WO2011014464A2 - Lead for use in rf field - Google Patents

Abstract

A lead such as used for pacemaking or neural stimulation has a current block IC which blocks current when RF is detected. In this way problems of tissue heating are reduced or eliminated when the patient carrying such a lead is given an MRI. The lead may also detect localized temperature, to be reported to equipment external to the lead. This may permit more judicious use of the RF energy, for example during an MRI session. Localized and spurious tissue heating nearby to an electrode of a lead, due to RF energy from an MRI, is reduced or eliminated by detecting the RF field, deriving energy from the field, and actively driving a node that is coupled to such an electrode so as to minimize voltage excursions at the electrode. A state is maintained or set within electronics nearby to an electrode of a lead, so that a can which connects with the lead is able to learn whether an MRI event has occurred since the last time that the can had sent commands to the electronics to set particular internal states. For a lead that delivers pacing pulses, a pacing detector is provided in addition to an MRI detector, and the blocking action is carried out most of the time but is disabled briefly for each pacing pulse. During a time of presence of RF energy, a bucking circuit attempts, to the extent possible, to drive a node toward blood potential, the node capacitively coupled with electrodes of the lead.

Description

Lead for use in RF field Cross-reference to related applications

This patent application claims the benefit of US application number 61/230,505, filed July 31, 2009, and US application number 61/296,853, filed January 20, 2010, and US application number

61/316,378, filed March 22, 2010, each of which is incorporated herein by reference for all purposes.

Background

Implanted leads and MRIs don't go well together, and this is a problem.

Leads can be effectively used in human subjects to permit, for example, the delivery of pacing signals to heart tissue, or the delivery of defibrillation signals to heart tissue, or the collection of EKG signals from heart tissue. Because leads offer these and many other diagnostic and therapeutic benefits, it happens more and more often that a lead is implanted within a human subject, and remains within the human subject for months or years. The lead is usually employed in connection with a "can" which resides at one end of the lead. The can contains circuitry and a power source and is designed to achieve some particular result or results. A can might be a pacemaker, or might be a defibrillator, or might be a neural stimulator, for example.

Magnetic Resonance Imaging (MRI) has proven to be a very helpful diagnostic and research tool. With MRI, the subject is placed within a strong magnetic field, and a pulse of strong RF (radio frequency) energy is applied. For particular nuclei, and under certain combinations of magnetic field strength and RF energy, a sensitive circuit may be able to detect energy re-emitted by the nuclei. A gradient in the magnetic field may facilitate imaging, in which the detected re-emitted energy permits the MRI system to develop images of (for example) human tissue. Atypical RF field might be 64 MHz or 128 MHz in frequency. If an implanted lead (such as a pacemaker lead or neural stimulation lead) is present in a patient at the time that an MRI is taking place, the result can be damage to tissue, for example due to localized heating of the tissue, because of voltages induced in the lead. The induced voltages are electrically coupled to tissues of the patient at the implanted device (sometimes called "the can") and at the one or more electrodes of the lead. An MRI scan may last six minutes or even thirty minutes.

Various approaches have been tried, such as the use of discrete inductors, to try to reduce or block the induced currents in the lead. This approach has many drawbacks including the fact that such inductors are bulky. An exemplary lead might be two millimeters in diameter, and it is difficult to fit such inductors into such a form factor.

As mentioned above, it turns out that if a subject in whom a lead has been implanted is subjected to magnetic resonance imaging, tissue heating is sometimes observed at points where the lead is in electrical contact with tissue. The explanation is that some of the RF energy couples to all or portions of the lead, developing potentials at (for example) electrodes along the lead. The potentials cause current flows through the tissue. The current flows can give rise to localized heating of the tissue.

Some leads have sophisticated satellites along their length. In each satellite is a microchip which is able to receive control messages from the can. The control message may instruct the chip to (for example) connect a particular electrode (among several electrodes) to a conductor running along the length of the lead. The chip will thus maintain one or more internal states, so that for some period of hours or days after receipt of the control message, the chip continues to connect (for example) the particular electrode to the particular conductor.

If such a lead happens to have been implanted into a subject that is subjected to MRI, there is the risk that the function of the chip will be disrupted, and the internal states may fail to be maintained. This is an undesirable result, because then the can will be unable to carry out its intended functions. Similar undesirable losses of states might happen in the event of external defibrillation (where typically 40 Joules might be discharged within the patient), or in the event of X-rays, or in the event of

electrocautery.

As will be discussed below, one approach to address the problem of localized tissue heating is to provide "blocking chips", integrated circuits that cut off the normal pass-through of the Sl and S2 signals along the length of the lead. While the blocking chips (as will be discussed below) can help with reducing the unwanted coupling between the lead and the ambient RF field, such cutting-off of conductivity of the Sl and S2 signals would mean that ordinary "pacing" signals would likewise get cut off. It would be very helpful if some approach could be devised that would permit a continued benefit of pacing signals, while nonetheless eliminating or nearly eliminating the possible localized tissue heating.

It would be very desirable if an approach could be found to reduce or eliminate spurious tissue heating at an electrode of a lead, due to RF energy from an MRI. It would also be very desirable if an approach could be found to permit the lead to continue to function properly even during the potential disruptions due to RF energy from an MRI or other high-energy external stimulus.

Summary of the invention

As will be described in greater detail below, several embodiments or aspects of the invention have been explored, and may each be used alone or in combination with others of the described embodiments or aspects of the invention.

According to one aspect of the invention, a lead such as used for pacemaking or neural stimulation has a current block IC which blocks current when RF is detected. In this way problems of tissue heating are reduced or eliminated when the patient carrying such a lead is given an MRI. According to a related aspect of the invention, an arrangement is employed that uses one or more coils (inductors) together with one or more blocking chips of the type just mentioned. As will be discussed in some detail below, actual experimentation has shown that the combination of a coil together with a blocking chip is sometimes able to reduce unwanted tissue heating to a greater extent than the blocking chip taken by itself.

The lead may also detect localized temperature, to be reported to equipment external to the lead. This may permit more judicious use of the RF energy, for example during an MRI session.

Localized and spurious tissue heating nearby to an electrode of a lead, due to RF energy from an MRI, is reduced or eliminated by detecting the RF field, deriving energy from the field, and actively driving a node that is coupled to such an electrode so as to minimize voltage excursions at the electrode. A state is maintained or set within electronics nearby to an electrode of a lead, so that a can which connects with the lead is able to learn whether an MRI event has occurred since the last time that the can had sent commands to the electronics to set particular internal states.

For a lead that delivers pacing pulses, a pacing detector is provided in addition to an MRI detector, and the blocking action is carried out most of the time but is disabled briefly for each pacing pulse.

Description of the drawing

The invention will be described with respect to a drawing in several figures, of which:

Fig. 1 shows a first exemplary lead;

Fig. 2 shows a second exemplary lead;

Fig. 3 shows an exemplary chip from a very high level;

Fig. 4 shows the chip of Fig. 3 in more detail including an RF detector 67;

Fig. 5 shows an exemplary RF detector 67 in some detail;

Fig. 6 shows an exemplary chip in plan view with some layers portrayed;

Fig. 7 shows the exemplary chip of Fig. 6 in plan view with additional layers portrayed;

Fig. 8 shows the exemplary chip of Fig. 7 in cross section;

Fig. 9 shows an arrangement with two coaxial coils, one inside the other;

Fig. 10 shows an arrangement with one conductor, the conductor being a coil, in which a return path passes through tissue; Fig. 11 shows an arrangement with two leads, each having a coil;

Fig. 12 shows a blocking chip with memory function;

Fig. 13 shows the chip of Fig. 12 in greater detail;

Fig. 14 shows a portion 134 of a lead according to the prior art;

Fig. 15 shows in functional block diagram form a chip 121 according to the prior art;

Fig. 16 shows apparatus including a can 133 and a lead according to the prior art;

Fig. 17 shows in functional block diagram form a chip 132 according to the invention;

Fig. 18 shows a chip 132 in vivo with an exemplary electrode B in addition to electrodes EO and El.

