WO2008015679A2

WO2008015679A2 - System and method for monitoring displacements of in vivo objects

Info

Publication number: WO2008015679A2
Application number: PCT/IL2007/000966
Authority: WO
Inventors: David Mendes; Ruth Beer; Emanuel Mendes; Avi Ariav; Issakhar Regev; Jacob Stern
Original assignee: Intellimedi Ltd.
Priority date: 2006-08-03
Filing date: 2007-08-02
Publication date: 2008-02-07
Also published as: WO2008015679A3; WO2008015679A8

Abstract

A system for monitoring displacements and/or rotational motion of in vivo objects is provided. The system comprises one or more magnetically coupled resonant electric circuits (RECs), a transmitter for exciting the pairs of RECs to oscillate, a receiver for receiving electromagnetic signals emitted by the oscillating RECs and a processor for deriving frequencies associated with the received signals. Each REC of a pair is respectively attached to an in vivo object, such as a segment of a bone. Relative translational and/or rotational motion of a pair of segments of a bone or bones result in varying resonance frequencies of the respective RECs. Monitoring is accomplished by comparing currently measured resonance frequencies with pre-stored frequencies. In a case in which variations in the measured frequencies do not exceed a predefined threshold a stationary state is determined.

Description

SYSTEM AND METHOD FOR MONITORING DISPLACEMENTS

OF IN VIVO OBJECTS

FIELD OF THE INVENTION The present invention relates to systems and methods for monitoring displacement between in vivo objects such as, artificial joint members, vertebrae, and segments of fractured or dissected bones. More specifically, embodiments of the present invention relate to systems capable of detecting variations of the distance, and/or relative orientations, between for example, vertebrae, or segments of a bone thus enabling to determine, relative displacement and/or rotations between them.

BACKGROUND OF THE INVENTION

The accuracy of monitoring the progress or regression of clinical syndromes regarding bone healing, bone union, bone growth and bone absorption is particularly important in orthopedic procedures. Exemplary procedures are surgery of the spine, trauma of the bones, chest bone (sternum) surgery and heart and lung surgical procedures. Early accurate diagnosis of the nature of post operative course is of paramount importance for providing data that may herald the necessity of modifying the treatment by medications that prevent bone absorption, such as biphosphonates (Wilkinson JM et al. J. Orthop. Res. 23(1): 1-8, 2005) and promote bone formation such as BMP (Clin Orthop. 417:50-61 , 2003) and Teriparatide (J Clin Endocrinol Metab. 2004 Dec 21) or by further surgery in order to avoid failure and to save suffering from the patient. Presently, monitoring is done by imaging modalities such as X Rays (Roentgen), CT, MRI, nuclear imaging and/or Scintimetry which frequently are of insufficient quality due to low resolution, subjective interpretation (observer dependant), disruption by metals and confusion by difficult anatomic structures. Normally such imaging systems are located remote from most of the clinical physicians, therefore results are often delayed, are more expensive and most of the technologies carry the risk of irradiation.

The Orthopedic literature and massages from conventions repeatedly express the existing frustration due to lack of sophisticated monitoring means [(Otterberg ET, Hauser DL, Siddiqui M, Bragdon BS and Harris WH, The Harvard Orthopaedic Journal 2:100-103),(Shaver MS, Brown TD, HiINs SL and Callaghan JJ (1997) JBJS 79A : 690-700), ( Bonicoli F., Proceeding the International CeramTec Symposium :50, 2000)]. A plea is expressed for a reliable modality that will provide early diagnosis in real time, objective, accurate, dynamic, available to the physician and easy to use, of high resolution and low cost for the follow up of patients following surgery and traumatic procedures.

Systems for measuring in vivo distances by detecting changes in the resonance frequency of a resonant circuit and or by analyzing features of the amplitude envelope of the oscillations of such circuits are known. Exemplary US Patents 6,583,630 and 6,656,135 respectively disclose such systems for measuring in vivo distances between two objects. Both disclosed systems include: a resonant circuit; a magnetic element having a predetermined structure, geometrical shape and magnetic properties; a transmitter operable to transmit an electromagnetic pulse; a receiver operable to detect oscillations emitted by the resonant circuit in response to said electromagnetic pulse, and an analyzer operable to analyze an amplitude envelope feature and/or the frequency of the oscillations, to thereby determine a distance between the resonant circuit and the magnetic element.

