JP6157976B2 - Magnetic resonance imaging apparatus and method - Google Patents

Description

  The present invention relates to a magnetic resonance imaging apparatus and method, and more particularly to an image reconstruction technique thereof.

  A magnetic resonance imaging (hereinafter referred to as “MRI”) apparatus measures a nuclear magnetic resonance (NMR) signal generated by a nuclear spin constituting a subject, particularly a human tissue, It is an apparatus that images the form and function of the abdomen, limbs, etc. two-dimensionally or three-dimensionally. In imaging, the NMR signal is given different phase encoding depending on the gradient magnetic field and is frequency-encoded to be measured as time series data. The measured NMR signal is reconstructed into an image by two-dimensional or three-dimensional Fourier transform. In particular, in a measurement technique called a spin echo imaging sequence, signal measurement is repeatedly performed in a relatively long time, and an image is reconstructed by performing two-dimensional or three-dimensional Fourier transform on the signal that has been measured.

  For this reason, since the reconstruction process is usually performed after the measurement is started and all echo signals are acquired, it takes a long time to obtain the post-measurement image. On the other hand, Patent Document 1 describes a technique for creating and displaying an image every time an echo signal is acquired. Further, in Patent Document 2, every time an echo signal is acquired, a first-dimensional (frequency encoding direction) Fourier transform is performed, and processing after acquiring all echo data is only a two-dimensional (phase encoding direction) Fourier transform. A technique for shortening the time until an image after measurement is obtained is described.

JP 2008-55023 A Japanese Patent Laid-Open No. 3-12130

  However, in Patent Document 1, image reconstruction processing (two-dimensional Fourier transform) is performed for each echo signal, but the storage target and the addition target are Raw Data (echo signal group), and every time the echo signal is received, It is necessary to perform a two-dimensional Fourier transform on all received echo signals. For this reason, the time until an image equivalent to the conventional technique is acquired is not shortened. On the other hand, in Patent Document 2, the time required to acquire an image equivalent to the conventional technique can be shortened by the calculation time of the first-dimensional Fourier transform, but the time required for the second-dimensional Fourier transform is shortened. Absent.

  An object of the present invention is to provide an MRI apparatus and method that solves the above-described problems and realizes a reduction in time from completion of final echo acquisition to completion of image reconstruction.

  In order to achieve the above object, in the present invention, a magnetic field generator that generates a uniform static magnetic field in a space that accommodates the subject, a gradient magnetic field superimposed on the static magnetic field, and a high-frequency magnetic field that irradiates the subject; A measurement unit that measures a nuclear magnetic resonance signal generated from the subject as an echo signal, a signal processing unit that images the measured echo signal, a control unit that controls the magnetic field generation unit, the measurement unit, and the signal processing unit, The control unit performs a Fourier transform of only one received echo signal, holds the obtained converted data, and cumulatively adds the obtained converted data every time the reception of the echo signal is repeated, and performs image reconstruction processing. A magnetic resonance imaging apparatus configured to perform is provided.

  In order to achieve the above object, according to the present invention, there is provided a magnetic resonance imaging method comprising a uniform static magnetic field in a space accommodating the subject, a gradient magnetic field superimposed on the static magnetic field, and a high frequency applied to the subject. A magnetic field is generated, a nuclear magnetic resonance signal generated from the subject is measured as an echo signal, and when the measured echo signal is imaged, a Fourier transform of only one received echo signal is performed, and converted data obtained. And a magnetic resonance imaging method for performing image reconstruction processing by accumulating the obtained conversion data every time reception of an echo signal is repeated.

  According to the present invention, it is possible to provide an MRI apparatus and method that can obtain the effect of reducing the amount of calculation after the acquisition of final data and can reduce the time from the start of measurement to the completion of image reconstruction.

It is a figure for demonstrating an example to the whole structure of the MRI apparatus based on Example 1. FIG. It is a figure which shows the measurement flowchart in the MRI apparatus of Example 1. FIG. FIG. 7 is a diagram for explaining an image reconstruction sequence and a calculation amount after obtaining final data according to the first embodiment. It is a figure for demonstrating the calculation amount after the conventional image reconstruction sequence and final data acquisition. It is a figure for demonstrating the comparison of the computational complexity after the final data acquisition in Example 1 and a prior art.

