Weeks 1-6 (written notes)

Question 1

(A) What is the main difference between electromagnetic waves and mechanical waves?

(B) What are two kinds of mechanical waves?

Answer 1

Electromagnetic waves (e.g. light/Xray) do not require a medium for propagation. Mechanical waves do require a deformable elastic medium for propagation.

1 - Transverse (such as waves in ocean)
2 - Longitudinal (e.g. echo - particle motion in same plane as wave direction)

Question 2

Explain the propagation of sound through a medium in terms of compression and refraction

Answer 2

The compression wave causes a force that pushed a molecule toward the next one, thus compressing the “spring”. This force then pushed onto the next molecule. As the wave passes, an area of refraction (reduced pressure) occurs immediately behind the wave.

Question 3

Explain why speed of sound varies through different materials.

Answer 3

One reason is due to the different weight of molecules making up the material. Heavier molecules transmit sound with lower velocity, due to the inertia of heavier molecules. I.e. the molecular “spring” compresses more before the following molecule moves away, and this takes more time. Vice versa for lighter molecules.

The second reason is due to differences in strength of intermolecular forces. In more compressible materials (e.g. air) the sound transmits at lower velocity. The preceding molecules must move further before enough force is applied to the next molecule (due to weaker intermolecular forces). Conversely, with stiffer (less compressive) materials such as bone, the sound transmits at a high velocity.

Question 4

What is the difference between power and intensity in ultrasound?

Answer 4

Power is the rate of doing work. Electrical power from the machine is converted into sound (in mW for echo). Intensity is power per area (mW/cm^2). It is the intensity of the beam that affects the strength of an echo, and therefore the brightness on the monitor. The intensity of the beam is not constant (e.g. higher intensity at focal point - where the beam’s power is compressed into a smaller area).

Question 5

How can a sonographer increase the intensity of the beam?

Answer 5

1. Increase the power output - this is a control on the machine.
2. Decrease the area of the beam by moving the focus to the area of interest

(Remember intensity = power/area)

Question 6

Explain the relationship between echo intensity and pixel brightness.

Answer 6

NON-LINEAR

–> A doubling of intensity is required each time to produce equal changes in display brightness. It therefore requires a greater increase in intensity to cause a single shade in colour as brightness increases. This is why we use logarithms (to create a linear representation…. the decibel)

Question 7

What is the decibel in ultrasound applications?

Answer 7

Used to compare differences in intensity/power/amplitude of two beams (or two different parts of the beam. Decibels are relative units, and two intensities are required for calculation.

Question 8

What are two kinds of sound-tissue interactions that are important to understand for image formation?

Answer 8

1 - Refraction
2 - Attenuation

Question 9

What are the types of attenuation in sound-tissue interaction?

Answer 9

1. Absorption
2. Divergence
3. Scattering
4. Reflection

Question 10

Briefly describe refraction in ultrasound.

Answer 10

Refraction is the change in direction of an ultrasound beam when it crosses a boundary at an angle. Refraction obeys “Snells Law”.

Question 11

Explain Snell’s Law, provide examples.

Answer 11

Snells law is used to understand beam refraction. The angle of refraction is measured from the normal of the interface (perpendicular to interface). When the beam is travelling from a higher velocity medium into a lower velocity, the beam bends back towards the perpendicular. When travelling from lower velocity to higher velocity medium, it will bend the other way, and approach 90 degrees. When the angle reaches 90 degrees, no sound is transmitted. This is the critical angle.

Question 12

Describe the term critical angle?

Answer 12

Refraction of an ultrasound beam, whereby the beam is travelling from a lower velocity medium into a higher velocity. The critical angle is where the in beam refraction, the beam changes direction to reach 90 degrees from perpendicular to the interface, and no sound is transmitted through the interface.

Question 13

What are the factors that beam attenuation depend on?

Answer 13

Material involved (a coefficient), the distance travelled, and the frequency of the beam.

Question 14

1 Absorption is a form of ____?_____ in ultrasound. It is a result of ________. It directly removed ultrasound energy from the beam, which is converted to ____.