Any system and method providing for conveniently monitoring the displacements and/or orientational changes of segments of the bones induced by a progress or regression of clinical syndromes and surgery results in bones, that is simple to manufacture and convenient in operating is therefore beneficial. BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a simplified block diagram of a system for monitoring displacements of in vivo objects according to the present invention;

Fig. 2 is a synthetic description of the signals generated by a system of the invention;

Fig. 3 is an exemplary frequency-distance profile of a system for monitoring displacements of in vivo objects according to the invention;

Figs 4 - 6 schematically describe placements of resonant electric circuits of a system for monitoring displacements according to the present invention respectively applied onto three different bones;

Fig. 7 presents typical frequency-distance profiles measured for a pair of resonant electric circuits one of which is relatively inclined towards the other in different angles respectively;

Fig. 8 is a graph of the resonance frequency versus distance measured for two exemplary pairs of resonant electric circuits affixed to two segments of sternum suspended in saline solution;

DETAILED DESCRIPTION OF THE PRESENT INVENTION

In accordance with the present invention a system and method for monitoring displacements and or rotations of in vivo objects is provided. Such system is referred hereinafter by either system of the invention or system for monitoring displacements. A system of the invention has at least one pair of magnetically coupled resonant electric circuits (RECs); a transmitter operable in exciting oscillations in each of these RECs; a receiver operable in receiving the electromagnetic radiation emitted by the simultaneously oscillating RECs, and a processor for signal and data processing and for deriving resonance frequencies and further analyzing the measurements' results. An operator interface linked to the processor provides for displaying and inputting data to, or by, an operator as known. Such a pair of RECs respectively attached to a pair of in vivo objects, is first irradiated by means of the transmitter. Any relative motion of these objects effect the magnetic coupling between the RECs, thereby modifying features of the signals received. The system of the invention provides for determining whether such modified signals are associated with relative displacements and/or rotations of the objects to be monitored.

Reference is first made to Figs 1 - 7. In Fig. 1 a simplified block diagram of a system for monitoring in accordance with the present invention is shown. System for monitoring displacements 10 has one or more pairs, such as pair 12, of magnetically coupled but electrically isolated RECs 14. RECs 14 are affixed unto in vivo objects whose relative displacements and/or rotations are to be monitored, such as segments 16 of a broken bone. Transmitter 18 having either a programmable pulse generator or CW generator operating at a specified band of frequencies feeds transmitting antenna 20 to radiate electromagnetic waves. Such transmissions excite the two coupled RECs 14 to simultaneously oscillate. The electromagnetic radiation induced by these oscillations is received in receiver unit 22 by means of dedicated receiving antenna 24. Sampling and digitizing unit 26, which is synchronized with transmitter 18, samples the received signals at a predefined sampling rate and digitizes them at a predefined dynamic range. The digitized data are temporarily stored in a buffer memory of sampling and digitizing unit 26 to be transferred to processor 28 for further processing and analysis. Operator interface 29 provides for inputting setup and/or working parameters, activating the system and for displaying an operator with the results of the measurements. Optional link to a computer of a back office, not shown, provides for uploading data related to the measurements and downloading system working parameters, as known. A common laptop computer implements processor 28 and operator interface 29 according to an embodiment of the present invention.

Coupling a pair of resonant circuit is effected by disposing them such that the distance between their centers does not exceed a predefined threshold whose magnitude is dependent on their relative orientation. The resonance frequency of each REC of a pair of magnetically coupled RECs is varying with their relative distance and orientation. Therefore a displacement and/or rotation of any member of such pair relative to the other causes the resonance frequencies to respectively change. Monitoring such in vivo displacements and/or rotations is accomplished in accordance with the present invention by comparing currently measured resonance frequencies with pre-stored frequencies associated with this pair of RECs.

