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Ultrasound Research Scanner for Real-time Synthetic Aperture Data Acquisition
Jensen, Jørgen Arendt; Holm, Ole; Jensen, Lars Joost; Bendsen, Henrik; Nikolov, Svetoslav; Tomov,
Borislav Gueorguiev; Munk, Peter; Hansen, Martin; Salomonsen, Kent; Gormsen, Kim
Total number of authors:
13
Published in:
I E E E Transactions on Ultrasonics, Ferroelectrics and Frequency Control
Link to article, DOI:
10.1109/TUFFC.2005.1503974
Publication date:
2005
Document Version
Publisher's PDF, also known as Version of record
Link back to DTU Orbit
Citation (APA):
Jensen, J. A., Holm, O., Jensen, L. J., Bendsen, H., Nikolov, S., Tomov, B. G., Munk, P., Hansen, M.,
Salomonsen, K., Gormsen, K., Hansen, J., Pedersen, H. M., & Gammelmark, K. (2005). Ultrasound Research
Scanner for Real-time Synthetic Aperture Data Acquisition. I E E E Transactions on Ultrasonics, Ferroelectrics
and Frequency Control, 52(5), 881-891. https://doi.org/10.1109/TUFFC.2005.1503974
ieee transactions on ultrasonics, ferroelectrics, and frequency control, vol. 52, no. 5, may 2005 881
Ultrasound Research Scanner for Real-time
Synthetic Aperture Data Acquisition
Jørgen Arendt Jensen, Senior Member, IEEE, Ole Holm, Lars Joost Jensen, Henrik Bendsen,
Svetoslav Ivanov Nikolov, Borislav Gueorguiev Tomov, Peter Munk, Martin Hansen, Kent Salomonsen,
Johnny Hansen, Kim Gormsen, Henrik Møller Pedersen, and Kim L. Gammelmark, Student Member, IEEE
Abstract—Conventional ultrasound systems acquire ul-
trasound data sequentially one image line at a time. The
architecture of these systems is therefore also sequential
in nature and processes most of the data in a sequential
pipeline. This often makes it difficult to implement radically
different imaging strategies on the platforms and makes
the scanners less accessible for research purposes. A system
designed for imaging research flexibility is the prime con-
cern. The possibility of sending out arbitrary signals and
the storage of data from all transducer elements for 5 to
10 seconds allows clinical evaluation of synthetic aperture
and 3D imaging. This paper describes a real-time system
specifically designed for research purposes.
The system can acquire multichannel data in real-time
from multi-element ultrasound transducers, and can per-
form some real-time processing on the acquired data. The
system is capable of performing real-time beamforming for
conventional imaging methods using linear, phased, and
convex arrays. Image acquisition modes can be intermixed,
and this makes it possible to perform initial trials in a clin-
ical environment with new imaging modalities for synthetic
aperture imaging, 2D and 3D B-mode, and velocity imaging
using advanced coded emissions.
The system can be used with 128-element transducers
and can excite 128 transducer elements and receive and
sample data from 64 channels simultaneously at 40 MHz
with 12-bit precision. Two-to-one multiplexing in receive
can be used to cover 128 receive channels. Data can be
beamformed in real time using the system’s 80 signal pro-
cessing units, or it can be stored directly in RAM. The
system has 16 Gbytes RAM and can, thus, store more than
3.4 seconds of multichannel data. It is fully software pro-
grammable and its signal processing units can also be recon-
figured under software control. The control of the system
is done over a 100-Mbits/s Ethernet using C and Matlab.
Programs for doing, e.g.,B-modeimagingcanbewritten
directly in Matlab and executed on the system over the net
from any workstation running Matlab. The overall system
concept is presented along with its implementation and ex-
amples of B-mode and in vivo synthetic aperture flow imag-
ing.
I. Introduction
M
odern ultrasound scanners employ digital signal
processing to generate high quality images which are
Manuscript received February 4, 2003; accepted October 12, 2004.
