Week 11 - Instruments: Imaging Anatomy, Motion and Flow With Principle 2
Learning Objectives
Compare Physical Operation Principle 1 and Principle 2.
Explain how Principle 2 sonographic instruments work.
List the functions of the beam former.
List the functions of the image former.
List improvements in anatomic imaging with Principle 2.
List improvements in motion and flow imaging with Principle 2.
Two Principles of Operation
Operating Principle 1: Discusses instruments that receive echo voltages from the transducer and display anatomic, motion, and flow information.
Operating Principle 2: Describes instruments of systems which operate on an entirely different principle.
Comparing Principle 1 with Principle 2
Principle 1:
There is a one-to-one correspondence between the echo stream from an emitted ultrasound pulse and its displayed scan line.
The inherent time constraints associated with this serial line-by-line approach limit various characteristics of the image and force trade-offs between one characteristic and another.
Improving detail resolution can degrade penetration or temporal resolution.
Higher frequency = reduced penetration.
Principle 2:
Fewer pulses are required, and transmission focusing is not necessary, yet the entire image is in focus.
Excellent detail resolution throughout.
Interdependence in Principle 1 is avoided by using many elements in the transducer to send broader, weakly focused, or unfocused beams of ultrasound into the anatomy.
Fewer beams = higher frame rates = increased temporal resolution.
Contrast resolution and penetration are improved, and artifacts are reduced.
Principle 2 Instruments
Principle 1 and Principle 2 are both composed of:
Beam former
Signal processor
Display
With Principle 2, the image processor is replaced by the image former.
Transducer in Principle 2
An echo from a given location arrives at the elements of a transducer in a specific pattern or sequence depending on its originating location.
Echoes from different origination locations arrive at the elements with different sequences.
This information can be used to sort out what arrived from each pixel location.
This is the retrospective beam-forming process that is accomplished in the image former computationally.
Retrospective Beam Formation Analogy
Raindrops falling on a puddle are an example of the computational challenge with virtual beam-forming.
The waves emanating from each drop travel away to be combined with all the others.
If they can be sorted out as they arrive at the bottom of the photo, the location of their origins and strengths can be individually determined.
This is analogous to what the retrospective computational processing is in Principle 2.
Beam Former in Principle 2
Beam former performs similar functions as with Principle 1.
Directing, focusing, and apodizing the reception beam are now performed by the image former.
Signal Processor in Principle 2
Voltages go through the beam former to the signal processor, where they are processed to a suitable form for input to the image former.
Functions of the signal processor are similar to those for Principle 1.
Functions of Image Former in Principle 2
Because the beam former is emitting weakly focused or unfocused pulses, echoes from all portions of the illuminated region from a given transmitted pulse arrive at various times at all the elements when in reception mode. In other words, the echoes arrive at the transducer elements mixed up and must be sorted out to determine the echo information for each location in the image.
Massive, high-speed, parallel processing sorts out the echoes, placing them in image memory in their proper location.
This enables rapid image acquisition with high detail resolution, contrast resolution, and very high frame rates (excellent temporal resolution).
Retrospective beam-forming process.
Virtual Beam Forming
Physical ultrasound beam forming is still necessary but no longer coupled directly to displayed scan lines.
Computed and displayed scan lines can be thought of as produced by virtual beams.
The number of virtual beams is greater than the number of physical beams.
Accomplished by massive, computational postprocessing utilizing Graphics Processing Units (GPUs).
Because each transmitted pulse provides much of the echo information for the image, the frame rate is increased significantly, improving temporal resolution significantly.
After the entire echo content of the image is acquired, electronic reception focus is applied to each pixel.
The resulting images are similar to what extremely thin, laser-like ultrasound beams would produce and can be described as an expansion of reception focus to each pixel in the acquired image.
Retrospective, computed beam forming can be imagined in transmission or reception forms.
Virtual transmission beam: An imaginary laser-thin transmitted ultrasound beam that can be thought of as producing the excellent detail resolution throughout the image.
Virtual reception beam: An imaginary reception beam that can be thought of as determining the echo produced at each pixel location in the image.
There is no direct relationship between the number of scan lines and the number of pulses required to produce the image.
The number of virtual beams and the time to produce them is determined by the computational, retrospective, beam-forming approach used in the image former and the speed of the GPUs.
Image Resolution
Principle 2 yields improvement in nearly every aspect of sonographic, anatomic imaging.
Improves detail, contrast, and temporal resolution and penetration.
Improvements in Anatomic Imaging with Principle 2
Detail resolution improved dramatically.
Laser-thin virtual beam.
Entire image in focus.
Contrast resolution improved.
Section thickness ↓
Temporal resolution improved significantly.
Broad physical beam (fewer pulses required).
No multiple focus needed.
Frame rates >1000 s^{-1}
Real-time volume imaging (4D).
Quantitative shear-wave elastography.
Artifacts reduced.
Speed correction throughout the image.
Section thickness reduced.
Motion and Flow Imaging
Simultaneous grey-scale, color-Doppler, and spectral-Doppler presentations, rather than with time-sharing between them as with Principle 1.
This increases the speed with which these presentations are updated.
High frame rates.
Retrospective sample volumes.
Multiple spectral displays.
In Summary
There are two operating principles in current commercially available sonographic systems. Some operate on Principle 1, and others operate on Principle 2.
With Principle 2, the image processor is replaced by the image former.
In the image former, massive parallel processing is a sophisticated processing technique that enables rapid image acquisition with great detail resolution, contrast resolution, and very high frame rates, enabling excellent temporal resolution.
Virtual beam forming improves detail, contrast, and temporal resolutions, sensitivity, and penetration.
Virtual beam forming enables simultaneous grey-scale, colour and spectral Doppler presentations and retrospective sample volumes with multiple spectral displays.