EP1968440A2

EP1968440A2 - Reducing susceptibility artifacts in mri using composite materials

Info

Publication number: EP1968440A2
Application number: EP06801986A
Authority: EP
Inventors: Steven M. Conolly
Original assignee: Individual
Current assignee: Individual
Priority date: 2005-08-18
Filing date: 2006-08-18
Publication date: 2008-09-17
Also published as: EP1968440A4; WO2007022499A3; WO2007022499A2

Abstract

A susceptibility-matching composite material (200) composed of a highly diamagnetic material and a filler material reduces susceptibility artifacts in MRI. The composite, for example, may be formed by embedding highly diamagnetic particles (e.g., graphite powder) in a lightweight filler material (e.g., stryofoam or expanded polystyrene foam or EVA foam) such that the net susceptibility of the composite (200) matches human tissue (206) or other test substance to be imaged. The filler is preferably a non-conductive, low-density material that is either deformable or formed as a deformable collection of small beads packaged in a container so that the composite (200) conforms to the surface of a body (206) pressed against it.

Description

REDUCING SUSCEPTIBILITY ARTIFACTS IN MRI USING COMPOSITE MATERIALS

FIELD OF THE INVENTION

The present invention relates generally to magnetic resonance imaging. More specifically, it relates to devices and techniques for reducing imaging artifacts due to susceptibility differences at tissue-air interfaces.

BACKGROUND OF THE INVENTION

Magnetic resonance imaging (MRI) is a non-invasive technique developed to produce images of the interior of the human body for medical diagnosis and research. MRI involves exposing a body to a strong magnetic field (called the BO field) which aligns the spins of nuclei in the body into parallel and anti-parallel states, modulating the field at a characteristic radio frequency to induce in specific atomic nuclei resonant transitions between parallel and anti-parallel states, detecting resulting changes in the magnetic field caused by such transitions, and reconstructing an image from the detected changes. Further details of the physical principles underlying MRI are provided in US Pat. 6,294,972, which is incorporated herein by reference.

In order to produce high quality images, it is generally desirable in MRI that the static BO field be both strong and homogenous. MRI devices using a stronger BO field have a greater signal to noise ratio (SNR). A homogeneous BO field provides a uniform imaging response. A conventional MRI device is thus designed to produce a strong (typically 1.5 T), almost perfectly homogeneous BO field. However, the introduction of a body into the MRI device during use creates new inhomogeneities in the BO field due to magnetic susceptibility differences between materials such as air, tissue, and bone. These susceptibility differences distort the BO field and result in "susceptibility artifacts" in the MRI images. One of the largest of these susceptibility differences is found near air-tissue interfaces. Air has a magnetic susceptibility about 10 parts- per-million (ppm) higher than human tissue, which is a sufficiently large difference to produce significant image degradation in regions near air-tissue interfaces. Consequently, susceptibility artifacts are especially problematic when attempting to image tissue near the skin, lungs, mouth, or sinuses.

One well known technique used to help produce BO field homogeneity is shimming. In active shimming, MRI devices incorporate both shim coils which are used to actively adjust the magnetic field over the imaging region. The shim coils are designed to create magnetic field patterns (typically Legendre polynomials) that are independent and orthogonal over a spherical phantom. It is common to have shim teπns such as a main field shim, the three linear gradients, and up to a dozen nonlinear terms. By using coils that generate orthogonal field patterns, one can, in principle, optimize the current sources for each shim coil independently. However, independence is not guaranteed over a human sample (since humans are not spherical), so the optimization procedure can be time consuming. As a result, the so-called "higher-order" shimming is often bypassed in the interest of patient throughput. In fact, higher order shimming is sometimes unavailable on standard MRJ scanners. Another problem is that some smaller air pockets create localized field pattern changes that are too steep and localized to be removed effectively by the relatively smooth shim coil field patterns. In addition to these problems, the field shimming problem becomes significantly harder at higher field strengths because higher BO fields will induce proportionately higher variations due to susceptibility variations. Thus, active shimming becomes even more difficult in newer MRI devices using higher BO fields up to 3 T, 4 T, and 7 T.

