Patent Description:
Typically, magnetic resonance imaging (MRI) systems can generate a magnetic fringe field external to a measurement volume of the MRI system. The magnetic fringe field can, for example, interfere with electronic equipment in the fringe field and/or attract metallic objects positioned in a vicinity of the MRI system. The metallic objects attracted by the magnetic fringe field can, for example, enter into the measurement volume of the MRI system, thereby endangering a patient undergoing a MRI scan. Accordingly, MRI devices are typically deployed in a dedicated MRI room.

Deployment of MRI systems in a dedicated room can be expensive and can require dedicated space within a hospital, doctor's office and/or other institution using MRI systems. Therefore, it can be desirable to deploy MRI systems without requiring a dedicated MRI room. <CIT> discloses a shield for a MR magnet comprising a cylindrical shell of magnetic material surrounding the MR magnet. The cylindrical shell is situated so that its longitudinal axis is coaxial with the magnetic axis of the MR magnet. Two disk shaped end caps of magnetic material are secured to either end of the cylindrical shell to form the housing. The end caps each define a central aperture extending longitudinally through the disk, with the radial extent of each of the apertures sized so that the perturbation of the field in the working volume of the MR magnet due to the cylindrical shell is compensated for. <CIT> discloses a magnet system for providing a localized substantially homogeneous field for use in medical magnetic resonance imaging and its incorporation into a method of siting. The magnet system includes a vacuum vessel defining a bore for receiving the subject of the imaging and having a first end and a second end. A cryogenic containment vessel is supported within the vacuum vessel and a single coil is disposed within the containment vessel for providing the magnetic field. The magnet system also includes a shield assembly of ferromagnetic material positioned closely adjacent the outside of the vacuum vessel and including first and second end assemblies and a plurality of axially extending beam-like members interconnecting the end assemblies. The coil comprises a winding made up of thousands of turns of small superconductive wire and which is configured to compensate for the effect of the presence of the shield to provide the substantially homogeneous field at a predetermined location in the bore. The components of the shield assembly are adapted for mounting to said vacuum vessel after siting. This permits simplified siting of the magnet system in that the vacuum vessel with the components inside of it can be sited first and the components of the shield assembly thereafter mounted on the vacuum vessel. A method of manufacturing the magnet system is also disclosed.

An aspect of the present invention is recited by claim <NUM>, with the dependent claims reciting optional features.

According to the invention, there is provided a magnetic resonance imaging (MRI) system, as defined in claim <NUM>, the system comprising: a magnetic field device (<NUM>) configured to generate a magnetic field within a measurement volume (<NUM>) and to generate a magnetic fringe field external to the measurement volume (<NUM>), wherein the measurement volume (<NUM>) is configured to accommodate at least a portion of a patient; a ferromagnetic housing (<NUM>) configured to envelop the magnetic field device (<NUM>), the housing (<NUM>) having a first portion (<NUM>) and a second portion (<NUM>) wherein: the housing (<NUM>) includes a proximal end (320a) and a distal end (320b) along a longitudinal axis (<NUM>) of the MRI system (<NUM>), the housing (<NUM>) further includes a housing opening (<NUM>) at the proximal end (320a) for enabling insertion of at least a portion of a patient within the measurement volume (<NUM>), the second portion (<NUM>) of the housing (<NUM>) includes at least a portion of the proximal end (320a) of the housing (<NUM>), the first portion (<NUM>) of the housing (<NUM>) includes at least a portion of the distal end (320b) of the housing (<NUM>); and a plate (<NUM>), having a plate opening (<NUM>), positioned external to the housing (<NUM>) adjacent the second portion (<NUM>) of the housing (<NUM>) at a predetermined non-zero distance from the proximal end (320a) of the housing (<NUM>), wherein the plate opening (<NUM>) is aligned with a corresponding housing opening (<NUM>) in the housing (<NUM>) to allow insertion of at least a portion of the patient therethrough, and wherein the plate (<NUM>) comprises a metal alloy, the metal alloy comprises a ferromagnetic material and wherein thickness of the first portion (<NUM>) of the housing (<NUM>) is less than the thickness of the second portion (<NUM>) of the housing (<NUM>), such that the plate (<NUM>) and different thicknesses of the first and second portions (<NUM>, <NUM>) cause the magnetic fringe field to be reduced in a region adjacent the plate opening (<NUM>) compared to an opposite region adjacent the distal end (320b) of the housing (<NUM>) such that the generated magnetic fringe field is asymmetric with respect to a center of the measurement volume (<NUM>).

