17-03-2014, 08:45 PM
Abstract— Coded excitation has been studied and applied to
medical ultrasound for several years. The advantages of using
coded excitation compared to the conventional pulses are
improved SNR, minimized exposure of patients to potential
biological effects, and the ability of using even higher
ultrasonic frequencies. However, the major problem associated
with coded excitation is the high side-lobe level which
introduces the side-lobe artifacts. In this paper, we have
utilized the adaptive beamforming to suppress the side-lobe
effects and thus enhance the resolution in a coded excitation
ultrasound imaging system. Simulation results showed that by
applying minimum variance adaptive beamformer to a chirpexcited
ultrasound imaging system, significant improvement in
both SNR and resolution can be achieved.
Keywords- Ultrasound medical imaging, Code excitation,
Adaptive beamforming
I. INTRODUCTION
Coded signals have been used successfully in many
engineering disciplines such as radars and mobile
communication systems. But it was successfully introduced
for clinical ultrasound only within the last 10 years. A major
difference between ultrasound and radar is the propagation
medium: strong frequency-dependent attenuation and
nonlinear propagation in tissue distort the received coded
waveform and may degrade code performance.
Coded excitation in medical ultrasound is primarily used
to either improve the signal-to-noise ratio (SNR) without
increasing the excitation voltage or lower the excitation
voltage without sacrificing the SNR [1]–[4]. Other
applications of coded excitation include increasing the frame
rate and improving resolution [5], enhancing the detection of
contrast agent [6], increasing the depth of field [7],
improving the SNR in finite amplitude distortion based
harmonic imaging [8], enhancing the generation of
harmonics by contrast agent micro-bubbles [9], [10], and
suppressing selected harmonic components in nonlinear
imaging [11].
The main idea behind coded excitation is to increase
emitted energy by elongating the transmitted waveform
(without increasing peak level) while still preserving
resolution by the time compression processing. Special
waveform modulation types e.g. linear frequency modulation
(chirp), binary codes (Barker) and binary complementary
codes (Golay) are characterized by a time localized narrow
peak of their autocorrelation function. Time compression is
performed on the received RF echoes by means of matched
filtration (cross-correlation with the transmitted waveform).
The performance of coded excitation systems generally
is characterized by the SNR improvement, the main-lobe
width (related to the axial resolution) and the side-lobe level
(related to the dynamic range and contrast resolution).
Consequently, the design of an efficient coded excitation
system can be summarized as a trade-off between SNR
maximization, range resolution and side-lobe minimization.
It was shown that in the case of linear FM signals, a SNR
improvement of 12 to 18 dB can be expected for large
imaging depths of attenuating media, without any depthdependent
filter compensation. In contrast, nonlinear FM
modulation and binary codes are shown to give a SNR
improvement of only 4 to 9 dB when processed with a
matched filter [12]. Also, the distant range side-lobes which
are associated with the ripples of the spectrum amplitude can
be removed by amplitude or phase predistortion of the
transmitted signals. However, the major problem associated
with coded excitation is the high side-lobe level which
introduces the side-lobe artifacts [13].
Medical ultrasound imaging is conventionally done by
insonifying the imaged medium with focused beams. The
backscattered echoes are beamformed using delay-and-sum
operations that cannot completely eliminate the contribution
of signals backscattered by structures off the imaging beam
to the beamsum. Several groups have presented
implementations of the adaptive beamforming to ultrasound
imaging in order to filter out off-axis signals [14-16]. The
resulted images exhibit enhanced contrast and resolution
when compared to their DAS counterparts, which gives them
a significantly different overall aspect.
In this paper, we utilize adaptive beamforming in coded
excitation system to enhance the resolution which was
reduced due to pulse compression in decoding process. Thus,
we achieve in improvement in terms of both SNR and
resolution, simultaneously and this may lead to ultrasound
imaging systems with high-quality and safety considerations.
medical ultrasound for several years. The advantages of using
coded excitation compared to the conventional pulses are
improved SNR, minimized exposure of patients to potential
biological effects, and the ability of using even higher
ultrasonic frequencies. However, the major problem associated
with coded excitation is the high side-lobe level which
introduces the side-lobe artifacts. In this paper, we have
utilized the adaptive beamforming to suppress the side-lobe
effects and thus enhance the resolution in a coded excitation
ultrasound imaging system. Simulation results showed that by
applying minimum variance adaptive beamformer to a chirpexcited
ultrasound imaging system, significant improvement in
both SNR and resolution can be achieved.
Keywords- Ultrasound medical imaging, Code excitation,
Adaptive beamforming
I. INTRODUCTION
Coded signals have been used successfully in many
engineering disciplines such as radars and mobile
communication systems. But it was successfully introduced
for clinical ultrasound only within the last 10 years. A major
difference between ultrasound and radar is the propagation
medium: strong frequency-dependent attenuation and
nonlinear propagation in tissue distort the received coded
waveform and may degrade code performance.
Coded excitation in medical ultrasound is primarily used
to either improve the signal-to-noise ratio (SNR) without
increasing the excitation voltage or lower the excitation
voltage without sacrificing the SNR [1]–[4]. Other
applications of coded excitation include increasing the frame
rate and improving resolution [5], enhancing the detection of
contrast agent [6], increasing the depth of field [7],
improving the SNR in finite amplitude distortion based
harmonic imaging [8], enhancing the generation of
harmonics by contrast agent micro-bubbles [9], [10], and
suppressing selected harmonic components in nonlinear
imaging [11].
The main idea behind coded excitation is to increase
emitted energy by elongating the transmitted waveform
(without increasing peak level) while still preserving
resolution by the time compression processing. Special
waveform modulation types e.g. linear frequency modulation
(chirp), binary codes (Barker) and binary complementary
codes (Golay) are characterized by a time localized narrow
peak of their autocorrelation function. Time compression is
performed on the received RF echoes by means of matched
filtration (cross-correlation with the transmitted waveform).
The performance of coded excitation systems generally
is characterized by the SNR improvement, the main-lobe
width (related to the axial resolution) and the side-lobe level
(related to the dynamic range and contrast resolution).
Consequently, the design of an efficient coded excitation
system can be summarized as a trade-off between SNR
maximization, range resolution and side-lobe minimization.
It was shown that in the case of linear FM signals, a SNR
improvement of 12 to 18 dB can be expected for large
imaging depths of attenuating media, without any depthdependent
filter compensation. In contrast, nonlinear FM
modulation and binary codes are shown to give a SNR
improvement of only 4 to 9 dB when processed with a
matched filter [12]. Also, the distant range side-lobes which
are associated with the ripples of the spectrum amplitude can
be removed by amplitude or phase predistortion of the
transmitted signals. However, the major problem associated
with coded excitation is the high side-lobe level which
introduces the side-lobe artifacts [13].
Medical ultrasound imaging is conventionally done by
insonifying the imaged medium with focused beams. The
backscattered echoes are beamformed using delay-and-sum
operations that cannot completely eliminate the contribution
of signals backscattered by structures off the imaging beam
to the beamsum. Several groups have presented
implementations of the adaptive beamforming to ultrasound
imaging in order to filter out off-axis signals [14-16]. The
resulted images exhibit enhanced contrast and resolution
when compared to their DAS counterparts, which gives them
a significantly different overall aspect.
In this paper, we utilize adaptive beamforming in coded
excitation system to enhance the resolution which was
reduced due to pulse compression in decoding process. Thus,
we achieve in improvement in terms of both SNR and
resolution, simultaneously and this may lead to ultrasound
imaging systems with high-quality and safety considerations.