Systems and methods for passively shielding a magnetic field

A passive magnetic field shielding system for shielding a fringe magnetic field is described. The passive magnetic field shielding system includes a magnet configured to generate a uniform magnetic field, an imaging system with associated electronics coupled to the magnet, and a passive shield configured to reduce the strength of the fringing magnetic field to approximately five Gauss at a distance from the passive shield.

BACKGROUND OF THE INVENTION

This invention relates generally to magnetic shielding and more particularly, to systems and methods for passive magnetic shielding.

A magnetic field generator for Magnetic Resonance Imaging (MRI), hereinafter referred to as a magnet, having a strength of 1 Tesla (T) or higher generates a fringing magnetic fields beyond the physical dimensions of the magnet itself.

Such magnetic fields are potentially dangerous and may have adverse effects on people who are within a proximity of such a magnet. As a result, it is generally considered desirable to limit or curtail the fringing magnetic fields of such magnets in an MRI system of a relocatable unit, such as a mobile van or a relocatable building. It is generally considered desirable to curtail the fringing magnetic fields below approximately five Gauss beyond a certain distance, for example, 15 centimeters, from the outside side-wall of the mobile MRI unit carrying the magnet.

The mobile MRI unit used to carry the magnet in sensitive environments includes shields that shield the fringing magnetic fields generated by the magnet to satisfy the limit. However, if a higher strength magnet, having a value of, for example, 3 T, is used, the shields may not be effective for the fringing magnetic fields generated by the higher strength magnet to meet the limits using conventional methods while satisfying the weight limitations for the mobile MRI unit set by the department of transportation or any other governing body

BRIEF DESCRIPTION OF THE INVENTION

In one aspect, a passive magnetic field shielding system for shielding a fringe magnetic field is described. The passive magnetic field shielding system includes a magnet configured to generate a uniform magnetic field, an imaging system with associated electronics coupled to the magnet, and a passive shield configured to reduce the strength of the fringing magnetic field to approximately five Gauss at a distance from the passive shield.

In another aspect, a relocatable magnetic resonance imaging (MRI) unit is described. The relocatable MRI unit includes a magnet configured to generate a uniform magnetic field and a passive shield configured to reduce the strength of the fringing magnetic field to a threshold level outside a region included within the passive shield and within a pedestrian area.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1shows a mobile Magnetic Resonance Imaging (MRI) Unit100, such as a mobile van, where a passive magnetic field shielding system102and a magnetic field generating device, such as a magnet104, may be used, in accordance with an exemplary embodiment of the invention. Mobile MRI unit100includes an imaging system such as an MRI system. Further, the MRI system may include associated electronics, such as, a computer, an operator console, a display device, an analog-to-digital converter, a sequence memory circuit, gradient coil driving circuits, a gate modulation circuit, a radio frequency power amplifier, a radio frequency oscillating circuit, a phase detector, a preamplifier, gradient coils, a transmit coil, and a receive coil as described in U.S. Pat. No. 6,529,003 B2 and used for generating magnetic resonance images. Passive magnetic field shielding system102further includes a side-wall106a, a side-wall106b, and a base108, hereinafter collectively referred to as a passive shield for passive magnetic field shielding system102. Mobile MRI unit100, (including automobile components such as a set of tires, an engine, a suspension system and the like) and, passive magnetic field shielding system102, has a weight greater than 50,000 pounds, such as ranging from and including 50,000 pounds to and including 100,000 pounds. In various embodiments, mobile MRI unit100may include a mobile vehicle such as, for example, a van, a truck or a motor car, that includes passive magnetic shielding system102. Magnet104generates a fringing magnetic field with a maximum strength ranging from and including 0.2 Tesla (T) to 3.5 T. An area between mobile MRI unit100and an imaginary Gauss line110is a pedestrian restricted area that is proximate to mobile MRI unit100. A point112(point A) is located on an imaginary Gauss line110or within an area enclosed by the imaginary Gauss line110covering a perpendicular distance of approximately 6 inches, such as between 5.5 and 6.5 inches, from either side-wall106aor side-wall106boutside mobile MRI unit100. The area covering the perpendicular distance of approximately 6 inches lies along mobile MRI unit100. In one embodiment of the invention, point112is located at the same height as that of a center114of magnet104. Further, a point116(point B) lies on imaginary Gauss line110or within an area enclosed by imaginary Gauss line110, covering a radial distance of approximately two meters, such as between 1.5 meters and 2.5 meters, from center114.

