Electromagnetically detectable marked surgical object

Abstract: An apparatus and method for detecting surgical objects within the human body where the apparatus includes the surgical object utilized in surgical procedures including a marker made of a selected nonmagnetostrictive, soft magnetic material which will emit known specific selected harmonic frequencies when exposed to an alternating electromagnetic field. That emission will cause a change in the alternating electromagnetic field which can be correlated to the presence of only the selected nonmagnetostrictive, soft magnetic material.

Description: Background of the Invention 1. Field of the Invention The invention relates to a surgical object marked with a detection device for the surgical object within the human body. More specifically, the invention relates to a detection device which responds to an alternating electromagnetic field. 2. Description of the Prior Art Despite precautions, surgeons still occasionally leave surgical objects such as sponges and, less frequently, small surgical tools in their patients after an operation. Areas which are badly injured tend to have a great amount of blood which may cover the surgical objects, making the objects hard to locate. Also, objects may find their way under an organ. This is most likely to occur in surgical areas such as the abdomen which are large and have many organs. The prior art discloses use of X-ray opaque material positioned on the surgical devices in order that after the surgery is completed and the wound is closed, an X-ray can be taken to insure no surgical objects were left within the patient. Although this detection method is effective, it is cumbersome. Most operating rooms do not have X-ray machines. Hence, the patient must be taken to another room. There the patient often must be "moved from his gurney to an X-ray table for X-rays to be taken. If a surgical object is detected after an X-ray has been taken, the patient must be returned to the operating room. Then, the cavity or incision must be reopened to remove the surgical object and then reclosed. This second surgery can cause a great deal of trauma to the patient, preventing optimum healing. Examples of surgical sponges which are marked by radio opaque material are disclosed in United States Patent No. 2,190,432 to Lewison, United States Patent No. 2,698,270 to Mesek, United States Patent No. 4,185,626 to Jones et al., and United States Patent No. 4,205,680 to Marshall. A manual counting of the sponges after the surgery is completed is also used to prevent surgical objects from being left in body cavities. This is not a foolproof method. Fatigue, poor handwriting, and misreading of numbers will occur during operations lasting 4 to 12 hours when dealing with badly damaged patients. Consequently, miscounts occur as a result of human error. Greenberg in United States Patent No. 3,587,583 attempts to overcome the problems of leaving surgical objects within the body. He proposes to mark the surgical object with a permanently magnetized material. A surgeon performs an operation in the normal manner. Before closing the incision the surgeon probes for the presence of a surgical object with a magnetic field detector means which generates an electric signal which is modified in the presence of a magnetic field. If the marked object is present, the magnetic field of the magnetic marker is sensed by the magnetic field detector means, which modifies the electric signal. Yet, an operating room has many types of equipment which generate permanent magnetic fields. The presence of those fields can activate the magnetic field detector means, giving false detection. Because of its unreliability in an operating room, Greenberg's device is not a practical solution to the problem. In United States Patent No. 5,057,095, Fabian proposes to mark surgical instruments with a marker adapted to produce identifying signal characteristics when exposed to an alternating magnetic field. He discloses three types of resonant markers that are able to resonate at a certain preselected frequency. The first marker is a magnetomechanical device comprised of a permanent magnet overlaying a magnetostrictive metal strip in a plastic housing. The magnetostrictive strip vibrates when the marker is exposed to an alternating electomagnetic field, and its resonance is detected when the frequency of the applied field reaches a predetermined value. However, such devices are very sensitive to pressure and stress, which will inhibit them. Since a body cavity is under some pressure and the marker may be stressed during surgery, this type of marker is not reliable for use as a marker for surgical objects. The second proposed type is an electromechanical circuit comprised of an air coil, with or without a ferrite core, and a resonant structure such as a piezoelectric crystal. As the first type, this type of marker can be adversely affected by pressure and stress because its principle of detection relies on a mechanical resonance; therefore, a piezoelectric crystal type marker is also unsatisfactory. The third type of marker proposed by Fabian is an electromagnetic LCR circuit. This type of marker can be either built out of discrete components or made of a flexible printed circuit. In the former case, this unit is expensive to build and bulky, and it is impractial for surgical sponges. In the latter case, due to its high electrical resonance frequency this type of marker can be adversely affected by the presence of metal objects and conductive media. Because the human body is conductive, it is also impractical for surgical sponges. Consequently, none of the markers proposed by Fabian, nor the Greenberg marker, has been available on the market. US-A-4298862 