Abstract:
Placement of a medical device within or in proximity to a substantially fluid filled compartment within a human body is aided by pulse-echo ultrasound and a time-stretching technique. In one embodiment, a small ultrasound transducer is placed near the tip of a catheter intended to be positioned within a ventricle of the brain to drain cerebrospinal fluid. The transducer is electrically pulsed so as to emit sound waves, and then used to receive returning echoes which contain information about structures in front of the transducer. These returning echoes are electronically amplified and processed in such a way that the information represented by the echoes is spread out over a much longer period than the actual acquisition time. This processed information may be presented to an operator visually, via audio feedback and/or via tactile feedback. Embodiments are disclosed where the information is displayed visually through variations in the color or brightness of a light source over time. Further embodiments are disclosed where the information is conveyed through audio that is modulated in amplitude, frequency, or both. Further embodiments are disclosed where the information is conveyed through tactile feedback that is modulated in intensity or duration. Further embodiments are disclosed which combine audio, visual, and/or tactile feedback. Further embodiments incorporate a digital or graphical display to provide quantitative information.

Description:
[0001]    The invention relates to an apparatus and method for positioning a medical device within or in proximity to a fluid containing compartment within a human body. 
       BACKGROUND 
       [0002]    The use of ultrasound to gather information about structure within living bodies, including the brain, has been practiced since at least the early 1940&#39;s. Ultrasound waves may be launched into tissue by an electrically stimulated transducer, which is typically constructed from a piezoelectric material. Once launched from the transducer, ultrasound waves interact with structures that they encounter, and may be transmitted, reflected, or scattered depending on the physical characteristics of the structure encountered. In particular, reflections or “echoes” arise when the ultrasound waves transition between materials (in this case, usually tissues or fluids) that have different acoustic impedances. 
         [0003]    The use of ultrasound specifically to aid in placement of a medical device within or in proximity to a substantially fluid filled compartment within a human body is also not new. U.S. Pat. No. 5,690,117, which is incorporated by reference in its entirety, describes an ultrasound-tipped silastic intracranial catheter which also incorporates optical imaging. Standard diagnostic ultrasound systems are used routinely to aid in cannulization of blood vessels, lancing of cysts, placement of drain catheters, and so forth. 
         [0004]    The use of echolocation where echoes of sound waves reflect from remote structures over time scales that are perceptible to a human is also not new. The earliest SONAR systems consisted simply of a sound transducer, amplifier, and speaker, where a sound wave was launched into water from the transducer, and then the operator would listen to amplified signals from the transducer for echoes. The time it would take for an echo to return to the transducer gave an indication of the distance to the reflecting structure. This sort of technique is not directly usable in medical ultrasound because the distances are small (usually less than 10 cm), and the echoes return so quickly that they are not evident to unaided human perception. 
         [0005]    The use of echolocation or RADAR to determine the position of structures, and then coding that information as audio information that is “time stretched” is also not new. For example, many modern automobiles incorporate hazard sensors where echolocation or RADAR is used to detect obstacles, and feedback is given to the driver as a series of audio “beeps”—the closer the obstacle is, the faster the beeps occur, leading up to a continuous tone when the obstacle is within a pre-determined distance from the vehicle. In this case, while the difference in echo time between the vehicle and obstacles at different distances may be on the order of milliseconds, the period between beeps indicating distance may be on the order of a second, which is easily perceptible by a human. Thus, the echo time is “stretched” so as to be perceptible. 
         [0006]    In medicine, there is often a need to quickly and accurately place a medical device within or in proximity to a substantially fluid filled compartment within a human body. For example, it may be necessary to place a catheter in a ventricle of the brain to allow drainage of cerebrospinal fluid. In current practice, procedures such as this are often done without any form of guidance—the practitioner simply relies of anatomical landmarks to assist in placement. Mis-placement, or repeated attempts at placement of such a catheter can have devastating effects due to resulting brain injury, yet attempts to provide guidance have been commercially unsuccessful. Thus, a simple and effective form of guidance is needed. 
