Note: Descriptions are shown in the official language in which they were submitted.
.
SCAN ROOM FOR MAGNETIC RESONANCE II~AGER
. . ~ , . ,
This invention relates to magnetic resonance (MR)
¦ imagers generally and, in particular, to the scan room in
which the imager i5 located.
MR imagers utilize high magnetic fields and radio
frequency electromagnetic radiation to obtain images of the
lnterior of an object. Superconducting magnets are commonly
employed in MR for producing magnetic fields greater than
;`~ 1.5 kilogauss. Because of the fringe fields of such
10 magnets, persons with pacemaker~ are ln danger of being
deprogrammed even at considerable distances from the magnet.
Because of this danger, FDA recommendations specify that
fringe fields must be maintained below 5 gauss in uncon-
trolled areas~ Aside from persons with pacemakers, fringe
15 fields greater than 2 gauss can seriously distort cathode
ray tubes. This renders large areas around, above, and
below the MR imager unusable for many bu~iness and medical
activities.
In addition, successful MR imaging requires that the
20 field inside the MR magnet be stable and homogeneous to a
very high degree. Thus the movement of vehicles, elevators,
or other large steel objects near the imager can distort the
MR image significantly. Consequently, the MR imager must be
i
J ~ r;
in an isoiated are~ or it mus~ be shielded from such outside
magnetic disturbances. Even non-moving steel objects, such
as steel beams and steel reinforcement bars in concrete, can
disturb the homogeneity inside the magnet. In order to
offset the effects of non-moving steel objects, the super-
conducting magnet can be shimmed with precisely located
steel bars to obtain the necessary homogenei~y. It is,
I however, impossible to shim for movinq ferrous objects. MR
manufacturers commonly specify that the MR imager be located
at least 58 feet from parking lots, elevators, or other
locations where there is the potential for large moving
ferrous objects. -
Magnetic resonance information from the object being
- scanned consists of signals in the radiofrequency band
between 1 and 200 MHz, depending on the strength of the
magnet. Any external noise in this radiofrequency band,
such as radio stations, electric motors, vehicle ignition
systems, etc., reproduce as artifacts in the MR image.
Consequently, longer data acquisition times are required to
average the noise down to an acceptable level. The best one
can do is to reduce ambient radiofrequency noise below the
level produced by thermal noise in the object itself and the
noise in the RF preamplifier of the imager. The pri~r art
is to surround the MR imager with a shielded room of copper
plate or copper screen to obtain the necessary
radiofrequency electromagnetic attenuation. MR manufactur-
ers commonly specify a copper shielded enclosure providing
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at leas~ `80 dB rad-iofrequency attenuation between 1 and 100
MHz .
Still another problem in MR imaging is capacitive
coupling of electrical noise from power lines and ground
currents. The prior art solution is to create a Faraday
shield of copper plate or copper screen that is electrically
isolated or "floated" from the ground. MR manufacturers
I commonly specify that electrical power line noise be attenu-
ated by at least 100 dB at the MR imager operating frequen-
cy.
Thsrefore, the problem is to shield the environment
outside of the scan room from fringe magnetic fields o~ the
magnet, and to shield the im~ger from external d.c. and
time-varying magnetic fields~ as well as from radiofrequency
electromagnetic radiation and electrical power line noise.
This shielding must be accomplished without impairing the
homogeneity of the magnetic field inside the magnet of the
imager.
Prior Art
The prior art failed to take into collective account
all four MR shielding problems (magnetic fringe containment,
shielding from d.c. and time-varying external magnetic
fields, shielding from radiofxequency electromagnetic
radiation, and shielding from electrical power line noise)~
For example, copper is the preferred material for
radiofrequency shielding, but it has no capability for
confining magnetic fields.
