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Diagnostic Imaging Exam > Ultrasound > Flashcards

Ultrasound Flashcards

1
Q

What is the basic principle for ultrasound imaging?

A

Transducer fires acoustic waves into the subject where reflected echo is then collected, processed and finally displayed on a screen.

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2
Q

Which are the three modes of ultrasound discussed during lectures?

A

Three Main Modes of Imaging

  1. A-Mode Scan (Amplitude Mode)
  2. M-Mode Scan (Motion Mode)
  3. B-Mode Scan (Brightness Mode)
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3
Q

Describe A-mode for Ultrasound

A

Amplitude-Mode

  1. One dimensional scan
  2. Plot the amplitude of the backscattered echo versus the time after transmission of the ultrasound pulse
  3. By firing the transducer on a repetitive basis, a success of signals can be displaced on an oscilloscope.
  4. The time between successive firing is called the repetition time .

(Commonly used for heart valves?)

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4
Q

Describe M-Mode for Ultrasound.

A

Motion-Mode

  1. Display a continuous series of A-scan
  2. Successive A-mode signals are displaced in successive columns.
  3. Motions of objects along the transducer axis is revealed as bright traces moving up and down across the image.
  4. Useful when looking at heart valve motion
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5
Q

Describe B-Mode for Ultrasound.

A

Brightness-Mode

  1. The B-mode scan is created by scanning a single transducer beam in a plane by moving the transducer in one direction (e.g. x-axis) while its beam is aimed down the z-axis.
  2. The B-mode image is created by brightness-modulation along a column using the corresponding A-mode signal.

Compound B-mode

  • The operator could ‘fix’ certain directions or orientations in order to produce a tomographic scan containing data from only one cross-sectional slice.
  • When multiple views of the same tissue are included in a single B-mode image, this is referred to as compound B-mode scanning.
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6
Q

Explain Compound B-Mode for Ultrasound

A

Compound B-mode

  1. The operator could ‘fix’ certain directions or orientations in order to produce a tomographic scan containing data from only one cross-sectional slice.
  2. When multiple views of the same tissue are included in a single B-mode image, this is referred to as compound B-mode scanning.
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7
Q

Explain Pulse Repetition Rate (Ultrasound)

A

Pulse Repetition Rate

  • A new pulse can be generated only after all echoes from the previous pulse have died out. Therefore, T_R >= 2d_p/c.
  • Pulse Repetition Rate: f_R = 1/T_R
  • Suppose N pulses are required to generate an image. The image frame rate F is: F =1/(T_R*N)
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8
Q

How can an acoustic wave be steered?

A

Transmit Steering

Given a linear array of transducer, the wave can be steered to different directions by exciting each transducer at different time.

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9
Q

Difference between Steering and Focusing (Ultrasound). What are different types of focusing?

A

Steering VS Focusing

Differences compared to Steering:

  1. The delays are not simply multiple of a base delay
  2. The Transducers need not be fired in the same order as their geometric order

Different types of Focusing

  1. Beamforming
    1. Increase the transducer’s sensitivity along one particular direction (receiving mode)
  2. Dynamic Focusing
    1. Focus the sensitivity to a particular scattering event at one point
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10
Q

Explain Ultrasonic Transducer.

A

Ultrasonic Transducer

  • Use Piezoelectric crystals to both generate and receive Ultrasound.
  • Transmitter: Induced electric field, generate mechanical displacement, thus acoustic wave
  • Receiver: Mechanical Displacement generates electric potential
  • Common piezoelectric material use for medical ultrasound transducer: Lead zironate titanate (PZT)
  • Medical Transducer is usually shocked excited: Transient/impulse signals
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11
Q

What is Depth of Penetration for Ultrasound?

A

Depth of Penetration

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12
Q

Briefly explain the Doppler effect

A

Doppler Effect

  • The actual frequency that you can measure changes when the source and/or receiver move(s).
  • Example: Pitch of the siren is higher as the vehicle approaches and becomes lower as the vehicle passes and move away
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13
Q

Which are the three different situations where Doppler effect may occur?

A

Doppler Effect

3 Different Situations:

  1. Moving source
  2. Moving observer
  3. Moving scatterer
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14
Q

Explain the Doppler frequency

A

Doppler frequency

Doppler Frequency: the difference between the observed frequency and the source frequency. Varies for the different situations.

  1. Negative frequency: Source moving away
  2. Positive frequency: source moving towards the receiver
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15
Q

Brief background to Ultrasound

A

Ultrasound

  • Imaging via Acoustic Wave
  • Ultrasound: sound with frequencies higher than audio frequency (20kHz and above)
  • Medical Ultrasound: 1-10MHz (up to 70MHz)
  • Starting in the 1950s
  • Expanded in the 1970s
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16
Q

Explain the difference between Longitudinal VS Transverse waves.

A

Longitudinal VS Transverse

Acoustic Waves are pressure waves that propagate through matter via compression and expansion of the material: Longitudinal Waves!!

Transverse Wave: a moving wave that consists of oscillations occurring perpendicular to the direction of energy transfer, e.g. electromagnetic wave

17
Q

What characterize an acoustic wave?

A

Characterization of Acoustic Wave

  1. Speed of sound (wave propagation): c=sqrt(1/(\kappa\rho)
  2. Compressibility \kappa , density \rho
  3. Varies as a function of frequency and material
  4. About 330m/s in air

Particle velocity: v(x,y,z,t) temporal derivative of particle displacement

Particle velocity is not the same as speed

18
Q

Explain Acoustic Pressure

A

Characterization of Acoustic Wave

  • Acoustic Pressure p to particle velocity v .
  • p=Zv
  • where Z=\rho*c - characteristic impedance
  • Units
    • Speed: m/s,
    • Density: kg/m3
    • Z: kg/m2s •
  • Same analogy in circuit theory p = Zv p v
19
Q

Explain Plane Wave Assumption

A

Plane Wave Assumption

In general, acoustic wave is a 3D phenomenon

Plane Wave Assumpton

If the acoustic wave varies in only one spatial direction (z) and time, and remains to be constant in other directions (x and y) and the second derivative with respect to x and y = 0 then p(x,y,z,t)=p(z,t)

20
Q
A

Diagnostic Imaging Exam (8 decks)

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