       BRIEF SUMMARY OF THE INVENTION 
       [0007]    To address the above mentioned problems and others, the technology disclosed herein couples an ultrasound echo-location device with “time stretching” techniques to provide a simple and effective form of guidance for the placement of a medical device within or in proximity to a substantially fluid filled compartment within a human body. The apparatus constructed in accordance with an embodiment of the invention utilizes a rigid or minimally flexible hollow member with an ultrasound transducer mounted at its tip situated within an intracranial catheter to allow echolocation through the tip of the catheter. The ultrasound transducer is electrically coupled to both transmit and receive electronics via an electrical connection that may involve wires, the hollow member itself, or both the hollow member and one or more wires as conductors. Transmit electronics are provided to electrically excite the transducer so acoustic waves are generated. Receive electronics are provided to electrically amplify and process signals from the transducer that arise due to returning echoes from the transmitted waves. Processing of the received signals includes the derivation of a time-stretched signal that is representative of the originally received signal, but with features of the signal spread out over a time interval so that they are perceptible to a human. One or more audio, visual, or tactile outputs are provided in order to indicate the presence of a substantially fluid filled space, and give a qualitative or quantitative indication of the distance from the ultrasound transducer to this space. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]      FIG. 1  schematically illustrates a system according to an embodiment of the invention. 
           [0009]      FIG. 1   a  illustrates a returning echo signal with a fluid-filled space present. 
           [0010]      FIG. 1   b  illustrates an algorithm to determine whether or not a fluid filled space is present, and to determine the location of the space. 
           [0011]      FIG. 1   c  illustrates an algorithm to compute a “stretched” time corresponding to the middle of a fluid filled space. 
           [0012]      FIG. 1   d  illustrates an algorithm to track the position of an ultrasound transducer with respect to a fluid filled space in the presence of ring-down or other initial noise signals. 
           [0013]      FIG. 2  schematically illustrates the signal acquisition and transmission portion of a wirelessly coupled system according to an embodiment of the invention. 
           [0014]      FIG. 3  schematically illustrates the signal processing, control, and user interface portion of a wirelessly coupled system according to an embodiment of the invention. 
           [0015]      FIG. 4  illustrates an embodiment of this invention where all circuitry including a power source and user interface is incorporated in a housing that is directly coupled to a hollow member incorporating an ultrasound transducer. The hollow member is shown inserted into a catheter. 
           [0016]      FIG. 5  illustrates an embodiment of this invention where the hollow member incorporating the ultrasound transducer is coupled to circuitry including a power source and user interface through a flexible cable. 
           [0017]      FIG. 5   a  illustrates an embodiment of this invention where the hollow member incorporating the ultrasound transducer is coupled directly to a portion of circuitry which may include a power source and user interface, which is then coupled to additional circuitry which may contain a power source and user interface through a flexible cable. 
           [0018]      FIG. 6  shows a detail of the distal end of the hollow member incorporating an ultrasound transducer inserted into a catheter intended for drainage of cerebrospinal fluid which incorporates a feature that allows the hollow member to be pushed through the tip of the catheter. 
           [0019]      FIG. 7  shows a detail of the distal end of the hollow member incorporating an ultrasound transducer inserted into a catheter intended for drainage of cerebrospinal fluid with the hollow member incorporating an ultrasound transducer is pushed through the tip of the catheter. 
           [0020]      FIG. 8  shows a detail of the distal end of the hollow member incorporating an ultrasound transducer inserted into a catheter intended for drainage of cerebrospinal fluid which is open at the distal end allowing an unimpeded acoustic path to the front of the ultrasound transducer. 
           [0021]      FIG. 9  shows a detail of the distal end of the hollow member incorporating an ultrasound transducer inserted into a catheter intended for drainage of cerebrospinal fluid which is open at the distal end allowing an unimpeded acoustic path to the front of the ultrasound transducer. The tip of the catheter is designed so that a lip exists inside the tip of the catheter. The hollow member is designed so that a portion of it can push against the lip in the catheter during insertion, while still allowing the ultrasound transducer to be positioned in the open tip of the catheter. 
           [0022]      FIG. 10  shows a detail of an embodiment of the distal end of the hollow member incorporating an ultrasound transducer with electrical connections for the transducer and a protective coating. 
