JPS6234547A - Magnetic resonance imaging apparatus - 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] [Technical Field of the Invention] The present invention utilizes the phenomenon of magnetic resonance (hereinafter referred to as MR) to image and quantify the spin density or relaxation time distribution of a specific atomic nucleus present in a specimen. The present invention relates to a magnetic resonance imaging apparatus.

[Technical Background of the Invention] A magnetic resonance imaging apparatus (hereinafter referred to as an MRI apparatus) non-invasively obtains a proton density distribution in a cross section of a living body by utilizing the MR phenomenon that occurs when a magnetic field is applied to the living body. And M.R.
The I device uses an electrical multi-slice method that allows the slice position to be electrically moved without mechanically moving the living body.

An example of an MRI apparatus using the multi-slice method will be described with reference to FIG. 3. A transmitter 11 emits selective excitation pulses as pulsed high frequency waves. The magnet device 12 generates a static magnetic field 1-10 along the illustrated Z direction. The power supply device 3 serves as a power source for the magnet device 2. The probe head 14 applies a high-frequency pulsed magnetic field 111 to a subject (not shown) in the Y direction shown in the figure using R pulses for selective excitation from the transmitter 11, and detects echo signals from the subject. The amplifier 15 detects and amplifies the above-mentioned signal and outputs it.The image reconstruction device 16 reconstructs a tomographic image of the cross section of the object by combining the echo signals obtained from a plurality of directions.

At this time, the display unit 11, which outputs a plurality of tomographic images using the multi-slice method, displays the image reconstructed by the image reconstruction device 16. The first coil device 18 uses a pair of coils to cover an inclination (1) in the Z direction shown in the figure (around 0 oblique magnetic lift).
Generates 7.

A power source 19 serves as a power source for the first coil device 18 . The second coil device 20 generates gradient magnetic fields GX, GY having a gradient n1 in the X and Y directions shown in the figure.The power source 21 serves as a power source for the second coil device 20. The subject is then inserted into the magnet's phallus 12, and a uniform static magnetic field of -10 is applied. And system control device 2
A selective excitation pulse is generated from the transmitter 11 based on a timing signal from a selective excitation pulse setting control means (not shown) in step 2, and a high-frequency pulse is applied to the subject via the probe head 14. On the other hand, the power supply 19 operates based on a timing signal from a gradient magnetic field Gz setting control means (not shown) provided to the system control device 22, and a gradient magnetic field G/ is applied to the subject via the first coil device 18. Ru.

As a result, the selective excitation R [pulse carrier frequency W
Only a specific atomic nucleus within one cross section of the object is excited due to the relationship between the magnetic field 1-1o and the static magnetic field 1-1o. Then tilt la yang GX,
The power supply 21 operates based on a timing signal from a GY determination control means (not shown) and applies gradient magnetic fields GX and GY via the second coil device 20. Then, the probe head 14 detects an echo signal within the - cross section to which position information in each direction is added as phase information and frequency information by this gradient magnetic field Gx*GY. R pulses for selective excitation are generated after a predetermined relaxation time 1', and the multi-slice method collects echo signals from other slice planes within this relaxation time T. , a cross-sectional image of the slice plane is reconstructed by the image reconstruction device 16 using the echo signals and displayed on the display 17. The acquisition pulse sequence at d'3 in the multi-slice method as described above is shown in FIG. The acquisition pulse sequence for obtaining a single slice image as shown in (a) is formed by matching the number of slices with a time shift of 1 as shown in Fig. 4 (b). In this case, each excitation pulse are all R pulses for selective excitation. By the way, as an example of a collection pulse sequence with multiple excitation RF pulses per slice, we use the inversion recovery method (In
version recovery method, hereinafter referred to as I
When considering the pulse sequence of the R method), in the acquisition pulse sequence of Fig. 4(b), the inversion R "pulse P") (
i=1.2. It is known that the slice characteristics of . . . n slice numbers) have a large effect on image quality.

[Problems with Background Art] It is technically difficult to improve the slice characteristics of the first pulse in the acquisition pulse sequence using the multi-slice method, which inevitably leads to inaccurate measurement values and deterioration of image quality. Furthermore, in order to improve the slicing characteristics, a high-performance hardware system is required, resulting in an expensive device.

On the other hand, when a single slice is used to obtain a good image, there is a problem in that the data collection effect is reduced.

[Object of the Invention] The present invention has been made based on the above circumstances, and firstly, even if pulse P(1) is not selected in one slice with n=1, the second pulse in (#) is There is no problem if you select C as the excitation pulse. Now, in a multi-slice of two or more slices, it is generally not possible to deselect P(1). This is because unnecessary information will be mixed into other slice planes. SUMMARY OF THE INVENTION An object of the present invention is to provide a magnetic resonance imaging apparatus having an acquisition pulse sequence capable of obtaining a good image even in two or more multi-slices without improving the slice characteristics of the first pulse. It is something.

[Summary of the Invention] The outline of the present invention for achieving the above-mentioned objects is to use non-selective excitation in a magnetic resonance apparatus that collects and images a specific nuclear density distribution or relaxation time in a cross section of an object using a multi-slice method. Pulse detection means for sequentially collecting the first acquisition pulse sequence by the multi-slice method obtained by constant control of the pulse and the carrier frequency, and setting control of the RF pulse for non-selective excitation and the carrier frequency pulse detection means for sequentially collecting a second collection pulse sequence obtained by changing the order of the multi-slice method; The present invention is characterized by comprising an averaging means for adding and averaging the echo signals, and an image reconstruction means for reconstructing a tomographic image regarding the slice plane using the averaged echo signals.

[Embodiments of the Invention] Hereinafter, embodiments of the present invention will be described with reference to the drawings.

In FIG. 1, reference numeral 1 denotes a subject, which is placed in a uniform static magnetic field generated by a static magnetic field generating means (not shown) such as a magnet. 2 is a gradient magnetic field power supply, and a gradient magnetic field coil 3
Together, they form a gradient magnetic field system that generates gradient magnetic fields. A transmitter 4 is connected to the transmitter/receiver coil 5, applies a high-frequency pulse magnetic field to the subject 1 using an excitation RF pulse from the transmitter 4, and sends an echo signal from the subject 1 to the receiver 6. Receive. 7 is an A/D converter;
Data obtained by digitizing the echo signal is input to the averaging device 8. 9 is a data processing device, which converts the averaged data into numerical values and images. 10 is a system control device that controls the entire system.

Next, the operation of the device having the above configuration will be explained.

First, in the single slice pulse sequence shown in FIG. 2(a), the time interval TD between P(1) and pulse sequence (1) is an important parameter. This is because the tA degree distribution of the obtained image is approximately expressed by the following equation.

t) (1-2e-TD/T 1 ) ・
...(1) where ρ is the spin density and T1 is the #l relaxation time constant.

In the conventional multi-slice, the TD is constant for each slice as shown in FIG. 2(b), but the image quality deteriorates depending on the slice characteristics of the pulse P(i).

Therefore, in the present invention, the TD is not constant as shown in FIG. 2(C), but can be expressed in the form of τ for the TD, and by reversing the order of each slice and averaging, the density distribution of the image can be determined. can be expressed as the following equation.

(, Q (12orp T D ” K J / T
i > +p (1-2e ”+KJ/T1) )/2
=ρ(1-2cosh(KJ/T1),.-TD/
Ti), 1. (2) It has been confirmed by calculation that the errors in (2) and (1) above are quite small when considering parameters applied in, for example, a diagnostic MRI apparatus.

In this way, the present invention is controlled by the system controller 10 so that the non-selective excitation R The echo signals (#■, #■, #■) of the first acquisition pulse sequence shown in A in Figure 2 (C) obtained by the multi-slice method obtained by setting and controlling the R lower pulse for non-selective excitation. The echo signals (#■, #■, #■) of the second collection pulse series shown in B in Figure 2 (C) obtained by setting and controlling the carrier frequency and reversing the order are A/D. A converter 7 converts the signal into a digital signal, an averaging device 8 performs averaging, and a data processing device 9 reconstructs a tomographic image regarding the slice plane to display the image.

Although one embodiment of the present invention has been described above, the present invention is not limited to the above embodiment, and it is possible to implement it with appropriate modifications within the scope of the gist of the present invention. Not even. For example, equations (1) and (2) above take into account only the minimum parameters necessary to explain the present invention, but even if you consider the more detailed equations using other parameters, they are essentially the same. It is. Moreover, it is applicable not only to the IR multi-slice method but also to other similar methods.

``Effects of the Invention The present invention makes it possible to obtain good images without improving the slice characteristics of the first pulse. It has the effect of

[Brief explanation of drawings]

Fig. 1 is a block diagram of the device of the present invention, Fig. 2 (a) is an explanatory diagram of a single slice acquisition pulse sequence in the IR method, and Fig. 2 (b) is an illustration of a multi-slice acquisition in the normal IR method. FIG. 2(C) is an explanatory diagram of a pulse sequence for multi-slice acquisition in the IR method according to the present invention. FIG. 3 is a general block diagram of an MRI apparatus.
) is an explanatory diagram of a basic data acquisition pulse sequence of a single slice, and FIG. 4(i)) is an explanatory diagram of a basic data acquisition pulse sequence of a continuous slice. 4... Transmitting/receiving coil, 6... Receiver, 8... Adding and averaging device, 9... Data processing device. Agent Patent Attorney Nori Kei Chika Yudo Condolences for Human Rehabilitation 1 Fig. 2 Fig. 1 Go - 艮■ ---------→ Condolence 3

Claims (1)

[Claims] In a magnetic resonance apparatus that collects and images a specific nuclear density distribution or relaxation time distribution in a cross section of an object using a multi-slice method, the multi-slice method is obtained by setting and controlling the non-selective excitation RF pulse and the carrier frequency. Pulse detection means that sequentially collects a first collection pulse sequence, and sequentially collects a second collection pulse sequence using a multi-slice method obtained by changing the order by setting and controlling the non-selective excitation RF pulse and the carrier frequency. pulse detection means for adding and averaging the echo signals obtained by the first collection pulse sequence and the echo signals obtained by the second collection pulse sequence, and using the averaged echo signals. 1. A magnetic resonance imaging apparatus comprising: image reconstruction means for reconstructing a tomographic image regarding a slice plane.