Artifact removal from an electric signal
The invention relates to a method and apparatus for suppressing interference in an electric signal, particularly for suppressing interference in an electrocardiogram (ECG) signal in connection with magnetic resonance imaging (MRI). In order to improve the accuracy of the suppression, the electric signal is first sampled at a high sampling frequency, whereby a first sequence of samples is obtained. Some of the samples in the first sequence of samples are then selected on the basis of predetermined criteria. The first sequence is then downsampled using the selected samples, whereby a second sequence of samples is obtained. The second sequence forms a digital presentation of the electric signal in which the interference is suppressed.
1. A method for suppressing interference in an electrical signal, the method comprising the steps of:
sampling the electrical signal at a first sampling frequency to obtain a first sequence of samples;
identifying some of the samples in the first sequence of samples on the basis of predetermined criteria; and
downsampling at least the identified samples of said first sequence to obtain a second sequence of samples from the identified samples of said first sequence, the second sequence of samples forming a digital presentation of the electrical signal in which the interference is suppressed.
2. A method according to claim 1 further comprising the step of generating information about the interference content in the samples and wherein the identification step is further defined as identifying samples in the first sequence of samples on the basis of their interference content.
3. A method according to claim 2 wherein the generating step is further defined as attaching a flag to each of the samples to indicate whether the sample is free of interference or not.
4. A method according to claim 3 wherein the downsampling is carried out using samples with a flag indicating that the corresponding sample is free of interference.
5. The method according to claim 4 further defined as providing all samples of said first sequence of samples for downsampling.
6. The method according to claim 2 wherein the identifying step is further defined as identifying samples that are artifact free.
7. The method according to claim 2 further defined as providing the identified samples for downsampling to form said second sequence of samples.
8. A method according to claim 1 further comprising the step of comparing the samples of the first sequence with a reference signal and wherein the identification step is performed based on the comparing step.
9. A method according to claim 8 further comprising the step of forming the reference signal from the first sequence of samples by limiting a slew rate in the sequence.
10. A method according to claim 8 wherein said reference signal comprises an external reference signal.
11. A method according to claim 1 wherein the downsampling step is further defined as downsampling the identified samples.
12. The method according to claim 1 wherein a frequency of downsampling is less than said first sampling frequency.
13. A method according to claim 1 wherein the downsampling step includes low pass filtering of the first sequence of samples.
14. A method according to claim 1 further comprising the step of low pass filtering the second sequence of samples.
15. A method according to claim 1 further comprising the step of suppressing power supply hum in the first sequence of samples by updating values of the samples according to the amount of power supply hum.
16. A method according to claim 15 wherein the suppressing step includes the steps of:
estimating the power supply hum based on the first sequence of samples; and
deducting the estimated power supply hum from the first sequence to update the values of the samples.
17. A method according to claim 1 further defined as one for suppressing interference in an ECG electric signal.
18. An apparatus for suppressing interference in an electrical signal, the apparatus comprising:
sampling means for sampling the electrical signal at a first sampling frequency to obtain a first sequence of samples;
identification means for identifying some of the samples in the first sequence of samples on the basis of predetermined criteria; and
downsampling means operatively connected to said identification means for downsampling at least the identified samples of said first sequence to obtain a second sequence of samples from the identified samples of said first sequence and for forming a digital presentation of the electrical signal in which the interference is suppressed from said second sequence of signals.
19. An apparatus according to claim 18 wherein the identification means is further defined as identifying samples in the first sequence of samples on the basis of their interference content.
20. An apparatus according to claim 19 wherein the identification means is further defined as identifying samples that are artifact free.
21. An apparatus according to claim 18 wherein said identification means further comprises means for comparing the samples of the first sequence with a reference signal so that the identification is based on the comparison.
22. An apparatus according to claim 18 wherein the downsampling means is further defined as downsampling the identified samples.
23. An apparatus according to claim 18 wherein said downsampling means is further defined as carrying out said downsampling at a frequency of downsampling that is less than said first sampling frequency.
24. An apparatus according to claim 18 further comprising:
means for estimating the power supply hum based on the first sequence of samples; and
means for updating the samples of the first sequence based on the estimated amount of power supply hum.