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TECHNICAL FIELD OF THE INVENTION
The present invention relates to a process for visualizing tissue
metabolism in a subject using a magnetic resonance imaging system and
oxygen-17. Oxygen-17 is injected into the peritoneal cavity of a subject
as a gas or contained in microbubbles.
BACKGROUND OF THE INVENTION
Magnetic resonance imaging systems rely on the tendency of atomic nuclei
possessing magnetic moments to align their spins with an external magnetic
field. Because only nuclei with odd numbers of nucleons have a magnetic
moment, only those nuclei can be detected and imaged using magnetic
resonance. At present, hydrogen with one nucleon, a proton, in its nucleus
is the element of choice for diagnostic tissue imaging.
Magnetic resonance imaging data obtained using non-metabolically derived
hydrogen, although useful in providing information on tissue perfusion
(blood flow to that tissue) and structure, are of limited use in detecting
the metabolism of those tissues. Visualization of tissue metabolism using
magnetic resonance imaging can be obtained by imaging H.sub.2 O formed
during aerobic metabolism.
Under aerobic conditions, H.sub.2 O is formed as a byproduct of oxygen
consumption. The metabolic formation of H.sub.2 O can be detected using
isotopes of oxygen. The most common isotope of oxygen, oxygen-16, has an
even number of nucleons and, thus, cannot be imaged in a magnetic imaging
system. Another isotope of oxygen, oxygen-15, is unstable with a short
half life (radioactive) and its use would expose a subject to potentially
harmful radiation.
The oxygen isotope, oxygen-17 (.sup.17 O.sub.2, is stable, has an odd
nucleon number and is suitable for use in magnetic resonance imaging.
Further, because .sup.17 O.sub.2 can be detected by a proton magnetic
resonance imaging in the form of H.sub.2.sup.17 O, the use of .sup.17
O.sub.2 provides data on the metabolic state of imaged tissues.
A magnetic resonance imaging process using .sup.17 O.sub.2 has been
previously reported. In accordance with that process, .sup.17 O.sub.2 is
administered intravenously in an artificial blood composition comprising
perfluorohydrocarbons as the oxygen carrier. See U.S. Pat. No. 4,996,041,
the disclosure of which is incorporated herein by reference. Because of
the limited oxygen-carrying capacity of perfluorohydrocarbons, that
process requires loading the patient with large volumes of the artificial
blood composition. Further, the effects of artificial blood compositions
per se on tissue metabolism are not yet known.
PCT Patent Publication No. WO 91/07990 reports the use of an inhalant gas
containing .sup.17 O.sub.2 as a nuclear magnetic imaging agent. That
process requires large volumes of expensive .sup.17 O.sub.2 gas and is
limited in its use to subjects having normal respiratory function. Large
volumes of inhalant gas are needed in that process because only a small
portion of the inhaled gas comes in contact with the blood.
Oxygen absorption into the blood can occur through from the peritoneal
cavity. Wilks, S., J. Appl. Physics, 14:311 (1939) and Van Liew et al.,
Microvascular Research, 1:257 (1969). Not only does the peritoneal cavity
offer a large surface area for absorption (equivalent to that of skin),
but also the membrane surfaces in the peritoneal cavity (the peritoneum
and the omentum) are readily supplied with capillary vessels that provide
ready access to the blood. Indeed, anoxic animals (animals with a
deficiency in blood oxygen tension) can be successfully oxygenated with
oxygen delivered into the peritoneal cavity. Bilge et al., Biomaterials,
Artificial Cells, Artificial Organs, 17(4):413 (1989).
BRIEF SUMMARY OF THE INVENTION
The present invention provides a process of visualizing tissue metabolism
of a subject comprising injecting .sup.17 O.sub.2 into the peritoneal
cavity of a subject and detecting metabolically formed H.sub.2.sup.17 O
using a magnetic resonance imaging system. A benefit of the process of the
present invention is the provision of an efficient process of introducing
.sup.17 O.sub.2 into tissues for imaging in a magnetic imaging system to
detect localized metabolic activity under physiological conditions by
monitoring the in vivo metabolism of oxygen via the production and
detection of H.sub.2.sup.17 O. A further benefit of the present invention
is the provision of an efficient process of introducing .sup.17 O.sub.2
into tissues for imaging in a magnetic imaging system to detect localized
metabolic activity in subjects having respiratory dysfunction.
