BME 513 Ultrasound Definitions

get a hint

hint

1st Principle of Ultrasound

1st Principle of Ultrasound

Ultrasound is a dpeth-based modality that uses the time of flight to determine depth of the anatomy being imaged

2nd Principle of Ultrasound

Ultrasound images come from reflections on medium boundaries, with high reflectivity surfaces having higher intensity reflections

Use of acoustic coupling gel in ultrasound

necessary to prevent air pockets between ultrasound transducer and skin so there are no high impedance boundaries (allows direct contact with the skin)

Attenuation

loss in ultrasound intensity as it travels through a medium, either through acoustic scattering or through attenuation

3rd Principle of Ultrasound

Greater imaging depth and higher frequency results in greater attenuation of the signal

State the ultrasound phenomena in the indicated area (specular reflection, transmission, scattering, enhancement, shadowing)

A: Specular Reflection

B: Transmission

C: Scattering

State the ultrasound phenomena in the indicated area (specular reflection, transmission, scattering, enhancement, shadowing)

A: Transmission

B: Scattering

C: Enhancement

D: Scattering (NOT SHADOWING)

State the ultrasound phenomena in the indicated area (specular reflection, transmission, scattering, enhancement, shadowing)

A: Scattering

B: Shadowing

C: Transmission

State the ultrasound phenomena in the indicated area (specular reflection, transmission, scattering, enhancement, shadowing)

A: Scattering

B: Transmission

C: Specular Reflection

D: Shadowing

E: Shadowing

Ultrasound transducer thickness formula

thickness = wavelength/2 = 4000/(2*f)

Purpose of Damping Block in Ultrasound

• Placed behind transducer to absorb ultrasound

• Used to limit the signal’s spatial pulse length (SPL)

Spatial Pulse Length (SPL)

duration of emitted ultrasound signal

Axial resolution formula

• Resolution in the direction of the beam

• = SPL/2

Purpose of matching layer in Ultrasound

• minimizes difference in acoustic impedance between transducer and skin/tissue

• optimal thickness is 1/4 wavelength

Lateral resolution formula

• Resolution perpendicular to beam direction

• = (transducer diameter)/2 in the far-field

4 Components of ultrasound DAQ

1. Beam former

2. pulser: provides voltage to piezoelectric element

3. transmit/receive switch

4. receiver

Pulse-Echo Operation

• Basic mode of ultrasound with an initial pulse followed by a listening ‘echo’ phase

• Each pulse sequence produces one A-line of data

Pulse repetition frequency (PRF)

• Number of times transducer is pulsed per second

• inverse of PRP

Pulse repetition period (PRP)

• inverse of PRF

Duty cycle definition

fraction of time for which the ultrasound pulse is “on”

3 display modes of ultrasound

1. A-mode: displays 2D line plot of amplitude vs. distance

2. B-mode: displays 1D brightness vs. distance

3. M-mode: 2D plot of B-mode over time, typically used for moving organs

Dynamic Receive Focusing

change the time delay for received signals depending on imaging depth

Transmit Focusing

Outside transducers fire before the inside transducers for a central focus point

Types of 2D transducer arrays

1. Linear arrays: subset of elements fire at a time, produces a single A-line and shifts laterally

2. Phased arrays: elements activated at different times to steer the beam left/right

Derivation of Doppler Shift formula

Preferred doppler angle

30-60 degrees

Continuous doppler ultrasound

2 transducers used with one to transmit continuously and another for detection

Pulsed doppler ultrasound

• A single transducer in pulse-echo format but with longer SPL

• phase change between pulses measured is used to estimate doppler shift

Duplex scanning

combine 2D B-mode imaging with pulsed Doppler to measure flow rates

Color-Flow Imaging

• 2D visual display of moving blood superimposed on the image

  • Red flows toward transducer

  • Blue flows away from transducer

Spectral Doppler Interpretation

• spectrum of frequencies received by the Doppler ultrasound which may represent laminar or turbulent flow

Velocity based artifacts

1. speed propagation

2. refraction

Reflection based artifacts

1. reverberation

   1. comet tail

   2. ring down

2. Multipath reflection

3. side-lobes

4. ambiguity

Attenuation based artifacts

1. shadowing

2. enhancement

methods of image enhancement

1. spatial compounding

   • ultrasound obtained from multiple angles to produce image

2. Ultrasound contrast agents

   • microbubble contrast

3. Harmonic imaging

   • tune in receive to include harmonic frequencies

Acoustic power (ultrasound power) SPTA & SPPA

• spatial peak-temporal average intensity (SPTA)

  • indicates thermal ultrasound effects

• spatial peak-pulse average intensity (SPPA)

  • indicates bioeffects and cavitation risk

Thermal index

Ratio of acoustical power to raise tissue temperature by 1 degree Celsius

Mechanical index

estimates the likelihood of rarefractions resulting in bubble formation