Synthetic time profiles representing the actual electric signals transmitted and received by an exemplary system of the invention employing pulsed transmissions are shown in Fig. 2. Plot 30 is a time profile of a transmitted DC pulse, plot 32 is a time profile of an oscillation excited in any of the coupled RECs. Plot 34 is the time profile of the sampled signal received. The units of both axes of the graph are arbitrary. Furthermore the scaling of the amplitude varies in the three graphs shown in Fig. 2. Namely the peak to peak (p.t.p.) amplitudes of the actual oscillations are significantly larger than the respective values of the received signals, whereas the amplitude of the transmitted pulses is significantly larger compared to the maximal p.t.p amplitude of the oscillations. The displacements in the amplitude values of the different time profiles shown are artificially introduced. The transmitted power per a single pulse, pulses' widths, the number of transmitted pulses within one session of measurements and the respective pulse repetition rates are programmable in accordance with a preferred embodiment of the present invention. Optionally an automatic gain control (AGC) or an automatic level control (ALC) as well as tuning of the pulses' width are automatically applied by means of the operating program installed in the system's processor. The pulse widths, repetition rates, and the number of pulses in a session are selected according to the method of the present invention such that the signal to noise ratio of the measured frequencies is promoted.

A while following the point in time in which a transmitted pulse ends the frequency of the received signal emitted by the excited pair of RECs approaches the average of the resonance frequencies of each of the RECs. Therefore in order to avoid blurring of the frequency measurements the receiver is normally blocked whilst a pulse is transmitted. Preferably such blocking continues for a while after the transmitted pulse ends, as is shown in Fig. 2. Blocking is accomplished either by means of a dedicated electrical circuit that is synchronized with the transmitted pulses, or by programming the sampling and digitizing unit to delay the sampling process of a received signal by a predefined time interval. Typical blocking times are of a few dozens of microseconds, whereas typical pulse widths are within a few microseconds.

The received signal of a pair of magnetically coupled RECs can be described by a monotonic wave whose frequency equals the arithmetic average of both respective resonance frequencies whereas its amplitude is modulated. In a case in which both RECs oscillate at the same amplitude the modulation frequency equals half of the difference in the resonance frequencies. Therefore by properly selecting the sampling rates as well as the ends of the time intervals along which a signal is sampled and further averaging the measured frequencies, a frequency of the received signal is determined. The frequencies of the RECs are derived from the values of such measured frequencies. The arithmetic average of the resonance frequencies of each REC of a pair of RECs is referred hereinafter as the resonance frequency of the pair.

A transmitter capable for generating pulsed CW signals is in accordance with the present invention. The frequency of the CW generator is selected to be close to the resonance frequency of a pair of RECs. The selected pulse width is significantly larger compared to the respective cycle time according to an embodiment of the present invention. The receiver is similarly blocked in such case for a predefined time interval following the end of a transmitted pulse. The derivation of the resonance frequencies by means of such system is similar to the method described above. In cases in which the transmitter generates CW signals each of the RECs of an irradiated pair are forced to oscillate at the same frequency which equals the frequency of the transmitter. The level of these oscillations increases as the transmitter frequency approaches the resonance frequency of the respective REC. Therefore the level of the received signal reaches its maximal value at a frequency equal, or closely equal, to the resonance frequency of the respective pair. Therefore, by sweeping the frequency of a transmitted CW signal having a fixed pre-selected amplitude, within a predefined frequency band, and further detecting the frequency in which the received signals gets their maximal value, this frequency can be determined, as known.

RESONANT ELECTRIC CIRCUITS

A REC of a system of the invention such as any common RLC circuit consists of inductor having a predefined inductance, a capacitor of a predefined capacity and a predefined ohmic resistance. To maximize the quality factor (Q factor) of a REQ normally the ohmic resistance of the wire is the only resistor employed. Typical Q factors of RECs of a system of the invention can reach and/or exceed the value of 150 close to their resonance frequencies.

The inductors are designed according to a preferred embodiment of the present invention to have a predefined diameter in the range of 1 - 3 mm; a predefined length in the range of 5 - 15 mm and a core of ferrite onto which the electrical wire is coiled. An inductor of a REC of a system of the invention typically has a predefined number of windings, such that a predefined inductance is achieved, which is in the range of a few milli Henrys. The wire, which is a common cooper wire typically utilized for manufacturing inductors, is preferably coiled on the surface of the ferrite core, as known, in layers each of which includes dozens of windings. The terminals of an inductor are electrically connected to the terminals of a common capacitor whose capacity is in the range of a number of hundreds of pF, but less than 2000 pF. A REC is wrapped or coated with an electrically insulating layer made of biocompatible material, such as parylene C.

