Ultrasound Computer Tomography System Research Articles

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

A potential method for future breast cancer screening is 3D 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 paper, we present a new transducer array design to be utilized in a third generation 3D 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 2D 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 3D 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.

Ultrasound computed tomography based on full waveform inversion with source directivity calibration

Ultrasound computed tomography based on full waveform inversion has the potential to provide high-resolution images of human tissues in a quantitative manner. A successful ultrasound computed tomography system requires the decent knowledge of acquisition array, including the spatial position and the directivity of each transducer, to meet the high-level demand of clinical applications. The conventional full waveform inversion algorithm assumes a point source with the omni-directional emission. Such assumption does not hold when the directivity of emitting transducer is not negligible. For a practical implementation, an efficient and accurate self-checking evaluation of directivity is crucial prior to the reconstruction of images. We propose to measure the directivity of each emitting transducer using the full-matrix captured data obtained with a water-immersed and target-free experiment. We introduce the weighted virtual point-source array to act as the proxy of emitting transducer during the numerical simulation. The weights of different points in the virtual array can be calculated from the observed data using the gradient-based local optimization method. Although the full waveform imaging method relies on the finite-difference solver of wave equation, such directivity estimation benefits from the introduction of analytical solver. The trick significantly reduces the numerical cost, enabling an automatic directivity self-check at boot. We verify the feasibility, efficiency, and accuracy of the virtual array method through simulated and experimental tests. For the experimental test, we also illustrate that full waveform inversion with directivity calibration can reduce the artifacts introduced by the conventional point source assumption, improving the quality of reconstructed images..

Sound speed imaging of small animal organs by ultrasound computed tomography

Sound speed is an important acoustic parameter for tissue characterization. Herein we developed an ultrasound computed tomography (USCT) system for <i>ex vivo</i> sound speed imaging and evaluation of small animal organs. The proposed USCT system employs a 256-element ring array transducer and allows simultaneous signal transmission and reception for all channels. The method does not require complicated sample preparation procedures and can yield accurate measurement results. Experimental results show that sound speeds of excised rat brain, heart, liver, spleen, and kidney measured by the method are close to published data. This work demonstrates a new method for sound speed imaging and holds potential for <i>in vivo</i> applications.

An Ultrasound Tomography Method for Monitoring CO2 Capture Process Involving Stirring and CaCO3 Precipitation.

In this work, an ultrasound computed tomography (USCT) system was employed to investigate the fast-kinetic reactive crystallization process of calcium carbonate. USCT measurements and reconstruction provided key insights into the bulk particle distribution inside the stirred tank reactor and could be used to estimate the settling rate and settling time of the particles. To establish the utility of the USCT system for dynamical crystallization processes, first, the experimental imaging tasks were carried out with the stirred solid beads, as well as the feeding and stirring of the CaCO3 crystals. The feeding region, the mixing process, and the particles settling time could be detected from USCT data. Reactive crystallization experiments for CO2 capture were then conducted. Moreover, there was further potential for quantitative characterization of the suspension density in this process. USCT-based reconstructions were investigated for several experimental scenarios and operating conditions. This study demonstrates a real-time monitoring and fault detection application of USCT for reactive crystallization processes. As a robust noninvasive and nonintrusive tool, real-time signal analysis and reconstruction can be beneficial in the development of monitoring and control systems with real-world applications for crystallization processes. A diverse range of experimental studies shown here demonstrate the versatility of the USCT system in process application, hoping to unlock the commercial and industrial utility of the USCT devices.

Handwriting With Sound-Speed Imaging Using Ultrasound Computed Tomography

This letter proposes the use of ultrasound computed tomography (USCT) for handwriting by imaging sound velocity changes in a medium caused by a rod. The rod was used as a writing object, and the tomographic instrument was able to process the data over time with a temporal frequency of 4 frames per second. Tests are carried out in writing various words and letters using USCT data and automatic imaging software. The work shows the functional imaging capability offered by the USCT system combining qualitative and temporal imaging features.

