Ultrasound 2

Question 1

interactions with ultrasound

Answer 1

• attenuation
• scattering
• transmission
• reflection
• refraction
• diffraction
• absorption

Question 2

transmission

Answer 2

occurs when US wave travels through tissue without any reflection/ scattering

Question 3

when is transmission applied

Answer 3

at early stages of US development in stand-off pads/water bags for superficial scanning before invention of higher freq transducers

Question 4

when does reflection occur

Answer 4

when an ultrasound wave encounters an interface between two tissues of different acoustic properties (eg different acoustic impedance)

Question 5

what does the percentage of beam reflected back at tissue depend on

Answer 5

• the tissue acoustic impedance
• beam angle of incidence

Question 6

what does the strength of reflected signal depend on

Answer 6

difference in acoustic impedance at the interface between tissues

Question 7

the higher the angle of incidence …

Answer 7

the less amount of reflected sound

Question 8

what are the two types of reflection

Answer 8

• specular reflectors
• diffuse reflectors

Question 9

what are specular reflectors

Answer 9

they reflect ultrasound beam very efficiently in a defined direction

Question 10

conditions of a specular reflector

Answer 10

• surface of tissue must be smooth
• size of tissue is more than wavelength of beam

Question 11

what should the incident and reflected angle be in specular reflection

Answer 11

they should be equal

Question 12

what happens is sound strikes a boundary at 90 degrees

Answer 12

it will be reflected straight back to the probe

Question 13

the bigger the difference between the tissues acoustic impedance …

Answer 13

the more will be reflected

Question 14

when does scatter/ diffuse reflection occur?

Answer 14

• when US meets irregular/ uneven surface
• size of tissue less than wavelength of US beam
• sound waves scattered in all different directions
• produce low amplitude echo that affects image quality

Question 15

what is refraction

Answer 15

the bending of sound waves as they pass from one medium to another

Question 16

when will the beam change direction

Answer 16

when US hits angle other than 90 degrees and there is difference in speed of sound between tissues

Question 17

how does wavelength change in refraction

Answer 17

it changes to accomodate a new velocity in second medium

Question 18

what does refraction cause

Answer 18

• artifacts
• spatial distortion

Question 19

what causes refraction

Answer 19

change in acoustic impedance across the boundary, degree of refraction depends on angle at which sound waves strike boundary and differences in acoustic impedance between two media

Question 20

clinical importance of attenuation

Answer 20

it influences depth in tissue from which useful information can be obtained
- in turn affects transducer selection

Question 20

what is diffraction

Answer 20

as US beam travels further from probe through tissue, it will diverge due to effect of diffraction
- energy becomes widespread which reduces its density

Question 20

what is an US artefact

Answer 20

• does not correspond to actual position in tissue
• demonstrates brightness that is not indicative of it’s properties

Question 20

the more echoes that return to the probe ..

Answer 20

• better attenuation
• better image

Question 21

acoustic shadowing

Answer 21

structures in body can cause shadowing/ produce dark area
- can be caused by reflection/ attenuation

Question 22

how is acoustic shadowing caused

Answer 22

when transducer encounters a structure that strongly reflects or absorbs sound waves that cause structure to appear dark
- sound waves can’‘t penetrate through a shadow

Question 23

what is acoustic enhancement

Answer 23

structures that are less absorptive/ reflected transmit more US through them

Question 24

what is the bladder used for in US

Answer 24

a window for better visualisation of pelvic structures
- helps transmit US waves more effectively

Question 25

what is reverberation

Answer 25

occurs between 2 parallel surfaces and displayed as multiple reflections ‘stacked’ on image
- sound has bounced between 2 surfaces repeatedly