Jezzard (WO 2003-062847) describes a passive shimming technique used in MRI to reduce susceptibility artifacts at air-tissue interfaces. The technique uses passive shims made of a highly diamagnetic material to locally alter the magnetic field to restore homogeneity. The highly diamagnetic material, pyrolytic graphite (PG), is commercially sold in the form of a pyrolytic graphite sheet (PGS) or as a solid crystal. The material is very anisotropic. Although its cross plane susceptibility is close to human tissue at -9 ppm, PGS has a large through-plane diamagnetic susceptibility of -595 ppm. Shimming on a per patient basis is difficult using this technique, however, since PGS is hard and cannot be reshaped to compensate for bodily variations between different patients.

Another approach to reducing susceptibility artifacts is to position susceptibility matching materials directly at the tissue-air interfaces to eliminate the susceptibility differences there. For example, the air around the body could be replaced with a material that is closely matched in susceptibility to human tissue. In an ideal case, one would like to have a diamagnetically doped gas that has a susceptibility equal to that of tissue, carries adequate oxygen for respiration, is inexpensive, nonconductive, and is biologically inert. Unfortunately, there is no known gas with this combination of properties. Indeed, there is no gas at all with the requisite diamagnetic behavior, safe or otherwise. Alternatively, one may use liquid or solid susceptibility matching materials. Liquid gel pads, for example, are commercially available for this purpose. The pads contain a gel material (essentially water) whose diamagnetic susceptibility is closely matched with that of tissue. These pads are positioned against specific areas of the body surface to reduce susceptibility artifacts there. There are two significant drawbacks of the gel pads, however. First, the density of the gel is similar to that of water, which makes them fairly heavy and cumbersome. Second, since the pads are placed between the body and the RF coil, they act as a shield between the RF coil and the area to be imaged, reducing signal strength. Because the pads need to remain sufficiently thick to remove artifacts, the shielding is to some extent unavoidable. It would not be safe to place the RF coils inside the gels, as this would make the coils overly heavy, and the fluid would cause problems with high voltages on the capacitors and inductors of the RF coil during transmit phase.

SUMMARY OF THE INVENTION In one aspect, the present invention provides a susceptibility-matching composite material for reducing susceptibility artifacts in MRI. The material is lightweight, inexpensive, nonconductive, matches the magnetic susceptibility of the human tissue, and may be shaped to an external body part (such as the foot, chin, shoulder, head, knee, spine). The composite material is three to sixty times less dense than the gel pads, making it much easier to handle. Because the composite material is dry and largely nonconductive, RF coils may be embedded within the material to achieve excellent filling factors while minimizing susceptibility artifacts in specific body parts. The composite material enjoys the advantage that its overall shape can be altered to shim specific bodily regions. Using this composite material allows high quality MRI scans to be obtained with at least an order of magnitude reduction in susceptibility artifacts near the surface of the body, such as the foot, breast, spine, head, knee, chin, carotid, shoulder, and prostate. This composite is also advantageous in fast imaging with high. field MRI scanners and may be useful in providing more robust frequency selective fat separation or fat suppression techniques. According to one aspect of the invention, susceptibility artifacts in magnetic resonance imaging of a portion of a human body can be reduced by positioning the portion of the human body and a diamagnetic material in a magnetic field generated by a magnetic resonance imaging device. The diamagnetic material is characterized in that it is a composite material made of a filler material and a collection of diamagnetic particles uniformly distributed through the filler material. The composite material preferably has a bulk magnetic susceptibility within 10% or less of the bulk magnetic susceptibility of human tissue, and more preferably within 1% or less. When the composite material is in contact with a surface of the portion of the human body to be imaged, the MRJ scan is perfoπned. Because of the susceptibility-matching properties of the material, susceptibility artifacts are reduced or eliminated in the resulting image. Field variations persist but they largely exist in the composite material, essentially moving the field interface region to an area outside the human. Hence the imaging area no longer suffers from susceptibility artifacts.

