KEYWORDS: Super resolution, Signal attenuation, Ultrasonography, Ultrasonics, Inverse scattering, Image resolution, Tissues, Medical imaging, Tomography, Current controlled current source
Routine applications of ultrasound imaging in medical diagnostics combine array technology and bemaforming (BF) algorithms for image formation. Although BF is very robust, it discards a signicant proportion of the information encoded in ultrasonic signals. Therefore, BF can reconstruct some of the geometrical features of an
object but with resolution limited by the transmitted wavelength according to the Rayleigh criterion. Recent studies have shown that imaging formation techniques based on inverse scattering rather than standard BF can overcome the Rayleigh limit to achieve subwavelength resolution. This allows for high resolution images to be obtained at relatively large wavelengths that can penetrate deep into highly attenuative media such as human tissue. In this paper we present the rst experimental demonstration of subwavelength resolution in glycerol whose ultrasonic attenuation is in the order of 1 dB/cm/MHz and which is comparable to the attenuation observed in tissue. Using a commercial clinical scanner and strands of human hairs we show that the inverse scattering approach outperforms current sonography revealing features that are undetected by sonography.
Breast ultrasound tomography has the potential to improve the cost, safety and reliability of breast cancer screening and diagnosis over the gold-standard of mammography. Vital to achieving this potential is the development of imaging algorithms to unravel the complex anatomy of the breast and its mechanical properties. The solution most commonly relied upon is Time-of-Flight Tomography but this exhibits low resolution due to the presence of diffraction effects. Iterative full-wave inversion methods present one solution to achieve higher resolution, but these are slow and are not guaranteed to converge to the correct solution. Presented here is HARBUT, the Hybrid Algorithm for Robust Breast Ultrasound Tomography, which utilises the complementary strengths of Time-of-Flight and Diffraction Tomography resulting in a direct, fast, robust and accurate high resolution method of reconstructing the sound-speed through the breast. The new algorithm is shown to produce accurate reconstructions with realistic data from a complex 3D simulation, with masses as small as 4mm being clearly visible.
It is difficult for ultrasound to image small targets such as breast microcalcifications. Synthetic aperture ultrasound imaging
has recently developed as a promising tool to improve the capabilities of medical ultrasound. We use two different tissueequivalent
phantoms to study the imaging capabilities of a real-time synthetic aperture ultrasound system for imaging
small targets. The InnerVision ultrasound system DAS009 is an investigational system for real-time synthetic aperture
ultrasound imaging. We use the system to image the two phantoms, and compare the images with those obtained from
clinical scanners Acuson Sequoia 512 and Siemens S2000. Our results show that synthetic aperture ultrasound imaging
produces images with higher resolution and less image artifacts than Acuson Sequoia 512 and Siemens S2000. In addition,
we study the effects of sound speed on synthetic aperture ultrasound imaging and demonstrate that an accurate sound speed
is very important for imaging small targets.
Breast ultrasound tomography (BUST) is currently being developed for the early detection of cancer. Images of mechanical properties across the breast are formed using ultrasonic signals transmitted through the breast immersed in water. This is possible because the perturbation to the free propagation of ultrasound induced by the presence of the breast encodes information about the material properties of the breast. To achieve high sensitivity with BUST it is therefore crucial that the perturbation is most sensitive to the internal lesions inside the breast. However, the perturbation is also affected by the shape of the breast which can cause significant and undesired refraction effects. To understand the effect of breast shape on the transmission of ultrasound, this paper investigates transmission through a cone with properties similar to that of breast tissue. We show that it is possible to identify two regimes of transmission depending on the physical properties of the cone and transducers. While the first regime is suitable for BUST measurements, the second regime is highly affected by the breast shape and measurements are not reliable for accurate reconstructions. We provide a physical approximation that describes transmission in the first regime and which will aid the design of future BUST systems.
Ultrasound tomography is an attractive imaging method for the detection of breast cancer. The complex anatomy
of the breast with its different spatial scales and material property contrasts make accurate reconstructions very
challenging. This paper proposes a hybrid approach whereby Travel-of-Flight and Diffraction Tomography are combined together to achieve high-resolution and high-accuracy sound-speed reconstructions. The method is validated with several numerical phantoms.
