Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
MAGNETIC MICRO-PARTICLES AS CONTRAST
AGENTS IN NUCLE M MAGNETIC RESONANCE IMAGING
BY
TRUMAN BROWN
BACKGROUND OF THE INVENTION
The pre~ent invention relates to an improvement in
nuclear magnetic resonance (NMR) imaging and in
particular to the use of tis3ue specific magnetic
micro-particle~ a3 contrast enhancing agents in NMR
imaging.
NMR spectroscopy has been used for a number of years
as an analytical technique for organic chemical ~tructure
determinations. This technique is ba~ed on the magnetic
properties of nuclei containing odd numbers of proton3
and neutrons. Nuclei po~sess an angular momentum which,
in conjunction with the charge thereof, produces a
magnetic field, the axi~ of which i9 directed along the
spin axis of each nucleus. The application of a magnetic
field to the nuclei cause alteration of the natural spin
direceions, which become aligned either with or against
the applied field. The nuclei also prece~s or rotate
about their axe3 at a characteristic frequency.
The rotational angle may be changed by abYorption of
electro-magnetic energy through a phenomen known as
resonance, which involve~ impres3ing on the nuclei a
3econd magnetic field of appropriate frequency to match
that of their normal precession. When the ~econd
magnetic field i3 interrupted or terminated, the nuclei
return to their initial precession state, generating
radio signals having a detectable decay rate or
relaxation time. The ~pin-lattice relaxation time (Tl)
is the rate at which spin energy is converted into
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thermal energy and transferred to the aggregate of atoms
or molecules under ~tudy. The qpin-spin relaxatlon time
(T2) is the rate at which spin energy of nuclei in a
high energy state is transferred to neighboring nuclei.
The radio signal generated by any given nucleus will have
a characteri~tic relaxation time depending on its
chemical environment.
More recently, NMR technique has been further
developed and refined to provide a potentially valuable
diagnostic to~l, enabling visualization of body tissue.
~It is possible to distinguish between abnormal and normal
tissue, e.g. a lesion and surrounding normal tis~ue, on
the basis of differences in spin-lattice and/or ~pin-~pin
relaxation times. Remarkably clear images are obtainable
that can aid in the characterization of pathologic or
physiologic proceqses within various organs, including
the brain, kidneys, and heart.
In carrying out NMR imaging, a suitable radio
frequency pulse sequence ~saturation-recovery,
inversion-recovery or spin-echo) is used to induce a
signal in nearby receiver coils The signal's exact
frequency, phase and strength are determined through a
mathematical function known as a Fourier transform. The
signals are a3~iqned shading intensity enabling a
computer to convert the re~ults into an image of the
targeted organ ~ystem.
There have been numerous reports over the laqt
several year~ that contrast agents may contribute
significantly to the diagnostic utility of NMR imaging.
Contrast agents enhance the difference in signal observed
between magnetically similar but histologlcally
diqsimilar ti~sue, giving images oE quperior contrast
that prbvide maximum diagnostic information.
o~
The mechanism of action of the specific magnetic
micro-particles u~ed as NMR contrast enhancing agents in
the present invention is to ~e distinguished from
iodinated contrast media typically used in radiography.
Magnetic micro-particles alter the magnetic environment
in the target organ system, ~o a~ to enhance proton
relaxation by decreasing the spin-spin relaxation time.
Contrast enhancement is thus produced indirectly by the
effect of the magnetic micro-particle on neighborlng
nuclei. On the other hand, iodinated contrast media used
i~n radiography generally have a high electron density and
effect contrast directly by absorption of x-rays. Such
agentR generally do not affect proton NMR signals, and
therefore, do not function as contrast media in NMR
imaging. Moreover, unlike iodinated contrast media, the
NMR contra~t agents described herein are not observed
directly on the image, since the magnetic effect produced
on neighboring nuclei is the mean~ of contrast
enhancement.
In order for a substance to be acceptable a~ an NMR
contrast-enhancing agent a number of critica must be `
satisfied. The contrast agent should be easily
manufactured from relatively inexpensive ~tarting
materials, chemically ~table and readily stored in a form
suitable for administration. More importantly, the
contra~t agent should have a reasonably strong influence
on proton relaxation time~ in relatively low
concentrations, but be essentially free of tox~c effects
or other undesirable side effects in appropriate dosages.
In addition, the contrast agent should be
tlssue-specific, i.e. permit selective ti~sue targetting.
Furthermore, the contra~t agent must remain stable in
vivo and`ultimately be deactivated or excreted.