In some embodiments of the invention, the diamagnetic particles are graphite particles e.g., pyrolytic graphite powder, crushed PG, or cut pyrolytic graphite sheet (PGS). In some embodiments, the filler material is a deformable, non-conductive, low-density material such as foam (e.g., stryofoam or expanded polystyrene foam, or an EVA foam). The composite material may be packaged in flexible material container suitable for placement against specific body parts. The composite material may be used as a liner within a container similar to a removable cast so that a limb (such as a foot, leg, hand, or arm) is surrounded by the composite material when the limb is placed in the container. The composite may also be used as a liner in a helmet used when imaging the head. The composite may also be formed into the shape of foam ear plugs that may be placed within the ear. The composite may also be used within the mouth. Although the composite has the advantage of being deformable if needed, it may also be useful in stiffer forms, such as a gantry table pad upon which the body rests during an MRI scan. In some embodiments, the composite may have RF coils embedded in it. The composite may be packaged in a manner similar to bean bags, which could readily conform to the body part. Here the graphite may be embedded in "balls" of filler material or on the surface of the filler material. The principle of effective operation is the same for each — the bulk volume susceptibility of the composite matches that of human tissue to within 10%, or preferably, to within 1%. BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. IA and IB illustrate how a susceptibility-matched composite of highly diamagnetic material and a light filler material leads to a reduction of susceptibility artifacts in an MRI image through a comparison of such a composite with other materials. FIG. 2 illustrates a composite material in the form of small composite beads contained within an envelope whose inner surface is deformable so that it conforms to the shape of a body part and whose outer surface is designed to retain the shape of a cylinder. FIG. 3 illustrates a composite material formed to serve as ear plugs. FIG. 4 illustrates a composite material in the form of a wrappable pad. FIG. 5 illustrates a composite material in the form of a gantry pad.

DETAILED DESCRIPTION

In preferred embodiments of the present invention, a composite susceptibility-matching material is provided for reducing susceptibility artifacts in MRI imaging. The composite has a low- density filler material such as foam with particles of a highly diamagnetic material embedded or mixed homogeneously within or around the filler. The filler material is preferably low-density, substantially non-conducting, and inexpensive. A low density material in this context is defined as a material preferably having less than 0.5 specific density. The filler material preferably possess magnetic susceptibility very similar to air (e.g., within 10% or less, and preferably within 1% or less). Examples of such filler materials include plastics, styrofoam, balsa wood, aero gels, foams, EVA foams, and expanded polystyrene foams. In preferred embodiments, the diamagnetic particles are primarily a graphite powder, pyrolytic graphite, another highly diamagnetic material, or some mixture of such materials. Thus, the diamagnetic particles used in the composite may be a mixture of different diamagnetic materials.

Pyrolytic graphite sheet (PGS) is commercially available (e.g., from Digikey.com) and has a magnetic susceptibility approximately 66 times stronger than that of human tissue. The average susceptibility χ_aVc_ra_ge for a composite material containing diamagnetic particles of susceptibility is

Xavcragc =/ XPGS / (1 + CX χ_PGs), where/is the volume fraction of the PGS particles, and α is the demagnetizing factor determined by the shape of the PGS particles (e.g., spherical particles have a « 1/3). Because the magnitude of X_PGS is very small, however, we can approximate

Xavcragc <=•/ 5(PGS-

In other words, the shape factor is not significant in this case, so any shape of PGS particles should yield the same bulk susceptibility. Thus, a homogeneous volume reduction of PGS particles within the filler material by a factor of 66 can be used to create a composite with bulk diamagnetic properties very similar to human tissue. Because PGS has an in-plane thermal and electrical conductivity similar to copper, it is desirable to ensure that the particles of the powder are sufficiently small to break up conducting paths in the PGS, which may otherwise create noise in the receiver coil.

