JP2004248823A - Magnetic resonance imaging apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a magnetic resonance imaging apparatus which carries out the tomography of the subjects with a high accuracy utilizing the nuclear magnetic resonance. <P>SOLUTION: The fMRI unit 1000 for detecting the detection signals attributed to the nuclear magnetic resonance is provided with a magnetostatic field coil 100, an inclined magnetic field coil 102 and an RF coil 104 to apply the magnetic fields to the subject 10 and a tomography control part 200 which applies an oscillating magnetic field to the RF coil 104 to generate the tomographic images from the detection signals. The tomography control part 200 comprises a tuning value memory part 220 for storing the calibration value of the resonance frequency obtained by the tuning processing of the magnetic resonance imaging apparatus prior to the measurement of the tomographic images for the subject 10, a frequency shift level detection part 300 which detects the hourly frequency shift levels of the resonance frequency to update the calibration value and an image reconstruction part 260 for the reconstruction of the tomographic images based on the detection signals referring to the calibration value. <P>COPYRIGHT: (C)2004,JPO&NCIPI

Description

【００７６】
（２）信号増幅部２５０からの信号そのものからの検出
傾斜磁場の印加が行われていない場合の検出信号は、原理的にチューニング時の検出信号と等価であるため、この信号をフーリエ変換する（スペクトル解析する）ことによって、中心周波数を検出し、その移動量を求めることがきる。
【００７７】
なお、上記２つの方法とも、実際には、一回の周波数シフトの検出結果だけに基づくのでは、時間的に緩やかに変化する問題としている周波数シフトの影響と、突発的な体動による影響を分離できないため、少なくとも数回のシフト情報を利用して、緩やかに変化する成分を抽出することが望ましい。このような「緩やかに変化する成分の抽出」には、メディアンフィルタ処理や移動平均処理などを利用することができる。
【００７８】
［中心周波数シフト量と信号強度変化の関係］
以下では、画像データから中心周波数のシフト量を検出する場合について、中心周波数シフト量と信号強度変化の関係について説明する。
【００７９】
まず、図７は、食用のハムを測定試料として、意図的に中心周波数ｆ_０にオフセットをかけた場合の信号強度の変化を示す図である。
【００８０】
ハムは、水分と脂肪分がＺ軸方向に均一に分布していると考えられるので、断層面での画像シフト量を正確に求めやすいと考えられる。
【００８１】
図７では、Ｆａｔ−ｓａｔパルスが付加されている場合（図中、ＦＳ（＋））では、中心周波数ｆ_０のオフセットに応じて、信号強度が大きく変化しているのに対し、Ｆａｔ−ｓａｔパルスが付加されていない場合（図中、ＦＳ（−））では、中心周波数ｆ_０のオフセットに応じて、信号強度はほとんど変化していない。
【００８２】
また、図７において、マイナス側にオフセットをかけているのは、中心周波数ｆ_０が上昇することに相当するが、Ｆａｔ−ｓａｔパルスが付加されている場合（図中、ＦＳ（＋））では、中心周波数ｆ_０が上昇の初期では、わずかに信号強度が減少し、そののち、信号強度が増加していることが分かる。
【００８３】
次に、図８は、Ｆａｔ−ｓａｔパルスが付加されている場合（図中、ＦＳ（＋））と付加されていない場合（図中、ＦＳ（−））とで、スキャン回数に対する、相対信号強度（図中、ＦＳ（＋）＿ｓｉｇｎａｌまたはＦＳ（−）＿ｓｉｇｎａｌ）とイメージシフト量（図中、ＦＳ（＋）＿ｉｍａｇｅ ｓｈｉｆｔまたはＦＳ（−）＿ｉｍａｇｅ ｓｈｉｆｔ）の関係を示す図である。
【００８４】
図８で、多数回のスキャンを行なった場合に、Ｆａｔ−ｓａｔパルスがある場合とない場合で、信号の相対強度変化に傾向の差が見られる。
【００８５】
図９は、図８に示したイメージシフト量から換算した中心周波数ｆ_０のオフセットに基づいて図７に示した関係により予測される信号強度の変化と、図８に示した実測された信号強度の変化とを比較して示す図である。
【００８６】
図９より、予測される変化と実測値とが良く一致していることが分かる。
すなわち、中心周波数ｆ_０のシフトが、観測される信号レベルの変動とよく対応すると結論できる。
【００８７】