'L~ 9~
. - 4 -
It i`s weli known in the art that ferrous metals can
also be used for electro~agnetic shielding. (See Electron-
- ics Dictionary, 4th Edition, copyright 1978 by McGraw-Hill,
Inc.). In the case of MR imaging, however, it was thought
that a mass of ferromagnetic shielding near the magnet would
have a deleterious effect on the homogenei~y of ~he magnetic
field inside the magnet. For example, the si~e planning
guide for MR imaging systems manufactured ~y Diasonics, NMR
Division, 533 Ca~ot Road, South San Francisco, CA 94080,
states that the construction materials of the scan room must
be non-ferrous. Moreover, it was not appreciat~d in the
prior art that a complete ferromagnetically-shielded room
could also be used to confine the fringe magnetic field
produced by ~he MR magnet.
Summary of the Invention
Contrary to the teachings of the prior art, and in
accordance with this invention, it has been discovered that
a scan room of substantially solid steel, which is symmet-
j rically placed with respect to the magnet, will contain the
five gauss line of the magnetic field within the scan room,provide the required magnetic, RF electromagnetic, and
electric shielding, shield the imager against large moving
ferrous objects, and yet provide a magnetic field homogene-
ity inside the magnet to the extent that it can be readily
shimmed for producing proper images. The site dedicated to
imaging can then be placed in hospitals and other populated
areas, and space can be more economically obtained in
,'Lfrc~
-- 5
desirable locations. It is an object of this invention to
provide such a room.
The invention in one broad aspect pertalns to a scan room
that is large enough to completely enclose a full body magnetic
resonance (MR) imager and that will reduce to an acceptable level
outside the room the strength of the fringe magnetic fields
produced by the imager and shield the imager from magnetic and
electric fields produced outside the room while allowing the
magnet of the imager to be shimmed to produce a field
sufficiently homogenous to obtain the desired images. The scan
room comprises side walls of welded steel plates encircling the
MR imager and spaced therefrom, the plates being made of a steel
having sufficient thickness to reduce the fringe magnetic field
outside the walls of the room to an acceptable level. Means is
provided for electrically insulating the plates from earth ground
and means is provided for connecting the room to earth ground.
Another aspect of the invention comprehends a scan room that
is large enough to completely enclose a full body magnetic
resonance (MR) imager and that will reduce to an acceptable level
outside the room the strength of the fringe magnetic fields
produced by the imager and shield the imager from magnetic and
electric fields produced outside the room while allowing the
magnet of the imager to be shimmed to produce a field
sufficiently homogeneous to obtain the desired images. The scan
room comprises side walls of welded steel plates symmetrically
encircling the MR device and spaced therefrom, a floor of welded
steel plates extending between the side walls below the imager,
and a ceiling of welded steel plates extending between the side
walls above the imager. The plates are made of a steel having
sufficient thickness to reduce the fringe magnetic field outside
-- 6
the housing to below five gauss and means is provided for
electrically insulating the plates from earth ground e~cept at
one point. An opening in -the side walls providing i,ngress and
egress to the room and the irnager, and door means is provided for
closing the opening.
More specifically, this invention seeks to provide a scan
room that will shield the imager from RF radiation of from 1 MHz
to 100 MHz generated outside the scan room by attenuating these
electromagnetic fields by 120 dB or more. The scan room will
also attenuate electrical powex line noise by at least 100 dB at
the MR operating frequency. This will enable one to obtain
superior MR images in shorter data acquisition times.
This invention also seeks to provide a steel scan room for
an MR imager that is made primarily of relatively low cost steels
lS that attenuate the fringe field intensity to the range where
relatively high-cost steels with high magnetic permeability can
be employed with economic advantage. A further advantage of this
invention is to show how the high magnetic permeability steels
can be used only at selected areas to keep material and labor
costs to a minimum.
These and other aspects, advantages and features of this
invention will be apparent to those skilled in the art from a
consideration of the specification including the attached
drawings and appended claims.
In the Drawings
Figure 1 is a sectional view of a rectangular scan room
constructed in accordance with this invention taken along line
1--1 of Figure 2.
Figure 2 is a front view of the scan room of Figure 1
looking in the direction of the arrows 2--2.
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Figure 3 is a vertical section through one side of the scan
room of Figure 1 showing the construction of the ceiling, side
walls and floor or base of the scan room.
