Transcript Session 7 - Pulse Echo Imaging Systems

Pulse Echo Imaging Systems
To date, we have discussed:
Behavior of sound waves
Interaction with tissue
Now we are going to learn what must
happen to the echoes returning to the
transducer to give us a visual display
Pulse Echo Imaging Systems
Integrated components that permit the
reception & image display of the echo
voltages from the transducer
Use the returning echoes

Strength
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Direction

Arrival time
DISPLAY
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DAC
POST PROCESSOR
IMAGE MEMORY
PROCESSOR
PRE-PROCESSOR
SCAN CONVERTER
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COMPRESSION
DETECTION
FILTER
SUMMER
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ECHO
DELAYS
PULSER
PULSE
DELAYS
T/R
ADCS
AMPS
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Pulse Echo Imaging Systems
beam former
signal processor
image processor
display
Beam Former
Responsible for:
Electronic beam scanning
Steering, focusing
Apodization
Aperture functions with arrays
Functions of a Beam Former
1.
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generate voltage that drive the transducer
determine PRF, coding, frequency, and intensity
scan, focus and apodize the transmitted beam
amplify the returning echo voltages
compensate the attenuation
digitize the echo voltage stream
direct, focus and apodize the reception beam
sends the digitized echo voltages to the signal
processor
Beam Former consists of:
Pulser
Pulse delays
Transmit/receive switch
Amplifiers
Analog-to-digital converters
Echo delays
Summer
Pulser
1. Produces electric voltages to drive
the transducer forming the beam
A. Transducer then produces US pulses
that travel into the patient
2. Initial voltages are 1-2 cycles of
electric pulses
Pulser
Voltage pulse frequency determines
resulting US transducer pulse frequency

Pulse range: 4 - 15 kHz
PRF is the # of voltage pulses/sec sent
to the transducer an = # US pulses/sec
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
US PRF = voltage PRF
1 US pulse is produced from each
voltage pulse
High PRF is wanted to receive display
information at a rapid rate, however, it must be
restricted enough to provide proper display of
returning echoes to avoid echo misplacement.
Echoes for deeper structures take longer to
return & that all the returning echoes from 1
pulse must be received before the next pulse is
emitted if echo misplacement is to be avoided.
SO…
Reducing the PRF & the # of images generated
each second is required
Output Power Control
Transducer’s intensity is increased by 
the voltage to the transducer with the
output power control (aka – overall power,
power, output, input)
 the output power increases the
amplitude of the transmitted sound beam
& the received echoes
 the output by 3 dBs, doubles the
intensity of the sound beam & increases
the ultrasonic exposure to the patient!
INPUT
Output Power Control
The greater the voltage amplitude
produced by the pulser = the greater the
US pulse intensity produced by the
transducer & ranges up to 100 V
Output level shown on the display in a
percentage or decibels relative to
maximum (100% or 0 dB) output
Output Power Control
 output by 3 dBs = ½ the sound beam intensity
Lower output reduces received echo amplitude.
