Abstract:
A system and method for monitoring patient physiological information during an MRI scan sequence is provided. The system includes a monitoring device configured to sense physiological information from a patient. The physiological information may include electrocardiograph signals, electro-anatomical mapping signals, or other information concerning a physiological condition of the patient. The system further includes a control circuit connected to receive signals from the monitoring device and to coordinate output of the electrode during an MRI scan.

Description:
CROSS REFERENCE 
       [0001]    The present application is based on, claims priority to, and incorporates herein by reference U.S. Provisional Application Ser. No. 61/477,528, entitled, “SYSTEM AND METHOD FOR ACQUIRING PATIENT PHYSIOLOGICAL INFORMATION DURING AN MRI SCAN,” filed Apr. 20, 2011, and U.S. Provisional Application No. 61/478,281, entitled, “SYSTEM AND METHOD FOR ACQUIRING PATIENT PHYSIOLOGICAL INFORMATION DURING AN MRI SCAN,” filed Apr. 22, 2011. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    The field of the invention is medical devices and methods for their use. More particularly, the invention relates to physiological monitoring of a patient, such as electrocardiogram monitoring, electro-anatomic mapping, and electro encephalography (EEG), during a medical imaging process. 
         [0003]    The electrocardiogram (ECG) is a widely used clinical tool for cardiac physiological monitoring and for the real-time diagnosis of heart conditions. Surface ECG monitoring, in which the ECG electrodes are attached to a patient&#39;s skin, is one type of ECG monitoring that is often used with medical imaging processes. Frequently, patients with cardiac conditions will undergo, for example, a magnetic resonance imaging (MRI) scan in which ECG information is monitored during the scan sequence and used to assist in MRI image acquisition. The ECG information that is obtained may be used, for example, for properly synchronizing cardiac MRI scans, which is a process typically referred to as “gating.” Because cardiovascular anatomy is continuously moving throughout the MRI scan, synchronization or “gating” of MRI data acquisition with the cardiac cycle allows for improved imaging of the cardiovascular anatomy at the various phases of the cardiac cycle. Surface ECG monitoring is also frequently used for physiological monitoring of patients who are being scanned for multiple indications (brain, knee, abdomen, etc.), or who are undergoing therapeutic interventions inside the MRI (even interventions not within the cardiovascular system). This is especially true for patients who are anesthetized during the scan or who have a history of heart disease or of stroke. 
         [0004]    Surface ECG monitoring inside an MRI scanner presents several challenges that can affect the quality, and thus the usability, of the ECG signals and MRI data acquired. First, the ECG electrodes can experience unwanted voltages that are induced during the ramping up and down of the magnetic field gradients used in the MRI scan. Due to the changing magnetic field caused by the gradient rampings, electric charges induced in the ECG electrodes can have amplitudes up to a few volts. Other voltages can be induced in the ECG electrodes by system control signals sent to the scanner, such as those used to bias diodes in radio-frequency excitation coils prior to radio-frequency transmission. The above induced voltages can be many times larger than the voltage levels of the true ECG signals and, therefore, can saturate the ECG acquisition equipment, making the true ECG signals more difficult to accurately filter out and detect. In addition, the higher voltages can potentially damage the ECG acquisition equipment, which is configured to ordinarily detect only measurements in the range of a few milli-volts. Additionally, the induced voltages can have a frequency content of anywhere between about 100-10,000 Hz, which can render it difficult to remove the unwanted induced signals with simple frequency-based filters. Similarly, the ECG electrodes and leads may conduct radio frequency (RF) fields, such as 64 MHz for 1.5 T or 127 MHz for 3 T, which are induced into the ECG electrodes and leads by the MRI scan sequence&#39;s RF pulses. These RF-induced signals cause further noise and, thus, also deteriorate the quality of the signals acquired by the ECG electrodes. Not only do these phenomena negatively affect the acquisition of ECG information, they can interfere with acquiring the desired imaging data. 
         [0005]    Attempts have been made to address the issues confronting use of surface ECG monitoring inside an MRI scanner, but have met with only limited success. Most attempted solutions have simply taken the approach of using a smaller number of ECG electrodes (typically around 3 to 5 electrodes) that are closely distanced from each other in order to, in theory, reduce the induced RF and gradient voltages. However, 12-lead ECG surface monitoring arrangements, in which electrodes are placed at designated positions on the torso, is the preferred and most widely used system for monitoring heart condition. Reducing the number of electrodes to only 3 to 5, and arranging the electrodes much more closely than the standard 12-lead arrangement, causes the quality and usefulness of the ECG signals to be severely deteriorated. As a result, such approaches are useful only to perform MRI scan synchronization and do not provide physiological monitoring-quality ECG traces. 
