Patent Publication Number: US-8111162-B2

Title: Method, apparatus and article for detection of transponder tagged objects, for example during surgery

Description:
CROSS-REFERENCE(S) TO RELATED APPLICATION(S) 
     This application claims the benefit under 35 U.S.C. §119(e) of U.S. Provisional patent application Ser. No. 61/056,787, filed May 28, 2008 and U.S. Provisional patent application Ser. No. 61/091,667, filed Aug. 25, 2008. 
    
    
     BACKGROUND 
     1. Field 
     This disclosure generally relates to the detection of the presence or absence of objects tagged with transponders, which may, for example, allow the detection of surgical objects during surgery. 
     2. Description of the Related Art 
     It is often useful or important to be able to determine the presence or absence of an object. 
     For example, it is important to determine whether objects associated with surgery are present in a patient&#39;s body before completion of the surgery. Such objects may take a variety of forms. For example, the objects may take the form of instruments, for instance scalpels, scissors, forceps, hemostats, and/or clamps. Also for example, the objects may take the form of related accessories and/or disposable objects, for instance surgical sponges, gauzes, and/or pads. Failure to locate an object before closing the patient may require additional surgery, and in some instances may have serious adverse medical consequences. 
     Some hospitals have instituted procedures which include checklists or requiring multiple counts to be performed to track the use and return of objects during surgery. Such a manual approach is inefficient, requiring the time of highly trained personnel, and is prone to error. 
     Another approach employs transponders and a wireless interrogation and detection system. Such an approach employs wireless transponders which are attached to various objects used during surgery. The interrogation and detection system includes a transmitter that emits pulsed wideband wireless signals (e.g., radio or microwave frequency) and a detector for detecting wireless signals returned by the transponders in response to the emitted pulsed wideband signals. Such an automated system may advantageously increase accuracy while reducing the amount of time required of highly trained and highly compensated personnel. Examples of such an approach are discussed in U.S. Pat. No. 6,026,818, issued Feb. 22, 2000, and U.S. patent Publication No. US 2004/0250819, published Dec. 16, 2004. 
     Commercial implementation of such an automated system requires that the overall system be cost competitive and highly accurate. In particular, false negatives must be avoided to ensure that objects are not mistakenly left in the patient. Some facilities may wish to install a single interrogation and detection system in each surgery theater, while other facilities may move an interrogation and detection system between multiple surgical theaters. In either case, the overall system will require a large number of transponders, since at least one transponder is carried, attached or otherwise coupled to each object which may or will be used in surgery. Consequently, the transponders must be inexpensive. However, inexpensive transponders typically have a relatively large variation in the frequency of signals they emit, making it difficult to accurately detect the signals returned by the transponders. This may be particularly difficult in some environments which are noisy with respect to the particular resonant frequencies of the transponders. Consequently, a new approach to detection of the presence and absence of transponder that facilitates the use of inexpensive transponders is highly desirable. 
     BRIEF SUMMARY 
     At least one embodiment may be summarized as a method of operation of a transponder detection device, including, during each of a plurality of detection cycles, receiving electromagnetic signals during a noise detection portion of the detection cycle; determining a noise value indicative of a noise level that corresponds to a highest one of a number N of samples or measurements of the electromagnetic signals received during the noise detection portion of the detection cycle, where the number N is greater than one; adjusting a signal detection threshold based at least in part on the determined noise value of at least one of the detection cycles; emitting at least one electromagnetic interrogation signal during a transmit portion of the detection cycle; receiving electromagnetic signals during a receive response portion of the detection cycle that follows the transmit portion of the detection cycle; and determining the presence or absence of a transponder based at least in part on a number M of samples or measurements of the electromagnetic signals received during the detection cycle and the adjusted signal detection threshold, where the number M is greater than one. The received signals may be unmodulated electromagnetic signals. 
     The method may further include determining a noise value indicative of a noise level based at least in part on the electromagnetic signals received during the noise detection portion of the detection cycle including setting the noise value based on the highest one of six samples or measurements of the electromagnetic signal received during the noise detection portion of the detection cycle. The method may further include adjusting a signal detection threshold based at least in part on the determined noise value of at least one of the detection cycles including adjusting the signal detection threshold based at least in part on a first number of determined noise values indicative of a noise level during at least one noise detection portion that occurred before the receive response portion of a first one of the detection cycles and a second number of determined noise values indicative of a noise level during at least one noise detection portion that occurred after the receive response portion of the first one of the detection cycles. The method may further include adjusting a signal detection threshold based at least in part on the determined noise value of at least one of the detection cycles including adjusting the signal detection threshold to be approximately twice an average of at least one of the first and the second number of determined noise values. The method may further include adjusting a signal detection threshold based at least in part on the determined noise value of at least one of the detection cycles including adjusting the signal detection threshold to be approximately twice a greatest one of at least one of the first and the second number of determined noise values. The method may further include determining the presence or absence of a transponder including comparing a maximum value of a plurality of matched filter outputs with the adjusted signal threshold. The method may further include adjusting a signal detection threshold based at least in part on the determined noise value of at least one of the detection cycles including adjusting the signal detection threshold to be approximately twice the determined noise value. The method may further include adjusting a signal detection threshold based at least in part on the determined noise value of at least one of the detection cycles including adjusting the signal detection threshold to be the large of approximately twice the determined noise value or a defined threshold value. The method may further include the defined threshold value being approximately 0.5 mV. The method may further include a ratio of N:M being at least equal to 4. The method may further include N being equal to about 200 and M being equal to about 800. The method may further include determining if an output of at least one matched filter during the noise detection portion of the detection cycle exceeds a noise fault threshold indicative of a noise fault. The method may further include determining if the output of the at least one matched filter during the noise detection portion of the detection cycle exceeds the noise fault threshold for a defined period of time; and terminating the detection cycle in response to the output of the at least one matched filter exceeding the noise fault threshold for the defined period of time. The method may further include emitting at least one electromagnetic interrogation signal during a transmit portion of the detection cycle including emitting at least one electromagnetic interrogation signal in each of a 136 KHz band, a 139 KHz band, a 142 KHz band, a 145 KHz band, a 148 KHz band, a 151 KHz band and a 154 KHz band. The method may further include ignoring any electromagnetic signals received during a recovery portion of the detection cycle that precedes the receive response portion of the detection cycle. The method may further include dumping energy from an antenna circuit during a dump portion of the detection cycle that precedes the recovery portion of the detection cycle. The method may further include, for each successive pair of detection cycles, varying a time between a start of a first one of the successive pairs of the detection cycles and a start of a next successive one of the pairs of the detection cycles. The method may further include the transmit and the receive response portions each occurring during an interrogation portion of the detection cycle, which follows the noise detection portion of the detection cycle. The method may further include emitting at least one electromagnetic interrogation signal during a transmit portion of the detection cycle including emitting a first wide band electromagnetic interrogation signal during a first transmit portion of the detection cycle and emitting at least a second wide band electromagnetic interrogation signal during a second transmit portion of the detection cycle. Determining the presence or absence of a transponder may be based at least in part on a frequency of the electromagnetic signals received during the detection cycle being within a defined frequency range. The defined frequency range may extend from about 137 KHz to about 160 KHz. Determining the presence or absence of a transponder may be based at least in part on a Q value of the electromagnetic signals received during the detection cycle. Determining the presence or absence of a transponder may be based at least in part on a Q value of the electromagnetic signals received during the detection cycle being at least equal to a threshold Q value. The threshold Q value may be 35. 
     At least one embodiment may be summarized as a transponder detection system, including a transmitter to transmit electromagnetic interrogation signals during at least one transmit portion of each of a plurality of detection cycles; a receiver to receive electromagnetic signals during a noise detection portion and during a receive response portion of each of the detection cycles; a noise level determiner that determines a noise level that corresponds to a highest one of a number N of samples or measurements of the electromagnetic signals received during the noise detection portion of the detection cycles, the noise detection portion temporally spaced from the transmit portions such that transponders are not responding to the electromagnetic interrogation signals; a detection threshold adjuster to adjust a detection threshold of the transponder detection system based at least in part on at least one value indicative of at least one of the noise levels; and a presence or absence determiner to determine the presence or absence of the transponders based at least in part on a number M of samples or measurements of the received electromagnetic signals and the adjusted detection threshold. The received signals may be unmodulated electromagnetic signals. 
     The noise level determiner may be configured to set the noise value based on the highest one of six samples or measurements of the electromagnetic signal received during the noise detection portion of the detection cycle. The detection threshold adjuster may be configured to adjust the signal detection threshold based at least in part on a first number of determined noise values indicative of a noise level during at least one noise detection portion that occurred before the receive response portion of a first one of the detection cycles and a second number of determined noise values indicative of a noise level during at least one noise detection portion that occurred after the receive response portion of the first one of the detection cycles. The transponder detection system may include a comparator coupled to determine if the output of the at least one matched filter during the noise detection portion of the detection cycle exceeds the noise fault threshold for a defined period of time; and wherein the controller is configured to terminate the detection cycle in response to the output of the at least one matched filter exceeding the noise fault threshold for the defined period of time. The controller may be configured to ignore any electromagnetic signals received during a recovery portion of the detection cycle that precedes the receive response portion of the detection cycle; to dump energy from an antenna circuit during a dump portion of the detection cycle that precedes the recovery portion of the detection cycle; and to vary a time between a start of a first one of the successive pairs of the detection cycles and a start of a next successive one of the pairs of the detection cycles. The transmitter may include a frequency adjuster configured to adjust a center frequency of the electromagnetic interrogation signals between successive transmit portions of the detection cycle. The presence or absence determiner may determine presence or absence be based at least in part on a frequency of the electromagnetic signals received during the detection cycle being within a defined frequency range. The defined frequency range may extend from about 137 KHz to about 160 KHz. The presence or absence determiner may determine presence or absence based at least in part on a Q value of the electromagnetic signals received during the detection cycle. The presence or absence determiner may determine the presence or absence of a transponder based at least in part on a Q value of the electromagnetic signals received during the detection cycle being at least equal to a threshold Q value. The threshold Q value may be 35. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S) 