Ultrasound tomography reconstruction algorithm using two opposite rotatable linear arrays for breast cancer diagnosis

This article presents a new ultrasound transmission tomography reconstruction algorithm based on a isometric fan-shaped beam along the straight trajectory for an original breast ultrasound imaging system with the cylindrical motion of opposite linear arrays. The ultrasound computed tomography system and measurement scheme are described in detail. The reconstruction algorithm is derived. The numerical breast phantom model is built, including eight structures representing the adipose tissue, glandular tissue, cysts, fibroadenomas, cancer, and calcifications. Using one array as a transmitter and the other one as a receiver, the attenuation distribution in a medium interposed between them is reconstructed. However, limited angle tomography is usually plagued by artifacts along the depth of the image that cause shape distortion to be ellipses, which had an impact on the accurate judgment of the size and shape. B-mode ultrasound images are used as prior information to aid in improving the image quality of limited aperture reconstructions. Simulation results show that the number, location, size, and shape of cancer, cyst, fibroadenoma, and calcification in the numerical breast phantom are reconstructed. Moreover, the quantitative analysis of reconstructed images demonstrates that the reconstruction performance of the region of interest has been greatly improved, which verifies the feasibility of the algorithm using two opposite linear arrays. The reconstruction images are produced by the designed ultrasound tomography reconstruction algorithm, allowing the assessment of the morphology, orientation, internal structure, and margins of lesions in the breast tissue. The ability to create good attenuation images using the cylindrical motion of opposite linear arrays allows reconstructing the three-dimensional image for breast cancer detection and characterization studies.

Study on Image Improvement of Ultrasound Computed Tomography System with Carving Imaging Technology

Study on Image Improvement of Ultrasound Computed Tomography System with Carving Imaging Technology

Coherence Factor-Like Beamforming for Ultrasound Computed Tomography

Ultrasound computed tomography (USCT) has important clinical application prospect in breast cancer screening and early diagnosis. In this paper, six kinds of coherence factor-like beamforming methods have been applied to improve the image quality for USCT, including coherence factor (CF), phase coherence factor (PCF), sign coherence factor (SCF), phasor dispersion based coherence factor (PDCF), spatial smoothed coherence factor (SSCF) and spatio-temporally smoothed coherence factor (STSCF). The mentioned methods were verified with the radio-frequency (RF) data of the breast phantom captured by the USCT system developed in the Medical Ultrasound Laboratory. The ring-type transducer has 1024 elements with a center frequency of 2.5 MHz. Experimental results show that the reconstructed images of the breast phantom by the CF gets the highest contrast to noise ratio (CNR), but overall image brightness reduces significantly. PCF gets the lowest variance and provides a more homogenous background. STSCF beamforming method can improve the robustness of the PCF and having the ability to suppress clutter while significant removal of black region artifacts. For practical application, these coherence factor-like beamforming methods can be implemented with low computational complexity.

A Prior-Information-Based Combination Solution for Picking the Difference of Time-of-Flight in Ultrasound Computed Tomography

Sound speed imaging is one modal of ultrasound computed tomography (USCT) which is helpful for early breast disease diagnosis. One of the most critical processes of sound speed reconstruction is time-of-flight picking. As each of the traditional time-of-flight picking methods has shortcomings for real data, in this study, a practical priorinformation-based combination (PIBC) solution for picking the difference of time-of-flight between the reference data and the object data (DTOF) is proposed to enhance the reconstruction accuracy and uniformity. By using DTOF, some system bias will be effectively alleviated. Firstly, by analyzing the signal-amplitude, the "penetrating-through-the-object" and the "bypassing-the-object" signals are distinguished. Then for the "penetrating-throughthe-object" signals, based on the 'majority rule,' the consistency of DTOF picked by different methods are calculated as a basis to combine the advantages of different picking methods; for the "bypassing-the-object" signals, the DTOF closest to zero is chosen. Finally, the DTOFs are post-processed to suppress the noise by a median filter and to fix the deficiency of the system by an interpolation operator. The new solution is verified by in vitro breast phantom experiment conducted on the home-made USCT system "Lucid." The proposed PIBC solution can quantitatively decrease Root Mean Squared Error (RMSE) and Mean Squared Error (MSE) of DTOF picking and enhance the image quality of reconstructed sound speed images with higher accuracy and uniformity. This work is significant for ray-based sound speed reconstruction and can provide a fine initial solution for high-resolution wave-based reconstruction.

