Ultrasound Beamformation and Image Reconstruction
AAPM 2009 Conference
G. Wayne Moore, B.Sc., MA, FASE
What we will cover
• Historical Perspective of Ultrasound Technology
• Ultrasound and Tissue Water
• Partition of Ultrasonic Imaging Events
• Image Quality Advances
• Ultrasound System Architecture Changes
• Beamformation Analysis and Test Device
• Modern Approaches to Solving Old Imaging Problems
• Newer Approaches to Image Reconstruction – 2D Matrix Arrays
• Test Fixture and 3D Volumetric Image Reconstruction Analysis
• What’s Coming in the Future for Ultrasound
We’
Water””!
We’re Just “Bags of Water
Ultrasound and Tissue Water
• An archetypal person has about 40 L of
body fluid
• 25 L are inside 75 trillion cells
• 15 L are outside the cells (interstitial spaces)
• 5 L in blood volume
– 2 L of RBCs
– 3 L of plasma
Still the law!
1540m/sec
6.5B
6.5
Bsec/cm
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Partition of Ultrasonic Imaging Events
The scanhead or
Transducer
History of Modern Diagnostic Ultrasound
Transmit signal out
Echo signal return
2
Specialized Signal
Processing
Gray Scale
Image
Processing
1
TissueTissueultrasound
Doppler
Image
Processing
3
6
In
co
mp
lex
≅t
ity
Color/Gray scale
display
Mass Storage:
VCR
CD
5
PACS
LAN
4
1979
2009
•A-Mode
•M-Mode
•B-Scan
•Mechanically Steered Real-time
•Linear Array Real-time
•Zero-crossing Doppler
•Spectral Doppler (FFT)
•Phased Array Real-time
•CW Doppler in Probe – Bernoulli Equation
•Color Flow Doppler
•Triplex modes of Operation
•Pseudo-Color
•Color “Angio” & Power Doppler
•Harmonic Imaging & Variants
•Contrast Media Imaging
•Spatial Compounding
•3D Imaging – Volumetric
•4D Imaging – Steered Curved Array
•Phase Aberration Correction
3D Volumetric cardiac image
derived from 2D Matrix Array
with 2,500 elements
pseudo Color display identifies
proximal and distal echoes
Some 3D Clinical Advantages:
Advantages
1985
1997
No geometric assumptions
No mental interpolations
(cognitive reconstruction)
Provides pre-surgical perspective
Provides post-surgical perspective
Pyramidal volume of ultrasonic data
2
+
Evolution of Ultrasound System Architecture
Proprietary
Hardware
Centric Design
Proprietary Hardware Centric
Hybrid Centric
Proprietary Software Centric
Software + Generalized Backend Centric
Single Platform
Microsoft XP Embedded
Software Platform
Specialized Front End
Generalized Back End
Proprietary Software
Centric Design
TGC Application
The digital beamformer
function has historically
resided in the main console
of the ultrasound system
Shift from analog to digital beamformers
PA
ADC
PA
ADC
PA
ADC
PA
ADC
PA
ADC
PA
ADC
PA
ADC
PA
ADC
PA
ADC
128 Element
Transducer
PA = Pre amplifier
ADC = Analog to digital converter
Dynamic Time Delays
Change of System Technologies Since 1990
=
Σ
10 bits 36 MHz
1024 Values
60 dB Dynamic Range
3
Digital beamforming 2009
Higher element counts were once
restricted by the need to have a
single wire in the probe cable for each
element in the array. Multiplexing the
elements allowed for transducers with
higher element counts and a lower
number of transmit channels in the
beamformer. Current 2D matrix
arrays with element counts of 2,500
necessitated a different approach to
beamforming, and where the
beamforming, or at least parts of it
would take place.
Beamformation Analysis
Text--fixture and setText
set-up
High speed serial interface that provides control signals for the sub-beamformer
e.g., defining delays for each unique element
x
Sub-beamforming
performed in the
probe
x
(f)
x(f)
x(f)
100 Channels feeding
& receiving
2,500 elements
Would be normally what
a “standard” front end
would expect to see
Time sampling is digital, all else is analog
Commercially available
ultrasound probe and
system testing device
using a single crystal to
both receive signals from
the probe and transmit
signals into the system
via the probe.