Answer 14

Attenuation

Result of internal frictional forces.

Directly removes ultrasound energy from the beam, which is converted to heat.

Question 15

What does beam absorption depend on?

Answer 15

1. The material itself
   (A) Viscosity (more viscous = more engery expended to move molecules = more absoprtion)
   (B) Molecule relaxation time (if slow to return to realxed position, molecules are still returning when next wave hits. More energy is required to stop the movement and reverse it, than if the molecule is at rest
2. Frequency of the beam
   (A) increased freq = faster molecules are moved = more energy expended)
   (B) increased freq = shortner wavelength = less time to return to position (relaxation time)
3. Depth of the tissue (further travel = further absoprtion

Question 16

What is Divergence?

What is another word for Divergence?

Answer 16

The spreading of an ultrasound beam as it moves further from the source

Diffraction

Question 17

What happens with increased divergence?

Answer 17

Increased diffraction (spread) = increased attenuation.

Remember, intensity = power per area. As area increased, intensity decreases

Question 18

How does beam frequency impact beam absorption (attenuation)

Answer 18

Two ways
1 - A higher frequency will mean the molecules are moved faster, and more energy is expended as heat. Therefore more absoptionn attenuation.
2 - With higher frequency/shorter wavelength, the molecules have less tie to return to their rest position. This can mean molecules are still returning to rest the next wave hits, and it takes more energy to stop the movement and reverse it.

Question 19

Give 5 examples of when beam divergence occurs?

Answer 19

1. In the far field of a non-focussed transducer
2. Beyonf the focal zone of a focussed tranducer
3. After refraction from a convex interface
4. After refraction at a curved interface
5. After passing through a small aperture

Question 19

Can divergence occur with reflected wavefronts?

Answer 19

Yes, divergence attenuation can occur on a pulsed wave and a returning echo wave

Question 20

What is scattering in ultrasound?

Answer 20

A form of attenuation

The dispersion of the u/s beam in many directions. It occurs when the u/w wave strikes a very small object.

Question 21

Discuss how scattering is affected by wavelength? What is this called? Provide an example.

Answer 21

When the interface is very small (i.e. much smaller than the wavelength), scattering occurs equally in all directions.

This is called Rayleigh Scattering

E.g. Occurs from blood cells (v small diameter relative to wavelength)

Question 22

Rayleigh Scattering is ________ dependent.
Explain the relationship.

Answer 22

Frequency

The intensity of the reflected echo is approximately proportional to the fourth power of the frequency
I ~= F^4

Question 23

How does scattering affect image formation?

Answer 23

A) With scattering only a very small portion of the beam returns to the transducer and is used as part of the image

B) The many scattered waves interact with each other (through interference) to form backscatter patterns (This contributes to the organ texture we see on the image)

Question 24

Reflection is a form of ______ and is the major contributor to _____ What are the two types of reflection?

Answer 24

Attenuation Image formation Non-specular (diffuse) and specular

Question 25

What is non-specular reflection?

Answer 25

Occurs when a sound wave strikes a rough or irregular surface. The echo returned back to the transducer is SMALL, but is NOT angle dependent of the incident beam (i.e. the sound beam can strike the surface at a wide range of angles and some echoes will return to the transducer).

Question 26

What is specular reflection?

Answer 26

Occurs when a sound wave strikes a large smooth surface. The reflection from a specular reflector is very LARGE, however det4ection is highly dependent on the angle of incidence. If that beam does not strike at or close to 90deg, the reflected echo will not travel back to the transducer (therefore not be detected)

Question 27

The percentage of an US beam reflected back to the transducer depends on .....

Answer 27

1. The angle of incidence 2. The reflecting surface texture 3. The acoustic property differences of the two tissues (we can use acoustic impedance of different materials to calculate)

Question 28

Explain acoustic impedance and how is it calculated in ultrasound? What are its units of measurement?

