Medical Ultrasound TechnologyState of the art explained :Medical Ultrasound
Technology(move mouse over a question
and click to select)
1. What means
crystal pitch and what is it determined by ?2. Why do we need so
many elements in the probes ?3. What is the
advantage of having higher numbers of channels ?
4. What is channel folding ?5. What means Broad Band
Technology or Multi Frequency Technology ?
6. What is the meaning of
the following expressions :* Dynamically variable aperture
* Electronic focusing
* Swept filter
* Apodization>
Spatial - Amplitude - Contrast resolution>
Temporal - Burst shape Ultrasonix Ultrasonic Transducer - Axial resolution>
Frequency - Spectrum - Lateral resolution7.
What means Multiple Beam Acquisition or Quad Acquisition ?
8. What is a so called
"Digital" Ultrasound System ?
9. What is a Digital
Beamformer ?
10. What is the meaning
of the following expressions :*
Confocal Imaging*
Compound Scanning*
Trapezoidal Scanning
1. What means crystal pitch and what is it determined by ?Ultrasonix Ultrasonic Transducer
Pitch
The crystal is cut into
very small elements that can be controlled independently.
The pitch is the width
of one element including a gap.
Directional propagation
of the sound depends on width of element
If sideway phasing
is required (as with phased array probe or with linear probe supporting
oblique CFM scan), the pitch has to be small in order to direct sufficient
energy in an angle to the probe surface.
The
Law : For 45° phasing,
pitch should ideally < l/2 For a Phased
Array probe with 90° sector capability (45° phasing) the following
applies (by example) :
l/2 = V/2F = 1’540m/2x2.5MHz
Frequency
PitchChannelsProbe footprintFor 2.5 MHz
For Linear Array probes
with oblique scan capability:Depending on the desired
oblique scan angle the pitch should be at least equal to l.
In the ideal situation
it should be 0.7 x l for 30° phasing.
Aperture is the size of
the active crystal width used to generate one single beam.Ultrasonix Ultrasonic Transducer
For the best focusing at
any given depth (lateral resolution!) the aperture is one of the parameter
to be controlled.
The smaller the F-number the tighter the focal point for better lateral resolution.
Ideal situation
will be with F 1.
The Law F-number = Focal
depth / Diameter of Aperture
2. Why do we need so many elements in the probe ?
The smaller the elements
are cut de more energy is available to phase the beam in oblique direction
(refer to point 1.) and de better we can eliminate side lobes and grating
lobes (elimination of image artefacts). However to get a sufficient size
aperture for an optimal focus in the far field, a larger number of elements
is required.
3. What is the advantage of
having high numbers of channels ?
The number of channels
determines how many elements can be activated for any one given beam and
therefore determines the active aperture. With larger aperture a better
focus can be achieved also in the far field.
For all imaging modes that
don’t require phasing, this means individual controlling of every single
element, every two symmetrically arranged elements are connected together
and controlled simultaneously by one channel. Through this the aperture
can be doubled with the same number of channels.
5. What means Broad Band Technology or Multi Frequency Technology ?
To apply any one of the
mentioned technologies in the ultrasound system,Ultrasonix Ultrasonic Transducer the transducer elements
must be capable to transmit and receive a wide frequency range from say
2.0 to 5.0 MHz such that with one probe higher or lower frequencies can
be processed depending on the application. Band width is generally stated
in %, e.g. 80% band width means the 6 dB point (half of max. energy) is
40% up and 40% down from centre frequency. Different manufacturer use different
names for this techniques. (e.g. broad band or multi hertz technology)
6. What is the meaning of the
following expressions :
Dynamically variable
aperture?
During the reception period
the aperture is continual increased by switching more element to the active
aperture in order to adjust for the smallest possible F-number.
Controlling the transmit
and receive focus through individual delays switched to each element during
transmit and during reception period to generate a parabolic shaped wave
front with a predetermined focal point. During reception this delay switching
is done dynamically to continuously adjust the focus to the position of
the instantaneous reception.
During the reception period
the filters are continually swept from a higher frequencies in the near
field to a lower frequencies in the far field.
This is to improve the axial
resolution in the near field but still maintain good penetration in the
far field.
Apodization?-Through apodization
the signal on every individual element is controlled with respect to amplitude,
wave shape and frequency.
This is done for the transmit signal in the transmission
phase as well as for the received signals during reception period in order
to further optimise the beam profile and beam spectrum for better spatial,
axial and contrast resolution.- Spatial - Amplitude:Ultrasonix Ultrasonic Transducer
Depending on the position of the element within the aperture the transmit
pulse applied has an individually, precisely defined amplitude.
