Abstract:
The invention relates to a sensor ( 23 ) adapted for a sensor and guide wire assembly for intravascular measurements in a living body, wherein the sensor ( 23 ) comprises a pressure sensitive part ( 24 ) and an electronic part ( 25 ), said pressure sensitive part ( 24 ) comprising a first chip ( 26 ) provided with at least one pressure sensitive device ( 27 ) and at least one piezoelectric element ( 35 ), and the electronic part ( 25 ) comprising a second chip ( 28 ) provided with at least one electric circuit, and wherein the pressure sensitive part ( 24 ) and electronic part ( 25 ) are spatially separated from each other and are electrically connected with at least one electrical lead ( 29 ).

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to sensor and guide wire assemblies, in which a sensor is mounted at the distal end of a guide wire for intravascular measurements of physiological variables in a living body, and in particular to the design and arrangement of the sensor. 
     BACKGROUND OF THE INVENTION 
     Sensor and guide wire assemblies in which a sensor is mounted at the distal end of a guide wire are known. In U.S. Pat. No. Re. 35,648, which is assigned to the present assignee, an example of such a sensor and guide wire assembly is disclosed, where a sensor guide comprises a sensor element, an electronic unit, a signal transmitting cable connecting the sensor element to the electronic unit, a flexible tube having the cable and the sensor element disposed therein, a solid metal wire (also called a core wire), and a coil attached to the distal end of the solid wire. The sensor element comprises a pressure sensitive device, e.g. a membrane, with piezoresistive elements connected in a Wheatstone bridge-type of arrangement mounted thereon. An exemplifying electrical circuit arrangement can also be found in the present applicant&#39;s U.S. Pat. No. 6,343,514. As an alternative, the pressure sensitive device can also be in the form of a resonant structure, as is disclosed in the present applicant&#39;s U.S. Pat. Nos. 6,182,513 and 6,461,301. Instead of using cables to connect a sensor element to an electronic unit, other ways of receiving sensor signals can be employed. U.S. Pat. Nos. 6,615,067 and 6,692,446, which are assigned to the present assignee, disclose sensor systems for signal transmission via body tissues and passive biotelemetry, respectively. 
     As is recognized in U.S. Pat. Nos. 6,112,598 and 6,167,763, which also are assigned to the present assignee, a potential problem with this kind of guide wire mounted sensor is the occurrence of so-called bending artefacts. A bending artefact is a change in the output signal from the sensor that is induced by a bending of the guide wire, rather than being induced by a change in the physical environment surrounding the sensor. For a sensor and guide wire assembly like the one disclosed in Re. 35,648, this means that when the guide wire is bent, the bending of the guide wire imposes a strain on the sensor element, which thereby is deflected or stretched (or contracted). The deflection of the sensor element is then transferred to a deformation of the pressure sensitive device; and, according to well-known principles, the output from the Wheatstone bridge will thereby be affected by the bending of the guide wire. 
     According to U.S. Pat. Nos. 6,112,598 and 6,167,763, a solution to this problem is to mount the sensor element in a cantilevering fashion such that the pressure sensitive end of the sensor element does not contact any structure other than its mount. These two patents disclose several embodiments with different ways of mounting the sensor element such that bending forces are not exerted on the pressure sensitive end of the sensor element. A common feature of these embodiments is that an elongated, essentially rectangular sensor chip is mounted in a recess in the core wire in such a way that the proximal end of the chip is attached to the core wire, while the distal end of the sensor chip protrudes into the recess such that a clearance is provided below the distal portion of the chip where the pressure sensitive device (e.g. a membrane) is provided. 
     In the U.S. application Ser. No. 10/611,661, which is assigned to the present assignee, a principally different solution is presented. Here it is the design of the sensor element itself—rather than the mounting arrangement and design of the core wire—that provides the resistance against bending artefacts. According to Ser. No. 10/611,661, a sensor element comprises a mounting base, which provides for the desired cantilevered mounting of the sensor element. 
     In U.S. application Ser. No. 10/622,136, which is assigned to the present assignee, another design of a sensor element is disclosed, wherein the sensor element is provided with a recess that acts as a hinge or articulation, which constitutes a border between a first end portion and a second end portion of the sensor element. This recess prevents deformations of the second end portion from being transferred to the first end portion where the pressure sensitive device (e.g. a membrane) is arranged. 
     Although a sensor and guide wire assembly provided with a sensor chip designed and mounted according to the teachings of U.S. Pat. Nos. 6,112,598 and 6,167,763 in practise has proven to work well, the design of a sensor and guide wire assembly can be improved, not least from a manufacturing point of view. 
     SUMMARY OF THE INVENTION 
     As mentioned above, the sensor element according to the prior art comprises an elongated, essentially rectangular chip with a membrane made from polysilicon provided thereon. To achieve the desired resistance against bending artefacts, this chip can be designed and mounted in different ways. A common feature with the known designs is that the chip, including the pressure-sensitive membrane and the electric circuitry, is provided as one unit. The sensor element has thereby an elongated shape, with a length on the order of a millimeter. As already may have been appreciated from the discussion above, a shorter sensor chip would be less sensitive to bending artefacts. To simply reduce the chip length would, however, encounter several difficulties, not least in the manufacturing process. 
     An object of the present invention is to provide a new and improved design for a sensor arrangement so that, when the sensor is mounted in a sensor and guide wire assembly, the sensor and guide wire assembly will have the same or better characteristics regarding resistance against bending artefacts. Preferably, the sensor and guide wire assembly as well as the sensor chip should at the same time be easier and thereby cheaper to manufacture. 
     Another object of the invention is to provide a sensor design that facilitates the integration of more complex electronic circuitry in the sensor. With a more sophisticated electronic circuit, which, for example, includes components for signal conditioning and processing, improved signal characteristics and a more reliable sensor performance can be achieved. 
     A further object of the invention is to facilitate the incorporation of more delicate pressure sensitive devices, such as resonating structures, in the sensor. 
     These objects are achieved by a sensor arrangement and a sensor and guide wire assembly according to the independent claim(s). Preferred embodiments are set forth in the dependent claim(s). 
     A sensor and guide wire assembly comprises a sensor which, according to the prior art, is in the form of a generally rectangular and rather thin sensor chip with a pressure sensitive device provided thereon. The pressure sensitive device can be in the form of a membrane, which covers a small cavity in the upper side at a first end portion of the sensor chip and which has piezoresistive elements mounted thereon. According to the invention, this first portion is spatially separated from a second part of the sensor. A sensor thereby comprises a pressure sensitive part, which has a pressure sensitive device, such as a membrane, provided thereon, and at least one piezoresistive element mounted on the membrane. The second part of the sensor is also referred to as the electronic part, and includes, in a first embodiment of the invention, at least one electric circuit including at least one electric resistor and connection pads. In other embodiments of the invention, the electronic part can comprise a printed circuit with electronic logic and different signal processing elements. In a sensor and guide wire assembly, the first and second parts are spatially separated with, for example, a few millimeters and are electrically connected with at least one electric lead. The length of the pressure sensitive part—which is the part of a sensor that is potentially sensitive to bending artefacts—can thereby be reduced, which, in turn, makes it less sensitive to such bending artefacts. The length of the electronic part can, on the other hand, be increased, if this is desirable in order to incorporate more functionality in the electronic circuitry arranged thereon. Another advantage with physical division of the sensor in an electronic part and a pressure sensitive part is that these two parts easily can be manufactured by different techniques and even by different manufacturers. The two parts are then electrically connected during the assembly of the sensor and guide wire assembly. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates the general design of a sensor and guide wire assembly according to the prior art. 
         FIG. 2  illustrates schematically a portion of a sensor and guide wire assembly comprising a sensor according to the present invention. 
         FIG. 3  illustrates an exemplifying coupling arrangement to be used together with a sensor according to the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     For better understanding of the context in which a sensor according to the present invention is going to be used, a sensor and guide wire assembly  1  of a conventional design is illustrated in  FIG. 1 . The sensor guide  1  comprises a hollow tube  2 , a core wire  3 , a first coil  4 , a second coil  5 , a jacket or sleeve  6 , a dome-shaped tip  7 , a sensor element  8 , and one or several electrical leads  9 . The proximal end of the first coil  4  is attached to the distal end of the hollow tube  2 , while the distal end of the first coil  4  is attached to the proximal end of the jacket  6 . The proximal end of the second coil  5  is connected to the distal end of the jacket  6 , and the dome-shaped tip  7  is attached to the distal end of the second coil  5 . The core wire  3  is at least partly disposed inside the hollow tube  2  such that the distal portion of the core wire  3  extends out of the hollow tube  2  and into the second coil  5 . The sensor element  8  is mounted on the core wire  3  at the position of the jacket  6 , and is through the electrical leads  9  connected to an electronic unit (not shown in the figure). The sensor element  8  comprises a pressure sensitive device in the form of a membrane  10  (not visible in the figure), which through an aperture  11  in the jacket  6  is in contact with a medium, such as blood, surrounding the distal portion of the sensor guide  1 . As is well known in the art, the dimensions as well as other properties of guide wires adapted for introduction into the artery can vary considerable based on the type of procedure being performed, the particular patient, etc. The corresponding ranges of dimensions are also applicable to a sensor guide whose distal end is provided with a sensor element. In one conventional design of a sensor guide like the sensor guide  1  shown in  FIG. 1 , the diameter of the tube  2  is about 0.014 inches (0.36 mm) and the dimensions of element  8  are 1340×180×100 μm (length×width×height). 
