Abstract:
A probe for insertion into the body of a subject includes a sensor, a first microcircuit, which stores first calibration data with respect to the sensor, and a first connector at the proximal end of the probe. A probe adapter includes a second connector, which mates with the first connector, a signal processing circuit, which is processes the sensor signal, and a second microcircuit, which stores second calibration data with respect to the signal processing circuit. A microcontroller in the adapter receives the first and second calibration data and computes combined calibration data. The adapter includes a third connector, which mates with a fourth connector on a console. The console includes signal analysis circuitry, which analyzes the processed signal using the combined calibration data provided by the probe adapter.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to invasive systems for medical diagnosis and treatment, and specifically to calibration of probes and sensors that are used in such systems. 
     BACKGROUND OF THE INVENTION 
     Tracking the position of probes within the body is required for many medical procedures. For example, various systems have been developed for determining the position coordinates (location and/or orientation) of an object in the body based on magnetic field sensing. These systems use sensors affixed to the object to measure the relative strengths of externally-generated magnetic fields and to derive from these measurements the position of the object. (The term “position” as used in the present patent application and in the claims refers to any set of spatial coordinates, including either location coordinates, angular orientation coordinates, or both.) Methods for magnetic-based position sensing are disclosed, for example, in U.S. Pat. Nos. 5,391,199, 5,443,489, and 6,788,967 to Ben-Haim, in U.S. Pat. No. 6,690,963 to Ben-Haim, et al., in U.S. Pat. No. 5,558,091 to Acker et al., in U.S. Pat. No. 6,172,499 to Ashe, and in U.S. Pat. No. 6,177,792 to Govari, all of whose disclosures are incorporated herein by reference. 
     When accurate position measurements are required, the probe may be calibrated in advance. An exemplary calibration process is described in U.S. Pat. No. 6,266,551 to Osadchy et al., whose disclosure is incorporated herein by reference. In the embodiments described in this patent, a device used to determine the location and orientation of a catheter inside the body comprises a plurality of coils adjacent to the distal end of the catheter. The catheter further comprises an electronic microcircuit adjacent to the proximal end of the catheter, which stores information relating to the calibration of the device. The microcircuit comprises a read/write memory component, such as an EEPROM, EPROM, PROM, Flash ROM or non-volatile RAM, and the information is stored in digital form. The calibration information includes data relating to the relative displacement of the distal tip of the catheter from the coils. The calibration information may also include data relating to deviation of the coils from orthogonality, or data relating to the respective gains of the coils, or a combination of these data. 
     U.S. Pat. No. 6,370,411 to Osadchy et al., whose disclosure is incorporated herein by reference, describes a catheter assembly for connection to a control console. The catheter assembly comprises two parts: a catheter of minimal complexity which is inserted into a patient&#39;s body, and a connection cable which connects between the proximal end of the catheter and the console. The catheter comprises a microcircuit which may carry calibration data that is specific to the catheter. The cable comprises an access circuit, which receives the information from the catheter and passes it in a suitable form to the console. Preferably, the cable operates with all catheters of a specific model or type, and therefore when a catheter is replaced, there is no need to replace the cable. The cable comprises an additional microcircuit in which information characteristic of one or more models of catheters associated with the cable is stored. The additional microcircuit may also include calibration information for the access circuit and amplifiers within the cable. The calibration information of the amplifiers may include, for example, their zero-gain, DC offset and linearity. 
     SUMMARY OF THE INVENTION 
     Embodiments of the present invention provide convenient methods for generating, storing and computing calibration information with respect to an invasive medical probe. 
     In disclosed embodiments, the probe connects via a suitable mating connector to an adapter, which in turn connects, via another mating connector, to a console. The probe comprises a sensor and a probe microcircuit, which stores sensor calibration data. The adapter comprises a signal processing circuit for processing a signal that is output by the sensor. The adapter comprises its own microcircuit, which stores calibration data with respect to the signal processing circuit. A microcontroller in the adapter computes combined calibration data based on the data from both of the microcircuits. Signal analysis circuitry in the console receives the processed signal and analyzes this signal using the combined calibration data provided by the probe adapter. 
