Electricity: measuring and testing – Particle precession resonance
Reexamination Certificate
2000-05-04
2001-10-23
Arana, Louis (Department: 2862)
Electricity: measuring and testing
Particle precession resonance
C324S096000
Reexamination Certificate
active
06307367
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is directed to a nuclear magnetic resonance imaging apparatus having a device for measuring magnetic field strengths.
2. Description of the Prior Art
Among other things, magnetic field strengths of nuclear magnetic resonance signals for generating magnetic resonance images are measured in a magnetic resonance apparatus. To that end, the apparatus has, for example, a reception antenna fashioned as an electrical coil. In order to avoid local current densities in the reception antenna that would place a patient at risk during an RF transmission phase as a result of resonances, particularly given a local reception coil, the reception antenna is detuned or turned off in a complicated way during an RF transmission phase. Further, the coil must necessarily be fashioned of electrically conductive materials and thus disturbs the homogeneity of the basic magnetic field that is important for the image quality. Further, a gradient system of the apparatus generates rapidly switched magnetic gradient fields in the nuclear magnetic resonance apparatus for generating images.
The abstract of Japanese Application 13 16 676 discloses a device for optical magnetic field measurement based on the Faraday effect, wherein an immediate digitalization of measured results takes place without interposition of electrical analog-to-digital converters. To that end, a light source emits light that is supplied via a mirror arrangement and a polarization device to three optical paths that are formed of magneto-optical material, exhibiting a length ratio of 1:2:4 relative to one another, and which are exposed to a magnetic field to be measured. The light emerging from the paths passes through another polarization device having electrical outputs connected to comparators. An angular difference of, for example, 45° thereby exists between the polarization planes of the polarization devices. The electrical output levels of the comparators form a discrete quantity bit-by-bit whose value corresponds to the field strength of the magnetic field to be measured.
German PS 33 26 736 discloses a magnetic field measuring device likewise based on the Faraday effect and containing a Faraday cell. In one embodiment, the Faraday cell is arranged in a solenoid-like coil arrangement, and in a further embodiment, the Faraday cell is fashioned as an optical monomode fiber.
European Application 0 086 373 discloses a magnetic field measuring device based on the Faraday effect that is optimized in view of space requirements. Lead glass, bismuth silicon oxide, bismuth germanium oxide or yttrium iron silicate (Y
3
Fe
5
O
12
) are thereby proposed as magneto-optical materials.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a magnetic resonance imaging apparatus with an improved device for measuring magnetic field strengths.
The object is inventively achieved in a magnetic resonance imaging apparatus according to the invention having a light source that emits linearly polarized light, a Faraday cell that contains magneto-optically active material and that has at least one light output, with the linearly polarized light emitted by the light source entering into the Faraday cell and traversing a prescribable number of optical path lengths therein and subsequently emerging via the light output, at least one analyzer for the linearly polarized light emerging from the Faraday cell, with the polarization plane of the analyzer being adjustable by a prescribable rotational angle relative to the polarization plane of the linearly polarized light emerging from the light source, and at least one light sensor that acquires the light emerging from the analyzer.
In contrast to known magnetic resonance apparatuses wherein, for example, a local reception antenna for the reception of nuclear magnetic resonance signals is fashioned as a coil and must therefore be detuned relative to the whole-body antenna (transmission antenna) emitting the RF pulses in order to avoid local current inductions at the patient, no resonances between the reception and transmission antenna can occur during operation of the inventive nuclear magnetic resonance apparatus. Current inductions at the patient that could lead to burns thus also do not occur. A shutoff of the reception amplifier when transmitting the RF pulse as well as a detuning for resonance reasons are therefore not required. This considerably shortens the examination times and the circuit-oriented outlay is minimized.
The above-described device is reactance-free in view of the electromagnetic compatibility so it can even be used as the reception antenna.
As explained above as an example, the above-described device can be employed for measuring magnetic field strengths of nuclear magnetic resonance signals. The device also can be employed for measuring magnetic gradient fields.
Binary information is already obtained with the aforementioned device given a Faraday cell with only one optical output. In this case, the word length amounts to one bit, and a simple yes
o information content is made available.
A “yes” level is obtained when the light sensor receives light, i.e. when a magnetic field of sufficient strength is present that causes the polarization plane of the light passing through the Faraday cell to be rotated so that the polarization plane (rotational angle &phgr;
k
) of the linearly polarized light emerging from the Faraday cell coincides with the polarization plane (rotational angle &agr;
k
) of the analyzer following the Faraday cell. In this case, the light sensor receives the light passing through the analyzer.
The rotation of the polarization plane of the light passing through the Faraday cell is thereby defined as
&phgr;
k
[rad]=K·l
k
·H,
wherein K is the electro-optical material constant for a specific wavelength of the light (in nm), l
k
is the optical path length and H is the magnetic field strength acting on the Faraday cell.
A “no” level means that the magnetic field lies below the predetermined threshold, an adequate rotation of the polarization plane therefore does not occur at the light passing through the Faraday cell, and the light sensor therefore receives no light.
A “yes” level is thus obtained when &phgr;
k
=&agr;
k
and a “no” level is obtained given &phgr;
k
≠&agr;
k
.
The above-described device thus enables a direct digital measurement of the magnetic field strength, so that a time-consuming conversion of measured values with analog-to-digital converters is eliminated.
The above-described device also enables a nearly reactance-free measurement of the magnetic field strength, since the light that passes through the Faraday cell does not influence the magnetic field strength to be measured. As a result, the field strengths of high-frequency magnetic fields can be measured with the inventive device.
In an embodiment, the light emerging from the analyzer can be transmitted with a light waveguide having high electromagnetic compatibility before being acquired by the light sensor. As a result, the location of the actual conversion of light into an electrical-digital quantity can be conducted in electromagnetically compatible fashion in a region wherein no direct interaction of the fields generated by the (converted) electrical signal with components of the nuclear magnetic resonance apparatus occurs.
The following implementation of the device is especially well-suited for a digital measurement of the magnetic field strength with a higher resolution. The optical path length l
k
is geometrically graduated according to the following relationship:
l
k
=2
−k
·l
0
,
whereby k=0 . . . N−1 and l
0
is a longest magneto-optically active light path in the Faraday cell within which the Faraday effect occurs and leads to a rotation of the polarization plane of the light passing through the Faraday cell. The number of light outputs is referenced N.
The longest optical path length l
k
=l
0
(k=0) supplies the least significant b
Arana Louis
Schiff & Hardin & Waite
Siemens Aktiengesellschaft
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