Physics Of Ultrasound Flashcards

1
Q

Imaging modes

A
  • A-mode — amplitude mode
  • B-mode — brightness mode
  • brightness of image = strength of echo
  • depth = position

• M-mode — motion mode

  • B-mode vs time
  • x-axis —> time ; y-axis —> depth
  • brightness of reflector = strength of echo
  • best temporal resolution

• 2D grayscale

  • multiple M-mode scan line
  • multiple pulses from multiple crystals
  • real time 2D images on screen
  • 2D phased array

•3D imaging
- cone / pyramid of M-mode scan lines

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2
Q

Basic U/S Physics

A
  • sound = mechanical longitudinal wave
  • medium vibrates as sound travels L—>R
  • sound waves = compressions and rarefactions in the molecules of the medium
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3
Q

Time (seconds) —> sound source

A

•Period — time to complete a single cycle

  • period = 1 / frequency
  • similar to wavelength but a measure of time

•Frequency — number of cycles per second

  • frequency = 1 / period
  • u/s > 20 KHz
  • increased frequency —> better axial resolution but more attenuation

•Pulse duration — time to complete a single pulse

  • similar to SPL but a measure of time
  • # of cycles x period

•Pulse repetition period — time from beginning of one pulse to beginning of the next

  • PRP = pulse duration + listening time
  • deeper the structure —> longer the listening time —> longer the PRP

•Pulse repetition frequency — number of pulses per second

  • PRF = 1 / PRP
  • analogous to frequency
  • 1/2 PRF = Nyquist limit [max doppler shift before aliasing]
  • important for temporal resolution (higher PRF —> higher frame rate —> better resolution)
  • depth dependent since it includes listening time
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4
Q

Length (distance) —> sound source & medium

A
  • Wavelength — length of a single cycle
  • Spatial pulse length — length of a single pulse
  • important for axial resolution
  • 1/2 SPL = axial resolution
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5
Q

Strength —> sound source

A

•Amplitude — difference between average acoustic and max acoustic variable
-acoustic variables: pressure, density, temperature, distance

•Power — amount of work done per unit time
- proportional to amplitude^2

•Intensity — power per unit area

  • proportional to amplitude^2
  • most important measure for bio effects (thermal, cavitation)
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6
Q

Velocity —> medium only

A

•stiffness —> proportional to velocity

•density —> inversely proportional to velocity
- paradoxically sound usually travels faster in higher density material because they tend to be stiffer and stiffness differences are usually larger than density differences

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7
Q

Attenuation

A
  • As u/s travels through tissue the strength / amplitude decreases
  • higher frequency attenuated more than lower frequency
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8
Q

Impedance

A
  • acoustic resistance to sound traveling through a medium
  • Z = P x V [p = density , v = velocity]
  • determined by the medium only
  • important for reflection — with perpendicular incidence, reflection only occurs when there is different impedance
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9
Q

Transducers

A

•converts one form of energy to another
•piezoelectric crystals convert voltage to u/s and back to voltage
- voltage applied to crystals —> vibrations —> sound pulses —> sends out —> bounce & reflect back —> vibrations —> voltage difference —> image —> displayed on screen
•distance related to time
•brightness related to strength

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10
Q

Backing Material

A

•damping material
•decreases ringing —> short pulses —> decreases SPL —> improved axial resolution
•increases band width
•decreases Q factor (QF = RF / BW)
- unitless # that represents ability of machine to emit a ‘clean’ pulse w/ narrow BW
- imaging transducers have a low QF (high BW , short SPL)
- therapeutic transducers having a high QF
- short pulses have a broad range of frequency = high BW , low QF
•decreases sensitivity to reflected echoes

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11
Q

Matching Layer

A
  • larger the difference in Z between two media the larger the reflection at the interface of those media
  • has Z between mucosa and PZT
  • prevents a large reflection at interface of mucosa and probe
  • Z : PZT > matching layer > gel > mucosa
  • Z = velocity x density [V determined by stiffness & density]
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12
Q

Transducer Frequency

A

•different for CW and PW
•CW —> frequency = electrical frequency of the voltage
•PW —> determined by the u/s system
- crystal properties determine the resonant frequency (thickness & velocity)
- RF = V / 2T

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13
Q

Band Width

A
  • difference between highest and lowest frequency pulse
  • damping —> increase band width
  • shorter the SPL the higher the BW
  • QF = RF / BW …
  • RF = V / 2T …
  • QF = (V / 2T) / BW
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14
Q
Near Field (Fresnel Zone)
Far Field (Fraunhofer Zone)
Focal Zone (near the focus)
A

•Ln = r^2/lambda
Ln —> length ,
r —> crystal radius
Lambda —> wavelength
- larger crystal radius —> longer focal length
- higher frequency / lower wavelength —> longer focal length

•crystal diameter and divergence

  • crystal diameter decreases —> divergence increases
  • smaller d —> shorter focal length
  • smaller d —> more divergence

•crystal frequency and divergence

  • higher frequency / lower wavelength —> deeper focal length
  • higher frequency —> less divergence
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15
Q

Focusing

A

•Methods:

  • lens
  • curved PZT crystal
  • focusing mirror
  • electronic (used in TEE)

•Effects:

  • improved lateral resolution
  • shorter focal zone
  • shallower focal depth = near zone
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16
Q

Resolution — ability to accurately image structures

A

•Spatial (LARD, LATA, Elevational) — ability to accurately create images of small structures in their correct anatomical position
-LARD resolution — distinguish 2 structures close together front-to-back [Longitudinal, Axial = 1/2 SPL, Radial, Range - pulsed u/s, Depth]

•Temporal — ability to accurately determine the position of an anatomic structure a particular instant in time
determined by 2 things:
- how much something moves
- frame rate —> # frames / s = # images / s
— # pulses (foci) / scan line
— line density (# lines / frame)
— imaging depth (listening time)
— sector width
— PRF proportional to frame rate (as PRF increases —> temporal resolution improves)
-M-mode has the greatest temporal resolution

17
Q

Axial resolution (pulse length)

A
  • along z-axis
  • axial resolution = 1/2 SPL
  • discern two objects front to back (in tandem)
  • higher frequency = shorter SPL = better axial resolution
18
Q

Lateral resolution (beam width)

A
  • along x-axis
  • discern two objects side by side
  • dependent on beam width — narrower the beam the better the lateral resolution
19
Q

Elevational resolution (beam height)

A

•along the y-axis

20
Q

5 functions of the receiver

A

•Amplification — enlargement of returning signals (overall gain)
•Compensation — makes all echoes from similar structures appear with similar brightness (time/depth gain compensation)
•Compression — reduces the total range of signal from smaller to largest
- maintains relative strengths of signals but reduces the differences in voltage
- inverse of dynamic range
•Demodulation — changes the shape of the electrical signal to make it recognizable by the image screen
- rectification turns all negative voltages positive
- smoothing smooths out the signal
•Rejection — very low level signals considered noise and ignored

21
Q

Artifacts

A
  • U/S assumes: sounds travels in a straight line; reflections are along the main axis of the beam; intensity of reflection corresponds to the reflectors scattering strength; sound travels at 1540 m/s; imaging plane extremely thing; sounds travels to reflector and back
  • Reverberation — multiple equally spaced reflections (ring down & comet tail)
  • Refraction
  • Side lobes & grating lobes
  • Acoustic shadowing — echo dropout
  • Mirror imaging