This work was supported by grants 9700883 and 9700563 from the
Danish Science Foundation, by B-K Medical A/S, and by grant EF-
782 from the Danish Academy of Technical Sciences.
J.A.Jensen,S.I.Nikolov,B.G.Tomov,P.Munk,H.M.Pedersen,
and K. L. Gammelmark are with the Center for Fast Ultrasound
Imaging, Ørsted•DTU, Technical University of Denmark, DK-2800
Kgs. Lyngby, Denmark (e-mail: jaj@oersted.dtu.dk).
O. Holm, L. J. Jensen, H. Bendsen, M. Hansen, K. Salomonsen, J.
Hansen, K. Gormsen, and H. M. Pedersen are with the IO Technolo-
gies A/S, Carl Jacobsens Vej 16, DK-2500 Valby, Denmark.
dynamically focused in receive. Processing has to be made
on data streams in the Gbytes per second range, and this
necessitates the use of dedicated chips to perform process-
ing in real time to keep the cost, space, and power con-
sumption moderate. Fundamental changes to the signal
processing are, thus, difficult or impossible to make.
The current scanners perform image acquisition sequen-
tially one line at a time, and the frame rate is, thus, lim-
ited by the speed of sound. When imaging flow, several
emissions have to be made in the same direction, and this
limits the frame rate, especially for large depths and large
color flow sectors. For a 100-line image at 15-cm depth,
the frame rate can be below 6 Hz, which is unacceptable
for cardiac imaging.
New imaging techniques based on synthetic aperture
(SA) imaging have therefore been suggested and investi-
gated [1]–[3]. The methods can potentially increase both
resolution and frame rate, since the images are recon-
structed from RF data from the individual transducer ele-
ments. Hereby a perfectly focused image in both transmit
and receive can be made. Research in real-time 3D imaging
is also underway [4], [5]. The purpose is to make systems
that in real time can display a pyramidal volume of the
heart, where different slices hereafter can be visualized.
It has been intensely discussed whether SA imaging
could give better images, and how they will be affected
by tissue motion and limited signal-to-noise ratio. It has
also been stated that flow imaging cannot be performed
with SA methods. A fundamental problem in SA imag-
ing is the poor signal-to-noise ratio in the images, since
a single element is used for emission. This gives a much
lower emitted energy compared to using the full aperture
in conventional imaging and therefore limits the depth of
penetration. A potential solution to this is the use of coded
excitation, and several groups are working on employing
coded signals to enhance the signal-to-noise ratio [6]–[8].
Coded imaging can also be used to further increase the
frame rate [9]–[11]. It has also recently been shown that SA
imaging can be used for velocity estimation [12], [13]. This
shows that SA systems can be made with all of the func-
tionality of conventional systems. There are, thus, good
reasons for constructing an experimental system capable
of measuring SA data in vivo to evaluate the proposed
methods. It is the purpose of this paper to describe such a
system and give some examples from its use, demonstrat-
ing that many of the problems in SA imaging mentioned
above can be solved.
0885–3010/$20.00
c
2005 IEEE
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882 ieee transactions on ultrasonics, ferroelectrics, and frequency control, vol. 52, no. 5, may 2005
All of the above techniques require digital signal pro-
cessing on signals from individual transducer elements.
The most advanced techniques also use different codes that
change as a function of element and emission number, so
that a fully flexible emissions system is needed. For re-
search purposes both demands can be difficult to fulfill
with commercial scanners, since they are often highly in-
tegrated, and it is difficult to access individual signals.
Programming commercial scanners for new imaging tech-
niques is often also either cumbersome or impossible. It is,
thus, beneficial to develop a dedicated research system that
can acquire, store, process, and display ultrasound images
from multi-element transducers for any kind of imaging
strategy.