In some arrangements, at least one of the thickness of the second portion of the housing, the distance between the plate and the housing and dimensions of the plate are predetermined to reduce the magnetic fringe field at the plate opening to a predetermined value.

In some arrangements, the second portion of the housing comprises the housing opening.

In some arrangements, the plate is coupled to the housing using a non-ferromagnetic and a non-paramagnetic material.

In some arrangements, the system is positioned within a desired room and wherein the plate is coupled to at least one of: a floor of the room, a ceiling of the room or any combination thereof.

In some arrangements, a shape of the plate is selected from the group consisting of: a square, a rectangular, and an oval.

In some arrangements, the plate has a thickness of <NUM> millimeters.

In some arrangements, dimensions of the plate and the distance of the plate from the housing are predetermined based on a desired magnetic fringe field at the plate opening.

It will be appreciated that, for simplicity and clarity of illustration, elements shown in the figures have not necessarily been drawn to scale.

The subject matter regarded as the invention is defined by the appended claims.

The invention, however, both as to organization and method of operation, together with objects, features, and advantages thereof, can be understood by reference to the following detailed description when read with the accompanying drawings in which:.

In the following description, various aspects of the present invention are described. For purposes of explanation, specific configurations and details are set forth in order to provide a thorough understanding of the present invention. However, it will also be apparent to one skilled in the art that the present invention may be practiced without the specific details presented herein, while staying within the scope of the invention defined by the appended claims.

Furthermore, well known features may have been omitted or simplified in order not to obscure the present invention. With specific reference to the drawings, it is stressed that the particulars shown are by way of example and for purposes of illustrative discussion of the present invention only, and are presented in the cause of providing what is believed to be the most useful and readily understood description of the principles and conceptual aspects of the invention. In this regard, no attempt is made to show structural details of the invention in more detail than is necessary for a fundamental understanding of the invention, the description taken with the drawings making apparent to those skilled in the art how the several forms of the invention may be embodied in practice.

Before at least one embodiment of the invention is explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of the components set forth in the following description or illustrated in the drawings. The invention is applicable to other embodiments that may be practiced or carried out in various ways as well as to combinations of the disclosed embodiments, as long as said ways and combinations fall within the scope of the invention defined by the appended claims.

Reference is now made to <FIG>, which schematically illustrate a magnetic resonance imaging (MRI) system <NUM> according to an example outside the scope of the invention.

Illustrations 100a and 100b in <FIG> show a perspective view and a side cross-sectional view of the MRI system, respectively. Illustrations 100c and 100d in <FIG> show a top view and a side view of the MRI system <NUM>, respectively, and a region <NUM> surrounding the MRI system <NUM>, in which the magnetic fringe field is greater than a predetermined value (e.g., a magnetic field value based on the magnetic field strength that can cause metallic objects to move towards the MRI system <NUM>).

The MRI system <NUM> can include a magnetic field device <NUM>. The magnetic field device <NUM> can generate a magnetic field within a measurement volume <NUM> and can generate a magnetic fringe field external to the measurement volume <NUM>. The magnetic field device can include multiple magnets <NUM> (e.g., permanent magnets). The magnetic field device can include multiple ferromagnetic elements <NUM> (e.g., pole pieces) that can be positioned adjacent to at least one of the multiple magnets <NUM> (e.g., as shown in <FIG>). In some examples, the measurement volume <NUM> is adapted to accommodate at least a portion (e.g., a head) of a patient (not shown).

The MRI system <NUM> can include a housing <NUM> that can be made of a ferromagnetic material. The housing <NUM> can include a proximal end 120a and a distal end 120b along a longitudinal axis <NUM> of the MRI system <NUM>. The housing <NUM> can envelope the magnetic field device <NUM>. The housing <NUM> can include a housing opening <NUM> at the proximal end 120a. The housing opening <NUM> can enable insertion of the at least portion of the patient (e.g., the head) within the measurement volume <NUM> of the magnetic field device <NUM>.

In some examples, the housing <NUM> includes additional openings, for example, a first additional opening 123a, a second additional opening 123b and/or a third additional opening 123c. The first additional opening 123a can be positioned at the distal end 120b of the housing <NUM> and/or can be coaxially aligned with the housing opening <NUM> (e.g., as shown in <FIG>). The second additional opening 123b and/or the third additional opening 123c can be positioned at various locations of the housing <NUM>, for example as shown in <FIG>. The additional openings 123a-c can, for example, reduce a weight of the housing <NUM>.

