Electricity: measuring and testing – Particle precession resonance – Spectrometer components
Reexamination Certificate
2001-02-15
2002-07-16
Arana, Louis (Department: 2862)
Electricity: measuring and testing
Particle precession resonance
Spectrometer components
C324S318000
Reexamination Certificate
active
06420873
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates generally to magnetic resonance imaging equipment, and more particularly to a method for reducing transient noise that interferes with the desired signal and may decrease the quality of the image that is produced.
Magnetic resonance imaging, or “MRI,” is an excellent medical diagnostic tool that has been around for several decades. The details of MRI are well-known and need not be repeated herein. In general, MRI involves placing a subject, such as a person, in a magnetic field of known strength. The hydrogen atoms in the subject, which are typically the atoms that are used for imaging in current MRI machines, will have a resonant frequency that is directly proportional to the applied magnetic field. By “shaping” the static magnetic field through the use of gradient coils, it is possible to produce a static magnetic field of known quantity at a single isolated region within the subject. This region is generally referred to as a voxel, and may be on the order of one cubic millimeter. By imaging thousands of these individual voxels, an overall image of the subject can be recreated.
The imaging of an individual voxel involves applying a radio frequency to the subject that corresponds to the resonant frequency of the voxel undergoing imaging. This resonant frequency is also known as the Larmor frequency. A certain number of hydrogen atoms in the voxel being imaged will absorb energy from the radio signal, which will cause them to switch spin states from a low energy state to a high energy state. After the radio signal is terminated, a certain number of hydrogen atoms in the high energy state will relax back to the low energy state, giving off a signal of known frequency during this relaxation process. By detecting this emitted signal, it is possible to determine the relative hydrogen content of the voxel being imaged. If the subject being imaged is a human, the different concentrations of hydrogen in the different human tissues will produce different signals for the voxels of different tissues. The different signals allow an image to be reconstructed such that it corresponds to the different tissues in the human body.
The signal emitted by the hydrogen atoms when relaxing from a high energy state to a low energy state is detected by a receiving antenna or coil that is positioned around the subject being imaged. In the case of MRI's designed for imaging humans, the receiving antenna or coil is generally cylindrically shaped with the person positioned in the center of the cylinder. The MRI machine may contain a number of different coils of different size, location, and configuration in order to image different parts of the human body. In addition to the signals emitted by the relaxing hydrogen atoms, the detector coils or antennas will sense additional noise or interference signals. These noise or interference signals are desirably removed from the detected signal in order to produce a better image.
One prior art method for reducing the noise or interference in the receiving antennas is disclosed in U.S. Pat. No. 5,525,906 issued to Crawford et al., the disclosure of which is hereby incorporated herein by reference. In this method, which is depicted in block diagram in
FIG. 3
herein, the signal from the receiving antenna is split into a detect path signal
1020
and a receive path signal
1022
. The detect path
1020
passes through a band pass filter
1024
which removes broad band thermal noise from the detect path signal
1020
. The detect path signal
1020
then passes through an amplifier
1026
before being input into a notch or band reject filter
1028
. Notch filter
1028
is designed to reject all frequencies that occur within the desired signal frequency range, which has a known bandwidth. The output
1030
of filter
1028
will thus consist of unfiltered noise. The unfiltered noise
1030
is input into a comparator
1032
which compares this signal to a voltage threshold
1034
. If the unfiltered noise signal
1030
exceeds the voltage threshold
1034
, comparator
1032
outputs a signal at
1036
that causes switch SW
1
to open, thereby blanking the output
1038
. If the unfiltered noise signal
1030
does not exceed the voltage threshold
1034
, the comparator outputs signal
1036
, which leaves switch SW
1
closed such that the receive signal
1022
is passed through to output
1038
, after passing through delay filter
1040
. The purpose of delay filter
1040
is to delay the signal on the receive path
1022
from reaching switch SW
1
prior to comparator output signal
1036
reaching switch SW
1
. Such a system is described in more detail in the U.S. Pat. No. 5,525,906, particularly in reference to
FIGS. 3 and 4
in the corresponding disclosure therein. While this prior art method has been successful in producing images of higher clarity, the need still exists for improved imaging techniques.
SUMMARY OF THE INVENTION
Accordingly, the present invention provides an improved method and apparatus for increasing the quality of MRI images. The present invention achieves this improved quality by providing an improved method for detecting transient noise that is generated in the MRI system.
According to one embodiment of the present invention, a method is provided for detecting interference in an MRI signal received from an MRI receiving antenna. The method comprises detecting a parameter of the MRI signal that varies as the envelope of the MRI signal varies and filtering the MRI signal to thereby produce a filtered parameter signal. The filtered parameter signal is them compared to a reference signal. The MRI signal is determined to likely include interference if the filtered parameter signal exceeds the reference signal.
According to another aspect of the invention, a method is provided for detecting transient interference in an MRI signal that comprises detecting an envelope of the MRI signal and filtering out low frequency components of the MRI signal to thereby produce a filtered envelope signal. The filtered envelope signal is compared to a reference signal and it is determined that the MRI signal includes interference if the filtered envelope signal exceeds the reference signal.
According to still another aspect of the invention, an interference detection system is provided for detecting interference in an MRI signal received from an MRI receiving antenna. The system includes a filter designed to remove low frequency components within the MRI signal, and an envelope detector that detects the envelope of the MRI signal. The filter and envelope detector produce in combination a filtered envelope signal. A comparator compares the filtered envelope signal to a reference signal and outputs an interference signal if the filtered envelope signal exceeds the reference signal.
According to yet another aspect of the invention, an interference detection system is provided for detecting interference in an MRI signal received from an MRI receiving antenna. The system comprises a filter designed to remove low frequencies within the MRI signal and a detector that detects a parameter that varies as the envelope of the MRI signal varies. The filter and detector produce in combination a filtered parameter signal. A comparator compares the filtered parameter signal to a reference signal and outputs an interference signal if said filtered parameter exceeds the reference signal.
In still other aspects of the invention, the parameter detector and/or the envelope detector may comprises a detector log video amplifier. The system may include a blanking switch controlled in a manner to blank the MRI signal if the comparator outputs the interference signal. The system may further includes a retriggerable multivibrator that is activated by the interference signal.
The methods and systems of the present invention provide improved clarity in MRI images by more accurately discerning whether or not the signal in an MRI receiving coil is corrupted by transient noise. By more accurately determining whether transient interference is present,
Arana Louis
Netcom, Inc.
Van Dyke Gardner, Linn & Burkhart, LLP
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