     In an exemplary embodiment, the sensor outputs a position signal, and the signal processing circuit comprises an amplifier, which amplifies the position signal. The console uses the combined calibration data to compute accurate position coordinates of the probe, corrected for deviations due both to the sensor and to the amplifier. The adapter is made to be compatible—in terms of both hardware and software configuration—with legacy probes that comprise both sensor and amplifier and have only a single microcircuit with overall calibration data for the catheter. The console may thus be used, without hardware or software modification, both with such legacy probes and with probes that connect to the console through the adapter. 
     There is therefore provided, in accordance with an embodiment of the present invention, medical apparatus, including: 
     a probe, having a proximal end and a distal end, which is adapted for insertion into the body of a subject, the probe including a sensor, which outputs a sensor signal; a first microcircuit, which stores first calibration data with respect to the sensor; and a first connector at the proximal end of the probe, electrically coupled at least to the sensor; 
     a probe adapter, including a second connector, which is arranged to mate with the first connector; a signal processing circuit, which is coupled to process the sensor signal so as to output a processed signal; a second microcircuit, which stores second calibration data with respect to the signal processing circuit; a microcontroller, which is arranged to receive the first and second calibration data from the first and second microcircuits, respectively, and to compute combined calibration data based on the first and second calibration data; and a third connector, electrically coupled at least to the signal processing circuit; and 
     a console, including a fourth connector, which is arranged to mate with the third connector; and signal analysis circuitry, which is coupled to receive at least the processed signal from the fourth connector and is arranged to analyze the processed signal using the combined calibration data provided by the probe adapter. 
     In a some embodiments, the sensor includes a position sensor, and the signal analysis circuitry is operative to determine coordinates of the distal end of the probe by analyzing the processed signal. In one embodiment, the position sensor is operative to generate the sensor signal responsively to a magnetic field applied externally to the body. 
     In a disclosed embodiment, the probe includes a catheter for insertion into a heart of the subject. 
     In some embodiments, the probe is a first type of probe, and the processed signal is a first processed signal, and the console is further operable in conjunction with a second type of probe, which is adapted to mate with the fourth connector and to convey at least a second processed signal to the fourth connector, and which includes a memory circuit, which stores third calibration data in a predetermined address space, which is accessed by the signal analysis circuitry in analyzing the second processed signal, and the microcontroller is operative to place the combined calibration data in the predetermined address space for reading by the processing circuitry. Typically, the console is operable in conjunction with both of the first and second types of probe without hardware or software modification according to probe type. 
     In one embodiment, the first calibration data are indicative of a sensitivity of the sensor and of a phase deviation introduced by the probe, and the signal processing circuit includes an amplifier, and the second calibration data are indicative of a gain of the amplifier and a second phase deviation introduced by the amplifier. 
     Typically, at least the first and second connectors include shielding against magnetic interference. 
     There is also provided, in accordance with an embodiment of the present invention, a method for performing an invasive medical procedure, including: 
     providing a probe, having a proximal end and a distal end, which includes a sensor, which outputs a sensor signal, a first microcircuit, which stores first calibration data with respect to the sensor, and a first connector at the proximal end of the probe, electrically coupled at least to the sensor; 
     connecting the first connector of the probe to a second connector of a probe adapter, which includes a signal processing circuit, which is coupled to process the sensor signal so as to output a processed signal, and a second microcircuit, which stores second calibration data with respect to the signal processing circuit, and a third connector, electrically coupled at least to the signal processing circuit; 
     using a microcontroller in the adapter, reading the first and second calibration data from the first and second microcircuits, respectively, and computing combined calibration data based on the first and second calibration data; 
     connecting the third connector of the adapter to a fourth connector of a console, which includes signal analysis circuitry; 
     inserting the probe into a body of a subject; and 
     using the signal analysis circuitry, receiving via the fourth connector at least the processed signal from the adapter and analyzing the processed signal using the combined calibration data provided by the probe adapter while the probe is in the body. 