An illustrative example of the effectiveness of volume-reduced PGS material in reducing susceptibility artifacts will now be discussed with reference to FIG. IA. Narrow (1 mm) strips of PGS material 10 were glued to low density balsa wood filler layers 12 to form a vertically stacked PGS-wood composite block 18 encased in a plastic protective cube whose edges have length 2.5 cm. The strip separation distance in each layer and the layer thicknesses are selected to create a net volume reduction by a factor of 66 so that the PGS composite block accurately mimics the susceptibility of human tissue. The orientation of the strips is alternated between adjacent layers of balsa wood to homogenize the blurring effect in 3D rather than 2D.

For comparative purposes, a solid piece of wood 14 was encased in a plastic cube to form a wood block 20, and an air-filled block 22 was formed by filling a similar plastic block with air 16. The figure shows an MRI image of the three blocks within a cylindrical container of water doped with MnC^. The composite block 20 is oriented so that the strip planes are perpendicular to the BO field. Bright regions 24 adjacent to the water-cube interfaces are characteristic dipole field patterns that represent image artifacts due to susceptibility differences. As clearly illustrated in the figure, the field pattern around the PGS composite cube is significantly smaller than the patterns surrounding the air cube 22 and wood cube 20. Hence, this simple PGS composite is very effective at removing the characteristic susceptibility artifact at the interface between the cube and the surrounding water. PGS is pure crystal since the material is synthesized by chemical vapor deposition. But PGS is currently quite expensive, so in many embodiments it is preferable to use a pyrolytic graphite powder formed by crushing mined graphite crystal, which is relatively inexpensive. Although PG powders are commercially available, some have been found to have magnetic contaminants.

It is important to ensure that the PG powder, flake, or crushed particles have the correct susceptibility. It is well-known in the PG industry that natural flakes can be heated to extreme temperatures (e.g., above 2000 C) which results in materials that are very low in impurities as a result of vaporization of "ash" constituents during high temperature heat treatment. Most natural graphite contains some iron due to the presence of iron in the rock in which it is formed.

To obtain a sufficiently pure PG powder, it may be preferable to crush heat-treated PG crystal. One then embeds or disperses the powder in a low-density filler matrix to form the composite. For example, FIG. IB is an image of B0-field intensity comparing field patterns for water 28, air 30, and a composite 32 formed from PG powder in a polyurethane foam matrix. The figure clearly shows that the susceptibility artifact near the surface of the composite 32 is greatly reduced compared to air 30, and is close to that of water 28. In the image, all three phantoms are immersed in water. To account for the randomization of orientation of the PG powder, we found mathematically that the concentration should be increased by a factor of 2 compared to PGS, resulting in a net volume dilution factor of approximately 30-32. Taking into account the full anisotropy of the PG powder, one can show that a volume fraction of 1/31 gives a susceptibility almost exactly matching that of water. Most human tissues are extremely close to the susceptibility of water, and the volume fraction can be further tuned to match human tissue even more accurately. In addition to being inexpensive, another advantage of the PG powder is that it is not directionally dependent once the crystal particle orientations are randomized in the filler (i.e., the composite has isotropic magnetic susceptibility even though the crystals are anisotropic). As an alternative to embedding in a foam matrix, the PG powder may also be distributed between tiny styrofoam beads. Care should be taken to ensure that the graphite powder has no magnetic contaminants. The size and separation of the particles determines how close local field distortions exist within the subject. This is due to the blurring effect of magnetic field sources. The field a distance 1 cm away from a complex magnetic field source is convolved with a function of width 1 cm; at 10 cm it is convolved with a function of width 10 cm. Hence, microvariations within the composite are largely blurred out a distance approximately equal to the separation between the particles. Hence it is preferred that the graphite particles and their separation be small relative to a pixel size. For example, a 100 micron particle size or smaller would be preferred.

