JPH0311226B2 - - Google Patents

Description

[Detailed Description of the Invention] (Industrial Application Field) The present invention relates to the phenomenon of nuclear magnetic resonance (hereinafter referred to as nuclear magnetic resonance).
This article relates to a method for preventing the occurrence of artifacts due to side effects when using 180° pulses in NMR tomography devices that use 180° pulses to observe the internal tissue of objects as tomographic images. be.

(Prior art) In an NMR tomographic imaging device, there are two methods to observe the signals emitted by spins excited by a 90° pulse: one is to observe the FID signal immediately after excitation, and the other is to generate spin echoes and observe them. There is, but
Recently, a method of observing echo signals is common. Echoes can be generated by reversing the gradient magnetic field or by using a 180° pulse.
The 180° pulse method has the advantage of not being affected by the inhomogeneity of the main magnetic field, and is widely used.

(Problem to be solved by the invention) However, when using a 180° pulse, the spins do not rotate exactly 180° due to factors such as pulse width errors and biased RF intensity distribution. In addition to this, unnecessary signals from outside the slice plane (hereinafter referred to as parasitic signals) may be mixed in, causing artifacts in the image.

On the other hand, in order to prevent the generation of parasitic signals by disturbing the phase of the spins that generate parasitic signals,
There are methods such as applying a spoiler pulse immediately before or after the 180° pulse, but the spoiler pulse also has side effects such as weakening the original signal, so it is not a perfect countermeasure.

Furthermore, since the parasitic signal appears temporally away from the center of the echo, methods such as not using the signal in that part may be considered, but in that case, the spatial resolution will decrease. Furthermore, it is not possible to completely remove the parasitic signal that remains at the center of the echo, making it impossible to completely eliminate the artifact.

In view of these points, the purpose of the present invention is to develop an NMR system that can almost completely eliminate artifacts without any side effects such as a decrease in signal strength or spatial resolution.
An object of the present invention is to provide a method for preventing artifacts in a tomographic imaging apparatus.

(Means for Solving the Problem) In order to achieve such an objective, the present invention extracts correction data by averaging scan data over different views, and uses it to calculate the scan data of each view. It is characterized by adding correction to.

(Example) The present invention will be explained in detail below using the drawings. FIG. 1 is a block diagram of the main parts of an NMR tomographic imaging apparatus for carrying out the present invention. In the figure, 1 is a magnet assembly, inside which a space (hole) is provided for inserting an object, and a main magnetic field coil that surrounds this space and applies a constant magnetic field to the object. , gradient magnetic field coils for generating gradient magnetic fields (x gradient magnetic field coils, y gradient magnetic field coils, z gradient magnetic field coils configured to be able to individually generate gradient magnetic fields), and atomic nuclei in the target object. An RF transmitting coil that provides a high-frequency pulse to excite the spins of the object and the NMR from the object.
A receiving coil and the like for detecting signals are arranged.

The main magnetic field coil, Gx, Gy, and Gz gradient magnetic field coils, RF transmitting coil, and NMR signal receiving coil are each connected to a main magnetic field power supply 2, a Gx, Gy, and Gz gradient magnetic field driver 3, an RF power amplifier 4, and a preamplifier. 5. 10 is a sequence memory circuit that controls the generation sequence of gradient magnetic fields and high-frequency magnetic fields and converts the obtained NMR signals into an A/D converter.
Control when to convert.

6 is a gate modulation circuit, and 7 is an RF oscillation circuit that generates a high frequency signal. The gate modulation circuit 6 modulates the high frequency signal output by the RF oscillation circuit 7 using the timing signal from the sequence storage circuit 10,
Generates high frequency pulses. This high frequency pulse
RF power amplifier 4.

8 is a phase detector which refers to the output signal of the RF oscillation circuit 7, detects it with a receiving coil, and sends it to the preamplifier 5.
Detects the phase of the NMR signal sent via the

11 is an A/D converter that converts the NMR signal obtained through the phase detector 8 from analog to digital.

13 is a computer that sends and receives information to and from the operation console 12, rewrites the contents of the sequence storage circuit 10 to realize various scan sequences, and calculates resonance from observation data input from the A/D converter. It is configured to be able to perform operations such as reconstructing the distribution of energy-related information into an image. This reconstructed image is displayed on the display device 9.

The operation in such a configuration will be explained next.
Here, the case of the spin warp method will be explained as an example.

Under the control of the sequence storage circuit 10, it operates in a sequence as shown in FIG.

While the main magnetic field coil is energized by the main magnetic field power supply 2 and a static magnetic field Ho is applied to the object, the gate modulation circuit 6 is opened by the sequence storage circuit 10, and the magnetic field is modulated into a predetermined shape (in this case, Gaussian shape). Applying the RF signal to the transmitting coil via the RF power amplifier 4,
Apply a 90° pulse to the object.