       In the drawings, identical reference numbers identify similar elements or acts. The sizes and relative positions of elements in the drawings are not necessarily drawn to scale. For example, the shapes of various elements and angles are not drawn to scale, and some of these elements are arbitrarily enlarged and positioned to improve drawing legibility. Further, the particular shapes of the elements as drawn, are not intended to convey any information regarding the actual shape of the particular elements, and have been solely selected for ease of recognition in the drawings. 
         FIG. 1  is a schematic diagram showing a surgical environment illustrating a medical provider using an interrogation and detection system to detect an object tagged with a transponder in a patient, according to one illustrated embodiment. 
         FIG. 2A  is a schematic diagram of a transponder, according to one illustrated embodiment. 
         FIG. 2B  is a schematic diagram of a transponder, according to another illustrated embodiment. 
         FIG. 2C  is a schematic diagram of a transponder, according to a further illustrated embodiment. 
         FIG. 2D  is a side elevational view of a transponder, according to yet a further illustrated embodiment. 
         FIG. 2E  is an end view of the transponder of  FIG. 2D . 
         FIG. 2F  is a cross-sectional view of the transponder of  FIG. 2D , taken along section line  2 F. 
         FIG. 2G  is an isometric view of a ferrite core of the transponder of  FIG. 2D . 
         FIG. 3A  is an exploded view of a wand of the interrogation and detection system, according to one illustrated embodiment. 
         FIG. 3B  is an isometric view of the wand of  FIG. 3A . 
         FIG. 4  is an isometric view of a controller of the interrogation and detection system, according to one illustrated embodiment. 
         FIG. 5  is a schematic diagram of a control system of the interrogation and detection system, according to one illustrated embodiment. 
         FIG. 6  is a schematic diagram of a software configuration of the interrogation and detection system, according to one illustrated embodiment. 
         FIGS. 7A-7I  are an electrical schematic diagram of the interrogation and detection system including a control circuit and antenna, according to one illustrated embodiment. 
         FIG. 8  is a timing diagram illustrating a method of frequency hopping, according to one illustrated embodiment. 
         FIG. 9  is a timing diagram illustrating pulsed timing, according to one illustrated embodiment. 
         FIG. 10  is a timing diagram showing activation of a pair of transistors of the control circuit in a push-pull configuration to drive the antenna, according to one illustrated embodiment. 
         FIG. 11  is a schematic diagram of a model circuit that models how a transponder responds to the transmitted interrogation signals, according to one illustrated embodiment. 
         FIG. 12  is a graph of a simulated transponder response signal based on the model circuit of  FIG. 11  where a frequency of the response signal matches a frequency of the interrogation signal, according to one illustrated embodiment. 
         FIG. 13  is a graph of a measured transponder response signal where a frequency of the response signal matches a frequency of the interrogation signal, according to one illustrated embodiment. 
         FIG. 14  is a graph of a simulated transponder response signal based on the model circuit of  FIG. 11  where a frequency of the interrogation signal is higher than a frequency of the response signal, according to one illustrated embodiment. 
         FIG. 15  is a graph of a measured transponder response signal where a frequency of the interrogation signal is higher than a frequency of the response signal, according to one illustrated embodiment. 
         FIG. 16  is a graph showing noise and filter levels for noise filtering, according to one illustrative embodiment. 
         FIG. 17  is a flow diagram of a method of operating an interrogation and control system, according to one illustrated embodiment. 
         FIG. 18  is a graph showing detection range versus noise achievable by a current embodiment and previous embodiments, for comparison purposes. 
         FIG. 19  is a flow diagram showing a method of operating an interrogation and detection system according to one illustrated embodiment, including receiving electromagnetic signals, for example unmodulated electromagnetic signals, determining a noise value, adjusting signal detection threshold, emitting interrogations signals, receiving electromagnetic signals, and determining a presence or absence of a transponder based at least in part on the adjusted signal detection threshold. 
         FIG. 20  is a flow diagram showing a method of determining a noise value in an interrogation and detection system, according to one illustrated embodiment. 
         FIG. 21  is a flow diagram showing a method of adjusting the signal detection threshold in an interrogation and detection system including, according to one illustrated embodiment. 
         FIG. 22  is a flow diagram showing a method of adjusting the signal detection threshold in an interrogation and detection system, according to one illustrated embodiment. 
         FIG. 23  is a flow diagram showing a method of adjusting the signal detection threshold in an interrogation and detection system, according to one illustrated embodiment. 
         FIG. 24  is a flow diagram showing a method of determining the presence or absence of a transponder in an interrogation and detection system, according to one illustrated embodiment. 
         FIG. 25  is a flow diagram showing a method of adjusting the signal detection threshold in an interrogation and detection system, according to one illustrated embodiment. 
         FIG. 26  is a flow diagram showing a method of adjusting the signal detection threshold in an interrogation and detection system, according to one illustrated embodiment. 
         FIG. 27  is a flow diagram showing a method of determining an occurrence of a noise fault in an interrogation and detection system, according to one illustrated embodiment. 
         FIG. 28  is a flow diagram showing a method of determining an occurrence of a noise fault in an interrogation and detection system, according to another illustrated embodiment. 
         FIG. 29  is a flow diagram showing a method of emitting electromagnetic interrogation signal(s) in an interrogation and detection system, according to one illustrated embodiment. 
         FIG. 30  is a flow diagram showing a method of eliminating false positives in an interrogation and detection system, according to one illustrated embodiment. 
         FIG. 31  is a flow diagram showing a method of eliminating false positives in an interrogation and detection system, according to another illustrated embodiment. 
         FIG. 32  is a flow diagram showing a method of de-correlating response signals in an interrogation and detection system, according to one illustrated embodiment. 
         FIG. 33  is a flow diagram showing a method of emitting electromagnetic interrogation signal(s) in an interrogation and detection system, according to one illustrated embodiment. 
         FIG. 34  is a flow diagram showing a method of determining the presence or absence of a transponder employing a frequency domain spectrum in an interrogation and detection system, according to one illustrated embodiment. 
         FIG. 35  is a flow diagram showing a method of determining the presence or absence of a transponder employing a Q value in an interrogation and detection system, according to one illustrated embodiment. 
         FIG. 36  is a flow diagram showing a method of determining the presence or absence of a transponder employing a Q value in an interrogation and detection system, according to another illustrated embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     In the following description, certain specific details are set forth in order to provide a thorough understanding of various disclosed embodiments. However, one skilled in the relevant art will recognize that embodiments may be practiced without one or more of these specific details, or with other methods, components, materials, etc. In other instances, well-known structures associated with transmitters, receivers, or transceivers have not been shown or described in detail to avoid unnecessarily obscuring descriptions of the embodiments. 
     Unless the context requires otherwise, throughout the specification and claims which follow, the word “comprise” and variations thereof, such as, “comprises” and “comprising” are to be construed in an open, inclusive sense, that is as “including, but not limited to.” 
     Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment. Thus, the appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment. Further more, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments. 
     As used in this specification and the appended claims, the singular forms “a,” “an,” and “the” include plural referents unless the content clearly dictates otherwise. It should also be noted that the term “or” is generally employed in its sense including “and/or” unless the content clearly dictates otherwise. 
     The headings and Abstract of the Disclosure provided herein are for convenience only and do not interpret the scope or meaning of the embodiments. 
       FIG. 1  shows a surgical environment  10  in which a medical provider  12  operates an interrogation and detection system  14  to ascertain the presence or absence of objects  16  in, or on, a patient  18 . The interrogation and detection system  14  may include a controller  20 , and an antenna  22  coupled to the controller  20  by one or more communication paths, for example coaxial cable  24 . The antenna  22  may take the form of a hand-held wand  22   a.    
     The object  16  may take a variety of forms, for example instruments, accessories and/or disposable objects useful in performing surgical procedures. For instance, the object  16  may take the form of scalpels, scissors, forceps, hemostats, and/or clamps. Also for example, the objects  16  may take the form of surgical sponges, gauze and/or padding. The object  16  is tagged, carrying, attached or otherwise coupled to a transponder  26 . Embodiments of the interrogation and detection system  14  disclosed herein are particularly suited to operate with transponders  26  which are not accurately tuned to a chosen or selected resonant frequency. Consequently, the transponders  26  due not require high manufacturing tolerances or expensive materials, and thus may be inexpensive to manufacture. 
     In use, the medical provider  12  may position the wand  22   a  proximate the patient  18  in order to detect the presence or absence of the transponder  26 . and hence an object  16  The medical provider  12  may in some embodiments move the wand  22   a  along and/or across the body of the patient  18 . In some embodiments, the wand  22   a  may be sized to fit at least partially in a body cavity  28  of the patient  18 . 
       FIG. 2A  shows a transponder  26   a  according to one illustrated embodiment. 
     The transponder  26   a  includes a miniature ferrite rod  30  with a conductive coil  32  wrapped about an exterior surface thereof to form an inductor (L), and a capacitor (C)  34  coupled to the conductive coil  32  to form a series LC circuit. The conductive coil  32  may, for example, take the form of a spiral wound conductive wire with an electrically insulative sheath or sleeve. The transponder  26   a  may include an encapsulation  36  that encapsulates the ferrite rod  30 , conductive coil  32 , and capacitor  34 . The encapsulant  36  may be a bio-inert plastic, that protects the ferrite rod  30 , conductive coil  32  and/or capacitor  34  from pressure and/or from fluids, for example bodily fluids. 
     In some embodiments, the ferrite rod  30  may include a passage  38  sized to receive a physical coupler, for example a bonding tie or string  40 . The bonding tie or string  40  may take the form of an elastomeric x-ray opaque flexible elongated member, that may be used to attach the transponder  26   a  to various types of objects  16 , for example surgical sponges. The transponder  26   a  may have a length of about 8 millimeters and a diameter of about 2 millimeters. Employing such small dimensions ensures that the transponder  26   a  does not impede deformation of objects  16  such as sponges. The transponder  26   a  may include an optional diode (not shown), to protect against over-voltage occurrences caused by other electronic instruments. 
       FIG. 2B  shows a transponder  26   b , according to another illustrated embodiment. 
