Ultrasound Physics and Instrumentation (MRD535)

Ultrasound Physics and Instrumentation (MRD535) - Imaging Applications

• The presentation covers ultrasound physics and instrumentation, specifically focusing on imaging applications.
• Learning objectives include describing the principles, physics, instrumentation, accessories, and image recording in ultrasonography, along with explaining the principles of ultrasonography, including ultrasound physics.
• This presentation also outlines different display modes for ultrasonography.

Display Modes

• A-mode (Amplitude mode): One-dimensional presentation of reflected sound waves. Returning echoes are shown as spikes. The vertical axis represents signal amplitude, and the horizontal axis shows echo delay (depth).
• B-mode (Brightness mode): Displays signals from returning echoes as dots with varying intensities. The brightness of a dot corresponds to the size of the echo; larger echoes are brighter; non-reflectors are dark. Dots are positioned along a time base, representing distance from the transducer. Combined dot lines form a two-dimensional (2-D) image.
• M-mode (Motion mode): Used to create an electronic trace of a moving object along the ultrasound beam's path. The transducer is fixed relative to the moving structure. Returning echoes are displayed as dots with varying intensities along a time base (similar to B-mode). This is useful for observing cardiac motion.
• Real-time mode: Rapid B-mode scanning generates images of a selected cross-section repeatedly. The rapid acquisition and viewing rate mimics continuous motion.
• Doppler mode: Used to study blood flow and cardiac movements. A constant frequency ultrasound beam interacts with a moving boundary (e.g., a red blood cell). The transducer detects shifted frequencies (Doppler shift). Approaching structures have a higher frequency shift, and receding structures have a lower frequency shift. Velocity of the moving reflector is related to the Doppler shift. Color is used to indicate the direction of flow (red = toward, blue = away ).

Doppler Effect

• The Doppler effect is the apparent change in frequency of a wave (e.g., sound, light) due to relative motion between the source of the wave and the observer.
• When the source is moving toward the observer, the frequency appears higher. When moving away, the frequency appears lower.
• This effect is not caused by a change in the source frequency but is due to the change in the distance of consecutive wave fronts.
• The Doppler effect can be observed in various types of waves.

Doppler Mode Details

• Doppler mode is useful for examining blood flow and cardiac movements.
• A moving acoustic boundary (e.g., red blood cell) causes a Doppler shift in the returning echoes that the transducer detects.
• Color in Doppler images indicates the direction of blood flow.
• The Doppler shift is related to the velocity of the moving reflector and the direction of motion. Stronger shifts reflect higher velocities.

Doppler Shift Formula

• The formula for Doppler shift (fd) is: fd = fr – ft = 2(ft)(v)(cosθ)/c
  • fd = Doppler shift
  • fr = Received frequency
  • ft = Transmitted frequency
  • v = Source velocity (blood)
  • c = Speed of sound
  • θ = Angle between the ultrasound beam and the blood flow direction

Types of Doppler

• Continuous-wave (CW) Doppler: Ultrasound waves are continuously emitted, and echoes are analyzed continuously. It requires separate transmit and receive elements. It cannot determine the precise location of the reflectors within the flow path.
• Pulse-wave (PW) Doppler: Ultrasound is transmitted in pulses, and echoes are analyzed between pulses. This allows for the determination of reflectors' location and the calculation of distance. It has the downside of not accurately showing higher velocities.
• Color flow Doppler: A combination of PW Doppler and B-mode. Blood flow direction and velocity are indicated by colors, superimposed on the B-mode image.
• Power Doppler: Only displays the intensity of reflections, not the direction of flow. Useful for detecting very low flow rates.

Exercises & Examples

• Included examples of calculations for Doppler shift.
• Images of various Doppler modes (B-mode, PW, Color, Power) are also given to show real-life applications.

This quiz explores ultrasound physics and instrumentation with a focus on imaging applications. It covers key principles, various display modes like A-mode and B-mode, and image recording techniques used in ultrasonography. Enhance your understanding of this vital field in medical imaging.