Patent Description:
A wide range of medical procedures involve placing objects, such as sensors, tubes, catheters, dispensing devices, and implants, within the body. Real-time imaging methods are often used to assist doctors in visualizing the object and its surroundings during these procedures. In most situations, however, real-time three-dimensional imaging is not possible or desirable. Instead, systems for obtaining real-time spatial coordinates of the internal object are often utilized.

<CIT>, describes a hybrid magnetic-based and impedance-based position sensing system. The system includes a probe adapted to be introduced into a body cavity of a subject.

<CIT>, describes a system for determining the position of a work piece within a cavity of an opaque body. The system claims to use a transducer that interacts with a primary field, and several transducers that interact with a secondary field.

<CIT>, describes a system for determining the position of a catheter inside the body of a patient. A correction function is determined from the difference between calibration positions derived from received location signals and known, true calibration positions, whereupon catheter positions, derived from received position signals, are corrected in subsequent measurement stages according to the correction function.

<CIT> describes a method of registering two or more localization systems utilizing unique coordinate frames to a common coordinate frames including measuring position information for one or more reference locations in each coordinate frame. For each reference location, a fiducial grouping is created from the respective position measurements. The fiducial groupings are used to generate a mapping function that transforms position measurements expressed relative to the second coordinate frame to the first coordinate frame. Each localization system may also measure position information for a respective fixed reference localization element. Divergence between these fixed reference localization elements in the common coordinate system may be used to monitor, signal, and correct for anomalies such as dislodgement and drift.

The disclosure is herein described, by way of example only, with reference to the accompanying drawings, wherein:.

Various diagnostic and therapeutic procedures involve mapping of the electrical potential on the inner surface of a cardiac chamber. Electrical mapping can be performed, for example, by inserting a medical probe (e.g., a catheter), whose distal end is fitted with a position sensor and a mapping electrode, into the cardiac chamber. The cardiac chamber is mapped by positioning the probe at multiple points on the inner chamber surface. At each point, the electrical potential is measured using the electrode, and the distal end position is measured using the position sensor. The measurements are typically presented as a map of the electrical potential distribution over the cardiac chamber surface.

While positioning the medical probe within the cardiac chamber, impedance-based and/or magnetic-based position sensing systems can be used to determine a location of the probe within the cardiac chamber. Location sensing systems, such as those described in <CIT>, can determine a location of the probe by using locations of a set of three adhesive skin patches (also referred to herein as patches) that are affixed to a back of a patient. Location measurements received from the patches can be used to define a rigid body in a body coordinate system, and to determine a location of the probe within the rigid body. The body coordinate system can be updated as the adhesive skin patches move due to normal patient activities such as breathing.

Typically, the adhesive skin patches move and have respective locations that are consistent with one another so that the rigid body referred to above does not deform, but there may be instances when movement of one or more of the patches results in each of the one or more patches having a location that is not consistent with locations of the remaining patches. Embodiments provide methods and systems for detecting and correcting an inconsistent location of one or more of the adhesive skin patches.

In a disclosed embodiment, the inconsistent location comprises a physical location of one of the adhesive skin patches. For example, if the patient is lying on a table, the one adhesive skin patch may "stick" to the table as the patient moves. In an alternative embodiment, the inconsistent location comprises apparent locations of a plurality of the patches. For example, the positioning system may be based on magnetic sensors, and magnetic interference may cause an "apparent" movement (i.e., not a physical movement) of the plurality of the patches to their respective apparent inconsistent locations.

<FIG> is a schematic pictorial illustration of a medical system <NUM>, and <FIG> is a schematic illustration of a probe used in the system, in accordance with an embodiment of the present invention. System <NUM> may be based, for example, on the CARTO® system, produced by Biosense Webster Inc. (Diamond Bar, California). System <NUM> comprises a medical probe <NUM>, such as a catheter, and a control console <NUM>. In embodiments described hereinbelow, it is assumed that probe <NUM> is used for diagnostic or therapeutic treatment, such as performing ablation of heart tissue in a heart <NUM>. Alternatively, probe <NUM> may be used, mutatis mutandis, for other therapeutic and/or diagnostic purposes in the heart or in other body organs.