discloses the use of a ductile strip of amorphous ferromagnetic material as a magnetic theft detection system. The strip is capable of producing magnetic fields at frequencies which are harmonics of the frequency of an incident field. The strip would however be unsuitable for use in an operating theatre, since in that environment there are other materials and equipment present which could produce harmonics detectable by the detection circuitry. In United States Patent No. 5,045,071, McCormick teaches about the use of magnetic materials for accurately locating the position of a catheter which has been inserted into a blood vessel. At column 9, lines 12-16, the patent cross references U.S. Patents 4,416,289, 4,431,005 and 4,445,501 for an explanation of the general method of detection. At column 5, lines 41-52, the '005 patent explains that a distortion of the magnetic field indicates the presence of the catheter. Thus, the McCormick patent teaches that merely a change in the magnetic field is a sufficient indicator of the position of the marked object. However, McCormick's measurements can be affected by the presence of other nearby magnetic and conductive materials. Hence, McCormick's technique can and likely will provide "false positives" as to the presence or the position of the marked object. Thus, there is a need for a marking method and apparatus for detecting surgical objects within the human body utilizing a material that can be readily identified before the patient leaves the operating suite. 3. Techniques for Detecting Electromagnetic Material There are different ways of providing and detecting what we can call generically an "electromagnetic marker." The cited prior art references all use materials which respond to an electromagnetic field. In order for a material to respond to an electromagnetic field and therefore to create "detectable changes" of the electromagnetic field, a material has to have at least one of the physical properties of electrical conductivity, moderate to high magnetic permeability, and magnetostriction (in general associated with moderate magnetic permeability). Moderate magnetic permeability is defined as a permeability comprised of between 5,000 and 20,000 and high magnetic permeability as a permeability above 20,000. In each case, the response to the electromagnetic field and, therefore, the creation of "detectable changes" of the electromagnetic field are heavily dependent upon the geometry and size of the marker. In addition, the response to the electromagnetic field depends upon the intensity and frequency of the electromagnetic field. In general, a magnetic material subject to an electromagnetic field of known and fixed frequency f o responds to the applied electromagnetic field by creating "changes" of the intensity of the applied field and by creating harmonics of the frequency f o . If the material is electrically conductive, it responds by creating not only "changes" of the intensity of the applied field but also "changes" of the phase of the field. In addition, if the material is magnetostrictive, the electromagnetic field creates strains or stress in the material, and the material responds to it by creating a frequency-dependent "change" of the intensity and of the phase of the applied field. Therefore, there are three methods of detection. First, one can simply look for a change of intensity and/or phase in an applied magnetic field, a method which can only be used for detection of the position of an object at a distance comparable to the size of the object. McCormick uses this method. Second, one could look for the frequency of the applied field to reach a predetermined value that is the electromechnical resonance frequency of the marker. Fabian discloses a magnetomechanical device which uses this technique. Finally, one could look for particular harmonics generated by a material in the presence of an applied magnetic field. This method has never been used in a medical environment. Indeed, the teaching of Heltemes in United States Patent No. 4,857,891 indicates that the art has generally failed to recognize that "open-strip" markers made of selected nonmagnetostrictive materials which generate specific harmonic frequencies upon application of a unidirectional electromagnetic field can be used to identify the presence of particular articles. Heltemes discloses a Magnetic Marker for Electronic Article Surveillance Systems having multiple filaments randomly dispersed in a sheet-like substrate so as to be substantially parallel to the plane thereof. "The filaments are selected of low coercive force, high permeability material, and the random orientation results in certain filaments intersecting with them being magnetically coupled to other filaments to thereby collect and concentrate lines of flux associated with an applied field of an EAS system into filaments parallel to the field." To take advantage of a high magnetic permeability material a marker has to be elongated, and preferably in the form of a fiber or long strip with an aspect ratio (length/square root of cross-sectional area) of at least 200. Helteme complies only partially to this requirement, but randomly distributes the fibers. In this respect Heltemes defeats this purpose because the applied electromagnetic field has to be parallel to the magnetic fibers to generate a high enough level of high harmonics to be recognized as marker specific. Moreover, Heltemes' marker is not very well suited for generating high harmonics. Consequently, Heltemes, like others in the prior art, failed to recognize that markers could be created for detection of surgical objects which generate specific, detectable, selected harmonic frequencies. The Invention The invention