Ultrasonic techniques for neurostimulator control
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates implantable techniques for electrical stimulation of an organism, and more particularly, relates to such techniques for electrical stimulation of a tissue adjacent to a spinal column. 
     2. Description of the Related Art 
     Electrical leads for electrical stimulation of a spinal cord or tissue adjacent a spinal cord have been made in the past and are disclosed, for example, in U.S. Pat. Nos. 4,044,774; 5,143,067; 5,095,905; 5,031,618 and 5,121,754. Experience has shown that positional sensitivity has continued to be a problem for users of such spinal cord stimulators. Over a period of time, the stimulator may change its position with respect to the spinal column, and such positioning changes adversely affect the stimulation provided by the lead unless there is a corresponding change in the magnitude of the stimulation. 
     A nerve stimulator sold by Medtronic, Inc. under the name ITREL II has a &#34;magnet amplitude&#34; mode which allows the user to manually toggle the amplitude of the electrical stimulation between pre-programmed high and low amplitude states. U.S. Pat. No. 5,031,618 (Mullett), assigned to Medtronic, Inc., discloses a system that uses a position sensor, such as a mercury switch, to modulate the output of a neurostimulator. Neither of the foregoing systems is capable of measuring the distance between the stimulator electrodes and the spinal cord which is a key parameter affecting stimulation thresholds in the spinal cord. See, &#34;MR Assessment of the Normal Position of the Spinal Cord in the Spinal Canal&#34; by J. Holsheimer, et al., American Journal of Neuroradiology (May, 1994). 
     U.S. Pat. No. 4,706,681 (Breyer et al.) discloses marker transducers which respond to ultrasonic signals to generate electrical signals which localize the marker transducers. However, the transducers do not determine this location with respect to tissue. 
     SUMMARY OF THE INVENTION 
     The invention may take the form of apparatus for stimulation of tissue adjacent the spinal cord of a vertebrate. In such an environment, a preferred embodiment of the invention includes an ultrasonic transducer module implanted adjacent the spinal cord for producing an ultrasonic wave that creates a corresponding ultrasonic echo wave reflected from a predetermined portion of the spinal cord. The transducer module also generates a distance signal in response to the ultrasonic echo wave. The value of the distance signal is related to the distance between the transducer module and the spinal cord. A stimulation driver generates a stimulation signal suitable for stimulating the spinal cord or adjacent tissue. A drive amplifier responsive to the stimulation signal and the distance signal generates a drive signal that increases in value as the distance between the transducer module and the spinal cord increases and decreases in value as the distance between the transducer module and the spinal cord decreases. One or more stimulation electrodes can be implanted adjacent the spinal cord, and the drive signal is transmitted to the electrodes so that the uniformity of stimulation received by the spinal cord or adjacent tissue is increased in spite of changes in the relative distance between the transducer module and the spinal cord. By using the foregoing techniques, the accuracy and uniformity of the stimulation provided to the spinal cord or adjacent tissue can be increased even if the transducer module changes position relative to the spinal cord. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     These and other advantages and features of the invention will become apparent upon reading the following detailed description and referring to the accompanying drawings in which like numbers refer to like parts throughout and in which: 
     FIG. 1 is a diagrammatic representation of a preferred embodiment of the invention for stimulating tissue adjacent a spinal cord together with a representation of an exemplary spinal cord located adjacent the apparatus; and 
     FIG. 2 is a diagrammatic cross-section of the spinal cord shown in FIG. 1 illustrating the placement of the preferred embodiment with respect to the spinal cord. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIGS. 1 and 2, a stimulation lead 10 is placed in the epidural space EPS of a human spinal column S near the dura DU which is located at a distance D from the dorsal columns DC. The dura DU is separated from the dorsal columns by a subdural space SDS. Other parts of spinal column S include the gray matter GM, the bony vertebral body BVB and the vertebral arch VA (FIG. 2). 