Fig. 19 is a cross-sectional view of the semiconductor fabrication arrangement of Fig. 20;

Fig. 20 is a plan view of a semiconductor fabrication arrangement for an integrated circuit according to the invention, with a node M4 employed to facilitate driving electrodes away from voltage excursions;

Fig. 21 shows in schematic depiction an exemplary RF detector 171 according to one embodiment of the invention;

Fig. 22 shows in schematic depiction an exemplary pacing detector 176 according to one embodiment of the invention; and

Fig. 23 shows in functional block diagram form a blocking chip 179 making use of not only the RF detector of Fig. 21 but also the pacing detector of Fig. 22. Where possible, like reference designations have been employed in the figures to denote like elements or structures.

Detailed description

Fig. 1 shows a first exemplary lead 41 according to the invention. The lead connects at 42 with an implantable device 43 such as a pacemaker or neural stimulator. It should be appreciated that the teachings and benefits of the invention are not tied to a particular type or class of implantable device 43. An MRI current block IC (integrated circuit) 45 is shown connected with the device 43 by means of conductors 44. The current block IC 45 is shown connected with a hardwired electrode 47 by means of conductors 46, and is in turn connected with hardwired electrode 49 by means of conductor 48.

In this embodiment, the current block IC 45 will, in its passive or quiescent state, provide connectivity between the two conductors 44 and respective two conductors 46. When RF is detected, switches within the IC 45 open, so that conductors 44 are not connected with conductors 46. Later, when the RF goes away, the switches within the IC 45 close again, so that conductors 44 are again connected with conductors 46.

In this way, the potentially harmful effects stemming from an MRI procedure are reduced and perhaps eliminated, because voltages induced into the conductors 44 by the RF do not pass through to the electrodes 47, 49.

It will be appreciated that while Fig. 1 shows a single current blocking IC 45, it is not required that there be only a single such IC. It might prove optimal to have a plurality of such ICs, distributed at locations along the lead. For example some leads may be 80 cm or more in length, and such a lead might have two or more such ICs along its length.

Fig. 2 shows a second exemplary lead 50 according to the invention. The lead connects at 42 with an implantable device 43 as before. An MRI current block IC (integrated circuit) 52 is shown connected with the device 43 by means of conductors 51. The current block IC 52 is shown connected with a multiplexed electrode 54 by means of conductors 53, and is in turn connected with yet another multiplexed electrode 56 by means of conductors 55, and so on.

In this embodiment, the current block IC 52 will, in its passive or quiescent state, provide connectivity between the two conductors 51 and respective two conductors 53. When RF is detected, switches within the IC 52 open, so that conductors 51 are not connected with conductors 53. Later, when the RF goes away, the switches within the IC 52 close again, so that conductors 51 are again connected with conductors 53.

Again, in this way, the potentially harmful effects stemming from an MRI procedure are reduced and perhaps eliminated, because voltages induced into the conductors 51 by the RF do not pass through to the electrodes at 54, 56, 58.

In the portrayed embodiment the current block chip 52 is separate from any of the multiplexed electrodes such as 54 (each of which includes its own IC). But it would be possible instead to have the current block functionality built in to one or more of the multiplexing ICs at 54, 56, 58.

In a lead having multiplexed electrodes, typically there will be control signals by which the "can" configures and controls the multiplexer chips at 54, 56, 58. Such control signals in an exemplary embodiment may be in the range of a megahertz at half a volt.

Fig. 3 shows an exemplary chip 45 from a very high level. Incoming contacts 61, 62 are for example connected to the implantable device 63 of Figs. 1 and 2. Outgoing contacts 63, 64 are connected in turn to an electrode 47 or 54 as described above. A "blood" electrode 65 may be provided at the chip 45. This electrode may facilitate developing and extracting power for use within the chip 45 from the conductors connected to contacts 61, 62.

It should be appreciated that the blood electrode 65 might not be necessary; a body-tissue reference for developing working voltages may be gotten from the electrodes such as 47 or 54, mentioned above.

It should be appreciated that while the invention is portrayed in a context of two conductors in and two more conductors out (a "two-wire lead), the teachings and benefits of the invention offer themselves even if a lead were to have only a single conductor.

The two conductors in a two-conductor lead are traditionally labeled Sl and S2, and this terminology is shown in some of the figures including Fig. 3.

Fig. 4 shows the chip 45 of Fig. 3 in more detail. A power extraction circuit 66 extracts power from contacts 61, 62 (from the "can") relative to a "blood" potential defined by contact 65. The developed voltages are provided for example by line 70 to other circuits such as RF detector 67 and switch circuits 68, 69. The RF detector 67 controls switch 68 by means of control line 71 and controls switch 69 by means of control line 72. Sl and S2 output contacts 63 and 64 are shown, which connect to distal elements such as electrodes 47 or 51.

The chip 45 may thus be a five-terminal device. If the lead has only one conductor, then the chip 45 would lose two terminals and might become a three-terminal device.

If the blood electrode at contact 65 were to prove unnecessary then the chip 45 could be a four-terminal (for two conductors in the lead) or a two-terminal (for a one-conductor lead) device.

The RF detector 67 will define thresholds such that a control signal such as the above-mentioned low- power control signals at one megahertz would not trigger the RF detector. A typical trigger threshold for the RF detector 67 might be 50 MHz.

The attenuation provided by the chip 45 in use may be 10 to 1 in ratio. When conducting (which is most of the time), the chip 45 transmits signals as faint as 10 mV from one side (the proximal side) to the other side (the distal side) and vice versa.

Modeling suggests that one induced potential would sometimes be a common-mode potential (meaning little or no difference between Sl and S2). The modeling suggests that another induced potential would be from a differential-mode current. This might happen, for example, due to non-identical lengths for the Sl and S2 lines. Fig. 5 shows an exemplary RF detector 67 in some detail. Conductors 61, 62 are the conductors that might pick up voltages from nearby RF sources such as MRI. These conductors are capacitively coupled at 81, 82 (perhaps 1 pF each) with resistances 200K (83, 84) to ground reference. This serves as a high-pass filter feeding rectifiers 85, 86. The developed DC accumulates at RC circuit 87, 88 (perhaps 4 pF and 300 megohms). A threshold device 89 is the discriminant having an output indicative of received RF levels. Its output goes through drivers 90, 91 to lines 71, 72 as mentioned in connection with Fig. 4.

More can be said about the ground reference of the detector 67 in Fig. 5. The power generation in the detector 67, and the development of a ground reference in the detector 67, can be done in a way that maximizes detection of RF regardless of whether the coupling of the lead to the RF field is more nearly common mode or is more nearly differential mode. The design of the power generation and ground reference can be such that when the RF coupling is mostly common mode, it will get detected

(developing a potential in capacitor 87 in Fig. 5) relative to the blood electrode 65. If, on the other hand, the RF coupling is mostly differential mode, then one of S 1 or S2 will turn out to be the lower of the potentials, and will become the ground reference. Stated differently, the developed potential in capacitor 87 will then be relative to the lower of Sl or S2 as a ground reference.

Fig. 6 shows an exemplary chip 45 in plan view with some layers portrayed. Contacts 61, 62, 63, 64, 65 each lie upon a substrate as shown in Fig. 6. protective metal layers 102, 103, 104, 105 are laid out, helping to shield and protect much of the circuitry below (close to the substrate in Fig. 6).