以上により、中心周波数ｆ_０の変動（たとえば、画像のシフト）をモニタしながら、中心周波数の変動値を相殺するように、チューニング時の中心周波数値を更新すれば、長時間にわたって、安定したｆＭＲＩ測定が可能である。
【００８８】
今回開示された実施の形態はすべての点で例示であって制限的なものではないと考えられるべきである。本発明の範囲は上記した説明ではなくて特許請求の範囲によって示され、特許請求の範囲と均等の意味および範囲内でのすべての変更が含まれることが意図される。
【００８９】
【発明の効果】
以上説明したとおり、本発明の磁気共鳴画像化装置に従えば、中心周波数ｆ_０の変動をモニタしながら、中心周波数の変動値を相殺するように、チューニング時の中心周波数値を更新するので、ベースラインの変動を抑制しつつ、長時間にわたって、安定した磁気共鳴画像の測定が可能である。
【図面の簡単な説明】
【図１】本発明に係る磁気共鳴画像化装置の一例のｆＭＲＩ装置１０００の構成を示す機能ブロック図である。
【図２】ｆＭＲＩ装置１０００の処理の流れを説明するためのフローチャートである。
【図３】人体とファントムとで、観測される検出信号波形を示す図である。
【図４】周波数空間において、観測されるデータのトラジェクトリを示す図である。
【図５】図４に対応して再構成された画像空間を示す図である。
【図６】イメージシフト量と中心波数値ｆ_０のシフト量との関係の実測データである。
【図７】ハムを測定試料として、意図的に中心周波数ｆ_０にオフセットをかけた場合の信号強度の変化を示す図である。
【図８】Ｆａｔ−ｓａｔパルスが付加されている場合と付加されていない場合とで、スキャン回数に対する相対信号強度とイメージシフト量の関係を示す図である。
【図９】中心周波数ｆ_０のオフセットに基づいて予測される信号強度の変化と、実測された信号強度の変化とを比較して示す図である。
【図１０】核磁気共鳴現象を利用したＭＲＩ装置の構成を示す機能ブロック図である。
【図１１】ｆＭＲＩの測定シーケンスの概要を示すタイミングチャートである。
【符号の説明】
１０ 被験者、１２ 台部、１００ 静磁場コイル、１０２ 傾斜磁場コイル、１０４，１０６ ＲＦコイル、２００ 断層撮影制御部、２１０ 入力部、２２０ チューニング値記憶部、２３０ 制御部、２４０ ＲＦパルス送信部、２５０ 信号増幅部、２６０ 画像再構成部、２７０ 表示部、３００ 周波数シフト検出部、１０００ ｆＭＲＩ装置。[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a configuration of a magnetic resonance imaging (MRI) apparatus for performing tomography of a living body.
[0002]
[Prior art]
2. Description of the Related Art As a method for imaging a cross section of the brain or whole body of a living body, a magnetic resonance imaging method using a nuclear magnetic resonance phenomenon of an atom in a living body, particularly a hydrogen atom, has been used for clinical image diagnosis of humans.
[0003]
When applied to the human body, the magnetic resonance imaging has, for example, the following features as compared with “X-ray CT” which is a similar tomography in the human body.
[0004]
(1) An image having a concentration corresponding to the distribution of hydrogen atoms and the signal relaxation time (reflecting the strength of atomic bonding) can be obtained. For this reason, light and shade according to the difference in the properties of the tissue are exhibited, and the difference in the tissue is easily observed.
[0005]
(2) The magnetic field has no absorption by bone. Therefore, it is easy to observe a region surrounded by the bone (intracranial, spinal cord, etc.).
[0006]
(3) Since it does not harm the human body unlike X-rays, it can be widely used.
[0007]
Such magnetic resonance imaging utilizes the magnetic properties of hydrogen nuclei (protons) which are most contained in each cell of the human body and have the greatest magnetism. The motion of a spin angular momentum, which is responsible for the magnetism of hydrogen nuclei, in a magnetic field can be classically compared to the precession of a coma.
