1,720,952 research outputs found
Full-Waveform Inversion for Breast Ultrasound
No full textBreast cancer is the most common type of cancer for women and in developed countries it forms one of their largest threats. Many studies have shown that early detection by screening is important for achieving a successful treatment and reducing the mortality rate. Nowadaysmammography is the gold standard for breast cancer screening. However, mammography has several drawbacks including the use of ionizing radiation, a painful procedure, and poor performance with dense breasts. Magnetic resonance imaging (MRI) could form an alternative as it has some powerful features. However, the high examination and equipment costs as well as the use of contrast agents limits its applicability. Another potential alternative for breast cancer screening is ultrasound. Ultrasound has the advantage over mammography orMRI that it is safe, cheap and patient-friendly. With ultrasound, a tumor can be detected since healthy breast tissues and cancerous tissues have different acoustic properties. All these features make ultrasound a promising candidate as a screening modality for breast cancer. Hand-held ultrasound scans are frequently used for breast imaging in hospitals. With these scanners reflectivity images are generated. These images typically show the boundaries between different tissues. Even when these exams are conducted by trained radiologists operator-dependency occurs. To eliminate this, automated full-breast ultrasound scanners have been developed where the transducer slides over the breast. However, as the imaging principle remains the same, only reflectivity images are generated. To avoid significant breast deformation as well as to scan the breast from as many sides as possible water-bath scanning systems have been developed. These systems have the additional advantage that both reflection and transmission measurements are obtained. This mixture of different measurement types make it feasible to obtain better images by employing advanced processing techniques. One promising imaging method is full-waveform inversion (FWI). FWI aims to match a modeled wavefield to a measured wavefield by adjusting the acoustic medium parameters. A minimization problem is constructed and solved to this aim. As a result, images showing quantitative information about the different tissues are obtained. This quantitative information aids to the characterization and identification of the different tissues. However, there are some challenges when applying FWI. One of the biggest challenges is its computational complexity. By the inclusion of wave phenomena such as diffraction, refraction, scattering and dispersion - needed to explain the measured data in great detail - the computational complexity of FWI has become significantly larger than conventional - mainly ray based - imaging methods. In this work, we investigate the applicability of contrast source inversion (CSI) as an FWI method for breast ultrasound. To this end, we first introduce our full-waveform forward modeling method which is based on solving an integral equation. With a synthetic example,we investigate howeach mediumparameter (compressibility, density, and attenuation) affects the scattered pressure field. The obtained results show that attenuation, in contrast to compressibility and density, has only little effect on the wavefield for frequencies below 1MHz. From that we conclude, that for these frequencies only attenuation can be neglected in our inversion. We also compare the results from our full-waveform modeling method with results obtained after commonly made approximations such as Born, ray-based and paraxial approximations. We observe from the presented numerical results that with each approximation important phenomena normally present in the full-wave data are absent. For this reason, we recommend to use a full-wave modeling method to compute synthetic measurement data.ImPhys/Medical Imagin
Multi-parameter inversion with the aid of particle velocity field reconstruction
No full textMulti-parameter inversion for medical ultrasound leads to an improved tissue classification. In general, simultaneous reconstruction of volume density of mass and compressibility would require knowledge of the particle velocity field along with the pressure field. However, in practice the particle velocity field is not measured. Here, the authors propose a method for multi-parameter inversion where the particle velocity field is reconstructed from the measured pressure field. To this end, the measured pressure field is described using outward propagating Hankel functions. For a synthetic setup, it is shown that the reconstructed particle velocity field matches the forward modelled particle velocity field. Next, the reconstructed particle velocity field is used together with the synthetically measured pressure field to reconstruct density and compressibility profiles with the aid of contrast source inversion. Finally, comparing the reconstructed speed of sound profiles obtained via single-parameter versus multi-parameter inversion shows that multi-parameter outperforms single-parameter inversion with respect to accuracy and stability. Accepted Author ManuscriptImPhys/Medical Imagin
Redatuming of 2-D Wavefields Measured on an Arbitrary-Shaped Closed Aperture
No full textWhole breast ultrasound scanning systems are used to screen a women’s breast for suspicious lesions. Typically, the transducers are located at fixed positions at relatively large distances from the breast to avoid any contact with the breast. Unfortunately, these large distances give rise to large spatial domains to be imaged. These large domains hamper the applicability of imaging by inversion. To reduce the size of the spatial computational domain, we present a two-dimensional redatuming method based on Hankel decomposition of the measured field. With this method, the field measured over an arbitrary-shaped closed curve can be redatumed to a new curve enclosing a smaller spatial domain. Additional advantages of the proposed method are that it allows to account for the finite size and orientation of a transducer and that it is robust to noise. The proposed method is successfully validated using synthetic and measured data and the results show that the recorded field can be redatumed to any position in the embedding.Accepted Author ManuscriptImPhys/Medical Imagin
Delft Breast Ultrasound: The development of a water-cup ultrasound breast scanning system
No full textBreast cancer is impacting over 1.5 million women each year. Early detection is essential in decreasing the mortality rate. Mammography is currently the golden standard for breast screening. However, this method is not ideal and water-bath ultrasound breast scanning is considered to be a promising screening modality. This type of scanning system is able to obtain ultrasound reflection and transmission data. In this study, a water-cup breast scanning setup is built. The setup consists of a plastic cup with four acoustic windows and four phased array ultrasound probes. The water-cup is used to execute a phantom study. The phantom study gives insight into the operability of the built water-cup. The differences between reflection and transmission data are observed and utilised. One imaging algorithm and two inversion algorithms are employed to reconstruct an image of the phantom. The images of the different algorithms are compared.Furthermore, a method is proposed that reduces the computation time of a reflection data based Born inversion algorithm. In the end, the water-cup is used to image an ex-vivo human breast. Finally, the limitations of the built water-cup are stated. In addition to manufacturing the water-cup, a pulse-echo-scan of an ex-vivo human breast is made. This pulse-echo-scan is compared with the cross-sectional photograph of the breast. Due to the pudding-like structure of the breast, it is challenging to make a cross-section at the same position as the pulse-echo-scan is made. The pulse-echo-scan is made at the Pathology Department of the Erasmus MC. The ex-vivo human breast study helps to understand what structures are present in the breast. This knowledge can be used to build a new measurement setup and optimize the imaging algorithms for finding tumours.Delft Breast Ultrasoun
Contrast source inversion on experimental data: Initial results
No full textQuantitative images showing the speed of sound proffle of the breast may be obtained by employing full-waveform inversion (FWI) methods on the measured data. These reconstruction methods work well for both dense and normal breasts. Contrast source inversion (CSI) is a frequency domain FWI method. In literature, many examples of successful application of CSI for breast imaging can be found. However, all these works are based on simulated data. In this work, we will present our first results obtained with employing CSI on experimental data. CSI was developed by Delft University of Technology and the experimental data was provided by FUJIFILM Healthcare Corporation. The experimental data is obtained using a ring-shaped transducer which scans a breast-mimicking gelatine phantom. Our initial results obtained with CSI look promising; all inclusions within the phantom are accurately reconstructed.ImPhys/Medical ImagingImPhys/Computational Imagin
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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