JP2889871B1 - Magnetic resonance diagnostic equipment - Google Patents

Abstract

An object of the present invention is to accurately extract only information reflecting a temperature change inside a subject. An object of the present invention is to apply a high-frequency magnetic field and a gradient magnetic field to a subject placed in a uniform static magnetic field, and to perform magnetic resonance from a chemical group whose chemical shift in the subject shows temperature dependence. In a magnetic resonance diagnostic apparatus that collects signals, a computer system 12 that calculates the temperature inside a subject for each localized area from a magnetic resonance signal and evaluates the accuracy of temperature measurement based on the time change of the amplitude value of the magnetic resonance signal is used. Have.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a magnetic resonance phenomenon for assisting diagnosis of a disease manifesting a thermal disorder and for monitoring a rise in temperature inside a subject when collecting images and spectral data. The present invention relates to a magnetic resonance diagnostic apparatus that obtains a physical state inside a subject, in particular, a temperature distribution inside the subject using the method, and displays the acquired temperature distribution.

[0002]

2. Description of the Related Art Generally, an MRI (Magnetic R)
With a magnetic resonance diagnostic apparatus commonly referred to as “esonance imaging,” it is possible to diagnose a lesion that has been difficult to depict by X-ray CT. However, since diagnosis is mainly based on morphological images obtained, depending on the disease,
It is not unusual for a lesion to be detected for the first time in a serious condition where the condition has worsened.

On the other hand, recently, attempts have been made to obtain functional images and metabolic images in a living body using MRI. Brain function MRI, which detects a change in cerebral blood flow due to stimulation in order to evaluate brain function, is a prime example, and an early diagnosis of ischemic disease has been attempted by using diffusion-weighted images.

[0004] On the other hand, since the temperature in a living body reflects many physiological functions, it has been pointed out that a disease can be diagnosed early by non-invasively measuring the temperature.
In addition to blood circulation disorders, pain and inflammatory diseases, the temperature distribution in the living body differs from the normal state depending on the tumor.
There is a possibility that the disease can be diagnosed earlier than the diagnosis using the morphological image acquired by I or the like.

In order to non-invasively measure a temperature change in a living body, various image measurement methods have been tried. A method using a relaxation time and a diffusion coefficient as temperature-dependent parameters that can be measured by X-ray CT, ultrasonic waves, microwaves, and MRI has been proposed. However, it has been shown that it is difficult to obtain an accurate temperature distribution because the temperature dependence of physical parameters measured by these methods differs depending on the substance.

Recently, the temperature dependence of chemical shifts has attracted attention as a temperature-dependent parameter that can be measured by MRI. In particular, since the chemical shift of protons in water shows almost constant temperature dependence (-0.01 ppm / ° C) regardless of the substance, it is not necessary to acquire a temperature calibration curve for each substance in advance, and it is not practical. (Y.I.).
Shihara et al. Proc. 11th
Ann. Scientific Meeting SM
RM, 4803, 1992). The living body is about 60%
It is also advantageous in terms of signal detection sensitivity because it is composed of water.

As a typical example of this method, a FE (Field Echo) pulse sequence shown in FIG. 8 has been proposed. From two phase images θ collected immediately before and after a temperature change process, The temperature change ΔT of both can be calculated according to the following equation. The suffix “after” indicates after a temperature change, and “before” indicates before a temperature change. ΔT (r) = T (r) after−T (r) before = {θ (r) after · θ (r) before)} / α · γ · τ · B0 Equation (1) r: space vector α: Constants related to the temperature dependence of water proton chemical shift γ: Nuclear magnetic rotation ratio τ: Echo time B0: Static magnetic field strength Based on the same principle, in order to speed up the measurement, orthogonal excitation is performed using a local excitation method. A method of detecting a signal from a columnar area limited with respect to two axes and obtaining a temperature change image of a spatial one-dimensional distribution has also been attempted.

