WO1992001362A1

WO1992001362A1 - Construction of shielded room for biomagnetic measurements

Info

Publication number: WO1992001362A1
Application number: PCT/US1991/004775
Authority: WO
Inventors: Frank W. Vanesky; James R. Marsden; David S. Buchanan
Original assignee: Biomagnetic Technologies, Inc.
Priority date: 1990-07-12
Filing date: 1991-07-10
Publication date: 1992-01-23

Abstract

A shielded room (20) is constructed by mechanically attaching electrically conducting facing sheets (32) to the beams (30) of a frame in a manner such that there is a continuous electrical current flow path (44) between the facing sheets. The mechanical interfaces in the current flow path are sealed with a layer of copper or zinc (34) at the interface, the copper or zinc layer being preferably applied to one or both of the pieces that meet at the interface by a spraying technique such as plasma spraying. The copper or zinc connector seal ensures a continuous current flow path along the length of the mechanical interface, and prevents leakage of electromagnetic energy into the interior of the room. A magnetic field barrier such as sheets (48) of mu-metal can be supported on the assembled structure. The room (20) is readily assembled with mechanical fasteners (42), which permits it to be later disassembled and moved if necessary.

Description

Construction of Shielded Room For Biomagnetic Measurements

Technical Field This invention relates to shielded rooms that produce an internal environment free of external electromagnetic and magnetic fields and to the measurement of biomagnetic fields within such rooms, and, more particularly, to the construction of the rooms. Background Art

The biomagneto eter is an instrument that has been developed to measure magnetic fields produced by the body, particularly the brain. The magnetic fields produced by the body are very small and difficult to measure. Typically, the strength of the magnetic field produced by the brain is about 0.00000001 Gauss. By comparison, the strength of the earth's magnetic field is about 0.5 Gauss, or over a million times larger than the strength of the magnetic field of the brain. Most electrical equipment also produces magnetic fields, in many cases much larger than that of the earth. Electromagnetic signals travelling through the environment can also interfere with the taking of magnetic measurements. It is apparent that, unless special precautions are taken, it is difficult or impossible to make magnetic measurements of the human body because the external influences such as the earth's magnetism, nearby apparatus, and electromagnetic signals can completely mask the magnetic fields produced by the body.

SUBSTITUTE SHEET The biomagnetometer includes a very sensitive detector of magnetic signals. The currently most widely used detector is a Superconducting Quantum Interference Device or SQUID, which is sufficiently sensitive to detect magnetic signals produced by the brain. (See, for example, US Patents 4,386,361 and 4,403,189, whose disclosures are incorporated by reference, for descriptions of two types of SQUIDs. ) This detector and its associated equipment require special operating conditions such as a cryogenic dewar, and cannot be placed into the body or attached directly to the surface of the body.

The present biomagnetometer usually includes a chair or table upon which the subject is positioned, and a structure which supports the SQUID in a cryogenic environment and in proximity with the head of the subject. Special electronics are used to filter out external effects, see for example, US Patents 3,980,076 and 4,079,730, whose disclosures are herein incorporated by reference. The electronics filters out a portion of the external noise, but in some regimes is not entirely successful. The electronics is also costly and can constitute a major portion of the cost of the system.

There is another possibility for reducing the adverse effect of the external magnetic field, which can be. used in place of, or preferably in addition to, the electronic signal processing. In this approach, the subject and detector are placed into a magnetically quiet enclosure that shields the subject and the detector from the external electromagnetic and magnetic fields. The magnitude of the static magnetic field within the

