WO2004097444A2

WO2004097444A2 - Processing of ecg signals in a mri apparatus

Info

Publication number: WO2004097444A2
Application number: PCT/GB2004/001829
Authority: WO
Inventors: Paul Joseph Cassidy
Original assignee: Isis Innovation Ltd
Priority date: 2003-04-29
Filing date: 2004-04-28
Publication date: 2004-11-11
Also published as: WO2004097444A3

Abstract

The invention relates to a method of signal processing in a system (1) in which a varying electromagnetic field is applied to a living being. In the method and system, physiological signals of the being are captured, and these signals are monitored for cross-talk due to variations in the electromagnetic field. In the event that cross-talk is sensed, the captured signal is blanked for a predetermined period of time.

Description

SIGNAL PROCESSING

The present invention relates to a method of signal processing in a system, a method of reducing movement artefacts in an imaging system, a method of imaging an object subject to movement, a signal processing device and a software product.

A number of situations occur in which it is necessary to acquire data from physiological signals of a living being which is being subjected to a varying electromagnetic field. Including among such situations are those were the living being is subjected to an imaging field, and where the physiological signal is used to determine events such as respiration or cardiac activity of the living being so that measures can be taken to reduce artefacts due to the event.

Where a high electrical, magnetic or similar field is applied to a living body, the field itself or the variation of the field may give rise to cross talk with the physiological signal.

Embodiments of the invention allow for cross-talk effects to be minimised.

Embodiments of the invention are also advantageous where signal processing is to be used to cope with the effects of movement in a living being.

According to a first aspect of the present invention there is provided a method of signal processing in a system in which a varying electromagnetic field is applied to a living being, and physiological signals of the being are captured, the method comprising monitoring the physiological signals for cross-talk due to variations in the electromagnetic field, and blanking, for a predetermined period of time, the captured signal when cross-talk is sensed.

The method may further comprise blanking the physiological signals for a user- controlled period of time.

The electromagnetic field may comprise a gradient field of the MRI system and the physiological signals comprise an ECG signal and/or a respiratory signal.

In a second aspect the invention relates to a method of reducing of movement artefacts in an imaging system in which signals are applied to a living being to produce a response, and the response is analysed to provide information representative of parameter of the being, the method comprising: deriving data indicative of movement of the being; analysing the said data to determine a first time window in which it is predicted that a forthcoming movement is expected to occur and a second time window in which it is predicted that movement is not expected to occur; applying the said signals to the being during the first and second windows; and analysing the response only to signals applied during the second window.

The imaging system may comprise a MRI system, wherein the signals applied to the being comprise a gradient field of the MRI system.

The data indicative of the movement of the being may be derived from at least one of the ECG signal and a respiratory signal.

Where the movement is respiratory, the data indicative of respiratory movement of the being may be derived from a movement sensor.

The movement sensor may comprise an induction loop.

The method may further comprise providing a user-defined sub-window of said second time window and analysing said signals only during said user defined sub-window.

In a third aspect, the invention relates to a method of imaging an object subject to movement using a device applying a varying electromagnetic field to the object to provide an imaging signal, in which an electromagnetic signal from the object is captured, and in which the movement is at least quasi-periodic and is related to a parameter of the electromagnetic signal, the method comprising monitoring the electromagnetic signal for the appearance of variations corresponding to the variation of the electromagnetic field; blanking the electromagnetic signal for a period following the occurrence of a variation corresponding to a variation of the varying electromagnetic field; deriving data indicative of said movement from the remaining electromagnetic signal; using the time occurrence of said data to adaptively form a time window in which said movement is predicted to occur; and during the time window disregarding said response.

In a fourth aspect, the invention relates to a signal processing device for a MRI system, the system being operable to apply a high field gradient signal and to derive an image signal, the signal processing device having a first input for an ECG signal, a second input for the image signal, wherein the device comprises discrimination circuitry for deriving from an ECG signal at said first input a respiration signal representing the occurrence of respiration; timing circuitry for predicting, based upon the occurrence of previous respiration signals, the occurrence instant of a next respiration signal, and gating circuitry connected to said second input and responsive to an output of said timing circuitry for gating out said image signal for a time window including said occurrence instant.

