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An improved method for detecting passive pills (telemetric measurement of physiological parameters)
This content has been downloaded from IOPscience. Please scroll down to see the full text. 1976 Phys. Med. Biol. 21 577 (http://iopscience.iop.org/0031-9155/21/4/009) View the table of contents for this issue, or go to the journal homepage for more
Download details: IP Address: 130.237.165.40 This content was downloaded on 01/10/2015 at 01:59
Please note that terms and conditions apply.
PHYS. MED. BIOL., 1976, VOL.
21,
NO.
4,577-588.
@
1976
An Improved Method for Detecting Passive Pills T. A. DELCHAR and M. J. A. SMITH Department of Physics, University of Warwick, Coventry Received 14 October 1975, infinal f o r m 12 March 1976 ABSTRACT.One of the principaldisadvantages of the passive pill as a telemetric method for measuring various physiological parameters has been its restricted range. The reasons for the restricted range with existing detection methods are discussed. An improved method using a locking spectrometer based on third-order phase-sensitive detection is described and its performance is assessed. A significant increase in the usable range of a high sensitivity passive pill is obtained.
1. Introduction
The progressive development of microminiature electronic components in recent years hasenabled considerable improvements to be made inthe performance of the endoradiosonde or active pill since the pioneering efforts of Mackay and Jacobson (1957). It is now possible, using this method of telemetry, to obtain continuous information on internal physiological parameters such as pressure, temperature and pH by the insertion or ingestion of a suitable active pill. However, the need to process and to transmit the relevant information and the requirement, therefore,for an internal power supply anda switch has meant that active pills are relatively large (typically 10 mm in diameter and up t o 30 mm in length), areexpensive to manufacture and arelimited in thetime for which they can operate. An alternative method of obtaining the required information, also without the use of connecting wires, is the passive pill (Farrar, Zworykin and Baum 1957) which consists of a single electrical resonant circuit with the resonant frequency controlled by a transducer sensitive t o the appropriate parameter. The pill is coupled by mutual inductance to a tuned receiving coil which both excit'es and measures the frequency of the pill's resonance. The passive pill does not require an internalpower supply and contains a minimal number of circuit components, often only an inductor and a capacitor. It follows that it can be small, inexpensive to manufacture and, therefore, disposable. It can also have an almost indefinite operational life provided that its outer membrane is made would be particularly impermeable t o body fluids. Thelatteradvantage important when, for example,measurements of intracranialpressureare required. The two major disadvantages which have been foreseen for the passive pill are its restricted range,that is the maximum distance between the pill and the receiving coil for a given accuracyof measuring the resonant frequency, and its failure t o operate when the magnetic axes of the pill and the receiving coil are orthogonal.
T.A . Delchar and
578
M. J . A . Smith
The primary purpose of this paper is to show that themaximum range of the passive pill, for a given range of angles between the magnetic axes, may be increased significantly by the use of odd-order phase-sensitive detection in the high-pass derivative filter mode (Smith 1975). Methods for measuring the resonant frequency of a passive pill Anymethodfordetermining the resonantfrequency f,, of a passive pill coupled by mutual inductance to the tuned circuit of a variable frequency oscillator involves a measurement of the frequency of the oscillator when the coupling is a maximum. The value of a pa'rticular method may, therefore, be judged by the accuracy with which it performs this measurement. In the direct method, maximum coupling corresponding to maximum power loss in thereceiving coil is assumed to occur when the outputfrom the oscillator is a minimum. This assumptionis, however, often inappropriate andcan leadto inaccuracies well before the noise limit of the system is reached. The origin of the inaccuracies may be illustrated qualitatively as follows. An inherent property of any amplitude-limited oscillator based on a tuned circuit is thevariation which occurs in the output of the oscillator asits frequency is varied. The variation, which will be referred to as the frequencydependent background component bV(f ), may be represent'ed by curve (a)in fig. 1. Line ( d ) in fig. 1 represents the frequency-independent component " V of 2.
Oscillation arnplliude
bV
Fig. 1. Frequency variation of the components of oscillation amplitude in an amplitudelimited oscillatorcoupled t o an electricalresonance. Thefrequency fp at the intersection of the axes corresponds to the resonantfrequency.
the oscillation amplitude and curve ( b ) the component p V ( f )produced when a resonant pill is allowed to couple to the oscillator. The sum of all three components is represented by curve (c). I n t h e case of strong coupling, " V (f) changes with frequency nearto f, a t a much greaterrate thanb V (f ). Maximum coupling then corresponds closely to minimumoscillation amplitude and fp may be determined with acceptable accuracy over a range of values of f p . bV may be removed after rectification by introducing a DC offset voltage and does not affect the accuracy of the measurement provided that the gain of the
An Improved Method for Detecting Passive Pills
579
system remains constant. I n t h ecase of weak coupling, p V ( f ) can change with frequency near to f,a t a rate comparable with b V ( f ) with the result that a minimum output from the oscillator nolonger corresponds exactly to maximum the magnitude of the coupling. Moreover, a.s f p variesdue to achangein physiological parameter being measured, the relationship between minimum output and maximum coupling also varies. I n t h e extreme case of very weak coupling, which is the situationof interest in practice, nowell defined minimum isobtained. For other than an unrealistically sharp pill resonance(high Qfactor)and asmallrange of variation of fp, the directmethod is clearly unsatisfactory as the error in measuring f p is caused by a large frequencydependent background component generated by the oscillator rather than by system noise. The problem is essentially oneof pattern recognition and itis well known that the accuracy of the directmethodmay beimprovedbymodulatingt'he frequency of the oscillator by asmall amount and using aphase-sensitive detector with an in-phase reference at the modulating frequency. The phasesensitive detector is connected to the oscillator through an amplifier tuned to the modulation frequency and a diode which provides a signal voltage proportional to the oscillation amplitude. I n some oscillators, notably the grid-dip oscillator, the diode is unnecessaryas efficient rectification is carried out by the same active element which generates the oscillations. For small modulation amplitudes (up to about0.4 times the half width Sf, of the pill resonance), the output of the phase-sensitive detector is proportional to the first derivative of the resonance. f,is, therefore, equalto thefrequency of the oscillator when the contribution from the pill to the output from the phase-sensitive detector is zero. The value of this method lies in the fact that all the terms in fig. 1 are differentiated once with respect to frequency. I n particular, b V is eliminated while the rate atwhich d ( b V ( f ) ) / d fchanges with frequency in the region of f p is now lower relative to d ( P V ( f))/d,fthan before differentiation. I n addition t'o the improved discrimination which first-order phase-sensitive detection can provide against the frequency-dependent background component, the pill component of the outputof the phase-sensitive detector in theregion of f p is proportional in both magnitude and sign to the deviation in frequency between the oscillator and f,. This form of output is particularly convenientas it may be used directly to lock the frequency of the oscillator to f,. Alt'hough first-order phase-sensitive detection offers a worthwhile improvement over the direct method for determining the point of maximum coupling between the pill and theoscillator, the error in f p for typical values of both the Q-factor of the pill (Q, 50-120) and the frequency sweep range ( 5 20Sf,) is stilllimited bythe frequency-dependentbackgroundcomponentfrom the oscillator rather than by system noise. It would seem logical a t this point to consider if phase-sensitive detection a t a multiple m (greater t'han one) of the modulation frequency f, can give a further reduction in b V (f ) as a result of high-order differentiation. A detailed analysis of this method when m is odd has been given by Smith (1975) and it is shown that the outputfrom a phasesensitive detector operating with an in-phase reference a t mf, contains odd
580
T . A . Delchar and
N . J.