[0008]
In the following, the principle of nuclear magnetic resonance will be briefly summarized in this intuitive classical model for explaining the background of the present invention.
[0009]
The direction of the spin angular momentum of hydrogen nuclei as described above (the direction of the axis of rotation of the coma) is random in an environment without a magnetic field, but is oriented in the direction of the line of magnetic force when a static magnetic field is applied.
[0010]
When an oscillating magnetic field is further superimposed in this state, the frequency of the oscillating magnetic field becomes the resonance frequency f determined by the strength of the static magnetic field.₀= ΓB₀If / 2π (γ: coefficient specific to a substance), energy moves toward the nucleus due to resonance, and the direction of the magnetization vector changes (precession increases). When the oscillating magnetic field is turned off in this state, the precession returns to the direction in the static magnetic field while returning the inclination angle. An NMR signal can be obtained by externally detecting this process using an antenna coil.
[0011]
Such a resonance frequency f₀Means that the static magnetic field strength is B₀When (T), 42.6 × B₀(MHz).
[0012]
Further, in magnetic resonance imaging, it is also possible to visualize the active site of the brain in response to an external stimulus or the like by using the fact that a detected signal changes according to a change in blood flow. Such a magnetic resonance imaging method is particularly called fMRI (functional MRI).
[0013]
In the fMRI, an ordinary MRI apparatus equipped with hardware and software necessary for fMRI measurement is used as the apparatus.
[0014]
Here, the reason that a change in blood flow causes a change in NMR signal intensity is based on the fact that oxygenated and deoxygenated hemoglobin in blood have different magnetic properties. Oxygenated hemoglobin has the property of a diamagnetic substance and does not affect the relaxation time of hydrogen atoms in water around it, whereas deoxygenated hemoglobin is a paramagnetic substance and changes the surrounding magnetic field. Therefore, when the brain is stimulated and the local blood flow increases and the oxygenated hemoglobin increases, the change can be detected as an MRI signal. As the stimulus to the subject, for example, a visual stimulus or an auditory stimulus is used.
[0015]
FIG. 10 is a functional block diagram showing a configuration of an MRI apparatus utilizing such a nuclear magnetic resonance phenomenon.
[0016]
Referring to FIG. 10, the MRI apparatus includes a pedestal 12 for supporting the subject 10, a static magnetic field coil 100 for generating the above-described static magnetic field, and an observation section (slice) of the subject as described later. Gradient coil 102 for giving information of the position of the slice and the position in the slice to the observation signal, an RF coil 104 for outputting an electromagnetic wave for applying an oscillating magnetic field to the nucleus to be observed, and an atomic nucleus to be observed And a tomographic control unit 200 for controlling the coils 100 to 104 and generating a tomographic image based on the signal received by the RF coil 106. .
[0017]
Further, the tomography control unit 200 includes an input unit 210 for inputting an instruction or the like from a user and, based on the instruction from the input unit 210, for example, for each measurement for each subject 10, the RF coil 104 An adjustment value of the intensity of the applied electromagnetic field and the signal intensity detected by the RF coil 106 and a value f of a measured resonance frequency (hereinafter, referred to as a center frequency)₀And a tuning value storage unit 220 for storing a result of adjustment (calibration value) of the adjustment as a tuning value. Here, such tuning is performed for each subject 10 because the environment of the magnetic field in the coil is slightly changed by the subject 10, and therefore it is necessary to adjust the environment.
[0018]
The tomography control unit 200 further includes a control unit 230 for controlling the tuning operation and the measurement operation as described above, and an RF pulse for applying an RF pulse to the RF coil 104 under the control of the control unit 230. A transmitting unit 240, a signal amplifying unit 250 for amplifying a signal from the RF coil 106, information such as a calibration value of a center frequency stored in a tuning value storage unit 220, and a detection signal from the signal amplifying unit 250. By performing a Fourier transform process based on the above, an image reconstruction unit 260 for reconstructing a cross-sectional image to be observed and a cross-sectional image reconstructed based on information from the image reconstruction unit 260 are displayed. And a display unit 270 for performing the operation.
[0019]
Here, more specifically, the static magnetic field coil 100 is composed of, for example, four air-core coils, and generates a uniform magnetic field inside by combining the four air-core coils to give orientation to spins of hydrogen nuclei in the body of the subject 10. .