In various embodiments of the invention, passive magnetic field shielding system102is configured to reduce the fringing magnetic field of magnet104to approximately 5 Gauss, such as between 4.5 and 5.5 Gauss, within the area covering a perpendicular distance of approximately 6 inches from either side-wall106aor side-wall106b.

FIG. 2shows a side-wall arrangement200of each side-wall106aand106b, in accordance with an exemplary embodiment of the invention. Side-wall arrangement200includes a first plate202a, a second plate202b, a third plate202c, a fourth plate202d, a fifth plate204, a sixth plate206and a seventh plate208, which are parallel to one another. First plate202a, second plate202b, third plate202c, and fourth plate202dare hereinafter referred to as a first set of plates202.

In an exemplary embodiment of the invention, first plate202a, second plate202b, third plate202c, and fourth plate202dare of equal height, width, and length, and are placed parallel to each other. In accordance with an alternative embodiment, fourth plate202dis shorter than first plate202a, second plate202b, and third plate202c. Further, fifth plate204is longer than first set of plates202, and sixth plate206is longer than fifth plate204and is attached to seventh plate208. Seventh plate208is longer than sixth plate206. Fifth plate204, sixth plate206and seventh plate208have the same height and width. Further, fourth plate202d, fifth plate204, sixth plate206, and seventh plate208are arranged in order of increasing length because the fringing magnetic field is the greatest near center114of magnet104and gradually decreases along the length (z-axis) of magnet104. It is noted that the heights of first set of plates202, fifth plate204, sixth plate206, and seventh plate208are measured along an x-axis, the widths of first set of plates202, fifth plate204, sixth plate206, and seventh plate208are measured along a y-axis, and the lengths of first set of plates202, fifth plate204, sixth plate206, and seventh plate208are measured along a z-axis.

First set of plates202, fifth plate204, sixth plate206and seventh plate208, are bolted together. Optionally, various other methods of attachment known in the art may be used for attaching first set of plates202, fifth plate204, sixth plate206and seventh plate208. Examples of these methods include, mechanical interlocking, gluing (using adhesive substrates), chemical bonding, welding (friction and diffusion methods), brazing procedures, and soldering processes. In various embodiments, at least one plate in set of plates202is a ferrous plate.

FIG. 3shows a top view of a shield arrangement300, in accordance with an exemplary embodiment of the invention. Shield arrangement300includes side-wall arrangement200. Further, a set of back plates310including a first back plate310a, a second back plate310b, and a third back plate310c, are attached, such as, bolted, to a back end of side-wall arrangement200. First back plate310a, second back plate310b, and third back plate310care attached, such as, bolted to each other. A front plate312is attached, such as, for example, bolted to a front end of side-wall arrangement200. Set of back plates310and front plate312are of same height as that of side-wall arrangement200. Each plate in set of back plates310and front plate312are of the same length. Further, an intersection of an axis314(y-axis) and an axis316(z-axis) form center114of magnet104.

It is noted that the length of first plate202a, second plate202b, third plate202c, fourth plate202d, fifth plate204, sixth plate206is measured along axis316. In an embodiment of the invention, each of first plate202a, second plate202b, third plate202c, fourth plate202d, fifth plate204and sixth plate206extends equally in a perpendicular direction from the axis314.

