Ultrasonic detection of restenosis in stents

An analyzer apparatus and method is provided for analyzing restenosis associated with a stent implanted within a living body. The apparatus includes an input for receiving ultrasonic data from an ultrasonic imaging apparatus; digital memory for storing the ultrasonic data at least temporarily; a processor for analyzing the ultrasonic data, the processor being configured to analyze the data in accordance with at least one predefined criteria to diagnose a degree of restenosis experienced by the stent; and an output for outputting information indicative of the diagnosis.

TECHNICAL FIELD
 The present invention relates generally to non-invasive diagnoses of
 medical implant devices, and more particularly to ultrasonic detection of
 restenosis in a stent.
 BACKGROUND OF THE INVENTION
 Various types of medical implant devices have been developed over the
 years. In many instances, such devices enable humans to live longer, more
 comfortable lives. Implant devices such as pacemakers, artificial joints,
 valves, grafts, stents, etc. provide a patient with the opportunity to
 lead a normal life even in the face of major heart, reconstructive, or
 other type surgery, for example.
 It has been found, however, that the introduction of such implant devices
 can sometimes lead to complications. For example, the human body may
 reject the implant device which can ultimately lead to infection or other
 types of complications. Alternatively, the implant device may malfunction
 or become inoperative. Therefore, it is desirable to be able to monitor
 the condition of the implant device. On the other hand, it is highly
 undesirable to have to perform invasive surgery in order to evaluate the
 condition of the device.
 Still further, it is desirable to be able to monitor conditions related to
 the use of implant devices. For example, in heart patients it may be
 helpful to know the amount of blood flowing through a stent in order to
 evaluate the health of the patient. Again, however, it is undesirable to
 have to perform invasive surgery in order to evaluate such conditions.
 Techniques have been developed which enable the function of an implant
 device to be monitored remotely from outside the body of the patient.
 These techniques involve including one or more sensors in the device for
 sensing the condition of the device. The device further includes a small
 transceiver for processing the output of the sensors and transmitting a
 signal based on the output. Such signal typically is a radio frequency
 signal which is received by a receiver from outside the body of the
 patient. The receiver then processes the signal in order to monitor the
 function of the device.
 While such conventional techniques may be effective in avoiding the need to
 perform invasive surgery, there are however several drawbacks associated
 therewith. For example, the transceiver included in the implant device
 typically includes complex electrical circuitry such as mixers,
 amplifiers, microprocessors, etc. for receiving an interrogation signal
 and for transmitting a response signal based on the output of the sensors.
 Such complex circuitry has a relatively high cost associated therewith. In
 addition, the complexity of the circuitry increases the likelihood that
 the device itself may be defective. This would then require further
 invasive surgery and could even result in physical harm to the patient.
 Still another shortcoming with conventional implant devices with sensors
 included therein is power concerns. Some type of circuit for providing
 power to the transceiver is necessary. The circuit may be a built-in power
 source such as a battery, or a circuit which derives operating power from
 an external excitation signal using magnetic or electromagnetic coupling.
 In either case, again the complexity of the circuit and/or the need to
 replace the battery periodically adds to the cost of the device and
 increases the opportunity for failure or defects.
 In view of the aforementioned shortcomings associated with conventional
 implant devices, there is a strong need in the art for a medical implant
 system and method, particularly with respect to a stent, which can
 remotely interrogate the stent but which does not require complex
 electrical circuitry such as mixers, amplifiers, microprocessors, etc.
 There is a strong need for a stent which carries out its function within a
 human or other living animal, and can be remotely interrogated simply and
 reliably. Moreover, there is a strong need for a stent which does not rely
 on complex energy conversion circuits in order to operate.
 SUMMARY OF THE INVENTION
 According to one aspect of the invention, a diagnostic system is provided.
 The system includes a stent implantable within a blood vessel of a living
 animal and operatively configured to prevent the vessel from collapsing.
