Encoded Total Focusing Method for Improving Data Acquisition Rate
C´esar Guti´errez-Fern´andez, Ana Jim´enez and Carlos Juli´an Mart´ın-Arguedas
Electronics Department, University of Alcal´a, Ctra. Madrid-Barcelona km. 33.600, Madrid 28805, Spain
Ultrasound Imaging, CDMA, Acquisition Rate.
Synthetic aperture imaging techniques are capable to obtain high quality images, fully focused in both trans-
mission and reception. However, these techniques require to perform so many emissions as elements in the
array to acquire RF data. This requirement decreases acquisition rate and can result in tissue motion artifacts
because of the phase misalignments between signals acquired in different emissions. Such inconvenience
claims for alternatives that reduce the total number of emissions needed to obtain the data. This work pro-
poses the use of Code Division Multiple Access (CDMA) techniques to attain this goal. By encoding the
ultrasound excitation signal emitted, through a pesudo-random Kasami code, several elements can emit simul-
taneously and the amount of data acquired in every emission increases. The encoded proposal attains a Kx
speed up in acquisition rate compared to conventional total focusing method, being K the number of Kasami
sequences available in the set.
The use of ultrasound imaging systems based on ar-
rays has been widespread during the last decades,
standing out as a safer and cheaper alternative to other
imaging diagnosis techniques. Conventional Phased
Array (PA) imaging systems fire all the elements of
the array in every excitacion, acquiring a line of the
image in every emission. The transmit beam must be
prefocused so images obtained blur out of the focus.
Synthetic Aperture Focusing Techniques (SAFT) rep-
resent an alternative to obtain fully focused images
(Jensen et al., 2006). In the basic SAFT approach the
transducer elements are activated sequentially acting
as both transmitter and receiver. One signal per emis-
sion is acquired and stored in memory. Once every
element has been activated and there are N signals
in memory, a beamforming process is applied to dy-
namically focus, both in emission and reception, ev-
ery pixel.
There are other SAFT approaches depending on
the number of active elements used in transmission
and reception. The SAFT method evaluated in this
work is the Total Focusing Method (TFM) (Lock-
wood and Foster, 1995), where a single element acts
as emitter and the echo data is recorded by the full
array. One of the drawbacks of SAFT techniques is
sensitivity to tissue motion during acquisition pro-
cess. Since Radio Frequency (RF) data is obtained
from multiple firings, motion artifacts may arise due
to phase misalignments between data from different
emissions. There are some solutions (Lokke Gam-
melmark and Jensen, 2003; Lokke Gammelmark,
2004) in the literature that propose motion compensa-
tion based on cross-correlation of reference signals to
find the shift in position or phase of the tissue. These
proposals are capable to estimate motion tissue in 2D,
however their high computational cost is an inconve-
nience for real-time imaging sytems.
Another way to overcome motion artifacts is to re-
duce the acquisition time needed to form the image.
In this regard, encoding techniques can allows us to
achievethis goal by reducing the number of emissions
needed to acquire the data.
Encoding techniques and pulse compression have
been used in ultrasonic imaging mainly to enlarge
penetration depth and to improve SNR without in-
creasing the voltage excitation by using linear fre-
quency modulation (FM)(O’Donnel, 1992; Toosi and
Behnam, 2009). Some works have also studied
the application of encoding through binary codes.
(EMoo-Ho et al., 2002) propose Golay encoding to
obtain signal to ratio (SNR) improvement without re-
ducing frame rate. (Kim and Song, 2004) present
a linear array emitting Golay codes in combination
with chirp, achieving simultaneous multi-zone focus-
ing along several scan lines. (Romero-Laorden et al.,
2012) propose the use of Golay encoding in minimum
redundancy SAFT solutions to overcome their limited
Gutiérrez Fernández C., Jiménez A. and Martín-Arguedas C..
Encoded Total Focusing Method for Improving Data Acquisition Rate.
DOI: 10.5220/0005255300950099
In Proceedings of the International Conference on Bioimaging (BIOIMAGING-2015), pages 95-99
ISBN: 978-989-758-072-7
2015 SCITEPRESS (Science and Technology Publications, Lda.)
Coded signals can be also used to differenti-
ate simultaneous emissions. These techniques are
known as Code Division Multiple Access (CDMA).