SUBSTIT enclosure is reduced from about 0.5 Gauss or more, to less than about .001 Gauss. With this reduction in the ambient static magnetic field, the biomagnetic events of interest can be measured more readily, and the signal processing required to achieve usable information is greatly reduced. Magnetically shielded enclosures have been known, as for example the design described in US Patent 3,557,777, whose disclosure is herein incorporated by reference. In this approach, concentric layers of a high permeability metal and a metallic conductor are supported on a frame. To permit construction of the room at remote sites, it is conventional practice to form the high permeability material and the metallic conductor material as sheets that are assembled to the frame. The '777 patent indicates that the layers of shielding sheets are simply fastened to a wooden frame with screws. While this practice may have been sufficient with the biomagnetic measurement technologies available in the 1960's, current practice with better biomagnetic measurement equipment has determined that the interfaces between the sheets must be sealed more positively to prevent field leakage to the interior of the shielded room. In current construction practice, the edges of the sheets of metallic conductor material are welded to each other to form a continuous shielding surface after assembly to the frame. The welded construction reduces the possibility of leakage of electromagnetic energy through gaps between the sheets, as even a slight leakage can significantly interfere with the current biomagnetic measurements.

SUBSTITUTE SHEET While operable, such welded enclosures cannot be readily disassembled for movement at a later time, as to a new facility. The welding operation must be carefully performed and checked, so that the preparation of each such enclosure is essentially a custom operation, requiring long lead times.

In another technique, the metallic conductor sheets are coated with copper by electrodeposition or similar technique, and fastened to each other with mechanical fasteners. While an enclosure prepared by this technique can be disassembled when necessary, .the preparation of large structural sheets of aluminum coated with copper is an expensive process.

Accordingly, there exists a need for an improved magnetically shielded enclosure which has a low level of electromagnetic and magnetic noise in its interior, which can be disassembled if necessary, and which is less expensive to construct than existing enclosures. The present invention fulfills this need, and further provides related advantages. Disclosure of Invention The present invention provides an approach for constructing a shielded room which can be assembled from prefabricated pieces on .site, and if desired at a later time, readily disassembled and reassembled at another site. The room reduces electromagnetic energy and magnetic fields inside the enclosure to acceptably low levels for the making of biomagnetic measurements, without the necessity of welding the structure and at reduced cost as compared with prior types of construction. In accordance with the invention, a shielded

SUBSTITUTE SHEET room comprises a frame of beams enclosing a volume sufficiently large to admit a person to the interior thereof; a plurality of facing sheets sized to fill the openings between the beams and form the walls of the room, each facing sheet being made of an electrically conductive material; fastener means for mechanically and removably fastening each facing sheet to the beams of the frame along the entire length of each edge of each facing sheet, at least one of the beams and the fastener means being electrically conductive to form a current flow -path having mechanical interfaces, between adjacent facing sheets; and a connector seal of a layer of a material selected from the group consisting of a copper-based alloy and a zinc-based alloy along the length of each mechanical interface in the current flow path between two adjacent facing sheets. The pure material or alloy that forms the connector seal is preferably applied by a spray metallizing process such as plasma spray or other technique that permits the it to be applied easily and inexpensively. The connector seal can thus be reapplied or touched up as necessary when the room is reassembled after a prior use and disassembly. Additionally, there are preferably provided sheets of a high magnetic permeability material that are overlaid on either the inside or the outside, or most preferably both the inside and the outside, of the electrically conductive facing sheets.

The beams of the frame and the facing sheets are preferably made of aluminum or an aluminum alloy. These metallic conductors shield the inside of the room against electromagnetic signals in the environment. In the past, it was

SUBSTITUTE SHEET conventional practice to weld the edges of the aluminum sheets together to ensure a conducting path and continuous shield. A gap in the shield could lead to leakage of electromagnetic energy into the interior of the room. A welded structure cannot be readily disassembled, however.

It has not been previously possible to construct a mechanically fastened shielded room structure made of aluminum facing sheets because aluminum oxide forms on the surface of the aluminum and acts as an insulating barrier in the current flow path between adjacent facing sheets. The exclusion of electromagnetic energy and electromagnetic noise of low frequency from the interior of the shielded room then slowly deteriorates over time, gradually increasing the level of noise within the room. As noted previously, the magnitude of the biomagnetic signals is so small that even very small levels of noise interfere with the measurements.

One possible solution is to construct the facing plates from copper or a copper alloy, but this approach is unacceptable because of the increased cost and weight of the facing sheets. Another possible approach is to place a copper layer over the outside of the aluminum facing sheets to act as an electromagnetic shield, but the facing sheets must still be welded together.