Development of Single‐Fiber Piezocomposite Transducers for 3‐D Ultrasound Computer Tomography

Ultrasound Computer Tomography (USCT) medical imaging is a promising approach for early detection of breast cancer. At Karlsruhe Institute of Technology (KIT) a 3‐D USCT system is developed. The region‐of‐interest (ROI) of 10 × 10 × 10 cm3 volume is surrounded by a aperture of 2014 semi‐spherical positioned ultrasound transducers. Results from a first patient study reveals the requirement of a significantly increased ROI to cover bodily variations. Design considerations and simulations show a demand for circular transducers with a diameter of ca. 500 µm, increasing the opening angle of the transducers to ca. 60°. Piezofiber composite technology is predestinated to simply provide circular transducers of the required dimensions. Moreover, piezocomposites based on single PZT (lead zirconate titanate Pb[ZrxTi1‐x]O3) fibers enable a cost‐effective and series‐production alternative to currently used dice‐and‐fill composites. A transducer design is presented which utilizes individually arranged single piezoceramic fibers with 460 μm in diameter within piezocomposite discs. As a result, fibers are independently addressable as single transducer elements allowing for the desired transducer arrangement. The electrical performance of each piezoceramic fiber is determined proofing a strong dependence both of the coupling coefficient and the resonance frequency from the transducer thickness. In further processing, the piezocomposite discs are connected to printed circuits, integrated into a cylindrical housing, and backfilled with polyurethane. Ultrasound characteristics such as sound pressure and opening angle are evaluated quantitatively. The results show that the transducer opening angles lie in the expected range, that the desired center frequency is achieved and that the bandwidth could be preserved compared to former dice‐and‐fill transducers.

An Innovative Practical Automatic Segmentation of Ultrasound Computer Tomography Images Acquired from USCT System

A 3D ultrasound computer tomography (USCT) device with a nearly isotropic and spatially invariant 3D point spread function has been constructed at Institute for Data Processing and Electronic (IPE), Karlsruhe Institute of Technology (KIT). This device is currently applied in clinical studies for breast cancer screening. In this paper, a new method to develop an automated segmentation algorithm for USCT acquired images is proposed. The method employs distance regularized level set evolutionary (DRLSE) active contours along with surface fitting extrapolation and 3D binary mask generation for fully automatic segmentation outcome. In the first stage of the proposed algorithm, DRLSE is applied to those 3D USCT slice images which contain breast and are less affected by noise and ring artifacts named as Cat2. The DRLSE segmentation results are employed to extrapolate the rest of slice images known as Cat1. To overcome defectively segmented slice images, a 3D binary mask is generated out of USCT attenuation images. The 3D binary mask is multiplied by the DRLSE-based segmentation results to form finally segmented 3D USCT images. The method was tested on 12 clinical dataset images. According to F-measure criterion, the proposed method shows higher performance than the previously proposed semiautomatic segmentation one.

Estimation of mechanical stiffness by finite element analysis of ultrasound computed tomography (UCT-FEA); a comparison with X-ray µCT based FEA in cancellous bone replica models

