Nuclear magnetic resonance imaging device

A nuclear magnetic resonance imaging device has a magnetic structure which defines a cavity for housing at least part of the body of a patient with a magnetic field which permeates at least part of the cavity, an opening provides access to the imaging cavity from outside of the magnetic structure and electrically conductive and electrically grounded shielding provided at the opening is movable between a first inactive configuration wherein the opening is substantially open and an active configuration in which the opening is at least partially closed by the shielding.

FIELD OF THE INVENTION

The invention relates to a Nuclear Magnetic Resonance Imaging device having at least one rigid shielding member for at least partially closing one or more open sides, which can be moved between a first open position and a second (at least partially closed) position, the at least one rigid shielding member preferably being made of an electrically conducting material or coated with layers made of an electrically conducting material.

BACKGROUND OF THE INVENTION

An object of the present invention is to provide improvements in the electromagnetic shielding of MRI scanners by at least closing parts of the openings of the scanners which do not need to be open to allow the body under examination to access a cavity of the device where the imaging occurs.

SUMMARY OF THE INVENTION

Particularly in MRI devices which are of the so called dedicated type, where the MRI scanner is relatively small so that the entire body of the patient cannot be housed inside the imaging cavity and also where the static magnetic field generated within the imaging cavity is of low or medium field strength, it is desirable to reduce the infiltration of electromagnetic noise inside the cavity and in the receiving coil. MRI signals are relatively low in strength and electromagnetic noise can reduce the quality of the acquired image. One solution known in the art to address this situation consists of at least reducing the opening of the imaging cavity by using electromagnetic shields which are grounded together with the electromagnetic shields provided in the scanner itself. Other kinds of shields have also been used. One optimum solution provides grounding the body of the patient under examination through the shields.

Devices having only one or two opposite openings for accessing the imaging cavity of the scanner can have simple solutions. However, problems arise when the scanner is not annular in shape. Scanners which have a “C-shape” or a “U-shape” have three open sides and it is particularly difficult to shield the imaging cavity of these devices. These kinds of MRI scanners allow different body parts of the patient to be positioned in the scanner. For positioning the different body parts in the scanner, different portions of the three open sides of the scanner are occupied by the patient so that a very flexible and also rapidly movable shielding member is needed.

The present invention provides for an MRI apparatus which has in combination a shielding member which can rapidly assume different configurations to partly close the open sides of the imaging cavity and with relatively simple devices. Furthermore, it is an object of the present invention to provide an MRI device having a shielding member which can maintain a temporary configuration for imaging a certain body part or anatomical region thereby reducing the time consuming process of closing the opening with an electromagnetic shield.

DETAILED DESCRIPTION OF THE INVENTION

Referring toFIGS. 1 to 3, a Nuclear Magnetic Resonance Imaging device comprises a magnetic structure1. The magnetic structure1shown in these figures is C-shaped and defines an imaging cavity with three open sides about its perimeter. The cavity is defined by an upper side101, a lower side201and a vertical side301. These three sides101,201, and301cover the magnetic structure and other operating members, and are made of an appropriate material, e.g. of plastic or the like, and the imaging cavity has a recess401, in a predetermined area, for accommodating a fastening base of a receiving coil (not illustrated). The magnetic structure enclosed by the cover is formed by two spaced apart plates of ferromagnetic material and forms a yoke which is connected together by a vertical wall. Two poles covering an intermediate layer of magnetized material are supported to the inner sides of the horizontal plates.

The free end edges of the upper and lower sides101and201are rounded, preferably with a semicircular profile.

A table2is associated with the magnetic structure1. The table2consists of a first part102and a second part202. The first part102has a supporting surface with a hollow region302which has a shape that is complementary and a size corresponding to the lower side201of the magnet1. Although the first part102can have any outer shape, it preferably has a circular segment shape with an angular extension of more than 180°, so as to fully surround the central U-shaped hollow, preferably with a circular rounded portion. The first part102of the table is designed to be fitted around the side201of the magnet1, which complements the supporting surface. The first part102of the table is supported by at least a pair of wheeled legs (not shown), which are provided at least in the end side of the first part that is configured to connect to the second part202of the table which is also supported by one of more legs3having wheels4. The one or more legs3are provided on the end side of the second part202of the table opposite to the end which connects to the first part102.