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SUMMARY OF T~E INVENTION
In accordance with the present invention there is
provided an improved method for producing an image of
animal ti~sue by nuclear magnetic re~onance imaging,
which compri~es binding to the tissue whose image i~
de~ired a magnetic micro-particle comprising magnetic
material coupled to a 3ub~tance having binding af~inity
for the animal tis~ue, in an amount effective for causing
a 3ubstantial reduction in the relaxation time of nearby
nuclei, thereby effecting contrast enhancement in the
~e~ulting image. The magnetic material i8 conveniently
coupled to the subAtance having binding affinity for the
target ti~ue by coating the magnetic material with a
bio-compatible polymer ha~ing reactive functional group~
and forming chemical bond~ between the functional groups
and the substance having binding affinity for the target
ti~sue.
The magnetic micro-particle used a~ a contrast-
enhancing agent in the practice of the present invention
fulfills all ~he criteria noted above. The contrast
agent ls made from commercially available and relatively
inexpensive magnetic material~, which are readily coupled
to various tis~ue-specific binding substance3 by
procedure~ well known to those ~killed in art. Polymer
coated magnetic particles are extremely ~table and may be
easily readied for administration by coupling to the
~ubstance having binding affinity for the target tis~ue.
The degree of magnetization of the magnetic
micro-particles, when magnetically polarized, will cause
a ~ubstantial local perturbation to the steady magnetic
field applied to the ~arget tis~ue, the detail~ of the
perturbation depending on the magnetic material selected
and the~particle ~ize thereof. Thi~ high degree of
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magnetization exerts an extremely ~trong influence on
nuclear relaxation time. The magnetic effect of this
contra~t agent cau~e~ magnetic pha~e perturbation in
nuclei up to about 20 micron~ or more removed from the
contrast agent, permitting extremely small amount~ of the
agent to be u3ed, e.g. on th,e order of one to ten
magnetic micro-particles per cell, thus providing a NMR
contra~t agent oE exceptional sensitivity, enabling the
detection of the distribution and transfer of
neurotransmitter substances iand low level hormone
receptor~. The u~e of bio-compatible coating~ on the
magnetic particle~ renders them free of toxic or other
undersirable side effects. The contra~t agent of the
present invention also enable~ selective tissue targeting
by rea~on of the coupling thereto of a substance having
binding affinity for the target tissue. With regard to
deactivation of the contra~t agent, it i3 believed that
over time it will be taken up by macrophaqe~ and strored
in the body.
The contrast agent of the pre~ent invention may be
used to particular advantage in tumor imaging by the u~e
of magnetic micro-particle~ comprising magnetic material
coupled eO antibody which bind3 specifically to
tumor-associated antigen. Other feature~ and advantages
of the inYention will become apparent to those skilled in
the art from the following detailed descripeion thereof.
DETAILED DESCRIPTION OF THE INVENTION
Various magnetic materials may be used in preparing
the magnetic micro-particles employed in the present
invention. As used herein, the term ~magnetic materials"
refers to those materials exhibiting ferromagnetism, and
include3 materials having a high magnetic su~ceptibity
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such that their internal magnetic field~ are on the order
of about one thousand gauss. Ferromagnetic material3
useful in the pre~ent invention are characterized by
having domains of magnetism which become aligned with an
external magnetic field, thus producing a high internal
magentic field. Suitable magnetic material includs
iron, cobalt, nickel and manganese. Compounds or alloy~
of the~e elements which exhibit ferromagnetism, e.g.
magnetite (Fe304) or Mn-Cu alloy may also be used.
The size of the magnetic micro-particles will depend
on the target organ system. In general, the average
particle size will be greater than few hundred Angstrom
units, but leqs than about one micron.
The substance having tissue-binding affinity that is
selected for coupling to the magnetic micro-particles
will be determined by the particular tissue whose image
is desired. Examples of such substances include
antibodies, preferably monoclonal antibodies,
neurotransmitters, hormones, matabolite~, enzymes,
toxins, and natural or ~ynthetic drugs. Analogues of
these substances may also be employed, if desired. As
used herein the term "analogue" refers to synthetic
materials that elicit a physioloqical response comparable
to that of the natural substance.
A~ previously noted, the sub~tance having
tissue-binding affinity may be coupled to the magnetic
micro-particles according to procedures well known in the
art. This may be most conveniently accomplished by
providing the magnetic micro-particles with a polymer
coating having reactive functional 9rOUpB~ and attachinq
the tissue-apecific ~ubstance to the polymer coating by
chemical bonding to the functional groups of the polymer.
