Electricity: measuring and testing – Particle precession resonance
Reexamination Certificate
2000-05-18
2002-11-26
Arana, Louis (Department: 2862)
Electricity: measuring and testing
Particle precession resonance
C062S637000
Reexamination Certificate
active
06486666
ABSTRACT:
METHOD AND APPARATUS
The invention relates to a method and apparatus for determining the degree of polarisation of nuclear spin polarised gases, in particular 3He or
129
Xe.
Nuclear spin polarised gases, such as the helium isotope with mass number 3 (
3
He) or the isotope of xenon with mass number 129 (
129
Xe) and gases containing the fluorine, carbon or phosphorus isotopes
19
F,
13
C or
31
P, are required for a wide variety of basic physics research experiments.
In the medical field, such isotopes have particularly been discussed for use in nuclear spin tomography (magnetic resonance imaging), for example of the lung. (See for example WO95/27438, WO97/37239, Bachert et al., Mag. Res. Med. 36: 192-196 (1996) and Ebert et al., The Lancet 347: 1297-1299 (1996)). In addition, Noël et al., J. Phys. III France 6: 1127-1132 (1996) discloses a
4
He magnetometer used to detect the static magnetic field produced by optically pumped
3
He nuclei submitted to RF discharge. Similarly, Cohen-Tannoudji et al., Phys. Rev. Letts. 22: 758-760 (1969) discloses the use of a sensitive low-field magnetometer to detect the static magnetic field produced by optically pumped
3
He nuclei in a vapor. To be useful in nuclear spin tomography, the nuclear spin polarised gases require a degree of polarisation P of spin I of the atomic nucleus, or the nuclear magnetic dipole moment &mgr;
I
connected therewith, which is about 4-5 orders of magnitude larger than P
Boltzman
the degree of polarisation of the gas in its relaxed state in normal thermal equilibrium in the magnetic field B
T
of the mr imaging apparatus. P
Boltzmann
is related to the Boltzmann constant, the magnetic dipole energy −&mgr;
I
B
T
and thermal energy kT by:
P
Boltzmann
=tan
h(&mgr;
I
B
T
/kT
) (1)
(where k=Boltzmann constant, and T=absolute temperature in Kelvin).
Where P
Boltzmann
<<1, then it approximates to &mgr;
I
B
T
/kT.
Since routinely B
T
=1.5T and T=300K for the hydrogen isotope
1
H used in tissue tomography, it has a P
Boltzmann
of only 5×10
−6
, but in gas tomography a P>1×10
−2
(i.e. 1%) is required. The requirement for such an extremely high P is mainly due to the low concentration of gas atoms in comparison to that of hydrogen in tissue. Gases with such degrees of polarisation (normally referred to as “hyperpolarised gases”) can be prepared using various known methods, advantageously by optical pumping or by polarization transfer.
In addition, large amounts of hyperpolarised gas, for example of the size of an intake of breath (0.5 to 1 litre) must be prepared for use.
Particularly high degrees of polarisation—for example >30%—in simultaneously high production amounts, for example 0.5 liters/h, may be achieved by compression of an optically pumped gas. This method is described in the following publications:
Eckert et al., Nuclear Instruments and Methods in Physics Research A 320: 53-65 (1992);
Becker et al., J. Neutron Research 5: 1-10 (1996);
Surkau et al., Nuclear Instruments and Methods in Physics Research A 384: 444-450 (1997); and
Heil et al., Physics Letters A 201: 337-343 (1995).
The extremely costly production of hyperpolarised gases, for example using the methods described above, generally involves production at a site remote from the place of use. As a result, they must be transported from the place of production to the user. Since a wide variety of relaxation processes (e.g. wall relaxation, relaxation in inhomogeneous, external, stray magnetic fields, etc.) causes the gas to depolarise to a greater or lesser extent, it is necessary to determine the degree of polarisation before using the hyperpolarised gas, for example in medical imaging.
One problem is that this must be determined as precisely as possible despite stray fields or applied fields. Further, the determination should be performable by relatively inexperienced personnel, ie. personnel who are not experts in the physics of hyperpolarized gases.
The present invention solves the above problem by providing a method for determining the degree of polarisation of nuclear spin polarised gases by exploiting the fact that nuclear spin polarisation of gases produces magnetic fields B
d
in the nanoTesla to microTesla (nT to &mgr;T) range.
Thus viewed from one aspect the invention provides a method of determining the degree of polarisation (P) of a nuclear spin polarised gas in a container, said method comprising determining the magnetic field B
d
of the polarised gas using a magnetic field sensor and then determining therefrom the degree of polarisation of the gas.
In the method of the invention, the shape and size of the container into which the polarised gas is placed is important. Thus, the magnetic field B
d
, which is dependent on the degree of polarisation of the gases, may be determined using a magnetic field sensor, e.g. a magnetometer, rather than a nuclear magnetic resonance polarimeter as has been used in the past, and the absolute degree of polarisation can be determined from B
d
by taking into consideration the geometric shape of the container in which the gas is placed, the type of gas and its density, and the arrangement of the sensor relative thereto.
If, as is preferred, the container in which the gas is received is spherical in shape, then the magnetic field has a field gradient like that formed by a point dipole.
Thus for a spherical container, the magnetic field B
d
of the polarised gas on the equitorial outer surface of the container deriving from the orientated nuclear magnetic dipole moment of the nuclear spin gas is:
B
d
=
-
P
·
n
·
R
3
μ
0
3
r
3
·
μ
N
(
2
)
where P represents the degree of polarisation to be determined and n the particle density of the gas. The factor R
3
/3r
3
is termed the geometry factor, and depends on the shape of the container and thus on the volume in which the nuclear spin polarised gas is dispersed. R represents the radius of the sphere and r the distance of the sensor from the centre point of the container sphere perpendicular to the dipole axis. &mgr;
0
=1.257×10
−6
Vs/Am, i.e. the permeability of vacuum, and &mgr;
N
=1.075×10
−26
Am
2
, the nuclear dipole moment of the gas (in this case
3
He)
Similar equations to equation (2) may be generated for containers which are non-spherical.
The geometric factor also takes the position of the magnetic field measuring apparatus relative to the direction of the magnetic field of the gas into consideration. If the field emerges from the poles of the container, the sensor is positioned in the equatorial plane of the spherical gas container.
Different geometric factors must be used for different container geometries, as in each case there is a different calculable field gradient of magnetic field B
d
. If the geometric factor, the distance from the measuring sensor and the particle density of the-nuclear spin polarised gas in the container are known, then equation (2) can be used to determine the absolute degree of polarisation P from the B
d
determined using the measuring apparatus.
As an example, assuming a degree of polarisation P=50% and a particle density n=10
20
/cm
3
, then the field at the edge of the sphere (r=R) has a value B
d
=0.22 &mgr;T. This value is of the order of 1 thousandth of the homogeneous magnetic field caused by the polarisation, similar to, for example, transport fields of 0.3 mT, for example, or external stray fields.
In a preferred implementation of the invention, it is proposed that the sensor comprises a very sensitive magnetic field sensor. In this respect, SQUIDs or more preferably sensors operating on the Forster principle can be considered. Förster sensors operate on the principle of saturable-core magnetometers. The measuring element of saturable-core magnetometers essentially consist of one or more narrow cores of highly permeable materials (&mgr;-metal or ferrite) with almost linear B(H) behaviour.
In a varia
Aidam Elke
Ebert Michael
Grossmann Tino
Heil Werner
Otten Ernst-Wilhelm
Arana Louis
Hellspin Polarisierte Gase GmbH
LandOfFree
Method and apparatus for measuring the degree of... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method and apparatus for measuring the degree of..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method and apparatus for measuring the degree of... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2945656