[0020]
The RF coil 104 emits high frequency to excite atomic nuclei in the body of the subject 10, and the RF coil 106 detects a detection signal (echo signal) caused by the generated nuclear magnetic resonance.
[0021]
The gradient magnetic field coil 102 includes three sets of gradient coils, not shown, of X, Y, and Z. The Z coil, when excited, inclines the magnetic field strength in the Z direction to limit the resonance surface. Immediately after the application of the magnetic field, a short-time gradient is applied to apply a phase modulation proportional to the Y coordinate to the detection signal (phase encoding). The X coil then applies a gradient at the time of data collection to make the detection signal proportional to the X coordinate. Gives a modulated frequency (frequency encoding).
[0022]
That is, when a high-frequency electromagnetic field having a resonance frequency is applied to the subject 10 in a state where a Z-axis gradient magnetic field is applied to a static magnetic field through the RF coil 104, hydrogen nuclei in a portion where the magnetic field strength is in a resonance condition are generated. , Are selectively excited and begin to resonate. Hydrogen nuclei in a portion that satisfies the resonance condition (for example, a slice of the subject 10 having a predetermined thickness) are excited, and the spins rotate together. When the excitation pulse is stopped, an electromagnetic wave emitted from the spinning spin induces a signal in the RF coil 106, and this signal is detected for a while. With this signal, the tissue containing water in the body of the subject 10 is observed. Then, in order to know the signal transmission position, a signal is detected by applying a gradient magnetic field of X and Y.
[0023]
The control unit 230 measures the detection signal while repeatedly providing the excitation signal, and the image reconstruction unit 260 reduces the resonance frequency to the X coordinate by the first Fourier transform calculation, and performs the Y coordinate on the second Fourier transform. Is restored to obtain an image, and the corresponding image is displayed on the display unit 270.
[0024]
The above description is essentially the same as the operation of a normal MRI apparatus.
Next, the operation sequence when operating as fMRI will be described in further detail.
[0025]
In order to capture an image of a brain function, a sequence called echo planar imaging (EPI) is employed.
[0026]
FIG. 11 is a timing chart showing an outline of such an fMRI measurement sequence.
[0027]
The same stimulation task is repeatedly given to the subject 10 at predetermined intervals at predetermined intervals. At this time, while the tomography continues in time series for the same section in the brain, a change in the EPI signal during the stimulation task period is detected in that section.
[0028]
Such a change in signal strength is caused by a change in blood flow in the brain, as described above. However, such a change in signal strength is of the order of several percent, and in order to obtain a sufficient SN ratio, the same stimulus task is repeated a plurality of times, and the change pattern of the EPI signal at each time is determined at the start of the task. And average. By such processing, a response in the brain to the stimulus is visualized. That is, it is possible to specify a site where activity is activated in the brain when performing a specific task.
[0029]
The principles of image generation by such echo planar imaging and clinical applications are described in Non-Patent Documents 1 and 2.
[0030]
On the other hand, as a result of observing using such an EPI sequence, fMRI requires a longer time scan than simply photographing a cross-sectional shape with NMR.
[0031]
However, if the measurement is repeated in the order of 10 minutes or 20 minutes after tuning is performed before the measurement, a phenomenon that the level of the signal baseline fluctuates is confirmed. Such fluctuations are called "low frequency drift". Here, the signal base line is, for example, a base level of a signal when a measurement sample is optimized with a calibration sample called a phantom. The effect of “low frequency drift” on fMRI measurement is described in Non-Patent Document 3.
[0032]
[Non-patent document 1]
Koichi Oshio, "EPI Revised", Journal of the Japan Society for Magnetic Resonance Medicine, Vol. 19, No. 1 (1999), p. 1-5
[0033]
[Non-patent document 2]
Kiyotaka Suzuki, "Echo Planar Imaging", Journal of the Japan Society for Magnetic Resonance Medicine, Vol. 19, No. 1 (1999), p. 7-18
[0034]
[Non-Patent Document 3]
Ann M. Smith, et al., "Investigation of Low Frequency Drift in fMRI Signal", NeuroImage 9, p. 526-533 (1999) Article ID nig. 1999.435
[0035]
[Problems to be solved by the invention]
As described above, since a signal change to be observed in fMRI is extremely weak, the existence of such a low-frequency drift defines a limit of measurement accuracy.
