Abstract:
An improved transient interference detector and suppressor for an MRI system detects the presence of transient interference in an MRI signal by detecting the envelope of the MRI signal and comparing the rate of change of the envelope to a reference signal. When the rate of change of the envelope exceeds the reference signal, a transient interference detection is made and appropriate action may be taken. When the rate of change of the envelope is less than the reference signal, no transient interference detection is made. The reference signal is set at a level slightly above a level corresponding to the average thermal noise in order to substantially prevent the minor, random fluctuations in the thermal noise from falsely triggering the detection of a transient interference event.

Description:
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&#39;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, appropriate steps can be taken from preventing these transient interference signals from being used to produce image data. These and other advantages of the present invention will be apparent to one skilled in the art in light of the following specification when read in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of an MRI system according to one aspect of the invention; 
     FIG. 2 is block diagram of one embodiment of the interference detector and suppressor of FIG. 1.; and 
     FIG. 3 is a block diagram of a prior art interference detector and suppressor. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention will now be described with reference to the accompanying drawings wherein like reference numerals correspond to like elements in the several drawings. A block diagram of an MRI system  10  is depicted in FIG. 1 . MRI system  10  includes a magnet assembly  12 , the details of which are not part of the present invention. As an illustrative example, magnet assembly  12  may include a polarizing magnet  14  and a radio frequency (RF) coil or antenna  16 , both of which generally surround a patient being imaged. RF coil  16  may be used to both transmit RF signals and detect the MRI image signals, or separate coils may be used for transmission and detection. The MRI image signals that are detected by RF coil  16  are first typically passed to a low noise amplifier or pre-amplifier  18 , which may have a gain of 30 dB and a noise figure of about ½ dB, although other values can be used. From amplifier  16 , the signals are passed along line  20  to interference detector and suppressor  22 , which is part of the present invention. After passing through interference detector  22 , the signals are sent along a line  24  to a signal processing and image construction module  26 , which may comprise a number of different components such as a down converter, computers, computer terminals, monitors, and memory devices. Signal processing and image construction module  26  forms no part of the present invention, and the details of one example of such a module can be found in U.S. Pat. No. 5,525,906, the disclosure of which is incorporated herein by reference. 
     Interference detector and suppressor  22  determines whether the MRI signals coming in on line  20  likely contain transient interference that would improperly be interpreted as image, or desired, signals. In general overview, interference detector and suppressor  22  operates by recognizing that the transient interference, such as sparks, will virtually always have an envelope that varies at a significantly greater rate than the other signals in the system. By detecting a high rate of envelope change in the incoming signals, interference detector and suppressor  22  determines that the incoming signal is likely corrupted with transient interference, and the appropriate action is taken. 
     The signals coming in on line  20  are made of three different types of signals: (1) the desired signals, which are used to generate images, (2) thermal noise, which is always present, and (3) transient interference, which occurs sporadically and usually is the result of sparks, or other temporary interference events. The desired signals have a known frequency range that is narrow with respect to the other two signals. The desired signals are centered around the Larmor frequency and may have a bandwidth of approximately two-hundred to four-hundred kilohertz. The thermal noise is present at all frequency levels but is limited in MRI system  10  by the bandwidths of receiving antenna  16  and low noise amplifier  18 . Receiving antenna  16  and amplifier  18  are generally tuned to have a maximum gain over a relatively narrow bandwidth around the desired frequency range, while having substantial gain over a wider frequency range of several tens of megahertz (typically 40 MHz). The thermal noise therefore has a much wider bandwidth than the desired signals. The transient interference or noise spikes usually manifest themselves as impulse events, such as small sparks that are near enough to receiving antenna  16  to induce a voltage in antenna  16 . As such, the noise spikes have a very broad frequency component. 
     The voltage induced in receiving antenna  16  by the sparks will be transferred to interference detector and suppressor  22  according to the impulse response of antenna  16  and amplifier  18 . According to linear system theory, this impulse response may be described as a carrier frequency with an associated envelope wherein the envelope will have characteristics that are dominated by the bandwidth of antenna  16  and amplifier  18 . Because antenna  16  and amplifier  18  have a much wider frequency response than the desired signal&#39;s bandwidth (typically by a factor of 100 to 1000), the envelope of the noise spike will exhibit a rate of change that is much faster than the envelope of the desired signal. The envelope of the noise spike will also change much faster than the envelope of the thermal noise, which remains nearly constant. 
     The detailed operation of interference detector and suppressor  22  is best explained with reference to FIG.  2 . Interference detector and suppressor  22  has line  20  as its input, which is split between a detect path  28  and a signal path  30 . Signal path  30  is fed into a switch  32  which allows signal path  30  to be directly coupled to output  24  when switch  32  is closed. In this embodiment, switch  32  is closed when no interference is detected, and opened when interference is detected. The opening of switch  32  is referred to as blanking as it prevents the MRI signal on signal path  30  from being fed into signal processing and image construction module  26 . Switch  32  is controlled by an interference detector  34  placed along detect path  28 . Interference detector  34 , in the illustrated embodiment, includes a detector log video amplifier (DLVA)  36 , a high pass filter  38 , and a comparator  40 . In the current embodiment, DLVA  36  is a 0.1 GHz to 2.5 GHz, 70 dB Logarithmic Detector/Controller model AD8313 sold by Analog Devices, Inc. which has a place of business in Norwood, Mass. Other detector log video amplifiers can, of course, be used within the scope of the invention. The details of the model AD8313 can be found in the accompanying technical data sheet (revision B) published by Analog Devices, Inc., and downloadable from the web site http://www.analog.com the disclosure of which is hereby incorporated herein by reference. Although other high pass filters may be used, high pass filter  38  is a 500 KHz, three pole filter in the current embodiment. Comparator  40  may be any circuit that produces an output based upon the comparison of two inputs. 