In the above equation (1), the temperature change is calculated based on the phase difference because θ (τ) contains information on chemical shifts of water protons accompanying the temperature change and the non-uniformity of the magnetic field at each position. This is because the phase proportional to is added, and the effect of the magnetic field inhomogeneity is removed.

Such information on the relative temperature change is obtained by the method described in Japanese Patent Application No. 8-80290 in the case of diagnosing a disease based on the temperature change when a thermal load is applied, or in the case of hyperthermia. It is reported to be very useful as a temperature monitor at the time of performing the measurement and a heat generation monitor in a living body due to a high-frequency magnetic field at the time of using a magnetic resonance diagnostic apparatus.

[0010]

In the above method, a phase change based on a temperature change in the target area is detected.
When the temperature change in the measurement region is uniform, the magnetization at each position in the region receives a uniform phase change, so that an accurate temperature change can be measured. However, when a temperature distribution occurs within one voxel of the measurement region, that is, when the temperature variation is large within one voxel, a situation occurs in which the magnetization phases vary within one voxel and cancel each other out.
There has been a problem that the phase change due to this lowers the accuracy of the temperature measurement.

As described above, since it is necessary to perform a process of subtracting the two phase images in order to remove the influence of the magnetic field inhomogeneity inside the subject, the two phase image data for which the difference process is performed are required. During the acquisition of factors other than temperature change, for example, a change in magnetic field inhomogeneity due to movement of the subject, or a change in phase information due to blood flow, as if a very large temperature change It was measured as if it had occurred, which was an obstacle in performing a diagnosis or monitoring a temperature change. The present invention has been made in view of such a point, and an object of the present invention is to accurately extract only information reflecting a temperature change inside a subject.

[0012]

According to the magnetic resonance diagnostic apparatus of the present invention, a high-frequency magnetic field and a gradient magnetic field are applied to a subject placed in a uniform static magnetic field, and a chemical reaction in the subject is performed. In a magnetic resonance diagnostic apparatus for collecting a magnetic resonance signal from a chemical group whose shift indicates temperature dependence, means for calculating a temperature inside a subject for each localized area from the magnetic resonance signal, and an amplitude value of the magnetic resonance signal Means for evaluating the accuracy of temperature measurement based on a time change of the temperature.

According to a second aspect of the present invention, when the time change of the amplitude value of the magnetic resonance signal is larger than a predetermined threshold value,
There is further provided means for reducing the local area. As described in claim 3, the temperature change of the amplitude value is given based on the amplitude value estimated from the temperature dependence of the relaxation time of the target nuclide.

According to a fourth aspect of the present invention, the temperature change of the amplitude value is obtained by changing the amplitude value of the magnetic resonance signal acquired at the same echo time as the acquired magnetic resonance signal by the spin echo method under a steady state. Is given on a standard basis.

According to a fifth aspect of the present invention, a specific localized area in which the time change of the amplitude value of the magnetic resonance signal is larger than a predetermined threshold is excluded from the calculation and / or output of the temperature or the temperature difference. Means are further provided.

[0016] As described in claim 6, the calculating means includes:
A temperature or a difference between temperature changes at a plurality of positions is calculated from the magnetic resonance signal. (Operation) According to the first aspect, the accuracy of the temperature measurement can be evaluated based on the time change of the amplitude value of the magnetic resonance signal.

According to the present invention, when the temporal change of the amplitude value of the magnetic resonance signal is larger than a predetermined threshold value, the limited area is reduced to reduce a decrease in accuracy due to temperature variation in the limited area. Can be.

According to the third aspect, since the amplitude value estimated from the temperature dependence of the relaxation time of the target nuclide does not include the amplitude component due to the temperature variation in the limited area, sufficient evaluation accuracy can be obtained. it can.

According to the fourth aspect, the amplitude value of the magnetic resonance signal acquired in the same echo time as the acquired magnetic resonance signal by the spin echo method under a steady state has a variation in temperature within the limited area. Since the amplitude component due to is canceled, sufficient evaluation accuracy can be obtained.