     The present invention will be more fully understood from the following detailed description of the embodiments thereof, taken together with the drawings in which: 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic, pictorial illustration of a catheter-based medical system, in accordance with an embodiment of the present invention; 
         FIGS. 2A and 2B  are block diagrams that schematically show circuitry used in catheters and in a console in the system of  FIG. 1 , in accordance with an embodiment of the present invention; and 
         FIG. 3  is a flow chart that schematically illustrates a method for catheter calibration, in accordance with an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
       FIG. 1  is a schematic, pictorial illustration of a system  20  for cardiac catheterization, in accordance with an embodiment of the present invention. System  20  may be based, for example, on the CARTO™ system, produced by Biosense Webster Inc. (Diamond Bar, Calif.). This system comprises an invasive probe in the form of a catheter  22  and a control console  24 . The catheter is typically provided to users as a disposable unit, with a connector  26 , typically a plug, that mates with a corresponding connector  28 , typically a receptacle, on the console. In the context of the present patent application and in the claims, the term “connector” is used in the conventional sense, to mean any sort of electrical plug or similar device that can be readily connected and disconnected in the field without technical operations such as soldering or crimping. 
     Catheter  22  comprises an insertion tube whose distal end  30  is designed to be passed through the vascular system and into a chamber of the heart. Typically, the distal end of the catheter comprises a functional element  32  near a distal tip  34  of the catheter for performing therapeutic and/or diagnostic functions. For example, element  32  may comprise an electrode or an ultrasound transducer. 
     Catheter  22  also contains a position sensor  36 , which is used in determining position coordinates of distal end  30 . In the CARTO system, the position sensor comprises three coils, which output signals in response to an externally-applied magnetic field. These signals are amplified by a signal processing circuit  48  in the catheter. Typically, circuit  48  is located for convenience in a handle  38  of the catheter, which also includes controls  40  for steering the catheter. The amplified signals that are output by circuit  48  pass through a cable  42  to console  24 , via connectors  26  and  28 . The console processes the signals to determine the coordinates of distal tip  34  and displays the result on a user interface screen  44 . The user may interact with the console by means of a user input device  46 , such as a keyboard. Further details of the theory and operation of magnetic position sensing systems of this type are provided in the patents cited in the Background of the Invention. 
     Signal processing circuit  48 , cable  42  and connector  26  are costly components. In order to reduce the cost of the disposable part of system  20 , an alternative catheter  50  is produced so as to permit these components to be reused over multiple procedures without sterilization. Catheter  50  comprises a connector  52  that mates with a corresponding connector  54  of an adapter  51 . The catheter comprises a termination circuit  56 , whose functions are described hereinbelow. Adapter  51  comprises signal processing circuitry  58  for amplifying the signals from sensor  36 , as well as cable  42  and connector  26 . This latter connector is plug-compatible with connector  28 , so that catheter  50  (in conjunction with adapter  51 ) may be used interchangeably with catheter  22  in system  20 . Typically, catheter  50  is a single-use device, while adapter  51  is reusable. In the embodiment shown in  FIG. 1 , connectors  52  and  54  are mechanically configured to form a sort of “split handle,” but other mechanical configurations may also be used to achieve the same electrical functionality. For example, in one alternative embodiment, some or all of circuitry  58  is located in or near connector  26 . 
       FIG. 2A  is a block diagram that schematically shows details of catheter  22  and console  24 , in accordance with an embodiment of the present invention. Sensor  36  comprises three non-concentric coils  60 ,  62  and  64 , which are aligned along mutually-orthogonal axes. Coil wires  66  are connected via cables  68  to amplifiers  72  in signal processing circuit  48 . Typically, cable shields  70  are grounded to a suitable ground connection (not shown) in circuit  48 . The amplified signals produced by amplifiers  72  pass through cable  42 , via connectors  26  and  28 , to a front end circuit  74  in console  24 . The front end circuit typically filters and digitizes the signals and passes the resulting digital samples to a central processing unit (CPU)  76 , which processes the samples in order to compute the location and orientation coordinates of distal tip  34 . 
