JP2009011476A - Magnetic resonance imaging apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To predict a temperature in consideration of a distribution of heat generation of a coil, a distribution of the amount of cooling, and a distribution of heat generation of an eddy current without considering a heating value and the amount of cooling of a gradient magnetic field coil to be uniform. <P>SOLUTION: The magnetic resonance imaging apparatus includes a static magnetic field generating means for imparting a static magnetic field to a subject, a gradient magnetic field generating means for imparting a gradient magnetic field to the subject by energizing the gradient magnetic field coil, the transmission system for irradiating the subject with a high-frequency magnetic field, the reception system for detecting echo signals released from the subject, a signal processing system for performing image operation by using echo signals detected by the reception system and control of operation of the entire apparatus, a means for displaying images, a means for entering imaging conditions, a means for measuring an initial temperature of the gradient magnetic field coil, and a means for predicting the temperature of the gradient magnetic field coil at the time of imaging from the initial temperature and the imaging conditions, wherein the temperature measuring means is placed in a position of showing the highest temperature measured in advance inside the gradient magnetic field coil. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a magnetic resonance imaging (hereinafter referred to as “MRI”) apparatus, and more particularly to an MRI apparatus having a function of predicting a temperature increase due to heat generation of a gradient magnetic field coil.

  MRI equipment measures NMR signals generated by nuclear spins that make up the body of a subject, especially human body, and images its head, abdomen, extremities, etc. in two or three dimensions. Device. In imaging, the NMR signal is given different phase encoding depending on the gradient magnetic field and is frequency-encoded and measured as time-series data. The measured NMR signal is reconstructed into an image by two-dimensional or three-dimensional Fourier transform.

  The gradient magnetic field is generated by energizing the gradient magnetic field coil, and at the same time, the coil itself generates heat. For the heat generation of the coil, a method of removing heat by flowing a cooling liquid in a cooling pipe arranged in the gradient magnetic field coil is used. However, as high-speed imaging methods are adopted to shorten the time for restraining the subject and improve the time resolution, the current flowing through the gradient magnetic field coil increases, and it is not possible to catch up only with heat removal by the coolant. There is a case. Such heat generation may cause the temperature of the gradient coil and its peripheral devices to exceed an allowable value, causing a failure of the gradient coil and its peripheral devices.

Therefore, in Patent Document 1, the calorific value of the gradient magnetic field coil and the RF shield arranged in the vicinity thereof is calculated from the set imaging conditions, and after taking into account the heat amount and initial temperature to be eliminated by the cooling system, the gradient The temperature of the magnetic field coil is calculated, and after the calculation result is compared with a preset threshold, permission of the set imaging condition or review of the imaging condition is displayed.

Japanese Patent Laid-Open No. 2003-319919.

  However, in Patent Document 1, since the temperature of the entire gradient magnetic field coil is calculated on the assumption that the temperature distribution in the gradient magnetic field coil is uniform, consideration is given to the fact that there are locations where the temperature exceeds the calculated value. It wasn't.

  Accordingly, an object of the present invention is to provide a magnetic resonance imaging apparatus capable of accurately knowing the maximum temperature reached by the gradient coil according to the set imaging conditions.

  In order to achieve the above object, the present invention is configured as follows.

  Static magnetic field generating means for applying a static magnetic field to the subject, gradient magnetic field generating means for applying a gradient magnetic field to the subject by energizing the gradient magnetic field coil, and causing nuclear magnetic resonance in the nucleus of the biological tissue of the subject A transmission system for irradiating a high-frequency magnetic field, a reception system for detecting an echo signal emitted by the nuclear magnetic resonance, an image calculation using the echo signal detected by the reception system, and an operation control of the entire apparatus. Based on the signal processing system to be performed, the display means for displaying the obtained image, the input means for inputting the imaging conditions, the temperature measuring means for measuring the initial temperature of the gradient coil, the initial temperature and the imaging conditions And a temperature prediction unit that predicts the temperature of the gradient magnetic field coil at the time of imaging. The temperature measurement unit includes a pre-measured maximum temperature in the gradient magnetic field coil. It is equipped to position shown.

