the present invention relates to an apparatus and a method for image guided insertion and removal of a cannula or needle . many specific details of certain embodiments of the invention are set forth in the following description and in fig1 through 20b to provide a thorough understanding of such embodiments . one skilled in the art , however , will understand that the present invention may have additional embodiments , or that the present invention may be practiced without several of the details described in the following description . fig1 a is a schematic cross - section view of a needle / cannula guide device 10 according to an embodiment of the invention . the needle / cannula guide device 10 includes a v - block 12 that supports a needle or cannula 18 . the v - block 12 includes two opposing sections that are coupled to each other at an apex . magnetic strips 16 are positioned on an exterior portion of the v - block 12 that magnetically retain the cannula 18 within the v - block 12 . accordingly , the v - block 12 may be fabricated from a suitably non - magnetic material , so that magnetic fields generated by the magnet strips 16 retain the metal needle 18 in the v - block 12 . the non - magnetic material of the v - block 12 may be comprised of a low friction polymeric material such as , for example , teflon ®, nylon ®, or delrin ®. alternatively , it may be comprised of a ferromagnetic material that may similarly convey the magnetic fields generated by the magnets 16 . the magnets 16 may be fixedly coupled to the v - block 12 . alternately , the magnets 16 may be removably coupled to the v - block 12 . fig1 b is a schematic cross - section view of a needle / cannula guide device 10 a according to another embodiment of the invention . many of the details of the present embodiment have been described in detail in connection with the embodiment shown in fig1 a , and in the interest of brevity , will not be described further . the guide device 10 a includes a foil wrapper 20 or other suitable wrapper materials that substantially encloses the cannula 18 . the wrapper 20 may be subjected to sterilization procedures so that the assembly 10 a may be sterilized by autoclaving , irradiation , or other known chemical processes . the foil wrapper 20 is generally sealably coupled to the v - block 12 so that the cannula 18 is substantially isolated from contaminants , yet is configured to be easily removed from the v - block 12 . fig1 c , d , and e illustrate alternate embodiments of the cannula guide devices 10 and 10 a , as shown in fig1 a and fig1 b , respectively . fig1 c is a plan view of the devices 10 and 10 a where the cannula 18 is positioned in the v - block 12 and is held in position by the magnets 16 , which extend uninterrupted along a length of the v - block 12 . fig1 d is a plan view of the devices 10 and 10 a that shows a first set of magnets 16 a positioned on first selected portions of the v - block 12 , and a second set of magnets 16 b that are positioned on second selected portions of the v - block 12 . as shown in fig1 d , the second set 16 b may be positioned between the first set 16 a . fig1 e is a plan view of the devices 10 and 10 a that shows magnets 16 a interruptably positioned on the v - block 12 . although the magnets 16 , 16 a and 16 b are generally depicted in fig1 c , fig1 d and fig1 e as rectangular , it is understood that the magnets 16 , 16 a and 16 b may have any regular shape . fig2 a and 2b are cross sectional and plan views , respectively , of a cannula guide device 20 a according to another embodiment of the invention . in fig2 a , the v - block 12 includes four magnet strips 24 , positioned on each arm of the v - block 12 that are used to generate a retaining force on the needle 18 . referring now also to fig2 b , the placement of the magnets 24 on the v - block 12 advantageously permit the v - block 12 to accommodate a variety of needle diameters . fig3 a and 3b are cross sectional and plan views , respectively , of a cannula guide device 20 b according to still another embodiment of the invention . the device 20 b includes magnets 24 b that are operable to generate an attractive force that is different from magnets 24 a . accordingly , the magnets 24 b may generate a greater attractive force on the needle 18 than the magnets 24 a . alternately , the magnets 24 a may generate a greater attractive than the magnets 24 b . fig4 a and 4b are cross sectional and plan views , respectively , of a cannula guide device 20 c according to still yet another embodiment of the invention . the device 20 c includes a unitary magnet strips 27 having regions that generate different attractive forces on the needle 18 . accordingly , the unitary magnetic strips 27 include a first magnetic strip portion 26 a and a second magnetic strip portion 26 b . the attractive force generated by the portion 26 a may be greater than the attractive force generated by the portion 26 b , or the attractive force generated by the portion 26 b may be greater than the attractive force generated by the portion 26 a . fig5 a and 5b are isometric views , respectively , of magnetic strips 30 a and 30 b that may be removably coupled to the v - block 12 ( fig1 a ). the magnetic strips 30 a and 30 b include a tab 34 configured to apply a pulling force to the strips 30 a and 30 b . referring now in particular to fig5 a , a unitary magnetic element 32 is positioned on the strip 30 a that generates a relatively uniform attractive force on the needle 18 ( not shown ). magnetic strip 30 b shown in fig5 b includes a magnetic element 36 