Chapter 14 Pulsed Echo Instrumentation

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Chapter 14Pulsed Echo Instrumentation
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Pulsed Echo Instrumentation

• Ultrasound system has two major functions
• Preparation and transmission of electrical
  signals to the transducer, which creates a sound
  beam
• Reception of electrical signals from the
  transducer, with subsequent processing into
  clinically meaningful images and sounds

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Pulsed Echo Instrumentation

• Six major components
• Transducer
• Pulser and Beam Former
• Receiver
• Display
• Storage
• Master Synchronizer

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Pulsed Echo Instrumentation

• Transducer
• Transmission transforms electrical energy into
  acoustic energy
• Reception converts returning acoustic energy
  into electrical energy
• Pulser Beam Former
• Creates controls electrical signal sent to the
  transducer to generate sound pulses
• Pulser determines the amplitude, pulse repetition
  period pulse repetition frequency
• Beam former determines the firing delay pattern
  for phased array systems

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Pulsed Echo Instrumentation

• Receiver
• Transforms electrical signals from the transducer
  into a form suitable for display
• Display
• Presents the processed data
• May be one of several different display formats
  or devices including the following
• A cathode ray tube (CRT)
• Transparency
• Spectral

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Pulsed Echo Instrumentation

• Storage
• Archives the ultrasound studies
• Storage media may include
• Videotape
• Paper printouts
• Photographs
• Transparent film
• CD-ROM, DVD, MOD
• Computer hard drive
• Master synchronizer
• Maintains organizes proper timing and
  interaction of the systems components

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Pulser

• Functions during transmission
• Creates electrical signals which excite the
  transducers PZT crystals
• Adjustable by the sonographer
• Ranges for near 0 to 500 volts
• Changes in voltage modify the brightness of the
  entire image
• Low system output
• Active element vibrates gently
• Reflected echoes are weak
• Image appears dark
• High system output
• Active elements vibrate more forcefully
• Reflected echoes are stronger
• Image appear brighter

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Pulser

• Synonyms
• Output gain
• Acoustic power
• Pulser power
• Energy output
• Transmitter output
• Power
• Gain (do NOT use)
• Measurements appearing on the screen relating to
  power
• Thermal Index (TI)
• An indicator of thermal mechanism activity
  (estimated temperature rise) a value equal to
  transducer acoustic output power divided by the
  estimated power required to raise tissue
  temperature by 1 C.
• Mechanical Index (MI)
• An indicator of nonthermal mechanism activity
  equal to the peak rarefactional pressure divided
  by the square root of the center frequency of the
  pulse bandwidth
• These terms are discussed more completely in
  chapter 22.

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Pulser

• Effect on image
• Changing or modifying the transducer output
  changes all pulses transmitted into the body
  all of the reflections received from anatomic
  structures
• The brightness of the entire image changes

Output Too Low
Output - Appropriate
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Pulser

• Noise
• Random persistent disturbances which obscure or
  reduce the clarity of a signal
• Contaminates images with low-level, undesirable
  signals
• Signal-to-Noise Ratio
• Comparison of the meaningful information (signal)
  present in an image to the amount of
  contamination (noise)
• Signal-to-Noise Ratio High
• Signal is stronger than the noise
• Image quality is high
• Signal-to-Noise Ratio Low
• Signal strength is closer to or less than the
  strength of the noise
• Image quality is lower, containing larger amounts
  of contaminated information and has less
  diagnostic value

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High vs. Low Signal-to-Noise Ratio

• Transducer Output vs. Noise
• Noise levels generally remain constant
• Increasing transducer output power is the only
  way to improve (increase) the signal-to-noise
  ratio
• Low output noise is more likely to degrade the
  image
• Raising the output power also raises (improves)
  the signal-to-noise ratio

High Signal-to-Noise Ratio
Low Signal-to-Noise Ratio
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Pulser

• Pulser determines the time between one voltage
  spike and the next, the PRP and, therefore, PRF
  are determined by the pulser
• PRP PRF determine the maximum imaging depth
  (depth of view)
• Short PRP Higher PRF
• Less listening time
• Superficial imaging
• Long PRP Lower PRF
• More listening time
• Deeper imaging
• Adjustable Yes change the depth of view

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Pulser
Shallow Higher PRF
Deep Lower PRF
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Beam Former

• Reception
• Establishes correct time delays used for dynamic
  receive focusing
• Controls dynamic aperture by varying the number
  of PZT crystals used during reception
• Digital beam former most modern
• Advantages
• Only require software programming for
  modifications updates
• More flexible than older analog systems
• Extremely stable
• No mechanical parts to fall out of calibration or
  wear out
• Versatile
• Capable of using transducers with a wide range of
  frequencies

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Beam Former

• Part of the transmitter
• Functions with array transducers during
  transmission reception
• Transmission
• Distributes a single electrical spike from the
  pulser to numerous active elements of an array
  transducer
• Coordinates the electrical signals sent to each
  active element to optimize the ultrasound beam
• Adjusts electrical spike voltages (apodization)
  to reduce lobe artifacts

Pulse Direction
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Beam Former

• Switch
• A special transmit-receive switch
• Protects the sensitive electrical components in
  the receiver from the high voltages created
  during transmission
• Directs electrical signals from the transducer to
  the appropriate electronic and processing
  components within the ultrasound system

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Channel

• A single channel consists of
• A single PZT active element
• Electronics in the beam former/pulser
• Wire that connects them
• The number of channels in the ultrasound system
  determines the number of elements in an array
  transducer which can be excited simultaneously
• Most modern systems have between 32 and 256
  channels

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Receiver

• Prepares the information for display on a CRT
  monitor which are contained in the tiny
  electrical signals which have been created by
  reflected pulses from the body having returned to
  the PZT element
• Order of receiver operations
• Amplification
• Compensation
• Compression
• Demodulation
• Reject

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Amplification

• First function of the receiver
• Each signal returning from the transducer is made
  larger
• Each signal undergoes an equal amount of
  amplification
• Effect on Image
• All electrical signals in the receiver are
  affected identically
• Entire image is made brighter or darker
• Does not improve the signal-to-noise ratio as
  amplifiers cannot distinguish between meaningful
  diagnostic information and electronic noise

Higher Amplification
Lower Amplification
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Amplification

• Adjustable
• Yes
• Units
• dB
• The final signal leaving the receiver is compared
  to the initial signal strength entering the
  receiver
• Typical values
• 60 100 dB
• Synonyms
• Receiver gain
• Preamplification
• The process of improving the quality of a signal
  before it is amplified
• Occurs as close to the active elements as is
  practical
• Prevents electronic noise from contaminating tiny
  signals created by the active elements of the
  transducer

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Compensation

• Second function of the receiver
• Corrects for the effects of attenuation
• Creates an image that is uniformly bright from
  top to bottom
• In a properly compensated image, a pair of
  identical reflectors at different depths are
  displayed with the same brightness

Improper Compensation
Proper Compensation
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Compensation

• Adjustable Yes
• Series of slider bars of knobs
• Units dB
• Effect on Image
• Treats echoes differently, depending on depth
  from which they arise
• Synonyms
• Time Gain Compensation
• TGC
• Depth Gain Compensation
• DGC
• Swept Gain

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Compensation TGC Curve

• Near gain
• Superficial depths, small amount of compensation
• Delay
• Depth at which variable compensation begins
• Slope
• Compensation corrects for the effects of
  attenuation due to increasing depth
• Knee
• Reflections are maximally compensated by the
  system
• Far gain
• Indicates the maximum amount of compensation that
  the receiver can provide

• X-axis
• Amount of compensation
• Y-axis
• Depth

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TGC
A typical TGC slope
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Lateral Gain Control (LGC)
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Pre-compensated TGC Profiles

• A TGC profile applied internally according to a
  best guess

This slope represents a compensated profile which
required slight adjustment
This slope represents a compensated profile which
was ideal
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Compression

• Third function of the receiver
• Maintains an images grayscale content within the
  range of detection of the human eye
• The human eye can only distinguish 20 shades of
  gray
• Maintains electrical signal levels within the
  accuracy range of the systems electronics
• Performed without altering the ranking between
  the signals
• Largest signal remains the largest
• Smallest signal remains the smallest

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Compression

• Adjustable Yes, two kinds
• One, built into the system design
• Second, which is user adjustable
• Effect on Image
• Modifies (alters) the grayscale mapping of the
  image
• Synonyms
• Log compression, dynamic range
• Units
• Decibels (dB)

Less Compression
Greater Compression
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Compression

• Important to clinical ultrasound because
• Most meaningful backscattered signals from
  biologic tissues are very weak
• The sonographer must be able to see differences
  in these weak reflections
• Comparisons of different strong reflections are
  not clinically important because they do not
  arise from different biologic tissues

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Demodulation

• Fourth function of the receiver
• Two part process which changes electrical signals
  within the receiver into a form more suitable for
  CRT display
• Rectification
• Smoothing
• Adjustable
• No
• Effect on Image
• None
• Purpose is to change the form of the electrical
  signal

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Demodulation - Rectification

• Rectification changes all of the negative
  voltages into positive voltages
• It corrects for, or eliminates, negative voltages

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Demodulation - Smoothing

• Smoothing, aka enveloping, places a smooth line
  around the bumps and evens them out

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Reject

• The fifth function of the receiver
• Allows the sonographer to control whether
  low-level grayscale information within the data
  will appear on the image
• Synonyms
• Threshold or Suppression
• Adjustable
• Yes
• Two levels
• One built into the system
• One is user adjustable
• Effect on Image
• Affects all low-level signals on the image
• Does not affect bright echoes

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Reject
Without Reject
With Reject
Bright reflections in both images are identical.
With reject, the weak reflections are eliminated.
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Summary Receiver Functions
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Dynamic Frequency Tuning

• When utilizing a wide bandwidth
• Highest frequencies are utilized to create the
  superficial portions of the image
• Lower frequency reflections are filtered out
• Intermediate frequencies are utilized to create
  the middle portions of the image
• Lowest frequencies are utilized to create the
  deeper portions of the image
• Higher frequency reflections have disappeared due
  to attenuation

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Output Power vs. Receiver Gain

• Both modify the brightness of the entire image
• Output Power
• Adjusts the strength of the sound pulse sent into
  the body via the transducer
• More powerful pulse leads to the entire image
  being brighter
• Increasing output power subjects the patient to
  greater levels of sound energy
• Therefore, the possibility of bioeffects is
  enhanced
• Image is too bright
• Reflectors bloom
• Lateral axial resolution degrades
• Image is too dark
• Increasing the power creates a brighter image
• Meaningful diagnostic echoes become stronger
  while the noise level remains unchanged
• Improves the signal-to-noise ratio

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Receiver Gain

• Alters the strength of the voltages in the
  ultrasound receiver after reception by the
  transducer
• Amplification does not alter the patients
  exposure to sound energy
• Therefore, does not increase or decrease the
  likelihood of bioeffects
• Higher amplification leads to a brighter image
• Does not alter the signal-to-noise ratio
• Both the meaningful signals and the noise are
  treated identically
• Lower amplification leads to a darker image

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ALARA

• Too bright vs. too dark
• Which adjustments should be made first, and why?
• ALARA Principle
• As Low As Reasonably Achievable
• When changes to output power or receiver gain can
  improve the diagnostic quality of an image, the
  first and best choice will minimize the patient
  ultrasound exposure.
• Image is too dark
• Increase the receiver gain first
• Does not increase patient exposure
• Image is too bright
• Decrease the output first
• Decreases patient exposure

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Output Power vs. Receiver Gain Summary