Abstract:
A method and apparatus for measuring changes in intracranial pressure (ICP) utilizing the variation of the surface wave propagation parameters of the patient&#39;s skull to determine the change in ICP. In one embodiment, the method comprises the steps of transmitting an ultrasonic bulk compressional wave onto the surface of the skull at a predetermined angle with respect to the skull so as to produce a surface wave, receiving the surface wave at an angle with respect to the skull which is substantially the same as the predetermined angle and at a location that is a predetermined distance from where the ultrasonic bulk compressional wave was transmitted upon the skull, determining the retardation or advancement in phase of the received surface wave with respect to a reference phase, and processing the determined retardation or advancement in phase to determine circumferential expansion or contraction of the skull and utilizing the determined circumferential change to determine the change in intracranial pressure.

Description:
ORIGIN OF THE INVENTION  
       [0001] The invention described herein was made by employees of the United States Government and may be used by or for the Government for governmental purposes without the payment of any royalties thereon or therefor. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Technical Field of the Invention  
           [0003]    The present invention relates broadly to the field of apparatuses and methods for determining changes in intracranial pressure (“ICP”) by measuring the effects of these changes on a patient&#39;s skull.  
           [0004]    2. Related Art and Problem to be Solved  
           [0005]    Monitoring circumferential expansion of the patient&#39;s skull due to changes in ICP is of significant diagnostic and post-operative importance for patients with cranial injuries, pathologies, or other conditions that may affect the pressure of the subarachnoidal fluid around the brain, and for patients who have undergone brain surgery.  
           [0006]    Known art methods and techniques for measuring changes in circumferential expansion of the patient&#39;s skull due to changes in ICP frequently involve launching an ultrasonic bulk compressional wave through the cranium. This technique can require that the position of the transducer be essentially absolutely stable, relative to the patient&#39;s skull, during data acquisition. However, one significant problem with the aforesaid known art method is the difficulty in keeping the transducers immobile relative to the patient&#39;s skull for long periods of time. Thus, inaccurate or inconclusive data may result due to transducer movement.  
           [0007]    It is an object of the present invention to provide a non-invasive method and apparatus for determining changes in ICP in a manner which does not require the transmission of a wave through the cranium.  
           [0008]    It is another object of the present invention to provide a new method and apparatus for determining changes in ICP by measuring changes in circumferential expansion or contraction of the patient&#39;s skull.  
           [0009]    It is another object of the present invention to provide a method and apparatus for determining changes in ICP, that provides substantially absolute stability of the transducer, relative to the patient&#39;s skull, during data acquisition.  
           [0010]    Other objects and advantages of the present invention will in part be obvious and will in part be apparent from the specification.  
         SUMMARY OF THE INVENTION  
         [0011]    The apparatus and method of the present invention use the variation of the surface wave propagation parameters of the patient&#39;s skull to determine the change in ICP.  
           [0012]    In accordance with one embodiment of the invention, the apparatus comprises a measuring device, transmit and receive transducers, and transmit and receive angle blocks wherein the transmit and receive transducers are mounted to the transmit and receive angle blocks, respectively. The transmit and receive angle blocks each have surfaces that are configured to contact the skull. The apparatus further includes a mounting strut to which the transmit and receive angle blocks are movably attached. The mounting strut is adjustable in overall length so as to accommodate skulls of varying sizes, and to provide stability to the transducers.  
           [0013]    In one embodiment, the measuring device is configured as a constant frequency pulsed phase-locked loop (“CFPPLL”) measuring device.  
           [0014]    The mounting strut can be adjusted on the skull so that the surface wave can be launched over a relatively small propagation path, for example, so the surface wave can travel across the forehead, or across a section of the skull containing a suture, or any other appropriate segment of the skull. Further, it is within the scope of the present invention that multiple pairs of tranducers (and angle blocks) could be used to launch (and receive) multiple surface waves over more than one propagation path. In this way, a differential comparison could then be made between the measured changes in ICP relative to each path. However, in at least one embodiment, each path may need to be calibrated separately and/or have different measurement interpretive algoritluns associated therewith (see below).  