The same
principle is applied during reception with a precisely defined individual
signal amplification for each element depending on it’s position within
the aperture.
This has a dramatic effect on beam focusing which improves
contrast resolution.- Temporal -
Burst shape: Same as described above but instead of the amplitude the burst wave
shape is individually controlled which improves axial/contrast resolution.-
Frequency - Spectrum:
Same as described above but the frequency is individually controlled for
each element. This results in improvement of lateral resolution.
7. What means Multiple Beam
Acquisition or Quad Acquisition ?
Instead of receiving one
beam at the time in a sequential mode several adjacent beams are received
and processed simultaneously. This means that a multiple of ultrasound
data is acquired that can be processed for :
- Dramatically higher frame
rate for temporal resolution
- Higher line density for
better lateral resolution
- Higher degree of image
correlation for improved details and contrast resolution
Of course all three
parameters can be weighted according to the application and clinical requirements.
8. What is a so called "Digital" Ultrasound System ?
A small note: Please be informed that at time of writing this technical
script, technology was on a much different level of nowadays, > 2004.
The reason for publishment of this script on this website is for educational
reasons only.
Every single system on
the market can be called a digital system because every system has certain
blocks that function digitally exclusively.
If we look at the entire control
circuitry or the image memory so we know that these parts function all
digitally in every system available today.
However one major block, the
actual acoustical beam former was untill not very long ago in all ultrasound
systems functioning on a analogue basis.
Since this digital beam
former systems have hit the market many competitors speak about digital
systems, partially to confuse the customers and partially because they
are confused themselves.
It is clear that in a long run digital beam formerUltrasonix Ultrasonic Transducer
will be part of every ultrasound system.
At present this technology is
only available in high end and high cost equipment as the components used
are still very costly.
With digital video entering into the consumer market
however it is expected that these components will become more affordable
and that digital beam formers will also be available in mid and lower range
systems in the near future.
Main blocks of
the signal path of an ultrasound system:
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9. What is a Digital Beam Former ?
A beam former is the major
part of the electronic array ultrasound system that controls all the acoustically
relevant functions such as electronic focusing, aperture, apodization and
so on ... which are all important to get the best possible beam profile.
In an analogue beam former this block is full of multiplexer switching
devices, analogue delay lines and attenuators.
A Digital Beam Former in
comparison is an extremely powerful, highly specialised, super fast number
crunching computer system.
It performs all the function of a beam former
in an order of magnitude faster, more precise and can do many more functions
in addition.
Through it’s enormous flexibility it can be programmed to
do functions that have never been available before or in other words, it
is like a programmable ultrasound computer.
As the DBF processes all ultrasound
signals in a pure digital form, the echo signals coming from every element
must be individually digitised.
Therefore every channel has its own A/D
converter. Specially designed super high integrated components, so called
ASICS, are used for the signal processing.
All this runs under the control
of DSP’s which are specialized Digital Signal Processors with a tremendous
computing power (up to 8'000 mops = mega operations per second).
With the
vast potential to control the acoustic beam and the flexibility inherent
to Digital Beam Forming a new area in ultrasound imaging has started.
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10. What is the meaning of the following expressions :
Confocal Imaging
Confocal imaging is a method
to obtain (almost) continuous transmit focus for the best lateral resolution
throughout the image depth.
Multiple transmit focus in a sequential fashion
however affects the frame rate dramatically.
With confocal imaging different
focal cells are acquired in a non-sequential mode with pulses that are
separated laterally such that no overlap of reverberation fields occur
and hence no dead time is required.
In addition, in areas of over-sampling,SIUI Ultrasound Probe
such as in the near field, scans are omitted.
This results in more focal
points over the entire depth giving better lateral resolution and contrast
resolution at a considerably higher frame rate.
Real time Compound Scanning
Real time compounding with
a linear probe is achieved by combining two images that are obtained through
scanning one image oblique to the left and the other image oblique to the
right.
In this way, the structuresUltrasonix Ultrasonic Transducer
not perpendicular to a standard 90° beam are much better delineated
and clearly defined.
Trapezoidal Scanning
Trapezoidal scan or linear
phased scan is a regular linear scanning where as on either side of the
image the field of view is enlarged by phasing to the side.
The advantage
of this method is a wide near field and an enlarged far field.
The same principles applies
for Phased Convex scanning but using a convex probe.