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The fundamental and harmonic components are individually compressed by a matched filter or by a
bespoke harmonic matched filter. This used to be rare, occurring 1 in 30,000 pregnancies; however,
with the increase in assisted reproductive techniques, the overall incidence has increased to
approximately 1 in 7000 pregnancies. HDL-C levels are inversely related to CV event rates. The
thick blue line is the FChT spectrum of the chirp with no time delay. The sub-wavelength scatterer is
located at 60 mm. 162 Page 195. The hyperparameter optimization script is built upon Keras Tuner.
NIH Resource on Medical Ultrasonic Transducer Technology, Univ. The autocorrelation function can
also be defined as the inverse Fourier transform of the power spectral density, P (f). Conventional
Fourier-type techniques and new spectral estimation methods will be discussed in this section
including their advantages and disadvantages for processing chirp signals. Although a 100 B-mode
frames were used to calculate the bicoherence index, it does not increase the total imaging duration
when compared with the existing perfusion imaging techniques. Better resolution can be achieved by
decreasing the pulse length, which is limited by the ultrasonic frequency. The nonlinear signals were
detected using a Polyvinylidene Fluoride (PVDF) needle hydrophone with an active element
diameter of 0.2 mm (Precision Acoustics Ltd., Dorchester, UK). The peak negative pressure level of
each waveform was 1.125 MPa at 2.25 MHz with a MI of 0.75 at the focus of the transducer. If non-
overlapping windows are used, a significant part of the signal is ignored due to the small values of
the windows near the edges. Most of the topics can be adjusted to the students qualifications and
wishes. The repetition of each sample of the finite duration time series is mapped into discrete
frequency bins by using the discrete Fourier transform. Their results showed that the harmonic
generation occurs even for ?sw ? 1. The enamel, dentine and restora- tion material appear in a
different colour, since they all have different density values. Therefore, the proposed contact imaging
method is suitable for imaging the buccal and palatal surfaces of the tooth. Share to Twitter Share to
Facebook Share to Pinterest. As cross-sectional imaging, ultrasound (US) is well suitable for AI
technology to standardize imaging protocols and improve diagnostic accuracy. Imaging modalities
useful in diagnosis include ultrasound, computed tomography, and magnetic resonance imaging.
Examples were given through the applications on different problems encoun- tered in chirp coded
excitation for medical ultrasound imaging, including hard-tissue 185 Page 218. The front panel of the
UARP is removed for this photograph. This frequency information can be used to calculate the
distribution of the signal’s energy on the time-frequency plane, which will be explained in the
following sections. Combining the third, fourth and fifth order harmonics will not only improve the
axial resolution but will also improve the SNR, where the SNR of the superharmonic component
becomes comparable to the second harmonic component. Filtering process must be repeated
individually for each harmonic component, which will increase the redundancy, computation time
and accu- mulated error. For an ultrasound system; pulse duration is usually adjusted according to
power requirements and necessary penetration depth, and the bandwidth is maximised to increase the
resolution. Understanding Peptic Ulcers Causes, Symptoms, and Treatment Options at Gokuld. The
dashed lines on the bottom figure show windows applied in the fractional Fourier domain. 82 Page
115. Narrow-band signals are good candidates for this approach as the spectral boundaries of
fundamental and second harmonic component are well separated; i.e. the second harmonic does not
overlap with the fundamental component.
Micro-CT, also known as the X-ray microtomography, is used for validation of the ultrasound
measurements for both techniques. Suri ISBN: 978-1-4614-7424-1 (Print) 978-1-4614-7425-8
(Online), Pages 155-171. 2014. Experiments were performed by using linear chirp excitation with
different band- widths to observe the effect of spectral overlap between the higher harmonics.
Therefore, scallop loss was not taken into consideration while choosing a window function. 202
Page 235. The centre frequency and bandwidth of the desired harmonic matched filter were set by
multiplying the centre frequency and bandwidth of the excitation signal with that harmonic number
according to the second order distortion model or square law (Arshadi et al., 2007; 118 Page 151.
Use of this site means you accept the use of cookies. Long duration excitation provides better
penetration and improved SNR by increasing the excitation energy without changing the peak
pressure level. The fundamental and harmonic components are individually compressed by a
matched filter or by a bespoke harmonic matched filter. In order to improve the penetration, the
energy of the excitation signal must be in- creased by increasing either the duration or amplitude of
the pulse. The signal in blue corresponds to the central line of the second harmonic image given in
Figure 6.14. The signal in red corresponds to the central line of the second harmonic image given in
Figure 6.15. 113 Page 146. The mainlobe width of the compressed signal was the same as with the
autocorrelation function of the excitation waveform, but an increase in sidelobe levels was observed.
