EP0324797A1

EP0324797A1 - Nuclear magnetic resonance imaging system

Info

Publication number: EP0324797A1
Application number: EP87907622A
Authority: EP
Inventors: Diane Dessalles-Martin; Guy Aubert; Martine Collet
Original assignee: General Electric CGR SA; Thomson CGR
Current assignee: General Electric CGR SA
Priority date: 1986-11-14
Filing date: 1987-11-13
Publication date: 1989-07-26
Also published as: US5029287A; FR2606624A1; JPH02500957A; CA1287106C; WO1988003646A1

Abstract

Installation d'imagerie par résonance magnétique nucléaire dans laquelle des moniteurs (15, 16), du type à tube cathodique, sont soustraits à l'action d'un champ magnétique intense (BO). A cette fin, l'installation (1) conforme à l'invention comporte une bobine plate (20) qui engendre un second champ magnétique (Bc) de sens (5) contraire à celui du champ magnétique intense (BO).Installation for nuclear magnetic resonance imaging in which monitors (15, 16), of the cathode-ray tube type, are shielded from the action of an intense magnetic field (BO). To this end, the installation (1) according to the invention comprises a flat coil (20) which generates a second magnetic field (Bc) of direction (5) opposite to that of the intense magnetic field (BO).

Description

NUCLEAR MAGNETIC RESONANCE IMAGING INSTALLATION

The present invention relates to a nuclear magnetic resonance imaging installation. The invention particularly relates to means for removing from the action of a magnetic field instruments placed in the environment of the apparatus which produces the magnetic field, as is the case in particular for monitors of the cathode screen type. which are used in the operating console of the nuclear magnetic resonance imaging device. A nuclear magnetic resonance imaging device, or MRI device, in particular comprises a magnet which produces in the device a constant, intense magnetic field, to which the body of a patient to be examined is subjected.

The implementation of the MRI machine is carried out by one or more operators who act at the level of an operating console to define the various parameters, and control the various operations which are necessary to obtain the desired image. To this end, an operating console generally comprises two monitors: a first monitor called an image-monitor and intended to represent the image obtained; a second monitor called monitor -dialogue, in particular allows an operator to adjust the different parameters. The use of the monitor-dialog is greatly facilitated when the latter is placed under a touch screen; but it should be noted that the use of the touch screen is really only interesting if the dialog-monitor is itself a color-monitor.

In general, the MRI machine itself is located in a first room which constitutes a Faraday cage. The operating console is located in an adjoining room, and the partition separating the two rooms is provided with a window which allows operators or doctors not to lose sight of the patient during the entire time when the latter is installed in the MRI machine, so as to possibly intervene very quickly with the patient in the event of an incident. For this reason, but also with a view to reducing the space required for premises, it is sought to place the operating console at a relatively short distance from the MRI machine, for example within 10 meters.

The problem which then arises is that of the use of monitors, when the latter are subjected to the action of the magnetic field produced by the MRI machine.

Indeed, installed in a magnetic field, a monitor presents disturbances which can make it unusable: thus for example, with a black and white monitor placed in a magnetic field of the order of 3 gauss, it is impossible to use correctly a touch screen ; with a color monitor, the constraint is even greater, and the latter is practically no longer usable when it is in a field greater than about 0.5 gauss. The intense field produced by the MRI machine can reach several thousand gauss in the machine itself, so that in the control room adjoining the examination room, the magnetic field leakage , even at ten meters, is still much higher than the values quoted above. Consequently, we note that in the prior art, only monitors of the black and white type can be used with the operating console, less than ten meters from the MRI machine, and again on the condition that these monitors are contained in shielding boxes. The use of shielding, arranged for example around a monitor, in addition to not allowing the use of a color monitor within ten meters of the MRI machine, has the additional disadvantage require adjustment of black and white devices: in fact, despite the use of shielding, the residual stray field requires adjusting the deflection coils specific to the monitor to make it usable; this drawback is particularly important since this adjustment can only be carried out when the black and white monitor is removed from its shielding, and that the result of the adjustment is only visible when the monitor is again reintroduced into its housing. shielding; so that several successive adjustment operations are necessary before obtaining the optimum adjustment. The present invention particularly aims to allow the use of at least one monitor, black and white or color, in a nuclear magnetic resonance imaging installation, at a distance from the MRI machine much lower than in the prior art for the same field strength, and to allow the commissioning of the monitor or monitors in a much simpler and safer manner than in the prior art. It should be noted that, under substantially the same conditions, the invention can be applied to the protection with respect to the magnetic field, of different instruments such as for example magnetic cassettes whose use and storage near the MRI machine can cause problems.