An operator <NUM> inserts probe <NUM> through the vascular system of a patient <NUM> so that distal end <NUM> (<FIG>) of probe <NUM> enters a chamber of heart <NUM>. In the configuration shown in <FIG>, operator <NUM> uses a fluoroscopy unit <NUM> to visualize distal end <NUM> inside heart <NUM>. Fluoroscopy unit <NUM> comprises an X-ray source <NUM>, positioned above patient <NUM>, which transmits X-rays through the patient. A flat panel detector <NUM>, positioned below patient <NUM>, comprises a scintillator layer <NUM> which converts the X-rays which pass through patient <NUM> into light, and a sensor layer <NUM> which converts the light into electrical signals. Sensor layer <NUM> typically comprises a two dimensional array of photodiodes, where each photodiode generates an electrical signal in proportion to the light detected by the photodiode.

Control console <NUM> comprises a processor <NUM> that converts the electrical signals from fluoroscopy unit <NUM> into an image <NUM>, which the processor presents as information regarding the procedure on a display <NUM>. Display <NUM> is assumed, by way of example, to comprise a cathode ray tube (CRT) display or a flat panel display such as a liquid crystal display (LCD), light emitting diode (LED) display or a plasma display. However other display devices can also be employed to implement embodiments of the present invention. In some embodiments, display <NUM> may comprise a touchscreen configured to accept inputs from operator <NUM>, in addition to presenting image <NUM>.

System <NUM> can use magnetic position sensing to determine position coordinates of distal end <NUM> inside heart <NUM>. In configurations where system <NUM> uses magnetic based position sensing, console <NUM> comprises a driver circuit <NUM> which drives field generators <NUM> to generate magnetic fields within the body of patient <NUM>. Typically, field generators <NUM> comprise coils, which are placed below the patient at known positions external to patient <NUM>. These coils generate magnetic fields in a predefined working volume that contains heart <NUM>. A magnetic field sensor <NUM> (also referred to herein as position sensor <NUM>) within distal end <NUM> of probe <NUM> generates electrical signals in response to the magnetic fields from the coils, thereby enabling processor <NUM> to determine the position of distal end <NUM> within the cardiac chamber. Magnetic position tracking techniques are described, for example, in <CIT>, <CIT>, <CIT>, <CIT>, <CIT>, <CIT> and<CIT>.

Additionally, system <NUM> can use impedance-based position sensing to determine position coordinates of distal end <NUM> inside heart <NUM>. In configurations where system <NUM> uses impedance-based position sensing, position sensor <NUM> is configured as a probe electrode, typically formed on an insulating exterior surface <NUM> of the distal end, and console <NUM> is connected by a cable <NUM> to body surface electrodes, which comprise three primary adhesive skin patches <NUM> and one or more ancillary adhesive skin patches <NUM>. In some embodiments, primary adhesive skin patches <NUM> are affixed to a back <NUM> of patient <NUM>, and the one or more ancillary adhesive skin patches are affixed to a front <NUM> of the patient. In operation, processor <NUM> can determine position coordinates of probe <NUM> inside heart <NUM> based on the impedance measured between the probe electrode and patches <NUM> and <NUM>. Impedance-based position tracking techniques are described, for example, in <CIT>, <CIT>and <CIT>.

Each patch <NUM> and <NUM> also comprises magnetic field sensors (e.g., coils) that can measure the magnetic fields produced by field generators <NUM>, and convey the magnetic field measurements to console <NUM>. Based on the measurements received from patches <NUM> and <NUM>, processor <NUM> can determine current positions for each of the primary and the ancillary adhesive skin patches. Both magnetic-based and impedance-based systems described hereinabove generate signals which vary according to the position of distal end <NUM>.

Processor <NUM> receives and processes the signals generated by position sensor <NUM> in order to determine position coordinates of distal end <NUM>, typically including both location and orientation coordinates. The method of position sensing described hereinabove is implemented in the above-mentioned CARTO™ system and is described in detail in the patents and patent applications cited above.

Processor <NUM> typically comprises a general-purpose computer, with suitable front end and interface circuits for receiving signals from probe <NUM> and controlling the other components of console <NUM>. Processor <NUM> may be programmed in software to carry out the functions that are described herein. The software may be downloaded to console <NUM> in electronic form, over a network, for example, or it may be provided on non-transitory tangible media, such as optical, magnetic or electronic memory media. Alternatively, some or all of the functions of processor <NUM> may be carried out by dedicated or programmable digital hardware components.