In preferred embodiments, graphite powder is used in addition to, or instead of PGS. Graphite powder has the advantage that it is extremely inexpensive and isotropic once randomized. The average susceptibility of graphite powder, however, is about 19 to 33 times that of tissue), so composites using graphite will require a larger volume fraction of diamagnetic particles in the foam (specifically 19 to 33 times volume dilution) than composites using PGS alone (which requires a volume dilution of about 60-66 fold). In preferred embodiments, a composite is made of a high purity graphite powder suspended in a low density filler such as styrofoam or expanded polystyrene (EPS) foam or EVA foam. For example, raw polystyrene beads may be mixed with graphite powder then the mixture is heated (e.g., with steam or pentane) to create EPS-graphite foam which can then be formed as desired, e.g., into small (i.e., 1 mm diameter) beads. These beads of styrofoam-graphite may then be enclosed in a containment envelope to form a susceptibility-matching composite package.

The air gaps between beads increase the volume, reducing the net susceptibility. This, however, is easily corrected for since the susceptibility of the air and the filler material are virtually identical at about 0.36 ppm. Hence, by increasing the graphite density in the beads, one can account for the "sphere packing" factor. The packing fraction for real EPS styrofoam spheres is easily determined experimentally for the particular filler material, but is expected to be approximately equal to the ideal sphere-packing fraction of 0.68. So, by setting the volume fraction inside the PG microparticles to be about 1/(32*0.68) (i.e., about 1/22) the net volume faction will be about 1/22*0.68= 1/32. Hence, to account for sphere packing gaps of air, we simply increase the concentration of PG microparticles from 1/32 to 1/22 (approximately) to match human tissue susceptibility. Note that some routine experimentation and adjustment may be necessary to determine the actual sphere packing factor, since electrostatic interactions and surface friction may cause deviations from the theoretical sphere packing fraction. In order to reduce the space between the EPS-graphite beads and the skin surface, it may be preferable to use smaller beads (smaller than approximately 2 mm) enclosed in a thin packaging envelope or fabric. This "bean-bag" like device could conform to a body part quickly and conveniently. It is important to keep in mind that the bulk volume fraction has not changed with this bean-bag type construction. The bulk fraction of pyrolytic graphite remains about 1/31. However, the density does not need to be perfectly homogeneous throughout the foam. This fact allows for air gaps, which in turn allows for a composite material that can be tailored to the individual patient's body part being imaged.

There are four composite manufacturing tolerances that are worthy of discussion: the dilution factor, the susceptibilities of the graphite powder and the filler materials, the sampling pattern of the graphite powder within the filler material, and the orientation of the graphite. A susceptibility matching error of 1 ppm (about 10% difference from tissue susceptibility) is considered excellent, while an error of 0.1 ppm (about 1% difference) is considered nearly perfect. There are also a few research applications (e.g., in vivo temperature monitoring of RF thermal ablation in the prostate) that would benefit from more precise susceptibility matching.

Dilution and Susceptibility Tolerance The tolerance on the magnetic susceptibility of the filler material depends only on the product of the volume fraction,/ and the susceptibility of the diamagnetic particles: χ_avera_ge =/ X_?G- Hence, the product of the dilution factor and the susceptibility must be accurate to about +/- 5% in order to be within 1 ppm of human tissue (+/- 1/2 ppm out of -9 ppm). For example, we could set the tolerance of each to about 2.5%. In addition, one can measure the susceptibility of an individual batch of graphite before processing and adjust the volume fraction accordingly. Hence, it is not difficult to match human tissue to within about 1%, which would be nearly perfect for today's clinical applications. Manufacturing tolerances of 0.01% are common in some areas, and these could be useful for emerging research applications.