 receiver gain will compensate for this
 intensity =  patient bioeffects
Example
Reducing output by 50% (-3 dB)
with a 5-MHz transducer = a
reduction in penetration by 5%
(i.e. 12.0 to 11.4 cm) or the
difference shown by the 2
arrows
Changing the output power
changes the biological effects
that occur in tissue
For this reason, use
minimum output power &
maximum receiver gain
Beam Former consists of:
Pulser
Pulse delays
Amplifiers
Transmit/receive switch
Analog-to-digital converters
Echo delays
Summer
Pulse Delays
- performs the sequencing & phasing
for beam scanning, steering, focusing,
aperture, and apodization of array
transducers
Pulse Delays
Receives 1 input from the pulser but provides
multiple outputs to the transducer elements
Each elements in the array needs a different
delay to form the ultrasound beam
Each delay & element combination is called a
transmission channel
 the # of channels allows more precise
control of beam characteristics
DISPLAY
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POST PROCESSOR
IMAGE MEMORY
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PRE-PROCESSOR
SCAN CONVERTER
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COMPRESSION
DETECTION
FILTER
SUMMER
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ECHO
DELAYS
PULSER
PULSE
DELAYS
T/R
ADC
AMP
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Beam Former consists of:
Pulser
Pulse delays
Transmit/receive switch
Amplifiers
Analog-to-digital converters
Echo delays
Summer
T/R Switch (transmit/receive)
Directs the driving voltages from the pulser &
pulse delays to the transducer during
transmission
Directs the returning echo voltages from the
transducer to the amplifiers during reception
Protects sensitive amplifiers input components
from the large driving voltages of the pulser
Beam Former consists of:
Pulser
Pulse delays
Transmit/receive switch
Amplifiers
Analog-to-digital converters
Echo delays
Summer
Amplifiers
-increases amplitude
Converts small voltages received from the
transducer elements to larger ones usable for
processing and storage
This amplification is called gain
1 amplifier for each channel in the beam former
Gain - ratio of amplifier output power to input power
(output power ÷ input power)
Power ratio = voltage ratio2 & is in dB
Example: - input voltage amplitude is 2 mV
& output voltage amplitude is 200 mV
voltage ratio = 200/2, or 100.
The power ratio is (100)2 = 10,000
= a gain of 40 dB
Gain Control
• Determines amount of amplification that
occurs in the amplifier
• Too little gain - weak echoes are not imaged
• Too much gain, saturation occurs;
i.e. - echoes appear bright & differences in
echo strength are not seen
GAIN
GAIN
Amplifiers
Have 60 - 100 dB range of gain
Transducer voltages range from a few
microvolts (μV) for blood to a few hundred
millivolts (mV) from bone or gas
I.E. - 60 dB gain amplifier - output power is
1,000,000 X the input power & the output
voltage is 1,000 X the input. with a 10-μV
voltage input, the output voltage is 10 mV.
If the gain is increased to 100 dB, the
output voltage increases to 1 V
Recall:
3 dB = a power gain of X 2
10 dB = X 10.
Values can also be combined;
13 dB corresponds to (X 2 X 10) or X 20
Compensation
Amplifier compensates for the weaker
echoes to return from a distance due
to the attenuation that limits the depth
we can image
time gain compensation [TGC] &
depth gain compensation [DGC]
equalizes differences in received
echo amplitudes caused by different
reflector depths
Compensation
If reflectors with equal reflection coefficients
vary in distance from the transducer, then
the reflectors will not result in echoes of
equal amplitude arriving at the transducer
due to attenuation
Longer path lengths result in greater
attenuation & later arrival times
Compensation
These amplitudes must be adjusted to
compensate for differences in attenuation
so we can display echoes from similar
reflectors at a similar brightness
Voltages from echoes arriving later are
amplified to a greater degree than earlier
ones. The later an echo arrives, the farther
it traveled, the weaker it is and the more it
needs to be amplified.
TGC
When properly used, it appears as if
there had been no attenuation
Increase of gain with depth is
commonly called the TGC slope
TGC slope is in decibels of gain per
centimeter of depth
TGC
Sonographer adjusts the TGC to compensate for
the attenuation of the tissues being imaged to
achieve, an average uniform brightness
throughout the image
Typical TGC amplifiers compensate for about 60
dB of attenuation
TGC
Beam Former consists of:
Pulser
Pulse delays
Transmit/receive switch
Amplifiers
Analog-to-digital converters
Echo delays
Summer
Analog-to-Digital Converters (Digitizers)
Analog echo signal is in a foreign
language to the digital scan converter
that only reads numbers or digits
ADCs convert the analog voltages
representing echoes to numbers for
digital signal processing and storage
Beam Former consists of:
Pulser
Pulse delays
Transmit/receive switch
Amplifiers
Analog-to-digital converters
Echo delays
Summer
Echo Delays
Digitized echo voltages pass through
digital delay lines to accomplish reception
dynamic focus & steering functions
Beam Former consists of:
Pulser
Pulse delays
Transmit/receive switch
Amplifiers
Analog-to-digital converters
Echo delays
Summer
Summer
The properly delayed channel signal
components are added together
(summed) to produce the scan line
Reception apodization & dynamic
aperture functions are also performed
Pulse Echo Imaging Systems
beam former
signal processor
image processor
display
Signal Processor
Filter
Detection
Compression
Signal Processor
Bandpass filtering - digital filtering
Amplitude detection (RF to video) –
detection of the amplified, compensated,
summed, returning echoes
Compression (dynamic range reduction)
compression of the amplified,
compensated, summed, returning echoes
Signal Processor
Filter
Detection
Compression
Filtering
aka reject, threshold, suppression
Amplifiers with an electronic filter, called
a bandpass filter, are used to reduce
noise in the electronics.
