Abstract:
A method and apparatus is disclosed for processing, in a magnetic resonance imaging system, a magnetic resonance signal including transient spike noise. The method includes receiving an initial signal related to the magnetic resonance signal, the initial signal including a carrier signal modulated by a modulation signal and further including a transient spike noise component. The method further includes determining an envelope signal indicative of an envelope of the initial signal, such that the envelope is indicative of the modulation signal and further indicative of the transient spike noise component. The method additionally includes filtering the envelope signal by way of a high-pass filter to remove information relating to the modulation signal from the envelope signal and to produce a filtered envelope signal indicative of the transient spike noise component. The method further includes comparing a comparison signal related to the filtered envelope signal with a threshold to produce a noise indication signal, and modifying the magnetic resonance signal based upon the noise indication signal.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The field of the invention is nuclear magnetic resonance imaging (MRI) methods and systems. More particularly, the invention relates to a method and system for processing nuclear magnetic resonance (NMR) signals acquired during a scan in order to remove transient spike noise from the NMR signals and thereby eliminate artifacts produced by such noise in the reconstructed image.  
           [0002]    When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B 0 ), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to an additional magnetic field (excitation field B 1 ) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, M z , may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment M t . A radio-frequency (RF) signal, which is also denoted the nuclear magnetic resonance (NMR) signal, is emitted by the excited spins after the excitation signal B 1  is terminated, and this NMR signal may be received and processed to form an image.  
           [0003]    MRI scanners include a large magnet assembly for producing the uniform polarizing field B 0  in a bore which is large enough to receive a patient. An RF coil surrounds the patient and is switched between a transmitter and receiver to produce the excitation field B 1  and to receive the resulting NMR signal. Additionally, three sets of gradient coils surround the RF coil to produce magnetic field gradients G x , G y  and G z , and a shield is disposed therebetween to isolate the RF coil so that its uniform field is not disrupted. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which the magnetic field gradients are switched on and off according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.  
           [0004]    The NMR signals are very small and extraordinary measures are taken to shield the MRI system from external RF noise and to eliminate sources of internal noise. Nevertheless, generators of short-duration noise pulses persist and may elude location and elimination. These noise pulses are referred to as “spike noise”, “impulse noise” or “white pixels”, and lead to image artifacts with such vernacular names as corduroy and zebra artifacts. Sources of such noise include arcing due to partial discharges from intermittent electrical contacts or electrostatic discharge, and harmonics of fast transients such as those caused by ground loops. When such noise sources occur regularly, their source can be located and measures can be taken to eliminate them. This “hardening” process occurs at any new MRI installation, and eventually all the short-duration noise sources are eliminated except those which are intermittent and defy cost-effective diagnosis.  
           [0005]    A number of strategies have been employed to mitigate the effects of intermittent noise sources. Such methods include the examination of the acquired NMR signals to locate noise spikes or the examination of the reconstructed image to locate the effects of such noise. These prior methods work when the noise spike occurs in NMR signals that are heavily phase or frequency encoded (i.e. on the edges of k-space), but they do not perform well when the noise spike occurs in NMR signals near the center of k-space. In the latter case the NMR signal magnitude is quite large and it is more difficult to discern signal from noise. Noise spikes detected by such methods are sometimes removed by interpolating between the adjacent values.  
           [0006]    Another strategy, which is more effective in detecting and eliminating short-duration noise spikes near the center of k-space, was disclosed in U.S. Pat. No. 5,525,906 entitled “Detection and Elimination of Wide Bandwidth Noise in MRI Signals.” In that strategy, the NMR signal is processed by a noise filter or Transient Noise Suppression (TNS) system that includes a noise detector. The noise detector has a bandstop filter that is tuned to stop the NMR signals but to pass a range of frequencies outside the NMR imaging frequency band that include at least some of the spike noise. Because a considerable portion of the energy of short-duration spikes is located outside of the NMR imaging frequency band, the bandstop filter effectively isolates the spike noise from the NMR imaging frequency information. The bandstop filter thus provides an output signal that is an indication of the level of spike noise independent of the NMR imaging frequency information.  
           [0007]    The magnitude of the output signal from the bandstop filter is then compared with a noise reference level at a comparator. When the magnitude of the output signal exceeds the noise reference level, a noise indication signal is produced (or is changed in its level) indicating that there is noise due to short-duration spikes. The noise indication signal can then be used to eliminate noise from the entire NMR signal by blanking out portions of the NMR signal whenever noise is detected, before the NMR signal is provided to an image reconstructor.  
