Tore Bjastad

Coherent Plane Wave Compounding for Very High Frame Rate Ultrasonography of Rapidly Moving Targets

Coherent plane wave compounding is a promising technique for achieving very high frame rate imaging without compromising image quality or penetration. However, this approach relies on the hypothesis that the imaged object is not moving during the compounded scan sequence, which is not the case in cardiovascular imaging. This work investigates the effect of tissue motion on retrospective transmit focusing in coherent compounded plane wave imaging (PWI). Two compound scan sequences were studied based on a linear and alternating sequence of tilted plane waves, with different timing characteristics. Simulation studies revealed potentially severe degradations in the retrospective focusing process, where both radial and lateral resolution was reduced, lateral shifts of the imaged medium were introduced, and losses in signal-to-noise ratio (SNR) were inferred. For myocardial imaging, physiological tissue displacements were on the order of half a wavelength, leading to SNR losses up to 35 dB, and reductions of contrast by 40 dB. No significant difference was observed between the different tilt sequences. A motion compensation technique based on cross-correlation was introduced, which significantly recovered the losses in SNR and contrast for physiological tissue velocities. Worst case losses in SNR and contrast were recovered by 35 dB and 27-35 dB, respectively. The effects of motion were demonstrated in vivo when imaging a rat heart. Using PWI, very high frame rates up to 463 fps were achieved at high image quality, but a motion correction scheme was then required.

Parallel beamforming using synthetic transmit beams

Parallel beamforming is frequently used to increase the acquisition rate of medical ultrasound imaging. However, such imaging systems will not be spatially shift invariant due to significant variation across adjacent beams. This paper investigates a few methods of parallel beamforming that aims at eliminating this flaw and restoring the shift invariance property. The beam-to-beam variations occur because the transmit and receive beams are not aligned. The underlying idea of the main method presented here is to generate additional synthetic transmit beams (STB) through interpolation of the received, unfocused signal at each array element prior to beamforming. Now each of the parallel receive beams can be aligned perfectly with a transmit beam - synthetic or real - thus eliminating the distortion caused by misalignment. The proposed method was compared to the other compensation methods through a simulation study based on the ultrasound simulation software Field II. The results have been verified with in vitro experiments. The simulations were done with parameters similar to a standard cardiac examination with two parallel receive beams and a transmit-line spacing corresponding to the Rayleigh criterion, wavelength times f-number (ulambdamiddotf#). From the results presented, it is clear that straightforward parallel beamforming reduces the spatial shift invariance property of an ultrasound imaging system. The proposed method of using synthetic transmit beams seems to restore this important property, enabling higher acquisition rates without loss of image quality

Multi-line transmission in 3-D with reduced crosstalk artifacts: a proof of concept study

Multi-line transmission (MLT) is a technique in which ultrasound pulses for several directions are transmitted simultaneously. The purpose is increased frame rate, which is especially important in 3-D echocardiography. Compared with techniques purely based on parallel beamformation, MLT avoids the need for reducing the transmit aperture and thus maintains a high harmonic signal level. The main disadvantage is that artifacts are caused by cross-talk between the simultaneous beams. In a conventional MLT implementation, simultaneous transmits would be spaced regularly in the azimuth and elevation planes. However, using rectangular geometry arrays, most of the acoustic side-lobe energy is concentrated along these planes. The results in this work show that the crosstalks can be pushed below the typical display range of 50 dB used in cardiac applications if the parallel transmit directions are aligned along the transverse diagonal of the array. Dispositions with 2 to 5 MLT for a typical cardiac 2-D phased-array were investigated using simulation software. Using the proposed alignment, the maximal crosstalk artifact amplitudes decreased 20 to 30 dB compared with conventional MLT dispositions. In water-tank measurements, side-lobe levels of a commercially available rectangular probe were 15 to 25 dB lower along the transverse diagonal, confirming that similar suppressions can be expected using actual transducers.

Narrowed beam width in newer ultrasound machines shortens measurements in the lateral direction: fetal measurement charts may be obsolete.

Fetal ultrasound measurements are made in axial, lateral and oblique directions. Lateral resolution is influenced by the beam width of the ultrasound system. To improve lateral resolution and image quality, the beam width has been made narrower; consequently, measurements in the lateral direction are affected and apparently made shorter, approaching the true length. The aims of this study were to explore our database to reveal time‐dependent shortening of ultrasound measurements made in the lateral direction, and to assess the extent of beam‐width changes by comparing beam‐width measurements made on old and new ultrasound machines.

Sensitivity of minimum variance beamforming to tissue aberrations

Minimum variance beamformers adapt to the received data. In the case of perfect data, this beamformer can produce images with better point and edge definitions than conventional delay-and-sum beamformers. But in medical ultrasound imaging, the quality of the data is often corrupted by aberration. As higher resolution often is associated with less robustness, a study using a 1D phase aberrator was done to find how the minimum variance beamformer performs with realistic phase aberrations. We found that with proper subaperture smoothing and regularization, the minimum variance beamformer will give better or similar performance compared to the delay-and-sum beamformer for aberrations typically found in the human body.