           [0023]      FIG. 11  shows a detail of another embodiment of the distal end of the hollow member incorporating an ultrasound transducer with electrical connections for the transducer and a protective coating. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0024]    Referring now to  FIG. 1  there is shown an ultrasound transducer  10  coupled to transmit/receive switch circuit  11 . In this embodiment, transmit/receive circuit  11  is shown without control connections to either signal processor  15  or system controller  22 , though such control connections may be incorporated in other embodiments. A signal generator  14  is connected to transmit amplifier  12 . In this embodiment, transmit amplifier  12  is shown as a variable gain amplifier with the gain being controlled by signal processor  15 , though in other embodiments amplifier  12  may be a fixed gain amplifier, or may be controlled by system controller  22 . The transmit amplifier  12  is connected to the transmit port of the transmit/receive switch circuit  11 . The receive port of the transmit/receive switch circuit  11  is connected to receive amplifier  13  which Is connected to signal processor  15 . In this embodiment, receive amplifier  13  is shown as a variable gain amplifier with the gain being controlled by signal processor  15 , though in other embodiments amplifier  13  may be a fixed gain amplifier, or may be controlled by system controller  22 . 
         [0025]    Signal processor  15  is connected to audio processor  16 , display processor  17 , and tactile processor  18 , all of which are intended to drive elements of a user interface providing information to a user. Audio processor  16  is connected to audio transducer  19  which can produce audible information. Display processor  17  is connected to visual display  20 , which can produce visual information. Tactile processor  18  is connected to tactile mechanism  21 , which can produce information that can be sensed by touch. 
         [0026]    In a preferred embodiment, audio transducer  19  is a piezoelectric transducer. In another embodiment, audio transducer  19  may be an electromagnetic speaker. It will be appreciated that many different audio transducer technologies may be utilized to fulfill the intent of the present invention. 
         [0027]    In a preferred embodiment, visual display  20  is made up of one or more light emitting diodes (LEDs). In one such embodiment, a single LED may be used to indicate the presence or absence of fluid and/or distance to the fluid, to the user through modulation of the brightness of the emitted light. In another such embodiment, two or more LEDs may be used to indicate the presence or absence of fluid to the user through modulation of the combined color of the light emitted by the LEDs. In another such embodiment, two or more LEDs may be used as an indicator of fluid through modulation of both brightness and color. In another such embodiment, many LEDs may be arranged so that an image may be formed which conveys information regarding the presence or absence of fluid. In such an embodiment, the image may be textual, graphical, or both. In a further embodiment, visual display  20  is made up of one or more liquid crystal displays. It will be appreciated that many different display technologies may be utilized to fulfill the intent of the present invention. 
         [0028]    In a preferred embodiment, tactile mechanism  21  is an electromechanical vibration device, such as an asymmetrical weight attached to an electric motor. In one such embodiment, the speed of the vibration may be used to indicate the presence or absence of fluid and/or distance to the fluid. In another such embodiment, the intensity of the vibration may be used as an indicator. In another such embodiment, the speed or intensity of vibration may be varied over time as an indicator. As an example of such an embodiment, the presence or absence of vibration indicates the presence or absence of fluid in front of the ultrasound transducer, while the time between short bursts of vibration indicates distance to the fluid from the ultrasound transducer face. In another embodiment, a solenoid may be used to move a mechanism to provide tactile feedback. It will be appreciated that many different tactile feedback technologies, including haptic technologies, may be utilized to fulfill the intent of the present invention. 
         [0029]    Audio processor  16 , display processor  17 , tactile processor  18 , visual display  20  and tactile mechanism  21  are shown as connected to system controller  22 . In any particular embodiment of the present invention, these connections may or may not be required. 
         [0030]    User inputs  25  provide means for the user to enable, disable, configure, or provide other input to the system. In a preferred embodiment, user inputs  25  incorporate electromechanical switches. In other embodiments, user inputs  25  may incorporate potentiometers, rotary encoders, or proximity sensors. It will be appreciated that many different input technologies may be utilized to fulfill the intent of the present invention. 
         [0031]    Power source  24  supplies power to elements which require it. The power source is preferably a rechargeable battery, though it may be a non-rechargeable battery, another electrical storage device such as a capacitor, or a power supply drawing energy from mains or an electrical energy generation apparatus. 