Sampling Pattern Tolerance

The tolerance for the sampling pattern of the composite material's homogeneity is best understood from a Green's function (or convolution) analysis. At a distance r from any magnetic source, the magnetic fields fall off as the convolution of the magnetic field source with a kernel whose width is proportional to the distance r. For a composite material, the variations in the composite due to discrete graphite flakes will be blurred out nearly perfectly at a distance equal to the composite array spacing. Hence, if we set up the composite material with discrete spacing of 1 mm, then we should expect fully homogeneous magnetic field behavior about 1-2 mm from the surface. Hence, by adjusting the lattice spacing to be small relative to pixel resolution this effect can be made nearly negligible. In fact, it is likely that any randomized flaking pattern that preserves orientation of the PGS (or largely randomizes it) will provide the most practical method of PGS composite manufacturing. Little if any spatial sampling problems exist with powders in an EPS foam, since the powder particle size is much smaller than a pixel.

Orientation Tolerance

Orientation tolerance issues apply to PGS composites due to the angular dependence on the susceptibility of PGS. If we neglect the relatively small susceptibility of the in-plane PGS, then we simply have an average susceptibility that varies as / X_PGS |cosθ|, where θ is the angle between the PGS normal and the Bo field. To keep the net susceptibility within 10% tolerance, we simply need cosθ > 0.9, which translates to |θ| < 25 degrees. This is a very relaxed manufacturing constraint. Indeed, 8 degree orientation tolerance translates to 1% tolerance on average susceptibility, which is quite practical. Note that it may be advantageous to process the PGS composite in a uniform magnetic field to maintain proper orientation. This would be especially true if the filler material undergoes a phase transition from a liquid or gel to a solid.

Since graphite powder is isotropic, however, there is no orientation issue with a composite using graphite powder. Thus, graphite is highly preferable to PGS composite in this respect. While creating the powder graphite composite, care should be taken to ensure randomization of the graphite by thorough mixing. This will ensure insensitivity to orientation dependence and ensure uniform dispersion of the PG throughout the composite PG foam.

Tolerance Summary

In view of the above, susceptibility-matching composites according to embodiments of the invention can be made to have susceptibilities within 1% of human tissue, reducing shifts of ~300 Hz at 1.5 T to ~3 Hz shift at the skin, producing negligible image artifacts. The high level of homogeneity attained using these composites can be helpful for demanding applications like RF ablation monitoring in an interventional setting, since a 15 C temperature increase induces only about 3 Hz frequency shift at 0.5 T, which is quite small relative to 100 Hz variations due to air. It also may be helpful for routine applications such as robust water-fat separation, or robust fat saturation with selective RF pulses, or SSFP at 3 T. At 1.5 T, fat and water are separated in resonance frequency by about 245 Hz. Air-tissue interfaces experience +/- 320 Hz field shifts which are enough to cause poor water-fat separation using most clinically accepted methods (spectral saturation, three-point Dixon, Chemsat, etc). By using the PG foam with excellent field matching, this ± 320 Hz field shift can be reduced to just ± 32 Hz with 10% tolerance (or preferably to ± 3.2 Hz with 1% tolerance). These classic methods of fat suppression and/or water-fat imaging will be far more robust with deviations of 32 Hz or 3.2 Hz. Essentially these methods will never fail with such a perfect field variation. It is common for them to fail in current clinical practice, and this can be a significant clinical confoundant.

In various preferred embodiments, small (i.e., 1 mm diameter or less) EPS or EVA foam- graphite composite beads 200 are contained within an envelope 202 that is at least partly composed of a flexible material (e.g., a fabric, rubber, or other membrane) 204, as shown in FIG. 2. The graphite density in the beads is adjusted to account for the sphere packing factor so that the overall volume concentration of the PG graphite mix (including the air voids) is precisely matched to human magnetic susceptibility. Preferably, the composite beads behave as an ultralight liquid or sand to provide high conformability to the individual geometry of the surface of the body part 206 being imaged. In practice, when pressed against the body being imaged, the deformable envelope surface 204 conforms to the shape of the body 206 because the beads 200 behave like an incompressible fluid within the flexible envelope 204. In some embodiments, an outside portion of the envelope not contacting the body may be rigid or semi-rigid hull 208 that approximates a cylindrical, spherical, or hemispherical shape, e.g., by using a semi-rigid supporting structure for the envelope, or using a semi-rigid material for the envelope itself. Using a cylindrical or spherical shell as an outside hull of the package helps to reduce image artifacts at the outside surface of the composite package. In some embodiments, the envelope may be configured in two or more parts attached with a hinge 212 or other mechanism that allows the device to be opened and closed around a body part. The envelope may also be similar to a removable cast having an outside shell and an inner padding that contains composite beads which conform to the body part in the cast.