         [0006]    Other attempted solutions have involved the use of software filtering or digital signal processing of the acquired ECG signals to remove or suppress the RF and gradient induced components in the ECG leads, but these have met with limited success and also do not provide physiological monitoring-quality traces. Noise caused by gradient ramping and RF transmission during an MRI scan sequence is intrinsically a difficult problem to address with software and digital signal processing, because the gradient noise component in ECG leads can be on the order of a thousand times stronger than the true ECG signal component. In addition, because the gradient fields applied by MR systems constantly change to a significant degree in terms of magnitude, direction, frequency, and duration (due to different requirements of each imaging sequence), it is difficult or impossible for signal-processing algorithms to adapt to the large variety of potential gradient noise. 
         [0007]    Electro-Anatomic-Mapping (EAM) is a relatively newer clinical tool than traditional surface ECG monitoring, in which ECG data is collected at various positions inside the body, including on the walls of the cardiac chambers. One of the distinguishing differences from surface ECG monitoring is that positional information is also acquired, sometimes on the same conductive lines as the ones that transfer the ECG signals from the electrodes to the receiver. This positional information is acquired, for example, by inducing electrical currents from surface electrodes and sampling them using catheters that have multiple electrodes on their shaft and that are moved inside the body. The positional tracking signals are generally electromagnetic signals at higher frequencies (5-10 kHz) than those found in conventional ECG (0-300 Hz), so they are easily separated by the EAM receiver. The common practice is to display the position of the catheter electrodes and the ECG voltage at those points, which defines the EAM map. Available EAM systems include the NavX® systems offered by St. Jude Medical, Inc. and the CarTo systems offered by Biosense Webster, Inc. 
         [0008]    However, like surface ECG monitoring, EAM mapping inside an MRI scanner also presents several issues that can affect signal quality. First, the ECG component of EAM signals encounters the same RF and gradient induced noise problems as discussed above. Second, the positional localization component of the EAM signals suffer from the induced voltages caused by the gradient ramps as well, since the noise created by the gradient ramps is within the same reception band (5-10 kHz) of the localization voltages. For example, one type of NavX® system operates using a 5.8 kHz signal, while others operate with 8 kHz signals. 
         [0009]    These issues confronting the use of EAM inside an MRI scanner have either not been addressed at all, or have only been addressed by an unsatisfactory solution. In fact, the only techniques currently in accepted use that offer positional information in MRI scanners are either based on passive tracking (i.e., using the MRI images themselves for following interventional devices) or use MRI techniques for active tracking (e.g., MR-tracking or MRI-gradient tracking). These solutions do not work outside the MRI scanner, so they cannot be used to monitor a patient during transfer in and out of the scanner, during portions of the procedure that are conducted outside the MRI bore, or during periods in which the patient is inside the scanner, but no images are being acquired. 
         [0010]    Moreover, the issues confronting acquisition of ECG and EAM information during an MRI scan can also negatively impact the use of other physiological monitoring tools, such as pulse oximeters, blood pressure cuffs, and respiratory monitors. Each of these clinical tools may include the use of electrodes and leads that can also be susceptible to the same unwanted currents and voltages induced by gradient ramping and RF transmission occurring during an MRI scan. 
         [0011]    It would therefore be desirable to provide a system and method for reducing or avoiding the negative effects of RF and gradient induced voltages on ECG and EAM signals (and other physiological monitoring signals), to allow for acquisition of physiological monitoring-quality ECG and/or EAM signals from inside an MRI scanner during a scan. Similarly, it would be further desirable if the system and method allowed for acquisition of such signals both inside and outside the scanner, as well as both during periods in which images are being acquired and when they are not being acquired. 
       SUMMARY OF THE INVENTION 
       [0012]    The present invention overcomes the aforementioned drawbacks by providing a system that includes circuits for disconnecting output of physiological monitoring devices during periods of an MRI scan sequence that can induce unwanted voltages in the monitoring devices or otherwise cause noise in the output of the devices, while permitting communication of the devices&#39; output during other periods of the scan sequence. In particular, the system utilizes a signal gating switch circuit that is responsive to control signals from components of the MRI system that are responsible for causing the induced voltages and/or noise. 
         [0013]    It is an aspect of the invention to provide an MRI system that comprises a magnet system, a plurality of gradient coils, an RF system, a patient monitoring system, and a control circuit. The magnet system is configured to generate a polarizing magnetic field about all or part of a patient arranged in the MRI system, the gradient coils are configured to apply a gradient field to the polarizing magnetic field, and the RF system is configured to apply an excitation field to the subject and acquire resulting MR image data. The patient monitoring system is arranged to acquire a physiological condition of the patient and to communicate signals representative of the physiological condition, such as ECG signals and/or EAM signals. The control circuit is configured to coordinate acquisition of the physiological condition signals during a scan with operation of either or both of the gradient coils and the RF system. 
