referring now to the drawings , and more particularly to fig1 to 6 , there is shown an embodiment of a pressure guidewire 10 . the general design for the pressure guidewire shown in fig1 herein , although it is made of fewer parts than prior art pressure guidewires , it is also made of same four sections , namely , the proximal section 12 , the middle section 14 , the sensor housing section 15 and the tip section 16 . although , as it will become clear hereinafter , the first three sections 12 , 14 and 15 are to some extent made of the continuity of the same shaft tube 18 , the above sections of shaft tube 18 may also be called herein after as proximal section 12 , middle section 14 and sensor housing section 15 . according to an embodiment , the proximal section 12 is made of a stainless steel hypotube , with an od of about 0 . 014 ″ and by way of non limiting example with an id of about 0 . 009 ″. the proximal section is used to push other more distal sections of the pressure guidewire within the vasculature . the proximal section resides within the guiding catheter at one end , with the other end exiting the patient through the introducer ( not shown ), therefore allowing the physician to remotely control the pressure guidewire within the blood vessel , such as pushing and torquing the wire . according to an embodiment , the length of the sensor housing 15 is in a range between 1 mm to 3 . 5 mm . according to an embodiment , the length of the sensor housing 15 is 2 . 5 mm . the middle section is the one that faces the most challenging trade - offs . the middle section 14 must not damage the vessel and hence it must be fairly flexible . it must however transmit the torque for better navigability , be stiff enough to deliver a good pushability and provide a good support for an angioplasty balloon . the middle section can be made by extending the proximal section 12 further , but it would be too stiff to navigate within the vessels . on the other hand , the middle section 14 can be softened by cutting the tube as a spiral or other cutting patterns as known by those skilled in the art . those skilled in the art also know that such cut patterns are achieved using laser , etching and other processes . spiral cutting the tube would result in a very soft section whether the pitch is high or low , and would not deliver any of the required tensile force and torque response . other non continuous cut patterns can provide adequate tensile force , but the stiffness is controlled by adjusting the pitch and cut pattern . it is however difficult to provide a smooth and continuous variation of stiffness . there is also a critical safety challenges when torquing such a cut patterned guidewire as the torque induced stresses are concentrated in narrow cut regions of the guidewire , hence potentially provoking guidewire failure . using the device described hereinafter it is possible to control safely the mechanical performance of the middle section 14 by way of a ) extending the proximal section 12 further ; b ) cut this extended section to soften it ; and c ) combining this cut section with an additional inner hypotube 20 overlapping the whole cut region of middle section 14 , the inner hypotube geometry being chosen such that the desired mechanical characteristics are achieved . the middle section 14 is the continuity of the proximal section 12 , where the portion corresponding to the 27 cm middle section 14 of the shaft tube 18 is cut , such as a spiral cut shaft tubing 22 according to one embodiment . the spiral cut section of the shaft tubing 22 does not provide any significant tensile strength nor does it provide significant bending or torque strength . the inner hypotube 20 is inserted within the shaft tube 18 such that it overlaps the middle section 14 , where the tubing is cut . the outside diameter ( od ) of the inner hypotube 20 fits the inside diameter ( id ) of shaft tube 18 and can be of 0 . 009 ″ in an embodiment . the id of the inner hypotube 20 must accommodates the sensor lead wire , or communication means ( not shown in fig1 ), as is the case of the id of the shaft tube 18 . sensor communication means can be made fairly small , especially if optical fiber is used , so the inner hypotube 20 can provide the desired mechanical properties . by way of another non - limiting example , inner hypotube id can be of 0 . 005 ″. one problem if such an inner hypotube 20 was used alone is that it may not provide adequate resistance against kinking . in the absence of the spiral cut shaft tubing overlay , or other cut pattern , a more conventional design would involve the addition of an elastomer coating to bring the outer diameter of the inner tube to a similar diameter as that of the shaft tube 18 ( i . e ., the stainless steel hypotube ). however , this would not improve the resistance against kinking , which is a safety consideration . another safety issue is the risk of leaving parts inside the patient as a result of joint failure between inner tube and proximal tube or sensor housing . in case of a joint failure in more conventional designs , such as the one proposed in u . s . pat . no . 