US: Instrumentation

Question 1

What phenomenon is responsible for the production of ultrasound waves in a transducer?

Answer 1

Piezoelectric effect.

Question 2

Briefly explain the Piezoelectric effect.

Answer 2

Some crystals create a potential difference when a pressure is applied.
Conversely, if a potential difference is placed across the crystal they will expand.

Question 3

How is the pulse length from the transducer related to the bandwidth of the emitted ultrasound beam?

Answer 3

Bandwidth is inversely proportional to the pulse duration. An infinitely long pulse would be needed to create a single frequency ultrasound beam.

Question 4

What are the basic components of an ultrasound transducer?

Answer 4

Electrodes
Piezoelectric crystal
Matching layer
Backing layer
Lens

Question 5

What is the function of the electrodes in an Ultrasound transducer?

Answer 5

Create the potential difference required to stimulate the piezoelectric crystal.

Question 6

What is the function of the Piezoelectric crystal in an Ultrasound transducer?

Answer 6

Creates the sound wave when a potential difference is applied.

Question 7

What is the function of the matching layer in an Ultrasound transducer?

Answer 7

Allows reflection between within the matching layer to add up constructively when entering the lens, so reduces loss in signal.

Question 8

What is the function of the backing layer in an Ultrasound transducer?

Answer 8

Reduces “ringing” of crystal after the potential difference is applied so that the pulse repetition frequency can be increased, and the length of the ultrasound pulse is reduced increasing axial resolution.

Question 9

What is the function of the lens in an Ultrasound transducer?

Answer 9

Focusses the ultrasound beam to a narrow point so that the slice thickness is reduced.

Question 10

What determines the axial resolution of an ultrasound transducer?

Answer 10

The pulse duration.

Question 11

What determines the lateral resolution of an ultrasound transducer?

Answer 11

The beam width at that depth.

Question 12

How is electronic focusing performed in ultrasound transducers?

Answer 12

Each array element has a different time delay added to it, with the largest being in the middle.
The delay in the emission of the wave from the array element causes the superposition of the waves to coincide as a focused wave.
On reflection the same time delay holds back the signal from the individual elements by the required amount of time so that the sum together producing an image.

Question 13

Explain how time-gain compensation works.

Answer 13

The gain can be individually selected and applied at different times after the initial pulse has been emitted. This means that the signal from different depths of the tissue can be individually increased or decreased depending on the clinical relevance.
TGC is generally used to increase signal from deeper within the tissue, while reducing the signal from the superficial tissue.

Question 14

What is the equation relating the transmitted and received frequencies from doppler ultrasound?

Answer 14

f(r) = f(t)2(v/c)cos(0)
v is the velocity of the object
c is the speed of sound in tissue (1540m/s)
cos(0) is the angle between the ultrasound probe and the direction of movement of the object.