Abstract:
In an endoscope, the tip which contains passages for viewing and light guiding is made of plastics material, for MR compatibility purposes, and an r.f. receive coil of the endoscope is provided with a fiducial having its own r.f. receive coil, to make it possible to track movement of the r.f. receive coil.

Description:
BACKGROUND 
     This invention relates to endoscopes suitable for magnetic resonance (MR) imaging. 
     Endoscopes are medical instruments suitable for insertion into a body cavity, and may include a viewing system, a light guide, an opening for injecting air or water, an opening to which a vacuum can be applied, and an opening for tools etc. 
     Endoscopes are usually in the form of flexible tubes which have axial passages for cables with which the tip of the endoscope may be steered, as well as for the services referred to in the preceding paragraph. In one construction, a series of outwardly-dished disks are contained in a sheath (EP-A-0 165 718). 
     While endoscopes produce visible images of the interior of the cavities under investigation, medical examination often requires an image of the tissue behind the tip of the endoscope. For this reason, it has been proposed to provide endoscopes with MR receive coils, in order to acquire MR signals with reasonably high signal-to-noise ratio from the tissue in the immediate vicinity of the endoscope (EP-A-0 850 595). 
     Such endoscopes are not at the present time commercially available, because of two principal difficulties in implementing such devices. 
     The first difficulty is that a conventional endoscope typically includes metals e.g. stainless steel for the steering cables, a metallic flat wire spiral for the sheath. Such materials would distort the powerful main magnetic field which underlies MR imaging, and hence the resulting MR image produced would be distorted. Secondly, currents, detrimental to the patient, would be induced in such materials when the r.f. excitation pulse, used to excite the resonance, was applied. 
     The second difficulty is that such an endoscope may not be fixed in position when the MR image is being built up. For example, the endoscope could be within a stomach cavity, and involuntary muscular movement of the walls could cause the endoscope to move around. An MR image requires collection of information over a period of time. For example, in order to spatially encode a two-dimensional slice, a series of r.f. excitations takes place, after each of which a phase-encode magnetic field gradient of a different magnitude is applied before a read-out pulse in the presence of an orthogonal magnetic field gradient, takes place. Any movement of the r.f. receive coil during the data collection would cause artifacts to appear in the MR image, which is calculated on the assumption that the r.f. receive coil remains fixed in position relative to the tissue during the building up of the MR image from the various pulses corresponding to the respective phase-encode gradients. 
     SUMMARY 
     The invention provides an endoscope which includes an r.f. receive coil, wherein the tip of the endoscope which contains passages for viewing means and light guide means is made of plastics material, and wherein a bendable portion of the endoscope connected to the tip includes disks of plastics material inside a sleeve of non-metallic material, the disks having openings through which steering cables of non-metallic material pass. 
     Such an endoscope is MR compatible, in the sense that the materials do not interfere with the main magnetic field, nor create current inducing loops. 
     The tip of the endoscope may be made from polyether etherketone (PEEK). The sleeve and/or the steering cable may be made from polyethylene (for example Dyneema, made by DSM High Performance Fibers B.V., Holland, a high performance gel spun polyethylene fiber), glass, carbon, nylon (a family of polyamide polymers) or aramid (a class of aromatic polyamide fibres, such as Kevlar or Twaron). 
     The bendable portion of the endoscope is connected to a flexible portion, for insertion into the patient. This insertion portion may be connected by a non-metallic umbilical, of at least 2½ metres, preferably at least 4 metres in length, to a services cabinet. 
     The invention also provides an endoscope which includes an r.f. receive coil, and a fiducial fixed relative to the r.f. receive coil which is provided with its own r.f. receive coil. 
     It then becomes possible to track the movement of the r.f. receive coil so that movement of the r.f. receive coil can be compensated for when building up the MR image. 
