Pulsed Echo Instrumentation Flashcards

Pulsed Echo Instrumentation

Overview

• Ultrasound systems generate sound pulses, receive reflections, and present information.
• Two main functions:
  • Transmit electrical signals to the transducer, creating a sound beam.
  • Receive electrical signals from the transducer, converting them into images and sounds.
• Meaningful images are obtained to diagnose abnormalities.

Major Components

• Transducer: Converts electric energy to acoustic energy during transmission and acoustic energy back into electrical energy during reception.
• Pulser and Beam Former: Creates and controls electrical signals sent to the transducer.
  • Pulser: Determines amplitude, pulse repetition period (PRP), and pulse repetition frequency (PRF).
  • Beam Former: Determines firing delay patterns for phased array systems.
• Receiver: Transforms electrical signals from the transducer into a suitable form for display.
• Display: Presents processed data.
• Storage: Archives ultrasound studies on media like hard drives, CDs, DVDs, photos, and USB drives.
• Master Synchronizer: Maintains timing and interaction of system components.

Pulser

• Creates electrical signals that excite the transducer's PZT crystals to create sound beams.
• Functions during transmission.
• Transducer Output: Sonographer can adjust the magnitude of the pulser's electrical voltage spike.
  • Ranges from 0-100 volts.
  • Changes in pulser voltage modify the brightness of the entire image.
• Low voltage: weak sound beam, dark image.
• High voltage: strong sound beam, bright image.

Pulser Voltage Terminology

• Also called:
  • Output gain
  • Acoustic power
  • Pulser power
  • Energy output
  • Transmitter output
  • Power
  • Avoid using "gain" as it is vague.

Effect of Pulser Voltage on Image

• Modifying transducer output changes all pulses transmitted.
• Changes all reflections received.
• Brightness of the entire image changes.
• Lower pulser voltages are desirable to decrease acoustic energy and minimize patient exposure and minimize bioeffects.
• Output power alone cannot make an image of uniform brightness from top to bottom.

Noise and Signal-to-Noise Ratio

• Noise: Random disturbance obscuring a signal's clarity.
• Signal-to-Noise Ratio: Comparison of meaningful information (signal) to contamination (noise).
• High signal-to-noise ratio: Strong signal, high-quality image.
• Low signal-to-noise ratio: Signal strength close to noise strength, contaminated image, less diagnostic value.
• Increasing output power increases the signal-to-noise ratio and improves image quality.
• Noise levels generally remain constant.

Pulse Repetition Period (PRP)

• The pulser controls the time between voltage spikes.
• PRP and PRF are reciprocals.
• Short PRP = high PRF = superficial imaging (less listening time).
• Long PRP = low PRF = deeper imaging (more listening time).
• Adjustable by sonographer.
• PRP determines maximum imaging depth.

Imaging Depth

• Shallow Imaging: Shorter listening time, shorter PRP, higher PRF.
• Deep Imaging: Longer listening time, longer PRP, lower PRF.

Beam Former

• Functions with array transducers during transmission and reception.
• Distributes the pulser's electrical spike to array elements.
• Adjusts electrical spike voltages to reduce lobe artifacts (apodization).
• Establishes time delays for dynamic receive focusing.
• Controls dynamic aperture by varying the number of PZT crystals used.

Digital Beam Former

• Produces signals in digital format.
• Advantages:
  • System modifications via software programming.
  • Extremely stable.
  • Versatile: capable of using transducers with a wide range of frequencies.

Switch (Transmit/Receive)

• Protects sensitive receiver components from high voltages during pulse creation.
• Directs electrical signals to appropriate processing components.

Channel

• Composed of:
  1. Single PZT element
  2. Electronics in the beam former/pulser
  3. Wire connecting them
• Number of channels determines the number of elements that can be excited simultaneously.
• Most systems have between 32 and 256 channels.

Receiver

• Prepares reflection information for display.
• Five operations in correct order:
  • Amplification
  • Compensation
  • Compression
  • Demodulation
  • Reject

Amplification (Receiver Gain)

• Each signal is made larger by an equal amount.
• Required because electrical signals are too low to display.
• Adjustable (Yes).
• Units: decibels (dB)- relative unit of measure.
• Typical values: 60-100 dB.
• All signals are affected identically.
• Does not improve signal-to-noise ratio.

Preamplification

• Improves signal quality before amplification.
• Occurs close to active elements.
• Prevents electronic noise contamination.

Compensation

• Sound waves attenuate (weaken) as they travel.
• Corrects for attenuation to create a uniformly bright image, depth gain compensation.
• Adjustable (Yes- TGCs).
• Units: decibels (dB).
• Treats echoes differently based on depth.
• Synonyms: Time-gain compensation (TGC), depth-gain compensation (DGC), swept gain.

Anatomy of a TGC Curve

• X-axis: amount of compensation.
• Y-axis: reflector depth.
• Near gain: small, constant compensation for superficial depths.
• Delay: depth at which variable compensation begins.
• Slope: corrects for increasing attenuation with depth.
• Knee: depth of maximum compensation.
• Far gain: maximum compensation receiver can provide.

Compression

• Keeps signal levels within the system's accuracy range.
• Keeps gray scale content within the eye's detection range (about 20 shades).
• Performed without altering the ranking between signals.
• User-controlled compression modifies the gray scale mapping.
• Synonyms: log compression, dynamic range.
• Units: decibels (dB)- relative unit that compares one signal to another.
• Important clinically because the most meaningful backscattered signals from biologic tissues are very weak and we must be able to see the differences in the weak reflections.

Demodulation

• Two-part process to change electrical signals into a displayable form.
• Adjustable? No is built into the ultrasound system.
• Effects on the Image: none.
• Rectification: converts negative voltages to positive voltages.
• Smoothing (Enveloping): places a smooth line around the bumps.

Reject

• Controls whether low-level gray scale information is displayed.
• Synonyms: threshold, suppression.
• Adjustable (Yes).
• Affects all low-level signals, but not bright echoes.

Receiver Function Summary

Dynamic Frequency Tuning

• Good axial resolution is achieved with shorter pulses.
• Pulse damping material helps to create these short pulses and creates a wide range of frequencies which are described as wide bandwidth or broadband.
• Systems use the high-frequency part of reflected pulses for superficial portions of the image (superior axial resolution).
• The Lower frequency portion of the bandwidth is used to create the deeper portions of the image because Higher frequency components have disappeared at great depths as a result of attenuation.

Output Power vs. Receiver Gain

• Output Power: Alters the strength of the sound pulse.
  • A powerful pulse causes all returning echoes from the body to be stronger and the entire image is brighter. If too bright, axial and lateral resolution is degraded.
  • Improves the signal-to-noise ratio.
  • Affects patient exposure and can lead to bioeffects.
• Receiver Gain (Amplification): Alters the strength of voltages during reception.
  • Higher amplification = brighter image.
  • Does not alter the signal-to-noise ratio.
  • Does not affect patient exposure - no bioeffect concerns.

ALARA Principle

• As Low As Reasonably Achievable.
• Minimize patient's ultrasound exposure.
• If image is too dark – increase receiver gain.
• If image is too bright – decrease the output power.

Output Power vs. Receiver Gain Summary