         [0032]    In an embodiment, system controller  22  triggers signal generator  14  to produce an electrical waveform that is amplified by transmit amplifier  12 , passes through transmit/receive switch circuit  11 , and excites transducer  10  so that it vibrates and produces acoustic waves that travel through tissue that is in contact with the transducer. Transmit/receive switch circuit  11  substantially blocks the amplified transmit signal from entering receive amplifier  13 . The transmit/receive switch circuit may be a diode-based directional circuit which is well understood and has been used in diagnostic ultrasound systems for many years. Alternatively, the switch circuit may be a solid-state switch which is controlled by either the system controller  22  or the signal processor  15 . The frequency of the acoustic waves produced in the system may vary greatly, but will preferably be in the range of 1 to 20 MHz. The number of wave cycles transmitted in a pulse may also vary greatly, but will preferably be in the range of 1 to 20 cycles. The pulse repetition frequency of the system may also vary greatly, but will preferably be in the range of 1 to 20000 Hz. 
         [0033]    Echoes returning to the transducer  10  from the tissue coupled to the transducer are converted to an electrical waveform by the transducer  10  and passed through the transmit/receive switch circuit  11  to the receive amplifier  13 . The amplified signal is then passed to signal processor  15  to be analyzed. In a preferred embodiment, signal processor  15  is a digital signal processor, incorporating an analog to digital converter so that the output of the receive amplifier  13  may be converted to digital form. In another embodiment, signal processor  15  is an analog signal processor, which may be as simple as a threshold detector that determines whether or not the amplitude of the received signal has exceeded a predetermined or adaptive limit. In a preferred embodiment, signal processor  15  controls the gain of the receive amplifier  13  in order to implement time-gain compensation (TGC). TGC is a well understood concept that has been incorporated in diagnostic ultrasound systems for many years, where gain is increased over time to compensate for the loss of energy in acoustic waves as they travel longer distances through tissue or fluid. 
         [0034]    The signal processor  15  is responsible for analyzing the signal representing returning echoes primarily to determine whether a fluid filled space is present in front of the transducer  10 , and if so, to convert this information to a form more suitable for feedback to the user.  FIG. 1   a  illustrates the time-amplitude properties of a typical signal indicating the presence of a fluid-filled space. It will be appreciated that in such an illustration, time also represents distance traveled by an acoustic wave. An assumption is generally made for the speed of sound in tissue, which is about 1540 meters per second. The signal begins at point  155 , which corresponds to the time when the system excites the ultrasound transducer to create the acoustic waves that will be used to search for a fluid filled space. A strong initial signal  150  may be present due to effects such as ring-down of the transducer. This initial signal is not relevant to fluid detection and should be minimized. The next portion of the signal  151  represents low level echoes from features within the tissue. Echo feature  152  is a higher amplitude feature representing the transition from tissue to fluid, while the next portion of the signal  153  is essentially echo free and represents the fluid. Echo feature  154  is a higher amplitude feature representing the transition from fluid to tissue, while the next portion of the signal  155  represents additional echoes from tissue features beyond the fluid compartment. The combination of features  152 ,  153 , and  154  form the signature of a fluid filled space, that is, an initial echo feature indicating transition from tissue to fluid followed by a relatively echo free region, followed by a trailing echo feature indicating transition from fluid to tissue. 
         [0035]    Because the signal features  152  and  154  are generally distinguished by their relatively larger magnitude from surrounding features in the echo signal, an algorithm may be implemented to identify the fluid space automatically.  FIG. 1   b  illustrates such an algorithm. In this embodiment, the algorithm is implemented by first setting a binary detection signal  166  to its inactive state, and then performing envelope detection  160  on the echo signal to yield a relatively smoothed version of the returning echo signal. A filtering operation may be added before or after envelope detection in order to improve the signal&#39;s suitability for analysis. In a preferred embodiment, a band limiting filter  161  is provided before the envelope detection, and a transient filter  162  followed by a smoothing filter  163  is provided after envelope detection. It will be appreciated that such filters may implement many different types of processing. Processing block  164  attempts to identify an initial signal feature corresponding to signal feature  152  by examining portions of the echo signal in sequence from its beginning until a feature is found with amplitude significantly greater than its surrounding regions, and with duration consistent with a tissue to fluid interface. In one embodiment, such a feature is considered to be found if its amplitude exceeds twice the amplitude of the surrounding signal regions, and its duration exceeds the pulse length of the ultrasound signal used to interrogate the tissue and fluid. If this signal feature is found, processing block  165  attempts to identify a subsequent signal feature corresponding to feature  154  by examining further portions of the echo signal in sequence until a feature is found with amplitude significantly greater than its surrounding regions, and with duration consistent with a tissue to fluid interface. If this pattern of signal features is found, a fluid filled space is considered to have been found and a binary detection signal  166  is turned to its active state. 