that also includes magnetic portions 36 a and 36 b that are configured to generate different attractive forces on the needle 18 ( not shown ). the magnetic strips 30 a and 30 b may also include an adhesive material that is operable to retain the strips 30 a and 30 b onto external surfaces of the v - block 12 . fig6 a and 6b are respective isometric views of needle guidance devices 40 a and 40 b . in fig6 a , the needle guidance device 40 a includes the magnetic strips 30 a as shown in fig5 a that are positioned on the exterior of the v - block 12 . the attractive force of the magnetic strips 30 a magnetically holds the needle 18 within an inner portion of the v - block 12 . in fig6 b , the needle guidance device 40 b includes the magnetic strip 30 b of fig5 b positioned on the v - block 12 . fig7 a - 7e are isometric views of the needle guidance device 40 a that will be used to a method of using the needle guidance device 40 a according to another embodiment of the invention . fig7 a and fig7 b show a first selected one of the magnetic strips 30 a being progressively removed from the v - block 12 . the first selected one of the strips 30 a may be removed by a user by grasping the tab 34 and applying a pulling force on the tab 34 in the direction shown . accordingly , the attractive force on the needle 18 is also progressively reduced . a selected length of the strip 30 a may be removed so that a desired attractive force acting on the needle 18 is attained . referring now to fig7 c , a second selected one of the strips 30 a may be removed by grasping the tab 34 and applying a pulling force on the tab 34 in a suitable direction . as a result , the attractive force on the needle 18 is still further reduced . although fig7 a through 7c show a single magnetic strip applied to external surfaces of the v - block 12 , more than one magnetic strip may be present on an external surface of the v - block 12 . referring now to fig7 d , when the first selected strip and the second selected strip are removed to a desired degree , the needle 18 may be separated from the v - block 12 . as shown in fig7 e , the magnetic strips 30 a may be positioned on the v - block 12 so that the strips 30 a are oriented oppositely to those shown in fig7 a through 7d . fig8 a - 8c are respective cross sectional views of a needle guidance device 50 according to yet another embodiment of the invention . the needle guidance device 50 includes a pair of opposing metal cores 54 having a gap 58 a and a gap 58 b between the ferromagnetic cores 54 . the metal cores 54 are generally semi - circularly shaped and may be made of any metal or metal alloy suitable for conveying a magnetic field , such as a ferromagnetic or ferrite material . a magnet 56 is removably positionable within a selected one of the gaps 58 a and 58 b . for purposes of illustration , the magnet 56 is positioned in the gap 56 a . when the magnet 56 is positioned within a selected one of the gaps 58 a and 58 b , a magnetic field is communicated along the cores 54 from the gap 58 a to the gap 58 b . the gap 58 b is configured to accept a needle 18 so that the needle 18 will be retained in the gap 58 b by the magnetic fields communicated from gap 56 a . as shown in fig8 a , the lines of the magnetic force are conveyed across the space 58 b . referring briefly now to fig8 b , the needle 18 is held within the gap 58 b . accordingly , the needle 18 will be retained within the gap 58 b while the magnet 56 is positioned within gap 58 a . the gap 58 b progressively narrows to accommodate needles having variable diameters . turning now to fig8 c , as the magnet 56 is moved outwardly from the gap 58 a of the needle guidance device 50 , the magnetic field spanning the gap 58 b is correspondingly reduced . accordingly , the needle 18 positioned within the gap 58 b may be gradually released from the needle guidance device 50 . fig9 a - 9d are respective cross sectional views of a needle guidance device 60 according to yet still another embodiment of the invention . with reference now to fig9 a , the needle guidance device 60 includes a magnet 66 that is configured to be rotated within the gap 58 a . in fig9 a , the magnet 66 is shown in a first position so that the magnetic lines of force are communicated along the ferromagnetic cores 54 . accordingly , a magnetic field is established within the gap 58 b , so that the needle 18 is retained within the gap 58 b , as shown in fig9 b . in fig9 c , the magnet 66 is rotated to a second position so that the magnetic lines of force are generally directed away from the ferromagnetic cores 54 . accordingly , the attractive force that retains the needle 18 within the gap 58 b is reduced so that the needle 18 may be moved away from the gap 58 b . fig1 a is an isometric view of the needle guidance device 50 of fig8 a through 8c . in this schematic view , the needle 18 is held into the gap 58 b by the magnetic field generated by the magnet 56 . the needle 18 is retained from moving through the gap 58 b and into an internal region of the device 50 by providing beveled walls within the gap 58 b that have a minimum distance “ d ” so that the beveled walls interfere with further movement of the needle 18 through the gap 58 b since the distance “ d ” is generally selected to be smaller than a diameter of the needle 18 . referring now to fig1 b , method of disengagement of the needle 18 from the gap 58 b is shown . the disengagement of the needle 18 from the needle guidance device 50 includes moving the magnet 56 upwardly and away from the cores 54 . correspondingly , a