Figure 4 is a sectional view on an enlarged scale taken
along line 4--4 of Figure 3.
Figure 5 is a sectional view on an enlarged scale taken
along line 5--5 of Figure 3 showing the connection between the
end of the curved ceiling and the end wall of the room.
Figures 6 and 7 are vertical sectional views of alternate
connections between the edge of the curved ceiling and the top of
the side walls.
-- 7 --
>
~ Figure 8 is a sectiona:L view through a wave guide
,~ extending through a side wall of the scan room through which
' utilities t air-conditioning and the like can pass into and
out of the room.
; 5 Figure 9 is a sectional vlew through a window in the
front wall of the scan room through which activities in the
scan room can ~e observed from outside.
t Figure 10 is a sectional view taken along line 10--10
of Figure 13.
Figure 11 is a horizontal section of the door provided
in the front wall of the scan room and the latch that holds
the door shut.
Figure 12 is a view looking in the direction of arrows
12-~12 of Figure 11.
` 15 Figure 13 is a view in elevation of the rear of the
scan room.
Figures 14, 15, and 16 are sectional views taken along
lines 14--14, 15--15, 16--16 of Figure 13.
Figure 17 is a graph showing the wall thickness
required to reduce the fringe magnetic field to 4 gauss at
the outside of the walls of the scan room with a 10
kilogauss magnet using cold-rolled steel for the walls of
the scan room.
Figure 18 is a graph showing DC permeability for
un-annealed steels.
Figure 19 is a graph of the field strength versus
distance for a 10 kilogauss superconducting magnet.
; 8 -
.
Figure 20 is ~a graph compaxing ~he permeability of a
highly permeable steel w~th that of cold-rolled steel.
Figure 21 shows the fringe field strength for 3.5
kilogauss magnet located inside a rectangular scan room to
show how the wall thickness of the scan room can be reduced
in certain areas to show how the walls can be varied in
thickness while main'aining ~he five gauss line inside the
walls of the scan room.
Figure 22 is a cross~sectional view through a portion
of the wall of the scan room designed to allow plates of
high permeability to be added or removed as required to
selectively increase and decrease the wall thickness and its
ability to contain the fringe magnetic field of the magnet.
Figure 23 is a curve of the actual measurements of the
magnetic field outside a scan room constructed in accordance
with this invention as the magnet current increases to the
operating current of 60 amperes for a 3~5 kilogauss magnet.
In general, for a spherical shell of wall thickness
and radius y , constructed of material with permeability
~, the attenuation factor ~ is approximately
~ -. 1 +
Thus a higher permeability and wall thickness attenuate
more.
As shown in Figure 21, the fringe magnetic fields of
the magnet of an MR imager are generally ellipsoid in shape
and extend out further from the poles of the magnet then
fxom the`sides. ~he ideal room shape would thus be an
ellipsoid ~hat matched ~he ~ield contours. The thickness of
the walls required to a~tenuate the fringe magnetic fields
vary with the distance the walls are from the magnet.
~-- 5 Another factor is whether the walls axe facing the parallel
fields, which are those parallei to a line extending through
the poles of the magnet, or whether they are facing fields
¦ that are perpendicular to the magnetic axis, as shown in
Figure 17. Obviously, fro~ ~he shape of the fringe fields,
as shown in Figure 21, a scan room that is cylindrical in
~ cross-section and positioned with its longitudinal axis
- parallel to the magnetic axis would be a desirable shape for
the scan room. Such a room cculd be constructed from a
cylindrical steel tank or large steel duct.
In most cases, however, the most practical shape for a
scan room will be one that is rectangular in cxoss-section,
such as the scan room shown in Figures 1 through 16. Such a
room has what is conventionally called side walls, end
walls, a ceiling, and a floor. A scan room that is cylin-
drical in shape would have just a side and end walls. So it
is intended in this specific~tion that "wall" is a generic
term that includes ceilings and floors as well as end walls
and side walls.