Although FIG. 2 illustrates the specific case of a foot, it is clear that the envelope may be easily adapted for use with a hand, and elbow or knee (e.g., by providing an opening at the bottom of the cylindrical hull shown for the limb to exit), a head (e.g., by providing a hemispherical hull so that the device fits on a head like a helmet), a back/spine (e.g., by providing a horizontal gantry pad for a patient to lay upon), and various other body parts. It will also be appreciated that the flexible envelope 204 may be used without semi-rigid hull 208 as a bean-bag like composite that can be wrapped around a body part and held in place by gravity, hook-and-loop fasteners, straps, or various other devices. In alternate embodiments, the composite beads 200 may be replaced by a solid foam composite whose surface is shaped during manufacture to approximate the expected shape of the body 206. Although the solid foam is defoπnable, it provides more support than the conforming beads. Such solid foam composites may be preferred to fit softer regions of tissue, such as the breast, abdomen, or thigh. Such solid foam embodiments in most cases would not need a semi-rigid hull 208 since the foam itself provides adequate structural support.

Preferred embodiments may include an integrated RF coil or RF coil array 210 in the composite that fits snugly around or against a body part (such as a foot, hand, knee, breast, head, spine, inner ear, chin, mouth, shoulders, etc.). Such devices provide excellent RF coil filling factor as well as excellent magnetic field homogeneity. RF coil arrays are well known to improve spatial coverage with excellent signal to noise ratio. They are also critical for certain high-speed imaging applications such as SENSE and SMASH. Finally the combination of a coil array with the susceptibility matching composite should provide a very homogeneous region over each small coil. This combination would enable the use of certain fast-imaging pulse sequences that are known to suffer from susceptibility artifacts (e.g., echo planar MRI, spiral MRI, balanced Steady State Free Precession, RARE or Fast Spin Echo, and fast gradient recalled echo imaging).

Another preferred embodiment is a diamagnetically doped ear plug 300, as shown in FIG. 3. The plug 300 could be formed of a continuous compressible doped foam, or as an envelope containing doped beads. The ear plug can double as an acoustic noise dampener while reducing field artifacts near the ear canal.

In another embodiment, the composite takes the form of a flexible diamagnetically doped foam pad 400, as shown in the cross-sectional illustration of FIG. 4. The pad may be wrapped around a body part multiple times to provide the desired net thickness. Preferably, the pad is about lcm to 3cm in thickness. It could have a ramped thickness, allowing tight conformity close to the body part, and providing speedier attainment of desired outer radius. Once wrapped, it may be secured using any of various types of fasteners. A similar but larger and thicker pad 500 may be used in an unrolled configuration as shown in FIG. 5. A patient can lay on a flexible composite foam pad 500 positioned on a MRI scanner gantry, for example, to reduce artifacts at the surfaces of the underside of the patient in contact with the pad. The pad 500 may be flat, or it may be shaped to conform to an approximate expected shape of a body lying on the pad. The pad may include extensions that conform to specific parts of the body, including shoulder pads, chin pads, a bowl shaped region to support the bottom of the head, or positioning pads under the knee. The PG-foam pad may also double as the attachment device for securing the RF coil to the patient, perhaps in conjunction with a hook-and-loop fastener for securing. This will ensure that the MRI technologist uses the PG foam strap and prevents poor imaging when the MRI technologist is too rushed to remember to use the susceptibility matching material.