Bandpass filter permits a range of
frequencies (its bandwidth) & rejects
those above & below the bandwidth
Signal Processor
Filter
Detection
Compression
Detection
- Conversion of echo voltages from
radio frequency (RF) form to video
form while retaining the echo voltage
amplitudes (AKA - demodulation)
Demodulation consists of
1. Rectification - process where the
negative half of the signal is
eliminated (turns all of the negative
voltages into positive voltages)
2. Smoothing or enveloping is the
process where all the peaks are
smoothed into one signal
Rectification
Smoothing
Signal Processor
Filter
Detection
Compression
Compression
Process of  the differences between the
smallest & largest echo amplitudes
Dynamic range - ratio of the largest to the
smallest amplitude that a system can
handle & is in dB
Example
An amplifier is sensitive to voltage amplitudes
ranging from .01 mV - 1,000 mV
ratio of voltage extremes = 1000/0.01, or 100,000
Power ratio is equal to the square of the voltage
ratio: (100,000)2, or 10,000,000,000
dynamic range of the amplifier is 100 dB
Dynamic Range Control
Sonographer adjusts this control to
perform compression
A smaller dynamic range setting presents
a higher- contrast image
Compression Control
Compression Control
Pulse Echo Imaging Systems
beam former
signal processor
image processor
display
DISPLAY
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POST PROCESSOR
IMAGE MEMORY
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PRE-PROCESSOR
SCAN CONVERTER
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COMPRESSION
DETECTION
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SUMMER
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ECHO
DELAYS
PULSER
PULSE
DELAYS
T/R
ADCS
AMPS
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Image Processor
Scan Converter
Pre-processor
Image Memory Processor
Post Processor
Digital – Analog Converter
Image Processor
- converts digitized, filtered, detected,
& compressed scan-line data into
images that are processed before and
after storage in image memory for
presentation on the monitor display
Image Processor Functions
1. converts the scan line data into a usable
format for image display on the monitor
2. processes the image data before
(pre-processing), during, & after
(post-processing) memory storage
Image Processor
Scan Converter
Pre-processor
Image Memory Processor
Post Processor
Digital – Analog Converter
Scan Converter
- Reformats echo data into an image format for
image processing, storage, & display
- In a fraction of a second, it corresponds each
echo from every pulse emitted from the
transducer to a scan line on the display monitor
yielding 1 frame of image information
- Process is repeated several times/second to
produce a rapid sequence of frames stored in
memory & presented on the display; known as
real-time
Scan Conversion
Scan Conversion
Digital scan converter uses a computer
and computer memory to digitize images
Digitizing images - converts echo data
into binary numbers (0’s and 1’s) &
stores them in memory
Binary numbers are later “converted” for
display of the ultrasound unit monitor
Scan Conversion
Advantage: provides stability, uniformity,
accuracy & storage

Analog scan converter - early type of
converter for gray-scale imaging
Image Processor
Scan Converter
Pre-processor
Image Memory Processor
Post Processor
Digital – Analog Converter
Preprocessing
Signal & image processing done before
storage of echo data in memory
Examples - persistence, panoramic
imaging, spatial compounding & 3D
Broader view of preprocessing includes
functions performed before scan