KEYWORDS: Breast, Ultrasonography, Super resolution, Transducers, Image resolution, Mammography, Reflection, Diffraction, Chemical elements, In vivo imaging
Ultrasound image resolution and quality need to be significantly improved for breast microcalcification detection. Super-resolution
imaging with the factorization method has recently been developed as a promising tool to break through the
resolution limit of conventional imaging. In addition, wave-equation reflection imaging has become an effective method
to reduce image speckles by properly handling ultrasound scattering/diffraction from breast heterogeneities during image
reconstruction. We explore the capabilities of a novel super-resolution ultrasound imaging method and a wave-equation
reflection imaging scheme for detecting breast microcalcifications. Super-resolution imaging uses the singular value decomposition
and a factorization scheme to achieve an image resolution that is not possible for conventional ultrasound
imaging. Wave-equation reflection imaging employs a solution to the acoustic-wave equation in heterogeneous media
to backpropagate ultrasound scattering/diffraction waves to scatters and reconstruct images of heterogeneities. We construct
numerical breast phantoms using in vivo breast images, and use a finite-difference wave-equation scheme to generate
ultrasound data scattered from inclusions that mimic microcalcifications. We demonstrate that microcalcifications can
be detected at full spatial resolution using the super-resolution ultrasound imaging and wave-equation reflection imaging methods.
Ultrasound is commonly used as an adjunct to mammography for diagnostic evaluation of suspicions arising
from breast cancer screening. As an alternative to conventional sonography that uses hand-held transducers,
toroidal array probes that encircle the breast immersed in a water bath have been investigated for ultrasound
tomography. This paper introduces a new method for three-dimensional synthetic aperture diffraction tomography
that maximizes the resolution in the scanning direction and provides quantitative reconstructions of the
acoustic properties of the object. The method is validated by means of numerical simulations.
Needle biopsy under ultrasound guidance is routinely used in clinical applications. However, in order to track the
position of the needle as it penetrates the tissue a particular alignment between the ultrasound probe and needle
must be kept, thus requiring highly skilled radiologists. In this paper we present a new technique which leads to
the detection of the needle regardless of its orientation relative to the imaging probe. We discuss the fundamental
aspects of the method and present some preliminary results that show the potential of the technique.
The development of ultrasound tomography for the detection of breast cancer could have a major impact on the
effectiveness of current diagnostic tools. Here, the potential of ultrasound tomography is investigated by means
of a new generation of toroidal ultrasound arrays that can measure both the signals reflected and transmitted
through human breast, simultaneously. Experiments performed on phantoms and human breast in vivo are
used to compare continuous wave (CW) insonification versus wideband (WB) excitation. It is shown that while
transmission diffraction tomography has little benefit from WB excitation, reflection tomography is greatly
improved due to the low signal-to-noise ratio of reflection measurements.
KEYWORDS: Multiple scattering, Fermium, Frequency modulation, Scattering, Sensors, Signal to noise ratio, Ultrasound tomography, Image resolution, Distortion, Super resolution
Ultrasound tomography attempts to retrieve the structure of an object by exploiting the interaction of acoustic
waves with the object. A fundamental limit of ultrasound tomography is that features cannot be resolved if they
are spaced less than λ/2 apart, where λ is wavelength of the probing wave, regardless of the degree of accuracy of
the measurements. Therefore, since the attenuation of the probing wave with propagation distance increases as
λ decreases, resolution has to be traded against imaging depth. Recently, it has been shown that the λ/2 limit is
a consequence of the Born approximation (implicit in the imaging algorithms currently employed) which neglects
the distortion of the probing wavefield as it travels through the medium to be imaged. On the other hand, such a
distortion, which is due to the multiple scattering phenomenon, can encode unlimited resolution in the radiating
component of the scattered field. Previously, a resolution better than λ/3 has been reported in these proceedings
[F. Simonetti, pp. 126 (2006)] in the case of elastic wave probing. In this paper, we demonstrate experimentally
a resolution better than λ/4 for objects immersed in a water bath probed by means of a ring array which excites
and detects pressure waves in a full view configuration.
KEYWORDS: Super resolution, Spatial frequencies, Imaging systems, Image resolution, Scattering, Inverse problems, Near field scanning optical microscopy, Signal detection, Detection and tracking algorithms, Phased arrays
For more than a century the possibility of imaging the structure of a medium with diffracting wavefields has been limited by the tradeoff between resolution and imaging depth. While long wavelengths can penetrate deep into a medium, the resolution limit precludes the possibility of observing subwavelength structures. Recent progress in microscopy has shown that by exploiting the super-oscillatory properties of evanescent fields, resolution several orders of magnitude smaller than the wavelength can be achieved so leading to Near-field Scanning Optical Microscopy. Based on a similar argument, this paper investigates the possibility of obtaining super resolution in the far-field (here far-field refers to a distance greater than λ, which would enable high resolution imaging at relatively large depth. The theoretical principles which result in the resolution limit are reviewed and a new strategy to overcome it is proposed. An advanced imaging algorithm for linear and two-dimensional array probing systems is presented and its capability of resolving targets as close as λ/3 is demonstrated experimentally, the targets being at several wavelength distance from the array. The results show that the method is superior to conventional techniques such as Synthetic Aperture Focusing, Synthetic Phased Arrays and Time Reversal.
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