provides a surgical object in accordance with claim 1. I therefore provide a marker for a surgical object which responds to the presence of an alternating electromagnetic field. That response is detectable as pulses of radio frequencies. The elongated member or members are preferably encapsulated with biocompatible material. Preferably, the magnetic material is an amorphous metal. However, crystalline materials can be used if the encapsulation material has a high flexibility and plasticity. I further provide a method for detecting a surgical object within an area, which may be a part of the human body, which includes marking the surgical object with my marker, which will emit energy in the form of known frequencies when it is exposed to an alternating electromagnetic field. Detection is made using a pulse detection technique similar to that used in magnetic resonance imaging (MRI), a common and safe diagnostic procedure. A patient is exposed to an alternating electromagnetic field of about 3-4 Oe (oersteds) for a specific time. This field will cause the elongated member to become magnetised and consequently to generate harmonics of the applied alternating electromagnetic field. A detector placed near the patient will detect the reflected waveform from the marker as sharp signal peaks at the specific harmonics of the frequency of the applied alternating electromagnetic field. My marker is particularly useful for surgical sponges. However, other surgical tools (e.g., forceps, scalpel), plastic or rubber/polymer surgical implement or equipment used during surgery (e.g., plastic drain, suction tubes), and implants, either temporary or permanent, (e.g., artificial veins, artificial arteries, knee replacement) may also be used. When the surgical object is a surgical sponge, the marker can be woven into the sponge. I also provide an improved method of attaching this device to a surgical sponge. When the surgical object is a surgical tool such as a forceps, the marker is placed on the tool by an adhesive means. In the case where the surgical object is an implant, the method of detecting the surgical object can be used as a diagnosis of past medical history of implants. Each type of implant can contain a specific material or combination of materials that emits a specific signal that corresponds to that surgical object. Thus, if the patient is incoherent, the doctor can diagnose the patient's past medical history of implants. I prefer to perform the excitation and detection steps prior to the closing of the surgical incision. I can thereby prevent the usual infection that would go along with leaving the surgical object within a body and the trauma which corresponds with reopening the incision to remove the surgical object. Other details, objects and advantages of the invention will become apparent as the following description of a present preferred embodiment thereof and a present preferred method of practicing the same proceeds. Brief Description of the Drawings In the accompanying drawings I have shown a present preferred embodiment of the invention in which: Figure 1 is a perspective view partially cut away of a present preferred marker; Figure 2 is a perspective view partially cut away of a second preferred marker; Figure 3 is a perspective view of a surgical sponge having one and optionally two markers woven through the sponge; Figure 4 is a diagram of a detector of my marker; Figure 5 is a graph of responses of a detector in the presence of my marker; and Figure 6 is a perspective view of a pair of forceps having a marker positioned on the exterior handle surface. Detailed Description of the Preferred Embodiments As shown in Figure 1, my marker 2 is comprised of an elongated body 10 of soft magnetic material which is nonmagnetostrictive. It is characteristic of many ferromagnetic materials that even the slightest applied mechanical strain tends to cold work the material and degrade its permeability and other magnetic properties. Nonmagnetostrictive magnetic materials are insensitive to strain; they have the required magnetic properties for my marker while they are also insensitive to work hardening. Amorphous materials are very difficult to cold work. Preferably, the material of my marker is a nonmagnetostrictive amorphous material with very soft magnetic properties. It should have high magnetic permeability, a low coercive field, and induction saturation should be as high as possible. One suitable material is sold by Allied under the trademark "Metglas." The composition of the alloy is recited in United States Patent No. 4,298,862 to Gregor et al. Thus, for example, Allied compositions identified as types 2826MB@ or 2705M may be preferably used. Another suitable amorphous material is Vitrovac 6025Z soft magnetic amorphous alloy with zero magnetostriction sold by Vacuumschmelze GmbH of Hanau, Germany. Although the marker body could be any dimension and shape, I prefer to make the elongated body 10 as a ribbon or a strip or a set of parallel ribbons, strips, fibers or filaments. The elongated geometry of these members allows me to take advantage of the high magnetic permeability of the materials. Preferably, the elongated members will have an aspect ratio (length/square root of cross-sectional area) of at least 200. Such a sufficiently large value for the aspect ratio will assure generation of a detectable signal according to the teachings of U.S. Pat. No. 3,665,449. If the direction of an applied electromagnetic field is essentially parallel to the marker, and the intensity of the applied electromagnetic field is greater than a minimum field or threshold, then the marker will generate high harmonics. That is, the marker will emit a spectrum of harmonics whose intensity decreases