5 , 085 , 223 and patent appl . no . us2010 / 0318000 , the risk of leaving the distal parts of the guidewire within the blood vessel in case of joint failure is quit important . the proposed design mitigates this risk by providing a spiral cut pattern external tube that covers an inner hypotube , the inner tube providing most of the mechanical characteristics of the middle section . in case of joint failure resulting from applying a pulling force too strongly , the spiral cut tube will collapse over and grip the inner hypotube 20 , thereby bringing along the distal portion of the guidewire . the spiral cut shaft tubing 22 , when provided with an inner hypotube 20 , delivers the desired mechanical characteristics . stiffness ( flexibility ), torque transfer , pushability and support are provided by the inner hypotube 20 . inner hypotube 20 dimensions are easily adapted to provide optimal mechanical performance . on the other hand , kink resistance , distal parts safety retainer , and guidewire outside diameter continuity are provided by the spiral cut shaft tubing 22 . it is worth mentioning that kink resistance provided by the spiral cut section is useful mostly in cases where the inner hypotube is not nitinol , e . g ., it is useful when inner hypotube is stainless steel . as shown by fig4 , it is also desirable to shape the outside diameter of the inner tube , especially over the last 1 to 3 cm , in order to further improve trackability . in this case , the external diameter of the distal portion of the inner hypotube 20 can be slightly grinded to taper its outer diameter 40 and hence , optimizing mechanical response . the very end 41 of the inner hypotube 20 , the portion that fits within the sensor housing section , should be enlarged , e . g ., to the same diameter as its proximal section , so as to assure a good joint with the internal wall of the sensor housing section 15 ( see fig1 , 5 and 6 ). the very end 41 of the inner hypotube being enlarged assures a better joint for at least two reasons . one is that the gap between the inner hypotube and the internal surface of the sensor housing is minimized , which in turns optimizes the shear strength of the adhesive or soldering in between the sensor housing internal diameter and outside diameter of the very end 41 of inner hypotube . another important reason is that an enlarged portion will have a much lower internal stress . when using nitinol , the risk of reaching a stress level corresponding to the first plateau of nitinol stress to strain curve as known by those skilled in the art , is much lower . when such first plateau is reached , the strain induced within a nitinol tubing is increasing dramatically , which may provoke a delamination of the adhesive or soldering , which in turn may cause the failure of the joint . one method of joining the inner hypotube 20 to the ends of middle section 14 of shaft tube 18 is shown in fig2 . a window 32 is provided through the wall of the shaft tube 18 , where the inner hypotube 20 is to be welded . the inner hypotube 20 is slid past the window and welded to the middle section 14 using a laser beam or other suitable welding or joining means . the laser beam melts and joins together the edges of the window 32 to the inner hypotube 20 and therefore , secures the two parts together . it is also possible to use the same window 32 to apply adhesive at the joint between the inner and outer tubes . it is also possible to weld the tubes together without such window by heating the outer surface of the outer tube such that it welds to inner tube . such heat can be generated using a laser , an electron beam or another heat source . similar methods apply for joining the inner hypotube 20 to the other end of middle section 14 , where another window 26 ( fig3 ) is provided at the distal end of middle section 14 of shaft tube 18 . according to an embodiment , the length of windows 32 and 26 is in a range between 0 . 2 mm to 0 . 5 mm . according to an embodiment , the length of windows 32 and 26 is 0 . 3 mm . both ends of inner hypotube 20 extend a little further past the position of windows 26 and 32 . without the presence of an inner hypotube 20 within the cut middle section 14 , blood may leak inside the shaft tube 18 , which constitutes a fairly large volume , which in turn may cause biocompatibility issues . the presence of the inner hypotube 20 seals off the inside of the shaft tube 18 where it is cut . the sensor housing section 15 is made of the very last 2 to 3 mm of the shaft tube 18 . the spiral cut , or other cut patterns , stops some 2 to 3 mm before the end of the shaft tube 18 , where the sensor 34 ( see fig3 ) is to be fitted . a sensor joining window 28 is provided to bond or to fix the sensor 34 within the sensor housing section 15 . blood pressure is applied to sensor 34 via opening 30 . in the present description , sensor 34 comprises a pressure sensor . according to an embodiment , the length of the opening 30 is in a range between 0 . 2 mm to 0 . 5 mm . according to an embodiment , the length of the opening 30 is 0 . 3 mm . according to an embodiment , the diameter of the sensor joining window 28 is in a range between 0 . 1 mm to 0 . 3 mm . according to an embodiment , the diameter of the sensor joining window 28 is 0 . 