  According to the present invention, it is possible to accurately know the maximum temperature reached by the gradient coil according to the set imaging condition before imaging, and thus it is possible to prevent a failure of the gradient coil and its peripheral devices.

{Embodiment 1}
Hereinafter, preferred embodiments of the MRI apparatus of the present invention will be described in detail with reference to the accompanying drawings.

  FIG. 1 shows the MRI apparatus of this embodiment. The MRI apparatus forms a uniform magnetic field region 4 by a magnet 1 that generates a uniform static magnetic field space. In order to provide the position information of the subject superimposed on this static magnetic field, a gradient magnetic field coil 2 that generates a gradient magnetic field whose magnetic field strength changes linearly in three axial directions orthogonal to each other is provided closer to the uniform space 4 than the magnet 1 Be placed. An RF coil 3 for transmitting an electromagnetic wave having a resonance frequency of proton and a receiving coil 5 for receiving an NMR signal of the examination site are arranged on the uniform space side of the gradient magnetic field coil 2. Although not explicitly shown, a shim coil for correcting a static magnetic field or a gradient magnetic field may be provided.

  An X gradient magnetic field power source 6, a Y gradient magnetic field power source 7, and a Z gradient magnetic field power source 8 are connected to the gradient magnetic field coil 2, respectively, and the gradient magnetic field coil 2 and the gradient magnetic field power sources 6 to 8 constitute a gradient magnetic field generating means. ing. The RF coil 3 is connected to a high-frequency transmitter 9 that supplies high-frequency power, and the reception coil 5 is connected to a high-frequency receiver 10 that amplifies the received signal to constitute a high-frequency transmission / reception system.

  The gradient magnetic field power supplies 6 to 8 and the high-frequency transceivers 9 and 10 are connected to a PC 11 for controlling the operation thereof. The PC 11 also has a function of performing an arithmetic process on the NMR signal amplified by the high-frequency receiver 10 and constructing an image. The PC 11 is connected to a monitor 12 that displays a screen for an operator to set measurement conditions, pulse sequence selection, and the like.

  The PC 11 has a control unit 100 that controls the gradient magnetic field power supplies 6 to 8 and the high-frequency transceivers 9 and 10, a temperature prediction unit 101 that predicts the temperature of the gradient coil, and a storage unit 102 that stores imaging conditions and the like. is doing.

  In the vicinity of the gradient magnetic field coil 2, an RF shield for shielding the influence of the RF pulse emitted from the RF coil 3 and a cooling pipe for flowing a coolant for removing heat generated by the gradient magnetic field coil 2 are arranged. ing. Further, the gradient magnetic field coil 2 is provided with a temperature sensor 103 at a heat spot described later. The temperature sensor 103 is connected to the PC 11 and the output of the temperature sensor 103 is transmitted to the temperature prediction unit 101 of the PC 11.

  The temperature of the gradient magnetic field coil 2 is determined by the heat generated by the gradient magnetic field coil 2, the heat generated by the eddy current generated in the RF shield, and the cooling efficiency by the coolant. The following describes the three factors.

  The heat generation of the gradient magnetic field coil 2 is due to Joule heat generated by the current flowing through the coil, and is determined by the electrical resistance of the coil and the current flowing through the coil. The coil has a pattern for generating a predetermined spatial magnetic field distribution, and since the conductor density is sparse and dense, the spatial distribution of the calorific value is non-uniform. In addition, the X coil that generates the gradient magnetic field in the X axis direction and the Y coil that generates the gradient magnetic field in the Y axis direction have the same coil pattern, but the Y coil is arranged by being rotated 90 ° with respect to the X coil, and the Z axis The Z coil that generates the gradient magnetic field in the direction has a pattern shape different from that of the X coil and the Y coil. Therefore, the heat generation distribution is different for each of the three coils.