Based on the signals received from probe <NUM> and other components of system <NUM>, processor <NUM> drives display <NUM> to update image <NUM> to present a current position of distal end <NUM> in the patient's body, as well as status information and guidance regarding the procedure that is in progress. Processor <NUM> stores data representing image <NUM> in a memory <NUM>. In some embodiments, operator <NUM> can manipulate image <NUM> using one or more input devices <NUM>. In embodiments, where display <NUM> comprises a touchscreen display, operator <NUM> can manipulate image <NUM> via the touchscreen display.

In the configuration shown in <FIG>, probe <NUM> also comprises a force sensor <NUM> contained within distal end <NUM> and an ablation electrode <NUM> mounted on a distal tip <NUM> of probe <NUM>. Force sensor <NUM> measures a force applied by distal tip <NUM> on the endocardial tissue of heart <NUM> by generating a signal to the console that is indicative of the force exerted by the distal tip on the endocardial tissue. In one embodiment, the force sensor may comprise a magnetic field transmitter and receiver connected by a spring in distal tip <NUM>, and may generate an indication of the force based on measuring the deflection of the spring. Further details of this sort of probe and force sensor are described in <CIT> and <CIT>. Alternatively, distal end <NUM> may comprise another type of force sensor.

Electrode <NUM> typically comprises one or more thin metal layers formed over exterior surface <NUM> of distal end <NUM>. Console <NUM> also comprises a radio frequency (RF) ablation module <NUM>. Processor <NUM> uses ablation module <NUM> to monitor and control ablation parameters such as the level of ablation power applied via electrode <NUM>. Ablation module <NUM> may also monitor and control the duration of the ablation that is provided.

<FIG> is a flow diagram that illustrates a method of correcting an inconsistent physical location of a single primary adhesive skin patch <NUM> by using location measurements from ancillary patches <NUM>, and <FIG>, referred to collectively as <FIG>, are schematic diagrams illustrating rigid bodies <NUM>-<NUM> that are constructed from locations <NUM>-<NUM> of the primary and the ancillary skin patches. In the example shown in <FIG>, locations <NUM>-<NUM> comprise three-dimensional coordinates in a coordinate system <NUM> comprising an X-axis <NUM>, a Y-axis <NUM>, and a Z-axis <NUM>.

In embodiments described hereinbelow, locations <NUM>-<NUM> are indicative of spatial relationships that correspond to rigid bodies <NUM>-<NUM>. Thus, in the example shown in <FIG>, locations <NUM>, <NUM>, <NUM> are indicative of first spatial relationships which define rigid body <NUM>, locations <NUM>, <NUM>, <NUM> are indicative of second spatial relationships which define rigid body <NUM>, locations <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM> are indicative of third spatial relationships which define rigid body <NUM>, and locations <NUM>, <NUM>, <NUM>, <NUM> and <NUM> are indicative of fourth spatial relationships which define rigid body <NUM>. In embodiments described herein, rigid body <NUM> may also be referred to as a first rigid body, rigid body <NUM> may also be referred to as a second rigid body, rigid body <NUM> may also be referred to as a third rigid body, and rigid body <NUM> may also be referred to as a fourth rigid body.

In an initial step <NUM>, operator <NUM> affixes primary adhesive skin patches <NUM> to back <NUM> of patient <NUM>, and affixes ancillary skin patches <NUM> to front <NUM> of the patient. In a first receive step <NUM>, processor <NUM> receives, at a first time, first position-dependent signals from patches <NUM> and <NUM>. In the flow diagram shown in <FIG>, primary patches <NUM> may be referred to as back patches, and ancillary patches <NUM> may be referred to as front patches.

In a first compute step <NUM>, processor <NUM> computes respective first location coordinates <NUM>, <NUM>, <NUM> for patches <NUM>, and respective first location coordinates <NUM>, <NUM>, <NUM> for patches <NUM>. In a first identification step <NUM>, processor <NUM> identifies the first spatial relationships between patches <NUM>, using, as shown in <FIG>, the respective first location coordinates of locations <NUM>, <NUM> and <NUM> of the primary adhesive skin patches, i.e., as rigid body <NUM>.

