Ultrasound Beamforming and Image Formation
Ultrasound Beamforming and
Image Formation
Jeremy J. Dahl
Duke University Page 1
Ultrasound Beamforming and Image Formation
Overview
➠ Ultrasound Concepts
➠ Beamforming
➠ Image Formation
➠ Absorption and TGC
➠ Advanced Beamforming Techniques
➛ Synthetic Receive Aperture
➛ Parallel Beamforming
➛ Spatial Compounding
➛ Adaptive Beamforming
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Ultrasound Beamforming and Image Formation
Ultrasonic Imaging
➠ Use acoustic (pressure) waves to form images
➠ Frequency range: 1-20 MHz
➠ Tomographic view: imaging plane is orthogonal to the surface
➠ Pulse-echo imaging
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Ultrasound Beamforming and Image Formation
Ultrasound System
Transducer
Scan Conversion and Display
Signal Processing
Beamformer
− IQ Computation − Magnitude Calculation − Compression − Filtering − Flow Processing − Image Mode Processing
− Summation − Geometric Focal Delays − A/D Conversion − TGC
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Ultrasound Beamforming and Image Formation
Coordinate System
Elevation (y)
Azimuthal (x)
Axial (z)
θ
TransducerElements
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Ultrasound Beamforming and Image Formation
➠ Ultrasound Concepts
☞ BEAMFORMING
➠ Image Formation
➠ Absorption and TGC
➠ Advanced Beamforming Techniques
➛ Synthetic Receive Aperture
➛ Parallel Beamforming
➛ Spatial Compounding
➛ Adaptive Beamforming
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Ultrasound Beamforming and Image Formation
Transmit Beamforming
τ 1
τ 2
τ 3
τ 4
τ 5
System TimeDelays Scattering Medium
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Ultrasound Beamforming and Image Formation
Receive Beamforming
τ 1
τ 2
τ 3
τ 4
τ 5
ScatteringMedium
Σ
AlignmentSignal
Summed RF Data(RF Line out)
DelaysSystem Time
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Ultrasound Beamforming and Image Formation
� �� �� �� � � �� �
PhasedLinear
Beams
Transducer Array
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Ultrasound Beamforming and Image Formation
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Ultrasound Beamforming and Image Formation
Fixed Focus Beamforming
Azimuthal Span (mm)
Dep
th (
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Azimuthal Span (mm)
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th (
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Ultrasound Beamforming and Image Formation
Fixed Focus Beamforming
(mm)
Dep
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Ultrasound Beamforming and Image Formation
Dynamic-Receive Beamforming
System Time Delays
Propagation Direction
Transducer
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Ultrasound Beamforming and Image Formation
Dynamic-Receive Beamforming
Azimuthal Span (mm)
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th (
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Ultrasound Beamforming and Image Formation
Dynamic-Receive Beamforming
(mm)
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Ultrasound Beamforming and Image Formation
Aperture Growth and Apodization
Dep
th
ApodizationWeight:
ApertureGrowth:
Time: t t t1 2 3
0
1
0
1
0
1
Unused TransducerElements
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Ultrasound Beamforming and Image Formation
Aperture Growth and Apodization
Azimuthal Span (mm)
Dep
th (
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−1 0 1
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Azimuthal Span (mm)
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th (
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Ultrasound Beamforming and Image Formation
Aperture Growth and Apodization
(mm)
Dep
th (
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th (
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Ultrasound Beamforming and Image Formation
➠ Ultrasound Concepts
➠ Beamforming
☞ IMAGE FORMATION
➠ Absorption and TGC
➠ Advanced Beamforming Techniques
➛ Synthetic Receive Aperture
➛ Parallel Beamforming
➛ Spatial Compounding
➛ Adaptive Beamforming
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Ultrasound Beamforming and Image Formation
Radio-Frequency (RF) Image
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Ultrasound Beamforming and Image Formation
Envelope Detection
Envelope
Signal with Carrier Frequency
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Ultrasound Beamforming and Image Formation
Envelope Detection
π fsin 2 0
fπcos 2 0 +RF Line in
Q
Filter
Filter
processing filtersTo other post−
andMapping
ICompression
2 2
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Ultrasound Beamforming and Image Formation
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Ultrasound Beamforming and Image Formation
Compression and Gray Scale Mapping
➠ The dynamic range of the envelope detected signals is still to large to provide
useful images. Bright targets can drown out the low signals of important
structures.
➠ Compression and gray scale mapping techniques are used to reduce the
dynamic range.
0 0.2 0.4 0.6 0.8 10
0.2
0.4
0.6
0.8
1
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Ultrasound Beamforming and Image Formation
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Ultrasound Beamforming and Image Formation
➠ Ultrasound Concepts
➠ Beamforming
➠ Image Formation
☞ ABSORPTION AND TGC
➠ Advanced Beamforming Techniques
➛ Synthetic Receive Aperture
➛ Parallel Beamforming
➛ Spatial Compounding
➛ Adaptive Beamforming
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Ultrasound Beamforming and Image Formation
Absorption
➠ Not all of the transmitted ultrasonic energy is reflected. In fact, most of the
transmitted energy is absorbed by the tissue. The typical rate of absorption
of ultrasonic energy is 0.5 decibels per centimeter per Megahertz.
➠ For example, an acoustical pulse at 5 MHz that travels 10 cm into tissue loses
25 dB of it’s signal strength (in other words, is about 1/18th of the original
amplitude).
