Advnaced B-Mode Ultrasound Scanning

Question 1

What are three ways to improve the B-mode frame rate?

Answer 1

• Decrease depth
• Decrease sector width
• Decrease line density

Question 2

What is the time per line per cm depth?

Answer 2

13 us/cm

Question 3

How is the time per frame calculated?

Answer 3

time per frame = time per line x lines per frame

Question 4

How does spatial compounding work?

Answer 4

• 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.

Question 5

What are the advantages of spatial compounding?

Answer 5

• Multiple viewing angles produce uncorrelated artefact patterns reducing speckle, acoustic drop-out and clutter.
• Better margin definition and soft tissue contrast in breast imaging.

Question 6

What is frequency compounding?

Answer 6

• 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.

Question 7

How does coded excitation improve penetration without increasing acoustic output?

Answer 7

• 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.

Question 8

How is the code embedded in the pulse in coded excitation?

Answer 8

• Phase of segment represent binary 1 or 0.
• Positive phase = 1
• Negative phase = 0

Question 9

How is the code extracted during coded excitation?

Answer 9

• 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.

Question 10

How can range lobes be reduced when using coded excitation?

Answer 10

• Range lobes can be cancelled out by using a second pulse that has a complimentary code.

Question 11

What are the advantages of using coded excitation?

Answer 11

• Good axial resolution despite long pulse lengths

- Improves SNR and so penetration

Question 12

What are the disadvantages of coded excitation?

Answer 12

• Halved frame rate

- Increased TI

Question 13

How does adaptive image processing reduce speckle and artefacts?

Answer 13

• 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.

Question 14

What is affected if the speed of sound in tissue is not 1,540 m/s?

Answer 14

• Focusing

- Measurements

Question 15

How does the Zonare SSC algorithm automatically detect the optimal speed of sound?

Answer 15

• 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.

Question 16

What improvements are gained using an optimal speed of sound?

Answer 16

• Improved Lateral Resolution
• Reduced Clutter
• Enhances Contrast

Question 17

What are the two types of ultrasound elastography? Which is qualitative and which is quantitative?

Answer 17

• Strain Imaging: Qualitative

- Shear Wave Imaging: Quantitative

Question 18

How does the shear wave velocity differ to the longitudinal wave velocity?

Answer 18

c_s=1-10 m/s

c_l= 1500 m/s

Question 19

What are the equations for the shear and longitudinal wave velocities?

Answer 19

c_l = (K/p)^1/2
c_s= (G/p)^1/2

K = bulk modulus
G = shear modulus
p = density

Question 20

What are the advantages and disadvantages of elastography?

Answer 20

+ 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.

Question 21

Briefly describe strain elastography,

Answer 21

• 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.

Question 22

Briefly describe Point Shear Wave (Acoustic Radiation Force Impulse, ARFI) Elastography.

Answer 22

• 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

Question 23

How does ARFI differ from true Shear Wave Elastography?

Answer 23

• 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.

Question 24

What are the limitations of “True” SWE?

Answer 24

• 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.