Intro Flashcards
How many shades of grey do most ultrasound systems have?
64
What is axial resolution?
Resolution along the length of the ultrasound beam
What does axial resolution depend on?
The pulse length
(this is equal to the number of cycles * wavelength)
What is the relationship between mechanical index (MI) and frequency ?
MI is inversely proportional to the square root of frequency
What are diffuse reflectors?
Reflectors that scatter the sound in all directions
What is gain?
The degree of amplification of the returning echo
What is PRF?
Pulse repetition frequency
- the speed at which echoes are transmitted and recieved
What is the piezoelectric effect?
The ability of a material to generate an electrical current in response to an applied pressure
What does acoustic impedance depend on?
How tightly or loosely packed a material’s particles are. i.e. a material with tightly packed particles will have a high acoustic impedance.
- acoustic impedance measures the resistance a material has to sound wave propagation through it
What is the equation for acoustic impedance? (Z)
Z = excess pressure / particle velocity
- i.e. the higher the acoustic impedance, the faster sound will travel through the medium
What are the two regions of the ultrasound beam?
- A near field (cylindrical shape)
- A far field (cone shape)
How does moving a particle from the periphery to the centre of the ultrasound beam impact its appearance
It increases its brightness
What are side lobes?
Some of the energy from the transducer radiates at various angles to the transducer face - known as side lobes
How do side beams impact image quality?
15% of energy of the ultrasound beams will be in the side lobes
- will be mis-registered on the image
What are the two dimensions of the ultrasound beam?
- Beam width
- Slice thickness
How does beam width impact spatial resolution?
Decreased beam width increases spatial resolution
How many crystal elements does a transducer usually have?
128
What are the main components of the ultrasound transducer?
- housing assembly
- electrical connections
- piezoelectric element
- backing layer
- acoustic lens
- impedance matching layer
What determines a transducers operating frequency?
The thickness of the piezoelectric element
What is resonant frequency?
Frequency at which the transducer works at maximum efficiency
- occurs when thickness of the crystal corresponds to half a wavelength
What is the purpose of the acoustic lens in a transducer?
Improves image (lateral) resolution by reducing beam width of the transducer
What is axial resolution? And how can it be increased?
Resolution along the axis of the beam
- shorter pulse length gives better resolution
What factors affect spatial pulse length and therefore axial resolution?
Frequency - lower frequency = lower pulse length
Transducer design - Increased damping makes pulse length shorter
How does increasing the field of view impact axial resolution?
Increased field of view decreases axial resoluion
What does lateral resolution depend on?
Beam width
- a narrow beam width gives a good spatial resolution
What is contrast resolution?
The ability of the imaging system to differentiate between body tissues and display them as different shades of grey.
What is temporal resolution and what is it determined by?
The ability of the imaging system to display events which occur at different times as separate images
- it is determined by frame rate
What is the equation for frame rate?
Frame rate = PRF / lines per frame
What is PRF dependent on?
Depth
- increased image depth results in lower PRF
When can you use multiple focal zones?
If there is little patient movement
How can temporal resolution be improved?
Reducing the sector angle and depth and selecting only a single focal zone
What is attenuation?
The decreases in amplitude and intensity of a sound wave as it travels through the body
What are the 5 main reasons why TGC is needed to amplify distant echoes?
- Absorption
- Reflection
- Scattering
- Refraction
- Divergence
What is the main factor causing attenuation?
Absorption
What is specular reflection?
Reflection of a smooth surface
What is diffuse reflection?
Reflection in multiple directions off a rough surface
When is scattering most likely to occur?
When an interface is equivalent to one wavelength in size
How does increased frequency impact scattering?
Higher frequency causes more scattering
What are the 3 main forms of artefact?
- Images that are not actually there
- Objects missing from image
- Misregistered structures
How can you try to reduce reverberation artefacts?
Use more gel
Reduce gain
Move position of transducer
What is acoustic shadowing?
An hypoechoic area behind an area of strongly attenuating tissue
- usually occurs at interfaces with large acoustic mis-matches
What is acoustic enhancement?
A localized area of increased echo amplitude behind an area of low attenuation
- often behind fluid-filled structures e.g. cyst, bladder
What is edge shadowing?
A combination of refraction and reflection occurring at the edge of rounded structures
What is the equation for flow rate (Q)?
Q = change in Pressure / Resistance
