JPH0326973B2 - - Google Patents

Description

Detailed Description of the Invention (Industrial Application Field) The present invention relates to a nuclear magnetic resonance tomography apparatus (hereinafter referred to as NMR-CT Regarding)
More specifically, when detecting a nuclear magnetic resonance signal (hereinafter referred to as an NMR signal) by a multi-slice scan using a phase detection means, the frequency of the reference signal of the phase detection means is shifted and the multi-slice is applied to the central axis of the static magnetic field system. This paper relates to NMR-CT in which the scan axes of the two are aligned.

(Prior art) NMR-CT uses a static magnetic field coil that creates a uniform static magnetic field H ₀ and a magnetic field in the same direction as the static magnetic field H ₀ in x, y,
A magnet section consisting of a gradient magnetic field coil that creates a magnetic field with a linear gradient in each direction of z, and a high-frequency pulse (high-frequency electromagnetic wave) is applied to a subject placed within the magnetic field formed by the magnet section, and the magnetic field from the subject is It has a transmitting/receiving section that detects an NMR signal, and a control/image processing section that controls the operation of the transmitting/receiving section and the magnet section, processes detected data, and displays an image, and the like.
The control/image processing section usually has a computer-based configuration, and receives the detected data through an A/D converter equipped with a low-pass filter.

In the above configuration, a static magnetic field of a predetermined strength is formed in a predetermined space (place where the subject is installed) by the static magnetic field coil. On the other hand, under the control of the control/image processing section, the transmitting/receiving section uses an excitation pulse sequence (90° pulse and 180° pulse) using the spin echo method (SE method) as shown in Figure 4. A sequence that occurs at a predetermined timing.
(see Figure A) and a gradient magnetic field sequence (a sequence in which gradient magnetic fields G _x , G _y and G _z are generated at a predetermined timing; see Figure 4 B, C and D) by the Fourier transform method are output, and the NMR at that time is output. Signal (4th
(see figure E). In addition, the control/image processing unit processes a large number of data collected according to the above sequence.
Image reconstruction processing is performed using the NMR signal, that is, raw data, and a tomographic image is displayed.

By the way, the raw data mentioned above is usually collected using fixed coordinate axes determined by the hardware of the static magnetic field system (the central axis of the static magnetic field system, also referred to as the machine system coordinate axis).
The test is performed using multi-slices, with the subject's body axis aligned with the body axis, and the tilt angle θ set to a desired value.
At this time, as shown in FIG.
It is preferable to collect data while moving a to 1e in the direction of the fixed coordinate axis 2 in order to maintain image quality. This is due to the following reasons.

In actual NMR-CT, the origin of the gradient magnetic field is fixed, so when a multi-slice scan is performed with the tilt angle θ, each slice plane (image) 1a to 1e is
The center of the image moves on an axis (scan system coordinate axis) 4 that is inclined at a tilt angle θ with respect to the fixed coordinate axis 2 (in the figure, 3 indicates the subject). As this movement progresses, a part of the slice plane comes out of the subject 3, as shown in FIG. When an image is reconstructed using data from such a slice plane, the reconstructed image may have omissions or aliasing due to signals from outside the imaging area, resulting in a decrease in image quality.

Therefore, conventional NMR-CT is equipped with a means to reduce the amount of warp, a means to increase the number of views, a means to increase sampling by widening the band of the low-pass filter, etc., to suppress the protrusion of the slice plane and to provide a predetermined number of views. This data collection prevents deterioration in image quality.

(Problems to be Solved by the Invention) However, in the case of conventional NMR-CT, although the image quality is improved by each of the above-mentioned means, the resolution deteriorates, the scan time increases, and the There is a problem that the amount of memory for data increases.

The present invention has been made in view of the above points, and its purpose is to improve image quality without deteriorating performance such as scan time and resolution when collecting multi-slice data by tilting. Our goal is to provide NMR-CT that can be used.

(Means for solving the problems) The NMR-CT of the present invention that achieves the above objectives is
When an NMR signal resulting from a multi-slice scan is detected by a phase detection means, the frequency of a reference signal of the phase detection means is shifted to align the scan axis of the multi-slice with the central axis of the static magnetic field system.

(Example) Hereinafter, the present invention will be described in detail with reference to the drawings.

FIG. 1 is a configuration diagram showing an embodiment of the present invention. The magnet assembly 11 has a space (hole) into which a subject is inserted, and a static magnetic field coil that applies a constant static magnetic field to the subject and a gradient magnetic field surrounding this space. Gradient magnetic field coils (gradient magnetic field coils are
x, y, and z axes), an RF transmitting coil that provides high-frequency pulses to excite the spin of the atomic nuclei within the subject, and a
Receiving coils and the like for detecting NMR signals are arranged. The static magnetic field coil, gradient magnetic field coil, RF transmitting coil, and receiving coil are connected to a static magnetic field power source (main magnetic field power source) 12, a gradient magnetic field drive circuit 13, an FR power amplifier 14, and a preamplifier 15, respectively. The sequence storage circuit 16 has means for generating a sequence signal for collecting multi-slice scan data based on the Fourier method using a pulse sequence of the SE method under a desired tilt angle according to a command from the computer 21, Gradient magnetic field drive circuit 13, gate modulation circuit 17, RF oscillation circuit (synthesizer)
18, a phase detector 19 and an A/D converter 20, respectively. Gate modulation circuit 1
7 modulates the high frequency signal from the RF oscillation circuit 18 using the timing signal from the sequence storage circuit 16 and supplies it to the RF power booster 14 . The RF oscillation circuit 18 is activated when the gate of the phase detector 19 is opened.
When receiving NMR signals (during data collection),
The frequency of the reference signal output to the phase detector 19 is shifted by a predetermined amount Δf with respect to the center frequency f ₀ for each slice plane. The phase detector 19 is the RF oscillation circuit 1
8, the output signal of the preamplifier 5 (NMR signal detected by the receiving coil) is subjected to phase detection and input to the A/D converter 20. A/D
The converter 20 is obtained via a phase detector 19
The NMR signal is converted from analog to digital and input to the computer 21. The computer 21 has a sequence storage circuit 1 in order to exchange information with the operation console 22 and perform various scan sequences.
6 Switching operations and rewriting memory, A/
Image reconstruction calculations are performed using data from the D converter 20.

