Topic 12: Elastography

Question 1

There are different modes of elastography. These can be classified by the following parameters

Answer 1

Origin of the force Time variation of the force Monitored parameter

Question 2

what are the two main steps required in order to carry out a strain elastogram?

Answer 2

1. Apply a force (stress) 2. Measure the displacements (stress)

Question 3

What is radiation force based elastography?

Answer 3

The tissue is displaced using acoustic radiation force.

Question 4

The acoustic radiation force impulse imaging?

Answer 4

In ARFI imaging, the on-axis response to the push pulse is monitored. The displacement is measured by comparing A-mode scan lines taken before and after the push pulse is applied.

Question 5

Shear wave elasticity imaging?

Answer 5

In SWEI, the shear speed is monitored based on time of flight using either multiple tracking locations or multiple push pulses. The shear sound speed c_s is related to the shear modulus by

Question 6

What is the advantage of acoustic radiation force impulse elastography compared to an elastogram?

Answer 6

The advantage over a normal elastogram is that the applied force does not decay with distance, as the location of the push pulse can be localised using electronic focusing.

Question 7

Supersonic shear imaging?

Answer 7

It is similar to SWEI however instead of a single push pulse it uses a multitude of push pulses with an increasing focal depth.

Question 8

What are the advantages of shear wave imaging (SSI and SWEI) compared to an elastogram?

Answer 8

1) The image is quantitative 2) Additional information inside hard lesions can be obtained, as the shear wave penetrates into the inclusions.

Question 9

Ultrasound Imaging

Answer 9

• Real-time images of soft tissue anatomy
• High spatial resolution
• Deep penetration
• Physiological information via measurement of blood flow

Question 10

Disadvantage s of ultrasound?

Answer 10

Weak contrast for certain important targets - e.g. microvasculature

Limited specificity: weak sensitivity to chemical differences

Question 11

Imaging using light - or optical imaging- has many benefits and disadvantages. Outline the main benefits and disadvantages of optical imaging for clinical use.

Answer 11

Pros:

1. High intrinsic contrast based upon optical absorption and scattering
2. Spectroscopic specificity - chemical information
3. Functional imaging of physiological parameters e.g. blood oxygenation

Cons:

1. Imaging depth/spatial resolution severely limited by strong optical scattering

Question 12

What is photoacoustic tomography?

Answer 12

Short laser pulse delivered through transparent US sensor

Scattered photons diffusely illuminate target region

Photons absorbed by tissue chromophores results in a 3D distribution of absorbed energy

Absorbed energy causes pressure to rise

Pressure rise propagates out as acoustic waves

Acoustic waves detected by US sensor array

Question 13

What timescale does the absorption of light occur over?

Answer 13

femtoscale (10^-15)

Question 14

What is thermal confinement?

Answer 14

If the laser pulse duration is much shorter than the thermal relaxation time, of the heated region then heat diffusion can be neglected as everything happens too fast for the heat to have diffused any significant distance.

Question 15

What is pressure confinement

Answer 15

f the laser pulse duration, tp, is much shorter than the time is takes the acoustic wave to propagate across the heated region, then the pressure will build up during the laser pulse.

Question 16

What are these conditions with respect to pressure confinement?

Answer 16

The first is called the initial acoustic pressure distribution.

The second is equivalent to stating that the initial particle velocity is zero.