradation through poor lateral resolution.
We have the option of selecting a preferred depth or range of tissue depths and accepting only echoes that arrive from those depths. we can cover the entire tissue depth of interest by transmitting not one but several ultrasound pulses per image line. For successive pulses we can focus at increasing depths in transmission (Fig 1). Focusing is also applied in reception but, by rejecting echoes from other depths (i.e. those outside the selected focal zone), we can gradually build up a composite image over the whole depth range.
What are the limitations of multiple zone focusing?
It produces an extended focal zone (Fig 2) and improves image quality but, unlike dynamic (swept) focusing in reception, it carries a performance penalty because it increases the time needed to acquire one image frame (and so reduces the frame rate). Thus the operator has a choice between good lateral spatial resolution and good temporal resolution.
What is dynamic aperture control?
To maintain the same beam width at the focus for increasing focal length, the synthetic aperture must be increased. Thus smaller groups of transducer elements are used for short focal lengths whereas larger groups are used at greater depths.
What are the typical number of elements in a linear array?
Typically there may be 256-512 elements that are electrically and acoustically isolated from each other. The elements are about 0.5 mm in width and about 5 mm in height.
How is a phased array different?
The construction of a phased array is similar to that of a linear or curvilinear array except that there are fewer elements and all of them are excited nearly (but not exactly) simultaneously to produce a single ultrasound beam. This beam is steered in such a way that it is swept through an arc to produce a sector-shaped FOV with an apex at the transducer
How is beam steering achieved in pahased arrays?
Beam steering is achieved with electronic delays that steadily increase (or decrease) from one end of the array to the other. The greater the difference in the delays, the greater the deviation of the beam from 90° to the transducer face. The beam is swept by rapidly changing the value of the delay difference. By making the differences unequal, the beam may be focused within the scan plane.