           [0008]    Although TNS systems are more effective at eliminating noise due to short-duration spikes near the center of k-space than the other systems mentioned above, TNS systems are highly frequency dependent. In particular, the stop band of the bandstop filter in a TNS system must be carefully set so that the filter passes the ranges of frequencies above and below the NMR imaging frequency band and not the NMR imaging frequency band itself. If the pass band of the bandstop filter encompasses the NMR imaging frequency band, the TNS system may mistake the high-magnitude signal components containing the imaging information for high-magnitude noise spikes, and inappropriately blank out portions of the NMR signal that contain useful information rather than noise. The high sensitivity of TNS systems to frequency is undesirable insofar as TNS systems must as a result be carefully and accurately implemented in order for the systems to properly remove noise due to short-duration spikes.  
           [0009]    The high frequency sensitivity of TNS systems is also undesirable because it makes it necessary to configure a TNS system differently depending upon the frequency of operation of the MRI system (particularly the frequency of the polarizing field B 0 ) in which it is implemented. Given the wide variety of MRI systems, and given that some MRI systems can operate at a variety of different frequencies, TNS systems must be repeatedly configured. Given that the tuning of TNS systems to MRI systems, and performance verification, can be costly, the high frequency sensitivity of TNS systems increases the cost of MRI systems overall and places an undesirable constraint on the design of new MRI systems, particularly those that operate at multiple frequencies.  
           [0010]    It would therefore be advantageous if a system could be developed for eliminating noise due to short-duration spikes from NMR signals and thereby mitigating the appearance of undesirable image artifacts from images created by MRI systems. It would particularly be advantageous if such a system could be developed that was successful in eliminating noise due to short-duration spikes even where the spikes were near the center of k-space. It would additionally be advantageous if such a system was not overly frequency sensitive in its operation, such that it could be easily implemented in a variety of MRI systems having a variety of frequencies of operation, or in MRI systems that operated at multiple frequencies of operation. It would further be advantageous if the system was low in cost and could be easily implemented.  
         BRIEF SUMMARY OF THE INVENTION  
         [0011]    The present invention relates to, in a magnetic resonance imaging system, a method of processing a magnetic resonance signal including transient spike noise. The method includes receiving an initial signal related to the magnetic resonance signal, the initial signal including a carrier signal modulated by a modulation signal and further including a transient spike noise component. The method further includes determining an envelope signal indicative of an envelope of the initial signal, such that the envelope is indicative of the modulation signal and further indicative of the transient spike noise component. The method additionally includes filtering the envelope signal by way of a high-pass filter to remove information relating to the modulation signal from the envelope signal and to produce a filtered envelope signal indicative of the transient spike noise component. The method further includes comparing a comparison signal related to the filtered envelope signal with a threshold to produce a noise indication signal, and modifying the magnetic resonance signal based upon the noise indication signal.  
           [0012]    The present invention also relates to a method of processing a magnetic resonance signal including transient spike noise, in a magnetic resonance imaging system. The method includes receiving an initial signal related to the magnetic resonance signal, where the initial signal includes a carrier signal modulated by a modulation signal and further includes a transient spike noise component. The method additionally includes determining an envelope signal indicative of an envelope of the initial signal, such that the envelope is indicative of the modulation signal and further indicative of the transient spike noise component. The method also includes filtering the envelope signal by way of a filter to remove information relating to the modulation signal from the envelope signal and to produce a filtered envelope signal indicative of the transient spike noise component. The method further includes processing the magnetic resonance signal based upon the filtered envelope signal.  
           [0013]    The present invention additionally relates to, in a magnetic resonance imaging system, a system for processing a magnetic resonance signal having a transient spike noise component. The system includes an envelope detector that receives an initial signal related to a magnetic resonance signal and provides an envelope signal in response to the initial signal. The system further includes a high-pass filter that is coupled to the envelope detector, receives the envelope signal and provides a filtered envelope signal in response to the envelope signal. The high-pass filter isolates signal components corresponding to transient spike noise in the initial signal from signal components corresponding to magnetic resonance imaging information in the initial signal. The system additionally includes a comparison device that is coupled to the high-pass filter, receives a comparison signal related to the filtered envelope signal and provides a noise indication signal based upon the filtered envelope signal. The system further includes a modification device that is coupled to the comparison device and modifies the magnetic resonance signal based upon the noise indication signal, in order to reduce the transient spike noise component in the magnetic resonance signal and produce an improved magnetic resonance signal.  
