Hyperpolarized solid materials with long spin relaxation times for use as imaging agents in magnetic resonance imaging

Abstract

An imaging agent is disclosed for use in nuclear magnetic resonance imaging. The imaging agent includes a first substance and a second substance. The first substance includes at least one atom having non-zero nuclear spin providing a polarized magnetic orientation. The second substance is bound to the first substance and inhibits physical contact between the at least one atom and other atoms and molecules to thereby inhibit spin relaxation of the polarized magnetic orientation of the at least one atom.

Description

BACKGROUND

The invention generally relates to the field of nuclear magnetic resonance (NMR), and relates in particular to magnetic resonance imaging (MRI).

MRI systems generally provide for diagnostic imaging of regions within a subject by detecting the precession of the magnetic moments of atomic nuclei in an applied external magnetic field. Spatial selectivity, allowing imaging, is achieved by matching the frequency of an applied radio-frequency (rf) oscillating field to the precession frequency of the nuclei in a quasi-static field. By introducing controlled gradients in the quasi-static applied field, specific slices of the subject can be selectively brought into resonance. By a variety of methods of controlling these gradients in multiple directions, as well as controlling the pulsed application of the rf resonant fields, three-dimensional images representing various properties of the nuclear precession can be detected, giving information about the density of nuclei, their environment, and their relaxation processes. By appropriate choice of the magnitude of the applied quasi-static field and the rf frequency, different nuclei can be imaged. Typically, in medical applications of MRI, it is the nuclei of hydrogen atoms, i.e., protons, that are imaged. This is, of course, not the only possibility. Information about the environment surrounding the nuclei of interest can be obtained by monitoring the relaxation process whereby the precessional motion of the nuclei is damped, either by the relaxation of the nuclear moment orientation returning to alignment with the quasi-static field following a tipping pulse (on a time scale T1), or by the dephasing of the precession due to environmental effects that cause more or less rapid precession, relative to the applied rf frequency (on a time scale T2).

Conventional MRI contrast agents, such as those based on gadolinium compounds, operate by locally altering the T1 or T2 relaxation processes of protons. Typically, this relies on the magnetic properties of the contrast agent, which alters the local magnetic environment of protons. In this case, when images display either of these relaxation times as a function of position in the subject, the location of the contrast agent shows up in the image, providing diagnostic information.

An alternative approach to MRI imaging is to introduce into the subject an imaging agent, the nuclei of which themselves are imaged by the techniques described above. That is, rather than affecting the local environment of the protons in the body and thereby providing contrast in a proton image, the imaging agent is itself imaged. Such imaging agents include substances that have non-zero nuclear spin such as ³He, ¹²⁹Xe, ³¹P, ²⁹Si, ¹³C and others. The nuclei in these substances may be polarized by various methods (including optically or using sizable applied magnetic fields at room or low temperature), orienting a significant fraction of the nuclei in the agent (hyperpolarizing), before introduction into the body, and then introducing the polarized material into the body. Once in the body, a strong imaging signal is obtained due to the high degree of polarization of the imaging agent. Also there is only a small background signal from the body, as the imaging agent has a resonant frequency that does not excite protons in the body. For example, U.S. Pat. No. 5,545,396 discloses the use of hyperpolarized noble gases for MRI.

Many proposed agents for hyperpolarized MRI have short relaxation (T1) times, requiring that the material be quickly transferred from the hyperpolarizing apparatus to the body, and imaged very soon after introduction into the body, often on the time scale of tens of seconds. For a number of applications, it is desirable to use an imaging agent with longer T1 times. For example, U.S. Pat. No. 6,453,188 discloses a method of providing magnetic resonance imaging using a hyperpolarized gas that provides a T1 time of several minutes and possibly up to sixteen minutes (1000s). Compared to gases, solid—or liquid materials usually lose their hyperpolarization rapidly. Hyperpolarized substances are, therefore, typically used as gases. Protecting even the hyperpolarized gas from losing its magnetic orientation, however, is also difficult in certain applications. For example, U.S. Published Patent Application No. 2003/0009126 discloses the use of a specialized container for collecting and transporting ³He and ¹²⁹Xe gas while minimizing contact induced spin relaxation. U.S. Pat. No. 6,488,910 discloses providing ¹²⁹Xe gas or ³He gas in microbubbles that are then introduced into the body. The gas is provided in the microbubbles for the purpose of increasing the T1 time of the gas. The relaxation time of such gas, however, is still limited.