reduction in magnetic holding force occurs within the gap 58 b so that the needle 18 may be removed from the needle guidance device 50 . fig1 c shows an alternate method for disengagement of the needle 18 from the needle guidance device 50 . moving the magnet 56 longitudinally along the gap 58 a so that the magnetic force across the gap 58 b is proportionately reduced effects the disengagement of the needle 18 . depending upon the relative strength of the magnet 56 , the composition of the cores 54 and the material used to fabricate the needle , a user removing the magnet 56 may find that the magnetic holding force is sufficiently reduced to permit non - injurious disengagement of the needle 18 from the gap 58 b of the needle guidance device 50 when the magnet 56 is only partially disengaged from the gap 58 a . alternately , the user may be required to completely remove the magnet 56 from the gap 58 a in order to release the needle 18 from the device 50 . fig1 a is an isometric view of the needle guidance device 60 that shows the needle 18 held in position by the rotating magnet 66 . in this case , the rotatable magnet 66 is in the vertical position within the gap 58 a , and the magnetic forces hold the needle 18 within the gap 58 b . fig1 b shows a completion of the disengagement process from fig1 a . the rotatable magnet 66 is rotated to a horizontal position as indicated by the crosshatched arrow within the gap 58 a . this rotation causes either a reduction of retentive magnetic forces spanning across the gap 58 b or generation of repulsive forces . as indicated by the downward arrow , the needle 18 becomes disengagable from the needle guidance device 60 and eventually separates from the gap 58 b . fig1 a is an isometric view of a needle guidance device 70 , according to another embodiment of the invention . the device 70 includes two ferromagnetic core assemblies 54 that are longitudinally spaced apart and share a common movable permanent magnet 56 configured to engage respective gaps 58 a in the core assemblies 54 . the magnet 56 may either be slidably disengaged from each ferromagnetic core assembly 54 either longitudinally or it may be removed from the gap 58 a by moving the magnet 56 in a radial direction and away from the core assemblies 54 . in either event , the progressive removal of permanent magnet 56 from the respective gaps 58 a causes a progressive reduction in magnetic fields across the gaps 58 b . accordingly , a user may advantageously select a suitable retentive force for the needle 18 . fig1 b shows a disengagement of the operation in the orthogonal displacement . here , the needle guidance device 70 is in a disengagement process where the permanent magnet 56 is removed 90 ° orthogonal to the spaces 58 a , to each ferrite core assembly 54 . removal as previously mentioned of a permanent magnet 56 causes a diminution magnetic retentive forces across the gap 58 b resulting in a progressively easier disengagement force to be affected to the needle 18 . fig1 a shows a needle guidance 80 being an electromagnetic alternate embodiment to the permanent magnet embodiment 70 . this electromagnetic embodiment 80 includes a dc power assembly that has a power source 82 , a variable resistor 84 connected to the power source 82 , in communication with a coil winding ( not shown — see fig1 b below ) electrically connected with the source 82 and resistor 84 via a wire 86 . the wire 86 is connected with the coil winding ( not shown ) that is wrapped within the groove 158 of the electromagnet 156 . the electromagnet 156 is a non - permanent electromagnet that respectfully occupies the spaces 58 a of metal cores 54 . the dashed arrow 84 a within the variable resistor 84 shows a resistor position when there is sufficient power that is delivered to the core winding occupying the grove 158 to induce a magnetic field of sufficient strength to hold the needle 18 across respective gaps 58 b of each iron or other metal core assembly 54 that is able to convey the magnetic flux fields generated by the electromagnet 156 . reducing the power indicated by the solid arrow 84 b resistor position progressively causes a reduction of magnetic force due to the diminution of current and / or voltage applied to the windings occupying the grove 158 . eventually the magnetic power is progressively lessened such that an applied disengagement force by a user permits the removal or non - injurious disengagement of the needle 18 , as indicated by the downward arrow , from the gaps 58 b of the guidance device 80 . fig1 b is an isometric view schematically depicting the electromagnet of fig1 a . within the grooves 158 of the he electromagnet 156 is a coil winding 88 . application of electrical power by the dc power supply 82 through the variable resistor 84 results in a magnetic force generated by the electromagnet 156 in proportion to the amount of electrical power delivered to the coil winding 88 . north , n and south , s poles are formed along the electromagnet 156 . as the power is gradually lessened between the 84 a and 84 b resistor positions , the retentive magnetic force field generated along the electromagnet 156 is accordingly lessened . as previously described for the removal of the magnetic strip embodiments and the permanent magnets and the electromagnet needle guidance devices as previously described provides a means for holding a selected cannula such that the cannula is controllably restricted in motion substantially along one dimension . the user may either manipulate the amount of magnetic strips to vary the magnetic power by the