           [0014]    The present invention additionally relates to a magnetic resonance imaging system. The system includes an operator console, a computer system coupled to the operator console and including a memory, a system control coupled to the computer system and including a transceiver, and a magnet assembly coupled to the system control that produces varying magnetic fields as determined by the system control. The transceiver receives a magnetic resonance signal from the magnet assembly, detects an envelope of an initial signal related to the magnetic resonance signal to produce an envelope signal, and filters the envelope signal to isolate a transient spike noise component of the envelope signal and produce a filtered envelope signal indicative of the transient spike noise component. The transceiver further processes the magnetic resonance signal based upon the filtered envelope signal to eliminate transient spike noise from the magnetic resonance signal and to produce an improved magnetic resonance signal. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]    [0015]FIG. 1 is a block diagram of an MRI system which employs the present invention;  
         [0016]    [0016]FIG. 2 is an electrical block diagram of a transceiver which forms part of the MRI system of FIG. 1;  
         [0017]    [0017]FIG. 3 is an electrical block diagram of a TNS system that forms part of the transceiver of FIG. 2; and  
         [0018]    [0018]FIG. 4 is an electrical block diagram of a noise detector which forms part of the TNS system of FIG. 3. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0019]    Referring first to FIG. 1, there is shown the major components of a preferred MRI system  100  that incorporates the present invention. The operation of the MRI system  100  is controlled from an operator console  101  which includes a keyboard and control panel  102  and a display  104 . The console  101  communicates through a link  116  with a separate computer system  107  that enables an operator to control the production and display of images on the display  104 . The computer system  107  includes a number of modules which communicate with each other through a backplane  117 . These include an image processor module  106 , a CPU module  108  and a memory module  113 , known in the art as a frame buffer for storing image data arrays. The computer system  107  is linked to a disk storage  111  and a tape drive  112  for storage of image data and programs, and it communicates with a separate system control  122  through a high speed serial link  115 .  
         [0020]    The system control  122  includes a set of modules connected together by an additional backplane  118 . These include a CPU module  119  and a pulse generator module  121 , which connects to the operator console  101  through a serial link  125 . It is through this link  125  that the system control  122  receives commands from the operator that indicate the scan sequence to be performed. The pulse generator module  121  operates the system components to carry out the desired scan sequence. It produces data that indicates the timing, strength and shape of the RF pulses that are to be produced, and the timing of and length of the data acquisition window. The pulse generator module  121  connects to a set of gradient amplifiers  127 , to indicate the timing and shape of the gradient pulses to be produced during the scan. The pulse generator module  121  also receives patient data from a physiological acquisition controller  129  that receives signals from a number of different sensors connected to the patient, such as ECG signals from electrodes or respiratory signals from a bellows. And finally, the pulse generator module  121  connects to a scan room interface circuit  133  that receives signals from various sensors associated with the condition of the patient and the magnet system. It is also through the scan room interface circuit  133  that a patient positioning system  134  receives commands to move the patient to the desired position for the scan.  
         [0021]    The gradient waveforms produced by the pulse generator module  121  are applied to a gradient amplifier system  127  comprised of G x , G y  and G z  amplifiers. Each gradient amplifier excites a corresponding gradient coil in an assembly generally designated  139  to produce the magnetic field gradients used for position encoding acquired signals. The gradient coil assembly  139  forms part of a magnet assembly  141  which includes a polarizing magnet  140  and a whole-body RF coil  152 . A transceiver module  150  in the system control  122  produces pulses which are amplified by an RF power amplifier  151  and coupled to the RF coil  152  by a transmit/receive switch  154 .  
         [0022]    The resulting signals radiated by the excited nuclei in the patient may be sensed by the same RF coil  152  and coupled through the transmit/receive switch  154  to a preamplifier  153 . The amplified NMR signals are demodulated, filtered, and digitized in the receiver section of the transceiver  150 . The transmit/receive switch  154  is controlled by a signal from the pulse generator module  121  to electrically connect the RF power amplifier  151  to the coil  152  during the transmit mode and to connect the preamplifier  153  during the receive mode. The transmit/receive switch  154  also enables a separate RF coil (for example, a head coil or surface coil) to be used in either the transmit or receive mode.  
