Joergen Arendt Jensen

Virtual ultrasound sources in high-resolution ultrasound imaging

This paper investigates the concept of virtual source elements. It suggests a common framework for increasing the resolution, and penetration depth of several imaging modalities by applying synthetic aperture focusing (SAF). SAF is used either as a post focusing procedure on the beamformed data, or directly on the raw signals from the transducer elements. Both approaches increase the resolution. The paper shows that in one imaging situation, there can co-exist different virtual sources for the same scan line - one in the azimuth plane, and another in the elevation. This property is used in a two stage beamforming procedure for 3D ultrasound imaging. The position of the virtual source, and the created waveform are investigated with simulation, and with pulse-echo measurements. There is good agreement between the estimated wavefront and the theoretically fitted one. Several examples of the use of virtual source elements are considered. Using SAF on data acquired for a conventional linear array imaging improves the penetration depth for the particular imaging situation from 80 to 110 mm. The independent use of virtual source elements in the elevation plane decreases the respective size of the point spread function at 100 mm below the transducer from 7mm to 2 mm.

Multi-element synthetic transmit aperture imaging using temporal encoding

A new method to increase the signal-to-noise-ratio (SNR) of synthetic transmit aperture (STA) imaging is investigated. The new approach is called temporally Encoded Multi-Element STA imaging (EMESTA). It utilizes multiple elements to emulate a single transmit element, and the conventional short excitation pulses are replaced by linear FM signals. Simulations using Field II and measurements are compared to linear array imaging. A theoretical analysis shows a possible improvement in SNR of 17 dB. Simulations are done using an 8.5 MHz linear array transducer with 128 elements. Spatial resolution results show better performance for EMESTA imaging after the linear array focus. Both methods have similar contrast performance. Measurements are performed using our experimental multi-channel ultrasound scanning system, RASMUS. The designed linear FM signal obtains temporal sidelobes below -55 dB, and SNR investigations show improvements of 4-12 dB. The depth performance is investigated using a multi-target phantom. Results show a 30 mm increase in penetration depth with improved spatial resolution. In conclusion, EMESTA imaging significantly increases the SNR of STA imaging, exceeding that of linear array imaging.

Comparison between different encoding schemes for synthetic aperture imaging

Synthetic transmit aperture ultrasound (STAU) imaging can create images with as low as 2 emissions, making it attractive for 3D real-time imaging. Two are the major problems to be solved: (1) complexity of the hardware involved, and (2) poor image quality due to low signal to noise ratio (SNR). We have solved the first problem by building a scanner capable of acquiring data using STAU in real-time. The SNR is increased by using encoded signals, which make it possible to send more energy in the body, while reserving the spatial and contrast resolution. The performance of temporal, spatial and spatio-temporal encoding was investigated. Experiments on wire phantom in water were carried out to quantify the gain from the different encodings. The gain in SNR using an FM modulated pulse is 12 dB. The penetration depth of the images was studied using tissue mimicking phantom with frequency dependent attenuation of 0.5 dB/(cm MHz). The combination of spatial and temporal encoding have highest penetration depth. Images to a depth of 110 mm, can successfully be made with contrast resolution comparable to that of a linear array image. The in-vivo scans show that the motion artifacts do not significantly influence the performance of the STAU.

Tissue motion in blood velocity estimation and its simulation

Determination of blood velocities for color flow mapping systems involves both stationary echo cancelling and velocity estimation. Often the stationary echo cancelling filter is the limiting factor in color flow mapping and the optimization and further development of this filter is crucial to the improvement of color flow imaging. Optimization based on in-vivo data is difficult since the blood and tissue signals cannot be accurately distinguished and the correct extend of the vessel under investigation is often unknown. This study introduces a model for the simulation of blood velocity data in which tissue motion is included. Tissue motion from breathing, heart beat, and vessel pulsation were determined based on in-vivo RF-data obtained from 10 healthy volunteers. The measurements were taken at the carotid artery at one condition and in the liver at three conditions. Each measurement was repeated 10 times to cover the whole cardiac cycle and a total of 400 independent RF measurements of 950 pulse echo lines were recorded. The motion of the tissue surrounding the hepatic vein from superficial breathing had a peak velocity of 6.2/spl plusmn/3.4 mm/s over the cardiac cycle, when averaged over the 10 volunteers. The motion due to the heart, when the volunteer was asked to hold his breath, gave a peak velocity of 4.2/spl plusmn/1.7 mm/s. The movement of the carotid artery wall due to changing blood pressure had a peak velocity of 8.9/spl plusmn/3.7 mm/s over the cardiac cycle. The variations are due to differences in heart rhythm, breathing, and anatomy. All three of these motions are handled independently by the simulation program, which also includes a parametric model for the pulsatile velocity in the elastic vessel. The model can be used for optimizing both color flow mapping and spectral display systems.

