M 5 & 6: Harmonics and Contrast Flashcards
define harmonics
essentially beam dynamics, its the result of the sound wave propagating through tissue
2 X the fundamental or driving frequency
what creates harmonics
wave distortion, specifically changes in wave compressibility and density which occur as the US beam propagates and produces multiple harmonic frequencies
what does wave distortion depend on
intensity of the beam, distance traveled and nature of the tissue
are harmonic frequencies linear
why
no
they are non-linear due to their dependencies (harmonic frequency is what we get back)
are fundamental frequencies linear
why
yes
the fundamental frequency is what we send out is the ‘leading edge’ so when it strikes the molecules for the first time they were previously stationary so theres no distortion
where does harmonics work the best
with higher intensities since there is more attenuation
how does bulk modulus change w/ increasing density
how is velocity effected
increases
increases
if only density increases how does velocity change
decreases
how does wave distortion and the compression of a sound wave work
at the peak amplitude theres increased density and stiffness (compressed) w/ faster velocities, and at the lowest amplitude theres decrease density, stiffness (expansion) and slower velocities…. the difference in the velocities cause the particles to move closer to one another and the wave is compressed, producing a higher frequency/harmonic frequency
3 ways harmonics improves the image
- the harmonic beam is narrower since they are best produced from the most intense part of the beam, e.g. better lateral resolution than the fundamental frquency
- grating lobes are eliminated b/c they’re too weak to produce harmonics
- reverb is greatly reduced/eliminated at the face of the probe (main bang) b/c harmonics/wave distortion doesn’t happen until deeper into the tissue (also helped my the matching layer)
how is penetration effect w/ harmonics
it’s reduced due to more attenuations so we need to use more power when scanning w/ harmonics
how does harmonics effect the contrast of the image
will appear to have more contrast
function of bandpass filtration
electronically eliminates the fundamental frequency and allows the harmonic frequency to pass through the beam former
whats the downside of bandpass filtration
the fundamental and second harmonic bandwidths both must fit w/in the overall probe bandwidth w/o overlapping…. this means that the individual bandwidths need to be narrower which will cause the SPL to be longer, resulting in worse axial resolution
what happens if we keep the SPL short and try to use the bandpass filter
the bandwidth of the fundamental and harmonic frequencies will overlap and the bandpass filter will cut out part of the harmonic frequency and the image is not formed properly
what is pulse inversion
a technique that can filter out the fundamental frequency and leave only the harmonic signal while keeping a wide bandwidth (as there’s no need to fit both signals in the overall probe bandwidth)
how does pulse inversion work
2 pulses are sent out, the initial pulse, followed by the inverse of that pulse…. since the fundamental frequencies are linear, when the echos return from both pulses, they will cancel out.
but the process of harmonics is non-linear so the 2 returning echos won’t cancel out and we have a resultant signal
which type of resolution is compromised w/ pulse inversion
which type is improved or maintained
temporal is reduced (but not significant)….. b/c we have to send out 2 pulses instead of 1
axial is maintained and lateral is improved
what are contrast agents
what are they used for
liquid suspensions injected into the blood stream, most are micro bubbles of gas w/in a shell
improve the intensity of returning echos for enhanced visualization of blood flow or tissue (lesion detection and characterization)
how does contrast increase the intensity of returning echos
through stable cavitation, the gas bubbles resonate and produces its own sound source
requirements for contrast agents (size, stability)
they need to be small enough to pass through capillaries but large enough to give back echos
must be stable enough to make it through the heart for several cycles to provide enough imaging time
why do the gas bubbles in contrast give off strong reflection
due to impedance mismatch b/w the gas and blood
purpose of shell used in contrast agents
keeps the gas from dissolving in the blood
what happens to the gas bubbles in contrast if we scan at 100% power
75%?