         [0023]    The NMR signals picked up by the RF coil  152  are digitized by the transceiver module  150  and transferred to a memory module  160  in the system control  122 . When the scan is completed and an entire array of data has been acquired in the memory module  160 , an array processor  161  operates to Fourier transform the data into an array of image data. This image data is conveyed through the serial link  115  to the computer system  107  where it is stored in the disk memory  111 . In response to commands received from the operator console  101 , this image data may be archived on the tape drive  112 , or it may be further processed by the image processor  106  and conveyed to the operator console  101  and presented on the display  104 .  
         [0024]    Referring particularly to FIGS. 1 and 2, the transceiver  150  produces the RF excitation field B 1  through the RF power amplifier  151  at a coil  152 A and receives the resulting signal induced in a coil  152 B. As indicated above, the coils  152 A and B may be separate as shown in FIG. 2, or may be a single wholebody coil as shown in FIG. 1. The base, or carrier, frequency of the RF excitation field is produced under control of a frequency synthesizer  200 , which receives a set of digital signals from the CPU module  119  and pulse generator module  121 . These digital signals indicate the frequency and phase of the RF carrier signal produced at an output  201 . The commanded RF carrier is applied to a modulator and up converter  202  where its amplitude is modulated in response to a signal also received from the pulse generator module  121 . This signal defines the envelope of the RF excitation pulse to be produced and is produced in the module  121  by sequentially reading out a series of stored digital values. These stored digital values may, in turn, be changed from the operator console  101  to enable any desired RF pulse envelope to be produced. The magnitude of the RF excitation pulse produced at output  205  is attenuated by an exciter attenuator circuit  206  that receives a digital command (TA) from the backplane  118 . The attenuated RF excitation pulses are applied to the RF power amplifier  151  that drives the RF coil  152 A. For a more detailed description of this portion of the transceiver  122 , reference is made to U.S. Pat. No. 4,952,877, which is incorporated herein by reference.  
         [0025]    Referring still to FIGS. 1 and 2, the NMR signal produced by the subject is picked up by the receiver coil  152 B and applied through the preamplifier  153  to the input of a receiver attenuator  207 . This NMR signal typically occupies a bandwidth of less than one megahertz (MHz), centered on the Larmor frequency of the spin species being imaged. In a 1.5 Tesla system imaging hydrogen, for example, the Larmor frequency is 63.86 MHz. The receiver attenuator  207  further modifies the amplitude of this NMR signal by an amount determined by a digital attenuation signal (RA) received from the backplane  118 . In addition, the receiver attenuator  207  includes a new Transient Noise Suppression (TNS) system or noise filter  300  (shown in FIG. 3), which practices the present invention and which will be described in detail below with reference to FIGS. 3 and 4.  
         [0026]    The received NMR signal is at or around the Larmor frequency, and this high frequency signal is down converted in a two step process by a down converter  208 . The down converter  208  first mixes the NMR signal with the carrier signal on output  201  and then mixes the resulting difference signal with the 2.5 MHz reference signal on line  204 . The down converted NMR signal is applied to the input of an analog-to-digital (A/D) converter  209  that samples and digitizes the analog signal. The digitized samples are applied to a digital detector and signal processor  210  which produces 16-bit in-phase (I) values and 16-bit quadrature (Q) values corresponding to the received signal. The resulting stream of digitized I and Q values of the received signal are output through the backplane  118  to the memory module  160  where they are employed to reconstruct an image.  
         [0027]    The 2.5 MHz reference signal of line  204 , as well as a 250 kHz sampling signal used to control the sampling of the A/D converter  209  and the 5, 10 and 60 MHz reference signals are produced by a reference frequency generator  203  from a common 20 MHz master clock signal. For a more detailed description of the receiver, reference is made to U.S. Pat. No. 4,992,736 which is incorporated herein by reference.  
