Ultrasound Interactions

Question 1

Reflection and Scattering back to the transducer produce?

Answer 1

RECIEVED SIGNAL

Question 2

What’s the formula for acoustic impedance?

Answer 2

Tissue density x speed of sound

Question 3

What is Acoustic Impedance (Z)?

Answer 3

Describes how much resistance an ultrasound beam encounters as it passes through a tissue.

Question 4

Specular reflections occur from large smooth surface, give an example.

Answer 4

Fetal skull, renal capsule

Question 5

What is acoustic Intensity?

Answer 5

Is the power per unit area. This has particular importance in measuring the effect of ultrasound in heating tissue.

Question 6

Range of intensities is measured in what scale?

Answer 6

Decibel Scale dB

Question 7

Give an example of specular reflection in practice.

Answer 7

The insonation beam should be parallel to the tissue to get accurate measurements. For example in Nuchal translucency, a horizontal NT needs to be achieved to visualise its border. The reflection received by the transducer is more resulting in proper visualisation of the NT and achieving accurate measurements.

Question 8

What are the interactions an ultrasound image is created from?

Answer 8

Ultrasound Image is created from Specular reflection, non-specular reflection and scattering.
The amplitude of the echoes depends on the strength of the scattering and depends on
1. Difference in Acoustic impedance for example, fat or soft tissue.
2. Difference in Sizes (scatterers, large difference in bone or small difference in blood.)
3. The orientation of the beam

Question 9

Describe Ultrasound Limitation on Bone.

Answer 9

Ultrasound is strongly attenuated by bone and gas due to almost total reflection of ultrasound. This causes shadowing and since no ultrasound beam gets transmitted it is difficult to see deeper structure through the bone or gas (bowel).

Refraction and attenuation also cause limitations in accuracy and quality of image. We can’t keep scanning deeper structure as the ultrasound beams keeps getting attenuated and scattered, the energy is loss and this degrades the ultrasound image.

Question 10

When does refraction occur?

Answer 10

Refraction occurs when the insonation beam is on an oblique angle. This can causes distortion of the true position of the target tissue producing a false image.

Question 11

Intensity of ultrasound beam decreases with depth due to:

Answer 11

1. Scatter
2. Reflection
3. Beam divergence
4. Absorption (heat)
5. Refraction

Question 12

What is absorption?

Answer 12

Incomplete transfer of mechanical energy from compression and refraction. Energy transferred to other energy states (e.g. heat) and not recovered.

Question 13

Describe how pulse length wave affects axial resolution.

Answer 13

Pulse length wave affects axial resolution because if you have a shorter wave length the reflected pulse are separated, the pulse will have enough time to be reflected back from the targets followed by another shorter pulses. However on longer pulses when they are reflected back they merged, due to this it’s difficult to see distinct edges. Therefore a higher frequency is better at axial resolution.