According to the fifth aspect, the specific limited area in which the time change of the amplitude value of the magnetic resonance signal is larger than the predetermined threshold value is excluded from at least one of the calculation and the output of the temperature or the temperature difference. Therefore, for example, a region where the phase value changes due to the influence of the blood flow or cerebrospinal fluid, or a region where the change in the magnetic field inhomogeneity due to the movement of the subject appears remarkably, that is, a region that is considered to contain a lot of errors is output And the possibility of misdiagnosis can be eliminated.

[0021]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, a magnetic resonance diagnostic apparatus according to the present invention will be described in detail with reference to the drawings. FIG.
1 is a block diagram illustrating a configuration of a magnetic resonance diagnosis apparatus according to the present embodiment. In FIG. 1, a static magnetic field and a gradient magnetic field coil 2 and a shim coil 4 provided inside the static magnetic field magnet 1 provide a uniform static magnetic field and three orthogonal axes (x,
y, x) and a linear gradient magnetic field. The gradient coil 2 is driven by a gradient coil power supply 5, and the shim coil 4 is driven by a shim coil power supply 6. The probe coil 3 provided inside the gradient magnetic field coil 2 includes a high-frequency coil and a tuning portion for tuning the resonance frequency of the high-frequency coil, and tunes at the resonance frequency of the target nuclide, for example, the proton of water. You can do it.

The transmitting section 7 supplies a high-frequency current pulse of the frequency to the probe coil 3 in order to generate a high-frequency magnetic field pulse of a frequency corresponding to the resonance frequency of the target nuclide from the probe coil 3. The receiving unit 9 receives a magnetic resonance signal generated from the magnetization spin of the target nuclide in the subject via the probe coil 3, amplifies the signal, and detects the signal. The probe coil 3 may be used for both transmission and reception, or may separately include a transmission-only coil and a reception-only coil. The data collection unit 11 converts the magnetic resonance signal received by the reception unit 9 into a digital signal,
Transfer to

The sequence control section 10 executes a gradient magnetic field coil power supply 5, a shim coil power supply 6, a transmission section 7, a reception section 9, and a data collection section 11 to execute a pulse sequence.
Control. The sequence controller 10 is controlled by the computer system 12. The computer system 12 is controlled by a command from the console 13.

The computer system 12 includes a data collection unit 11
A process such as Fourier transform is applied to the magnetic resonance signal from the computer, and the temperature is calculated for each limited area (voxel) based on the result. This data is sent to the image display 14 and displayed.

Further, the computer system 12 evaluates the accuracy of the temperature measurement, changes the size of the voxel based on the evaluation result, and excludes voxels with inaccuracy from calculation or output of the calculation result. Has functions. Hereinafter, these functions will be described.

FIG. 2 shows the procedure of the former function, that is, the function of evaluating the accuracy of temperature measurement and changing the size of the voxel based on the evaluation result. here,
A case where a one-dimensional temperature change in space is measured will be described as an example.

First, in the procedure (a), the pulse sequence shown in FIG. 3 is executed in a steady state. In the pulse sequence of FIG. 3, as shown in the equation (1), the read gradient magnetic field Gr is used to encode the temperature information into the magnetization phase.
The application timing of (X) is shifted by τ (= tl−t2) compared to the case of measuring a normal spin echo signal. During the period from the excitation pulse (90 °) to the acquisition of the echo, the signal is mainly T2 due to the inhomogeneity of the magnetic field.
Attenuates due to the effect of ^* . Since the attenuation of the signal value also occurs when a non-uniform temperature distribution occurs in the voxel, if the temperature distribution exists in the voxel,
In addition to the originally existing magnetic field inhomogeneity, a T2 ^* effect appears due to this temperature distribution inhomogeneity. Next, in step (b), the amplitude value Sr of the magnetic resonance signal in the steady state is calculated and set as an initial value.

Then, the pulse sequence is executed in the procedure (c), the amplitude St is calculated in the procedure (d) from the magnetic resonance signal obtained thereby, and then the change in the amplitude from the initial value is calculated in the procedure (e). Is obtained from “Sr / St” or “| Sr−St |”.