  Embodiments of a magnetic resonance imaging (MRI) apparatus of the present invention will be described below with reference to the accompanying drawings. Note that components having the same function are denoted by the same reference symbols throughout the drawings for describing the embodiments of the present invention, and the repetitive description thereof is omitted.

  An overall outline of an example of the MRI apparatus according to each embodiment will be described with reference to FIG. FIG. 1 is a block diagram illustrating an example of the overall configuration of the MRI apparatus according to the present embodiment. This MRI apparatus uses a nuclear magnetic resonance (NMR) phenomenon to obtain a tomographic image of a subject. As shown in FIG. 1, the MRI apparatus includes a static magnetic field generation system 2, a gradient magnetic field generation system 3, A transmission system 5 that generates a high-frequency magnetic field that irradiates the subject, a reception system 6 that functions as a measurement unit that measures an NMR signal generated from the subject as an echo signal, a signal processing unit 7, a sequencer 4, and a central processing And a central processing unit (CPU) 8. The static magnetic field generation system 2, the gradient magnetic field generation system 3, and the transmission system 5 constitute a magnetic field generation unit.

  The static magnetic field generation system 2 generates a uniform static magnetic field in the direction perpendicular to the body axis in the space around the subject 1 if the vertical magnetic field method is used, and in the body axis direction if the horizontal magnetic field method is used. Thus, a permanent magnet type, normal conduction type or superconducting type static magnetic field generation source is arranged around the subject 1.

  The gradient magnetic field generation system 3 includes a gradient magnetic field coil 9 that applies gradient magnetic fields in the three-axis directions of X, Y, and Z, which are coordinate systems (stationary coordinate systems) of the MRI apparatus, and gradient magnetic fields that drive the respective gradient magnetic field coils. A gradient power supply Gx, Gy, Gz is applied in the X, Y, and Z triaxial directions by driving the gradient magnetic field power supply 10 of each coil in accordance with a command from the sequencer 4 described later. . At the time of imaging, a slice direction gradient magnetic field pulse (Gs) is applied in a direction orthogonal to the slice plane (imaging cross section) to set a slice plane for the subject 1, and the remaining two orthogonal to the slice plane and orthogonal to each other. A phase encoding direction gradient magnetic field pulse (Gp) and a frequency encoding direction gradient magnetic field pulse (Gf) are applied in one direction, and position information in each direction is encoded in the echo signal.

  The sequencer 4 is a control unit that repeatedly applies a high-frequency magnetic field pulse (hereinafter referred to as “RF pulse”) and a gradient magnetic field pulse in a predetermined pulse sequence. The sequencer 4 operates under the control of the CPU 8 and collects tomographic image data of the subject 1. Various commands necessary for the transmission are sent to the transmission system 5, the gradient magnetic field generation system 3, and the reception system 6. The sequencer 4 and the CPU 8 are collectively referred to as a control unit.

  The transmission system 5 irradiates the subject 1 with an RF pulse in order to cause nuclear magnetic resonance to occur in the nuclear spins of the atoms constituting the living tissue of the subject 1, and includes a high-frequency oscillator 11, a modulator 12, and a high-frequency amplifier. 13 and a high frequency coil (transmission coil) 14a on the transmission side. The RF pulse output from the high-frequency oscillator 11 is amplitude-modulated by the modulator 12 at a timing according to a command from the sequencer 4, and after the amplitude-modulated RF pulse is amplified by the high-frequency amplifier 13, the RF pulse is arranged close to the subject 1. By supplying to the high frequency coil 14a, the subject 1 is irradiated with the RF pulse.