         [0036]    It will be appreciated that knowledge of anatomy relevant to a specific application may be applied to improve detection performance. For example, in an application where the present invention is being used to guide the placement of a ventricular drainage catheter, it is known that in the vast majority of cases the ventricle will not be found in the first two centimeters of tissue, nor will it be found deeper than 8 centimeters from the brain surface. Therefore, in one embodiment, signal features indicating fluid spaces outside these distances are ignored for detection purposes. 
         [0037]    If a fluid filled space is detected, its approximate distance from the face of the ultrasound transducer and its approximate extent in the direction of the ultrasound beam is easily determined by computing the time intervals between the initial ultrasound pulse and the returning echoes from signal features corresponding to features  152  and  154  in  FIG. 1   a . The time points representing features  152  and  154  are preferably the midpoint of their extent in time, but may also be chosen to be anywhere within, immediately before, or immediately after their extent in time. Once these three time points are determined, the current position of the ultrasound transducer being used to guide the medical device relative to the fluid filled space may be determined and used to provide feedback to a user. Processing blocks  164  and  165  determine these two time points, store them, and make them available for subsequent processing through outputs  168  and  169 . In a practical implementation, the outputs  168  and  169  may represent time in any suitable scale, for example, samples in a digital system, or as a number representing microseconds. The algorithm illustrated in  FIG. 1   b  thus provides an indication as to whether or not a fluid filled space is present in front of the ultrasound transducer, and if so, at what depth in front of the transducer the leading and trailing edge of this space lies. 
         [0038]    Detection of a fluid-filled space can happen within tens of microseconds. If we assume that ultrasound travels through tissue at 1540 meters per second, or approximately 1.54 millimeters per microsecond, then a fluid filled space 4 cm from the ultrasound transducer would give an initial echo corresponding to feature  152  in  FIG. 1   a  in about 52 microseconds. In a case where the extent of the fluid filled space in the direction of the ultrasound beam is about 1 cm, the echo corresponding to feature  154  in  FIG. 1   a  would arrive about 12 microseconds later, depending on the speed of sound in the fluid. Events occurring in such short timeframes are generally not perceptible by human, so some mechanism must be employed to spread their presentation out over time, space, or both so as to make them perceptible. A simple example of this is the oscilloscope, which measures an electrical waveform that may have megahertz or gigahertz bandwidth, but then allows the user to examine details of the signal on a display where microseconds or nanoseconds of signal information are spread out over a display and held for a time so that the signal information is perceptible. Oscilloscopes were used in early diagnostic ultrasound systems, and in fact could be used to produce displays similar to that shown in  FIG. 1   a . Ultrasound imaging systems generally encode signal amplitude as brightness and time as distance in a display. A series of echo signals are processed and accumulated to produce a single image. 
         [0039]    Previous investigators have used oscilloscopes or oscilloscope-like displays to present information similar to that shown in  FIG. 1   a  to aid in finding fluid filled spaces within tissue for many years. Recently, this technique has been demonstrated for the detection of cerebrospinal fluid in the brain, and proposed to aid the correct placement of a ventricular drainage catheter in the brain. A limitation of this technique is that it requires a user to interpret this oscilloscope-like display, which may not be intuitive. It is an object of the present invention to overcome this limitation by simplifying the presentation of echo information so that it is more easily understood, specifically by identifying fluid filled spaces automatically, using time-stretching techniques to present information to a user in an easier to understand format, using audio, visual, and tactile feedback, either singly or in combination. 
         [0040]      FIG. 1   c  illustrates an algorithm for performing the above described time stretching operation utilizing the outputs of the algorithm of  FIG. 1   b . First, processing block  180  computes a time point corresponding to the distance half way between the leading edge of the fluid filled space and the trailing edge of the fluid filled space using outputs  168  and  169 . This represents the approximate time that it would take an acoustic wave to travel from the transducer to the middle of the fluid filled space and back to the transducer. Multiplier  181  then multiplies this time by a stretching factor that will make this time perceptible to a human. In a preferred implementation, this stretching factor is chosen to be about 20000, so that a time of 50 microseconds is stretched out to one second. Output  182  represents this “stretched” time. 