The artefacts appear between multiple signals at Wigner distribution as an interference between the
cross- terms (Cohen, 1989). Therefore, significant speckle reduction can be achieved by
superharmonic imaging and more details can be visualized. This change in the window shape creates
problems for pulse compression with matched filters, which will be explained in the next section.
The dashed arrows show the behaviour of each transformation on the time-frequency plane. 37 Page
70. The second generation contrast agents were encapsulated microbubbles, which were more stable
to be carried in the systemic circulation. The structure of the dental tissues absorbs and scatters the
sound wave due to roughness and the irregular curved shape of the tooth. The mainlobe width of the
compressed signal was wider than the autocorrelation function of the excitation waveform, especially
below ?15 dB. After filling the cavities, the restorative material was cured by UV light and the
surfaces of the fillings were flattened using dental instruments. The FrFT can still be employed to
sep- arate overlapping harmonic chirp components and suppress the compression sidelobe artefacts,
but the signal processing becomes quite different to the case of hard-tissue imaging since the problem
is not the separation of consecutive reflections any more. The use of human teeth for this study was
approved by the Leeds Dental Institute Skeletal Tissues Bank which is in turn approved by the Na-
tional Health Service Research Ethics Committee (UK) (ref: 07H130693), according to the Human
Tissues Act 2004 (UK). However, in order to calculate the optimal transform order for the discrete
FrFT, the resolution of the signal both in time and frequency must be known. For preprocessing of
the test images the same preprocessing steps and scripts are used. Palencia, “A direct calculation of
moments of the Sample Variance”, Mathematics and Computers in Simulation, Volume 82, Issue 5,
January, Pages 790-804. 2012. The proposed solution in this work is to use LFM chirp excitation by
exploiting the Fractional Fourier transform (FrFT). The combined effect of ultrasound and
microbubble on RAGE plasmid transfection in HCAECs was evaluated by detecting protein
expression of RAGE by western blot. After observing non-correlated phase behaviour between
microbubble generated har- monics and the fundamental component, the bispectral analysis was used
to separate the second harmonic response of tissue and microbubbles. The main reason of its success
is that it provides a low-cost way to help diagnosing and can be used in many medical applications.
The image sensitivity can be described as the signal-to-noise ratio. In this study, experimental
measurements were performed with a tissue phantom, where 100 B-mode images were captured and
processed to reduce the variance.
The red dashed line is the autocorrelation function of the excitation waveform, which is given for
comparison. Figure 8.13 shows the waveform compressed in the FrFT domain. Ultrasonographically,
the uterus and adnexas are evaluated in sagittal and transverse orientations, which allows for direct
evaluation and three-dimensional measurement of any identified extrauterine mass. By moving the
transducer along the surface of the tooth being examined and combining these individual A-scan
lines with the spatial information an ultrasound image can be formed. In statistics and signal
processing the bispectral analysis has many applications, although there are only a few examples in
ultrasound (Hillis et al., 2006). In this study, the bispectral analysis was used for the first time to
evaluate the harmonic generation of ultrasound contrast agents. Its response after pulse compression
will be evaluated in the next section by considering sidelobe performance and resolution. We use
cookies to create the best experience for you. For in-vivo tooth measurements, the expected error will
be similar with the ex-vivo measurements performed in this study. It is also possible to reduce the
sidelobe levels after compression by applying different windows on the matched filter as described
in section 2.3.1. In this approach, there is always a trade-off between axial pulse width and
sidelobes, where it is not possible 128 Page 161. However, the spectral overlap between the higher-
order harmonics reduces the image dynamic range. The name of the FChT comes from its unique
fan-shaped transformation kernel, which is best explained visually. Figure 3.5 represents the
simplified behaviours of the Fourier transform, the FrFT, and the FChT on the time-frequency plane.
