Abstract:
A probe system for measuring fluid flow in a conduit, such as a blood vessel with ultrasound transit time or similar measurement methods. The probe system having a probe body with a space to receive in a secure but detachable fashion a pliable soft insert. The insert has a central lumen or aperture which is sized to securely but detachably fit around a vessel or conduit without squeezing or in any way altering the conduit during application or use. The insert is acoustically matched with the vessel or conduit and fluid flowing therein to thereby minimize distortion or attenuation of ultra sound waves generated to assess flow. In a further aspect a set of inserts with varying sized lumens or apertures are provided to match with vessels or conduits of varying size. The system among other things increases accuracy of flow measurements while minimizing trauma to the vessel or conduit.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application claims priority under 35 USC § 119 (e) from U.S. provisional application Ser. No. 60/881926 filed Jan. 23 2007 titled Acoustically Compatible Elastometic Cuff Insert for Ultrasound Probes or Disposable Insert for a Perivascular Probe 
     
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0002]    Not applicable. 
       REFERENCE TO A “SEQUENCE LISTING” 
       [0003]    Not applicable. 
       FIELD OF THE INVENTION 
       [0004]    The present invention relates to ultrasonic probes used to measure fluid flow, more particularly it relates to an ultrasonic probe with a single or multiple use insert that secures the probe to a conduit and provides an acoustical path with minimal distortion of ultrasound transmissions generated to measure flow. 
       BACKGROUND OF THE INVENTION 
       [0005]    Use of ultrasound to measure and access flow in a conduit or a blood vessel has been well known in the art for years. U.S. Pat. No. 4,227,407, which is incorporated herein by reference, describes a system that measures volume flow with transit time ultrasound. 
         [0006]    In the typical transit-time ultrasound flow sensor system flow is measured by the passage time of an ultrasound signal between two transducers where the signal passes through the flowing stream of fluid in a conduit or vessel on its passage from one transducer to the other. These measurements are used to determine flow volume by one of the two following methods: differential or common-mode transit time as follows: a) Differential Transit Time: the flow of liquid shortens the ultrasound transit time in downstream direction, and lengthens the transit time in upstream direction. The difference between alternate measurements of upstream and downstream transit times can thus be used as a measure of flow rate through the conduit. b) Common-mode transit time: the average value of a downstream and upstream transit time is a measure of the acoustical velocity of all the media between transmitting and receiving transducers. By introducing a change in this liquid&#39;s acoustical velocity (e.g. via the introduction of a bolus of a different liquid, or a momentary change in temperature) it can thus be used as an indicator dilution sensor (see, for example, the methods disclosed in U.S. Pat. Nos. 5,453,576 and 5,595,182, herein incorporated by reference). 
         [0007]    All such sensors can measure flow parameters in conduits by employing ultrasound transit-time principles of operation with full flow illumination, wherein the flow cross-section is practically fully and homogeneously illuminated by an ultrasonic beam (Cornelis Drost, U.S. Pat. No. 4,227,407; Shkarlet Yuri, U.S. Pat. No. 6,098,466 incorporated herein by reference). 
         [0008]    Other methods exist which are also used to measure flow including those based on electromagnetic sensing, Doppler ultrasonic methodologies and some that use Laser Doppler systems. 
         [0009]    A typical ultrasonic transit time device (UTT) consists of 2 to 4 transducers which alternate between send and receive modes. When an electrical pulse stimulates a transducer in send mode, an acoustic wave is broadcast towards a transducer in receive mode which is properly aligned to receive such a signal. The ultrasonic paths, which are defined by the transducers&#39; height, width and orientation, will encompass the entire conduit in which the fluid is flowing so that an accurate full volume flow measurement is possible. 
         [0010]    As most fluid conduits are round and most ultrasound transducers, in particular those used for transit time ultrasound readings, have a flat wave generation surface, a varying volume of space typically exists between the transducer and the conduit. Air is a very poor medium which to transmit ultrasound wave through so this space between the transducer and conduit needs to be filled with a saline solution, an acoustic couplant, protective wrapping or if a chronic implant in an animal or human patient by tissue in-growth between the transducer and conduit. However, the saline solution, acoustic couplant or wrapping often can get displaced over time; this is especially true when placed near a beating heart or some other moving part of the body. Also, the protective wrapping is often not acoustically transparent and tissue in-growth takes time to grow in, leaving a time period where accurate measurements are not available. 