           [0015]    In at least one embodiment, the measuring device generates an electrical tone burst which is inputted into the transmit transducer. The transmit transducer converts the electrical tone burst into a sound wave, such as an ultrasound bulk compressional wave that passes through the transmit angle block. The bulk compressional wave is then emitted into the skin and subcutaneous tissue that surrounds the skull at a predetermined angle so as to create a surface wave upon the skull. The surface wave travels over a propagation path and through the tissue that contacts the receive angle block. The receive angle block receives the surface wave at an angle that is generally the same as the aforementioned predetermined angle. The receive angle block converts the surface wave into an ultrasonic bulk compressional wave which is then received by the receive transducer. The receive transducer converts the ultrasonic bulk compressional wave back into an electrical tone burst which is inputted into the measurement device.  
           [0016]    The transmit and receive transducers could take various forms, for example: piezoelectric, magnetostrictive, or electrodynamic. Moreover, bone is mildly piezoelectric, therefore, in another possible embodiment, one could induce and detect a surface wave with electromagnetic transducers and essentially avoid potential problems with skin tissue perfusion variability.  
           [0017]    In at least one embodiment, the measurement device compares the phase of the electrical tone burst outputted by the receive transducer to a reference phase in order to determine if there is a difference in phase. As the skull responds to the changes in intracranial pressure, the velocity of the surface wave changes thereby resulting in retardation or advancement of the phase of the surface wave received by the receive angle block. Thus, the measurement device determines whether there has been any phase retardation or advancement. The measurement device outputs a data signal that represents the measured phase change.  
           [0018]    It is within the scope of the present invention that a variety of surface waves could be propagated on the skull. Examples of these surface waves may include, Rayleigh, Rayleigh-type, Generalized Rayleigh, Leaky (or psuedo-), Bleustein-Gulynev, Shear Horizontal, Lamb, Generalized Lamb, Love, and Stoneley waves. Additionally, as used herein, the term “sound wave” is used to refer to the propogation of a disturbance, resultant from the disturbing of substantially any medium.  
           [0019]    The apparatus of the present invention can further include a processing device that receives the data signal outputted by the measurement device and performs measurement interpretative algorithms on the data signal in order to determine the degree of circumferential expansion (or contraction) of the skull due to changes in ICP. This degree of circumferential expansion (or contraction) can be determined as a change to the total circumference of the skull or as a change to a circumferential arc portion of the skull.  
           [0020]    In at least one embodiment, the measurement interpretive algorithms can implement a biological time constant (for example, representing pulse rate, respiration, etc.) during interpretation of the data contained in the data signal outputted by the measuring device. In one embodiment, the processing device can be realized by a computer program that is configured to provide functions as filtering, integrating, averaging, etc. In another embodiment, the processing device is configured with electronic components that can provide the aforesaid functions.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0021]    The features of the invention believed to be novel and the elements characteristic of the invention are set forth with particularity in the appended claims. The figures are for illustration purposes only and are not drawn to scale. The invention itself, however, both as to organization and method of operation, may best be understood by reference to the detailed description which follows taken in conjunction with the accompanying drawings in which:  
         [0022]    [0022]FIG. 1 is a block diagram of an apparatus of the present invention; and  
         [0023]    [0023]FIG. 2 is a schematic diagram of one embodiment of a measuring device that can be used in the apparatus of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0024]    In describing the embodiments of the present invention, reference will be made herein to FIGS. 1 and 2 of the drawings in which like numerals refer to like features of the invention.  
         [0025]    Referring to FIG. 1, there is shown apparatus  10  of the present invention. Apparatus  10  comprises measurement device  11 . In one embodiment, the measurement device can be configured as a CFPPLL described in commonly owned U.S. Pat. No. 5,214,955, the disclosure of which is herein incorporated by reference as if set forth in its entirety. The CFPPLL is shown in FIG. 2 and is briefly described in the ensuing description in order to facilitate understanding of the present invention. The CFPPLL is configured to operate in the “pitch-catch” mode which is described in the aforementioned U.S. Pat. No. 5,214,955. In another embodiment, the measurement device  11  might comprise a variable frequency pulsed phase-locked loop (“VFPPLL”), for example as described in the article by Yost, et al.,  Fundamental Aspects of Pulse Phase-locked Loop Technology-based Methods for Measurement of Ultrasonic Velocity , J. Acoust. Soc. Am. 91, 1456-1468 (1992), which article is incorporated herein by reference as if set forth in its entirety.  
