WO2008114195A2

WO2008114195A2 - Rf receiver for an mri system comprising a susceptibility matched padding device

Info

Publication number: WO2008114195A2
Application number: PCT/IB2008/050987
Authority: WO
Inventors: Miha Fuderer
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2007-03-20
Filing date: 2008-03-17
Publication date: 2008-09-25
Also published as: WO2008114195A3

Abstract

The present invention relates generally to the MRI technique. More particularly, the present invention relates to an RF receiver (1) for an MRI system (2). Furthermore the present invention relates to a method of examining an irregular shaped object (6) by means of an MRI system (1). In order to provide a simple and reliable technique for obtaining better image quality in MRI of irregular shaped objects (6), an RF receiver (1) is suggested, the receiver (1) comprising a padding device (3) adapted to be placed on said object (6) in a way that a more regular outer object shape (13) is obtained, said padding device (3) comprising material having a magnetic susceptibility similar to the magnetic susceptibility of said object (6), the receiver (1) further comprising a number of receiver elements (5) mounted in or on said padding device (3).

Description

RF receiver for an MRI system

FIELD OF THE INVENTION

The present invention relates generally to the magnetic resonance imaging (MRI) technique. More particularly, the present invention relates to a radio frequency (RF) receiver for an MRI system. Furthermore the present invention relates to a method of examining an irregular shaped object by means of an MRI system.

BACKGROUND OF THE INVENTION

MRI uses the magnetic properties of material and its interaction with both a large static magnetic field (the Bo field), and radio waves (i.e. an oscillating electromagnetic field; the Bi field), to produce MR signals, which subsequently are used to obtain highly detailed images of an object to be examined, e.g. of parts of a human body. In an MRI system an RF receiver is employed to acquire said MR signals, which are induced in a number of receiver elements (antenna elements, coil elements).

MRI imaging is vulnerable to artifacts resulting from inhomogeneities in the Bo field. These artifacts are the result of variation in magnetic susceptibility, and are most severe in regions where air-tissue transitions exist, in particular at the external boundary of the object to be imaged. Susceptibility artifacts can cause geometrical distortions in the image as well as loss of signal. Furthermore, the presence of inhomogeneity makes it more difficult to make use of the chemical shift; most notably, fat suppression becomes more difficult. The extent of these artifacts increases with the Bo field. In case of clinical MRI of the human body, a rather difficult area in that regard is the human neck area, which has a relatively complex irregular shape (broad shoulders, short cylindrical neck, round head). Similar problems occur on other regions of the human body, e.g. around the fingers of a hand. It is an object of the present invention to provide a simple and reliable technique for chemical shift selective magnetic resonance imaging of irregular shaped objects. SUMMARY OF THE INVENTION

This object is achieved according to the invention by an RF receiver for an MRI system, the receiver comprising a padding device adapted to be placed on an irregular shaped examination object in a way that a more regular outer object shape is obtained, said padding device comprising material having magnetic properties similar to the magnetic properties of said object, the receiver further comprising a number of receiver elements mounted in or on said padding device.

The object of the present invention is also achieved by a method of examining an irregular shaped object by means of an MRI system, the MRI system comprising an RF receiver, said receiver comprising a padding device and a number of receiver elements mounted in or on said padding device, said padding device comprising material having magnetic properties similar to the magnetic properties of said object, the method comprising the step of placing the padding device on said object in a way that a more regular outer object shape is obtained. A core idea of the invention is to correct the inhomogeneities of the Bo field caused by susceptibility variations at the external boundary of the object to be imaged. This correction is achieved according to the present invention by creating a more homogeneous Bo field within the resulting object. The boundary effect of Bo is shifted away from the imaged object. As a result, the whole region (i.e. the "resulting" object), where the susceptibility is more or less constant, is more smoothly bounded.

For this purpose a padding device is employed, said device comprising material having magnetic properties similar to the magnetic properties of the imaged object. With the use of the padding device, the original object to be imaged, e.g. the human neck, is modified in a way that a new "resulting" object is created. This resulting object comprises the original object and the padding device. A main aspect of the invention is that with the padding device the resulting object shows a more regular outer geometric shape.

With the present invention, it is possible in a simple and reliable way to significantly improve chemical shift selective MR imaging of irregular shaped objects, in particular of objects with a high susceptibility variation. Chemical shift artifacts can be reliably corrected. Because of the homogeneous B₀ field, a very good fat-suppression can be obtained. Furthermore, both the geometrical accuracy and the total signal to noise ration can be improved and susceptibility artifacts can be corrected.

These and other aspects of the invention will be further elaborated on the basis of the following embodiments which are defined in the dependent claims. It has been found, that inhomogeneities of the Bo field caused by susceptibility variations are less of a problem in object areas that can be regarded as more or less cylindrical over an extended length. The same results can be observed in cases where the object shows a spherical shape. Thus, according to a preferred embodiment of the invention, the padding device is adapted to obtain a cylindrical or spherical "resulting" object shape, i.e. the resulting object preferably having a circular or ellipsoidal cross section.

For obtaining such a shape, the padding device is, according to a preferred embodiment of the invention, adapted to be at least partly surrounding the object. For example, the padding device is preferably adapted to be (at least partly) wrapped around a part of a human body, e.g. around a neck, an ankle or a finger of the hand.

According to a preferred embodiment of the invention, the padding device exhibits some flexibility to adapt to the object's shape (i.e. the outer circumferences of the imaged object), e.g. to the anatomy of patients. For this purpose the padding device preferably shows a flexible inner shape. In case of MRI of the human neck, the cavities around the human neck are preferably completely padded (filled up) with the padding device, i.e. the padding fills out the complete space between the shoulder and the lower head.

For obtaining such a flexible inner shape of the padding device, the padding device is preferably an (at least partly deformable) container filled with a granular substance or the like, according to a preferred embodiment of the invention. For filling the container, different materials can be used. One of them is water, having a very similar susceptibility as human tissue; however, water also resonates to MRI, which may be a problem for some applications. Another material of desirable magnetic properties is salt (NaCl), which can be used in applications where its weight does not pose a patient-comfort problem. PVC granules are a more patient-friendly alternative, but their susceptibility is slightly lower than that of human tissue. According to a preferred embodiment of the invention (granules of) a mixture of Styrofoam and polycrystalline graphite is used. The content of the graphite, which magnetic susceptibility is 20 times that of water, can be tuned to give the mixture the most appropriate magnetic property. In the mixture preferably about 2-10 % (percent per volume) polycrystalline graphite is used. Even more preferably the part of polycrystalline graphite is about 5 % (percent per volume). The light weight Styrofoam is preferably used because of its material properties, which provide an excellent patient comfort.

Alternatively, the padding device does not contain a suitable material, but consists of such a material, e.g., PVC, or another plastic mixed with graphite. The term "mixing" is not to be understood as co-adding graphite granules to plastic granules, but in the sense of "melding", i.e. the mixing is performed at a microscopic level. Regardless of the design of the padding device, the use of such a material leads to an average magnetic susceptibility within the padding device, which preferably is similar to an average magnetic susceptibility of the object's material, e.g. the average magnetic susceptibility of human tissue.

According to a preferred embodiment of the invention, the receiver elements (and preferably the associated electronics) are integrated into the padding device. Alternatively, the receiver elements are mounted on the outside of the padding device, either on the inner surface (i.e. between the padding device and the imaged object) or the outer surface (i.e. outside the "resulting" object).

BRIEF DESCRIPTION OF THE FIGURES

These and other aspects of the invention will be described in detail hereinafter, by way of example, with reference to the following embodiment and the accompanying drawings; in which:

Fig.1 shows a first schematic illustration of a patient and an RF receiver (side view), and

Fig. 2 shows a second schematic illustration of a patient and an RF receiver (cross sectional view along arrows AA on Fig. 1 with parts of the patient being translucent).

DETAILED DESCRIPTION OF THE EMBODIMENTS

In Figs. 1 and 2 a patient 4 wearing an RF receiver 1 according to the present invention is shown for use in a MRI system 2. Illustrated are the patient's neck 6, head 7, and shoulders 8. Other MRI system components are not shown. However, a standard MRI system can be used with said receiver 2.

