Ultrasound Flashcards
1
Q
Sound waves
A
Mechanical radiation, propagating by vibration of molecules of a medium
- manifests as longitudinal pressure (compression) wave and transverse (shear) wave
- sound production requires a vibrating source
2
Q
Compression and rarefaction
A
- back and forth displacement of the source squuezes and pulls on particles in the medium
3
Q
Basic acoustics
A
- the wavelength is the distance between two points of compression
- the frequency is how many waves in a particular time
- C = dependent on the medium there will be a different speed of sound
4
Q
What determines speed?
A
- material properties - density (p) and Bulk modulus (stiffness (B))
- the higher the density the slower the speed
- the more squishy the tissue is, the slower the speed
- Fat, blood, muscle and liver are around 1500m/s, whereas bone is much faster at 4080m/s and air is much slower at 343m/s
- average is 1540m/s
5
Q
Interactions of US with tissue
A
- Absorption - US energy is converted to heat
- Reflection - some energy reflected from a boundary
- interference - artefacts in the image
- refraction - some or all energy is diverted from its original path
- diffraction - natural divergence of beam from sound source (defocusing)
- Scatter - some energy is dispersed in all directions
6
Q
Absorption
A
- Ultrasound energy is converted to heat
- amplitude is reduced
- depends on tissue properties, frequency and the distance it has to travel
- if you are scanning the abdomen, you need a lower frequency than if you are scanning the neck as you need to get the extra distance
7
Q
Reflection and Transmission
A
- reflections occur at tissue boundaries where there is a change in acoustic impedance (Z)
- The density and speed of sound in the tissue determines the impedance
- the bigger the difference in impedance, the bigger the reflection
8
Q
Types of reflectors
A
- Specular reflectors - returns echoes to the transducer only when the sound beam is perpendicular to the interface
>straight, flat interfaces
>if you hit the reflector at an angle, the angle of reflection is going off the plane and so wont be picked up by the scanner - Scattering
>much smaller interfaces within solid organs - scatter echoes in all directions
9
Q
Interference
A
- Constructive = two waves meet and their crests line up - increase amplitude
- Destructive = two waves meet but one peak meets a trough - results in lower amplitude
- causes speckle in the image
10
Q
Attenuation and Decibels
A
- attenuation is the loss of power or amplitude of the ultrasound signal as it passes through tissue
- Attenuation = Absorption + Scatter + Reflection
- Attenuation loss measured in decibels (a ratio)
11
Q
Frequency vs Attenuation
A
- attenuation is nearly proprtional to frequency - the higher the freq., the more loss of sound
- in soft tissue, attenuation ~1dB/cm/MHz
12
Q
US Machine
A
- transmitter - energises transducer
- transducer - piezoelectric ceramics converts electrical energy to mechanical vibrations
- receiver, ADC and processer - to detect and amplify backscattered energy
- image display - presents US image
13
Q
Transducers
A
- transducer contains multiple elements (120-250) each with its own electrodes
- linear - sound waves parallel to each other giving rectangular image. (+ = good near field resolution. - = artefacts when applied to curved part of body as gives air gaps between skin and transducer)
- curved - a compromise of linear and sector. Density of scan lines decreases with increaseing distance from transducer
- phased - sector image so can look inbetween ribs
- annular (old)
14
Q
ultrasound generation
A
- piezoelectric element inside
- pass a current across it and it will vibrate (expand). if you hit it, it will generate an electric current
- generally use ceramic material, but new materials have been researched recently
- damping material behind element allows mot current to go forwards
- matching layer means you minimise reflection
15
Q
Continuous and Pulsed wave
A
- Continuous - just turn it on, giving continuous doppler tone - can measure blood flow in vessels
> need to have a different receiver however as first is ringing at all times, cannot receive as well - Tone burst - used to break up haematomas (much higher power than imaging)
- Short pulse - ring it, then stop it by reversing polarity - gives high spatial resolution (main use in imaging)
16
Q
Pulse-echo principles (A-scan)
A
- amplitude scanning
- most has gone into the body, and a little comes back
- then goes further into the body, hits another interface and another bit is reflected
- bu this point, some from the first burst has come back to the detector, generating a tiny electric current in the detector (peak on graph)
- bit later, second echo comes back, a little weaker as it has had to travel further - lost energy from attenuation/absorption (smaller peak on graph)
17
Q
B-mode image
A
- brightness mode
- rather than a graphical display with points on a graph, you get an image display with different dots of different brightnesses
- large number of pulse-echo lines
- each echo is displayed along a line as a dot
- the brightness of each dot is deteremined by the strength of the echo
- the distance down the display relates to its depth below the transducer
- image builds up from left to right, sweeping across, then starts again
- now with digital imaging we can image from different points simultaneously and so dont get sweeping as frame rate is faster
18
Q
Transmit focussing
A
- Want the beam to be narrow so that we know where the echo has come from (not spread out in all directions)
- by delaying the transmission, you can get it to focus at different depths
- all the wave fronts will come together at a single point