 The stent may be a typical commercially available stent or one specially
 designed to exhibit a mechanical transfer function which, in response to
 mechanical excitation, causes the structure to produce an acoustic signal
 having a characteristic which is modulated in relation to the presence of
 restenosis within the stent. The system further includes an exciter for
 acoustically transferring mechanical energy to the stent from outside the
 living animal, and a receiver located outside the living animal which
 detects the acoustic signal produced by the structure, processes the
 acoustic signal in relation to the mechanical transfer function, and
 provides an output indicative of the parameter based on the processed
 acoustic signal.
 The system may be based on existing ultrasonic imaging equipment, or can
 comprise a system designed specifically for analyzing a stent. A
 combination of software and/or hardware is provided for analyzing
 ultrasonic data reflected or reradiated from the stent in response to
 ultrasonic pulses. The data is digitized and processed using one or more
 algorithms such as a Fast Fourier Transform (FFT), wavelets, etc. By
 analyzing response parameters such as amplitude, harmonic content, phase
 and/or modulus data as a function of frequency, for example, it has been
 found that the degree of restenosis within the stent may be diagnosed. A
 signature database for storing response data for one or more standard
 stents of different sizes, types, manufacturers, etc., is provided. The
 system uses a pattern recognition or matching algorithm to identify the
 particular stent within the body, and uses such information to normalize
 the acquired data, set baselines, etc.
 A feature of the invention is that it can be implemented with limited
 hardware and/or software in combination with conventional ultrasonic
 imaging equipment. Alternatively, the present invention may be carried out
 as an entirely new system configured specifically for the detection of
 restenosis in stents.
 To the accomplishment of the foregoing and related ends, the invention,
 then, comprises the features hereinafter fully described and particularly
 pointed out in the claims. The following description and the annexed
 drawings set forth in detail certain illustrative embodiments of the
 invention. These embodiments are indicative, however, of but a few of the
 various ways in which the principles of the invention may be employed.
 Other objects, advantages and novel features of the invention will become
 apparent from the following detailed description of the invention when
 considered in conjunction with the drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS
 The present invention will now be described with reference to the drawings,
 wherein like reference numerals are used to refer to like elements
 throughout.
 Referring initially to FIG. 1, a system for remotely interrogating a
 medical implant device according to the invention is generally designated
 30. The system 30 includes a medical implant device 32 which is implanted
 in a living animal such as a human patient 34. As is discussed in more
 detail below, the medical implant device 32 can be any of a wide variety
 of different types of devices including, for example, a stent, graft,
 artificial joint, etc. In the preferred embodiment, the device is a stent.
 The device 32 preferably is configured to carry out or assist in carrying
 out a function within the,patient 34. For example, in the case of a stent
 the device 32 prevents the closing of an arterial wall and permits the
 flow of blood therethrough. In the case of a graft, the device 32 serves
 to couple blood flow between two separate ends of a blood vessel. The
 device 32 may instead consist of an artificial hip or knee which
 facilitates movement of the leg of the patient 34. Other type devices
 include, but are not limited to, a hemodialysis shunt and spinal brace,
 for example.
 The system 30 further includes an acoustic analyzer 36 for remotely
 powering and/or interrogating the implant device 32 in order to evaluate
 the device function. The analyzer 36 in the exemplary embodiment includes
 a broadband acoustic source/detector unit 38 which is positioned outside
 the patient 34 in close proximity to the implant device 32. As will be
 discussed in more detail below, the source/detector unit 38 serves to
 excite the device 32 with acoustic energy. The acoustic energy is used to
 evaluate the mechanical transfer function of the device 32. The
 source/detector unit 38 may then receive acoustic signals reradiated
 and/or reflected by the device 32 in response to the excitation. Such
 signals can then be processed by the analyzer 36 to detect a parameter of
 interest (e.g., amount of restenosis, etc.).
