7. Ultrasound Flashcards
What is ultrasound?
mechanical oscillation or wave with a frequency (pitch) exceeding the upper limit of the audible range of the human ear, which is 20 000 Hz.
Describe Propagation of ultrasound
Ultrasound propagates with a velocity (c) characteristic for the medium, and it displays wave properties (see refraction).
What is damping of ultrasound?
Part of the energy that propagates in ultrasound is dissipated in the medium in the form of heat, hence transmitted intensity is attenuated
→ The damping is used to characterize the decrease of intensity
Formula for damping of ultrasound?
where J0 is the incident intensity and J is the intensity after passing through the layer.
Reflection of ultrasound
At the boundary surface of different media, part of the incident ultrasound wave is (1)___ and part of it (2)__
- reflected (reflection)
- penetrates into the medium
Reflection of ultrasound
If the direction of propagation of the incident ultrasound wave is not perpendicular to the boundary surface (skewed incidence)
→ then the ultrasound wave will ____
It will change its direction of propagation (refraction)
What is Acoustic impedance (Z )?
- An important parameter that characterizes the acoustic properties of the medium.
- It is defined as the product of the medium density (ρ) and the velocity (c) of the ultrasound in that medium (Z = ρ · c).
- The surface of a medium is called boundary surface (or interface) if the acoustic impedance changes.
What is the formula for Acoustic impedance (Z )?
Z = ρ x c
- ρ is the density of the medium
- c is the velocity of the ultrasound in the medium.
What is the reflectivity (R)?
The measure of reflection from a surface, which is defined as the ratio of the reflected (JR) and the incident (J0) intensity
Reflection of ultrasound
The greater the acoustic impedance difference between two bounding media, the greater the ___
the intensity of reflected ultrasound
Reflection of ultrasound
On the liquid–gas or solid–gas interface ultrasound is ___
totally reflected
Reflection of ultrasound
On the liquid–gas or solid–gas interface ultrasound is totally reflected.
Because of this, a coupling medium (gel, water or oil) used in medical ultrasonography that helps to____
transfer the acoustic energy from the transducer into the body and back again.
Reflection of ultrasound
On the liquid–gas or solid–gas interface ultrasound is totally reflected.
Because of this, a coupling medium (gel, water or oil) is used in medical ultrasonography that helps to transfer (1)___ from the (2)___ into the body and back again.
- the acoustic energy
- transducer
Reflection of ultrasound
On the liquid–gas or solid–gas interface ultrasound is totally reflected.
Because of this, a coupling medium (gel, water or oil) is used in medical ultrasonography that helps to transfer (1)___ from the (2)___ into the body and back again.
- the acoustic energy
- transducer
What is direct piezoelectric effect?
Piezoelectric insulators (e.g., crystals) accumulate electric charge and become electrically polarized when mechanically strained (by pressure or tension).
→ A measurable potential difference builds up on the electrodes placed on the opposite sides of the strained crystal.
Detection of mechanical oscillations caused by ultrasound is based on ___ (which effect?)
Piezoelectric effect
-> Piezoelectric insulators (e.g., crystals) accumulate electric charge and become electrically polarized when mechanically strained (by pressure or tension).
→ A measurable potential difference builds up on the electrodes placed on the opposite sides of the strained crystal.
inverse piezoelectric effect
If voltage is applied on the electrodes of the piezoelectric material, it becomes deformed (contracted or stretched), hence the __ of the crystal changes
thickness
Ultrasound radiation is generated based on which effect?
Inverse piezoelectric effect
Ultrasound source
______, which is converted into ultrasound, is produced by a sine wave oscillator
Alternating voltage of high frequency
Ultrasound source
Alternating voltage of high frequency, which is converted into ultrasound, is produced by a ___
sine wave oscillator
(Sine wave oscillators are used as references or test waveforms by many circuits. A pure sine wave has only a single or fundamental frequency—ideally no harmonics are present. Thus, a sine wave may be the input to a device or circuit, with the output harmonics measured to determine the amount of distortion.)
Ultrasound source
Alternating voltage of high frequency, which is converted into ___, is produced by a transducer
ultrasound
Ultrasound source
What is a transducer?
a device that converts electric signals into mechanical, and vice versa.
Ultrasound source
Structure of transducer?
It contains piezoelectric materials equipped with electrodes
Ultrasound source
The transducer converts the electrical energy into mechanical, thereby generating an (1)___ (via the (2)____) which is directed into the investigated part of the body
- ultrasound pulse
- inverse piezoelectric effect
How to detect ultrasound?
- In the “silent” phase that follows pulse transmission the transducer “switches into receiver mode”
- Waits for the echo signals reflected from the tissue surfaces of the examined body part.
- echoes of reduced intensity reach and deform the transducer material
- As a consequence of electric polarization in the transducer, the ultrasound is converted back into the electrical oscillation (via the direct piezoelectric effect)
- This signal is then conducted through a cable into the electronic amplifier
Detection of ultrasound
In the “silent” phase that follows pulse transmission the transducer “switches into receiver mode” and waits for the ___of the examined body part
echo signals
Detection of ultrasound
As a consequence of (1)___ in the transducer, the ultrasound is converted back into the (2)___ (via the (3)___), and this signal is then conducted through a cable into the (4)___
- electric polarization
- electrical oscillation
- direct piezoelectric effect
- electronic amplifier
Because the frequency of the reflected ultrasound changes if the ____ (Doppler effect, see later), the frequency spectrum of the echo contains further information as well.
reflecting surface is in motion
What is Doppler effect?
Change in the frequency (and wavelength) of the wave as a result of the relative motion of the source and the observer
(1)_____ (transmission) and (2)____(reception) changes (3)___ in the transducer.
