Nick Bottenus

Synthetic aperture focusing for short-lag spatial coherence imaging

It has been demonstrated that short-lag spatial coherence (SLSC) ultrasound imaging can provide improved speckle SNR and lesion CNR compared with conventional B-mode images, especially in the presence of noise and clutter. Application of the van Cittert-Zernike theorem predicts that coherence among the ultrasound echoes received across an array is reduced significantly away from the transmit focal depth, leading to a limited axial depth of field in SLSC images. Transmit focus throughout the field of view can be achieved using synthetic aperture methods to combine multiple transmit events into a single final image. A synthetic aperture can be formed with either focused or diverging transmit beams. We explore the application of these methods to form synthetically focused channel data to create SLSC images with an extended axial depth of field. An analytical expression of SLSC image brightness through depth is derived for the dynamic receive focus case. Experimental results in a phantom and in vivo are presented and compared with dynamic receive focused SLSC images, demonstrating improved SNR and CNR away from the transmit focus and an axial depth of field four to five times longer.

Equivalence of time and aperture domain additive noise in ultrasound coherence

Ultrasonic echoes backscattered from diffuse media, recorded by an array transducer and appropriately focused, demonstrate coherence predicted by the van Cittert-Zernike theorem. Additive noise signals from off-axis scattering, reverberation, phase aberration, and electronic (thermal) noise can all superimpose incoherent or partially coherent signals onto the recorded echoes, altering the measured coherence. An expression is derived to describe the effect of uncorrelated random channel noise in terms of the noise-to-signal ratio. Equivalent descriptions are made in the aperture dimension to describe uncorrelated magnitude and phase apodizations of the array. Binary apodization is specifically described as an example of magnitude apodization and adjustments are presented to minimize the artifacts caused by finite signal length. The effects of additive noise are explored in short-lag spatial coherence imaging, an image formation technique that integrates the calculated coherence curve of acquired signals up to a small fraction of the array length for each lateral and axial location. A derivation of the expected contrast as a function of noise-to-signal ratio is provided and validation is performed in simulation.

A synthetic aperture study of aperture size in the presence of noise and in vivo clutter

Conventional wisdom in ultrasonic array design drives development towards larger arrays because of the inverse relationship between aperture size and resolution. We propose a method using synthetic aperture beamforming to study image quality as a function of aperture size in simulation, in a phantom and in vivo. A single data acquisition can be beamformed to produce matched images with a range of aperture sizes, even in the presence of target motion. In this framework we evaluate the reliability of typical image quality metrics – speckle signal-tonoise ratio, contrast and contrast-to-noise ratio – for use in in vivo studies. Phantom and simulation studies are in good agreement in that there exists a point of diminishing returns in image quality at larger aperture sizes. We demonstrate challenges in applying and interpreting these metrics in vivo, showing results in hypoechoic vasculature regions. We explore the use of speckle brightness to describe image quality in the presence of in vivo clutter and underlying tissue inhomogeneities.

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.

Synthetic tracked aperture ultrasound imaging: Design, simulation, and experimental evaluation

Abstract. Ultrasonography is a widely used imaging modality to visualize anatomical structures due to its low cost and ease of use; however, it is challenging to acquire acceptable image quality in deep tissue. Synthetic aperture (SA) is a technique used to increase image resolution by synthesizing information from multiple subapertures, but the resolution improvement is limited by the physical size of the array transducer. With a large F-number, it is difficult to achieve high resolution in deep regions without extending the effective aperture size. We propose a method to extend the available aperture size for SA—called synthetic tracked aperture ultrasound (STRATUS) imaging—by sweeping an ultrasound transducer while tracking its orientation and location. Tracking information of the ultrasound probe is used to synthesize the signals received at different positions. Considering the practical implementation, we estimated the effect of tracking and ultrasound calibration error to the quality of the final beamformed image through simulation. In addition, to experimentally validate this approach, a 6 degree-of-freedom robot arm was used as a mechanical tracker to hold an ultrasound transducer and to apply in-plane lateral translational motion. Results indicate that STRATUS imaging with robotic tracking has the potential to improve ultrasound image quality.

