JPH07303619A - Method for compensating eddy current in mri apparatus and mri apparatus - Google Patents

Abstract

PURPOSE:To provide constantly the proper eddy current compensation volume, irrespective of the temperature of the heat shield, etc. CONSTITUTION:The temperature sensor 30 measures the temperature T of the heat shield P just before the start of scan or once or at regular intervals during the scan and outputs it to the eddy current compensation volume setting section 14. The eddy current compensation volume setting section 14, which memorizes the correlative curve of the temperature T of the heat shield and the eddy current compensation volume S, picks up the eddy current compensation volume S corresponding to the temperature T of the heat shield given by the temperature sensor 30 based upon the correlative curve and outputs said volume to the gradient magnetic field driving circuit 3. The gradient magnetic field driving circuit 3 drives the gradient coil 57 and produces the ideal gradient magnetic field by adding the eddy current compensation volume S to the gradient magnetic field. Accordingly, even during the period when the temperature of the heat shield P, etc., shifts from the temperature of the open air to the steady state at the very low temperature, the deterioration of image quality due to eddy current can be prevented.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION This invention is applied to MRI (Magnetic R
The invention relates to an eddy current correction method in an esonance imaging apparatus and an MRI apparatus. More specifically, it is possible to prevent deterioration of image quality regardless of temperature fluctuation of a heat shield.
The present invention relates to an eddy current correction method in an RI apparatus and an MRI apparatus that implements the method.

[0002]

2. Description of the Related Art FIG. 5 is a sectional view showing an example of a magnet assembly of a conventional MRI apparatus. In this magnet assembly 51, the static magnetic field coil 53 is housed in the helium bath 54 and cooled to a cryogenic temperature (for example, 4.2K) by the refrigerator 52. The helium tank 54 is
The heat shields P and Q are insulated from the outside air (for example, 23 ° C.). The warm bore vacuum vessel 56 includes the static magnetic field coil 53, a helium bath 54, and heat shields P and Q.
And accommodate. In the bore of the magnet assembly 51, a gradient magnetic field coil 57 that generates a gradient magnetic field, an RF (Radio Frequency) pulse transmission to the subject H, and an NMR (Nuclear Magnetic Resonanc) from the subject H.
e) An RF coil 58 for receiving signals is installed. Reference numeral 59 is a bridge that supports the subject H.

In the magnet assembly 51, the metallic heat shields P and Q have extremely low temperatures (for example, 80 each).
(K, 20K), the electric resistance is very small. Therefore, an eddy current caused by the gradient magnetic field flows, and the eddy current distorts the static magnetic field, deteriorating the image quality. Similarly, eddy currents are also generated in the conductors other than the heat shields P and Q, and the eddy currents distort the static magnetic field and deteriorate the image quality. (For simplicity, all eddy currents are generated. Is referred to as "heat shield etc."). Therefore, as shown in FIG. 6, a gradient magnetic field Kc is applied to the ideal gradient magnetic field Ki to which an eddy current correction amount Sc for canceling the influence of the eddy current is added. When this gradient magnetic field Kc is applied, it means that the ideal gradient magnetic field Ki is applied, and the deterioration of the image quality due to the eddy current can be prevented. The eddy current correction amount Sc is set by measuring the eddy current amount when the heat shield or the like is cooled to an extremely low temperature and is in a steady state.

[0004]

When the operation of the refrigerator is stopped due to the maintenance and inspection of the MRI apparatus or the occurrence of a failure, the temperature of the heat shield or the like rises. Next, when the MRI apparatus is operated, the temperature t of the heat shield or the like is as shown in FIG.
The temperature gradually changes to the cryogenic temperature tc in the steady state. The steady state time Dc is, for example, about one week. By the way, as shown in FIG. 8, the eddy current amount is small (I1) when the temperature t of the heat shield or the like is high (t1), and is large (Ic) when the temperature t of the heat shield or the like is low (tc). ,
When the temperature t of the heat shield or the like is high (t1), the influence of the eddy current is small.

However, in the conventional MRI apparatus, the amount of eddy current (I) when the heat shield and the like are cooled to an extremely low temperature (tc)
Since c) was measured and the eddy current correction amount was set, if the temperature of the heat shield or the like is high, the correction is overcorrected as shown in FIG. 9, and the applied gradient magnetic field Kr is substantially equal to the ideal gradient magnetic field Ki. It will be far apart. Therefore, there is a problem that the image quality deteriorates if the eddy current correction is performed during the period until the steady state is reached (for example, about one week).
Therefore, an object of the present invention is to implement an eddy current correction method in an MRI apparatus capable of preventing deterioration of image quality regardless of temperature fluctuations of a heat shield and the like, and to implement the method.
It is to provide an RI device.