In the frequency domain all reflections completely overlap with each other, since they all have the
same bandwidth and the centre frequency. Therefore, in this study a higher-order spectral analysis
method was proposed to analyse the nonlinear microbubble behaviour. However, the FChT can
achieve the optimum compression in terms of both sidelobe level and axial resolution. 7.5
Conclusions The main advantage of the superharmonic imaging over second harmonic imaging is the
enhanced spatial resolution. Although the detection threshold is improved, it is observed that the
duration of the excitation signal is longer than the duration of time of flight in the restoration, which
causes signal overlapping between consecutive internal reflections. Therefore, pulse inversion can
only remove the image artefacts, but can- not further improve the spatial resolution and sidelobe
levels for coded excitation. In the imaging department, ultrasound is also useful for determining
neuaxial blocks in people especially children. Note that the frequency domain is not the traditional
frequency axis, since the time-frequency plane is warped by the FChT. However, when the peak is
not at the centre of a frequency bin, there is an error between the displayed and the actual level. In
addition to this, they maintain extensive records of. In this study, the tooth sample was always stored
in an aqueous solution in order to eliminate the drying effect. The alignment of the transducer and
the needle hydrophone is performed by a CNC system. 94 Page 127. To improve the lateral res-
olution, the aperture size of the ultrasound probe or the excitation frequency1 must be increased. The
experi- mental setup, ultrasound measurements and the signal processing for this study were
performed by Sevan Harput. In ultrasound contrast imaging, long duration excitation is preferred
since it triggers nonlinear microbubble oscillations and induces a stronger response from
microbubbles. Arguably the most widely used imaging technologies in medicine. After repositioning
the tooth sample with a different angle consecutive measurement is performed. 58 Page 91. The
autocorrelation reveals the similarities between the observations performed at dif- ferent times and
the existing frequencies in the signal can be estimated by using these time delays between the
observations. A mismatched filter can also be designed to rectify the phase corruption introduced by
the receiver hardware and transducer element. The compression ratio for a LFM chirp is the time-
bandwidth product of the signal. Therefore, the FrFT is proposed as a filtering tool, with wideband
chirp coded excitation, for the extraction of the overlapped second harmonic component. 6.2
Materials and Methods In order to validate the proposed method, experiments were conducted both
in trans- mission and pulse-echo configurations.
Therefore, scallop loss was not taken into consideration while choosing a window function. 202
Page 235. For this reason, the phase coded signals will perform poorly for ultrasound applications
due to limitations on both signal duration and bandwidth. Potential risks involved in the ablation
procedure are not uncommon and could be catastrophic when out of control. The bispectral analysis
was proposed as an auxiliary tool for the second harmonic ultrasound imaging applications with high
SNR and low CTR. The microbub- ble response is considered to be nonlinear, because the oscillation
amplitude and the number of oscillation modes are increased. Tristan Vega, S. Aja-Fernandez, M.
Martin-Fernandez, C. Also generation of subharmonics and ultraharmonics increases the sidelobe
levels after compression, due to the increased amount of spectral overlap. But interpolation may be
necessary after warp- ing the time-frequency plane by the Fan Chirp transform, which can result in
degraded resolution, aliasing or extra computational load. 1.3 Objectives of this work and the
organization of the thesis The chirp coded excitation has been proven to be useful in medical
ultrasound imaging. In summary, there is no agreement on the probabilistic distributions that model
the different nature of tissues in US. Therefore, the crossover points between the spectral peaks must
be less than 0.5, which corresponds to ?6 dB points. Third, the filtered signals are transformed back
to the time domain. By calculating the transform order according to the signal parameters of the
second harmonic component, further reduction in sidelobe levels are achieved for second harmonic
imaging with chirp excitation. 115 Page 148. However, there are certain predisposing factors which
can be additive. The processed echo signal from 48 elements was combined by using a delay-and-
sum beamforming technique. Nevertheless, the design of an efficient coded excitation system is not
trivial. After verifying the validity of the proposed filtering technique, new measurements were
performed by using the UARP on a wire phantom. About Python workflow for automatic prostate
segmentation in transrectal ultrasound 3d images by using deep learning v-net model. The FrFT
technique becomes more effective, if the phase distortions induced by experimental equipment
needed to be eliminated. 4.3 Experiments To verify the accuracy of the proposed coded excitation
scheme and the FrFT technique, measurements were first performed on a tooth phantom of known
dimensions. Superharmonic imaging provides improved spatial resolution by combining the third,
fourth and fifth harmonics of the nonlinear received signal. The dynamic range is the ratio between
the largest and the smallest echoes can be represented in the image. First, the time domain signal is
transformed into fractional Fourier domain, where the compression is achieved. When the two
images are compared, the effectiveness of the filtering in the fractional Fourier domain becomes
obvious. Increasing the bandwidth will cause over- lapping between the second harmonic
component and the fundamental component of the received signal. The conventional method of
detection and classification of brain tumor is by human inspection with the use of medical resonant
brain images. The FrFT performs more efficiently than these multiple excitation methods, where it
can be used as a filtering tool with a wideband chirp excitation in order to extract the overlapped
second harmonic component without reducing the system frame-rate and bandwidth. Freear,
“Performance Evaluation of Nonlinear Fre- quency Modulated Signals in Ultrasound Harmonic
Imaging,” IEEE Int. Methods: Liver fibrosis models of rats were created by intraperitoneal injection
of thioacetamide over 12 weeks. They are also strong ultrasound scatterers; behaving linear at low
pressure levels and nonlinear at higher pressures. Therefore, zero padding in time domain was
performed on the recorded signals in order to increase spectral resolution after FFT for all
recordings. The fundamental and harmonic components are individually compressed by a matched
filter or by a bespoke harmonic matched fil- ter.
