Patent Publication Number: US-2019192044-A1

Title: System and method for magnetic occult lesion localization and imaging

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Patent Application Ser. No. 62/385,945, filed on Sep. 9, 2016, and entitled “SYSTEM AND METHOD FOR MAGNETIC OCCULT LESION LOCALIZATION AND IMAGING,” which is herein incorporated by reference in its entirety. 
    
    
     BACKGROUND 
     In contemporary breast cancer management, greater than 70 percent of breast cancer patients are eligible for and select breast-conserving therapy. The combination of early detection from screening and improvements in adjuvant therapies has translated into improvements in overall survival. However, the patient experience and treatment efficiency during the therapeutic process requires dramatic improvement. 
     Breast conserving surgery typically includes a surgical procedure whereby the tumor and a rim of surrounding normal tissue are removed. Currently, options for guiding the accurate excision of non-palpable lesions are unsatisfactory in terms of patient experience, healthcare system resource utilization, and cost-effectiveness. The main two approaches used for guidance of breast conserving surgery are wire localized breast biopsy (“WLBB”) and radioactive seed localization (“RSL”). 
     WLBB involves the implantation of a hooked wire on the day of surgery under mammographic or ultrasound guidance to mark the center and/or borders of the lesion. The patient is required to remain in the hospital with the wire protruding from the breast for several hours with minimal anesthetic. This is not only painful for the patient, but can also cause wires to dislodge as the patient waits for excision. Furthermore, if the wire is implanted under mammographic compression, the positioning of the wire rarely corresponds with supine surgical orientation, and its trajectory often requires surgical incision placement that is suboptimal for cosmesis. The path of the wire often results in the excision of more tissue than necessary. 
     RSL has more recently been adopted as an alternative approach to WLBB where a radioactive seed is used to mark the center and/or borders of the tumor. The implanted seeds are contained entirely within the breast, thereby preventing their movement with respect to the lesion. The surgeon uses a hand-held gamma ray detector to localize the seed and guide excision. While this addresses many of the patient flow and comfort issues with WLBB, the main obstacle with this technique is that the implanted seeds are radioactive, therefore requiring significant investment and vigilance for handling equipment, regulatory approvals and monitoring, specialized personnel and training, as well as administrative expenses. This process is also associated with marginally increased radiation exposure of staff and patients. 
     Thus, there remains a need for a system and method for guiding breast conserving surgeries, and other surgical excisions and procedures, in which less invasive, non-radioactive localization of the lesion or tumor are implemented. 
     SUMMARY OF THE DISCLOSURE 
     The present disclosure provides a magnetic detector system for localizing a magnetic seed that generates a magnetic field. The detector system generally includes a detector probe, a processor, and an output. The detector probe can include a housing extending along a central axis from a distal end to a proximal end, a first magnetic sensor arranged at the proximal end of the housing, and a second magnetic sensor arranged at the distal end of the housing. The first magnetic sensor and the second magnetic sensor detect a magnetic field generated by a magnetic seed and in response thereto generate signal data representative of the magnetic field. The processor can be in communication with the first magnetic sensor and the second magnetic sensor to receive the signal data therefrom and to process the signal data to compute a location of the magnetic seed. Processing the signal data includes accounting for an anisotropic geometry of the magnetic field generated by the magnetic seed. The output provides feedback to a user based on the computed location of the magnetic seed. 