The magnetic structure1may be provided with a member41for shielding electromagnetic noise, which is configured in the form of a rigid oscillating frame or shield frame141which carries shielding panels or shielding curtains that are slidable about the frame.

In the illustrated embodiments, the shield frame141and the shielding panels or curtains can have any desired amount of extension in order to close partially or completely one, two or all three open sides of the magnetic structure. Obviously, shielding panels or curtains with different amounts of extension may be also provided either alternatively or in combination.

As shown in the figures, the shield frame141is U-shaped and has a shape which is complementary to the outside perimeter of the upper and lower sides of the magnetic structure1. The shield frame141is hinged so as to be able to oscillate about a horizontal axis, parallel to the closed vertical side301. In the preferred embodiment, the axis is substantially at the same level as or at a slightly higher level than the upper side101of the magnetic structure1.

The shield frame141is formed by two “U-shaped” guides which are held at a distance corresponding substantially to the distance between the two horizontal upper and lower sides101,201of the magnetic structure1by transverse elements241that connect the two “U-shaped” guides at their corresponding ends. The distance of the “U-shaped” guides is such that by oscillating the frame downward into the area of the open sides, the frame substantially encompasses the edges of the upper and lower sides of the magnetic structure defining the cavity and abuts to the peripheral surfaces of two plates which form the upper and lower sides. In the preferred embodiment, the shield frame141is made of or provided with an electrically conducting material or is coated by a layer made of an electrically conducting material.

In the present embodiment, electrical connections which interact with complementary electrical connections disposed in a corresponding position on a contact surface of the first part102of the table and/or on the side201of the magnet are provided at the peripheral edges and possibly also at the faces of the “U-shaped” frame141which overlay the peripheral edges of the upper plates of the magnetic structure1at which the frame is hinged. Along the “U-shaped” guides, at least one shielding plate or one shielding curtain may be provided. The shielding plate and the shielding curtain each engages with their edges the upper and the lower guides of the “U” shaped frame141. Furthermore the shielding plate and the shielding curtain are preferably made of or layered with electrically conductive material which ends with electric contact elements at the edges engaged in the guides. These elements cooperate with electrical contact elements provided in the two guides. The contact elements are configured to allow sliding and thereby also provide an electrically conductive contact between the shielding panel or the shielding curtain and the guides.

FIG. 2illustrates a shielding panel50in the form of a rigid but flexible thin plate. The shielding panel can slide along the guides in which it is engaged with its upper and lower edges. InFIG. 2, the shielding panel50is shown with continuous lines in a first position and with discontinuous lines in a centered position at the rounded central part of the “U-shaped” guides.

FIGS. 3A and 3Billustrate an embodiment of the electrically conductive contacts which can be provided inside the guides. In this embodiment, a row of tongues52are cut out from a track51of electrically conductive material. The tongues52are connected at one longitudinal end to the track50and they further have a convex shape with which they protrude from the track surface toward the center of the guide. Such contact elements can be provided on both side walls of the guides and the panel50can slide between each pair of facing tongues52which come into electrical contact with a continuously electrically conductive track on each side of the panel, as illustrated inFIG. 3B.

With reference now toFIGS. 4A to 4C, a cross sectional view shows the panel engaged in the two opposite guides provided with the electrically conductive tongues. According to a further improvement, as illustrated inFIGS. 4A to 4C, the shielding panel50may be engageable and disengageable with the guides. This can be achieved in several ways. One of the several ways is illustrated inFIGS. 4A to 4C. Due to the fact that the panel is substantially vertical, at least while in the active position of the shield, i.e. when the shield is swung downwards, the dimension of the panel perpendicular to the guides is less than the distance of the bottom wall of the guides, but sufficient to be brought into an engaged position in which the two opposite edges of the shielding panel still remain engaged between the rows of electrically conductive contact elements52on the lateral sides of the guides. Advantageously, these contact elements, in this example the tongues52, are positioned near the side of the guide opposite to their bottom. At a certain distance of the lower edge of the panel, limiting lateral protrusions150are provided which limit the depth of penetration of the lower edge of the shielding panel50in the lower guide at a level at which the upper edge of the shielding panel still is engaged between the two facing rows of electrically conductive contact elements, i.e. the tongues52. These protrusions150abut against an upper side of the lower guide along which the limiting protrusions slide when the shielding panel is displaced.

