Patent Publication Number: US-2022226609-A1

Title: System and Method for Medical Device Position Guidance

Description:
RELATED APPLICATIONS 
     This application is a continuation of U.S. application Ser. No. 16/377,329, filed on Apr. 8, 2019, the entirety of which is hereby incorporated by reference for all purposes. 
    
    
     FIELD OF THE INVENTION 
     The subject matter of the present invention relates generally to a medical device position guidance system and method. 
     BACKGROUND 
     Physicians and other health care providers frequently use catheters to treat patients. The known catheters include a tube which is inserted into the human body. Certain catheters are inserted through the patient&#39;s nose or mouth for treating the gastrointestinal tract. These catheters, sometimes known as enteral catheters, typically include feeding tubes. The feeding tube lies in the stomach or intestines, and a feeding bag delivers liquid nutrient, liquid medicine or a combination of the two to the patient. 
     Other types of catheters are inserted into the patient&#39;s veins or arteries for treating the cardiovascular system. These intravascular catheters include, among others, central venous catheters, peripheral venous catheters and the peripherally inserted central catheters. These catheters include a relatively small tube that passes through the patient&#39;s veins or arteries. Depending on the application, the health care provider can use an intravascular catheter to remove blood vessel blockages, place inserts into blood vessels and provide patients with injections of medications, drugs, fluids, nutrients, or blood products over a period of time, sometimes several weeks or more. 
     When using these known enteral and intravascular catheters, it is important to place the end of the catheter at the proper location within the human body. Erroneous placement of the catheter tip may injure or harm the patient. For example, if the health care provider erroneously places an enteral catheter into the patient&#39;s lungs, liquid may be introduced into the lungs with harmful results. If the health care provider erroneously places an intravascular catheter into the wrong blood vessel of the cardiovascular system, the patient may experience infection, injury or a harmful blockage. 
     With feeding tubes in particular, it is also prudent to check that the exit aperture of the feeding tube (typically located at the distal end/tip of the tube) remains in its desired location over the period of treatment, e.g., feeding. Protocols that address this requirement in enteral feeding tubes include frequent monitoring for the appropriate pH of fluids extracted from the feeding tube when not carrying nutritional liquids and careful patient monitoring to ensure nutritional uptake is as expected. 
     In some cases, health care providers use X-ray machines to gather information about the location of catheters within the body. There are several disadvantages with using X-ray machines. For example, these machines are relatively large and heavy, consume a relatively large amount of energy and expose the patient to a relatively high degree of X-ray radiation. Also, these machines are typically not readily accessible for use because, due to their size, they are usually installed in a special X-ray room. This room can be far away from the patient&#39;s room. Therefore, health care providers can find it inconvenient to use these machines for performing catheter insertion procedures. Furthermore, it can be inconvenient to transport these machines to a patient&#39;s home for home care catheter procedures. Moreover, even X-rays are not necessarily conclusive as to the location of the catheter tip, as the natural and continuous movement of the internal organs can make it difficult for the physician interpreting the X-ray to be sure of the actual location of the distal end of the catheter. 
     Another existing catheter locating means involves using an electromagnetic coil positioned inside the catheter and an electromagnetic coil locating receiver outside of the patient&#39;s body. The electromagnetic coil is generally incorporated into a stylet or guide wire which is inserted within the catheter. The coil locating receiver can be used to determine the distance the coil is from the receiver and its depth in the patient&#39;s body and can communicate with a display to show a reference image of a non-subject body and an image of the coil located on the display with the reference image. However, these systems also have several disadvantages. For example, the coil locating receiver is a large device that must rest in a precise location outside the patient&#39;s body and does not permit for adjustments due to each individual patient&#39;s anatomical size or shape. However, a patient undergoing a feeding tube placement will be agitated and sudden movements are expected, which can move the coil locating receiver, thus increasing the likelihood of positional errors or complications in locating the catheter. Additionally, these existing systems can only display the coil location over a reference image of a non-subject (i.e., a generic patient) body without reference to the individual patient&#39;s particular anatomy. Therefore, health care providers can estimate the positioning of the catheter using the electromagnetic coil and coil locating receiver but cannot estimate or view the specific patient&#39;s anatomy. 
     Consequently, there is a need for a medical device, e.g., catheter, position guidance system that is adaptable to patients of all sizes to ensure more accurate catheter placement. In particular, a medical device position guidance system that provides a stationary frame of reference with the patient and determines patient anatomical shape and size would also be useful. 