  The receiving system 6 detects an echo signal (NMR signal) emitted by nuclear magnetic resonance of nuclear spins constituting the living tissue of the subject 1. The receiving system 6 receives a high-frequency coil (receiving coil) 14 b on the receiving side and a signal amplifier 15. And a quadrature phase detector 16 and an A / D converter 17. After the NMR signal of the response of the subject 1 induced by the electromagnetic wave irradiated from the high frequency coil 14a on the transmission side is detected by the high frequency coil 14b arranged close to the subject 1 and amplified by the signal amplifier 15, The signals are divided into two orthogonal signals by the quadrature phase detector 16 at a timing according to a command from the sequencer 4, converted into digital quantities by the A / D converter 17, and sent to the signal processing unit 7.

  The signal processing unit 7 performs various data processing and display and storage of processing results, and includes an external storage device such as an optical disk 19 and a magnetic disk 18 and a display 20 including a CRT. When data from the reception system 6 is input to the CPU 8, the CPU 8 executes processing such as signal processing and image reconstruction, and displays the tomographic image of the subject 1 as a result on the display 20 and an external storage device. On the magnetic disk 18 or the like. Note that the processing such as signal processing and image reconstruction may be realized by a dedicated circuit or the like provided separately from the CPU 8.

  The operation unit 25 inputs various control information of the MRI apparatus and control information of processing performed by the signal processing unit 7 and includes a trackball or mouse 23 and a keyboard 24. The operation unit 25 is arranged in the vicinity of the display 20, and the operator controls various processes of the MRI apparatus interactively through the operation unit 25 while looking at the display 20.

  In FIG. 1, the high-frequency coil 14a and the gradient magnetic field coil 9 on the transmission side are opposed to the subject 1 in the static magnetic field space of the static magnetic field generation system 2 into which the subject 1 is inserted. If the horizontal magnetic field method is used, the subject 1 is installed so as to surround it. The high-frequency coil 14b on the receiving side is installed so as to face or surround the subject 1.

  At present, the radionuclide to be imaged by the MRI apparatus is a hydrogen nucleus (proton) which is a main constituent material of the subject as widely used clinically. Information on the spatial distribution of the proton density and the spatial distribution of the relaxation time of the excited state is imaged, thereby imaging the form or function of the human head, abdomen, limbs, etc. two-dimensionally or three-dimensionally.

  In the first embodiment, a magnetic field generator that generates a uniform static magnetic field in a space that accommodates the subject, a gradient magnetic field superimposed on the static magnetic field, and a high-frequency magnetic field that irradiates the subject, and a nuclear magnetism generated from the subject A measurement unit that measures the resonance signal as an echo signal, a signal processing unit that images the measured echo signal, and a control unit that controls the magnetic field generation unit, the measurement unit, and the signal processing unit. The present invention relates to a magnetic resonance imaging apparatus configured to perform Fourier transform of only one echo signal, hold the obtained conversion data, and cumulatively add the obtained conversion data and perform image reconstruction processing every time reception of the echo signal is repeated. Is.

  In a preferred configuration of the present embodiment, the control unit performs a Fourier transform in the frequency encoding direction of one echo signal as a Fourier transform, and a Fourier transform in the phase encoding direction for the obtained converted data (1 × frequency encoding number M). By performing the above, control is performed so as to obtain intermediate data. The Fourier transform in the frequency encode direction is a fast Fourier transform, and the Fourier transform in the phase encode direction is a discrete Fourier transform.

  The MRI apparatus according to the first embodiment will be described with reference to a measurement flowchart executed by the MRI apparatus in FIG. 2 and an image reconstruction sequence in FIG. 3 and a diagram for explaining a calculation amount after obtaining final data. Hereinafter, description will be made with reference to the flowchart of FIG. FIG. 3 schematically shows an image reconstruction processing sequence according to the present embodiment. For comparison, FIG. 4 schematically shows a conventional image reconstruction processing sequence. FIG. 3A shows a conversion process repeated every time an echo signal is received, intermediate data obtained as a result, and an image after final data acquisition obtained by accumulating the intermediate data. FIG. 3B shows a one-dimensional fast Fourier transform (1DFFT) in the frequency encoding direction, a one-dimensional discrete Fourier transform (1DDFT) in the phase encoding direction, and addition with intermediate data up to the previous time, each time an echo signal is acquired. Indicates processing. FIG. 3C schematically shows processing of a one-dimensional discrete Fourier transform (1DDFT) in the phase encoding direction.