As shown in Figure 1, MR imager 10 is positioned in the
geometric center of the scan room. Platform 12 supports the
patient in thé proper position to obtain the desired image
or images. The MR imager is surrounded completely on all
sides by solid steel walls made up of abutting steel plates
i
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- 1 0
~;~
tha~ are`connecte~ electrically together to provide no
opening through which magnetic ~ringe fields or RF radiation
can escape or enter the room. In the preferred ernbodiment,
~ the abutting plates are connected by weld metal that pro-
:~ 5 vides a good electrical connection between the plates and
`~ structural integrity. The breaks in this solid steel
enclosure are limited, preferably to the openings through
which utilities, such as air-conditioning, electrical
conduits, waterlines or the like ex~end into the room, the
door that must be provided to allow people to enter and
j leave the room, and, preferably, a window ~o allow the
people outside to look in and the people inside to look out.
The construction of the door, window, and other openings in
the otherwise solid steel room that provide these functions
while insuring that no fringe magnetic field or RF radiation
escapes or enters the room will be described in detail
below.
Generally, the scan room has side walls 14a and 14b,
j flat front and back walls, 16a and 16b, floor 18, and
arcuate roof or ceiling 20 that curves upwardly away from
~ thé imager in a direction perpendicular to the magnetic axis
; of the MR imager. As shown in Figure 3, floor 18 rests on
concrete slab 22. This will generally be the case because
the steel room and the imager are both heavy. Above the
concrete base is layer 24 of asbestos board a material that
has a high eléctrical resistance. On top of the transite
base at spaced intervals are strips 26 of steel plate,
preferably made of stainless steel. The space between the
'`i -- 1 1 --
strips of stainless steel is filled with floor stone 28,
which is covered by steel plate 30. This layer of steel is
made up of a plurality of plates welded together since steel
plate is usually supplied in strips 48" wide. The steel
5 floor is then covered by vinyl sheet tile or some other
~ suitable flooring. Preferably, although it is not shown in
the drawings, a sheet of water impervious material that also
has a high electrical resistance, such as polyethylene is
placed between the slab and the asbestos board layer to
provide a moisture barrier between the slab and the floor of
the room. The combination of the relatively thin sheet of
~ polyethylene and the relati~ely thick layer of asbestos
; board that increases the space between the metal in the
floor and the ground results in a xesistance to ground in
excess of 50,000 ohms. The room is then grounded at only
one point, 31, to prevent ground loop noise.
The side and end walls ar~ generally of the same con-
~ struction. In Figure 3, side wall 14a is made up of a solid
3 wall of steel. If the thickness of the w211 iS going to be
say 1/4~ or 1/2', then steel plate of that thickness can be
used. Here again, the wall will be made up of a plurality
of plates welded together. But if the thickness is going to
be greater than that, then preferably the wall will be made
up of a plurality of overlapped plates. For example, in the
scan room that has been constructed, the wall thickness was
specified to be one inch thick. For convenience and ease in
construction, the walls were made up of four layers of steel
plate ~" thick. In such an arrangement, the welded joints
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~ - 12 -
.
between àdjacent plates should be gtaggered so that the
chance of RF radiation or magnetic fields escaping from or
entering into the room through the welds i5 reduced. In the
drawing, the wall is shown solid for convenience.
The solid steel end and side walls are supported by a
plurality of spaced hollow steel columns that are rectangu-
lax in cross-section. These columns are designated by the
¦ number 32. A relatively narrow foot plate 34 is welded to
floor plate 30 to support columns 32 and side wall 14a. The-
side wall also should be welded to foot plate 34 to provide
a sealed joint between the bottom of the wall and the floor
of the room.
Extending between the top of the side walls in the
direction that ceiling 20 curves are a plurality of tie rods
34. These tie rods extend through the walls adjacent their
upper ends, through one of the rectangular columns, and are
secured in place by nuts 36 as shown in Figure 6. For
aesthetic purposes, inner wall 38 made out of sheet rock or
some such material is positioned to cover the walls and
provide a more pleasing aspect to the room. In the same
manner, an acoustic tiled ceiling 40 can be suspended from
steel ceiling 20 as shown in Figure 3. The acoustic tile
reduces noise reverberation in the room produced by switch-
ing the gradient fields in the MR imager. Also, sheat rock
wall 42 can be located outside the room to ~over the steel
construction of the room so that the casual observer would
have no idea that he was entering or leaving a solid steel
room.