Nevertheless, there has been a problem that the cause of such low-frequency drift has not been clearly identified in the past, and as a result, sufficient measures have not been taken.
[0036]
The present invention has been made to solve the above-described problems, and an object of the present invention is to provide a magnetic recording apparatus capable of performing tomography of a measurement target with high accuracy using nuclear magnetic resonance. It is to provide a resonance imaging device.
[0037]
[Means for Solving the Problems]
In order to achieve such an object, the magnetic resonance imaging apparatus of the present invention detects a detection signal caused by nuclear magnetic resonance from a measurement target, and generates a tomographic image of the measurement target. In a magnetic resonance imaging apparatus, a static magnetic field applying means for applying a static magnetic field to the object to be measured, and, within a selected cross section of the object to be measured, the position information of the nucleus that emits the detection signal, A gradient magnetic field applying means for applying a magnetic field modulated as a detection signal has to the object to be measured, and applying an oscillating magnetic field to the object to be measured and detecting the detection signal from the object to be measured An oscillating magnetic field transmitting and receiving unit for providing the oscillating magnetic field to the oscillating magnetic field transmitting and receiving unit, and receiving the detection signal to generate the tomographic image. Before measuring the tomographic image of the measured object, a calibration value storage means for storing at least a calibration value of a resonance frequency obtained by a calibration process for the magnetic resonance imaging apparatus, and a time-dependent frequency of the resonance frequency Frequency shift amount detecting means for detecting a shift amount and updating the calibration value, and image reconstructing means for reconstructing the tomographic image based on the detection signal with reference to the calibration value. Including.
[0038]
Preferably, the frequency shift amount detecting means detects a temporal displacement of the tomographic image output from the image reconstructing means, and converts the positional shift into the frequency shift amount.
[0039]
Preferably, the frequency shift amount detecting means compares the centroid position of each of the plurality of tomographic images corresponding to the same cross-section reconstructed at different times, so that the positional shift of the tomographic image over time is compared. Is detected.
[0040]
Preferably, the frequency shift amount detecting means detects the frequency shift amount by measuring the detection signal in a state where the gradient magnetic field applying means is inactive.
[0041]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, embodiments of the present invention will be described with reference to the drawings.
[0042]
[Configuration and Operation of Magnetic Resonance Imaging Apparatus]
FIG. 1 is a functional block diagram showing a configuration of an fMRI apparatus 1000 as an example of a magnetic resonance imaging apparatus according to the present invention.
[0043]
In the configuration of the fMRI apparatus 1000 shown in FIG. 1, the same parts as those of the MRI apparatus shown in FIG. 10 are denoted by the same reference numerals, and description thereof will not be repeated.
[0044]
Note that the RF coil 104 and the RF coil 106 may be separate coils, or the same coil may perform both transmission and reception.
[0045]
The difference between the configuration of the fMRI apparatus 1000 shown in FIG. 1 and the configuration of the MRI apparatus shown in FIG. 10 is that the center frequency f is based on the signal from the signal amplification section 250 or the tomographic image from the image reconstruction section 260.₀Is provided with a frequency shift detecting unit 300 for detecting the amount of shift with time.
[0046]
The control unit 230 sets the center frequency f set in the tuning performed before the measurement on the subject 10 and stored in the tuning value storage unit 220.₀Is updated at any time based on the frequency shift amount detected by the frequency shift detection unit 300.
[0047]
A characteristic part of the present invention is that the center frequency f₀And the software for controlling the updating process performed by the control unit 230 as described above. However, a part or all of such processing may be performed by hardware to shorten the processing time.
[0048]
FIG. 2 is a flowchart for explaining the flow of processing of the fMRI apparatus 1000. It is assumed that the static magnetic field is applied by the static magnetic field coil 100 regardless of the presence or absence of the subject 10 in the measurement system.