Thus for disengaging the shielding panel from the guides, the panel can be shifted upwardly until the lower edge can slip out of the lower guide, then the panel can be inclined laterally, bringing the lower edge away from the lower guide, so that by sliding the panel downward also the upper edge can be disengaged from the upper guide. In order to engage a shielding panel into the guides, one can carry out movements which are the reverse of the ones described above for disengaging the shielding panel from the guides.

This further improvement allows one to provide a series of differently dimensioned shielding panels (referring to their dimension in the direction of sliding along the guides). The differently dimensioned panels can be used alternatively to one another or in combination, it being possible to engage two or more panels in the guides to form a shielding wall inside the “U-shaped” frame which closes as much as possible of the open sides of the magnetic structure, leaving open only passages having dimensions that are limited to the strictly necessary dimensions for introducing a body or an anatomical part of a body inside the cavity of the magnetic structure.

Thus the opening of the shielding member can be varied according to the dimensions of the patient and to the morphology of the anatomical part which has to be placed into the imaging cavity of the magnetic structure. Furthermore due to the fact that the entire frame41of the shielding member can be swung upwardly, the imaging cavity can be completely opened without the need to change the position of the shielding panel, so that it is possible to access the cavity of the magnetic structure for carrying out operations without changing the position of the shielding panel or shielding panels.

The electrical contact of the frame of the shield member to ground (i.e., to earth potential) can be obtained through the hinges which can be made of electrically conductive material. In addition, the electrical contact of the frame to ground can be obtained by further electrically conductive contact elements which are positioned on the contact surfaces of the frame141which abut against contact surfaces at the periphery of the upper plate of the magnetic structure, at the patient table or at the lower plate of the magnetic structure and at the transverse wall connecting the upper and lower plates. The contact surfaces can be provided as illustrated inFIG. 15Awith analogous contact elements as the one described for the guide and illustrated inFIG. 3.

Alternatively, the contact elements on the contact surface of the frame of the shielding member or the contact surfaces at the periphery of the upper plate of the magnetic structure, at the patient table or at the lower plate of the magnetic structure and at the transverse wall connecting the upper and lower plates, may be formed by an electrically conductive track which is continuous while the other contact surface carries the contact elements according toFIG. 15A.

By providing several shielding panels which can be engaged together in the frame of the shielding member, each shielding panel can be provided with electrically conductive elements at edges oriented transversally with respect to the sliding direction, so that an electrical contact can also be established between adjacent shielding panels. The contact elements can be a peripheral edge or band of electrically conductive material or contact elements can be provided which are made according to the example ofFIGS. 6A to 6C.

FIGS. 5A to 5Cillustrate a variation of the shielding panel. In this case, the shielding panel is formed by several staves250which are hinged one to the other. In this embodiment, for example, the staves may be hinged by an external coating layer of plastic material which forms a hinge film between each two adjacent staves. The staves and/or at least one of the layers can be made or layered with electrically conductive material. Alternatively, or in combination, a further layer of electrically conductive material can be provided as an intermediate layer between the staves and one or both of the external coating layers. Also, in this example, the shielding panel is provided with electrically conductive contact elements or with an electrically conductive track at one, and preferably at both, edges engaged in the guides of the frame of the shielding member.

The electrical contact elements of the guides can be made according to the previously described example ofFIG. 3. The staved shielding panel according toFIGS. 5A to 5Ccan also be engageable and disengageable in and from the frame of the shielding member in a similar way as described in connection with the embodiment ofFIGS. 4A to 4C.