slowly with the order of the harmonics in the spectrum. According to the invention, the 9th or the 11th harmonics are detectable. Additionally I prefer the ribbon 10 to have a width less than 2 mm, a thickness ranging between 0.01 to 0.03 mm and a length of about 5 to 7 cm. Body 10 could be longer or shorter. The width and thickness of the ribbon 10 can be selected to obtain flexibility so that when the marker is attached to an object such as a surgical sponge, it will not adversely affect the flexibility of the object. I prefer to encapsulate the elongated body with a polymeric material that is compatible with the human body. Suitable coating materials are nylon and delrin plastic. The coating 12 prevents the body from oxidizing or reacting with body fluids and covers any sharp edges or corners of the ribbon 10. In order to maximize the ratio of the length to the square root of the cross-sectional area, it has been found desirable that the material be as thin and as narrow as practical, depending upon off-setting production cost considerations. It is evident that fibers possess the required geometry. The metallic fiber which I prefer to use can be made in accordance with the teachings of U.S. Patents No. 5,003,291, 5,015,992, 5,015,993 and 5,027,886 by Strom-Olsen and Rudkowski. These fibers are nonmagnetostrictive, amorphous magnetic materials with very high magnetic permeability. Typically, the diameter of such fibers ranges between .01 and .04 mm. I further provide marking means in the form of a flexible marker made of fiber laminated between two layers of polymeric materials. In order for detection to occur, one to approximately ten fibers should be placed parallel to each other, and each fiber should be approximately 20 to 100 mm in length. Such a multiple fiber marker assembly is shown in Figure 2. As shown, the marker of this invention comprises two layers 23 in the form of a web or ribbon of polymeric materials. I provide a plurality of fibers 11a, llb and llc arranged parallel to each other and secured between the two layers 23 by adhesion or lamination means. As mentioned earlier and for the same reasons, I prefer to make use of a polymeric material that is compatible with the human body. However, a marker made of a ribbon or fiber should be less than the length or the width of the surgical object or not so long that it would require significant bending or multiple folding for attachment to the object to be marked. When one is outside this preferred range, a response signal from the marker will be less characteristic and therefore more difficult to identify. Although my markers would be X-ray detectable, one could impregnate the coating or layers with an X-ray opaque material such as a barium compound to improve detectability. Another possibility is to use an X-ray opaque coating material which is currently being used on medical products. Use of such material not only improves the detectability of the marker, but should also reduce the time needed to obtain government and hospital approval of use of the marker. One could also use nonmagnetostrictive crystalline magnetic materials such as Permalloy alloy for the marker body 10. If a crystalline material is used, the cover layer of polymeric material should be designed to minimize the transfer of mechanical stress induced upon bending or folding the marker. The surgical sponge 4 shown in Figure 3 is a gauze sponge having a marker 2 woven throughout the sponge. It is understood that other means of attachment or securing the marker 2 to the surgical sponge 4 can be used. The marker can be connected to the sponge by means of pressure, heat, adhesive or the like. Either the marker of Figure 1 or the marker of Figure 2 could be used. I prefer to use markers which are shorter than a length or a diagonal of the sponge to avoid bending or folding. Optionally, I may use a second marker 32, shown in chainline, positioned at a right angle to the first marker 2. This configuration assures that the sponge will be detected regardless of its orientation with respect to the detector. One could use more than two markers on an object. But, as the number of markers increases, the response of the marked object to a detector will likely be less distinctive. Therefore, I prefer to use one or two markers positioned as in Figure 3. This arrangement provides a distinctive response such as shown in Figure 5. It is relatively easy to detect an object marked with my marker. Once the surgery procedure is completed, the surgeon exposes the surgical cavity to an alternating electromagnetic field, using the detection system. Preferably, the patient will be on a nonmagnetic gurney or examination table. A very low magnetic field of approximately 3 to 4 Oe is all that is required. Such a field is relatively easy to establish and will not harm the patient or the equipment which is normally found in an operating room. If desired, one may use as little as 1.5 Oe and as much as 6 Oe or higher. However, it is normally not practical to exceed 10 Oe because such high fields would interfere with other equipment present in an operating room. This magnetic field will cause the marker body attached to any surgical objects in the surgical cavity to emit specific harmonic frequencies corresponding to the selected marker material. That emission will cause a change in the alternating electromagnetic field, which change can be correlated to the presence of only the selected nonmagnetostrictive, soft magnetic marker material. The detection system can be designed to measure all changes in the applied electromagnetic field and then look for the specific change which would be caused by the presence of the marker. Alternatively, the detection system could be designed