         [0011]    While the shape of a biological conduit can be estimated accurately, the outer diameter can vary significantly from individual to individual and thus cannot be estimated until the individual is opened up and the vessel examined. Thus, up to the present the only solution in the prior art was and is to determine an outer diameter of a vessel by visual observation once the patient or animal is opened up during a surgical procedure and the vein or artery is exposed. Thus, it is not possible to be certain that the appropriately-sized flow probes will be on hand during a procedure. Given the potentially wide variation in exterior vein or artery diameter it is not currently economically feasible to have a large number of flow probes of different sizes sterilized and on hand during each surgery to ensure a proper fit. Additionally, one cannot over-emphasize the need for eliminating any air space between the probe&#39;s transducer surface and the exterior of the vein or artery. In order to obtain any useable results a proper fit that eliminates any potential air pockets which can cause unwanted reflection of the ultrasonic wave must be established between transducers and the artery or vein selected for flow measurement. A proper fit will also support the vessel and minimize the amount of movement of the vessel when readings are taken. One problem that occurs that often prevents this are body fluids that can seep into the space between the probe and vessel, these can cause false readings of flow. Additionally vessels are susceptible to rupture when they are subjected to rubbing along a high friction surface, even when rounded. The current practice used to protect a vessel during flow probe installation is to wrap the vessel with a padding or mesh. There are probes that have adjustable pockets to hold the vessel; however, these tend to be cumbersome and difficult to use. 
         [0012]    In certain instances, the application of the flow probe is limited by the health of the vessel. Any squeezing of the vessel can release plaque, which will migrate along the vessel and potentially cause clots. For applications where this is an issue, a probe must designed to be easily installed without disturbing the vessel. More importantly it must be capable of being removed without altering or damaging the vessel. 
         [0013]    For use in quick spot measurements of flow, an ideal flow probe will be properly sized to the size of the vessel, quickly placed over the vessel, measurements taken, and then easily removed without disrupting the vessel. The current art lacks in the ability to perform this process without either squeezing a vessel or having large gaps that exist between transducers of the probe and the vessel or artery from which flow measurements are to be obtained. 
       SUMMARY 
       [0014]    Thus, it is an objective of the present invention to solve the problems mentioned above and provide a system with a probe that is easy to install and provides an accurate and correct fit around a conduit or vessel without gaps, thus providing an attenuation-free as possible acoustic connection between the probe and vessel. It is a further objective to provide an apparatus to improve the safety and effectiveness of ultrasonic transit-time flow measurement. 
         [0015]    The present invention and its various aspects achieves these and other objectives by providing a system that employs a disposable cuff insert which correctly positions a perivascular probe along an axis perpendicular to a fluid conduit, such as an artery or vein, without influencing the vessel in any way. An insert that securely fits into the interior space of the probe has an opening through its center that allows the insert to securely surround a vein or artery of an outside diameter equivalent to the opening in the center of the insert. Inserts with varying openings through their center allow for selection of an insert with an opening that is properly sized to securely fit around veins or arteries of varying size. The ultrasonic path between transducers is then comprised only of the cuff insert which is ultrasonically matched to the conduit, reducing ultrasonic reflections and the need for an acoustic couplant. 
         [0016]    In another variation of the present invention it provides an insert for a perivascular probe with: a) a probe insert with a body made of a pliable flexible material having a lumen surface formed on an interior portion of the insert, the lumen surface ending at two opposing openings and thereby defining an aperture through the insert, which aperture is sized such that the lumen surface can be securely, snugly and detachably fitted to a portion of an exterior surface of a fluid conduit with a specific exterior dimension, the insert also including a split region to facilitate fitting of the insert to the fluid conduit; b) the probe insert having an exterior surface configured to securely but detachably fit within an interior space of a probe body, the probe body having appropriately placed within it at least two ultrasonic transducers configured to exchange transmissions there between, which transmissions provide full flow illumination of the interior of a conduit positioned against the lumen surface of the insert, when the insert is positioned within the probe; and c) wherein the pliable flexible material of the insert is ultrasonically matched to material making up a conduit held by the insert and fluid flowing in the conduit to thereby eliminate distortion of ultrasonic transmissions passing through the conduit. 
         [0017]    In yet another variation it provides a modular perivascular probe system with: a) a probe body forming an interior pocket to hold an insert in a secure but detachable and snug airtight fit; b) the probe body having at least two transducers positioned within itself to exchange ultrasonic transmissions there between; c) an insert made of a pliable and flexible material having an exterior surface configured to fit in a snug airtight fashion within the pocket formed by the probe body; d) the insert having an aperture there through formed by a lumen surface in an interior of the insert, the lumen surface ending at two opposing openings; the lumen surface is sized such that the lumen surface can be securely, snugly and detachably fitted around a portion of an exterior surface of a fluid conduit of a specific size, in an interior of the insert to thereby create an aperture there through; e) the lumen surface being configured to hold a vessel in a position that ultra sonic transmissions between the two transducers fully illuminate flow of liquid in the conduit; f) wherein the pliable flexible material of the insert is ultrasonically matched to material making up the conduit held by the insert and fluid flowing in the conduit to thereby eliminate distortion of ultrasonic transmissions passing through the conduit; and g) wherein the at least two transducers are connected by a communication link to a cpu, which cpu is programmed to control the operation of the at least two transducers and obtain signal information from signals transmitted between the at least two transducers to thereby obtain information regarding fluid flowing in the fluid conduit. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0018]    The invention will be better understood by an examination of the following description, together with the accompanying drawings, in which: 
           [0019]      FIG. 1  is a perspective view of a preferred embodiment of the insert cuff and probe body of the present invention positioned adjacent to each other; 
           [0020]      FIG. 2  is a face view of a preferred embodiment of an insert cuff of the present invention secured in the pocket of the probe body; 
           [0021]      FIG. 3  is a face view of a preferred embodiment of the probe body of the present invention; 
           [0022]      FIG. 4  is a view of the probe body of  FIG. 3  along lines IV-IV; 
           [0023]      FIG. 5  is a perspective view of a preferred embodiment of the insert of the present invention; 
           [0024]      FIG. 6  is a perspective view of a preferred embodiment of the insert of the present invention being secured around a vessel; 
           [0025]      FIG. 7  is a perspective view of a vessel with an insert secured around it and the insert positioned in the pocket formed by a probe body: 
           [0026]      FIG. 8  is a perspective view of a vessel positioned in the pocket of a probe body without the insert; 
           [0027]      FIG. 9  is a view of the bottoms of the probe body and insert adjacent to each other; 
           [0028]      FIG. 10  is a view of the vessel with insert around it and insert in the probe body of  FIG. 7  from the bottom from the perspective indicated by line X-X; 
           [0029]      FIG. 11  is a front view of the vessel secured in the insert with the insert positioned in a probe body; 
           [0030]      FIG. 11A  is a schematic diagram of the electrical circuitry of the present invention; 
           [0031]      FIG. 12  schematic diagram of the configuration of the vessel secured in the insert with the insert in the probe body of a cross-section perspective viewed along line XII-XII of  FIG. 10 ; 
           [0032]      FIG. 13  is a graph demonstrating what happens to a ultrasound wave that is incident at an oblique angle on a boundary between two ultrasound transmissive media that have different acoustic impedances and velocities of sound; 
           [0033]      FIG. 14  is a graph demonstrating what happens to a ultrasound wave that is incident at a perpendicular angle on a boundary between two ultrasound transmissive media that have different acoustic impedances and velocities of sound; 
           [0034]      FIG. 15  is a schematic view of some of the components of system of the present invention and a conduit; 
           [0035]      FIG. 16  is another schematic view of some of the components of system of the present invention and a conduit; 
           [0036]      FIG. 17  is view of a set of inserts and a probe body with which they would be used; and 
           [0037]      FIG. 18  is view of the set of inserts depicted in  FIG. 17  as they would appear in a probe body. 
           [0038]      FIG. 19  is a front view of another variation of the probe and insert of the present invention; 
           [0039]      FIG. 19A  is a side view of the probe in  FIG. 19  along line  1 XXA- 1 XXA; 
           [0040]      FIG. 20  is a front view of another variation of the probe and insert of the present invention; 
           [0041]      FIG. 21  is a side view of another embodiment of the present invention; 
           [0042]      FIG. 22  is a top partial schematic view of the probe and insert of  FIG. 21  along line XX 1 -XX 1 ; 
           [0043]      FIG. 23  is a top view of an insert of the variation of the invention depicted in  FIGS. 21 and 22 ; and  FIG. 24  is a perspective view of the insert depicted in  FIG. 23 . 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0044]    The present invention provides an apparatus and method for accurate, efficient and cost effective measurements of fluid flow such as blood in conduits and vessels of varying size. In its preferred embodiment the apparatus is used in conjunction with ultrasonic transit-time measurements in conduits and vessels such as arteries and veins.  FIG. 1  provides a perspective view of an insert cuff  21  of the present invention adjacent to a probe body  23 .  FIG. 2  provides a face view with insert cuff  21  positioned in probe body  23 . As will be discussed in detail below aperture  25  in insert cuff  21  would be secured around a vessel or conduit such that lumen surface  27  of insert  21 , which forms aperture  25 , would be detachably abutted against the outside wall of the vessel or conduit before insert  21  is inserted into probe body  23  as depicted in  FIG. 2 . 