         [0014]    It is another aspect of the invention to provide a patient monitoring system that includes at least one electrode configured to sense physiological information from a patient and an MRI scanner interface configured to receive signals from an MRI scanner concerning operation of at least one of a gradient system and an RF system of the MRI scanner The patient monitoring system also includes a control circuit connected to receive signals from the electrode(s) and to regulate use of the output of the electrode(s) during an MRI scan. In the system, regulation by the control circuit is controlled according to the signals received by the MRI scanner interface. 
         [0015]    It is a further aspect of the invention to provide a method for acquiring patient physiological information during an MRI scan. The method includes the steps of receiving signals concerning operation of a gradient system and/or an RF system of an MRI scanner during a scan sequence, receiving patient monitoring signals acquired by a physiological monitoring device at least partially arranged inside the MRI scanner during the scan sequence, and coordinating operation of the physiological monitoring device based on the signals concerning operation of the at least one of a gradient system and an RF system of an MRI scanner, to at least one of impede receipt of the patient monitoring signals during a portion of the scan sequence and disregard patient monitoring signals during the portion of the scan sequence. 
         [0016]    The foregoing and other aspects and advantages of the invention will appear from the following description. In the description, reference is made to the accompanying drawings which form a part hereof, and in which there is shown by way of illustration a preferred embodiment of the invention. Such embodiment does not necessarily represent the full scope of the invention, however, and reference is made therefore to the claims and herein for interpreting the scope of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]      FIG. 1  is a block diagram of an exemplary magnetic resonance imaging (“MRI”) system in which ECG/EAM signal gating control and processing circuits are integrated with the MRI system in accordance with the present invention; 
           [0018]      FIG. 2  is a block diagram of an exemplary magnetic resonance imaging (“MRI”) system employed in combination with an exemplary ECG/EAM system in accordance with the present invention; 
           [0019]      FIG. 3  is a block diagram of a representative ECG/EAM switching and processing circuit in accordance with the present invention; 
           [0020]      FIG. 4  is a chart depicting gradient waveforms from an exemplary MRI scan and exemplary ECG traces. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0021]    Referring particularly to  FIG. 1 , the present invention is employed in a magnetic resonance imaging (“MRI”) system. The MRI system includes a workstation  102  having a display  104  and a keyboard  106 . The workstation  102  includes a processor  108 , such as a commercially available programmable machine running a commercially available operating system. The workstation  102  provides the operator interface that enables scan prescriptions to be entered into the MRI system. The workstation  102  is coupled to four servers: a pulse sequence server  110 ; a data acquisition server  112 ; a data processing server  114 , and a data store server  116 . The workstation  102  and each server  110 ,  112 ,  114  and  116  are connected to communicate with each other. 
         [0022]    The pulse sequence server  110  functions in response to instructions downloaded from the workstation  102  to operate a gradient system  118  and a radio frequency (“RE”) system  120 . Gradient waveforms necessary to perform the prescribed scan are produced and applied to the gradient system  118 , which excites gradient coils in a gradient coil assembly  122  to produce the magnetic field gradients G x , G y , and G z  used for position encoding MR signals. The gradient coil assembly  122  forms a part of a magnet assembly  124  that includes a polarizing magnet  126  and a whole-body RF coil  128 . 
         [0023]    RF excitation waveforms are applied to the RF coil  128 , or a separate local coil (not shown in  FIG. 1 ), by the RF system  120  to perform the prescribed magnetic resonance pulse sequence. Responsive MR signals detected by the RE coil  128 , or a separate local coil (not shown in  FIG. 1 ), are received by the RF system  120 , amplified, demodulated, filtered, and digitized under direction of commands produced by the pulse sequence server  110 . The RF system  120  includes an RF transmitter for producing a wide variety of RF pulses used in MR pulse sequences. The RF transmitter is responsive to the scan prescription and direction from the pulse sequence server  110  to produce RF pulses of the desired frequency, phase, and pulse amplitude waveform. The generated RF pulses may be applied to the whole body RF coil  128  or to one or more local coils or coil arrays (not shown in  FIG. 1 ). 
         [0024]    The RF system  120  also includes one or more RF receiver channels. Each RF receiver channel includes an RF amplifier that amplifies the MR signal received by the coil  128  to which it is connected, and a detector that detects and digitizes the I and Q quadrature components of the received MR signal. The magnitude of the received MR signal may thus be determined at any sampled point by the square root of the sum of the squares of the I and Q components: 
         [0000]        M =√{square root over (I 2   +Q   2 )}  Eqn. (1);
 