         [0041]    Referring now again to  FIG. 1 , and understanding that the algorithms described above and illustrated in  FIG. 1   b  and  FIG. 1   c  are implemented by signal processor  15 , the outputs  166  and  182  are provided to audio processor  16 , display processor  17 , and actuator processor  18 . In a preferred embodiment, audio processor  16  generates an electrical signal representing a short audible tone or “beep” which repeats at the temporal rate indicated by output  182  if output  166  is active, otherwise no signal is generated. This electrical signal is then used to generate a series of audible beeps through audio transducer  19 . In this preferred embodiment, display processor  17  generates an electrical signal suitable for driving a LED so that it flashes at the temporal rate indicated by output  182  if output  166  is active, otherwise no signal is generated. This electrical signal is then used to drive visual display  20 , which in this case is simply a LED, so that it flashes as described above. In this preferred embodiment, actuator processor  18  generates an electrical signal suitable for driving a solenoid with a spring-return plunger of sufficient mass so that its motion is perceptible by touch in a pulsed fashion at a temporal rate indicated by output  182  if output  166  is active, otherwise no signal is generated. This electrical signal is then used to drive tactile mechanism  21 , which in this case is simply the solenoid described above. Thus, in this embodiment, if a fluid filled space is present so that its center is about 4 centimeters in front of the ultrasound transducer  10 , audio transducer  19  will beep at 1 Hz, visual display  20  will flash at 1 Hz, and tactile mechanism  21  will make a physical movement perceptible by touch at 1 Hz. As the ultrasound transducer  10  is moved closer to the fluid filled space, the rate of audio, visual, and tactile feedback will increase. 
         [0042]    It will be appreciated that any of the audio, visual, or tactile feedback mechanisms may be enabled or disabled in any combination by the system controller  22 . It will also be appreciated that, in any particular implementation, mechanisms for audio, visual, or tactile feedback may or may not be present, so long as at least one such feedback mechanism is present. It will also be appreciated that, in any particular implementation, mechanisms for audio, visual, or tactile feedback may be used to provide different types of feedback at the same time. In one embodiment, for example, visual display  20  may display a red light if a fluid filled space is not detected and a green light if such a space is detected, while audio processor and transducer  16  and  19  provide a series of beeps indicating distance to the fluid filled space if such a space is detected, and tactile mechanism  21  provides feedback through physical motion when the ultrasound transducer is determined to be in the fluid filled space. It will be apparent that the mechanisms for audio, visual, and tactile feedback may be used many such combinations to provide feedback to the user. 
         [0043]    It will be appreciated that, as the ultrasound transducer  10  is moved closer to the fluid filled space, at some point the signal feature  152  will blend with signal feature  150  and may become indistinguishable therefrom. If the ultrasound transducer is moved through the fluid filled space, ultimately signal feature  154  will blend with signal feature  150  and may become indistinguishable therefrom.  FIG. 1   d  further illustrates how this situation may be addressed in practice. 
         [0044]    In  FIG. 1   d , features corresponding to the signal features identified in  FIG. 1   a  are referenced. The algorithm is initialized in box  210 , which clears any indication that a fluid filled space has been reached and clears any stored location in processing block  213 . Decision block  211  indicates whether signal features  152  and  154  of  FIG. 1   a  have been found as described herein, indicating the presence of a fluid filled space. If these signal features have not been found, the algorithm returns to its initialization state  210 . If the signal features have been found, processing block  212  determines the end of signal feature  150 . In an embodiment, this determination may be made by a threshold test after block  163  of  FIG. 1   b . Processing block  213  determines the beginning of signal feature  154 . In an embodiment, this determination may be made by a threshold test as may be performed in block  164  of  FIG. 1   b . Processing block  213  then compares the location of signal feature  154  to any previously stored location. If the current location is within a pre-determined distance from the previously stored location, tracking is considered successful and feature  154  is indicated to have been identified. If the stored location information is cleared, as would be the case in the first iteration through the algorithm, feature  154  would also be indicated as having been found. If the location of signal feature  154  is outside of the pre-determined distance from the previously stored location, tracking is considered unsuccessful, and feature  154  is indicated as not having been identified. Decision block  214  determines whether or not the start of signal feature  154  has been found in processing block  213 . If it has not been found, tracking is lost and the algorithm returns to its initialization state  210 . If it has been found, decision block  215  determines whether the beginning of signal feature  154  is within a pre-set tolerance of the end of signal feature  150 . If it is, an indication is given to the user that the fluid-filled space has been reached. If not, the algorithm returns to processing block  154 . 