(Step 101)
First, measurement is performed in which signal measurement is repeated in a relatively long time, such as a spin echo imaging sequence. When the echo signal is received, the process proceeds to step 102.

(Step 102)
A one-dimensional fast Fourier transform (1DFFT) in the frequency encoding direction of only one acquired echo signal is performed, and a result of 1 × frequency encoding number M is acquired. The calculation amount of the one-dimensional fast Fourier transform (1DFFT) in the frequency encoding direction is the number of multiplications M / 2 * log (M) and the number of additions M * log (M).

(Step 103)
The one-dimensional discrete Fourier transform (1DDFT) of each point in the phase encoding direction is performed on the result of the one-dimensional fast Fourier transform (1DFFT) in the frequency encoding direction in step 102, and an image of phase encoding number N × frequency encoding number M is performed. Acquire data as intermediate data. At this time, the data value in the phase encoding direction other than that point is regarded as 0, and the data value 0 is not calculated.

  Usually, the one-dimensional discrete Fourier transform (1DDFT) is given by the following equation.

N = number of phase encodes (number of pixels in the phase encode direction),
k = 0, 1, 2,..., N−1 (array of pixels in the phase encoding direction),
If n = 0, 1, 2,..., N−1 (data array in the phase encoding direction), the data in the phase encoding direction obtained by one echo is only one point, and phase encoding directions other than the one point are obtained. The data value of can be regarded as zero.

  In this case, except for one point, x (n) becomes 0 and no addition is required. Therefore, a one-dimensional discrete Fourier transform (1DDFT) in one phase encoding direction can obtain an equivalent result by the following equation.

k = 0, 1, 2,..., N-1 (image pixel array), n _an = data array order in the phase encoding direction (row number: constant)
As a result, the amount of calculation per pixel becomes one multiplication, and the number of multiplications in the one-dimensional discrete Fourier transform of one point of the data of one echo is N times because it is divided into pixels of the number N of phase encodes in the phase encoding direction. It becomes. Since this is performed at each point of the data of one echo (the number of frequency encodings is M), the amount of calculation is the number of multiplications M * N.

(Step 104)
In the case of the second and subsequent echoes, the intermediate data (phase encoding number N × frequency encoding number M) stored in step 105 is added to the intermediate data (phase encoding number N × frequency encoding number M) obtained in step 103. to add.

  Since each pixel of phase encoding number N × frequency encoding number M is added, the amount of calculation is the number of additions N * M.

(Step 105)
The result of step 104 is stored as new intermediate data.

  From the above, the amount of calculation up to step 104 is the number of multiplications M / 2 * log (M) + M * N and the number of additions M * log (M) + N * M, which is the amount of calculation required for each echo. . However, it is a condition that the processing for each echo so far is completed within one iteration. For example, in the case of a spin-echo imaging sequence, this condition can be satisfied, and it is possible to repeat reception of echo signals as a spin-echo imaging sequence and perform image reconstruction processing using the repetition time. it can.

(Step 106)
If the acquisition of all echo signals is completed and the measurement is completed, the routine proceeds to step 107. If all the echo signals are not acquired and the measurement is not completed, the process proceeds to step 101.

(Step 107)
The final added data obtained as a result of step 104 is displayed as a reconstructed image, and the measurement is terminated.

  According to the first embodiment described above, when the number of phase encodings is N and the number of frequency encodings is M, the processing after obtaining the final echo is a one-dimensional fast Fourier transform in the frequency encoding direction of only one echo (calculation amount: number of multiplications M / 2). * Log (M), number of additions M * log (M)), and one-dimensional discrete Fourier transform (computation amount: number of multiplications M * N) of each point in the phase encoding direction for the result and intermediate data up to the previous time Addition (calculation amount: number of additions M * N).

