106 research outputs found
Adaptive parameter selection scheme for nonlinear simulations using field II with operator splitting
No full textObjectives: Understanding and simulating nonlinear pulse propagation is of increasing interest in current ultrasound research. This article gives an adaptive parameter selection scheme for selecting parameters needed when the Field II linear propagation simulator is combined with operator splitting to enable nonlinear propagation simulations.Methods: The general Field II/operator splitting technique was introduced in [1, 2], requiring the selection of various geometrical parameters associated with virtual propagating planes. At that stage these needed to be selected essentially by an iterative trial-and-error technique for a particular transducer geometry, frequency and focussing scheme. Whilst careful parameter selection could provide satisfactory results over a limited propagation range for a given transducer, excitation and focussing scheme, an automated parameter selection method for the general case was still lacking.Results: The paper provides an adaptive method for selecting the required parameters as a function of the varying harmonic and spatial properties of the nonlinear pulse as it propagates through the medium.Conclusions: The method is fully derived and the significant improvement in the simulator’s performance is demonstrated by comparison to previous results in [1, 2]
System identification approach to contrast imaging with single pulse emission
No full textObjectives: Contrast imaging methods such as pulse inversion exploit the difference in scattering responses between linear tissue and nonlinear contrast agents. Such methods emit two or more pulses separated either temporally or spatially, at the cost of either reduced frame rate or reduced spatial resolution as compared to the theoretical values possible from a single pulse emission. This article proposes a method for imaging contrast agents specifically using single pulse emission only.Methods: A system identification approach is taken, first fitting a least squares linear transfer function model between emitted and received pulses, and then obtaining a set of predicted received pulse values based on the emitted pulse and the linear least squares model.Results: In the case of the reflector being linear (tissue), the linear least squares model accurately predicts the received pulse and the pulse prediction error is insignificant. In the case of a nonlinear reflector (contrast agent), the linear model cannot accurately predict the received pulse and the pulse prediction error is significant.Conclusions: These results are parallel to the insignificant and significant residual pulses obtained respectively in pulse inversion, but the method here requires only a single emission to obtain the discrimination. Results for typical contrast agent responses are given
Computation of steered nonlinear fields using offset KZK axes
No full textIntroduction/Background: Commercial medical scanners utilise electronic beam steering and the second harmonic signals generated by tissue nonlinear processes to form images at the second harmonic frequency. Furthermore, characteristics of nonlinearly generated harmonics within the beam itself contribute to improvements in lateral resolution and quality of the reconstructed image. Fully understanding these harmonic features offers new possibilities for redesigning or optimising array transducers to attain better imaging performances, as well as in other areas such as contrast agent responses and
microbubble destruction strategies for drug delivery. However, such an understanding is hindered at present by the computational difficulty in accurately predicting all the nonlinear characteristics of steered beams. This paper addresses this issue, presenting a KZK based method for analysing beams
steered at arbitrary angles.Method: The parabolic KZK equation is often used to study nonlinear characteristics of medical ultrasound beams. This equation is traditionally applied in a propagation direction perpendicular to the surface of the transducer, and has been shown to model the pulse propagation well along the central axis of the transducer [1]. However, its accurate angle of application is restricted to within approximately only 16 degrees away from the central transducer axis due to the underlying approximations employed in the derivation of the KZK equation.