         [0025]    and the phase of the received MR signal may also be determined: 
         [0000]    
       
         
           
             
               
                 
                   φ 
                   = 
                   
                     
                       
                         tan 
                         
                           - 
                           1 
                         
                       
                        
                       
                         ( 
                         
                           Q 
                           I 
                         
                         ) 
                       
                     
                     . 
                   
                 
               
               
                 
                   Eqn 
                   . 
                   
                       
                   
                    
                   
                     ( 
                     2 
                     ) 
                   
                 
               
             
           
         
       
     
         [0026]    The pulse sequence server  110  also connects to a scan room interface circuit  132  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  132  that a patient positioning system  134  receives commands to move the patient to desired positions during the scan. 
         [0027]    The digitized MR signal samples produced by the RF system  120  are received by the data acquisition server  112 . The data acquisition server  112  operates in response to instructions downloaded from the workstation  102  to receive the real-time MR data and provide buffer storage, such that no data is lost by data overrun. In some scans, the data acquisition server  112  does little more than pass the acquired MR data to the data processor server  114 . However, in scans that require information derived from acquired MR data to control the further performance of the scan, the data acquisition server  112  is programmed to produce such information and convey it to the pulse sequence server  110 . For example, during prescans, MR data is acquired and used to calibrate the pulse sequence performed by the pulse sequence server  110 . Also, navigator signals may be acquired during a scan and used to adjust the operating parameters of the RF system  120  or the gradient system  118 , or to control the view order in which k-space is sampled. The data acquisition server  112  may also be employed to process MR signals used to detect the arrival of contrast agent in a magnetic resonance angiography (“MRA”) scan. In all these examples, the data acquisition server  112  acquires MR data and processes it in real-time to produce information that is used to control the scan. 
         [0028]    The data processing server  114  receives MR data from the data acquisition server  112  and processes it in accordance with instructions downloaded from the workstation  102 . Such processing may include, for example: Fourier transformation of raw k-space MR data to produce two or three-dimensional images; the application of filters to a reconstructed image; the performance of a back projection image reconstruction of acquired MR data; the generation of functional MR images; and the calculation of motion or flow images. 
         [0029]    Images reconstructed by the data processing server  114  are conveyed back to the workstation  102  where they are stored. Real-time images are stored in a data base memory cache (not shown in  FIG. 1 ), from which they may be output to operator display  104  or a display  136  that is located near the magnet assembly  124  for use by attending physicians. Batch mode images or selected real time images are stored in a host database on disc storage  138 . When such images have been reconstructed and transferred to storage, the data processing server  114  notifies the data store server  116  on the workstation  102 . The workstation  102  may be used by an operator to archive the images, produce films, or send the images via a network to other facilities. 
         [0030]    In some scans, the pulse sequence server  110  may optionally receive physiological patient monitoring signals from a number of different sensors connected to the patient, such as ECG, EEG, or EAM signals from electrodes and associated interventional catheters, pulse oximeter signals, signals from a blood pressure cuff, or respiratory signals from a bellows or other respiratory monitoring device. In the embodiment depicted in  FIG. 1 , ECG, EEG, or EAM signals are acquired from a patient inside the MRI scanner during a scan and are transmitted via an ECG/EAM control circuit  135  to an ECG/EAM signal receiver or processor  130 . Preferably, the ECG or EAM signals are conducted using low-noise cables, to reduce interference inside the scan room. The processor  130  conditions, filters, and/or processes the raw signals and ultimately transmits ECG and/or EAM information to the workstation  102  and/or the pulse sequence server  110 . Alternatively, the functionality of the processor  130  may be integrated with the workstation  102  or other component of the MRI system, or may include a commercial ECG or EAM receiving and processing system such as, for example, a Cardiolab-IT Electrophysiology Recording System offered by GE Healthcare or the NavX® system offered by St. Jude Medical, Inc. 