A polymer coating may be applied to the magnetic material
by a redox polymerization proce3s wherein a metal oxide,
e.g. magnetite serves as a source of reducing agent in
the redox activation system. Such a process i3 describecl
in U.S. Patent No. ~,157,323 t*e~ ~e-d~sc~s~rF-uf--
~h~ ~e~r-a-t~ ~ i~-t-he-~r~t--~p~lic~ti~_by -
Eefer~en~-aff-~h~h-se-t--f-~r-lh-he~e-~-i~ full. Polymer
coating~ may also be applied to magnetic substrates
according to the procedure described in ~.S. Patent No.
4,070,246. Polymer-coated nnagnetic micro-particles are
commercially available from a number of sources.
Bio-compatible polymer~ are preferred for use in the
present invention. The term "bio-compatible polymer~ i9
intended to qignify a polymer that doe~ not produce any
significant toxic effect or other undesirable effect on
the test subject to whom the magnetic micro-particles are
administered. Bio-compatible polymers may be either
synthetic or naturally occuring, representatiYe examples
of such polymers being albumin and polystyrene. In
addition to facilitating the coupling of the
tissue-specific substance, the bio-compatible polymer
coating serves to prevent adverse physiological effect~
caused by the magnetic material and to prevent
deterioration of the magnetic material by chemicals
present in the environment in which the magnetic
micro-particles are u~ed.
The ti~sue-Apecific substance may be coupled to the
polymer-coated magnetic material by methods familiar to
those ~killed in art, ~uch as by covalent bonding, as
noted above, or by ionic or hydrogen bonding. Suitable
methods for thi~ purpose are disclosed in the
aforementioned U.S. Patent No. 4,157,323.
The presently preferred mode of administration of
the magnetic micro-particles is by intravenous injection,
although other suitable technique~ for introducing the
contrast agent into the area of NM~ imaging examination
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may also be used, if desired.
The dosage at which the magnetic micro-particles i~
admini~tered will vary depending on the test subject and
the nature of the ti~sue ~ought to be imaged. While it
S i9 contemplated that the pre~ent invention will
ultimately be used in NMR imaging of human ti~sue, ~uch
use ha3 not yet been undertaken. However, the present
invention ha3 considerable utility in NMR imaging of the
organ systems of lower animals for differention of normal
and pathologic tissue, characterization of pathologic
tissue, characterization of physiologic or pathologic
phenomena, and the like.
The pulse sequence used in obtaing the NMR image in
accordance with the present invention should be one which
has high sensitivity to spin-spin relaxation parameters.
Although the scientific principle underlying the
present invention has not been thoroughly investigated,
and is not completely understood, it i3 believed that the
magr.etic micro-particle functions as a ~mall magnetic
dipole which produces a magnetic field that causes
perturbation of the nuclei of water molecules surrounding
the magnetic micro-particles in the tissue under
examination. The magnetic field will dissipate by the
third power of the distance from the magnetic
micro-particle. Thus, aq water molecule~ diffuse through
the varying magnetic field cau~ed by the presence of the
magnetic micro-particles, their magnetic pha~e is
seriously perturbed, causing a ~ubstantial reduction in
the apparent ~pin-spin relaxation time, and thereby
enhancing contrast in the resultant NMR image.
The procedures for obtaining an image of body tis~ue
using NMR are well known to those ~killed in the art.
Basic concepts of NMR imaging are set forth in Kaufman et
al (eds.) Nuclear Magnetic Resonance Imaging In Medicine~
New York: Igaku-Shoin, 1981. A number of commercial
supplier~ exist for NMR imaging apparatus~
Regarding specific applications, the present
invention may be used for imaging tumors. Before
carrying out this particular application, however, the
primary tumor is removed and the tumor antigen is used
for the production of monoclonoal antibodies specific the
tumor associated antigen. Thereafter, polymer coated
magnetic micro-particles are derivatized with the
monoclonal antibody. The derivatized magnetic
micro~particles are then administered to the te~t subject
and after a time sufficient for the magnetic
micro-particles to bind to the tumor a~sociated antigen,
the NMR image is obtained. The present invention may
also be used in obtaining images of the distribution and
transfer of specific neurotran~mitter~ in nervous ti~sue,
by coupling a neurotransmitter or a neurotransmitter
analogue ~o magnetic micro-particles, which are then
administered to the test subject and an NMR image of the
nervous tissue i~ obtained. Another application of the
present invention is in imaging the interaction between
hormones and hormone-binding tissues. In this
application the magnetic micro-particle comprises
magnetic material coupled to a hormone or hormone
analogue which binds specifically to receptors associated
with the hormone-binding tissue. The image produced by
these and other applications of the pre~ent ~nvention
should provide valuable information for diagnosising many
dieases.
While certain embodiments of the present invention
have been described hereinabove, it i~ not intended to
limit the invention to such embodiments, but various
modifications and may be made therein and thereto without
departing from the spirit and scope of the invention as
set forth in the following claims.