- Wave-packet release time
- detection time
- periodically
What is pulse-echo method?
- The distance (d ) between the reflecting boundary surface and the transducer can be calculated from the time (Δt) elapsed between the emission of the wave-packet-like ultrasound pulse into the medium and the return of its echo
- provided that the velocity of the sound (c) is known in the given medium as follows…
Diagnostic ultrasound imaging methods (modes)
Describe One-dimensional A-image (amplitude modulation)
- A single transducer produces one ultrasound beam that propagates in a straight line.
- In the image…
- the horizontal axis represents time or the corresponding distance
- the vertical axis denotes the intensity of the reflected echo signal
Diagnostic ultrasound imaging methods (modes)
Describe One-dimensiona B-image (brightness modulation)
- a single transducer produces one ultrasound beam that propagates in a straight line.
- The brightness of pixels on the horizontal time scale is proportional to the amplitude of the reflected ultrasound signal.
- Brightness is encoded usually on a gray scale
Diagnostic ultrasound imaging methods (modes)
Describe Two-dimensional B-image (2D, brightness modulated)
- An array of several transducers are specially synchronized in a way that the direction of the wavefront can be changed, and a plane (of fan shape) is scanned.
- A series of one- dimensional B-images at different angles appears on the screen
- Real image is reconstructed of one section of the scanned region of the biological sample.
Diagnostic ultrasound imaging methods (modes)
Describe M-image (motion)
- In the 2D B-image one direction is chosen, and a sequence of one-dimensional B-images is plotted vertically as a function of time
- We obtain information about the position of the given surface as a function of time.
Diagnostic ultrasound imaging methods (modes)
Describe reconstructed 3D image (tomography)
- Three-dimensional image of the biological object can be reconstructed from the 2D B-images of a large number of parallel planes scanned, that can be processed further by a computer.
- The 3D-rendered dataset can be manipulated orientationally (i.e., turned around arbitrary axes), or different image sections can be calculated.
Diagnostic ultrasound imaging methods (modes)
Describe Reconstructed 4D-image
Sequence of several reconstructed 3D images, where the 4th dimension is time.
In this method an ultrasound movie is created.
The Doppler effect is a change in the frequency (and wavelength) of the wave as a result of the relative motion of the source and the observer.
Similarly to the phenomenon described above, the frequency of the ultrasound reflected from a ___
moving boundary surface
Doppler effect
The relationship between Doppler shift and the relative velocity
The Doppler shift ( f – f0) is proportional to the relative velocity ( v/c) and the incident frequency.
Doppler methods
Describe Doppler time-velocity image
At the selected section of the indicated direction in the 2D B-image, Doppler-frequency-shift that corresponds to the velocity of the observed surfaces is plotted as a function of time (Fig. 11, right).
Doppler methods
Describe Color-coded Doppler image
The color-coded velocity of the moving body parts is superimposed on the grayscale 2D B-image (Fig. 11, left).
Doppler methods
Describe Doppler flow meter
it allows the measurement of the velocity of blood flow in larger blood vessels according to ..
- v is the velocity of blood flow
- c is the ultrasound velocity in the medium
- f – f0 is the Doppler shift
- Θ is the angle between the ultrasound beam and the axis of the blood flow
The radar principle – distance measurement by ultrasound – US-A-image
Describe
Using a special ultrasound-head
→ short ultrasound pulses are emitted in air towards a reflecting surface, and the same ultrasound head detects the echo signal.
→ Both the signal of the emitted ultrasound and the reflected echo signal are visualized on the screen of the oscilloscope
Measurements using a water-filled phantom (model)
Place the (1)____ (Fig. 15.), and observe the (2)___ of different layers.
- water-filled rubber gloves into the phantom
- images of different sections
Measurements using a water-filled phantom (model)
Place the air-filled rubber gloves into the phantom, and note the ___
- water-filled rubber gloves into the phantom
- images of different sections
Measurements using a water-filled phantom (model)
Turn the device into M mode and observe the (1)___ of the ruler that is moved back and forth in the water of the phantom.
Turn the device into Doppler mode and observe the (2)___of the ruler movement (3)____(listen to the sound corresponding to this movement by loudspeakers).
- M-image
- velocity changes
- in time
Describe time sharing mode of transducer
pulses instead of continuous wave ultrasound
Definition of axial resolving limit
- depends on the pulse length. → The axial resolving limit is the half of the pulse length.
- Pulse length is inversly proportional to the frequency.
(Axial resolution is the ability to see the two structures that are side by side as separate and distinct when parallel to the beam. So a higher frequency and short pulse length will provide a better axial image.)
Definition of lateral resolving limit
- the minimum separation of two interfaces aligned along a direction perpendicular to the ultrasound beam.
- It depends on the beam width (or beam shape)
(Lateral resolution is the image generated when the two structures lying side by side are perpendicular to the beam. This is directly related to the width of the ultrasound beam. Narrower the beam better is the resolution. The width of the beam is inversely related to the frequency.)
Focusing
→ increases the (1)___
→ reduces the (2)___
- divergence of the beam in the far field regime
- depth sharpness.
What is the beating phenomenon? Which effect does it relate to?
The phenomenon in which the beating frequency equals to the difference of the two interfering frequency → related to Doppler effect
4 Doppler curves
Describe this Doppler curve
flow can be represented by one velocity in each moment
Describe this Doppler curve
flow can be represented by a velocity distribution in each moment
What is compressibility?
relative volume decrease over pressure
What does this image indicate?
Speed of ultrasound in various materials.
What is is proportional to frequency in the diagnostic range?
Absorption coefficient
What is specific attenuation
for soft tissues?
homogeneous tissue model
- Ratio of damping to the product of frequency and layer thickness
