JPH0316859B2 - - Google Patents

Description

[Detailed Description of the Invention] (Industrial Application Field) The present invention relates to an NMR imaging device (nuclear magnetic resonance image display device) that grasps the density distribution of target atomic nuclei in a specimen using nuclear magnetic resonance phenomena. ,
More specifically, information about edges in the original image is extracted from the collected raw data,
The present invention relates to an NMR imaging apparatus that obtains a predicted image of ringing caused by the information and the truncation frequency of the raw data, and subtracts the predicted image from the original image to create a ringing-corrected image.

(Prior art) NMR imaging equipment uses a static magnetic field coil that creates a uniform static magnetic field H ₀ and a gradient that creates a magnetic field that has a linear gradient in each of the x, y, and z directions in the same direction as the static magnetic field H ₀ . A magnet section consisting of a magnetic field coil applies high-frequency pulses (high-frequency electromagnetic waves) to a subject placed within the magnetic field formed by the magnet section, and generates a high-frequency pulse (high-frequency electromagnetic wave) from the subject.
It has a transmitting/receiving section that detects an NMR signal, 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.

In the above NMR imaging device, the control/image processing section uses the saturation recovery method.
Recovery method: SR method) and iterative recovery method (Inversion method)
Outputs a pulse sequence based on Recovery method (IR method). Also, data collection is performed based on the Fourier transform method, and image reconstruction is performed. At this time, expanding the data collection area to include high spatial frequency components will worsen the S/N ratio and increase the scan time, so data collection is limited to the range necessary to obtain the desired resolution. (data censoring occurs). By the way, it is widely known that ringing occurs when truncation is performed in a finite term of a Fourier series. In other words, the scan data (Raw) is the Fourier series expansion of the NMR image.
In image reconstruction using data), rigging occurs due to the finite data sampling range. For this reason, when reconstructing an image using data collected based on the Fourier transform method described above, a window function as shown in FIG. 4 is used to gradually reduce high frequency components to zero. As a result, ringing due to data truncation does not occur in the image.

(Problem to be solved by the invention) However, in conventional NMR imaging devices, images are reconstructed by applying a window function that gradually reduces high-frequency components to zero, which degrades spatial resolution. There is a problem.

The present invention has been made in view of the above, and an object thereof is to provide an NMR imaging apparatus that suppresses ringing and improves image quality while minimizing deterioration of spatial resolution.

(Means for Solving the Problems) The NMR imaging apparatus of the present invention that achieves the above object extracts information regarding edges in the original image determined from the collected raw data, and uses the information and the truncation frequency of the raw data. A predicted image of ringing caused by the above is obtained, and the predicted image is subtracted from the original image to create a ringing-corrected image.

(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 1 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. A gradient magnetic field coil is used to generate x,
), an RF transmitting coil that provides a high-frequency pulse to excite the spin of the atomic nucleus 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 NMR signal receiving coil are connected to a main magnetic field power supply 2, a gradient magnetic field driver 3, an RF power amplifier 4, and a preamplifier 5, respectively. The sequence storage circuit 10 has means for scanning with a pulse sequence of the Fourier method according to instructions from the computer 13, and means for controlling the timing when A/D converting the detected NMR signal. The drive circuit 3, gate modulation circuit 6, and A/D converter 11 are operated. Gate modulation circuit 6 modulates the high frequency signal from RF oscillation circuit 7 using the timing signal from sequence storage circuit 10 and supplies it to RF power amplifier 4 . The phase detector 8 uses the output of the RF oscillation circuit 7 as a reference signal,
The NMR signal detected by the receiving coil and sent via the preamplifier 5 is subjected to phase detection and is provided to the A/D converter 11. The A/D converter 11 converts the NMR signal obtained through the phase detector 8 into analog-to-digital conversion and provides the converted signal to the computer 13 . Calculator 13
In order to exchange signals with the operation console 12 and to realize a scan sequence, it switches the operation of the sequence storage circuit 10 and rewrites the contents, and performs resonance using the data from the A/D converter 11. It performs calculations to reconstruct the distribution of energy-related information into an image, and outputs the reconstructed image data to the display device 9. Image reconstruction calculations include means for obtaining an original image from collected raw data (known means), extracting information about edges in the original image, and removing ringing caused by the information and the truncation frequency of the raw data. This is performed by means for obtaining a predicted image of the original image, and means for subtracting the predicted image from the original image to create a ringing-corrected image.

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

The sequence storage circuit 10 scans in accordance with instructions from the computer 13 in the sequence shown in FIG.