         [0026]    As shown in FIG. 2, the CFPPLL includes a fixed frequency oscillator  12 , such as a phase-stable synthesizer. The fixed frequency oscillator  12  sends its constant frequency output to three different blocks: the logic and timing circuit  14 , a buffer amplifier-phase shift circuit  16 , and a tone-burst formation gate  18 . The logic and timing circuit  14  uses this signal as a clock and counts down to determine the timing on its outputs. One output is the transmit or Tx gate to form the electrical tone burst. The logic and timing circuit  14  also contains a setting so that the operator can set the gate for as many cycles as is desired.  
         [0027]    The electrical tone burst is amplified by amplifier  20  and delivered to output  22  which can be connected to a transmit transducer which is described in the ensuing description (see FIG. 1). As will be described in the ensuing description, after the electrical tone burst is received by the transmit transducer, the electrical tone burst is converted to a bulk compressional wave and is introduced to the surface of the skull which produces a surface wave. The surface wave travels a predetermined distance along the skull and is received by a receive transducer which is described in the ensuing description. The signals received by the receive transducer are inputted into input  23  of measurement device  11 . A receive or Rx gate signal enables gate  26  to pass the received signal to preamplifier  28 . The amplified received signal then proceeds to a mixer  30  where the amplified received signal is phase-compared to the reference signal provided from the fixed frequency oscillator  12 . Unless these two signals are at quadrature, a voltage level, which constitutes an error signal, is generated by the mixer  30  and passed to output  31  and a low-pass filter  32 . The output of low-pass filter  32  is inputted into a sample and hold  34 , which is activated by the logic and timing circuit  14 . The sample and hold  34  holds the level of voltage until the next pulse-echo cycle (P-Ec). Thus, the sample and hold output is updated at each P-Ec. This output voltage is fed to an integrator circuit  36  whose voltage output is delivered to the phase shift circuit  16  after passing through an adder circuit  38 . The adder circuit  38  also receives a voltage adjustment from an initial phase adjustment  40 , which may be manually operated by turning a dial. In at least one embodiment, during initial set up the operator thus changes the voltage output of the mixer  30 , if necessary, by using the phase adjuster and adder circuit until the sample and hold voltage output is zero, which occurs at quadrature of the echo and the signal from the main frequency oscillator.  
         [0028]    Thus, any change in the acoustic parameters of the surface wave propagation path defined by the skull of a patient (and the location of the transmit and receive transducers) will result in a change in the quadrature condition by an alteration in the voltage to the phase-shift circuit. One way to quantify a phase is to observe the voltage applied to the phase shifter  16 . There are a variety of ways in which to quantify the phase shift. In order to measure changes in velocity, the results must be measured in terms of the phase shift caused by changes in the traversal time of the acoustical signal as the acoustic signal travels along the propagation path defined by the skull. This is affected by change in phase in either the electrical or acoustical parameters. One can use this fact to calibrate by either calibrating the phase shift circuit against input voltage and reading the input voltage changes, or inserting a known, adjustable phase shifter, such as a calibrated line stretcher (not shown, but described in U.S. Pat. No. 5,214,955, the disclosure of which patent is herein incorporated by reference) and adjusting until the same input voltage to the phase shifter is obtained.  
         [0029]    In at least one alternate embodiment, measurement device  11  is configured as the electronic apparatus described in commonly owned U.S. Pat. No. 5,617,873, the disclosure of which is herein incorporated by reference, and indicated by numeral  30  therein.  
         [0030]    Referring to FIG. 1 herein, apparatus  10  further includes mounting strut  50 . Mounting strut  50  comprises center section  52  and movable end portions  54  and  56 . End portions  54  and  56  are pivotally connected to center section  52  by pivot pins  58 . Mounting strut  50  further includes transmit and receive angle blocks  60  and  62 , respectively, that are attached to end portions  54  and  56 , respectively. Transmit and receive angle blocks  60  and  62  have surfaces  64  and  66 , respectively, that are configured for contact with a patient&#39;s head. In one embodiment, mounting strut  50  is configured to have a predetermined degree of elasticity so as to provide firm contact between surfaces  64 ,  66 , and the head of a patient. In one embodiment, mounting strut  50  is configured so that end portions  54  and  56  are urged inward towards the patient&#39;s head so as to facilitate firm contact between surfaces  64 ,  66 , and the head of a patient.  