     SUMMARY 
     The present invention is directed to a medical device position guidance system. The system includes a processor, a plurality of external detector devices, a display device, and a memory device. The plurality of external detector devices are configured to be positioned in a predetermined external arrangement on a subject. Each detector device is operatively coupled to the processor. Each of the plurality of external detector devices are configured to interrogate each other to determine a distance between each of the plurality of detector devices. The memory device stores instructions which when executed by the processor, cause the processor to: (i) receive signals relating to the distance between each of the plurality of detector devices from each of the plurality of detector devices to determine a three-dimensional volume between the plurality of external detector devices; (ii) using the received data from the plurality of external detector devices and the three-dimensional volume to determine an anatomical shape, size, and/or orientation of the subject within the three-dimensional volume; and (iii) cause the display device to display the anatomical shape, size, and/or orientation of the subject within the three-dimensional volume. 
     In one particular embodiment, the plurality of external detector devices includes a first external detector device, a second external detector device, and a third external detector device. Moreover, the predetermined external arrangement can be based on at least one bony landmark of the subject. Further, the predetermined external arrangement can include the first external detector device configured to be placed on a right upper landmark of the subject, the second external detector device configured to be placed on a left upper landmark of the subject, and the third external detector device configured to be placed on a central landmark of the subject. In addition, the system can be configured to maintain a stationary frame of reference relative to the subject. 
     In another embodiment, the determined anatomical shape and size of the subject comprises an external anatomical shape and size of the subject. Moreover, the memory device can further include information defining a pre-defined anthropometric relationship between the external anatomical shape and size of the subject and the internal anatomical shape and size of the subject, further wherein the display can be configured to display the internal anatomical shape and size of the subject within the three-dimensional volume. Further, the internal anatomical shape and size of the subject within the three-dimensional volume can include internal organs within the three-dimensional volume displayed in approximate size and location within the three-dimensional volume. 
     In an additional embodiment, the system can further include a medical device configured to be placed within the subject, wherein the medical device includes an electromagnetic sensor configured to be placed within the subject. The processor can be configured to: (i) determine a distance between the electromagnetic sensor and each of the plurality of external detector devices; and (ii) cause the display device to display a position of the electromagnetic sensor in relation to the anatomical shape, size, and/or orientation of the subject within the three-dimensional volume. 
     In a further embodiment, each of the plurality of external detector devices can include a housing that is configured to be affixed to the subject. Moreover, the housing of each of the plurality of external detector devices can be configured to be adhesively affixed to the subject. 
     In yet another embodiment, each of the plurality of external detector devices can further include a wireless communication device configured to communicate wirelessly with the processor. 
     In still another embodiment, each of the plurality of external detector can be configured to communicate with the processor via a wired connection. 
     In one more embodiment, each of the plurality of external detector devices can include an electromagnetic emitter and/or an electromagnetic receiver. 
     The present invention is further directed to a method of noninvasively determining a size and shape of a subject. The method includes steps of: placing a plurality of external detector devices in a predetermined external arrangement on the external anatomy of the subject; measuring a distance between each of the plurality of external detector devices to determine a three-dimensional volume; determining an anatomical shape and size of the subject within the three-dimensional volume; and displaying the anatomical shape and size of the subject within the three-dimensional volume on a display device. 
     In one particular embodiment, the plurality of external detector devices can include a first external detector device, a second external detector device, and a third external detector device, and the step of measuring can include the first external detector device, the second external detector device, and the third external detector device interrogating each other to triangulate the three-dimensional volume. 
     Moreover, the predetermined external arrangement can include the first external detector device placed on a right upper landmark of the subject, the second external detector device placed on a left upper landmark of the subject, and the third external detector device placed on a central landmark of the subject. 
     Further, the step of displaying the anatomical shape and size of the subject can include displaying the shape of at least one internal organ in approximate size and location within the three-dimensional volume. 
     In another embodiment, the method can further include steps of: inserting a feeding tube into the subject, wherein the feeding tube includes an electromagnetic sensor in an insertion end of the feeding tube; determining a distance between the electromagnetic sensor and each of the plurality of external detector devices; and displaying a position of the electromagnetic sensor in relation to the anatomical shape and size of the subject within the three-dimensional volume on the display device. 
     In an additional embodiment, each of the plurality of external detector devices can include an electromagnetic emitter and/or an electromagnetic receiver. 
     In one more embodiment, the step of placing a plurality of external detector devices in a predetermined external arrangement on the subject includes affixing each of the plurality of external detector devices to the subject. Moreover, the external detector devices affixed to the subject can maintain a stationary frame of reference relative to the subject. 
     These and other features, aspects, and advantages of the present invention will become better understood with reference to the following description and appended claims. The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A full and enabling disclosure of the present invention, including the best mode thereof, directed to one of ordinary skill in the art, is set forth in the specification, which makes reference to the appended figures, in which: 
         FIG. 1A  illustrates a block diagram of a medical device position guidance system according to one particular embodiment of the present invention; 
         FIG. 1B  illustrates an emitter/receiver of the medical device position guidance system of  FIG. 1A ; 
         FIG. 2  illustrates a predetermined arrangement of detector devices according to one embodiment of the medical device position guidance system of  FIG. 1A ; 
         FIG. 3  illustrates anatomical landmarks of a human body; 
         FIG. 4  illustrates a perspective view of a housing of an external detector device of the medical device position guidance system of the present invention; 
         FIGS. 5A-B  illustrate bottom views of the housing of  FIG. 4 ; 
         FIG. 6  illustrates an invasive medical device of the medical device position guidance system of  FIG. 1A ; 
         FIG. 7  illustrates a block diagram of a medical device position guidance system according to an alternative embodiment of the present invention; and 
         FIG. 8  illustrates a method of determining a size and shape of a subject using the medical device position guidance system of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Reference now will be made in detail to embodiments of the invention, one or more examples of which are illustrated in the drawings. Each example is provided by way of explanation of the invention, not limitation of the invention. In fact, it will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the scope or spirit of the invention. For instance, features illustrated or described as part of one embodiment can be used with another embodiment to yield a still further embodiment. Thus, it is intended that the present invention covers such modifications and variations as come within the scope of the appended claims and their equivalents. 
     As used herein, the terms “about,” “approximately,” or “generally,” when used to modify a value, indicates that the value can be raised or lowered by 5% and remain within the disclosed embodiment. 
     Generally speaking, the present invention is directed to a medical device position guidance system. The medical device position guidance system includes a plurality of external detector devices configured to be positioned in a predetermined external arrangement on a subject. The system additionally includes a processor, and each detector device is operatively coupled to the processor. Each of the plurality of external detector devices are configured to interrogate each other to determine a distance between each of the plurality of detector devices. The system further includes a display device and a memory device. The memory device stores instructions which when executed by the processor, cause the processor to: (i) receive signals relating to the distance between each of the plurality of detector devices from each of the plurality of detector devices to determine a three-dimensional volume between the plurality of external detector devices; (ii) using the received data from the plurality of external detector devices and the three-dimensional volume to determine an anatomical shape, size, and/or orientation of the subject within the three-dimensional volume; and (iii) cause the display device to display the anatomical shape, size, and/or orientation of the subject within the three-dimensional volume. The present invention is also directed to a method of determining the size and shape of a subject using the medical device position guidance system. Because of the specific components of the medical device position guidance system, the present inventors have found that the patient&#39;s anatomical shape and size can be more accurately determined and represented for use in the insertion of an invasive medical device into the patient&#39;s body due to the stationary frame of reference of the external devices relative to the patient. Moreover, the present inventors have found that the medical device position guidance system of the present invention can reduce complications that can arise due to positional errors resulting from the movement of a patient during use of the medical device position guidance system. 
     The specific features of the medical device position guidance system of the present invention may be better understood with reference to  FIGS. 1A-7 . 
     Referring now to the drawings,  FIG. 1A  illustrates one embodiment of a medical device position guidance system  100 . The medical device position guidance system  100  includes (a) an apparatus  150  having a housing which supports a controller or processor  120  and a display device  140 ; (b) a plurality of non-invasive external detector devices  110  electronically coupled to the processor by a wire, cable, signal data connection, signal carrier or wireless connection; (c) a power source  160  coupled to the apparatus  150 ; and optionally (d) an invasive medical device  200  in communication with the plurality of external detector devices  110  and operatively coupled to the apparatus  150  by a wire, cable, cord or electrical extension, which, in turn, is operatively coupled to the processor  120 . Each of the plurality of external detector devices  110  are configured to be positioned in a distributed arrangement on a surface of a subject  10  (see  FIG. 2 ) which is a mammal, such as a human. Although the illustrated example depicts a human, it should be appreciated that medical device position guidance system  100  could be used with any mammals such as domestic animals. In general, and referring to  FIGS. 1A-B  and  4 , the plurality of noninvasive external detector devices  110  each includes a housing  112  which supports an electromagnetic field emitter/receiver  180  operably coupled to the processor  120 , where the processor  120  is coupled to a memory device  130 . According to the embodiment, the medical device position guidance system  100  is operable to provide audiovisual information about the shape, size, and orientation of a subject&#39;s anatomy through a wired or wireless connection between the plurality of external detector devices  110  and a computer  150  having a processor  120  and a display device  140 . The medical device position guidance system  100  can be further operable to provide audiovisual information about the position and orientation of the invasive medical device  200  relative to the plurality of external detector devices  110  and the patient&#39;s detected anatomy, through a wired or wireless connection between the plurality of external detector devices  110 , the invasive medical device  200 , and the computer  150  having a processor  120  and a display device  140 . 
     As illustrated in  FIGS. 1A, 2 and 4-5 , each of the external detector devices  110  includes a housing  112  surrounding an electromagnetic emitter and/or receiver  180 . The housing  112  can include an upper surface  114 , a lower surface  116 , and at least one side surface  118  extending from the upper surface to the lower surface. For example, as shown in  FIG. 4 , the upper surface  114  and the lower surface  116  can be circular or oval in shape and have a continuous side surface  118  extending therebetween, forming a generally cylindrical-shaped housing  112 . In another embodiment (not shown), the upper surface  114  and the lower surface  116  can be rectangular in shape and can have four side surfaces  118  extending therebetween corresponding to each of the sides of the rectangle. However, the external shape of the housing  112  of each external detector device  110  is of little consequence to the way in which the actual electromagnetic emitter and/or receiver  180  works. As such, the housing  112  can have any other suitable external shape based on a particular application of the medical device position guidance system  100 . 
     The housing  112  of each external detector device  110  can have a footprint (i.e., shape and size of the lower surface  116 ) that is generally comparable to standard electrocardiogram leads. For example, the housing  112  can have a diameter D extending across the widest portion of the upper surface  114  or lower surface  116  that is in a range from about 0.5 inches (1.25 cm) to about 5 inches (13 cm), or any value or range therebetween, such as from about 1 inch (2.5 cm) to about 3 inches (7.6 cm), for example from about 1.5 inches (3.8 cm) to about 2.5 inches (6.4 cm). The at least one side surface  118  of the housing  112  can have a height H in a range from about 0.25 inches (0.63 cm) to about 2 inches (5.1 cm), or any value or range therebetween, such as from 0.3 inches (0.76 cm) to about 1 inch (2.5 cm), for example about 0.5 inches (1.25 cm). In addition, each of the external detector devices  110  can be lightweight. 
     As shown in  FIGS. 4 and 5A -B, each external detector device  110  can further include a fixation mechanism  115  that is configured to affix the external detector device  110  to the subject. In a preferred embodiment, the external detector device  110  can be directly affixed to the subject&#39;s body  10  by the fixation mechanism  115  so that the external detector device  110  maintains a fixed reference point in relation to the subject  10 . Thus, when the subject  10  moves, the external detector device  110  moves with the subject  10  to maintain a static frame of reference with respect to the particular patient. The fixation mechanism  115  can be positioned on the lower surface  116  of the external detector device housing  112 . For example, the fixation mechanism  115  can include an adhesive material  117  that is configured to affix the external detector device  110  to the skin of the subject, a patch on the subject&#39;s body, or a garment worn by the subject. The adhesive material  117  can be an adhesive substrate that can be adhesive on both sides such that it adheres to the lower surface  116  of the housing  112  on one side and to a subject&#39;s body or garment on the other side. When the fixation mechanism  115  is adhesive material  117  adhered to the lower surface  118  of the housing  112 , the external detector device  110  can additionally include a peelable protective sheet  119  covering the entire adhesive material  117 . The peelable protective sheet  119  can be removed prior to affixing the adhesive  117  to the subject  10  or the subject&#39;s garment. Optionally, a used adhesive substrate  117  can be removed from the housing  112  and discarded, and a new adhesive substrate  117  can be applied. Alternatively, the adhesive material  117  can be any suitable adhesive arrangement which is capable of releasably adhering the housing  112  to the subject&#39;s skin or garment. In other embodiments, the fixation mechanism  115  can include a clip, pin, magnet, hook and loop system, or any other suitable means for affixing the external detector device  110  to a subject&#39;s body or garment. By using a fixation mechanism  115  on each external detector device  110  that can affix the external detector device  110  to the subject&#39;s body or garment, the frame of reference of each external detector device  110  can remain stationary with the subject&#39;s body. Thus, the likelihood of positional errors when using the medical device position guidance system  100  can be reduced as compared to other guidance systems because there can be fewer complications arising due to movement of the subject&#39;s body. 
