Ultrasonic imaging is often used to estimate blood flow velocity. Currently, estimates are carried out using Doppler-based techniques. However, there are a number of shortcomings such as the limited spatial resolution and the inability to estimate longitudinal flows. Thus, alternative methods have been proposed to overcome them. Difficulties are notably encountered with high-frequency imaging systems that use swept-scan techniques. In this article, we propose to compare four vector velocity estimation methods that are complementary to Doppler, focusing on 40 MHz, high-frequency imaging. The goal of this study is to evaluate which method could circumvent the limitations of Doppler methods for evaluation of microcirculation, in the vessels having diameter on the order of 1 mm. We used two region-based approaches, one decorrelation-based approach and one spatiotemporal approach. Each method has been applied to seven flow sequences with various orientations and mean velocities. Four sequences were simulated with a system approach based on a 3D set of moving scatterers. Three experimental sequences were carried out by injecting blood-mimicking fluid within a gelatin phantom and then acquiring images with Visualsonics, Vevo 660 system. From velocity estimates, several performance criteria such as the normalized mean error or the normalized mean standard deviation were defined to compare the performance of the four estimators. The results show that region-based methods are the most accurate exhibiting mean errors less than 10% and mean standard deviation less than 13%. However, region-based approaches are those that require the highest calculative cost compared to the decorrelation-based method, which is the fastest. Finally, the spatiotemporal approach appeared to be a trade-off in terms of computational complexity and accuracy of estimates. It provides estimates with errors less than 10% for mean velocity and the CPU time is approximately 17 s for a ROI of size 40 * 80 pixels. 相似文献
Control of surface states of titanium dioxide nanoparticles using 2-(3,4-dihydroxyphenyl)ethylamine (dopamine) and 3,4-dihydrophenylacetic acid, which act as ligands to the undercoordinated surface sites (carrier traps), is demonstrated by electrochemical techniques. The deepest traps were found to be most reactive and are selectively removed by the addition of the ligands which enhances the kinetics of electron accumulation in the film. Furthermore, a shift in the Fermi level to more positive potentials was detected for electrodes modified with the negatively charged ligand (3,4-dihydrophenylacetic acid) compared to that of electrodes modified with the positively charged ligand (dopamine). The presence of the negative charge on the ligand also contributed to the underpotential of hydrogen evolution on 3,4-dihydrophenylacetic acid-modified electrodes. 相似文献
We cryptanalyse here two variants of the McEliece cryptosystem based on quasi-cyclic codes. Both aim at reducing the key size
by restricting the public and secret generator matrices to be in quasi-cyclic form. The first variant considers subcodes of
a primitive BCH code. The aforementioned constraint on the public and secret keys implies to choose very structured permutations.
We prove that this variant is not secure by producing many linear equations that the entries of the secret permutation matrix
have to satisfy by using the fact that the secret code is a subcode of a known BCH code. This attack has been implemented
and in all experiments we have performed the solution space of the linear system was of dimension one and revealed the permutation
matrix. The other variant uses quasi-cyclic low density parity-check (LDPC) codes. This scheme was devised to be immune against
general attacks working for McEliece type cryptosystems based on LDPC codes by choosing in the McEliece scheme more general
one-to-one mappings than permutation matrices. We suggest here a structural attack exploiting the quasi-cyclic structure of
the code and a certain weakness in the choice of the linear transformations that hide the generator matrix of the code. This
cryptanalysis adopts a polynomial-oriented approach and basically consists in searching for two polynomials of low weight
such that their product is a public polynomial. Our analysis shows that with high probability a parity-check matrix of a punctured
version of the secret code can be recovered with time complexity O(n3) where n is the length of the considered code. The complete reconstruction of the secret parity-check matrix of the quasi-cyclic LDPC
codes requires the search of codewords of low weight which can be done with about 237 operations for the specific parameters proposed. 相似文献
