Chapter 17 Ultrasound I Flashcards
what are sound waves
a pressure disturbance that travel away from their source
how are US waves transmitted through tissue?
waves of alternating compression and rarefaction
US velocity
product of wavelength and frequency
frequency
cycles/s
number of oscillations at a fixed point along the wave in each second
measured in Hz (oscillation/s)
wavelength
distance between successive wave crests
period
reciprocal of frequency
time between successive oscillations
what are harmonic frequencies
integral multiples of a fundamental frequency
frequency of audible sound
15 Hz to 20 kHz
what instruments produce high vs low frequency?
small instrument = high frequency
large instrument = low frequency
US frequencies
> 20 kHz
infrasound frequencies
< 20 Hz
frequencies used for US in clinic
1-20 MHz
pros and cons of high vs low US frequency
low frequency = better penetration
high frequency = axial resolution
when are sound waves formed?
electrical energy is converted into mechanical energy to form a wave of varying pressure
what does US wavelength depend on?
material compressability
US wavelength in soft tissue at 1.5 MHz
1 mm in soft tissue
0.2 mm in air
3 mm in bone
how does frequency affect wavelength?
US wavelength decreases with increasing frequency
wavelenght at 15 MHz in soft tissue
0.1 mm
how long are the US pulses?
about 2 wavelengths long
what determines the axial resolution?
length of US pulse
resolution is the wavelength (i.e. half the pulse length)
does sound velocity depend on frequency?
NO
-since velocity is fixed, frequency and wavelength are inversely related
what does sound velocity depend on?
type of material
materials that are not highlt compressible have high sound velocity (bone)
compressible materials (air) have low sound velocities
average velocity of sound in soft tissue
1540 m/s
velocity of sound in fat
5% lower than it is in soft tissue
-leads to artifacts
velocity of sound in air
~ 343 m/s
how are relative intensities in US expressed?
dB
what is dB
log scale
negative dB = signal attenuation and vice versa
-10% is -10 dB
-1 % is -20 dB
-0.1 % is - 30 dB
-+20 dB is 100 fold increase
-doubling the intensity is + 3 dB
-halving the intensity is -3 dB
what is acoustic impedance, Z
product of density and sound velocity’
expressed in rayls
what has low acoustic impedance
air and lung
-low density and low sound velocity
what has high acoustic impedance
bone and piezoelectric crystal
-high density and high sound velocity
what does fraction of US reflected at interface depend on?
acoustic impedance of the two tissues on both sides of the interface
when there are big differences in impedance, most of the US energy is reflected
when the acoustic impedances are similar, most of the US is transmitted and echoes are weaker
specular vs non-specular reflections
specular = occur from large smooth surfaces
-angle of incidence is equal to angle of reflection
non-specular = occur from rough surfaces
-don’t contribute to US image because any echoes reaching the transducer are weak
what is US echo
specular reflection travelling back to a transducer
used to create US images
what must transmitted and reflected US intensities add up to?
unity
why is gel applied between the skin and the transducer?
tissue/air interfaces reflect 100% of the incident beam
gel displaces the air and minimizes the large reflections so US can be transmitted into patient
do bone/tissue interfaces reflect a large amount of the US intensity?
yes
can you image through air or bone?
No, they reflect too much of the US energy
fat/tissue interface- more reflection or transmission
mostly transmission
when does scattering occur?
when US encounters objects that are smaller than the US wavelength
what organs contain many scattering sites?
kidney
pancreas
spleen
liver
reflected intensity for different materials or tissues
air > 99%
lung 50 %
bone 40%
fat 0.8%
muscle < 0.1 %
what is speckle
scattering of US beam from small reflectors characteristic of the tissue structure
what is hyperechoic speckle
hypoechoic speckle
high scatter amplitude relative to background signal
low scatter intensity relative to background signal
what organs contain almost no scatter (show black)
bladder
cysts
what is refraction
change in direction of US beam when passing from one tissue to another
-due to differences in speed of sound in the two tissues
what changes with refraction?
frequency stays same but wavelength and speed velocity changes
angle of refraction
when velocity of sound in tissue 2 is > tissue 1, transmission angle is > than angle of incidence
why does refraction cause artifacts?
US machines assume straight line propagation
refraction results in artifacts
what kind of artifacts does refraction cause?
spatial distortions
straight sticks partially immersed in water appear bent because of diffraction of light that has different velocities in air and water
refraction can also result in shadows because the beam is deflected from its normal path
what is attenuation in US?
loss of US energy because of scattering and absorption
-absorbed sound energy is converted to heat
how is US energy attenuated?
-exponential
-proportional to frequency
-doubling the distance travelled in tissue doubles the attenuation
do fluids have low or high attenuation in US?
low
attenuation coefficient for clinical imaging
0.5 dB/cm/MHz
what organs have very high attenuation
lung and bone
what is a transducer?
device that converts from one form of energy into another
transducers in US
piezoelectric
convert electrical enegy into US and vice cersa
typically PZT (lead-zirconate-titanate)
explain the mechanics of the transducer
high frequency voltage oscillations are produced by scanner electronics and sent to the US transducer over so-axial cables
transducer crystals do not conduct electricity but each side is coated with a thin layer of silver which acts as electrodes
Non-conducting crystals change shape in resoonse to voltages at these electrodes
crystal shape changes increase and decrease the pressure in front of the transducer, producing US waves
when the crystal is subjected to pressure changes by the returning US echoes, the pressure changes are converted back into electrical energy signals
-voltage signals are returning echoes are transferred from the receiver to a computer, which creates US images
broad bandwidth transducers
generate more than 1 frequency
operators select the exam frequency
resonance frequency of transducer
frequency at which piezoelectric transducer is most efficient in converting electric energy to acoustic energy
-determined by piezoelectric element thickness
transducer crystal thickness
-usually 1/2 the wavelength at resonance
-high frequency transducers are thin (ex 0.1 mm at 7.5 MHz)
-low frequency transducers are thick (ex 0.5 mm at 1.5 MHz)
what kind of US is used in clinic?
pulsed
what is used to create the short pulses?
blocks of damping material placed behind the transducers reduce vibration to shorten pulses
dampened transducer vs non dampened
dampened generates broad range of frequencies
transducers without damping generate pure frequency
-pure frequencies produce very long pulses and vice versa
what is the impedance of the matching material on front surface of transducer?
intermediate between that of transducer and tissue
what is thickness of the matching layer?
1/4 the wavelength of sound in that material (quarter wave matching)
size of piezoelectric element
width is usually < 1/2 wavelength
vertical heights are several mm
what region is used for US imaging?
near field
fresnel zone
what is length of near field proportional to?
effective transducer size
doubling the transducer effective size will quadruple the length of the near field
where does the far field start?
where the near field ends
what is far field
US beam diverges and intensity falls of fast
Fraunhofer zone
-US imaging doesnt extent into far field
what are side lobes and what do they do
small beams of reduced intensity emitted at angles to the primary beam
-presence of side lobes can give rise to artifacts
-multi-element arrays may also have grating lobes, similar to side lobes, that also cause artifacts
what does focusing US beam do?
convergence and narrowing of the beam
improves lateral resolution
focusing with acoustic lens
use a transducer face that is concave
beam will be narrowed at pre-determined distance from transducer
placing a concave acoustic lense on the surface of the transducer will also accomplish this
focusing using a phased array
focal depth can be varied for each pulse using different patterns of element acivation
generate several images with different focal depths then cherry pick best parts of each pulse to create composite image
what is focal depth
point at which beam is at its narrowest
region over which beam is relatively narrow for focused US
focal zone
what is high intensity focused US
applies US energy to locally heat and destroy diseased tissue through ablation
-uses low frequencies to improve penetration
linear array
one line of sight’receive
receive echoes along single line
group of elements (rather than single element) increases near field
next line of sight is obtained by firing another group of elements that are displaced by one or two elements
complete frame is obtained by firing groups of elements from one end of the linear array to the other end
-128 to 256 elements
-rectangular FOV
-used in peds to visualize superficial structures
-limited FOV is determined by size of linear array
convex arrays
operates same way as linear aary; scan lines perpendicular to transducer surface
-naturally creates image with an arc
-trapezoidal FOV
-fits better on abdomen
-wider FOV than linear array
-lateral resolution < 1 mm
-however, beams separate at depth, creating gaps
-increasing gaps decreases lateral resolution
annular array
crystal is cut into rings like cross section of an onion
-focused by applying electrical pulse to each element in turn
-individual pulses combine to create a composite pulse, focused to a point at a specific depth
-focal point depth depends on time delay between electrical pulses applied to transducer elements
-varying delays permits US beam to be focused to different depths
-annular arrays allow US pulses to be focused in 2 dimensions instead of just one
-US beam produced by annular array transducers is symmetrical: produces thinner scan slice than other types of arrays
phased arrays
96 elements
have many small transducers: each can be pulsed independently
-varying the time delays to individual elements results in a beam at a set angle (steers beam direction electronically)
-US beam is swept through similar to a search light
-images are generated by detecting echoes along each line of sight
-data from multiple beams are put together to generate slices
-US images obtained with phased arrays originate from a single point
-use phased arrays when there is a limited acoustic window, like ribs
is electronic focusing or convex lens better?
electronic focusing introduces flexibility that improves image quality
but using electronic focusing will result in reduced frame rate, lower line density, or reduced FOV
how much US energy is reflect at fat/tissue interface?
< 1 %
where can electronic focusing be applied?
any array that has multiple elements (linear, convex, phased)
-only phased arrays steer the beam with electronic focusing
velocity of sound in air vs tissue
330 m/s vs 1540 m/s
high acoustic impedance material
ex. bone
has high density and high sound velocity
transucer thickness
half the wavelength
what will doubling the transducer element diameter do to the near zone distance?
will quadruple near zone distance
what does use of focusing in US do?
results in narrower beams, thus improving lateral resolution
width and height of individual elements in multi-element arrays
width of half a wavelength and height of a few mm
why are linear arrays used to image infants?
large FOV is not required
superficial regions will be more visible
where can you apply electronic focusing?
to any array that has multiple elements