FIG. 4shows an isometric view of a shield layout402, which is an exemplary embodiment of passive magnetic field shielding system102. Shield layout402includes side-wall106a, side-wall106b, a foot shield or base section402a, a foot shield or base section402b, a foot shield or base section402cand set of back plates310, hereinafter collectively referred to as passive shield for shield layout402.

In an embodiment of the invention, magnet104may be placed on a base406of mobile MRI unit100that is located at a perpendicular height above base sections402a,402band402c. In various embodiments of the invention, the perpendicular height is maintained to enable shielding of an area that includes point112(point A inFIG. 1) and point116(point B inFIG. 1) from center114of magnet104. The perpendicular height may be approximately 3 inches, such as between 2.5 and 3.5 inches.

Base section402ais attached, such as, bolted or welded, to side-wall106a. Similarly, base section402bis attached to set of back plates310and base section402cis attached to side-wall106b.

FIG. 5shows a front view of shield layout402in accordance withFIG. 4. An axis510(x-axis) and axis314(y-axis) pass through center114of magnet104. In various embodiments, the width of base sections402aand402c(measured along axis314) are identical. In various embodiments, the heights of side-wall106aand side-wall106b(measured along axis510) are identical.

FIG. 6shows a side view of base section402ain accordance with various embodiments of the invention. Base section402aincludes five plates, namely a plate602a, a plate602b, a plate602c, a plate604and a plate606. In an embodiment of the invention, the width of plates602a,602b,602c,604and606(measured along axis510) are identical. In another embodiment of the invention, the length of each of plate602a,602band602c(measured along axis316) is identical and is less than the length of plates604and606. Further, the length of plate606is greatest among the five plates in base section402a. It is noted that plates602a,602b,602cand604extend equally in both directions from axis510. The graded arrangement of plates in base section402aenables shielding of an area along the length of magnet104in z-direction. In an embodiment of the invention, the dimensions of base section402aare identical to that of base section402c.

FIG. 7shows an isometric view of a shield layout700, which is an exemplary embodiment of passive magnetic field shielding system102. Shield layout700includes side-wall106a, side-wall106b, a base section702a, a base section702b, a base section702cand set of back plates310, hereinafter collectively referred to as passive shield for shield layout700. An extension or dropped side-wall706aand an extension or dropped side-wall706bare attached, such as bolted or welded, to base sections702aand702crespectively, to shield an area covering a radial distance that includes point112(point A inFIG. 1) and point116(point B inFIG. 1).

FIG. 8shows a front view of shield layout700in accordance with various embodiments of the invention. In various embodiments, the length of dropped side-walls706aand706bare equal to the length of first plate202ain side-wall arrangement200. Further, the heights (along axis510) of dropped side-walls706aand706bare equal. Further, dropped side-wall706aand dropped side-wall706bmay lie outside a controlled environment enclosed within mobile MRI unit100.

FIG. 9is a side view of base section702ain accordance with various embodiments of the invention. Base section702aincludes five plates, namely a plate902a, a plate902b, a plate902c, a plate904and a plate906. The width of each plate (measured along axis510)902a,902b,902c,904and906in base section702ais identical. The length of plate902a,902band902c(measured along axis316) is identical and is less than that of plates904and906. Further, the length of plate906is greatest among the five plates in base section702a. It is noted that plates902a,902b,902cand904extend equally in both directions from axis510. The graded arrangement of plates in base section402aenables shielding of an area along the length of magnet104in z-direction. In an embodiment of the invention, the dimensions of base section902aare identical to that of base section902c.

FIG. 10shows an isometric view of a shield layout1000, which is an exemplary embodiment of passive magnetic field shielding system102. Shield layout1000includes side-wall106a, side-wall106b, a base section1002a, a base section1002b, a base section1002c, a base section1002d, a base section1002e, and set of back plates310, hereinafter referred to as passive shield for shield layout1000. Shield layout1000further includes an extension or foot shield section1006aand an extension or a foot shield section1006b. In an embodiment of the invention, magnet104may be placed (on a base of mobile MRI unit100) at the same height as that of base sections1002a,1002b,1002c,1002dand1002e.