A REC is further coated with relatively thick cover layer providing for strengthening its mechanical structure and securing the REC from the in vivo chemical environment. Optionally an adhering material typically utilized in arthroplasty is utilized for such coating. Any glue normally utilized in osteotomy, such as Simplex P^(TM), Speedset Radiopaque Bone Cement^(TM), Refobacin Plus Bone Cement^(TM), Biomet Plus Bone Cement^(TM), Orthoset 1 Radiopaque Bone Cement^(TM), is suitable as long as such coating when is cured provides for mechanically strengthening the structure of a REC and promotes its sustainability to the in vivo chemical and mechanical environment. Alternatively such cover is made of a biocompatible plastic resin such as polyurethane.

ATTACHING RECS TO BONES

The value of the mutual inductance associated with any of the RECs of a pair increases as their separation decreases. The mutual inductance reaches its extreme values for each separation distance at a collinear configuration. Each REC of a pair is respectively attached to a segment of a pair of bones or segments of bones whose relative displacements and/or rotations are to be monitored. Attaching according to a preferred embodiment of the present invention is accomplished such that the RECs' axes are not collinear but parallel. The axes of the RECs of an attached pair preferably point in opposite directions such that the values of their mutual inductance are maximized for this configuration. In Fig. 3 exemplary resonance frequency - distance profile 40 measured by employing pulsed transmissions according to a preferred embodiment of the present invention is shown. The resonance frequency of a pair of RECs the relative orientations of which are fixed, or varying within a relatively small range of rotational angles, decreases as their relative distance increases. Similarly the resonance frequency of a pair decreases as the angle between their axes increases while their separation remains fixed. Therefore by monitoring changes in the measured resonance frequency of a pair of RECs its RECs respectively attached to segment of a bone or bones, displacements and/or rotations of the respective segments of the bone or bones are determined. The resonance frequency of a pair of RECs monotonically decreases, as the distance between the respective segments of the bone or bones increases. Similarly the difference between extreme values of the resonance frequency of a pair of RECs measured along a predefined time interval increases as the amplitude of the rotational motion of the respective segments of bone or bones increases. However a smaller distance between the RECs of a pair or lower amplitude of their relative rotations results in an increased resonance frequency and/or decreased difference between the extreme frequency values measured within a relatively short time.

Attaching each of the RECs to a segment of bone is accomplished by means of any of the aforementioned adhering material or bones' glues. Optionally the RECs are introduced into conforming niches or bores drilled into and/or through the bone prior to their attaching. Optionally the cover of RECs is threaded such that the REC is screwable into a drilled bore disposed on the surface of a segment of bone. Alternatively a segment of the surface of the cover layer is threaded and/or textured providing for its attaching into an inner surface of a drilled bore by pressing. In cases in which a number of pairs of RECs are attached to the same segment of bone, the axes of different pairs are preferably such oriented that they are not mutually parallel, to thereby promote the sensitivity of detecting relative rotations. EXEMPLARY APPLICATIONS

In Figs 4 - 6 three potential applications are schematically described respectively. Pairs of RECs, such as pair 50 and 52, of a system for monitoring in vivo displacements and/or rotations are respectively attached to two halves of a sternum. Such configuration provides for monitoring the healing of a sternum following for example an open chest surgery. Pair of RECs 60 provides for monitoring displacements and/or rotations of vertebra 62 relative to vertebra 64 in a spine. Similarly pair of RECs 70 provides for monitoring displacements and/or rotations of segment 72 relative to the bone of skull 74 following craniotomy. Similarly the relative displacements and/or rotations of segments of fractured or dissected bones, prostheses attached to a segment of bone or artificial joints can be monitored by means of a system of the invention. The progress of healing of bone trauma and/or dissection can be determined by monitoring the changes in the respective resonance frequency of a pair of RECs. In cases in which such changes do not exceed a respective threshold the respective segments of the bone or bones are regarded stationary thereby healing is determined according to the method of the present invention.