     The transponder  26   b  includes a single loop of conductive material  42 , for example a loop of conductive wire forming an inductor (L), coupled in series to a capacitor  44  (C) to form an LC series circuit. The loop of conductive material  42  and capacitor  44  may be encapsulated in an elastomeric coating or sleeve  46 . The dimensions of the transponder  26   b  may be similar to the dimensions of the transponder  26   a . In some embodiments, the dimensions of the transponder  26   b  are greater than the dimensions of the transponder  26   a . The transponder  26   b  is highly flexible, and thus may provide its own thread-like or string-like attachment to various types of objects  16 . 
       FIG. 2C  shows a transponder  26   c  according to a further embodiment. 
     The transponder  26   c  includes a dumbbell-shaped ferrite rod  48  having broad end portions  48   a ,  48   b , and a narrow intermediate portion  48   c  which is wrapped by a conductive coil  50 . The broad portions  48   a ,  48   b  contain the conductive coils  50 . Such a design may provide stronger and/or more reliable signal emission than transponders  26   a ,  26   b  fashioned with cylindrical ferrite rods. The transponder  26   c  may optionally include an encapsulant  52 . Further details regarding the transponder  26   c  may be found in U.S. Provisional patent application No. 60/811,376 filed Jun. 6, 2006. In some embodiments, the transponder  26   c  may be formed as a fusiform-shaped object, with truncated ends. The fusiform shape may be advantageous over cylindrical shaped transponders  26   a ,  26   b  in reducing the likelihood of close parallel alignment of the transponders  26   a ,  26   b , which may produce transponder-to-transponder interaction and interference. 
       FIGS. 2D-2G  show a transponder  26   d  according to yet a further embodiment. 
     The transponder  26   d  includes a ferrite core  53 , inductor (L)  54 , and capacitor©  55  electrically coupled to the inductor  54  to form an LC series circuit. The transponder  26   d  also includes a capsule  56  with a cavity  57  open at one end to receive the ferrite core  53 , inductor  54  and capacitor  55 , as well as a lid  58  to close the open end of the capsule  56 . 
     The ferrite core  53  may, for example, take the form of a soft ferrite drum, and may, for example, be formed of Nickel Zinc. Suitable ferrite cores  53  may be commercially available from TAK FERRITE as part no. L8A DR3X9 B=1.8 F=6 or from HUAH YOW under part no. 10R030090-77S. The drum may have a pair of larger diameter end portions  53   a ,  53   b , with a smaller diameter intermediate portion  53   c  therebetween. 
     The inductor  54  may take the form of magnet wire wrapped around the intermediate portion  53   c  of the ferrite core  53 . The magnet wire may, for example, have a dimension of approximately 41 American Wire Gauge (AWG), although some embodiments may employ wires or conductors of larger or small gauges. Suitable inductors  54  may be commercially available from ELEKTISOLA under part no. PN-155 or from ROSEN under part no. 2UEW-F. The inductor may, for example, include approximately 432 turns, over approximately 6.5 layers, although some embodiments may include a greater or lesser number of turns and/or layers. The transponder  26   d  may include tape and/or epoxy enveloping the inductor  54 . Suitable tape may be commercially available from 3M under part nos. 1298, 1350-1 or PLEO1P801, while suitable epoxy may be commercially available from LOCKTITE under part no. 3211. 
     The capacitor  55  may, for example, take the form of a ceramic capacitor. The capacitor  55  may, for example, have a capacitance of 470 PF, 100V, with a Quality factor of Q&gt;2200@1 MHz. Suitable capacitors  55  may be commercially available from SANJV DIELECTRIC under part no. 0805NPO471J101 or from FENG HUA under part no. 0805CG471J101 NT. 
     The capsule  56  and lid  58  may, for example, be formed of a polypropylene. Suitable capsules  56  and lids  58  may be commercially available from WEITHE ELECTRON (HK) COMPANY, under part specification CASE 4.3×12.6. The combination of the capsule  56  and lid  58  may, for example, have a length of approximately 12.8 mm and a diameter of 4.4 mm. Circuit bonds may, for example, employ UNITED RESINS CORP. part no. 63001500 CIRCUIT BOND LV, while solder may take the form of a lead free 96.5% Ag/3% Sn/0.5 Cu solder. 
     The transponders  26  may be attached to hemostats, scissors, certain forms of forceps, and the like. In some embodiments, the transponders  26  may be coupled to the object  16  by way of a clamp or holder. In some embodiments, the transponders  26  may be retained within a cavity of the holder. In some embodiments, the holder may be fashioned of a durable deformable material, such as surgical grade polymer, which may be deformed to clamp securely onto the finger or thumbhole of an instrument. In other embodiments, the transponders  26  may be attached to objects  16  by way of pouches fashioned of sheet material (e.g., surgical fabric) surrounding the transponder  26 . The transponder  26  is retained within the pouch, and in some embodiments the pouch may be sewn or otherwise sealed. Sealing may be done with adhesive, hot glue, clamping, grommeting, or the like. 
       FIGS. 3A and 3B  show a wand  22   a , according to one illustrated embodiment. 
     The wand  22   a  may include a first housing structure  60   a  and a second housing structure  60   b  which mates to the first housing structure  60   a  to form a housing  60 . The housing  60  may include an annular portion  60   c  and a handle portion  60   d  extending from the annular portion. The handle portion may be sized and dimensioned to be gripped by the hand of a medical provider  12  ( FIG. 1 ). In some embodiments, the housing portions  60   a ,  60   b  may be identical in shape to one another. 
     The housing  60  may define one or more cavities  62  sized and dimensioned to receive the antenna  22 . The antenna  22  may, for example, take the form of an annulus or air-coil formed of coils of conductive material, for example wire. In one embodiment, the antenna  22  includes 10 turns evenly spaced between an inner diameter of about 11 inches and an outer diameter of about 14 inches. The antenna  22  acts as an inductor. 
     The wand  22   a  may include a coupling member  64  which may be positioned in the cavity in the handle portion  60   d  to provide a connector to communicatively couple to an end of the coaxial cable  24  to the antenna  22 . The coupling member  64  may take the form of a standard coaxial connector. Some embodiments may employ other types of communications pathways between the controller  20  and the antenna  22 , and thus may employ other types of coupling members or connectors. 
     In some embodiments, the wand  22   a  may include one or more user interface devices, for example one or more visual indicators to provide visual indications to the medical provider  12 . Such may, for example, take the form of one or more light emitting diodes, which may produce one or more different colors. Such user interface devices may additionally, or alternatively include a speaker or other transducer, operable to produce a sound or other sensory indication, for example a tactile sensation. Such user interface devices may be configured to provide sensory feedback to the medical provider  12  indicative of an operating condition of the interrogation and detection system  14 . For example, such may indicate when the interrogation and detection system  14  is operating, when the presence of a transponder  26  has been identified, and/or when an error has occurred. Locating user interface devices on the wand  22   a  may be advantageous since the medical provider  12  will typically focus their attention on the wand  22   a  while scanning the patient  18 . 
       FIG. 4  shows the controller  20  according to one illustrated embodiment. 
     The controller  20  includes an input port  70  with an appropriate coupling member, for example a connector to allow an end of the coaxial cable  24  to be communicatively coupled to the controller  20 . As noted above, some embodiments may employ other communications pathways between the controller  20  and the antenna  22 , hence other types of coupling members or connectors may be employed. The controller  20  may also include a power switch (not illustrated in  FIG. 4 ), for example, positioned on a back or rear of the controller  20 . The controller  20  may further include a power cord (not shown) to couple the controller  20  to a suitable power supply. The power supply may, for example take the form of a standard wall outlet or any other power supply or source. The controller  20  may further include one or more user interface devices for providing information to a user. For example, the controller  20  may include one or more visual indicators  134 , for instance one or more light emitting diodes (LEDs) and/or liquid crystal displays. Additionally, or alternatively, the controller  20  may include one or more speakers  130  or other transducers operable to produce sound or tactile sensations. The controller  20  forms a transmitter and receiver, or transceiver, to transmit interrogation signals and receive responses to those signals, as well as to receive electromagnetic signals which may be indicative of noise. 
       FIG. 5  shows a control system  100  of the interrogation and detection system  14 , according to one illustrated embodiment. 
     The control system  100  includes a field programmable gate array (FPGA) board  102 , analog board  104  and display board  106 , communicatively coupled to one another. 
     The FPGA board includes an FPGA  108 , configuration jumpers  110 , RS-232 drivers  112 , oscillator  114 , random access memory (RAM)  116 , flash memory  118 , and voltage monitoring (VMON) analog-to-digital converter (ADC)  120 . 
     The FPGA  108  may take the form of a Xilinx Spartan3 FPGA, which runs FPGA and application software. As explained below, on power up, the FPGA reads the configuration information and application software program from the flash memory  118 . 
     The configuration jumpers  110  are used to select the application software configuration. 
     The RS-232 drivers  112  are used to allow the application software to communicate using serial RS-232 data for factory test and diagnostics. 
     The oscillator  114  sets the clock frequency for the operation of the FPGA  108 . The oscillator  114  may, for example, take the form of 40 MHz oscillator, although other frequencies are possible. 
     The RAM  116  is connected to the FPGA  108  and is available for use by the application software. The application software uses this memory space for storage of both the executable program and program data. The RAM  116  may, for example, have a capacity of 1 MB. 
     The flash memory  118  contains both the FPGA configuration data and the binary application program. On power up the FPGA  108  reads the flash memory to configure the FPGA  108  and to copy the application program binary data from the flash memory  118  to the RAM  102 . 
     The voltage monitor ADC  120  is connected to the FPGA  108  and controlled by the application software to monitor a power supply and regulated voltage forms in controller electronics. 
     The analog board  104  includes transmit control circuits  122 , capacitor selection circuits  124 , wand detection circuit  126 , signal ADC  128 , audible beeper  130  and self-test signal  132 . 
     The transmit control circuits  122  on the analog board  104  are controlled by signals from the FPGA  108  to generate a transmit waveform. These signals are denominated as LO_FET_ON and HI_FET_ON, which control the transmit or drive transistors Q 1 , Q 2  ( FIG. 7A ) along with a signal denominated as DUMP_ON which controls a dump TRIAC ( FIG. 7A ). 
     Optional capacitor selection circuits  124  on the analog board  104  are controlled by the signals from the FPGA  108  to tune the drive circuit to match an inductance of the antenna  22 . 
     The wand detection circuit  126  detects when a wand  22   a  is connected to the controller  20 . The output of the wand detection circuit  126  drives a signal denominated as the LOOP_LEVEL_OUT signal, which is an input to the FPGA  108 . 
     The signal ADC  128  is used as a receiver to sample the signals received at the antenna  22   a  from the transponders  26  ( FIGS. 2A-2C ). The signal ADC  128  may, for example, operate at a 1 MHz sample rate and may have 12-bits of resolution. The FPGA board  102  generates the timing and control signals for the signal ADC  128 , which signal are denominated as ADC_CTRL, CS 1 , SCLK, SD 0 . 
     The audible speaker or beeper  130  can be controlled by the FPGA  108  to emit sounds to indicate various states, modes or operating conditions to the medical provider  12  ( FIG. 1 ). 
     The FPGA  108  can cause the generation of the self test signal  132  on the analog board  104  at the signal ADC  128 . Self-testing may be performed at start up, and/or at other times, for example periodically or in response to the occurrence of certain conditions or exceptions. 
     The display board  106  includes user interface elements, for example a number of light emitting diodes (LEDs)  134 . The FPGA board  102  can control the LEDs  134  on the display board  106 . The display board  106  also includes a user selectable activation switch, denominated as front panel button  136 . The front panel button  136  is connected to the display board  106  which allow the FPGA  108  to monitor when the front panel button  136  is activated (e.g., pressed). 
       FIG. 6  shows a software configuration  200  of the interrogation and detection system  14 , according to one illustrated embodiment. 