     As shown in  FIG. 2 , the plurality of external detector devices  110  are configured to be positioned on the external anatomy of a subject  10  in a predetermined arrangement. The predetermined arrangement of the external detector devices  110  can be specific to a particular medical device being positioned in the subject  10 . The predetermined arrangement can include multiple predetermined external fixation points on the subject&#39;s external anatomy, where each of the predetermined external fixation points are distributed or separated from each other as shown in  FIG. 2 . The predetermined external fixation points can be based on well-known external anatomical landmarks. In some embodiments, the well-known external anatomical landmarks can be bony landmarks, as the bony landmarks can be located visually or palpated on subjects of any shape or size regardless of physical presentation of the subject, such as the presence of adipose tissue, edema, or other tissues. 
     For example, as illustrated in  FIG. 2 , when the medical device position guidance system  100  is used to determine a subject&#39;s upper anatomy such as for inserting an enteral catheter (feeding tube), three external detector devices  110  can be positioned on the subject  10 . For instance, one device  110  can be placed at a right upper landmark, such as the right midclavicular line  52 , one device  110  can be placed at a left upper landmark, such as the left midclavicular line  54 , and one device  110  can be placed at a central landmark, such as the xiphoid process  20 . As illustrated in  FIGS. 2 and 3 , the xiphoid process  20  is the cartilaginous section at the lower end of the sternum  30  which is generally positioned along the mid-sagittal line  50  and which is not attached to any ribs  32  and is gradually ossified in adult humans. The right and left midclavicular lines  52  and  54  are each imaginary lines which are generally parallel to the mid-sagittal line  50  and pass downwards over the trunk of the human body  10  through the midpoint of the right and left clavicle bones  16  and  18 , respectively. However, the midclavicular lines  52  and  54  and the xyphoid process  20  are not the only landmarks that could be used for this purpose. There may be other points of the body to which the predetermined arrangement of the plurality of external detector devices  110  could be reliably co-located or located with a predetermined offset for use in a reliable position guidance system. 
     As illustrated in  FIGS. 1A-B  and  7 , each external detector device  110  includes an electromagnetic emitter and/or receiver  180 . In one embodiment, each external detector device may include an electromagnetic emitter  182  formed through a plurality of coils  182  of wire(s) connected to a power source  160 . The power source  160  can be a wired or wireless connection to a power source  160  of the apparatus  150  or can be a battery  192  within the external detector device  110 . When the power source sends electrical current to the coils  182 , the coils  182  can transmit a signal or electromagnetic field capable of being detected by an electromagnetic receiver. Although the coils  182  are disclosed as one example of a magnetic field emitter  182 , it should be appreciated that the electromagnetic emitter  182  can include any suitable mechanism or device which generates or produces magnetic energy or a magnetic field, such as a permanent magnet, resistive magnet, or superconducting magnet. 
     As shown in  FIGS. 1A and 7 , each external detector device  110  can additionally or alternatively include an electromagnetic receiver  184  that can detect an electromagnetic field or signal generated by an electromagnetic emitter, such as the electromagnetic emitters  182  of the other external detector devices  110 . The electromagnetic receivers  184  can each include at least one receiver coil  184 , such as three receiver coils  184 , that are operable to receive an induced current and detect the induced voltage in response to a magnetic field generated by an electromagnetic field emitter  182  when the magnetic field is directed toward and reaches the receiver coil(s)  182 . It should be appreciated that the receiver coils  184  may be any suitable structures capable of receiving an induced current in response to a generated magnetic field. In some embodiments, each of the plurality of external detector devices  110  can include both an electromagnetic emitter  182  and an electromagnetic receiver  184  as part of the emitter/receiver  180 . Additionally, there can be shielding  186  within the emitter/receiver  180  between the electromagnetic emitter  182  and the electromagnetic receiver  184 . The shielding  186  can prevent signal interference between the electromagnetic emitter  182  and the electromagnetic receiver  184  within the emitter/receiver  180 . For example, the shielding  186  can be a barrier between the electromagnetic emitter  182  and the electromagnetic receiver  184  that can be made of conductive or magnetic materials. 
