Ultrasound Imaging

Question 1

What is the typical frequency of ultrasounds used in practice?

Answer 1

Typically 5-10 MHz

Can be 2-18 MHz

Question 2

How do sound waves differ from X-rays?

Answer 2

Need a material to travel through
Velocity depends on material
Have properties of a wave including reflection, refraction, interference

Question 3

How are ultrasounds produced and what is this process called?

Answer 3

Piezoelectric effect
Voltage applied across crystal - crystal deforms due to its piezoelectric properties - emission of high-frequency sound wave

Question 4

What is acoustic impedance?

Answer 4

Acoustic impedance = density of tissues x speed of sound in tissue

Question 5

What affects the proportion of reflection of ultrasound waves in a patient?

Answer 5

Depends on different in acoustic impedance

Relatively little reflection at soft tissue boundaries / larger % at soft tissue/bone interface

Question 6

What is the difference between specular and non-specular reflection?

Answer 6

Specular = beam hits large smooth surface
Non-specular = beam hits small structures e.g. inside of liver, re-radiated in all directions (weak echoes), gives texture to organs

Question 7

How are ultrasound signals received and used to display an image?

Answer 7

Sound waves reflected from various acoustic interfaces within the body - the echoes deform the crystal, resulting in production of electrical signals - electrical signals displayed as image on screen

Question 8

What are two display modes of ultrasound images?

Answer 8

B mode (brightness)
M mode (motion)

Question 9

Describe the B mode of image display.

Answer 9

Images a slice through the patient, image built up from lots of lines
Brightness depends on amplitude of signal, position depends on time for signal to return
See movement in ‘real time’
Need to scan organs in >1 plane

Question 10

Describe the M mode of image display.

Answer 10

Used in cardiac work
B mode image used to position a single line, movement of points along line followed
Image displayed as position vs. time
Continually updated, giving trace of movement

Question 11

How is ultrasound imaging carried out?

Answer 11

Choose area of body overlying region of interest, avoiding bone / gas where possible
Clip hair
Clean skin (surgical spirit may damage transducer)
Apply liberal quantities of acoustic gel
Place transducer over region of interest

Question 12

What 3 things should we consider when choosing a transducer?

Answer 12

Type
‘Footprint’
Frequency

Question 13

What are the 3 types of transducers?

Answer 13

Phased array
Linear array
Microconvex/convex

Question 14

Describe a phased array transducer.

Answer 14

Beam is steered electronically
Cone-shaped image
Easy to manipulate
Small contact area
Wide field at depth

Question 15

Describe a linear array transducer.

Answer 15

Multiple elements
Triggered in groups
Rectangular image
Large contact area
Large field of view near skin - good for superficial structures

Question 16

Describe microconvex/convex transducers.

Answer 16

Elements arranged in a curve
Wider cone-shaped image
Easy to manipulate
Small contact area
Wide field at depth

Question 17

Why is a smaller wavelength preferred for ultrasound?

Answer 17

Velocity = frequency x wavelength
Velocity is constant within soft tissues
As frequency increases, wavelength decreases
Better resolution if wavelength and therefore pulse length is smaller

Question 18

Why can we not just use the smallest wavelength possible for ultrasounds?

Answer 18

High frequency gives good image resolution
But sound attenuation is proportional to frequency
So sound does not penetrate so far into body

Question 19

What are the dis/advantages of high frequency ultrasounds?

Answer 19

7.5-18+ MHz
Good resolution
Cannot image deeper structures in larger animals

Question 20

What are the dis/advantages of low frequency ultrasounds?

Answer 20

2.5-5 MHz
Can image deeper structures and larger animals
Poorer resolution