Let p1, p2, p3 be primes. This is the second article in a series of three on the (p1, p2, p3)-generation of the finite projective special unitary and linear groups PSU3(pn), PSL3(pn), where we say a noncyclic group is (p1, p2, p3)-generated if it is a homomorphic image of the triangle group Tp1, p2, p3. This paper is concerned with the case where p1 = 2 and p2 = p3. We determine for any prime p2 the prime powers pn such that PSU3(pn) (respectively, PSL3(pn)) is a quotient of T = T2, p2, p2. We also derive the limit of the probability that a randomly chosen homomorphism in Hom(T, PSU3(pn)) (respectively, Hom(T, PSL3(pn))) is surjective as pn tends to infinity. 相似文献
This paper reports the feasibility of nano-oxide precipitate formation in Fe–Cr alloy by ion implantation synthesis. High contents of Al+ and O+ ions were implanted into thin films of high purity Fe–10%Cr alloy at room temperature and were studied by transmission electron microscopy (TEM) and atom probe tomography (APT). In contrast, to the common two-stage implantation/annealing scheme of precipitate ensemble synthesis by ion beams, cluster formation took place at the implantation stage in our study, requiring no subsequent high-temperature annealing. The post-implantation microstructural examination revealed in the as-implanted thin foil an array of precipitates with diameters in the range of 3–30?nm. The precipitate number density distribution was found to depend on the foil thickness. The precipitate enrichment with both Al and O was confirmed by the energy-filtered TEM analysis. Judging from the electron diffraction pattern and high-resolution TEM analysis, the crystal lattice of precipitates corresponds to some cubic modification of aluminium-rich oxide or pure aluminium oxide. The precipitate lattice alignment with the host matrix was revealed for at least a part of precipitates. The analysis of APT data using cluster detection algorithm indicates the presence of local zones enriched in Al and O, even in those areas of as-implanted samples where no clusters were visible by TEM. 相似文献
Among alternative techniques to overcome the difficulties associated with thermal regeneration, non-thermal plasma can be considered as one of the most promising technology. The coke trapped in the zeolite micropores can be oxidized at room temperature with a low energy consumption using a dielectric barrier discharge reactor with a pin to plate geometry. The influence of various experimental parameters for coke removal efficiency and ozone production was investigated: input power, gap between the two electrodes, gas flow rate, catalyst mass and compactness. We showed that the efficiency was not strongly increased by increasing the deposited power from 23 to 36 W, but it depends strongly on the mass, so the depth of the wafer. The elimination of coke becomes more difficult as soon as the compactness is increased. The removal of coke is not uniform within the wafer, the one localized into the depth of the wafer is difficult to remove and requires higher input power (>?30 W).
Microsecond to millisecond timescale backbone dynamics of the amyloid core residues in Y145Stop human prion protein (PrP) fibrils were investigated by using 15N rotating frame (R1ρ) relaxation dispersion solid-state nuclear magnetic resonance spectroscopy over a wide range of spin-lock fields. Numerical simulations enabled the experimental relaxation dispersion profiles for most of the fibril core residues to be modelled by using a two-state exchange process with a common exchange rate of 1000 s−1, corresponding to protein backbone motion on the timescale of 1 ms, and an excited-state population of 2 %. We also found that the relaxation dispersion profiles for several amino acids positioned near the edges of the most structured regions of the amyloid core were better modelled by assuming somewhat higher excited-state populations (∼5–15 %) and faster exchange rate constants, corresponding to protein backbone motions on the timescale of ∼100–300 μs. The slow backbone dynamics of the core residues were evaluated in the context of the structural model of human Y145Stop PrP amyloid. 相似文献
Borate glass particles and microspheres with size distributions in the range of approximately 100–400 μm, were loosely compacted and sintered for 10 min at 600 °C to form a porous, three-dimensional construct (porosity 25–40%). Conversion of the borate glass to hydroxyapatite was investigated by soaking the constructs in a solution of K2HPO4 (0.25 M) at 37 °C and with a pH value of 9.0, and measuring the weight loss of the constructs as a function of time. Almost full conversion of the borate glass to hydroxyapatite was achieved in less than 6 days. X-ray diffraction revealed an initially amorphous product that subsequently crystallized to hydroxyapatite. The biocompatibility of the porous constructs was investigated by in vitro cell culture with human mesenchymal stem cells derived from bone marrow (bMSC) and human mesenchymal stem cell derived osteoblasts (MSC-Ob). The cells adhered to the scaffolds and the MSC-Obs produced alkaline phosphatase which is an indication of osteogenic differentiation. The data suggest strong bioactive characteristics for the borate glass constructs and the potential use of the constructs as scaffolds for tissue engineering of bone. 相似文献