It will be appreciated that Fig. 6 is not necessarily to scale, so as to permit portraying some details. Gaps 106 for example may be ten microns across.

Fig. 7 shows the exemplary chip 45 of Fig. 6 in plan view with a additional layer portrayed. The additional layer is metal regions 61a, 62a, 63a, 64a, and 65a, each conductively coupled with respective contact 61, 62, 63, 64, 65. The metal regions do not lie directly upon all things that appear to be directly below in Fig. 7. Instead, insulating layers are present, lying in many instances between the metal elements 102, 103, 104, 105 on the one hand and the metal elements 61a, 62a, 63a, 64a, and 65a. Tabs at 61a, 62a, 63a, 64a, 65a allow welded connections to the chip 45.

Fig. 8 shows the exemplary chip 45 of Fig. 7 in cross section. Substrate 111 may be seen. Metal conductors 112 exemplify the shield conductors 102, 103, 104, 105. Insulating layer elements 113 are shown as well, said insulating elements 113 omitted for clarity from Figs. 6 and 7. Grossly larger metal conductors 114 are also shown, exemplifying top metal elements 61a, 62a, 63 a, 64a and 65 a.

A lead may thus have a proximal end connectable to equipment external to the lead, having a first electrode at a position along the lead, having a second electrode at a position between the proximal end and the position of the first electrode. The lead may further comprise an RF detector. A first conductor may extend along a length of the lead away from the second electrode in a first direction. The lead may further comprise a second conductor extending along a length of the lead away from the second electrode in a second direction. A first switch is provided which connects the first conductor with the second conductor. When RF is detected the switch opens, and later when there is no longer RF, the switch again closes.

There may be third and fourth conductors, extending in two directions, and a counterpart switch likewise responsive to detected RF. The detection of RF may be defined as comprising detecting RF for a duration exceeding a predefined interval, or may be defined as comprising detecting RF at a power level exceeding a predefined threshold, or a combination of both.

According to a related aspect of the invention, an arrangement is employed that uses one or more coils (inductors) together with one or more blocking chips of the type just mentioned. As will be discussed in some detail below, actual experimentation has shown that the combination of a coil together with a blocking chip is sometimes able to reduce unwanted tissue heating to a greater extent than the blocking chip taken by itself.

Fig. 9 shows an arrangement with two coaxial coils 191 and 192, one inside the other. This

arrangement has a can 43 and two coils located between the proximal end 42 and a first chip 45, with other chips 47, 49 positioned distal to the first chip 45. Fig. 10 shows an arrangement with one conductor, the conductor being a coil 191, in which a return path passes through tissue. The can 43 has a housing in electrical contact with tissue. The chips 45, 47, 49 also are in electrical contact with tissue.

Fig. 11 shows an arrangement with two leads, each having a respective coil. One lead has coil 191 A and chips 45 A and so on. The other lead has coil 19 IB and chips 45B and so on. This arrangement is particularly suitable for neural pain therapy where the leads are implanted to either side of a section of a spine of a patient.

The coil acts as an inductor within the field. The coil absorbs some of the ambient RF, converting it into waste heat, and the heat is distributed along the length of the coil. This is in some ways preferable to having a straight conductor, since with the straight conductor, the ambient RF that gets picked up is likely to give rise to currents that would cause localized tissue heating at particular points such as electrode locations.

It should be noted that the turns of the coil 191 (or 191 A or 191B) are not necessarily insulated from each other. Contrariwise, it is thought preferable if adjacent turns are able to be in at least loose electrical connectivity with each other. In such an arrangement, each instance of adjacent turns touching each other presents an opportunity for dissipation of some heat at that location. This promotes a distribution of heating along the length of the coil (as mentioned above). A practical consequence is that the actual measurable temperature rise along the coil is so small that it is nearly immeasurable. Another practical consequence is that the blocking capability of the blocking chip, together with the results achieved by the coil, lead to greatly reduced localized heating at points nearby to the blocking chip.

With actual experimentation, it has been observed that the coil reduces the RF energy reaching the blocking chip or chips, and that the blocking chip or chips further reduce the RF energy reaching electrodes.

Some actual experimentation results are now described. If a straight wire is employed with passive electrodes, during an exposure interval, a tissue heating of 45 degrees Celsius was seen in at least one location along the wire. In this case, a flip angle of 170 degrees was used in the MRI environment.

If a coil is employed (without the help of one or more RF blocking chips) then what was seen was a reduction in heating by a factor of about five. * * *

With a single coil and an RF blocking chip, the reduction in unwanted heating has worked out to about a factor of eleven. This is thus a factor of two better than if the coil is used by itself to attempt to reduce unwanted heating.

The result when the coil and blocking chip are both used, the unwanted heating has worked out to maybe half of a degree Celsius. This is at a noise level.

The coil may have ten to twenty turns per centimeter along its axis. The conductor of the coil is preferably not insulated, and as a result, adjacent turns may well be in electrical contact to some extent. This leads to a distribution of inductances and capacitances and resistances along the axial length of the coil.

It is thought that the structure of the coil interrupts a skin effect that would otherwise exist if the conductor were a conductive cylinder providing continuous conduction along its length.

It is noted that the RF -protective benefits of the coil or coils are greater when a can or pulse generator 43 is attached to the lead than when it is not attached. This is thought to be due, in part, to the fact that the can comes close to shorting two conductors together (at RF) due to protective devices located within the can.

The chips 45, 47, 49 can also provide electrode multiplexing as described in the US application number 61/230,505. This permits a system such as that of Fig. 11 to deliver configurable connectivity to a large number of addressable electrodes (perhaps sixteen in number) while providing RF blocking. In this arrangement only two coils 191 A, 191B are needed. Consider what would happen if one were to attempt to provide RF blocking solely through the use of coils, and without the assistance of the multiplexing capability just mentioned. If the pulse generator 43 were to have a distinct conductor to each of the (perhaps sixteen) electrodes, this would require that sixteen coils be provided. But the cross-section for sixteen coils is so wide as to be wholly unworkable in any realistic implantable electrode array system.

The decision whether to employ the blocking chips alone, or the blocking chips in combination with inductors, is of course influenced by the particular circumstances. In some applications (for example in particular parts of the body) it may be impossible to accommodate the increased diameter of the lead with the inductor. In other applications, it may be acceptable for the lead to have the increased diameter.

A third embodiment of the invention presents itself, as will now be discussed. Consider a chip 75 as shown in Fig. 12. The chip 75 connects with conductors Sl and S2 which run along the length of some or all of the lead. Chip 75 is shown in more detail in Fig. 13. RF detector 76 is shown, preferably capacitively coupled to Sl and S2. Capacitive coupling is thought to be optimal, since in a normal environment (when there is no MRI happening) the capacitors largely isolate the RF detector 76. At RF, however, the capacitors are close to a "straight wire" or metallic coupling between the RF detector 76 and the conductors Sl and S2. The RF detector 76 derives power (here called VH and VL) to power other components in the chip. VH is defined as the highest voltage anywhere in the chip, and VL the lowest voltage anywhere in the chip. VH and VL provide DC power for the rest of the chip. Block 76 has a third output 77 which is called "MRI Detect". The MRI Detect signal means that RF energy has been detected within some bandwidth, for some duration of time. It is thought preferable if the VH and VL lines are developed and stable prior to the event of the MRI Detect signal being asserted.

The MRI Detect signal 77 can pass to switches 79 which open, isolating the S 1 and S2 pass-through conductors illustrated to the right-hand side of the figure in Fig. 13. In this way the chip 75 can fulfill the RF blocking function described above.