A . Smith
derivatives of the resonance shape of order m and above when large sinusoidal modulationamplitudes ( Sf,) are used (high-pass,derivative filter mode). The most important conclusions from the analysis are, first of all, that, for QP in the approximate range 50-120, m = 3 is sufficient to reduce the magnitude of the frequency-dependentbackgroundcomponentbyafurtherlargefactor (typically in excess of lo4) compared with the situation when m = 1. I n most cases this is sufficient to reduce t'he background component below the noise level. Secondly, the fact'or by which the magnitude of the pill component in t'he small region of f, is reduced by increasing m from 1 to 3 is,bycontrast, (typically 4). Thirdly, althoughthe sign of successive odd derivatives alternates, the combined output has thesame form in the region off, as thefirst derivative and retains the very desirable property of first derivative recording t'hat the output is proportional to the frequency deviation from f,. The method of third-order phase-sensitive detection in the high-pass derivative filter mode should, therefore, be capable of increasing the range of the passive pill for a given accuracy in the determination off, as the method is limited by noise and not by a frequency-dependent background generated by the pillsensing oscillator. I n certain circumstances, pa'rticularly where low values of Qp and system noise are expected, it may be necessary to consider using fifth-order detection with large modulation amplitudes. N
3.
Choice of operating frequency
Before describing a suitable third-order phase-sensitive detection systemand an associated pill, it is necessary to choose an optimum valuefor fp or, a t least, suggest a rangeof possible operating frequencies. A number of different criteria have been adopted previously to determine fp dependingon the particular environment of the pill and the availablerange. For example, the elegant passive pressure pills developed by Collins (1 967)for studying glaucoma in the rabbit and byOlsen, Collins, Loughborough, Richards, Adams and Pinto (1967) for measuring intracranial pressure operated in the region of 95 MHz. This relatively high frequency enabled air-cored inductors tuned by self-capacitance to be used and ensured that thepills were physically small. There are, however, a number of reasons why a lower frequency is preferable for a passive pill intended for more general use particularlywhenasensitive and accurate detection method is available. The first and probably the prime reason is related directly to the expected maximum range. If, as is clearly desirable, the pill is to be used in the gut, the ideal minimum diameterof the receiving coil should be such that it can be placed around the patient. There is then no orientation of the pill a t which f p cannot be measured by a suitable adjustment of the position of the coil. Diameters approaching one metre are required and, as one turn is the minimum possible for the receiving coil, f p should be less than about 10 MHz. I n making this estimate for an upper limit to f,, it is assumed that a minimum tuning capacitance for the coil of approximately 200 p F is required if capacitance changes due
An Improved Method for Detecting Passive Pills
581
to thepresence and movement of the patient are not change to the frequency of the oscillator to any significant extent. An estimate for a lower limit to f p may be obtained by recognizing that the magnitude of the received signal is proportional to the loaded Q-factor ( Q ) of the air-cored receiving coil, that is the Q-factor when the coil is connected t o the oscillator. From this point of view Q should be as high as possible. However, a contraryargument for a lower Q follows from the requirement that the presence or movement of a patient inside the coil should not cause an unduly large change inthe oscillation amplitude. A suitable compromise may be obtained by designing the coil so that it has an unloaded Q-factor greater than 100 and by adjusting the level of positive feedback inthe oscillator with the coil connected so that theloaded Q is about a factor of two or three times less than the unloadedvalue.Experience gained from constructingvarious receiving coils of about 1 m diameter tuned t'o frequencies between 50 kHz and 10 MHz suggests that 100 kHz is a suitablelower limit forfpon the basis of the suggested compromise. More specifically the magnitude of the coupling between the two resonant circuits and, therefore, the magnitude of the signal available for the measurement of f,,is proportional to k2QQp where k is the coefficient of coupling between the circuits and depends on the geometry of the arrangement and the demagnetizing factor of the core used in the pill. k is related to the mutual where L, and L inductance M between the coils by theexpression 144 = k(Lp L)* are the self-inductances of the pill and the receiving coils respectively. It is important to note that f p does not appear explicitly in the expression for the magnitude of the coupling. A second reasonfor using alargediameter receiving coil and, hence, for preferring a frequency below 10 MHz is that variations in the coupling as the spatial position of the pill varies are less marked than in the case of a small is diamet'er ( 5 20 cm) coil. Thispoint is relevant where asmalldiameter possible and might be thought preferable as, for example, when an implanted pill is to be used to measure intracranial pressure. Indeed, by using a large diameter receiving coil inthissituation,the operation of the pill may be monitored during implantation with minimum risk of interference with the implantation procedure. A third reason in supportof frequencies below 10 MHz is that thepenetration of induction fields into conducting matter tendst o be greater as the frequency is lowered. Further reasons for preferring frequencies below about 10 MHz are apparent when the design of the pill is considered. The ferrite core which is necessary at these frequencies for a highQp enables k to be increased compared with apill of the same size without the ferrite. As the volume of the core is small compared with that enclosed by the receiving coil, the increase in k depends on the demagnetizing factor rather than therelative permeability (assumedB 1)of the core and is, therefore, controlled by its shape. An increase in k by a factor up to about 5 (equivalent to anincrease in received signal of 25 times) can be expected in practice.
T.A . Delchar and M . J . A . Smith
582
Another reason in favour of these relatively low frequencies is that they allow the tuning capacitor in the pill t'o exceed about 1 0 0 p P . f p is then well defined by this capacitance and, as in t'he case of the receiving coil, is not 'pulled' by variable stray capacitance between the pill and its environment. Frequency pulling would be expected with t'he type of pill used by Collins (1967). Pillsoperating below 10 MHz can also beconstructed so t'lzat Q, is a'lmost independent of the conductance of t'heir environment. Degradation of QP occurswhen the electric field gradient along the winding of t'he pill is allowed tointeractwith highlyconductingsurroundings andit is clearly importa'nt to prevent the occurrence of this effect particularly when the pill is used inthegut. A lower limitfor fp can be obtainedby considering the frequency a t which Qp falls below an arbitrary minimum value and is unlikely to be less than thecorresponding limit forthe receiving coil because of the use of ferrite as the core for the pill. If the above reasons are accepted, they provide a strong case in favour of an operating frequency below 10 MHz for a general purpose passive pill, particularly for one intended for use with humans and animals of a similar size. The preferred frequency range probably lies between 0.1 and 10 MHz. However, it should be emphasized that this conclusion depends t o a large extent on the use of a method of measuring the resonant frequency of the pill which is considerably more sensit'ive for a given accura,cy t'han those available hitherto. 4. Locking spectrometer A block diagram of t'he locking spectrometer which has been used successfully to increase the detection range of passive pills is shown in fig. 2 . The "
Sweep Generotor -3 H z
l L
fp++$
Oscillator
7
C
f,
fm"3f,
.