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

Under a uniform static magnetic field H ₀ by the static magnetic field power supply 12,
A signal from the sequence storage circuit 16 causes the gradient magnetic field drive circuit 13, the gate modulation circuit 17, and
The RF oscillation circuit 18 generates gradient magnetic fields and high-frequency pulses according to the Fourier transform method using a pulse sequence based on the SE method. As a result, multi-slice scanning is performed. The operating waveforms at this time are as shown in FIGS. 2A to 2D, and are basically the same as those in FIGS. 4A to 4D. Periods A and B in FIG. 2 represent energization in slice A and slice B planes, and the NMR signal shows spin echoes from each slice plane.

On the other hand, during data collection (FIG. 2), the RF oscillation circuit 18 supplies a reference signal of a different frequency to the phase detection circuit 19 for each scanned slice plane. The reference signal has a frequency shifted by Δf with respect to the center frequency f ₀ , and for example, the frequencies at slice planes A and B are f ₁ and f ₂ , respectively (FIG. 2G).

In FIG. 2, TE represents echo time and TR represents repetition time.

Next, correction of the center of the slice plane (image) using the above shift amount will be specifically explained.

In the tilt multi-slice, the frequency of the excitation pulse obtained from the RF oscillation circuit 18 changes for each slice. Also, when collecting data
The frequency of the RF oscillation circuit 18 (the frequency of the signal given to the phase detector 19) also changes for each slice.
The above-mentioned shift amount Δf at this time is expressed by the following equation in terms of the slice plane as shown in FIG.

Δf = γ_{・ G} ), Coronal, etc. slice direction and tilt axis (machine system coordinates x-axis, y-axis, z
the frequency f of the reference signal determined corresponding to
By setting f=f ₀ ±Δf, it is possible to obtain an image in which the moving direction of the image center is corrected in the direction of the machine system coordinate axes. That is, by the above operation, the origin of the readout gradient is shifted by the amount by which the image center is shifted to the machine system coordinate axes, and the moving distance of the image center can be set arbitrarily.

Therefore, the slice plane in the tilt multi-slice scan can be moved as shown in FIG. 5 to prevent it from protruding from the subject.

Note that the present invention is not limited to the above-mentioned embodiments, but can be applied to any scan method that belongs to the two-dimensional Fourier method.
Further, an RF oscillation circuit (synthesizer) may be provided for each transmission and reception.

(Effects of the Invention) As explained above, according to the NMR-CT of the present invention, when an NMR signal by multi-slice scan is detected by the phase detection means, the frequency of the reference signal of the phase detection means is shifted, and the Since the multi-slice scan axis is aligned with the central axis of the magnetic field system, when collecting multi-slice data by tilting, image quality can be improved without deteriorating performance such as scan time or resolution.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing an embodiment of the present invention, FIGS. 2 and 3 are explanatory diagrams of the operation in an embodiment of the present invention, and FIG. 4 is an SE method pulse sequence based on Fourier. The explanatory diagrams of the sequence, FIGS. 5 and 6, are explanatory diagrams of slice planes in tilt multi-slice. 1a to 1e...Slice plane, 2...Fixed coordinate axes (machine system coordinate axes), 3...Object, 4...Scan system coordinate axes, 11...Magnet assembly, 12...Static magnetic field power supply, 13...Gradient Magnetic field drive circuit, 14...RF power amplifier, 15...Preamplifier, 16...Sequence storage circuit, 17...Gate modulation circuit, 18...RF oscillation circuit, 19...
Phase detection circuit, 20...A/D converter, 21...
Calculator, 22...operation console.

Claims (1)

[Claims] 1. Applying a gradient magnetic field and high-frequency electromagnetic waves to a subject placed in a static magnetic field according to a predetermined sequence, and collecting nuclear magnetic resonance signals by multi-slice scanning via a phase detection means. In the nuclear magnetic resonance tomography apparatus, the frequency of the reference signal of the phase detection means is shifted by an amount Δf calculated by the following formula with respect to the center frequency f ₀ for each slice, so that the center axis of the static magnetic field system is shifted. A nuclear magnetic resonance tomography apparatus characterized by comprising means for aligning scan axes of multi-slices. Δf=γ・G _x・l・tanθ However, γ...Magnetic rotation ratio G _x ...Reading gradient θ...Tilt angle l...Length from scan center to center of slice surface