permanent magnets or adjust power to electromagnets so that a user may progressively overcome the retentive forces still applied to the needle 18 and effect the extraction or disengagement of the needle 18 from the respective needle guidance devices in a non - injurious way from a patient or other subject . fig1 - 20b are partial isometric views that depict various embodiments of the present invention coupled to an ultrasound transceiver 100 . in the description that follows , it is understood that the various embodiments may be removably coupled to the ultrasound transceiver 100 , or they may be permanently coupled to the transceiver 100 . it is also understood that , although an ultrasound transceiver is described in the following description and shown in the following figures , the various embodiments may also be incorporated into other imaging devices . fig1 is a partial isometric side view the v - block 40 a of fig6 a and fig6 b coupled to an ultrasound transceiver 101 to form an assembly 100 . the ultrasound transceiver 101 has the needle guidance device 40 a coupled to a transducer housing 104 of the transceiver 101 using a bridge 108 . the needle guidance device 40 a may be fixedly coupled to the housing 104 , or the device 40 a may be removably coupled to the housing 104 . in either case , the transceiver 100 also includes a trigger 102 , a display 103 , a handle 106 , and a transducer dome 112 . upon pressing the trigger 102 , an ultrasound scancone 116 emanates from the transducer dome 112 that penetrates a subject or patient . the scancone 116 is comprised of a radial array of scan planes 118 . within the scanplane 118 are scanlines ( not shown ) that may be evenly or unevenly spaced . alternatively , the scancone 116 may be comprised of an array of wedged distributed scancones or an array of 3d - distributed scanlines that are not necessarily confined to a given scan plane 118 . as shown , the scancone 116 is radiates about the transducer axis 11 that bisects the transducer housing 104 and dome 112 . fig1 is a partial isometric , side view of the needle guidance device 50 of fig8 a , fig8 b and fig8 c coupled to the ultrasound transceiver 101 to form an assembly 120 . the ultrasound transceiver 101 has the needle guidance device 50 mounted to the transducer housing 104 using the bridge 108 of fig1 . the device 50 may be fixedly or removably coupled to the housing 104 . a scan cone 116 is similarly projected from the transceiver 101 . various aiming aids may be placed on the needle guidance device 50 to assist a user in aiming the insertion of a needle that is held by a magnetic force to slide within the gap 58 b . fig1 is a partial isometric view of a needle guidance device 90 that may be removably coupled to the housing 104 of an ultrasound transceiver 101 , according to another embodiment of the invention . the needle guidance device 90 is attached to an engagement wedge 92 . the engagement wedge 92 slidably and removably attaches with the slot holder 94 that is positioned on a selected portion of the housing 104 . various aiming aids may be placed on the needle guidance device 90 to assist a user in aiming the insertion of a needle that is held by a magnetic force to slide within the gap 58 b . fig1 is a partial isometric view of a needle guidance device 130 according to another embodiment of the invention . the device 130 is configured to be positioned within a transceiver housing 132 . a pair of magnets 134 and 136 are positioned on a rotational shaft 137 that projects into the housing 132 . the magnets 134 and 136 provide an attractive force on the needle 18 when the magnets 134 and 136 are aligned with the needle 18 . when the magnets 134 and 136 are rotated away from alignment ( by manually rotating a wheel 139 coupled to the shaft 138 ) with the needle 18 , the attractive force on the needle 18 is reduced , thus allowing the needle 18 to be moved relative to the housing 132 . fig1 a is a side view of an ultrasound scanner having a magnetic guide assembly 144 , according to an embodiment of the invention . the guidance assembly 144 includes the transceiver 101 in which a needle 18 with reservoir 19 is held within a ferrite housing 144 . the ferrite housing 144 is secured to transducer housing 104 by a clip - on clasp 142 . fig1 b is an isometric view and exploded view of components of the assembly 144 of fig1 a . in the exploded view , the guidance assembly 144 is seen in greater detail . the ferrite housing 144 receives ferrite cores 146 and 150 . rotable within the space defined by the ferrite core 146 and gap 58 a of ferrite cores 150 is a rotatable magnet 148 . located between the clip - on clasp 142 and the ferrite housing 144 is an articulating bridge 143 . the articulating bridge 143 allows the user to alter the entry angle of the needle 18 into the patient relative to the transducer axis 11 as illustrated in fig1 . rotating the magnet 148 alters the magnetic holding power to gap 58 b between ferrite cores 150 . fig1 a is a side view of alternate embodiment shown in fig1 a that uses a sliding magnet . a guidance assembly 170 includes the transceiver 101 in which a needle 18 with reservoir 19 is held within a ferrite housing 145 . the ferrite housing 145 is secured to transducer housing 104 by a clip - on clasp 142 and articulating bridge 143 . the ferrite housing 145 is configured to receive three components . fig1 b is an isometric view and exploded view of the components of the device 170 of fig1 a . in the exploded view the guidance assembly 170 is seen in greater detail . the ferrite