JPH0620444B2 - Magnetic resonance imager - Google Patents

Description

TECHNICAL FIELD OF THE INVENTION The present invention utilizes a magnetic resonance (hereinafter MR) phenomenon to image and quantify the spin density or relaxation time distribution of specific nuclei present in a subject. Magnetic resonance imaging apparatus.

[Technical Background of the Invention] A magnetic noise imaging device (hereinafter referred to as an MRI device) non-invasively obtains a proton density distribution in a living body cross section by utilizing MR development generated by applying a magnetic field to the living body. And MR
The I-device employs an electrical multi-slice method that can electrically move the slice position without mechanically moving the living body.

An example of the MRI apparatus using the multi-slice method will be described with reference to FIG. 3. The transmitter 11 emits a selective excitation pulse as a pulsed high frequency wave. The magnet device 12 generates a static magnetic field H ₀ along the Z direction shown. The power supply device 3 serves as a power supply for the magnet device 12. The probe head 14 uses the RF pulse for selective excitation from the transmitter 11 to generate a high-frequency pulse magnetic field H in the Y direction in the figure with respect to the subject (not shown).
₁ is applied and the echo signal from the subject is detected. The amplifier 15 detects and amplifies the echo and outputs it.
The image reconstructing device 16 processes echo signals obtained from a plurality of directions to reconstruct a tomographic image of the cross section of the subject. At this time, a plurality of tomographic images are output by the multi-slice method. The display unit 17 displays the image reconstructed by the image reconstructing device 16. The first coil device 18 generates a gradient magnetic field G _Z which is inclined in the Z direction in the figure by a pair of coils. Power supply 1
9 serves as a power source for the first coil device 18. The second coil device 20 generates gradient magnetic fields G _X and G _Y having gradients in the X and Y directions shown. The power source 21 serves as a power source for the second coil device 20. Then, the subject is inserted and arranged inside the magnet device 12, and the uniform static magnetic field H ₀ is applied thereto. Then, 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 the system controller 22, 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 the timing signal from the gradient magnetic field G _Z setting control means (not shown) in the system controller 22, and the gradient magnetic field G _Z is applied to the subject through the first coil device 18. . This allows
Carrier frequency W _{0 of the} RF pulse for selective excitation and static magnetic field H
_Due to the relationship with ₀ , only specific nuclei within one cross section of the subject are excited. Next, the power supply 21 operates based on the timing signal from the gradient magnetic field G _X , G _Y setting control means (not shown), and the gradient magnetic field G _X , G _{Y is} passed through the second coil device 20.
Is applied. Then, the probe head 14 detects an echo signal in the one cross section in which position information in each direction is added as phase information and frequency information by the gradient magnetic fields G _X and G _Y. The selective excitation RF pulse is generated after a predetermined relaxation time T,
In the multi-slice method, echo signals from other slice planes are sampled within this relaxation time T, and the image reconstruction device 16 reconstructs a cross-sectional image relating to the slice planes using the echo signals, and a display device. Display on 17.
The acquired pulse sequence in the multi-slice method as described above corresponds to the number of slices while time-shifting the acquired pulse sequence for obtaining a single slice image as shown in FIG. 4 (a) as shown in FIG. 4 (b). They are also formed together, and in this case, all the excitation pulses are RF pulses for selective excitation.

By the way, as an example of a collection pulse sequence having a plurality of excitation RF pulses per single slice, the inversion recovery method (In
version recovery method, abbreviated as IR below
Method), the inversion RF pulse P ⁽ⁱ⁾ (i) in the acquisition pulse sequence of FIG.
It is known that the slice characteristics of = 1, 2, ..., N: slice number) have a great influence on the image quality.

[Problems of the Background Art] It is technically difficult to improve the characteristics of the slice of the first pulse in the collected pulse sequence by the multi-slice method, and it is inevitable that measurement values become inaccurate and image quality deteriorates. In addition, a high-performance hardware system is required to improve the slice characteristics, resulting in an expensive device. On the other hand, when a single slice is used to obtain a good image, the data acquisition effect is reduced.

[Object of the Invention] The present invention has been made based on the above circumstances. First, in the case of one slice of n = 1, even if the pulse P ⁽¹⁾ is not selected, the second pulse in (#) There is no problem if is used as the selective excitation pulse. Now, it is generally impossible to deselect P ⁽¹⁾ in two or more multi-slices. This is because unnecessary information mixes with other slice planes. It is an object of the present invention to provide a magnetic resonance imaging apparatus having a collection pulse sequence that can obtain a good image without improving the slice characteristics of the first pulse even in the case of two or more multi-slices. It is a thing.

[Outline of the Invention] An outline of the present invention for achieving the above object is to provide a magnetic resonance apparatus for collecting and imaging a specific nuclear density distribution or relaxation time distribution in a cross section of a subject by a multi-slice method,
Pulse detection means for sequentially collecting the first acquisition pulse sequence by the multi-slice method obtained by setting and controlling the non-selective excitation RF pulse and the carrier frequency, and the non-selective excitation RF
Pulse detection means for sequentially sampling the second acquisition pulse sequence by the multi-slice method obtained by changing the order by setting control of the pulse and carrier frequency, echo signal obtained by the first acquisition pulse sequence, and And an image reconstructing unit for reconstructing a cross-sectional image on the slice plane by using the echo signals obtained by the arithmetic mean. It is what

Embodiments of the Invention Embodiments of the present invention will be described below with reference to the drawings.

In FIG. 1, reference numeral 1 denotes a subject, which is arranged in a uniform static magnetic field generated by static magnetic field generating means (not shown) such as a magnet. Reference numeral 2 is a gradient magnetic field power supply, and gradient magnetic field coil 3
Together with this, a gradient magnetic field system for generating a gradient magnetic field is formed. A transmitter 4 is connected to the transmission / reception coil 5, applies a high-frequency pulse magnetic field to the subject 1 by an exciting RF pulse from the transmitter 4, and sends an echo signal from the subject 1 to the receiver 6. To receive. An A / D converter 7 inputs the digitized data of the echo signal to the averaging device 8. Reference numeral 9 is a data processing device, which digitizes and averages the data obtained by addition and averaging. Reference numeral 10 denotes a system control device that controls the entire system.

Next, the operation of the device configured as described above will be described. First, in the single-slice pulse sequence of FIG. 2 (a), P ₍₁₎
The time interval TD between the pulse sequence (#) and the pulse sequence (#) is an important parameter. This is because the density distribution of the obtained image is approximately represented by the following equation.

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

In the conventional multi-slice, the TD is constant for each slice as shown in FIG. 2B, but the image quality is deteriorated due to the characteristics of the slice of the pulse P (i). Therefore, in the present invention, T
D is not constant as shown in FIG. 2 (c) and can be expressed in the form of TD + Kτ, and the density distribution of the image quality is expressed by the following equation by averaging with the order of each slice reversed. You can

It can be confirmed by calculation that the errors of (2) and (1) above are fairly small considering the parameters applied in the MRI apparatus for diagnosis. According to the present invention, the system controller 10 causes the RF pulse for non-selective excitation to be generated from the transmitter 4 with the relaxation time TD + Kτ.
By the multi-slice method obtained by controlling the setting of the non-selective excitation pulse and the carrier frequency.
The echo signals (#, #,) of the first acquisition pulse sequence shown in A of FIG. 6C, the R lower pulse for non-selective excitation, and the carrier frequency are set and controlled to obtain the reverse order. Two
The echo signals (# ,,) of the second collected pulse sequence shown in B of FIG. 6C are converted into digital signals by the A / D converter 7, and the averaging device 8 performs averaging, and the data processing device 9 Is used to reconstruct a tomographic image relating to the slice plane and display the image.

Although one embodiment of the present invention has been described above, the present invention is not limited to the above-mentioned embodiment, and it can be appropriately modified and implemented within the scope of the gist of the present invention. There is no end. For example, the above equations (1) and (2) consider only the minimum parameters necessary for the explanation of the present invention, but even if the equations are considered in more detail using other parameters, they are essentially the same. is there. In addition, the method is not limited to the IR method multi-slice, and can be applied to other similar methods.

EFFECTS OF THE INVENTION The present invention has an effect that a good image can be obtained without improving the slice characteristics of the first pulse, and thus an excellent image can be obtained by a lower-cost device. .

[Brief description of drawings]

FIG. 1 is a block diagram of the device of the present invention, and FIG. 2 (a) is an IR.
Fig. 2 (b) is an explanatory view of a normal IR method multi-slice acquisition pulse sequence, and Fig. 2 (c) is an IR method multi-slice collection according to the present invention. FIG. 3 is a block diagram of a general MRI apparatus, FIG. 4 (a) is an explanatory view of a basic data acquisition pulse sequence of a single slice, and FIG. 4 (b) is a multi-slice diagram. It is explanatory drawing of a basic data collection pulse series. 4 ... Transmit / receive coil, 6 ... Receiver, 8 ... Addition averaging device, 9 ... Data processing device.

Claims (1)

[Claims] 1. A magnetic resonance apparatus for collecting and imaging a specific nuclear density distribution or relaxation time distribution in a cross section of a subject by a multi-slice method, which is obtained by setting and controlling a non-selective excitation RF pulse and a carrier frequency. Pulse acquisition means for sequentially sampling the first acquisition pulse sequence by the multi-slice method, and second acquisition by the multi-slice method obtained by changing the order by setting and controlling the RF pulse for non-selective excitation and the carrier frequency. Pulse detection means for sequentially sampling the pulse series, arithmetic mean for arithmetically averaging the echo signals obtained by the first collection pulse series and echo signals obtained by the second collection pulse series, and arithmetic mean An image reconstructing means for reconstructing a tomographic image on a slice plane using an echo signal.