JP2545871B2 - MRI equipment - Google Patents

Description

Detailed Description of the Invention [Industrial applications]

The present invention relates to an MRI apparatus that performs imaging using nuclear magnetic resonance (NMR) development.

[Prior art]

In an MRI apparatus, image reconstruction is usually performed by a two-dimensional (or three-dimensional) Fourier transform method. The two-dimensional Fourier transform method will be explained. By applying a gradient magnetic field whose magnetic field strength is inclined in one direction within a specific slice plane that is selectively excited, frequency encoding of position information in that direction is performed and an NMR signal is received. . The position information orthogonal to the above direction is extracted by phase encoding performed by applying a gradient magnetic field whose magnetic field intensity is inclined in a direction perpendicular to the gradient direction of the frequency encoding gradient magnetic field. That is, the above-described excitation / reception sequence is repeated many times while changing the phase encoding amount to collect the data amount necessary for image reconstruction.

[Problems to be Solved by the Invention] As described above, in the MRI apparatus, it is necessary to repeat the excitation / reception sequence many times while collecting the data while changing the phase encoding amount, so that the imaging time is long. There is a problem. For example, in the case of reconstructing an image having N × N pixels, it is necessary to change the phase encoding amount and repeat the N times of excitation / reception sequences, so if the repetition time is TR, then N × N It takes TR time. It is an object of the present invention to provide an MRI device that can shorten the imaging time.

[Means for solving problems]

The MRI apparatus according to the present invention obtains a signal intensity of data collected in each pulse sequence and pulse sequence control means for controlling the pulse sequence of excitation / reception to be repeated while gradually increasing the phase encoding amount from 0. A means for stopping the repetition of the above pulse sequence after the intensity becomes small enough to be regarded as noise, and a predetermined value as the data to be collected in the stopped pulse sequence, and the Fourier data together with the data collected so far. Means for exchanging and reconstructing an image.

[Action]

The strength of the MR signal generally decreases as it decenters in both the frequency and phase directions. Therefore, when the excitation / reception pulse sequence is repeated while gradually increasing the phase encoding amount from 0 and data is collected for each pulse sequence from the center of the phase encoding, the signal strength decreases each time. become. When this signal strength drops too much, it can be equated with noise and is useless to collect as data. Therefore, the signal strength is calculated for each pulse sequence,
If the repetition of the pulse sequence after that is stopped when the intensity becomes small enough to be regarded as noise, useless data collection is avoided and the time spent therefor is eliminated, so that the imaging time can be shortened.

【Example】

In one embodiment of the present invention, as shown in FIG. 1, an MR signal obtained by detection after reception is sampled by the A / D converter 1 and converted into a digital signal. This signal is taken into the CPU 3 via the buffer 2 and is further sent to the memory 4 for storage. The pulse sequence controller 5 gradually increases the phase encoding amount from 0,
It controls to repeat the pulse sequence of excitation / reception. Here, the data obtained by sampling the MR reception is represented by a complex number S (t, l). t is a function of time when sampling the MR signal obtained by one reception. Further, l is a function of the amount of phase encoding and takes a different value for each pulse sequence. If the pixels of the image to be reconstructed are N × N, t is an integer from −N / 2 to (N / 2) −1, and l is also from −N / 2 to (N / 2) −1. Is an integer. Therefore, the data represented by the complex number S (t, l) is arranged two-dimensionally as shown in FIG. 2, and if this is subjected to two-dimensional Fourier transform, the image is reconstructed. To be done. In this embodiment, the pulse sequence control device 5 gradually increases the phase encoding amount as described above, and the excitation / reception pulse sequence is repeated, and the pulse sequence is collected from the data of l = 0. First, l
When the data of = 0 is obtained, the signal strength Mo is obtained by the CPU 3. This signal strength is determined by the maximum signal strength or the total power. This data is written in the memory 4. Next, a pulse sequence of excitation / reception is performed with l = 1, −1 to collect data, and this data is stored in the memory 4
And the signal strength M1 at this time is similarly obtained by the CPU 3. In this way, the data is collected one after another and the signal strength is obtained, but the CPU 3 simultaneously determines whether or not the signal strength Mk of l = k, -k has decreased. Here, for example, this determination is made based on the ratio of the signal strength from 1 = 0 to 1 = k−1 to the sum ΣM, Mk / ΣM.
If this ratio is large, the pulse sequence controller 5 controls to perform the next pulse sequence of excitation / reception of the phase coating amount, and data collection up to l = N is performed. When this ratio becomes small before 1 = N is reached, the CPU 3 sends a stop command to the pulse sequence controller 5. As a result, the pulse sequence for excitation / reception is not performed after this time. In this case, the DC component is written in the memory 4 as the data that should be obtained by the pulse sequence that is no longer performed. In this manner, in any case, when all the data in the array shown in FIG. 2 are aligned, the two-dimensional Fourier transform device 6 performs the two-dimensional Fourier transform to obtain a reconstructed image. In this embodiment, the signal strength of l = k, -k is obtained, but the signal strength of the surrounding area may be obtained to detect the decrease. In addition, the phase encoding can be applied to the half encoding method by performing only one of positive and negative. Further, although the case where the method is applied to the two-dimensional Fourier transform method has been described above, 3
It is also applicable to the dimensional Fourier transform method by starting each of the two phase encodings from 0. In the above, the decrease in signal strength is determined during shooting, but it is also possible to predict the decrease in signal strength before shooting and stop the pulse sequence based on that.

【The invention's effect】

According to the MRI apparatus of this invention, the imaging time can be shortened,
As a result, it is possible to contribute to the reduction of the restraint time of the subject and the improvement of the throughput of the apparatus.

[Brief description of drawings]

FIG. 1 is a block diagram of an embodiment of the present invention, and FIG. 2 is a diagram showing a two-dimensional arrangement of data to be collected. 1 ... A / D converter, 2 ... buffer, 3 ... CPU, 4 ...
Memory, 5 ... Pulse sequence control device, 6 ... Two-dimensional Fourier calculation device.

Claims (1)

(57) [Claims] 1. A pulse sequence control means for controlling so as to repeat a pulse sequence for excitation / reception while gradually increasing a phase encoding amount from 0, and a signal strength of data collected in each pulse sequence is obtained, and the strength is noise. Means for stopping the repetition of the above pulse sequence after it becomes small enough to be regarded as
An MRI apparatus comprising: means for giving a predetermined value as data to be collected in a stopped pulse sequence, and performing Fourier transform and image reconstruction with the data collected up to that point.