In one aspect, the present invention is directed to a process of
visualizing tissue metabolism in a subject comprising the steps of:
a) injecting a gas containing an effective imaging amount of .sup.17
O.sub.2 into the peritoneal cavity of the subject;
b) maintaining the subject for a time period sufficient for the .sup.17
O.sub.2 to be (i) absorbed into the blood stream of the subject, (ii)
distributed throughout the tissues of the subject, and (iii) converted to
H.sub.2.sup.17 O; and
c) detecting the H.sub.2.sup.17 O with a magnetic resonance imaging system
thereby visualizing the tissue metabolism.
The gas containing .sup.17 O.sub.2 is air oxygen carbon dioxide or a
mixture of oxygen and carbon dioxide. In a preferred embodiment, the gas
is a mixture of about 50 percent by volume oxygen and about 50 percent by
volume carbon dioxide. The carbon dioxide can itself contain .sup.17
O.sub.2 and have the formula C.sup.17 O.sub.2.
In another aspect, the present invention contemplates a process of
visualizing tissue metabolism in a subject comprising the steps of:
a) injecting microbubbles of substantially uniform diameter that contain an
effective imaging amount of .sup.17 O.sub.2 into the peritoneal cavity of
the subject;
b) maintaining the subject for a time period sufficient for the
microbubbles containing an effective imaging amount of .sup.17 O.sub.2 to
be (i) absorbed into the blood of the subject, (ii) distributed throughout
the tissues of the subject and (iii) for the .sup.17 O.sub.2 to be
converted to H.sub.2.sup.17 O; and
c) detecting the H.sub.2.sup.17 O with a magnetic resonance imaging system
thereby visualizing the tissue metabolism.
The microbubbles are formed by subjecting a viscous solution in an
atmosphere of .sup.17 O.sub.2 to frequency energy in the range of from
about 5,000 Hz to about 30,000 Hz for a time period sufficient to form,
but not stabilize the microbubbles. The viscous solution is preferably an
aqueous protein solution comprising from about 2 percent by weight to
about 10 percent by weight of albumin. In a more preferred embodiment, the
viscous solution comprises an aqueous protein solution of about 5 percent
by weight of albumin.
DETAILED DESCRIPTION OF THE INVENTION
A Gas Containing .sup.17 O.sub.2
The present invention relates to a process of visualizing tissue metabolism
using magnetic resonance imaging of H.sub.2.sup.17 O. In accordance with
that process, a gas containing an effective imaging amount of .sup.17
O.sub.2 is injected into the peritoneal cavity of a subject; the subject
is maintained for a period of time sufficient for the .sup.17 O.sub.2 to
be absorbed into the blood stream, distributed to tissues throughout the
subject, and converted to H.sub.2.sup.17 O. The H.sub.2.sup.17 O formed in
a particular tissue is visualized by imaging the tissue with a magnetic
resonance imaging system.
As used herein, the term "subject" refers to a mammal and includes human as
well as non-human mammals.
The gas containing .sup.17 O.sub.2 can be air (a mixture of about 20
percent by volume oxygen and about 80 percent by volume nitrogen) oxygen
(.sup.16 O.sub.2 or .sup.18 O.sub.2), carbon dioxide or a mixture of
carbon dioxide and oxygen. Each of those gases can contain from about zero
to about 10 percent by volume water as a vapor. Preferably, the water
vapor comprises from about 4 percent by volume to about 8 percent by
volume of the gas. Gases for use in the present invention are commercially
available. By way of example, a mixtures of .sup.18 O.sub.2 and .sup.17
O.sub.2 is commercially available from Isotec Inc., Miamisburg, Ohio.
The gas contains an effective imaging amount of .sup.17 O.sub.2. An
effective imaging amount of .sup.17 O.sub.2 is that amount necessary to
provide tissue visualization of formed H.sub.2.sup.17 O with magnetic
resonance imaging. Means for determining an effective imaging amount in a
particular subject will depend, as is well known in the art, on the nature
of the gas used, the mass of the subject being imaged, the sensitivity of
the magnetic resonance imaging system and the like.
In a preferred embodiment, the gas containing .sup.17 O.sub.2 is a mixture
of oxygen and carbon dioxide. The oxygen component of such a mixture can
comprise any combination of .sup.17 O.sub.2, .sup.18 O.sub.2 and .sup.17
O.sub.2 so long as an injected volume of the mixture provides an effective
imaging amount of .sup.17 O.sub.2. Additionally or alternatively, the
carbon dioxide component of such a mixture can contain any combination of
C.sup.16 O.sub.2 and C.sup.17 O.sub.2 so long as an injected volume of the
mixture provides an effective imaging amount of .sup.17 O.sub.2.
The advantage of using carbon dioxide is that CO.sub.2 is absorbed from the
peritoneal cavity into the blood stream more readily than oxygen. Further,
carbon dioxide enhances the absorption of oxygen across the membranes
lining the peritoneal cavity.
The volume of gas injected into the peritoneal cavity of a particular
subject is selected inter alia on the basis of subject size and gas
composition. The only limitation on injected volume is that the particular
volume selected not adversely affect the subject and that the volume
contain an effective imaging amount of .sup.17 O.sub.2.
After injection of a gas containing .sup.17 O.sub.2, the subject is
maintained for a time period sufficient for (1) the injected
.sup.17O.sub.2 to be absorbed from the peritoneal cavity into the blood
(2) the absorbed .sup.17 O.sub.2 to be distributed throughout the subject
and enter the tissues of the subject and (3) the .sup.17 O.sub.2 in the
tissue to be converted to H.sub.2.sup.17 O. Typically, a sufficient time
period is from about 20 minutes to about 90 minutes and, preferably from
about 20 minutes to about 60 minutes.
Tissue H.sub.2.sup.17 O is visualized by imaging that tissue with a
magnetic resonance imaging system. The visualization of tissue
H.sub.2.sup.17 O can be accomplished with commercially available magnetic
imaging systems such as a General Electric 1.5 T Signa imaging system [1H
resonant frequency 63.9 megahertz (MHz)]. Commercially available magnetic
resonance imaging systems are typically characterized by the magnetic
field strength used, with a field strength of 2.0 Telsa as the current
maximum and 0.2 Telsa as the current minimum.
For a given field strength, each detected nucleus has a characteristic
frequency. For example, at a field strength of 1.0 Telsa, the resonance
frequency for hydrogen is 42.57 MHz; for phosphorus-31 is 17.24 MHz; and
for sodium-23 is 11.26 MHz.
B. Microbubbles Containing .sup.17 O.sub.2
In another aspect, the present invention relates to a process of
visualizing tissue metabolism using magnetic imaging of H.sub.2.sup.17 O
comprising the steps of:
a) injecting microbubbles of substantially uniform diameter that contain an
effective imaging amount of .sup.17 O.sub.2 into the peritoneal cavity of
a subject, wherein said microbubbles are formed by subjecting a viscous
solution in an atmosphere of .sup.17 O.sub.2 to frequency energy in the
range of from about 5,000 Hz to about 30,000 Hz for a time period
sufficient to form said microbubbles;
b) maintaining said subject for a time period sufficient for said
microbubbles containing an effective imaging amount of .sup.17 O.sub.2 to
be (i) absorbed into the blood stream of said subject, (ii) distributed
throughout the tissues of said subject, and (iii) for said .sup.17 O.sub.2
to be converted to H.sub.2.sup.17 O; and
c) detecting said H.sub.2.sup.17 O with a magnetic resonance imaging system
thereby visualizing said tissue metabolism.
Microbubbles containing .sup.17 O.sub.2 are formed by introducing .sup.17
O.sub.2 into a viscous solution by subjecting the viscous solution to high
frequency ultrasonic energy of from about 5,000 Hz to about 30,000 Hz for
a time period sufficient to form microbubbles having a diameter of from
about 2 microns to about 20 microns and, preferably from about 2 microns
to about 4 microns. The time period depends as is well known in the art
upon the particular ultrasonic energy used. A procedure for forming
microbubbles from viscous solutions can be found in U.S. Pat. Nos.
4,572,203 and 4,774,958, the disclosures of which are incorporated herein
by reference.
Exemplary viscous solutions include aqueous media having dissolved or
suspended therein from about 40 percent by weight to about 80 percent by
weight of a biocompatible polymer such as dextrose or sorbitol. U.S. Pat.
Nos. 4,572,203 and 4,774,958.
In a preferred embodiment, a viscous solution is an aqueous protein
solution comprising from about 2 percent by weight to about 10 percent by
weight of a biocompatible protein such as albumin. Preferably the aqueous
protein solution comprises about 5 percent by weight albumin. Microbubbles
formed from such a 5 percent by weight albumin solution have a diameter of
from about 2 microns to about 4 microns. U.S. Pat. No. 4,774,958.
The viscous solution can further comprise nutrients such as glucose and
electrolytes such as sodium, chloride, potassium, calcium and the like.
The high frequency energy level used to form microbubbles is selected so as
to form unstable microbubbles of a uniform diameter, which microbubbles
break up after injection into the peritoneal cavity releasing .sup.17
O.sub.2 into the peritoneal cavity.
Thus, unlike the method of microbubble formation disclosed in U.S. Pat.
Nos. 4,572,203 and 4,774,958, the method of microbubble formation used
with the present invention does not involve heat or chemical denaturation
and stabilization of formed microbubbles.
In a preferred embodiment, microbubbles for use in the present invention
are formed using high frequency energy in the range of from about 5,000
Hz. to about 15,000 Hz.
The following example illustrates a particular embodiment of the present
invention and is not limiting of the specification and claims in any way.
EXAMPLE
Example 1
Magnetic Resonance Imaging of Rat Brain
A 400 gram Sprague Dawley rat was anesthetized with sodium pentobarbital
(30 mg/kg). About 30 milliliters (ml) of a 50 percent by volume mixture of
carbon dioxide and oxygen (50 percent by volume .sup.17 O.sub.2 and 50
percent by volume C.sup.16 O.sub.2) was injected into the peritoneal
cavity. Volume changes were analyzed every twenty minutes for a period of
100 minutes after injection. Visualization of H.sub.2.sup.17 O in brain
tissue was monitored over the same period of time.
Direct measurement of volume changes indicated that 28 percent of the
oxygen from the gas mixture of carbon dioxide and oxygen was absorbed by
60 minutes after injection.
Magnetic resonance images of the rat brain were performed using a 1.5 Telsa
GE Signa system. Imaging was enhanced by the use of a 10 cm solenoid coil
placed orthogonal to the field of the Signa. Dilutions of H.sub.2.sup.17 O
in 5mm tubes were placed in the field as references to observe any field
changes that might occur during imaging.
Scout images of the brain at repetition times of 3000 milliseconds (ms) and
excitation times of 60 ms provided heavily "T.sub.2 weighted" images. The
slice thickness of the images was 3mm and the images were acquired over a
period of 11 minutes. Measurements of contrast at two reference sites in
the hypothalamus and cortex were made.
The presence of H.sub.2.sup.17 O was observed by magnetic resonance imaging
40 minutes after injection. Ninety minutes after injection, no
H.sub.2.sup.17 O could be visualized in the brain.
Although the present invention has been described in terms of certain
preferred embodiments, and exemplified with respect thereto, one skilled
in the art will readily appreciate that various modifications, changes,
omissions and substitutions can be made without departing from the spirit
thereof.
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