Kutay F. Ustuner

Ultrasonic imaging system and method with SNR adaptive processing

An ultrasonic imaging system includes a processor that varies the receive signal path parameters as a function of the signal to noise ratio of the echo signal. Background noise information is either acquired by imaging with the transmitters turned off, or estimated by using the known differences in bandwidth and correlation lengths of the signal and noise, or computed based on a system noise model using currently prevailing system parameters. Acquired receive signals are then processed as a nonlinear function of the comparison between the acquired receive signal and the background noise values. The family of SNR adaptive processors includes the SNR adaptive filtering in at least one of the azimuth, elevation and time dimensions, SNR adaptive high pass, band pass, and whitening filtering, SNR adaptive synthesis, and SNR adaptive compounding.

Acoustic reciprocity of spatial coherence in ultrasound imaging

A conventional ultrasound image is formed by transmitting a focused wave into tissue, time-shifting the backscattered echoes received on an array transducer, and summing the resulting signals. The van Cittert-Zernike theorem predicts a particular similarity, or coherence, of these focused signals across the receiving array. Many groups have used an estimate of the coherence to augment or replace the B-mode image in an effort to suppress noise and stationary clutter echo signals, but this measurement requires access to individual receive channel data. Most clinical systems have efficient pipelines for producing focused and summed RF data without any direct way to individually address the receive channels. We describe a method for performing coherence measurements that is more accessible for a wide range of coherence-based imaging. The reciprocity of the transmit and receive apertures in the context of coherence is derived and equivalence of the coherence function is validated experimentally using a research scanner. The proposed method is implemented on a commercial ultrasound system and in vivo short-lag spatial coherence imaging is demonstrated using only summed RF data. The components beyond the acquisition hardware and beamformer necessary to produce a real-time ultrasound coherence imaging system are discussed.

Aberration correction using broad transmit beams

A new tissue aberration correction method is described that is compatible with imaging techniques that use weakly focused, unfocused or defocused transmit beams. This method is based on the principle of reciprocity. Rather than relying on a focused transmit beam to create a virtual point source, we use a focused receive beam to do the same. The data acquisition scheme is to transmit with a virtual element, receive echoes with all the elements and store the channel data, and form a receive focus at a point of interest. As the transmit virtual element number is varied, the beamsum from receive focusing also varies. The ensemble of receive beamsums are processed (e.g. cross-correlated) to extract information about aberration. More generally, dynamic and parallel receive beams are formed to produce single-transmit images. These are correlated to improve the stability of aberration estimation. The estimated aberration information can be used to compensate the receive focus, and the process can be iterated without acquiring new data. Aberration for any insonified region can be estimated by steering the receive focus to that region. A synthetic image is formed with correction applied on both receive beamformation and transmit synthesis.

Pulse-echo beamformer with high lateral and temporal resolution and depth-independent lateral response

With conventional beamformation techniques, in order to increase the depth of field one must sacrifice either lateral or temporal resolution. An increase in the transmit F-number results in an increase in the depth of field at the expense of lateral resolution. The use of multiple transmit/receive events per line, on the other hand, compromises temporal resolution. We describe a beamformation technique that is not prone to such tradeoffs. The object is insonified with a field that is cosine-modulated laterally, and the rate of growth of the receive aperture is tuned to this lateral modulation frequency. The resultant round-trip lateral bandwidth is optimal: it equals twice the maximum achievable one-way lateral bandwidth. The advantage of this technique is that this full bandwidth is realized at all depths, and with only a single transmit/receive event. A generalization of this technique allows a full frame of full-bandwidth image to be acquired using only two transmit/receive events.

Adaptive Imaging Using an Optimal Receive Aperture Size

Sidelobe contribution from off-axis targets degrades image quality in a coherent array imaging system. In ultrasound imaging, focusing errors resulting from sound-velocity inhomogeneities in human tissue — also known as phase aberrations — reduce the coherence of the received signals and elevate the sidelobe level. This paper proposes an adaptive receive-aperture technique based on thresholding of the coherence factor (CF). The CF describes the coherence of the received array signals and can be used as an index of focusing quality. This paper demonstrates that thresholding of the CF allows the mainlobe-dominated signals to be distinguished from the sidelobe-dominated signals, after which the receive-aperture size at each imaging position can be optimally determined so as to enhance the mainlobe-dominated signals and suppress the sidelobe-dominated signals. Thus, image quality degradation resulting from sound-velocity inhomogeneities can be reduced. Simulations and measured ultrasound data are used to evaluate the efficacy of the proposed technique. The characteristics of the proposed technique including the effects of the signal-to-noise ratio (SNR) and the transmit focal depth, and speckle reduction are discussed. The proposed technique is also compared with the parallel adaptive receive compensation algorithm and shown to produce a better improvement in image quality.

Ultrasound coherence imaging using hardware receive beamforming and broad transmit beams

Conventional B-mode ultrasound images suffer from clutter composed of reverberation and aberration that exhibit only partial spatial coherence, making it possible to suppress these confounding signals using coherence- based imaging techniques. Coherence is typically measured by transmitting a focused wave into the tissue and computing the covariance of the returned echo signals across all combinations of receive channel pairs. We mathematically and experimentally prove the equivalence of the coherence measured as a function of transmit channel and as a function of receive channel. This forms the basis for an alternative method of coherence measurement using a synthetic aperture technique to store focused and summed receive channel data as a function of transmit channel. This technique avoids the need for access to individual receive channel data and is compatible with the existing signal pipeline on common commercial clinical scanners. We demonstrate in vivo short-lag spatial coherence imaging of the human liver to produce images with reduced clutter, using an ACUSON SC2000 ultrasound system to acquire data and perform full synthetic aperture focusing. The possibility to trade-off image quality for acquisition time is also presented in an effort to make the proposed sequences more accessible for real-time imaging.