CDMA have been successfully used for indoor po-
sitioning and obstacle detection in robotics applica-
tions (De Marziani et al., 2012; Diego et al., 2012;
Klaus-Werner et al., 1998). In these works, a multi-
user scenario is considered, assigning a different code
to each user. Then simultaneous emissions and re-
ceptions from different users are possible, since each
code is used as an user identifier.
Multiple access techniques have been already ap-
plied in ultrasonic imaging. (Shen, 1996) proposes to
emit a pseudo-orthogonal m-sequence in each direc-
tion, acquiring several lines at the same time thanks
to a bank of filters in reception stage. (Gran and
Jensen, 2008) propose the use of pseudo-random
codes in synthetic transmit aperture (STA) to separate
the echoes originating from two different transmitters
and improve SNR. Also (Diego et al., 2012) present
a design of an ultrasonic phased array with M emit-
ters and a single receiver based on encoded excita-
tion with loosely synchronous sequences for obstacle
detection, which scans the whole environment with a
single emission.
This work suggest a new method based on us-
ing CDMA techniques in TFM imaging to reduce
the emissions needed to acquire RF data. By encod-
ing the transmission with pseudo-orthogonal Kasami
codes several elements of the array can be excited si-
multaneously. Echoes coming back from the simul-
taneous emissions can be distinguished in reception
thanks to the suitable correlation properties of the
codes. Thereby the proposal allows to increase the
amount of data acquired by a single emission, acceler-
ating the acquisition process and making the imaging
system less sensible to motion artifacts.
Kasami sequences are widespread in CDMA tech-
niques due to their suitable auto-correlation (AC) and
cross-correlation (CC) functions. The small set of
Kasami sequences used in this work is composed by
K = 2
sequences of length L = 2
1, where X
must be even. If the length of the sequences in-
creases, the number of sequences available also in-
creases. However, as it was exposed (Guti´errez-
Fern´andez et al., 2014), in ultrasound imaging appli-
cations code length is limited due to the blind-zone
problem. Therefore, 63 bits sequences are used in
this work, which can be used to scan distances begin-
ning from 5 cm. With this code length the Kasami set
is composed by K=8 pseudo-orthogonal sequences.
Generation of this Kasami sequences is based on the
algorithm proposed by (P´erez et al., 2009), which al-
lows to select those sequences with lower cross-talk
values in asynchronous environments.
In Total Focusing method a set of NxN signals is used
to obtain the highest image quality. A scheme of the
acquisition process is shown in Figure 1. In every
emission, a single element is fired and all elements re-
ceive echo data. For a N-element array this process is
repeated N times, storing NxN recordings to compose
the image. Once stored in memory, these recordings
can be properly delayed to bring into focus all image
Figure 1: Acquisition process in Total Focusing Method for
a 1-D N-element array. In every emission, a single element
is fired and all elements receive echo data, acquiring N sig-
nals. This process is repeated for each array element, stor-
ing NxN recordings to compose the image.
TFM has the same T/R matrix and effective aper-
ture than conventional PA (Lockwood and Foster,
1995), but it is fully focused, so the highest image
quality is reached. Moreover, TFM reduces the elec-
tronics complexityin the emission stage by multiplex-
ing the same electronic in every emission. However,
as it has been aforementioned in section 1, TFM is
affected by motion artifacts due to there are phase
misalignments between signals acquired in different
emissions. Therefore, it is clear that a reduction in
the number of emissions needed to acquire RF data is
desirable to properly scan fast moving tissues.
The acquisition scheme of the proposal is shown in
Figure 2, when a 1-D, N-element array and a set
of K Kasami sequences, C
,··· ,C
, are con-
sidered. In every emission there are K active ele-
ments, each one driven by a different Kasami code,
,··· ,C
. Orthogonality between codes as-
signed to each selected element makes possible that
all of them can emit simultaneously. The elements
activated in each emission are selected to maximize
the separation among simultaneous emissions, since
there is a separation of S-element between consecu-
tive active elements. In reception, the full array is
used to record N signals. Then, these signals are cor-
related with each one of the Kasami codes used in
emission, allowing us to distinguish echoes received
from the K simultaneous emissions. After correla-
tion K groups of N signals are obtained and stored
in memory. This process is repeated until the com-
plete set of NxN signals to compose the image is ob-
Figure 2: Acquisition process for one emission with the sys-
tem proposed, combining CDMA and TFM techniques. A
1-D, N-element array and a set of K Kasami sequences,
,··· ,C
, are considered. There are K active el-
ements in every emission, each one driven by a different
Kasami code, C
,... ,C
. The full array is used in
reception. KxN signals are acquired simultaneously in one
emission. A correlation process between the N signals and
the K codes emitted is carried out, obtaining KxN signals.
tained. Consequently, only S = N/K emissions must
be performed in this proposal, attaining a Kx speed up
in frame rate (imag/sec) respect to the classical TFM
technique, where N emissions are needed (Lockwood
and Foster, 1995).
