JPH1043156A - Nuclear magnetic resonance imaging device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To know the temperature based on the heating of a gradient magnetic field coil without using any temperature measuring means equipped with a sensor such as thermocoupler or thermistor. SOLUTION: In order to photograph the tomographic image of a reagent A, the reagent A is sent into a gantry 3. Then, when an operator manipulates a console 1, sets photographic conditions and executes scan, an arithmetic circuit 7 provided at a system controller 4 calculates a calorific parameter on the set photographic conditions and a voltage generating means 8 generates a voltage corresponding to the calculated calorific parameter and supplies the generated voltage to a time constant circuit 9 having a time constant corresponding to temperature increase characteristics during the scan of a device and temperature decrease (radiation) characteristics during the stop of scan. The time constant circuit 9 outputs the temperature data of the device, these data are applied to a display 2, and temperature information is reported to the operator.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a nuclear magnetic resonance imaging apparatus for imaging a desired tomographic plane of a subject using nuclear magnetic resonance, and more particularly to a temperature rise caused by heat generated from a gradient magnetic field generating means. The present invention relates to a nuclear magnetic resonance imaging apparatus having a function capable of grasping without using a temperature measuring means.

[0002]

2. Description of the Related Art A nuclear magnetic resonance imaging apparatus (hereinafter referred to as MR)
The device I) calculates the density distribution, relaxation time distribution, and the like of nuclear spins at a desired examination site in a subject by performing arithmetic processing on a signal measured using a nuclear magnetic resonance phenomenon. The signal is arithmetically processed and displayed as an image as a tomographic image of the inspection site. The static magnetic field generating magnetic circuit that applies a uniform magnetic field to the subject, a gradient magnetic field coil that applies a gradient magnetic field, and the tissue of the subject are High-frequency pulse transmitting means for applying a high-frequency magnetic field to cause nuclear magnetic resonance in the nuclei of the constituent atoms, nuclear magnetic resonance signal detecting means for detecting a signal by the nuclear magnetic resonance, and an image using the nuclear magnetic resonance signal Computing means for performing a reconstruction computation.

Meanwhile, in recent MRI apparatuses, it has become indispensable to equip a two-dimensional TOF angiography measurement function for measuring and imaging a blood flow in addition to high-speed imaging and improvement in patient throughput. This type of angio measurement uses a gradient echo method that uses reversal of the gradient magnetic field for spin imaging, and uses a sequence that adds a rephase function, and uses an imaging method that scans for a long time with a very short repetition time and thin slice thickness. The intensity and the frequency of use of the gradient magnetic field used for this purpose are several times higher than the conventional imaging methods.

Therefore, the current applied to the gradient magnetic field coil also increases in proportion to the gradient magnetic field used, and the problem of heat is increasing. That is, when a large current flows through the gradient magnetic field coil, heat is generated by the resistance of the coil wire, and the temperature in the room where the MRI apparatus is arranged increases. When the gradient coil is driven continuously, the temperature of the gradient coil may rise to 100 ° C. or more. In such a case, there is a risk that the gradient coil may be burned. In addition, during the diagnosis of the subject, if the temperature in the room where the MRI apparatus is located reaches a temperature that causes the subject to feel uncomfortable,
Diagnosis must be discontinued to protect the patient.

On the other hand, in an MRI apparatus using a permanent magnet as a static magnetic field generating magnetic circuit, the intensity of the static magnetic field tends to change due to a change in ambient temperature. As described above, when the magnitude of the static magnetic field changes due to the influence of temperature, a gradient magnetic field generated by a gradient magnetic field coil is added to the static magnetic field to make the position correspond to the magnitude of the magnetic field. NM having this resonance frequency
An error occurs in the operation of detecting the R signal and specifying the position. Since this displacement of the position detection causes image distortion and blur, the permanent magnet type M
In the RI system, temperature control is performed to prevent temperature change,
The temperature of the magnet is always kept constant.