E
RF
Dlode
OscIllotor f
t Dlgltal f- meter
-
DC
control
Tuned arnpiifler
3 fm
-
-
-
1. /S1
In phose
-
OY
PSD
L
I
lf-d12m;I; Phose lock
X 0
Y-f 0
Y - f id)
Fig. 2. Block diagram of theimproved locking spectrometer in theopen-loop ( S 1 open) coupled to a pill of resonant frequencyfp.
nlode
An Improved Method for Detecting Passive Pills
583
spectrometerisbasedonthethird-orderphase-sensitivedetectionsystem operating in the high-pass derivative filter mode and also incorporates the other design criteria discussed above. The receiving coil L consists of two turns of 3 mm diameter coppertube held in an enclosed insulating former of 60 cm diameter and is attached to the RF oscillator to form a single sterilizable unit. The RF oscillator is based on an emitter follower with variable positive feedback between the emitter and a capacit,at'ive divider network across L. The frequency of oscillation is determined by a voltage-controlled capacitance across L and may be varied by & 7% about 4.2 MHz (equivalent to a range of f p of approximately 0.6 MHz or 20 8f for Qp = 7 0 ) . This operating frequencyarises nat'urally fromthe criteria set out insection 3 and is not necessarily the optimum for t'hisequipment.The associated variation in the oscillation amplitude across L , which is caused by the compensat'ing non-linearity of the emitter follower as the impedance of t'he resonant receiving circuitvaries, is typically k 0.05% about 7 VRMS. This change in oscillation amplitude is the frequency-dependent background component b V ( f ) referred to previously andits highvalue is averyreal indication of the need for high-order differentiation. A modulationfrequency f, of 10 kHz enables the pill resonance to be displayed on an oscilloscope with reasonably fast scan speeds. The modulation is obtained by applying a pure sinusoidal voltage a t f, to a second voltagecontrolled capacitance across L. It is particularly important in the design of the locking spectrometer to ensure t'hat no third harmonic component off, is present in theoscillations across L if the low background component characteristic of third-order phase-sensitive detection is to be obtained. Any component a t 3f, will produce the largeaddit'ionalbackgroundassociatedwithfirst just satisfies this derivative recording a t 3fn,. The present arrangement only criterion as a small component at 3f, is generated by t'henon-linear elements in the circuit. I n future designs it is recommended that the waveform applied to the voltage-controlled capacitance is corrected electronically so as to ensure a pure sinusoidal frequency modulation of f p . The spectrometer may be operated in the open-loop mode as shown in fig. 2 (S1 open) and thedifferentiated shape of the pill resonance displayed either on an oscilloscope (at 3 Hz) or an X Y chart recorder connected between X and Y. The oscilloscope display is particularly useful for adjusting the orientation and position of L to obtain the maximum signal and for otheroperations involved in setting up the spectrometer.However, for recording time variations in f p and, t'herefore, in the physiological parameter of interest, i t is generally more convenient to close the loop via the DC cont'rol unit (S1 closed). I n this locked mode, the value of f p may be recorded a t Y - t ( d ) in digital form suitable for computer analysis by choosing a frequency meter with this facility. Alternatively, a continuous (analogue) plot of f p with time may be obtained by of the phase-locked loop (PLL) connectinga Y -t recorder totheoutput frequency discriminator a t Y -t. The DC control unit, which completes the locking loop by connecting the phase-sensitivedetector tothe frequencydeterminingvoltage-controlledcapacitance across L, containsproportional,
584
T . A . Delchar and N . J . A . Smith
differential and integral elements. Integral control is necessary as it provides the capacitance with an offset voltage which compensates automatically for a non-infinite loop gain for Dcllow frequency shifts in fp and enables the high accuracy of the restof the system to be realized. The final time constant of the DC control unit is typically about 30 ms. The functions of the remaining units in the spectrometer should be apparent from fig. 2 without further comment.
5. Operation of the locking spectrometer
The operationof the locking spectrometer is relatively straightforward andis not expected to require highly trained personnel, With the spectrometer in the open-loop mode (S1 open) and a time constant of 1 ms in the phase-sensitive detector (PSD) and the PLL discriminator, the RF oscillator is swept repetitively over its maximum range(4.2 f-0.3 MHz) by a low frequency ( 3 Hz) triangular waveform from t'he sweep generator a'nd the out'putof the PSD is observed on the Y-plates of the mediumpersistence oscilloscope. As theX-plates of the oscilloscope are connected to the output of t'he PLL discriminator, the Xdisplacement is directly proportional to the RF frequency. The position of L , the reference phase and the magnitude of the positivefeedback in the RF oscillator may now be adjusted to give the largest amplitude differentiated resonance (approximating to a broadened third derivative) on the oscilloscope consistentwith a horizontalbaseline and a symmetricalresonanceshape. After some experience with making these adjustments, the operator will find that they correspond closely to the optimum adjustment of the spectrometer, particularly of the reference phase. The gain of t'he tunedamplifier may thenbe set t'o a predetermined level. If required, a record of the resonance shape may be obtained byreducing the frequency of the triangula'r waveformDo a convenient value and byconnecting the X and Y axes of an X Y recorder to thePSD and the PLL discriminator respectively. The time constants of the PSD and the PLL discriminator should be increased by amounts justsufficient to avoid distortion of the resonance shape. This open-loop mode of operation may be useful for determining f p (the pointof zero amplitude at thecentre of the resonance) when the coupling between the pill and the receiving coil is weak provided that f p is not expected to change during the time required to obtain the t'race. I n most cases the closed-loop mode of operation will be found to be more useful. This is achieved by progressivelyreducing the magnit'ude of the triangular waveform and adjusting the centre frequency of the RF oscillator until it is single valued and corresponds approximately to the cent're of the resonance as seen on the oscilloscope. Provided that the circuit parameters in the DC control unit have been correctly pre-set, S1 may now- be closed and the frequency of the oscillator will equalf,. If the P - t recorder is connected to the PLL discriminator (or the digital converter to the frequency meter), the time variation of f p and, hence, the time variation of the required physiological parameter for a properly calibrated pill will be obtained.
An Improved Method for Detecting Passive Pills
585
6. The passive pill Although this paper is concerned primarily with the method of measuring the resonant frequency of a passive pill rather than with the pill itself or with the results of clinical trials, a brief description of the construction of the actual pressure sensitive pill which was used to test the method is clearly desirable. The usually accepted range for a general purpose pressure transducer for medical use is about 300 m Hg (Eversden 1973). This figure is greater than the pressure changes which are expected in vivo as it allows for daily variations in ambient pressure which might otherwise cause the pill to limit. Clearly, if the pill is to operate with maximum sensitivity, a change in pressure of 300 mm Hg should give a corresponding change in frequency which closely matches the available frequency range of the locking spectrometer (0.6 MHz). Experiments withboth capacitive and inductivetransducers confirmed thatthis large frequencyvariation could only be achievedconvenientlybyvarying the inductance of the pill. In thevariable inductance pill, the external pressure acts on apartiallygas-controlled flexible diaphragm and changes the separation between two ferrite cylinders of differing diameters (fig. 3). Approximately 13 Perspexsleeve
Ferrltebody
Sllastic 382 rubber sheath
Fer
wlndtng
winding
Fig. 3. Section through a passive pill.
turns of 36 SWG enamelled copper wire is wound onthe ferrite body and located in a groove in the ferrite in order to avoid degradation of Q, when the pill is placed in a conducting environment. This winding is connected to a further coil comprising four turns of 36 swa enamelled copper wire, mounted on the end of the ferritebodyandaroundthe movableferriteslug. Both windings are connected in the same sense. The larger of the two ferrite cylindersis bored out to receive the 185 p F tuning capacitor. The outer surface of the pill, including the diaphragm, is acastcylindrical sheath of Silastic 382 medical grade elastomer. The pill is 7 mm in diameter, 18 mm long and weighs 1.72 g. A typical pill of this construction has a maximum diaphragm displacement of 1 mm and gives a t least the required sensitivity ( 2 kHz (mm Hg)-l) in its operational frequency range with Qp 70 and a response time less than 30 ms.