     Although not shown in the figure, the sensor element  8  further comprises an electrical circuitry, which in a Wheatstone bridge-type of arrangement is connected to one or several piezoresistive elements provided on the membrane  10 . As is well known in the art, a certain pressure exerted on the membrane  10  from the surrounding medium will thereby correspond to a certain stretching or deflection of the membrane  10  and thereby to a certain resistance of the piezoresistive elements mounted thereon and, in turn, to a certain output from the sensor element  8 . It should therefore be clear that it is highly preferable that this output from the sensor element  8  does not change due to factors that are not related to a real change in the physical properties of the surrounding medium. As was mentioned above, one such factor is so-called bending artefacts, the source of which is that a bending of the sensor guide  1  is transferred to a deformation of the membrane  10 . Here, the discussion above about piezoresistive elements coupled in a Wheatstone bridge-type of arrangement should only be seen as an illustrative exemplification; in short, the basic problem is that a pressure sensitive device, such as a membrane, can be influenced by a bending of a sensor guide. 
     To remedy the potentially adverse effects from bending artefacts, the present invention provides a new design of a sensor to be used in a sensor and guide wire assembly.  FIG. 2  shows schematically a portion of a sensor and guide wire assembly  21  comprising a core wire  22  and a sensor  23  according to the present invention. The sensor  23  comprises essentially two parts: a pressure sensitive part  24  and an electronic part  25 . The pressure sensitive part  24  comprises a small chip  26 , in which a cavity has been formed. The cavity is covered by a pressure sensitive device in the form of a membrane  27 , on the surface of which at least one piezoresistive element is arranged (not shown in the figure). When the sensor  23  is used in a sensor and guide wire assembly, the pressure prevailing in the ambient medium will create a deflection of the membrane  27 , which, in turn, changes the resistance of the piezoresistive element and accordingly the output of the sensor  23 . The electronic part  25  includes at least one electric circuit, which is provided at the surface of a chip  28 . The electronic part  25  is electrically connected to the pressure sensitive part  24  by at least one electric lead  29 . An exemplifying coupling arrangement will be discussed in more detail in conjunction with the description of  FIG. 3 . 
     According to the invention, the electronic part  25  is disposed in the vicinity of the pressure sensitive part  24 , and is electrically connected to the pressure sensitive part  24  with said at least one electrical lead  29 . In this embodiment, the spatial separation between the pressure sensitive part  24  and the electronic part  25  is small, for example on the order of a few millimeters or even fractions of a millimeter, but also a larger spatial separation is conceivable. With the inventive division of the sensor  23  into a pressure sensitive part  24  and an electronic part  25 , the pressure sensitive part  24  can be made very small since the number of electrical components, e.g. resistors, that have to be fitted onto the surface of the chip  26  is reduced in comparison with the known technical solutions. A smaller (shorter) pressure sensitive part is correspondingly less prone to bending artefacts, as has been outlined above. The length of the electronic part  25 , which is insensitive, or at least comparatively insensitive, to bending artefacts, can, on the other hand, be increased to include more components, i.e. the components not provided in the pressure sensitive part  24 , or further components to include more functionality in the sensor  23 , or to provide the sensor  23  with better output characteristics. By dividing a sensor into one chip that comprises a pressure sensitive device and another chip that only contains elements that are pressure insensitive, the two chips can be manufactured by different methods and even by different manufacturers. The two chips are then mounted separately during the assembly of a sensor and guide wire assembly, and are electrically connected by at least one electrical lead. 
     An exemplifying coupling arrangement for a sensor according to the present invention is schematically illustrated in  FIG. 3 . This coupling arrangement is based on the previously discussed Wheatstone bridge-type of coupling; and for the sake of clarity, the number of electrical components provided at a pressure sensitive part  31  and an electronic part  32 , respectively, has been minimized, but it should be understood that a much more sophisticated circuit solution can be implemented. This is in particular the case for the electronic part  32 , whose size and thereby the available space for electrical components can be increased in accordance with an object of the invention. 