     In one embodiment, each external detector device  110  can be electrically connected to the apparatus  150  via a wire, cable, or other connection to receive power from the apparatus  150  and to communicate with the processor  120 . Alternatively, each external detector device  110  can have a wireless configuration including a battery  192  that provides a voltage to the electromagnetic emitter/receiver  180  and a wireless communication chip  190  configured to communicate with the processor  120 . Optionally, the wireless communication chip  190  can include a processor (not shown). The wireless communication chip  190  can be any suitable form of wireless communication capable of sending and receiving digital signals from the processor  120  of the control apparatus  150 . 
     When the plurality of external detector devices  110  are positioned in the predetermined arrangement on the subject  10  based on predetermined external landmarks, the locations of the landmarks can provide adequate separation of the external detector devices  110  on the subject to enable the electromagnetic emitters  182  and receivers  184  of each external detector device  110  to interrogate each other, i.e., for the emitters  182  to emit an electromagnetic field and for the receivers  184  detect the magnetic fields emitted by the respective emitters  182  of the other external detector devices  110 . Each external detector device  110  can send one or more signals to the processor  120  detailing the detected coil  184  voltage. Each external detector device  110  can also send one or more signals to the processor  120  detailing the drive signals used to generate the electromagnetic fields with the emitters  182 . The processor  120  can compare each of the detected coil voltages and the drive signals used to create the electromagnetic fields to assess and calculate the distance and the relative angular orientation between each of receivers  184  of the external detector devices  110  to define an electromagnetic three-dimensional volume. Using algorithms stored in the memory  130 , the processor  120  can use data collected about the electromagnetic three-dimensional volume to derive the subject&#39;s external and internal anatomical shape and size within the three-dimensional volume. 
     For example, as shown in the embodiment illustrated in  FIGS. 1A, 2 and 7 , the medical device position guidance system  100  can include three external detector devices  110  configured to triangulate and define the subject&#39;s upper external anatomy shape and size within the three-dimensional volume. This embodiment including three external detector devices  110  can be beneficial because each of the three external detector devices  110  can form one of three points in space in order to define a single plane, such as an X-Y plane. The determination of an X-Y plane can allow the determination of a distance in the Z-direction. Thus, using three external detector devices  110  can enable the determination of the three-dimensional volume. The three points defined by the three external detector devices  110  can thereby define the patient size and relative anatomical locations within the three-dimensional volume. 
     The memory  130  can store algorithms defining a generally known pre-defined anthropometric relationship between external anatomy and the internal anatomy, e.g. organs within a subject&#39;s body. The processor  120  can execute these algorithms to relate the subject&#39;s external anatomy, as detected by the external detector devices  110 , to approximate the shape and size of the internal organs associated with that external anatomy. In the embodiment illustrated in  FIG. 4 , the upper external anatomy shape and size can be used to calculate the shape and size of the lungs, esophagus and stomach. The memory  130  can further store image processing algorithms which the processor  120  can execute in order to visually render a graphical representation of the shapes of the lungs  12  and  14 , esophagus  22  and stomach  24  in approximate size and location within the three-dimensional volume and depict the rendered graphical representation of the internal anatomy to scale on a suitable monitor or display  140 , e.g. as illustrated in  FIG. 2 . Thus, the medical device position guidance system  100  can render a graphical representation of the subject&#39;s internal anatomy prior to insertion of the invasive medical device  200  to enable the accurate placement of the invasive medical device  200  in the proper location within the body. 
     The medical device position guidance system  100  can additionally include an invasive medical device  200  having an electromagnetic emitter and/or receiver  210 . For example, in the embodiment illustrated in  FIG. 6 , the invasive medical device can be a catheter  200  that is suitable for enteral nutrition having at least one lumen  208  (passageway extending from proximal to caudal/distal end) or multiple lumens. In one embodiment, the catheter  200  can include markings  205  on the outer wall  206  of the catheter  200  indicating the length of the catheter  200  that has been inserted. An electromagnetic emitter and/or receiver  212  can be located near the tip of the catheter. The electromagnetic emitter and/or receiver  212  can be a coil  212  adjacent to or embedded within the wall  206  of the catheter  200 . The electromagnetic emitter and/or receiver  212  is operatively connected to the processor  120 , for example, by at least one wire, cable, cord or electrical extension  214 , or other wireless connection (not shown). For example, the electromagnetic emitter and/or receiver  212  can connect to a wire or pair of wires  214  which run the length of the catheter  200 . In one embodiment, the pair of wires  214  can be incorporated into a metal stylet (not shown) used for inserting the catheter  200  into the subject&#39;s gastrointestinal tract. The pair of wires  214  includes at least one signal-carrying wire and can be bound together at the ends of the wires and may be encapsulated in material known to be suitable for its intended use. In a wireless embodiment, the electromagnetic emitter/receiver  212  can connect via wire(s) to a processor of the catheter assembly (not shown), which can then wirelessly receive and/or output information through any suitable wireless communication means  220 , such as receiving and/or sending information through an antenna in the form of modulated electromagnetic waves or radio waves to an antenna on the apparatus. 