to measure only that change or a surrounding band of changes of the type which would be caused by the presence of the selected marker material. A number of techniques and variety of devices could be used to detect the presence of the magnetized marker body attached to a surgical object left in the patient. Those techniques and devices should be apparent to those skilled in the art. The detection system circuitry with which the marker 2 is associated can be any system capable of (1) generating within an interrogation zone an incident alternating electromagnetic field, and (2) detecting magnetic field variations at selected harmonic frequencies produced in the vicinity of the interrogation zone by the presence of the marker therewithin. Such systems typically include means for transmitting a varying electrical current from an oscillator and amplifier through conductive coils that form a frame antenna capable of developing an alternating magnetic field. A fairly simple detector is illustrated in Figure 4. A flat search coil 14 has a first section 13 wound in a clockwise direction from points A to B and a second section 15 with the same number of windings running in a counter-clockwise direction from points B to C. A frame inductor antenna 16 is placed near the search coil. If an AC current is passed through the inductor antenna, an alternating magnetic field will be created. That field will induce a voltage through the search coil. One can detect the voltage from points A to B and plot it on coordinates 34 in Figure 5 as a sine wave 23. The voltage from points B to C can be plotted as a sine wave 25, 180 degrees out of phase from sine wave 25. If the two waves are plotted simultaneously, they will cancel each other and yield a straight line over the time-axis. In the event a magnetized marker moves within the interrogation zone of coil 14, it will change the AC magnetic field received by the coil 14 and modify waves 23 and 25. For a single marker, that modification can be seen as a series of peaks 27 also shown in Figure 5 on coordinates 35. The points along the time-axis where peaks occur depend upon the size and composition of the marker. If too many markers are used, the peaks would flatten and approach a sine wave. Therefore, I prefer not to use more than two markers, preferably oriented as in Figure 3. The marker produces peaks at particular points along the x-axis. Thus, the detector looks for a response at those intervals. Only if a response occurs at the chosen points is a detection made. It is possible that equipment in the operating room, such as CRTs, will generate electromagnetic fields which will cause a detector response. However, the chances that such interference will produce peaks at the selected intervals is small. Hence, false detections are remote possibilities, and they can be eliminated by predetecting and electronically cancelling the signals of such equipment. My markers can be used in other surgical equipment such as forceps, scalpels and hemostats. Figure 6 shows a pair of forceps 6 which have a marker positioned along the handle. My markers can also be used to mark surgical implants. Hence, a physician could learn about an implant in the patient's past medical history using my detection method. The area under suspicion of an implant will be excited at a specific frequency. If an implant exists in that area, it will emit energy over a specific spectrum of frequency which corresponds to the specific implant material. For this diagnostic process to work, a standard must be used for all implants. Standard material should be used for each specific implant differing the material from one implant to another. In that event, the response will identify the specific implant. I have shown and described the present preferred embodiment of the invention. It is to be distinctly understood that the invention is not limited thereto, but may be otherwise variously embodied within the scope of the following claims.

Claims: A surgical object (4,6) marked to detect its presence in a human body following a surgical operation, comprising a marker (2) attached to the surgical object, including at least one elongated member (10) of a substantially nonmagnetostrictive soft magnetic material, wherein when the marker is exposed to an alternating electromagnetic field, the nonmagnetostrictive material generates detectable high harmonics in the 9 th or the 11 th order of the alternating field, the specific intensities of the high harmonics being characteristic of the magnetic material and therefore of the marker, thereby causing a change in the alternating magnetic field which change can be correlated to the presence of the marker and the absence of such a change indicating the absence of the marker. A surgical object according to claim 1 wherein the or each elongated member comprises an amorphous material. A surgical object according to claim 1 wherein the or each elongated member comprises a crystalline material. A surgical object according to any of claims 1 to 3 including a plurality of substantially parallel elongated members. A surgical object according to any of claims 1 to 4, wherein the elongated member or members are surrounded by a biocompatible coating. A surgical object according to any preceding claim, wherein the biocompatible coating is radio opaque. A surgical object according to any preceding claim, wherein the or each elongated member has an aspect ratio of at least 200. A surgical object according to any preceding claim, being a surgical sponge (4), with the marker woven into the sponge.

Inventors: SZYMAITIS DENNIS W, SZYMAITIS, DENNIS W.
Applicants: SZYMAITIS DENNIS W, SZYMAITIS, DENNIS W.
Application year: 1995
Main classification: A61B19
IPC codes: A61B19