Advnaced B-Mode Ultrasound Scanning Flashcards
What are three ways to improve the B-mode frame rate?
- Decrease depth
- Decrease sector width
- Decrease line density
What is the time per line per cm depth?
13 us/cm
How is the time per frame calculated?
time per frame = time per line x lines per frame
How does spatial compounding work?
- Successive frames acuired with beam steered at -theta through 0 to +theta (at least three, but up to 9 different angles)
- Data from succesive frames averages and displayed as one image.
What are the advantages of spatial compounding?
- Multiple viewing angles produce uncorrelated artefact patterns reducing speckle, acoustic drop-out and clutter.
- Better margin definition and soft tissue contrast in breast imaging.
What is frequency compounding?
- Frequency compounding takes the beamformed RF data and filters into three narrower frequency components.
- These components are demodulated to give a B-mode image, and averaged to produce a single B-mode image with reduced speckle.
How does coded excitation improve penetration without increasing acoustic output?
- Using a longer transmit pulse with a digital coded embedded in it.
- The code can then be extracted from the received pulse using a matched filter, allowing smaller signals to be distinguished from the noise.
How is the code embedded in the pulse in coded excitation?
- Phase of segment represent binary 1 or 0.
- Positive phase = 1
- Negative phase = 0
How is the code extracted during coded excitation?
- Assigning a value of +1 to positive phases and -1 to negative phases, as we step the received signal trough the match filter we multiply any overlapping segments together, and sum the total.
- When the code overlaps the filter entirely, we get the highest signal, we are also likely to get some non-zero values at other locations called range lobes.
How can range lobes be reduced when using coded excitation?
- Range lobes can be cancelled out by using a second pulse that has a complimentary code.
What are the advantages of using coded excitation?
- Good axial resolution despite long pulse lengths
- Improves SNR and so penetration
What are the disadvantages of coded excitation?
- Halved frame rate
- Increased TI
How does adaptive image processing reduce speckle and artefacts?
- Attempts to distinguish between real image features and artefacts
- Analysis & filtering performed on multiple downsampled images which are then combined to form the output image.
- Smooths along interfaces and enhances egdes
- Smooths isotropically in uniform areas.
What is affected if the speed of sound in tissue is not 1,540 m/s?
- Focusing
- Measurements
How does the Zonare SSC algorithm automatically detect the optimal speed of sound?
- Uses wide transmit zones - not narrow beams.
- RF data lines stored in memory.
- Data lines are processed multiple times with differing SoS assumptions (1400-1650 m/s)
- Focusing is assessed in each image using Fourier Transform
- Image with the sharpest focusing has the “correct” speed of sound.
What improvements are gained using an optimal speed of sound?
- Improved Lateral Resolution
- Reduced Clutter
- Enhances Contrast
What are the two types of ultrasound elastography? Which is qualitative and which is quantitative?
- Strain Imaging: Qualitative
- Shear Wave Imaging: Quantitative
How does the shear wave velocity differ to the longitudinal wave velocity?
c_s=1-10 m/s
c_l= 1500 m/s
What are the equations for the shear and longitudinal wave velocities?
c_l = (K/p)^1/2 c_s= (G/p)^1/2
K = bulk modulus G = shear modulus p = density
What are the advantages and disadvantages of elastography?
+ Shear modulus has a larger range (10^3 - 10^5 Pa) in tissue compared to bulk modulus (2 x 10^9 Pa) so gives higher contrast.
- Shear waves are more rapidly attenuated in tissue.
Briefly describe strain elastography,
- Pressure applied to transducer causes shear deformation in the tissue which propagates at c_s.
- Speckle provides image markers that follow the deformation.
- RF lines of current images compared with those from previous images.
- Axial strain estimated by taking gradient of line of axial displacement.
- Persistence applied across images to improve SNR.
Briefly describe Point Shear Wave (Acoustic Radiation Force Impulse, ARFI) Elastography.
- Small axial displacement created using a long (0.05-1 ms) focused pulse.
- Displacements of a few microns decay away over milliseconds.
- Speed of shear wave can be tracked in ROI.
- Young’s Modulus e = 3p(c_s)^2
How does ARFI differ from true Shear Wave Elastography?
- In true SWE, ARFI cone is swept down the acoustic axis faster that the shear wave speed creating a Mach cone.
- Mach cone is cylindrical so decays less rapidly.
- Ultra-fast scan produces images at a 20 kHz frame rate (plane wave transmitted - focusing on receive only).
- 2D map of arrival time of the shear wave is created, and process is repeated for several ARFI pusles.
What are the limitations of “True” SWE?
- Frame rate is 3-4 Hz, persistence must be applied.
- Penetration is about 3 cm for linear, and 8 cm for curved arrays.
- Maximum ROI is 3 cm box.