This restriction causes a problem for studying steered beams, since typical steering ranges up to the order of 45 degrees away from the transducer centreline. To overcome this problem whilst at the same time exploiting the traditional KZK benefits, we develop an iterative method for displacing the KZK axes away from the central transducer axis to investigate arbitrary beam or field angles of interest. Results/Conclusions: The implemented algorithm is based on a time domain solution of the KZK equation, on a standard 3GHz PC with 2GB RAM producing runtimes in the order of a few hours per investigation angle. The transducer used is a 64 element linear phased array operating at 1.7MHz (height 10.5 mm, width 0.27mm, kerf 0.065mm) with beam steering at 0 and 45 degrees. Validation of the basic KZK algorithm without steering has been given previously at linear pressures (54kPa) against both experimental measurements and Field II and at nonlinear pressures (700kPa) against measurements described previously in [1]. The KZK axes are swept over a range of angles covering the full spectrum 499 of -90 to +90 degrees relative to transducer centreline, in order to investigate the entire emitted field for both steered and unsteered beams. Element pitch is then increased to investigate grating lobes. All results are decomposed into first and second harmonic fields to compare relative frequency domain properties. The overall contribution is a better characterisation of linear and nonlinear field characteristics at high angles than is possible with the traditional KZK approach
Connection between X-Waves, Fourier-Bessel series and optimal modelling aperture for circular symmetric arrays
No full textThis paper addresses various unresolved issues raised in publications [1], [2], and [3], in connection with the study and application of limited-diffraction and non-diffracting beams. Nondiffracting beams have the property of a constant radial profile with propagation distance, subject to an infinite aperture source, and the related theories advanced in the context of medical imaging have resulted in the possibility of extremely high fame rates. However, all the fundamental theory assumes an infinite-aperture source being available. In reality this is not possible, and when nondiffracting beams are implemented on finite source apertures they become limited diffraction beams. [1], [2], [3] have studied the use of and numerical differences between nondiffracting theory and limited diffraction implementation of such beams for field computation and tuning. In [1], [2] it was shown that an iterative technique could be applied to extend the effective modelling aperture from the physical limits of the transducer (limited diffraction basis functions) towards an infinite modelling aperture (Bessel beams
/ X-waves), to both tune and compute the emitted field from any given circular symmetric flat array with linear propagation conditions. This technique involved the concept of a modelling aperture spanning the gap between the physical limit of the transducer and infinity, which, when increased iteratively resulted in convergence of the corresponding computed field and source driving function. However, the technique relied on a combination of iterative combinations and 137 numerical field convergence to within a pre-selected limit in order to terminate the computations at an appropriate point.
In this paper, a formal mathematical connection between the limited-aperture (limited diffraction) basis functions and the nondiffracting infinite aperture theory (Bessel beams / X-Waves) is established as a function of the increasing modelling aperture. The result is that a specific optimal modelling aperture may then be specified as a function of frequency spectrum, spatial field extent to be investigated, and pulse repetition frequency. Consequently the previous iterative technique may be replaced by a single one-shot computation to achieve the same result. As a result, the new technique is significantly more efficient than the previous technique and the specific saving in computation depends on the particular transducer considered, but typically computational reductions are in the order of 50%. The global contribution of the paper is twofold : firstly a formal mathematical connection between limited diffraction beams and nondiffracting beams as function of increasing modelling aperture, and secondly the derivation of the optimal modelling aperture required for computation and tuning of circular symmetric fields with minimal computational demands
Ultrasonographer experience vs. power Doppler index for ovarian tumor discrimination
No full textObjectives: To assess the performance of the power Doppler index
(PDI) compared to subjective judgment of the ultrasonographer in
the preoperative diagnosis of ovarian malignancy.
Methods: Eighty-two successive complex adnexal masses were
examined prospectively with power Doppler before surgical
treatment. The investigation was multicentric and included in a
large contrast-enhanced ultrasound European study. A vascularity
index based on the number of colored pixels, was estimated on
selected frames (defined region of interest covering the entire tumor)
of the tumors using an in-house color quantifying program. The
sensitivity and specificity of PDI, the resistance index and subjective
visual scoringwere compared using receiver operating characteristic
(ROC) curves.