FIGS. 5A to 5Cillustrate different positioning of the shielding panel in the frame of the shielding member. In the present example, two staved panels are provided which can be engaged together in the frame of the shielding member in order to obtain different configurations of partially closing the open sides of the magnetic structure. InFIG. 5A, a frontal opening is left free substantially corresponding to the arched part of the frame of the shielding member. The width of the opening is illustrated with dotted lines and the lateral limits are illustrated with a continuous line. The open part of the shielding member is indicated by “O”. Considering the above described embodiments, it appears clearly that the open sides of the shielding member are not completely closed by the body under examination which has normally rounded shapes. In this case, as it will be described later on, the remaining open spaces between the body part passing through the openings left free in the shielding member and the shielding frame can be completely closed by providing soft cushion-like elements having an outer layer of electrically conductive material and which can provide for an electrical contact between the frame, the edges of the shielding panel and the part of the body passing through the opening. To this end, the frame of the shielding member can have further electrically conductive contact elements which are positioned on the surfaces oriented against the internal part of the frame. The electrically conductive track on the lower guide will provide for a direct contact with the part of the body which lays on it, although depending on the morphology an electrically conductive insert in the form of a cushion can be provided also between the lower guide and the body part or from the upper side of the guides. The electrically conductive inserts are pressed between the part of the body passing through the openings left free in the shielding member and the corresponding parts of the frame of the shielding member and the edge or edges of the shielding panels.

Alternatively or in combination, the shielding panels can be provided with at least one edge which is transverse to the sliding direction and which is shaped according to the morphology of the body part which has to pass through the opened side. An example is shown inFIGS. 6A and 6C. In this case, a shaped edge350may be made by a soft compressible element which has an external electrically conductive layer which is in electrical contact with the electrically conductive material of the shielding panel. Thus the shaped edge of the shielding panel can be easily adapted to the different morphologies of the same body part due to dimensional differences among different patients. The shaped edge can also exert a certain action of compression against the part of the body passing through the opening in the shielding member in order to provide for a better electrical contact between the human body and the shielding member. The shaped edge of the shielding panel can be made in the form of a shielding pillow as described above or it can be made by a tubular element which is secured to the edge of the shielding panel and which is made of elastic material such as rubber or the like. The tube may be made of electrically conductive rubber or have an external layer of electrically conductive material such as electrically conductive tissue.

Due to the fact that the shielding panels can be engaged and disengaged in and from the frame of the shielding member, several shielding panels having differently shaped edges for fitting and electrically contacting the part of the body passing through the opening in the shielding member can be provided. These shielding panels can have a short extension in the direction of sliding so that they can be used as end shielding panels in combination with further shielding panels forming the major or central part of the shielding member.

Furthermore, different panels50having different dimensions in the direction of sliding can be provided as illustrated inFIG. 6C. The shielding panels having the shaped edge350and the other shielding panels having different dimensions in the direction of sliding can have contact edges450which are of electrically conductive material, and which while abutting against the facing edge of an adjacent panel, provide for the continuity in the electrical conduction between adjacent panels of the shield.

As illustrated inFIG. 6C, alternatively a shielding panel or curtain can be provided with an opening which is defined by an electrically conductive edge350similar to the shaped edge.

FIGS. 7A and 7Billustrate a further embodiment of the shielding panel provided in combination with the frame of the shielding member. In this case the shielding panel has a bellows or folding door type construction. Several adjacent strips55are hinged together at their facing edges by mechanical hinge elements or by flexible bridges of material. In the present embodiment, the flexible bridges are formed by the same material as the strips and folding lines are provided at the bridges consisting in a reduction of the thickness of the material or in partial cuts. The strips engage the upper and the lower guides with their shorter ends. In a compact arrangement, the shielding curtain or panel is folded on itself in such a way that the strips are oriented transversally to the sliding direction defined by the guides and the strips are laying one against the other with their larger surfaces. The shielding panel according to this embodiment can thus be shortened or lengthened between a minimum and a maximum length in the sliding direction. In addition, the shielding panel according to this embodiment can be displaced along the guides of the frame of the shielding member.