4
Green LED
Red LED
Fundamental Imaging
sample rate gaps in
phased array sector
display
Test Device – Block Diagram
Power On Timer
200 mSec
ON
OFF
Threshold timer
10 mSec
Amplifier
Comparator
Transducer
+
Threshold Level
Target
3-5 cm
5-7 cm
7-9 cm
10MHz 3-5cm
5MHz 5-7cm
26µsec = 2cm
26µsec = 2cm
2nd Harmonic Imaging
Harmonic Image
Fundamental Image
Generator
10 MHz
5 MHz
2.5 MHz
Target Transmitter
2.5MHz 7-9cm
26µsec = 2cm
An example of expanding
aperture focusing used to
maintain spatial resolution
throughout depth of field
Active aperture changes as
a function of depth
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Old imaging problems, new imaging approaches
Transducer
Scanning Field
Shadowing
Object
Shadow
Still the law!
1540m/sec
6.5B
6.5
Bsec/cm
Objects that are very reflective or
very attenuating remove
ultrasound from the image area
behind the mass producing
shadows. These events mark a
specific interaction between
ultrasound and tissues, and a
property of the shadowing
tissues.
Shadowing in Propagation Events
Probe
Transducer
Scanning Field
Refractive
Surface
Refractive
Shadow
Circular or cystic structures can
produce refraction at the edge of
the structure, bending the beam
into the object, producing
shadows extending from the edge
of the object. This happens with a
change in propagation velocity,
but does not happen if there are
no velocity changes.
Ultrasound Refraction at the Edge of a Circular Object
Breast
Lesion
Normal Scan Lines
SonoCT
Scan Lines
Spatial Compounding
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Limited Fields of View
Spatial
Compounding
Using Multiple
Apertures and
Angles
SieScape by Siemens
Slice
Motion
With a structure identifying algorithm, the system
locates and positions major structures, calculating the
structure location in the continuous slicing scan of the
transducer. This technique is equivalent to compound
B-scanning without a scanning arm.
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Complex Beamforming in 4D Systems
20 Degree Angle
Text--fixture and setText
set-up for 4D Image Formation Analysis
Using a Philips iE33 Ultrasound System and X7X7-2t probe
In order to create “real-time” 4D ultrasound images
(i.e., three spatial dimensions plus motion), volume
datasets must be acquired, processed and
displayed at a rate higher than the capacity of the
human eye to retain a visual impression
60 Degree Angle
Bench SetSet-up
Nickel (PVDF)
Tektronix Scope
RF Power Amplifier
Probe/Nickel Fixture
2D Matrix Array 2,500 elements
Frame rate paces random access to the array
8
TEE/Nickel Interface
Position/Pressure
Control Mechanism
3
2
3
2
1
2
3
2
3
X7-2t
5x5 element sub-aperture
Philips X7-2t probe
2500 elements
100 channels
Inside the X7X7-2t
Potted Passive
Components
Array Carrier
CCD
X7--2t
Inside the X7
Flex to Array
Interface
Flex Circuit
Backing
Material
Gold Foil
I/O connection for
Integrated Circuit
Tip Collar
Coax to
Flex Circuit
Interface
Transducer
Array
NonNon-insulated Bm
thick bonding wires*
9
System Interface
Input signal from Nickel
4Bsec burst @ 5MHz
* common mode rejection ratio
Battery powered
Diff PrePre-amplifier
good high freq
detection and CMRR*
Detecting xmit, generates
target, gets amplified by
the RF amp
Ultrasound system pulse
Noise feed through
from RF amp
Nickel pulse
Noise + Acoustic
Acoustic signal only
Receive out of ultrasound system
Nickel transmit pulse matches every other
system transmit creating one line gaps in
sector display shown above
10
3D image rotated
Nickel transmit
pulse displayed
across all slices
½ sector switching
3D image Nickel transmit pulse displayed
Ni Sys
Ni Sys
Signal In Front
Mixed mode (Color Flow Doppler and Imaging)
Reduced frame rate approximately matches every
Nickel pulse thereby “filling in” each sector line
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Signal In Middle
So What is Coming Next?
Signal In Back
Major innovation in ultrasound will slow, but…
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…ultrasound still must remain competitive with other
so…
…
advanced imaging modalities such as MR, so
(1) 2D Matrix Arrays will be developed for broader clinical application
For example, extensions of ultrasound
applications based on clinical need and
cost effectiveness
Standard Imaging
System Connector
Standard Ultrasound Cable
(2) Design and optimization of capacitive micro-machined ultrasonic
transducers – silicon substrates may not be the avenue
1. Wrist-Mounted
Connector
(3) Extended sub-beamformer + CMUTs
2. Flat Flex Cable
3. Small-profile,
finger-mounted array
END PRESENTATION
Thank You
Nobility of Soul Never Kept a Business in the Black
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