In a second receive step <NUM>, processor <NUM> receives, at a second time subsequent to the first time, second position-dependent signals from patches <NUM> and <NUM>. In a second compute step <NUM>, processor <NUM> computes respective second location coordinates <NUM>, <NUM>, <NUM> for patches <NUM> and respective second location coordinates <NUM>, <NUM>, <NUM> for patches <NUM>. In a second identification step <NUM>, processor <NUM> identifies the second spatial relationships between patches <NUM>, using, as shown in <FIG>, the respective second location coordinates of locations <NUM>, <NUM> and <NUM> of primary adhesive skin patches <NUM>, i.e., as rigid body <NUM>.

In a detection step <NUM>, processor <NUM> detects a discrepancy between the first and the second spatial relationships. The discrepancy is caused by a change of location of only one primary patch <NUM> relative to the other primary patches. The detected discrepancy indicates that the second location of the only one primary patch is inconsistent with the second locations of the remaining primary patches <NUM>.

In the present example, the inconsistent location is a result of a physical movement of the only one primary patch <NUM> from location <NUM> (<FIG>) to location <NUM> (<FIG>) not being consistent with movements of the remaining primary patches <NUM> from locations <NUM> and <NUM> to locations <NUM> and <NUM> (i.e., both locations <NUM> and <NUM> comprise physical locations of the only one primary patch). For example, processor <NUM> may detect the discrepancy between the first and the second spatial relationships by detecting that rigid body <NUM> and rigid body <NUM> are no longer congruent, and that the non-congruency is effectively caused by the movement of only one of the patch locations defining the bodies. In other words, by detecting the incongruence between rigid bodies <NUM> and <NUM>, processor <NUM> detects a discrepancy between the first and the second spatial relationships caused by a given patch <NUM> that has first location <NUM> and the other patches <NUM> that have respective first locations <NUM> and <NUM>.

In a third identification step <NUM>, processor <NUM> identifies the third spatial relationships between patches <NUM> and <NUM>, using, as shown in <FIG>, the respective first location coordinates indicated by locations <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, and <NUM> of the primary and the ancillary skin patches, i.e., as rigid body <NUM>.

During a medical procedure, processor <NUM> receives signals from all of the primary and the ancillary adhesive skin patches. Typically, as shown in <FIG>, the processor defines rigid bodies <NUM> and <NUM> based on respective locations of primary patches <NUM>. In embodiments, upon detecting an inconsistent movement/location of a given patch <NUM>, processor <NUM> can calculate a correction for location <NUM> of the given patch by using locations of ancillary patches <NUM> and the remaining primary patches to create rigid bodies <NUM> (<FIG>), <NUM> (<FIG>) and <NUM> (<FIG>), as explained hereinbelow.

In a fourth identification step <NUM>, processor <NUM> identifies the fourth spatial relationships between patches <NUM> and the other patches <NUM> (i.e., the fourth spatial relationships do not include the given patch <NUM> that moved inconsistently), using, as shown in <FIG>, the respective second location coordinates of locations <NUM>, <NUM>, <NUM>, <NUM> and <NUM> of the primary and the ancillary adhesive skin patches, i.e., as rigid body <NUM>.

In a calculation step <NUM>, processor <NUM> calculates, based on the spatial relationships, a location correction for the only one primary patch. In some embodiments, the spatial relationships comprise the third and the fourth spatial relationships. Finally, in an application step <NUM>, processor <NUM> applies the location correction to the second location of the only one primary patch, thereby determining a corrected second location for the only one primary patch, and the method ends. In some embodiments, processor <NUM> applies the location correction while using the second location coordinates of patches <NUM> in order to track an object such as probe <NUM> in the patient's body.

To calculate the location correction using the third and the fourth spatial relationships (i.e., rigid bodies <NUM> and <NUM>), processor <NUM> can determine corrected second location <NUM> for the only one primary patch by determining, based on rigid body <NUM>, an expected second location (i.e., the corrected second location) for the only one primary patch in rigid body <NUM> (as indicated by an arrow <NUM>), thereby defining rigid body <NUM>. Location <NUM> comprises a three-dimensional coordinates in coordinate system <NUM>.

Once processor <NUM> has calculated the location correction for the only one primary patch, processor <NUM> can apply the location correction to subsequent signals indicating subsequent locations of the only one primary patch. Therefore, upon processor <NUM> receiving, at a third time subsequent to the second time, third position-dependent signals from the only one primary patch, the processor can compute, based on the third position-dependent signals, third location coordinates for the only one primary patch, and apply the location correction to the third location of the only one primary patch, thereby determining a corrected third location for the only one primary patch.