Answer 28

Acoustic impendence is an acoustic property of a material, used to understand the percentage of ultrasound that is reflected and transmitted at an interface. It is the product of the materials density, and the velocity of ultrasound in the material (Z = P V) where p = density of material, V = velocity of sound in material Rayls (aka kg.m^-2.s^-1x10^-6)

Question 29

What does a reflection coefficient represent?

Answer 29

The fraction of the ultrasound beam that is reflected at an interface.

Question 30

What is the average U/S beam attenuation? How to we account for attenuation in image formation?

Answer 30

On average, an U/S beam intensity is attenuated by 1dB/cm/MHz in soft tissue, therefore echoes returning from deeper structures become progressively weaker and weaker. These echoes would be represented on the B-Mode image monitor much darker (less brightness). To overcome this, we used time gain compensation to amplify the deeper echoes more than the closer echoes, which can produce equal brightness from similar interfaces regardless of there depth.

Question 31

What is the purpose of time gain compensation?

Answer 31

To produce equal brightness of echoes from similar interfaces, regardless of their depth in the patient (i.e. amplify deeper echoes)

Question 32

There are several aspects of TGC. What are these?

Answer 32

- Near gain - Slope - Far gain - The delay

Question 33

What is near gain vs far gain

Answer 33

Near gain represents the amount of gain applied to the closest echoes. Since these echoes are relatively strong, minimal gain is required. Far gain is the amount of gain applied to distant echoes, where normally higher gain is required to produce equal amplitude/brightness of echoes.

Question 34

What is the slope adjustment in TGC?

Answer 34

Slope adjusts the rate at which the amplification is increased for deeper echoes. Tissues which attenuate the signals more will require an increase in the TGC slope. For example, a fatty liver.

Question 35

What is the delay adjustment in TGC?

Answer 35

The delay control regulates the time (depth) at which the TGC begins to be applied. A delay would be useful when the superficial echoes are of no interest (such as strong echoes from the skin surface) and in fact, amplification would be detrimental to the image.

Question 35

Discuss the two main systems of TGC control in US

Answer 35

(A) 3 or 4 function systems (older U/S systems). These have a single control for each of a number of the following adjustments (near gain, slope, fair gain, delay) (b) Segmental system (modern system). There is separate controls for each of a number of individual depths in the patient. Most machines have presets, and the controls for TGC are designed just to adjust the settings for individual patients.

Question 36

List the main features of a simple transducer (which all transducers have)

Answer 36

- PZT crystal - Case - Electrodes - Dampening material - Matching layer

Question 37

What is the piezoelectric crystal and how does it work?

Answer 37

The PZT is the main functional components of the transducer. The crystal expands and contacts to send out ultrasound waves. This expansion/contraction occurs by the application of an AC voltage. The crystal expands/contracts and then "rings" (or resonates) like a bell for a short time. The frequency at which it resonates is called its resonant frequency. The two main functions 1. Convert electrical energy into mechanical energy (transmit echoes) 2. Concert mechanical energy into electrical energy (receive echoes)

Question 38

What is the critical dimension of a PZT crystal?

Answer 38

This is its thickness. The thickness of the crystal is 1/2 the wavelength of its resonant frequency.

Question 39

Do thicker crystals resonate at higher or lower frequencies? Why?

Answer 39

Lower. Because a crystals thickness is half the wavelength of its resonant frequency --> increased thickness = increased/longer wavelength = lower frequency.

Question 40

What is the role of the electrodes in a basic transducer?

Answer 40

The electrodes' role is to energise the PZT crystal. This may be a thing plating for gold or silver, with an electrical connector to the crystal.

Question 41

What is dampening material used for? Where is it found?

Answer 41

Dampening material is found in all basic U/S transducers. It is used to stop the resonance of the PZT crystal in order to produce short U/S pulses. If the crystal was able to ring down unhindered, a long ultrasound pulse would result, which is not ideal as short pulses are required for high resolution. The dampening material also stops the sound waves from passing back into the transducer and bouncing back into the crystal and causing artifact/noise.