         [0031]    The signal gating control circuit  135  may be responsive to a variety of control signals to coordinate ECG and EAM acquisition with operation of gradient system  118  and RF system  120 . As discussed above, the ramping up and down of the gradient fields as well as certain RF transmissions can seriously degrade the quality of output from ECG and EAM electrodes, as well as other physiological monitoring devices. Thus, during periods of an imaging sequence that cause induced voltages and noise in the output of the ECG and EAM electrodes (such as gradient ramping and RF transmission), output from the ECG and EAM electrodes can be disconnected or discarded by control circuit  135  so as to limit the effect of the unwanted noise on patient monitoring and MRI image acquisition. For example, in the embodiment shown, control circuit  135  is responsive to gating signals from the RF system  120  and a gradient ramp detection circuit  137  integrated with the gradient system  118 . These gating signals may be, for example, TTL (transistor-transistor logic) signals used to cause the control circuit  135  to alternately disconnect the output of the electrodes from the processor  130  or connect the output of the electrodes to the processor  130 . 
         [0032]    Accordingly, signal gating control circuit  135  may comprise a switch that alternately opens to disconnect or impede output of the ECG or EAM electrodes and closes to communicate output of the electrodes to processor  130 , in according with control signals from the gradient system  118  and RF system  120 . Such a switch may include solid-state, fast-response electronics to ensure a rapid disconnection and reconnection of the real-time ECG or EAM acquisition during and after the gradient ramps. Desirable switching delay times are less than 200 nsec, although it is understood that other switching times may be acceptable. In one embodiment, the switch could comprise a single pole double throw electronic switch. Thus, as depicted in  FIG. 1 , control circuit  135  may include a switch  139  connected between the patient electrodes and the processor  130 . 
         [0033]    Alternatively, as will also be discussed below with respect to  FIG. 2 , control circuit  135  may include a system for discarding or ignoring signals output by the electrodes (or other physiological monitoring tool) during periods of induced noise (such as during gradient ramping or RF transmission). In such an implementation, software algorithms may be used that analyze control signals from the gradient system  118  and RF system  120  as timing signals to blank, discard, or ignore ECG or EAM acquisition during periods of induced noise. Such software may be executed on processor  130 , or the functionality of control circuit  135  and processor  130  may both be integrated with workstation  104  or other computational component of the MRI system. Alternatively, control circuit  135  may comprise a software algorithm that is implemented by a digital signal processing circuit for removing output from the electrodes during periods of induced noise from the signal transmission stream to the processor  130 . In other words, control circuit  135  may include a switch, circuit, chip, or software algorithm for preventing use of signals output by patient physiological condition sensors during periods of noise induced by operation of the gradient system  118  and/or RF system  120  during an MRI scan. 
         [0034]    The gradient ramp detection circuit  137 , which provides a control signal to control circuit  135 , may be integrated with the gradient system  118  by being installed in the gradient cabinet as a separate circuit or may be included as part of the functionality of the gradient system  118  itself. In an alternative configuration, the gradient ramp detection circuit  137  may include a pickup coil positioned within the bore of the magnet assembly  124 . The ramp detection circuit  137  is connected to a real-time output of the gradient system  118 , so that the detection circuit  137  can monitor application of gradient fields during a scan sequence and output a signal to the control circuit  135  causing it to interrupt or disconnect output of the patient monitoring electrodes whenever the gradient field is being ramped up or ramped down. As discussed above, it is the ramping stages of gradient field application which can cause induced voltages in ECG and EAM electrodes. Thus, during steady state “on” and fully “off” stages of gradient field application, the gradient ramp detection circuit  137  outputs a signal to the control circuit  135  causing it to connect or communicate the output of the patient monitoring electrodes to the processor  130 . 
         [0035]    Likewise, the RF system  120  may also be connected to, or include, a detection circuit (not shown), but control of signal gating control circuit  135  according to operation of the RF system may simply be achieved using existing output lines of the RF system. For example, in many MRI systems, an unblank signal is output by the RF amplifier of the RF system  120  just prior to and/or during RF transmission which could also be used for control of the control circuit  135 . 
         [0036]    Alternatively, the signal gating control circuit  135  may be responsive to signals from the scan prescription workstation  102 , or the pulse sequence server  110  that indicate when the gradient system will be ramping up or down and/or when the RF system will be transmitting. The control circuit  135  may also be responsive to the presence of other noise-causing signals that are related to or accompany the operation of the gradient and/or RF system. For example, in certain GE MRI scanners, a 1000V signal is sent to the RF body coil to reverse bias the diodes on it, prior to the actual RF signal being sent from the RF amplifier. This 1000V signal can also create substantial noise within the bore of the magnet, such as in ECG and EAM electrodes, so the control circuit  135  can also disconnect output from such electrodes during application of the 1000V signal. In addition, the control circuit  135  may also be responsive to a simple user override. Furthermore, as stated above, the control circuit  135  may also gate the output of other physiological monitoring tools, such as pulse oximeters, blood pressure cuffs, respiratory monitors, and the like, which may also experience induced voltages and noise from gradient and RF operation during an MR scan. 
         [0037]    Once the gated output of the ECG or EAM electrodes reaches the processor  130 , the processor  130  then communicates the gated ECG or EAM information to the pulse sequence server  110  and/or workstation display  104 . Thus, the gated information can be used for displaying an ECG trace or an EAM image, as will be described in more detail below, and for MR image acquisition. That is, for cardiac-gated scans, pulse sequence server  110  can synchronize, or “gate,” the performance of the scan with the subject&#39;s heart beat or respiration. In other embodiments, the control circuit  135  may communicate ECG signals directly to the pulse sequence server  110  instead of, or in addition to, communication to the processor  130 . 
         [0038]    Referring now to  FIG. 2 , an additional embodiment is shown in which a separate, stand-alone ECG or EAM system  140  is employed in conjunction with a standard MRI system. The stand-alone system  140  may include, for example, a portable ECG unit offered by such companies as GE Healthcare, Siemens Healthcare, Philips Healthcare, Cardiac Science, Medrad, and Invivo. Using this arrangement, patient monitoring can seamlessly continue (1) outside of, and apart from, the MRI system, (2) inside the MRI system when no scanning is taking place, and (3) inside the MRI system during an MRI scanning sequence. In this embodiment, the gradient ramp detection circuit, the ECG/EAM signal gating control circuit, and the ECG/EAM receiver or processor are all integrated into the stand-alone system  140 . 
         [0039]    During operation, the ECG/EAM system  140  is connected via an MRI scanner interface to output lines of the gradient system  118  and the RF system  120 , to allow the internal gradient ramp detection circuit and signal gating control circuit to prevent or disregard ECG/EAM acquisition at the proper times. Thus, within the stand-alone ECG or EAM system  140 , a chip or circuit may be included for detecting periods of gradient ramping in real time and sending control signals to a signal gating control switch or circuit that connects or disconnects ECG/EAM acquisition in coordination with the detected periods of gradient ramping. Likewise, a chip or circuit may be included for detecting periods of RF transmission, or receiving signals indicating that RF transmission is taking place or about to take place, in real time, and sending control signals to a signal gating control switch or circuit that connects or disconnects ECG/EAM acquisition in coordination with the periods of RF transmission. Alternatively, the ECG/EAM system  140  may include software that allows the system to receive input regarding operation of the gradient system  118  and RF system  120 , calculate periods of gradient ramping and/or RF transmission, then ignore output from the ECG/EAM electrodes during those periods. In either case, the MRI system may be adapted or retrofit to include simple, removable connection points for plug-in leads so that the signals regarding operation of the gradient system  118  and RF system  120  may be communicated to the ECG/EAM system  140 . Alternatively, the ECG/EAM system  140  may simply be connected to the pulse sequence server  110  or workstation  104  to receive information concerning the pulse sequence prescriptions for the gradient and RF systems. 
         [0040]    Referring now to  FIG. 3 , a functional block diagram is shown, which depicts the operation and interconnection of certain aspects of the above-described embodiments in greater detail. In  FIG. 3 , a plurality of electrodes  141 , for example, a set of ECG or EAM electrodes and/or associated catheters (not shown), are connected through a delay/filter  142  to one or more signal gating control circuits  144 , illustrated in  FIG. 3  as SPDT switches. The delay/filter  142 , which may include a low-pass filter can be used to delay signals from the electrodes  141  before arriving at the control circuits  144  by several tens of microseconds, for example, approximately 40 microseconds. This delay allows, for example, the illustrated SPDT switches to react fast enough, even if a triggering pulse arrives a little late. That is, triggering, such as the above described TTL output, is turned ON when a gradient in the X, Y, or Z direction is slewed to a new value. Without such a delay there is the chance of some short spike, for example 20 microseconds in duration, of noise may get through the SPDT switch before it cuts off transmission. In addition, the low-pass filter broadens somewhat any very-rapid spikes. It also works as an anti-aliasing filter for the A/D converters in ECG or EAM recording systems. Similarly to the gradient detection circuitry  146  and associated blanking period extension  149 , signals may be acquired from an MRI RF cabinet  150  through an associated blanking period extension  151 . Thus, these two blanking extensions  149 ,  151 , which may both be variable with independent control, provide greater control over the switching and allow for control over associated delays. 