         [0028]    Referring particularly to FIG. 3, the preferred embodiment of the noise filter  300  of receiver attenuator  207 , in accordance with the present invention, receives the acquired NMR signal at an input  310  from the pre-amp  153 . The NMR signal is split at a splitter  305  and is applied through a line  335  to a noise detector  340  and through a line  315  to a time delay circuit  320 . As will be explained in more detail below, the noise detector  340  senses the presence of any short-duration noise spikes in the NMR signal and produces a noise indication signal on line  345 . The noise indication signal has a duration which is coincident with the short-duration noise spike. The time delay  320  imposes a delay on the applied NMR signal such that the delayed NMR signal produced on its output  325  coincides with the delay imposed by the noise detector  340 . Although in the conventional noise filter (or TNS system) discussed in the Background of the Invention, the desired time delay can be provided through the use of a bandpass filter that is tuned to pass only the NMR signals, the new noise filter  300  typically does not employ such a bandpass filter as the time delay  320 . That is, preferably the one or more time delay devices employed by the new noise filter  300  are not frequency sensitive in order to pass only the NMR signals.  
         [0029]    Referring still to FIG. 3, both the time delayed NMR signal and the noise indicating signal are applied to a blanking circuit  330 . The blanking circuit  330  is a gallium-arsenide semiconductor switch which is turned off by the noise indicating signal on line  345  to block any short-duration noise which is contained in the NMR imaging signal on output  325  to produce a noise-free signal at an output  350 . A characteristic of this switch of blanking circuit  330  is that it does not generate appreciable energy at frequencies near the Larmor frequency when it is operated. Since the noise spikes are short in duration and the blanking periods are correspondingly short, it has been discovered that this method of removing noise spikes from the NMR signal does not produce objectional image artifacts. Indeed, as long as the blanking period is less than one sample period of the A/D converter  209 , the image artifacts are negligible, even when it occurs during the acquisition of a view in central k-space. The blanking period should be as short as possible while still of sufficient duration to cover the noise spike. Thus, because of its simplicity and cost effectiveness, blanking out the noise is preferred over the other corrective methods such as interpolation or adding the inverse of the noise to the NMR signal.  
         [0030]    Referring particularly to FIG. 4, the noise detector  340  includes an amplifier  410 , an envelope detector  420 , a high-pass filter  430 , a second amplifier  440 , a comparator  450  and a trigger  460 . The NMR signal provided via the line  335  is amplified by the amplifier  410 , which outputs an amplified NMR signal  412 . In certain embodiments where the magnitude of the NMR signal is already at a desired level, the amplifier  410  is not required. The amplified NMR signal  412  typically is made up of a carrier signal (at the frequency of the RF carrier signal discussed above, e.g., the Larmour frequency) that is modulated by a modulation signal such that the signal  412  has an envelope, as shown in an exemplary signal fragment  414 . The received modulation frequency (or frequencies) of the amplified NMR signal  412  is typically not identical to the transmitted modulation frequency imposed by the modulator and up converter  202  discussed above, since the modulation of the signal in part reflects the characteristics of the patient being scanned and as well as other operational factors.  
         [0031]    Also as shown by the signal fragment  414 , the amplified NMR signal  412  sometimes includes one or more noise spikes  416 . The amplified NMR signal  412  is provided to the envelope detector  420 , which determines the envelope of the amplified NMR signal  412  and provides an envelope signal  425  indicative of that envelope. In one embodiment, the envelope detector  420  can be one of the model 8306, 8309 or 8310 Logarithmic Amplifiers manufactured by Analog Devices, Inc. of Norwood, Mass. However, the design of the envelope detector  420  can be any one of a number of designs currently known to those skilled in the art including, for example, a half-wave rectifier with an output terminal that is connected to ground by way of a capacitor. As shown in an exemplary signal fragment  424 , the envelope signal  425  generally conforms to the envelope determined by the received modulation frequency but further includes noise spikes  426  corresponding to the noise spikes  416 .  
         [0032]    The envelope signal  425  is provided to the high-pass filter  430 , which filters out the low-frequency components of the envelope signal. Because the frequencies that are associated with the noise spikes  426  in the envelope signal  425  are much higher than the frequencies associated with the envelope itself (e.g., the received modulation frequency), the high-pass filter  430  is able to filter out virtually all of the components of the envelope signal other than the signal components associated with the noise spikes  426 . The high-pass filter  430  can be designed to have the desired pass band utilizing any one of a number of conventional filter design techniques known to those skilled in the art. In alternate embodiments, the high-pass filter  430  can be replaced by other types of filters so long as the replacement filters still filter out the received modulation frequency signal components associated with the NMR imaging information, without filtering out all of the noise spike signal components.  