[0049]
Referring to FIG. 2, when the process is started, an initial tuning process of the device is performed for each subject 10 (step S100), and control unit 230 detects the intensity of the electromagnetic field applied from RF coil 104 and the detection by RF coil 106. The adjusted value of the signal strength to be measured and the measured center wave value f₀Is stored in the tuning value storage unit 220 as a tuning value (step S102).
[0050]
Subsequently, when the measurement for the subject 10 is started (Step S104), the control unit 230 controls the gradient magnetic field coil 102 to output the electromagnetic wave from the RF pulse transmission unit 240 from the RF coil 104 (Step S108). . The signal from the subject 10 is received by the RF coil 106, amplified by the signal amplifier 250 (Step S110), and sent to the image reconstruction unit 260.
[0051]
In step S108, the detection signal is phase-modulated by changing the intensity of the magnetic field of the Y coil among the gradient magnetic field coils 102 a plurality of times in an equal manner, and in step S110, when the detection signal is received, By changing the magnetic field generated by the X coil of the gradient coil 102, frequency modulation is applied to the detection signal.
[0052]
The image reconstructing unit 260 reconstructs an image using Fourier transform processing and displays the image on the display unit 270 (step S112).
[0053]
Subsequently, the center frequency f₀Is detected (step S114), and when a frequency shift is detected (step S116), the control unit 230 sets the center frequency f stored in the tuning value storage unit 220.₀Is updated based on the detected frequency shift amount (step S118), and the process returns to step S108.
[0054]
On the other hand, if no frequency shift is detected in step S116, subsequently, control unit 230 determines whether or not measurement has been completed (step S120). If measurement is to be continued, the process returns to step S108. Then, if it is determined in the sequence that the measurement is to be terminated, the process is terminated.
[0055]
[Principle of low frequency drift reduction]
Next, a mechanism will be described in which the above-described problem of the fluctuation of the baseline level is eliminated by performing the processing described with reference to FIGS. 1 and 2.
[0056]
FIG. 3 is a diagram showing a detection signal waveform observed between a human body and a phantom.
3, the vertical axis represents the amplitude of the detection signal, and the horizontal axis represents the frequency. Note that the frequency decreases from the right side to the left side of the figure.
[0057]
In FIG. 3, a predetermined frequency width (corresponding to about 1.5 msec on the time axis) is excited corresponding to the selected slice.
[0058]
In FIG. 3, the first thing to note is that the center frequency value f₀Moves during the measurement period for some reason compared to the time of tuning, it is observed as an image displacement in MRI.
[0059]
Next, in the phantom, only the peak due to water hydrogen is observed, whereas in the human body, the peak due to hydrogen in fat as well as water hydrogen is due to the chemical shift due to water hydrogen. Signal on the low frequency side separately from the signal from Therefore, when measuring the human body, a process of adding a fat suppression pulse (Fat-sat pulse) and masking a frequency band (corresponding to about 10 msec on the time axis) indicated by oblique lines in FIG. 3 is performed. .
[0060]
In a signal from a human body, when a Fat-sat pulse is added, the center frequency f₀, The area surrounded by the peak waveform greatly fluctuates in the frequency band corresponding to the selected slice.
[0061]
On the other hand, when the Fat-sat pulse is not added to the signal from the human body or the signal waveform from the phantom, the center frequency f₀Does not change in the area surrounded by the peak waveform in the frequency band corresponding to the selected slice.
[0062]
Therefore, when a Fat-sat pulse is added as a signal from the human body, the center frequency f₀Assuming that there is a variation in the baseline, it is thought that the observed baseline variation can be explained. Conversely, if there is no Fat-sat pulse, it is expected that the fluctuation of the baseline will be small even with a signal from the human body.
[0063]
Next, the center wave value f₀A brief description will be given of the fact that when is moved, it is observed as an image displacement on the MRI imaging screen.
[0064]
FIG. 4 is a diagram showing a trajectory of data observed in a frequency space.
[0065]
In FIG. 4, the direction of the vertical axis indicates a change in phase, and the interval of the vertical axis indicates the inter-echo time.
[0066]
FIG. 5 is a diagram showing an image space reconstructed corresponding to FIG.
In the image space, the vertical axis direction is observed as a phase difference.
[0067]
At this time, the frequency per pixel in the image space is determined by the data acquisition bandwidth BW (phase) in the phase direction.