25%?
the collapse and dissolve
the bubbles produce very intense harmonic echos which increase the contrast b/w the contrast agent and the tissue
bubbles produce resonant frequency
an MI of what value w/ contrast will cause the bubbles to pop
0.4 MI, which means 0.4 MI is 100% power in this case
what is Sono CT/compound imaging/spatial compounding
combination of beam steering and frame averaging that can improve SNR and help reduce artifacts
why does shadowing from a stone appear darker w/ harmonics
theres more attenuation (reflection, refraction, absorption, etc) w/ harmonics so the shadow looks darker
does CW or PW have a narrower bandwidth
CW (because we use a longer pulse, see doppler module)
size of the contrast agent
~ 7 micrometers, the same size, or smaller than a RBC
air reflects what % of sound
99.9%
when should you use harmonics
anytime penetration allows… as much as possible
describe coded excitation
newer technology that has the ability to visualize blood flow in 2D w/o the need for doppler and the artifact associated w/ it
how is the transmitted/received pulse different w/ coded excitation
advantages
the transmitted pulses are digitized and coded:
we send out a coded the pulse and the retuning echos will have the same code which tells the machine that the echo is real…. if the retuning pulse doesn’t have the code, it’s disregarded as noise.
reduces noise, improves image quality and SNR
which part of the receiver is important w/ coded excitation
compression: this refers to ignoring echos below our threshold level/noise level and only using the echos b/w the threshold and saturation level
helps get rid of fake echos and improve SNR
problem w/ compression and how does coded excitation account for this
real low level echos may be rejected as noise if they don’t meet the threshold… coded excitation will accept all echos that return as code so it will incorporate these weaker but real, low level echos in the image
describe the process of coded excitation for each scan line in the B-flow image
- transmit coded sound waves
- decoder enhances flow signal
- flow and tissue displayed as in B-mode
detecting what kind of reflectors in B-mode is key w/ coded excitation
blood reflectors
why are echos from blood rejected w/ conventional scanning
are do we resolve this w/ coded excitation
the echos are very weak and below our threshold
code the pulse so that real echos can be differentiated from noise and so that the beam former no longer relies on intensity of returning echos to determine noise
how is colour doppler added to our 2D image
its a separate image thats transposed onto the 2D, this is why FR goes down so much w/ colour
another name for coded excitation
b-flow
advantages of coded excitation compared to colour doppler
which gives us more info, colour doppler or coded excitation
- simultaneous tissue and flow w/o an over lay like colour doppler
- full field of view
- no separate firing of scan lines (only 1) so higher frame rate than colour doppler (one or the best benefits)
colour doppler… we can’t tell direction of flow w/ coded excitation
other advantages of coded excitation
- better sensitivity, suppresses unwanted signal components… allows you to scan w/ a higher frequency and still penetrate
- better spatial res, contrast and penetration which helps w/ technically difficult patients
how does coded excitation allow for better penetration
it can separate out the noise from the real weaker echos the are returning from deeper in the tissue that would have otherwise gotten cancelled out by the compression part of the receiver
2 types of harmonics
tissue or native harmonics
contrast harmonics
another name for fundamental frequency
characteristic frequency
which is more intense, fundamental or harmonic frequency
fundamental
are 3rd, 4th, and 5th harmonics also produced as a result of wave distortion
yes
relationship b/w the strength of the harmonic beam/energy and the fundamental wave
the harmonic energy generated is proportional to the fundamental energy squared
H proportional to F^2
(like intensity is proportional to amp^2)
does harmonics affect dynamic range
yes, it reduces it b/c the signals are weaker
name for the process of filtering the fundamental frequency at reception to avoid noise and clutter
wave shaping
what controls the amount of harmonics generated by tissue
transmit power, specifically the MI
Describe how wave shaping works
Another name for wave shaping
It controls the shape and duration of the transmitted pulse in order to filter out the fundamental frequency
Coded excitation
How must the frequency change to accomplish wave shaping/coding
Frequency must be lowered
Types of contrast agents
Oral, vascular or tissue specific
Is there a better contrast resolution with harmonics?
Why
Yes
We have less dynamic range so more contrast
Why don’t we use air as a contrast agent
Highly soluble in blood
Can sonographers inject contrast material?
No
What’s one way to enhance shadowing?
Turn off compound imaging, which sweeps the sound beam across the face of the probe and averages the frames to reduce shadowing and enhancement
Name for the peak and lowest amplitudes when related to density
Peak: compression
Lowest: rare-fraction
What happens when you decrease the distance b/w the peak refraction pressure?
you are increasing the frequency and producing harmonics (FOR SONOG CANADA)