     Lead 10 carries an ultrasonic transducer 20 capable of creating ultrasonic sound waves in response to an ultrasonic drive circuit 22 which generates an ultrasonic signal that is carried to transducer 20 over a conductor 24. Alternatively, transducer 20 could be separated from lead 10. 
     Ultrasonic waves W are carried through the dura and the subdural space from transducer 20 to dorsal columns DC which create corresponding reflected ultrasonic echo waves R, some of which reach a conventional ultrasonic receiver transducer 30 and cause it to create a corresponding electrical echo signal in response to the echo waves R. The echo signal is carried over a conductor 32 to an amplifier 34 which generates an amplified ultrasonic signal on a conductor 36 that is carried to a filter discriminator 38. Discriminator 38 generates a distance signal on a conductor 40 by minimizing the components of the amplified ultrasonic signal resulting from ultrasonic waves reflected from objects other than the dorsal columns. The drive signal on conductor 40 decreases in value as distance D between lead 10 and the dorsal columns increases and increases as distance D decreases. 
     Ultrasonic drive 22 may be operated continuously so that ultrasonic waves W are transmitted continuously towards dorsal columns DC and reflected echo waves R are continuously received by transducer 30. For this embodiment, the distance signal on conductor 40 is proportional to the amplitude of the reflected echo waves received by transducer 30. 
     Alternatively, ultrasonic drive 22 can be operated to produce time-spaced pulses that cause time-spaced ultrasonic waves W to be propagated toward dorsal columns DC. As a result, the reflected echo waves R also are received by transducer 30 in a time-spaced manner. In this embodiment, the amplitude of the drive signal on conductor 40 is inversely proportional to the period of reflected echo waves R, that is, the amount of time between the generation of a pulse wave by transducer 20 and the receipt of a corresponding reflected echo wave R by transducer 30. Drive 22 preferably applies a shock pulse to transducer 20. An exemplary circuit for generating such a shock pulse is described on page 97 of Physical Principles of Ultrasonic Diagnosis, by P. N. T. Wells (Academic Press 1969) which is incorporated by reference. 
     Filter discriminator 38 can be made by generating a time window and only analyzing the echo signals received during the time window. A circuit for generating such a time window is taught in &#34;An Ultrasonic Bladder-volume Sensor&#34;, by M. T. Rise et al., IEEE Transactions on Biomedical Engineering, Vol. BME-26, No. 12, Dec. 1979, which is incorporated by reference. 
     All of components 20-40 form an ultrasonic transducer module; components 30-40 comprise an ultrasonic receiver for generating the distance signal on conductor 40. 
     A stimulation driver 50 is provided for generating a stimulation signal suitable for electrical stimulation of the spinal cord or adjacent tissue. Such stimulation drivers are well known in the art of electrical stimulation. For example, an exemplary stimulation driver is manufactured by Medtronic, Inc. under the trademark Itrel II. 
     The stimulation signal produced by driver 50 is processed by an operational amplifier 60 having a negative input 62, a positive input 64, and an output 66. Input 64 is connected to ground potential and input 62 is connected through a resistor 68 to output 40 and through a resistor 70 to the output of stimulation driver 50. Resistors 68 and 70 form with amplifier 60 a negative feedback network so that increasing amplitude of the distance signal on conductor 40 proportionately reduces the amplitude of the drive signal on output 66. 
     The drive signal produced on output 66 is transmitted to conventional stimulation electrodes and 82 over conductors 84 and 86. 
     The amplitude of the drive signal on conductor 66 also may be controlled through a resistor 72 that is connected to a conventional voltage adjustment circuit 74 accessible by a physician. 
     Conductors 24, 32, 84 and 86 may be located inside lead 10 and may exit the skin of the patient in whom lead 10 is implanted. In this embodiment, components 22 and 34 through 66 may be located outside lead 10 and outside the body of the patient. 
     Those skilled in the art will recognize that the preferred embodiments may be altered without departing from the true spirit and scope of the invention as defined in the accompanying claims.