Pulse Echo Instrumentation
Pulse Echo Instrumentation
Chapter 14
There are six major components that
comprise this scenario
• Transducer
• Pulser and beam
former
• Receiver
• Display
• Storage media
• Master
Synchronizer
Other names for output
• Each manufacture has coined a term for this
– Output gain, acoustic power, transmitter output,
power, or gain
• Two terms were devised in order to standardize
this wide variation
– Thermal Index and Mechanical Index
– Both are mathematical number that are used to help
determine bioeffects
The creation and processing of the sound
wave is separated into two primary
categories
• Preparation and transmission of the electrical
signal to the transducer
• The reception of the returning signals and
processing of the sound into visual images
and audio
Pulser
• Functions during transmission sending the
pulses to the PZT
• Transducer output ranges from close to 0
to 500 volts and is technologist controlled
• Set low week signals are transmitted
• Set high and strong signals are
transmitted
• So what is the effect of the image
displayed when you modify the voltage?
Image effect
• It changes the brightness on the ultimately
displayed image
• It also changes the potential for bioeffects
• So the lower the pulse voltage should be
used as long as the imaged quality is not
compromised
• This output is controlled by the sonographer
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Noise
• Describes as undesirable low-level signals
that degrade the ultimate image
• A ration of the original signal to the
contamination noise is often used
• High ratios means there is a much better
signal than noise
• And of course the reverse is also true
• So what do you think we want in imaging?
PRP and the pulser
• The timing of the voltage to the PZT is
also determined by the pulser
– Longer delays longer pulses
– Shorter delays shorter pulse
• Since PRP is a reciprocal of PRF the
pulser also effects this as well
The Beam Former
• Is considered part of the transmitter
• Recall it transmits electrical pulses in a
specific pattern to create the phased
sector in an array transducer
• It also creates (adjusts) the electrical
signal that reduces side lobe artifacts
• What is this term called?
Output and noise
• Increasing the power output from the
transducer increases the signal to noise
ratio
• The image improves because the noise
remains constant
• It does have an effect on the energy
transmitted into the patient so there is a
trade of
The Image
• Recall PRP and PRF determine the max
imaging depth
• Short PRP has a high PRF and a shallow
imaging depth
• A long PRP has a longer PRF and imaging
depth is greater
• Is this controlled by the sonographer?
Beam Former continued
• Establishes the correct time delays for
dynamic receive focusing
• Also controls dynamic aperture
• An advanced microprocessor called a
“digital beam former” produces this signal
in digital format
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Digital beam former
The Switch
• Both transmit and receive
signals pass through.
• Prevents the high voltage
transmit signal from
interfering with the very
low receive signal
• Directs the transmit signal
from the beam former to
the transducer
• Directs the receive signal
form the transducer to the
processing components
• Advantages of this entity
• Requires only software
programming not
hardware
• Stable no mechanical
parts to calibrate or wear
• Versatile can be used
with an extreme wide
range of transducer
Frequencies
Amplification
• Channels are the
single PZT element
the electronics in the
pulser and the wire
that connects them
• Receiver the receiver
prepares the electrical
information for display
on the CRT and is
composed of five
elements
•
•
•
•
•
Amplification
Compensation
Compression
Demodulation
Reject
In this order
Amplification continued
• Measured in dB they
are a relative value
– The final signal is
compared to the
original signal strength
• Values range form 60
to 100 dB
• It is synonyms with
receiver gain
• Preamplification
improves the strength
quality before its
amplified
• Designed to prevent
electronic noise in the
system from
degrading the receive
signal
• In this process all the receive signals are
made larger equally
• The receive signals are the same they just
become either brighter or darker
depending on adjustments made
• Doesn’t alter the signal to noise ratio
• Adjustable by a control on the system
console
Compensation
• As sound travels through the body it
attenuates
• the deeper you image the more it
attenuates
• Why not up the power to get to the deeper
structures?
• A god image uses this process to create a
uniform image from near field to far field
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Compensation continued
• This is a form of variable amplification is
measured in dB
• Its effect depends on the depth the signal
originates
• This is a control utilized by the
sonographer.
• Common terms used for this are
– Time gain compensation (TGC), depth gain
compensation (DGC) and swept gain
Compression
• The human eye can only see a finite
number of gray shades
• Compressing ranks the signal strength
form the strongest to the weakest and
mathematically places into a category
• It does not alter the signal but keeps them
within the operators perception range as
well as the accuracy range of the system
electronics
Compression continued
• This process is important because we can
increase the display efficiency of the weak
signals
• While not changing the display intensity of
the strong signals
A TGC curve
• Near
• Delay – depth tat
compensation begins
• Slope - corrects
attenuation effects over
path length
• Knee – max
compensation used
• Far gain- max
compensation that can be
provided
Compression continued
• While one component of this process is
not adjustable another is and is known as
grayscale mapping
• Also called log compression or dynamic
range
• Again measured in dB and is a relative
unit comparing one signal to another
Demodulation
• A process by which
an electrical signal is
changed into a form
suitable for displaying
on an CRT
• The sign wave is
converted in all
positive voltages
through a process
called Rectification
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Demodulation continued
• Next the envelope is
smoothed to create
an even line
• The reason for this
change is so the
electrical signal is
more appropriate t
display on a CRT
there is no effect on
the actual image itself
Dynamic Frequency Tuning
• Signal pulses are dampened to create a
better axial image
• This process creates a variable frequency
wave
– there are some at the primary frequency
– still others at frequencies above and below
this
• What is the term used to describe this
frequency?
Reject
• A return signal is made up of many signals good
signals and bad signals some of the bad signals
are very week and can degrade the ultimate
image.
• Removing some of these low level bad signals
can improve the image. It does not effect the
bright echos
• To much reject applied and the actual image can
suffer
• The songrapher has some control but the
system also has built in designs
Patient exposure to Ultrasound
• Bioeffects is related to power output not
changes in amplification
• When possible always use the lowest
power possible.
• A principle known as ALARA should
always apply when making power output
changes in order to minimize patient
exposure
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