housing 145 receives two ferrite cores 172 and a slidable magnet 176 . the slidable magnet 176 is moveable within the space 56 a defined by the ferrite cores 172 . opposite the space 56 a is space 56 b that receives the needle 18 . the articulating bridge 143 allows the user to alter the entry angle of the needle 18 into the patient or subject relative to the transducer axis 11 as illustrated in fig1 . sliding the magnet 176 alters the magnetic holding power to gap 58 b between ferrite cores 172 . fig2 a is a side view of alternate embodiment of the device 170 of fig1 a utilizing a pulling magnet . a guidance assembly 180 includes the transceiver 101 in which a needle 18 with reservoir 19 is held within a ferrite housing 182 . the ferrite housing 182 is secured to transducer housing 104 by a clip - on clasp 142 and articulating bridge 143 . the ferrite housing 145 is configured to receive three components . fig2 b is an isometric view and exploded view of components of the device 180 of fig2 a . in the exploded view the guidance assembly 180 is seen in greater detail . the ferrite housing 182 receives two ferrite cores 188 and a trigger receiver 186 . the trigger receiver 186 receivers the trigger 190 that has a magnet frame 191 . the magnet frame 191 retains the magnet 192 . the magnet 192 is snap - fitted into the magnet frame 191 of the trigger 190 . the magnet - loaded trigger 190 is slidably placed into the trigger receiver 186 . the trigger receiver 186 guides the magnet - loaded trigger 190 within the gap 58 b defined by the two ferrite cores 188 . pulling the magnet - loaded trigger 190 alters the magnetic holding power to gap 58 b receiving the needle 18 located opposite the gap 58 a between ferrite cores 188 . an example embodiment includes a system and method using single or multiple cameras for tracking and displaying the movement of a needle or cannula before and / or during insertion into a blood vessel or other sub - dermal structure and subsequent movements therein . a needle or a cannula - fitted needle may be detachably mounted to an ultrasound transceiver in signal communication with a computer system and display configured to generate ultrasound - acquired images and process images received from the single or multiple cameras . along the external surfaces of the needle or cannula may be fitted optical reflectors that may be discernable in the camera images . the ultrasound transceiver may be secured against a subject &# 39 ; s dermal area adjacent to a sub - dermal region of interest ( roi ). optical signals may be reflected towards the single or multiple cameras by the needle or cannula embedded reflectors and conveyed to the computer system and display . the trajectories of the needle or cannula movements may be determined by data analysis of the reflector signals detected by the cameras . the trajectories of needle or cannula having one or more reflectors may be overlaid onto the ultrasound images to provide alignment coordinates for insertion of the needle or cannula fitted needle into the roi along a determined trajectory . an example embodiment of the present invention generally includes an ultrasound probe attached to a first camera and a second camera . the example embodiment also generally includes a processing and display generating system that may be in signal communication with the ultrasound probe , the first camera , and / or the second camera . typically , a user of the system scans tissue containing a target vein using the ultrasound probe and a cross - sectional image of the target vein may be displayed . the first camera captures and / or records a first image of a medical object to be inserted , such as a cannula for example , in a first direction and the second camera captures and / or records a second image of the cannula in a second direction orthogonal to the first direction . the first and / or the second images may be processed by the processing and display generating system along with the relative positions of the ultrasound probe , the first camera , and / or the second camera to determine the trajectory of the cannula . a representation of the determined trajectory of the cannula may be then displayed on the ultrasound image . fig1 is a diagram illustrating a side view of one embodiment of the present invention . a two - dimensional ( 2d ) ultrasound probe 10 may be attached to a first camera 14 that takes images in a first direction . the ultrasound probe 10 may be also attached to a second camera 18 via a member 16 . in other embodiments , the member 16 may link the first camera 14 to the second camera 18 or the member 16 may be absent , with the second camera 18 being directly attached to a specially configured ultrasound probe . the second camera 18 may be oriented such that the second camera 18 takes images in a second direction that may be orthogonal to the first direction of the images taken by the first camera 14 . the placement of the cameras 14 , 18 may be such that they can both take images of a cannula 20 when the cannula 20 may be placed before the cameras 14 , 18 . a needle may also be used in place of a cannula . the cameras 14 , 18 and the ultrasound probe 10 may be geometrically interlocked such that the cannula 20 trajectory can be related to an ultrasound image . in fig1 , the second camera 18 may be behind the cannula 20 when looking into the plane of the page . in an embodiment , the cameras 14 , 18 take images at a rapid frame rate of approximately 30 frames per second . the ultrasound probe 10 and / or the cameras 14 , 18 may be in signal communication with a processing and display generating system 61 described in fig7 and 8 below . in typical operation , a user first employs the ultrasound probe 10 and the processing and display generating system 61 to generate a cross - sectional image of a patient &# 39 ; s arm tissue containing a vein to be cannulated (“ target vein ”) 19 . this could be done by one of the methods disclosed in the patents , patent publications and / or patent applications which are herein incorporated by reference , such as , for example , u . s . patent application ser . no . 