Paul D. Freiburger

A comparative evaluation of several algorithms for phase aberration correction

A common framework is presented to classify several phase correction techniques. A subset of these techniques are evaluated through simulations which utilize 2-D phase aberration profiles measured in the breast. The techniques are compared based on their ability to reduce phase errors, stability, and sensitivity to noise and missing elements in the transducer array. Significant differences are observed in these measures of performance when the size and location of the aperture area used to generate a phase reference signal are varied. Techniques that utilize a small correction reference region are more susceptible to noise and missing elements than techniques which use larger reference regions. The algorithms encounter problems in 2-D phase correction when making the transition from one row to the next, due to the low interelement correlation at the transition points. It is shown that the magnitude of the interelement correlation is the key parameter governing phase correction performance.<<ETX>>

A speckle target adaptive imaging technique in the presence of distributed aberrations

Acoustic velocity inhomogeneities in tissue result in aberration of ultrasound images. These aberrations can be modeled as a near field thin phase screen or as a distributed aberrator. The effect of a near field thin phase screen is to time shift the received echo at each element, while distributed aberrators result in both pulse distortions and time shifts from element to element. Most current techniques for the correction of distributed aberrators are limited to application on point targets. A new technique is proposed which uses multiple transmits from spatially shifted transmit apertures (the translating transmit aperture algorithm), in conjunction with phase conjugate filters, to correct for distributed aberrations in the presence of speckle targets. The performance of the translating transmit aperture algorithm in improving the correlation between signals received by elements of different spatial separations is measured, and factors affecting the performance of this technique are investigated in simulation and experiment.

Method and apparatus for abberation correction in the presence of a distributed aberrator

Disclosed is an ultrasonic phase aberration correction method and apparatus. One preferred embodiment employs a translating transmit aperture for transmitting two separately transmitted pulses through two different transmit apertures but which are focused to the same predetermined target in the human body. A receiver element measuring a first echo from a first transmit pulse is matched with a receiver element measuring a second echo from a second transmit pulse from a translated aperture, the match corresponding to receivers with a separation equal and opposite to the spatial translations of the translated transmit apertures. The matched elements are compared for signal variations which estimate phase aberrations and especially improve on estimates of distributed aberrations. The transmit or receive signals can then be compensated for or adjusted based on the signal comparison and variation estimates. It is also disclosed that the use of phase spectra analysis with a translating aperture improves aberration correction. Further, the use of a translating aperture with cross-correlation techniques also indicates improved correlations.

Two Dimensional Ultrasonic Beam Distortion in the Breast: In Vivo Measurements and Effects

Two dimensional arrival time data was obtained for the propagation of ultrasound across the breasts of 7 female volunteers. These profiles were extracted through the use of cross-correlation measurements and a simulated annealing process that maintained phase closure while aligning the data. The phase aberration measured in two dimensions had a larger magnitude than previously reported phase aberration measured in one dimension in the breast. A point spread function generation computer program was used to demonstrate the system response degrading effects of the measured phase aberration and the usefulness of current one dimensional phase aberration correction techniques. The results indicate that two dimensional correction algorithms are necessary to restore the system performance losses due to phase aberration.

Pulsed wave doppler processing using aliased spectral data

A method for producing Doppler ultrasound data at a user-requested pulse repetition frequency (PRF) utilizing undersampled echo signals. Echo signals are created in response to Doppler pulses that are transmitted into the patient at a rate less than a desired PRF. The echo signals are analyzed in the time domain to determine a velocity of scatterers in an area of tissue defined by a range gate. From the velocity, the Doppler shift of the scatterers is determined. The echo signals are interpolated to produce a number of samples equal in number to that which would have been produced had the Doppler pulses been transmitted at the user-requested PRF. The interpolated echo signals are then analyzed in the frequency domain which produces a number of spectra indicative of the velocity and direction of the moving scatterers. From the Doppler shift determined, the correct spectra is selected and displayed for a user. In addition, the present invention can be used to increase the amplitude of the pulses transmitted into a patient by lowering the transmit pulse frequency so that the total amount of ultrasonic energy delivered to the patient remains the same. The larger amplitude transmit pulses produce echo signals having a better signal-to-noise ratio.

An evaluation of transducer design and algorithm performance for two dimensional phase aberration correction

In vivo measurements of phase aberration profiles in the breast are used to investigate the geometric requirements for phased arrays to be used in adaptive imaging. Simulation studies are then described which compare the performance of several proposed techniques for phase aberration correction. The results describe the impact of noise, dead elements and other factors on the performance of these systems.<<ETX>>

Parallel processing techniques for the speckle brightness phase aberration correction algorithm

The speckle brightness adaptive algorithm has previously been implemented in approximately real-time on low frequency, one-dimensional arrays. To increase the speed of this technique, a temporally parallel algorithm and a spatially parallel algorithm are described. Theoretical analyses, simulation results and experimental measurements are presented for these algorithms. Theoretical predictions indicate that these techniques increase the correction speed, but some decrease in the accuracy of the compensating phase estimate occurs. Simulation results indicate that these parallel algorithms perform well at removing the effects of phase aberration. Preliminary experimental measurements demonstrate the correction speed improvements achievable with these algorithms.

The Impact of Acoustic Velocity Variations on Target Detectability in Ultrasonic Images of the Breast

Experimentally obtained ultrasonic phase aberration profiles in the breast were used to investigate the impact of acoustic velocity variations on images of simulated breast lesions. The imaging properties of several phased-array transducers with varying frequencies and geometries were studied as phase error profiles of increasing magnitude were introduced. The targets studied were anechoic lesions of various sizes. The results indicate that phase aberrations significantly degrade the contrast of ultrasonic breast images, especially for high-resolution scanning systems.

A real-time adaptive ultrasonic imaging system.

The performance of an experimental, adaptive, phased array imaging system in improving the quality of abdominal organ images in healthy volunteers was assessed. Trials were conducted under usual clinical imaging conditions and when phase aberrators of known shape and magnitude were introduced into the imaging system. The system, which uses local target brightness as a quality factor, was able to improve clinical image quality when aberrators introduced externally were present and had a negligible effect on image quality when they were not. On several of the volunteers, the phase aberrations were measured directly across the abdomen. The implications of these results for the future clinical application of adaptive imaging systems and the future system modifications are reported.

Classifying ultrasound image regions by using characteristics of the ARFI induced tissue displacement temporal profile

Classifying an ultrasound image into regions with diagnostic information can directly assist clinicians and can also be used as a preprocessing stage in parametric image construction. In this work we propose to extract information from an Acoustic Radiation Force Impulse (ARFI) induced displacement temporal profile and use this information as input to a linear classifier to assign each image sample to one category, either fluid, high stiffness, low stiffness or undetermined. Three parameters were derived from the displacement profile, (1) signal to noise ratio (SNRD), (2) maximum displacement (Dmax), and (3) time-to-peak displacement (TTP). The proposed method was tested on phantoms which contain a fluid filled cyst and solid inclusions with various stiffness values, two diagnostically confirmed human breast simple cysts, and five Microwave ablated thermal lesions in ex-vivo bovine livers. Using 35.6kPa as separation between low and high stiffness, for phantom and breast simple cysts we obtained a mean classification accuracy of 93%; for thermal lesions, we obtained a mean classification accuracy of 81%.