Question 42

What is the purpose of a matching layer built inside the U/S transducer?

Answer 42

The acoustic impedance of the PZT crystal is very different to that of the skin. Therefore, a large % of sound intensity would be reflected at this interface and not transmitted. A matching layer is therefore placed immediately in front of the PZT crystal to match the acoustic impedance of the crystal and soft tissue. Modern impedance matched transducers have multiple matching layers to maximise transmission.

Question 42

How is a matching layer in a basic transducer calculated?

Answer 42

It is designed to be the geometric mean of the materials it is matching to maximise U/S transmission.

Question 43

What is a multi-element transducer? What are these transducers also called? Why are they used?

Answer 43

These are now most modern transducers. They have many small crystal elements for the formation of each ultrasound pulse. Each element produces a small wavefront, and they each combine to form a single wavefront. The individual elements must be acoustically and electronically isolated from each other. Known as ARRAY TRANSDUCER (as they form an array of elements) This technique allows for greater flexibility in beam formation.

Question 44

What are the types of array transducers?

Answer 44

Linear Convex Phased Annular

Question 45

Explain the types of wave interference? How is this considered in transducer manufacturing?

Answer 45

1. Constructive interference - waves are in-phase and add together (higher amplitude). 2. Destructive interference - waves are out-of-phase and negate each other (amplitude lower). 3. Complex interference (mix of both above) Transducers can be designed to optimise constructive interference to get a narrow and thin beam (increasing amplitude in the middle), and create timings to facilitate destructive interference on the outside of the beam for to optimise image.

Question 46

The rate at which pulses are emitted by the transducer is known as ___?

Answer 46

Pulse repetition frequency (PRP)

Question 47

What is the pulse repetition period? What is the main reason this is important for imaging?

Answer 47

The time from the start of one pulse, to the start of the next pulse. The PRP must be long enough to allow all required echoes to be received by the transducer, so ultimately, this is the DEPTH CONTROL on the machine (listening time)

Question 48

The length of time we wait for an echo to return can tell us ____

Answer 48

How deep the echo came from

Question 49

What is the difference between pulse duration and duty factor

Answer 49

Pulse duration is the time taken for one pulse in seconds. Duty factor is the FRACTION of time that the unit is transmitting (pulse duration divide PRP)

Question 50

What is spatial pulse length (SPL) and how is it determined?

Answer 50

The actual length of on pulse measured in metres. SPL is determined by the length of one cycle, so, the wavelength x number of cycles in one pulse.

Question 51

A pulsed ultrasound beam consists of sound waves of multiple frequencies. The range of frequencies is called _______. Where is this normally measured?

Answer 51

The bandwidth Usually measured at the half intensity (or amplitude) level.

Question 52

A shorter U/S pulse has a ____smaller or larger?_____ range of frequencies? What does this mean for the bandwidth?

Answer 52

Larger range Increased bandwidth

Question 53

What is the centre frequency of the beam? What happens to the centre frequency as the pulse passes through tissue?

Answer 53

The maximum intensity = resonant frequency of the crystal The centre frequencies change because higher frequency components are attenuated to a greater degree than the lower frequency components.

Question 54

The beam emitted from a typical diagnostic U/S transducer is not uniform. It will vary in both ____ and time ___.

Answer 54

Space and time (I.e. spatial variation and temporal variation)

Question 55

What is spatial variation?

Answer 55

This is the variation of intensity of an U/S beam over a distance. It varies with (a) distance from the transducer and (b) cross-sectional area of the beam

Question 56

What is the difference between spatial peak and spatial average

Answer 56

Spatial peak occurs along the axis of the beam at the focal point. The spatial average is the average intensity across the beam, at a specified distance from the transducer (usually the focal point)

Question 57

The spatial ___ can be up to _x the spatial ____

Answer 57

The spatial peak can be up to 25x the spatial average

Question 58

What is temporal variation? Briefly explain temporal variation with pulsed echo.