The device may further comprise processing circuitry connected to said first input for providing a control signal if the content of a signal at the first input has a component at the gradient frequency which exceeds a given threshold; and transmission circuitry having an input, a control node and an output, the input being connected to the first input, the control node being responsive to the control signal and the output being coupled to an input of the discrimination circuitry, the transmission circuitry being operable to pass a signal at the first input to the discrimination circuitry input in the absence of said control signal and to block said signal from said discrimination circuitry input in the presence of said control signal

The device may further comprise filter circuitry connected to the first input and having an output to the transmission circuitry.

The filter circuitry may have a first wide band filter path and a second narrow band filter path disposed substantially parallel to the first narrow band filter path, and the discriminator circuitry may comprise a first discriminator having an input connected to receive an output of said narrow band filter path and having an output for a logic signal indicative of said occurrence of respiration, and a second discriminator having an input connected to receive an output of said wide band filter path and having an output for a logic signal indicative of said occurrence of respiration.

There may be a third input for a motion sensor signal, and a selector connected to an input of the narrow band filter path, the selector being operable to select either said first input or said third input for application to said narrow band filter path.

The device may have logic circuitry receiving the logic outputs of the first and second discriminators and having a logic circuitry output node connected to the timing circuitry.

The logic circuitry may be operable to provide, at said logic circuitry output, an AND function of the outputs of the first and second discriminators.

The logic circuitry may be operable to provide, at said logic circuitry output, an OR function on the outputs of the first and second discriminators.

In a yet further aspect, the invention provides a signal processing device for an

MRI system of the fourth aspect, further comprising a pulse shaping circuit connected to receive a signal from said first input and to derive from an ECG signal applied to said first input a pulse train indicative of predetermined ECG events.

The timing circuitry may comprise counter circuitry having a clock input.

The counter circuitry may comprise an up counter and a down counter. The timing circuitry may comprise counter circuitry having a clock input, said counter circuitry comprising an up counter and a down counter and further comprising a pulse shaping circuit connected to receive a signal from said first input and having an output, said pulse shaping circuit being operable to derive from an ECG signal applied to said first input a pulse train indicative of predetermined ECG events, wherein said pulse train output is connected to said clock input of said counter circuitry.

In a final aspect the invention relates to a computer program product stored on a computer useable medium comprising readable program means for causing a computer to perform the method of any of the first to third aspects when the product is run on a computer.

Embodiments of the invention will now be described, by way of example only, with reference to the drawings in which

Figure 1 shows a schematic drawing of a small animal MRI system embodying this invention;

Figure 2 shows a block diagram for a signal processing device embodying the invention as shown in Figure 1 ; Figure 3 shows a processing circuit for ECG signals;

Figure 4 shows a dual counter circuit useable with Figure 2, and including an adaptive control override;

Figure 5a shows a timing diagram for the counter circuit of Figure 4;

Figure 5b shows a timing diagram for the complete circuit of Figure 4; Figure 6 shows a second dual counter circuit useable with Figure 2, and without an adaptive counter override; and

Figure 7 shows a timing diagram for Figure 6. In the various figures like reference signs refer to like parts.

Referring first to Figure 1, a small animal MRI imaging system 1 comprises an enclosure (2) for a small animal. The enclosure has electrodes (3a, 3b) for application to the animal, the electrodes being connected to an ECG signal line pair (3). The enclosure has a respiration sensing loop (4a) connected to a pair of respiratory loop signal lines (4). Respiration lines (4) and ECG lines (3) form inputs to a signal processing device (30) which will be more fully described herein with respect to Figure 2. Signal processing device (30) receives inputs (21, 22) from a sensing coil (20) which may be the gradient coil. A gradient signal generator (10) is connected to gradient coils (11) which, in use, apply gradient signals to a small animal disposed in the enclosure (2). Other components, such as RF coils, are omitted for simplicity from the figure. Output terminals (23, 24) of the signal processing device (30) convey output signals to a computer (21) for image formation. Other connections and inputs may be provided: specifically there may be a path connecting the gradient generator (10) to the computer (21) and to the signal processing device (30) if required.