     Detector log video amplifier  36  provides an output to high pass filter  38  that has a voltage corresponding to the power of the signal on detect path  28  that is input into DLVA  36 . For example, if the model AD8313 is used for DLVA  36 , it will output a voltage of approximately 0.6 volts, 0.8 volts, 1 volt, and 1.2 volts for input powers of −60 dBm, −50 dBm, −40 dBm, and −30 dBm, respectively (at approximately 900 MHz). The power of the input to DLVA  36  is a parameter that varies as the envelope of the input to DLVA  36  varies. Measuring changes in the power of the input to DLVA  36  therefore allows for changes in the envelope of the input to DLVA  36  to be detected. It will be understood by those skilled in the art that other devices could be used to detect the power of the incoming signal on detect path  28 , and that instead of measuring power, the envelope could be directly detected, or that other parameters that vary as the envelope varies could alternatively be detected. Because the envelope of interfering noise spikes will change at a much greater rate than the desired signals or the thermal noise, the output of DLVA  36  will vary greatly only when an interfering noise spike is present. High pass filter  38  distinguishes between the interfering noise spike and the desired signals and thermal noise by substantially filtering out the low frequency components of the output of DLVA  36  that are due to the desired signals and the thermal noise. 
     The output of high pass filter  38  is fed into a first input  44  of comparator  40  which compares its value to the value of a reference voltage  42  that is fed into a second input  46  of comparator  40 . If the value of first input  44  exceeds the value of second input  46 , comparator  40  outputs a high signal (an interference signal) at its output  48 . The output  48  of comparator  40  is optionally, though preferably, fed into a retriggerable multivibrator  50  which outputs a high signal (or blank signal) at  52  for a predetermined time period when its input receives a high signal. The purpose of multivibrator  50  is to provide a uniform blanking period when interference is detected by detector  34 . While the duration of the blanking signal output from multivibrator  50  can be varied as desired, the duration of the blanking signal is preferably equal to the sampling rate of an analog-to-digital (A/D) converter (not shown) that converts the analog signal on path  30  to a digital signal. This A/D converter is located in signal processing and image construction module  26 , and preferably converts the analog signal on path  30  to digital after the analog signal has been down-converted to lower frequencies. In the current embodiment, this sampling period is five microseconds, and multivibrator  50  outputs a five microsecond pulse every time its input goes high to thereby assure that the MRI signal on path  30  will be blanked for five microsecond increments. 
     Reference voltage  42  is preferably set at a value that is 6 to 10 dB above the thermal noise floor, although other values ranging from 2 to 20 dB and beyond can be used within the scope of the invention. In order to set reference voltage  42  a desired amount above the thermal noise floor, it is necessary to know the slope of the input/output characteristics of DLVA  36 . For example, if DLVA  36  has a 20 millivolt output for every one dBm of input power, then reference voltage  42  should be set at 120 millivolts (6 dBm×20 mV/dBm) to be 6 dB above the thermal noise floor. If it were desired to set reference voltage  42  at 10 dB above the thermal noise floor, it should be set at a value of 200 millivolts (10 dBm×20 mV/dBm). Setting the value of reference voltage  42  is therefore dependent only upon the slope of the input/output characteristics of DLVA  36  and the desired level above the thermal noise floor. This provides the advantage that detector  34  does not need to be re-adjusted or replaced when it is used in different temperature environments, or even when it is used with different antennas  16  that may have different thermal noise characteristics. While changing antennas or the temperature may cause a change in the average power of the thermal noise, these changes will be relatively slowly varying. As such, they will be filtered out by high pass filter  38 . For example, suppose the thermal noise floor initially presents −50 dBm of power to the input of DLVA  36 , causing DLVA  36  to output a signal of 0.8 volts. The 0.8 volt output of DLVA  36  will be generally constant (i.e. slowly changing) and therefore filtered out by high pass filter  38 . The same is true if the thermal noise floor changes to a power of −40 dBm, causing a change in the output of DLVA  36  from 0.8 to 1 volt, or some other value. Because this change is slow with respect to the envelope changes of the interference signal, it will be filtered out by high pass filter  38 . Detector  36  therefore provides the advantage that reference voltage  42  need not be changed once it is set, despite changes to the thermal noise brought about by changing temperatures or changing antennas. 
     Various modifications can be made to the embodiment described above without departing from the scope of the invention. One such change is reversing the order of high pass filter  38  and detector log video amplifier  36  such that detect path  28  is first fed into high pass filter  38  whose output is then fed into DLVA  36 . Another change is the addition of a delay filter in signal path  30  to create a delay in signal path  30  equal to the delay of detect path  28 . Another change is to use comparator output  48  to trigger corrective action other than blanking, such as, for example, re-scanning of the area corresponding to the corrupted signal. As still another change, a low pass filter might be inserted between the output of DLVA  36  and the input to high pass filter  38 . Such a low pass filter might be a single pole filter with a cut-off frequency of around ten megahertz. The low pass filter would help avoid false blanking due to random fluctuations in the thermal noise floor by filtering out any such high frequency random fluctuations. Further possible modifications include the use of a bandpass filter for filtering the input  20  into noise detector and suppressor  22 . Such a filter may be used to reduce thermal noise and remove inconsequential frequencies that are widely offset in frequency from the desired signals. A different type of envelope detector other than DLVA  36  may also be used. 
     While the present invention has been described in terms of the preferred embodiments depicted in the drawings and discussed in the above specification, along with several alternative embodiments, it will be understood by one skilled in the art that the present invention is not limited to these particular embodiments, but includes any and all such modifications that are within the spirit and the scope of the present invention as defined in the appended claims.