In the approach of the present invention, the connector seal of a copper alloy or a zinc alloy is applied to the surface of the facing sheet or the beam in the region that will become the mechanical interface between two pieces of the structure. For example, a layer of copper or zinc alloy may be applied to a border around the edges

SUBSTITUTE SHEET of one face of each of the aluminum facing sheets, or to a strip along each edge of the aluminum frame beams. The facing sheet is then mechanically fastened to the beam with a fastener, so that the copper connector seal is in the mechanical interface between the beam and the facing sheet.

A spray approach to applying the copper or zinc alloy to the underlying structure is preferred because the metal is selectively deposited quickly over the relatively narrow border area with a simple, readily available apparatus. A sufficient mechanical bond is formed to maintain the metal layer on the underlying metal, and it should be recognized that the bond need not be strong in tension because the fastener compresses the connector seal. After the room has been disassembled for subsequent assembly at another site, the connector seal can be touched up by the same spray approach, if necessary.

However, tests have demonstrated that touch-up is seldom required.

It has been found that this connector seal produces a long lasting, highly effective seal against leakage of electromagnetic energy through the mechanical interface. Alternatively stated, the connector seal produces a continuous conductive path all around the edge of each facing sheet to the next facing sheet, in this case through the frame beam. Another approach is to place the connector seal between the facing sheet and the fastener, where in this case the fastener extends continuously along the length of the edge of the facing sheet. The former approach of achieving the conductive path from one facing

SUBSTITUTE SHEET sheet, through the beam, and to the adjacent facing sheet is preferred, because then conventional short mechanical fasteners can be used. Thus, generally, a process for excluding electromagnetic energy from the interior of a room comprises the steps of erecting the room with walls made from facing sheets of an electrically conducting material; connecting an edge of a first facing sheet to an edge of a second facing sheet with a- removable fastener, there being at least one mechanical interface between the first and second facing sheets; and interposing a connector seal of a sprayed material selected from the group consisting of a copper-based alloy and a zinc-based alloy at each mechanical interface between the first and second facing sheets. However, the facing-sheets are joined together, the connector seal must be placed in the mechanical interfaces to ensure a seal against leakage of electromagnetic energy and a conductive path between the facing sheets, along the entire length of the edge of the sheet.

The approach of the invention permits the shielded room to be completely prefabricated and then assembled at a selected location. Testing has shown that the shielded room achieves satisfactory performance in excluding interfering signals for extended periods of time. The room can later be disassembled for moving or modification, and reassembled with at most the re- application of the connector seal. Other features and advantages of the invention will be apparent from the following more detailed description of the invention, taken in conjunction with the

BSTITUTE SHEET accompanying drawings, which illustrate, by way of example, the principles of the invention. Brief Description of Drawings

Figure 1 is a perspective view of a shielded room utilizing the constructional techniques of the invention;

Figure 2 is a sectional view of a joint in the walls of the room of Figure 1, taken along lines 2-2; Figure 3 is a perspective view of a facing sheet after application of the connector seal;

Figure 4 is a view similar to that of Figure 2, illustrating an alternative approach for the sealing arrangement; Figure 5 is a perspective view of a frame beam after application of a connector seal; and

Figure 6 is a schematic depiction of the operation of the biomagnetometer within the shielded room. Best Mode for Carrying Out the Invention

In accordance with a preferred embodiment of the invention, apparatus for detection of biomagnetic activity of a person comprises a shielded room having a frame of~beams enclosing a volume sufficiently large to admit a person to the interior thereof, plurality of facing sheets sized to fill the openings between the beams and form the walls of the room, each facing sheet being made of an electrically conductive material, fastener means for mechanically and removably fastening each facing sheet to the beams of the frame along the entire length of each edge of each facing sheet, at least one of the beams and the fastener means being electrically conductive to form a current flow path having mechanical

SUBSTITUTE SHEET interfaces, between adjacent facing sheets, and a connector seal of a layer of a material selected from the group consisting of a copper-based alloy and a zinc-based alloy along the length of each mechanical interface in the current flow path between two adjacent facing sheets; means for preventing penetration of external magnetic fields into the interior of the room; and means for performing measurements of biomagnetic activity of a person located within the room.