Importantly in this embodiment, the chip also includes measurement and storage block 78. In response to assertion of the MRI Detect line 77, this block will do the following:

• reset the temperature register (a register that is intended to store a value indicative of a

measured temperature); and

• store a number that is the maximum measured ambient temperature.

The temperature value measured and stored by block 78 can be an analog value, so it could be for example be measured with respect to a band-gap reference circuit that is running off a voltage regulator and a current regulator. The circuit can measure the temperature and record the maximum temperature, and then, when the MRI goes away, it can convert that, either analog or digital temperature, and store it in a register. Afterward, then, a command can be sent to the can 43 (for example, a pacemaker) that has appropriate communication circuitry inside it, and the can will interrogate the chip 75 and ask for a readout, using a talk-back circuit that communicates from the lead back up to the can. In this way the chip 75 tells the controller what the stored temperature in the register is.

As will be appreciated, if a patient who has a lead is to be given an MRI by a doctor, this will permit the doctor to put the patient into the MRI device, set the MRI device to a very low power setting, run the image at this low power setting, determining what the temperature is at the lead. If the temperature is in the expected range (presumably a very low amount of temperature rise), then it would be possible to run the regular scan, and at the end of that scan, it would be possible to know what the temperature excursion was.

This will permit taking a single MRI under circumstances permitting some confidence level that the MRI is not causing an undue amount of tissue heating.

Consider instead so-called "gated" MRI, where many scans are taken sequentially, each at a certain time after the last heartbeat. The idea is to try to image for example, the dynamic motion of a heart as a function of time.

Using the chip 75, it would be possible to check the temperature at the lead from time to time during the sequence of scans. It will thus be appreciated that the temperature measurements could be employed in several ways. For example, if the measured temperature were to exceed some predetermined threshold, a decision could be made to stop the scanning, or to increase the interval of time between scans.

As mentioned, one approach would be to collect a maximum temperature during a measurement interval. Alternatively, the chip 75 could be used to measure temperatures in real time, and to report the temperatures externally in real time. While it is thought to be optimal to store the maximum temperature, many of the aims of the invention are served even if what is stored is a mean temperature or a range of temperatures.

To provide a detailed description of another embodiment of the invention, it will be helpful to establish some terminology, drawing first upon Fig. 16. Fig. 16 shows apparatus including a can 133 and a lead according to the prior art. A portion 134 of the lead includes a satellite 122 and in an exemplary embodiment, at least one other satellite 135 is located elsewhere along the lead. Turning to Fig. 14, what is shown is the portion 134 of the lead according to the prior art. Conductors Sl and S2 run along the lead. Satellite 122 is shown in more detail in Fig. 14 than in Fig. 16. Satellite 122 contains an integrated circuit chip 121 according to the prior art, connected to conductors Sl and S2 and connected to electrodes EO and El. The conductors EO and El are disposed for contact with external material, such as tissue of a subject, for example a human subject.

Fig. 15 shows in functional block diagram form a chip 121 according to the prior art. The chip 121 has a power extraction module 123 which derives stable power supplies 125 from Sl and S2. A control module 124 receives commands from the can 133 (omitted for clarity in Fig. 2) and sends control signals 126 to other parts of the chip 121. The control module 124 receives power from the power extraction module 123. Importantly, chip 121 also has a switching fabric 127 which provides the ability to cross-connect any of one or more electrodes (connected at connection points labeled EO and Dl) to either of conductors Sl and S2. The switching fabric is composed in part of semiconductor switches 128 shown in Figs. 15 and 17 as circles.

At the frequency regimes in which the lead normally functions, the switches 128 may be modeled as having a very high impedance (when open) and having a relatively low impedance (when conducting). But importantly, at frequency regimes and power levels typical of MRI, the switching fabric 127 is most helpfully modeled to include parasitic capacitances 145. Such capacitances help to explain how it is that in the RF energy field of an MRI, sometimes there will be elevated tissue temperature in regions nearby to electrodes such as EO and El . The RF energy field is understood to couple to portions of conductors Sl or S2, developing currents and voltages which in turn couple to the electrodes EO and El, thereby giving rise to currents within the tissue nearby. The currents in turn may bring about elevated temperatures.

Such tissue heating is, of course, undesirable, and as mentioned above, it would be very desirable if an approach could be found to reduce or eliminate spurious tissue heating at an electrode of a lead, due to RF energy from an MRI.

In a typical prior-art system such as shown in Fig. 16, the can 133 will send commands to satellites 122, 135 to instruct the satellites 122, 125 to make particular connections. Each satellite, as mentioned above, has a chip 121 (Fig. 15) which maintains various internal states. The internal states maintained may for example include the desired open/closed positions of switches 128. In an exemplary embodiment the can 133 sends such commands relatively rarely, perhaps only every few days, or perhaps only in the doctor's office. (Message-passing and computations consume energy, and the can 133 has only a limited energy budget.)

Experience shows, however, that if the system of Fig. 16 finds itself within a strong RF field such as that employed in MRI (or other high-energy external stimuli), the internal stages of the chip 121 may be lost or disrupted. As mentioned above, it would also be very desirable if an approach could be found to permit the lead to continue to function properly even after the potential disruptions due to RF energy from an MRI.

In accordance with the invention, a node M4 is provided (shown in Figs. 17, 19, 20) which is capacitively coupled with the bonding pads 138, 139 (shown in Figs. 19, 20) of electrodes EO, El (shown in Figs. 17, 20). The node M4 is connected via line 142 to a driver 146 (Fig. 17) which, under circumstances detailed below, urges the node M4 toward particular potentials. Fig. 17, just mentioned, shows in functional block diagram form a chip 132 according to the invention. The chip 132 has all or nearly all of the circuitry and functionality of chip 121 (Fig. 15), and in addition has functionality portrayed in Fig. 17. The chip 132 has an RF (radio frequency) detector 129 which is quiescent in the absence of a strong RF field, and which generates power and control signals 130 in the presence of a strong RF field. Switching fabric 129 (which includes switches 128) is shown in Fig. 17. Importantly, a node M4 is present, which node is modeled as capacitively coupled (capacitances 141) with bonding pads for electrodes EO and El .

When RF is detected, driver 146 is activated. In an exemplary embodiment, driver 146 receives two inputs 143, 144. One input (say 144) is drawn directly from (for example) electrode EO and the other input (say 143) is a reference input, for example some highly filtered low-frequency signal lying between the Sl and S2 levels.

Yet another embodiment provides a "blood" electrode B (Fig. 18) which is in contact with surrounding material 145 such as tissue. The B electrode is, importantly, not very strongly coupled with conductors Sl and S2, and thus is fairly indicative of the neutral potential of the tissue 145. In this embodiment, the potential at electrode B serves as the reference input for driver 146.

If electrode EO is driven to "blood electrode" potential no current will flow between the two electrodes, by definition. More subtly, however, we can design the driver circuit so that the "bucking" takes place at higher frequencies such as RF, and that the "bucking" drive does not apply itself at the frequency domain nearby to DC, such as the frequency domain of the pacing pulses. This means that even if we drive the AC potential to zero, we can still allow the pacing pulse to pass.