The receiver 1 combines the function of MRI signal reception with the function of padding the patient 4 with material having comparable magnetic properties as human tissue, in order to minimize field- variations within the patient 4. Accordingly, the receiver 1 comprises a padding device 3 optimized for being wrapped around the patient's neck 6, and further comprises a number of receiver coils 5 (and associated electronics, not shown) integrated into the padding device 3. The padding device 3 is used according to the invention to provide a Bo homogeneity correction. As described in more detail below, the padding device 3 is adapted for the correction of susceptibility artifacts stemming e.g. from the cavities around the patient's neck 6. The padding device 3 is adapted to be placed on the neck 6 in a way that a more regular outer geometric shape of the resulting object 9 is obtained. In the illustrated embodiment, the resulting object 9 exhibits basically a cylindrical shape with an ellipsoidal cross section. In the illustrated embodiment of the invention, the padding device 3 has the shape of a collar, which almost completely surrounds the patient's neck 6. The padding device 3 exhibit a U-shape. After placing the padding device 3 in its working position, the U- base 10 is positioned beneath the neck 6, and the U-legs 11 are positioned on the left and right side of the neck 6. In this working position, as illustrated in Figs. 1 and 2, a "resulting" object 9 is created, comprising the neck region of the patient 4 as well as the padding device 3 wrapped around the neck 6.

The cavities around the neck 6 are preferably completely padded (filled up) with the padding device 3, i.e. the padding fills out (at least almost) the complete space around the neck 6 between the shoulder 8 and the lower head 7. Preferably the padding device 3 furthermore comprises some cushioning elements. In particular the U-base may comprise some flexible elements (not shown) made from foam or the like, which will allow for some patient comfort.

The padding device 3 shows a flexible inner shape 12 in order to match the shape of the neck 6. For this purpose the padding device 3 is a container filled with a granular substance (not shown) having magnetic susceptibility similar to the magnetic properties of human tissue. In this case a mixture of 95 % (percent per volume) Styrofoam granules and 5 % (percent per volume) of polycrystalline graphite is used as filling material. The outer shape 13 of the padding device 3 is basically rigid in order to obtain an regular overall cylindrical shape.

The receiver coils 5 are located within the padding device 3 in a way that a number of first receiver coils 5 are positioned basically in the U-base 10 of the padding device 3. Second receiver coils 5 are positioned in the U-legs 11 of the padding device 3, by this means surrounding the neck 56 of the patient 4.

It will be evident to those skilled in the art that the invention is not limited to the details of the foregoing illustrative embodiments, and that the present invention may be embodied in other specific forms without departing from the spirit or essential attributes thereof. The present embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein. It will furthermore be evident that the word "comprising" does not exclude other elements or steps, that the words "a" or "an" do not exclude a plurality, and that a single element, such as a computer system or another unit may fulfil the functions of several means recited in the claims. Any reference signs in the claims shall not be construed as limiting the claim concerned.

REFERENCE NUMERALS

1 RF receiver

2 MRI system

3 padding device

4 patient

5 receiver coil

6 neck

7 head

8 shoulder

9 resulting object

10 U-base

11 U-leg

12 inner shape

13 outer shape

Claims

CLAIMS:

1. An RF receiver (1) for an MRI system (2), the receiver (1) comprising

- a padding device (3) adapted to be placed on an irregular shaped examination object (6) in a way that a more regular outer object shape (13) is obtained, said padding device (3) comprising material having magnetic properties similar to the magnetic properties of said object (6), and

- a number of receiver elements (5) mounted in or on said padding device (3).

2. The RF receiver (1) as claimed in claim 1, wherein the padding device (3) is adapted to obtain a cylindrical or spherical outer object shape (13).

3. The RF receiver (1) as claimed in claim 1, wherein the padding device (3) is adapted to be at least partly surrounding the object (6).

4. The RF receiver (1) as claimed in claim 1, wherein the padding device (3) shows a flexible inner shape (12) in order to match the object's shape.

5. The RF receiver (1) as claimed in claim 1, wherein the padding device (3) is a container filled with a granular substance.

6. The RF receiver (1) as claimed in claim 5, wherein the substance comprises granules of a plastic material into which an amount of graphite has been mixed.

7. The RF receiver (1) as claimed in claim 1, wherein the receiver elements (5) are integrated into the padding device (3).

8. A method of examining an irregular shaped object (6) by means of an MRI system (2), the MRI system (2) comprising an RF receiver (1), said receiver (1) comprising a padding device (3) and a number of receiver elements (5) mounted in or on said padding device (3), said padding device (3) comprising material having magnetic properties similar to the magnetic properties of said object (6), the method comprising the step of placing the padding device (3) on said object (6) in a way that a more regular outer object shape (13) is obtained.