Base section1002ais located in front of foot shield section1006a, and base section1002bis located behind foot shield section1006a. Similarly, base section1002cis located in front of foot shield section1006b, and base section1002dis located behind foot shield section1006b. Base section1002eis attached, such as, bolted, to set of back plates310and lies between base1002band base1002d. Foot shield section1006ais attached, such as, bolted or welded, to side-wall106aand foot shield section1006bis attached, such as, bolted or welded, to side-wall106b. Similarly, base sections1002aand1002bare attached to side-wall106a, while base sections1002cand1002dare attached to side-wall106b. Further, foot shield section1006aand foot shield section1006blie in contact with the controlled environment of mobile MRI unit100. Accordingly, temperature of foot shield section1006aand foot shield1006bcan be controlled, for instance, by controlling a parameter, such as the temperature, of the controlled environment.

In various embodiments of the invention, foot shield sections1006aand1006bare included in shield layout1000to magnetically shield an area covering a radial distance that includes point116(point B inFIG. 1) from center114.

FIG. 11shows a front view of shield layout1000in accordance with an embodiment of the invention. Axis510(x-axis) and axis314(y-axis) pass through center114of magnet104. In an embodiment of the invention, the width of each foot shield section1006aand1006band each base section1002a,1002b,1002c, and1002d, measured along direction314, is identical.

FIG. 12shows foot shield1006a, in accordance with an embodiment of the invention. A first section1202is attached at a perpendicular distance from a second section1204, thereby making first section1202at a lower point than second section1204. First section1202is connected to second section1204by a set1206of parallel vertical plates. Set1206of parallel vertical plates includes a set1206aof parallel vertical plates and a set1206bof parallel vertical plates.

First section1202includes three plates, a plate1202a, a plate1202b, and a plate1202c, with identical dimensions and placed parallel to each other. Second section1204includes a first set1208of plates including a plate1208a, a plate1210a, and a plate1212aattached at a top end of set1206aof parallel vertical plates connecting a first end of first section1202to first set1208of plates. Further, second section1204includes a second set1214of plates including a plate1208b, a plate1210b, and a plate1212bwhich are attached at a top end of set1206bof parallel vertical plates connecting a second end of first section1202to second set1214of plates. Further, plates in second section1204are arranged in a graded structure. In various embodiments of the invention, the dimensions of plates1208a,1208b,1210aand1210bare identical. Plates1212aand1212bhave a width that is identical to that of plates1208a,1208b,1210aand1210b, but have a greater length. It is noted that the length is measured along axis316and the width is measured along axis510.

FIG. 13shows a foot shield1300, which is an exemplary embodiment of foot shield1006a. Foot shield1300includes first section1202and set1206of parallel vertical plates as described with reference toFIG. 12. Plates1208aand1208bare identical, but shorter in length (measured along z-axis) than plates1210aand1210brespectively. Additionally, the dimensions of plates1210aand1210bare also identical. Further, plates1212aand1212bare of equal length, but longer than plates1210aand plates1210brespectively. In various embodiments, foot shield1300is attached, such as, bolted, to each side-wall106aand106bof mobile MRI unit100.

FIG. 14shows a foot shield1400, which is an exemplary embodiment of foot shield1300shown inFIG. 13. Foot shield1400includes second section1204and set1206of parallel vertical plates as described with reference toFIG. 13. Additionally, first section1202is arranged in a graded section. Plates1202band1202care identical in dimensions and have a larger width (measured along y-axis) compared to plate1202a.

Foot shield1400is attached, such as, bolted, to each side-wall106aand106bof mobile MRI unit100. Further, foot shield1400may be attached such that an extension, which is the difference between the width of plates1202aand plate1202b, and lies inside mobile MRI unit100.