This paper describes such a research scanner, its de-
sign, implementation, and use in vivo. The primary pur-
pose of the scanner is to acquire complete RF data sets,
where the signals on the individual elements are stored
and can be used for later processing in development of al-
gorithms and imaging schemes. A large memory covering
several heart cycles is, thus, needed. It must be possible to
implement any kind of imaging, and a fully flexible trans-
mission system is needed. Not all mentioned methods can
be implemented in real time, since the resources needed
for real-time processing can be determined only after the
algorithm has been developed. A key requirement is there-
fore to store all data and then transport them to general
computer clusters for processing. To allow scanning on hu-
man volunteers, it is, however, necessary to have real-time
processing capabilities for orientation of the scanning, and
the various traditional scanning methods should be imple-
mented. The processing should be so flexible that some
of the new methods can be implemented or experimented
with for real-time implementation. Finally, the scanner
should be easy to program, allowing one to implement en-
tirely new scanning methods with minimal efforts. Such
a system can assist in the development of scanning tech-
niques and algorithms and facilitate preclinical trials. Also,
such a system makes it possible to investigate the role of
tissue motion in SA imaging and to quantify the emitted
intensities for the advanced imaging methods.
The demands on the system are further elaborated in
the next section. A description of the individual units of
the system is given in Section III, and the actual imple-
mentation is also shown. The programming model for the
system, and how phased array imaging can be done in few
lines of code, is shown in Section IV. Examples of clini-
cal use of the system on human volunteers is described in
Section V, and a summary of experiences with the system
and its further development is given in Section VI.
II. System Specification
The purpose of the system is to make the acquisition of
multichannel data in real-time from clinical multi-element
ultrasound transducers possible, and to enable real-time
or near real-time processing of the acquired data. The sys-
tem must be capable of performing the beam formation for
all currently available imaging methods, and this makes it
possible to carry out initial trials with new imaging modal-
ities for synthetic aperture imaging, 3D imaging, and 2D
and 3D velocity estimation. It must be capable of working
in a clinical environment to evaluate the performance of
various algorithms on human volunteers.
The system is specifically intended for research pur-
poses, and is not intended for commercial use. The size
of the system is, thus, not very important, and it is not
necessary to make it easily transportable. Also, it is used
only by experts and the user interface to the medical doc-
tor is of minor importance. Most efforts have been put on
the flexibility of the system and its ease of configuration
for new uses.
The function of the system is defined by the different
imaging methods for which it can be used. Each of the
imaging types is described below, and the consequences
for the system then given.
A. Linear Array Imaging
A linear array image is generated by a multi-element
transducer with 128 to 256 elements [14]. The beam is
moved by selecting, e.g., 64 adjacent elements and emitting
a focused beam from these. The focusing in receive is also
done by a number of elements, and dynamic focusing is
used. Apodization in both transmit and receive are often
applied. The number of active elements is usually 32 to 64.
The transducer frequency is from 2 to 10 MHz. Imaging is
done down to a depth of 30 cm.
Thus, the demand on the system for 64 channels is si-
multaneous sampling at 40 MHz. The focusing delay for
element i is given by
t
d
(i)=
|r
i
−r
p
|−|r
c
−r
p
|
c
, (1)
where c is speed of sound, r
p
is the imaging point, r
c
de-
notes the center element of the active aperture, and r
i
is
the location of the element. Assuming the point to be very
close to the transducer gives the maximum delay
t
d max
=
|r
i
|−|r
c
|
c
. (2)
A linear array often has λ = c/f
0
pitch, and the maximum
delay is then
t
d max
=
N
e
/2 · c/f
0
c
=
N
e
2f
0
, (3)
where f
0
is center frequency and N
e
is the number of active
elements. For 64 elements and f
0
=2MHz,t
d max
equals
16 µs.
The maximum time to sample one line to a depth of d
is given by
t
s
=
2d
c
. (4)
This gives t
s
= 430 µs, corresponding to 17,200 samples
at 40 MHz for a depth of 30 cm.
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jensen et al.: scanner for real-time synthetic aperture data acquisition 883
B. Phased Array Imaging
The beam is electronically swept over the imaging area
by using a 64-to-256 element array [15]. The beamforming
delays are used both for steering and focusing. The pitch is
reduced to λ/2 to avoid grating lobes, and a very conserva-
tive estimate of maximum delay is to assume the imaging
pointtobeattheedgeofthearray.Themaximumdelay
then is
t
d max
=
N
e
λ/2
c
=
N
e
2f
0
, (5)
which is equal to the demand for linear array imaging.