     As illustrated in  FIGS. 1A, 6 and 7 , the medical device position guidance system  100  works as follows when positioning an invasive medical device  200 . When the distal end  204  of the catheter  200  is inserted into the subject  10 , e.g., inserted through the subject&#39;s nose or mouth into the gastrointestinal tract, the electromagnetic sensor  212 , such as an electromagnetic  212  in the catheter  200  can detect an electromagnetic field generated by each of the emitters  182  of the plurality of external detector devices  110  and/or generate a magnetic field to be detected by each of the receivers  184  of the plurality of external detector devices  110 . The magnetic field(s) propagate through the tissue or skin of the subject and induce a voltage in the electromagnetic receiver of the catheter coil  212  or external detector device  110 . Information about the induced voltage is then sent to the processor  120  of the apparatus  150 . Using the algorithms stored in the memory  120 , the processor  120  compares each of the detected coil voltages and the drive signals used to create the electromagnetic fields to determine the relative position and orientation of the catheter tip  210  in relation to each of the external detector devices  110  along the x, y and z axes. Using the information continuously gathered from the plurality of external detector devices  110  to determine the subject&#39;s anatomical shape and size along with the relative position and orientation of the catheter tip  210 , the relative position and direction of the catheter tip within the subject can be graphically represented on the display. Moreover, the relative position and orientation data of the catheter tip  210  can show the depth of the catheter tip  210  in the z direction within the subject  10 . 
     Using the algorithms stored in the memory  120 , the processor  120  can produce indicator data based on the signals received and processed by the processor  120 . The processor  120  can also produce indicator data representative of the position of the catheter tip  210  in the form most useful to the user of the apparatus, e.g. a clinician. One such form is a graphical representation of the catheter  200  itself in approximate size and scale relative to the subject&#39;s anatomy, e.g. as shown in  FIG. 2 . To enable the processor  120  to graphically represent the catheter  200 , the catheter  200  can send a signal to the processor  120  containing information regarding the size and shape of the catheter  200 , or the memory  130  can store information regarding the size and shape of the catheter  200 , or a user can input information regarding the size and shape of the catheter into the processor  120 . Additionally or alternatively, the processor  120  can produce an indicator image, such as an arrow symbol that is an indication of the position and direction of the coil being detected. The indicator image can be superimposed on the graphical representation of the subject&#39;s internal anatomy. For example, when the subject&#39;s upper anatomy is graphically represented with images of the lungs  12  and  14 , esophagus  22  and stomach  24  in approximate shape and size as described above and as shown in  FIGS. 2-3 , the catheter tip  210  can be represented with an indicator image, e.g. an arrow, as it is inserted in the subject  10  to view the relative position and direction of the catheter tip  210  within the internal anatomy. The indicator image can be superimposed onto relevant two-dimensional views of the subject&#39;s body  10  or a three-dimensional representation of the subject&#39;s body  10 . For example, the two-dimensional views and/or the three-dimensional representation of the subject&#39;s body  10  can be shown in-scale with the size of the subject&#39;s body  10 , such as being scaled to the actual size of the subject&#39;s body  10 . 
     The depth measurements of the catheter tip  210  are a relative measure and not an absolute, but used in the appropriate way can greatly assist trained and experienced personnel in intubating a catheter  200  or other invasive medical device into a patient. The relative depths of the catheter tip  210  are taken of particular note since the ratio of change from person to person will likely be very small. In an example of an enteral feeding catheter intubation, when the catheter tip  210  passes below the xiphoid process  20  it is very deep (e.g., about 17 cm or about 6.5 inches below the plane of the external detector devices  110 . While the catheter tip  210  passes through the stomach  24 , its tip lies closer and less deep (e.g., about 14 cm or about 5.5 inches) and when passing under the mid-sagittal line  50  it is very shallow and closest to the surface of the subject  10  (e.g., about 8 cm or about 3 inches). In the first part of the duodenum of the small intestine  26  it is relatively shallow (e.g., about 10 cm or about 4 inches) and finally becomes very deep (e.g., about 17 cm or about 6.5 inches) when in the duodenum/jejunum of the small intestine  26 . After long term clinical use, an acceptable and reliable range of depths and ratios at the points or regions described (or others) can be developed and used along with the approximate calculations of the subject&#39;s internal anatomy shape and size for assisting clinical assessment of the correctness of the route taken by the catheter tip  210 . It can be seen that, although not proof of the exact location of the caudal/distal end of the catheter tip  210 , the displayed characteristics provide yet another aid to improving clinical decision making with respect to the positioning guidance and location of a catheter. 