Here, if the above-mentioned non-uniformity of the temperature distribution occurs within one voxel, the accurate temperature distribution cannot be measured. Therefore, from the viewpoint of diagnosis or temperature change monitoring, the temperature is accurately measured. It is necessary to evaluate whether the change has been measured. When the temperature in one voxel is uniform, an accurate temperature change can be detected from the phase of the magnetization, and the amplitude value of the magnetization becomes the same as in the steady state regardless of the presence or absence of the temperature change.

In consideration of this point, in the procedure (f), when the amplitude value in the course of the measurement changes greatly from the steady state, that is, when the parameter J indicates the threshold value R or less, 1 is set.
It can be evaluated that the temperature in the voxel is not accurately measured. This is because when the temperature is extremely varied in one voxel, the phases cancel each other out in one voxel, and the signal amplitude is significantly reduced.

In this case, in step (g), the voxel size is reduced by a predetermined size, and the same measurement is repeated, whereby a more accurate measurement of the temperature change becomes possible. This is because the smaller the voxel size, the smaller the temperature variation within the small voxel.

In order to detect a change in the amplitude value of the signal, the difference from the steady state (Sr-St) or the ratio (S
r / St), but it is not necessary to fix the reference signal value to the signal amplitude (St) detected in the steady state. Can also be specified as

Here, as shown in FIG. 4C, the amplitude value Stn of the magnetization also changes due to the change in the relaxation time due to the temperature change. At the same timing (echo time) as in FIG.
The measurement is performed using the spin echo signal Srn measured in the normal spin echo pulse sequence (t3 + t4 = t1 + t2, t3 = t4) shown in FIG. 4B, and the change ratio (Srn + 1 / Srn) of this signal and FIG. a) The change ratio of the signal including the temperature change information measured in (a) (Stn + 1 / S
By comparing tn), it is possible to determine that accurate temperature measurement has not been performed when these ratios have changed beyond the threshold value.

Here, in FIG. 4, the amplitude value of the reference signal is fixed to the value in the steady state (Sr0, St0). However, it is not necessary to fix the reference signal value to the steady state. , Change the current time (Srn, Stn)
An earlier signal (Srn-1, Stn-1) can be designated as a reference. This method can perform a more accurate determination than the above-described method based on only the signal value obtained from the pulse sequence in FIG. 3, but the method shown in FIG. 4 measures the spin echo signal every time. There is a need to. For this reason, FIG.
As shown in FIG. 4, the temperature measurement pulse sequence of FIG. 4A and the reference value measurement pulse sequence of FIG.
It is conceivable to execute it as a set. This allows
Since the spin echo signal and the signal including the temperature change information can be measured substantially simultaneously, effective diagnosis and temperature change monitoring can be performed.

Next, FIG. 6 shows the procedure of the latter function, that is, the function of excluding voxels with inaccuracy from calculation or output of calculation results. Here, a case where a one-dimensional spatial temperature change is measured will be described as an example.

First, in step (a), the pulse sequence for temperature measurement shown in FIG. 3 is repeatedly executed a plurality of times in a steady state, and in step (b), phase value data is calculated for each voxel. Next, in step (c), phase values at different observation times are compared.

In the procedure (d), when the calculated difference value is larger than the preset threshold value K, the voxel includes a blood vessel or a portion having large original non-uniformity. Therefore, it is determined that the voxel is likely to cause erroneous measurement due to slight movement of the living body, and the position information of this voxel is detected and stored as mask data Rp.

In the temperature change measurement processing after step (e),
The voxel specified by the mask data Rp is not displayed (blanked) as a target for temperature measurement and output.
According to such a series of procedures, it is possible to prevent a situation in which a temperature change has not occurred, and a movement of a living body determines that a temperature change has occurred, thereby preventing an erroneous diagnosis.