The techniques and materials of the present invention are useful not only in MRI imaging of the human body and other living tissue such as animals, but also in imaging various other substances. For example, the techniques can be used to minimize magnetic field variation at the air interface of a liquid-filled test tube. Specifically, a composite material is uniformly doped with a highly diamagnetic material (such as pyrolytic graphite or graphite powder) at a dilution factor designed to match the bulk magnetic susceptibility of the test substance (e.g., the liquid in a test tube). Uniform doping is defined in the present description to be doping that is sufficiently uniform that inhomogeneities in the diamagnetic particle distribution are not resolvable by the MRI imaging device. In the case of doped bead composites, beads manufactured with different degrees of doping may be purchased and then mixed on an as-needed basis in appropriate ratios to achieve a desired net susceptibility. If the diamagnetic agent is conductive (such as graphite) then it preferably takes the form of small pieces or a powder to prevent eddy current paths from introducing RF losses in the receive coil. An envelope similar to that shown in FIG. 2 may be filled with a bead composite that is susceptibility-matched to a test substance. The test substance (contained in a test tube or other container) is then surrounded by the susceptibility-matched composite to reduce image artifacts in MRI.

Claims

1. A method for reducing susceptibility artifacts in magnetic resonance imaging of a portion of a human body, the method comprising: positioning the portion of the human body and a diamagnetic material in a magnetic field generated by a magnetic resonance imaging device, wherein the diamagnetic material is a composite material made of a filler material and a collection of diamagnetic particles uniformly distributed through the filler material; and performing a magnetic resonance imaging scan when the composite material is in contact with a surface of the portion of the human body to be imaged.

2. The method of claim 1 wherein the composite material has a bulk magnetic susceptibility within 10% or less of the bulk magnetic susceptibility of human tissue.

3. The method of claim 1 wherein the composite material has a bulk magnetic susceptibility within 1% or less of the bulk magnetic susceptibility of human tissue.

4. The method of claim 1 wherein the diamagnetic particles are graphite particles.

5. The method of claim 1 wherein the diamagnetic particles comprise at least one material selected from the group consisting of pyrolytic graphite powder, crushed PG, and cut pyrolytic graphite sheet (PGS).

6. The method of claim 1 wherein the filler material is deformable, non-conductive, with a specific density below 0.5.

7. The method of claim 1 wherein the filler material is a type of foam.

8. The method of claim 7 wherein the filler material comprises a material selected from the group consisting of plastics, balsa wood, and aero gels.

9. The method of claim 1 wherein the composite material comprises an embedded RF coil.

10. A diamagnetic material for reducing susceptibility artifacts in magnetic resonance imaging of a portion of a human body, the diamagnetic material comprising a composite material made of a filler material and a collection of diamagnetic particles uniformly distributed through the filler material, wherein the diamagnetic material has a bulk magnetic susceptibility within 10% or less of the bulk magnetic susceptibility of human tissue.

11. The material of claim 10 wherein the composite material has a bulk magnetic susceptibility within 1% or less of the bulk magnetic susceptibility of human tissue.

12. The material of claim 10 wherein the diamagnetic particles are graphite particles.

13. The material of claim 10 wherein the graphite particles comprise at least one material selected from the group consisting of pyrolytic graphite powder, crushed PG, and cut pyrolytic graphite sheet (PGS).

14. The material of claim 10 wherein the filler material is deformable, non-conductive, with a specific density below 0.5.

15. The material of claim 10 wherein the filler material is a type of foam.

16. The material of claim 10 wherein the filler material is selected from the group consisting of plastics, balsa wood, and aero gels.

17. The material of claim 10 wherein the composite material comprises an embedded RF coil.

18. A composite material comprising a soft foam and pyrolytic graphite powder, wherein the pyrolytic graphite powder is uniformly distributed through the soft foam at a bulk volume fraction of approximately 1/31.