Answer 58

The variation of intensity of an ultrasound beam with time. Pulse echo u/s uses short pulses of sound, separated with long periods of rest. During the time between pulses, intensity falls to zero. Temporal variation even occurs during a pulse (cycling change in intensity within each pulse)

Question 59

List some of the times that beam intensity can be measured (i.e with respect to temporal variation)

Answer 59

- Temporal peal: the maximum amplitude/intensity of the pulse - Pulse average: the average intensity over the duration of a single pulse - Temporal average: The average intensity taken over one whole on/off beam cycle (including rest time).. much lower

Question 60

What is the temporal average? How is it calculated?

Answer 60

The average intensity taken over one whole on/off beam cycle (including rest time). It can be up to 1000x less than the temporal peak due to the large rest time between pulses. TA intensity = PA intensity x Duty Factor (duty factor = pulse duration/PRP)

Question 61

Describe the near field in beam profiling?

Answer 61

Characterised by a non-diverging beam (sometimes called frenzel zone). This is the LENGTH in a non-focussed transducer.

Question 62

What are two factors that affect the near field and far field calculations?

Answer 62

Size of transducer (diameter) and the operating frequency of the transducer (specifically wavelength)

Question 63

Describe the far field in beam profiling?

Answer 63

Characterised by a diverging beam (sometimes called the fraunhofer zone). This is the ANGLE of divergence in a non-focussed transducer.

Question 64

What are side lobes?

Answer 64

Weak "off shoot" beams of sound of low intensity. They occur in all planes due to the complex interference of the wavelets emitted, and give off artifacts that degrade the image

Question 65

What is the 3rd dimension of an U/S beam that is not apparent in the 2D image created?

Answer 65

Slice thickness

Question 66

Considering beam width, what are the two common definitions for intensity?

Answer 66

The 3 DB edge - The beam width edge is taken as the point in intensity that the beam has fallen to 1/2 its max value The 10dB edge (more common) - The beam width edge is taken as the point in intensity that the beam has fallen to 1 10th its max value

Question 67

What are grating lobes?

Answer 67

A special type of side lobe occurring in array transducers. They occur only in the scan plane and tend to be of higher intensity then conventional side lobes - they can often cause artifact.

Question 68

What can transducer manufactures do to reduce grating lobes?

Answer 68

(A) Apodisation - Controls the intensity across the beam, by decreasing the voltage of the pulse to outer elements of a firing group. Therefore wavelets from outer elements have less intensity (and also reduced sensitivity on receive of echo) (B) Decrease the element spacing - Reduced spacing can allow the grating lobe angle to reach 90deg, where the lobe artifact would tranvel "backwards". There is a limit to how much this can be done, so elements can also be SUBDICED.

Question 69

What is element subdicing?

Answer 69

Where normal crystal elements are "subdiced" into multiple smaller elements, each transmitting its own wavelet, acting like an individual crystal. A technique used to reduce grating lobe artifact

Question 70

What are the four ways in which beam focussing is accomplished?

Answer 70

1. Curved crystal 2. Lens 3. Focussed mirror 4. Electronic (Arrays) - very common

Question 71

With beam focussing, what will decreasing the radius of curvature do?

Answer 71

- Decrease the focal depth - Decrease the beam width at the focal depth

Question 72

We focus the beam to improve what kind of resolution?

Answer 72

Lateral resolution

Question 73

How is the focal zone defined?

Answer 73

The depth range of the beam that is less than twice its minimum width. It is the portion of the beam with the highest intensity (decreased area)

Question 74

What is another name for electronic focussing

Answer 74

Array focussing

Question 75

How does electronic focussing work?

Answer 75

By slightly delaying the firing time of the inner elements of an array, the combined individual wavelets form a beamfront (Hyugens principle) which is focussed. A delay circuit is used to achieve this.

Question 76

What happens when the delays are increased in electronic focussing?

Answer 76

The radius of curvature decreased, therefore the focal point moves closer to the transducer (shorter focal zone). At the same time, increased focussing beams the focal zone is narrower.

Question 77

What is the benefit of modern transducers having multiple focal zones? What is the compromise?