[0068]
The data acquisition bandwidth BW (phase) in the phase direction is expressed as follows.
BW (phase) = 1 / Inter echo time
At this time, the frequency per pixel (f_pix) Is as follows.
[0069]
f_pix= BW (phase) / (number of phase matrices)
Therefore, the image shift amount I_shiftIs as follows.
[0070]
I_shift= Δf₀/ F_pix
Where Δf₀Is the central wave value f₀Is the shift amount.
[0071]
FIG. 6 shows the image shift amount (ΔShift) and the central wave value f.₀Is actual measurement data of the relationship with the shift amount (Δf). The image shift amount and the variation of the center frequency are in direct proportion.
[0072]
Therefore, as a method of detecting the frequency shift by the frequency shift detection unit 300, for example, there is the following method.
[0073]
(1) Detection from image data
The amount of image shift is measured by detecting a positional shift between two images of the same slice based on the actual measurement values shown in FIG. 6, and is converted into a frequency shift amount.
[0074]
At this time, the method of determining the image shift amount is not particularly limited. For example, the movement amount of the imaging target can be obtained from the difference in the center of gravity Xc of the two binarized images represented by the following equation. .
[0075]
(Equation 1)

[0076]
(2) Detection from the signal itself from the signal amplification unit 250
Since the detection signal when no gradient magnetic field is applied is in principle equivalent to the detection signal at the time of tuning, this signal is subjected to Fourier transform (spectral analysis) to detect the center frequency. The amount of movement can be obtained.
[0077]
In addition, in the above two methods, in fact, based on only one frequency shift detection result, the influence of the frequency shift, which is a problem that changes slowly with time, and the influence of sudden body movement are considered. Since separation is not possible, it is desirable to extract a slowly changing component using at least several times of shift information. For such “extraction of slowly changing components”, a median filter process, a moving average process, or the like can be used.
[0078]
[Relationship between center frequency shift amount and signal strength change]
In the following, a description will be given of the relationship between the center frequency shift amount and the change in signal strength when detecting the shift amount of the center frequency from the image data.
[0079]
First, FIG. 7 shows that the center frequency f₀FIG. 6 is a diagram showing a change in signal strength when an offset is applied to the.
[0080]
Since it is considered that ham has a uniform distribution of water and fat in the Z-axis direction, it is considered that the amount of image shift on the tomographic plane can be easily obtained accurately.
[0081]
In FIG. 7, when a Fat-sat pulse is added (FS (+) in the figure), the center frequency f₀, The signal intensity greatly changes in accordance with the offset, whereas when the Fat-sat pulse is not added (FS (−) in the figure), the center frequency f₀, The signal strength hardly changes in accordance with the offset.
[0082]
In FIG. 7, the offset on the negative side is the center frequency f.₀Rise, but when a Fat-sat pulse is added (FS (+) in the figure), the center frequency f₀It can be seen that at the beginning of the rise, the signal strength slightly decreases, and then the signal strength increases.
[0083]
Next, FIG. 8 shows the relative signal with respect to the number of scans when the Fat-sat pulse is added (FS (+) in the figure) and when it is not added (FS (-) in the figure). It is a figure which shows intensity | strength (FS (+) _ signal or FS (-) _ signal in a figure) and image shift amount (FS (+) _ image shift or FS (-) _ image shift in a figure).
[0084]
In FIG. 8, when a large number of scans are performed, there is a difference in the relative intensity change of the signal between the presence and absence of the Fat-sat pulse.
[0085]
FIG. 9 shows the center frequency f converted from the image shift amount shown in FIG.₀FIG. 9 is a diagram showing a comparison between a change in signal strength predicted based on the relationship shown in FIG. 7 based on the offset shown in FIG. 7 and a change in signal strength actually measured shown in FIG. 8.
[0086]
From FIG. 9, it can be seen that the predicted change and the measured value match well.
That is, the center frequency f₀It can be concluded that the shifts correspond well with the observed signal level variations.
[0087]
From the above, the center frequency f₀If the center frequency value at the time of tuning is updated so as to cancel out the fluctuation value of the center frequency while monitoring the fluctuation (for example, the shift of the image), stable fMRI measurement can be performed for a long time.