As described above, in the MRI apparatus, it is necessary to measure the temperature. Therefore, the temperature measuring means is provided inside or outside the room in which the MRI apparatus is arranged, and the control device is controlled by the output of the temperature measuring means. And the measurement results of the temperature measuring means are displayed.

[0007]

As described above, MRI
In the apparatus, a temperature measuring means is provided inside or outside the room where the MRI apparatus is arranged. As a temperature sensor of the temperature measuring means, a sensor whose resistance value changes depending on the temperature such as a thermocouple or thermistor is usually used. It is considered to be used. Such a temperature sensor such as a thermocouple or a thermistor detects a temperature change as a change in a resistance value, so that it is necessary to supply a current to the sensor. In an MRI apparatus, such a current causes a strong noise at the time of imaging. Therefore, such a sensor cannot be used in the MRI apparatus, and the temperature of the air of the temperature control apparatus discharged from the shield room to the outside is reduced. Non-electrical temperature sensors that measure or utilize pressure fluctuations due to air expansion have been used. However, with such a non-electrical temperature measuring means, an error occurs in the temperature measurement, and the temperature management such as protection of the gradient coil and elimination of discomfort of the subject cannot be sufficiently performed.

Further, when it is necessary to measure the temperature with high precision, a temperature sensor such as a thermocouple or a thermistor has been used despite the influence on the image. But,
In a temperature measurement method using a temperature sensor such as a thermocouple or a thermistor, not only a clear image cannot be obtained, but also an induction current is generated in the sensor by a high-frequency magnetic field irradiated by an MRI apparatus during imaging. At the time of occurrence, there is also a problem that the temperature detected by the sensor is deviated.

In view of the above circumstances, the present invention measures or measures the temperature of a nuclear magnetic resonance imaging apparatus including a gradient coil without using a temperature measuring means having a high-precision sensor such as a thermocouple or a thermistor. An object of the present invention is to provide a nuclear magnetic resonance imaging apparatus capable of grasping.

[0010]

In order to achieve the above object, a nuclear magnetic resonance imaging apparatus according to the present invention comprises a console for setting an imaging condition and instructing a start of imaging, and a console. A calorie parameter under the imaging conditions is obtained, a voltage generation means for generating a voltage corresponding to the parameter at the start of imaging, a voltage generated by the voltage generation means is supplied, and a time constant circuit for outputting device temperature information is provided. The time constant circuit has a time constant corresponding to a temperature rise during photographing of the apparatus and a heat radiation characteristic when photographing is stopped.

According to the nuclear magnetic resonance imaging apparatus of the present invention, when an operator performs imaging (scanning), the voltage generating means generates a voltage related to a calorific value parameter corresponding to imaging conditions set on the console. The voltage generated by the voltage generating means is supplied to a time constant circuit, and its output rises with the progress of the photographing according to the supplied voltage, and falls with the lapse of time while the photographing is stopped.

The time constant of the time constant circuit is set in accordance with the temperature rise characteristic due to heat generation of the gradient magnetic field coil at the time of photographing by actual measurement of the actual apparatus and the heat radiation characteristic at the time of stopping photographing.

Accordingly, the output signal of the time constant circuit outputs the current temperature information of the apparatus during the photographing and during the stop of the photographing, and supplies the information to the display to display the temperature. Can be displayed. In addition, if the output signal of the time constant circuit is supplied to the alarm circuit, the operator is warned that the temperature of the device has exceeded the allowable temperature, to call attention, or to stop imaging or prohibit imaging. Can be controlled.

[0014]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the MRI apparatus of the present invention will be described below with reference to the drawings. FIG. 1 shows the MR of the present invention.
FIG. 1 is a basic configuration diagram of one embodiment of an I apparatus. 1 is a console for an operator to set imaging (scan) conditions and instruct execution of imaging (scan), 2 is a display, and 3 is a gantry. A magnetic circuit for generating a static magnetic field that applies a uniform magnetic field to the subject, a gradient coil that applies a gradient magnetic field, and a high frequency to cause the atomic nuclei of the atoms constituting the tissue of the subject to perform nuclear magnetic resonance (not shown) A high-frequency pulse transmitting means for applying a magnetic field and a nuclear magnetic resonance signal receiving means for detecting a signal by the nuclear magnetic resonance are provided.

