ultrasound Flashcards
4 ways ultrasound can be attenuated
reflection
refraction
scatter
absorption
equation for velocity
velocity = frequency x wavelength
range of sound frequencies in medical ultrasound
2 MHz - 20 MHz
human hearing range
50 Hz - 20 kHz
low frequency sound waves in relation to penetration
they can penetrate deeper into the human body compared to high frequency
low frequency waves in relation to resolution
has lower anatomical resolution
abdomen ultrasound frequency
3-5 MHz - good penetration, but poor resolution
small and superficial parts
5-10 MHz
skin, eyes ultrasound frequency
10-20 MHz
- poor penetration, but good resolution
x-ray vs ultrasound oscillation
x-rays - transverse waves and oscillation in their amplitude is at right angles to the direction of travel.
ultrasound - longitudinal wave and oscillation in its amplitude is in the same direction of travel
sound waves need to pass through a…..
Medium such as air, water or solids
acoustic impedance
is the resistance that the sound waves experience when passing through matter/tissue
practical significance of acoustic impedance
- impedance is high in bone, but less in soft tissue
- allows us to see a range of structures
- sound reflection is stronger at boundaries with different impedance values.
How can a sonographer improve the visualisation of deep structures in the body
- use a lower frequency to increase penetration - however, this decreases the resolution
- perhaps the use of different probes - adjusts beam focus
- time gain compensation
time gain compensation
- reduces the impact of wave attenuation by tissues through the increased intensity of the received signal in proportion to the depth
acoustic shadowing
occurs at boundaries between significantly different tissue impedances - which leads to signal loss and a dark appearance.
an example of acoustic shadowing
- decreased signal behind calcifications. e.g. a gull stone in the gall bladder, as the gull bladder is fluid-filled and a gull stone is dense -high difference in acoustic impedances
useful artefacts
- stones/ calcifications
- fluid structures like cysts can have acoustic enhancement behind them - brighter image response
how are ultrasound waves produced in a transducer
- piezoelectric effect - when an electrical current is passed through the crystals, they vibrate to a set frequency and emit sound
crystals in the probe acts as a transducer and emits and receives sound waves.
how are ultrasound waves received in a transducer
- when sound waves return back from the body, the crystals start to vibrate too - giving us an electrical return signal
- the changes in sound waves sent and received gives us the differences on our image.
Why do sound waves travel faster through denser materials
because the vibrations of sound pass faster through molecules that are packed close together
acoustic impedance and sound reflection
- if the acoustic impedance is between tissues is large, then a lot of sound is reflected and not much sound penetrates deeper
when does reflection work best?
when the tissue boundary is at right angles to the probe
what happens when the tissue boundary is not at right angles to the probe
the refelcted sound is not received by a simple probe
some sound is refracted
what is refraction
- where the angleof incident sound is not at 90 degrees to the boundary
- sound velocity changes between the tissues and therefore the direction of travel changes
ultrasound scatter
reflectors such as Blood cells and non-smooth tissue interfaces vase sound to fan out in all directions
- this causes speckle or irregularity in the image
a bleed will show up with
multiple small echoes that come from the blood cells and platelets - may look grainy
what happens as ultrasound passes through the body
The ultrasound is attenuated (reduction in beam intensity) with increasing depth in the body
what do decibels measure
the relative difference between 2 sound intensities
e.g. intensity between a produced sound beam and a returning echo
types of image resolution
lateral resolution and axial resolution
lateral resolution (at right angles to the beam)
depends on the diameter of the ultrasound beam - best with narrow beam
axial resolution (in direction of the beam)
depends on the sound frequency - high frequency = good resolution
types of ultrasound scan
- A Mode
- B Mode
- M Mode
- 3D and 4D
- Doppler
types of ultrasound scan
- A Mode
- B Mode
- M Mode
- 3D and 4D
- Doppler
types of ultrasound scan
- A Mode
- B Mode
- M Mode
- 3D and 4D
- Doppler
types of ultrasound scan
- A Mode
- B Mode
- M Mode
- 3D and 4D
- Doppler
The piezoelectric effect
- when an electrical current is applied across the crystal, it resonates sending out a sound wave
- when a force is applied perpendicular to the crystal, an electrical charge is produced
- the frequency of the sound is determined by the thickness of the crystal
A mode scan - Amplitude modulated
- produces a graph whose height shows the strength of reflection over time time
B Mode scan - brightness modulated
this produces a 2D greyscale image based on the strength of reflected sound echos, according to depth - most common scan
M Mode
Shows motion (e.g. heart valves), over time
3D and 4D scan
- volume and video images
Doppler scan
- ## change in sound frequency shows the speed and direction of blood flow
Duplex ultrasound
- Combines Doppler colour-coded blood flow direction
- red towards the probe and blue away from the probe
- flow speed with a 2D B-mode anatomical scan
rotating transducer
- produces an arc or fan shaped image section
linear array transducer
- consists of row elements and produces a straight sided image section
Array transducers
can be focused to either examine either shallow or deep tissues
Anechoic
no echoes
Exhogenic
- Brighter than (compared to something next to it)
- E.g. Bone is echogenic
hypoechoic
less echoes than surroung structures
Hyperechoic
higher level of echoes then surrounding structures
cystic
no echoes with echo enhancement behind
complex mass
components of solid/cystic tissue
solid mass
internal echoes, no enhancement behind
homogenous
same appearances/texture, smooth throughout .I.e liver/uterus
heterogeneous
different irregular pattern, different echo characteristics
biological effects of ultrasound
Thermal effects
mechanical effects
factors affecting temperature increases
frequency of the ultrasound beam - heating increases with F
power in watts
Attenuuation (acoustic impedance) of tissue - heating is greatest at bone
scan time
factors affecting mechanical indices
peak ultrasound beam pressure - related to power
frequency of the ultrasound beam - mechanical effects decrease with F
Beam focus depth - mechanical effects decrease with focus depth
Ultrasound ALARA
- use minimum power output
keep scanning time to a minimum
do not rest the transducer on the skin surface when not scanning
Artefacts – misleading or incorrect information
- caused by:
- the nature if the tissue
- the operator
Equipment malfunction
Artefacts: Assumptions made by the machine
- the beam being infinitely thin
propagation being in a straight line
the speed of sound being exactly 1540 m/s
the brightness of the echo being directly related to the reflectivity of the target
propagation
- the speed at which a sound wave travels through a medium