  Heat generation due to eddy current is due to Joule heat of eddy current generated in the surrounding conductor when the gradient magnetic field fluctuates in time, and the gradient magnetic field strength, gradient magnetic field It is determined by the rise time, fall time, number of pulses per unit time, conductor area, and conductor resistance. Here, the conductor refers to a gradient magnetic field coil, a metal portion of an RF shield, or the like. The magnetic field strength and direction in the conductor vary depending on the location, and the conductor also has a spatial distribution, so the amount of heat generated by the eddy current also has a spatial distribution. Heat generation due to the eddy current occurs when the gradient magnetic field fluctuates with time, that is, when the gradient magnetic field rises and falls, and thus exhibits a behavior different from that of the gradient magnetic field coil 2 described above.

  The cooling efficiency by the cooling liquid is determined by the flow rate and temperature of the cooling liquid, the piping structure of the cooling pipe, the distance between the gradient coil and the cooling pipe, and the thermal conductivity around the coil. The piping structure of the cooling pipe is designed in consideration of pressure loss, heat generation distribution, etc., but it is difficult to arrange it evenly on the entire coil surface due to the relationship with peripheral equipment, and spatial distribution occurs in the cooling efficiency. .

  As described above, the distribution of heat generation and cooling efficiency depends on the current conditions such as the peak value of the current flowing through the gradient coil, the current axis, the rise time and fall time of the gradient magnetic field, the number of pulses per unit time, and the current conduction time. Therefore, the temperature distribution of the gradient magnetic field coil 2 also changes depending on the energization conditions, and the position showing the maximum temperature also changes.

  That is, in the embodiment of the present invention, the temperature change of each heat spot is predicted after the position indicating the maximum temperature (hereinafter referred to as a heat spot) is specified in advance for each energization condition determined by the imaging condition, and according to the prediction result. To determine whether to permit the imaging condition. The specific implementation method will be described below.

  The predicted temperature T predicted by the temperature prediction unit 101 provided in the PC 11 is compared with the allowable temperature Tmax. If the predicted temperature T is equal to or lower than the allowable temperature Tmax, imaging is started, and the predicted temperature T seems to exceed the allowable temperature Tmax. If so, a screen for changing the measurement conditions is displayed.

  The upper limit temperature Tmax is set to a temperature at which the resin does not deteriorate, for example, 70 °. If the allowable temperature is exceeded, the resin deteriorates, causing a gradient coil failure. Therefore, the temperature prediction unit 101 always predicts the maximum temperature position of the gradient coil.

  At the time of shipment, a position (hereinafter referred to as a heat spot) showing the maximum temperature under each condition is specified by conducting an energization test with respect to the energized conditions assumed. As described above, the temperature of the gradient magnetic field coil is determined by the coil heat generation amount, the eddy current heat generation amount, and the cooling efficiency by the coolant. Therefore, a direct current is supplied to the gradient magnetic field coil in a state where cooling water is supplied, and a temperature distribution determined by the amount of heat generated by the coil and the amount of cooling is measured. With the cooling water flowing, a pulse current is applied to the gradient magnetic field coil, and the temperature distribution determined by the eddy current heat generation amount, the coil heat generation amount, and the cooling amount is measured.

  For the measurement of the temperature distribution, for example, a wide area can be easily measured by using thermography. The energization conditions during temperature distribution measurement may be set by combining the maximum DC current value that can be output by the gradient magnetic field power supplies 6 to 8, the fastest gradient magnetic field rise time and fall time, and the maximum number of pulses per unit time. . The position of the heat spot is specified for each of the plurality of energization conditions combined in this way, and a temperature sensor 103 is provided at the position of each specified heat spot.

  The amount of heat generated by the eddy current is determined by the peak value of the current flowing through the gradient magnetic field coil, the gradient magnetic field rise time, the fall time, and the number of pulses per unit time. Therefore, by making the energization amount at the time of the direct current and the pulse current the same, the temperature distribution due to the heat generation of only the eddy current can be confirmed. That is, the temperature distribution due to the eddy current heat generation is obtained by subtracting the direct current application result from the pulse current application result.

  The sequence parameters (crest value, energization axis, gradient magnetic field rise time, fall time, number of pulses per unit time, energization time, etc.) are varied to determine the distribution of heat generation and eddy current heat generation due to coil energization.

