Ultrasound

Question 1

describe a sound wave

Answer 1

Sound is a longitudinal wave that requires a medium for propagation.

This wave consists of a series of compressions and rarefactions.

Question 2

What happens to medium particles with the soundwaves?

Answer 2

The particles in the medium simply oscillate back and forth as the sound wave propagates through the tissue

Question 3

The range of ultrasound frequnecies

Answer 3

2 - 10 MHz

Question 4

What does the higher the frequency mean?

Answer 4

the more superficial the wavelength travels into soft tissue

Question 5

What is acoustic impedance?

Answer 5

It is a measure of how “easy” it is for sound to pass through a medium

Question 6

What effect will a big difference in acoustic impedance e.g., tissue and air?

Answer 6

It will result in strong reflection of the soundwave

Question 7

Ultrasound is a wave motion. True or False?

Answer 7

True

Question 8

How does refraction of ultrasound obey similar laws to light?

Answer 8

Sound changes
velocity in going from one medium to another (wavelength changes).

Question 9

What does refraction result in?

Answer 9

The beam bends as it passes from one medium to another
Beam axis no longer lies beneath transducer axis. Usually does not cause significant artefact

Question 10

What is diffraction?

Answer 10

the bending of the ultrasound beam into the shadow of a
strong absorber
It occurs at the absorber edge e.g. gall stone
Can cause minor artefact

Question 11

What is the basis of ultrasound

Answer 11

Images are constructed by computing the time it takes for the beam to
travel from the transducer and return from a reflecting surface.

This gives us our depth data (based on the velocity of sound in tissue)

The magnitude of the echo modulates the brightness of the display

Question 12

What is a transducer?

Answer 12

A device which converts one form of energy to another form

Question 13

What is used for the ultrasound transducer?

Answer 13

Piezoelectric crystal

Question 14

How does this crystal work for the transducer?

Answer 14

By applying a voltage across the crystal the molecular dipoles
change orientation, changing the shape of the crystal.

The reciprocal effect also happens. By mechanically altering the
shape of the crystal a small voltage is generated across the crystal.
Thus the piezoelectric crystal can be used to generate and detect an ultrasound signal

Question 15

What two modes can the transducer have?

Answer 15

1. Transmit more
2. Receive mode

Question 16

What occurs in transmit mode?

Answer 16

a high pulsed voltage is applied to the transducer

Question 17

What occurs in receive mode?

Answer 17

the small voltage generated by deformation of the crystal
is detected and amplified

Question 18

What is the backing block of of the transducer?

Answer 18

resin/metallic powder composite which dampens reverberations in the crystal and absorbs
backward transmission of the ultrasound pulse

Question 19

Why is there a plastic matching layer positioned between the crystal and the skin in the transducer?

Answer 19

As there is a huge difference in acoustic impedance between the
transducer crystal and the tissues, it improves sound transmission

Question 20

What does the thickness of the matching layer depend on?

Answer 20

For optimum transmission the matching layer has a thickness of a quarter of the
wavelength

Question 21

What is the use of the acoustic insulator in the transducer?

Answer 21

An acoustic insulator such as rubberized cork separates the crystal and backing block from the casing. This prevents artefacts due to casing vibrations

Question 22

Why is ultrasound gel used?

Answer 22

to make good contact between the probe and the skin by eliminating the air layer. If we did not use ultrasound gel one would get almost 100% reflection at the transducer/air interphase

Question 23

What kind of wave is used in ultrasound?

Answer 23

Pulse wave!

Question 24

How many wavelengths should an ultrasound pulse have?

Answer 24

Ideally, 2-3 wavelengths

Question 25

“The deeper the area of interest in ultrasound, the ____ the spatial resolution is”

Answer 25

Worse

Question 26

What is pulse repetition frequency?

Answer 26

This is the number of pulses per second and is set by a master clock in the ultrasound unit.

Question 27

What drives the transducer?

Answer 27

an electrical pulse generator

Question 28

By increasing the PRF we increase ____ but shortens ____

Answer 28

Image information rate (Echoes received), shortens the receive time

Question 29

What is the image depth?

Answer 29

The time between pulses must be greater than the time for the “return trip” (travel of
pulse into tissue and time for to receive returning echo) which is equivalent to twice the
image depth

Question 30

What determines spatial resolution in ultrasound?

Answer 30

Spatial Pulse Length (SPL)

Question 31

What determines the maximum depth which can be imaged?

Answer 31

PRF

Question 32

The range of frequencies within a pulse is called…

Answer 32

a Bandwidth

Question 33

By increasing the SPL we _______ the bandwith

Answer 33

Decrease

Question 34

What is specular reflection?

Answer 34

occurs when the boundary between two structures is smooth. It is responsible for
generating the echoes that define organ boundaries

Question 35

Nonspecular reflection

Answer 35

occurs when the surface irregularities are similar in size
to the beam’s wavelength. echoes are must smaller than in specular reflection

Question 36

How does scatter occur in ultrasound?

Answer 36

When sound waves encounter structures that are much smaller than their wavelength they are scattered more or less equally in all directions.

The scattered echoes interfere and produce a” Speckle” appearance which is characteristic of the tissue structure, could be mistook for pathology

Question 37

What is an important artefact in ultrasound?

Answer 37

Mirror artefact

Question 38

What is mirror artefact?

Answer 38

When the reflecting surface meets the beam at a large angle of
incidence (e.g. diaphragm)
Part of the beam is not reflected directly back to the transducer. This can result in a mirror image of a structure