2 mm . according to an embodiment , the pressure sensor comprises an optical pressure sensor . a radiopaque marker band 36 may be located in the sensor housing section 15 to help in localizing the pressure guidewire in the vessels . the marker band , or otherwise similar non radiopaque band , main purpose is however to allow easy assembly of the sensor within the sensor housing . it is indeed preferable to avoid the application of hard adhesive directly on sensor head . the marker band 36 can be bonded to the pressure sensor communication means ( e . g ., optical fiber ) as a pre - assembly in an environment allowing the control of adhesive flow . the sensor 34 is secured within pressure guidewire by bonding the marker band 36 , or other non radiopaque band , to the sensor housing section 15 using sensor joining window 28 . a tip section 16 ( fig5 ) is provided at the end of the sensor housing section 15 . a conventional atraumatic tip section 16 shown in fig3 can be used as there is no sensor lead wire to be passed therethrough . having similar materials for shaft tube 18 and inner hypotube 20 , such as stainless steel , makes the above welding process fairly simple and reliable . it may however be desirable to use dissimilar materials that cannot easily be welded together . for example , it is desirable to use shaft tube 18 made of stainless steel for providing a good pushability , while it is also desirable to use nitinol for the inner hypotube 20 for its superior yield strength and lower elastic modulus . it is well known by those skilled in the art that those two materials are not easily welded together , at least not without an intermediate material . the present description also discloses a method for joining together those dissimilar materials . fig6 shows one method of joining an inner hypotube made of nitinol to a stainless steel middle section 14 hypotube . the inner hypotube 52 is slid within the shaft tube 18 to overlap the cut section , and fed through past the windows 26 and 32 ( only window 26 shown in fig6 ). a small piece 50 of nitinol that fits in the windows is machined , inserted into the window 32 to get in contact with the inner hypotube 52 once in place , and laser welded to the inner hypotube 52 , interlocking the two parts together . the small piece 50 then prevents the inner hypotube 52 from sliding within shaft tube 18 . a small amount of adhesive can also be added to eliminate any movement of the small piece 50 relative to shaft tube windows 28 and 32 . another method of joining the above inner hypotube 52 to stainless steel shaft tube 18 is to use an intermediate material , for example nickel , that can be welded to both nitinol and stainless steel . a small piece 50 made of nickel can be laser welded to inner hypotube 52 first . a small hole in the center 51 can be provided to promote heat transfer to inner hypotube 52 . the inner hypotube 52 is then interlocked to proximal section 12 as described above . the edges of the small piece 50 of nickel can then be laser welded to shaft tube 18 . in this case , no direct welding of nitinol to stainless steel occurs and therefore , no brittle interface is created . another method involves the use of adhesive to bond the nitinol inner hypotube 20 to the stainless steel shaft tube 18 . the same parts can also be soldered together and by any other methods known by those skilled in the art . it can be appreciated that the design is made with one single uniform part , from proximal section 12 to the sensor housing section 15 , and therefore the guidewire 10 is very smooth with minimum mechanical steps . the manufacturability of this design is quite easy and very efficient as the number of parts is minimized . the mechanical properties can be optimized by varying the respective wall thicknesses of inner hypotube 20 and shaft tube 18 . the pitch of the spiral cut or other laser cut pattern can be varied ideally from a larger pitch to a shorter one when approaching the sensor housing section 15 . sharp turns are expected to arise in the region of the sensor housing section 15 , near the tip of the guidewire 10 and therefore , it would be best if the pitch of the laser cut pattern was reduced near the sensor housing section 15 to allow a smoother bending of the cut shaft tubing 22 . for improved safety , the spiral cut pattern provides a retainer for distal portions of guidewire . risks of leaving parts inside patient body are therefore minimized . there will be an optical connector ( not shown ) at the very proximal end of the guidewire 10 . as known by those skilled in the art , above described pressure guidewire can be coated with different material such as teflon or hydrophilic coating so as to reduce friction against wall of vessels and / or guiding catheter . now referring to fig7 , there is shown blood vessels with a cut - out portion in which a pressure guidewire 10 is inserted . while preferred embodiments have been described above and illustrated in the accompanying drawings , it will be evident to those skilled in the art that modifications may be made without departing from this disclosure . such modifications are considered as possible variants comprised in the scope of the disclosure .
the present document describes a pressure guidewire comprising : a shaft tube with a proximal section ; a middle section extending from the proximal section of the shaft tube , the middle section having greater flexibility than the proximal section ; an inner hypotube installed substantially within the middle section for optimal mechanical properties ; a pressure sensor with a communication means routed through the middle section and the proximal section ; and a sensor housing for receiving the pressure sensor . there are also described methods for joining the inner hypotube to the shaft tube .