The impact of aberration on high frame rate cardiac B-mode imaging

In echocardiography, especially in 3D echocardiography, achieving high frame rates is a major challenge. A suggested solution is parallel receive beamforming. With out any compensation, this approach is known to produce block-like artifacts, where each block corresponds to one parallel receive group. In this work, in vitro imaging, in vivo imaging, and simulations were used to investigate the artifacts. In vitro, imaging a tissue phantom, the artifacts were successfully compensated for. However, in vivo, imaging the heart, the compensation techniques no longer sufficed and the artifacts persisted. With in vivo imaging, aberrating tissue layers are present between the heart and the probe. To investigate the effects of aberration on a parallel receive sys tem, an in vitro experiment was performed with and without a silicon phase aberrator in front of the probe. The aberrator caused the artifacts to appear even when co techniques were applied. Simulations confirmed the measured results and indicated that distorted beam profiles and decorrelation between parallel receive groups caused the artifacts. To quantify the magnitude of the artifacts, a correlation-based indicator was developed. The indicator separated images with and without artifacts and confirmed that the artifacts appeared from the combination of parallel receive beams and aberration.

Parallel beamforming using synthetic transmit beams [biomedical ultrasound imaging]

Ultrasound images generated using conventional parallel beamforming contains stationary stripes due to beam-to-beam variations across adjacent beams. Such an imaging system is thus shift variant. This paper investigates a method which eliminates this flaw and restores the shift invariant property. The beam-to-beam variations occur because the transmit and receive beams are not aligned. The underlying idea is then to generate additional synthetic transmit beams (STB) through interpolation of the received, unfocused signal at each array element prior to beamforming. Now each of the parallel receive beams can be aligned perfectly with a transmit beam-synthetic or real-thus eliminating the distortion caused by misalignment. To investigate the performance of the method a simulation study has been conducted. The simulations were done with parameters similar to a standard cardiac examination. Using the proposed method the variation was reduced to 1.5 dB. The simulations were confirmed by results front in vitro experiments where vertical line artifacts were unnoticeable when using the proposed method.

Synthetic transmit beam technique in an aberrating environment

Parallel beamforming is a commonly used method for increasing the frame rate in ultrasound imaging systems. By receiving in several directions for each transmission, the frame rate is increased. However, this method also introduces blocklike artifacts in the B-mode image, due to the reception offsets when compared with the transmission direction. The synthetic transmit beam technique (STB) has been previously proposed as a compensation technique when addressing these artifacts. Previous work by Hergum et al. investigated the performance of this method in regard to the case of 2 parallel beams in tissue mimicking phantoms without aberrations. This study is a continuation of that work in which this method is tested in an aberrating environment using 4 parallel beams. Several quantitative and qualitative performance aspects of this method have been investigated such as lateral shift invariance, beam-to-beam correlation fluctuations, speckle- tracking performance, improvements from higher order STB interpolation and beam profile shape preservation, as well as perceived image quality improvements. The results were obtained from simulations, in vivo measurements, and in vitro measurements. The results showed that aberration amplified the image artifacts for regular parallel beamforming, which resulted in more shift variance, lower beam-to-beam correlation, higher speckle- tracking error, and more variation in beam profile shape. Compared with regular parallel beamforming, STB resulted in a significantly better image quality and a higher score in all measuring methods. The improvements from using STB were largest in cases involving aberration. Using STB, the variation in beam-to-beam correlation was reduced from 30% to 1%, and the standard deviation of the speckle-tracking error was reduced from 8% to 1.5%.

Error analysis of subaperture processing in 1-D ultrasound arrays

To simplify the medical ultrasound system and reduce the cost, several techniques have been proposed to reduce the interconnections between the ultrasound probe and the back-end console. Among them, subaperture processing (SAP) is the most straightforward approach and is widely used in commercial products. This paper reviews the most important error sources of SAP, such as static focusing, delay quantization, linear delay profile, and coarse apodization, and the impacts introduced by these errors are shown. We propose to use main lobe coherence loss as a simple classification of the quality of the beam profile for a given design. This figure-ofmerit (FoM) is evaluated by simulations with a 1-D ultrasound subaperture array setup. The analytical expressions and the coherence loss can work as a quick guideline in subaperture design by equalizing the merit degradations from different error sources, as well as minimizing the average or maximum loss over ranges. For the evaluated 1-D array example, a good balance between errors and cost was achieved using a subaperture size of 5 elements, focus at 40 mm range, and a delay quantization step corresponding to a phase of π/4.

3D Tissue Doppler imaging with ultra high frame rate

High frame rate TDI, may be used to study propagation of mechanical waves. In this paper we present a 3D TDI modality which gives ~500 FPS. The high frame rate was achieved by using few transmit beams, plane wave, and packet size 1 on transmit. ECG trigged acquisition was used to acquire 3D B-mode in one cardiac cycle and 3D TDI in the next. Normal acquisition was used for 3D B-mode. Recording both 3D B-mode and 3D TDI allows us to produce a left ventricle mesh, by using GE 4D Auto LVQ, and map the calculated velocities onto the mesh. A feasibility study, where we compared with GE tri-plane TVI, was performed. Peak velocities from 6 positions in the base of the left ventricle were measured. Data from 5 normal subjects was recorded. Our results show that the differences between 3D TDI and the first tri-plane recording were comparable to the differences between the two tri-plane recordings.