The transducer frequency is from 2 to 10 MHz. Investi-
gations are done to a depth of 20 cm.
Thus, the demand on the system for 128 channels is
sampling at 40 MHz. The demands on focusing delays,
sampling time, and storage are the same as for linear array
imaging.
C. Flow Estimation, Spectrum
Beamforming is done in one direction with either a lin-
ear or a phased array [16]. The flow signal from blood has
40 dB less power than that from stationary tissue. The
dynamic range of the flow signal is 30 dB. The effective
number of bits must be 12 or more when the signals from
all channels have been combined. The pulse emitted can
have from 4 to 16 periods of the center frequency of the
transducer or a coded signal can be employed.
D. Flow Imaging
Imaging is done by pulsing repeatedly in one direction
and then changing the direction to generate an image [17].
An image can therefore be assembled from up to 1000 pulse
emissions.
E. Three-Dimensional Imaging
A matrix element transducer is used with, e.g.,40×40
elements [4], [18]. Only some of the elements are used for
transmit and receive. The area of the elements is small,
and pulsing should be done with 100 to 300 volts. Coded
signals should be used. Coded pulses with up to 64 cycle
periods must be possible with a high-amplitude accuracy.
This corresponds to emission over a period of 32 µswitha
sampling frequency of 40 MHz and an accuracy of 12 bits.
The maximum delay conservatively corresponds to the
transmission time over the aperture. Assuming λ/2pitch
gives
t
d max
=
√
2N
e
λ/2
c
=
N
e
√
2f
0
, (6)
which for 64 × 64 elements and f
0
=2MHzgives23µs.
Parallel lines are generated by using parallel beamform-
ers and reusing data from one pulse emission. The system
must be capable of reading the data sampled from one
element a number of times, and using different phasing
schemes for each cycle through the data. This gives a very
high demand on the processing that might not be obtain-
able by the system.
F. Synthetic Aperture Imaging
A standard linear or phased array multi-element trans-
ducer is used. Pulsing is done on a few elements and the
received response is acquired for all elements [19], [20]. The
image is then reconstructed from only a small number of
pulse emissions by using the data from all elements.
This type of imaging needs large amounts of storage
and the ability to reuse the data for the different imag-
ing directions. The system must be capable of storing 3
seconds of data for all channels. For a 12-bit resolution at
40 MHz, 64 channels, and 3 seconds of data, this amounts
to a total storage demand of more than 14 Gbytes.
Theprocessingofthedatacannotbedoneinthesys-
tem due to the very large demand on processing. This is
solved by storing the data in real time and the transferring
them to a multiprocessor system for storage and image re-
construction.
G. Synthetic Aperture Flow Imaging
The data are acquired in the same way as for synthetic
aperture imaging, but the focusing is done continuously
for all points in the image [21]. Hereby the data for flow
estimation are present for all image locations for all time.
It is, thus, very important that the image measurement be
continuous over the whole data acquisition time.
H. Fast Coded Synthetic Aperture Imaging
Here the image is acquired by sending different codes on
the individual elements for different emissions [9]–[11]. The
image is then created by processing the received signals
using different filters to separate out the individual signals.
The system must, thus, be capable of sending an arbi-
trary code that is different for the different channels, and
that can change from emission to emission.
In general the demands for all of these imaging modes
can be condensed into a few generic demands. The trans-
mission system should be capable of sending an arbi-
trary signal on each element. The signals can be differ-
ent from element to element and from emission to emis-
sion. The transmission system must be capable of focus-
ing the transmission differently for emission to emission.