According to the invention, an installation for nuclear magnetic resonance imaging, comprising an apparatus producing an intense magnetic field, the apparatus being at a given distance from at least one instrument whose operation can be disturbed by the intense magnetic field, is characterized in that it comprises means for producing a second magnetic field of direction substantially opposite to that of the intense magnetic field, so that the second magnetic field tends to cancel the first intense magnetic field at the level of the instrument.

The invention will be better understood thanks to the following description, given by way of nonlimiting example, and to the two appended figures, among which:

FIG. 1 schematically shows an installation for nuclear magnetic resonance imaging according to the invention: - Figure 2 is a front view of a flat coil and an operating console shown in Figure 1.

FIG. 1 schematically shows a nuclear magnetic resonance imaging installation 1 or MRI installation, intended, in the nonlimiting example described, for use in the context of medical diagnosis. According to a current implementation in a hospital site, the installation of MRI 1 is shared between two adjoining rooms, the first of which is an examination room 2 and the second is a control room 3; the examination room 2 contains the MRI machine 4 proper, symbolized in FIG. 1 by a rectangle arranged along a longitudinal axis Z; in practice, the MRI machine 4 may have the general shape of a circular cylinder centered around the longitudinal axis Z, and the inner part of which along the longitudinal axis Z is intended to receive a patient (not represented) . The MRI machine 4 comprises in a conventional manner a coil or a magnet (not shown) producing an intense magnetic field B ₀ , homogeneous and constant inside the MRI machine 4, and particularly in a volume of interest materialized in FIG. 1 by a length of interest L having a center 0. The direction of the field B _n , inside the MRI machine 4, is for example that shown by the arrow 7, so that the MRI machine 4 can symbolize a magnet, one side 8 of which is located towards the control room 3 constitutes a north face N and the second side 9 of which faces the examination room 3 is a South face S. The direction of the field B _n is that of the longitudinal axis Z, and its Intensity which is constant at all points of the length L is several thousand gauss over this length L, for example 5000 gauss or 0 , 5 tesla.

The control room 3 contains an operating console 10 using which operators (not shown) carry out the operations necessary to obtain the image of the patient. A partition 11 which separates the examination room 2 from the control room 3 incorporates a communication door

60, and a window 12 through which the operators monitor the patient throughout the examination. According to a layout which is preferred, when the shape of the available premises allows it, the plane of the window 12 is substantially perpendicular to that of the longitudinal axis Z of the MRI machine 4, and the console 10 is substantially parallel to the window

12; this arrangement allows operators located at the front

13 of the console 10, to manipulate the controls of the console 10 without losing sight of the patient.

As previously mentioned, an operating console 10 includes at least one monitor. In the nonlimiting example described, the operating console 10 comprises a first and a second monitor 15, 16 arranged one above the other, so that the first monitor 15 being in FIG. 1 in a plane deeper than the latter, it is only partially represented. The screens 17 of the monitors 15, 16 are oriented substantially towards the front 13 of the console 10, and form a volume which is particularly sensitive to the intense magnetic field B_, this particularly sensitive volume being represented in FIG. 1 by a sensitive length L2 formed between the screens 17, and the rear 19 of the monitors 15, 16.

According to a characteristic of the invention, the MRI installation 1 comprises means for producing a second magnetic field B, antagonistic to the first field B. In the nonlimiting example described, the means for producing the second magnetic field B, antagonist of the first magnetic field B-, comprise a coil 20 of the flat coil type. The flat coil 20 is traversed by a current (not shown) whose direction is such that a first face 21 of the flat coil 20 oriented towards the MRI machine 4 constitutes a North N pole, and that its second face 22 constitutes a South S pole.

In the nonlimiting example described, the flat coil 20 being centered on the longitudinal axis Z, the second magnetic field B is also oriented on the axis Z but its direction on the axis Z, shown by the second arrow 5, is contrary to the first direction 7 of the first field B _n . As a result, by adjusting the intensity of the second field B, we tend to compensate or even to cancel the first field B ,, particularly at the sensitive length L2.

