JPS62231648A - Nmr imaging method - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application This invention relates to a method of performing imaging (reconstructing a distribution image of NMR parameters) using the NMR (nuclear magnetic resonance) phenomenon.

BACKGROUND OF THE INVENTION Conventionally, a two-dimensional Fourier transform method has been known as an NMR imaging method. In this method, data is collected using a pulse sequence as shown in FIG. 5, for example, in terms of an orthogonal three-axis system of x, y, and z. First, 906 pulses of a certain frequency are applied while applying a gradient magnetic field Gz whose magnetic field strength is inclined in the Z direction, and a certain surface perpendicular to the Z axis at a position in the Z direction corresponding to the frequency is selectively excited. Thereafter, a 180° pulse is applied to generate NMR48, which is sampled to collect data. that time,
A gradient magnetic field Gx whose magnetic field strength is tilted in the X direction and a gradient magnetic field GV whose magnetic field strength is tilted in the Y direction are applied.

The gradient magnetic field Gx performs frequency encoding and adds position information in the X direction to the NMR signal. The gradient magnetic field cy performs phase encoding and adds position information in the Y direction to the NMR signal. And the gradient magnetic field Gx
is the same each time, and only the gradient magnetic field GV is changed each time, and the pulse sequence is repeated many times. Since the data obtained with one pulse sequence can be considered as projection data on one line in the X direction of the selected surface, this can be called one line of data. The data thus collected is subjected to two-dimensional Fourier transformation to reconstruct an NMR parameter distribution image.

For example, when reconstructing an image with a 256x258 matrix of pixels, one pulse sequence
Consists of 256 pieces of data sampled at 56 points.
Line data is obtained, and this pulse sequence is repeated 256 times to collect 256 lines of data.

In this way, in the conventional two-dimensional Fourier transform method, if the image is a matrix of N×N pixels, data is collected by repeating the pulse sequence N times, and the N
It is necessary to collect N lines of data having N lines of data, and the number of repetitions of the pulse sequence is uniquely determined in accordance with the number of pixels.

Therefore, it usually takes several minutes to collect all the data necessary to obtain one image.

Problems to be Solved by the Invention However, even if the number of repetitions of the pulse sequence is uniformly determined in this way, it may end up merely collecting noise, which is disadvantageous in that it takes time. That is, in the collected data array as shown in Figure 3, the central phase (
When the data of 1 line A of phase encoding No.) is taken out, its profile becomes as shown in FIG.

From this Figure 4, we can see that the NMR signal component decreases as it moves away from the center, and the profile in the 0 phase direction has a similar profile where the ratio of noise components is higher than the signal component at the edges, and the ratio of noise is higher at the edges. ing. Therefore, it can be seen that it is wasteful to spend time collecting such data because the data collected in a sequence with a large amount of phase encoding has a higher proportion of noise components.

An object of the present invention is to provide an NMR imaging method that can reduce the number of repetitions of a pulse sequence and shorten the overall data acquisition time without deteriorating image quality.

Means for Solving the Problems In the NMR imaging method of the present invention, when repeating a pulse sequence many times, the amount of 1-phase encoding is gradually increased from O to repeat the sequence, and the data obtained in each sequence is When the noise component included in becomes large, the repetition of the above sequence is finished, a predetermined value is given as data to be collected in the subsequent sequence, and the image is reconstructed by two-dimensional Fourier transformation. shall be.

When the amount of active phase encoding increases, the NMR signal component decreases and the noise component increases. Therefore, even if data is not collected in a sequence with increased noise components, no signal is substantially lost, and therefore the reconstructed image does not deteriorate.

Therefore, the total data collection time can be shortened by reducing the number of repetitions of the pulse sequence without deteriorating the quality of one image.