  On the other hand, in the image reconstruction processing sequence of the prior art shown in FIG. 4, a first-dimensional (frequency encoding direction) Fourier transform is performed every time an echo signal is acquired, and a two-dimensional (phase encoding direction) Fourier is obtained after acquiring all data. In the case of performing the conversion, as shown in FIG. 4B, after obtaining the final RAW data, a one-dimensional fast Fourier transform in the frequency encoding direction for one echo is performed and placed in the memory. Since the one-dimensional fast Fourier transformation in the phase encoding direction is performed on the one-dimensional fast Fourier transform result in the frequency encoding direction arranged in the memory for each echo, the amount of calculation after obtaining the final echo is the number of multiplications (M / 2) * log (M) + (M * N) / 2 * log (N), the number of additions M * log (M) + M * N * log (N). In the configuration of this embodiment, the number of multiplications is M / 2 * log (M) + M * N, and the number of additions is M * log (M) + M * N. Therefore, the number of multiplications / additions can be reduced as compared with the prior art. Can do.

  FIG. 5 shows a comparative example of the calculation amount after the final data acquisition in the configuration of the present embodiment and the prior art. The left side of the figure shows the amount of multiplication, and the right side shows the amount of addition. Both show the amount of calculation after the final data acquisition when the frequency encoding number M is fixed to 256 and the phase encoding number N is changed from 0 to 256. The dotted line is a conventional example, and the solid line is an example. This case is shown.

  As can be seen from the figure, by reducing the amount of calculation by the configuration of the present embodiment, it is possible to shorten the time required to acquire an image equivalent to the conventional technique. Further, since the Fourier transform in the phase encoding direction does not use the fast Fourier transform (FFT), the number of data in the phase encoding direction is determined according to the region of interest (ROI) without being restricted by the number of data of 2n. By making it arbitrarily determinable, it is possible to eliminate unnecessary repetition and to shorten the measurement time. Furthermore, since 1TR (repetition time) is required to acquire one row of data, the number of data in the phase encoding direction N × TR becomes the measurement time, and the number of data in the phase encoding direction can be arbitrarily determined according to the region of interest. Since unnecessary repetition is not necessary, measurement time can be shortened.

  Next, Example 2 will be described. The difference from the first embodiment is application to multi-slice and multi-echo measurement. Hereinafter, only different portions will be described, and description of the same portions will be omitted. In the first embodiment, the multi-slice and the multi-echo measurement are not mentioned. However, in the multi-slice and the multi-echo measurement, if there is an echo interval such that the processing for each echo of the first embodiment is completed, the multi-slice and the multi-echo It can be applied to echo measurement, and further image reconstruction speed can be increased.

  According to the second embodiment, an effect of reducing the amount of calculation proportional to the number of multi-slices and the number of multi-echoes can be obtained, and the image reconstruction can be further speeded up.

    Next, Example 3 will be described. The difference from the first and second embodiments is parallel processing of image reconstruction. Hereinafter, only different portions will be described, and description of the same portions will be omitted. In the second embodiment, an echo interval is required to complete the processing for each echo of the first embodiment in multi-slice and multi-echo measurement, but each echo has an independent data holding unit and calculation processing unit. Thus, calculation is possible independently for each echo acquired at the same slice and the same echo time (TE), and the application conditions of the multi-slice and multi-echo measurement of the second embodiment are the application conditions of the first embodiment. It can be applied by satisfying the repetition time such that the processing for each echo is completed.

  According to the third embodiment, the application conditions of multi-slice and multi-echo measurement are relaxed, the present invention can be applied to a larger number of slices and multi-echo, and further image reconstruction speed can be increased.

  Although various embodiments of the present invention have been described above, the present invention is not limited to the above-described embodiments, and includes various modifications. For example, the above-described embodiment has been described in detail for better understanding of the present invention, and is not necessarily limited to a two-dimensional space. When applied to a three-dimensional space, the same applies to another one-dimensional space. Can be applied, and the amount of calculation can be further reduced.

  Further, a part of the configuration of a certain embodiment can be replaced with the configuration of another embodiment, and the configuration of another embodiment can be added to the configuration of a certain embodiment. It is also possible to add, delete, and replace other configurations for a part of.

