Abstract:
An invasive probe for mapping the walls of a lumen employs a real-time tracking means and a wall distance measurement means. As the probe is advanced within the lumen, the real-time tracking means provides three-dimensional coordinates of the probe&#39;s position and orientation. Concurrent with probe localization, the distance between the probe and the lumen walls is measured. Both the probe position and the wall distance measurement are sent to a data acquisition system which in turn provides a graphic or numeric display to the operator. Probe tracking can be performed with radio-frequency, magnetic resonance, ultrasonic techniques or the like. If desired, lumen wall distance measurements can be performed with magnetic resonance or ultrasound methods. Lumen wall distance measurements can also be performed with mechanical devices such as balloons and/or expanding structures.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to medical imaging systems such as magnetic resonance, X-ray and ultrasound scanners, and more particularly to imaging systems designed to provide images of lumens within the body. 
     2. Discussion of Prior Art 
     Several methods are currently available for the in-vivo imaging of vessels and other lumens within the body. These include X-ray angiography, MR angiography and ultrasonic imaging. When X-ray procedures are employed to image lumens within the body, an X-ray opaque substance must be introduced into the lumen. For X-ray angiography, an iodinated contrast agent is typically injected into the bloodstream. For imaging the colon, on the other hand, a solution containing a Barium salt is frequently introduced into the patient. These contrast agents permit the visualization of the shape of the lumen by providing visual contrast between the inside of the lumen (which absorbs the X-rays) and the surrounding tissue (which is transparent to X-rays). Undesirable aspects of X-ray methods include the use of ionizing radiation, the use of toxic contrast agents and the difficulty of acquiring three-dimensional information without using a Computed Axial Tomography (CAT) scanner. 
     Magnetic resonance (MR) can also be used to make images of lumens within the body. Image contrast can be based upon velocity-induced phase shifts (as in phase-contrast MR angiography) or upon differences in T 1  caused by the injection of a T 1  relaxation agent. While MR imaging has the potential to discriminate between different types of lesions in a lumen wall and in many situations can be used to make diagnostic quality angiograms, the inherent low signal-to-noise ratio of MR imaging limits its spatial resolution. 
     In some parts of the body, ultrasonic imaging can be used to determine the shape of a lumen with a greater resolution that that available with magnetic resonance. Ultrasonic imaging can be acquired from outside the body using a hand-held probe applied next to the skin, or from inside the body using an ultrasonic imaging catheter. In both forms of ultrasonic imaging, the probe position and orientation are manipulated by the operator to maximize image quality and utility. Unfortunately, the exact position and orientation of the probe is not easily incorporated into the ultrasound image, since images are typically obtained without reference to an external or anatomical landmark. 
     Operator dependency is particularly severe when a vascular ultrasound probe is used. In these procedures, the probe is not held by the operator. Instead, the probe is placed at the end of a catheter which is inserted into a blood vessel. The catheter is manipulated by the operator, but must be followed with an X-ray fluoroscope to insure proper placement and orientation. The ultrasonic images collected by the probe is typically a cross-section of the vessel, but since the orientation of the probe can only be known with the X-ray fluoroscopic image, the ultrasonic image by itself cannot be used to provide information regarding the larger features of the vessel. 
     One alternative to using an X-ray image to locate an ultrasound catheter is to monitor the insertion depth of the catheter. This approach permits the reconstruction of data along the length of the vessel. Since no information is obtained about the orientation of the catheter within the vessel, however, the vessel can not be properly reconstructed into an image which shows the vessel&#39;s curvature and morphology. Reconstruction of ultrasonic images into larger data sets in which insertion depth is exclusively used to provide spatial information will be inherently and irreversibly distorted, particularly in regions of vessel curvature. 
     Several methods exist to follow the location of an invasive device within the body. These methods include MR tracking as disclosed in “Tracking System and Pulse Sequences to Monitor the Position of a Device Using Magnetic Resonance”, C. L. Dumoulin, S. P. Souza and R. D. Darrow (U.S. Pat. No. 5,307,808) and radio frequency tracking as disclosed in “Tracking System to Follow the Position and Orientation of a Device Using Radio-Frequency Fields”, C. L. Dumoulin, J. F. Schenck, and P. B. Roemer (U.S. Pat. No. 5,377,678). While these methods provide an instantaneous measurement of device location, they are not able to provide information about the diameter of a lumen. 
     What is needed is a means for acquiring high resolution images of luminal features such as location and wall composition within the body. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The features of the invention believed to be novel are set forth with particularity in the appended claims. The invention itself, however, both as to organization and method of operation, together with further objects and advantages thereof, may be best understood by reference to the following description taken in conjunction with the accompanying drawing in which: 
     FIG. 1 is a simplified block diagram of an MR tracking system used to follow an invasive device in real-time (Prior Art). 
     FIG. 2 is a simplified block diagram of an RF tracking system used to follow an invasive device in real-time (Prior Art). 
     FIG. 3 is a simplified block diagram of a luminal mapping system according to the present invention for tracking a luminal probe in three dimensions, determining radial distances between the lumen wall and the probe, and creating a 3D image map. 
     FIG. 4 is a simplified block diagram of a luminal mapping system according to the present invention in which ultrasound is used to determine the distance to the lumen wall. 
     FIG. 5 is a simplified block diagram of a luminal mapping system according to the present invention in which an MR coil is used to determine the distance to the lumen wall. 
     FIG. 6 is a simplified block diagram of a luminal mapping system according to the present invention in which a mechanical spiral coil is used to determine the distance to the lumen wall. 
     FIG. 7 is a simplified block diagram of a luminal mapping system according to the present invention in which an inflatable balloon is used to determine the distance to the lumen wall. 
    
    
     SUMMARY OF THE INVENTION 
     A system for acquisition of 3D images of a lumen of a subject makes use of a distance determination means incorporated into an insertion end of an invasive device. The distance determination device may be an ultrasonic beam which rotates to sense distance to lumen surface radially around the catheter, a mechanical expanding spring, or an inflatable balloon. 
     At least one device locating means is embedded in the insertion end of the invasive device. 
     A tracking means, being either radio-frequency (RF), or magnetic resonance (MR) tracking equipment, determines the instantaneous 3D location of the device locating means. 
     A signal interpretation device is coupled to the distance determination means and converts signals from the distance determination means into a measurement of the diameter of the lumen at its current 3D location. 
     The diameters and associated 3D locations are stored in a storage device until the lumen is measured over a desired area. 
     A 3D map of the lumen is created by the signal interpretation device by displaying the 3D locations and their associated diameters on the display device. 
     An operator may interact with the system to select the surfaces displayed, the viewpoint, color coding, and other display parameters. 
     OBJECTS OF THE INVENTION 
     It is an object of the present invention to provide a 3D map of the dimensions of a lumen of a subject. 
     It is another object of the present invention to diagnose luminal dysfunction employing a 3D luminal map of a subject. 
     DETAILED DESCRIPTION OF THE INVENTION 
     Systems for creating a 3D map of cavities within a subject  1  according to the present invention are shown in FIGS. 1, and  2 . These track the real-time location of an invasive device  320 , such as a catheter within subject  1 . 
     An operator  3 , typically a Physician, inserts an invasive device  320  into a lumen of subject  1 . Invasive device  320  has an element which is tracked by a tracking means. For magnetic resonance (MR) tracking, the tracked device maybe an MR coil, or a plurality of MR coils. These may be either receive or transmit coils. The tracked device may also be a quantity of a material which is imaged well in an MR image, such as copper sulfate solution. 
     The tracking means, as shown in FIG. 1, includes a magnet assembly  101  having RF and gradient coils, system electronics  340  and a display  380 . The acquired MR signal is processed by the MR electronics  340  which interpret the signal into a location or plurality of locations which are tracked. 
     With RF tracking, as shown in FIG. 2, the tracked device may be an RF coil, or a plurality of RF coils attached to the invasive device  320 . An external coil  201  operates to transmit an RF signal which is received by the RF coils attached to the invasive device  320 . System electronics  350  interpret the signals to determine the location and orientation of invasive device  320  in real time. The location of invasive device  320  is displayed on a display  380 . 
     In the alternative, external coil  201  may be a receive coil and the RF coils attached to invasive device  320  may be transmit coils. 
     In FIG. 3, a system  300  for the acquisition of a luminal image of a lumen  310  of subject  1  is shown. Lumen  310  may be a vessel, intestine, esophagus, stomach, or other cavity within the subject to be imaged. Lumen  310  may also include other body cavities, such as the abdominal cavity which are only accessible through an incision. Invasive device  320 , incorporating a distance determination means  330 , is inserted into lumen  310  and is used for determining a distance from the invasive device  320  to the inside wall of the lumen  310 . The invasive device  320  is tracked by a device tracking means  360  which may be magnetic resonance (MR) tracking, or radio frequency (RF) tracking, ultrasonic tracking or other conventional tracking technology. The invasive device may be moved further in or retracted from luminal cavity  310  and therefore its displacement along the luminal cavity is measured as D. 
     A signal propagation means  340  connects the means for distance determination  330  to external equipment such as a signal interpretation device  351 . Signal propagation means  340  may be, depending on the type of transducer used, an electric cable, or a fiber optic line. 
     Signal interpretation device  351  converts the signal from distance measurement means  330  into an actual distance measurement. The device tracking means  360  also provides the current location of invasive device  320  and therefore the location of distance determining means  330  to signal interpretation device  351 . 
     As this information is collected it is stored in a storage device  355  for later reconstruction of a representation of the lumen wall. An operator may move invasive device inward or outward in order to attain information on different locations D within lumen  310 . Distance measurement means  330  is designed to collect distance information shown as R for different angular displacements θ with reference to distance measurement device  330 . Preferably, distance measurements R for different values of θ are made within a short time period of each other such that distance measurement device moves very little between measurements to provide accurate measurements. 
     Operator  3  may then interact with a user interface  365  attached to signal interpretation device  351  in order to request that signal interpretation device  351  display the stored representation of lumen  310  on a display device  380 . 
     FIG. 4 shows an ultrasonic embodiment  400  of invasive device  320  of FIG. 3 in a more detailed diagram. The embodiment of FIG. 4 employs a piezo-electric ultrasound transducer  420  to produce an ultrasound beam B. This ultrasound beam B reflects off of a planar acoustic mirror  430  angled such that the beam passes outside of the catheter and intersects the lumen wall  310 . In this embodiment housing of invasive device  320  is comprised of an acousto-transparent material thereby allowing the ultrasonic beam to pass through it with little attenuation. In an alternative embodiment the acoustic material may be made in a circular window passing around the perimeter of the invasive device  320 . The planar acoustic mirror  430  is attached to a flexible rotating shaft  440 . The rotating shaft turns and consequently turns the planar acoustic mirror  430  causing the ultrasound beam to be reflected at different angular variations θ around the perimeter of invasive device  320 . 
     Ultrasonic beam B reflected from lumen wall  310  again reflects off of acoustic mirror  430  and is received by piezo-electric ultrasound transducer  420 . 
     The signal from transducer  420  is passed back through the invasive device to signal interpretation device  351  determining the instantaneous distance R at a plurality of angular variations θ around invasive device  320 . 
     A first device locating means  461  and a second device locating means  463  are attached to invasive device  320  at known locations relative to planar acoustic mirror  430 . These device locating means are related to the device tracking means  360  such that they are targets which are tracked by device tracking means  360 . For example, these may be small RF coils which may tracked by an MR tracking device  360 . In still another alternative embodiment,  461 ,  463  may be RF coils which are tracked by an RF tracking means. The MR tracking means and RF tracking means are known in the art. 
     In FIG. 5, an MR luminal probe  500  is shown. In this embodiment an MR receive coil  520  is shown connected to a coaxial cable  540  which propagates signals detected by the MR receive coil to outside equipment. 
     MR receive coil  520  may or may not be tuned to the Larmor frequency of tissue of subject  1  desired to be imaged. A tuned coil provides a more sensitive receptor to MR signals, however, and it may be desirable to incorporate tuning capacitors  521 ,  523  and  525 . This may make MR receive coil  520  larger and more bulky. 
     If desired, a matching capacitor  522  can be used to match the receive coil. Again, a first device locating means  561  and a second device locating means  563  are used to track the location of the invasive device  320 . Only one of the devices is required to determine location, however, two or more device also provide an orientation of the invasive device. 
     Receive coil  520  receives MR signals from lumen  310  and can provide a localized image of the lumen using MR imaging sequences. These images, combined with the information D of the depth within the lumen the invasive device may be used to provide a three-dimensional image of the inside of lumen  310 . 
     In FIG. 6, a mechanical luminal probe  700  is shown which measures lumen diameter. Invasive device  320  employs a rotating shaft  720  which runs the length of the invasive device and connects to a spiral spring  730 . Spiral spring is rolled into a small diameter such that it may move freely through lumen  310 . At a selected position within the lumen rotating shaft  720  is rotated to allow spiral spring  730  to expand until a difference in torsional force is sensed. Rotating shaft  720  may be rotated manually by an operator or it may be rotated by a motor  770  designed to sense a difference in torsional force. An elastic sock  740  such as a latex balloon may be used to cover spiral spring  730  and rotating shaft  720  to ease maneuvering through lumen  310 . 
     Again, at least one device locating means  761  is employed to determine the location within the lumen of the luminal probe. 
     In FIG. 7, an inflatable luminal probe  800  is shown in which an inflatable balloon  820  expands when a metered amount of fluid is pumped into balloon  820  through pipe  830  which is connected to a metering pump  850 . The fluid may be water, water solutions, air, or other gasses. 
     Metering pump  850  keeps track of the volume of fluid which is pumped into inflatable balloon  820 . This information is passed then to signal interpretation device which has been pre-calibrated to determine a diameter based upon the volume of fluid pumped into balloon  820 . 
     Since inflatable luminal mapping probe  800  completely blocks the lumen, there may be a need for a bypass such as when it is employed in vessels. In this case, an inlet  860  is employed which passes a biological fluid, such as blood, through the length of the probe housing and out through an outlet  870 . This inlet and outlet bypass may be employed on any of the other luminal mapping probe embodiments above which substantially block the lumen and require a bypass of a fluid. 
     Again, a device locating means  861  is tracked by tracking means  360 . 
     While several presently preferred embodiments of the novel invention have been described in detail herein, many modifications and variations will now become apparent to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and variations as fall within the true spirit of the invention.