11 / 460 , 182 filed jul . 26 , 2006 . the user then identifies the target vein 19 in the image using methods such as simple compression which differentiates between arteries and / or veins by using the fact that veins collapse easily while arteries do not . after the user has identified the target vein 19 , the ultrasound probe 10 may be affixed to the patient &# 39 ; s arm over the previously identified target vein 19 using a magnetic tape material 12 , for example . the ultrasound probe 10 and the processing and display generating system 61 continue to generate a 2d cross - sectional image of the tissue containing the target vein 19 . images from the cameras 14 , 18 may be provided to the processing and display generating system 61 as the cannula 20 may be approaching and / or entering the arm of the patient . the processing and display generating system 61 locates the cannula 20 in the images provided by the cameras 14 , 18 and determines the projected location at which the cannula 20 will penetrate the cross - sectional ultrasound image being displayed . the trajectory of the cannula 20 may be determined in some embodiments by using image processing to identify bright spots corresponding to micro reflectors previously machined into the shaft of the cannula 20 or a needle used alone or in combination with the cannula 20 . image processing uses the bright spots to determine the angles of the cannula 20 relative to the cameras 14 , 18 and then generates a projected trajectory by using the determined angles and / or the known positions of the cameras 14 , 18 in relation to the ultrasound probe 10 . in other embodiments , determination of the cannula 20 trajectory may be performed using edge - detection algorithms in combination with the known positions of the cameras 14 , 18 in relation to the ultrasound probe 10 , for example . the projected location may be indicated on the displayed image as a computer - generated cross - hair 66 ( shown in fig7 ), the intersection of which may be where the cannula 20 is projected to penetrate the image . in other embodiments , the projected location may be depicted using a representation other than a cross - hair . when the cannula 20 does penetrate the cross - sectional plane of the scan produced by the ultrasound probe 10 , the ultrasound image confirms that the cannula 20 penetrated at the location of the cross - hair 66 . this gives the user a real - time ultrasound image of the target vein 19 with an overlaid real - time computer - generated image of the position in the ultrasound image that the cannula 20 will penetrate . this allows the user to adjust the location and / or angle of the cannula 20 before and / or during insertion to increase the likelihood they will penetrate the target vein 19 . in other embodiments , the ultrasound image and / or the computer - generated cross - hair may be displayed in near real - time . in an example embodiment , this allows a user to employ normal “ free ” insertion procedures while having the added knowledge of knowing where the cannula 20 trajectory will lead . fig2 is a diagram illustrating a top view of the embodiment shown in fig1 . it is more easily seen from this view that the second camera 18 may be positioned behind the cannula 20 . the positioning of the cameras 14 , 18 relative to the cannula 20 allows the cameras 14 , 18 to capture images of the cannula 20 from two different directions , thus making it easier to determine the trajectory of the cannula 20 . fig3 is diagram showing additional detail for a needle shaft 22 to be used with one embodiment of the invention . the needle shaft 22 includes a plurality of micro corner reflectors 24 . the micro corner reflectors 24 may be cut into , or otherwise affixed to or embedded in , the needle shaft 22 at defined intervals δl in symmetrical patterns about the circumference of the needle shaft 22 . the micro corner reflectors 24 could be cut with a laser , for example . fig4 a and 4b are diagrams showing close - up views of surface features of the needle shaft 22 shown in fig3 . fig4 a shows a first input ray with a first incident angle of approximately 90 ° striking one of the micro corner reflectors 24 on the needle shaft 22 . a first output ray is shown exiting the micro corner reflector 24 in a direction toward the source of the first input ray . fig4 b shows a second input ray with a second incident angle other than 90 ° striking a micro corner reflector 25 on the needle shaft 22 . a second output ray is shown exiting the micro corner reflector 25 in a direction toward the source of the second input ray . fig4 a and 4b illustrate that the micro corner reflectors 24 , 25 are useful because they tend to reflect an output ray in the direction from which an input ray originated . fig5 is a diagram showing imaging components for use with the needle shaft 22 shown in fig3 in accordance with an example embodiment of the invention . the imaging components are shown to include a first light source 26 , a second light source 28 , a lens 