U.S. Published Patent Application No. 2004/0024307 discloses the use of para-hydrogen labeled imaging agents using non-zero nuclear spin atoms such as ¹³C, ¹⁵N, ²⁹Si, ¹⁹F, ³Li, ¹H, and ³¹P in a host molecule using enriched hydrogen. The hydrogenated imaging agent is then employed in a soluble form in a liquid or solvent for MRI and is disclosed to have a T1 time of preferably about 1000 s or longer.

U.S. Published Patent Application No. 2005/0136002 discloses the use of a particulate contrast agent that includes non-zero nuclear spin atoms such as ¹⁹F, ¹³C, ¹⁵N, ²⁹Si, ³¹P, ¹²⁹Xe and ³He. The particulate composition is disclosed to respond to physiological conditions in a subject to provide improved imaging by changing contrast characteristics. The responses to physiological conditions are disclosed to include melting, or changing the viscosity or chemical composition of the subject. The spin relaxation times, however, are generally disclosed to be less than 1 s.

There is a need, therefore, for a contrast agent or imaging agent that provides greater flexibility in designing relaxation times during nuclear magnetic resonance imaging.

SUMMARY OF THE INVENTION

An imaging agent is provided in accordance with an embodiment of the invention for use in nuclear magnetic resonance imaging that includes a first substance and a second substance. The first substance includes at least one atom having non-zero nuclear spin. The second substance is bound to the first substance and provides shielding for the first substance from the environment in a way that allows long nuclear spin relaxation of the first substance, allowing the at least one atom of the first substance to be hyperpolarized with a long relaxation time.

In accordance with various embodiments, the first substance includes at least one of ²⁹Si, ¹³C, ¹⁹F, ³¹P, ¹²⁹Xe or ³He, and the imaging agent may be provided in solid form as a powder.

BRIEF DESCRIPTION OF THE DRAWINGS

The following description may be further understood with reference to the accompanying drawings in which:

FIG. 1 shows an illustrative diagrammatic view of a molecule of an imaging agent in accordance with an embodiment of the invention;

FIG. 2 shows an illustrative diagrammatic view of a molecule of an imaging agent in accordance with another embodiment of the invention;

FIG. 3 shows an illustrative diagrammatic view of a particle of an imaging agent in accordance with another embodiment of the invention; and

FIG. 4 shows an illustrative diagrammatic view of the particle shown in FIG. 3 rotated 90 degrees.

The drawings are shown for illustrative purposes and are not to scale.

DETAILED DESCRIPTION OF THE INVENTION

Applicant has discovered, that although liquids and solids typically have short relaxation times, certain solid materials can be manufactured that result in extremely long T1 times, and that these materials can make excellent hyperpolarized contrast agents. Applicant has discovered that a long T1 contrast agent for hyperpolarized MRI application may be formed using a material whose host substance has no nuclear spin and include with it an impurity or dopant that has a nonzero nuclear spin. For example, naturally occurring Si or C, are composed mostly of ²⁸Si and ¹²C respectively, which ate zero nuclear atoms, but also contain ²⁹Si and ¹³C, respectively, which have nuclear spin ½. The concentration of the nonzero-spin component in the zero-nuclear-spin host can be controlled by various methods to range from a few percent, up to 80-90%. The nonzero spin substance can also be a dopant material in the host, such as ³¹P in Si. In another embodiment, either natural or artificial concentrations of ¹³C in a ¹²C constitute such a system. Other embodiments include ²⁹Si in silica or quartz.

Relaxation times (T1) of the nonzero nuclear component in these solid materials are found to extended considerably beyond other hyperpolarized imaging agents, including gases, with T1 times of at least 30 minutes to an hour and even four hours or beyond at room temperature. The solid may be provided as a powder or, the powder can be suspended in a liquid. Powder sizes ranging from nanoparticles, a few nanometers in diameter, to micron diameters and beyond are possible. These materials appear to be biocompatible, although the powder size should be considered carefully for compatibility with biological systems.