At RF, the coupling between node M4 and electrodes EO, El being modeled as largely capacitive, is a very low impedance. (In one simulation the capacitance 141 is modeled at about 6.4 picofarads.) At normal operating frequency regimes, by contrast, the node M4 is not particularly coupled with any of the electrodes EO, El, and importantly, its presence does not particularly cause unwanted crosstalk or coupling between electrodes EO and El. The design goal for driver 146 is, of course, to try to pump current into node M4 in such a way as to minimize voltage excursions at (for example) electrode EO during the RF regime. If the system does successfully reduce or eliminate voltage at (for example) electrode EO, then it will likely bring about this same desired result at the other electrodes (for example) El, since at RF, the electrodes are at low impedance relative to each other through the node M4. Making this point in a different way, it is likely to be unnecessary to provide and allocate a distinct driver 146 for each of the electrodes EO, El. A single driver is likely to suffice.

It will be appreciated that the "bucking", that is, the driving of node M4, should not take place during a time when a pacing pulse is happening. So the bucking activity would be turned off if a pacing detector (such as detector 176 in Fig. 22) were detecting that pacing is happening.

Fig. 19 is a cross-section view of a semiconductor fabrication arrangement for an integrated circuit according to the invention, with the node M4 employed to facilitate driving electrodes away from voltage excursions. An insulating layer 140 has conductive node M4 on one side, and has bonding pads 138, 139 on the other side, which in the exemplary embodiments here will connect to electrodes EO, El. It will be appreciated that the semiconductor fabrication elements depicted in Fig. 19 are not drawn to scale, and that many fabrication elements have been omitted for clarity From Fig. 19.

Fig. 20 is a plan view of the semiconductor fabrication arrangement of Fig. 19. Bonding pads 136, 137 are provided to connect to conductors Sl, S2. Bonding pads 138, 139 are provided to connect to electrodes EO, El. Node M4 is overlaid above (or below, depending upon perspective) the bonding pads 138, 139, giving rise to capacitive couplings as mentioned above. Main circuitry core 147 is connected with the bonding pads 136, 137, 138, and 139, and is connected (through a though- connection) to node M4.

Although the embodiment described here has two electrodes EO, El, it will be appreciated that the teachings of the invention offer themselves equally well with other numbers of electrodes such as four electrodes.

As mentioned above, Fig. 18 shows a chip 132 in vivo in tissue 145 with an exemplary electrode B in addition to electrodes EO and El.

Returning to Fig. 17, a module 124 is shown which receives RF energy when such energy is present. The module 124 stores energy in memory cell 131, which is intended to maintain its state even if all other internal states within chip 132 are lost due to the presence of the high-energy, high-frequency RF field. Module 124 will later respond to a query from can 133 (Fig. 16) to report that the RF event (typically an MRI event) had occurred.

Importantly, this arrangement permits the can 133 to learn that an MRI event (or other high-energy external event) has occurred. Thus informed, the can 133 is able to send commands to the satellites 122, 125 to reset their internal states to desired conditions. The can 133 may also send a command to the module 124 for clearing the state of memory cell 131. The can 133 is also able to take any other desired actions, such as for example communicating the fact of the MRI or other event to equipment that is external to the can 133 (and that is omitted for clarity in Fig. 16).

One of the ways in which the module 124 can inform the can 133 of the event detection (the "on" state of memory cell 131) is by emitting some recognizable signal on Sl and S2 in response to some easily recognized event such as a pacing pulse from the can 133. The can 133 could watch for that signal each time it emits a pacing pulse. If the signal is noted, then the can 133 would be aware that at least one chip 132 needed to have its internal states reset to desired values. The can 133 would then reset the states of one or more of the chips 132 until the condition had been remedied.

As was discussed above, the MRI environment can give rise to a situation in which a lead resonates or otherwise couples with the RF energy of the MRI environment. Such resonance or coupling can bring about nonzero currents through adjacent tissue, and the currents can bring about localized warming. This prompts, for example, the "blocking chip" approach of Figs. 4 and 13. In the blocking chip approach, a detected MRI environment prompts opening of switches 68, 69 (in Fig. 4) or switches 79 (in Fig. 13). The opening of the switches breaks up the long lead into two or more shorter structures, which hopefully exhibit reduced coupling to the RF energy bathing the region.

One drawback to this, however, is that the original purpose of the lead may have been therapeutic, for example delivering pacing pulses to a heart. Opening the switches in Fig. 4 or Fig. 13 means that the pacing pulses would not reach their intended destination, namely particular regions of muscle tissue in the heart.

Turning ahead to Fig. 23, we see a chip 179 of which one or more may be present along the length of a lead. We see that the lead is at least a four-terminal device, with Sl and S2 inputs at 181 and 182, and Sl and S2 pass-through terminals 183, 184 which pass the Sl and S2 signals distally to the rest of the lead and perhaps to another of the chips 179. Switches 79 are familiar to the reader from previous discussion of Figs. 4 and 13. MRI detector 171 is also familiar to the reader, detecting ambient RF at or above some power level and lasting longer than some duration. Detector 171, as mentioned above, receives RF energy and develops power supplies (here called VH and VL) for the rest of the chip, and then (preferably after the power supplies are stable) asserts an "MRI detect" signal 185 indicative of the detected RF ambient.

Importantly for the embodiment of the invention now being discussed, there is also provided a pacing detect circuit 176. This circuit detects a pacing pulse that is incoming on Sl, S2 and asserts a signal 186 indicative of detection of the pacing pulse. Lines 185, 186 reach logic and timing circuitry 179 which have an output 187 which controls switches 79. Briefly, the result of this is that the pacing pulses do get passed along to the pass-through terminals 183, 184 and thence to the distal portion of the lead. During times when the pacing pulse is not happening, and when the RF environment is present, the switches 79 are opened, blocking or reducing the RF coupling or resonance.

The pacing pulse represents a relatively short duty cycle. This means that during an MRI event, the switches 79 can be open almost all of the time. This greatly reduces tissue heating, while preserving the opportunity for pacing pulses to continue to reach their intended destination.

A typical pacing pulse duration might be one millisecond in duration. This would permit the switches 79 to be opened (in the "blocking" position) for about 999 milliseconds out of each second. It will thus be appreciated that turning off the blocking for the duration of a pacing pulse only permits perhaps a tenth of a percent of the tissue heating to take place when compared with the tissue heating if no blocking at all is performed. Making this same point differently, the embodiment being discussed here satisfies not only the goal of reducing tissue heating, but also permits pacing pulses to get through, while still delivering 99.9 percent of the benefit that would have accrued had the pacing pass-through that is described here not been provided.

If 99.9 percent is not close enough to 100 percent, then a possible approach would be to make the pacing pulse shorter in duration while passing more energy during the pulse. The general goal is to deliver some number of joules per pulse, and this could be done in one millisecond or half a millisecond or a quarter of a millisecond.

Turning to Fig. 21, what we see is an exemplary MRI detection circuit 171. Potentials if any from conductors Sl, S2 pass through high-pass filters 172 and are rectified. The DC if any that is developed reaches RC circuit 173 about which more will be said later. This develops an MRI logic level 185 as well as an MRI drive output 175. The MRI drive output powers the pacing detection 176. The notion here is that there is no need to expend limited power resources trying to detect pacing pulses (in circuit 176) unless MRI is happening. Stated differently, there is no need to selectively turn off blocking unless the blocking is on.

The alert reader will perceive much in common between the RF detector 67 of Fig. 5 and the RF detector 171 of Fig. 21. Each of the detectors applies a high-pass filter to each of Sl and Sl, rectifies any detected high frequency energy, and permits a capacitor to charge up, eventually yielding a result namely the assertion of a logic line indicative of detection of RF. Each relies upon an understood ground reference which might be the lower of Sl and S2 or might be the above-mentioned "blood" reference electrode.