         [0033]    The output of the high-pass filter  430  therefore is a filtered envelope signal  435  including, for the most part, only signal components corresponding to the noise spikes  416 , as shown in an exemplary signal fragment  434 . Thus, by way of the process of envelope detection and subsequent filtering, a signal essentially indicative of only the noise spikes  416  is generated. This filtered envelope signal  435  is in turn provided to the second amplifier  440 , which in turn amplifies the filtered envelope signal  435  to provide a comparator input signal  445  to the comparator  450 . The second amplifier  440  may not be required in alternate embodiments where the signal strength of the filtered envelope signal  435  is already within desired levels.  
         [0034]    The magnitude of the comparator input signal  445  is compared with the magnitude of a noise reference  447  by the comparator  450 . The comparator  450 , which can be an operational amplifier, operates as a threshold detector; that is, the comparator determines whether the comparator input signal  445  exceeds the noise reference  447 , which is a noise threshold or DC voltage threshold (VT). The noise reference  447  is a function of the “noise floor” for the particular system, receiver coil, subject and NMR pulse sequence being used. The noise reference  447  is set to a magnitude that is greater than the noise floor when no short-duration noise spikes are occurring. This level can be set manually during the set-up and calibration of the MRI system  100  and altered automatically as a function of the receive coil and the pulse sequence selected by the operator. The comparator input signal  445  can include a DC offset.  
         [0035]    The comparator  450  produces a noise indication signal  455  that pulses on when the comparator input signal  445  exceeds the noise reference  447 , as shown in a signal fragment  454 . This noise indication signal  455  is provided to a trigger  460  (or monostable multi-vibrator or “monoshot”). The trigger  460  operates to output pulses on line  345 , as shown by a signal fragment  464 . The widths of the pulses vary in dependence upon the widths of the pulses of the noise indication signal  455 . The pulses output by the trigger  460 , however, only take on widths that are multiples of a fixed width (duration) determined by a time constant circuit that is coupled to (or part of) the trigger. The trigger  460  in some embodiments is enabled by a DATA IN window signal (not shown) that controls data acquisition. Because short-duration noise is known to be produced by a number of system transitions that occur when data is not acquired, the use of the DATA IN window signal insures that only those noise events that occur during image data acquisition are registered. A counter (not shown) is also coupled to the trigger  460 , in some embodiments, to count and store the total number of registered noise events that have occurred. This number can be periodically read by maintenance personnel and used to evaluate system performance.  
         [0036]    The new noise detector  340  of FIG. 4 is implemented as part of the new noise filter  300  (or TNS system) in MRI system  100  in place of a conventional noise detector as discussed above. The new noise detector  340 , unlike the conventional noise detector, allows for broad band detection of transient spike noise, as follows. As discussed, the conventional noise detector must be carefully configured so that the bandstop filter of the noise detector does not pass signals in the NMR imaging frequency band. The NMR imaging frequency band includes frequencies in the range extending approximately from the RF carrier minus the received modulation frequency to the RF carrier plus the received modulation frequency. The signals in the NMR imaging frequency band are removed by the bandstop filter so that spike noise, which exists in a broader range of frequencies above and below the NMR imaging frequency band, can be distinguished from the NMR imaging information and thus detected.  
         [0037]    In contrast, the new noise detector  340 , by detecting the envelope of the NMR signals, effectively down-converts the information in the NMR imaging frequency band and removes the RF carrier. That is, signal components in the NMR imaging frequency band around the RF carrier are converted to the much lower frequency range extending from 0 Hz up to approximately the received modulation frequency (e.g., 1 MHz). Thus, the resulting envelope signal  425  effectively includes, in addition to spike noise at a wide range of frequencies, only low-frequency signal components corresponding to the NMR imaging information. Because of this down-conversion of the NMR signals, the range of frequencies at which there is no NMR imaging information only exists above the frequency range defined by the received modulation frequency, as opposed to both above and below the NMR imaging frequency band. Consequently, the noise detector  340  employs the high-pass filter  430  in place of a bandstop filter.  