Turning to Figure 2, a signal processing device (30) receives the ECG signal input (3) at a first pair of input nodes (31a, 31b) and the respiratory loop signals a second pair of input nodes (35a, 35b). The ECG signals are converted from differential to single-ended by a first converter (131) and the respiration signals by a second converter (135). The single-ended output (132) of the first converter (131) is connected to the input of a first discriminator circuit (32) having a first discriminator output (33). The single-ended ECG signal (132) is also connected to a first broadband filter (34) having a first filter output (36). The respiratory loop single-ended signal (136) at the output of the second converter (135) and the single-ended ECG signal (132) are fed as inputs of a selector circuit (37) having a selector output signal (38) which consists of one of the single-ended respiratory loop and the single-ended ECG signals. The selector output (38) is fed to a narrow band filter circuit (39) having a narrow band filter output (40). The first filter output (36) and the narrow band filter output (40) are fed to respective transmission gate circuits (41) and (42) which, when closed pass their signals respectively to first node (43) and second node (44). The transmission gates (41) and (42) are opened either by the output (33) of the discriminator circuit (32) or by a user-defined pulse at an input (45). The first node (43) is connected as the input to two circuits, the first of which is a threshold and pulse shaping circuit (46) having a threshold and pulse shaping circuit output (47), whose signal rises synchronously with rises in the ECG signal. This ECG synchronised signal is used to clock a counter circuit (50). The first node (43) is also supplied to a second discriminator circuit (48) having a second filter output (49). The second node (44) is connected to a third discriminator circuit (51) having a third discriminator output (52). The second filter output (49) and third discriminator output (52) are connected via a logic circuit (53) to the counter circuit (50) at the input (54). The output (55) of the counter circuit (50) is connected to a processing circuit (56) which is shown for simplicity as a gating circuit operating on the sensed signal (21, 22) from the coil (20). The output (56) of the processing circuitry is fed to output node (23, 24) of the detecting circuit.

In use the single-ended ECG signal derived at line (132) is processed by discriminator circuitry (32) to provide a gating output signal at the first discriminator output (33) to open the transmission gate path, to prevent transmission if the AC content of the ECG signal is above a predetermined threshold. This threshold is selected such that when the gradient content of the ECG signal is high enough to interfere with the operation of the device, a gating output is provided. The gating output signal is a pulse of predetermined length, produced for example from a monostable vibrator. The single-ended ECG signal (132) is also fed through the first broadband filter circuitry (34), which includes a mains frequency notch filter, to the transmission gate (41) which is opened by the gating output signal on (33) for a predetermined period of time if the gradient content of the ECG signal is too high. Otherwise the gate is closed so providing the filtered signal to the first node (43). A user-defined input at node (45) can cause the gate (41) to be opened for a time determined by the user if necessary. The signal at the first node (43) is fed via the pulse shaping circuit (46) to provide an ECG-synchronous pulse of level switchable for clocking the counter circuit (50). It is also fed via the second discriminator circuit (48) which is configured to provide a pulse output indicative of the onset of a signal at nodes (3 la, 3 lb) that represent a respiration event.

The selector (37) allows the signal at node (38) to be either the ECG signal (132) or the signal (136) provided by the respiration-sensing loop. The signal selected by selector (37) is narrow-band filtered, and fed to the second transmission gate (42), opened in concert with the first gate (41). If the gate remains closed, the filtered signal (40) is fed via the second node (44) to third discriminator (51) which provides an output pulse when input signal representing a respiration event are received. Thus the device (30) provides at the logic circuit input (53) either a respiration signal derived via narrow band filtering of an ECG signal (via filter circuitry (39)) and a respiration signal derived from wide band filtering of the ECG signal (via wide band filter (34)), or a respiration signal derived from a wide band filtered ECG signal and a respiration signal derived from a narrow band filtered respiration loop signal.

The logic circuit (53) allows either an OR function or an AND function of these inputs. Its output is used to load dual counter circuit (50). For adaptive control, one counter counts down from a loaded value while the other counts up. Determination of the count operation occurs at the next respiration signal and thus the counts in the counters represent respectively the time between the last two respiration signals and the time since the last respiration signal has occurred. These values are used to set two windows, in the present embodiment a first fixed window having a given number of ECG pulses timed to overlap with a respiration event and a second window corresponding the next expected gap between the respiration events.

Turning now to Figure 3, the blanking signal generator (32) receives an input, here ECG input 132. The ECG signal is passed to a differentiator (200) having an output (210) connected as input to an absolute value circuit (201) which acts as a rectifier. The absolute value circuit (201) has an output (211) connected to a threshold comparator (202). In the present embodiment which is designed to operate at TTL levels of 0 volts and 5 volts the threshold comparator (202) is set to trigger at an input level of around 200 milivolts.