While embodiments described herein use three ancillary patches <NUM> to correct an inconsistent movement of only one primary patch <NUM>, configurations comprising any number of ancillary patches <NUM> whose respective location measurements can be used to define rigid bodies <NUM>, <NUM> and <NUM> are considered to be within the scope of the present disclosure. Therefore, in embodiments, at least four adhesive patches (i.e., three primary patches <NUM> and at least one ancillary patch <NUM>) may be affixed to patient <NUM>.

<FIG> is a flow diagram that illustrates a method of correcting inconsistent locations of a plurality of primary adhesive skin patch <NUM>, and <FIG>, referred to collectively as <FIG>, are schematic diagrams illustrating first patch location coordinates <NUM>-<NUM>, second patch location coordinates <NUM>-<NUM> and corrected second patch location coordinates <NUM>-<NUM>, in accordance with an embodiment of the present invention.

In the example shown in <FIG>, locations <NUM>-<NUM> comprise three-dimensional coordinates in a coordinate system <NUM> comprising an X-axis <NUM>, a Y-axis <NUM>, and a Z-axis <NUM>. In embodiments described herein, locations <NUM>-<NUM> are indicative of first spatial relationships represented by a rigid body <NUM>, and locations <NUM>-<NUM> are indicative of second spatial relationships indicated by a rigid body <NUM>.

In an initial step <NUM>, operator <NUM> affixes primary adhesive skin patches <NUM> to back <NUM> of patient <NUM>, and in a first receive step <NUM>, processor <NUM> receives, at a first time, first position-dependent signals from patches <NUM>. The first position-dependent signals are generated using the magnetic position sensing referred to above. In the present invention, the first position-dependent signals also indicate a first magnetic interference level for each primary patch <NUM>. In the example shown in <FIG>, the magnetic interference level(s) typically provide a measure of a proximity of X-ray source <NUM> to field generators <NUM>. In the flow diagram shown in <FIG>, primary patches <NUM> are also referred to as back patches.

In a first compute step <NUM>, processor <NUM> computes respective first location coordinates and computes a first magnetic interference index (i.e., a value) based on the first magnetic interference levels. In a first identification step <NUM>, processor <NUM> identifies the first spatial relationships between primary patches <NUM>, using, as shown in <FIG>, the respective first location coordinates of locations <NUM>, <NUM> and <NUM> of the primary adhesive patches, i.e., as rigid body <NUM>.

In a second receive step <NUM>, processor <NUM> receives, at a second time subsequent to the first time, second position-dependent signals from primary patches <NUM>. In the present invention, the second position-dependent signals also indicate a second magnetic interference level for each primary patch <NUM>.

In a second compute step <NUM>, processor <NUM> computes respective second location coordinates and respective second magnetic interference levels for each primary patch <NUM>, and computes a second magnetic interference index based on the second magnetic interference levels. In a second identification step <NUM>, the processor identifies the second spatial relationships between primary patches <NUM>, using, as shown in <FIG>, the respective second location coordinates of locations <NUM>, <NUM> and <NUM> of the primary adhesive skin patches, i.e., as rigid body <NUM>.

In a detection step <NUM>, processor <NUM> detects a discrepancy between the first and the second magnetic indices and a discrepancy between the first and the second spatial relationships of a plurality of primary patches <NUM> relative to the other primary patches. The detected discrepancy indicates that the second locations of a plurality of primary patches <NUM> are inconsistent with the second locations of the remaining primary patches <NUM>.

In the present example, location <NUM> comprises a physical first location of a first given primary patch <NUM>, location <NUM> comprises a physical first location of a second given primary patch <NUM>, location <NUM> comprises an apparent second location of the first given primary patch, and location <NUM> comprises an apparent second location of the second given primary patch. In embodiments of the present invention, the inconsistent (i.e., apparent) locations are a result of a difference between the first and the second magnetic field measurements, the difference causing an apparent movement of the first and the second given primary patches from locations <NUM>, <NUM> and <NUM> (<FIG>) to locations <NUM>, <NUM> and <NUM> (<FIG>). In some embodiments, processor <NUM> can detect the discrepancy between the first and the second spatial relationships by detecting a difference between rigid body <NUM> and rigid body <NUM>.