         [0045]    In its preferred embodiment insert cuff  21  is produced by an injection molding process and is made of a pliable and elastic rubber like material that is acoustically matched to the conduit or vessel it is positioned around the fluid flowing in that conduit. The preferred embodiment of the present invention measures blood flow in a conduit or vessel, thus the material insert  21  must be acoustically matched and biocompatible with blood and the blood vessel around which it will be positioned. Acoustically matched means that the material of insert  21  and the blood vessel and blood flowing in the vessel must have the same or a very close acoustic impedance and acoustic velocity. Such a properly chosen material for insert  21  must not distort sound waves or focus the acoustical field on the center of the flow lumen formed by aperture  25 , but rather maintain “full flow illumination”. Prior art, described in U.S. Pat. No. 7,194,919, incorporated herein by reference outlines the requirements of an ideal material that provides full flow illumination. 
         [0046]      FIG. 3  provides a raised view of probe body  23  with its two legs  29  and  31  that are connected by superstructure  33 . Space  35  is formed between legs  29  and  31  to receive insert cuff  21 ,  FIGS. 1 and 2 . In the preferred embodiment of the present invention probe body  23  is made of a rigid plastic like material such as a biocompatible epoxy.  FIG. 4  provides view along line IV-IV of  FIG. 3  wherein one looks into space  35  formed by legs  29  and  31  in probe body  23 . In the preferred embodiment of the present invention transducers  41  and  45  are positioned within leg  29  and transducers  45  and  47  are positioned in leg  31 . Transducers  41  and  45  are positioned to exchange transmission of ultrasound waves across space  35  and Transducers  43  and  47  are positioned to exchange transmission of ultrasound waves across space  35 . 
         [0047]    Referring to  FIG. 5  insert  21  has a split  37  that runs from the exterior surface  39  of insert  21  to lumen surface  27 . As depicted in  FIG. 6  split  37  allows the opening up of insert  21  because it is a pliable rubber like material and thus positioned over conduit or vessel  53  without squeezing or disturbing vessel  53 . Insert  21  is selected such that aperture  25  has approximately the same diameter as the outside diameter of the vessel or conduit around it will be secured. 
         [0048]    After insert  21  is secured in a detachable fashion around vessel  53  it is inserted into probe body  23  as depicted in  FIG. 7 . As depicted in  FIG. 7  insert  21  is securely positioned around vessel  41  with lumen surface  27  in full contact with vessel  41 . In turn insert  21  is secured inside probe body  23 .  FIG. 8  is provided to emphasize the function of insert  21  in that  FIG. 8  shows vessel  53  in probe body  23  without insert  21 . As can be seen space  48  is that area that would normally be filled up with the insert cuff. The arrangement depicted in  FIG. 8  is non-functional since it leaves a gap  48  with air between the transducers of probe body  23  and vessel  53 . 
         [0049]      FIG. 9  provides a side by side view of the bottom of the probe body  23  along line IV-IV of  FIG. 3  and a view of the bottom of the insert  21  along line IX-IX of  FIG. 5 . In comparing in  FIG. 9  the view of insert  21  with the view of space or pocket  35  formed in probe body  23  it can be seen that the insert is sized to snugly fit into space or pocket  35 . In order to properly practice the present invention a sealed generally air tight fit with very few or no air bubbles between the exterior surfaces  49 A,  49 B,  49 C and  49 D of insert  21  and interior surfaces  51 A,  51 B,  51 C and  51 D of probe body  23  must be achieved when insert  21  is placed in space or pocket  35  within probe body  23 . 
         [0050]    Accuracy of the flow measurements taken by the present invention is one of the paramount goals. As noted above the invention measures flow with transit time ultrasound measurements. To help achieve accuracy in its measurements the present invention relies on planar ultrasound transducers sized to fully illuminate a complete cross-sectional area of the vessel. This requires production by the transducer in transmit mode of a substantially coherent planar wave of ultrasound as wide as or wider than the vessel under study wherein sufficient coherence of the wave and wave front of the generated wave is maintained along the acoustic path between the transmitting transducer and the receiving transducer, such that all parts of the ultrasound wave front arrive substantially in phase at the receiving transducer. Some of the features of the present invention that help achieve this goal of full coherent flow illumination of the vessel or conduit are: a) providing a transducer wide enough to generate an ultrasound wave that covers an entire cross-sectional area of the vessel or conduit, b) positioning the transmission face of the transducers so that the acoustic wave is perpendicular to the boundary between the probe body and the insert and such that the advancing ultrasound wave front will present a flat planar face that is parallel to this boundary between the probe body with embedded transducer and the insert, c) assuring there is a snug airtight fit between the surface of the probe body where the transducer is located and the adjacent portion of the insert, and d) matching the acoustic impedance and acoustic velocity of the insert to the vessel or conduit and the fluid flowing in the vessel or conduit to minimize reflection, refraction and acoustic focusing of the ultrasound waves at the boundaries between insert  21  and vessel or conduit  53 . 
         [0051]    Referring to  FIG. 10  the ultrasonic wave path  57  between transducer  41  and transducer  45  cuts across vessel  53  and the ultrasonic wave path  59  between transducer  43  and transducer  47  cuts across vessel  53 .  FIG. 11  is a raised cross-sectional view of vessel  53 , probe body  23  and insert  21  along line XI-XI in  FIG. 10 . As depicted in  FIG. 11  transducers  43  and  45  generate ultrasound waves that have paths respectively  57  and  59 , which when taken in conjunction with the course of the paths in  FIG. 10  it can be seen that they fully illuminate a complete cross-section of conduit or vessel  53  a required by the present invention. 
         [0052]    Referring to  FIG. 10  again it can be seen that the transmission faces  41 T and  45 T of each of transducers  41  and  45  face each other and are parallel to each other. Additionally, the boundary between probe body  23  and insert  21  formed by the abutting of surfaces  49 C and  51 C adjacent to transducer  41  is parallel to transmission surface  41 T of transducer  41 . Likewise the boundary between probe body  23  and insert  21  formed by the abutting of surface  49 B and  51 B adjacent to transducer  45  is parallel to transmission surface  45 T of transducer  45 . Likewise with respect to transducers  43  and  47 , transmission surface  43 T is parallel to interior surface  51 D of the probe body, which in turn is parallel to exterior surface  49 D of the insert which in turn is parallel to exterior surface  49 A of the insert, which in turn is parallel to exterior surface  51 A of the probe body, which finally is parallel to transmission surface  47 T of transducer  47 . 
         [0053]    Transducers  41 ,  43 ,  45  and  47  are all individually electrically connected  38  to a flow meter  40   FIG. 11 ,  FIG. 4  as well as  FIG. 11A ,  FIG. 11A  being a schematic diagram of the electrical and ultrasound connections of the invention. As depicted in  FIG. 11  all of the individual electrical connections  38  are bundled together in electrical lead  39 . Referring back to  FIG. 11A  signals from flowmeter activate the various transducers which generate ultrasound beams  42  and  44  which pass back and forth between the paired transducers  41  and  45  beam path beam path  42  and transducers  43  and  47  beam path  44 . The ultrasound signal received by the transducer of the pair in receive mode converts the received ultrasound signal back into an electrical signal and sends it over its individual connection  38  to flowmeter  40 . Flowmeter  40  than analyzes the signal and based on that signal or several received signals from each of the transducers determines flow rate. Flowmeter  40  is a dedicated computer with CPU, memory, graphic or electronic display, signal interface with the transducers and appropriate software that analyzes and stores the results. U.S. Pat. No. 4,227,407 previously incorporated by reference go into detail on the specific methods of calculating. Transonic Systems Inc. makes a T400 research flowmeter and HT 300 clinical flowmeter that would work with the probes as disclosed herein. In an alternative arrangement a general purpose computer running appropriate software with standard digital to analogue converter to connect to the transducers could be used instead of dedicated flowmeter. 
         [0054]      FIG. 12  provides a cut away schematic, not to scale, view along line XII-XII of  FIG. 10 . In  FIG. 12  a side view of transducers  45  appears adjacent to a side edge view of boundary  65  formed by surface  51 B of probe  23  and surface  49 B of insert. Also, a side view of transducers  41  appears adjacent to a side edge view of boundary  67  formed by surface  51 C of probe  23  and surface  49 C of insert. An oblique angle view of vessel  53  appears. As can be seen from this schematic diagram transducer transmission-reception surface  45 T is parallel to boundary  65  which in turn is parallel to boundary  67  which in turn is parallel to transmission-reception surface  41 T. Thus, if transducer  45  generates a planar ultrasound wave indicated by wave front  63 , wave front  63  (depicted at multiple positions in  FIG. 12  to show its movement) will pass through boundary  65  without refraction since the waves of wave front  63  are perpendicular to boundary  65 , likewise it will pass through insert  21  and then through boundary  67  to eventually arrive at transmission-reception surface  41 T of transducer  41  in a fairly coherent form with a fairly planar wave front do to this structural feature of parallel surfaces. (It is also due to the acoustic matching of the insert to the vessel fluid flowing in the vessel, which will be discussed in detail a few paragraphs below after the present discussion.) Arrows  50  in  FIG. 12  are representative of fluid flowing in conduit or vessel  53 , such as blood. Although, may not be specifically depicted every time in the drawings stated every time with references to conduits or vessels when discussing measurements of fluids flowing this can be presumed. 
         [0055]    Planar wave front  63  and thus the ultrasound waves of which it consists, since these waves are arriving at boundary  65  with an orientation perpendicular to boundary  65  planar wave front  63  as it passes through boundary  65 , will maintain its planar shape, coherence and homogeneity. This can be explained by Snell&#39;s law: 
         [0000]    
       