     The software may include application software  202  that is responsible for operating the electronics controller  20  ( FIGS. 1 and 4 ). The application software  202  controls the timing for generating transmit pulses, processes sampled data to detect transponders  26  ( FIGS. 2A-2C ), and indicates status to the user with the display LED&#39;s  134  ( FIG. 5 ) on the display board  106  and/or via the audible speaker or beeper  130  on the analog board  104 . The application software  202  is stored in the flash memory  118  ( FIG. 5 ) and transferred into the RAM  116  by a boot loader  204 . 
     The boot loader  204  is automatically loaded when the FPGA  108  is configured, and starts execution after a processor core  206  is reset. The boot loader  204  is responsible for transferring the application software  202  from the flash memory  118  to the external RAM  116 . 
     The processor platform  208  is configured into the FPGA  108  ( FIG. 5 ) on power up from the configuration information stored in the flash memory  118 . The processor platform  208  implements a custom microprocessor with a processor core  206 , peripherals  210   a - 210   n , and custom logic  212 . 
     The processor core  206  may take the form of a soft processor core supplied by XILINX under the name MICROBLAZE, that implements a 32-bit processor including memory cashes and a floating point unit. A soft core processor is one that is implemented by interconnected FPGA logic cells instead of by a traditional processor logic. The processor core  206  is connected to the internal FPGA peripherals  210   a - 210   n  using a 32-bit processor bus  211  called the On-Chip Peripheral Bus. The XILINX supplied peripherals for the MICROBLAZE processor core  206  include external memory interfaces, timers, and general purpose I/O. 
     The custom logic  212  to create the transmit signals, sample the ADC, and accumulate the transponder return signals is designed as a peripheral to the processor core  206 . The custom logic  212  is the part of the design of the FPGA  108 . 
       FIGS. 7A-7I  show a control circuit  300  according to one illustrated embodiment. The control circuit  300  is used to drive the antenna  22  to excite or interrogate transponders  26  ( FIGS. 2A-2C ), and to receive or detect and process signals received by the antenna  22  from the transponders  26 . Thus, the control circuit  300  forms a transmitter and receiver, or transceiver. 
     The control circuit  300  includes a transmitter circuit  302  formed by a pair of drive transistors (e.g., field effect transistors) Q 1 , Q 2  operated in a push-pull configuration between a high voltage rail (e.g., 24 V) and a low voltage rail (e.g., GND). The drive transistors Q 1 , Q 2  are responsive to respective drive signals DRIVE_HI, DRIVE_LO, which are applied to the gates of the respective drive transistors Q 1 , Q 2 . The drive transistors Q 1 , Q 2  are coupled to the antenna  22  by a non-switched capacitor C 8  and the coaxial cable  24 . The antenna  22  and capacitor C 8 , as well as capacitance provided by the coaxial cable  24 , form an LC circuit. 
     Optionally, the control circuit  300  may also include a dynamic tuning circuit  304 . The dynamic tuning circuit  304  selectively adjusts the capacitance of the LC circuit. In the illustrated embodiment, the dynamic tuning circuit  304  includes a number of switched capacitors C 33 -C 36  and relays U 9 , U 10 . The relays U 9 , U 10  are operated to selectively couple the switched capacitors C 33 -C 36  in series with the non-switched capacitor C 8 , thereby adjusting the LC characteristics of the LC circuit, and allowing fine tuning of the LC circuit around center frequencies or center channels of a number of wide band frequency bands, as described in more detail below. 
       FIG. 8  illustrates a detection cycle  400  that employs an approach that optimizes signal to noise ratio (SNR) by a receiver portion, according to one illustrated embodiment. Such may, for example, advantageously increase range or increase sensitivity at a given range. 
     One embodiment is optimized based on having an overall detection cycle that performs well for transponders with resonant frequencies from approximately 136 KHz to approximately 154 KHz, and which has a pulse timing that is consistent with hardware limitations. An optimal SNR may be achieved by, for example, transmitting a single wideband frequency pulse. 
     The application software  202  ( FIG. 6 ) implements the detection cycle  400  using transmission or interrogation in a frequency band centered around a center channel or frequency. In the illustrated embodiment, the application software  202  sequences through a non-measurement portion (i.e., gap)  400   a , and two distinct measurement portions, denominated as a noise detection portion  400   b  and an interrogation or signal detection portion  400   c , each detection cycle  400 . In at least one embodiment, the detection cycle  400  may, for example, be approximately 275 milliseconds, the gap portion may be approximately 10 milliseconds, the noise portion approximately 37 milliseconds and the interrogation or signal portion approximately 228 milliseconds. 
     During the noise detection portion  400   b , which may, for example be a first measurement portion of each detection cycle  400 , ambient or background noise is measured or sampled, providing a value indicative of a level of ambient or background noise for the particular environment. The noise measurements or samples are taken or captured at a time sufficiently after excitement of the transponders  26  by the interrogation signal emitted by the transmitter such that the transponders  26  are substantially not resonating or responding to any previous excitation by interrogation signals. In particular, a number N of measurements or samples are taken during the noise detection or first measurement portion  400   b.    
     During the interrogation portion  400   c , which may, for example take the form of the second measurement portion of each detection cycle  400 , responses by transponders  26  are measured or sampled. The response measurements or samples are taken with the transmitter transmitting or at a time sufficiently close to excitement of the transponders  26  by the interrogation signal emitted by the transmitter such that the transponders  26  are still substantially resonating or responding to the interrogation signal. In particular, a number M of measurements or samples are taken during the interrogation or second measurement portion  400   c.    
     While the interrogation portion  400   c  is illustrated as one contiguous or continuous portion  400   c , in some embodiments the interrogation portion  400   c  may take the form of two or more separate portions or intervals. Each of the portions  400   c  may employ the same transmit frequency band, for example centered around 145 KHz. Other center channels or frequencies may for example be 136 KHz, 139 KHz, 142 KHz, 145 KHz, 148 KHz, 151 KHZ and/or 154 KHz, or any other frequency suitable for exciting the transponder to resonate. Some embodiments may employ frequency hopping, for example transmitting a different center channel or frequency for each of a plurality of interrogation portions  400   c  of each detection cycle  400 . Such is discussed further in U.S. provisional patent application Ser. No. 60/892,208, filed Feb. 28, 2007 and U.S. non-provisional application Ser. No. 11/743,104, filed May 1, 2007. 
     The gap portion  400   a  may provide time for the response of the transponders  26  to the interrogation signal to decay sufficiently to allow measurement of noise. 
     Some embodiments may arrange the gap  400   a , the noise detection portion  400   b  and/or the interrogation portion  400   c , or parts thereof, in a different order. 
     In one embodiment, the time to accumulate the noise sample or value indicative of a noise level may, for example, be approximately 37 milliseconds, and the time to accumulate the transponder signal measurement approximately 228 milliseconds. Along with a gap  400   a  of approximately 10 milliseconds between the signal and noise portions, the time for a single detection cycle  400  would be approximately 275 milliseconds. As noted above, the transmitter is OFF during the noise measurement portion  410   b  of each detection cycle to allow the receiver to measure ambient noise, and the signal measurement portion  410   c  is taken with the transmitter transmitting a wideband interrogation signal about the particular center channel or frequency. 
     The noise samples collected by the receiver may be accumulated and a highest one or more of multiple samples or measurements over one or more detection cycles selected or used to prevent unwarranted fluctuations. The noise level may be determined by a noise level determiner, which may be implemented as part of the controller or as a dedicated circuit, and which may be implemented as a hardwired circuit or as a programmed circuit which may include a processor or other programmable element (e.g., FPGA, ASIC). The response signals from the transponder  26  may be accumulated and/or averaged or integrated over one detection cycle or over multiple detection cycles. 
     The number N of noise measurements or samples and/or the number M of response measurements or samples may be selected to achieve a desired ratio of N to M, in order to achieve or maintain a desired signal to noise ratio. For example, obtaining 200 noise measurements or samples and 800 response measurements or samples each detection cycle results in an SNR of approximately 2 (e.g., the square root of the 800 divided by 200). While an SNR as low as 1.1:1 may be sufficient in some embodiments, an SNR approaching 2:1 ensures sufficient differentiation to eliminate or reduce the possibility of false positives to an acceptable level for the particular applications envisioned herein. Any known hardware and software accumulators, summer, integrators and/or other hardware or software may be suitable. 
       FIG. 9  illustrates pulse timing, according to one illustrated embodiment. 
     The custom logic in the FPGA  108  generates the timing and control signals for each pulse  410 . During a transmit portion  410   a  of the pulse  410 , the logic of the FPGA  108  drives the drive transistor control lines to generate the transmit signal. The FPGA logic controls the frequency of the transmit signal. During a dump portion  410   b  of the pulse  410 , the logic of the FPGA  108  drives the gate of the dump TRIAC T 1  to quickly drain the transmit energy from the antenna  22  in order to allow detection of the response signal form the transponder  26 , if any. A recovery portion  410   c  of the pulse  410  implemented by the controller allows receive filters and amplifiers to recover from the transmitted pulse before detecting the response signal from the transponder  26 , if any. During the receive response portion  410   d  of the pulse  410 , the FPGA  108  controls the signal ADC  128  to sample the response signal from the transponder  26 , if any. The signal ADC  128  may, for example, sample at a 1 MHz sample rate with a 12-bit resolution. A dither portion  410   e  of the pulse  410  implemented by the controller has a random variable length of time, and may, for example be generated by a pseudo-noise (PN) sequence generator. Adding a random length of time between pulses de-correlates the response signal received from the transponder  26  from constant frequency sources of interference, if any. 
     For example, within each of 228 millisecond signal measurement intervals  400   c  discussed above, the custom logic of the FPGA  108  ( FIG. 5 ) accumulates the received signals from, for example 800 pulses. 
       FIG. 10  shows signal timing for driving the drive transistors Q 1 , Q 2  ( FIG. 7A ), according to one illustrated embodiment. 
     The custom logic in the FPGA  108  ( FIG. 5 ) generates the signals  420   a ,  420   b  to drive the drive transistors Q 1 , Q 2  ( FIG. 7A ) during the transmit portion  410   a  ( FIG. 9 ) of the pulse  410 . A transmit (TX) period value is used by the logic of the FPGA  108  to set the transmit frequency. The low transistor (e.g., Low FET) Q 2  turns ON at the beginning of the transmit period. The Low FET off value controls when the low transistor (e.g., Low FET) Q 2  is turned OFF. The low transistor Q 2  is turned OFF before the high transistor (e.g., High FET) Q 1  is turned ON to avoid a short circuit through the transistors Q 1 , Q 2 . The High FET on value controls when the high transistor (e.g., High FET) Q 1  is turned ON. The High FET Off value controls when the high transistor Q 1  is turned OFF. The high transistor is turned OFF before the low transistor Q 2  is turned ON to avoid a short circuit through the transistors Q 1 , Q 2 . For example, to achieve a transmit frequency of 144.9 KHz, the transmit period should be set to 6.9 psec. Also for example, a suitable duration that both the low and high transistors Q 1 , Q 2  are OFF may be set to 400 nsec. 
     The ADC converts the signal received from the transponder  26 , if any, from analog to digital. Such conversion may, for example, be performed at a sampling rate of 1 MHz with a 12-bit data resolution. The sampled ADC data is then accumulated together or integrated, for example over 800 measurements or samples, to compute the total summed response signal received from the transponder  26 , if any. 
     The accumulated or integrated received signal is match filtered with both in-phase and quadrature reference signals to determine the signal magnitude. The received receive signal is matched filtered with a plurality of reference signals, for example with the seven reference signals, for instance as shown in Table 1 below. Some embodiments, may employ match filtering before accumulating or integrating the received signal. 
     