         [0031]    In another embodiment, mounting strut  50  is configured so that center section  52  is comprised of two portions that are slidably engaged with each other so that the overall length of center section  50  can be adjusted so as to accommodate skulls of varying sizes. Such a feature can be used to vary the length of the propagation path over the patient&#39;s skull thus enabling the surface wave to be launched over a relatively small propagation path, for example, the patient&#39;s forehead or any other appropriate segment of the patient&#39;s skull.  
         [0032]    Referring to FIG. 1, apparatus  10  further comprises transmit and receive transducers  70  and  72  that are attached to transmit and receive angle blocks  60  and  62 , respectively. Transmit transducer  70  is electrically connected to output  22  of measuring device  11 , for example, by wire or cable  74 . Similarly, receive transducer  72  is electrically connected to input  23  of measuring device  11 , for example, by wire or cable  76 .  
         [0033]    Apparatus  10  can further include processing device  80  that receives the signals outputted at output  31  of measuring device  11 . Processing device  80  can implement measurement interpretive algorithms in order to generate a biological time constant which is used in the analysis of the data extracted from the signal outputted at output  31  of measuring device  11 . In one embodiment, processing device  80  includes electronic components and circuitry such as microprocessors, timing circuitry, data storage devices (e.g. RAM, ROM, cache, etc.), filter circuits, and other processing circuits for effecting implementation of the aforementioned measurement interpretive algorithms. In another embodiment, processor device  80  is configured as a computer that implements a computer program that effects generation of the aforesaid biological time constant.  
         [0034]    Operation  
         [0035]    Referring to FIG. 1, one possible mode of operation is as follows. Measurement device  11  outputs an electrical tone burst through output  22  which is carried by wire  74  and is inputted into transmit transducer  70 . Transducer  70  converts the electrical tone burst into an ultrasonic bulk compressional wave. The ultrasonic bulk compressional wave passes through the transmit angle block  60  and is directed to the interface between surface  64  and the skin of the patient&#39;s skull. The ultrasonic bulk compressional wave passes into the subcutaneous tissue of the patient&#39;s skull at a predetermined angle, thereby producing a surface wave which travels along the skull. The surface wave traverses the skull along a propagation path and passes through the interface between the skin of the patient&#39;s skull and surface  66  of receive angle block  62 . The receive angle block  62  receives the surface wave and passes the surface wave to the receive transducer  72 . The angle at which the surface wave is received by the receive angle block is substantially the same as the predetermined angle described in the foregoing description. The receive angle block converts the surface wave into a bulk compressional wave. The receive transducer  72  converts the bulk compressional wave into an electrical waveform (i.e. an electrical tone burst). The electrical waveform is carried by wire or cable  76  to input  23  of measurement device  11 .  
         [0036]    As the patient&#39;s skull responds to ICP, the velocity of the surface wave changes thereby causing retardation or advancement of the phase of the received surface wave. Measuring device  11  measures the phase of the electrical waveform outputted by receive transducer  72  with respect to a reference signal in order to determine the change in phase between the two signals. Measuring device  11  outputs a signal at output  31  which contains data that indicates phase difference. The signal outputted at output  31  is inputted into processing device  80  which implements the measurement interpretive algorithms that determine the changes in circumferential expansion, or contraction, of the patient&#39;s skull due to changes in ICP. This circumferential expansion or contraction, for example, can be calculated either in the form of total skull circumference change or in reference to changes in a specific circumferential arc portion of the skull. These determined circumferential changes can then be utilized to determine changes in ICP.  
         [0037]    The principles, embodiments, and modes of operation of the present invention have been described in the foregoing specification. The invention which is intended to be protected herein should not, however, be construed as limited to the particular forms disclosed, as these are to be regarded as illustrative rather than restrictive. Variations or changes may be made by those skilled in the art without departing from the spirit of the invention. Accordingly, the foregoing detailed description should be considered exemplary in nature and not limited to the scope and spirit of the invention as set forth in the attached claims.  
         [0038]    Thus, having described the invention, what is claimed is: