Adaptive filtering of physiological signals in physiologically gated magnetic resonance imaging

• US 4,991,587 A
• Filed: 11/21/1989
• Issued: 02/12/1991
• Est. Priority Date: 08/09/1985
• Status: Expired due to Term

• Current Assignee: Picker International, Inc. (Koninklijke Philips N.V.)
• Original Assignee: Picker International, Inc. (Koninklijke Philips N.V.)
• Inventors: Rogers, James J., Blakeley, Douglas M.
• Primary Examiner(s): Smith, Ruth S.
• Application Number:
  US07/439,855
• Time in Patent Office: 448 Days
• Field of Search: 128/653 R, 128/653 A, 128/653 SC, 128/69 S, 128/696, 128/670, 128/671, 128/700, 128/708, 128/901, 324/318, 324/322
• US Class Current: 600/483
• CPC Class Codes: A61B 5/055 involving electronic [EMR] or nuclear [NMR] magnetic resonance, e.g. magnetic resonance imaging
• CPC Class Codes: A61B 5/1135 by monitoring thoracic expansion
• CPC Class Codes: A61B 5/352 Detecting R peaks, e.g. for synchronising diagnostic apparatus; Estimating R-R interval
• CPC Class Codes: A61B 5/7239 using differentiation including higher order derivatives
• CPC Class Codes: A61B 5/7285 for synchronising or triggering a physiological measurement or image acquisition with a physiological event or waveform, e.g. an ECG signal A61B6/5205, A61B6/541, A61B8/5207, A61B8/543 take precedence
• CPC Class Codes: A61B 6/541 involving acquisition triggered by a physiological signal
• CPC Class Codes: G01R 33/56 Image enhancement or correction, e.g. subtraction or averaging techniques , e.g. improvement of signal-to-noise ratio and resolution
• CPC Class Codes: G01R 33/5673 Gating or triggering based on a physiological signal other than an MR signal, e.g. ECG gating or motion monitoring using optical systems for monitoring the motion of a fiducial marker
• CPC Class Codes: Y10S 128/901 Suppression of noise in electric signal

Abstract

A patient (B) is disposed in a region of interest of a magnetic resonance apparatus (A). During an imaging sequence, changing magnetic field gradients and radio frequency pulses are applied to the region of interest. The changing magnetic field gradients induce a corresponding electrical response in the patient. Electrodes (40) of a cardiac monitor (C) sense the electrocardiographic signal of the patient as well as the electrical response to the magnetic field gradient changes and produces an output signal having a cardiac component and a noise component. The bandwidth of the noise component varies in accordance with the changes of the magnetic field gradients. An adaptive filter (80) filters the output signal to remove the changing magnetic field gradient induced noise. The bandwidth of the filter function with which the output signal is filtered is varied or adjusted in accordance with the magnetic field gradient changes. The magnetic field gradient changes are monitored (84), their rate of change determined (88), and the bandwidth of the filter function is adjusted (92) in accordance with the rate of change. Alternately, the magnetic field gradient changes are determined a priori from a knowledge of the imaging sequence. As the imaging sequence progresses, a look-up table (150) is addressed to retrieve preprogrammed filter function information for various views or stages of the imaging sequence.

Claims

1. A non-invasive diagnostic apparatus comprising:

a means for non-invasively examining an internal region of a subject and generating diagnostic signals indicative thereof;

a monitoring means for monitoring a condition of the subject generating output signals indicative thereof, the non-invasive examination means causing a noise signal component that changes with changing examination parameters during the non-invasive examination, the noise component being superimposed on the output signal such that the output signal includes both noise and monitored condition information;

means for detecting the changing examination parameters

an adaptive filter means for filtering the output signal with a filter that is adjusted in accordance with the detected changing examination parameters.

2. The apparatus as set forth in claim 1 wherein the noise component has a bandwidth that changes with the changing examination parameters and further including a filter changing means for changing a bandwidth of the filter in accordance with the noise component bandwidth.

3. A non-invasive diagnostic apparatus comprising:

means for subjecting a region of interest of a patient to changing magnetic fields and radio frequency signals in order to induce magnetic resonance of selected nuclei in the region of interest;

a cardiac monitoring means operatively connected with the patient for monitoring electrocardiographic signals, the changing magnetic fields inducing noise signals which are superimposed on the electrocardiographic signals, such that an output signal of the cardiac monitoring means includes both noise and cardiac cycle information;

means for detecting the changing magnetic fields;

an adaptive filter means for filtering the output signal with a filter function that is adjusted in accordance with the detected changing magnetic fields, such that the filter function is adaptively changed with variations in the changing magnetic fields;

a receiving means for receiving magnetic resonance signals from the region of interest;

a sequence control means for controlling the application of magnetic field gradients and radio frequency pulses and processing received magnetic resonance signals into diagnostic information, the control means being operatively connected with the filter means to control at least one of diagnostic information processing and magnetic field gradient and radio frequency pulse application in accordance with the adaptively filtered cardiac information received from the adaptive filter means.