The filters 172 are designed to try to avoid responding to "friendly" signals such as control signals that go from the can to the chip, and to avoid responding to pacing pulses and other desired signals. In the present design environment, it was chosen to pass RF starting at about 2.5 megahertz through the filters 172. MRI RF fields in the tens of megahertz will thus be able to pass through filters 172 and will develop charging current to charge the capacitor in RC circuit 173.

Turning to Fig. 22, what we see is an exemplary pacing detection circuit 176. Potentials if any from conductors Sl, S2 pass through filters 177 and may satisfy the thresholds defined for DC comparators 188 as shown. If both comparators are satisfied, then logic yields pacing-detect signal 186. Filters 177 are low-pass filters that are intended to block most or all of the RF energy in the MRI field, while passing the DC of the pacing pulse. In this circuit, what is tested for is a particular DC voltage difference between Sl and S2.

More can be said about the duration of the "pacing window", namely the length of time during which the blocking is turned off to allow the pacing to take place. The pacing pulse will typically comprise an excursion in one polarity followed by an excursion in the other polarity. The goal is that the pulse is "charge balanced" meaning that the area under the curve (current plotted against time) nets to zero. If the pacing-detect circuit together with the timing and logic circuit 179 were to close the window too soon, then the second excursion might get cut off and charge balance would not be maintained.

The logic and timing circuitry 179 can carry out its work in any of several ways. One approach is to measure the duration of the first excursion of the pacing pulse, and then to keep the window open for two or three times that measured duration, to allow plenty of time for the second excursion to run its course, thereby preserving the charge balance. A similar approach is to start a timer or one-shot because of the detection of the start of a pacing pulse, and to configure the timer or one-shot so that it keeps the window open for some interval of time that is expected to permit both excursions to take place.

A different approach is to provide two detection circuits along the lines of circuit 176, one of which detects the first excursion and the other of which detects the second excursion, and to keep the window open whenever either of the two detectors has its output. This will likewise preserve or largely preserve the charge balance. Such a "biphasic detector" will permit the pulse (sometimes termed a "biphasic pulse") to finish before the blocking of RF is resumed.

It will be appreciated that other detection circuits could be devised which would serve the purposes set forth herein, and use of such other detection circuits would not depart from the invention.

The above discussion is directed to the case where the can is a piece of equipment (such as a cardiac pacemaker) over which the designer of the lead has little or no control. If, however, the designer of the lead has the opportunity to participate in the design of the can, or indeed if the designer of the lead is also the designer of the can, then a yet different approach could be employed. The can, when emitting the pacing pulse, could superimpose upon it a predetermined signal such as a one-megahertz signal. The chip or chips in the lead could watch for that particular superimposed signal, for example with a bandpass filter, and could open the "pacing window" specifically in reliance upon detection of that signal.

This embodiment of the invention has been discussed in the particular context of cardiac pacemaking. The teachings of this embodiment of the invention could, however, offer their benefits for other types of stimulation, such as neural stimulation. It might for example be desired to carry out adjustments in the neural stimulation (using for example a multiplexed lead) while an MRI is taking place, so as to help with the physical placement (or virtual placement using the multiplexing) in real time during the MRI.

Those skilled in the art will have no difficulty whatsoever devising myriad obvious variants and improvements upon the invention as described herein, all of which are intended to be encompassed within the claims which follow.

Claims

1. A method for use with a lead implanted in external material, the lead having a proximal end connectable to equipment external to the lead, the lead having a first electrode at a position along the lead, the lead having a second electrode at a position between the proximal end and the position of the first electrode, the lead further comprising an RF detector, the lead comprising a first conductor extending along a length of the lead away from the second electrode in a first direction, and comprising a second conductor extending along a length of the lead away from the second electrode in a second direction, the lead further comprising a first switch connecting the first conductor with the second conductor, the method comprising the steps of: detecting RF by means of the RF detector; and responsive to the detection of RF, opening the first switch, whereby the first conductor ceases to be connected with the second conductor.

2. The method of claim 1 wherein the lead further comprises a detector of a signal at a lower frequency than the RF, the method further characterized in that the opening of the first switch does not happen when the signal is detected.

3. The method of claim 2 wherein the signal is a cardiac pacing pulse.

4. The method of claim 1 further comprising the steps of: responsive to a ceased detection of RF, closing the first switch, whereby the first conductor resumes connection with the second conductor.

5. The method of claim 1 wherein the lead further comprises a third conductor extending along a length of the lead away from the second electrode in the first direction, and comprises a fourth conductor extending along a length of the lead away from the second electrode in the second direction, the lead further comprising a second switch connecting the third conductor with the fourth conductor, the method further characterized in that: responsive to the detection of RF, the second switch is opened, whereby the third conductor ceases to be connected with the fourth conductor.

6. The method of claim 5 further comprising the steps of: responsive to the ceased detection of RF, closing the second switch, whereby the third conductor resumes connection with the fourth conductor.

7. The method of claim 1 wherein the step of detecting RF by means of the RF detector comprises detecting RF for a duration exceeding a predefined interval.

8. The method of claim 1 wherein the step of detecting RF by means of the RF detector comprises detecting RF at a power level exceeding a predefined threshold.

9. The method of claim 8 wherein the step of detecting RF by means of the RF detector further comprises detecting RF at a power level exceeding a predefined threshold.

10. Apparatus comprising: a lead, the lead having a proximal end connectable to equipment external to the lead, the lead having a first electrode at a position along the lead, the lead having a second electrode at a position between the proximal end and the position of the first electrode, the lead further comprising an RF detector, the lead comprising a first conductor extending along a length of the lead away from the second electrode in a first direction, the lead comprising a second conductor extending along a length of the lead away from the second electrode in a second direction, the lead further comprising a first switch connecting the first conductor with the second conductor, the first switch responsive to detection of RF by opening the first switch, whereby the first conductor ceases to be connected with the second conductor, the first switch responsive to a ceased detection of RF by closing the first switch, whereby the first conductor resumes connection with the second conductor.

11. The apparatus of claim 10 wherein the lead further comprises a detector of a signal at a lower frequency than the RF; the first switch further characterized in that the opening of the first switch does not happen when the signal is detected.

12. The apparatus of claim 11 wherein the signal is a cardiac pacing pulse.

13. The apparatus of claim 10 wherein the lead further comprises a third conductor extending along a length of the lead away from the second electrode in the first direction, and a fourth conductor extending along a length of the lead away from the second electrode in the second direction, and a second switch connecting the third conductor with the fourth conductor, the second switch responsive to detection of RF by opening the second switch, whereby the third conductor ceases to be connected with the fourth conductor, the second switch responsive to a ceased detection of RF by closing the second switch, whereby the third conductor resumes connection with the fourth conductor.

14. The apparatus of claim 10 further characterized in that the detection of RF to which the first switch responds comprises detecting RF for a duration exceeding a predefined interval.

15. The apparatus of claim 10 further characterized in that the detection of RF to which the first switch responds comprises detecting RF at a power level exceeding a predefined threshold.

16. The apparatus of claim 14 further characterized in that the detection of RF to which the first switch responds further comprises detecting RF at a power level exceeding a predefined threshold.

17. The apparatus of claim 10 further comprising a sterile wrapping around the apparatus.

18. Apparatus comprising an integrated circuit chip, the chip having first, second, third, and fourth contacts connectable with conductors external to the chip, the chip further comprising an RF detector, the lead further comprising a first switch connecting the first contact with the second contact, the first switch responsive to detection of RF by opening the first switch, whereby the first contact ceases to be connected with the second contact, the first switch responsive to a ceased detection of RF by closing the first switch, whereby the first contact resumes connection with the second contact. the second switch responsive to detection of RF by opening the second switch, whereby the third contact ceases to be connected with the fourth contact, the second switch responsive to a ceased detection of RF by closing the second switch, whereby the third contact resumes connection with the fourth contact.