In some examples, the housing opening <NUM> has greater diameter as compared to additional openings 123a-c. In some examples, the housing <NUM> includes the housing opening <NUM> without additional openings 123a-c.

The MRI system <NUM> can generate magnetic field within the measurement volume <NUM> of the magnetic field device <NUM>. The magnetic field measured at a center 112a of the measurement volume <NUM> can range, for example, between <NUM>-<NUM> Gauss. The MRI system <NUM> can generate magnetic fringe field in a region <NUM> surrounding the MRI system <NUM>. The region <NUM> can indicate a region in which the magnetic fringe field is above a predetermined value, for example <NUM> Gauss. The magnetic fringe field generated by the MRI system <NUM> can be substantially symmetric with respect to the center 112a of the measurement volume <NUM> (e.g., as shown in <FIG>). The magnetic fringe field measured on the longitudinal axis <NUM> at a predetermined distance of <NUM> from the center 112a of the measurement volume <NUM> can range, for example, between <NUM>-<NUM> Gauss.

Reference is now made to <FIG>, which schematically illustrate a magnetic resonance imaging (MRI) system <NUM>, including an asymmetric housing <NUM>, according to an example outside the scope of the invention.

Illustrations 200a and 200b in <FIG> show a perspective view and a side cross-sectional view of the MRI system, respectively. Illustrations 200c and 200d in <FIG> show a top view and a side view of the MRI system <NUM>, respectively, and a region <NUM> surrounding the MRI system <NUM>, in which the magnetic fringe field is greater than a predetermined value.

The MRI system <NUM> can include a magnetic field device <NUM>. The magnetic field device <NUM> can generate a magnetic field within a measurement volume <NUM> and can generate a magnetic fringe field external to the measurement volume <NUM>. In various examples, the magnetic field device <NUM> and/or the measurement volume <NUM> are identical to the magnetic field device <NUM> and the measurement volume <NUM>, respectively, as described above with respect to <FIG>.

The magnetic field device <NUM> can include multiple magnets <NUM> (e.g., permanent magnets) and/or multiple ferromagnetic elements <NUM> (e.g., pole pieces) that can be positioned adjacent to at least one of the multiple magnets <NUM> (e.g., as shown in <FIG>). In various examples, magnets <NUM> and/or ferromagnetic elements <NUM> are identical to magnets <NUM> and ferromagnetic elements <NUM>, respectively, as described above with respect to <FIG>.

The MRI system <NUM> can include a housing <NUM> that can be made of a ferromagnetic material. The housing <NUM> can include a proximal end 220a and a distal end 220b along a longitudinal axis <NUM> of the MRI system <NUM>. The housing <NUM> can envelope the magnetic field device <NUM>. The housing <NUM> can include a housing opening <NUM> at the proximal end 220a. The housing opening <NUM> can enable insertion of at least a portion of a patient (e.g., a head) within the measurement volume <NUM> of the magnetic field device <NUM>. In some examples, the housing opening <NUM> is identical to the housing opening <NUM>, as described above with respect to <FIG>. In various examples, the housing <NUM> includes additional openings (e.g., a first additional opening 223a, a second additional opening 223b and/or a third additional opening 223c) that can be identical to the additional openings <NUM> (e.g., the first additional opening 123a, the second additional opening 123b and/or the third additional opening 123c), as described above with respect to <FIG>. In some examples, the housing <NUM> includes the housing opening <NUM> without additional openings 223a-c.

The housing <NUM> can include a first portion <NUM> and/or a second portion <NUM>, where a thickness of the first portion <NUM> can be different from a thickness of the second portion <NUM> (e.g., as shown in <FIG>). In some examples, the second portion <NUM> includes at least a portion of the proximal end 220a of the housing <NUM> (e.g., around the housing opening <NUM>). In various examples, the thickness of the first portion <NUM> and/or the thickness of the second portion <NUM> are predetermined to reduce the magnetic fringe field external to the housing <NUM>. In some examples, the thickness of the second portion <NUM> is greater than the thickness of the first portion <NUM> (e.g., as shown in <FIG>). For example, the thickness of the first portion <NUM> can range between <NUM>-<NUM> and/or the thickness of the second portion <NUM> can range between <NUM>-<NUM>.