Figure 1 illustrates a shielded room 20 having a door 22 therethrough. A person 24 is positioned inside the shielded room 20, with one or more biomagnetometer assemblies 26 positioned for making biomagnetic measurements of the person. An optional three-axis magnetic field cancellation coil 28 is positioned around and outside of the room 20. By way of illustration of the dimensions but not of limitation, the height of the room 20 is typically about 8 feet.

The general construction of the^~~shielded room is a frame of I-beams 30 that are bolted together. A number of facing sheets 32 are fastened to the I-beams 30 to^" form the walls that enclose the room 20. The I-beams 30 and the facing sheets 32 are electrically conducting materials, preferably an aluminum alloy such as 1100 or 6061 alloy, although the construction is not so limited. The conductive structure provides a shield against electromagnetic energy fields external to the shielded room 20 and eddy currents that might be created within the walls. However, for the shielding to be effective to the high level of exclusion required in this field, there may not be any significant leakage paths through the joint

SUBSTITUTE SHEET areas between the facing sheets 32. The typical lateral dimensions of the facing sheets 32 are 4 feet by 10 feet.

Figure 2 is a sectional view through the wall of the room 20, including the facing structure and the I-beam 30. The facing sheet 32 of electrically conductive material, here an aluminum alloy, has a connection seal 34 along an edge 36 of at least one of the faces 38. The connection seal 34 is a layer of copper or a copper-based or zinc-based alloy, preferably copper, deposited upon the face 38, to a thickness that is not critical but is preferably about 0.010 inches. (As used herein, the term "copper-based alloy" includes pure copper and also alloys of copper. The term "zinc-based alloy" includes pure zinc and also alloys of zinc.) The copper-based or zinc- based alloy is preferably deposited by flame or plasma spraying in air, with conventional equipment used for this purpose. The spraying is preferably not accomplished in vacuum, as this would require a vacuum chamber large enough to hold the facing sheets. The facing sheet 32 is illustrated ^~in perspective view in Figure 3, showing the connection seal 34 around the edges thereof.

The facing sheets 32 have bolt holes 40 drilled through the sheet 32 and the layer that forms the connection seal 34. The facing sheets 32 are fastened to the I-beam 30 with adjacent facing sheets in a generally end-to-end abutting arrangement, as illustrated in Figure 2. Operability of the facing sheets 32 for shielding against electromagnetic energy is not dependent upon reaching a closely abutting end-to-end relationship between the adjacent facing sheets. Fasteners, here illustrated as conventional bolts 42, hold the facing sheets 32 to the I-beam 30.

When the bolts 42 are tightened, a continuous electrically conductive path from a first facing sheet, through the connector seal on the first facing sheet and the mechanical interface between the first facing sheet and the I-beam, through the I-beam, through the connector seal on a second facing sheet and the mechanical interface between the I-beam and the second facing sheet, and into the second facing sheet is completed, here indicated schematically by the line 44. This conduction path is complete along the entire length of the edge of the facing sheet and the I- beam, even though there are discrete fasteners along the length of the edge.

Figure 2 also shows an outside facing sheet 46 of a high-magnetic permeability material and an inwardly facing sheet 48 of a high-magnetic permeability material. The high-magnetic permeability material in each case is preferably mu-metal, which has a composition of about 77 weight percent nickel, 5 weight percent copper, 1.5 weight percent chromium, balance iron. This material, when properly processed, is used to exclude magnetic fields from the inside of the room 20. The mu-metal sheets are supported from the I-beam 30, with one layer on the outside and one layer on the inside.

Figure 4 shows an alternative embodiment to that of Figure 2, wherein the connection seal 34 has been deposited on the face of the I-beam 30 and on the facing sheet 32. The coated I-beam 30 is illustrated in Figure 5. The other elements of

SUBSTITUTE SHEET Figure 4 are similar in form and function to those of Figure 2. A conductive path is formed between the facing sheets through the I-beam, again along the entire length of the I-beam. (The sheets 46 and 48 are omitted from Figure 4 for visual clarity, but would be present in use.)