In a calculation step <NUM>, processor <NUM> calculates, based on the first location coordinates, location corrections for the plurality of primary patches. In some embodiments, the location correction for a given patch <NUM> comprises a distance and orientation from the second location of the given patch to the first location of the given patch (or vice versa). Finally, in an application step <NUM>, processor <NUM> applies the location corrections to the second locations of the plurality of the primary patches, thereby determining corrected second locations for the plurality of the primary patches, and the method ends.

In the example shown in <FIG>, based on the distances and the orientations are indicated by arrows <NUM>, <NUM> and <NUM>, processor <NUM> determines corrected second locations <NUM>, <NUM> and <NUM> for the plurality of the primary patches. Locations <NUM>, <NUM> and <NUM> comprise three-dimensional coordinates in coordinate system <NUM>. In embodiments where the detected movement of patches <NUM> is caused by magnetic interference (i.e., the detected movement is apparent), then the corrected location coordinates are in accordance with the first location coordinates. Therefore, in the example shown in <FIG>, location <NUM> is in accordance with location <NUM>, location <NUM> is in accordance with location <NUM>, and location <NUM> is in accordance with location <NUM>.

Once processor <NUM> has calculated the location correction for patches <NUM>, processor <NUM> can apply the location correction to subsequent signals indicating subsequent locations of the back patches. Therefore, upon processor <NUM> receiving, at a third time subsequent to the second time, third position-dependent signals from patches <NUM>, the processor can compute, based on the third position-dependent signals, third location coordinates for the back patches, and apply the location correction to the third locations of the back patches, thereby determining a corrected third location for patches <NUM>.

In embodiments of the present invention, processor <NUM> can track an object (e.g., probe <NUM>) in the patient's body relative to the respective location coordinates of patches <NUM> while applying the respective location corrections to the respective location coordinates of the patches. Additionally, while embodiments described herein use three primary patches <NUM> whose respective location measurements can be used to define rigid bodies <NUM>-<NUM> and <NUM>-<NUM>, configurations comprising more than three patches <NUM> are considered to be within the scope of the present invention.

It will be understood that the description above provides two embodiments for locating and correcting inconsistent second locations of one or more patches <NUM>. In a first embodiment, as described supra in the description referencing <FIG> and <FIG>, processor <NUM> detects an inconsistent second location for only one patch <NUM>, but does not detect a discrepancy in the magnetic interference index between the first and the second times. In the present invention, as described supra in the description referencing <FIG> and <FIG>, processor <NUM> detects respective inconsistent second locations for a plurality of patches <NUM> while detecting a discrepancy in the magnetic interference index between the first and the second times.

Claim 1:
A method of correcting inconsistent locations of back patches caused by magnetic interference, the method comprising:
affixing (<NUM>) back patches (<NUM>) to the back of a patient;
receiving (<NUM>), at a first time, from each of the back patches, a first position-dependent signal generated using a magnetic field sensor of the back patch that is configured to measure magnetic fields produced by external field generators and a first magnetic interference level signal;
computing (<NUM>) first location coordinates in a three-dimensional coordinate system (<NUM>) comprising an X-axis (<NUM>), a Y-axis (<NUM>), and a Z-axis (<NUM>) for each of the back patches using the first position-dependent signals and computing a first magnetic interference index for the back patches using the first magnetic interference level signals;
identifying (<NUM>) first spatial relationships between the back patches using the first location coordinates of the back patches as a first rigid body (<NUM>);
receiving (<NUM>), at a second time subsequent to the first time, from each of the back patches, a second position-dependent signal generated using the magnetic field sensor of the back patch and a second magnetic interference level signal;
computing (<NUM>) second location coordinates in the coordinate system (<NUM>) for each of the back patches using the second position-dependent signals and computing a second magnetic interference index for the back patches using the second magnetic interference level signals;
identifying (<NUM>) second spatial relationships between the back patches (<NUM>) using the second location coordinates of the back patches as a second rigid body (<NUM>);
detecting (<NUM>) a discrepancy between the first and the second magnetic interference indices and a discrepancy between the first and the second spatial relationships of a plurality of the back patches relative to the remaining of the back patches, wherein the detected discrepancy indicates that the second location coordinates of the plurality of the back patches are inconsistent with the second location coordinates of the remaining of the back patches;
calculating (<NUM>), based on the first location coordinates, location corrections for the plurality of the back patches; and
applying (<NUM>) the calculated location correction to the second location coordinates of the plurality of the back patches, thereby determining corrected second location coordinates for the plurality of back patches.