In various examples, the second portion <NUM> includes at least a portion of the proximal end 220a (e.g., around the housing opening <NUM>) and/or at least a portion of at least one of distal end 220b and/or side portions of housing <NUM> (e.g., around at least some of the additional openings 223a-c). In various examples, housing <NUM> includes two (e.g., the first portion <NUM> and the second portion <NUM>) or more portions (not shown), where each of the portions has different thickness.

The MRI system <NUM> can generate magnetic field within the measurement volume <NUM> of the magnetic field device <NUM>. The magnetic field measured at a center 212a of the measurement volume <NUM> can range, for example, between <NUM>-<NUM> Gauss. The MRI system <NUM> can generate magnetic fringe field in a region <NUM> surrounding the MRI system <NUM>. The region <NUM> can indicate a region in which the magnetic fringe field is above a predetermined value, for example <NUM> Gauss. The magnetic fringe field generated by the MRI system <NUM> can be asymmetric with respect to the center 212a of the measurement volume <NUM>. For example, the region <NUM> can include a zone <NUM> (e.g., indicated by a dashed pattern in <FIG>) positioned adjacent to the second portion <NUM> at the proximal end 220a of the housing <NUM>, in which the magnetic fringe field can be smaller as compared to the magnetic fringe field in corresponding opposite zone of the region <NUM> poisoned adjacent to the distal end 220b of the housing <NUM>. The magnetic fringe field measured on the longitudinal axis <NUM> at a predetermined distance of <NUM> from the center 212a of the measurement volume <NUM> can range, for example, between <NUM>-<NUM> Gauss.

Reference is now made to <FIG>, which schematically illustrate a magnetic resonance imaging (MRI) system <NUM>, including an asymmetric housing <NUM> and a plate <NUM> positioned at a predetermined distance from the housing <NUM>, according to some embodiments of the invention.

Illustrations 300a and 300b in <FIG> show a perspective view and a side cross-sectional view of the MRI system, respectively. Illustrations 300c and 300d in <FIG> show a top view and a side view of the MRI system <NUM>, respectively, and a region <NUM> surrounding the MRI system <NUM>, in which the magnetic fringe field is greater than a predetermined value.

The MRI system <NUM> includes a magnetic field device <NUM>. The magnetic field device <NUM> is configured to generate a magnetic field within a measurement volume <NUM> and can generate a magnetic fringe field external to the measurement volume <NUM>. In various embodiments, the magnetic field device <NUM> and/or the measurement volume <NUM> are identical to the magnetic field device <NUM> and the measurement volume <NUM>, respectively, as described above with respect to <FIG>, and/or identical to the magnetic field device <NUM> and the measurement volume <NUM>, respectively, as described above with respect to <FIG>.

The magnetic field device <NUM> can include multiple magnets <NUM> (e.g., permanent magnets) and/or multiple ferromagnetic elements <NUM> (e.g., pole pieces) that can be positioned adjacent to at least one of the multiple magnets <NUM> (e.g., as shown in <FIG>). In various embodiments, magnets <NUM> and/or ferromagnetic elements <NUM> are identical to magnets <NUM> and ferromagnetic elements <NUM>, respectively, as described above with respect to <FIG>, and/or identical to magnets <NUM> and ferromagnetic elements <NUM>, respectively, as described above with respect to <FIG>.

The MRI system <NUM> includes a housing <NUM> made of a ferromagnetic material. The housing <NUM> includes a proximal end 320a and a distal end 320b along a longitudinal axis <NUM> of the MRI system <NUM>. The housing <NUM> is configured to envelop the magnetic field device <NUM>. The housing <NUM> includes a housing opening <NUM> at the proximal end 320a. The housing opening <NUM> enables insertion of at least a portion of a patient (e.g., a head) within the measurement volume <NUM> of the magnetic field device <NUM>. In some embodiments, the housing opening <NUM> is identical to the housing opening <NUM>, as described above with respect to <FIG>, and/or identical to the housing opening <NUM>, as described above with respect to <FIG>. In various embodiments, the housing <NUM> includes additional openings <NUM> (e.g., a first additional opening 323a, a second additional opening 323b and/or a third additional opening 323c) that can be identical to the additional openings <NUM> (e.g., the first additional opening 123a, the second additional opening 123b and/or the third additional opening 123c), as described above with respect to <FIG>, and/or identical to the additional openings <NUM> (e.g., the first additional opening 223a, the second additional opening 223b and/or the third additional opening 223c), as described above with respect to <FIG>. In some embodiments, the housing <NUM> includes the housing opening <NUM> without additional openings <NUM>.

The housing <NUM> includes a first portion <NUM> and/or a second portion <NUM>, where a thickness of the first portion <NUM> is different from a thickness of the second portion <NUM> (e.g., as shown in <FIG>). According to the invention, the second portion <NUM> includes at least a portion of the proximal end 320a of the housing <NUM>. According to the invention, the thickness of the second portion <NUM> is greater than the thickness of the first portion <NUM> (e.g., as shown in <FIG>). In various embodiments, the housing <NUM>, the first portion <NUM> and/or the second portion <NUM> are identical to the housing <NUM>, the first portion <NUM> and the second portion <NUM>, respectively, as described above with respect to <FIG>. For example, the thickness of the first portion <NUM> can range between <NUM>-<NUM> and/or the thickness of the second portion <NUM> can range between <NUM>-<NUM>.

The MRI system <NUM> includes a plate <NUM>. The plate <NUM> is positioned external to the housing <NUM> at a predetermined non-zero distance <NUM> from the proximal end 320a of the housing. The plate <NUM> includes plate opening <NUM>. The plate opening <NUM> is aligned with the housing opening <NUM> to allow insertion of the at least portion of the patient (e.g., the head) therethrough into the measurement volume <NUM>. The plate <NUM> is made of a metal alloy that includes ferromagnetic material.

The plate <NUM> can be coupled to the housing <NUM> of the MRI system <NUM> using, for example, a non-ferromagnetic and/or a non-paramagnetic material (e.g., aluminum). In various embodiments, the MRI system <NUM> is positioned within a desired room and/or the plate <NUM> is coupled to at least one of: a floor of the room, a ceiling of the room or any combination thereof (not shown).

In various embodiments, the thickness of the second potion <NUM> of the housing <NUM>, dimensions of the plate <NUM> and/or the distance <NUM> between the plate <NUM> and the proximal end 320a of the housing <NUM> are predetermined based on a desired weight of the plate and/or based on a desired magnetic fringe field at the plate opening <NUM>. For example, the distance <NUM> can range between <NUM>-<NUM> and/or the plate <NUM> can have a substantially rectangular shape having a height ranging between <NUM>-<NUM>, a width ranging between <NUM>-<NUM> and/or a thickness ranging between <NUM>-<NUM>. In various embodiments, a shape of the plate <NUM> is selected from the group consisting of a square, a rectangular, and/or an oval.

The MRI system <NUM> is configured to generate magnetic field within the measurement volume <NUM> of the magnetic field device <NUM>. The magnetic field measured at a center 312a of the measurement volume <NUM> can range, for example, between <NUM>-<NUM> Gauss. The MRI system <NUM> can generate magnetic fringe field in a region <NUM> surrounding the MRI system <NUM>. The region <NUM> can indicate a region in which the magnetic fringe field is above a predetermined value, for example <NUM> Gauss. The magnetic fringe field generated by the MRI system <NUM> is asymmetric with respect to the center 312a of the measurement volume <NUM>. For example, the region <NUM> can include a zone <NUM> (e.g., indicated by a dashed pattern in <FIG>) positioned adjacent to the plate <NUM>, in which the magnetic fringe field is smaller as compared to the magnetic fringe field in corresponding opposite zone of the region <NUM> poisoned adjacent to the distal end 320b of the housing <NUM>. The magnetic fringe field measured on the longitudinal axis <NUM> at a predetermined distance of <NUM> from the center 312a of the measurement volume <NUM> and adjacent to the plate <NUM> can range, for example, between <NUM>-<NUM> Gauss, while the magnetic fringe field at corresponding opposite point adjacent to the distal end 320b of the housing <NUM> can range, for example, between <NUM>-<NUM> Gauss.

Reference is now made back to <FIG>. In various examples, MRI system that includes a housing having varying thickness (e.g., the housing <NUM> of the MRI system <NUM>, as described above with respect to <FIG>) generates magnetic fringe field that can be lower by -<NUM>% and/or reduces a region that exhibits the magnetic fringe field adjacent to a thicker portion of the housing (e.g., the zone <NUM> in the region <NUM>, as described above with respect to <FIG>) as compared to MRI system that includes housing having uniform thickness (e.g., the housing <NUM> of the MRI system <NUM>, as described above with respect to <FIG>).