As shown at 10 in FIG. 1, a molecule may include central atom of a non-zero spin substance such as ²⁹Si, ¹³c, ³¹P, ¹²⁹Xe or ³He that is surrounded by atoms of zero-spin substances such as ²⁸Si or ¹²C, or compounds such as ²⁸SiO₂. As shown at 12 in FIG. 2, a molecule in accordance with another embodiment of the invention may include a non-zero spin substance that is surrounded by a zero spin substance. A molecule 14 including zero spin atoms 16 and non-zero spin atoms 18 as shown in FIG. 3. When the molecule 14 is rotated (e.g., 90 degrees) the magnetic orientations of the non-zero spin atoms 18 continue to be oriented in the same direction as shown in FIG. 4.

For particular crystal structures, such as those found naturally occurring in Si or C (diamond) the electronic environment of the nonzero spin component is isotropic, so that weak coupling of electrons to nuclei does not have any preferred orientation. This means that the direction of the nuclear magnetic moment of the nonzero spin component is not locked to the crystal axes of the material, or the particle of material. As a result, even when the individual particles tumble, the nuclear magnetic moment holds its hyperpolarized orientation.

In further embodiments, a binding material may also be included on the surface of the solid imaging agent powder (nanoparticle) to bind to certain biological materials with specificity. As an example, by coating the nanoparticle imaging agent with a substance that preferentially adheres to certain cancer cells, the location of these cells in the body can be imaged by MRI. Nanoparticles of solid material such as silicon, diamond, or silica may be functionalized on the surfaces to cause them to attach to a wide range of specific proteins, cells, or organs, while maintaining long T1 times. This allows specific biological surfaces or processes to be tagged by the hyperpolarized materials.

Various techniques may be used to produce the hyperpolarization of the material prior to introducing it into the biological system or medical patient. These include, but are not limited to, optical dynamical nuclear polarization, various Overhauser techniques (solid effect, thermal effect) and polarization in high magnetic fields and/or low temperatures, including fields produced with permanent magnets.

A wide range of potential applications of hyperpolarized powders with long (up to multi-hour) T1 times include bolus injection into the bloodstream for angiography or examining ruptures in the blood-brain barrier, placement of the hyperpolarized material into body cavities (oral cavity, sinus, gut, esophagus, colon, vagina) allowing, for instance, hyperpolarized virtual colonoscopy and related diagnostics, lung imaging using hyperpolarized inhalants (10-100 nm particles in aerosol).

Those skilled in the art will appreciate that numerous modifications and variations may be made to the above disclosed embodiments without departing from the spirit and scope of the invention.

Claims

1. A method of detecting a silicon particle within a patient, the method comprising introducing a silicon particle into a patient, wherein the silicon particle comprises a hyperpolarized 29Si nuclear spin and the size of the silicon particle is between about 10 nm and about 10 μm; and detecting the 29Si nuclear spin by nuclear magnetic resonance.

2. The method of claim 1, wherein in the step of introducing, the silicon particle is suspended in a liquid.

3. The method of claim 1, wherein the spin relaxation (T1) time of the 29Si nuclear spin is at least 30 minutes at room temperature.

4. The method of claim 1, wherein the spin relaxation (T1) time of the 29Si nuclear spin is at least one hour at room temperature.

5. The method of claim 1, wherein the silicon particle includes a binding material on its surface that binds to a biological material.

6. The method of claim 1, wherein the step of detecting comprises imaging the silicon particle by nuclear magnetic resonance imaging.

7. The method of claim 6, wherein the silicon particle is imaged within a bloodstream of the patient.

8. The method of claim 6, wherein the silicon particle is imaged within a body cavity of the patient.

9. The method of claim 6, wherein the silicon particle is imaged within a lung cavity of the patient.

10. The method of claim 1, wherein in the step of introducing, the silicon particle is part of a powder.

11. The method of claim 1, wherein the size of the silicon particle is between about 10 nm and about 100 nm.

12. The method of claim 1, wherein the silicon particle is made of naturally occurring silicon.

13. The method of claim 1, wherein the silicon particle comprises 28SiO2.