[0006]

An eddy current correction method in an MRI apparatus of the present invention adds an eddy current correction amount in order to cancel the influence of an eddy current generated in a heat shield or the like which keeps a static magnetic field coil at a low temperature. In an MRI apparatus that applies a gradient magnetic field, the temperature of at least one of the heat shields (all parts where eddy currents are generated) is measured, and the eddy current correction amount is changed based on the measured temperature. This is a structural feature.

Further, the MRI apparatus of the present invention is an MRI apparatus for applying a gradient magnetic field to which an eddy current correction amount is added in order to cancel the influence of an eddy current generated in a heat shield or the like which keeps a static magnetic field coil at a low temperature. , Temperature measuring means for measuring the temperature of at least one of the heat shield and the like (all parts where eddy currents occur), and eddy current correction amount setting means for changing the eddy current correction amount based on the measured temperature It is characterized in that it is provided with.

[0008]

With the eddy current correction method and the MRI device in the MRI apparatus of the present invention, the temperature of at least one portion of a heat shield or the like (all parts where the eddy current is generated) is measured, and the eddy current correction is performed based on the measured temperature. Change the amount. Therefore, regardless of the temperature of the heat shield or the like, it becomes possible to always provide an appropriate eddy current correction amount and prevent deterioration of image quality.

[0009]

The present invention will be described in more detail with reference to the embodiments shown in the drawings. The present invention is not limited to this. FIG. 1 is a block diagram of an embodiment of the MRI apparatus of the present invention. In this MR device 100, the magnet assembly 1 has a bore (space portion) for inserting a subject therein, and an RF for exciting spins of atomic nuclei in the subject by surrounding the bore. An RF coil 58 that transmits a pulse and receives an NMR signal from the subject, and a gradient magnetic field coil 57 that generates a gradient magnetic field (the gradient coil includes coils for reading, phase encoding, and slice selection axes). And a static magnetic field coil 53 that applies a constant static magnetic field to the subject, heat shields P and Q that insulate the static magnetic field coil 53 from the outside air, and a temperature T (hereinafter, heat) of the heat shields P and Q. A temperature sensor 30 for measuring the temperature T of the shield) is arranged. FIG. 2 shows the arrangement.
In FIG. 2, 52 is a refrigerator and 54 is a helium tank. Reference numeral 59 is a bridge that supports the subject H.
The gradient magnetic field coil 57 and the RF coil 58 are connected to the gradient magnetic field drive circuit 3, the RF power amplifier 4 and the preamplifier 5, respectively. Further, the temperature sensor 30 is
It is connected to the eddy current correction amount setting unit 14 and outputs the heat shield temperature T measured once or periodically before the start of scanning or during scanning to the eddy current correction amount setting unit 14. The eddy current correction amount setting unit 14 controls the heat shield temperature T
And the eddy current correction amount S are stored, and the eddy current correction amount S corresponding to the heat shield temperature T given from the temperature sensor 30 is extracted based on the relation curve and is sent to the gradient magnetic field drive circuit 3. Output. FIG. 3 schematically shows the relationship curve. This relational curve is obtained in advance based on actual measurement, simulation or theoretical calculation.

The sequence storage circuit 8 operates the gradient magnetic field drive circuit 3 and the gate modulation circuit 9 based on the stored data acquisition pulse sequence in accordance with a command from the computer 7. The gradient magnetic field driving circuit 3 drives the gradient magnetic field coil 57 to generate a gradient magnetic field. This gradient magnetic field is a gradient magnetic field obtained by adding the eddy current correction amount S to the ideal gradient magnetic field (Ki in FIG. 4).
The gate modulation circuit 9 modulates the high frequency output signal from the RF oscillation circuit 10 into a pulsed signal having a predetermined timing and a predetermined envelope, and outputs the pulsed signal as an RF pulse.
In addition, after power amplification by the RF power amplifier 4, it is applied to the RF coil 58 of the magnet assembly 1 to excite a target region.