     The present disclosure also provides a kit for localization of an implantable magnetic seed. The kit generally includes an introducer device, a detector probe, a processor, and an output. The introducer device includes a needle and a plunger. The needle is composed of a non-magnetic material and has a lumen that extends from a distal end to a proximal end of the needle. The lumen of the needle is sized to receive a magnetic seed for implantation in a subject. The plunger is also composed of a non-magnetic material and is arranged within the lumen of the needle. The plunger is sized to be received by the lumen of the needle such that when the plunger is translated along a length of the lumen air is allowed to flow past the plunger so as not to generate a vacuum effect in the lumen. The detector probe includes a housing extending along a central axis from a distal end to a proximal end, a first magnetic sensor arranged at the proximal end of the housing, and a second magnetic sensor arranged at the distal end of the housing. The first magnetic sensor and the second magnetic sensor detect a magnetic field generated by the magnetic seed and in response thereto generate signal data representative of the magnetic field. The processor is in communication with the first magnetic sensor and the second magnetic sensor to receive the signal data therefrom and to process the signal data to compute a location of the magnetic seed. Processing the signal data includes accounting for an anisotropic geometry of the magnetic seed. The output provides feedback to a user based on the computed location of the magnetic seed. 
     The foregoing and other aspects and advantages of the present disclosure will appear from the following description. In the description, reference is made to the accompanying drawings that form a part hereof, and in which there is shown by way of illustration a preferred embodiment. This embodiment does not necessarily represent the full scope of the invention, however, and reference is made therefore to the claims and herein for interpreting the scope of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of an example magnetic occult lesion localization and imaging (“MOLLI”) system. 
         FIG. 2  is an example of a magnetic seed that can be localized with the MOLLI system of the present disclosure. 
         FIG. 3  is an example of the magnetic seed illustrating a bio-compatible coating over a magnetic material core. 
         FIG. 4  is an example magnetic vector field diagram for a magnetic seed that generates an anisotropic magnetic vector field. 
         FIG. 5  is an example magnetic flux density diagram for a magnetic seed that generates an anisotropic magnetic field with an anisotropic magnetic flux density distribution. 
         FIG. 6  is an example cross sectional view of a detector probe for detecting magnetic seeds implanted in a subject. 
         FIG. 7  is an example cutaway view of the distal end of the detector probe of  FIG. 6 . 
         FIG. 8  is an example cutaway view of the proximal end of the detector probe of  FIG. 6 . 
         FIG. 9  is an example of a detector probe having an array set of magnetic sensors at its distal end and a single magnetic sensor at its proximal end. 
         FIG. 10  is an example of an introducer device for implanting a magnetic seed in a subject. 
         FIG. 11  is a cross sectional view of the introducer device of  FIG. 9 . 
     
    
    
     DETAILED DESCRIPTION 
     Described here are systems and methods for marking the location and extent of an anatomical region-of-interest, such as a tumor, using magnetic seeds whose position and orientation are measured or otherwise detected using a detection device that includes two or more magnetic sensors. The system described here generally includes magnetic seeds that are implanted into a subject to mark the center, boundaries, or both, of an anatomical region-of-interest, such as a tumor. In one example application, the magnetic seeds can be implanted to mark the boundary of a breast tumor; however, other clinical applications will be apparent to those skilled in the art. 
     According to the systems and methods of the present disclosure, one or more non-radioactive, magnetic seeds are implanted to mark and define the center and extent of an anatomical region of interest, such as a tumor or other lesion. Using a magnetic sensor-based detector system, a clinician (e.g., a surgeon) can accurately identify the location of the magnetic seeds prior to any incision. In use for marking the location of a breast tumor, the clinician can plan out a surgery to allow for the best achievable cosmetic result, while ensuring optimal oncologic outcomes. 
     As shown in  FIG. 1 , an example magnetic occult lesion localization and imaging (“MOLLI”) system  10  is shown. The system  10  generally includes one or more magnetic seeds  12  that are implanted into an anatomical region-of-interest  14  in a subject  16 . The region-of-interest  14  may include a tumor. In some embodiments, one or more of the magnetic seeds  12  may also be positioned on a skin surface of the subject  16 . 
