Ultrasound Beamforming and Image Formationpeople.duke.edu/~jjd/RSNA_USbeamforming.pdf · Ultrasound...

Ultrasound Beamforming and Image Formation

Ultrasound Beamforming and

Image Formation

Jeremy J. Dahl

Duke University Page 1

Overview

➠ Ultrasound Concepts

➠ Beamforming

➠ Image Formation

➠ Absorption and TGC

➠ Advanced Beamforming Techniques

➛ Synthetic Receive Aperture

➛ Parallel Beamforming

➛ Spatial Compounding

➛ Adaptive Beamforming

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

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

Coordinate System

Elevation (y)

Azimuthal (x)

Axial (z)

TransducerElements

☞ BEAMFORMING

➠ Image Formation

Transmit Beamforming

System TimeDelays Scattering Medium

Receive Beamforming

ScatteringMedium

AlignmentSignal

Summed RF Data(RF Line out)

DelaysSystem Time

� ��

PhasedLinear

Transducer Array

Fixed Focus Beamforming

Azimuthal Span (mm)

−1 0 1

35−1 0 1

Azimuthal Span (mm)

Fixed Focus Beamforming

−5 0 5

(mm)−5 0 5

Dynamic-Receive Beamforming

System Time Delays

Propagation Direction

Transducer

Azimuthal Span (mm)

−1 0 1

35−1 0 1

Azimuthal Span (mm)

−5 0 5

(mm)−5 0 5

Aperture Growth and Apodization

ApodizationWeight:

ApertureGrowth:

Time: t t t1 2 3

Unused TransducerElements

Azimuthal Span (mm)

−1 0 1

35−1 0 1

Azimuthal Span (mm)

−5 0 5

(mm)−5 0 5

➠ Beamforming

☞ IMAGE FORMATION

Radio-Frequency (RF) Image

Envelope Detection

Envelope

Signal with Carrier Frequency

Envelope Detection

π fsin 2 0

fπcos 2 0 +RF Line in

Filter

processing filtersTo other post−

andMapping

ICompression

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

➠ Beamforming

➠ Image Formation

☞ ABSORPTION AND TGC

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.

5.7 MHz 8.0 MHz 10.0 MHz

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.

Without TGC With TGC

Advanced Beamforming Techniques

➠ Synthetic Receive Aperture

➠ Parallel Beamforming

➠ Spatial Compounding

➠ Adaptive Beamforming

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.

Second TransmitFirst Transmit

Transmitting

Receiving

Transmitting and Receiving

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.

TransmitBeam

Receive Beams

Transducer

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.

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

pNormalp Spatial Compounding

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.

➠ 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.

➠ Some of the effects of aberration

➛ Reduced image brightness

➛ Loss in resolution

➛ Obscured targets

➛ Image artifacts

Control Aberrated

➠ 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

The Near-Field Phase Screen Model

Scattering Medium

Phase ErrorSignal Misalignment −

Summed RF

Phase−ScreenNear Field

System TimeDelays

A Distributed Aberration Model

Summed RFData

System TimeDelays

Mid−RangePhase−Screen

Propagation of Distorted Wavefronts

ScatteringMedium

Aberrated Corrected

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.

Thank You

Questions?

Ultrasound Beamforming and Image Formationpeople.duke.edu/~jjd/RSNA_USbeamforming.pdf · Ultrasound...

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