         
           
             
               
                 sin 
                 
                   θ 
                   1 
                 
               
               
                 V 
                 
                   L 
                   1 
                 
               
             
             = 
             
               
                 sin 
                 
                   θ 
                   2 
                 
               
               
                 V 
                 
                   L 
                   2 
                 
               
             
           
         
       
     
         [0000]    as it is applied to sound waves. When a sound wave arrives at a boundary between two different materials depending on the acoustic velocity and impedance of each material and the velocity of sound in each material three possible things can occur: a) the wave is in whole or part reflected back into the material it has just traveled through, b) the wave can in whole or part pass through and continue on in the same direction in the new material or c) it can in whole or part pass through and be refracted in the new material. The equation for acoustic impedance is Z=ρV, where Z is the impedance, ρ is the density of the media and V is the velocity of sound in the media. Generally, differences in acoustic impedance Z between the two different materials is primarily determinative of the amount reflected at the boundary between the two materials as opposed to passing through the boundary. The closer the impedance of the two materials is matched the more of the sound waves signal strength passes through rather to the new medium rather than being reflected back. The amount the sound wave is refracted as it passes through the boundary between the two materials is dependent primarily on the difference in velocity of sound in each of the two materials, the greater the difference in the velocity of sound the greater the refraction of the ultrasound waves. However, if the ultra sound wave passes through the boundary at an angle perpendicular to the boundary no refraction will occur as defined above in Snell&#39;s law. 
         [0056]    The effect of Snell&#39;s law described above is illustrated by  FIGS. 13 and 14 .  FIG. 13  shows that when the direction of ultrasonic wave V L1  arrives at an oblique angle θ 1  (measured from the y-axis) to a boundary, the x-axis, between sound transmissive materials M 1  and M 2  each of which have different acoustic velocities and different acoustic impedances the portion of ultrasonic wave V L2  that passes into medium M 2  is going to diverge (be refracted) from the direction of ultrasonic wave V L1  at a different angle greater angle from the y-axis, θ 2 . On the other hand as depicted in  FIG. 14  if the ultrasonic wave V L1  arrives at a perpendicular angle to the boundary, the x-axis, between materials M 1  and M 2  the portion that passes into material M 2  continues in the same direction as V L1 . To improve performance and lessen reflection V L1 , the epoxy  71   FIG. 10  is selected to have an acoustic impedance Z 1  of the epoxy that is equivalent to 
         [0000]    
       
         
           
             
               Z 
               1 
             
             = 
             
               
                 
                   ( 
                   
                     Z 
                     4 
                   
                   ) 
                 
                 2 
               
               
                 Z 
                 2 
               
             
           
         
       