➠ Absorption is frequency dependent: The higher the frequency, the greater
the absorption. Although resolution is better at the higher frequencies, the
penetration of the ultrasound signal is not as good as the low frequencies.
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Ultrasound Beamforming and Image Formation
5.7 MHz 8.0 MHz 10.0 MHz
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Ultrasound Beamforming and Image Formation
Time-Gain Compensation (TGC)
➠ Time-gain compensation is used to counteract the effects of absorption. Gain
is applied to the signal as a function of time (or distance).
➠ Manufacturers apply pre-determined TGC to the ultrasonic signals, however
still allow the user some control of the gain with depth.
➠ Gain can be applied down to reasonable depths depending on the frequency.
At some point, however, the SNR of the signal is so low that applying any
TGC only serves to amplify noise.
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Ultrasound Beamforming and Image Formation
Without TGC With TGC
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Ultrasound Beamforming and Image Formation
Advanced Beamforming Techniques
➠ Synthetic Receive Aperture
➠ Parallel Beamforming
➠ Spatial Compounding
➠ Adaptive Beamforming
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Ultrasound Beamforming and Image Formation
Synthetic Receive Aperture
➠ Synthetic receive aperture imaging emulates a larger transducer when a sys-
tem’s available beamforming channel count is smaller than the number of
elements in the transducer.
➠ The beamforming is considered synthetic because multiple transmits are
used to construct the beam as if it were received on the entire transducer
at once.
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Ultrasound Beamforming and Image Formation
Second TransmitFirst Transmit
Transmitting
Receiving
Transmitting and Receiving
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Ultrasound Beamforming and Image Formation
Parallel Receive Beamforming
➠ Parallel receive beamforming, also known as “Explososcanning,” is a method
of beamforming that forms multiple receive beams from a single transmit
event.
➠ In parallel receive beamforming, a broad transmit beam is fired, and multiple
receive beams are formed within the bounds of the transmit beam.
➠ Parallel receive beamforming is used to increase frame rate. This is most
useful when the imaging deep within tissue, or real-time 3-D imaging is de-
sired.
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Ultrasound Beamforming and Image Formation
TransmitBeam
Receive Beams
Transducer
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Ultrasound Beamforming and Image Formation
Spatial Compounding
➠ All ultrasound images suffer from coherent noise, called speckle. Speckle
results from the constructive and destructive wave interference of reflections
from sub-resolution scatterers, and gives the image a grainy appearance.
➠ Speckle reduces the visible resolution by a factor of 10.
➠ Spatial compounding is a means by which the effects of speckle can be re-
duced.
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Ultrasound Beamforming and Image Formation
Spatial Compounding
➠ In spatial compounding, multiple images of the same target are averaged in
order to reduce the coherent noise.
➠ Each image must contain uncorrelated speckle patterns.
➠ Many ways to obtain uncorrelated speckle patterns:
➛ Divide the transducer into small sub-apertures
➛ Change the steering angle of the beams
➛ Physically translate the transducer
➛ Change the transmit frequency
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Ultrasound Beamforming and Image Formation
pNormalp Spatial Compounding
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Ultrasound Beamforming and Image Formation
Adaptive Beamforming
➠ Up to this point, we’ve assumed that the sound speed in human tissue is a
constant (1540 m/s).
➠ This is just an average of the soft tissue sound speed.
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Ultrasound Beamforming and Image Formation
Adaptive Beamforming
➠ Because the sound speed can change from tissue to tissue, AND because
the thickness of these tissues vary from location to location, the sound wave
used for ultrasonic imaging can become distorted.
➠ The distortion in the sound wave is called ABERRATION in adaptive beam-
forming.
➠ In adaptive beamforming (also called adaptive imaging) we attempt to com-
pensate the beamformer for the aberration.
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Ultrasound Beamforming and Image Formation
Adaptive Beamforming
➠ Some of the effects of aberration
➛ Reduced image brightness
➛ Loss in resolution
➛ Obscured targets
➛ Image artifacts
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Ultrasound Beamforming and Image Formation
Control Aberrated
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Ultrasound Beamforming and Image Formation
Adaptive Beamforming
➠ Many methods have been created to compensate for aberration.
➠ They generally fall into two classes, based on the model of aberration used:
➛ Near-field phase-screen models
➛ Distributed aberration models
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Ultrasound Beamforming and Image Formation
The Near-Field Phase Screen Model
Σ
τ
τ
τ
τ
τ
5
4
3
2
1
Scattering Medium
Phase ErrorSignal Misalignment −
Summed RF
Phase−ScreenNear Field
System TimeDelays
Data
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Ultrasound Beamforming and Image Formation
A Distributed Aberration Model
Σ
τ
τ
τ
τ
τ
5
4
3
2
1
Summed RFData
System TimeDelays
Mid−RangePhase−Screen
Propagation of Distorted Wavefronts
ScatteringMedium
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Ultrasound Beamforming and Image Formation
Aberrated Corrected
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Ultrasound Beamforming and Image Formation
Challenges in Adaptive Beamforming
➠ Requires access to the channel signals. Most manufacturers do not provide
access to these signals. In addition, the volume of data created by the chan-
nel signals is extremely large.
➠ Significant computational effort.
➠ Low frame rates - relatively few have attempted to make adaptive beamform-
ing work with real-time imaging.
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Ultrasound Beamforming and Image Formation
Thank You
Questions?
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