T.A . Delchar and
X .J.
A . Smith
Thisperformancecomparesmostfavourably with, forexample, theactive device reported by Watson (1974). Higher values of Q, are possible for somewhat lower pressure sensitivities. It should be mentioned that pressure pills have been constructed in this laboratory with totalvolumes approaching two thirdsof the volume of the pill described above. pH andtemperature sensitive pills have also been constructed. Work is in progress at thepresent time t'o compensat'e for the finite temperature sensitivity of the partially gas-controlled pressure pill and also to ensure t'hat diffusion across the silicone membrane forming the diaphragmis commensurat'e with the long times for which the pill may be in contact with body fluids. 7. Performance of the locking spectrometer and the passive pill
A detailed discussion of the satisfactory agreement which is obtained between the theoretical and actual performance of the locking spectrometer would be out of context in this paper and the reader is referred to the paper by Smith (1978) for further information. The actual performance of the locking spectrometer and pressure pill described above is probably best demonstrated by statingthat f p can be measured with a minimum noise-limited precision of 17; of Sf, or better (approximately equivalent to 0.15 mm Hg in 300 mm Hg) for all values of the angle between the magnetic axes of the pill and the receiving coil up to 9" away from the orthogonal (zero signal) direction, This performanceis achieved with the pill at the centre of the receiving coil and a spectrometer response time of 10 ms. f p remains within the precision limit for modest variations in any of the circuit parameters of the spectrometer, for any position of the pill within the plane of the receiving coil and for the pill immersed in various conducting solutions including HC1of p H = 1.8. Although the orientation of the pill to the ort'hogonal direction has to be increased as it is moved away from the plane of t'he receiving coil, t'he performance is approximately that quoted above throughout' t'he volume ( lo5 cm3) of a sphere defined by t'he receiving coil as a great circle. It should be emphasized that theaccuracy of a pressure measurement usingthe pill is limited at thepresent time by the stabilityof the pill rather than by the precision of the locking spectrometer. The above performance is obtained with a receiving coil of radius U = 30 cm. As, in the authors'experience, a similar performance over a0.6 MHz range of f, using first-order phase-sensitive detection requires a 5 7 cm for the same pill (core radius b = 2.5 mm),the lockingspectrometerbasedonthird-order detectionclearlyoffersaveryworthwhileimprovement. For a pill inthe optimum orientation at t'he centre of the receiving coil, the magnitude of the signalinducedin the receiving coil is proportional to k2 and, therefore, to (b/a)3 for any order of detection. Consequent'ly a decrease in b/a by a factor of approximately 4.3 represents a corresponding decrease by a factor of about 80 in the magnitude of the induced signal which may be detected with the given precision. This bias in favour of third-order detection is even greater when the minimum distance between the pill and receiving coil is restricted so that itlies
An Improved Method for Detecting Passive Pills
587
between the radius of the receiving coils appropriate to first- and third-order detection. In thecase of first-order detection where the smaller radius receiving coil is necessary, the pill cannot be positioned at the centre of the coil. The magnitude of the induced signal is then proportional to [ab/(a2+ z2)I3where z is the perpendicular distance between the pill and thecentre of the receiving coil, and is clearly less than in the case when z = 0. The performanceof the prototypepressure pill measured with the third-order locking spectrometer is illustrated by fig. 4. Fig. 4 shows the variation in f p as a function of the staticpressure exerted onthe water in which the pill is immersed. The non-linearity of the curve is a consequence of the relative geometry of the particular ferrite cores used in the pill and may be reduced at the expense of
71
L'31
L32
130
L26
L26
L2L
L22
Resonant frequency [MHz)
Fig. 4 . Resonant frequency f p of the prototype pressure pill measured by the locking spectrometer as a function of the applied static pressure in a physiological range.
sensitivity. As the slope of the curve in fig. 4 is nowhere less than the required 2 kHz (mm Hg)-l, a linear characteristic is a real possibility in future designs. The frequency response of the pill is essentially flat to at least 30 Hz. 8.
Conclusions
The high cost, limitedlife and relatively largesize of the endoradiosonde have been important factors in restricting the use of this form of telemetry for the measurement of various physiological parameters.Althoughthealternative passive pill overcomes these disadvantages, its use has been restricted by its short range if it is to operate over a reasonable range of orientations. The locking spectrometer described in this paper provides a significant increase in the rangeof the passive pill and itshould now be possible to measure accurately the frequency of a pill situated at any point within the human body. We are grateful for the encouragement given by members of the Research Committee of the Warwickshire Postgraduate Medical Centre and by M r . B. Williams of the Midland Centre for Neurosurgery and Neurology. 21
588
An Improved Method for Detecting Passive Pills
RJ~UMI? Une mbthode am6lior6e de detection des pilules passives L’un des principaux inconvbnients du comprimb passif, en tant que mirthode de t616m6trie pour relever divers parambtres physiologiques, Btait, jusqu’8 prbsent, sa portbe limit6e. L’exposB examine les raisons de cette port6e limitbe avec les m6thodes de sondage actuelles. Une mitthode ambliorbe, utilisant un spectrombtre blocable, bas6 sur le sondage sensible aux phases de troisibme ordre, est decrite et ses performances sont 6valu6es. On obtient un accroissement sensible de la portBe utile d’un cornprim6 passif B haute sensibilite.
ZUSAMMENFASSUNO; Verbesserte Methode zum Kachweis passiver Pillen Einerder wesentlichen Sachteile der passiven Pille alstelemetrische Methode zur Messung verschiedener physiologischer Parameter ist ihre beschrankte Reichweite gewesen. Die Griinde fur diese beschrankte Reichweite in Verbindungmit bestehenden Nachweismethoden werden erortert. Beschrieben wird eine verbesserte Methode unter Verwendung eines Sperrspektrometers auf der Grundlage von phasenempfindlichem Kachweis dritten Grades und dessen Arbeit eingeschatzt. Das Ergebnis ist eine betriichtliche Zunahme der nutzbaren Reichweite einer hochempfindlichen passiven Pille.
REFERENCES COLLINS,C. C., 1967, IEEE Trans. Bio-Med. Eng., 14, 74-83. EVERSDEN,I. D., 1973, Biomed. Eng., 8, 192-197. FARRAR,J. T.,ZWORYKIN,V. K., and BAUM,J., 1957, Science, K.Y., 125, 97.5-976. MACKAY,R. S.,and JACOBSON, B., 1957,Lancet, 272, 1224-1225. W. B., RICHARDS, V., ADAMS,J. E . , and OLSEN,E. R., COLLISS, C. C., LOUGHBOROUGH, PISTO,D. W., 1967, Am. J . Surg., 113, 727-729. SMITH,M. J. A., 1975, J . Phys. E : S c i . Instrum., 8, 1058-1062. H. C., and MELDRUM,S.J., 1974, Phys. Ned. WATSON,B. W., CURRIE, J. C. M., RIDDLE, Biol., 19, 86-96.
