Understanding Ultrasound Physics

Understanding Ultrasound Physics

Pulsed Echo Instrumentation

Synonyms for Pulser Voltage
  • Due to the lack of industry-wide standards, pulser voltage in ultrasound systems is referred to by various terms:
    • Output gain
    • Acoustic power
    • Pulser power
    • Energy output
    • Transmitter output
    • Power
    • Gain (Note: the term 'gain' is particularly vague and should be avoided)
Standard Measurements in Ultrasound Systems
  • Two key measurements for standardizing transducer output:
    • Thermal Index (TI)
    • Mechanical Index (MI)
  • TI and MI are further discussed in Chapter 24, Bioeffects.
Adjustable Transducer Output
  • The sonographer can adjust transducer output using a control on the console.
Effect on Image Brightness
  • Modifying transducer output impacts:
    • All pulses transmitted to the body
    • All reflections received from anatomical structure boundaries
  • Overall image brightness alters with transducer output settings.
  • Lower pulser voltages are preferred for clinical practice:
    • They decrease transmitted acoustic energy.
    • They minimize patient exposure to ultrasonic energy.
    • They reduce the likelihood of bioeffects.
    • Note: Output power alone cannot ensure uniform brightness in images from top to bottom.
Understanding Noise
  • Definition of Noise: Noise is a random and persistent disturbance that obscures or reduces a signal's clarity.
  • Noise can contaminate ultrasound images with low-level undesirable signals and can act to degrade image quality.
Enhancing Signal-to-Noise Ratio
  • Increasing output power is a common method to improve signal-to-noise ratio:
    • As output power increases, image quality improves as signal strength increases relative to noise.
    • Noise levels typically remain constant despite variations in output power.

Pulse Repetition Period (PRP)

PRP and Pulse Repetition Frequency (PRF)
  • PRP controls the time between one voltage spike and the next.
  • Recall: PRP and PRF are reciprocals:
    • PRF = rac{1}{PRP}
PRP Effects on Imaging Depth
  • Short PRP:
    • High PRF
    • System spends less time listening
    • Results in superficial imaging
  • Long PRP:
    • Low PRF
    • System listens for a longer duration
    • Results in deeper imaging
  • Adjustability: The sonographer can control the depth of view via console settings.
Depth of View Table

  • Shallow Imaging:

    • Shorter listening time
    • Shorter pulse repetition period
    • Higher PRF

  • Deep Imaging:

    • Longer listening time
    • Longer pulse repetition period
    • Lower PRF

Beam Former Functionality

  • The beam former, part of the transmitter, acts with array transducers during transmission and reception.
  • Functionality:
    • Receives a single electrical spike from the pulser and distributes it to multiple active elements in an array transducer.
    • Coordinates complex electrical signals sent to each element to optimize the ultrasound beam.
    • Adjusts electrical spike voltages to reduce artifacts via apodization.
    • During reception, establishes correct time delays for dynamic receive focusing.
    • Controls dynamic aperture by varying the active PZT crystals number.
  • Modern Beam Formers: Use digital technology for signal processing, providing state-of-the-art performance.
Advantages of Digital Beam Formers
  • Flexibility in modifications and updates through software, rather than hardware redesign.
  • Stability with fewer mechanical parts that may fall out of calibration.
  • Versatility with a range of usable transducer frequencies.

Switch Function in the Beam Former

  • The "transmit-receive" switch is crucial during transmission and reception.
  • It protects sensitive receiver components from high voltages during transmission while directing low voltages back into the ultrasound system from transducers.

Key Components of an Ultrasound System

  1. Transducer: Converts electrical energy to acoustic energy and vice versa.
  2. Pulser and Beam Former: Generate and control electrical signals for sound pulses.
  3. Receiver: Prepares electrical signals for display.
  4. Display: Shows processed data in various formats.
  5. Storage: Archives ultrasound studies on various media.
  6. Master Synchronizer: Manages timing and interaction of components.

Receiver Functions

Amplification
  • Definition: The first function of the receiver, making signals larger.
  • Each signal undergoes the same amount of amplification.
  • Effects:
    • Impacts the overall brightness of the image.
    • Does not distinguish between signal and noise.
    • Not capable of final brightness uniformity throughout the image.
  • Adjustable: Yes, through controls on the console.
  • Units: Measured in decibels (dB).
Compensation
  • Purpose: Corrects for sound wave attenuation, ensuring uniform brightness from top to bottom of the image.
  • Adjustable: Yes.
  • Units: Measured in decibels (dB).
Compression
  • Purpose: Enhances gray scale information. It allows visualization of tissue differences.
  • Adjustable: Yes, both integral and user-controlled types exist.
  • Units: Often reported in decibels (dB).
Demodulation
  • Purpose: Converts electrical signals into a format suitable for display, involving:
    • Rectification: Converts negative voltages into positive.
    • Smoothing: Evens out the signal, eliminating small variations.
  • Adjustable: No, it is built into the system.
Reject
  • Purpose: Controls low-level signals displayed in the image, differentiating between meaningful reflections and noise.
  • Adjustable: Yes, with two levels of reject commonly available.

Output Power vs. Receiver Gain

Understanding Output Power
  • Affects the strength of the sound pulse that transducers send.
  • High output power results in brighter images but may degrade resolution if set too high.
  • Concerns about patient exposure to sound energy arise from changes to output power.
Understanding Receiver Gain
  • Alters voltage strength created by transducer during signal reception.
  • Doesn't influence patient exposure and does not improve signal-to-noise ratio through equal treatment of signals and noise.