The mechanical integrity of a bone is determined by its quantity and quality. Conventional mechanical testing is the ‘gold standard’ for assessing bone strength, although not applicable in vivo since it is inherently invasive and destructive. A mechanical test measurement of stiffness (N mm−1) provides an accurate estimate of strength, although again inappropriate in vivo. Several non-destructive, non-invasive, in vivo techniques have been developed and clinically implemented to serve as surrogates for bone strength assessment including dual-energy X-ray absorptiometry along with axial and peripheral quantitative computed tomography, and quantitative ultrasound. Finite element analysis (FEA) is a computer simulation method that predicts the behaviour of a structure such as a bone under mechanical loading, being previously combined with in vivo bone imaging, reporting higher predictions of mechanical integrity than imaging alone.We hypothesised that ultrasound computed tomography (UCT) may be combined with FEA, thereby predicting the stiffness of bone. The objective of this study was to apply finite element analysis to UCT derived attenuation images of trabecular bone replica samples, thereby providing an estimate of mechanical stiffness that could be compared with both a gold standard mechanical test and a surrogate X-ray µCT-FEA.Replica bone samples were 3D-printed from four anatomical sites (femoral head, lumbar spine, calcaneus and iliac crest), with two cylindrical volumes of interest extracted from each sample. Each replica sample was scanned by X-ray µCT and a bespoke UCT system, from which finite element analysis was performed to estimate mechanical stiffness. The samples were then mechanically tested, yielding the gold standard stiffness value.The coefficient of determination (R2) to estimate mechanical test derived stiffness was 99% for µCT-FEA and 84% for UCT-FEA. In conclusion, UCT-FEA is a promising tool for estimating the mechanical integrity of a bone. This study demonstrated that UCT-FEA, based upon quantitative attenuation images, provided a comparable estimation of gold standard mechanical-test stiffness and therefore has significant potential clinical utility for osteoporotic fracture risk assessment and quantitative assessment of musculoskeletal tissues.

Highly precise acoustic calibration method of ring-shaped ultrasound transducer array for plane-wave-based ultrasound tomography

Ultrasound computed tomography (USCT) is promising for a non-invasive, painless, operator-independent and quantitative system for breast-cancer screening. Assembly error, production tolerance, and aging-degradation variations of the hardwire components, particularly of plane-wave-based USCT systems, may hamper cost effectiveness, precise imaging, and robust operation. The plane wave is transmitted from a ring-shaped transducer array for receiving the signal at a high signal-to-noise-ratio and fast aperture synthesis. There are four signal-delay components: response delays in the transmitters and receivers and propagation delays depending on the positions of the transducer elements and their directivity. We developed a highly precise calibration method for calibrating these delay components and evaluated it with our prototype plane-wave-based USCT system. Our calibration method was found to be effective in reducing delay errors. Gaps and curves were eliminated from the plane wave, and echo images of wires were sharpened in the entire imaging area.

Development of High-speed Iterative Refraction Calibration Method of Ultrasound Computed Tomography System for Breast Cancer Screening

Development of High-speed Iterative Refraction Calibration Method of Ultrasound Computed Tomography System for Breast Cancer Screening

Tissue boundary roughness index for evaluating malignancy of cancer in ultrasound-computed tomography system

Ultrasound-computed tomography (USCT) is a promising candidate for a radiation free, painless, and quantitative modality for breast cancer examination. We developed a USCT prototype with a ring-shaped transducer array (frequency: 1.5 MHz) that moves along the ring axis for scanning the entire breast. Distributions of speed and attenuation of ultrasound are computed from ultrasound transmitted through tissue. Although prior research shows that there is a correlation between malignancy and the speed of sound and attenuation values, in some cases both benign and malignant tumors have similar values. In this study, we propose to quantify the boundary roughness of a tumor from the spatial power distributions of reflected ultrasounds that are transmitted from different apertures of the ring transducer. In conventional ultrasound echography, the boundary roughness of a tumor is an important indicator of malignancy. However, roughness is judged by radiologists based on B-mode images and is a subjective index. We simulated the ultrasound reflection signal from boundaries of different roughness. The simulation result indicates that a spatial power concentration has a significant correlation with boundary roughness. We confirmed through gel phantom experiments that the index distinguishes rough and smooth boundaries, which is not evident in B-mode images.