Tests have been conducted with a joint such as shown in Figure 4 to establish that it has initially, and maintains, an electrical conductivity sufficient to shield the interior of the room 20 from external electromagnetic energy. Specimens were prepared with the configuration of Figure 4, and then subjected to an ageing test. Four different metallic coatings were used: flame sprayed zinc, flame sprayed copper, flame sprayed nickel silver, and flame sprayed phosphorus bronze. The joints were formed, and then each joint was subjected to an 18 day ageing test. In this test, each joint was heated from 35°C to 65°C and then cooled to 35°C over a four hour period. Each joint was subjected to a total of 104 total cycles. The purpose of this testing is to accelerate any failure of the joint in the sense of loss of conductivity. The conductivity of each joint was measured before and after ageing. The copper and zinc coated joints did not deteriorate during ageing; that is, their conductivities were unchanged after ageing as compared with prior to ageing. The phosphorus bronze-coated joint suffered a decrease in conductivity by a factor of 2.5, and the nickel-silver suffered a decrease in conductivity by a factor of 7.0. Thus, while the copper, zinc, and phosphorus bronze alloys are all acceptable, the copper-coated and zinc-coated sealed joints are preferred.

ITUTE SHEET In the preferred approach, the shielded room of the invention is preferably used in conjunction with biomagnetic measurements as shown in Figure 1 and also Figure 6. Referring to Figure 6, a biomagnetometer 60 includes a plurality of magnetic sensing coils 62 for measuring small magnetic fields. The output signal of each magnetic sensing coil 62 is detected by a detector, preferably a superconducting quantum interference device (SQUID) 64. Both the magnetic sensing coil 62 and the SQUID 64 are maintained at a cryogenic operating temperature within a liquid helium dewar 66.

The magnetic signals from the body of the person 24 are picked up by the magnetic sensing coils 62 in the dewar 66, and the signals are detected by the SQUIDs 64. The SQUIDs 64 detect the magnetic field values as electrical currents that are processed in an electronics system 68 and stored in a computer 70 as a function of time, for display and study.

The, general structure of the biomagnetometer 26, including the magnetic sensing coils 62, the SQUIDs 64, the dewars 66, the electronics 68, and the compaiter 70 are known in the art. See for example US patents 4,793,355; 3,980,076;

4,389,612; 4,079,730; 4,386,361; and 4,403,189, whose disclosures are incorporated by references.

The approach -of the invention has the important advantage that the sealing between the sheets of the electromagnetic shielding can be accomplished by a mechanical seal and fastener, so that the room can be readily disassembled and then later reassembled. It is not necessary to weld the facing sheets together and then later separate them in order to disassemble the room, and then reweld the facing sheet.

Although particular embodiments of the invention has been described in detail for the purpose of illustration, various modifications may be made without departing from the spirit and scope of the invention. Accordingly, the invention is not to be limited except as by the appended claims.

SUBSTITUTE SHEET

Claims

1. A shielded room, comprising: a frame of beams enclosing a volume sufficiently large to admit a person to the 5 interior thereof; a plurality of facing sheets sized to fill the openings between the beams and form the walls of the room, each facing sheet being made of an electrically conductive material; 0 fastener means for mechanically and removably fastening each facing sheet to the beams of the frame along the entire length of each edge of each facing sheet, at least one of the beams and the fastener means being electrically 5 conductive to form a current flow path having mechanical interfaces, between adjacent facing sheets; and a connector seal of a layer of a sprayed material selected from the group consisting of a 0 copper-based alloy and a zinc-based alloy along the length of each mechanical interface in the current flow path between two adjacent facing sheets. —

___^ 2. The shielded room of claim 1, wherein the 5 connector seal is formed as a strip of the connector seal material along each edge of at least one face of each facing sheet.

3. The shielded room of claim 1, wherein the connector seal is formed as a strip of the 0 connector seal material on the beams of the frame.

