Youzuo Lin

ULTRASOUND WAVEFORM TOMOGRAPHY WITH WAVE-ENERGY-BASED PRECONDITIONING

Ultrasound waveform tomography using the conjugate gradient method produces images with different qualities in different regions of the imaging domain, partly because the ultrasound wave energy is dominant around transducer elements. In addition, this uneven distribution of the wave energy slows down the convergence of the inversion. Using the Hessian matrix to scale the gradients in waveform inversion can reduce the artifacts caused by the geometrical spreading and the defocusing effect resulting from the incomplete data coverage. However, it is computationally expensive to calculate the Hessian matrix. We develop a new ultrasound waveform tomography method that weights the gradient with the ultrasound wave energies of the forward and backward propagation wavefields. Our new method balances the wave energy distribution throughout the entire imaging domain. This method scales the gradients using the square root of the wave energy of forward propagated wavefields from sources and that of backpropagated synthetic wavefields from receivers. We numerically demonstrate that this new ultrasound waveform tomography method improves sound-speed reconstructions of breast tumors and accelerates the convergence of ultrasound waveform tomography.

Efficient implementation of ultrasound waveform tomography using source encoding

Ultrasound waveform tomography takes wave propagation effects into account during image reconstruction, and has the potential to produce accurate estimates of the sound speeds of small breast tumors. However, waveform tomography is computationally time-consuming for large datasets acquired using a synthetic-aperture ultrasound tomography system that consists of hundreds to thousands of transducer elements. We introduce a source encoding approach to ultrasound waveform tomography to significantly improve the computational efficiency. The method simultaneously simulates ultrasound waveforms emitted from multiple transducer elements. To distinguish the effect of different sources, we apply a random phase to each source. The random phase helps eliminate the unwanted cross interferences produced by different sources. This approach greatly reduces the computational time of ultrasound waveform tomography to one tenth of that for the original waveform tomography, and makes it feasible for ultrasound waveform tomography in clinical applications.

Breast ultrasound tomography with two parallel transducer arrays: preliminary clinical results

Ultrasound tomography has great potential to provide quantitative estimations of physical properties of breast tumors for accurate characterization of breast cancer. We design and manufacture a new synthetic-aperture breast ultrasound tomography system with two parallel transducer arrays. The distance of these two transducer arrays is adjustable for scanning breasts with different sizes. The ultrasound transducer arrays are translated vertically to scan the entire breast slice by slice and acquires ultrasound transmission and reflection data for whole-breast ultrasound imaging and tomographic reconstructions. We use the system to acquire patient data at the University of New Mexico Hospital for clinical studies. We present some preliminary imaging results of in vivo patient ultrasound data. Our preliminary clinical imaging results show promising of our breast ultrasound tomography system with two parallel transducer arrays for breast cancer imaging and characterization.

Ultrasound bent-ray tomography with a modified total-variation regularization scheme

The sound-speed distribution of the breast can be used for characterizing breast tumors, because they typically have a higher sound speed than normal breast tissue. This is understood to be the result of remodeling of the extracellular matrix surrounding tumors. Breast sound-speed distribution can be reconstructed using ultrasound bent-ray tomography (USRT). We have recently demonstrated that USRT, using arrival times of both transmission and reflection data, significantly improves image quality. To further improve the robustness of tomographic reconstructions, we develop a USRT method using a modified total-variation (MTV) regularization scheme. Regularization is often used in solving inverse problems by introducing restrictions such as for smoothness. Tikhonov regularization is a widely used regularization scheme that tends to smooth tomographic images, but oversmoothing can obscure critical diagnostic detail such as tumor margins. Total-variation (TV) regularization is another common regularization scheme that preserves tumor margins, but at the cost of increased image noise. Our new USRT with MTV regularization is a Tikhonov-TV hybrid, reducing image noise while preserving margins. We apply our new method to ultrasound transmission data from numerical phantoms, and compare the results with those obtained using Tikhonov regularization.

Ultrasound waveform tomography with the total-variation regularization for detection of small breast tumors

Waveform tomography has the potential to quantitatively reconstruct the sound speed values of breast tumors. It is difficult to obtain quantitative values of the sound speed of breast tumors when their sizes are in the order of, or smaller than, the ultrasound wavelength. Because of the ill-posedness of the full-waveform inversion, regularization techniques are usually used to improve reconstruction. We develop an ultrasound waveform tomography method with the total-variation regularization to improve sound-speed reconstructions of small breast tumors. Our numerical examples demonstrate that our ultrasound waveform tomography with the total-variation regularization is a promising tool for quantitative estimation of the sound speed of small breast tumors.

