Ultrasound Flashcards
(34 cards)
What medical ultrasound consists of?
Is the use of high frequency ultrasound waves (~1 - 10 MHz) into the body, followed by the reception processing and parametric display of echoes returning from structures from within the body.
What are thei main characteristics of ultrasonic imaging?
Tomographic: images are cross-sections of the volume
Soft tissue imaging: it is not sueful bor zones with bone or gas.
Real time: very fast sampling rates
Non invasive
Unexpensive
Portable
Why can there be ultrasonic waves in tissue?
Following the lattice model of tissue, that is, that their elements are coupled, it has the two properties necessary for wave propagation:
- Elasticity (compressibility): any deviation from the equilibrium will tend to be corrected.
-Inertia (mass density): the correction will tend to overshoot, producing the need for a correction in the opposite direction.
What are some characteristic of ultrasonic waves?
They propagate as acoustic pressure waves (mechanical waves).
They are longitudinal and with a planar wavefront
They can be desccribed by the wave equation
The translation of individual particles is much, much smaller than the wavelength (~ 10 - 6 m vs. ~10 - 3 m
)
What is the wave equation?
It is a PDE:
d^2W/dz^2 = 1/c^2 * d^2W/dt^2, where c, the sound propagation speed, corresponds to 1/ sqrt(kp), where p is the tissue density and k is the tissue compressibility
What is ultrasound? What are some of it applications and at which frequencies?
Ultrasound is any soundwave above 20 kHz. at around !MHz we have medical and destructive applciations, above 2MHz we have Diagnostic and NDE
What are the main interactions US has with tissue?
The only non-specular interaction is scattering, which is angle independent. It occurs when the soundwave strikes particles smaller than its wavelength, echoing several waves with smaller amplitude in all directions
Specular interactions are refraction reflection and transmission, and they occur when they interact with particles larger than the wavelength.
What is speckle’s nosie?
The classic US noise present in this type of imaging, attributed to non-specular scattering
How can blood flow be measured with soundwaves?
By sending sounwaves and reading their echoes, considering the doppler effect, the flow direction can be stablished.
How can be ultrasound intensity modelled?
By an exponential function:
I(z) = I(z=0)*exp(-uz), where u corresponds to the attenuation coefficient of the tissue.
How does the attenuation coefficient vary with tissue?
The attenuation coefficient increase approximately linearly with the frequency of the sound.
What is a US transducer? What is its configuration?
A US transducer is the instrument used for sending the soundwaves. It is made by a piezoelectric crystal disk connected to a contact wire. Thus, if a voltage is applied, the crystal will deform, and vice versa, if it receives pressure, a voltage will be sent.
The crystal it is fixed to a damping material on the inside, and a plastic layer to the outside. Everything is encompased between a metal case, and in between the damping material and the contact wire there is an acoustic insulator.
What are the main characteristics of the used crystal in US imaging?
It is a piezoelectric disk with the diameter much greater that its thickness, so it produces logitudinal waves.
The diameter is usually ~1.3 mm, and the natural frequency can be calculated as f=c_crystal/diameter, where c_crystal being the speed of sound in the crystal, approximately 4000 m/s. Thus, f ~= 1.5 MHz
How the beam geometry in US imaging can be modelled?
We can consider that in the transducer there is a large amoun of points of source, each emitting a spherical wave. Then the total pressure wave is the sum of these.
What are the names of the near field and far field of the beam?
Fresnel zone and Fraunhofer zone
What is beam focusing? What is DOF and Focal distance?
The use of curved-face transducers to change the beams geometry.
DOF means depth of focus, the distance for which the beam loses 50% relative to the focal point.
Focal distance is that where the lateral beam width is the narrowest.
What are lineal sequential arrays?
Whar are the advantages and disadvantages
The array of ~64-512 piezoelectric crystal for the configuration of a US transfucer. Crystals are grouped into groups of ~3-4 elements.
The advanteage is that you don’t need mechanical sterring and focusing, but at the cost of increased complexity and electronics, the presence of grating lobes, and a trade-off between lateral resolution and depth of focus.
What are linear phased arrays, dynamic focusing and beamforming?
Linear phased arrays are arrays where the beams transmitted or received can be delayed in time, in order to steer the beam and change the wavefront to a curve
What is TGC?
Time gain compensation consist of the amplification of signals that took more time to echo, given that these signals, coming from tissue that is further away, got scattered and absorbed by a greater factor compared to the signals coming from tissue close to th etransducer.
What are advantages and disadvantages of annular arrays?
They allow for 2D lateral focusing, but they have to be manually steered.
What is the advantage of multidimensional arrays?
The extra rows allow for the steering and focusing in the elevation dimension, allowing true 3D imaging
What are the axes considered in US imaging? How does the spatial resolution changes in these?
Axial: in the direction of the wave. The smaller the frequency, the freater the spatial resolution.
Lateral: degine the tomographic plane with the axial. Better resolution with more focused beams.
What and how affects the SNR?
the more intense the pulse, the better snr
the more frequency, worse snr
the greater depht of focus, better SNR
What and how affects the CNR?
the noise is proportional to the electronics bandwitdh
the more speckle, the mor noise
clutter, rregarding the signal arising from side lobes, grating lobes, multipath reverberation, tissue motion, and other acoustic phenomena