In various embodiments of the invention, plates in set1206, first section1202or second section1204may be connected by various means of mechanical attachment, such as bolting or fastening. Optionally, plates in set1206, first section1202or second section1204may be substituted by a single block with identical dimensions.

In various embodiments of the invention, the material used in the manufacture of passive magnetic shielding system102may include oriented steel, perm endure, and/or a low carbon steel such as, for example, 1006 grade carbon steel or 1010 grade carbon steel. However, the exact arrangement of the shield is determined by the actual material used. For example, when permendure is used in side-wall arrangement200, plate202amay be removed from side-wall arrangement200and still be capable of providing effective magnetic shielding to point112and point116at imaginary Gauss line110. Permendure material has high saturation magnetization and favorable permeability. Further, oriented steel has a high permeability and moderate saturation magnetization.

FIG. 15shows a graph of a fringing magnetic field1502versus a distance1504(along axis316) from center114of magnet104corresponding to various embodiments of the invention. A unit on axis1504is in centimeters, and a unit on axis1502is in Gauss.

Curve1506(Design A) corresponds to shield layout402, curve1508(Design B) to shield layout700, and curve1510(Design C) to shield layout1000. In various embodiments, shield layout402has the heaviest weight, shield layout700has a weight less heavier than the heaviest weight, and shield layout1510has a weight less heavier than the weight of shield layout700.

For curve1506, a point1506aindicates a peak fringing magnetic field of approximately 4.3 gauss, such as between 4.1 gauss and 4.4 gauss, at a distance of approximately 40 cm, such as between 38 cm and 42 cm from centre114at a first side of magnet104. Similarly, a point1506bindicates a peak fringing magnetic field of approximately 4.3 gauss at a distance of approximately 40 cm, such as between 38 cm and 42 cm from centre114at a second side of magnet104.

For curve1508, a point1508aindicates a peak fringing magnetic field of approximately 4.8 gauss, such as between 4.7 gauss and 4.9 gauss at a distance of approximately 150 cm, such as between 148 cm and 152 cm from centre114at the first side of magnet104. Similarly, a point1508bindicates a peak fringing magnetic field of approximately 4.8 gauss at a distance of approximately 150 cm, such as between 148 cm and 152 cm from centre114at the second side of magnet104.

For curve1510, the approximate 5-Gauss limit for the fringing magnetic field is achieved at a distance of 142.24 cm from center114and is shown by a point1510acorresponding to a distance on the first side of magnet104and a point1510bcorresponding to a distance on the second side of magnet104.

The fringing magnetic field keeps on decreasing on either side of magnet104with an increase in the distance from center114in magnet104. Accordingly, the fringing magnetic field at a distance of 500 cm on either side of magnet104is approximately 0.2 Gauss in all three embodiments.

A technical effect of various embodiments of the invention is that they provide an improved system for achieving passive magnetic shielding for a magnet generating a fringing magnetic field with a maximum strength ranging from and including 0.2 T to 3.5 T. Passive magnetic shielding systems also weigh less. The reduction in weight and the increase in magnetic shielding are due to the use of an optimum material and design to construct the passive magnetic shielding system.

In various embodiments of the invention, the selection of the material of passive magnetic shielding system102may be varied according to the fringing magnetic field strength desired. Optionally, the number of plates attached in first section1202, second section1204or set1206may be reduced or increased. Similarly, the number of plates in side-wall arrangement200may be reduced depending on the field strength of the magnet. Although the preceding embodiments are discussed with respect to medical imaging, it is understood that the image acquisition and processing methodology described herein is not limited to medical applications, but may be utilized in non-medical applications as well.

As used herein, an element or step recited in the singular and preceded with the word “a” or “an” should be understood as not excluding plural elements or steps, unless such exclusion is explicitly recited. Furthermore, references to “an” or “one” “embodiment” of the present invention are not intended to be interpreted as excluding the existence of additional embodiments that also incorporate the recited features.