         [0041]    For ECG-only systems, these lines will contain ECG signals, which generally are on the order of a few milli-volts at a few hundred Hz. For EAM systems, these lines may contain ECG signals as well as positional signals at a few kHz. 
         [0042]    As discussed above, single pole double throw (SPDT) switches may be used to gate the communication of the electrodes&#39; output. In the embodiment depicted in  FIG. 3 , the control circuits 144  are responsive to control signals from the MRI RF system  150  and MRI gradient system  148 . (However, as discussed above, a software or signal processing approach may be taken in lieu of, or even in combination with, a switch, to disregard physiological condition acquisition during periods of induced noise. In addition, a digital or analog “sample and hold” circuit may be used in a complement to the control circuits  144  in order to replace the disregarded signal, and supply the acquisition system, with data acquired immediately before the switch disconnects). The gradient detection circuit  146  is employed to monitor gradient waveforms output by the gradient system  148  in real time to determine periods when gradient fields in the X, Y, or Z direction are being ramped up or ramped down. The blanking period extension or blanking circuit  149  may be included. The blanking extension, which may be variable in duration, allows the TTL pulse reactions to be extended by a variable amount of time, for example, 0-6 milliseconds, if there is still some MRI noise that would otherwise go through the switch after it reopened. A circuit having an OR switch  152  may be used to perform an “OR” function, allowing the switches to be disconnected whenever gradient ramping is occurring OR when RF transmission is occurring. This “OR” switch  152  may be integrated with the control circuits 144  and/or both circuits may be integrated into a stand-alone ECG or EAM system. Also, the control circuits 144  may be formed by two or more SPDT circuits arranged in series, for example, to behave like an “OR,” but without the OR switch. As discussed above, however, an “OR” functionality is not necessarily required, since embodiments of the invention may gate electrode output according to only one noise-inducing MR function (e.g., gating according to only gradient ramping, according to only RF transmission, or according to only some other MR function like 1000V diode biasing signals). Note that the invention also embodies the possibility of placement of delay lines and/or signal filters before control circuits  144 . Such delay lines allow for removing components of the large voltages that may occur somewhat (several microseconds) before the OR switch  152  sends a command to the control circuits  144  to disconnect signal transmission, since these delay lines delay the arrival of the large signals until the switch is activated. 
         [0043]    Downstream of the control circuits 144 , the gated electrode outputs  154  are communicated to a receiver/processor  156  during periods of the MRI scan sequence at which no gradient fields are being ramped up or down (i.e., all X, Y, and Z gradients are either at a steady state “on” or are fully “off”) and/or no RF transmission is taking place. Thus, the control circuits 144  (or, in other embodiments, a signal gating software algorithm or digital signal processing circuit) prevent the receiver/processor  156  from using output of the ECG or EAM electrodes that was affected by noise caused by induced voltages. 
         [0044]    In an EAM system, as shown, the receiver/processor  156  includes a signal splitter  158  to split each gated electrode output  154 . One of the split signals for each gated electrode output  154  is then sent through a low-pass filter  160  to separate out the ECG component of the electrode output  154 , while the other split signal is sent through a band-pass filter  162  to separate out the EAM positional component of the electrode output  154 . In an ECG-only system, though a signal splitter  158  is not necessary, it may be still be desirable to use a low pass filter  160  to reduce stray noise, for example a minimum-phase low pass Butterworth filer. These filtering steps may be performed digitally in either a workstation or in the processing unit of an ECG/EAM unit (such as a GE Cardiolab unit). 