The sampling frequency should be 40 MHz and the res-
olution is determined by the dynamic range of the sys-
tem, which should be above 60 dB. A 12-bit digital-to-
analog converter (DAC) is, thus, sufficient. Coded wave-
forms should be emitted for up to 32 µs and different codes
should be emitted for different emissions and elements. For
100-image lines this gives 80 ksamples of memory for each
transducer element.
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884 ieee transactions on ultrasonics, ferroelectrics, and frequency control, vol. 52, no. 5, may 2005
Fig. 1. Overall diagram of system.
The receiving system should be capable of sampling full
RF data from all channels. The sampling frequency should
be 40 MHz and the resolution at least 12 bits. Sampling
should be possible over a couple of heart beats. Three sec-
onds of real-time data at 40 MHz with 2 bytes per sample
gives a demand of 229 Mbytes, so that a RAM of 256
Mbytes per channel is necessary. To process the data it
must be possible to dynamically focus the data in real
time. A focusing chip must be used for each channel, and
this chip must have memory with parameters for the fo-
cusing, and the system must be capable of summing the
data from the individual channels. Ideally the processing
should be easily reconfigurable, and it must be possible to
store the raw RF data, the focused data, or the focused
and summed data. The beamformed and summed data
should also be transported out of the system in real time
for further processing and display on a screen.
III. System Realization
The Remotely Accessible Software configurable Mul-
tichannel Ultrasound Sampling (RASMUS) system con-
sists of four distinct modules: transmitters, analog am-
plifiers (Rx/Tx amplifiers), receivers, and a sync/master
unit. The main blocks are depicted in Fig. 1. The connec-
tion to the transducer is through a 128-wire coaxial cable
through the Rx/Tx amplifiers. The transmitter sends the
signals through the transmit amplifiers, and the receiver
unit samples the amplified and buffered signals from the
Rx/Tx amplifiers. The sync/master unit holds a crystal
oscillator and controls the central timing of the scanning
process. The overall operation of the system is controlled
through a number of single-board PCs in the individual
units interconnected through a standard 100-Mbits/s Eth-
ernet. The waveforms and focusing delay data are trans-
mitted from the controlling PC to the transmitters and
Fig. 2. Main diagram of the transmitter board.
receiver boards. The data from the sampling are processed
by field programmable gate arrays (FPGAs) that can be
configured for specific signal processing tasks over the net.
One Focus FPGA is used for each element and a Sum
FPGA is placed for each eight elements. The processed
and summed signal can then be routed from Sum FPGA
to Sum FPGA through a cascade bus. The resulting signal
is read by one or more signal processors (ADSP) that can
be connected through serial link channels capable of trans-
mitting 40 Mbytes per second. The beamformed signal is
sent via the link channels to the PC for further processing
and display.
The following paragraphs detail the overall design of the
individual boards.
A. Transmitter
The overall diagram of the transmitter is shown in Fig. 2
and its layout is shown in Fig. 3. Each transmitter board
has 16 channels each having a 128-ksample pulse RAM
connected to a 40-MHz, 12-bit DAC. The pulse RAM is
controlled by two FPGAs, where the individual waveforms
are selected as a memory start address and a transmit de-
lay. The delay RAM holds the start address of the wave-
form in the pulse RAM and the corresponding delay for
each line. The delay RAM is implemented as 32 k × 32
bit SRAM. At the start of each line the pulse emission
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![Fig. 4. Main diagram of receiver board (from [22]).](/figures/fig-4-main-diagram-of-receiver-board-from-22-2or6y774.webp)
![Fig. 8. Linear array (left) and synthetic aperture scan (right) of nylon wires in a tissue mimicking phantom with an attenuation of 0.5 dB/(MHz cm) (from [8]).](/figures/fig-8-linear-array-left-and-synthetic-aperture-scan-right-of-oy0qchhe.webp)
![Fig. 9. In vivo color flow map image at a 77-degree flow angle for the jugular vein and carotid artery. The color scale indicates the velocity along the flow direction: red hues indicate forward flow and blue reverse flow (from [13]).](/figures/fig-9-in-vivo-color-flow-map-image-at-a-77-degree-flow-angle-5outi9u6.webp)