4. The apparatus as set forth in claim 3 wherein the means for detecting the magnetic fields includes:

means for monitoring magnetic field gradients applied to the region of interest; and

further including

means for determining a noise bandwidth corresponding to the changing gradients; and

means for adjusting the filter function in accordance with the corresponding noise bandwidth.

5. The apparatus as set forth in claim 3 wherein the sequence control means causes the means for subjecting a region of interest of a patient to changing magnetic fields to apply a preselected sequence of gradient pulses across image region and wherein the adaptive filter means includes a lookup table means which is addressed by the selected gradient pulses of each sequence to designate a corresponding filter function and a filter function source for supplying the filter function designated by the look-up table to the filter means for filtering the output signal.

6. The apparatus as set forth in claim 3 wherein the cardiac signal monitoring means is disposed within the changing magnetic fields closely adjacent the region of interest, the cardiac monitor means including an optic transmitter for converting the output signal to an optical signal which is transmitted to an optical receiving means disposed remote from the imaging region, the optical receiving means converting the optical signal back to an electrical signal, the optical receiving means being operatively connected with the adaptive filter means.

7. The apparatus as set forth in claim 6 further including:

a second anatomical condition monitoring means which generates an second anatomical condition signal;

a combining means for combining the cardiac cycle information with superimposed noise and the second anatomical condition signal, the combining means being operatively connected with the optic transmitter means such that the second anatomical condition information, cardiac cyclic information, and noise are combined to produce the optical signal;

a separating means operatively connected with the optical receiving means for separating a cardiac and noise component and a second anatomical condition component from the receiving means electrical signal.

8. A magnetic resonance imaging system for generating a magnetic resonance image representation of a selected region of interest of a subject, the system comprising:

a magnetic resonance apparatus including;

a main magnetic field means for creating a substantially uniform main magnetic field through the region of interest;

a gradient field means for causing magnetic field gradients across the region of interest;

a radio frequency transmitter for transmitting radio frequency signals into the region of interest for exciting magnetic resonance;

a receiving means for receiving magnetic resonance signals emanating from the region of interest;

a reconstruction means for reconstructing an image representation from the received magnetic resonance signals;

an anatomical condition monitoring means for monitoring an anatomical condition and producing an output signal that includes a monitored anatomical condition signal component and a noise component which noise component is attributable to the magnetic field gradients caused across the image region;

a filtering means for filtering the output signal with a selectable filter function for filtering the noise signal component from the anatomical condition signal component;

a gradient monitoring means operatively connected with a gradient field means for providing a gradient signal that varies in accordance with changes in the magnetic field gradients across the region of interest, the changes in the magnetic field gradients being indicative of a bandwidth of the noise component;

a filter function changing means in accordance with the gradient signal.

9. The system as set forth in claim 8 wherein the filter function of the filtering means includes a band pass filter function that is adjusted in accordance with the bandwidth of the noise component.

10. The system as set forth in claim 9 wherein the gradient signal addresses a preprogrammed look-up table to select a filter function corresponding to the applied magnetic field gradients, and a filter function source means for supplying the selected filter function to the filter means.

11. A method of non-invasive medical investigation comprising:

subjecting a region of interest of a subject to a main magnetic field, magnetic field gradients across the region of interest, and radio frequency signals to excite selected dipoles within the region of interest to resonance;

monitoring an anatomical condition of the patient and producing an output signal which includes an anatomical condition component and a noise component whose bandwidth varies in accordance with the applied magnetic field gradients;

detecting changes in the applied field gradients;

filtering the output signal with a filter function whose bandwidth varies in accordance with the detected changes to filter the noise component and pass the anatomical condition component;

monitoring magnetic resonance signals emanating from the resonating dipoles in the region of interest.

12. The method as set forth in claim 11 further including reconstructing an image representation from the magnetic resonance signals received from excited dipoles in the region of interest.

13. The method as set forth in claim 12 further including controlling at least one of the application of magnetic field gradients and RF signals and processing of magnetic resonance signals in accordance with the anatomical condition component.

14. The method as set forth in claim 13 wherein the anatomical condition monitoring step includes electrically sensing electrocardiographic signals from the subject and noise signals induced in the subject by changing the magnetic field gradients.