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An improved method for detecting passive pills (telemetric measurement of physiological parameters)
This content has been downloaded from IOPscience. Please scroll down to see the full text. 1976 Phys. Med. Biol. 21 577 (http://iopscience.iop.org/0031-9155/21/4/009) View the table of contents for this issue, or go to the journal homepage for more
Download details: IP Address: 130.237.165.40 This content was downloaded on 01/10/2015 at 01:59
Please note that terms and conditions apply.
PHYS. MED. BIOL., 1976, VOL.
21,
NO.
4,577-588.
@
1976
An Improved Method for Detecting Passive Pills T. A. DELCHAR and M. J. A. SMITH Department of Physics, University of Warwick, Coventry Received 14 October 1975, infinal f o r m 12 March 1976 ABSTRACT.One of the principaldisadvantages of the passive pill as a telemetric method for measuring various physiological parameters has been its restricted range. The reasons for the restricted range with existing detection methods are discussed. An improved method using a locking spectrometer based on third-order phase-sensitive detection is described and its performance is assessed. A significant increase in the usable range of a high sensitivity passive pill is obtained.
1. Introduction
The progressive development of microminiature electronic components in recent years hasenabled considerable improvements to be made inthe performance of the endoradiosonde or active pill since the pioneering efforts of Mackay and Jacobson (1957). It is now possible, using this method of telemetry, to obtain continuous information on internal physiological parameters such as pressure, temperature and pH by the insertion or ingestion of a suitable active pill. However, the need to process and to transmit the relevant information and the requirement, therefore,for an internal power supply anda switch has meant that active pills are relatively large (typically 10 mm in diameter and up t o 30 mm in length), areexpensive to manufacture and arelimited in thetime for which they can operate. An alternative method of obtaining the required information, also without the use of connecting wires, is the passive pill (Farrar, Zworykin and Baum 1957) which consists of a single electrical resonant circuit with the resonant frequency controlled by a transducer sensitive t o the appropriate parameter. The pill is coupled by mutual inductance to a tuned receiving coil which both excit'es and measures the frequency of the pill's resonance. The passive pill does not require an internalpower supply and contains a minimal number of circuit components, often only an inductor and a capacitor. It follows that it can be small, inexpensive to manufacture and, therefore, disposable. It can also have an almost indefinite operational life provided that its outer membrane is made would be particularly impermeable t o body fluids. Thelatteradvantage important when, for example,measurements of intracranialpressureare required. The two major disadvantages which have been foreseen for the passive pill are its restricted range,that is the maximum distance between the pill and the receiving coil for a given accuracyof measuring the resonant frequency, and its failure t o operate when the magnetic axes of the pill and the receiving coil are orthogonal.
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The primary purpose of this paper is to show that themaximum range of the passive pill, for a given range of angles between the magnetic axes, may be increased significantly by the use of odd-order phase-sensitive detection in the high-pass derivative filter mode (Smith 1975). Methods for measuring the resonant frequency of a passive pill Anymethodfordetermining the resonantfrequency f,, of a passive pill coupled by mutual inductance to the tuned circuit of a variable frequency oscillator involves a measurement of the frequency of the oscillator when the coupling is a maximum. The value of a pa'rticular method may, therefore, be judged by the accuracy with which it performs this measurement. In the direct method, maximum coupling corresponding to maximum power loss in thereceiving coil is assumed to occur when the outputfrom the oscillator is a minimum. This assumptionis, however, often inappropriate andcan leadto inaccuracies well before the noise limit of the system is reached. The origin of the inaccuracies may be illustrated qualitatively as follows. An inherent property of any amplitude-limited oscillator based on a tuned circuit is thevariation which occurs in the output of the oscillator asits frequency is varied. The variation, which will be referred to as the frequencydependent background component bV(f ), may be represent'ed by curve (a)in fig. 1. Line ( d ) in fig. 1 represents the frequency-independent component " V of 2.
Oscillation arnplliude
bV
Fig. 1. Frequency variation of the components of oscillation amplitude in an amplitudelimited oscillatorcoupled t o an electricalresonance. Thefrequency fp at the intersection of the axes corresponds to the resonantfrequency.
the oscillation amplitude and curve ( b ) the component p V ( f )produced when a resonant pill is allowed to couple to the oscillator. The sum of all three components is represented by curve (c). I n t h e case of strong coupling, " V (f) changes with frequency nearto f, a t a much greaterrate thanb V (f ). Maximum coupling then corresponds closely to minimumoscillation amplitude and fp may be determined with acceptable accuracy over a range of values of f p . bV may be removed after rectification by introducing a DC offset voltage and does not affect the accuracy of the measurement provided that the gain of the
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system remains constant. I n t h ecase of weak coupling, p V ( f ) can change with frequency near to f,a t a rate comparable with b V ( f ) with the result that a minimum output from the oscillator nolonger corresponds exactly to maximum the magnitude of the coupling. Moreover, a.s f p variesdue to achangein physiological parameter being measured, the relationship between minimum output and maximum coupling also varies. I n t h e extreme case of very weak coupling, which is the situationof interest in practice, nowell defined minimum isobtained. For other than an unrealistically sharp pill resonance(high Qfactor)and asmallrange of variation of fp, the directmethod is clearly unsatisfactory as the error in measuring f p is caused by a large frequencydependent background component generated by the oscillator rather than by system noise. The problem is essentially oneof pattern recognition and itis well known that the accuracy of the directmethodmay beimprovedbymodulatingt'he frequency of the oscillator by asmall amount and using aphase-sensitive detector with an in-phase reference at the modulating frequency. The phasesensitive detector is connected to the oscillator through an amplifier tuned to the modulation frequency and a diode which provides a signal voltage proportional to the oscillation amplitude. I n some oscillators, notably the grid-dip oscillator, the diode is unnecessaryas efficient rectification is carried out by the same active element which generates the oscillations. For small modulation amplitudes (up to about0.4 times the half width Sf, of the pill resonance), the output of the phase-sensitive detector is proportional to the first derivative of the resonance. f,is, therefore, equalto thefrequency of the oscillator when the contribution from the pill to the output from the phase-sensitive detector is zero. The value of this method lies in the fact that all the terms in fig. 1 are differentiated once with respect to frequency. I n particular, b V is eliminated while the rate atwhich d ( b V ( f ) ) / d fchanges with frequency in the region of f p is now lower relative to d ( P V ( f))/d,fthan before differentiation. I n addition t'o the improved discrimination which first-order phase-sensitive detection can provide against the frequency-dependent background component, the pill component of the outputof the phase-sensitive detector in theregion of f p is proportional in both magnitude and sign to the deviation in frequency between the oscillator and f,. This form of output is particularly convenientas it may be used directly to lock the frequency of the oscillator to f,. Alt'hough first-order phase-sensitive detection offers a worthwhile improvement over the direct method for determining the point of maximum coupling between the pill and theoscillator, the error in f p for typical values of both the Q-factor of the pill (Q, 50-120) and the frequency sweep range ( 5 20Sf,) is stilllimited bythe frequency-dependentbackgroundcomponentfrom the oscillator rather than by system noise. It would seem logical a t this point to consider if phase-sensitive detection a t a multiple m (greater t'han one) of the modulation frequency f, can give a further reduction in b V (f ) as a result of high-order differentiation. A detailed analysis of this method when m is odd has been given by Smith (1975) and it is shown that the outputfrom a phasesensitive detector operating with an in-phase reference a t mf, contains odd
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derivatives of the resonance shape of order m and above when large sinusoidal modulationamplitudes ( Sf,) are used (high-pass,derivative filter mode). The most important conclusions from the analysis are, first of all, that, for QP in the approximate range 50-120, m = 3 is sufficient to reduce the magnitude of the frequency-dependentbackgroundcomponentbyafurtherlargefactor (typically in excess of lo4) compared with the situation when m = 1. I n most cases this is sufficient to reduce t'he background component below the noise level. Secondly, the fact'or by which the magnitude of the pill component in t'he small region of f, is reduced by increasing m from 1 to 3 is,bycontrast, (typically 4). Thirdly, althoughthe sign of successive odd derivatives alternates, the combined output has thesame form in the region off, as thefirst derivative and retains the very desirable property of first derivative recording t'hat the output is proportional to the frequency deviation from f,. The method of third-order phase-sensitive detection in the high-pass derivative filter mode should, therefore, be capable of increasing the range of the passive pill for a given accuracy in the determination off, as the method is limited by noise and not by a frequency-dependent background generated by the pillsensing oscillator. I n certain circumstances, pa'rticularly where low values of Qp and system noise are expected, it may be necessary to consider using fifth-order detection with large modulation amplitudes. N
3.
