Ultrasound Tomography.

Ultrasound tomography (USCT) is a promising imaging modality, mainly aiming at early diagnosis of breast cancer. It provides three-dimensional, reproducible images of higher quality than conventional ultrasound methods and additionally offers quantitative information on tissue properties. This chapter provides an introduction to the background and history of USCT, followed by an overview of image reconstruction algorithms and system design. It concludes with a discussion of current and future applications as well as limitations and their potential solutions.

Single-Fiber Transducer Arrays for 3-D Ultrasound Computed Tomography: From Requirements to Results.

A potential method for future breast cancer screening is 3-D ultrasound computed tomography (USCT). The utilized image reconstruction algorithms require transducer characteristics fundamentally different from conventional transducer arrays, leading to the necessity of a custom design. This design has to provide random transducer positioning, isotropic sound emission as well as a large bandwidth and wide opening angle. In this article, we present a new transducer array design to be utilized in a third generation 3-D USCT system. Each system requires 128 cylindrical arrays, mounted into the shell of a hemispherical measurement vessel. Each new array contains a 0.6 mm thick disk with 18 single PZT fibers (0.46 mm diameter) embedded in a polymer matrix. Randomized positioning of the fibers is achieved with an arrange-and-fill process. The single-fiber disks are connected on both ends with a matching and backing disk using simple stacking and adhesives. This enables fast and scalable production. We characterized the acoustic field of 54 transducers with a hydrophone. Measurements in 2-D showed isotropic acoustic fields. The mean bandwidth and opening angle are 131% and 42°, respectively (both -10 dB). The large bandwidth arises from two resonances within the utilized frequency range. Parameter studies using different models showed that the realized design is already close to the achievable optimum for the transducer technology used. Two 3-D USCT systems were equipped with the new arrays. First images show promising results, with an increase in image contrast and a significant reduction of artifacts.

Ultrasonic Synthetic-Aperture Interface Imaging.

Synthetic-aperture (SA) imaging is a popular method to visualize the reflectivity of an object from ultrasonic reflections. The method yields an image of the (volume) contrast in acoustic impedance with respect to the embedding. Typically, constant mass density is assumed in the underlying derivation. Due to the band-limited nature of the recorded data, the image is blurred in space, which is quantified by the associated point spread function. SA volume imaging is valid under the Born approximation, where it is assumed that the contrast is weak. When objects are large with respect to the wavelength, it is questionable whether SA volume imaging should be the method-of-choice. Herein, we propose an alternative solution that we refer to as SA interface imaging. This approach yields a vector image of the discontinuities of acoustic impedance at the tissue interfaces. Constant wave speed is assumed in the underlying derivation. The image is blurred in space by a tensor, which we refer to as the interface spread function. SA interface imaging is valid under the Kirchhoff approximation, where it is assumed that the wavelength is small compared to the spatial dimensions of the interfaces. We compare the performance of volume and interface imaging on synthetic data and on experimental data of a gelatin cylinder with a radius of 75 wavelengths, submerged in water. As expected, the interface image peaks at the gelatin-water interface, while the volume image exposes a peak and trough on opposing sides of the interface.

Tailoring Materials for Mottronics: Excess Oxygen Doping of a Prototypical Mott Insulator.

The Mott transistor is a paradigm for a new class of electronic devices-often referred to by the term Mottronics-which are based on charge correlations between the electrons. Since correlation-induced insulating phases of most oxide compounds are usually very robust, new methods have to be developed to push such materials right to the boundary to the metallic phase in order to enable the metal-insulator transition to be switched by electric gating. Here, it is demonstrated that thin films of the prototypical Mott insulator LaTiO3 grown by pulsed laser deposition under oxygen atmosphere are readily tuned by excess oxygen doping across the line of the band-filling controlled Mott transition in the electronic phase diagram. The detected insulator to metal transition is characterized by a strong change in resistivity of several orders of magnitude. The use of suitable substrates and capping layers to inhibit oxygen diffusion facilitates full control of the oxygen content and renders the films stable against exposure to ambient conditions. These achievements represent a significant advancement in control and tuning of the electronic properties of LaTiO3+x thin films making it a promising channel material in future Mottronic devices.

Comparing different ultrasound imaging methods for breast cancer detection.

Ultrasound is frequently used to evaluate suspicious masses in breasts. These evaluations could be improved by taking advantage of advanced imaging algorithms, which become feasible for low frequencies if accurate knowledge about the phase and amplitude of the wave field illuminating the volume of interest is available. In this study, we compare five imaging and inversion methods: time-of-flight tomography, synthetic aperture focusing technique, backpropagation, Born inversion, and contrast source inversion. All methods are tested on the same full-wave synthetic data representing a 2-D scan using a circular array enclosing a cancerous breast submerged in water. Of the tested methods, only contrast source inversion yielded an accurate reconstruction of the speed-ofsound profile of the tumor and its surroundings, because only this method takes effects such as multiple scattering, refraction, and diffraction into account.

3D ultrasound computer tomography of the breast: a new era?

A promising candidate for imaging of breast cancer is ultrasound computer tomography (USCT). The main advantages of a USCT system are simultaneous recording of reproducible reflection, attenuation and speed of sound volumes, high image quality, and fast data acquisition. The here presented 3D USCT prototype realizes for the first time the full potential of such a device. It is ready for a clinical study. Full volumes of a breast can be acquired in four minutes. In this paper images acquired with a clinical breast phantom are presented. The resolution and imaged details of the reflectivity reconstruction are comparable to a 3 tesla MRI volume of the phantom. Image quality and resolution is isotropic in all three dimensions, confirming the successful implementation experimentally.

Sound-speed image reconstruction in sparse-aperture 3-D ultrasound transmission tomography.

The paper is focused on sound-speed image reconstruction in 3-D ultrasound transmission tomography. Along with ultrasound reflectivity and the attenuation coefficient, sound speed is an important parameter which is related to the type and pathological state of the imaged tissue. This is important in the intended application, breast cancer diagnosis. In contrast to 2-D ultrasound transmission tomography systems, a 3-D system can provide an isotropic spatial resolution in the x-, y-, and z-directions in reconstructed 3-D images of ultrasound parameters. Several challenges must, however, be addressed for 3-D systems-namely, a sparse transducer distribution, low signal-to-noise ratio, and higher computational complexity. These issues are addressed in terms of sound-speed image reconstruction, using edge-preserving regularized algebraic reconstruction in combination with synthetic aperture focusing. The critical points of the implementation are also discussed, because they are crucial to enable a complete 3-D image reconstruction. The methods were tested on a synthetic data set and on data sets measured with the Karlsruhe 3-D ultrasound computer tomography (USCT) I prototype using phantoms. The sound-speed estimates in the reconstructed volumes agreed with the reference values. The breast-phantom outlines and the lesion-mimicking objects were also detectable in the resulting sound-speed volumes.

Calibrating an ultrasonic computed tomography system using a time-of-flight based positioning algorithm.

This paper presents a method for geometrical and time-delay auto-calibration of an ultrasonic computed tomography (USCT) system. The algorithms used for the calibration are based on the principles similar to the global positioning system (GPS) navigation. Ultrasonic transmitters and receivers in USCT can be viewed like satellite transmitters and mobile receiver units in GPS. However, unlike in GPS, none of the positions of the transmitters or receivers in USCT are assumed to be known and all are the to-be-calibrated unknowns. The presented method is capable of calibrating the positions of all ultrasonic transducers and their individual time delays at once. No calibration phantoms are necessary.