     A detector probe  18  is used to detect or otherwise measure the position, orientation, or both, of the magnetic seeds  12 . The detector probe  18  generally includes a housing  20  that contains a first magnetic sensor  22  and a second magnetic sensor  24 . The housing  20  generally defines a hand-held structure such that the detector probe  18  can be held and used by a clinician in an operating room or other surgical or clinical environment. As one example, the housing  20  can generally extend from a proximal end to a distal end along an axis. The first magnetic sensor  22  can be positioned or otherwise arranged at the proximal end of the house  20  and the second magnetic sensor  24  can be positioned or otherwise arranged at the distal end of the housing. In some embodiments, the first magnetic sensor  22  and the second magnetic sensor  24  can be coaxially aligned along the axis of the detector probe  18 ; however, in other embodiments one of the sensors (e.g., the second magnetic sensor  24 ) can be offset from the axis of the housing  20  to provide a more ergonomic design of the detector probe  18 . 
     In some embodiments, the tip  25  of the detector probe  18  containing the second magnetic sensor  24  can be removable. In these configurations, the tip  25  can be interchanged with different tips having different magnetic sensors. For instance, as will be described below, one tip could include a single magnetic sensor while another tip could include more than one magnetic sensor, such as an array set of two or more magnetic sensors. Having a removable tip  25  also allows for easier sterilization since the tip  25  can be removed and separately sterilized rather than sterilizing the entire detector probe  18 . In some other implementations, the tip  25  can be made disposable, such that after a single use the tip  25  can be removed and replaced with a new, sterile tip  25 . In still other implementations, the detector probe  18  itself can be made to be disposable. 
     The detector probe  18  may also include other sensors, including additional magnetic sensors or one or more accelerometers, gyroscopes, temperature sensors, and so on. These other sensors can be positioned within the housing  20 , or may be positioned or otherwise arranged on an outer surface of the housing. As one example, one of these other sensors could be affixed to the outer surface of the housing  20 . 
     The detector probe  18  is in electrical communication with a computer system  26 , which generally operates the detector probe  18  and receives signal data from the magnetic sensors  22 ,  24 . The computer system  26  can also provide visual feedback, auditory feedback, or both, to a surgeon to assist the surgeon during a procedure. This feedback can be provided via an output  50 , which may include a display, a speaker, or so on. It is contemplated that the MOLLI system  10  can be integrated with or otherwise implement virtual reality systems, augmented reality systems, or both. 
     As one non-limiting example of visual feedback that can be provided to a user, the output  50  can include a display that displays one or more numerical values associated with the detected location of a magnetic seed  12 . For instance, the numerical values can represent distances between the detector probe  18  and a magnetic seed  12 , an error or uncertainty in the measured location of a magnetic seed  12 , or both. 
     As another non-limiting example of visual feedback that can be provided to a user, the output  50  can include a display to provide visual feedback integrating diagnostic images of the subject and the anatomical site to which magnetic seeds  12  will be or have been delivered. Examples of such diagnostic images include mammographic or other x-ray images, sonographic images, magnetic resonance images, or other images that may be organized in a central electronic repository, such as a picture archiving and communication system (“PACS”). In some implementations, the output  50  can include a display that provides a comparative view of diagnostic images and information from the signal data received from the magnetic sensors  22 ,  24 . As one example, the computer system  26  can generate display elements indicating the position and orientation of the magnetic seeds  12 , the detector probe  18 , or both, and can display these display elements overlaid on the diagnostic images. 
     As one non-limiting example of auditory feedback that can be provided to a user, the output  50  can include a speaker that receives an auditory signal from the computer system  26 . The auditory signal can indicate the presence of a magnetic seed  12  within the vicinity of the detector probe  18 . For instance, a characteristic of the auditory signal can change based on the relative distance between the detector probe  18  and the magnetic seed  12 . As one example, the pitch of the auditory signal can be changed. As another example, the auditory signal can include a series of chirps or other tones, with the repetition frequency of the chirps increasing or decreasing based on the relative distance between the detector probe  18  and the magnetic seed  12 . 