30 , and a sensor chip 32 . the first and / or second light sources 26 , 28 may be light emitting diodes ( leds ), for example . in an example embodiment , the light sources 26 , 28 are infra - red leds . use of an infra - red source is advantageous because it is not visible to the human eye , but when an image of the needle shaft 22 is recorded , the image can show strong bright dots where the micro corner reflectors 24 may be located because silicon sensor chips are sensitive to infra - red light and the micro corner reflectors 24 tend to reflect output rays in the direction from which input rays originate , as discussed with reference to fig4 a and 4b . in alternative embodiments , a single light source may be used . although not shown , the sensor chip 32 may be encased in a housing behind the lens 30 and the sensor chip 32 and light sources 26 , 28 may be in electrical communication with the processing and display generating system 61 shown in fig7 below . the sensor chip 32 and / or the lens 30 form a part of the first and second cameras 14 , 18 in some embodiments . in an example embodiment , the light sources 26 , 28 may be pulsed on at the time the sensor chip 32 captures an image . in other embodiments , the light sources 26 , 28 may be left on during video image capture . fig6 is a diagram showing a representation of an image 34 produced by the imaging components shown in fig5 . the image 34 may include a needle shaft image 36 that corresponds to a portion of the needle shaft 22 shown in fig5 . the image 34 also may include a series of bright dots 38 running along the center of the needle shaft image 36 that correspond to the micro corner reflectors 24 shown in fig5 . a center line 40 is shown in fig6 that runs through the center of the bright dots 38 . the center line 40 may not appear in the actual image generated by the imaging components , but is shown in the diagram to illustrate how an angle theta ( θ ) could be obtained by image processing to recognize the bright dots 38 and determine a line through them . the angle theta represents the degree to which the needle shaft 22 may be inclined with respect to a reference line 42 that may be related to the fixed position of the sensor chip 32 . fig7 is a system diagram of an embodiment of the present invention and shows additional detail for the processing and display generating system 61 in accordance with an example embodiment of the invention . the ultrasound probe 10 is shown connected to the processing and display generating system via m control lines and n data lines . the m and n variables are for convenience and appear simply to indicate that the connections may be composed of one or more transmission paths . the control lines allow the processing and display generating system 61 to direct the ultrasound probe 10 to properly perform an ultrasound scan and the data lines allow responses from the ultrasound scan to be transmitted to the processing and display generating system 61 . the first and second cameras 14 , 18 are also each shown to be connected to the processing and display generating system 61 via n lines . although the same variable n is used , it is simply indicating that one or more lines may be present , not that each device with a label of n lines has the same number of lines . the processing and display generating system 61 may be composed of a display 64 and a block 62 containing a computer , a digital signal processor ( dsp ), and analog to digital ( a / d ) converters . as discussed for fig1 , the display 64 can display a cross - sectional ultrasound image . the computer - generated cross hair 66 is shown over a representation of a cross - sectional view of the target vein 19 in fig7 . the cross hair 66 consists of an x - crosshair 68 and a z - crosshair 70 . the dsp and the computer in the block 62 use images from the first camera 14 to determine the plane in which the cannula 20 will penetrate the ultrasound image and then write the z - crosshair 70 on the ultrasound image provided to the display 64 . similarly , the dsp and the computer in the block 62 use images from the second camera 18 , which may be orthogonal to the images provided by the first camera 14 as discussed for fig1 , to write the x - crosshair 68 on the ultrasound image . in other embodiments , the dsp and the computer in the block 62 may use images from both the first camera 14 and the second camera 18 to write each of the x - crosshair 68 and the z - crosshair 70 on the ultrasound image . in still other examples , images from the cameras 14 , 18 may be used separately or in combination to write the crosshairs 68 , 70 or other representations of where the cannula 20 is projected to penetrate the ultrasound image . fig8 is a system diagram of an example embodiment showing additional detail for the block 62 shown in fig2 . the block 62 includes a first a / d converter 80 , a second a / d converter 82 , and a third a / d converter 84 . the first a / d converter 80 receives signals from the ultrasound probe 10 and converts them to digital information that may be provided to a dsp 86 . the second and third a / d converters 82 , 84 receive signals from the first and second cameras 14 , 18 respectively and convert the signals to digital information that may be provided to the dsp 86 . in alternative embodiments , some or all of the a / d converters are not present . for example , video from the cameras 14 , 18 may be provided to the dsp 86 directly in digital form rather than being created in analog form before passing through a / d converters 82 , 84 . the dsp 86 may