  Further, the above-described configuration, function, processing unit, and the like have been described as an example of creating a program that realizes part or all of them. Needless to say, it can be realized with this.

DESCRIPTION OF SYMBOLS 1 Subject 2 Static magnetic field generation system 3 Gradient magnetic field generation system 4 Sequencer 5 Transmission system 6 Reception system 7 Signal processing part 8 Central processing part (CPU)
9 Gradient Magnetic Field Coil 10 Gradient Magnetic Field Power Supply 11 High Frequency Transmitter 12 Modulator 13 High Frequency Amplifier 14a High Frequency Coil (Transmission Coil)
14b High frequency coil (receiver coil)
15 Signal amplifier 16 Quadrature detector 17 A / D converter 18 Magnetic disk 19 Optical disk 20 Display 21 ROM
22 RAM
23 Trackball or mouse 24 Keyboard

Claims (10)

A magnetic field generator for generating a uniform static magnetic field in a space for accommodating the subject, a gradient magnetic field superimposed on the static magnetic field, and a high-frequency magnetic field applied to the subject;
A measurement unit that measures a nuclear magnetic resonance signal generated from the subject as an echo signal;
A signal processing unit for imaging the measured echo signal;
A control unit for controlling the magnetic field generation unit, the measurement unit, and the signal processing unit,
The controller is
Performs Fourier transform of only one received echo signal, retains the obtained intermediate data, and controls to perform image reconstruction processing by accumulating the obtained intermediate data each time reception of the echo signal is repeated.
A magnetic resonance imaging apparatus. The magnetic resonance imaging apparatus according to claim 1,
The controller is
As the Fourier transform, a Fourier transform in the frequency encoding direction of the one echo signal is performed, and a Fourier transform in the phase encoding direction is performed on the obtained converted data (1 × frequency encoding number M), thereby obtaining the intermediate data. obtain,
A magnetic resonance imaging apparatus. The magnetic resonance imaging apparatus according to claim 2,
The Fourier transform in the frequency encoding direction is a fast Fourier transform, and the Fourier transform in the phase encoding direction is a discrete Fourier transform.
A magnetic resonance imaging apparatus. The magnetic resonance imaging apparatus according to claim 2,
The controller is
The number of transform data for performing Fourier transform in the phase encoding direction can be arbitrarily determined according to the region of interest.
A magnetic resonance imaging apparatus. The magnetic resonance imaging apparatus according to claim 1,
The controller is
Repetitive reception of the echo signal is performed as a spin echo imaging sequence, and image reconstruction processing is performed using the repetition time.
A magnetic resonance imaging apparatus. A magnetic resonance imaging method comprising:
A uniform static magnetic field in a space accommodating the subject, a gradient magnetic field superimposed on the static magnetic field, and a high-frequency magnetic field applied to the subject,
Measure the nuclear magnetic resonance signal generated from the subject as an echo signal,
When imaging the measured echo signal,
Performs Fourier transform of only one echo signal received, holds the obtained conversion data,
Every time reception of the echo signal is repeated, cumulative conversion of the obtained conversion data is performed to perform image reconstruction processing.
A magnetic resonance imaging method. The magnetic resonance imaging method according to claim 6,
As the Fourier transform, a Fourier transform in the frequency encoding direction of the one echo signal is performed, and a Fourier transform in the phase encoding direction is performed on the obtained converted data (1 × frequency encoding number M), thereby obtaining the intermediate data. obtain,
A magnetic resonance imaging method. The magnetic resonance imaging method according to claim 7, comprising:
The Fourier transform in the frequency encoding direction is a fast Fourier transform, and the Fourier transform in the phase encoding direction is a discrete Fourier transform.
A magnetic resonance imaging method. The magnetic resonance imaging method according to claim 7, comprising:
The number of transform data for performing the Fourier transform in the phase encoding direction can be arbitrarily determined according to the region of interest.
A magnetic resonance imaging method. The magnetic resonance imaging method according to claim 6,
Repetitive reception of the echo signal is performed as a spin echo system imaging sequence, and image reconstruction processing is performed using the repetition time.
A magnetic resonance imaging method.

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