In this method, however, temperature information cannot be obtained in a voxel in which the change in the magnetic field inhomogeneity due to the movement of the living body is remarkable. Therefore, as shown in FIG. In some cases, it is effective to specify voxels that are considered to be equally affected by a change in uniformity or a change in blood flow, and to calculate these differences. In particular, the above method is simple and effective when it is desired to detect a rise in temperature at a specified site, for example, when monitoring temperature for monitoring safety in a subject. The present invention can be implemented in various modifications without being limited to the embodiments described above.

[0040]

According to the present invention, the accuracy of the temperature measurement is evaluated based on the time change of the amplitude value of the magnetic resonance signal, and only the information reflecting the temperature change inside the subject can be accurately extracted. it can.

[Brief description of the drawings]

FIG. 1 is a configuration diagram of a magnetic resonance diagnostic apparatus according to an embodiment.

FIG. 2 is a diagram showing a procedure of a function of evaluating the accuracy of temperature measurement by the computer system of FIG. 1 and changing the size of a voxel based on the evaluation result.

FIG. 3 is a diagram showing a spatial one-dimensional temperature measurement pulse sequence used in the present embodiment.

FIG. 4 is a diagram showing a principle of compensating for a change in amplitude value caused by a relaxation time.

FIG. 5 is a diagram showing a pulse sequence for compensating for an amplitude value change caused by a change during relaxation and obtaining temperature change information.

FIG. 6 is a diagram showing a procedure of a function of excluding voxels having low accuracy from the calculation or the output of the calculation result by the computer system of FIG. 1;

FIG. 7 is a diagram showing the principle of a method for reducing the influence of erroneous measurement due to movement of a living body.

FIG. 8 is a diagram showing a conventional pulse sequence for temperature measurement.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Static magnetic field magnet, 2 ... Gradient magnetic field coil, 3 ... Probe coil, 4 ... Shim coil, 5 ... Gradient magnetic field coil power supply, 6 ... Shim coil power supply, 7 ... Transmission part, 9 ... Reception part, 10 ... Sequence control part, 11 ... data collection unit, 12 ... computer system, 13 ... console, 14 ... image display, 101 ... excitation high frequency pulse, 102 ... slice selection gradient magnetic field waveform, 103 ... readout gradient magnetic field waveform, 104 ... phase encoding gradient magnetic field waveform, 105: excitation high-frequency pulse, 106: inverted high-frequency pulse, 107: slice selection gradient magnetic field waveform, 108: read-out gradient magnetic field waveform, 109: magnetic resonance signal including temperature information.

Claims (6)

(57) [Claims] 1. An object placed in a uniform static magnetic field,
In a magnetic resonance diagnostic apparatus for applying a high-frequency magnetic field and a gradient magnetic field to collect a magnetic resonance signal from a chemical group whose chemical shift in the subject exhibits temperature dependence, the temperature inside the subject is limited from the magnetic resonance signal. A magnetic resonance diagnostic apparatus comprising: means for calculating for each region; and means for evaluating the accuracy of temperature measurement based on a temporal change in the amplitude value of the magnetic resonance signal. 2. The magnetic resonance diagnostic apparatus according to claim 1, further comprising means for reducing the localization area when a time change of the amplitude value of the magnetic resonance signal is larger than a predetermined threshold. . 3. The magnetic resonance diagnostic apparatus according to claim 1, wherein the temporal change in the amplitude value is given based on an amplitude value estimated from the temperature dependence of the relaxation time of the target nuclide. 4. The time-dependent change in the amplitude value is given based on the amplitude value of a magnetic resonance signal acquired at the same echo time as the magnetic resonance signal acquired by a spin echo method under a steady state. The magnetic resonance diagnostic apparatus according to claim 1, wherein: 5. The apparatus according to claim 1, further comprising a unit for excluding at least one of calculation and output of a temperature or a temperature difference with respect to a specific localized area in which a time change of an amplitude value of the magnetic resonance signal is larger than a predetermined threshold value. The magnetic resonance diagnostic apparatus according to claim 1, wherein: 6. The magnetic resonance diagnostic apparatus according to claim 1, wherein the calculating means calculates a temperature or a difference between temperature changes at a plurality of positions from the magnetic resonance signal.