conversion, such as TGC, Selective
Enhancement, Logarithmic Compression,
Image Update, and RES or Write Zoom
Preprocessing
Persistence
Averages the information on sequential
frames to provide a smoother image
Process reduces noise (speckle), 
dynamic range and contrast resolution
 frame rate – images appear to lag
behind the scanning
Low level – rapid moving structures
Hi level –slow-moving structures
Panoramic Imaging
Sequentially displays several frames of
image information on the monitor
Provides an image that has a wider field of
view than what the transducer could
normally produced
Aids in relating the location of different
structures on one image
Spatial Compounding
Accomplished by phasing the beam in
multiple directions & then averaging the
sequential frames to produce one frame
for monitor display
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 speckle
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 artifacts

 the chance of image information resulting
from normal incidence
3D
- computer processing of many parallel
2D scans that present on the monitor as a
3D image
Image presentation methods
Surface rendering (common in OB)
Transparent views
2D slices
Write zoom
(AKA - write magnification or regional expansion)
- zooms in or enlarges the image before it is
stored in the scan converter
1. Sonographer selects a Region of Interest
(ROI) in the image to be magnified
2. US system rescans on the ROI resulting in
new information collection
3. All the scan lines produced by the
transducer are now squeezed into the ROI.
With more pixels in the ROI, there is
 detail and spatial resolution
Preprocessing
Can only be done on an active or unfrozen
image & is accomplished BEFORE the echo
is stored in the memory
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If you can only use that button when you have a
live image it is a is a pre-processing function
If the button on your machine becomes inactive
once you press the freeze button, then it is a preprocessing function
Image Processor
Scan Converter
Pre-processor
Image Memory Processor
Post Processor
Digital – Analog Converter
Image Memory
After echo data are scan-converted into image form and
preprocessed, the 2D image frames are stored in image
memory (random access memory or RAM)
Storing each cross-sectional image in memory as the
sound beam is scanned through the anatomy permits
display of a single image (frame) out of the rapid
sequence of several frames acquired each second
Displaying 1 frame out of the sequence is known as the
freeze-frame
Most instruments store the last several frames acquired
prior to freezing. This is called the cine-loop, cine
review, or image review feature.
Computer Memory Terminology
Bit (binary digit) - smallest unit of digital
computer memory
Only two values in binary system - 1 & 0
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1 or ‘on’ is represented by a white echo
0 or ‘off’ is represented by a black echo
Computer Memory Terminology
Byte - eight bits of computer memory
Kilobyte - 1024 bytes (8192 bits)
Pixel (picture element) - smallest element
of a digital picture
a number is stored that corresponds to
the echo strength received in each of the
pixel locations in memory
 pixels/inch to create the image =
 image’s spatial & detail resolution
Converting binary digits to base 10
1. Label the columns or places for each digit in a
binary number are:
64
32 16 8
4
2
1
2. Write in the number underneath the columns.
For example, 10010:
64
32 16 8
4
2
1
1
0
0
1
0
3. Add the numbers that have a one underneath
them to find the answer in base ten.