       
         
           
               
             
               
                 TABLE 1 
               
               
                   
               
               
                 Match Frequency 
               
               
                   
               
             
            
               
                 136 KHz 
               
               
                 139 KHz 
               
               
                 142 KHz 
               
               
                 145 KHz 
               
               
                 148 KHz 
               
               
                 151 KHz 
               
               
                 154 KHz 
               
               
                   
               
            
           
         
       
     
     The maximum value for the matched filters (e.g., seven matched filters) with active transmit is compared with an adjusted detection threshold by a comparator. If the maximum value is greater than the detection threshold, then a response signal from a transponder  26  is considered as having been detected by the presence or absence determiner, and appropriate action is taken, such as discussed below with reference to  FIG. 17 . Noise filtering is further discussed further below, with reference to  FIG. 16 . The comparator may be part of the controller and/or the presence or absence determiner and may be implemented as part of the controller or as a dedicated circuit, and which may be implemented as a hardwired circuit or as a programmed circuit which may include a processor or other programmable element (e.g., FPGA, ASIC). 
     Noise faults may be detected as well as wand transmit voltage faults. Noise faults may be detected when the matched filter output during the noise detection portion is greater than a noise fault threshold (e.g., a threshold magnitude 2.7 mV over a threshold time, e.g., 7 seconds or threshold magnitude 7 mV over a time threshold of 7 seconds). Wand transmit voltage faults may be detected when the wand transmit voltage drops below a wand voltage fault threshold (e.g., 270 V Peak-to-Peak ). Two environmental faults in a row such as the above, may trigger an Environmental Error Mode, while two normal measurements in a row may return to a normal Scan Mode. Faults in general are discussed in more detail below. Alternatively, the interrogation and detection system may employ a fast Fourier transform approach ( FIG. 34 ) in lieu of match filtering. 
       FIG. 11  shows a model circuit  450  that models how a transponder responds to the transmitted interrogation signals, according to one illustrated embodiment. 
     The model includes a resistance R in series with an inductance L on one voltage rail, and a capacitance C coupled across the voltage rails. An input voltage V I  and an output voltage V O  are produced across the voltage rails. 
     The Laplace transform for the transponder response model are represented by equations 1 and 2. 
     