     As is seen from  FIG. 3 , the pressure sensitive part  31  comprises a chip  33 , in which a cavity has been formed. The cavity is covered by a membrane  34 , at the surface of which a piezoresistive element  35  has been provided. The piezoresistive element  35  is by a first electrical lead  36   a  connected to a first connection pad  37   a , and by a second electrical lead  36   b  to a second connection pad  37   b . The connection pads  37   a, b  are arranged at the surface of the chip  33  outside the membrane  34 . 
     In this basic embodiment, the electronic part  32  comprises a chip  38 , at the surface of which a resistor  39  and three connection pads  40   a - c  have been arranged. Two electrical leads  41   b  and  41   c  connect the resistor  39  to the connection pads  40   b  and  40   c , respectively. The connection pad  40   a  is by an electrical lead  42   a  connected to the connection pad  37   a  at the pressure sensitive part  31 , while an electrical lead  42   b  connects the connection pad  40   b  with the connection pad  37   b . It should now be appreciated that the electrical circuit provided at the pressure sensitive part  31  and electronic part  32  forms one half of a Wheatstone bridge. For the sake of completeness, the other half of the Wheatstone bridge, which is generally referenced with reference number  51 , has been illustrated in the upper portion of  FIG. 3 . This second or external half  51  of the Wheatstone bridge can be arranged within an external unit, such as a monitor, to which a sensor guide is connected and which also is used for numerically or graphically displaying information related to the sensor output. 
     The external half  51  of the Wheatstone bridge comprises a first resistance  52  and a second resistance  53 . The first resistance  52  is by an electrical lead  54   a  connected to the connection pad  40   a  of the electronic part  32 , while another electrical lead  54   b  connects the second resistance  53  to the connection pad  40   c . A positive excitation voltage E +  is applied over the resistances  52  and  53 , while a negative excitation voltage E is applied directly to the connection pad  40   b  of the electronic part  32 . A voltage difference S (i.e. a signal), which represents the resistance of the piezoresistive element  35  and thereby the pressure that the surrounding medium exerts on the membrane  34 , can thereby be obtained between the electrical leads  54   a  and  54   b.    
     It should, once again, be emphasized that the circuit arrangement described above is only an exemplifying arrangement. It is, for example, possible to provide a full Wheatstone bridge in a sensor according to the invention. In that case, the pressure sensitive part could comprise a piezoresistive element as described above, whereas the electronic part would include at least three resistors. In another arrangement, the pressure sensitive part could include also pressure insensitive resistors. A particular advantage with the present invention is the enhanced possibility to provide a more complex electrical circuit at an electronic part of a sensor. In that case, more sophisticated components, such as operational amplifiers, could be provided to improve the signal characteristics from the sensor. It is in particular possible that the electronic part comprises commercially available standard electronics, which, e.g., is provided as integrated circuits with the so-called CMOS technology. With a more sophisticated electrical circuit arrangement at the sensor side, the number of leads that connect a sensor to an external unit can be reduced, and even reduced to zero if a wireless signal transmission is employed. A wireless signal transmission is, for example, discussed in the above referenced patents. It is also possible to replace the piezoresistive element of the pressure sensitive part with another type of piezoelectrical component, for example a capacitive device which could be provided on the underside of a membrane and at the bottom of a recess, which is covered by that membrane, such that the capacitance of the capacitive device depends on the deflection of the membrane. The pressure sensitive device could also comprise a vibrating or resonating structure, whose vibration or resonance frequency is dependent on the pressure exerted by the ambient medium. 
     Although the present invention has been described with reference to a specific embodiment, also shown in the appended drawings, it will be apparent for those skilled in the art that many variations and modifications can be done within the scope of the invention as described in the specification and defined with reference to the claims below. It is, for example, possible to dispose an electronic part and a pressure sensitive part of a sensor in separate recesses in a core wire which is arranged inside a sensor and guide wire assembly. The invention can be used for intravascular measurements of other types of physiological variables such as temperature or flow, and is further applicable to direct as well as indirect measurements of such physiological variables. Also, features of the above-described embodiment may be combined with features of the U.S. patents and patent applications discussed in the background section above as well as with features of U.S. provisional applications 60/577,197 (filed Jun. 7, 2004 by Lars Tenerz and Sauli Tulkki) and 60/605,170 (filed Aug. 30, 2004 by Sauli Tulkki). The entire contents of all of these patents and applications are incorporated herein by reference.