be in data communication with a computer 88 that includes a central processing unit ( cpu ) 90 in data communication with a memory component 92 . the computer 88 may be in signal communication with the ultrasound probe 10 and may be able to control the ultrasound probe 10 using this connection . the computer 88 may be also connected to the display 64 and may produce a video signal used to drive the display 64 . in still other examples , other hardware components may be used . a field programmable gate array ( fpga ) may be used in place of the dsp , for example . or , an application specific integrated circuit ( asic ) may replace one or more components . fig9 is a flowchart of a process of displaying the trajectory of a cannula in accordance with an embodiment of the present invention . the process is illustrated as a set of operations shown as discrete blocks . the process may be implemented in any suitable hardware , software , firmware , or combination thereof . as such the process may be implemented in computer - executable instructions that can be transferred from one computer to a second computer via a communications medium . the order in which the operations are described is not to be necessarily construed as a limitation . first , at a block 100 , an ultrasound image of a vein cross - section may be produced and / or displayed . next , at a block 110 , the trajectory of a cannula may be determined . then , at a block 120 , the determined trajectory of the cannula may be displayed on the ultrasound image . fig1 is a flowchart of a process showing additional detail for the block 110 depicted in fig9 . the process is illustrated as a set of operations shown as discrete blocks . the process may be implemented in any suitable hardware , software , firmware , or combination thereof . as such the process may be implemented in computer - executable instructions that can be transferred from one computer to a second computer via a communications medium . the order in which the operations are described is not to be necessarily construed as a limitation . the block 110 includes a block 112 where a first image of a cannula may be recorded using a first camera . next , at a block 114 , a second image of the cannula orthogonal to the first image of the cannula may be recorded using a second camera . then , at a block 116 , the first and second images may be processed to determine the trajectory of the cannula . fig1 schematically depicts an alternative embodiment of a needle having a distribution of reflectors located near the bevel of the needle . a needle shaft 52 includes a bevel 54 that may be pointed for penetration into the skin to reach the lumen of a blood vessel . the needle shaft 52 also includes a plurality of micro corner reflectors 24 . the micro corner reflectors 24 may be cut into the needle shaft 52 at defined intervals δl in symmetrical patterns about the circumference of the needle shaft 52 . in an example , the micro corner reflectors 24 may be cut with a laser and serve to provide light reflective surfaces for monitoring the insertion and / or tracking of the trajectory of the bevel 54 into the blood vessel during the initial penetration stages of the needle 52 into the skin and / or tracking of the bevel 54 motion during guidance procedures . while the preferred embodiment of the invention has been illustrated and described , as noted above , many changes can be made without departing from the spirit and scope of the invention . for example , a three - dimensional ultrasound system could be used rather than a 2d system . in addition , different numbers of cameras could be used along with image processing that determines the cannula 20 trajectory based on the number of cameras used . the two cameras 14 , 18 could also be placed in a non - orthogonal relationship so long as the image processing was adjusted to properly determine the orientation and / or projected trajectory of the cannula 20 . the radiation emitting from the light sources 26 , 28 may be of a frequency and intensity that may be sufficiently penetrating in tissue to permit reflection of sub - dermal located reflectors 24 to the detector sensor 32 . the sensor 32 may be suitably filtered to optimize detection of sub - dermal reflected radiation from the reflectors 24 so that sub - dermal trajectory tracking of the needles 22 , 52 or cannulas 20 having one or more reflectors 24 may be achieved . also , an embodiment of the invention could be used for needles and / or other devices such as trocars , stylets , or catheters which are to be inserted in the body of a patient . additionally , an embodiment of the invention could be used in places other than arm veins . regions of the patient &# 39 ; s body other than an arm could be used and / or biological structures other than veins may be the focus of interest . while various embodiments of the invention have been illustrated and described , as noted above , many changes can be made without departing from the spirit and scope of the invention . for example , electromagnetic strips may be removably attached to v - blocks and the magnetic power controlled by an electric circuit applied to the electromagnetic strips . permanent magnets used in the various embodiments may be of any metal able to generate and communicate a magnetic force , for example , iron , iron alloys , and neodymnium based magnets . accordingly , the scope of the invention is not limited by the disclosure of the preferred embodiment . instead , the invention should be determined entirely by reference to the claims that follow .