In various embodiments, MRI system that includes a housing having varying thickness and a plate positioned adjacent to a thicker portion of the housing (e.g., the housing <NUM> and the plate <NUM> of the MRI system <NUM>, as described above with respect to <FIG>) reduces a region that exhibits the magnetic fringe field adjacent to the plate (e.g., the zone <NUM> in the region <NUM>, as described above with respect to <FIG>) and/or reduces the magnetic fringe field at a plate opening by ∼<NUM>% and ∼<NUM>% as compared to MRI system that includes housing having varying thickness only without a plate (e.g., the MRI system <NUM>, as described above with respect to <FIG>) and as compared to MRI system that includes housing having uniform thickness (e.g., the housing <NUM> of the MRI system <NUM>, as described above with respect to <FIG>), respectively.

In some embodiments, MRI system that includes a housing having varying thickness and a plate positioned adjacent to a thicker portion of the housing reduces the magnetic fringe field in a region adjacent to a plate opening by ∼<NUM>% as compared to corresponding opposite region adjacent to a distal end of the housing (e.g., the housing <NUM> and the plate <NUM> of the MRI system <NUM>, as described above with respect to <FIG>).

In some embodiments, the present invention can include substantially reducing a magnetic fringe field at a housing opening of an MRI system, while eliminating a need for a dedicated MRI room, such that, for example, medical equipment can be positioned adjacent to the housing opening without a risk of being attracted by the magnetic fringe field and/or providing a medical staff with a safe access to a patient undergoing a MRI scan.

In the above description, an embodiment is an example or implementation of the invention. The various appearances of "one embodiment", "an embodiment", "certain embodiments" or "some embodiments" do not necessarily all refer to the same embodiments. Although various features of the invention may be described in the context of a single embodiment, the features may also be provided separately or in any suitable combination, provided that said combination falls within the scope of the invention as defined by the appended claims.

Conversely, although the invention may be described herein in the context of separate embodiments for clarity, the invention may also be implemented in a single embodiment. Certain embodiments of the invention may include features from different embodiments disclosed above, and certain embodiments may incorporate elements from other embodiments disclosed above. The disclosure of elements of the invention in the context of a specific embodiment is not to be taken as limiting their use in the specific embodiment alone. Furthermore, it is to be understood that the invention can be carried out or practiced in various ways and that the invention can be implemented in certain embodiments other than the ones outlined in the description above, provided that the resulting subject matter falls within the scope of the appended claims.

Claim 1:
A magnetic resonance imaging (MRI) system (<NUM>), the system comprising:
a magnetic field device (<NUM>) configured to generate a magnetic field within a measurement volume (<NUM>) and to generate a magnetic fringe field external to the measurement volume (<NUM>), wherein the measurement volume (<NUM>) is configured to accommodate at least a portion of a patient;
a ferromagnetic housing (<NUM>) configured to envelop the magnetic field device (<NUM>), the housing (<NUM>) having a first portion (<NUM>) and a second portion (<NUM>) wherein:
the housing (<NUM>) includes a proximal end (320a) and a distal end (320b) along a longitudinal axis (<NUM>) of the MRI system (<NUM>),
the housing (<NUM>) further includes a housing opening (<NUM>) at the proximal end (320a) for enabling insertion of at least a portion of a patient within the measurement volume (<NUM>),
the second portion (<NUM>) of the housing (<NUM>) includes at least a portion of the proximal end (320a) of the housing (<NUM>),
the first portion (<NUM>) of the housing (<NUM>) includes at least a portion of the distal end (320b) of the housing (<NUM>); and
a plate (<NUM>), having a plate opening (<NUM>), positioned external to the housing (<NUM>) adjacent the second portion (<NUM>) of the housing (<NUM>) at a predetermined non-zero distance from the
proximal end (320a) of the housing (<NUM>), wherein the plate opening (<NUM>) is aligned with a corresponding housing opening (<NUM>) in the
housing (<NUM>) to allow insertion of at least a portion of the patient therethrough, and wherein the plate (<NUM>) comprises a metal alloy, the metal alloy comprises a ferromagnetic material and wherein thickness of the first portion (<NUM>) of the housing (<NUM>) is less than the thickness of the second portion (<NUM>) of the housing (<NUM>), such that the plate (<NUM>) and different thicknesses of the first and second portions (<NUM>, <NUM>) cause the magnetic fringe field to be reduced in a region adjacent the plate opening (<NUM>) compared to an opposite region adjacent the distal end (320b) of the housing (<NUM>) such that the generated magnetic fringe field is asymmetric with respect to a center of the measurement volume (<NUM>).