Yuichi Miwa

Two-directional phase aberration correction using a two-dimensional array

The aim of this paper is to show the effectiveness of two-directional phase aberration correction using real-time signals obtained from a human body. The authors constructed a two-dimensional array and the real-time data acquisition system for this purpose. The array has ten elements in both the scan and elevation directions. It is placed in contact with a human body and signals reflected from the subjects liver are digitized by parallel A/D converters and stored in memory. The data acquisition for each image is completed in 52.4 ms. A cross-sectional image of the liver is then constructed off-line with a computer. Images of the liver before and after the phase aberration correction show that the image was significantly improved after the compensation.

Subaperture decoding to enhance performance of coded excitation

Coded excitation requires long code to improve signal to noise ratio (SNR). However, in order to achieve a high lateral resolution, the beam forming is normally performed based on a low f-number and real-time dynamic focusing, which limits the maximum length of coded waves. Individual decoding before beamforming can solve this problem, but it demands an enormous size of hardware. In this study, a subaperture decoding technique is proposed to achieve a high SNR based on coded excitation without compromising lateral resolution. In this technique, the receiving aperture is divided into N subapertures, beamforming is performed for each subaperture, and each output is decoded, delayed and summed together to complete beamforming in whole aperture. Computer simulations were used to demonstrate this technique. When the transmission waveform was encoded by the 13-chip Barker code and decoded with a mismatched filter, and the reception f-number was 1, the proposed technique, with the number of subaperture of 16, lowered the range sidelobe level from -25 dB to -45 dB.

Non-cylindrical transmission focusing for large depth of field

In imaging with a microbubble contrast agent, which tends to make irreversible changes by transmission even at a low intensity, both transmission beamwidth and number of transmission per frame must be minimized. A transmission focusing technique based on a noncylindrical delay, which is applicable to a one-dimensional array, is proposed. Unlike X waves, the main beam propagates always coherently with the waves transmitted from the central part of the aperture. The non-cylindrical focusing delay was calculated by assuming a local focal length along the aperture, which increases gradually as the distance from the center increases. Numerical simulation of wave propagation predicted a large depth of field for a reasonably narrow main beam having longitudinally uniform intensity. The noncylindrical focusing was employed in a prototype beamformer, and the large depth of transmission field was confirmed with Shrielen measurements.

Magnetic resonance image denoising using multiple filters

We introduced and compared ten denoisingfilters which are all proposed during last fifteen years. Especially, the state-of-art denoisingalgorithms, NLM and BM3D, have attracted much attention. Several expansions are proposed to improve the noise reduction based on these two algorithms. On the other hand, optimal dictionaries, sparse representations and appropriate shapes of the transform’s support are also considered for the image denoising. The comparison among variousfiltersis implemented by measuring the SNR of a phantom image and denoising effectiveness of a clinical image. The computational time is finally evaluated.