Yukiko Ogura

Maximum-likelihood estimation of current-dipole parameters for data obtained using multichannel magnetometer

A method of reducing the influence of external noise magnetic field on the accuracy of estimating current dipole parameters is proposed. It utilizes the spatial correlation of external noises, and is applied to data measured using a multichannel magnetometer. Computer simulation demonstrates the effectiveness of the proposed method.<<ETX>>

Relationship between dipole parameter estimation errors and measurement conditions in magnetoencephalography

In this study, a model in a computer simulation uses a single current dipole in a spherical homogeneous medium. Dipole parameters are estimated using a moving dipole procedure. Signal-to-noise ratio (SNR) is defined as the square-root of the ratio of the average signal power to the average noise power over all measurement points. At SNR>20, accurate estimation can be carried out independently of dipole depth and coil size. At SNR<20, dipole depth influences estimation error. When the dipole is located near the center of the sphere, the measurement region should include both extrema of the magnetic field to minimize estimation error. However, when the dipole is not so deep, the position of the measurement region does not influence estimation error. When SNR<4, estimation error increases as coil size increases. Coil size minimizing estimation error is determined by the ratio of environmental magnetic field noise to electrical noise. For a constant size of measurement region, increasing the number of measurement points decreases estimation error to a certain level. This error level depends on SNR.<<ETX>>

Static imaging of a rotating object. Image quality improvement using J2 synthesis

Several studies on the imaging of rotating objects have been reported ( 1-3). Object rotation can be introduced to microscopic imaging for materials and for in vitro biological samples. One advantage of object rotation is that the imaging method is combined with magic-angle spinning to perform solid NMR imaging ( 1) . Recently, we proposed a new imaging method for rotating objects (3). The major advantage of this method is that it requires only one static field gradient applied in a fixed direction. However, an image of satisfactory quality has not yet been obtained using the proposed method. This is mainly because the image had a point spread function (PSF) equal to J:(x), which has large side lobes. Here, Jn(x) is the nth-order Bessel function of the first kind and n is an integer. As previously indicated by Macovski (4)) the image quality is expected to be improved by decreasing the PSF’s side lobes using J2 synthesis (5)) which is known to be an effective method in radio astronomy to generate an appropriate PSF. This paper describes an experimental investigation on image quality improvement using J2 synthesis. The reconstruction of the proposed imaging method has been explained previously (3). In that method, the coordinate frame fixed to the rotating object is defined as r . The received signal, F(t, tk), is multiplied by exp[iyrO. D(t, &)I. D(t, tk) is expressed as

A method of ultrasonic 3-D computed velocimetry

A method of ultrasonic three-dimensional (3-D) vector velocimetry, which is derived by extending the previously proposed two-dimensional (2-D) vector velocimetry, is presented. In the proposed method, the three vector components of velocity to be measured are defined as the velocity in the beam axial direction, and angle velocities in two transverse directions. To derive the three components of velocity, signals detected by a 2-D array transducer are first 2-D Fourier transformed in the spatial domain parallel to the 2-D array transducer and then are one-dimensional (1-D) Fourier transformed in the time domain. The advantage of the proposed method is that it uses a linear signal processing, so it can simultaneously measure individual velocities of multiple scatterers. Computer simulations clearly demonstrate the effectiveness of the proposed method.

Static imaging of a rotating object

Abstract A new method for obtaining a static image of a rotating object has been studied. This method requires only one static field gradient in a fixed direction. The reconstruction technique proposed by Macovski can be applied to this imaging method. The nth sideband in the discrete spectrum obtained using the proposed method represents the magnetization with a point spread function which is equal to Bessel function Jn2 This point spread function can be modified to an appropriate shape using J2 synthesis. To obtain acceptable image quality, all positive-order sidebands should be employed when reconstructing with J2 synthesis. The number of sidebands required increases as spatial resolution improves. The area values of the sidebands rather than their peak values should be used in reconstruction to avoid the influence of main field nonuniformity and field gradient nonlinearity in the proposed method. Experiments show that the rotation frequency has to be set large enough to avoid overlapping the envelopes of neighboring sidebands in the discrete spectrum.