     The computer system  26  can include one or more processors for receiving the signal data from the magnetic sensors  22 ,  24  and for processing the signal data to detect or otherwise measure a position, orientation, or both, of the magnetic seeds  12 . In some embodiments, the computer system  26  can include one or more processors that are arranged within the housing  20  of the detector probe  18 ; however, in other configurations the computer system  26  is physically separate from the detector probe  18 . The computer system  26  can also measure an error in the measured position, orientation, or both, of a magnetic seed  12  and can present this information to a user, such as by generating a visual, textual, or numerical display based on the measured uncertainty. The computer system  26  can also calibrate the detector probe  18 , and process the signal data to provide an assessment of the margin of the region-of-interest  14  (e.g., a tumor margin) or to implement bracketing of the region-of-interest  14 . 
     In some embodiments, the detector probe  18  may also include one or more trackers  28  used for tracking the detector probe  18  with a surgical navigation system  30 . Examples of such sensors include optical markers, infrared emitters, radio frequency emitters, ultrasound emitters, and so on, which may be detected by a suitable tracking system  32 , such as an optical tracking system, radio frequency tracking system, and so on. The trackers  28  may also include accelerometers, gyroscopes, and the like, for tracking the detector probe  18  using a surgical navigation system that is based on inertial sensors. 
     An introducer  34  is also provided for introducing the magnetic seeds  12  into the subject  16 . The introducer  34  has a generally non-magnetic construction, such that the introducer  34  does not interfere with accurate placement of the magnetic seeds  12 . 
     The MOLLI system  10  utilizes the magnetic sensors  22 ,  24  in the detector probe  18  to accurately locate the magnetic seeds  12  within a patient. Signal data measured by these magnetic sensors  22 ,  24  contain information about the magnetic field vector of the detected magnetic seeds  12 , and this signal data is provided to the computer system  26  where the signal data are converted into a distance measure and visual feedback, auditory feedback, or both, to guide the surgeon. 
     It is contemplated that the MOLLI system  10  can detect a magnetic seed  12  that is around 7 cm from the tip of the detector probe  18 . Based on this data, at a distance of 60 mm, magnetic seeds  12  can be detected with a one percent false positive/false negative rate. This added confidence will help ensure surgeons are able to accurately identify the target site. 
     An example magnetic seed  12  that can be implemented in accordance with the present disclosure is illustrated in  FIGS. 2 and 3 . In the example shown in  FIG. 2 , the magnetic seed  12  has a generally cylindrical shape; however, it will be appreciated that any other suitable shapes can be implemented, including spherical shapes, ellipsoidal shapes, rectangular shapes, and so on. Each magnetic seed  12  can be sized to fit in standard needles for implantation. As will be described below, a non-magnetic introducer device  34  can be used to accurately implant magnetic seeds  12 . 
     In general, the magnetic seeds  12  are constructed such that they generate an anisotropic magnetic field. In some embodiments, the magnetic seeds  12  also generate magnetic fields with anisotropic magnetic flux density distributions. 
     The magnetic seeds  12  are generally composed of a magnetic material  36  that is encapsulated in a bio-compatible shell  38 , as shown in  FIG. 3 . In some embodiments, the magnetic material is a rare-earth magnet composed of an alloy containing one or more rare-earth elements. As one example, the magnetic material can be a neodymium magnet, such as Nd 2 Fe 14 B (“NIB”) or other alloys containing neodymium. 
     The bio-compatible shell  38  can be composed of gold; however, it will be appreciated that the bio-compatible shell  38  can also be composed of other bio-compatible metallic and non-metallic materials, including bio-compatible polymers. In some embodiments, the bio-compatible shell  38  includes more than one layer. As one example, the bio-compatible shell  38  can include an inner layer composed of nickel, a second layer composed of copper, a third layer composed of nickel, and a fourth, outer layer composed of diX® parylene-C (Kisco Ltd.; Japan). 