- 13 -
Fig~re 4 shows the details of construction of the
connections between the end walls and the gide walls. In
the Figure, it is the connection between side wall 14a and
end wall 16b. The walls are shown as solid steel although,
as explained abo~e, they more likely will be made up a
number of overlapping plates. The edges are beveled as
shown and welded on the inside and outside.
Figure S shows the detail of the connection between
ceiling 20 and one of the end walls such as end wall 16b.
The ceiling is shown as beir.g made up of a plurality of
overlapping plates and end wall is shown as a solid plate.
- Here again they both could DC made up of ovexlapping plates.
Alternate connections between the edges of arcuate ceiling
20 and side wall 14a are shown in Figures 6 and 7. In
lS Figure 6, the ceiling is made up of a plurality of plates
and the side wall is a solid plate. The weld pattern shown
in Figure 6 is preferred to ensure a seal between the plates
that will prevent the ingress of RF radiation. In Figure 7,
with the side wall and ceiling both being made up of
overlapping plates, each layer of plates will be welded to
the corresponding layer in the ceiling as the room is
assembled.
Figure 8 shows the detail of a wave guide extending
through front wall 16a through which air can be pumped into
and out of the room. In order to prevent escape of RF
radiation, tubular member 50 must be made of an electrically
conductive material and shaped to provide the proper atten-
uation to such RF radiation as is produced by the imager or
- 14 -
that is likely to be on the outside of the room. To insure
the proper protection, here again from both the inside and
; outside radiation, the wave guide should be designed to
achieve an attenuation of at least 80 dB at a frequency of 1
MHz.
A window is provided in the otherwise solid steel room
so that people inside won't feel so isolated and people
outside can look in to see what's going on. The window must
be able to attenuate the ~F fields in the same manner that
the steel walls do. Such a window is shown in Figure 9.
The window is located along the axis of the magnet so the
patient is visible to the operator lo~ated outside~ It
; includes two-spaced glass panes 60 and 62 mounted in foam
rubber pads 64 and 66. Located between the panes are two
parallel copper screens 68 and 70 that attenuate the
electromagnetic energy passing through the glass panes.
These screens are soldered at each along their edges top and
bottom and sides to brass spacer 72. The spac-r in turn is
I soldered to a U-shaped member made up of angle 74 and plate
76, both of which are made of brass. The U-shaped memher is
attached to front wall 16a by mounting screws 78. Brass
face plates 80 and 82 are attached to the U-shaped member
and extend partially over glass panes 60 and 62. Strips 84
of tin copper braid are positioned between the U-shaped
member and front wall 16a to prevent leakage of any RF
signals into and out of the room. The window must be
relatively small so that magnetic fringe fields do not
extend out significantly from the window.
- 15 -
The `details for the ~oor of the scan room are shown in
Figures 11 and 12. The door includes a rectangular frame
made up of tubular steel members 100 that are rectangular in
cross-section. Covering the frame on both sides is outer
layer 102 of stainless steel, inner layer 104 of cold-rolled
steel, and middle layer 106 of coppe~. The door for a 3.5
kilogauss MR imager was made up of 44 ounce copper which is
¦ 1/16" thick as the inner layer. The stainless steel layer
was 1/16" thick stainless and the inner steel layer was 16
lo gauge cold-rolled steel that could be spot-welded to the
; metal frame. The edges of the door are covered by U-shaped
members 110 and 112 made of ~rass. The door is mounted for
pivotal movement into the room by hinges 114 that are
attached to L-shaped brass members 116. On the outsidel
L-shaped members 116 are connected to brass face plates 118
and the joint between these two members and front wall 16a
are sealed by the tin copper braid used with the window.
