Harmonics and Contrast Resolution Flashcards
Conventional approaches to overcoming poor signal
Low frequency probe
Different imaging
applications
higher transmit power
Invasive Technique
weak reflections from Rayleigh scattering
cause poor signal to noise ratio, meaning that signal can be both nondiagnostic or not detected
The idea of contrast imaging
not to increase the backscattering surface but use a contrast agent which helps increases the autistic impedance mismatch of blood
How to increase the amount of backscatter (two ways)
Increase the surface of the reflector
increase the acoustic impedance mismatch
Amount of reflection
Based on acoustic impedance mismatch
how does the contrast agent increase the acoustic impedance mismatch within the blood?
Result of high compressibility and low density of gas
Contrast agents improve the reflectivity of blood by how much
30DB
what does harmonics do
Transmit beams at a lower frequency and receive at a higher frequency
advantage of higher transmit frequency
Improved resolution
First harmonic
Fundamental frequency (operating frequency)
second-harmonic
frequency that is twice the fundamental freqency
Transmitting at a lower frequency allows for better ________while receiving a6t a higher frequency allows for better _______
Penetration, resolution
Harmonics
multiple of the operating frequency
2nd harmonic frequency
2*transmit frequency
If transmit frequency was 4, 2nd harmonic frequency would be 8
high frequency….
Inadequate penetration
low frequency….
Poor resolution
what are the two mechanisms for producing harmonic signals
- Non-linear propagation through tissue producing ‘native’ or ‘tissue harmonic imaging’
- Resonance of contrast agents producing contrast harmonic imaging
Broadband transducers
Make harmonic imaging possible
when harmonic imaging was first developed
Used contrast agents
Non-linear response of tissue
Distorts the transmitted wave, producing harmonic energy
what does harmonic imaging use
2nd harmonic
with compression and refraction
Density of the medium changes and results in a change in propagation velocity
Change in propagation velocity
Nonlinear
with compression
Propagation speed increases
with refraction
Propagation speed decreases
what occurs to the amplitude of each successive higher harmonic
Amplitude decreases
therefore the amplitude of the 2nd harmonic
is weaker then the fundamental signal
Harmonics are best near….
The focus
With increasing depth, there is what
Increasing beam attenuation, decreasing the pressure.
rate of decreasing harmonic energy will be faster than the fundamental frequency
In the near field and far field….
Fundamental imaging may outperform harmonic imaging
Attenuation of the harmonic signal is
higher because of increased frequency
Harmonic beam is ______compared to fundamental beam
Narrower
Narrow harmonic beam helps to improve what?
Lateral resolution
Beam intensity is _____in the nearfield, which reduces what?
Much weaker, artifacts
to produce harmonic imaging what must be prsent
enough depth
what is the cycle of events of harmonic imaging
- Fundamental beam sent into body
- Wave travels deep into body
- areas of density affect refraction and compression
- harmonic energy is produced
- transducer bandwidth detects harmonic imaging
how does harmonic imaging affect axial resolution
Degraded axial resolution
-longer pulse duration which means longer spatial pulse length and therefore a decreased axial resolution because there is an overlap of signals
Bandwidth is_________to PD
Inversely proportional
Narrower bandwidth requires a ______pulse and ______cycles in the wave
longer, increased
Advantage: decreased grating lobes
lower energy grating lobes produce much weaker harmonic signals, reducing the energy in the grating lobes
-Weaker grating lobes result in improved LATERAL resolution and less lateral translation of off axis energy into the main beam
clutter reduction
Major advantage of harmonic imaging
how does harmonic imaging help reduce clutter
reverb and clutter is created by large acoustic impedance mismatches from specular reflectors
-Harmonic signal is much weaker then fundamental signal which causes a large reduction in the strong signals responsible for artifacts
how does harmonic imaging eliminate clutter and reverb artifacts
Intensity of each reverberation is lower which produces less harmonic signal
most artifacts are caused by specular reflectors in near field, where the beam is still converging (therefore weaker harmonic energy)
Phase aberration
Distortion of the beam caused by differences in speed of sound in inhomogenous tissue
how is beam aberration reduced by harmonic imaging
- Focusing of lower frequencies is less affected y speed of sound then higher frequencies
- Higher frequencies=shorter wavelengths
- Less time delay and more destructive interference
Overall advantages to harmonic imaging
- Increased lateral resolution
- decreased phase aberration
- decreased clutter artifacts
- Decreased grating lobes
over disadvantages to harmonic imaging
-decreased axial resolution
Pulse/phased inversion harmonics
-Peak of the harmonic wave occurs at the same time as both the peak and the minima of the fundamental wave
how does pulse/phase inversion imaging work
By transmitting multiple lines with different phases and then adding the resulting lines together, the fundamental energy adds destructively while the harmonic data adds constructively. As a result the harmonic energy gets stronger and there is no need to degrade the axial resolution to help eliminate the fundamental energy.
