THI and CEUS Flashcards
Briefly describe the THI process.
- Transducer transmits at fundamental frequency (f0), but receives at 2nd harmonic (2*f0).
- Second harmonic is tissue generated and extracted from received echoes.
- Images are constructed by processing the tissue generated harmonic component.
What are some advantages and disadvantages of THI?
+ Better border delineation.
+ Less haze, clutter, and reverberation.
+ Better contrast resolution.
+ More useful of imaging larger patients (possibly??)
- Worse axial resolution and penetration (possibly??)
Why is THI not possible in a linear acoustic medium?
- Change in density is proportional to pressure change
- So there is a spatially constant compressibility in the medium.
- Propagation speed c=1/(pk)^1/2 is spatially constant
- So Frequency contents are constant.
How does the non-linear nature of tissue lead to acoustic deformation?
- Linear relationship is only true for infinitesimal disturbances.
- Changes in density are greater in rarefaction that in compression.
- Higher compressibility in trough leads to lower travelling speed which further leads to wave distortion.
What does the coefficient B/A represent, and what does it quantify?
- B/A is the 2nd order compressibility of a medium and specifies the medium’s elastic property.
- It is also known as the Acoustic Non-linearity parameter.
- B/A quantifies the non-linearity stress-strain relationship in a medium.
N.B: A is the first order compressibility, and is related to the speed of sound in the medium.
Apart from material, what does B/A depend on?
- Temperature
- Frequency
What is the typical range for B/A in tissues?
5-12
What are the two methods for achieving THI?
- Electronic Filtering THI (Conventional or Differential)
- Pulse Inversion THI
What problems arise from electronic filtering?
- Some of the fundamental frequencies may pass through the filter.
- Some of the harmonic frequencies may be omitted.
How does pulse inversion THI work?
- Transmit two pulses of opposite phase, ant different times.
- Sum of fundamental frequencies cancel due to 180 phase shift.
- Second harmonic are in phase (phase shift of 360) and so constructively interfere.
- Echoes can be summed in either time domain (after digitally storing the signal) or in the frequency domain, taking advantage of their phase spectra.
- This can also be applied along alternating scan lines.
What are the advantages and disadvantages of pulse inversion THI?
+ Shorter pulses can be used so bandwidth is wider than transducer.
+ Better rejection of fundamental component compared to filtering.
- Frame rate is reduced as each line requires two pulses.
Briefly explain how contrast agents are used in Ultrasound scanning.
- Contrast agents are protein shelled gas bubbles.
- The bubbles performs a forced vibration under isonation which is enhanced if matched to their resonant frequency.
- Echoes have strong harmonic, sub-harmonic and resonant frequency components.
- Micro-bubbles are similar diameter to RBCs (2-6 um)
- Bubbles scatter transmitted pulses and generate resonant pulses.
What are the differences between fundamental and harmonic CEUS?
Fundamental
- Imaging as in B-mode.
- Conventionally used in colour Doppler.
Harmonic
- Takes advantage of high agent-to-tissue B/A contrast.
Low and High MI applications.
Whats the difference between low and high MI CEHI?
Low MI CEHI
- Stable oscillation of bubbles.
- Use bubble generated 2nd harmonics.
- Important in colour flow studies.
High MI CEHI:
- Low Mi imaging followed by flash when bubbles are intentionally destroyed.
- Gives indication of perfusion of different organs.
What are potential risks of CEUS?
- Embolisms
- Allergic reactions
- Toxicity
- Biological effects of acoustic cavitation.