     Catheter  22  is calibrated in the factory in order to determine the combined sensitivity and phase offset of coils  60 ,  62 ,  64  and amplifiers  72 , as well as the exact location and angular skew of the coils relative to the distal tip of the catheter. The calibration data are then stored in a microcircuit memory  78 , such as an electrically-erasable programmable read-only memory (EEPROM) in handle  38 . When the catheter is subsequently connected to console  24  for clinical use, CPU  76  reads the calibration data from memory  78  via a bus  80  passing through cable  42 . Typically, the calibration data are arranged in a predetermined format, and the CPU is programmed to read the data from a certain address or range of addresses in the memory. The CPU uses the calibration data in determining accurate position coordinates of the catheter tip based on the sensor signals. The calibration process and the use of memory  78  to store the calibration parameters are described in detail in the above-mentioned U.S. Pat. No. 6,266,551. 
       FIG. 2B  is a block diagram that schematically shows details of catheter  50 , adapter  51  and console  24 , in accordance with an embodiment of the present invention. Sensor  36  in catheter  50  is identical to that in catheter  22 , and both catheters work with the same console  24 , as noted above. Cables  68  in catheter  50  connect to connector pins  82  of connector  52 . These pins mate with receptacles  84  of connector  54 . Connectors  52  and  54  typically also comprise ground connections  86  for grounding the circuits in catheter  50 . Typically, connectors  52  and  54  comprise magnetic shielding  85 , using μ-metal, for example, to reduce magnetic interference with the weak signals on pins  82 . 
     Both sensor coils  60 ,  62 ,  64  in catheter  50  and amplifiers  72  in adapter  51  contribute to the overall sensitivity and phase offset of the system. Since the adapter may be used with many different catheters, and a given catheter may be used with any adapter, the sensor coils and the amplifiers are calibrated separately. In other words, separate calibration data must be determined for each catheter and for each adapter. The appropriate calibration data for the given catheter and the given adapter are then combined when the catheter and adapter are used together in the field in order to determine the correct overall calibration factors to be applied by CPU  76  to the amplified sensor signals. 
     If console  24  was originally designed to operate with unitary catheters (such as catheter  22 , as shown in the preceding figures), however, then the console may be wired and programmed to receive only one set of calibration factors from the memory in the catheter. As noted above, in legacy catheters these calibration factors relate to the combined characteristics of the sensor coils and amplifiers. The console is not capable of receiving and using separate catheter and adapter calibration factors. 
     To address this problem, two memories  88  and  90  are used to contain the calibration data: memory  88  in catheter  50  and memory  90  in adapter  51 . These memories may comprise EEPROM chips or any other suitable type of non-volatile memory, such as EPROM or Flash memory. Catheter memory  88  contains calibration data with respect to sensor coils  60 ,  62 ,  64 . Adapter memory  90  contains calibration data relating to circuit  58 , and particularly to the characteristics of amplifiers  72 . 
     A microcontroller  92  in adapter  51  reads the calibration data from both of memories  88  and  90  and computes a combined set of calibration factors. The microcontroller then provides the combined calibration factors to the console in a manner that emulates the legacy interface of memory  78  in catheter  22  ( FIG. 2A ). For example, the microcontroller may write the combined calibration factors to the same address range in memory  90  as CPU  76  is programmed to access for this purpose, in the same format as is used for the calibration factors in memory  78 . Therefore, no modification is required to console  24  to enable it to receive and apply the calibration factors computed by microcontroller  92 . 
     Although sensor  36  is described above as a magnetic position sensor, the system configuration and methods described herein may also be applied in conjunction with other types of position sensors, such as ultrasonic and impedance-based position sensors. In impedance-based systems, for example impedance is measured between electrodes affixed to the catheter and electrodes placed on the body surface, and location coordinates are derived from the impedance measurements. Methods for impedance-based position sensing are disclosed, for example, in U.S. Pat. No. 5,983,126 to Wittkampf, in U.S. Pat. No. 6,456,864 to Swanson, and in U.S. Pat. No. 5,944,022 to Nardella, as well as in U.S. patent application Ser. No. 11/030,934 filed on Jan. 7, 2005, all of whose disclosures are incorporated herein by reference. 
