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