Reference numeral 4 denotes a system controller for controlling a gradient magnetic field coil and high-frequency transmitting / receiving means in accordance with imaging conditions set by an operator on the console 1 to execute a desired imaging pulse sequence, 5 a shield room, and 6 a bed It is. As shown in FIG. 2, the system controller 4 calculates a calorific value parameter from the pulse sequence of the imaging conditions set on the console 1, and a voltage generator 8 for generating a voltage corresponding to the calculated caloric value parameter.
And a time constant circuit 9 to which the voltage generated by the voltage generating means 8 is applied.
The display 2 converts the output signal of the time constant circuit 9 into temperature data and displays the temperature of the device.

Next, the calculation of the calorific value parameter by the arithmetic circuit 7 and the setting of the time constant of the time constant circuit 9 will be described. When an imaging condition is input to the console 1 by the operator, the arithmetic circuit 7 calculates a calorific value parameter Q by, for example, the following Expression 1 using a pulse sequence corresponding to the input imaging condition.

[0017]

(Equation 1) In Equation 1, the calorific value parameter Q is derived from two parameters, TR and the current value of the gradient magnetic field waveform, as shown in the waveform diagram of FIG.

TR is the repetition time of the pulse sequence. In the imaging by the MRI apparatus, the pulse sequence of this time TR is repeated. The current value is the amount of current flowing through the gradient coil determined by the type of pulse sequence, imaging range, slice thickness, and the like. MR
Since the temperature rise in the I device mainly depends on the amount of current flowing through the gradient coil, the calorific value parameter is derived based on this amount of current. Here, the heating value of the gradient magnetic field coil is obtained by multiplying the square of the current value by the winding resistance of the coil. Since this current is output in a pattern as shown in FIG. Satisfies Equation 1.

The calorific value parameter Q calculated by the arithmetic circuit 7
Is converted into a voltage by the voltage generating means 8 and output to the time constant circuit 9. Since the time constant circuit 9 includes the resistor R and the capacitor C connected in series, the output signal of the time constant circuit 9, that is, the terminal voltage of the capacitor C causes a transient phenomenon according to the time constant CR. In order to make the change in the terminal voltage of the capacitor C due to this transient phenomenon correspond to the change in the temperature of the MRI apparatus, a time constant CR is derived by the following method.

1) Measure the temperature rise and fall characteristics when scanning the calorific value parameter in the actual apparatus. 2) Substitute the measured temperature (° C.) and time (second) at each time measured in 1) into the following equation to obtain a time constant.

Actual measured temperature = (initial temperature−final temperature) e
^{-t / time constant} + final temperature The above equation is obtained by applying the following equation of the capacitor terminal voltage Vc in the CR series circuit to the equation for temperature rise. Vc = (initial value−final value) e− ^{t / CR} + final value In the above equation, “final temperature” is the upper limit of the allowable temperature of the apparatus at the time of executing the scan, or the lower temperature. This means the lowest temperature (ie, room temperature) at which the temperature decreases due to heat radiation when scanning stops. 3) Since the time constants obtained in 2) exist at each measurement time, it is desirable to take the average of each time constant as a temperature approximation model.

The operation of the MRI apparatus of the embodiment in which the time constant of the time constant circuit 9 is set as described above will be described. First, in order to capture a tomographic image of the subject A, the operator places the subject A on the movable bed 6 and sends the subject A into the gantry 3 as shown in FIG. Then, when the operator inputs imaging conditions to the console 1 and executes scanning, the arithmetic circuit 7 calculates a heat amount parameter Q in accordance with the pulse sequence of the input (set) imaging conditions, and calculates the heat amount parameter Q. The voltage generating means 8 generates a corresponding voltage, and supplies the voltage to the time constant circuit 9.