Choice of operating frequency
Before describing a suitable third-order phase-sensitive detection systemand an associated pill, it is necessary to choose an optimum valuefor fp or, a t least, suggest a rangeof possible operating frequencies. A number of different criteria have been adopted previously to determine fp dependingon the particular environment of the pill and the availablerange. For example, the elegant passive pressure pills developed by Collins (1 967)for studying glaucoma in the rabbit and byOlsen, Collins, Loughborough, Richards, Adams and Pinto (1967) for measuring intracranial pressure operated in the region of 95 MHz. This relatively high frequency enabled air-cored inductors tuned by self-capacitance to be used and ensured that thepills were physically small. There are, however, a number of reasons why a lower frequency is preferable for a passive pill intended for more general use particularlywhenasensitive and accurate detection method is available. The first and probably the prime reason is related directly to the expected maximum range. If, as is clearly desirable, the pill is to be used in the gut, the ideal minimum diameterof the receiving coil should be such that it can be placed around the patient. There is then no orientation of the pill a t which f p cannot be measured by a suitable adjustment of the position of the coil. Diameters approaching one metre are required and, as one turn is the minimum possible for the receiving coil, f p should be less than about 10 MHz. I n making this estimate for an upper limit to f,, it is assumed that a minimum tuning capacitance for the coil of approximately 200 p F is required if capacitance changes due
An Improved Method for Detecting Passive Pills
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to thepresence and movement of the patient are not change to the frequency of the oscillator to any significant extent. An estimate for a lower limit to f p may be obtained by recognizing that the magnitude of the received signal is proportional to the loaded Q-factor ( Q ) of the air-cored receiving coil, that is the Q-factor when the coil is connected t o the oscillator. From this point of view Q should be as high as possible. However, a contraryargument for a lower Q follows from the requirement that the presence or movement of a patient inside the coil should not cause an unduly large change inthe oscillation amplitude. A suitable compromise may be obtained by designing the coil so that it has an unloaded Q-factor greater than 100 and by adjusting the level of positive feedback inthe oscillator with the coil connected so that theloaded Q is about a factor of two or three times less than the unloadedvalue.Experience gained from constructingvarious receiving coils of about 1 m diameter tuned t'o frequencies between 50 kHz and 10 MHz suggests that 100 kHz is a suitablelower limit forfpon the basis of the suggested compromise. More specifically the magnitude of the coupling between the two resonant circuits and, therefore, the magnitude of the signal available for the measurement of f,,is proportional to k2QQp where k is the coefficient of coupling between the circuits and depends on the geometry of the arrangement and the demagnetizing factor of the core used in the pill. k is related to the mutual where L, and L inductance M between the coils by theexpression 144 = k(Lp L)* are the self-inductances of the pill and the receiving coils respectively. It is important to note that f p does not appear explicitly in the expression for the magnitude of the coupling. A second reasonfor using alargediameter receiving coil and, hence, for preferring a frequency below 10 MHz is that variations in the coupling as the spatial position of the pill varies are less marked than in the case of a small is diamet'er ( 5 20 cm) coil. Thispoint is relevant where asmalldiameter possible and might be thought preferable as, for example, when an implanted pill is to be used to measure intracranial pressure. Indeed, by using a large diameter receiving coil inthissituation,the operation of the pill may be monitored during implantation with minimum risk of interference with the implantation procedure. A third reason in supportof frequencies below 10 MHz is that thepenetration of induction fields into conducting matter tendst o be greater as the frequency is lowered. Further reasons for preferring frequencies below about 10 MHz are apparent when the design of the pill is considered. The ferrite core which is necessary at these frequencies for a highQp enables k to be increased compared with apill of the same size without the ferrite. As the volume of the core is small compared with that enclosed by the receiving coil, the increase in k depends on the demagnetizing factor rather than therelative permeability (assumedB 1)of the core and is, therefore, controlled by its shape. An increase in k by a factor up to about 5 (equivalent to anincrease in received signal of 25 times) can be expected in practice.
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Another reason in favour of these relatively low frequencies is that they allow the tuning capacitor in the pill t'o exceed about 1 0 0 p P . f p is then well defined by this capacitance and, as in t'he case of the receiving coil, is not 'pulled' by variable stray capacitance between the pill and its environment. Frequency pulling would be expected with t'he type of pill used by Collins (1967). Pillsoperating below 10 MHz can also beconstructed so t'lzat Q, is a'lmost independent of the conductance of t'heir environment. Degradation of QP occurswhen the electric field gradient along the winding of t'he pill is allowed tointeractwith highlyconductingsurroundings andit is clearly importa'nt to prevent the occurrence of this effect particularly when the pill is used inthegut. A lower limitfor fp can be obtainedby considering the frequency a t which Qp falls below an arbitrary minimum value and is unlikely to be less than thecorresponding limit forthe receiving coil because of the use of ferrite as the core for the pill. If the above reasons are accepted, they provide a strong case in favour of an operating frequency below 10 MHz for a general purpose passive pill, particularly for one intended for use with humans and animals of a similar size. The preferred frequency range probably lies between 0.1 and 10 MHz. However, it should be emphasized that this conclusion depends t o a large extent on the use of a method of measuring the resonant frequency of the pill which is considerably more sensit'ive for a given accura,cy t'han those available hitherto. 4. Locking spectrometer A block diagram of t'he locking spectrometer which has been used successfully to increase the detection range of passive pills is shown in fig. 2 . The "
Sweep Generotor -3 H z
l L
fp++$
Oscillator
7
C
f,
fm"3f,
.