Detection of breast microcalcifications using synthetic-aperture ultrasound

Ultrasound could be an attractive imaging modality for detecting breast microcalcifications, but it requires significant improvement in image resolution and quality. Recently, we have used tissue-equivalent phantoms to demonstrate that synthetic-aperture ultrasound has the potential to detect small targets. In this paper, we study the in vivo imaging capability of a real-time synthetic-aperture ultrasound system for detecting breast microcalcifications. This LANLs (Los Alamos National Laboratorys) custom built synthetic-aperture ultrasound system has a maximum frame rate of 25 Hz, and is one of the very first medical devices capable of acquiring synthetic-aperture ultrasound data and forming ultrasound images in real time, making the synthetic-aperture ultrasound feasible for clinical applications. We recruit patients whose screening mammograms show breast microcalcifications, and use LANLs synthetic-aperture ultrasound system to scan the regions with microcalcifications. Our preliminary in vivo patient imaging results demonstrate that synthetic-aperture ultrasound is a promising imaging modality for detecting breast microcalcifications.

Full-waveform inversion in the time domain with an energy-weighted gradient

When applying full-waveform inversion to surface seismic reflection data, one difficulty is that the deep region of the model is usually not reconstructed as well as the shallow region. We develop an energy-weighted gradient method for the time-domain full-waveform inversion to accelerate the convergence rate and improve reconstruction of the entire model without increasing the computational cost. Three different methods can alleviate the problem of poor reconstruction in the deep region of the model: the layer stripping, depth-weighting and pseudo-Hessian schemes. The first two approaches need to subjectively choose stripping depths and weighting functions. The third one scales the gradient with only the forward propagation wavefields from sources. However, the Hessian depends on wavefields from both sources and receivers. Our new energy-weighted method makes use of the energies of both forward and backward propagated wavefields from sources and receivers as weights to compute the gradient. We compare the reconstruction of our new method with those of the conjugate gradient and pseudo-Hessian methods, and demonstrate that our new method significantly improves the reconstruction of both the shallow and deep regions of the model.

Breast ultrasound waveform tomography: using both transmission and reflection data, and numerical virtual point sources

Ultrasound transmission tomography usually generates low-resolution breast images. We improve sound-speed reconstructions using ultrasound waveform tomography with both transmission and reflection data. We validate the improvement using computer-generated synthetic-aperture ultrasound transmission and reflection data for numerical breast phantoms. Our tomography results demonstrate that using both transmission and reflection data in ultrasound waveform tomography greatly enhances the resolution and accuracy of tomographic reconstructions compared to ultrasound waveform tomography using either transmission data or reflection data alone. To verify the capability of our novel ultrasound waveform tomography, we design and manufacture a new synthetic-aperture breast ultrasound tomography system with two parallel transducer arrays for clinical studies. The distance of the two transducer arrays is adjustable for accommodating different sizes of the breast. The parallel transducer arrays also allow us to easily scan the axillary region to evaluate the status of axillary lymph nodes and detect breast cancer in the axillary region. However, synthetic-aperture ultrasound reflection data acquired by firing each transducer element sequentially are usually much weaker than transmission data, and have much lower signal-to-noise ratios than the latter. We develop a numerical virtual-point-source method to enhance ultrasound reflection data using synthetic-aperture ultrasound data acquired by firing each transducer element sequentially. Synthetic-aperture ultrasound reflection data for a breast phantom obtained using our numerical virtual-point-source method reveals many coherent ultrasound reflection waveforms that are weak or invisible in the original synthetic-aperture ultrasound data. Ultrasound waveform tomography using both transmission and reflection data together with numerical virtual-point-source method has great potential to produce high-resolution tomographic reconstructions in clinical studies of breast ultrasound tomography.

Ultrasound waveform tomography with a spatially variant regularization scheme

Regularization is often needed in breast ultrasound waveform tomography to improve tomographic reconstructions. A global regularization parameter may lead to either over-regularization or under-regularization in different regions in the imaging domain. We develop a new ultrasound waveform tomography method with spatially-variant regularization. Our new method employs different regularization parameters in different regions of the breast so that each regularization parameter is optimal for the local region. Our numerical examples demonstrate the improvement of ultrasound waveform tomography using the spatially-variant modified total-variation regularization for sound-speed reconstructions of large and small breast tumors, particularly when their sizes are significantly different from one another.

Ultrasound waveform tomography with the second-order total-generalized-variation regularization

Ultrasound waveform tomography with the total-variation regularization could improve reconstructions of tumor margins, but the reconstructions usually contain unwanted blocky artifacts. We develop a new ultrasound waveform tomography method with a second-order total-generalized-variation regularization scheme to improve tomographic reconstructions of breast tumors and remove blocky artifacts in reconstruction results. We validate our new method using numerical phantom data and real phantom data acquired using our synthetic-aperture breast ultrasound tomography system with two parallel transducer arrays. Compared to reconstructions of ultrasound waveform tomography with modified total-variation regularization, our new ultrasound waveform tomography yields accurate sound-speed reconstruction results with significantly reduced artifacts.