       
         
           
             
               
                 
                   
                     
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                         V 
                         I 
                       
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                         s 
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                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   1 
                 
               
             
             
               
                 
                   
                     H 
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                       ( 
                       s 
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                   = 
                   
                     
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                         1 
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                             R 
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                   Equation 
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     Using the Laplace equations, the transponder response output can be simulated for various input signals. The simulated and measured transponder responses are shown in  FIGS. 12-15 . 
       FIG. 12  shows a graph of a simulated transponder response signal  460  based on the model circuit  450  ( FIG. 11 ), according to one illustrated embodiment. 
     The simulated transponder response signal  460  is produced by the transponder in response to excitation by an interrogation signal. The simulated transponder response signal  460  represents a response by the transponder at the same frequency as the frequency of the interrogation signal (e.g., 146 KHz). 
       FIG. 13  shows a graph of a measured transponder response signal  470 , according to one illustrated embodiment. 
     The measured transponder response signal  470  is produced by the transponder in response to excitation by an interrogation signal. The measured transponder response signal  470  represents a response by the transponder at the same frequency as the frequency of the interrogation signal (e.g., 146 KHz). 
       FIG. 14  shows a graph of a simulated transponder response signal  480  based on the model circuit  450  ( FIG. 11 ), according to one illustrated embodiment. 
     The simulated transponder response signal  480  is produced by the transponder in response to excitation by an interrogation signal. The simulated transponder response signal  480  represents a response by the transponder at a frequency (e.g., 146 KHz) to an interrogation signal that has a higher frequency (e.g., 156 KHz), for instance 10 KHz higher than the response signal. 
       FIG. 15  shows a graph of a measured transponder response signal  490 , according to one illustrated embodiment. 
     The measured transponder response signal  490  is produced by the transponder in response to excitation by an interrogation signal. The measured transponder response signal  490  represents a response by the transponder at a frequency (e.g., 146 KHz) to an interrogation signal that has a higher frequency (e.g., 156 KHz), for instance 10 KHz higher than the response signal. 
     The similarity between the simulated transponder response signals  460 ,  480  and the measured transponder response signals  470 ,  490  demonstrates that there is a good match using the circuit model  450 . 
       FIG. 16  is a graph showing an example of measured noise and the output of the noise filter corresponding to the measurements, according to one illustrative embodiment. 
     The noise filtering processes the measured or sampled noise values for each detection cycle to determine a stable noise floor value. The output of the noise filter may, for example, be the maximum of either the current noise measurement or a decayed value of the previous nose floor. 
     The output of the noise filter is an estimate of the current noise floor level after selecting the highest of a plurality (e.g., 6) of noise measurements or samples. The filtered noise floor may advantageously include samples collected, captured or measured both before and after a given signal sample is collected, captured or measured. Thus, for any sample of a given detection cycle the noise floor may include noise samples from the given detection cycle, as well as a next successive detection cycle. The filtered noise floor may additionally, or alternatively, include noise samples from one or more successively preceding detection cycles, as well as one or more successfully succeeding detection cycles. 
     The number of response measurements or samples averaged to create the full signal may, for example, be 800. Using the noise filter output as an adaptive detection threshold of the full signal provides an adaptive threshold that is estimated to be the square root of the quotient of 800 divided by 200, or approximately 2.0 times the estimated noise floor level for the full signal. This provides approximately twice the estimated noise floor. Performance in very low noise conditions may be further stabilized by using an adaptive threshold, implemented by a detection threshold adjuster, that is twice the filtered noise level (e.g., 0.5 mV). The adaptive threshold adjuster may be implemented as part of the controller or as a dedicated circuit, and may be implemented as a hardwired circuit or as a programmed circuit which may include a processor or other programmable element (e.g., FPGA, ASIC). 
       FIG. 17  shows a method  500  of operating the interrogation and detection system  14  according to one illustrated embodiment. 
     In response to detecting a disconnect of power, the interrogation and detection system  14  enters a Power OFF mode at  502 . For example, the Power OFF mode  502  may be entered when the controller  20  ( FIGS. 1 and 4 ) is unplugged or when the power switch on the controller  20  is turned OFF. In the Power OFF mode  502 , the Power LED  134   a  and other front panel LEDs  134  will be turned OFF (non-emitting). The software  200  is inoperative in the Power OFF mode  502 . 
     In response to detecting an application of power, the interrogation and detection system  14  enters a Power-Up mode  504 . The Power UP mode  502  may, for example, in response to the application of power to the controller  20  and turning ON the switch on the back of the controller. In the Power-Up mode  504 , a Power LED  134   a  may be turned ON or illuminated, and may remain ON or illuminated as long as the power is applied and the switch is in the ON state. In response to entering the Power UP mode  502 , the software  200  will perform software initialization, built in tests, and an audio/visual test. 
     If a fault is detected, the software  200  progresses to a System Fault Mode  506 . If no faults are detected, the software  200  may turn a System Ready LED green, and enter a Wand Detection Mode  508 . 
     In the System Fault mode  506 , the software  200  may cause an indication of the detection of a system fault by blinking a System Ready LED  134   b  yellow, and/or issuing a sequence of rapid beeps or other sounds. The corrective action for the System Fault Mode  506  is to cycle power to reinitiate the Power Up mode  504 . Continued failure indicates a failed controller  20 . 
     In the Wand Detection Mode  508 , the software  200  checks for a wand  22   a  connected to the controller  20 . The Wand Detection Mode  508  may be indicated by turning the System Ready LED  134   b  green and turning the Wand Ready LED  134   c  OFF. If no wand  22   a  is detected, the software  200  remains in the Wand Detection Mode. If a wand  22   a  is detected, the software  200  progresses to the Wand Initialization Mode  510 . 
     At the start of the Wand Initialization Mode  510 , after the detection of a wand  22   a , the software  200  may turn the Wand Ready LED  134   c  yellow and check for the presence of a fuse in the wand  22   a . If a fuse is found, the software  200  may attempt to blow the fuse and verify that the fuse was correctly blown. After the fuse is blown the software  200  may verify that wand  22   a  is operating within tolerances. The software  200  may indicate that the wand  22   a  is ready by turning the Wand Ready LED  134   c  green. The software  200  may also start a timer which will allow the wand  22   a  to be disconnected and reconnected to the controller for a period to time (e.g., 5 hours) after the fuse is blown. 
     The controller  20  may determine the adjustments or fine tuning to be made about the center frequencies or channels during Wand Initialization Mode  510 . In particular, the controller  20  may determine the particular frequency in each of the frequency bands that elicits the response with the highest voltage. The controller may determine such be varying the capacitance of the LC circuit using the switched capacitors C 33 -C 36  during the Wand Initialization Mode  510 . The particular combination of switched capacitors C 33 -C 36  which achieved the response with the highest voltage may then be automatically employed during the Scan Mode  514  (discussed below) to adjust or fine tune about the center frequency or channel in each broad band of transmission. Other approaches to determining the fine tuning may be employed. 
     If the software  200  does not successfully complete the Wand Initialization Mode  510 , the software  200  enters an Invalid Wand Mode  512 . If the software  200  successfully completes the Wand Initialization Mode  510 , the software  200  progresses to the Scan Mode  514  to automatically start scanning. 
     In the Invalid Wand Mode  512 , the software  200  may blink the Wand Ready LED  134   c  yellow and issues a slow beep pattern. 
     The Invalid Wand Mode may be entered in response to any of the following conditions:
         The wand  22   a  connected to the controller  20  is out of tolerance.   The controller  20  is unable to blow the fuse in the wand  22   a.      The wand  22   a  does not have a fuse and more than the set time period has past (e.g., 5 hours) since a fuse was blown.   The wand  22   a  does not have a fuse and the controller  20  has been restarted.   The wand  22   a  has been connected to the controller for more than the set time period (e.g., 5 hours).   The wand  22   a  is detuned due to close proximity to metal.       

     The corrective action for the Invalid Wand Mode  512  is to remove the invalid wand  22   a  and attach a new wand  22   a  to the controller  20  that contains a fuse or to reconnect the wand  22   a  while holding it in the air at least 2 feet away from large metallic objects. 
     The software  200  enters the Scan Mode  514  when the wand  22   a  is ready and the operator presses a Start/Stop button. The software  200  may issue a short three beep pattern via the speaker or beeper  130  when entering the Scan Mode  514  to identify the entry to the user. 
     In the Scan Mode  514 , the software  200  may continuously or periodically perform the following functions.
         Look for response signals from transponders  26     Monitor the noise level   Insure the wand  22   a  is connected and operating correctly   Blink the LED&#39;s in a circular pattern       

