Ultrasound

Question 1

What is an A-scan used for? (amplitude)

Answer 1

Find information about the depth and size of organs within a patient’s body. Used before a b scan

Question 2

(Us) Describe what happens when an alternating voltage is applied to a PZT transducer so that ultrasound is produced.

Answer 2

The Pz electric crystals can also detect ultrasound. They are squashed when waves hit them, causing an alternating electric field to be created and an alternating current (if connected in a circuit).

Question 3

Under what conditions will maximum energy conversion into ultrasound occur?

Answer 3

maximum energy transfer when frequency of applied voltage = natural
mechanical vibration frequency of the transducer
causing resonance

Question 4

Why is transmission improved by a coupling medium such as oil or gel?

Answer 4

A form of impedance matching, where the coupling medium has an acoustic impedance similar to that of body tissue. This prevents ultrasound pulses from being reflected from the surface.

Question 5

Why do the amplitude of reflected pulses vary on an A-scan?

Answer 5

Amplitude of pulses is attenuated as it penetrates deeper (caused by absorption and dispersion)
amplitude of echo pulses depends on the proportions of ultrasound which are
reflected and transmitted at each surface or boundary or depends on the acoustic impedences of the two materials at the interface.

Question 6

(Us) What is the modification to the crystal assembly of the piezoelectric crystal which makes is vibrate and emit short pulses?

Answer 6

Heavily damp the crystals to produce short pulses and increases resolution of the device

Question 7

what the ideal value of the acoustic impedance for a gel?

Answer 7

1.7 × 10^6 or equals that of skin / tissue

Question 8

the basic physical principles behind the A-scan

Answer 8

transmitter produces short pulses
at internal boundary some reflected, rest transmitted to next boundary
reflected pulse received by probe and signal sent to oscilloscope
oscilloscope sweep started when pulse is first transmitted

Question 9

how the results of an A-scan used to find the size of an organ

Answer 9

time taken between pulses from front and back of organ

Question 10

physical properties of a material which determine its acoustic impedance

Answer 10

density of the material and

speed of sound in the material

Question 11

State what is meant by resolution and explain why the wavelength of the ultrasound determines the resolution of the image

Answer 11

Ability to distinguish between objects that are close together.
Smallest angle that objects can subtend the observer and be seen
Shortening the wavelength of the ultra sound will increase the resolution of the image will be because the diffraction at the material boundary is less.

Question 12

Describe how ultrasound is used to produce an image of different tissues within the body

Answer 12

Rransducer swept across surface of skin
Emits pulsed ultrasound signal
Reflected at boundaries where acoustic impedance changes
Time for pulse to return is measured
Depth of boundary calculated / position of boundary is plotted
Equation relating to establishing depth (1/2 ct)
Acoustic impedance is resistance to passage of sound through the medium
Causes attenuation of ultrasound
Causes reflection of sound at a boundary
Is needed in order to produce image
Reduced by use of gel on skin

Question 13

What is acoustic impedance?

Answer 13

Resistance to passage of sound through the medium