[0088]
The embodiments disclosed this time are to be considered in all respects as illustrative and not restrictive. The scope of the present invention is defined by the terms of the claims, rather than the description above, and is intended to include any modifications within the scope and meaning equivalent to the terms of the claims.
[0089]
【The invention's effect】
As described above, according to the magnetic resonance imaging apparatus of the present invention, the center frequency f₀The center frequency value at the time of tuning is updated so that the fluctuation value of the center frequency is canceled while monitoring the fluctuation of the center frequency.Thus, stable measurement of the magnetic resonance image can be performed for a long time while suppressing the fluctuation of the baseline. It is possible.
[Brief description of the drawings]
FIG. 1 is a functional block diagram showing a configuration of an fMRI apparatus 1000 as an example of a magnetic resonance imaging apparatus according to the present invention.
FIG. 2 is a flowchart illustrating a flow of a process performed by the fMRI apparatus 1000.
FIG. 3 is a diagram showing detection signal waveforms observed between a human body and a phantom.
FIG. 4 is a diagram showing a trajectory of observed data in a frequency space.
FIG. 5 is a diagram showing an image space reconstructed corresponding to FIG. 4;
FIG. 6 shows the image shift amount and the center wave numerical value f.₀Is the actual measurement data of the relationship with the shift amount.
FIG. 7 shows that a center frequency f is intentionally measured using ham as a measurement sample.₀FIG. 6 is a diagram showing a change in signal strength when an offset is applied to the.
FIG. 8 is a diagram illustrating a relationship between a relative signal intensity and an image shift amount with respect to the number of scans when a Fat-sat pulse is added and when a Fat-sat pulse is not added.
FIG. 9 shows a center frequency f₀FIG. 6 is a diagram showing a comparison between a change in signal strength predicted based on the offset of the above and an actually measured change in signal strength.
FIG. 10 is a functional block diagram showing a configuration of an MRI apparatus using a nuclear magnetic resonance phenomenon.
FIG. 11 is a timing chart showing an outline of a measurement sequence of fMRI.
[Explanation of symbols]
10 subjects, 12 units, 100 static magnetic field coil, 102 gradient magnetic field coil, 104, 106 RF coil, 200 tomography control unit, 210 input unit, 220 tuning value storage unit, 230 control unit, 240 RF pulse transmission unit, 250 Signal amplification section, 260 image reconstruction section, 270 display section, 300 frequency shift detection section, 1000 fMRI apparatus.

Claims (4)

A magnetic resonance imaging apparatus for detecting a detection signal due to nuclear magnetic resonance from the measured object and generating a tomographic image of the measured object,
Static magnetic field applying means for applying a static magnetic field to the object to be measured,
Within the selected cross section of the object to be measured, gradient magnetic field applying means for applying to the object to be measured a magnetic field that is modulated so that the detection signal has position information of a nucleus that emits the detection signal,
Oscillating magnetic field transmitting and receiving means for applying an oscillating magnetic field to the object to be measured and detecting the detection signal from the object to be measured,
Providing the oscillating magnetic field to the oscillating magnetic field transmitting and receiving means, comprising a tomographic control means for receiving the detection signal and generating the tomographic image,
The tomographic control means,
Before the measurement of the tomographic image for the object to be measured, calibration value storage means for storing a calibration value of at least a resonance frequency obtained by a calibration process for the magnetic resonance imaging apparatus,
Frequency shift amount detecting means for detecting the frequency shift amount of the resonance frequency over time, and updating the calibration value,
Image reconstruction means for reconstructing the tomographic image based on the detection signal with reference to the calibration value. The magnetic resonance imaging apparatus according to claim 1, wherein the frequency shift amount detecting unit detects a temporal displacement of the tomographic image output from the image reconstructing unit and converts the positional shift into the frequency shift amount. The frequency shift amount detecting unit detects a temporal displacement of the tomographic image by comparing a center of gravity position of each of the plurality of tomographic images corresponding to the same section reconstructed at different times. The magnetic resonance imaging apparatus according to claim 2. The magnetic resonance imaging apparatus according to claim 1, wherein the frequency shift amount detecting unit detects the frequency shift amount by measuring the detection signal in a state where the gradient magnetic field applying unit is inactive.

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