Different simulations have been carried out to analyse
the advantages and drawbacks of the proposed tech-
nique in comparision with conventional TFM. An ar-
ray with N = 96 punctual elements, a pitch of half
wavelength and a frequency of 3 MHz are considered
for both cases. In all simulations punctual elements
with a flat frequency response has been considered.
A Gaussian envelope pulse with 80% of relative
bandwidth has been considered as excitation for con-
ventional TFM images. In the encoded proposal, the
excitation signals are Kasami sequences with L = 63
bits, so the Kasami set is formed by K = 8 pseudo-
orthogonalsequences. The sequencesare BPSK mod-
ulated to adapt the codes to the working frequency of
the transducer, a single carrier period per bit has been
5.1 Contrast Resolution
Figure 3 showsthe images obtained with conventional
(a) and encoded (b) TFM proposals when a medium
with a punctual reflector located at r = 158 mm and
θ = 0
is scanned. Each image has a resolution of
64x1200 pixels and it is fully focused, both in emis-
sion and reception, in each pixel. The encoded ap-
proach presents a background noise in the reflector
direction which reduces the image contrast in com-
parison with the conventional case. This background
noise is consequence of the interference inherent to
encoding techniques. The higher interference values
are 25 dB under the maximum response, these values
are mainly attributable to the AC interference.
5.2 Temporal Resolution
Conventional TFM needs to perform 96 emissions,
one per array element, to obtain the whole set of NxN
signals, while the enconded TFM requires only 12
emissions, therefore in the case shown in the figure
3 the proposal attains a 8x speed up in acquisition
rate respect to conventional TFM. The reduction of
the emissions needed to obtain RF data depends on
the number of pseudo-orthogonal codes availables in
the Kasami set. If the set is formed by K pseudo-
orthogonal codes, K elements can be fired simultane-
Figure 3: Images obtained with conventional (a) and encoded (b) TFM proposals when a medium with a punctual reflector
located at r = 158 mm and θ = 0
is scanned. Each image has a resolution of 64x1200 pixels. An array with N=96 elements
and pitch of half of wavelength is considered for both cases. Kasami sequences of L=63 bits are used to drive the elements in
the encoded approach.
ously in a single emission, so Kx speed up is reached.
This reduction in acquisition time can make the en-
coded TFM image system less sensible to motion ar-
tifacts and suitable to real-time applications.
This work has presented a proposal of synthetic aper-
ture technique TFM in conjunction with CDMA to
improve the acquisition rate. The central idea of the
proposal is that emission from multiple elements si-
multaneously can be achieved by driving them with
encoded signals, therefore, the RF data can be ac-
quired with fewer emissions. The excitation signals
used to drive the elements are a set of K pseudo-
orthogonal Kasami sequences.
Simulation results have shown that inherent inter-
ference of Kasami encoding adds a background noise,
reducing the image contrast respect to TFM reference
image. Further studies will be conduced to mini-
mize this interference by searching for other codes
with better correlation properties or applying post-
processing techniques.
The encoded TFM acquires the whole RF data
in only N/K emissions, whereas conventional TFM
requires N, one per array element. Consequently, a
Kx speed up in acquisition rate compared to conven-
tional method is attained. The improvement in acqui-
sition rates can be greater if the number of pseudo-
orthogonal codes available increases, since the num-
ber of elements that can emit simultaneously also in-
creases. The solution presented stands as a promising
alternative to make high quality TFM imaging sys-
tems less sensible to motion artifacts and capable to
reach acquisition time requirements in real-time ap-
This work has been granted by University of Al-
cal´a under cEYE project (CCG2013/EXP-043) and
the Spanish Ministry of Economy and Competitive-
ness under LORIS project (TIN2012-38080-C04-01)
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