     Furthermore, although the embodiments describe herein relate specifically to calibration and operation of a position sensor, memories  88  and  90  and microcontroller  92  may similarly be used in calibrating sensors of other types used in catheter  50  or in other sorts of probes. For example, assuming functional element  32  to be a sensor, such as a sensing electrode, a chemical sensor, a temperature sensor, a pressure sensor or an ultrasonic transducer, memory  88  may contain calibration data with respect to this sensor. Additionally or alternatively, the memory may contain calibration data with respect to an ultrasonic imaging transducer that is used in conjunction with a position sensor, as described, for example, in U.S. patent application Ser. No. 10/447,940, to Govari (published as US 2004/0254458 A1), whose disclosure is incorporated herein by reference. 
     Memories  88  and  90  may also be used to hold access control parameters, as described in the above-mentioned U.S. Pat. No. 6,266,551. These parameters may include, for example, an identification code, or a use counter or expiration time. Microcontroller  92  may read and process parameters that are stored in memories  88  and  90  and provide the result to CPU  76 . The CPU may then prevent operation of system  20  if the parameters indicate that an improper or, expired catheter or adapter has been connected to the console. 
       FIG. 3  is a flow chart that schematically illustrates a method for calibrating and processing signals produced by catheter  50 , in accordance with an embodiment of the present invention. Sensor  36  in catheter  50  is calibrated using a suitable calibration setup, at a sensor calibration step  100 . A jig and procedures that may be for this purpose are described in the above-mentioned U.S. Pat. No. 6,266,551. The sensor calibration parameters typically include the sensitivity and phase shift S ij   sensor , Φ ij   sensor  of each of coils  60 ,  62  and  64 , measured relative to an externally-applied magnetic field of known amplitude and phase. The calibration parameters may also include the spatial offset of each of the coils relative to distal tip  34  of catheter  50 , as well as the deviation of the coil axes from orthogonality. The calibration data are stored in memory  88 . 
     Amplifiers  72  in adapter  51  are calibrated, as well, at an adapter calibration step  102 . For this purpose, test signals may be applied to the inputs of the amplifiers (via connector  54 , for example), and the amplifier outputs may be measured in order to determine the amplifier gains and phase shifts A ij   n , φ ij   n . These results are stored in memory  90 . 
     When connectors  52  and  54  and coupled together, and system  20  is powered up, microcontroller  92  reads the calibration parameters from memories  88  and  90 , at a start-up step  104 . The microcontroller then computes combined calibration parameters for the catheter and adapter together, at a combined parameter computation step  106 . For example, the microcontroller may multiply each of the sensor coil sensitivity values by the gain of the corresponding amplifier to give a combined sensitivity value, and may sum the sensor phase shift with the phase shift of the amplifier to give a combined phase shift value. Alternatively, more complex computation algorithms may be applied to combine the parameters. 
     Microcontroller  92  writes the combined calibration parameter values to the appropriate target address space where CPU  76  expects to find the calibration parameters. For example, a range of addresses in memory  90  may be left available for this purpose. After writing the parameters to this range, microcontroller routes the control and data lines of memory  90  so as to enable CPU to read the parameters from the memory via bus  80 . For this purpose, the microcontroller may set internal switches within the microcontroller or set external switches (not shown) in circuit  58 . Alternatively, the microcontroller may save the combined calibration parameter values in an internal memory, which is mapped to the appropriate target address space of CPU  76 , and may emulate the operation of memory  78  when the CPU attempts to read the values. 
     CPU  76  reads the combined calibration parameter values from adapter  51 , at a parameter reading step  108 . The CPU then applies these values in processing the signals that it receives from catheter  50 . Operation of system  20  proceeds in an identical manner regardless of whether catheter  22  or catheter  50  is used. 
     Although the embodiments described above relate specifically to certain types of cardiac catheters, the principles of the present invention may similarly be applied to invasive medical probes and systems of other types. It will thus be appreciated that the embodiments described above are cited by way of example, and that the present invention is not limited to what has been particularly shown and described hereinabove. Rather, the scope of the present invention includes both combinations and subcombinations of the various features described hereinabove, as well as variations and modifications thereof which would occur to persons skilled in the art upon reading the foregoing description and which are not disclosed in the prior art.