Results: Histology identified 34 malignant tumors including 11
borderline tumors and 48 benign tumors. PDI was considerably
higher in malignant than in benign lesions: 21.88 (95% CI,
19.19 24.57) vs. 8.11 (95% CI, 6.09 10.19) for the benign tumors
and 11.63 (95% CI, 1.20 22.06) for borderline tumors. The PDI
cut-off value for differential diagnosis was set at 0.107 (10.7%
of the tumor colored). However, the sensitivity and specificity of
the PDI were quite low: 75% (95% CI, 72.9 90.7) and 60%
(95% CI, 56.3 92.5) respectively. Using logistic regression, visual
Doppler scoring alone (OR 6.14 (95% CI, 2.30 16.38), P = 0.0003)
or associated with gray-scale examination (OR 10.5 (95% CI,
3.19 34.54), P = 0.0001) performed better than PDI (OR 2.61
(95% CI, 1.06 6.43), P = 0.04).
Conclusions: The power Doppler index obtained using our color
quantifying software has discriminating power for adnexal masses,
but visual Doppler scoring performed best
Intravenous contrast ultrasound examination using contrast-tuned imaging (CnTI) and the contrast medium SonoVue for discrimination between benign and malignant adnexal masses with solid components.
No full textOBJECTIVE: To determine whether intravenous contrast ultrasound examination is superior to gray-scale or power Doppler ultrasound for discrimination between benign and malignant adnexal masses with complex ultrasound morphology.
METHODS: In an international multicenter study, 134 patients with an ovarian mass with solid components or a multilocular cyst with more than 10 cyst locules, underwent a standardized transvaginal ultrasound examination followed by contrast examination using the contrast-tuned imaging technique and intravenous injection of the contrast medium SonoVue(R). Time intensity curves were constructed, and peak intensity, area under the intensity curve, time to peak, sharpness and half wash-out time were calculated. The sensitivity and specificity with regard to malignancy were calculated and receiver-operating characteristics (ROC) curves were drawn for gray-scale, power Doppler and contrast variables and for pattern recognition (subjective assignment of a certainly benign, probably benign, uncertain or malignant diagnosis, using gray-scale and power Doppler ultrasound findings). The gold standard was the histological diagnosis of the surgically removed tumors.
RESULTS: After exclusions (surgical removal of the mass > 3 months after the ultrasound examination, technical problems), 72 adnexal masses with solid components were used in our statistical analyses. The values for peak contrast signal intensity and area under the contrast signal intensity curve in malignant tumors were significantly higher than those in borderline tumors and benign tumors, while those for the benign and borderline tumors were similar. The area under the ROC curve of the best contrast variable with regard to diagnosing borderline or invasive malignancy (0.84) was larger than that of the best gray-scale (0.75) and power Doppler ultrasound variable (0.79) but smaller than that of pattern recognition (0.93).
CONCLUSION: Findings on ultrasound contrast examination differed between benign and malignant tumors but there was a substantial overlap in contrast findings between benign and borderline tumors. It appears that ultrasound contrast examination is not superior to conventional ultrasound techniques, which also have difficulty in distinguishing between benign and borderline tumors, but can easily differentiate invasive malignancies from other tumors
Infuence des parametres individuels sur les velocites ophtalmiques mesurees dans un groupe temoins
No full textA levet set model for the numerical modeling of composite delamination with nonconforming mesh and minimal remeshing
No full textInternational audienceDelamination is one of the main failure processes for composite laminates. Modeling progressivedelamination of such materials is thus of particular interest. A new approach has been proposedby F. P. van der Meer et al. a few years ago, which allows for the use of elements larger than thecohesive zone. With this method, the crack front is represented implicitly using a level set fieldwhereas the cohesive zone is supposed to reduce to a line. Consequently, the crack front does nothave to remain aligned with the mesh and remeshing is no longer necessary. Weak discontinuitiesare inserted in the displacement field at crack front location, allowing for a sharp transition betweenthe cracked and uncracked parts. Crack growth is driven by an explicit energy-based relation inwhich the configurational force is defined as the jump in Eshelby tensor over the front. In thispaper, we investigate the case of isotropic multilayered membranes with several crack fronts evolvingindependently or interacting with each others
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