Answer 77

There can be a narrow beam path over the entire depth. However, this will require a reduced frame rate because of the extra time taken to produce each line of echoes.

Question 78

What is Dynamic Aperture?

Answer 78

A form of beam focussing to further reduce beam width close to the transducer, by having fewer crystal elements used for the echo pulses. It is effectively the same as reducing crystal diameter.

Question 79

What is dynamic recieve focussing?

Answer 79

Similar to array focussing, delays can be used to "hold up" the inner elements of a returning echo wavefront, so that the curved wave is processed at the same time. This allows a short high amplitude output to be produced.

Question 80

How can we tell how deep a structure/interface is from an echo that has returned?

Answer 80

Based on the time it took for the echo to return.

Question 81

How is depth altered in U/S

Answer 81

By adjusting the PRF - the rate at which the pulses are emitted from the transducer. I.e. deeper tissues require a longer time for the transducer to wait until all echoes are returned, therefore a lower PRF.

Question 82

What is A-Mode ultrasound

Answer 82

An older form of U/S, whereby the returning echoes were displayed on the screen as an amplitude variation, whereby amplitude represented echo strength. It could not produce images of anatomy.

Question 83

Increased line density = increased ______

Answer 83

lateral resolution

Question 84

What is line density and what does it affect?

Answer 84

A term used to describe how close the beam lines are. When line density increased, that will improve lateral resolution.

Question 85

Does line density change with depth?

Answer 85

Yes, in sector or convex transducers. Even with a large number of lines, the line density can become quite poor in the far field.

Question 86

What is compound imaging?

Answer 86

A form of B-Mode imaging that uses electronic steering to create several angled beam paths from each point. The echoes received from all the beam paths are combined to form a single frame. This helps to image interface that lay along (parallel to) an otherwise straight beam path.

Question 87

What would be one downside to compound imaging?

Answer 87

It can reduce shadowing and enhancement because the beam paths will be able to write into these areas. This is only an issue when shadowing is being looked for/ suspected, for example in breast u/s. It should be turned off if required.

Question 88

What is M-Mode Ultrasound? How would it be read on the monitor?

Answer 88

Used to geographically demonstrate any movement of organs or structures. It uses many of the principles of B-Mode ultrasound, but does not build up an image with multiple beam paths. If there is no movement, the lines a straight. If there is movement, it will be displayed like a wave, indicating movement towards or away from the transducer (depth on y axis, time on x axis)

Question 89

When might M-Mode ultrasound be used?

Answer 89

To detect foetal heart movement To measure heart value opening times and cusp displacement in echo

Question 90

Define frame rate

Answer 90

The rate at which B-Mode frames are processed

Question 91

What is real time imaging?

Answer 91

The propagation of rapid, sequential frames of images. The rate at which these B-Mode frames are produced is the frame rate.

Question 92

What two factors does frame rate depend on?

Answer 92

- The number of scan lines in each frame - The time required for each scan line

Question 93

Explain the ways that frame rate can be increased

Answer 93

1. Decreasing depth of penetration (less depth = less "wait" time) 2. Decreasing the number of scan lines (less scan lines = less time to produce entire frame). There are three ways scan line can be reduced (A) Decreasing the field of view (B) Decreasing the line density (C) Decreasing the number of focal zones (each focal zone is equivalent to another scan line)

Question 94

What changes will occur if the maximum depth of the field of view is increased?

Answer 94

- The image will extend over greater depth - The line density will remain the same, unless in a phased array transducer, density will decrease with increased depth. - The PRF will be reduced - The frame rate will be reduced (more anatomy seen with compromised frame rate)

Question 95

What will occur to the image if the sector angle is decreased?

Answer 95

The FOV will decrease AND either one of the following (depending on the machine/brand): The line density remains the same (same spatial image quality) and the frame rate will increase (because each frame takes a shorter time to produce - temporal image quality will improve - good for moving structures) OR The line density will increase (spatial image quality increases) but the frame rate will not change.