It should be noted that, in the spirit of the invention, the second opposing magnetic field B could also be produced in a different way, for example by one or more coils (not shown) arranged in the monitors 15, 16 themselves, or also by a solenoid (not shown) of great length which would be placed around the monitors 15, 16 or even around the console 10. Among these different means, the flat coil 20 has an important advantage in that it is reduced footprint, and therefore presents negligible discomfort. Indeed, the flat coil 20 can have a circular or square section, and its plane can be parallel to that of the window 12 and even applied against the latter as in the nonlimiting example presented in FIG. 1, this position of the flat coil 20 being particularly advantageous for presenting a reduced bulk; but the flat coil 20 could have, with respect to the monitors 15, 16, a different position, such as for example surrounding the monitors 15, 16 or even disposed on the other side of the latter relative to the MRI machine 4.

In the nonlimiting example described, the monitors 15, 16 are arranged along the longitudinal axis Z so that the second sensitive length L2 is coincident or substantially parallel to the longitudinal axis Z; the flat coil

20 being itself centered on the longitudinal axis Z and its plane being perpendicular to the latter. Under these conditions, the magnetic field can be made almost zero over the sensitive length L2, so that it is possible to use monitors 15, 16 of the black and white type or of the color type, without the need to place these monitors 15, 16 in a shielding box as in the prior art.

Indeed, assuming that the intense field B _Q at the center 0 of the length of interest L has a value of about 5000 gauss, its value on the axis Z at a distance D of the order of 7 meters is about 10 gauss, i.e. the sensitive length L2 would be subjected to a magnetic field of 10 gauss in the absence of the flat coil 20, which would make it impossible to use monitors 15, 16 of black and white type not provided with shielding, and completely impossible the use of color monitors with or without shielding.

Figure 2 shows the flat coil 20 and the operating console 10 viewed from the front of the operating console

10, as shown in FIG. 1 by a third arrow 43. FIG. 2 shows in a first plane the operating console 10, then in a deeper second plane the flat coil 20, and finally in an even more plane deep the partition 11 which comprises the window 12. The operating console

10 is carried above the ground 45 by a base 46. The operating console 10 comprises various conventional control and display members (not shown), and comprises the first and second monitors 15, 16. The first monitor 15 constitutes the dialogue monitor likely to include a touch screen, (not shown); the second monitor 16 disposed above the first constitutes the image monitor. These two monitors 15, 16 can each indifferently, in the MRI equipment of the invention, be of the black and white type or of the color type. The entire surface 47 presented by the two monitors 15, 16 parallel to the plane of the flat coil 20, is substantially centered on the center 26 of the flat coil

20. In the nonlimiting example described, the center 26 also represents the longitudinal axis Z of the MRI machine 4 shown in FIG. 1.

The flat coil 20 is formed from a conventional conductor forming turns 48 (shown in dotted lines) mounted on a frame 31 of non-magnetic material, for example wood. In the nonlimiting example described, the section 39 of the flat coil 20 given by the shape of the frame 31 is substantially square. The sides 50 of the frame have a third length L3 of the order of 2 meters in the nonlimiting example described, so that the average distance has between the center 26 of the flat coil 20, and its periphery represented by the frame 31 is of the order of 1 meter, that is to say half of the third length L3 of the sides 50.

Referring again to FIG. 1, the rear 8 of the console 10 is preferably very close to the second face 22 of the flat coil 20, a few centimeters for example, so that the rear of the monitors 15, 16 is also very close to it and can possibly penetrate into the flat coil 20, so as for example to be at the level of the median plane 27 of the latter; the flat coil 20 having a thickness E of a few centimeters, 5 centimeters for example. Since the monitors 15, 16 are generally always in the same position in the console 10, the console 10 and the flat coil 20 constitute an assembly which may have, relative to the MRI machine 4, a position different from that shown on the Figure 1, so as to adapt for example to the situation of existing sites: thus for example, one can optionally move the flat coil assembly 20-console 10 laterally along an axis 25 transverse to the longitudinal axis Z , so as to adapt in hospital configurations where it is not possible to have the operating console 10 facing the magnet or MRI machine 4, but this within relatively low limits of the order of 0.60 m on either side of the longitudinal axis Z. However, the flat coil 20-console 10 assembly can be moved much more with respect to the longitudinal axis Z, provided that it is oriented so that the plane of the flat coil 20 is substantially perpendicular ular to the local direction of the magnetic field: this arrangement is shown in FIG. 1 where the operating console 10 and the flat coil 20 are shown in dotted lines and marked respectively 10a and 20a, the flat coil 20a being perpendicular to one second local direction U of the intense magnetic field B _fi . In all cases, the value of the second magnetic field B can be adjusted to compensate, or even canceling, over the sensitive length L2, the intense magnetic field B _Q , by adjusting the value of the current flowing in the conductors (not shown) of the flat coil 20; the latter being powered by an adjustable power supply, in itself conventional (not shown).