4. The shielded room of claim 1, wherein the connector seal is formed of a material selected from the group consisting of substantially pure copper and substantially pure zinc.

TIT

5. The shielded room of claim 1, wherein the fastener means includes an electrically conductive fastener that simultaneously joins a first facing sheet and a second facing sheet to the frame, and the connector seal is placed between each facing sheet and the fastener.

6. The shielded room of claim 1, wherein the beams of the frame are electrically conductive, and the connector seal is placed between each facing sheet and the beam it contacts.

7. The shielded room of claim 1, wherein each of the beams is electrically conductive.

8. The shielded room of claim 1, wherein the beams are made of an aluminum alloy.

9. The shielded room of claim 1, wherein the facing sheets are made of an aluminum alloy.

10. The shielded room of claim 1, further including a plurality of sheets of a high permeability magnetic material fastened overlying the sheets of the electrically conductive material.

11. The shielded room of claim 1, wherein the connector seals are plasma sprayed metal.

12. A shielded room, comprising: a frame of beams enclosing a volume sufficiently large to admit a person to the interior thereof; a plurality of facing sheets sized to fill the openings between the beams and form the walls of the room, each facing sheet being made of an electrically conductive material; fastener means for mechanically and removably fastening each facing sheet to the beams of the frame along the entire length of each edge of each facing sheet, at least one of the beams

SUBSTITUTE SHEET and the fastener means being electrically conductive to form a current flow path having mechanical interfaces, between adjacent facing sheets; and 5 a connector seal of a layer of a material selected from the group consisting of a copper-based alloy and a zinc-based alloy along the length of each mechanical interface in the current flow path between two adjacent facing 10 sheets.

13. The shielded room of claim 12, wherein the beams of the frame are electrically conducting and the connector seal is placed on the edges of the faces of the facing sheets that contact the

15 beams.

14. The shielded room of claim 12, wherein the beams of the frame are electrically conducting, and the connector seal is placed on the portions of the beam that contact the facing

20 sheets.

_^

15. The shielded room of claim 12, wherein the fasteners are electrically conducting and extend from a first facing sheet to a second adjacent facing sheet, and the connector seal is

25 placed on the edges of the facing sheets that contact the fastener.

16. The shielded room of claim 12, wherein the fasteners are electrically conducting and extend from a first facing sheet to a second

30 adjacent facing sheet, and the connector seal is placed on the portions of the fasteners that contact the facing sheets.

17. The shielded room of claim 12, wherein the connector seal is a metal sprayed metal. 5 18. Apparatus for detection of biomagnetic

SUBSTITUTE SHEET activity of a person, comprising: a shielded room having a frame of beams enclosing a volume sufficiently large to admit a person to the interior thereof, a plurality of facing sheets sized to fill the openings between the beams and form the walls of the room, each facing sheet being made of an electrically conductive material, fastener means for mechanically and removably fastening each facing sheet to the beams of the frame along the entire length of each edge of each facing sheet, at least one of the beams and the fastener means being electrically conductive to form a current flow path having mechanical interfaces, between adjacent facing sheets, and a connector seal of a layer of a material selected from the group consisting of a copper-based alloy and a zinc- based alloy along the length of each mechanical interface in the current flow path between two adjacent facing sheets; means for preventing penetration of external magnetic fields into the interior of the room; and means for performing measurements of biomagnetic activity of a person located within the room.

19. A process for excluding electromagnetic energy from the interior of a room, comprising the steps of: erecting the room with walls made from facing sheets of an electrically conducting material; connecting an edge of a first facing sheet to an edge of a second facing sheet with a removable fastener, there being at least one

SUBSTITUTE SHEET mechanical interface between the first and second facing sheets; and interposing a connector seal of a sprayed material selected from the group consisting of a copper-based alloy and a zinc- based alloy at each mechanical interface between the first and second facing sheets.

20. The process of claim 19, wherein the step of interposing is accomplished by spraying a layer of the connector seal material onto a strip of each facing sheet that forms a mechanical interface, prior to the step of connecting.

SUBSTITUTE SHEET