E
RF
Dlode
OscIllotor f
t Dlgltal f- meter
-
DC
control
Tuned arnpiifler
3 fm
-
-
-
1. /S1
In phose
-
OY
PSD
L
I
lf-d12m;I; Phose lock
X 0
Y-f 0
Y - f id)
Fig. 2. Block diagram of theimproved locking spectrometer in theopen-loop ( S 1 open) coupled to a pill of resonant frequencyfp.
nlode
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spectrometerisbasedonthethird-orderphase-sensitivedetectionsystem operating in the high-pass derivative filter mode and also incorporates the other design criteria discussed above. The receiving coil L consists of two turns of 3 mm diameter coppertube held in an enclosed insulating former of 60 cm diameter and is attached to the RF oscillator to form a single sterilizable unit. The RF oscillator is based on an emitter follower with variable positive feedback between the emitter and a capacit,at'ive divider network across L. The frequency of oscillation is determined by a voltage-controlled capacitance across L and may be varied by & 7% about 4.2 MHz (equivalent to a range of f p of approximately 0.6 MHz or 20 8f for Qp = 7 0 ) . This operating frequencyarises nat'urally fromthe criteria set out insection 3 and is not necessarily the optimum for t'hisequipment.The associated variation in the oscillation amplitude across L , which is caused by the compensat'ing non-linearity of the emitter follower as the impedance of t'he resonant receiving circuitvaries, is typically k 0.05% about 7 VRMS. This change in oscillation amplitude is the frequency-dependent background component b V ( f ) referred to previously andits highvalue is averyreal indication of the need for high-order differentiation. A modulationfrequency f, of 10 kHz enables the pill resonance to be displayed on an oscilloscope with reasonably fast scan speeds. The modulation is obtained by applying a pure sinusoidal voltage a t f, to a second voltagecontrolled capacitance across L. It is particularly important in the design of the locking spectrometer to ensure t'hat no third harmonic component off, is present in theoscillations across L if the low background component characteristic of third-order phase-sensitive detection is to be obtained. Any component a t 3f, will produce the largeaddit'ionalbackgroundassociatedwithfirst just satisfies this derivative recording a t 3fn,. The present arrangement only criterion as a small component at 3f, is generated by t'henon-linear elements in the circuit. I n future designs it is recommended that the waveform applied to the voltage-controlled capacitance is corrected electronically so as to ensure a pure sinusoidal frequency modulation of f p . The spectrometer may be operated in the open-loop mode as shown in fig. 2 (S1 open) and thedifferentiated shape of the pill resonance displayed either on an oscilloscope (at 3 Hz) or an X Y chart recorder connected between X and Y. The oscilloscope display is particularly useful for adjusting the orientation and position of L to obtain the maximum signal and for otheroperations involved in setting up the spectrometer.However, for recording time variations in f p and, t'herefore, in the physiological parameter of interest, i t is generally more convenient to close the loop via the DC cont'rol unit (S1 closed). I n this locked mode, the value of f p may be recorded a t Y - t ( d ) in digital form suitable for computer analysis by choosing a frequency meter with this facility. Alternatively, a continuous (analogue) plot of f p with time may be obtained by of the phase-locked loop (PLL) connectinga Y -t recorder totheoutput frequency discriminator a t Y -t. The DC control unit, which completes the locking loop by connecting the phase-sensitivedetector tothe frequencydeterminingvoltage-controlledcapacitance across L, containsproportional,
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T . A . Delchar and N . J . A . Smith
differential and integral elements. Integral control is necessary as it provides the capacitance with an offset voltage which compensates automatically for a non-infinite loop gain for Dcllow frequency shifts in fp and enables the high accuracy of the restof the system to be realized. The final time constant of the DC control unit is typically about 30 ms. The functions of the remaining units in the spectrometer should be apparent from fig. 2 without further comment.
5. Operation of the locking spectrometer
The operationof the locking spectrometer is relatively straightforward andis not expected to require highly trained personnel, With the spectrometer in the open-loop mode (S1 open) and a time constant of 1 ms in the phase-sensitive detector (PSD) and the PLL discriminator, the RF oscillator is swept repetitively over its maximum range(4.2 f-0.3 MHz) by a low frequency ( 3 Hz) triangular waveform from t'he sweep generator a'nd the out'putof the PSD is observed on the Y-plates of the mediumpersistence oscilloscope. As theX-plates of the oscilloscope are connected to the output of t'he PLL discriminator, the Xdisplacement is directly proportional to the RF frequency. The position of L , the reference phase and the magnitude of the positivefeedback in the RF oscillator may now be adjusted to give the largest amplitude differentiated resonance (approximating to a broadened third derivative) on the oscilloscope consistentwith a horizontalbaseline and a symmetricalresonanceshape. After some experience with making these adjustments, the operator will find that they correspond closely to the optimum adjustment of the spectrometer, particularly of the reference phase. The gain of t'he tunedamplifier may thenbe set t'o a predetermined level. If required, a record of the resonance shape may be obtained byreducing the frequency of the triangula'r waveformDo a convenient value and byconnecting the X and Y axes of an X Y recorder to thePSD and the PLL discriminator respectively. The time constants of the PSD and the PLL discriminator should be increased by amounts justsufficient to avoid distortion of the resonance shape. This open-loop mode of operation may be useful for determining f p (the pointof zero amplitude at thecentre of the resonance) when the coupling between the pill and the receiving coil is weak provided that f p is not expected to change during the time required to obtain the t'race. I n most cases the closed-loop mode of operation will be found to be more useful. This is achieved by progressivelyreducing the magnit'ude of the triangular waveform and adjusting the centre frequency of the RF oscillator until it is single valued and corresponds approximately to the cent're of the resonance as seen on the oscilloscope. Provided that the circuit parameters in the DC control unit have been correctly pre-set, S1 may now- be closed and the frequency of the oscillator will equalf,. If the P - t recorder is connected to the PLL discriminator (or the digital converter to the frequency meter), the time variation of f p and, hence, the time variation of the required physiological parameter for a properly calibrated pill will be obtained.
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6. The passive pill Although this paper is concerned primarily with the method of measuring the resonant frequency of a passive pill rather than with the pill itself or with the results of clinical trials, a brief description of the construction of the actual pressure sensitive pill which was used to test the method is clearly desirable. The usually accepted range for a general purpose pressure transducer for medical use is about 300 m Hg (Eversden 1973). This figure is greater than the pressure changes which are expected in vivo as it allows for daily variations in ambient pressure which might otherwise cause the pill to limit. Clearly, if the pill is to operate with maximum sensitivity, a change in pressure of 300 mm Hg should give a corresponding change in frequency which closely matches the available frequency range of the locking spectrometer (0.6 MHz). Experiments withboth capacitive and inductivetransducers confirmed thatthis large frequencyvariation could only be achievedconvenientlybyvarying the inductance of the pill. In thevariable inductance pill, the external pressure acts on apartiallygas-controlled flexible diaphragm and changes the separation between two ferrite cylinders of differing diameters (fig. 3). Approximately 13 Perspexsleeve
Ferrltebody
Sllastic 382 rubber sheath
Fer
wlndtng
winding
Fig. 3. Section through a passive pill.
turns of 36 SWG enamelled copper wire is wound onthe ferrite body and located in a groove in the ferrite in order to avoid degradation of Q, when the pill is placed in a conducting environment. This winding is connected to a further coil comprising four turns of 36 swa enamelled copper wire, mounted on the end of the ferritebodyandaroundthe movableferriteslug. Both windings are connected in the same sense. The larger of the two ferrite cylindersis bored out to receive the 185 p F tuning capacitor. The outer surface of the pill, including the diaphragm, is acastcylindrical sheath of Silastic 382 medical grade elastomer. The pill is 7 mm in diameter, 18 mm long and weighs 1.72 g. A typical pill of this construction has a maximum diaphragm displacement of 1 mm and gives a t least the required sensitivity ( 2 kHz (mm Hg)-l) in its operational frequency range with Qp 70 and a response time less than 30 ms.