         [0038]    Because the spike noise includes a range of frequency components extending to frequencies well beyond the received modulation frequency, the stop band of the high-pass filter  430  does not need to be as carefully tailored to the frequency range of the received modulation signal components as the stop band of the bandstop filter is tailored to the NMR imaging frequency band in the conventional system. For example, in typical MRI systems, the RF carrier frequency can range from 8 MHz to 200 MHz, and the received modulation frequency band around the carrier frequency can vary from a bandwidth of 150 KHz to a bandwidth of 1 MHz. Although spike noise generally can include a broad frequency band, for example, ranging from 0 Hz all the way to 1 GHz, the operation of the receiver coil  152  and preamplifier  153  tend to narrow the spike noise frequency band around the carrier frequency, for example, a 10 MHz band.  
         [0039]    Consequently, for effective operation, the conventional noise filter (discussed in the Background of the Invention) with its bandstop filter needs to be able to filter out the 150 KHz to 1 MHz modulation frequency band centered about the appropriate RF carrier frequency, without overly attenuating the spike noise signal components outside that band. Given that even minor changes in the design or implementation of MRI systems (particularly changes which affect the polarizing field BO) can result in significant changes in the RF carrier frequency at which the received modulation frequency band is centered, the bandstop filter must be carefully configured/tuned with respect to each MRI system to guarantee proper operation of the noise filter.  
         [0040]    In contrast, the high-pass filter  430  of the new noise detector  340  typically needs to be configured only so that its stop band includes the received modulation frequency band after the amplified NMR signal  412  is down-converted by the envelope detector  420 . As noted, this frequency band of the envelope or received modulation signal (after down-conversion) typically extends from 0 Hz to somewhere in the range of 150 KHz to 1 MHz (depending upon the embodiment). Because the frequencies of the signal components of the spike noise can extend up to 10 MHz, the stop band of the high-pass filter  430  can extend significantly higher than 1 MHz. At the same time, because the envelope signal  425  is down-converted from the RF carrier frequency, the stop band of the high-pass filter  430  does not have to coincide with (or otherwise account for) the RF carrier frequency.  
         [0041]    The new noise filter  300  is therefore much less frequency sensitive than the conventional noise filter. A single configuration of the new noise filter  300  with a particular high-pass filter  430  can be employed in a variety of MRI systems having a variety of RF carrier frequencies or modulation frequencies, or in individual MRI systems that operate at multiple frequencies. The new noise filter  300  allows for broad band detection of transient noise spikes insofar as it is capable of detecting transient noise spikes for MRI systems having RF carrier frequencies or modulation frequencies within broad bands of frequencies.  
         [0042]    There are numerous variations that can be made from the preferred embodiment without departing from the spirit of the invention. The noise detector  340  can be placed at other locations in the NMR signal path and its noise indication signal can be employed in other ways to ameliorate the effects of short-duration noise spikes. For example, the noise detector  340  can be connected between the RF receive coil and the pre-amp  153 . Instead of blanking or correcting the NMR signal in its analog form, the noise indication signal could also be employed, in alternate embodiments, to correct the acquired NMR data in its digital form. This same noise indication signal can be used to “time stamp” the acquired NMR data so that it can be altered later in the image reconstruction process, or the noise indication signal can be used to alter the scan sequence. For example, if a noise spike is detected while acquiring a view from the central region of k-space, the view acquisition order can be altered to reacquire that view at a later time in the scan.  
         [0043]    Other variations are possible. For example, circuitry can be added to protect amplifiers and other components from over voltage and over current conditions. Rather than being fixed, the noise reference level can be controlled by an automatic gain circuit that adjusts the level as a function of NMR noise power. Further, rather than simply blanking out the noise spike, other signal correction methods are possible. For example, a negative impulse might be inserted into the NMR signal coincident with the noise spike to offset its effects, or a sample and hold circuit might be employed to apply the signal sample just prior to the noise spike during the detected noise spike interval. The NMR signal can be passed through a variable bandwidth, bandpass filter, and the noise indication signal employed to constrict the bandwidth to a fraction of the signal bandwidth. Yet another correction method is to capture the NMR signal carrier frequency prior to the noise spike with a phase locked loop, and to insert a signal of this frequency into the gap formed by the blocked noise spike.  
         [0044]    While the foregoing specification illustrates and describes the preferred embodiments of this invention, it is to be understood that the invention is not limited to the precise construction herein disclosed. The invention can be embodied in other specific forms without departing from the spirit or essential attributes of the invention. Accordingly, reference should be made to the following claims, rather than to the foregoing specification, as indicating the scope of the invention.