 The source/detector unit 38 is coupled via an electrical cable 40 to the
 main circuitry 42 included in the analyzer 36. The main circuitry 42
 includes suitable circuits for driving the source/detector unit 38 as
 described below, and for processing the output of the source/detector unit
 38 in order to provide an output to an operator (e.g., display 44).
 As will be better understood based on the description which follows, the
 present invention utilizes acoustic coupling between the source/detector
 unit 38 and the implant device 32. The device 32 is designed to respond to
 acoustic energy transmitted by the source/detector 38 in a manner which
 eliminates the need for complex electronics, power supplies, etc. within
 the device. In this manner, the device 32 can be a very simple, relatively
 low cost device which is less prone to failure. The device 32 does not
 require an active transmitter, mixer, amplifier, etc. as in other
 conventional devices. Moreover, the patient 34 is exposed to less high
 frequency radiation as compared to other types of remotely interrogated
 implant devices, thus improving the safety of the device.
 Referring now to FIG. 2, the acoustic analyzer 36 in accordance with the
 exemplary embodiment is illustrated in more detail. The source/detector
 unit 38 preferably is a hand-held sized device which is held by a doctor,
 nurse or medical assistant outside the body of the patient 34 in close
 proximity to the implant device 32. Since the system 30 is non-invasive,
 the source/detector unit 38 may be placed adjacent the implant device 32
 with the body of the patient (e.g., skin, muscle tissue, etc.), designated
 50, disposed therebetween.
 The analyzer 36 includes a data processing and control circuit 52 which is
 programmed to carry out the various control and computational functions
 described herein. More particularly, the circuit 52 provides a control
 signal on control bus 54. The control signal controls the frequency
 (within the acoustic frequency band) at which the source/detector 38
 excites the device 32 by emitting acoustical energy while positioned in
 close proximity to the device 32 as shown. In addition, the control
 circuit 52 provides a control signal on bus 54 in order to control whether
 the source/detector 38 is transmitting acoustic energy or receiving
 acoustic energy reradiated/reflected from the device 32 in response to
 being excited.
 The source/detector 38 receives acoustic energy from the device 32 based on
 the mechanical transfer function of the device 32, and converts the energy
 into an electrical signal on line 56. The signal on line 56 is input to a
 signal conditioning circuit 58 which conditions the signal prior to being
 input to the data processing and control circuit 52. As is discussed more
 fully below, the data processing and control circuit 52 processes and
 analyzes the signal on line 56 in order to determine a parameter
 associated with the device. For example, the excitation signal from the
 source/detector 38 is used to induce a mechanical resonance in the device
 32. The source/detector 38 then detects the response of the device 32 to
 such excitation by analyzing, for example, any harmonics which are present
 as determined by the acoustical energy radiated by the resonating device
 32. Alternatively, the circuit 52 may analyze the decay time associated
 with the mechanical resonance in response to excitation by the
 source/detector 38. Additionally, or in the alternative, the circuit 52
 may analyze other properties of the acoustic signal reradiated and/or
 reflected by the device 32 in response to the excitation signal (e.g.,
 changes in the Fourier Transform of the received signal).
 Features such as the presence of harmonics and/or the decay time of the
 received signal can be correlated to the function performed by the implant
 device. For example, the presence of harmonics in a stent 32 may increase
 or decrease as a function of the degree of restenosis which occurs within
 the stent. Thus, by monitoring the presence of harmonics over the course
 of periodic testing (e.g., trending), it is possible to track the build-up
 of restenosis. Similarly, a mechanical resonance decay time of the stent
 32 may increase or decrease as a function of the amount of restenosis
 present in the stent. Still further, the system 30 can analyze other
 changes in the mechanical transfer function itself and correlate such
 changes to the amount of restenosis. The scope of the present invention is
 intended to encompass any and all such correlations which may be found
 between the parameter of interest, the acoustic excitation and the
 response of the device 32.