Feasibility of Swept Synthetic Aperture Ultrasound Imaging

Ultrasound image quality is often inherently limited by the physical dimensions of the imaging transducer. We hypothesize that, by collecting synthetic aperture data sets over a range of aperture positions while precisely tracking the position and orientation of the transducer, we can synthesize large effective apertures to produce images with improved resolution and target detectability. We analyze the two largest limiting factors for coherent signal summation: aberration and mechanical uncertainty. Using an excised canine abdominal wall as a model phase screen, we experimentally observed an effective arrival time error ranging from 18.3 ns to 58 ns (root-mean-square error) across the swept positions. Through this clutter-generating tissue, we observed a 72.9% improvement in resolution with only a 3.75 dB increase in side lobe amplitude compared to the control case. We present a simulation model to study the effect of calibration and mechanical jitter errors on the synthesized point spread function. The relative effects of these errors in each imaging dimension are explored, showing the importance of orientation relative to the point spread function. We present a prototype device for performing swept synthetic aperture imaging using a conventional 1-D array transducer and ultrasound research scanner. Point target reconstruction error for a 44.2 degree sweep shows a reconstruction precision of 82.8 and 17.8 in the lateral and axial dimensions respectively, within the acceptable performance bounds of the simulation model. Improvements in resolution, contrast and contrast-to-noise ratio are demonstrated in vivo and in a fetal phantom.

Implementation of swept synthetic aperture imaging

Ultrasound imaging of deep targets is limited by the resolution of current ultrasound systems based on the available aperture size. We propose a system to synthesize an extended effective aperture in order to improve resolution and target detectability at depth using a precisely-tracked transducer swept across the region of interest. A Field II simulation was performed to demonstrate the swept aperture approach in both the spatial and frequency domains. The adaptively beam-formed system was tested experimentally using a volumetric transducer and an ex vivo canine abdominal layer to evaluate the impact of clutter-generating tissue on the resulting point spread function. Resolution was improved by 73% using a 30.8 degree sweep despite the presence of varying aberration across the array with an amplitude on the order of 100 ns. Slight variations were observed in the magnitude and position of side lobes compared to the control case, but overall image quality was not significantly degraded as compared by a simulation based on the experimental point spread function. We conclude that the swept aperture imaging system may be a valuable tool for synthesizing large effective apertures using conventional ultrasound hardware.

Application of synthetic aperture focusing to short-lag spatial coherence

It has been demonstrated that short-lag spatial coherence (SLSC) ultrasound imaging can provide improved SNR and CNR compared to conventional B-mode images, especially in the presence of noise and clutter. Application of the van Cittert-Zernike theorem predicts that coherence between the ultrasound echoes received across an array is reduced significantly away from the transmit focal depth, leading to a limited axial depth of eld in SLSC images. Transmit focus throughout the eld of view can be achieved using synthetic aperture methods to combine multiple transmit events into a single nal image. We propose the application of these methods to create synthetically focused channel data to be used to create an SLSC image that will have an extended axial depth of eld comparable to B-mode images. Experimental results in a phantom and in vivo are presented and compared to both B-mode images and dynamic receive focused SLSC images, demonstrating improved SNR and CNR away from the transmit focus and an enlarged axial depth of eld.

Evaluation of the transverse oscillation method using the cramer–rao lower bound [Correspondence]

The transverse oscillation method enables lateral displacement tracking by generating an oscillation orthogonal to the conventional RF signal. The widely varying methods used in the field to create such oscillations and perform displacement estimation make it difficult to compare the expected performance of alternative techniques. We derive closed-form expressions for the oscillating pressure fields produced by two common apodization functions-the rectangular and bi-lobed Gaussian apodizations-after heterodyning demodulation is applied to separate the orthogonally-oscillating signals. With these fields and spectra we present a form of the Cramer-Rao lower bound for ultrasonic signals that contains a spectrum shape term, allowing theoretical prediction of relative performance across different techniques and parameter choices. Simulations show good agreement with the trends predicted by the theoretical results for the chosen class of aperture functions. The simulations demonstrate the importance of frequency-space analysis in devising a transverse oscillation scheme and suggest that the study of other classes of aperture functions and field formation techniques should be continued to further improve the accuracy of lateral displacement tracking.

Evaluation of Large-Aperture Imaging Through the ex Vivo Human Abdominal Wall

Current clinical abdominal imaging arrays are designed to maximize angular field of view rather than the extent of the coherent aperture. We illustrate, in ex vivo experiments, the use of a large effective aperture to perform high-resolution imaging, even in the presence of abdominal wall-induced acoustic clutter and aberration. Point and lesion phantom targets were imaged through a water path and through three excised cadaver abdominal walls to create different clinically relevant clutter effects with matched imaging targets. A 7.36-cm effective aperture was used to image the targets at a depth of 6.4 cm, and image quality metrics were measured over a range of aperture sizes using synthetic aperture techniques. In all three cases, although degradation compared with the control was observed, lateral resolution improved with increasing aperture size without loss of contrast. Spatial compounding of the large-aperture data drastically improved lesion detectability and produced contrast-to-noise ratio improvements of 83%-106% compared with the large coherent aperture. These studies indicate the need for the development of large arrays for high-resolution abdominal diagnostic imaging.