According to a characteristic of the invention, the average distance a between the center 26 and the frame 31 is equal to or greater than the sensitive length L2 presented by the monitors 15, 16, so that the value of the second field B generated by the coil plate 20 varies relatively relatively on its axis. On the other hand, it should also be noted that by placing the flat coil 20 between the MRI machine 4 and the monitors 15, 16, the possible decrease in the value of the second magnetic field B along the sensitive length L2 is appreciably compensated by the decrease which the value of the intense field B presents on the direction of the axis Z when one moves away from the apparatus of MRI 4. Indeed, one notes that on the longitudinal axis Z, at distance D from the center O of the MRI machine 4 of the order of 5.5 m, the intense field B _Q has a value of 17 gauss, while at 5.8 m, the intense field B _{Q is} worth approximately 15 gauss. Consequently, by placing the monitors 15, 16, so that the sensitive length L2 is substantially opposite to the MRI machine 4, with respect to the median plane 26 of the flat coil 20, it is possible with a flat coil 20 whose radius or mean distance a is equal to or greater than the sensitive length L2, not only obtaining that the second opposing field B varies fairly weakly, but also obtaining that this variation is compensated by the variation of the first intense field B. u should be noted that for a specialist in nuclear magnetic resonance imaging, the solution of the invention, to the problem of the use of monitors subjected to an intense magnetic field, can surprise and even worry, because for this specialist everything magnetic field produced in the environment of an MRI machine can be superimposed on the intense field B _Q and risks destroying its homogeneity over the first length of interest L.

However, the calculations predict and tests have shown that the second compensation field B, produced by the flat coil 20 at a relatively short distance D from the center O of the MRI machine 4, brings only a slight modification of the value of the intense field B _Q over the length of interest L, this change in value resulting in operation only a slight displacement of the central section relative to the origin, that is to say relative to the center O. This modification of the value of the field B _Q is caught up during the calibration operation which is conventional for an MRI apparatus, and which makes it possible to redefine the position of the central section relative to the origin. Indeed, considering that the distance D is 6.8 meters between the flat coil 20 and the center 0 of the length of interest L, and that the flat coil 20 has the following characteristics given only by way of example: the frame 31 is a square whose sides 50 have a length L3 equal to 2a equal to 2 meters, ie a square of 2 x 2 m;

- the coil 20 comprises 100 turns traversed by a current of 16.5 amperes, that is 1650 amperes making it possible to obtain the value of the second field B necessary to compensate for the intense field B_ over the sensitive length L2; the flat coil 20 is centered on the Z axis, that is to say practically coaxial with the MRI machine 4;

- under these conditions, the field B produced by the coil 20 on its axis Z at a distance D is given by the following formula:

where μ _c is the permeability of the vacuum, = 4 x lθ ^"7 ; I is the current in the coil 20; a is the average distance between the center 26 of the coil and the frame 31; D the distance from the center 26 of the flat coil 20. It is found that for a distance D = 0, that is to say at the center 26 of the coil 20, the field B is equal to 9.3 gauss. We find that for a distance D = 6.8 meters, i.e. at the center 0 of the length of interest L, the value of the second compensation field B is 0.040 gauss, which may require d 'be compensated by the calibration of the MRI machine 4, as explained above. With regard to a possible first-order defect in the homogeneity of the field B _Q , measured for example along the longitudinal axis Z, which can be provided by the second compensation field B, its value is given by the derivative of the second field B on the Z axis, determined by the second formula which follows:

T / _m

-7 where ( ₀ = 4 l .10; 1 = 1650 amps; a is the average distance equal to 1; D is 6.8 meters and corresponds to the distance between the center of the flat coil and the center of the length of interest; T / m = tesla per meter.