 In order to interrogate/excite the stent 32 over a significant portion of
 its transform function frequency range, a broad band source/detector 38 is
 preferred. This provides for the greatest range of response and excitation
 of the device 32. Conventional ultrasound transducers with more limited
 bandwidth can also be used, although preferably after those frequencies in
 the mechanical transfer function of the device 32 having significant
 correlation to restenosis have been identified.
 FIG. 3 provides a perspective view of the source/detector 38 in relation to
 a stent type device 32 located in a blood vessel 59. As shown in FIG. 3,
 the source/detector 38 includes a two-dimensional (mxn) array 60 of
 miniature acoustic devices 62. Each device 62 is made up of an
 electro-acoustic transducer such as a piezoceramic device. In a transmit
 or excite mode, each device 62 is responsive to an electrical driving
 signal so as to emit an acoustic wave. Conversely, in a receive mode each
 device is designed to receive an acoustic wave and convert the received
 wave into an electrical signal. The level of the signal is based on the
 intensity of the received wave. Although the preferred embodiment utilizes
 an array 60 of piezoceramic devices 62, other type devices can also be
 used without departing from the scope of the invention.
 The devices 62 are arranged in a generally planar array. The active faces
 of the devices 62 are oriented in a common direction so as to be directed
 downward towards the implant device 32. A housing 64 (shown in cut-away)
 provides a protective enclosure for the source/detector 38, with an
 acoustic window provided in the housing 64 to allow acoustic waves to be
 emitted and received by the devices 62.
 As is illustrated in FIG. 4, an electrical input/output 66 of each device
 62 in the array 60 is hardwired together with the others in parallel. The
 input/outputs 66 are selectively connected via a switch 68 to either the
 output of a voltage controlled oscillator (VCO) 70 or a received signal
 line 72. During a transmit or excite mode, a control signal on line 74
 from the circuit 52 (FIG. 2) causes the switch 68 to couple the output of
 the oscillator 70 to the input/output 66 of each of the devices 62. At the
 same time, the circuit 52 provides a control voltage on line 76 to control
 the frequency of the VCO 70.
 The VCO 70 preferably is an oscillator which is designed to produce an
 output signal at any frequency within the acoustical range of 50 kilohertz
 (kHz) to 10 megahertz (MHz). Furthermore, it is desirable that each of the
 devices 62 provide a generally uniform response throughout the range.
 However, with existing piezoceramic devices 62 currently available, each
 device has a generally narrow band of operation (e.g., on the order of
 .+-.5% about its center operating frequency f.sub.op). Consequently, the
 array 60 in the present invention is made up of devices 62 selected with
 different operating frequencies f.sub.op uniformly distributed across the
 broadband acoustical range of 50 kHz to 10 MHz. As a result, the composite
 response of the devices 62 is generally uniform as represented in FIG. 5.
 In this manner, the array 60 is able to transmit and detect acoustic energy
 regardless of the particular frequency at which the device 32 is to be
 excited or at which the device 32 emits acoustic energy in response to
 excitation. The operating frequencies f.sub.op of the devices 62 are
 selected so that at least one device 62 is responsive to the excitation
 signal from the VCO 70 in order to emit an acoustic signal at each
 frequency. Similarly, at least one device 62 is responsive in the receive
 mode to detect the respective frequencies reradiated by the device 32,
 including any harmonics.
 In a further preferred embodiment, the devices 62 with the different
 operating frequencies f.sub.op are spatially distributed within the array
 60. Such spatial distribution preferably is selected so that the
 respective operating frequencies will be uniformly distributed across the
 array 60 and the overall frequency response of any region within the array
 60 will be the same as the other. For example, regions 80 and 82 each
 preferably contain a sufficient number of devices 62 with selected
 operating frequencies to exhibit the same response curve shown in FIG. 5.
 Therefore, it will be appreciated that the overall array 60 will function
 as a broadband source/detector generally independent of the particular
 region (e.g., 80 or 82) which is positioned immediately adjacent the
 device 32. The array 60 therefore will be operative throughout the entire
 acoustic frequency band of interest.