         [0045]    The receiver/processor  156  also includes, or is connected to, an ECG acquisition system  164  and/or an EAM system  166 . Because the electrodes&#39; output has been gated in coordination with operation of the gradient system  148  and RF system  150 , the output that is communicated to receiver/processor  156  and ECG acquisition system  164 ,  166  does not include gradient-induced noise. Accordingly, an ECG acquisition system  164  (such as a GE Healthcare Cardiolab-IT system) can sample only gradient and RF noise-free ECG signals, and display physiological monitoring-quality images of ECG traces. These images may be displayed on a screen of a stand-alone ECG unit, on the screen of an MRI system workstation, or both. Likewise, an EAM system  166  can sample only gradient and RF noise-free EAM positional signals, and can thus display full electrophysiological images without distortion or complication from gradient or RF noise. Correspondingly, because the output of the ECG/EAM electrodes is gated so as to reduce the effect of induced voltages, a full set of electrodes can be utilized in a normal monitoring arrangement, such as, for example, a standard 12-lead electrode arrangement. In other words, there is no longer a need to reduce the number of electrodes or the spacing of the electrodes to attempt to limit the amount of induced noise, because the periods of acquisition in which noise is induced by gradient ramping or RF transmission are gated or ignored. Also, a “sample and hold circuit” can be added to the system. This circuit may be a digital memory storage circuit that continuously acquires data into a circular buffer to feed prior-acquired data to the receiver/processor  156  during periods when signal transmission from the electrodes to is blocked. Alternatively, the “sample and hold circuit” can be comprised of analog components such as a capacitor placed between the physiologic signal and ground, and located between control circuit  144  and receiver/processor  156 . Advantageously, the positional and ECG data is constantly sent to the receivers/processors, although it may introduce some bias into the data. 
         [0046]    In addition to preventing induced voltages in electrode outputs from affecting image quality, the control circuits  144  can also be used for another purpose in EAM systems. During operation of an EAM system, some electrodes are used to transmit electric signals that are detected and sampled by electrodes in interventional catheters located inside a patient anatomy, such as a heart chamber. Because the transmission and reception of these electric signals can experience interference from the gradient and RF transmissions inside an MRI scanner, the control circuits 144  can also be used to prevent electrodes from transmitting during a gradient ramping period or an RF transmission period. In this case, the control circuits 144  would be gated according to the same control signals, and would still be connected to electrode leads, but the transmission of signals on the leads would be travelling in the opposite direction—to the patient from the receiver/processor  156 , rather than from the patient to the receiver/processor  156 . Likewise, preventing transmission of electric signals from EAM electrodes during certain portions of an MRI scan sequence also reduces the risk that the electric signals from the EAM electrodes could be picked up by the RF coil of the MRI scanner and affect image data acquisition of the MRI system. 
         [0047]    Referring now to  FIG. 4 , a set of graphs  4 A- 4 E of exemplary gradient waveforms and ECG signals is depicted to illustrate certain aspects of the invention. Graph  4 A shows a set of waveforms Gx, Gy, and Gz representing the timing and amplitude of gradient fields applied in the X, Y, and Z directions inside an MRI scanner during a scan sequence. As can be seen, the gradients are usually applied such that they have trapezoidal waveforms, with periods of ramping up  168  and periods of ramping down  170  between steady state “on” periods  172  and fully “off” periods  174 . The bottom line of graph  4 A depicts an exemplary “TTL” control signal which experiences a high level  176  whenever a gradient field in any direction is ramping up or ramping down, and a low level  178  at all other times. This control signal represents the output of a gradient ramping detection circuit, as described above, which is used to cause a signal gating switch to open (high levels  178 ) and close (low levels  178 ) according to the gradient ramping waveforms. 
         [0048]    Graph  4 B is an exemplary ECG signal acquired when no MRI scan sequence is taking place. Graph  4 C, in contrast, is an exemplary ECG signal acquired during an MRI scan, with no gating or other approach to compensate for induced voltages in the ECG electrodes. Graph  4 D is a close-up view of a segment of  FIG. 4C , with an exemplary TTL control signal (Graph  4 A) superimposed thereon. As can be seen, periods  180  when gradient ramping is taken place (as identified by high TTL levels “H”) cause false spikes  182  to be seen in the ECG signal. 
         [0049]    Graph  4 E is an exemplary ECG signal acquired during an MRI scan sequence, using the gradient gating aspects of the invention described above. As can be seen, especially in comparison to Graph  4 C, the ECG signal is far less affected by induced voltages, and is of a quality that can be easily used for normal physiological monitoring (e.g., the QRS points of the ECG trace can be accurately and easily determined). 
         [0050]    The present invention has been described in terms of one or more preferred embodiments, and it should be appreciated that many equivalents, alternatives, variations, and modifications, aside from those expressly stated, are possible and within the scope of the invention.