     When the operator or user pushes the Start/Stop button or the a scan maximum time interval (e.g., 4 minute) has been reached, the software  200  may issue a short three beep pattern and return to the Wand Ready Mode  516 . 
     When an appropriate response signal from a transponder  26  is detected while in Scan Mode  514 , the software  200  may turn ON an amber DETECT LEDs  134   d  and/or provide an audible alarm. The alarm may, for example, beep a continuous solid tone as long as the transponder is detected, with a minimum of beep duration of, for instance 0.5 second. 
     If the software  200  detects the wand  22   a  is disconnected while in the Scan Mode  514 , the software  200  enters the Scan Fault Mode  520 . In the Scan Fault Mode  520 , the software  200  may issue a sequence of rapid beeps and blink ON and OFF the amber DETECT LEDs  134   d . The Scan Fault Mode  520  can be cleared by pushing the Start/Stop button. The software  200  will automatically clear the scan fault mode  520  after 10 beeps. 
     While in the Scan Mode  514 , if excess noise or loss of transmit signal is detected, the software  200  will progress to the Environment Error Mode  522 . In the Environment Error Mode  522 , the software  200  may issue or produce an appropriate indication. For example, the software  200  may cause the production of a sequence of slow beeps and the blinking ON and OFF the green circle LEDs  134   e . The corrective action for the Environment Error Mode  522  is to reposition the wand  22   a  away from large metal objects or sources of electrical interference. The software  200  will automatically stop the scan if the environment error condition lasts for more than a set time or number of beeps (e.g., 5 beeps). 
       FIG. 18  shows a detection range versus noise performance  1802  achievable by the embodiment described herein, as well as the detection range versus noise performance  1804  achievable a “slow” version and the detection range versus noise performance  1806  achievable by a “fast” version of embodiments disclosed in U.S. non-provisional patent application Ser. No. 11/743,104, filed May 1, 2007. As can be appreciated from  FIG. 18 , the embodiment of the present disclosure is capable of achieving far superior performance, having greater detection range in even mildly noisy environments. Such is particularly advantageous in environments such as operating theaters, and substantially helps reduce false readings (e.g., false positives, false negatives). Thus, such may provide the level of performance demanded by hospitals and doctors. 
       FIG. 19  shows a method  1900  of operating an interrogation and detection system, according to one illustrated embodiment. The method  1900  may be implemented by any of the interrogation and detection system embodiments discussed above. 
     During each of a plurality of detection cycles, the interrogation and detection system performs a number of acts  1902 - 1914 . At  1902 , the interrogation and detection system receives electromagnetic signals, for example unmodulated electromagnetic signals, during a noise detection portion of the detection cycle. The below descriptions will be presented in terms of unmodulated electromagnetic signals due to the unique technical advantages realized by a system that employs simple resonant transponders without any on-board memory or storage, and from which information cannot be read from or written to. However, some embodiments may employ readable and/or writable transponders, for instance radio frequency identification (RFID) transponders or tags, which respond with a modulated electromagnetic signal that encodes information in the modulation. The various techniques described herein are applicable to such transponders and modulated electromagnetic signals. 
     At  1904 , the interrogation and detection system determines a noise value indicative of a noise level that corresponds to a highest one of a number N of samples or measurements of the unmodulated electromagnetic signals received during the noise detection portion of the detection cycle, where the number N is greater than one. At  1906 , the interrogation and detection system adjusts a signal detection threshold based at least in part on the determined noise value of at least one of the detection cycles. 
     At  1908 , a transmitter of the interrogation and detection system emits at least one electromagnetic interrogation signal during a transmit portion of the detection cycle. At  1910 , a receiver of the interrogation and detection system receives unmodulated electromagnetic signals during a receive response portion of the detection cycle that follows the transmit portion of the detection cycle. 
     At  1912 , a presence or absence determiner of the interrogation and detection system determines the presence or absence of a transponder based at least in part on a number M of samples or measurements of the unmodulated electromagnetic signals received during the detection cycle and the adjusted signal detection threshold, where the number M is greater than one. A ratio of N:M may be at least equal to 4. N may be equal to about 200 and M may be equal to about 800. The presence or absence determiner may be implemented as part of the controller or as a dedicated circuit, and may be implemented as a hardwired circuit or as a programmed circuit which may include a processor or other programmable element (e.g., FPGA, ASIC). 
       FIG. 20  shows a method  2000  of operating an interrogation and detection system, according to one illustrated embodiment. The method  2000  may be employed in determining a noise value  1904  of method  1900  ( FIG. 19 ). 
     At  2002 , a noise level determiner of the interrogation and detection system determines a noise value indicative of a noise level based at least in part on the unmodulated electromagnetic signals received during the noise detection portion of the detection cycle by setting the noise value based on the highest one of six samples or measurements of the unmodulated electromagnetic signal received during the noise detection portion of the detection cycle. The noise level determiner may be implemented as part of the controller or as a dedicated circuit, and may be implemented as a hardwired circuit or as a programmed circuit which may include a processor or other programmable element (e.g., FPGA, ASIC). 
       FIG. 21  shows a method  2100  of operating an interrogation and detection system, according to one illustrated embodiment. The method  2100  may be employed in adjusting the signal detection threshold  1906  of method  1900  ( FIG. 19 ). 
     At  2102 , a detection threshold adjuster of the interrogation and detection system adjusts the signal detection threshold by adjusting the signal detection threshold based at least in part on a first number of determined noise values indicative of a noise level during at least one noise detection portion that occurred before the receive response portion of a first one of the detection cycles and a second number of determined noise values indicative of a noise level during at least one noise detection portion that occurred after the receive response portion of the first one of the detection cycles. The detection threshold adjuster may be implemented as part of the controller or as a dedicated circuit, and may be implemented as a hardwired circuit or as a programmed circuit which may include a processor or other programmable element (e.g., FPGA, ASIC). 
       FIG. 22  shows a method  2200  of operating an interrogation and detection system, according to one illustrated embodiment. The method  2200  may be employed in adjusting the signal detection threshold  1906  of method  1900  ( FIG. 19 ). 
     At  2202 , a detection threshold adjuster of the interrogation and detection system adjusts the signal detection threshold based at least in part on the determined noise value of at least one of the detection cycles by adjusting the signal detection threshold to be approximately twice an average of at least one of the first and the second number of determined noise values. 
       FIG. 23  shows a method  2300  of operating an interrogation and detection system, according to one illustrated embodiment. The method  2300  may be employed in adjusting the signal detection threshold  1906  of method  1900  ( FIG. 19 ). 
     At  2302 , a detection threshold adjuster of he interrogation and detection system adjusts the signal detection threshold based at least in part on the determined noise value of at least one of the detection cycles by adjusting the signal detection threshold to be approximately twice a greatest one of at least one of the first and the second number of determined noise values. 
       FIG. 24  shows a method  2400  of operating an interrogation and detection system, according to one illustrated embodiment. The method  2400  may be employed in determining the presence or absence of a transponder  1912  of method  1900  ( FIG. 19 ). 
     At  2402 , a presence or absence determiner of the interrogation and detection system determines the presence or absence of a transponder by comparing a maximum value of a plurality of matched filter outputs with the adjusted signal threshold. 
       FIG. 25  shows a method  2500  of operating an interrogation and detection system, according to one illustrated embodiment. The method  2500  may be employed in adjusting the signal detection threshold  1906  of method  1900  ( FIG. 19 ). 
     At  2502 , a detection threshold adjuster of the interrogation and detection system adjusts the signal detection threshold based at least in part on the determined noise value of at least one of the detection cycles by adjusting the signal detection threshold to be approximately twice the determined noise value. 
       FIG. 26  shows a method  2600  of operating an interrogation and detection system, according to one illustrated embodiment. The method  2600  may be employed in adjusting the signal detection threshold  1906  of method  1900  ( FIG. 19 ). 
     At  2602 , a detection threshold adjuster of the interrogation and detection system adjusts the signal detection threshold based at least in part on the determined noise value of at least one of the detection cycles includes adjusting the signal detection threshold to be the large of approximately twice the determined noise value or a defined threshold value. The defined threshold value may for example be approximately 0.5 mV. 
       FIG. 27  shows a method  2700  of operating an interrogation and detection system, according to one illustrated embodiment. The method  2700  may be employed to prevent extraneous objects (e.g., metal table, EKG leads, etc.) from producing a positive result. 
     At  2702 , a comparator of the interrogation and detection system determines if an output of at least one matched filter during the noise detection portion of the detection cycle exceeds a noise fault threshold indicative of a noise fault. 
       FIG. 28  shows a method  2800  of operating an interrogation and detection system, according to one illustrated embodiment. The method  2800  may be employed in performing  2702  of method  2700  ( FIG. 27 ). 
     At  2802 , a comparator or controller of the interrogation and detection system determines if the output of the at least one matched filter during the noise detection portion of the detection cycle exceeds the noise fault threshold for a defined period of time. At  2804 , the interrogation and detection system terminates the detection cycle in response to the output of the at least one matched filter exceeding the noise fault threshold for the defined period of time. The comparator may be implemented as part of the controller or as a dedicated circuit, and may be implemented as a hardwired circuit or as a programmed circuit which may include a processor or other programmable element (e.g., FPGA, ASIC). 
       FIG. 29  shows a method  2900  of operating an interrogation and detection system, according to one illustrated embodiment. The method  2900  may be employed in performing  1908  of method  1900  ( FIG. 19 ). 
     At  2902 , a transmitter of the interrogation and detection system emits at least one electromagnetic interrogation signal during a transmit portion of the detection cycle by emitting at least one electromagnetic interrogation signal in each of a 136 KHz band, a 139 KHz band, a 142 KHz band, a 145 KHz band, a 148 KHz band, a 151 KHz band and a 154 KHz band. 
       FIG. 30  shows a method  3000  of operating an interrogation and detection system, according to one illustrated embodiment. The method  3000  may be useful in preventing false positives (i.e., tag detections) from being triggered by the transmission of the interrogation or excitement signals. 
