Ultrasound Physics Midterm 1-7

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140 Terms

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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).

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Frame

all scan lines next to each other make up an image.

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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.

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Grayscale

2D, B-mode.

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M-mode

motion mode.

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Doppler

uses high frequency sound waves to visualize blood flow in vessels.

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PW

PW spectral doppler, color doppler, power doppler.

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CW

CW spectral doppler.

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Depth

sets the field of view of image; shallow vs deep.

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Zoom

makes all or part of an image bigger.

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Overall gain too low

whole image too dark.

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Overall gain too high

whole image too bright.

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Time Gain Compensation (TGC)

changes the brightness of the image only at certain depths.

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Higher frequency

superficial structures; prettier pictures but worse penetration.

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Lower frequency

deeper structures; not as pretty picture but able to penetrate.

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ALARA

as low as reasonably achievable.

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Echogenicity

how dark or bright a particular area appears.

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Hyperechoic

relatively brighter than surrounding tissue.

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Hypoechoic

relatively darker than surrounding tissue.

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Anechoic

black; without echoes.

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Isoechoic

indistinguishable from background.

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Echogenic

creates echoes.

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Smaller → larger units

move LEFT.

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Larger → smaller units

move RIGHT.

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Sound

a pressure wave created by mechanical action.

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Sound is a mechanical, longitudinal wave

Soundwaves carry ENERGY; they vibrate forward & back.

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Sound cannot travel in a vacuum

needs a medium through which to travel.

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Sound travels in a straight line

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Longitudinal wave

sound beam is parallel to direction of particle motion.

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Acoustic variables

pressure, density & particle motion.

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Variables go through cycles

of increase and decrease as wave travels.

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Waves are made up of cycles

cycles of increase & decrease.

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In-phase waves

the peaks and troughs overlap each other on both waves.

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Create constructive interference

come together create one big wave.

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Amplitude

strength of the wave.

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Out-of-phase wave

the peak of one wave sits on the trough of the other and vice versa.

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Create destructive interference

cancel each other out.

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Source

machine/transducer.

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Medium

soft-tissue; what sound is traveling through.

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Frequency (f)

# of cycles per second.

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Hertz (Hz)

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Operating frequency (f) of transducer

CANNOT be changed by operator.

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Determines by thickness of element

Thicker element = lower frequency if thickness of an element does not change, then frequency does not change!

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Thinner frequency

Higher frequency

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Infrasound

<20 Hz

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Audible sound

20-20,000 Hz

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Ultrasound

>20,000 Hz

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Medical diagnostic ultrasound

2-20 MHz

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Penetration

Ability to image deeper

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Resolution

Pretty picture

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Period (T)

Time it takes for one cycle to occur

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Propagation speed

At what speed does the sound move through a medium

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Stiffness

Ability of an object to resist compression

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Density

Relative weight of material

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Propagation speed of sound through soft tissue

1540 m/s

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Wavelength

The distance over which one cycle occurs

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Impedance (z)

Acoustic resistance to sound in a medium

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Impedance mismatch

Difference between the impedances of two different media

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Pulsed-wave (PW) ultrasound

Multiple cycles are sent into the tissue at one time

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Pulse Repetitions Frequency (PRF)

The # of pulses produced in one second

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Pulse Repetition Period (PRP)

Time of the pulse including the dead/listening time

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Pulse Duration (PD)

The time of the pulse NOT including the dead/listening time

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Duty Factor (DF)

Ratio (%) of transmission time to the total time

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Spatial Pulse Length (SPL)

the length of one pulse

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Pulse Repetition Frequency (PRF)

# of pulses in one second

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Depth ambiguity

if machine sends out new wave before it receives the last one, it loses track of where the pulse came from

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Continuous wave ultrasound

Must have two transducer elements (one to continuously send & one to continuously receive)

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Intensity

What describes the strength of the beam

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Power

What describes the strength of the beam

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PD = nf

A formula relating pulse duration to frequency

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DF=PD (s)PRP (ms)

A formula relating duty factor to pulse duration and pulse repetition period

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Typical value of PRF

4,000 - 15,000 Hz

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Typical value of PD

0.1 - 0.15 us

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Typical value of SPL

0.1 - 2.5 mm

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PRP is directly related to depth

As depth increases, PRP increases

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PRF inversely related to depth

Longer the depth = slower (slower) the PRF

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Duty factor is unitless

DF is directly related to PD and inversely related to PRP

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CW: DF = 100%

Continuous wave ultrasound has a duty factor of 100%

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PW: DF = < 1%

Pulsed wave ultrasound has a duty factor less than 1%

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What happens to the length of pulse if PD is decreased?

PD decreases, SPL decreases

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What happens to SPL if frequency is increased?

SPL decreases

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Strength of the shock to the element

The voltage used to shock the element, the amplitude.

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Strength of driving voltage

The voltage applied to create the shock.

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Big shock

High amplitude.

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Weak shock

Low amplitude.

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Power units

Watts (W) or milliwatts (mW).

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Intensity formula

Intensity = power / area (I = PA).

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Intensity units

W/cm2 or mW/cm2.

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Stronger shock

Means stronger wave (higher amplitude).

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Range Equation

d=ct²; t = round trip time.

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Depth for soft tissue

d=0.77t.

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ROUND-TRIP TIME

If solving for depth & have ROUND-TRIP TIME, you do nothing.

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ONE-WAY TIME

If solving for depth & have a ONE-WAY TIME, you multiply by 2.

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A-Mode

Amplitude mode; height of spikes represents the strength of the reflector.

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B-Mode

Brightness mode; the brightness of the displayed dots corresponds to signal strength.

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Sector Image Shape

Pie-shaped; narrow near field of view made by sector probe.

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Linear Image Shape

Rectangular shape; image same width in near field as far field made by linear sequenced array.

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Curvilinear Image Shape

Wide near field of view; wider than sector made by a curvilinear transducer.

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Vector Image Shape

Flat top; trapezoidal shape made by a linear transducer or vector phased array.

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Transducers

Any device that converts one form of energy into another.