    
    
     DRAWINGS 
     One way of carrying out the invention will now be described in greater detail, by way of example, with reference to the accompanying drawings, in which: 
     FIG. 1 is a schematic side view of an endoscope in accordance with the invention; 
     FIG. 2 is an axial section of the front part of the endoscope of FIG. 1; 
     FIG. 3 is an axial sectional view of a part of the central region of the endoscope shown in FIG. 1; 
     FIG. 4 is a side view of the tip of the endoscope; 
     FIG. 5 is an end view of the tip taken through the section  5 , 5  of FIG. 4; 
     FIG. 6 is a side view of a central anchor region of the endoscope; 
     FIG. 7 is a sectional view taken along the lines  7 , 7  on FIG. 6; 
     FIG. 8 is a sectional view of a light guide opening into the end of the tip; 
     FIG. 9 is a sectional view of an air/water inlet which opens at the end of the tip; 
     FIG. 10 is an axial section of a viewing window which opens at the end of the tip; 
     FIG. 11 is a sectional view of a cavity for an MR pick-up coil which opens at the end of the tip; 
     FIG. 12 shows the former  15  of FIG. 11 bearing the pick-up coil and fiducial coil; and 
     FIG. 13 is a top view along the line A—A′ in FIG.  12 . 
    
    
     DESCRIPTION 
     The endoscope consists of a main insertion tube, a proximal body portion (not shown), an umbilical (not shown), and a connector to a services cabinet (not shown). 
     The umbilical is constructed of non-metallic materials like the insertion tube, and is 5 metres long, in order that the services cabinet, which will generally be metallic, can be kept well away from the magnetic resonance imaging apparatus, to which the receive coil of the endoscope is connected. 
     The proximal body portion is made of polyurethane mouldings, and is rigid plastics material, and contains the control knob used to steer and control the endoscope. 
     The insertion tube consists of a distal tip  1 , a steerable bendable section  2 , and a main bendable but not steerable insertion tube  4 . The section  2  and the section  4  are connected by a spiral anchor  3 . 
     The distal tip is made of plastics material, in fact, polyether etherketone (PEEK), in contrast to conventional endoscopes in which the tip is made of stainless steel. The steerable section comprises a number of dished-disks  5  which are apertured at  6  to receive steering cables  7  (only one of which is shown in FIG. 2 although four or five are provided. The dished-disks could be similar to those shown in FIGS. 5A and 5B of EP-A-0 165 718. The disks are made of plastics material, for example, of PEEK although nylon or acetal could be used. The steering cable may be of nonmetallic material, in place of the conventional stainless steel. The steering cable is contained within a nickel alloy sheath for example, Cromaloy. The spiral anchor  3  is also made of plastics material such as PEEK, and the bendable main insertion tube  4  is also composed of plastics disks having openings through which the steering cable passes. These plastics disks are made of acetal, such as DELRIN, but nylon or PEEK could be used. 
     The spiral anchor is the junction piece between the steerable section  2  and the non-steerable flexible section  4 . It is used to anchor the outer casings of the steering cables, which thus act as Bowden cables. The casings of the steering cables are spirally wound to form the casings, and are made of Cromaloy. The cables themselves are anchored to the distal tip  1 . Manipulation of controls on the proximal body portion cause the steerable portion  2  to steer in the desired way. 
     The steering cable may be made of polyethylene, such as Dyneema, a high performance gel spun polyethylene fiber, but could instead be made of glass, carbon, nylon or aramid. 
     The disks are contained in two sleeves, an inner sleeve of non-metallic braid for example Aramid to give it torsional rigidity, in place of the stainless steel or copper braid which has hitherto been used to give the insertion tube its torsional rigidity. The endoscope has an outer sleeve of non-metallic material such as EPDM (Ethylene propylene Diene Modified) over the steerable section  2  and an outer sleeve of flexible polyurethane over the insertion tube portion  4 . The inner sleeves could instead consist of fabric made of glass, carbon or nylon or polyethylene such as Dyneema. 
     Referring to FIGS. 4 and 5, the distal tip  1  of the endoscope has apertures  8   a  to  8   d  at its inner end to receive the service modules shown in FIGS. 8 to  11 , and these communicate with openings (not shown) in the distal end of the distal tip. 
     The light guide shown in FIG. 8 comprises lenses  9   a ,  9   b  to spread light sent along glass fibres  10  which are clamped in ferrule  11  of plastics material such as PEEK. The air/water channel shown in FIG. 9 provides a jet for cleaning the lens shown in FIG.  8  and the possibility of air to inflate a body cavity into which the endoscope is inserted. The hollow end  12  of the module shown in FIG. 9 is made of plastics material such as PEEK, connected to a tube  12   a  of EVA (ethylene-vinyl acetate copolymer). 
     The viewer shown in FIG. 10 comprises lenses and spacers  13  potted in Araldite within a ferrule of silicon brass in a channel of the distal tip. 
     Referring to FIG. 11, the r.f. pick-up coil accommodated in this cavity  14  in the distal end of the distal tip is not shown, but the former  15  on which the r.f. pick-up coil is mounted is shown schematically. 
     The r.f. pick-up coil on its former  15  is inserted into the cavity  14  and sealed in position by means of O-rings. The r.f. pick-up coil mounted on the former inductively couples to an r.f. receive coil  19  (FIG. 2) which surrounds the outer periphery of the end of the distal tip  1 , in the manner shown in EP-A-0 850 595. 
     Thus, r.f. signals detected by the r.f. receive coil surrounding the distal tip  1  induce signals in the r.f. pick-up coil mounted on former  15  and these signals are then transmitted via silicon brass pins  20  araldited into PEEK, along wires  18  back along the length of the endoscope which is then connected to one channel of magnetic resonance imaging apparatus. 
     In accordance with another aspect of the invention (FIGS. 11 to  13 ), a fiducial  17  including an MR visible sample is mounted on the former  15 , and the fiducial  17  is surrounded by an r.f. receive coil  16  which is also connected to the MR imaging apparatus via wires  21  extending along the service channel, but in this case a separate channel of the magnetic resonance imaging apparatus. 
     The fiducial coil  16  is mounted with its axis transverse to the long axis of the endoscope. It is thus orthogonal to the pick-up coils  15   a , and orthogonal also to the receive coil  19 , so that there is the minimum of coupling between the coil  16  on the one hand and the coils  15   a  and  19  on the other hand. 
     The assembly shown in FIGS. 12 and 13 is inserted into the cavity  14  from the open end with the cables  18 ,  21  on the side of the former first to enter the cavity. Other sitings of the fiducial and its coil within the cavity  14  are of course possible. 
     The r.f. pulse sequence of the magnetic resonance imaging apparatus is such that each r.f. excitation pulse for exciting the tissue to be imaged may be accompanied by one or more further pulses of small flip angle which do not therefore excite the tissue to any great extent but which do excite the magnetic resonant active fiducial  17 . Thus, when the r.f. receive coil surrounding the distal tip collects an echo signal, a signal is also collected by the receive coil  16 . Alternatively, the fiducial may be excited as described in UK Patent Application GB 2343251A (title: MRI gradient field probe; inventors: M. Burl and J. R. Young,) or may use the main excitation and/or similar pulses, instead of having a separate excitation pulse. 
     In fact, magnetic field gradients are applied in such a way as to spatially locate in the co-ordinate system of the MR imaging apparatus the fiducial, for each r.f. pulse applied to excite the tissue to be imaged. Three gradients may be applied one at a time, to locate the fiducial, as described by Dumoulin et al Magn Resonance Med 1993 29 411 if all these dimensions of its location have to be measured. In other instances, one dimension only may be needed, and the same r.f. pulse as is used to excite the tissue to be imaged can be used for the fiducial also. 
     Thus, in the processing of the r.f. information received concerning the tissue, a correction can be built in to reflect the movement undergone by the r.f. receive coil  19  as monitored by the fiducial  17 . 
     This then reduces or eliminates motion artifacts concerned with movement of the endoscope during data collection. 
     The tissue to be imaged may be excited by a coil surrounding the body of the person into whom the endoscope is being inserted, but the r.f. pick-up coil mounted on the former  15  could also operate to cause the r.f. coil  19  surrounding the distal tip to operate in a transmit sense as well as in a receive sense. 
     Equally, while the coil  16  surrounding the fiducial  17  could be receive only, it could if desired be transmit and receive. 
     The endoscope may be a colonoscope, laparascope, cystoscope, or gastroscope, or other type of endoscope. 
     The invention has been described with reference to the preferred embodiment. Obviously, modifications and alterations will occur to others upon reading and understanding the preceding description. It is intended that the invention be construed as including all such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof.