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Pulse-echo principle
send out a pulse (transmitting short pulses of sound waves into a medium) and wait for it to return (reflect off boundaries & internal structures within the body).
Frame
all scan lines next to each other make up an image.
Pathway for ultrasound image
piezoelectric element (ceramic inside the transducer) → vibrates to create sound wave → travels into the body → sound hits a reflector in the body → causes an echo and returns to the transducer → image formed.
Grayscale
2D, B-mode.
M-mode
motion mode.
Doppler
uses high frequency sound waves to visualize blood flow in vessels.
PW
PW spectral doppler, color doppler, power doppler.
CW
CW spectral doppler.
Depth
sets the field of view of image; shallow vs deep.
Zoom
makes all or part of an image bigger.
Overall gain too low
whole image too dark.
Overall gain too high
whole image too bright.
Time Gain Compensation (TGC)
changes the brightness of the image only at certain depths.
Higher frequency
superficial structures; prettier pictures but worse penetration.
Lower frequency
deeper structures; not as pretty picture but able to penetrate.
ALARA
as low as reasonably achievable.
Echogenicity
how dark or bright a particular area appears.
Hyperechoic
relatively brighter than surrounding tissue.
Hypoechoic
relatively darker than surrounding tissue.
Anechoic
black; without echoes.
Isoechoic
indistinguishable from background.
Echogenic
creates echoes.
Smaller → larger units
move LEFT.
Larger → smaller units
move RIGHT.
Sound
a pressure wave created by mechanical action.
Sound is a mechanical, longitudinal wave
Soundwaves carry ENERGY; they vibrate forward & back.
Sound cannot travel in a vacuum
needs a medium through which to travel.
Sound travels in a straight line
Longitudinal wave
sound beam is parallel to direction of particle motion.
Acoustic variables
pressure, density & particle motion.
Variables go through cycles
of increase and decrease as wave travels.
Waves are made up of cycles
cycles of increase & decrease.
In-phase waves
the peaks and troughs overlap each other on both waves.
Create constructive interference
come together create one big wave.
Amplitude
strength of the wave.
Out-of-phase wave
the peak of one wave sits on the trough of the other and vice versa.
Create destructive interference
cancel each other out.
Source
machine/transducer.
Medium
soft-tissue; what sound is traveling through.
Frequency (f)
# of cycles per second.
Hertz (Hz)
Operating frequency (f) of transducer
CANNOT be changed by operator.
Determines by thickness of element
Thicker element = lower frequency if thickness of an element does not change, then frequency does not change!
Thinner frequency
Higher frequency
Infrasound
<20 Hz
Audible sound
20-20,000 Hz
Ultrasound
>20,000 Hz
Medical diagnostic ultrasound
2-20 MHz
Penetration
Ability to image deeper
Resolution
Pretty picture
Period (T)
Time it takes for one cycle to occur
Propagation speed
At what speed does the sound move through a medium
Stiffness
Ability of an object to resist compression
Density
Relative weight of material
Propagation speed of sound through soft tissue
1540 m/s
Wavelength
The distance over which one cycle occurs
Impedance (z)
Acoustic resistance to sound in a medium
Impedance mismatch
Difference between the impedances of two different media
Pulsed-wave (PW) ultrasound
Multiple cycles are sent into the tissue at one time
Pulse Repetitions Frequency (PRF)
The # of pulses produced in one second
Pulse Repetition Period (PRP)
Time of the pulse including the dead/listening time
Pulse Duration (PD)
The time of the pulse NOT including the dead/listening time
Duty Factor (DF)
Ratio (%) of transmission time to the total time
Spatial Pulse Length (SPL)
the length of one pulse
Pulse Repetition Frequency (PRF)
# of pulses in one second
Depth ambiguity
if machine sends out new wave before it receives the last one, it loses track of where the pulse came from
Continuous wave ultrasound
Must have two transducer elements (one to continuously send & one to continuously receive)
Intensity
What describes the strength of the beam
Power
What describes the strength of the beam
PD = nf
A formula relating pulse duration to frequency
DF=PD (s)PRP (ms)
A formula relating duty factor to pulse duration and pulse repetition period
Typical value of PRF
4,000 - 15,000 Hz
Typical value of PD
0.1 - 0.15 us
Typical value of SPL
0.1 - 2.5 mm
PRP is directly related to depth
As depth increases, PRP increases
PRF inversely related to depth
Longer the depth = slower (slower) the PRF
Duty factor is unitless
DF is directly related to PD and inversely related to PRP
CW: DF = 100%
Continuous wave ultrasound has a duty factor of 100%
PW: DF = < 1%
Pulsed wave ultrasound has a duty factor less than 1%
What happens to the length of pulse if PD is decreased?
PD decreases, SPL decreases
What happens to SPL if frequency is increased?
SPL decreases
Strength of the shock to the element
The voltage used to shock the element, the amplitude.
Strength of driving voltage
The voltage applied to create the shock.
Big shock
High amplitude.
Weak shock
Low amplitude.
Power units
Watts (W) or milliwatts (mW).
Intensity formula
Intensity = power / area (I = PA).
Intensity units
W/cm2 or mW/cm2.
Stronger shock
Means stronger wave (higher amplitude).
Range Equation
d=ct²; t = round trip time.
Depth for soft tissue
d=0.77t.
ROUND-TRIP TIME
If solving for depth & have ROUND-TRIP TIME, you do nothing.
ONE-WAY TIME
If solving for depth & have a ONE-WAY TIME, you multiply by 2.
A-Mode
Amplitude mode; height of spikes represents the strength of the reflector.
B-Mode
Brightness mode; the brightness of the displayed dots corresponds to signal strength.
Sector Image Shape
Pie-shaped; narrow near field of view made by sector probe.
Linear Image Shape
Rectangular shape; image same width in near field as far field made by linear sequenced array.
Curvilinear Image Shape
Wide near field of view; wider than sector made by a curvilinear transducer.
Vector Image Shape
Flat top; trapezoidal shape made by a linear transducer or vector phased array.
Transducers
Any device that converts one form of energy into another.