     At  3002 , the controller of the interrogation and detection system ignores any unmodulated electromagnetic signals received during a recovery portion of the detection cycle that precedes the receive response portion of the detection cycle. 
       FIG. 31  shows a method  3100  of operating an interrogation and detection system, according to one illustrated embodiment. The method  3100  may be useful in preventing false positives (i.e., tag detections) from being triggered by energy left in the antenna after the transmission of the interrogation or excitement signals. 
     At  3102 , the controller of the interrogation and detection system dumps energy from an antenna circuit during a dump portion of the detection cycle that precedes the recovery portion of the detection cycle. 
       FIG. 32  shows a method  3200  of operating an interrogation and detection system, according to one illustrated embodiment. The method  32  may be useful in de-correlating the response signal received from the transponder from constant frequency sources of interference, if any. 
     At  3202 , a ditherer or controller of the interrogation and detection system varies a time between a start of a first one of the successive pairs of the detection cycles and a start of a next successive one of the pairs of the detection cycles. The ditherer may be implemented as part of the controller or as a dedicated circuit, and may be implemented as a hardwired circuit or as a programmed circuit which may include a processor or other programmable element (e.g., FPGA, ASIC). 
       FIG. 33  shows a method  3300  of operating an interrogation and detection system, according to one illustrated embodiment. The method  3300  may be employed in performing  1908  of the method  1900  ( FIG. 19 ). 
     The transmitter interrogation and detection system emits at least one electromagnetic interrogation signal during a transmit portion of the detection cycle by at  3302  emitting a first wide band electromagnetic interrogation signal during a first transmit portion of the detection cycle and at  3304  emitting at least a second wide band electromagnetic interrogation signal during a second transmit portion of the detection cycle. 
       FIG. 34  shows a method  3400  of operating an interrogation and detection system, according to one illustrated embodiment. The method  3400  may be employed in performing  1912  of the method  1900  ( FIG. 19 ), for example in lieu of the match filtering  2400 ,  2700 ,  2800  ( FIGS. 24 ,  27  and  28 , respectively). 
     At  3402 , a receiver or controller of the interrogation and detection system converts the received signal(s) from the time domain to the frequency domain spectrum. The interrogation and detection system may, for example, perform a Fourier transform, for instance a fast Fourier transform such as a 256 point fast Fourier transform. Suitable algorithms and/or sets of software code for performing such are available or can be written. 
     At  3404 , the receiver or controller of the interrogation and detection system searches the frequency domain spectrum to determine the object with the strongest resonance in a defined frequency band. For example, the interrogation and detection system may search the frequency domain spectrum from about 120 KHz to about 175 KHz. An amplitude of the resonant object may be computed as the sum of the resonant power plus and minus 2 fast Fourier transform bins from the peak resonance frequency. This approach may provide a more accurate measurement of power than simply using the peak value. The frequency of the resonant object may be computed using an interpolation approach. This approach may provide a more accurate determination of resonant frequency than simply using the fast Fourier bin number. 
     At  3406 , a presence or absence determiner of the interrogation and detection system determines the presence or absence of a transponder based at least in part on a frequency of the unmodulated electromagnetic signals received during the detection cycle being within a defined frequency range. The defined frequency range may extend from about 137 KHz to about 160 KHz. The presence or absence determiner may be implemented as part of the controller or as a dedicated circuit, and may be implemented as a hardwired circuit or as a programmed circuit which may include a processor or other programmable element (e.g., FPGA, ASIC). 
       FIG. 35  shows a method  3500  of operating an interrogation and detection system, according to one illustrated embodiment. The method  3500  may be employed in performing  1912  of the method  1900  ( FIG. 19 ). 
     At  3502 , a presence or absence determiner of the interrogation and detection system determines a Q value (i.e., Quality factor) of the resonant object from a signal decay slope for the received unmodulated electromagnetic signal(s) returned by the resonant object. The interrogation and detection system may, for example, use multiple windows, for instance five (5) window positions may provide suitable results. 
     At  3504 , the presence or absence determiner of the interrogation and detection system determines the presence or absence of a transponder based at least in part on a Q value of the unmodulated electromagnetic signal(s) received during the detection cycle. The interrogation and detection system may preferably employ the Q value determination in conjunction with determination based on the frequency  3402  ( FIG. 34 ) and on the determination based on the adjusted signal detection threshold  1914  ( FIG. 19 ). 
       FIG. 36  shows a method  3600  of operating an interrogation and detection system, according to one illustrated embodiment. The method  3600  may be employed in performing  1912  of the method  1900  ( FIG. 19 ). 
     At  3602 , a presence or absence determiner the interrogation and detection system determines the presence or absence of a transponder is based at least in part on a Q value of the unmodulated electromagnetic signals received during the detection cycle being at least equal to a threshold Q value. The threshold Q value may be 35. The interrogation and detection system may preferably employ the Q value determination in conjunction with determination based on the frequency  3402  ( FIG. 34 ) and on the determination based on the adjusted signal detection threshold  1914  ( FIG. 19 ). The presence or absence determiner may be implemented as part of the controller or as a dedicated circuit, and may be implemented as a hardwired circuit or as a programmed circuit which may include a processor or other programmable element (e.g., FPGA, ASIC). 
     Consequently, tag detection may advantageously be based on the received unmodulated electromagnetic signal(s) satisfying all three conditions: 1) measured amplitude is above a threshold, which may be an adjustable threshold, 2) measured frequency is between a lower limit and an upper limit, and 3) measured Q value is above a minimum Q threshold. Interference, for example from RFID tags or EKG cables, are rejected when any of the following three conditions are satisfied: a) measured frequency is below the lower frequency limit, b) measured frequency is above the upper frequency limit, or c) measured Q value is below the threshold Q value. Such may provided significantly superior results over previous approaches, preventing false positives which could otherwise cause a patient to remain open for longer period of time during surgery and tie up hospital personnel and resources. 
     The above description of illustrated embodiments, particularly the pulsed wide band frequency hopping with dynamic adjustment of the transmission frequency in the various frequency bands and the use of switched capacitors to achieve such, advantageously permit the use of inexpensive transponders which are not accurately tuned to a chosen or selected resonant frequency. This is in marked contrast to the approach typically taken with other types of resonant transponders (i.e., transponders without memory). Such approaches typically interrogate or excite the resonant transponder using narrow frequency bands centered closely on specific frequencies, to achieve a selected resonant response from a highly accurate transponder in order to differentiate signal from noise. This is also in marked contrast to the approach typically taken with radio frequency identification (RFID) tags whether active or passive, which also typically employ are narrow band to achieve a selected response from a highly accurate RFID tag. 
     The above description of illustrated embodiments, including what is described in the Abstract, is not intended to be exhaustive or to limit the embodiments to the precise forms disclosed. Although specific embodiments of and examples are described herein for illustrative purposes, various equivalent modifications can be made without departing from the spirit and scope of the disclosure, as will be recognized by those skilled in the relevant art. The teachings provided herein of the various embodiments can be applied to other transponders and interrogation and detection systems, not necessarily the exemplary surgical object transponders and interrogation and detection systems generally described above. 
     For instance, the foregoing detailed description has set forth various embodiments of the devices and/or processes via the use of block diagrams, schematics, and examples. Insofar as such block diagrams, schematics, and examples contain one or more functions and/or operations, it will be understood by those skilled in the art that each function and/or operation within such block diagrams, flowcharts, or examples can be implemented, individually and/or collectively, by a wide range of hardware, software, firmware, or virtually any combination thereof. In one embodiment, the present subject matter may be implemented via Application Specific Integrated Circuits (ASICs). However, those skilled in the art will recognize that the embodiments disclosed herein, in whole or in part, can be equivalently implemented in standard integrated circuits, as one or more computer programs running on one or more computers (e.g., as one or more programs running on one or more computer systems), as one or more programs running on one or more controllers (e.g., microcontrollers) as one or more programs running on one or more processors (e.g., microprocessors), as firmware, or as virtually any combination thereof, and that designing the circuitry and/or writing the code for the software and or firmware would be well within the skill of one of ordinary skill in the art in light of this disclosure. 
     In addition, those skilled in the art will appreciate that the mechanisms of taught herein are capable of being distributed as a program product in a variety of forms, and that an illustrative embodiment applies equally regardless of the particular type of signal bearing media used to actually carry out the distribution. Examples of signal bearing media include, but are not limited to, the following: recordable type media such as floppy disks, hard disk drives, CD ROMs, digital tape, and computer memory; and transmission type media such as digital and analog communication links using TDM or IP based communication links (e.g., packet links). 
     The various embodiments described above can be combined to provide further embodiments. U.S. Provisional patent application Ser. No. 61/056,787, filed May 28, 2008; U.S. Provisional patent application Ser. No. 61/091,667, filed Aug. 25, 2008; U.S. Provisional patent application No. 60/811,376 filed Jun. 6, 2006; U.S. Pat. No. 6,026,818, issued Feb. 22, 2000; U.S. patent Publication No. US 2004/0250819, published Dec. 16, 2004; U.S. provisional patent application Ser. No. 60/811,376, filed Jun. 6, 2006 and U.S. non-provisional patent application Ser. No. 11/743,104, filed May 1, 2007, are incorporated herein by reference, in their entirety. Aspects of the embodiments can be modified, if necessary, to employ systems, circuits and concepts of the various patents, applications and publications to provide yet further embodiments. 
     These and other changes can be made to the embodiments in light of the above-detailed description. In general, in the following claims, the terms used should not be construed to limit the claims to the specific embodiments disclosed in the specification and the claims, but should be construed to include all possible embodiments along with the full scope of equivalents to which such claims are entitled. Accordingly, the claims are not limited by the disclosure.