Optimization of the aperture and transducers of a three-dimensional ultrasound computer tomography system

In previous work we optimized the aperture of our 3D Ultrasound Computer Tomography (USCT) system with emphasis on reflection tomography. Based on the promising clinical results with speed of sound and attenuation images, the next generation aperture will be upgraded to contain also optimization for transmission tomography. The main changes to be implemented in aperture, transducers, and transducer arrays are: (1) Overall 3D aperture: due to the shape of the buoyant breast a simpler hemispherical aperture can be applied. (2) The diameter of the aperture will be increased and the diameter of the transducers will be decreased for further homogenization of the illumination. (3) The disjunctive sampling of the transducers will be increased and the transducers will be distributed randomly to enhance the uniformity of transmission tomography. (4) Transducers will be connected both as emitters and receivers to decrease the need for mechanical movement of the aperture.

Quantitative evaluation of polymer gel dosimeters by broadband ultrasound attenuation

Ultrasound has been examined previously as an alternative readout method for irradiated polymer gel dosimeters, with authors reporting varying dose response to ultrasound transmission measurements. In this current work we extend previous work to measure the broadband ultrasound attenuation (BUA) response of irradiated PAGAT gel dosimeters, using a novel ultrasound computed tomography system.

Latest Advances in Breast Sonography

With technological improvements in transducer technology and signal processing, breast sonography has become an essential modality in breast imaging. Technical advances aiming to improve the sensitivity and specificity of sonographic evaluation are still ungoing, and most of these advances are available for clinical use, while some are not yet ready for clinical application. In this article, recently published studies and patents reporting promising results using tissue harmonic imaging, compound imaging, extended field-of-view imaging, Doppler sonography, contrast-enhanced sonography, elastography, shearwave elastography, computer-aided diagnosis, automated-3D sonography, semi-automated scanning ultrasound device integrated with a digital breast tomosynthesis system, ultrasound computed tomography system and real-time virtual sonography with image fusion will be reviewed. Keywords: Automated-3D ultrasound, breast imaging, compound sonography, computer-aided diagnosis, contrast-enhanced sonography, Doppler sonography, elastography, Extended field-of-view imaging (EFV), signal processing, Tissue harmonic imaging (THI), transducer technology.

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.

TH‐C‐332‐10: Ultrasound Inverse‐Scatter Tomography of the Breast

Purpose: To present the design and performance of a new automated whole‐breast ultrasound computed tomography (USCT) system that employs 3D inverse‐scatter reconstruction. Method and Materials: The scanner (Techniscan Medical Systems, Inc., Salt Lake City, Utah) is installed at the University of California, San Diego to evaluate clinical performance including ability to detect and analyze breast masses. Patients lie prone on a table while opposing transmitter and receiver transducer arrays rotate 360° around the breast. Ultrasound plane waves (300 kHz–2MHz) are emitted every 2 degrees while the scattered beam is detected by a 960‐element six‐row transducer. In the same plane three B‐mode linear arrays acquire backscatter data from wideband signals (3.6–8.4 MHz). The arrays move up the breast in 2mm increments to scan the entire breast. Discrete frequency domain data is used by a proprietary 3D inverse‐scattering algorithm that incorporates multiple scattering within and between the planes. 2‐D coronal images of the entire breast are reconstructed as accurate quantitative maps of sound speed and attenuation along with aberration‐corrected high‐resolution reflection tomograms. Results: To date, more than 50 subjects were scanned with wide range in age (20–78), breast density and diagnostic outcome. Representative cases will be presented comparing mammography, sonography and multi‐planar USCT images. Included are benigns (cysts, fibroadenomas, fibrocystic disease) and biopsy‐proven malignancies (invasive ductal carcinomas, invasive tubular carcinoma and mixed lobular/ductal carcinoma). System performance and tissue characterization are excellent. Accuracy and linearity of sound speed measurements by USCT is very high (R2=0.988) over the range of 1400 to 1600 m/sec. Conclusion: The USCT system provides rapid, automated scanning with quantitative 3D images of the breast and substantially new information for characterizing breast masses. Conflict of Interest: Research sponsored in part by Techniscan Medical Systems, Inc.