At the same time as this 90° pulse is applied, a gradient magnetic field Gz in the z-axis direction (FIG. 2B) is applied to selectively excite spins within a specific slice plane.

Next, the gradient magnetic field coils are energized, and gradient magnetic fields Gy and Gx are applied as shown in C and D of FIG.
Perform phase encoding using Gy. The purpose of applying Gx during this period is to provide a phase difference between each spin in advance in order to observe the signal as an echo.

Next, apply a 180° pulse, then apply Gx,
An echo signal is generated as shown in FIG.

This spin echo signal is detected by a receiving coil and guided to a phase detector 8 via a preamplifier 5.
Phase detection is performed with reference to the RF oscillator signal. The phase-detected signal is digitally converted by an A/D converter 11 and sent to a computer 13.

The signal observed in this way corresponds to one line of the two-dimensional Fourier transform of the nuclear spin density distribution, so by collecting data while sequentially changing the amplitude of Gy and inversely transforming the two-dimensional Fourier transform, the You can get the image.

In this case, the NMR signal includes a parasitic signal B in addition to the original echo signal A, as shown in FIG. Since this parasitic signal B is often reproducible regardless of the view, if only the parasitic signal component can be separated by some method, it can be corrected by subtracting it from the data of each view. In general, scan data using the Fourier method, including spin warp, has very small signal components in views with a large amount of warp (phase encoding), and each view has a different waveform, so it is necessary to average an appropriate number of views. Then, almost completely only the parasitic signals can be extracted.

FIG. 3 shows the process of such correction. In the figure, data of only views with a large amount of warp among data D1 before correction is averaged to obtain data D2 in which only parasitic signals are extracted. Next, this correction data D2 is subtracted from the data D1 before correction to obtain data D3 in which the parasitic signal has been corrected. Such calculations are performed in the computer 13.

Note that the present invention is applicable not only to the spin warp method but also to all Fourier method scan data using a 180° pulse.

Furthermore, the projection restoration method
It can also be applied to reconstruction (abbreviated as PR method). Since there are no views with small signal components in the PR method, signal components remain in the average data D4 as shown in FIG. Here, if the parasitic signal and the original signal are far enough apart in time that they can be separated, correction can be made by cutting out and using only the parasitic signal using the window function D43 of the time window as shown in Figure 4. .

However, since there is usually a portion where the two signals overlap, complete correction cannot be made, but a considerable effect can be obtained by similar processing. In particular, it is desirable to provide several dedicated views for extracting only parasitic signals. Complete correction can be achieved by using a view with a large amount of warp in the pulse sequence of FIG. 2 as the dedicated view and using the average value of these views for correction. The scan time is only slightly extended by providing a dedicated view.

(Effects of the Invention) As explained above, according to the present invention, only the parasitic signal component due to the 180° pulse is extracted by averaging scan data over different views, and the reproducibility of the parasitic signal is utilized. By subtracting that component from the scan data, the artifacts caused by the 180° pulse can be almost completely removed without any side effects. Furthermore, since there is no need to repeatedly measure the same view during implementation, there is no increase in scan time, and even if a dedicated view is provided, the scan time only increases slightly.
Therefore, the effect is great in practical use.

[Brief explanation of drawings]

Fig. 1 is a block diagram of the main parts of an NMR tomographic imaging apparatus for carrying out the present invention, Fig. 2 is a waveform diagram for explaining the operation, and Figs. 3 and 4 are diagrams for explaining the correction process. It is an explanatory diagram. DESCRIPTION OF SYMBOLS 1... Magnet assembly, 2... Main magnetic field power supply, 3... Gradient magnetic field drive circuit, 4... RF power amplifier, 5... Preamplifier, 6... Gate modulation circuit, 7... RF oscillation circuit, 8... Phase detector, 9
... Display device, 10 ... Sequence memory circuit, 1
1... A/D converter, 12... Operation console,
13...Calculator.

Claims (1)

[Claims] 1. In an NMR tomography device that applies a 180° pulse to an object to generate a spin echo signal and measures it to obtain a tomographic image of the object, scan data is averaged over different views. By doing so, we obtain data that extracts the parasitic signal component caused by the 180° pulse, and by subtracting the parasitic signal component data from the scan data of each view, we remove artifacts caused by the 180° pulse in the tomographic image of the object. A method for preventing artifacts in an NMR tomographic imaging device, characterized in that: 2. The NMR according to claim 1, wherein the scan data is averaged over a plurality of different views having a large amount of phase encoding.
A method for preventing artifacts in a tomographic imaging device.