64
32 16 8
4
2
1
1
0
0
1
0
= 16 + 2 = 18
Computer Memory Terminology
Matrix - a table divided into many cubicles (pixels)
that stores the digitized scan line
Storage capacity varies with matrix size

512 X 512 matrix has 512 rows and 512 columns
or 262,144 pixels per matrix
The greater the number of pixels in the matrix,
the better the spatial resolution on the image

512 X 640 matrix has better resolution than a
512 X 512 matrix
Matrix – Pixel Relationship
PIXEL
MATRIX
Digital Memory
When digital memory consists of a single
matrix, each pixel storage location is only
a single number: a “0” or a “1”
is like an on-off switch and can operate
only in 2 conditions – off or on (“0” or “1”)
each pixel is assigned either a “0” or “1”
Single matrix only allows bistable (blackand-white) imaging
Multi-Bit Memory
To image several shades of
gray, (including black &
white) it is necessary to store
one of several numbers in
each memory location
This requires the memory to
have more than one matrix
Multi Bit Memory
Matrixes can be thought of as being layered
back-to-back
In a 4-bit memory, there are 4 matrixes back to
back; each pixel has 4 bits associated with it
In the binary system, this allows numbers from
0 to 15 to be stored (a 16-shade system)
4-bit memory = 16-shade memory
4-bit memory has 4 binary digits assigned to
each pixel
Adding in a 4-bit memory
1
2
3
4
Common ultrasound instruments today
use 8-bit memories
Human vision can only differentiate
about 100 gray shade levels
# of shades of gray displayed by a scan
converter is determined by:
2n, where n = # of bits
Example:
How many shades of gray are displayed by a
5-bit scan converter?
Using the equation above write 2 down 5 times
then multiply across:
2 X 2 X 2 X 2 X 2 = 32
32 possible shades of gray can be stored in
each pixel with a 5-bit scan converter
Contrast Resolution
ability of a gray-scale display to distinguish between
echoes of slightly different intensities
depends on the # of bits/pixel in the image memory
echo intensities are assigned a number, the dynamic
range of the echoes is equally distributed among the
number of gray levels the system is capable of
In 4- & 8-bit systems, the difference in dynamic range
covered by each shade would cause the same shade
to be assigned a different number in memory
Increasing the # of bits/pixel (more gray shades)
improves contrast resolution
3-bit VS 5-bit
Contrast Resolution comparison
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2
dB 10
11
2
dB 10
4
11
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12
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2 4 26
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Image Processor
Scan Converter
Pre-processor
Image Memory Processor
Post Processor
Digital – Analog Converter
Postprocessing
processing performed on image data
retrieved from memory
assigns specific display brightness to
numbers retrieved from memory
is controlled by the sonographer
Determining if a Function is
Preprocessing or Postprocessing
Postprocessing functions can be performed
on frozen or active images
If it cannot be performed on a frozen image it is a preprocessing function
Post-processing Functions
Read magnification or zoom
Measuring capabilities
Post process button
B color
Zoom
Sometimes called read magnification magnification of the image performed
with stored information
Pixels are enlarged to fill the screen
& do not improve spatial resolution
B Color
Postprocessing that improves contrast
resolution by assigning colors, rather than
gray shades, to different echo strengths
Human eye can differentiate more color
tints than gray shades, color displays offer
improved contrast resolution capability
Post Processing Curves
Preprogrammed, postprocessing brightness
assignment schemes (curves) are selectable by the
operator. Other postprocessing curve may be
designed as desired by the operator using panel
controls.
A linear assignment equally divides the display
brightness range among the stored gray levels of the
system. Other schemes may be used that allow
assignment of more of the brightness range to certain
portions of the stored-number range capability of the
system.
Use of Post processing curves can improve the
presentation and perception of small echo strength
differences stored in memory (improved contrast
resolution)
Image Processor
Scan Converter
Pre-processor
Image Memory Processor
Post Processor
Digital – Analog Converter
Digital to Analog Converter (DAC)
- Converts the digital signal back into an analog
signal for viewing on the monitor
-
Monitor only reads a video or analog signal so the
digital signal coming out of the memory must be
changed back
- Analog signal that emerges from the receiver
could go directly to the monitor if we only
wanted to see 1 echo scan line at a time. We
rather see whole frames of images.
- Entire signal path is Analog-Digital-Analog
Scan Line
1 pulse of US creates a
single scan line (from a
series of returning
echoes) in < 1 mSec
Echoes are presented in sequence on a scan line as they return from tissue.
(A) 1st echo is displayed. (B) 2nd echo is added. (C) More echoes are added.
(D) All the echoes from a single pulse have been received and displayed as a
completed scan line.
Pulse Echo Imaging Systems
beam former
signal processor
image processor
display