The output (212) of the threshold comparator (202) is connected as input to a monostable multi vibrator (203). The output of the monostable multi vibrator (203) provides the blanking pulse output (33).

In use the ECG signal is differentiated by the differentiator (200) which produces output signal (210) dependent on the AC content of the ECG signal. The absolute value circuit (201) receives this signal and provides as output a level depending on the magnitude of the signal (210). If the magnitude signal (211) exceeds 200 milivolts then the threshold comparator (202) has an output transition which triggers the monostable multi vibrator (203) to provide a blanking pulse at output (33).

The blanking system is either under internal or external control depending on the choice of the user. For internal control the duration of the blanking pulse is determined by a monostable multi vibrator and may be, for example, around 5ms. Once the blanking time has elapsed, re-blanking can occur almost immediately but not contiguously. For external control, the direct duration of the blanking pulse is determined by the user. Again once the blanking time has elapsed, re-blanking can occur almost immediately but not contiguously. However, the blanking time cannot exceed approximately 500ms. This time limit is performed, for example, by a second monostable multi vibrator which acts the time out the external control after 500ms. Thus inappropriate or erroneous blanking durations are avoided.

Referring now to Figure 4, an adaptive control circuit (50) contains an up counter (101) and a down counter (102). The up counter has a load input (201) and a clock input (202). The clock input is provided by the ECG signal (136) and the load input is responsive to the respiration signal (136) via a delay (203). The up counter (101) also has a data input (204) which provides all zeros for loading into the up counter. The down counter (102) receives a count value (205) from the up counter and, similarly to the up counter has a load input (211) and a clock input (212). The clock input is again provided on the ECG signal (132) and the load input is provided by the respiration signal (136), here without a delay. The count value (205) is also provided to a decoder circuit (104) and a counter output (206) of the down counter (102) is provided to a second decode circuit (103). The output of the decode circuit (104) provides a dummy scan output (220) and is also fed to a logic circuit (105) that receives the second output (207) of the second decode circuit (103). The output (208) of the logic circuit (105) is fed as clock input (222) to a second down counter (106) which also has a load input (221) connected to the respiration signal (136). A count input (209) is provided to the second down counter (106) and the output (210) of the second down counter (106) provides a normal scan output (210). The function and operation of circuit (50) of Figure 4 will now be described with reference to Figure 5a.

Referring to Figure 5a, the digitised respiration signal is used to load the up and down counters (101, 102) with the delay (203) being used to allow time for the count from the up counter (101) to load into the first down counter (102). The digitised ECG signal (132) is then used to clock both counters. As shown in

Figure 5 a, the first up counter is loaded with 0, but due to the delay (203) this resets to 0 slightly after the load pulse is applied to input (211) of the down counter (102) so that the down counter (102) receives the previously- stored count value of the up counter (101). In the case of the first respiration pulse shown in Figure 5 a, the first down counter is loaded with the value 10, the second respiration pulse causes loading of the value 14 from the up counter and the third causes the value 15 from the up counter to be loaded into the down counter. The output (220) from the first decode (104) provides a logic 1 output at the instant of the respiration signal and keeps this at logic 1 until the third

ECG pulse after respiration occurs. This logic 1 signal is used to provide the dummy scan outputs. During the dummy scan, normal scanning occurs but the results are not processed.

The second decoder (103) provides a logic 1 output from the instant at which the signal on line (220) returns to 0 until the down counter reaches the value 2. It therefore provides a window during which scanning can reliably be carried out on the expectation that (based upon previous cycles) respiration will occur around the time the down counter (102) reaches value 0.

Where the user input selective control circuitry is provided or used, the output (208) is connected to the second down counter (106). The second down counter is loaded by the respiration signal with a user-selected number of normal scans desired. The second down counter (106) is then clocked by the normal scan output (208) of the logic circuit (105) so that the counter output (210) will count up to the number of normal scans set by the user. The normal scans will be determinated either when the second down counter (106) reaches 0 or when the first down counter falls below 2.