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Thisperformancecomparesmostfavourably with, forexample, theactive device reported by Watson (1974). Higher values of Q, are possible for somewhat lower pressure sensitivities. It should be mentioned that pressure pills have been constructed in this laboratory with totalvolumes approaching two thirdsof the volume of the pill described above. pH andtemperature sensitive pills have also been constructed. Work is in progress at thepresent time t'o compensat'e for the finite temperature sensitivity of the partially gas-controlled pressure pill and also to ensure t'hat diffusion across the silicone membrane forming the diaphragmis commensurat'e with the long times for which the pill may be in contact with body fluids. 7. Performance of the locking spectrometer and the passive pill
A detailed discussion of the satisfactory agreement which is obtained between the theoretical and actual performance of the locking spectrometer would be out of context in this paper and the reader is referred to the paper by Smith (1978) for further information. The actual performance of the locking spectrometer and pressure pill described above is probably best demonstrated by statingthat f p can be measured with a minimum noise-limited precision of 17; of Sf, or better (approximately equivalent to 0.15 mm Hg in 300 mm Hg) for all values of the angle between the magnetic axes of the pill and the receiving coil up to 9" away from the orthogonal (zero signal) direction, This performanceis achieved with the pill at the centre of the receiving coil and a spectrometer response time of 10 ms. f p remains within the precision limit for modest variations in any of the circuit parameters of the spectrometer, for any position of the pill within the plane of the receiving coil and for the pill immersed in various conducting solutions including HC1of p H = 1.8. Although the orientation of the pill to the ort'hogonal direction has to be increased as it is moved away from the plane of t'he receiving coil, t'he performance is approximately that quoted above throughout' t'he volume ( lo5 cm3) of a sphere defined by t'he receiving coil as a great circle. It should be emphasized that theaccuracy of a pressure measurement usingthe pill is limited at thepresent time by the stabilityof the pill rather than by the precision of the locking spectrometer. The above performance is obtained with a receiving coil of radius U = 30 cm. As, in the authors'experience, a similar performance over a0.6 MHz range of f, using first-order phase-sensitive detection requires a 5 7 cm for the same pill (core radius b = 2.5 mm),the lockingspectrometerbasedonthird-order detectionclearlyoffersaveryworthwhileimprovement. For a pill inthe optimum orientation at t'he centre of the receiving coil, the magnitude of the signalinducedin the receiving coil is proportional to k2 and, therefore, to (b/a)3 for any order of detection. Consequent'ly a decrease in b/a by a factor of approximately 4.3 represents a corresponding decrease by a factor of about 80 in the magnitude of the induced signal which may be detected with the given precision. This bias in favour of third-order detection is even greater when the minimum distance between the pill and receiving coil is restricted so that itlies
An Improved Method for Detecting Passive Pills
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between the radius of the receiving coils appropriate to first- and third-order detection. In thecase of first-order detection where the smaller radius receiving coil is necessary, the pill cannot be positioned at the centre of the coil. The magnitude of the induced signal is then proportional to [ab/(a2+ z2)I3where z is the perpendicular distance between the pill and thecentre of the receiving coil, and is clearly less than in the case when z = 0. The performanceof the prototypepressure pill measured with the third-order locking spectrometer is illustrated by fig. 4. Fig. 4 shows the variation in f p as a function of the staticpressure exerted onthe water in which the pill is immersed. The non-linearity of the curve is a consequence of the relative geometry of the particular ferrite cores used in the pill and may be reduced at the expense of
71
L'31
L32
130
L26
L26
L2L
L22
Resonant frequency [MHz)
Fig. 4 . Resonant frequency f p of the prototype pressure pill measured by the locking spectrometer as a function of the applied static pressure in a physiological range.
sensitivity. As the slope of the curve in fig. 4 is nowhere less than the required 2 kHz (mm Hg)-l, a linear characteristic is a real possibility in future designs. The frequency response of the pill is essentially flat to at least 30 Hz. 8.
Conclusions
The high cost, limitedlife and relatively largesize of the endoradiosonde have been important factors in restricting the use of this form of telemetry for the measurement of various physiological parameters.Althoughthealternative passive pill overcomes these disadvantages, its use has been restricted by its short range if it is to operate over a reasonable range of orientations. The locking spectrometer described in this paper provides a significant increase in the rangeof the passive pill and itshould now be possible to measure accurately the frequency of a pill situated at any point within the human body. We are grateful for the encouragement given by members of the Research Committee of the Warwickshire Postgraduate Medical Centre and by M r . B. Williams of the Midland Centre for Neurosurgery and Neurology. 21
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RJ~UMI? Une mbthode am6lior6e de detection des pilules passives L’un des principaux inconvbnients du comprimb passif, en tant que mirthode de t616m6trie pour relever divers parambtres physiologiques, Btait, jusqu’8 prbsent, sa portbe limit6e. L’exposB examine les raisons de cette port6e limitbe avec les m6thodes de sondage actuelles. Une mitthode ambliorbe, utilisant un spectrombtre blocable, bas6 sur le sondage sensible aux phases de troisibme ordre, est decrite et ses performances sont 6valu6es. On obtient un accroissement sensible de la portBe utile d’un cornprim6 passif B haute sensibilite.
ZUSAMMENFASSUNO; Verbesserte Methode zum Kachweis passiver Pillen Einerder wesentlichen Sachteile der passiven Pille alstelemetrische Methode zur Messung verschiedener physiologischer Parameter ist ihre beschrankte Reichweite gewesen. Die Griinde fur diese beschrankte Reichweite in Verbindungmit bestehenden Nachweismethoden werden erortert. Beschrieben wird eine verbesserte Methode unter Verwendung eines Sperrspektrometers auf der Grundlage von phasenempfindlichem Kachweis dritten Grades und dessen Arbeit eingeschatzt. Das Ergebnis ist eine betriichtliche Zunahme der nutzbaren Reichweite einer hochempfindlichen passiven Pille.
REFERENCES COLLINS,C. C., 1967, IEEE Trans. Bio-Med. Eng., 14, 74-83. EVERSDEN,I. D., 1973, Biomed. Eng., 8, 192-197. FARRAR,J. T.,ZWORYKIN,V. K., and BAUM,J., 1957, Science, K.Y., 125, 97.5-976. MACKAY,R. S.,and JACOBSON, B., 1957,Lancet, 272, 1224-1225. W. B., RICHARDS, V., ADAMS,J. E . , and OLSEN,E. R., COLLISS, C. C., LOUGHBOROUGH, PISTO,D. W., 1967, Am. J . Surg., 113, 727-729. SMITH,M. J. A., 1975, J . Phys. E : S c i . Instrum., 8, 1058-1062. H. C., and MELDRUM,S.J., 1974, Phys. Ned. WATSON,B. W., CURRIE, J. C. M., RIDDLE, Biol., 19, 86-96.