     
         [0000]    where Z 2  is the acoustic impedance of insert  21  material and Z 4  is the acoustic impedance of transducer  47 . This is based on the following relationship: 
         [0000]        Z   4 =√{square root over ( Z   1   ×Z   2 )} 
         [0000]    which is a formula used to determine the best acoustic impedance matching between to materials to minimize attenuation and reflection of sound waves passing from one material where the sound waves are generated into a second material. 
         [0057]    As depicted in  FIGS. 9 and 10  and discussed at length above insert  21  is sized to fit snugly in pocket  35  of probe body  23  with an airtight fit between insert exterior surfaces  49 A,  49 B,  49 C and  49 D and matching exterior surfaces  51 A,  51 B,  51 C and  51 D. Probe body  23  is made of a hard substantially rigid material such as epoxy or other plastic like material while insert  21  is made of a pliable rubber like material. Thus, by properly sizing insert  21  with respect to pocket  35  of probe body  23  the necessary air tight fit can be achieved by proper manufacture of the insert and probe body. 
         [0058]    The problem of acoustic focusing is another problem the present invention deals with. Acoustic focusing refers to the comparable effect of a lens has on light as it passes between two different medium with different indexes of refraction. For example when light passes from air into a lens its rays or wave fronts are diverted from their direction of travel to a new direction; thus, when the light passes out the other side of the lens it may be focused on a point or area different from what it originally was directed towards prior to entering the lens. Likewise with respect to ultrasound waves the index of refraction with respect to optics is equivalent to the acoustic impedance and difference in velocity of sound between two different materials. Just as a bigger difference between the index of refraction in two different mediums causes light to be reflected or refracted more when passing between two mediums so too with sound passing between two different materials with different acoustic impedance and acoustic velocity. Referring to  FIG. 15  a schematic diagram of sound passing through an insert with a vessel and fluid flowing in it of significantly different acoustic velocities of sound. As can be seen vessel  75  and fluid  77  cause ultrasound waves  79  generated by transducer  81  and passing through insert  80  to bend and be focused at receiving transducer  83 . 
         [0059]    The present invention minimizes the “acoustic focusing” by ensuring that the acoustic velocity, speed of sound, in insert  21  is matched to that of the conduit and fluid flowing in the conduit. In the case of a preferred embodiment of the invention it involves matching the acoustic impedance and speed of sound in insert  21  such that it is the same or almost the same as that of blood and the veins and arteries of an animal and human.  FIG. 16  is a schematic diagram illustrating the effect that matching the acoustic velocity in the insert  85  to the conduit  75  and fluid  77  flowing in the conduit has on ultrasound waves  79  generated by transducer  81  and received by transducer  83 . As can be seen ultrasound waves  79  are not distorted in any significant way by passing through vessel  75  and fluid  77 . 
         [0060]    The choice of material for use in the single use insert is extremely important. The material must match the acoustical properties of the fluid which is to be measured; in most cases blood. The acoustical velocity of the material will have a dramatic effect on focusing of acoustical beams and overall probe reading. According to Snell&#39;s law, the larger the mismatch between velocities, the greater the refraction of waves between two materials. This was described in the patent. If the waves are greatly refracted, we lose “full flow illumination”. A secondary effect of acoustical velocity mismatch is that the flow measurement will be negatively impacted. Because of the curvilinear shape of the vessel, the number of waves which pass through each material is dependent upon position relative to the center of the vessel. For instance, the thickness of the insert is significantly thinner at its central axis as compared to the top of the lumen. If material acoustic velocities differ, the ultrasound transit time measurement will be off. The acoustical impedance match between two materials will determine the extent of reflection and transmission through the boundary. Because the acoustical impedance of a material can be determined by multiplying the density of the material with its acoustical velocity, two materials which have similar acoustical velocities will have similar impedances if their densities match. This invention stresses that an ideal material will have similar acoustical velocities and density to the fluid being measured. 
         [0061]    In the preferred embodiment of the present invention the insert cuff can be made of Pebax 3533 manufactured by the Arkema or Tecoflex® manufactured by the Thermdics company. Naturally, any other material that is flexible and rubber like could be used provided the acoustic impedance and velocity of sound in that material could be matched to the conduit and fluid in the conduit under investigation such as an arteries and veins and blood flowing in them. A third property of an ideal material for insert  21  is one that has a stable acoustic velocity over the range of operating temperatures that the insert will experience. In many materials their acoustic velocity changes significantly with temperature changes. However, a material like Pebax changes very little in the range of 20 to 40 degrees C., the typical operating range that the insert of the present invention will be operating under. This ensures that the probe and insert can be calibrated and the results relied on over a significant temperature range. 
         [0062]    The insert of the present invention contains no electrical components and given the type of material it is made of can be injection molded in high volumes and are extremely cheap to make in large numbers. Thus, the insert cuffs of the present invention are disposable and economical. Consequently, a surgeon will be able to have a number of insert cuffs with varying lumen diameters present and sterilized during the initial implantation and thus should be able to place an insert with the proper lumen size around the artery or vein to be monitored. Therefore, once the vessel is exposed, a proper size can be chosen so that the vessel is neither squeezed nor surrounded by open air or materials with varying acoustical properties. Adjustment to proper fit of the disposable insert cuff is as simple as choosing a cuff insert with a correctly sized lumen. 