  Thermography can also be used for the above temperature measurement. In order to further increase the measurement accuracy, a temperature sensor such as a platinum resistance or a thermocouple may be attached to the heat spot specified by thermography.

  A temperature prediction formula for predicting the temperature change of the heat spot will be described.

Coil heat generation ΔTc is T0 as the initial temperature, Tsat as the saturation temperature calculated by the power and thermal resistance supplied to the gradient coil, Ta as the ambient temperature, K as the thermal time constant of the coil, and t as the elapsed time. Can be represented by the formula (1).

Since the coil heat generation can be regarded as independent, equation (1) is established for each coil.Temperature rise (fall) ΔTx due to X-axis energization, temperature rise (fall) ΔTy due to Y-axis energization, temperature rise (fall) due to Z-axis energization Is ΔTz, the heat generation amount Tc of the entire coil can be expressed by the following equation (2).

Tc = ΔTx + ΔTy + ΔTz Equation (2)

On the other hand, the heat generation ΔTe due to eddy current is the saturation temperature calculated from the peak value, rise time, fall time, conductor resistance value, and heat conduction value of the conductor, Tsateddy, the initial temperature T0e of the conductor, and the eddy current generated in the conductor If the time constant of is Keddy, it can be expressed by equation (3).

Heat generation due to eddy currents can also be considered as independent eddy currents generated by the X, Y, and Z coils, so the temperature rise (fall) ΔTex of the conductor due to X-axis energization, ΔTey, the temperature rise (fall) of the conductor due to Y-axis energization, When the temperature rise (decrease) of the conductor due to the Z-axis energization is ΔTez, the heat generation Te of the entire coil can be expressed as in equation (4).

Te = ΔTex + ΔTey + ΔTez Equation (4)

Since the coil heat generation and the heat generation due to the eddy current can be regarded as independent from each other, the predicted temperature T in consideration of the coil heat generation amount and the eddy current heat generation amount is as shown in Equation (5).

T = Tc + Te formula (5)

Substituting temperature data, ambient temperature, peak value, gradient magnetic field rise time, and fall time into equations (1) to (5), temperature prediction parameters (thermal resistance value, thermal time constant, conductor corresponding to each axis) Heat conduction value, time constant of eddy current).

  The above calculation is performed for each heat spot, and a temperature prediction parameter corresponding to each heat spot is determined. The determined temperature prediction parameter is stored in the storage means 102 together with the imaging condition, the position of the heat spot, and the temperature prediction formula, and the peak value, the conduction axis, the rising time of the gradient magnetic field, the falling time, and the number of pulses per unit time When an energization condition such as an energization time is input, it is called from the temperature prediction unit 101 and used for calculation of temperature prediction.

  One embodiment of the operation of the MRI apparatus of the present invention in actual imaging will be described using the flowchart of FIG.

  In step S201, the operator sets a desired imaging condition via the PC 11. Energization conditions for the gradient magnetic field power supplies 6 to 8 are set according to the set imaging conditions.

  In step S202, the temperature sensor 103 measures the initial temperature of the gradient magnetic field coil 2 before energization of the gradient magnetic field power supplies 6-8. Note that the initial temperature of the gradient magnetic field coil 2 is not limited to that based on the measurement result of the temperature sensor 103, and may be set to room temperature if it is the first imaging of the day, or if it is the second or later, the temperature prediction unit 101. May be set to a predicted value.

  In step S203, based on the energization conditions set in S201 and the initial temperature of the gradient coil 2 measured or set in S202, the temperatures of all the heat spots are calculated using equations (1) to (5). Is predicted by the temperature prediction unit 101.

  In step S204, the temperatures of all the heat spots predicted in S203 are compared with a preset allowable temperature. If the predicted temperature is lower than the allowable temperature, the process proceeds to step S206. If the predicted temperature is equal to or higher than the allowable temperature, Proceed to step S205.

  In step S205, a condition resetting screen showing the energization condition range within the allowable value is displayed on the monitor 12, the operator is prompted to reset the imaging conditions, and the process returns to step 201.