     In some examples, the magnetic seeds  12  can be sintered from rare-earth metals. The sintering method of manufacturing for the magnetic seeds  12  allows for a stronger magnetic flux distribution than alternative techniques; however, due to the small geometry of the magnetic seeds  12  and variance in materials, it is possible that the flux densities of the magnetic seeds  12  to fluctuate (e.g., by 4-6 percent). This inter-seed variability can be accounted for within the anisotropy and distance algorithms; however, this minimal variance is also generally acceptable for the purposes of the MOLLI guidance system of the present disclosure. It is also contemplated that constructing the magnetic seeds  12  to have radial symmetry will mitigate errors attributable to intra-seed variance. 
     The magnetic seeds  12  used in the present disclosure are generally constrained in geometry by the introducer needles that are used to implant the magnetic seeds  12  into the region-of-interest  14 . As one example, for the magnetic seeds  12  to be inserted using standard sized needles commonly employed in radiology departments, the magnetic seeds  12  can be designed to have a diameter of 1.6 mm and a length of 3.2 mm along the longitudinal axis of the magnetic seed  12  (e.g., the cylindrical axis of the magnetic seed  12  illustrated in  FIG. 2 ). This geometry enables the field strength of the magnetic seeds  12  to be maximized while still remaining practical to implant. 
     The magnetic field generated by an anisotropic magnetic seed  12  is roughly similar in geometry to a conventional bar magnet. An example vector magnetic field distribution for a magnetic seed  12  is represented in  FIG. 4 , which demonstrates the perturbations and anisotropic response of the magnetic seed  12  construction. Notably, the vectors follow a toroidal pattern around the magnetic seed  12 , which represents anisotropy in the magnetic field. This anisotropic effect is characterized and accounted for during detection of the magnetic seeds  12  such that the detector probe  18  can accurately discretize the distance to the magnetic seeds  12 .  FIG. 5  illustrates an example representation of the magnetic flux density of a magnetic seed  12  demonstrating a nonlinear and anisotropic distribution of the magnetic field. Each annular ring in  FIG. 5  represents an increase in the strength of the flux density. 
     As shown in  FIG. 5 , the magnetic flux of the magnetic seeds  12  is not equivalent at the same distance axially versus radially. Because the system  10  calculates the distance of the magnetic seeds  12  from the detector probe  18  from the magnetic flux measured at the tip of the detector probe  18 , the orientation of the magnetic seed  12  will influence the measurement of the distance between the magnetic seed  12  and the detector probe  18 . 
     Thus, the anisotropic construction of the magnetic seeds  12  results in similar anisotropy in both their vector fields and flux density. This anisotropic effect can be quantified and this quantified information can be used in compensation algorithms to estimate the true distance between the tip of the detector probe  18  and a given magnetic seed  12 . The uncertainty in those measurements can also be estimated and reported. 
     For example, the MOLLI system described here can evaluate the uncertainty in the calculation of the distance between the detector probe  18  and a given magnetic seed  12 , and this information can then be displayed alongside a digital readout. It is contemplated that, for the example magnetic seed and detector probe designs described here, the magnitude of this error can vary between around 8 mm at the limit of magnetic seed detection (e.g., 7 cm from a magnetic seed) to less than 1 mm nearest a magnetic seed (e.g., 1 cm from a magnetic seed). 
     The estimation of the error in the seed-to-detector distance is dependent on the model used to account for the anisotropic construction of the magnetic seed  12 . Simple look up tables are unable to accurately estimate the error in the seed-to-detector distances because they do not account for the physical construction of the magnetic seed  12 . In these lookup-table approaches, the marker is assumed to be a single point in space with a homogenous magnetic field surrounding it, and thus no information about the structure of the marker is provided. Using lookup-tables with anisotropic magnetic seeds therefore does not allow for reliable estimation of the error in the distance of such magnetic seeds from a detector probe. As such, surgeons will not have confidence in the number that is presented. 