         [0063]    Sets of inserts with apertures formed by lumen surfaces of varying size designed to securely fit around the conduit or vessel could be made and used with one or two probe bodies.  FIG. 17  provides an example of a probe body  93  that would be matched with the set of inserts  91 A,  91 B,  91 C and  91 D.  FIG. 18  provides a view how each insert  91 A,  91 B,  91 C and  91 D might appear when inserted in probe body  93 .  FIGS. 17 and 18  or illustrative of the concept of a set of disposable insert. For use with animals of human sizes of the aperture  25  formed by lumen surface  27  might vary in 1 mm increments in diameter from 1 mm or 2 mm all the way up to inserts with apertures of 36 mm. A set might contain a set of inserts with apertures of diameters such as 8 mm, 10 mm, 12 mm, 14 mm, 16 mm, 20 mm, 24 mm, 28 mm, 32 mm and 36 mm. Also, it is possible to provide sets where the incremental change is smaller or bigger. Additionally, inserts could be specially made to very precise sizes down to a 10 th  of a millimeter with a set say of 17 mm, 17.25, 17.5, 17.75, and 18 mm or even smaller gradations such as 17.54 mm lumen. 
         [0064]    Thus, with the ability to make sets of inserts with finely graded aperture diameters a doctor or other health care professional will not squeeze or alter in any way a vein or artery when placing the flexible insert over the artery or vein. In the preferred embodiment, no wrapping or ultrasonic couplant is needed between insert and conduit. The transducers of the probe will maintain constant pressure against the cuff insert, ensuring minimal air pockets. 
         [0065]    In another variation of the preferred embodiment a flange  101   FIG. 1  can be added to lumen surface  27  at openings  103  on either side of insert  21  which with lumen surface  27  form aperture  25 . Flange  101  is an extension of insert material out from lumen surface  27  that makes the aperture wider than the width of insert  21 . Flange  101  thus would create an extension of the aperture that would aid in assuring insert  21  and thus probe body  23  are properly aligned with vertical axis  105  and horizontal axis  107   FIG. 7  of the insert and probe body. This would be another way of helping assure the transducers are properly aligned with vessel  53 . In a preferred embodiment the extensions can be thin no more than 1 mm thick and extend out from the insert from 2 mm to 3 mm. 
         [0066]    In the preferred embodiment insert  21  can be secured in detachable but secure fashion inside probe body  23 . One such a way is to simply make insert  21  with a slightly oversized fit between probe and insert provides a means to prevent the insert from moving relative to the probe. In another way suture holes  111   FIG. 2  are placed strategically in insert  21  which allow for sutures  113  to be secured through suture holes  111  and over notches  115 , to allow for extra support if needed. In another embodiment of the invention, the insert is held close against the transducer surface by clips. These clips maintain a force pressing the insert both towards the transducers and towards the bottom of the probe. They provide a mechanism to quickly lock down the insert to prevent it from moving during measurements as well as provide a method to rapidly remove the insert and probe when needed. In another embodiment, the probe itself contains a ledge that the insert is held under to maintain position. 
         [0067]    In a preferred embodiment of the invention, the insert cuff is disposable. They can be initially sterilized by a variety of methods including EtO, Sterrad and gamma radiation. 
         [0068]      FIG. 19  is a face view of an insert  21  and probe body  23  with another way to secure insert  21  in probe body  23  in a detachable but secure fashion. Clip  123  pivots at point  123 P between a closed position  123 C where it is secured in notch  127  to an open position  123 O. Likewise clip  125  pivots at point  125 P between a closed position  125 C where it is secured in notch  129  to an open position  125 O.  FIG. 19A  is a side view of probe body  23  of  FIG. 19  from position IXX-IXX. Clip  123  is in the open position  123 O and loops over the outside of probe body  23  between pivot points  123 P on either side of probe body  23 . As can be seen, in the open position  123 O insert  21  can be removed from probe body  23 . However, once clip  123  is snapped into notch  127  in the closed position  123 C it securely but detachably holds insert  21  in probe body  23 . Clips  123  and  125  can be made of surgical stainless steel, epoxy or any other type of rigid biocompatible rigid material. 
         [0069]      FIG. 20  provides another way of securely but detachably securing insert  21  in probe body  23  with the addition of lip or flange  135  at the base of leg  29  which hooks in and lip or flange  137  at the base of leg  31  which also hooks in. Insert  21  when placed in probe body would slip over lips  135  and  137  because of its flexible make up and be securely but detachably held by probe body  23 . 
         [0070]      FIG. 21  is a side view of another version of the probe insert with a two transducer set up, with both transducers set up on one side of the vessel and a reflector on the opposite. In  FIG. 21  insert  141  is secured in probe  143 . Probe  143  consists of a probe body  147  with support arm  149  terminating in reflector arm  151  which is perpendicular to support arm  149  to thereby hold insert  141  between it and probe body  147  and reflecting arm  151 . Aperture  155  runs through insert parallel to reflecting surface  151 R. Split  157  that runs the length of insert  141  parallel to Aperture  155  allows insert to be secured around conduit or vessel  159  in the same fashion as discussed above.  FIG. 22  is a top view of the probe body and insert of  FIG. 21  from view XXII-XXII. Arm  149  which is on the opposite side is in outline form. Additionally a first transducer  163  and a second transducer  167  can also be seen in outline form, they actually are embedded in probe body  147 . Transmissions between transducer  163  and  167  would pass along path ultrasound path  171 , the transmission coming off of and being received by transmission surfaces  163 T and  167 T with it reflecting off of acoustic reflecting surface  151 R. Transducer  163  connects via line  173  through probe stem  173  to a CPU or flowmeter, not shown. Likewise transducer  167  connects by line  175  through probe stem  173  to the CPU or flow meter. 
         [0071]      FIG. 23  is a top view of insert  141  and  FIG. 24  is a perspective view of the insert  141 . The material of both probe body  147  and insert  141  would be made of the same materials and in the same fashion as discussed above. 
         [0072]    While the invention has been particularly shown and described with reference to a preferred embodiment thereof, it will be understood by those skilled in the art that various changes in form and detail may be made to it without departing from the spirit and scope of the invention.