SIGNAL AND DATA PROCESSING

A typical sampling rate of the received signal according to a preferred embodiment of the present invention is within a few dozens of hundreds Hz. The samples are digitized with a common 14 bits A/D converter. Most of the signal processing including filtering is accomplished by the operating software of the systems processor. The measured frequency is derived, such as by means of FFT or a filter matched to the resonance frequencies of the pairs, in cases of pulsed transmissions. An adaptive matched filtering such as by an adaptive neural network filter is suitable for measuring the resonance frequencies of pairs in cases of CW transmissions. A measured resonance frequency of pairs of RECs is averaged according to a preferred embodiment of the present invention over a predefined number of repeatedly measured frequencies within a first time interval. An exemplary first time interval includes samples having signal to noise ratios (SNR) exceeding a predefined threshold. A process of sampling the data according to a preferred embodiment of the invention includes the following steps: (a) signals are repeatedly sampled at a predefined sampling rate along a time interval which is significantly longer compared to a cycle time of the modulated amplitude of the received signals; (b) averaged values of SNR are derived for successive time intervals shorter compared to a cycle time of the modulated amplitude, these intervals are contained within the first time interval; (c) a region in time in which such averaged SNR gets its maximal value is selected; (d) FFT computation is carried out considering a predefined number of samples received along this selected time region; (e) a frequency bin whose output value is maximal including its adjacent bins are selected; (f) a frequency value is derived by averaging the frequencies related to these selected frequency bins, averaging is carried out considering the values of the output of a selected frequency bin.

Such repeatedly measured frequencies as well as their averaged value associated with a specified pair of RECs are stored in a record associated with an identity of a patient in a database. Currently averaged frequency is compared to values of averaged frequencies related to this specific pair of RECs previously stored in the database. A deviation of the currently measured frequency off a respective stored frequency is determined. Cases in which the currently averaged frequencies are smaller compared to the respective averaged frequencies previously measured correspond to displacements in which the distance between the respective RECs is extended, and/or rotational movement by extended angles occurred. In such cases a notice to the operator is automatically displayed. Optionally an alert is automatically transmitted to the backward computer to which the system processor is linked and/or the responsible physician is similarly alerted in such cases. However cases in which the averaged frequencies increase compared to the previously stored values correspond to decreasing distances and/or extreme rotational angles. Such deviations repeatedly determined along the aforementioned time interval are compared to a predefined threshold associated with the specific bone or pair of bones and the respective pair of RECs. In cases in which the deviations are smaller compared to such threshold the pair of RECs is regarded stationary. A corresponding notice is displayed to the operator and optionally transferred to the backward computer.

EXAMPLE 1

A laboratory experiment for demonstrating the functional behavior of the resonance frequency of a pair of RECs of a system according to a preferred embodiment of the present invention is hereby described with reference to Fig. 7. A pair of RECs whose relative distances and orientations are controlled were irradiated by trains of electromagnetic pulses. The RECs were disposed one aside the other, such that the angles between their axes as well as their relative distance could be varied. Plot 80 is a frequency (f) - distance (d) profile measured for a fixed orientation in which both RECs are parallelly disposed such that their mutual inductances are maximal for each distance. Plot 82 is a frequency - distance profile measured for a fixed orientation in which the angle between the axes is of 10°. Plot 84 represents a frequency - distance profile measured at an inclination angle of 20°. The frequency scale is measured in kHz. The distances were measured between the surfaces of the cover layers of both RECs in mm. Therefore the values presented are off a minimal value which is close to the length of the diameter of the RECs.

At relatively large distances, the respective profiles corresponding to different inclination angles even those related to inclinations within a significant angular range such as of 40°, almost coincide. In such distances no practical determination of rotational motion can be accomplished. However one is able to detect rotational motion at smaller distances. Furthermore, the accuracy of the resonance frequency measured by means of a system of the invention is relatively high. Typical tolerances of the measured frequencies get up to a few tenths of percent and even lower. The determination whether displacements and/or rotations occur is based on frequency measurements carried out considering the same pairs of RECs along different time intervals. Such measurements are analogous in a way to measuring relational frequencies in which the natural deviations of the actual inductances, such as due to tolerances of manufacturing processes and/or the features of the materials utilized, from their respective nominal values are therefore irrelevant. Furthermore the functional dependence of the frequency on the distance as demonstrated reasonably conforms to the theory. The ratio between the mutual inductance Li₂ and the self inductance L is found conformal with a monotonically decrease as the inverse of the third power of the distance between the inductors. (The effective inductance corresponding to these experimental configurations is given by L+L₁₂).