     The systems and methods of the present disclosure, however, incorporate a physical model of the magnetic seeds  12  into the detection of the magnetic seeds  12 , and thus an uncertainty in those measurements can be accurately estimated and reported. Reporting a distance with an error estimate will provide confidence to the surgeon and will allow them to use this information in important clinical decision making. 
     As described above, the implanted magnetic seeds  12  are detected using a detector probe  18  that generally includes a first and second magnetic sensor  22 ,  24 . As an example, the magnetic sensors  22 ,  24  can be magnetometers. In one example, the detector probe  18  is constructed such that the first magnetic sensor  22  is arranged at the proximal end of the detector probe  18  and such that the second magnetic sensor  24  is arranged at the distal end of the detector probe  18 . 
     The detector probe  18  is designed to be insensitive to the Earth&#39;s magnetic field by using an in-built subtraction system that accounts for changes in the orientation of the detector probe  18  relative to the Earth&#39;s magnetic field. Although the magnetic sensors  22 ,  24  can be aligned along the central axis of the detector probe, in a preferred embodiment, one of the magnetic sensors can be offset from the central axis of the detector probe  18 . 
     As one example, the second magnetic sensor  24  can be offset to provide a more ergonomic design of the detector probe  18 . Such an arrangement is illustrated in  FIGS. 6-8 , which show a detector probe  18  in which the first magnetic sensor  22  and the second magnetic sensor  24  are not coaxial with the central axis  40  of the detector probe  18 . Because of the built-in ability to compensate for the Earth&#39;s magnetic field, unlike previous magnetic detector systems, the detector probe  18  does not have strict requirements or constraints on the alignment of the magnetic sensors  22 ,  24  with respect to each other and other arrangements and alignments of the magnetic sensors  22 ,  24  can be readily adapted. 
     The detector probe  18  is in electrical communication with the computer system  26 , as described above, via a cable  42  located at the distal end of the detector probe  18 . The cable  42  can include one or more electrical wires, and can also include one or more optical fibers. In general, the cable  42  provides electrical power to the detector probe  18  and also provides for communication of signal data measured by the magnetic sensors  22 ,  24  to the computer system  26 . In some other embodiments, the detector probe  18  can be in wireless communication with the computer system  26 , in which the cable  42  can be removed. Power can be provided to the detector probe  18  via an internal battery in these configurations. In other embodiments, the computer system  26  can be housed within the housing  20  of the detector probe  18 . For instance, as shown in  FIG. 6 , the computer system  26  can include a printed circuit board one which a hardware processor and a memory are arranged. In such configurations, the computer system  26  can be powered via cable  42 , or via an internal battery. 
     During operation, or before operation, of the detector probe  18 , the computer system  26  can perform a calibration procedure, in which each magnetic sensor  22 ,  24  is independently calibrated. The measurements provided by the magnetic sensors  22 ,  24  can then be fused to compensate for any misalignment. Through this calibration procedure, the magnetic sensors  22 ,  24  are placed in a common coordinate system, such that the location of the magnetic sensors  22 ,  24  is known relative to a common spatial reference point. These calibration values can be stored as calibration data in the computer system  26  for subsequent use by the computer system  26  and detector probe  18 . The detector probe  18  can be independently serially numbered and calibrated and the corresponding calibration data for the magnetic sensors  22 ,  24  can be stored in a memory (e.g., a non-volatile memory) contained within the detector probe  18 . In some instances, the computer system  26  can be contained within the detector probe  18  and the memory can form a part of the computer system  26 . 