         [0045]      FIG. 2  is a block diagram of a portion of the system illustrated in  FIG. 1 , but with a wireless transmitter  36  and receiver  37  added.  FIG. 3  shows the remainder of the elements of  FIG. 1  that are missing from  FIG. 2 , with the addition of wireless receiver  50  and transmitter  51 . The combination of the systems illustrated in  FIG. 2  and  FIG. 3  thus, in combination, may implement all of the functions discussed above for the system illustrated in  FIG. 1  by sending appropriate data across a wireless link formed with the wireless transmitters and receivers. In a preferred implementation, the wireless link is formed with low-power Bluetooth™ components, though it will be appreciated that many different forms of communication, including RF, optical, and ultrasonic means, may be used to form this link. It will also be appreciated that certain modifications to the architecture of  FIG. 1  may be made to most efficiently incorporate such a wireless link, for example, control over signal generator  14  by system controller  22  in  FIG. 1  may be implemented by passing such control information from system controller  52  wirelessly to signal processor  35  and then on to signal processor  34 . It will also be appreciated that the wireless link described above may be placed at many different locations in the architecture of  FIG. 1 , for example in another embodiment the wireless transmitter  36  may be connected directly to receive amplifier  35  and the signal processing function would be performed on the other side of the wireless link. In such an implementation, wireless receiver  36  may be connected directly to signal generator  34  so as to provide a means for triggering transmission of ultrasound pulses from the system controller  52 . It will also be appreciated that a similar implementation may incorporate certain signal processing functions on both sides of the wireless link, for example the band limiting filter function may be implemented in signal processor  35  of  FIG. 2 , while an additional signal processor added to the system illustrated in  FIG. 3  may implement detection and smoothing operations. 
         [0046]    The present invention is particularly suitable for guiding the placement of a catheter to drain cerebrospinal fluid from one of the ventricles of the brain.  FIG. 4  illustrates a physical implementation of the system illustrated in  FIG. 1 . Hollow member, or “stylet”  71  with an ultrasound transducer corresponding to transducer  10  of  FIG. 1  positioned at its distal end is shown inside a cut-away view of drainage catheter  70 . The drainage catheter will incorporate at least one drainage port in the side of the catheter near its distal end, but such ports are not shown in the illustration. The tip of drainage catheter  70  incorporates a “fishmouth” slit  72  that allows stylet  71  to be pushed through the otherwise closed end of the catheter  70  as is described further below. Stylet  71 , which is preferably constructed of an electrically conductive material and coupled to the face of the ultrasound transducer, also contains a wire connected to the rear of the ultrasound transducer, which in combination with the conductive stylet form an electrical circuit corresponding to connection  23  in  FIG. 1 . Stylet  71  is mechanically coupled to housing  73 , which contains electronics and a power source corresponding to the other elements in  FIG. 1 . In this embodiment, the power source is preferably a battery power source, though it may be another electrical storage device such as a capacitor, or a power supply drawing energy from mains or an electrical energy generation apparatus. Graphical display  76  and LED lamps  75  are shown, both corresponding to visual display  20  of  FIG. 1 , as are user inputs  74  corresponding to user inputs  25  of  FIG. 1 . 
         [0047]      FIG. 5  illustrates a similar embodiment to that of  FIG. 4 , where now the stylet  81  corresponding to stylet  71  of  FIG. 4  is connected by a small co-axial cable  82  to housing  83  which contains the electronics and power source. In this embodiment, the power source is preferably a rechargeable battery, though it may be a non-rechargeable battery, another electrical storage device such as a capacitor, or a power supply drawing energy from mains or an electrical energy generation apparatus. The proximal end of the catheter  80  corresponding to the catheter  70  of  FIG. 4  is shown, as are the graphical display  86  and LED lamps  85  corresponding to elements  76  and  75 , respectively, of  FIG. 4 . User inputs  84  corresponding to user inputs  74  of  FIG. 4  are also shown. 