The preamplifier 5 includes a magnet assembly 1
The NMR signal from the subject detected by the RF coil 58 is amplified and input to the phase detector 12. Phase detector 12
Uses the output of the RF oscillation circuit 10 as a reference signal, phase-detects the NMR signal from the preamplifier 5, and outputs the A / D converter 1
Give to one. The A / D converter 11 converts the analog signal after phase detection into a digital signal and inputs it to the computer 7. The computer 7 performs an image reconstruction operation on the digital signal from the A / D converter 11 to generate an image (proton density image) of a target area. This image is displayed on the display device 6. In addition, the computer 7 is a console 13
Responsible for overall control such as receiving information input from.

As shown in FIGS. 3 and 4, when the heat shield temperature T is high (T1), the eddy current correction amount S is small (S1), and when the heat shield temperature T is low (Tc).
Becomes large (Sc). On the other hand, as described above, the eddy current amount is small when the heat shield temperature T is high (T1) and is large when the heat shield temperature T is low (Tc). Therefore,
When the heat shield temperature T is high (T1) and low (T1)
c) is also an appropriate eddy current correction amount, and as shown in FIG.
The ideal gradient magnetic field Ki can always be applied substantially. That is, regardless of the heat shield temperature T, it is possible to always prevent the deterioration of the image quality due to the eddy current.

Therefore, according to the MRI apparatus 100,
When the MRI apparatus 100 is restarted after maintenance or a failure, the vortex is always swirled even during a period (for example, about one week) in which the temperature of the heat shield or the like gradually changes from the outside air temperature to the extremely low temperature of the steady state. It is possible to prevent the deterioration of the image quality due to the current.

Instead of the relational curve shown in FIG. 3, parameters (time constant, amplitude, etc.) of the eddy current correction amount S may be tabulated and stored in the eddy current correction amount setting unit 14.

[0015]

According to the eddy current correction method and the MRI apparatus in the MRI apparatus of the present invention, an appropriate eddy current correction amount can be always given regardless of the temperature of the heat shield and the like, and the image quality is deteriorated. Will be able to prevent. As a result, it is possible to prevent the deterioration of the image quality due to the eddy current even during the period when the temperature of the heat shield or the like gradually changes from the outside air temperature to the extremely low temperature in the steady state.

[Brief description of drawings]

FIG. 1 is a block diagram of an embodiment of an MRI apparatus of the present invention.

2 is a cross-sectional view of a magnet assembly of the MRI apparatus of FIG.

FIG. 3 is a schematic diagram of a relationship curve between a heat shield temperature and an eddy current correction amount.

FIG. 4 is a waveform diagram showing an ideal gradient magnetic field, an eddy current correction amount, a synthetic gradient magnetic field, and a substantial gradient magnetic field.

FIG. 5 is a sectional view of an example of a magnet assembly of a conventional MRI apparatus.

FIG. 6 is a waveform diagram showing an ideal gradient magnetic field, an eddy current correction amount, a synthetic gradient magnetic field, and a substantial gradient magnetic field.

FIG. 7 is a graph showing a change in temperature of a heat shield or the like from the outside air temperature to a cryogenic temperature in a steady state.

FIG. 8 is a graph showing the amount of eddy current corresponding to the temperature of a heat shield or the like.

FIG. 9 is a waveform diagram showing an ideal gradient magnetic field, an eddy current correction amount, a synthetic gradient magnetic field, and a substantial gradient magnetic field.

[Explanation of reference numerals] 100 MRI apparatus 1 Magnet assembly 3 Gradient magnetic field drive circuit 7 Computer 8 Sequence memory circuit 14 Eddy current correction amount setting unit 30 Temperature sensor 53 Static magnetic field coil 54 Helium tank P, Q heat shield

Claims (2)

[Claims] 1. An MRI apparatus for applying a gradient magnetic field to which an eddy current correction amount is added in order to cancel the influence of an eddy current generated in a heat shield or the like which keeps a static magnetic field coil at a low temperature. Is measured, and the eddy current correction amount is changed based on the measured temperature. 2. An MRI apparatus for applying a gradient magnetic field to which an eddy current correction amount is added in order to cancel the influence of an eddy current generated in a heat shield or the like that keeps a static magnetic field coil at a low temperature. An MRI apparatus comprising: a temperature measuring unit that measures the temperature at one location; and an eddy current correction amount setting unit that changes the eddy current correction amount based on the measured temperature.