Elastography

Question 1

What is the difference between diseased tissue and surrounding tissue?

Answer 1

The diseased tissue is typically stiffer

Question 2

What is elastography?

Answer 2

A term for methods that image stiffness (on basis of palpation)

Question 3

How is the stiffness estimated?

Answer 3

By applying a force (stress) and measuring the relative displacement (strain)

Question 4

What is the basic principle of strain elastography?

Answer 4

1. Apply a force (stress): compressing using transducer

2.Measure the displacement (strain): comparing A-mode scan lines before and after deformation (finding Δt)

3. Estimate the Young’s Modulus (stiffness): requires knowledge of the applied force

Question 5

What occurs after the tissue displacement is measured?

Answer 5

The displacement (time shift Δt) is converted to strain by taking the derivative with respect to depth and an image is displayed

Question 6

What does strain represent?

Answer 6

Relative deformation which is normalised: gives measure of mechanical properties relative to surrounding medium

soft objects: high strain (red)

hard objects: low strain (blue

Question 7

What decreases with depth?

Answer 7

Axial stress and so does strain (inclusions closer to the transducer will deform more relative to deeper objects with the same stiffness)

Question 8

What does deformation depend on?

Answer 8

Stiffness and Poisson’s ratio

Question 9

What does a compression in one direction lead to?

Answer 9

An expansion in another

Question 10

How can we find an image of stiffness (not strain)?

Answer 10

Strain values are compared to a reference position in the image at the same depth

Question 11

What does radiation force based elastography use?

Answer 11

Tissue displacement caused by acoustic radiation force (momentum transfer due to absorption)

Question 12

What does higher intensity push pulse generate?

Answer 12

Radiation force which causes displacement

Question 13

What is the frequency of the shear wave response of push pulse?

Answer 13

10 - 500 Hz which travels at 1 -10 m/s in soft tissue

Question 14

How is the acoustic radiation force impulse found?

Answer 14

Monitor on axis change in displacement over time due to push pulse

Question 15

How does shear wave elasticity imaging work?

Answer 15

Send a single push pulse and monitor shear wave speed based on time of flight using either multiple tracking locations (left) or multiple push pulses (right)

Extract shear wave speed from difference in arrival times

Question 16

What is c_estimated?

Answer 16

track beam spacing/arrival time difference

Question 17

What is result of shear wave elasticity imaging?

Answer 17

Often only a single value is given as the flight assumption is only valid over limited axial depth

Question 18

What does the shear wave elasticity imaging measure?

Answer 18

The velocity of generated shear wave

Question 19

What is the shear modulus, G?

Answer 19

G = shear stress/ shear strain

Question 20

What is the Young’s modulus in isotropic material?

Answer 20

E = 2G (1 + v)

v = Poisson’s ratio

Question 21

How are the different modes of ultrasound elastography classified?

Answer 21

Origin of force:

external , internal (acoustic radiation force)

Type of applied force:

quasi-static (compression of skin surface)
harmonic or impulsive

The type of monitoring parameter:

tissue displacement due to force
velocity of a shear wave generated by force
on-axis or off-axis compared to the applied force