Ultrasound 2 Flashcards
interactions with ultrasound
- attenuation
- scattering
- transmission
- reflection
- refraction
- diffraction
- absorption
transmission
occurs when US wave travels through tissue without any reflection/ scattering
when is transmission applied
at early stages of US development in stand-off pads/water bags for superficial scanning before invention of higher freq transducers
when does reflection occur
when an ultrasound wave encounters an interface between two tissues of different acoustic properties (eg different acoustic impedance)
what does the percentage of beam reflected back at tissue depend on
- the tissue acoustic impedance
- beam angle of incidence
what does the strength of reflected signal depend on
difference in acoustic impedance at the interface between tissues
the higher the angle of incidence …
the less amount of reflected sound
what are the two types of reflection
- specular reflectors
- diffuse reflectors
what are specular reflectors
they reflect ultrasound beam very efficiently in a defined direction
conditions of a specular reflector
- surface of tissue must be smooth
- size of tissue is more than wavelength of beam
what should the incident and reflected angle be in specular reflection
they should be equal
what happens is sound strikes a boundary at 90 degrees
it will be reflected straight back to the probe
the bigger the difference between the tissues acoustic impedance …
the more will be reflected
when does scatter/ diffuse reflection occur?
- 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
what is refraction
the bending of sound waves as they pass from one medium to another
when will the beam change direction
when US hits angle other than 90 degrees and there is difference in speed of sound between tissues
how does wavelength change in refraction
it changes to accomodate a new velocity in second medium
what does refraction cause
- artifacts
- spatial distortion
what causes refraction
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
clinical importance of attenuation
it influences depth in tissue from which useful information can be obtained
- in turn affects transducer selection
what is diffraction
as US beam travels further from probe through tissue, it will diverge due to effect of diffraction
- energy becomes widespread which reduces its density
what is an US artefact
- does not correspond to actual position in tissue
- demonstrates brightness that is not indicative of it’s properties
the more echoes that return to the probe ..
- better attenuation
- better image
acoustic shadowing
structures in body can cause shadowing/ produce dark area
- can be caused by reflection/ attenuation
how is acoustic shadowing caused
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
what is acoustic enhancement
structures that are less absorptive/ reflected transmit more US through them
what is the bladder used for in US
a window for better visualisation of pelvic structures
- helps transmit US waves more effectively
what is reverberation
occurs between 2 parallel surfaces and displayed as multiple reflections ‘stacked’ on image
- sound has bounced between 2 surfaces repeatedly