     It should not of course be forgotten that other clinical monitoring techniques may continue to be used thereby increasing the confidence of the clinician that the catheter is appropriately located, whether that be for enteral nutrition or other purposes. 
     The medical device position guidance system  100  can be used in a method  800  of noninvasively determining a size and shape of a subject  10 , as shown in  FIG. 8 . In step  802 , the medical device position guidance system  100  can be provided. For example, a medical device position guidance system  100  can include three external detector devices  110  as described in an embodiment above. In step  804 , the plurality of external detector devices can be placed in a predetermined external arrangement on the external anatomy of the subject. The predetermined external arrangement can include a right upper landmark, a left upper landmark and a central landmark. 
     Then, in step  806 , the emitter/receivers  180  of the external detector devices  110  can be activated to begin generating magnetic fields. For example, the processor  120  can send a drive signal to each of the emitter/receivers  180  that causes electric energy to be sent to the electromagnetic emitters  182 , thereby causing each of the electromagnetic emitters  182  to generate a magnetic field. 
     Next, in step  808 , a distance between each of the plurality of external detector devices  110  can be measured in order to determine a three-dimensional volume. Particularly, the electromagnetic emitters  182  can emit magnetic fields which induce a current in the electromagnetic receivers  184  of the respective other external detector devices  110 . Each emitter/receiver can send one or more signals to the processor  120  detailing the detected coil  184  voltage. Each external detector device  110  can also send one or more signals to the processor  120  detailing the drive signals used to generate the electromagnetic fields with the emitters  182 . The processor  120  can compares each of the detected coil voltages and the drive signals used to create the electromagnetic fields to assess and calculate the distance and the relative angular orientation between each of receivers  184  of the external detector devices  110  to define an electromagnetic three-dimensional volume. 
     In step  810 , the defined electromagnetic three-dimensional volume can be used to determine an anatomical shape and size of the subject within the three-dimensional volume. For example, using algorithms stored in the memory  130  including a known pre-defined anthropometric relationship between anatomical landmarks of the predetermined arrangement, the processor  120  can use data collected about the electromagnetic three-dimensional volume to derive the subject&#39;s external and internal anatomical shape and size within the three-dimensional volume. 
     Next, in step  812 , the anatomical shape and size of the subject within the three-dimensional volume can be displayed on a display device. For example, the anatomical shape and size of at least one internal organ can be displayed on the display device. 
     The method  800  can further include a step  814  of inserting a feeding tube  200  into the subject. The feeding tube can include an electromagnetic sensor  212  in an insertion end of the feeding tube  200 . Then, in step  816 , the system  100  can determine a distance between the electromagnetic sensor and each of the plurality of external detector devices  110 . Finally, in step  818 , the position of the electromagnetic sensor  212  in relation to the anatomical shape and size of the subject within the three-dimensional volume can be displayed on the display device  140 . For example, an indicator image can show the position and/or direction of the electromagnetic sensor  212  of the feeding tube  200  in relation to a two-dimensional or three-dimensional representation of the subject&#39;s body. The display can be shown in-scale with the patient&#39;s body. Using the indicator image generated in step  218 , a user or clinician can adjust the position of the feeding tube  200  until the user or clinician is comfortable with the location of the tip of the feeding tube  200  within the subject. 
     Although the above embodiments related to positioning an end of a catheter it should be appreciated that the medical device position guidance system is operable to assist in the placement of any medical device or invasive component into a mammal in the course of stent placement, ablation, blockage removal, heat treatment, surgical procedure, fluid delivery or any other suitable invasive procedure. It should be appreciated that any type of catheter may be used for any of the medical procedures described above. It should also be appreciated that any suitable invasive medical device can be used in place of a catheter. 
     This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they include structural elements that do not differ from the literal language of the claims or if they include equivalent structural elements with insubstantial differences from the literal language of the claims.