Ultrasound Beamforming and Image Formation Jeremy J. Dahl
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 Beamformer TGC A/D Conversion Geometric Focal Delays Summation Signal Processing IQ Computation Magnitude Calculation Compression Filtering Flow Processing Image Mode Processing Scan Conversion and Display
Coordinate System Elevation (y) Azimuthal (x) Transducer Elements θ Axial (z)
Ultrasound Concepts BEAMFORMING Image Formation Absorption and TGC Advanced Beamforming Techniques Synthetic Receive Aperture Parallel Beamforming Spatial Compounding Adaptive Beamforming
Transmit Beamforming τ 1 τ 5 τ 4 τ 3 τ 2 Transmit Beamforming Scattering Medium System Time Delays
Receive Beamforming Signal Alignment Scattering Medium τ 1 Summed RF Data (RF Line out) Σ τ 2 τ 3 τ 4 τ 5 System Time Delays
Ultrasound Beamforming and Image Formation Beams Linear Phased Transducer Array
Fixed Focus Beamforming 10 10 15 15 Depth (mm) 20 Depth (mm) 20 25 25 30 30 35 1 0 1 Azimuthal Span (mm) 35 1 0 1 Azimuthal Span (mm)
Fixed Focus Beamforming 12 14 16 12 14 16 Depth (mm) 18 20 22 24 Depth (mm) 18 20 22 24 26 28 30 5 0 5 (mm) 26 28 30 5 0 5 (mm)
Dynamic-Receive Beamforming System Time Delays Propagation Direction Transducer
Dynamic-Receive Beamforming 10 10 15 15 Depth (mm) 20 Depth (mm) 20 25 25 30 30 35 1 0 1 Azimuthal Span (mm) 35 1 0 1 Azimuthal Span (mm)
Dynamic-Receive Beamforming 12 14 16 12 14 16 Depth (mm) 18 20 22 24 Depth (mm) 18 20 22 24 26 28 30 5 0 5 (mm) 26 28 30 5 0 5 (mm)
Aperture Growth and Apodization Time: t1 t 2 t 3 Apodization Weight: 1 1 1 Aperture Growth: 0 0 0 Unused Transducer Elements
Aperture Growth and Apodization 10 10 15 15 Depth (mm) 20 Depth (mm) 20 25 25 30 30 35 1 0 1 Azimuthal Span (mm) 35 1 0 1 Azimuthal Span (mm)
Aperture Growth and Apodization 12 14 16 12 14 16 Depth (mm) 18 20 22 24 Depth (mm) 18 20 22 24 26 28 30 5 0 5 (mm) 26 28 30 5 0 5 (mm)
Ultrasound Concepts Beamforming IMAGE FORMATION Absorption and TGC Advanced Beamforming Techniques Synthetic Receive Aperture Parallel Beamforming Spatial Compounding Adaptive Beamforming
Radio-Frequency (RF) Image
Envelope Detection Envelope Signal with Carrier Frequency
Envelope Detection Filter RF Line in cos 2 π f 0 I 2 + Q 2 Compression and Mapping To other post processing filters Filter sin 2 π f 0
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. 1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 0 0.2 0.4 0.6 0.8 1
Ultrasound Concepts Beamforming Image Formation ABSORPTION AND TGC Advanced Beamforming Techniques Synthetic Receive Aperture Parallel Beamforming Spatial Compounding Adaptive Beamforming
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 system 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.
First Transmit Second 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 desired.
Transducer Transmit Beam Receive Beams
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 reduced.
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.
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 beamforming. In adaptive beamforming (also called adaptive imaging) we attempt to compensate the beamformer for the aberration.
Adaptive Beamforming Some of the effects of aberration Reduced image brightness Loss in resolution Obscured targets Image artifacts
Control Aberrated
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
The Near-Field Phase Screen Model Signal Misalignment Phase Error τ 1 Near Field Phase Screen Summed RF Data Σ τ τ τ τ 2 3 System Time Delays 4 5 Scattering Medium
A Distributed Aberration Model Mid Range Phase Screen Summed RF Data Σ τ τ τ τ τ System Time Delays 1 2 3 4 5 Propagation of Distorted Wavefronts Scattering Medium
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 channel signals is extremely large. Significant computational effort. Low frame rates - relatively few have attempted to make adaptive beamforming work with real-time imaging.
Thank You Questions?