19. The apparatus of claim 18 wherein the chip further comprises a detector of a signal at a lower frequency than the RF; the first switch further characterized in that the opening of the first switch does not happen when the signal is detected.

20. The apparatus of claim 19 wherein the signal is a cardiac pacing pulse.

21. The apparatus of claim 18 further characterized in that the detection of RF to which the first and second switches respond comprises detecting RF for a duration exceeding a predefined interval.

22. The apparatus of claim 18 further characterized in that the detection of RF to which the first and second switches respond comprises detecting RF at a power level exceeding a predefined threshold.

23. The apparatus of claim 16 further characterized in that the detection of RF to which the first and second switches respond further comprises detecting RF at a power level exceeding a predefined threshold.

24. A method for use with a lead implanted in external material, the lead having a proximal end connectable to equipment external to the lead, the lead having a first electrode at a position along the lead, the lead having a second electrode at a position between the proximal end and the position of the first electrode, the lead further comprising an RF detector, the lead comprising a first conductor extending along a length of the lead away from the second electrode in a first direction, and comprising a second conductor extending along a length of the lead away from the second electrode in a second direction, the lead further comprising a first switch connecting the first conductor with the second conductor, the method comprising the steps of: detecting RF by means of the RF detector; and responsive to the detection of RF, opening the first switch, whereby the first conductor ceases to be connected with the second conductor; wherein is provided between the proximal end and the first electrode a first coil, the coil providing electrical connectivity between the proximal end and the first switch.

25. The method of claim 24 wherein the lead further comprises a detector of a signal at a lower frequency than the RF, the method further characterized in that the opening of the first switch does not happen when the signal is detected.

26. The method of claim 25 wherein the signal is a cardiac pacing pulse.

27. The method of claim 24 wherein is further provided between the proximal end and the first electrode a second coil, the second coil providing electrical connectivity between the proximal end and the first switch, the second coil coaxial with and surrounding the first coil.

28. The method of claim 24 where in the first coil has turns in the range of 10 to 20 turns per centimeter of axial measure.

29. The method of claim 24 further comprising the steps of: responsive to a ceased detection of RF, closing the first switch, whereby the first conductor resumes connection with the second conductor.

30. The method of claim 24 wherein the lead further comprises a third conductor extending along a length of the lead away from the second electrode in the first direction, and comprises a fourth conductor extending along a length of the lead away from the second electrode in the second direction, the lead further comprising a second switch connecting the third conductor with the fourth conductor, the method further characterized in that: responsive to the detection of RF, the second switch is opened, whereby the third conductor ceases to be connected with the fourth conductor.

31. The method of claim 30 further comprising the steps of: responsive to the ceased detection of RF, closing the second switch, whereby the third conductor resumes connection with the fourth conductor.

32. The method of claim 24 wherein the step of detecting RF by means of the RF detector comprises detecting RF for a duration exceeding a predefined interval.

33. The method of claim 24 wherein the step of detecting RF by means of the RF detector comprises detecting RF at a power level exceeding a predefined threshold.

34. The method of claim 32 wherein the step of detecting RF by means of the RF detector further comprises detecting RF at a power level exceeding a predefined threshold.

35. An apparatus comprising: a lead, the lead having a proximal end connectable to equipment external to the lead, the lead having a first electrode at a position along the lead, the lead having a second electrode at a position between the proximal end and the position of the first electrode, the lead further comprising an RF detector, the lead comprising a first conductor extending along a length of the lead away from the second electrode in a first direction, the lead comprising a second conductor extending along a length of the lead away from the second electrode in a second direction, the lead further comprising a first switch connecting the first conductor with the second conductor, the first switch responsive to detection of RF by opening the first switch, whereby the first conductor ceases to be connected with the second conductor, the first switch responsive to a ceased detection of RF by closing the first switch, whereby the first conductor resumes connection with the second conductor. wherein is provided between the proximal end and the first electrode a first coil, the coil providing electrical connectivity between the proximal end and the first switch.

36. The apparatus of claim 35 wherein the lead further comprises a detector of a signal at a lower frequency than the RF; the first switch further characterized in that the opening of the first switch does not happen when the signal is detected.

37. The apparatus of claim 35 wherein the signal is a cardiac pacing pulse.

38. The apparatus of claim 35 wherein is further provided between the proximal end and the first electrode a second coil, the second coil providing electrical connectivity between the proximal end and the first switch, the second coil coaxial with and surrounding the first coil.

39. The apparatus of claim 35 where in the first coil has turns in the range of 10 to 20 turns per centimeter of axial measure.

40. A lead comprising at least one conductor running along its length, the lead disposed at a proximal end to connect by its at least one conductor with electronic equipment external to the lead, the lead further comprising a chip at a location other than at the proximal end, the chip electrically connected with the at least one conductor, the chip comprising: a detector detecting RF energy within a predetermined frequency bandwidth and with a duration exceeding a predetermined duration coupled with the at least one conductor, and yielding a signal indicative thereof; a temperature sensor sensing ambient temperature; a memory device responsive to the signal for storing information indicative of the sensed temperature; said chip responsive to an external stimulus for communicating the information indicative of the sensed temperature by means of the at least one conductor to equipment external to the lead.

41. The lead of claim 40 wherein the number of conductors running along the length of the lead is two, and wherein the chip is electrically connected with both of the conductors.

42. The lead of claim 40 wherein the chip further comprises a switch corresponding to each of the at least one conductor, the switch connecting the conductor proximal to the chip with the conductor distal to the chip, the switch responsive to the signal for opening the switch.

43. The lead of claim 40 further comprising a power extraction circuit extracting power from ambient RF energy, and delivering the power to the temperature sensor and to the memory device.

44. The lead of claim 40 further characterized in that the lead is sterile and is encased in sterile packaging.

45. A chip for use with a lead comprising at least one conductor running along its length, the chip comprising: a detector detecting RF energy within a predetermined frequency bandwidth and with a duration exceeding a predetermined duration coupled with the at least one conductor, and yielding a signal indicative thereof; a temperature sensor sensing ambient temperature; a memory device responsive to the signal for storing information indicative of the sensed temperature; said chip responsive to an external stimulus for communicating the information indicative of the sensed temperature by means of the at least one conductor to equipment external to the lead.

46. The chip of claim 45 wherein the number of conductors is two.

47. The chip of claim 45 wherein the chip further comprises a switch corresponding to each of the at least one conductor, the switch connecting the conductor proximal to the chip with the conductor distal to the chip, the switch responsive to the signal for opening the switch.

48. The chip of claim 45 further comprising a power extraction circuit extracting power from ambient RF energy, and delivering the power to the temperature sensor and to the memory device.

49. A method for use with a lead, the lead comprising at least one conductor running along its length, the lead disposed at a proximal end to connect by its at least one conductor with electronic equipment external to the lead, the lead further comprising a chip at a location other than at the proximal end, the chip electrically connected with the at least one conductor, method comprising: at the chip, detecting RF energy within a predetermined frequency bandwidth and with a duration exceeding a predetermined duration coupled with the at least one conductor; at the chip, sensing ambient temperature; at the chip, in response to the detected RF energy, storing information indicative of the sensed temperature; at the chip, responding to an external stimulus for communicating the information indicative of the sensed temperature by means of the at least one conductor to equipment external to the lead.

50. The method of claim 49 wherein the chip further comprises a switch corresponding to each of the at least one conductor, the switch connecting the conductor proximal to the chip with the conductor distal to the chip, the method further comprising opening the switch in response to the detected RF energy.