EXAMPLE 2

A laboratory experiment demonstrating the stability and repeatability of the measurements of the respective resonance frequency of two pairs of RECs of a system according to a preferred embodiment of the present invention was conducted while monitoring displacements of one segment of bone relative to another. A sternum of a pig was longitudinally bisected. Two pairs of RECs symmetrically disposed close to the edges of the cut were attached unto niches drilled through each segment of the sternum by means of bond cement as described hereinabove. The pairs of RECs are placed along the sternum as describe hereinabove with reference to Fig. 4 to which reference is again made. The segments of the bone and the RECs were immersed in saline solution to mimic in vivo environment. The setup of the experiment provides for controlling the distances between both edges of the cut along the sternum. Electromagnetic CW radiation emitted by the transmitter of the system respectively excited oscillations in each of both pairs. Distinguishing between the different pairs was achieved by respectively positioning the antenna at a close proximity to each of the pairs.

Reference is now made to Fig. 8 in which frequency- distance profiles of the respective pairs are respectively shown. Plot 90 is a frequency - distance profile measured for the pair of RECs that were initially disposed closer to each other at a minimal distance of 4 mm.. Plot 92 is a frequency - distance profile of the pair initially disposed at a minimal distance of 7 mm. The horizontal axis of the graph displays distances measured between the edges of both segments of sternum disposed along the cut. The differences in resonance frequencies respectively related to each pair being at the same distance separating between its RECs shown can be interpreted by deviations in the initial orientations as well as by differences in their inductances' values. The same results were obtained for measurements repeated a number of times within the same tolerances which are lower by far than the value of the threshold defining a stationary configuration corresponding to a "healed bone".

EXAMPLE 3

Another experiment in which both segment of sternum were mutually connected to each other by means of rings made of stainless steel, such as normally utilized in open chest surgery for reconnecting a bisected sternum, was further conducted. Two pairs of RECs where similarly attached to the sternum as described in EXAMPLE 2 above. The sternum segments and the RECs were similarly suspended in saline solution. Measurements of the resonance frequencies of both pairs were repeatedly taken along a specified time interval which corresponds to dozens of measuring sessions. The signal to noise ratio of the measured frequencies was somewhat lower compared to those demonstrated in the above mentioned experiment described in EXAMPLE 2 above. However reasonable stability and repeatability of the measurements were demonstrated. Furthermore the distribution of the measured resonance frequencies of the pairs were practically the same as measured in the above mentioned experiment and their values were considerably lower than the threshold for determining a stationary configuration.

Claims

1. A system for monitoring displacements of in vivo objects, said system comprising a) at least one pair of resonant electronic circuits (RECs) each having a respective resonance frequency, wherein said RECs magnetically coupled to each other, and wherein said RECs electrically isolated from each other; b) a transmitter for exciting said at least one pair of RECs to simultaneously oscillate; c) a receiver for receiving electromagnetic signals emitted by said oscillating pair of RECs, and wherein each of said RECs respectively comprises an inductor, and wherein said resonance frequency is dependent on the mutual inductance of the inductor of one of said RECs induced by the inductor of other REC.

2. A system as in claim 1 , said system further comprising a processor for deriving a frequency by which said pair of RECs oscillates.

3. A system as in claim 1 , wherein each of said RECs further comprises an electrically isolating layer.

4. A system as in claim 1 , wherein each of said RECs further comprises a cover layer.

5. A system as in claim 4, wherein said cover layer comprises an adhering material.

6. A system as in claim 4, wherein said cover layer is threaded such that each of said RECs is screwable into a bore drilled into a bone.

7. A system as in claim 4, wherein a segment of a surface of said cover layer is textured.

8. A method for monitoring displacements and or rotations of a first in vivo object relative to a second in vivo object, said method comprising the steps of a) affixing a resonant electric circuit (REC) to each of said first and second in vivo objects, wherein said RECs are magnetically coupled, and wherein said RECs are electrically isolated from each other; b) exciting said pair of RECs to simultaneously oscillate by irradiating them with electromagnetic waves; c) storing a frequency associated with said oscillations, and d) comparing said stored frequency to a pre-stored frequency.

9. A method as in claim 8, wherein each of said first and second objects is a segment of a bone.

10. A method as in claim 8, further comprising repeating said step b and c prior to step d.

11. A method as in claim 10, further comprising averaging said frequencies prior to said comparing.

12. A method as in claim 10, further comprising selecting regions in time in which a signal to noise ratio associated with said oscilations exceeds a predefined threshold.