Face plate 118 extends into the opening for the door and
supports beryllium copper finger springs 122 that engage the
door as shown in Figure 11 to prevent RF frequencies and
magnetic fields from entering or leaving the room. In the
same manner, on the opposite side of the door frame, plate
124 supports beryllium copper finger springs 122. Strike
plate 126 is attached to plate 124 and is engaged by latch
arm 128 when the door is closed as shown in Figure 11.
Latch arm 128 includes roller bearing 120 mounted adjacent
its end that actually engages strike plate 126. The roller
bearing reduces the friction between the latch arm and
- 16 -
strike plate ther~by allowing the latch arm ~o be rotated
into and out of engagement with the strike plate with a
minimum tox~ue. Handles 132 and 134 are located on the
inside and outside of the room, respectively, to allow the
door to be opened and closed from both sides.
The scan room is built without ~he MR imager in place
inside. Therefore provision must be made for moving the
I imager into position in the room after the room has been
built. The door normally used for people to move into and
' 10 out of the room is not large enough for this purpose.
; Therefore, provision is made for providing an opening in the
rear wali of the room large enough to allow the equipment to
be moved in and out, but which is not normally open and is
more or less semi-permanently in place most of the time.
Such a temporary door is shown in Figures 13 through 16. It
consists of solid steel panel 140 that is tack-welded in
place in an opening provided in rear wall 16b. Tack welds
are used to provide at least some resistance to the passage
i of magnetic and RF fields between the ends of the plates but
which can be easily cut when it comes times to open the back
of the room.
To provide the necessary seal to prevent leakage into
and out of the room, metal strips 142 of metal plate extend
around and overlap the joint between the panel 140 and back
wall 16b. These plates are welded at one end to the panel
as shown in Figure 14 and have an opening through which
bolts 144 can engage tapped holes 146 provided in the back
wall. The top of panel 140 is secured to wall 16b as shown
- 17 -
in Figure 16 by we~d 146 and also by metal strap 148 that is
bolted to columns 150 and 152 overlapping where these
columns have been cut so that these columns can continue to
support the end wall even though they are cut. The bottom
end of panel 140 is welded to the floor as shown in Figure
10 and is further connected to columns 150 and 152 by angle
154 that is bolted to base lSk. To remove the panel, the
i bolts are removed and the welds cut between the panel and
the back wall.
As stated above, a scan room was built in accordance
with this invention to house a 3.5 kilogauss MR imager. It
is located at 8968 Kirby Drive in Houston, Texas. Calcu-
lations prior to the construction of the room suggested that
it should be 30 feet long, 20 feet wide, the side walls
;~ 15 should be 9 feet high, and the ceiling should have a radius
of curvature of 26' 7-3/8" measured from the inside surfac~
of the ceiling. The side and end walls and the ceiling were
made of fsur overlapping layers of cold-rolled steel plate
~" thick to provide a total thickness o~ one inch of
1 20 cold-rolled steel. The floor underneath the imager as
stated above was constructed of solid steel plate 1/8" using
A36 steel. The floor did not require the same thickness of
metal that the walls and ceiling did because it is closer to
the magnet center than the ceiling. To symmetrize the
field, the floor under the magnet was non-magnetic stainless
steel. Copper sheet could also have been used. After the
room was constructed and the imager installed, measurements
of the magnetic field strength were made at the outside
I
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walls and`, in all çases, the magnetic field outside the room
was below S gauss. Figtlre 23 is a plat of the actual
measurements of magnetic field taken outside the west wall
of the room about shoulder high as the current supplied to
the magnet increased to 60 amperes. The Rirby site is
located within 25 feet of a parking lot and no disturbance
has been observed even when liquid helium i8 being delivered
i by a truck. By using the curved ceiling, the distance
`` between the ceiling and imager directly over the imager is
increased where the magnetic field would be the strongest.
This allows the thickness of the ceiling to be reduced
because of the increased distance between the ceiling and
the imager.