According to a further feature of the embodiment ofFIGS. 7A and 7B, rotatable locking elements can be provided, which cooperate with the upper and lower guides, at certain distances along the overall length of the shielding panel in the extended position. The rotatable locking elements help in maintaining the shielding panel in a certain condition of elongation. The locking elements are provided at the ends of a rod57which is rotatably supported by some of the strips55. Ends of the locking elements protrude inside the corresponding guide and over the corresponding edge of the strip or of the strips55by which the rod57is supported. The locking elements are secured at these protruding ends of the rotatable rod in a manner to rotate with the rod57. The locking elements are formed by small levers58which can be rotated in a position parallel to the sliding direction and in a position transverse to the sliding direction of the panel. At these positions they protrude laterally outwardly from the two sides of the panel abutting against at least a wall of the guides to lock the panel or at least the strip or strips55by which the rod57is supported relatively to a sliding in the guide. As shown inFIGS. 7A and 7B, it is preferable for one rod57with the associated locking lever58to be supported at the free edge of the first strip55forming the panel and at least one further rod57with its locking levers to be supported at the free edge of the last strip55. Further rods57with their associated locking levers are provided in intermediate positions of the shielding panel and preferably the rods are supported at the folding line or hinge between two adjacent strips55thus forming a hinge connecting the adjacent strips together.

As disclosed in combination with the preceding embodiments, the shielding panel according toFIG. 8is preferably made of electrically conductive material or is at least layered with a sheet or film of electrically conductive material. Furthermore, the electrical contact with the guides can be achieved by providing rotatable rods57and associated locking levers made of conductive material or having at least a layer of conductive material thus ensuring electrical contact between the shielding panel and the guides.

In an alternative construction of the shielding panel according to the present invention, a bellows or folding door type shielding panel is made of a shaped element of rubber or another elastic material. In its unstressed condition, the panel is in the compacted or closed condition corresponding to the condition of the minimum length in the sliding direction. The rods57can be embedded in the thickness of the rubber or elastic material by inserting the rods in the mold and injecting the rubber or elastic material. In the same way, a layer or a mesh of conductive elements such as electrically conductive wire mesh can be incorporated in the thickness of the rubber or of the elastic material.

Alternatively to the above described example, the shielding panel or the shielding curtain can be made of an elastic tissue or film which can be expanded in the direction of sliding in order to obtain differently sized shielding panels or curtains starting from a minimum length up to a maximum length defined by the elastic elongation capability of the specific elastic tissue or film.

FIGS. 8A and 8B,9,10,11andFIGS. 12A,12B,12C illustrate still another embodiment of the present invention in which the shielding member comprises a frame and at least one shielding curtain which is of the roll blind type.

In this embodiment, the transverse elements241connecting the ends of the upper and lower guides form a chamber, for example, if made tubular, the transverse elements house a roll60on which the shielding curtain50is wound. Either one or both of the transverse elements241may form a chamber for housing a roll60. The edges of the shielding curtain50engaged in the upper and lower guides may have electrical contact elements or tracks similar to the shielding panels50of the previous examples. These electrical contact elements or tracks provide an electrical contact with electrical contact elements in the guides, for example as shown and described according toFIGS. 3A and 3B.

The shielding curtain may be made of a foil or tissue which may be electrically conductive or layered with an electrically conductive layer. The shielding curtain can be also made of extensible material. Furthermore, at the leading edge, with reference to the sliding direction of unwinding the shielding curtain, a locking mechanism can be provided for locking the curtain in a certain position within the guides. This is preferably if the roll is provided in combination with spring elements which automatically drive the roll in the winding direction. The locking mechanism can be made similar to the locking mechanism of the shielding panel according to the embodiment ofFIGS. 7A and 7B. A rod57is rotatably mounted at the leading edge, for example, in a tubular hem made at the leading edge of the shielding curtain or in a tubular seat to which the leading edge of the shielding curtain is attached. The rod57carries at its ends (protruding inside the upper and lower guides) eccentric locking levers which can be rotated with the rod. The two locking levers58can assume a first position in which they do not interfere or come in contact with the walls of the upper and lower guides and a second position in which they come into contact with a wall of the upper and lower guides and generate a friction coupling to the guides which is sufficient to lock the shielding curtain in the unwind position against the force exerted by the automatic winding device associated to the roll60.

Preferably, as shown inFIGS. 12A to 12E, the rod has, at one or more intermediate positions, one or more radial extensions59which serve as handles for rotating the rod57and the locking levers58.

In the example ofFIGS. 12A to 12E, the leading edge does not carry one rod but instead carries two half rods each one being associated to only one locking lever cooperating with the upper or lower guide. The two half rods are coaxial and each carries one radial lever for rotating the rods.FIGS. 12A to 12Cillustrate a cross section according to a plane perpendicular to the upper and lower guides and below the position of the locking lever relatively to the guide as seen in the direction of the rod which corresponds to the position of the radial levers in the associate cross-section.