Embodiment In one embodiment of the present invention, as shown in FIG. 1, sampling is performed at N sampling points in one pulse sequence to obtain one line of data consisting of N data. This pulse sequence is repeated while gradually increasing the phase encoding amount from 0 in both the positive and negative directions. The magnitude of the noise component in the l-line data obtained in the initial pulse sequence of phase encoding amount O is detected, and thereafter compared with the magnitude of the signal component in the data of each line. Then, when the magnitude of the signal component in the l-line data collected in the M-th pulse sequence becomes as small as the magnitude of the noise component above, if the amount of phase encoding is increased beyond that, only noise will be obtained. It is assumed that there is no
The repetition of the subsequent pulse sequence ends.

Then, although N lines of data should originally be collected with NXN pixels, the repetition of the pulse sequence is discontinued when M lines of data, which is less than this, is collected.
Data on subsequent lines will also end up not being collected. Therefore, the DC component in the data of the collected M lines is first removed, and then O is given to all the data of the remaining lines to complete the NXN data array. Then, an image is reconstructed by performing two-dimensional Fourier transform on this NXN data array.

In this case, data on lines containing only noise components is simply not collected, so even if data collection is omitted, it does not mean that any necessary data will actually be missed.

M
The total data collection time was M/N.
shorten to

In the other embodiment shown in FIG. 2, the number of lines for data collection can be further reduced. In this case, the pulse sequence is repeated while gradually increasing the phase encoding amount only in the positive direction from O, and the pulse sequence in which the phase encoding amount is increased in the negative direction is not performed, and the same as above is performed. , when the magnitude of the signal component in one line of data collected in a certain pulse sequence becomes as small as the magnitude of the noise component in the first line with zero phase encoding, the subsequent pulse sequence is repeated. stop. Then, in the same situation, the process ends after only M72 tree lines of data are collected based on M72 pulse sequences.

In this case, the data to be collected with the negative phase encoding amount is calculated by inverting the actually collected M/2 line data and performing phase correction. The data of other uncollected lines give constant values as in the above embodiment. Then, a two-dimensional Fourier transform is performed to reconstruct the image.

In this embodiment, the data collection time will be reduced to M/2N.

Effects of the Invention According to the NMR imaging method of the present invention, the repetition of the pulse sequence is stopped and the data collection is completed when only the substantially necessary data is collected without being restricted by the number of pixel matrices of the reconstructed image. Therefore, data collection time can be shortened without deteriorating the quality of reconstructed images, and throughput can be improved.

[Brief explanation of drawings]

Fig. 1 is an arrangement diagram of data collected in one embodiment of the present invention, Fig. 2 is an arrangement diagram of data collected in another embodiment, and Fig. 3 is an arrangement diagram of data collected in another embodiment.
The figure is a data array diagram of a conventional example, and Figure 4 is line A in Figure 3.
FIG. 5 is a time chart showing an example of a general pulse sequence. 1st note Jl@sukeninashi 2nd item M3 phantom 1 appetizer animal paper, 9 items N i solid Sl''Ji scattering 1,

Claims (1)

[Claims] (1) By applying a gradient magnetic field whose magnetic field strength is gradient in one direction within the selected plane and performing frequency encoding in that direction, position information in that direction is added to the NMR signal, and at the same time, it is perpendicular to that direction. By applying a gradient magnetic field whose magnetic field strength is gradient in the direction in which the magnetic field is tilted, and performing phase encoding in that direction, position information in that direction is added to the NMR signal, and data sampling of the NMR signal is performed.The amount of frequency encoding is the same. In the NMR imaging method, in which only the phase encoding amount is changed and the data is repeated many times, the collected data is subjected to two-dimensional Fourier transform to reconstruct the NMR parameter distribution image.
The above sequence is repeated by gradually increasing the amount of phase encoding from 0, and when the noise component included in the data obtained in each sequence becomes large, the repetition of the above sequence is finished, and in subsequent sequences, An NMR imaging method characterized in that a predetermined value is given as data to be collected, and an image is reconstructed by performing two-dimensional Fourier transformation.