The patient gave birth normally at 40 weeks gestation. A matched filter2 is a linear time-invariant
filter that works by correlating a known signal with an unknown signal to detect the presence of the
known signal. The test results were graded by a teaching assistant according to the rule of 0.5 points
assigned to every feature and 2 points assigned to risk stratification, with a total of 100 points.
However, the authors observed that gold restorations transmit minimal acoustic energy to the tooth
behind the restoration due to their large acoustic impedance. The superharmonic component is
obtained by combining the pulse compressed third, fourth, and fifth harmonic components in time
domain.124 xx Page 22. The Fourier transform has a sinusoidal kernel2 that is suitable for analysing
monochromatic signals. The performance of the superharmonic for the chirp excitation with 40%
FBW was close to the theoretical expectations, where doubling the signal bandwidth should double
the axial resolution. There is also a thick-walled ring-shaped structure in the right adnexa (white
arrow) compatible with a tubal ectopic pregnancy, (b) image shows the ectopic gestational sac
corresponds to 7 weeks 5 days and has a fetal pole, (c) reveals blood flow related to the ectopic
pregnancy, (d) the power Doppler image demonstrates the characteristic ring of fire sign. Since the
medical probe bandwidth spans between 3 MHz and 8 MHz, the excitation signal was chosen to be
a LFM chirp sweeping the frequency range of 3?4 MHz so the second harmonic could be received at
the frequency range of 6? 8 MHz. A single RF line was selected from each frame for processing.
Augmentation is an essential step when working with a limited sample size. The captured data was
processed offline in Matlab (MathWorks Inc., Nat- ick, MA, USA). The trans- ducer is mounted to
the moving arm of the CNC positioning system. Methods: Forty-five volunteers were recruited and
divided into the group1 (?50 years old) and group 2 ( R and SWE-P R ) and shear wave dispersion
indexes (SWD-A R and SWD-P R ), which located at the anterior and posterior walls of the
common carotid artery (CCA), were obtained by USWD, and compared with pulse wave velocity
(PWV). In medical imaging with ultrasound, chirp excitation is used to increase the penetration
depth by increasing the total transmitted energy using longer pulse durations, while the resolution is
controlled by the signal bandwidth. 1The resolution in the direction of propagation. Palencia, “A
direct calculation of moments of the Sample Variance”, Mathematics and Computers in Simulation,
Volume 82, Issue 5, January, Pages 790-804. 2012. Terms Privacy Security Status Docs Contact
GitHub Pricing API Training Blog About. Using chirp coded excitation for second harmonic and
superharmonic imaging was suggested by M. Arif. M. Arif and Sevan Harput contributed
proportionally while solving the problems encountered during the experiments, calibrating the
instruments, performing the measurements, and processing the data. Chapter 3 explains the Linear
Canonical transform and two of its special cases used in this work; the Fractional Fourier transform
(FrFT) and the Fan Chirp trans- form (FChT). The FrFT performs more efficiently than these
multiple excitation methods, where it can be used as a filtering tool with a wideband chirp excitation
in order to extract the overlapped second harmonic component without reducing the system frame-
rate and bandwidth. NIH Resource on Medical Ultrasonic Transducer Technology, Univ. For this
example the signal length was 4096 samples and the window length was 256 samples. In this work,
bispectral analysis is used to separate the second harmonic response of tissue and microbubbles via
the phase relations of fundamental and harmonic components. By analysing the received echoes with
the FrFT, this work presents the ultrasonic non-destructive evaluation of dental restorations in human
teeth. 5.1 Introduction For dental imaging, conventional X-ray radiography and computed
tomography (CT) remain dominant while micro-CT has been used for in-vitro applications achieving
74 Page 107. The measure- ments were conducted in a tank containing deionized and degassed water
at 20?C. The SNR and penetration can be increased by using coded excitation and increasing the
total transmitted energy, but the available transducer bandwidth is the real limitation for the
superharmonic imaging. Tissue generates only higher order harmonics in a predictable manner, but
microbubbles have unique acoustic sig- natures that change with bubble size, encapsulation, gas core,
excitation pressure, and excitation frequency. The superharmonic components, as given in Figure 7.7,
processed by the previous method have higher sidelobe lev- els. The second harmonic matched filter
was designed with twice the centre 143 Page 176. This novel approach in ultrasound imaging is
speedily achieving extensive use with numerous applications. 3D surmounts the limitations of 2D
ultrasound imaging as this study will show.