Turning now to Figure 6, an alternative counter circuit which does not have the adapted override facility will now be described. This circuit (150) has a down counter (301) with a load input (311) and a clock input (312) and a further down counter (302) similarly having a load input (321) and a clock input (322). The first down counter (301) also has a borrow output (313) and the second down counter (302) has a borrow output (323). The respiration signal (136) is connected to a decision block (303) which receives an enabling signal (230) from the borrow output of the second down counter (302). When the enabling signal is at logic 1, the decision circuit (303) passes the respiration signal (136) to the load input (311) and (321) of the respective counters. When the enabling signal is in the disabled state, the load inputs are not provided. The ECG signal is used to clock the first down counter (301). An input (231) to the first down counter provided an input number of dummy scans. In the present embodiment it is selected as 3. This value is of course not essential to the invention and other numbers could be selected as desired. The output (232) of the first counter is connected to a decoder circuit (304). An input (233) is provided to the second down counter (302), this input constituting a user defined number of scans.

Operation of the circuit of Figure 6 will now be described with respect to Figure 7. It is assumed initially that the enabling signal on line (230) is initially at logic 1. It should, however, be borne in mind that that fact that the ECG signal is applied to the first counter will inevitably result in the borrow output (313) becoming active and therefore at some point this information will cause clocking of the second down counter (302) to start and at some point therefore the borrow output (323) of the second counter will turn to the enabled state.

The value of 3 is loaded by the load input (311) into the first counter (301) which counts down according to the ECG signal (132) from 3 to 0. The second down counter counts down from the user selected number of normal scans desired to 0. Since the borrow output of the second down counter (302) is in a non-enabled state at this time, further respiration pulses will be ignored. The output (234) of the decoder (304) will provide a logic 1 output 3 pulses after respiration by virtue of the first down counter (301). The output (313) provides the selection value for normal scan. For example three ECG pulses may occur at and around the respiration event with the remaining number of ECG events until the next respiration occurs being determined by the behaviour of the subject. The gradient and RF signals continue to be applied during the respiration event but the device serves to prevent measurements from being taken during those three ECG periods.

The effect is to provide two intervals, named herein "normal scanning" and "dummy scanning" without changing the behaviour of the stimulus signals. This operates to prevent transients from disturbing the results.

Use of counters and of the blanking strategy reduces the time delay as opposed to other devices aimed at achieving similar results which use a filtering strategy. The filtering strategy can cause propagation delays which may be entirely unacceptable.

The present embodiment uses the ECG pulse to clock the counter circuitry (50). It would of course be possible to instead use a separate clock source. This is, however, disadvantageous where the ECG provides sufficient resolution. It would of course be possible to use an external clock if the ECG pulse was relatively too slow.

Embodiments of the invention are especially useful for small animal cardiac MRI. The use of digital trigger signals to the MR scanner so that MR acquisitions synchronise to the cardiac and respiratory cycles can reduce the effects of motion artefacts on MR images without substantially increasing the volume and hence the accuracy of the MR device. Under certain circumstances it is not necessary to use the inductive loop motion sensor device. Other respiration signals may be used including only a signal derived from the ECG.

Claims

CLAIMS:

1. A method of signal processing in a system in which a varying electromagnetic field is applied to a living being, and physiological signals of the being are captured, the method comprising monitoring the physiological signals for cross-talk due to variations in the electromagnetic field, and blanking, for a predetermined period of time, the captured signal when crosstalk is sensed.

2. A method as claimed in claim 1, further comprising blanking the physiological signals for a user-controlled period of time.

3. A method of operating a MRI system, where the electromagnetic field is a gradient field of the MRI system and the physiological signals comprise at least one of an ECG signal and a respiratory signal, wherein the method is in accordance with claim 1 or 2.

4. A method of reducing of movement artefacts in an imaging system in which signals are applied to a living being to produce a response, and the response is analysed to provide information representative of parameter of the being, the method comprising: deriving data indicative of movement of the being; analysing the said data to determine a first time window in which it is predicted that a forthcoming movement is expected to occur and a second time window in which it is predicted that movement is not expected to occur; applying the said signals to the being during the first and second windows; and analysing the response only to signals applied during the second window.

5. A method as claimed in claim 4, wherein the imaging system comprises an MRI system, wherein the signals applied to the being comprise a gradient field of the MRI system.

6. A method as claimed in claim 4 or 5, wherein the data indicative of the movement of the being is derived from at least one of the ECG signal and a respiratory signal.

7. A method as claimed in claim 4,5 or 6 wherein the movement of the being is respiratory movement and the data indicative of the respiratory movement of the being is derived from a movement sensor.