     Although  FIGS. 6-8  depict a detector probe  18  with only two magnetic sensors  22 ,  24 , the detector probe  18  can be constructed to have more than two magnetic sensors. In some examples, more than two magnetic sensors can be arranged within the housing  20  of the detector probe  18 , while in other examples, one or more additional magnetic sensors can be affixed or otherwise arranged on the outer surface of the housing  20 . As one non-limiting example, the detector probe  18  can include one or more arrays of magnetic sensors. For instance, the detector probe  18  could include an array set of two distal magnetic sensors  24   a ,  24   b , and one proximal magnetic sensor  22 , as illustrated in  FIG. 9 . Other sensors can also be arranged within the housing  20  of the detector probe  18  or on the outer surface of the housing  20 . Examples of such other sensors include accelerometers, gyroscopes, and so on. One or more of the magnetic sensors  22 ,  24  can also be replaced with an array of such sensors. 
     By utilizing one or more arrays of magnetic sensors, or other sensors (e.g., accelerometers, gyroscopes) the direction from a magnetic seed  12  and the detector probe  18  can be determined and visualized. With the capability of measuring the directionality of the magnetic seeds  12  relative to the detector probe  18 , a digital collimation effect can be provided and switched on or off as desired by the clinician. When activated, the collimation will only provide an auditory or visual cue to the clinician when a magnetic seed  12  is within a viewing window of the tip of the detector probe  18 . Outside of this viewing window, the detector probe  18  will not trigger an auditory or visual cue, even if a magnetic seed  12  is detected as outside of that viewing window. This functionality allows the MOLLI system  10  to closely replicate the use and function of RSL probes. 
     The detector probe  18  is capable of resolving depth and can achieve a spatial resolution that is sufficient to detect and resolve magnetic seeds  12  that are close to each other. This capability allows for bracketing the region-of-interest  14  with magnetic seeds  12 , which is not possible with radioactive seeds. 
     As mentioned above, the MOLLI system  10  can operate with feedback from only two magnetic sensors  22 ,  24 . It is contemplated that with only two magnetic sensors  22 ,  24 , and no other sensors, the MOLLI system  10  can achieve a sensitivity and specificity of 95 percent at a depth of detection of 70 mm. 
     The MOLLI system  10  is designed to help expeditiously guide a surgeon to a magnetic seed  12  with relative ease. To help achieve this goal, the localization attainable by the MOLLI system  10  is accurate and precise, with a spatial resolution that is comparable or better than that of gamma probes used in radio-seed localization. 
     The introducer device  34  can be used to provide the magnetic seeds  12  to a location in the subject  16 . Preferably, the introducer  34  is composed of a non-magnetic material, such that the magnetic seeds  12  can be accurately positioned without interacting with the introducer device  34 . 
     As shown in  FIGS. 10 and 11 , the introducer device  34  generally includes a needle  44  having a lumen  46  that is sized to receive a magnetic seed  12 . In some embodiments, the lumen  46  is sized to be sufficiently larger than the magnetic seed  12  such that an air tight seal is not achieved when the magnetic seed  12  is positioned in the lumen  46  of the needle  44 . 
     The needle  44  is preferably composed of a non-magnetic material, as mentioned above. As one example, the needle  44  can be composed of titanium or a suitable titanium alloy. As another example, the needle  44  can be composed of stainless steel or a suitable stainless steel alloy. The needle  44  can also be composed of other magnetically inert metals, plastics, or so on. 
     A plunger  48  is located at the distal end of the needle  44  and is sized to be received by the lumen  46  of the needle  44 . The plunger  48  is in fluid communication with the magnetic seeds  12 , such that operation of the plunger  48  provides a force the pushes the magnetic seed  12  out of the lumen  46  at the open tip of the needle  44 . The plunger  48  is also sized such that when it is retracted in the lumen  46  after deploying a magnetic seed  12 , air is allowed to pass freely in the lumen  46 , thereby eliminating a vacuum effect that could otherwise interfere with the accurate placement of the magnetic seed  12 . In some other embodiments, the plunger  48  is sized to have an air tight fit with the inner surface of the lumen  46 , but a hole is formed in the plunger  48  such that air can flow past the plunger  48  to avoid creating a vacuum effect that could interfere with accurate placement of the magnetic seeds  12 . 