         [0048]      FIG. 5   a  illustrates a similar embodiment to that of  FIG. 4 , where now the stylet  201  corresponding to stylet  71  of  FIG. 4  is mechanically coupled to housing  202  containing electronics, which is in turn connected by cable  203  to housing  204  which contains electronics. It will be appreciated that housing  202  may actually be the same as hollow stylet  201 . In an embodiment, electrical tuning components such as one or more inductors or capacitors may be incorporated in close physical proximity to the transducer at the distal end of hollow stylet  201 . It will be appreciated that a power source may be incorporated in housing  202 , housing  204 , or in both. Preferably, a power source is incorporated in housing  204 , and any power required by electronics contained in housing  202  is supplied through cable  203 . In this embodiment, the power source is preferably a rechargeable battery, though it may be a non-rechargeable battery, another electrical storage device such as a capacitor, or a power supply drawing energy from mains or an electrical energy generation apparatus. The proximal end of the catheter  200  corresponding to the catheter  70  of  FIG. 4  is shown, as are the graphical display  205  and LED lamps  206  corresponding to elements  76  and  75 , respectively, of  FIG. 4 . User inputs  207  corresponding to user inputs  74  of  FIG. 4  are also shown. 
         [0049]      FIGS. 6 and 7  further illustrate the configuration of the catheter that allows the distal end of the stylet to be pushed through its end. In  FIG. 6 , stylet  91  incorporating the ultrasound transducer at its distal end is shown inside a cut-away view of a drainage catheter. As above, the drainage catheter will incorporate at least one drainage port in the side of the catheter near its distal end, but such ports are not shown in the illustration. A slit  92  is cut in the distal end of the catheter so that an opening may be created. In  FIG. 7 , the stylet  101  is now shown pushed through the slit at the distal end of the catheter  100 . 
         [0050]    It is possible to construct a catheter such as is illustrated in  FIG. 8 , where the distal end of the stylet  111  is always exposed through the open end of the catheter  110 . Such an implementation may have a problem with the catheter “riding up” on the stylet as it is inserted into the brain tissue. This problem may be addressed through an embodiment illustrated in  FIG. 9 . Here, stylet  122  incorporates a feature  123  so that its lateral dimension, or most commonly its diameter, is reduced near its distal end. This feature fits against a corresponding “lip” feature in the distal end of the catheter  120  so that the distal end of the stylet incorporating the ultrasound transducer  124  is exposed, but force may be applied to the catheter through the stylet. Preferably, the distal end of the catheter  120  is made from a material that is more rigid than the material of the body of the catheter  121 . 
         [0051]      FIG. 11  illustrates a construction detail of an embodiment of the ultrasound transducer  141  corresponding to transducer  10  of  FIG. 1  inside the stylet  140  corresponding to stylet  71  of  FIG. 4 . Here, the ultrasound transducer is most preferably constructed of a piezo-electric ceramic material, and is plated with a conductive metal, preferably silver or gold, on its front and back face. A wire  142  is bonded to the plated back face of the transducer, and the transducer is bonded inside the hollow stylet  140  with a glue. The plating on the back face of the transducer is configured so that it does not reach the edge of the transducer, so that it is not in electrical contact with the stylet. A small piece of conductive material  143  is bonded to the face of the transducer  141  and to the distal end of the stylet  140  so that the front face of the transducer is in electrical contact with the stylet. An acoustically transmissive protective coating  144 , which is preferably a latex or epoxy, is deposited at the tip of the stylet. It will be apparent that the conductive material  143  is not required if the plating of the front face of the transducer wraps around the edge of the transducer so that it is in electrical contact with the stylet. Thus, the ultrasound transducer  140  may be grounded through the conductive stylet  140 , and transmit and receive signals may be conveyed through wire  142 . 
         [0052]      FIG. 12  also illustrates a construction detail of an embodiment of the ultrasound transducer  190  corresponding to transducer  10  of  FIG. 1  inside the stylet  192  corresponding to stylet  71  of  FIG. 4 . Here, the plating on the front of the transducer wraps around the side of the transducer so that the additional conductive element is not required between the face of the transducer and protective layer  191 . An additional feature  193  is added to the hollow stylet which is a narrowing of the stylet near its distal end that acts as a back-stop for the transducer  190  to aid in positioning during manufacturing. 
         [0053]    This invention has been described in detail with reference to particular embodiments thereof, but it will be understood that various other modifications can be effected within the spirit and scope of the invention.