  In step S206, imaging is started based on the set imaging conditions. Note that the temperature measurement result of the heat spot may be displayed on the monitor 12 during the imaging.

According to the embodiment described above, since the location where the maximum temperature of the gradient coil reaches the maximum temperature can always be predicted and dealt with regardless of the energization condition, the temperature of the gradient coil does not exceed the allowable value. Therefore, it is possible to prevent failure due to deterioration of the resin due to temperature rise.
{Embodiment 2}

  In this embodiment, the temperature of the gradient magnetic field coil is predicted in response to a change in the installation environment. FIG. 3 shows a flowchart, and this embodiment will be described.

  In step S301, it is determined whether or not a preset period, for example, two months have passed. If the set period has elapsed, the process proceeds to S302, and if not, the flow ends.

  In step S302, the gradient magnetic field power supplies 6 to 8 are energized to the gradient coil 2 according to preset conditions, for example, the energization conditions used when specifying the position of the heat spot at the time of factory shipment in the first embodiment.

  In step S303, the temperature of the heat spot is measured by each temperature sensor 103 provided in the heat spot of the gradient coil 2.

In step S304, a temperature prediction parameter is calculated based on the comparison result of the temperature energized in S302, the temperature predicted value calculated using equations (1) to (5), and the temperature measured in S303.
In step S305, the temperature prediction parameter calculated in S304 is stored in the storage means 103, and the data is updated.

  When the control unit 100 of the PC 11 performs the above steps, temperature prediction according to the situation of the facility where the MRI apparatus is installed can be performed, and the difference between actual measurement and prediction is reduced. The data update operation may be performed manually by the operator as necessary. Also, temperature measurement may be performed between sequences so that noise is not caused by temperature measurement during imaging, and the temperature prediction parameter may be updated using the data.

  According to the present embodiment, temperature prediction can be performed more accurately, so that a margin for an error in predicted temperature can be set small. As a result, the standby time can be shortened, and the total imaging time can be shortened.

  Further, if the difference between the newly calculated parameter and the previous parameter is large, there is a possibility that the cooling efficiency has changed. Therefore, if the newly calculated parameter and the previous difference are larger than a preset threshold value, a warning screen is displayed on the monitor 12 to prompt the operator to check the gradient coil, cooling system, etc. good.

  Although specific embodiments of the present invention have been described above, the present invention is not limited to the above-described embodiments, and can be modified and implemented as necessary.

1 is an overall basic configuration diagram of an MRI apparatus according to the present invention. FIG. 3 is a flowchart of the first embodiment. FIG. 5 is a flowchart of the second embodiment.

Explanation of symbols

  1 magnet, 2 gradient coil, 3 RF coil, 4 uniform field, 5 receiver coil, 6 X gradient magnetic field power supply, 7 Y gradient magnetic field power supply, 8 Z gradient magnetic field power supply, 9 high frequency transmitter, 10 high frequency receiver, 11 PC, 12 monitor, 100 control unit, 101 temperature prediction unit, 102 storage means, 103 temperature sensor.

Claims (1)

Static magnetic field generating means for applying a static magnetic field to the subject, gradient magnetic field generating means for applying a gradient magnetic field to the subject by energizing the gradient magnetic field coil, and causing nuclear magnetic resonance in the nucleus of the biological tissue of the subject A transmission system for irradiating a high-frequency magnetic field, a reception system for detecting an echo signal emitted by the nuclear magnetic resonance, an image calculation using the echo signal detected by the reception system, and control of the operation of the entire apparatus. Based on the signal processing system to be performed, the display means for displaying the obtained image, the input means for inputting the imaging conditions, the temperature measuring means for measuring the initial temperature of the gradient coil, the initial temperature and the imaging conditions In a magnetic resonance imaging apparatus comprising temperature prediction means for predicting the temperature of the gradient coil during imaging,
The magnetic resonance imaging apparatus according to claim 1, wherein the temperature measuring means is provided at a position indicating the maximum temperature in the gradient magnetic field coil measured in advance.

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