FIG. 2 is an explanatory diagram of waveforms when data is collected using a pulse sequence using the Fourier method. uniform static magnetic field
Under H ₀ , a 90 _x ° pulse RF is applied (Figure 2 B) while applying a slice gradient (Figure 2 B). As a result, only spins within a specific slice plane of the subject are selectively excited. Next, there is a rephase gradient (Figure 2 (b)) to recover the phase shift of the spins that occurred during slicing, a dephase gradient (Figure 2 (d)) to generate an echo signal later, and a warp gradient (Figure 2 (h)). ), then cut all slopes and apply 180 _y ° pulse RF. This results in
The spins are reversed, and by subsequent application of a readout gradient (FIG. 2D) a spin echo signal is obtained (FIG. 2E). This spin echo signal is a two-dimensional Fourier transform of the spin distribution of the subject.
corresponds to the line. This line is determined by the product of the warp gradient magnitude and the magnetic field application time. Thereafter, the above sequence is repeated with different gradients, and data necessary for image reconstruction is collected from a predetermined area.

Scan data from a predetermined area is amplified and phase detected, then converted into a digital signal and input to a computer (memory) 13. Calculator 13
In this step, image reconstruction is performed based on the flow shown in FIG. Before entering into the explanation of the flow shown in FIG. 3, the basis for obtaining a desired image through the flow will be explained. Ringing can be thought of as a convolution of the Fourier transform of a rectangular wave with the same width as the raw data sampling range and the image. It is clear that this is approximately equal to the truncation frequency because ringing can be removed very effectively by averaging processing having a width equal to one wavelength of the truncation frequency. Further, ringing occurs from all parts of the image, but where the density is uniform, the out-of-phase ringing cancels each other out due to interference, and as a result, only the ringing that occurs from the edge parts is visible. It is clear that the ringing intensity is proportional to the edge size because the convolution integral is a linear operation. Next, the flow shown in FIG. 3 will be explained. The original image is obtained by performing reconstruction processing using the scan data (raw data) using a normal (known) method. Only edge portions are extracted from the original image to create an edge image (the edge image is created, for example, by differential processing of the image. At this time, pre-processing and post-processing are performed as necessary). This edge image includes information regarding the position (direction, length) of the edge and the size of the step (edge strength). Next, a ringing image is created from this edge image and the high-frequency truncated spatial frequency of the raw data. What makes it possible to estimate the ringing image is that the period of the ringing that occurs when band-limited raw data is Fourier transformed corresponds to its truncation frequency, and that the ringing that occurs originates from the edges and its intensity is This is because it has a characteristic that it is approximately proportional to the edge strength and attenuates as the distance from the edge increases. The estimation of this ringing image is
This is possible regardless of where the original data of the image is obtained. For example, when the original data of the image uses the entire rectangular or square area (ringing appears anisotropic), or when the original data is cut out into a circle and reconstructed (ringing appears isotropic). The above estimation is possible in any case. After obtaining the ringing image in this manner, the ringing image is subtracted from the original image to obtain a ringing-corrected image. Therefore, ringing can be suppressed without deteriorating resolution.

Note that the present invention is not limited to the above embodiments. For example, edges may be detected using other known methods. Further, the processing shown in FIG. 3 may be used in combination with filtering processing for original data or image data.

(Effects of the Invention) As explained above, according to the NMR imaging apparatus of the present invention, information regarding edges in the original image obtained from the collected raw data is extracted, and the information and the truncation frequency of the raw data are combined. In addition to obtaining a predicted image of the resulting ringing, the method also subtracts the predicted image from the original image to create a ringing-corrected image, thereby creating an image in which ringing is suppressed while minimizing deterioration in spatial resolution. can be obtained and the image quality can be improved.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing an embodiment of the present invention, FIGS. 2 and 3 are diagrams illustrating the operation of the embodiment of the present invention, and FIG. 4 is a diagram illustrating a window function. 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 detection circuit,
9...Display device, 10...Sequence storage circuit,
11...A/D converter, 12...operation console, 13...calculator.

Claims (1)

[Claims] 1. A gradient magnetic field and high-frequency electromagnetic waves are applied to a subject placed in a static magnetic field according to a sequence based on the Fourier transform method, and scan data of the subject based on the nuclear magnetic resonance phenomenon is collected. Reconstruct the image
In the NMR imaging apparatus, means for extracting information regarding edges in the original image obtained from the collected raw data and obtaining a predicted image of ringing caused by the information and the truncation frequency of the raw data; and means for subtracting the predicted image to create a ringing-corrected image.