     In some embodiments, the plunger  48  and needle  44  are constructed such that in use the plunger  48  is held in place while the needle  44  is retracted to place a magnetic seed  12 . In these embodiments, the plunger  48  is preferably designed to hold the magnetic seed  12  in place while the needle  44  is retracted. In some other embodiments, a lock  52  or other suitable retaining device is used to constrain the plunger  48  within the lumen  46  of the needle  44 . The lock  52  can be composed of silicone or other malleable rubber, plastic, or synthetic material. 
     In some configurations, the tip of the needle  44  can be sealed using bone wax or another suitable bio-compatible and bio-degradable material, so as to provide a temporary closure at the tip of the needle  44  that disallows the magnetic seeds  12  to exit the lumen  46  without operation of the plunger  48 . 
     The MOLLI system  10  generally operates by interrogating the volume around the tip of the detector probe  18  for a magnetic seed  12 . The magnetic flux of magnetic seed  12  is then measured and an algorithm used to determine the distance of the magnetic seed  12  from the tip of the detector probe  18 . This algorithm corrects and accounts for the anisotropy of the magnetic seeds  12 , as mentioned above, by incorporating a physical model of the magnetic seeds  12  into the calibration and detection algorithms. The distance calculation is the primary method of feedback for the surgeon as it is correlated to both the visual display, auditory feedback, and actual units displayed. As described above, directionality can also be measured and displayed to the surgeon. 
     In addition to providing the distance from the tip of the detector probe  18  to an implanted magnetic seed  12 , the MOLLI system  10  of the present disclosure is able to determine the distance and quantify the error in the distance measurement, which is described above. This capability of the MOLLI system  10  allows for the completeness of a surgery to be evaluated by ensuring that the cut edge of an excised specimen is at a specified distance from the magnetic seeds  12 , which enables the surgeon to plan the margin and grossly evaluate whether this margin was achieved intraoperatively. This intraoperative margin evaluation can reduce the incidence of re-excision in breast conserving surgeries. The margin evaluation method can also alert the surgeon to an area on their excised volume where the distance to the magnetic seed is less than the average, thereby allowing the surgeon to re-excise that portion of the surgical cavity to better ensure that a clear surgical margin can be achieved. 
     Conventional wire-guided localization used in breast conserving surgeries is often used to mark diffuse disease. In these circumstances, two wires may be located to mark the extent of disease and signify to the surgeon that the region between the two wires is the target area. Additionally, the indexes on the wire are also used to indicate that the region between a specific marker and the end of the wire should be removed. 
     The popularization of RSL has led to the use of iodione-125 marker seeds to localize a lesion for removal in a similar fashion to the original intent of wire-guided localizations. In recent years “bracketing” has become an additional use of RSL seeds to identify the broad regions that a radiologist has identified as suspicious and necessary for removal. 
     Conventionally, bracketing is utilized when the extent of disease will not be readily apparent to the surgical team. As an example, a clinically representative distance for bracketing is on the order of 50-70 mm. It is contemplated that the spatial resolution attainable with the MOLLI system  10  of the present disclosure will allow localization of magnetic seeds  12  as close as 10 mm. As such, there is little interaction between magnetic seeds  12  separated by clinically representative distances of greater than 40 mm, thereby allowing for bracketing to be implemented with the MOLLI system  10 . 
     The ability to identify the extent of disease with multiple magnetic seeds  12  prior to surgery is desirable. This ability will allow the surgeon to plan the procedure in order to completely excise the tumor with minimal excision of normal tissue. The MOLLI system  10  is capable of differentiating magnetic seeds spaced apart by 10 mm or more at 2 cm depth; as such, the MOLLI system  10  enables bracketing of lesions. Lesions that are smaller than 1 cm do not typically require bracketing given their limited volume. 
     The present invention has been described in terms of one or more preferred embodiments, and it should be appreciated that many equivalents, alternatives, variations, and modifications, aside from those expressly stated, are possible and within the scope of the invention.