8. A method as claimed in claim 7 wherein the movement sensor comprises an induction loop.

9. A method as claimed in any of claims 4 to 8, further comprising providing a user-defined sub-window of said second time window and analysing said signals only during said user defined sub-window.

10. A method of imaging an object subject to movement using a device applying a varying electromagnetic field to the object to provide an imaging signal, in which an electromagnetic signal from the object is captured, and in which the movement is at least quasi-periodic and is related to a parameter of the electromagnetic signal, the method comprising monitoring the electromagnetic signal for the appearance of variations corresponding to the variation of the electromagnetic field; blanking the electromagnetic signal for a period following the occurrence of a variation corresponding to a variation of the varying electromagnetic field; deriving data indicative of said movement from the remaining electromagnetic signal; using the time occurrence of said data to adaptively form a time window in which said movement is predicted to occur; and during the time window disregarding said response.

11. A signal processing device for a MRI system, the system being operable to apply a high field gradient signal and to derive an image signal, the signal processing device having a first input for an ECG signal, a second input for the image signal, wherein the device comprises discrimination circuitry for deriving from an ECG signal at said first input a respiration signal representing the occurrence of respiration; timing circuitry for predicting, based upon the occurrence of previous respiration signals, the occurrence instant of a next respiration signal, and gating circuitry connected to said second input and responsive to an output of said timing circuitry for gating out said image signal for a time window including said occurrence instant.

12. A signal processing device for an MRI system, as claimed in claim 11 the device further comprising processing circuitry connected to said first input for providing a control signal if the content of a signal at the first input has a component at the gradient frequency which exceeds a given threshold; and transmission circuitry having an input, a control node and an output, the input being connected to the first input, the control node being responsive to the control signal and the output being coupled to an input of the discrimination circuitry, the transmission circuitry being operable to pass a signal at the first input to the discrimination circuitry input in the absence of said control signal and to block said signal from said discrimination circuitry input in the presence of said control signal

13. A signal processing device for an MRI system as claimed in claim 12, further comprising filter circuitry connected to the first input and having an output to the transmission circuitry.

14. A signal processing device for an MRI system as claimed in claim 13, wherein said filter circuitry has a first wide band filter path and a second narrow band filter path disposed substantially parallel to the first narrow band filter path, said discriminator circuitry comprises a first discriminator having an input connected to receive an output of said narrow band filter path and having an output for a logic signal indicative of said occurrence of respiration, and a second discriminator having an input connected to receive an output of said wide band filter path and having an output for a logic signal indicative of said occurrence of respiration.

15. A signal processing device for an MRI system as claimed in claim 14, having a third input for a motion sensor signal, and a selector connected to an input of the narrow band filter path, the selector being operable to select either said first input or said third input for application to said narrow band filter path.

16. A signal processing device for an MRI system as claimed in claim 15, having logic circuitry receiving the logic outputs of the first and second discriminators and having a logic circuitry output node connected to the timing circuitry.

17. A signal processing device for an MRI system as claimed in claim 16, wherein the logic circuitry is operable to provide, at said logic circuitry output, an AND function of the outputs of the first and second discriminators.

18. A signal processing device for an MRI system as claimed in claim 16 or 17, wherein the logic circuitry is operable to provide, at said logic circuitry output, an OR function on the outputs of the first and second discriminators.

19. A signal processing device for an MRI system as claimed in any preceding claim, further comprising a pulse shaping circuit connected to receive a signal from said first input and to derive from an ECG signal applied to said first input a pulse train indicative of predetermined ECG events.

20. A signal processing device for an MRI system as claimed in any of claims 11 to 18, wherein the timing circuitry comprises counter circuitry having a clock input.

21. A signal processing device for an MRI system as claimed in claim 20, wherein the counter circuitry comprises an up counter and a down counter.

22. A signal processing device for an MRI system as claimed in any of claims 11-18, wherein the timing circuitry comprises counter circuitry having a clock input, said counter circuitry comprising an up counter and a down counter and further comprising a pulse shaping circuit connected to receive a signal from said first input and having an output, said pulse shaping circuit being operable to derive from an ECG signal applied to said first input a pulse train indicative of predetermined ECG events, wherein said pulse train output is connected to said clock input of said counter circuitry.

23. A computer program product stored on a computer useable medium comprising readable program means for causing a computer to perform the method of any of claims 1-10 when the product is run on a computer.