Effects of phase aberration on high frame rate imaging

Quantitative assessment of effects of phase aberration and noise on high-frame-rate imaging

The goal of this paper is to quantitatively study effects of phase aberration and noise on high-frame-rate (HFR) imaging using a set of traditional and new parameters. These parameters include the traditional -6-dB lateral resolution, and new parameters called the energy ratio (ER) and the sidelobe ratio (SR). ER is the ratio between the total energy of sidelobe and the total energy of mainlobe of a point spread function (PSF) of an imaging system. SR is the ratio between the peak value of the sidelobe and the peak value of the mainlobe of the PSF. In the paper, both simulation and experiment are conducted for a quantitative assessment and comparison of the effects of phase aberration and noise on the HFR and the conventional delay-and-sum (D&S) imaging methods with the set of parameters. In the HFR imaging method, steered plane waves (SPWs) and limited-diffraction beams (LDBs) are used in transmission, and received signals are processed with the Fast Fourier Transform to reconstruct images. In the D&S imaging method, beams focused at a fixed depth are used in transmission and dynamically focused beams are used in reception for image reconstruction. The simulation results show that the average differences between the -6-dB lateral beam widths of the HFR imaging and the D&S imaging methods are -0.1337mm for SPW and -0.1481mm for LDB, which means that the HFR imaging method has a higher lateral image resolution than the D&S imaging method since the values are negative. In experiments, the average differences are also negative, i.e., -0.2804mm for SPW and -0.3365mm for LDB. The results for the changes of ER and SR between the HFR and the D&S imaging methods have negative values, too. After introducing phase aberration and noise, both simulations and experiments show that the HFR imaging method has also less change in the -6-dB lateral resolution, ER, and SR as compared to the conventional D&S imaging method. This means that the HFR imaging method is less sensitive to the phase aberration and noise. Based on the study of the new parameters on the HFR and the D&S imaging methods, it is expected that the new parameters can also be applied to assess quality of other imaging methods.

Effects of phase aberration and noise on extended high frame rate imaging

Based on the high frame rate (HFR) imaging theories, an extended HFR imaging method has been developed recently in our lab where multiple limited-diffraction array beams or steered plane waves are used in transmissions to reconstruct a high quality image of an equivalent dynamic focusing in both transmissions and receptions. The method has the potential to simplify imaging systems because the fast Fourier transform and square-wave aperture weightings can be used. The method is also flexible in using different numbers of transmissions for a continuous trade-off between image quality and frame rate. In this paper, we study the effects of phase aberration and noise on the extended HFR imaging method with in vitro experiments and compare the results with those obtained with a conventional delay-and-sum (D&S) method of a fixed-transmission focus and a dynamically-focused reception. In the experiments, an ATS539 tissue-mimicking phantom and an Acuson V2 phase array transducer (128 elements, 2.5 MHz, and 0.15-mm pitch) were used. The transducer was driven by a homemade general-purpose HFR imaging system that was capable of producing both the limited-diffraction array beams and steeredplane waves and echo data were acquired with the same system and then transferred to a personal computer via a universal serial bus (USB) 2.0 link for image reconstructions. The phase aberration was introduced by adding random phase shifts to both transmission and reception beams. The random noise was added to the received radiofrequency echo data. Results show that the phase aberration and noise degrade both the extended HFR and the conventional delay-and-sum (D&S) imaging method. However, images reconstructed with the extended HFR imaging method have an overall higher quality than those with the D&S method given the phase aberration and noise models studied.

Extended high-frame rate imaging method with limited-diffraction beams

Fast three-dimensional (3-D) ultrasound imaging is a technical challenge. Previously, a high-frame rate (HFR) imaging theory was developed in which a pulsed plane wave was used in transmission, and limited-diffraction array beam weightings were applied to received echo signals to produce a spatial Fourier transform of object function for 3-D image reconstruction. In this paper, the theory is extended to include explicitly various transmission schemes such as multiple limited-diffraction array beams and steered plane waves. A relationship between the limited-diffraction array beam weighting of received echo signals and a 2-D Fourier transform of the same signals over a transducer aperture is established. To verify the extended theory, computer simulations, in vitro experiments on phantoms, and in vivo experiments on the human kidney and heart were performed. Results show that image resolution and contrast are increased over a large field of view as more and more limited-diffraction array beams with different parameters or plane waves steered at different angles are used in transmissions. Thus, the method provides a continuous compromise between image quality and image frame rate that is inversely proportional to the number of transmissions used to obtain a single frame of image. From both simulations and experiments, the extended theory holds a great promise for future HFR 3-D imaging.