Anna T. Fernandez

High resolution ultrasound beamforming using synthetic and adaptive imaging techniques

Ultrasound image quality can be improved by incorporating elevation information from high-order arrays and adjust the beamforming techniques to take advantage of the added information. We present the use of 1.75D ultrasound arrays for improving image quality with two approaches: 1) increasing the depth-of-field through synthetic transmit and receive imaging in elevation, and 2) implementing adaptive imaging techniques to remove tissue phase-aberrations to improve contrast and detail resolution. Phantom data is presented to show the increased visualization with synthetic elevation beamforming. Phantom and clinical results show the advantages of measuring and correcting images from two-dimensional phase aberration profiles using our 8 /spl times/ 128 1.75D array.

Synthetic elevation beamforming and image acquisition capabilities using an 8 /spl times/ 128 1.75D array

Ultrasound imaging can be improved with higher order arrays through elevation dynamic focusing in future, higher channel count systems. However, modifications to current system hardware could yield increased imaging depth-of-field with 1.75D arrays (arrays with individually addressable elements, several rows in elevation) through the use of synthetic elevation imaging. We describe synthetic elevation beamforming methods and its implementation with our 8 /spl times/ 128, 1.75D array (Tetrad Co., Englewood, CO). This array has been successfully interfaced with a Siemens Elegra scanner for summed RF and single channel RF data acquisition. Individual rows of the 8 /spl times/ 128 array can be controlled, allowing for different aperture configurations on transmit and receive beamforming. Advantages of using this array include finer elevation sampling, a larger array footprint for aberration measurements, and elevation focusing. We discuss system tradeoffs that occur in implementing synthetic receive and synthetic transmit/receive elevation focusing and show significant image quality improvements in simulation and phantom data results.

Aberration measurement and correction with a high resolution 1.75D array

Accurate measurement of tissue induced aberrations is necessary for effective adaptive ultrasound imaging. We acquired single channel RF data on a 6.7 MHz, 8/spl times/128 array (Tetrad Co.) operating at F/1.0 in azimuth and F/2.89 in elevation. This array was interfaced to a Siemens Elegra scanner, allowing for data acquisition during routine clinical scanning. Breast images in three patients and four volunteers (for a total of 16 scans), and thyroid and liver images in six volunteers (10 scans each) were taken. A least squares algorithm was employed to estimate the arrival time error induced by the tissue and to generate corrected images. In general, mild (20-40 nsec r.m.s.) and spatially stable aberration profiles were measured.

Two-dimensional phase aberration correction using an ultrasonic 1.75D array: case study on breast microcalcifications

Ultrasound phase aberration resulting from tissue velocity inhomogeneities reduce the focusing ability of ultrasound waves and degrade image quality. Two-dimensional phase aberration measurements and correction with higher-order 1.75D arrays are expected to reduce this problem. We implement a least-mean-squares measurement algorithm and discuss implementation decisions for phase aberration correction techniques. We present clinical results from using individual channel RF signals from an 8/spl times/128 1.75D array (Tetrad Co.) interfaced with a Siemens Elegra scanner. The average patient aberration measurement in the thyroid (9 patients) was 22.2/spl plusmn/4.6 ns r.m.s. amplitude with 4.8 mm/spl plusmn/1.3 mm FWHM autocorrelation length. Analysis of breast scans (8 patients) resulted in a patient average aberration measurement of 33.1/spl plusmn/7.0 ns r.m.s. amplitude and 6.4/spl plusmn/1.5 mm FWHM. The results show a statistically significant difference (p-value=0.01) in aberration measurements in clinical breast imaging patients in age groups 20-40 years old and 50-70 years old. We present results of receive-only aberration correction in clinical images and describe quantitative improvements and the results from using specific correction implementation techniques in a case study of breast microcalcifications after phase aberration correction.

Aberration estimation using FDORT: insights and improved method for speckle signals

Clinical ultrasound imaging is degraded by tissue velocity inhomogeneities that reduce resolution and contrast. The Focused DORT (French acronym for decomposition of the time reversal operator) method, here denoted FDORT, can be used as an aberration estimation method. FDORT uses per-channel received RF data obtained from several focused transmissions. An aberration profile is estimated using a singular value decomposition method. The advantage of FDORT over a crosscorrelation based method is its ability to identify individual wavefronts in complex RF regions. It also allows frequencydependent estimation. FDORT exhibits good results with point scatterers but significant residual rms errors (between applied and estimated aberrator) in speckle regions (typically 15ns for a 45ns aberration). This study aims to explain the behavior of FDORT in speckle by interpreting the FDORT matrix as a crossspectrum matrix of the backscattered signal, similar to the one studied by Måsøy et al. [JASA 117 (1) 2005]. This explanation is then used to improve the method; reducing both the bias and variance of the estimation. Originally, the first singular vector was used for the estimation: it contained the amplitude and delay law that maximized the speckle brightness. A bias results from the fact that the transmit itself is aberrated. We propose a method to reduce the bias to a linear shift by using a combination of all eigenvectors. A new scheme is introduced to reduce the variance, typically by a factor of 2. Finally, we introduce a non-biased estimator by forming a tensor from a full synthetic aperture data set. Its first singular vector maximizes the speckle brightness by correcting both transmit and receive this is equivalent to an FDORT estimation when the aberration is completely corrected in transmit and provides the lowest error. We performed Field-II (Jensen) simulations using a 45ns, 4mm FWHM near-field phase screen on a speckle phantom with point scatterers and cysts. Cyst contrast improvements and rms residual errors are computed for the different methods. In-vivo aberration estimation results are also presented. The theoretical understanding of the speckle behavior in FDORT has led to improved performance and further insights into using statistical-based approaches for aberration measurement. Keywords-aberration, medical, correlation matrix

Comparison of normal and harmonic ultrasonic imaging with a multirow array

Harmonic ultrasonic imaging is often qualitatively better in the clinical setting than normal ultrasonic imaging in the same frequency range. One possible factor in the differential image quality is the degree of aberration present in the imaged tissue. This work presents initial results of a quantitative comparison of normal and harmonic imaging in phantoms with aberration. We acquired single element RF data using an 8/spl times/128 array in normal (transmit frequency 6.7 MHz) and harmonic (transmit frequency 3.8 MHz) imaging modes. The multirow array allows a finer sampling of the aberrator as compared to a linear array. Estimates of apparent aberrator strength, apparent sound speed, and beam shape are compared for normal and harmonic data. Results demonstrate that the apparent aberration is generally greater for the normal mode than for the harmonic mode.

Adaptive imaging using an 8/spl times/128 array

We present initial results from a new 1.75D array. The advantages of this array include: 1) Improved elevation focusing, 2) More effective phase aberration measurement and use of adaptive imaging techniques due to the increased sampling in the elevation dimension, 3) The ability to obtain individual channel data, and 4) Harmonic imaging capability.