Velocity estimation using synthetic aperture imaging [blood flow]

Presented an approach for synthetic aperture blood flow ultrasound imaging. Estimates with a low bias and standard deviation can be obtained with as few as eight emissions. The performance of the new estimator is verified using both simulations and measurements. The results demonstrate that a fully functioning synthetic aperture scanner can be made.

Sampling system for in vivo ultrasound images

Newly developed algorithms for processing medical ultrasound images use the high frequency sampled transducer signal. This paper describes demands imposed on a sampling system suitable for acquiring such data and gives details about a prototype constructed. It acquires full clinical images at a sampling frequency of 20 MHz with a resolution of 12 bits. The prototype can be used for real time image processing. An example of a clinical in vivo image is shown and various aspects of the data acquisition process are discussed.

Should compression of coded waveforms be done before or after focusing

In medical ultrasound signal-to-noise ratio improvements of approximately 15-20 dB can be achieved by using coded waveforms. Exciting the transducer with an encoded waveform necessitates compression of the response which is computationally demanding. This paper investigates the possibility of reducing the workload without introducing errors. Ne - 1 compression filtrations (convolutions) can be saved by inverting the precedence of compression and beamforming (called post-compression), when N is the number of transducer elements. Post-compression with dynamic receive focusing will theoretically introduce errors. Simulations and measurements show that increasing the depth of the scatterers results in a decreased error. Transmit focus depth and the distance between focus points have a significant influence on the error. The size of the error is studied and a new scheme for correcting the error is proposed. The study is done by simulations in Field II and by measurements with our experimental scanner RASMUS. The measurements are done on a string phantom and in-vivo on the abdomen of a male volunteer.

Clinical comparison of pulse and chirp excitation

Coded excitation (CE) using frequency modulated signals (chirps) combined with modified matched filtering has earlier been presented showing promising results in simulations and in-vitro. In this study an experimental ultrasound system is evaluated in a clinical setting, where image sequences are assessed by skilled medical doctors. The effect on penetration depth and image quality were measured. A modified clinical scanner with a 4 MHz single element mechanical transducer, and external transmitter and receiver boards (RASMUS system) were used. The system allowed rapid toggling between chirp and short pulse excitation to simultaneously produce identical image sequences using both techniques. Nine healthy male volunteers were scanned in abdominal locations. All sequences were evaluated by 3 skilled medical doctors, blinded to each other and to the technique used. They assessed the depth (1) in which image quality decreased and (2) in which the image would be insufficient for clinical diagnosis. Furthermore they compared image quality in matching pairs of conventional and CE images. The average increase in penetration depth were almost 2 cm. Side-by-side comparison showed that coded image quality was consistently rated better; significant (p/spl les/0.05) when images were cropped at minimum the depth for good image quality and highly significant (p<0.001) when cropped at maximum depth sufficient for clinical diagnosis. We conclude that coded excitation with linear FM chirps improves penetration and image quality significantly in a clinical setting.

An improved minimum variance beamforming applied to plane-wave imaging in medical ultrasound

Minimum variance beamformer (MVB) is an adaptive beamformer which provides images with higher resolution and contrast in comparison with non-adaptive beamformers like delay and sum (DAS). It finds weight vector of beamformer by minimizing output power while keeping the desired signal unchanged. We used the eigen-based MVB and generalized coherence factor (GCF) to further improve the quality of MVB beamformed images. The eigen-based MVB projects the weight vector with a transformation matrix constructed from eigen-decomposing of the array covariance matrix that increases resolution and contrast. GCF is used to emphasis on coherence part of images that improves the resolution. Four different datasets provided by IUS 2016 beamforming challenge are used to evaluate the proposed method. In comparison with DAS with rectangular weight vector, our method improved contrast about 8.52 dB and 6.20 dB for simulation and experimental contrast phantoms, respectively. It also enhanced lateral (axial) resolution about 87% (40%) and 73% (21%) for simulated and experimental resolution phantoms, respectively.

Efficient focusing scheme for transverse velocity estimation using cross-correlation

The blood velocity can be estimated by cross-correlation of received RF signals, but only the velocity component along the beam direction is found. A previous paper showed that the complete velocity vector can be estimated, if received signals are focused along lines parallel to the direction of the flow. Here a weakly focused transmit field was used along with a simple delay-sum beamformer. A modified method for performing the focusing by employing a special calculation of the delays is introduced, so that a focused emission can be used. The velocity estimation was studied through extensive simulations with Field II. A 64-elements, 5 MHz linear array was used. A parabolic velocity profile with a peak velocity of 0.5 m/s was considered for different angles between the flow and the ultrasound beam and for different emit foci. At 60 degrees the relative standard deviation was 0.58% for a transmit focus at 40 mm. For 90 degrees the new approach gave a relative standard deviation of 8.3% with a focus at 40 mm and 8.0% at a transmit focus of 150 mm. Pulsatile flow in the femoral artery was also simulated. A purely transverse flow profile could be obtained with a relative standard deviation of less than 10% over the whole cardiac cycle, which is sufficient to show clinically relevant transverse color flow images.