Figure 17 shows how the wall thickness required to
achieve 4 gauss at the wall with a 10 kilogauss magnet using
cold-rolled steel varies with the distance that the wall is
from the magnet. For example, for a wall thickness of 1.2"
the side walls th~t are facing the perpendicu~ar component
of the magnetic axis would be required to be forty feet from
the magnet. The end walls would be approximately fif-
ty-seven feet. For a two inch wall thickness, the side
walls can move into thirty feet and the end walls to approx-
imately 47~ feet. This is a very large room and 10~5 of
steel. Cold-rolled steel, of course, i8 much cheaper than
steels having a high permeability. As shown in Figure 20,
there is a substantial difference between the permeability
of this steel and cold-rolled steel when the flux density in
the steel is between 4,000 and 8,000 gauss. sut the
.
-- 19 --
differenc`e drops off rapidly as the flux density increases
leaving little advantage between the high permeability and
very expensive steels and the cold-rolled skeel3.
Therefore, enough cold-rolled steel should be used to reduce
the flux density to where the maximum benefit is obtained
from the high permeability steel.
By using a careful mixture of the two different steels,
however, a substantial savings can be realized in total
weight of the room and its cost. For example, referring to
Figure 21 again, if the walls of the room are made of a
cold-rolled steel just thick enough to provide the minimum
wall required to confine the magnetic field at the locations
furthest from the magnet and then, at the areas where this
minimum wall thickness would not be su~ficient, adding a
la~er of high permeability steel to provide the necessary
attenuation at these points, a reduction of the total weight
of the room would result and provide a substantial savings
in construction cost.
An example of how this can be done is shown in Figure
22, here two layers 160 and 162 are welded together in
overlapping position with a small spacer between the two
metal plates. Spacer 164 can be any non-permeable material,
such as air, wood, chips and wallboard. The combined thick-
ness of the two plates 160 and 162 are designed to provide
the minimum wall required to maintain the magnetic field.
Then in the areas where this wall thickness is not adequate,
additional plates 166 and 168 of steel having a high per-
meability can be bolted in place at selected locations to
- 20 -
provide the necessary attenuation factor ~o that portion of
the room. As shown in Figure 22, ~hese plates can be
attached by studs 170 that are attached to the outside
surface of plate 142 and extend through holes in plates 166
and 168. The pla*es are held in place by nuts 152.
As shown in Figure 21 an ellipsoid magnetic fringe
field located inside a rectangular room causes the wall
thickness requirement to vary because of the changing
`` distance between various portions of the wall and the
10 magnet. As shown in the Figure, the center of ~he side
walls should be thicker than the walls adjacent the corners.
Field contours are computed as if no shielding were
present and after the particular shape of the room is
superimposed on the field contour, the field intensity at
15 the walls of the room are determined. The thickness of the
steel at each location can be then scaled according to the
field strength at that location using the B versus H (flux
density versus magnetic induction) curves for the steel or
steel composite. Such a curve is shown in Figure 18 relat-
20 ing to cola-rolled steel and a ste~l manufactured by U.S.
Steel called 283C.
Then, as stated above, the room can be built with a
minimum wall thickness and then the wall thickness augmented
wherever required and only where required by adding addi-
tional steel plates in the manner shown in Figure 22. In
addition, should the magnet be changed to one producing a
stronger field, the room can be adapted to it by adding
additional plates.
, - 21 -
: Any ,particula~ scan room should be designed to provide
only the protection desired. ~or example, the room in
Houston was on the ground floor of a multi-story building.
There was no need to protec~ anything in the ground below
the foundation so only the steel req~ired for symmetry was
usedO If the scan room is in a one-story building, it may
j be necessary to make only the side walls of steel.
i From the foregoing it will be seen that ~his invention
is one well adapted to attain all of the ends and objects
hereinabove set forth, toyether with other advantages which
are obvious and which are inherent to the apparatus and
r structure.
It will be understood that certain features and subcom-
~`` binations are of utility and may be employed without refer-
ence to other features and subcombinations. This is contem-
plated by and is within the scope of the claims.
Because many possible embodiments may be made of the
invention without departing from the scope thereof, it is to
! be understood that all matter herein set forth or shown in
the accompanying drawings is to be interpreted as illustra-
tive and not in a limiting sense.