51. A method for use with a lead, the lead comprising at least one conductor running along its length, the lead disposed at a proximal end to connect by its at least one conductor with electronic equipment external to the lead, the lead further comprising a chip at a location other than at the proximal end, the chip electrically connected with the at least one conductor, method comprising: subjecting the lead to RF energy; at the chip, detecting RF energy within a predetermined frequency bandwidth and with a duration exceeding a predetermined duration coupled with the at least one conductor; at the chip, sensing ambient temperature; at the chip, in response to the detected RF energy, storing information indicative of the sensed temperature; at the chip, responding to an external stimulus for communicating the information indicative of the sensed temperature by means of the at least one conductor to equipment external to the lead.

52. The method of claim 51 further comprising the step of ceasing the subjection of the lead to RF energy in response to the communicated information.

53. The method of claim 51 further comprising the step of delaying a subsequent subjection of the lead to RF energy in response to the communicated information.

54. A method for use with a lead, the lead having at least one satellite along its length, the lead further comprising at least two conductors along its length, the at least two conductors connected with the at least one satellite, the satellite comprising integrated circuity connected with the at least two conductors, the satellite further comprising at least one driven electrode connected with the integrated circuitry, the at least one driven electrode switchingly coupled with the at least two conductors, the integrated circuitry further comprising a node capacitively coupled with the at least one driven electrode, the node defining a node potential thereof, the driven electrode in contact with surrounding material; the method comprising the steps of: detecting an RF field; in response to the detection of the RF field, extracting first and second voltage levels from the at least two conductors, the first voltage level defining a higher voltage than the second voltage level; sensing a difference between the node potential and a reference potential; driving the node potential toward the reference potential by selectively coupling the first voltage level or the second voltage level to the node potential.

55. The method of claim 54 wherein the driving of the node potential is carried out at high frequencies and is not carried out at frequencies near to direct current.

56. The method of claim 54 wherein the lead further comprises a detector of a signal at a lower frequency than the RF, the method further characterized in that the driving of the node potential does not happen when the signal is detected.

57. The method of claim 56 wherein the signal is a cardiac pacing pulse.

58. The method of claim 54 wherein the satellite further comprises a reference electrode, the reference electrode in contact with the surrounding material, and wherein the reference potential is defined by the reference electrode.

59. Apparatus comprising: a lead, the lead having at least one satellite along its length, the lead further comprising at least two conductors along its length, the at least two conductors connected with the at least one satellite, the satellite comprising integrated circuitry connected with the at least two conductors, the satellite further comprising at least one driven electrode connected with the integrated circuitry, the at least one driven electrode switchingly coupled with the at least two conductors, the integrated circuitry further comprising a node capacitively coupled with the at least one driven electrode, the node defining a node potential thereof, the driven electrode disposed for contact with surrounding material; the integrated circuity further comprising an RF field detector; the integrated circuitry further comprising power extraction means responsive to detection of an RF field for extracting first and second voltage levels from the at least two conductors, the first voltage level defining a higher voltage than the second voltage level; the integrated circuitry further comprising a driver responsive to a difference between the node potential and a reference potential for driving the node potential toward the reference potential by selectively coupling the first voltage level or the second voltage level to the node potential.

60. The apparatus of claim 59 wherein the driver drives the node potential toward the reference potential at high frequencies and not at frequencies near to direct current.

61. The apparatus of claim 59 wherein the lead further comprises a detector of a signal at a lower frequency than the RF, the driver further characterized in that the driving of the node potential does not happen when the signal is detected.

62. The apparatus of claim 61 wherein the signal is a cardiac pacing pulse.

63. The apparatus of claim 59 wherein the satellite further comprises a reference electrode, the reference electrode disposed for contact with the surrounding material, and wherein the reference potential is defined by the reference electrode.

64. The apparatus of claim 59 wherein the number of driven electrodes is four, each of the driven electrodes is switchingly coupled with the at least two conductors, and each of the driven electrodes is capacitively coupled with the node.

65. Apparatus comprising: a lead, the lead having at least one satellite along its length, the lead further comprising at least two conductors along its length, the at least two conductors connected with the at least one satellite, the satellite comprising integrated circuitry connected with the at least two conductors, the integrated circuitry disposed to maintain first internal states in a first power regime, the integrated circuitry further comprising a high-energy field detector, the field defined as having higher power levels than the first power regime; the integrated circuitry further comprising power extraction means responsive to detection of a high- energy field for storing a second internal state within a cell; the integrated circuitry further comprising a communication means disposed, after the cessation of the high-energy field, for response to a query from external to the lead, with information indicative of the stored second internal state.

66. The apparatus of claim 65 wherein the communication means is further responsive to a command from external to the lead, for clearing the stored second internal state.

67. A method for use with apparatus comprising a lead, the lead having at least one satellite along its length, the lead further comprising at least two conductors along its length, the at least two conductors connected with the at least one satellite, the satellite comprising integrated circuity connected with the at least two conductors, the integrated circuitry disposed to maintain first internal states in a first power regime, the integrated circuity further comprising a high-energy field detector, the high-energy field defined as having higher power levels than the first power regime; the integrated circuitry further comprising power extraction means responsive to detection of a high-energy field for storing a second internal state within a cell; the integrated circuitry further comprising a communication means disposed, after the cessation of the high-energy field, for response to a query from external to the lead, with information indicative of the stored second internal state; the method comprising the steps of: detecting a high-energy field at the field detector; storing a second internal state within the cell indicative of the detection of the high-energy field; from external to the lead, querying the integrated circuitry; in response to the query, responding to the query with information indicative of the stored second internal state.

68. The method of claim 67 wherein the communication means is further responsive to a command from external to the lead, for clearing the stored second internal state, the method further comprising the steps of: from external to the lead, commanding the integrated circuitry; and in response to the command, responding to the command by clearing the stored internal state.

69. The method of claim 68 further comprising the step, performed after the query step and the response to query step, of commanding the integrated circuitry to set first internal states.

70. A method for use with a lead with a distal end and a proximal end, the lead having two conductors along its length, the lead having a chip at a point along the lead, the chip switchably connecting the conductors on the proximal side thereof with the respective conductors on the distal side thereof, the chip comprising an RF detector and a detector of a signal at a lower frequency than the RF, the method comprising the steps of: in the event that the signal is not detected and RF is detected, opening the switchable connection so that the distal conductors are not connected with the proximal conductors; in the event that the signal is detected and RF is detected, closing the switchable connection so that the distal conductors are connected with the proximal conductors; in the event that RF is not detected, closing the switchable connection so that the distal conductors are connected with the proximal conductors.

71. The method of claim 70 wherein the chip further comprises a memory, the method comprising the step of: in the event of a detection of RF, storing a state in the memory; and after the state is stored in the memory, communicating the state to equipment external to the lead.

72. A lead with a distal end and a proximal end, the lead having two conductors along its length, the lead having a chip at a point along the lead, the chip switchably connecting the conductors on the proximal side thereof with the respective conductors on the distal side thereof, the chip comprising an RF detector and a detector of a signal at a lower frequency than the RF; the switchable connection responsive to the event that the signal is not detected and RF is detected by opening the switchable connection so that the distal conductors are not connected with the proximal conductors; the switchable connection responsive to the event that the signal is detected and RF is detected by closing the switchable connection so that the distal conductors are connected with the proximal conductors;

The switchable connection responsive to the event that RF is not detected by closing the switchable connection so that the distal conductors are connected with the proximal conductors.

73. The apparatus of claim 72 wherein the chip further comprises a memory; the memory responsive to the event of a detection of RF for storing a state in the memory; and the memory disposed, after the state is stored in the memory, to communicate the state to equipment external to the lead.