Weeks 1-6 (written notes) Flashcards
(A) What is the main difference between electromagnetic waves and mechanical waves?
(B) What are two kinds of mechanical waves?
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)
Explain the propagation of sound through a medium in terms of compression and refraction
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.
Explain why speed of sound varies through different materials.
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.
What is the difference between power and intensity in ultrasound?
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).
How can a sonographer increase the intensity of the beam?
- Increase the power output - this is a control on the machine.
- Decrease the area of the beam by moving the focus to the area of interest
(Remember intensity = power/area)
Explain the relationship between echo intensity and pixel brightness.
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)
What is the decibel in ultrasound applications?
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.
What are two kinds of sound-tissue interactions that are important to understand for image formation?
1 - Refraction
2 - Attenuation
What are the types of attenuation in sound-tissue interaction?
- Absorption
- Divergence
- Scattering
- Reflection
Briefly describe refraction in ultrasound.
Refraction is the change in direction of an ultrasound beam when it crosses a boundary at an angle. Refraction obeys “Snells Law”.
Explain Snell’s Law, provide examples.
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.
Describe the term critical angle?
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.
What are the factors that beam attenuation depend on?
Material involved (a coefficient), the distance travelled, and the frequency of the beam.
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 ____.
Attenuation
Result of internal frictional forces.
Directly removes ultrasound energy from the beam, which is converted to heat.
What does beam absorption depend on?
- 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 - 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) - Depth of the tissue (further travel = further absoprtion
What is Divergence?
What is another word for Divergence?
The spreading of an ultrasound beam as it moves further from the source
Diffraction
What happens with increased divergence?
Increased diffraction (spread) = increased attenuation.
Remember, intensity = power per area. As area increased, intensity decreases
How does beam frequency impact beam absorption (attenuation)
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.
Give 5 examples of when beam divergence occurs?
- In the far field of a non-focussed transducer
- Beyonf the focal zone of a focussed tranducer
- After refraction from a convex interface
- After refraction at a curved interface
- After passing through a small aperture
Can divergence occur with reflected wavefronts?
Yes, divergence attenuation can occur on a pulsed wave and a returning echo wave
What is scattering in ultrasound?
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.
Discuss how scattering is affected by wavelength? What is this called? Provide an example.
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)
Rayleigh Scattering is ________ dependent.
Explain the relationship.
Frequency
The intensity of the reflected echo is approximately proportional to the fourth power of the frequency
I ~= F^4
How does scattering affect image formation?
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)
Reflection is a form of ______ and is the major contributor to _____
What are the two types of reflection?
Attenuation
Image formation
Non-specular (diffuse) and specular
What is non-specular reflection?
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).
What is specular reflection?
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)
The percentage of an US beam reflected back to the transducer depends on …..
- The angle of incidence
- The reflecting surface texture
- The acoustic property differences of the two tissues (we can use acoustic impedance of different materials to calculate)
Explain acoustic impedance and how is it calculated in ultrasound? What are its units of measurement?
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)
What does a reflection coefficient represent?
The fraction of the ultrasound beam that is reflected at an interface.
What is the average U/S beam attenuation? How to we account for attenuation in image formation?
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.
What is the purpose of time gain compensation?
To produce equal brightness of echoes from similar interfaces, regardless of their depth in the patient (i.e. amplify deeper echoes)
There are several aspects of TGC. What are these?
- Near gain
- Slope
- Far gain
- The delay
What is near gain vs far gain
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.
What is the slope adjustment in TGC?
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.
What is the delay adjustment in TGC?
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.
Discuss the two main systems of TGC control in US
(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.
List the main features of a simple transducer (which all transducers have)
- PZT crystal
- Case
- Electrodes
- Dampening material
- Matching layer
What is the piezoelectric crystal and how does it work?
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)