Therefore, the output of the time constant circuit 9, that is, the terminal voltage of the capacitor C is changed by the execution of the scan.
It rises at the time constant CR, and when the scan stops, the time constant CR
To descend. Since the time constant of the time constant circuit 9 is set so as to match the temperature rise characteristic during scanning of the actual device and the temperature decrease (heat radiation) characteristic after scanning is stopped, the output of the time constant circuit 9, that is, the capacitor The terminal voltage of C is
Since the device temperature information is shown, if this is input to the display 2 as temperature data, the display 2 displays the current temperature of the device, and the operator can know the temperature state of the device from the displayed value.

In the above embodiment, the temperature data of the time constant circuit is given to the display to display the temperature. However, the temperature data is used to sound an alarm or to set the living space of the subject in the gantry. Temperature control, such as sending air to the console, and by inputting temperature data to the console,
Control such as stopping imaging, temperature control of the gradient coil,
It can also be used for temperature control. Further, in the embodiment, the calorific value parameter is calculated by the above expression 1, but may be calculated by another arithmetic expression.

Note that some MRI apparatuses have a function of calculating a calorific value parameter from a pulse sequence based on the setting of imaging conditions and generating a voltage corresponding to the calculated calorific value parameter. If you use that function, a new calorific value parameter calculation means,
There is no need to provide voltage generating means for converting the calorific value parameter into a voltage.

Further, in the embodiment, the time constant circuit is constituted by the resistor R and the capacitor C. However, as long as the temperature rising characteristic at the time of scanning of the apparatus and the temperature decreasing (radiation) characteristic at the time of stopping the scanning can be reproduced, The time constant circuit may have any configuration, such as a combination of a counter and a timer. Further, in the embodiment, the system controller is provided with the function of calculating the calorific value parameter and the function of generating the voltage corresponding to the calculated parameter. However, these functions may be provided on the console to which the imaging conditions are input.

[0027]

According to the nuclear magnetic resonance imaging apparatus of the present invention, the temperature information of the apparatus including the periphery of the subject can be accurately obtained without using a temperature measuring means having a temperature sensor such as a thermocouple or a thermistor. It is possible to construct a system that can be known and that takes into consideration the livability of the subject.
In addition, since a temperature measurement unit serving as a noise source is not used, a high-quality image free from artifacts can be obtained. Further, since it is possible to know the temperature information of all the heating elements existing in the gantry including the gradient magnetic field coil, it is possible to appropriately cope with the heat generation problem of the gradient magnetic field coil and the like.

[Brief description of the drawings]

FIG. 1 is a schematic diagram showing one embodiment of a nuclear magnetic resonance imaging apparatus of the present invention.

FIG. 2 is a diagram showing one embodiment of a system controller of FIG. 1;

FIG. 3 is a diagram showing a gradient magnetic field waveform.

[Explanation of symbols]

1: Console 2: Display
3: Gantry 4: System controller 5: Shield room
6: bed 7: arithmetic circuit 8: voltage generating means
9: Time constant circuit

Claims (1)

[Claims] 1. A means for applying a uniform magnetic field to a subject, a means for applying a gradient magnetic field, and a high-frequency pulse for applying a high-frequency magnetic field to cause nuclear magnetic resonance in nuclei of atoms constituting the tissue of the subject A nuclear magnetic resonance imaging apparatus comprising: a transmitting unit; a nuclear magnetic resonance signal detecting unit configured to detect a signal based on the nuclear magnetic resonance; and a computing unit configured to perform an image reconstruction operation using the nuclear magnetic resonance signal. A calorific value parameter under the photographing conditions set in the above is obtained, a voltage generating means for generating a voltage corresponding thereto at the start of photographing, and a voltage generated by the voltage generating means are supplied,
A time constant number for outputting device temperature information at the time of shooting and at the time of stopping shooting, wherein the time constant circuit has a time constant corresponding to a temperature rise at the time of shooting of the device and a heat radiation characteristic at the time of stopping shooting. A nuclear magnetic resonance imaging apparatus comprising: