Electricity: measuring and testing – Particle precession resonance – Using a nuclear resonance spectrometer system
Reexamination Certificate
1999-07-08
2001-01-23
Patidar, Jay (Department: 2862)
Electricity: measuring and testing
Particle precession resonance
Using a nuclear resonance spectrometer system
C324S309000
Reexamination Certificate
active
06177796
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a radio-frequency signal receiving unit particularly for Nuclear Magnetic Resonance imaging machines, comprising:
a) a signal receiving coil (
1
), whose outputs (
101
,
201
) are connected to a signal amplifier (
2
)
b) the coil (
1
) being in balanced connection with the signal amplifier (
2
), i.e. connected thereto in such a manner that none of its two outputs (
101
,
201
) is permanently connected to the ground;
c) passive balancing means (
4
), the amplifier (
2
) being interposed between the coil outputs and the said passive balancing means (
4
);
d) the signal amplifier (
2
) comprising at least two active amplifying devices (
3
), each associated to an input to which one of the outputs (
101
,
201
) of the coil is connected,
e) active amplifying devices (
3
) having an output connected to a device for recombining useful signals and canceling out undesired signals, that is a device which is able to add signals in phase opposition and to cancel signals in phase from inputs.
In Nuclear Magnetic Resonance imaging machines and especially in “dedicated” machines, i.e. designed to detect images of individual parts of the body, the problem of effectively removing noise from the detected signal is particularly significant.
The mostly magnetic radio-frequency signal reemitted from the patient body is relatively feeble and combines with mostly electric noise components introduced by the body directly inside the detection volume: therefore, this noise must be canceled from the signal, which is also affected by thermal noise, so that is can be further amplified and that the data thereof can be correctly processed to reconstruct the image.
A well-known and effective solution to the problem of electromagnetic noise on the signals received by a receiving coil consists in using a Faraday cage, which is assembled in such a way as to enclose and contain the whole machine and the patient inside it, in order to provide effective shielding of the detected electromagnetic signals.
This choice certainly provides a solution to the problem of electromagnetic noise removal.
Nevertheless, the relatively big shape and size of a structure of this type make its construction particularly expensive and difficult, and anyway requiring plenty of time; the use of the machine would be more complex for the personnel and, further, if the machine is not properly integrated into an appropriate environment, it may induce a certain emotional impact on the patient and be psychologically unattractive.
A further solution consists in placing the patient body on the ground, while closing the apertures for entering and introducing the part of the body in the machine volume dedicated to Nuclear Magnetic Resonance imaging.
In this case, noise may be removed thanks to ground connection, since a specific path is provided for the noise signals picked up by the patient body: however, even this solution is not effective enough to satisfactorily remove the noise.
The electrical connection of the patient to the ground is relatively difficult due both to the anatomic structure and to the low conductivity of the human body.
In any case, the ideal ground connection conditions cannot be easily attained and a certain part of noise is equally transmitted inside the detection cavity.
Document U.S. Pat. No. 4,839,594 discloses a radio-frequency signal receiving unit showing the combination of features a) to e) listed above. The arrangement of U.S. Pat. No. 4,839,594 solves the problem of suppressing the electrically induced noise by the patient body. Nevertheless the amplifier introduces thermal noise, which amount is currently quantified by the so called Noise Figure. This thermal noise limits the overall signal to noise ratio that can be achieved by the system reducing the advantages aimed by the balanced coil configuration.
SUMMARY OF THE INVENTION
The invention aims to provide a balanced coil configuration as described above and according to the configuration of features a) to e), which is built in such a way to improve the overall signal to noise ratio, minimizing the noise Figure of the balanced amplifier.
The invention achieves the above mentioned goal by providing a balanced coil configuration showing in combination with the above listed features a) to e) the further feature f) consisting in the fact that the outputs of the receiving coil are connected to the inputs of the amplifier through balanced means for optimization of the noise figure of the amplifier.
The invention achieves the above objects by providing a unit of the type described herein in which the receiving coil is in balanced connection with the signal amplifier, i.e. connected thereto in such a way that none of its two outputs is permanently connected to the ground.
According to an advantageous embodiment the means for optimization of the amplifier noise figure are balanced impedance matching means of the coil.
Preferably the balanced impedance matching means are symmetric capacitive transformer or symmetric inductive transformer.
The signal amplifier has the further characteristic that it provides a signal power amplification with little added noise.
According to a further improvement, the signal amplifier to be used may be of the so-called “push-pull” very low-noise type.
A “push-pull” amplifier that may be used in this case consists of a circuit including at least two active amplifying devices, each associated to an input, and connected at their output to a device for recombining useful signals and canceling out undesired signals, that is a device which is able to add signals in phase opposition and to cancel signals in phase from inputs.
Circuits including at least two or more transistors, field effect transistors, or other active components in a particular differential configuration, connected to different passive components may be used as an active amplifier.
A transformer placed at the output of the circuit or a resonant section of transmission line or an operational amplifier say be provided as a device for recombining output signals from the at least two active amplifiers.
Said transformer having the function o a device for recombining signals from the two inputs after amplification acts as a passive “balun” after the first balanced amplification stage, thereby ensuring that the noise added to the signal has a very low power.
The secondary winding of the transformer is connected by one end to the ground and by the other end to the following signal amplifying and/or processing stages, usually all in non-balanced connection with respect to the ground potential.
The signal amplifier in use is miniaturized and/or on printed circuit or also in the form of a so-called “hybrid”, a circuit integration technology which is intermediate between miniaturized circuits and integrated circuits, and is mounted directly on the case or on the coating or on the supporting means and/or at the outputs of the receiving coil, as close as possible thereto; these arrangements are also necessary to improve the balanced connection condition of the coil.
By not using long connecting conductors and the associated impedance transformers, most problems of signal-to-noise ratio worsening are prevented.
The advantages of this combination consist in a relatively low cost of the amplifier, in a very easy implementation and use of the whole circuit, in an effective suppression of the noise introduced by the body, which is fast and safe since it occurs in the amplification stage and at least in the maintenance, and even improvement of the noise figure, which is comparable with the one obtained with the best currently available systems, i.e. in the attainment of an excellent signal-to-noise ratio.
The unit according to the invention is particularly effective in suppressing noise introduced, through the patient body, inside the volume dedicated to signal reception, thanks to the acknowledgement that the noise introduced by the body is mainly electric, hence most of the noise may be simply removed by using a combination based on an am
Burns Doane Swecker & Mathis L.L.P.
Esaote S.p.A.
Patidar Jay
Shrivastav Brij B.
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