FIGS. 12C and 12Erelate to a further variation which provides a panel according toFIGS. 1 to 7in which the same mechanism of the locking levers is provided and in which the locking levers can be further used as a mechanism for engaging and disengaging the shielding panel from the guides, which is an alternative to the one illustrated inFIGS. 4A to 4C. In this case, the locking levers and the associated rods can be provided at both edges of the panel (which are transverse to the sliding direction) and even at several intermediate positions so that they can also be used for providing electrical contact between the shielding panel and the shielding frame. In this case, the rod and the locking levers are of electrically conductive material and the guides are provided at the surfaces coming into contact with the locking levers with electrically conductive tracks or a similar arrangement. For allowing sliding of the panel by maintaining the panel surely engaged in the guides, the friction between the levers and the walls of the guides can be at least partially reduced by providing an intermediate angular position in which the locking levers still cooperate with the walls of the guides but with a reduced contact surface.

An example of the mechanism of the winding rod is illustrated inFIG. 13. A roller160is mounted on a roller mechanism comprising a spring260which is secured between the roller160and a stationary member on which the roller is rotatably mounted, such as for example the two end disks360which have an end pin such as a flat end pin460. The flat end pins460engage a flat hole which can be provided in flat brackets560which are secured inside the tubular transverse element241. The rod50is housed in a hem660of the leading edge of shielding cloth50.

The roller illustrated inFIG. 13is a typical roller blind mechanism which can be used without any considerable change in the present invention.

Obviously also other mechanisms can be used, for example, mechanisms of the known art having a self locking mechanism which stops the roller automatically and which can be released by further turning in the unwinding direction the shielding curtain or cloth before allowing the coil to rotate the roller in the winding direction. Such mechanisms are provided with ratchet gears which allow such functions.

FIG. 10illustrates a side view of the shielding member41according to the present embodiment and in which the shielding curtain is a net or similar structure made of electrically conductive material.

FIG. 10is a sectional view ofFIG. 9along a plane which is parallel to the guides and in which both transverse elements241, connecting the ends of the upper and lower guides together, house a roller blind type shielding curtain. One of the shielding curtains is unwound for a length corresponding to the straight part of the guides and is also locked in this position, while the other is completely wound inside the roller. It has to be appreciated that the locking mechanism at the leading edge of the shielding curtains also has the function of limiting the winding since it abuts against a slit in the wall of the transverse elements241through which the shielding curtain passes outside of the transverse element241. So in the completely wound up condition, the locking mechanism at the leading edge of the shielding curtains still remains outside the transverse element241, having a width greater than the width of the slit.

FIG. 11illustrates a variation in which, at least at the straight ends of the upper guide, the roller blind like shielding curtain is provided inside the straight part of the upper guide. In this case, the axis of the roller is parallel to the axis of the straight end of the upper guide. The rounded portion of the guide can be associated to sliding panels or curtains according to one or more of the preceding examples.

It is further worth noting that in this case the roll blind type shielding curtain can easily be provided with a non-rectilinear leading edge which can be shaped according to the morphology of the part to be examined which has to enter the apparatus, i.e. the imaging cavity of the magnetic structure1. According to the previous example ofFIG. 6A to 6C, the shielding curtain of the roller blind type can also be provided with shaped edges and/or with openings.

According to a further variation of the preceding embodiments, the upper and lower guides may be constructed similarly to conventional curtain guides as, for example, the one illustrated inFIG. 14. In this case the electrical contact between the shielding curtain or panel50can be ensured by the guiding track60which is made or coated with electrically conductive material and which is secured to the upper and lower parts of the frame141of the shielding member41. Also electrically conductive strips can be provided on the track. The track can be made of one piece or it can be formed by several segments which are connected mechanically and electrically by connection bridges61. At the end of the tracks, end stops63can be provided. The shielding panels or shielding curtains50can be connected to the guiding tracks60by carriers62which are electrically conductive and which are provided with rolls162engaging the track and with hooks262, clips362or rings462for securing the shielding panel or the shielding curtain50to the carrier62. In this case, the elements of the frame corresponding to the upper and lower guide can be entirely of electrically non-conductive material.

FIGS. 15A and 15Billustrate a further variation of the previous embodiments, in which the lower guide is not present.

In this case, the frame141of the shielding member comprises only the upper “U-shaped” guide which is hinged to the magnetic structure as in the previous example ofFIG. 1. The shielding panels50or the shielding curtains are suspended from the upper guide and at their lower edges electrical contact elements150cooperate directly with electrical contact elements on the table part102of the table2. The electrical contact elements150at the bottom edge of the shielding panel or shielding curtains50come into contact with an electrically conductive track502which surrounds the “U-shaped” opening302of the table102in which the lower side201of the magnetic structure penetrates when the table is brought against the magnetic structure1.

FIG. 15Billustrates this condition. Furthermore, inFIG. 15A, an example of electrical contact elements provided between the table part102and the lower side of the magnetic structure1is shown. In this example, which is not limiting, the two contact surfaces of the “U-shaped” opening302of the table102and the “U-shaped” peripheral surface of the lower side201of the magnetic structure1respectively bear an electrically conductive track402which is coincident with a row of elastic contact elements52made according to the example ofFIGS. 3A and 3B. Similar electrical contact elements are provided between the swingable “U-shaped” guide and the corresponding contact peripheral surface of the upper side101of the magnetic structure1. In this case, the contact elements52are provided on the internal surface of the “U-shaped” upper guide while the electrically conductive track402is provided on the peripheral complementary surface of the upper part101of the magnetic structure.

FIG. 16illustrates a variation in which the upper guide is not swingable but is stationary and secured to the peripheral surface of the upper side101of the magnetic structure.

FIG. 17is still another variation in which only the upper guide is provided in combination with a roller blind type shielding curtain or panel. In this case, both variations ofFIGS. 15A and 15Band ofFIG. 16are possible.

With reference now toFIG. 18, an annular magnetic structure is shown in a lateral view. In this case, the frame of the shielding member141is not secured in a swingable manner to the magnetic structure at its opening but is secured in a slidable manner within guides70. InFIG. 18, the shield closes part of the open side of the magnetic structure1, so that the opening “0” of the magnetic structure1is partially visible. Although this example does not illustrate a magnetic structure according to the previous examples, it is clear that a slidable shielding member can be provided also in combination with a magnetic structure as illustrated inFIG. 1or15A and1bor16and17. In this case, the transverse element241which connects the ends of the “U-shaped” guides or (with reference toFIGS. 15A and 15B) the ends of the upper guide, are engaged each one in one corresponding guide of two parallel vertical guides.

Further variations of the embodiments shown may comprise a shielding panel or curtain made of the Venetian blind kind. In this case, the frame141of the shielding member41has at least one upper element which corresponds to the Venetian blind classic upper rod. Alternatively, the Venetian blind-like shielding panel can be oriented vertically, and the ends of the strips forming the blind can be provided with one or more protrusions which engage either only one upper guide or one upper and one lower guide forming the frame.

A further variation relates to a magnetic structure1having at least one opening with a rounded or circular shape. In this case, the frame141of the shielding member41may be rectangular or square, while the one, three or four elements of the frame are aligned parallel or coincident with a tangential axis to the rounded or circular opening. The frame141inscribes or is a part of a square or rectangular element inscribing the opening of the magnetic structure1.

A further variation relates to the shielding of a magnetic structure having more than one open side and where each opening of each open side is separated from an adjacent open side. This case applies, for example, in magnetic structures having two spaced apart substantially parallel poles which are held in their spaced apart position and connected by columns. In this case, a corresponding shielding member is associated to each open side. The shielding members can be of the identical type or each open side of the magnetic structure can be provided with a different shielding member construction chosen among the different types described above. The reason for this can be found in the different shapes and/or functions of the open sides relative to the kind of part of the body which has to access the imaging cavity of the magnetic structure and pass through a specific one of the open sides of the magnetic structure.

The present invention is not intended to be limited to the above description and illustrations, but may be greatly varied, especially with regard to the construction of the devices, without departing from the guiding principles disclosed above and claimed below.