We find, _, ^c = 17. 10 T / m: this result indicates that the field variation over one meter at the center O of the MRI machine 4 is 0.017 gauss so that the gradient field strength on the Z axis is less than one part per million or 1 ppm in a current MRI machine, therefore less than the limit of measurement possibilities and can be neglected.

If the flat coil 20 is even closer to the center O of the MRI machine 4, the field gradient or inhomogeneity provides for the second field B over the length of interest L can be greater than 1 p. p. m. In this case, this inhomogeneity can be easily compensated for by an adjustment of the offset current of a gradient coil.

It is in fact known that MRI machines conventionally comprise, in addition to a coil which produces the intense field B _Q , coils of gradients which have the function of creating a field gradient along three orthogonal axes, one of which is the longitudinal axis Z. These gradient coils are generally mounted around the cylinder that constitutes the MRI machine, in a known manner; a single gradient coil 40 intended to act along the longitudinal axis Z being shown in Figure 1 for clarity thereof.

Field gradients are only established during one phase of the exam, so they are established on an impulse. By applying a continuous current to one or more of these gradient coils, called offset current, on which the pulses can be superimposed, we can correct first order inhomogeneities of the intense field B _Q , by an appropriate adjustment of this current, so that each of these gradient coils can constitute a means for compensating for an inhomogeneity of the field B _Q generated in the MRI apparatus 4 by the flat coil 20. Thus, by taking again the nonlimiting example described where the flat coil 20 is perpendicular to and centered on the longitudinal axis Z, it is possible to use the gradient coil 40 and to apply an offset current to it which makes it possible to correct first order homogeneity defects of the field B _n along the Z axis, including those generated by the operation of the flat coil 20. This description constitutes a non-litmitative example, showing how instruments can be removed from the action of a intense magnetic field, in particular instruments of the type comprising a cathode ray tube. The invention is particularly interesting in the context of a diagnostic MRI installation, but can also be applied to protect instruments in all installations using an intense magnetic field.

Claims

1. Installation for nuclear magnetic resonance imaging, comprising an apparatus (4) producing an intense magnetic field (B _Q ), the apparatus (4) being located at a given distance (D) from at least one instrument (15 , 16) to protect from the intense magnetic field (B „), the intense magnetic field (B _Q ) having a direction (7) given at the level of the instrument (15, 16), characterized in that it comprises means (20) to produce a second magnetic field (B) having a second direction (5) substantially opposite to the direction (7) of the first intense magnetic field (B _Q ) in order to tend to cancel the magnetic field (BO) at the level of l 'instrument (15, 16).

2. Installation according to claim 1, characterized in that the instrument (15, 16) is a monitor of the cathode-ray tube type.

3. Installation according to claim 2, characterized in that the monitor (15, 16) is of the color monitor type.

4. Installation according to claim 1, characterized in that the means (20) for producing a second magnetic field (B) comprise a coil (20) of the flat coil type.

5. Installation according to claim 4, the instrument (15, 16) having a length (L2) sensitive to the magnetic field, characterized in that an average distance (a) between a center (26) of the flat coil (24 ) and a peripheral part (31) of the flat coil (20) is equal to or greater than the sensitive length (L2).

6. Installation according to claim 4, characterized in that the plane of the flat coil (20) is substantially perpendicular to a direction (Z, U) of the intense magnetic field (B _Q ) at the level of the instrument (15, 16 ).

7. Installation according to claim 4, characterized in that the instrument (15, 16) is substantially centered on an axis (Z) of the flat coil (20).

8. Installation according to claim 5, characterized in that the flat coil (20) is disposed substantially between the device (4) and the sensitive length (L2).

9. Installation according to one of claims 5, or 6, or 7, characterized in that the flat coil (20) has a substantially square section.

10. Installation according to claim 6, characterized in that a first direction (Z) of the intense magnetic field (B _Q ) coincides with the longitudinal axis (Z) of the device (4), and in that the plane of the flat coil (20) is substantially centered on said longitudinal axis (Z)

11. Installation according to claim 1, characterized in that it further comprises means (40) for compensating for an inhomogeneity of the intense field (B _Q ) generated in the device (4) by the second magnetic field

(B _c )