 Briefly referring back to FIG. 3, the stent device 32 may be a conventional
 stent which generally consists of a cylindrical tube. The tube may be made
 of metal such as stainless steel, or another material such as plastic
 and/or a composite material. The tube wall may be uniform, helical, or
 some other geometry.
 Notably, the stent 32 will have a resonant frequency .omega..sub.R (or
 frequencies in the case of there being multiple resonant frequencies),
 based upon its physical configuration and material properties of the stent
 32. The inventors have recognized that if the stent 32 is excited by an
 acoustic pulse which has strong frequency component(s), .omega..sub.P, of
 its own in the neighborhood(s) of the resonant frequency or frequencies
 .omega..sub.R, the reradiated signal of the stent 32 will contain both
 sets of frequency components (i.e., .omega..sub.P and .omega..sub.R), and
 that the amplitude of these components, both absolutely and relative to
 one another, will be a function of the degree of damping of the sent 32
 due to restenosis. FIG. 7a is an example of such a function: It is a plot
 of the amplitude of the resonance frequency componet, .omega..sub.R, as a
 function of damping coefficient "a".
 FIG. 7b illustrates how the damping coefficient "a" varies with respect to
 degree of restenosis. In FIG. 7b, a level 0 restenosis represents no
 occlusion in the stent and the damping coefficient a is at a local
 minimum. A level 1 restenosis represents complete occlusion at which the
 damping coefficient a is at a local maximum. Thus, the stent 32 can be
 said to have a mechanical transfer function which varies in relation to
 the degree of restenosis.
 The amplitude distribution of the reradiated signal from the stent 32 in
 the frequency domain can be found from the Fourier transform of the
 reradiated signal.
 Thus, if a time domain measurement of the reradiated acoustic energy from
 the stent 32 is made and then Fourier transformed so that the power or
 amplitude at the frequency or frequencies .omega..sub.R is determined,
 then the damping coefficient a can be determined from FIG. 7a mentioned
 above, for example. The amount of occlusion or degree of restenosis can
 then be estimated via the correlation represented in FIG. 7b.
 FIG. 6 is a flowchart representing the above analysis as carried out by the
 system 30 in accordance with one embodiment of the invention. The data
 processing and control circuit 52 (FIG. 2) includes a microprocessor which
 is programmed to carry out the appropriate control and processing
 described herein. Such programming will be apparent to those having
 ordinary skill in the art based on the disclosure provided herein. Hence,
 further details regarding the particular programming have been omitted for
 sake of brevity.
 Beginning in step 100, the system 30 initializes itself by ascertaining the
 most suitable resonant frequency .omega..sub.R of the stent 32. In one
 embodiment this may be done by reference to a lookup table of resonances
 for different stents (e.g., known commercially available stents). Such
 data can be previously obtained empirically in laboratory tests.
 Alternatively, the source/detector unit 38 is held in close proximity to
 the patient's body with the array 60 facing the stent 32 (e.g., as
 represented in FIG. 1). The control circuit 52 (FIG. 2) systematically
 begins to sweep the output frequency of the VCO 70 through the acoustic
 frequency band in which the resonant frequency .omega..sub.R is expected
 to appear. The output of the VCO 70 is applied to the array so that the
 stent 32 is excited by the acoustic energy at the frequency of the VCO 70.
 The circuit 52 systematically samples the acoustic energy which is
 reradiated by the stent 32 at each frequency by controlling the switch 68.
 The energy level of the reradiated signal at each particular frequency is
 input to the circuit 52 from the signal conditioning circuit 58.
 Since the source/detector 38 is preferably broadband as noted above, at
 least one device 62 is operative at each frequency to transmit and receive
 the acoustic signal. The circuit 52, in step 100, determines at which
 frequency in the acoustic frequency band the reradiated acoustic energy is
 at its highest level as detected by the source/detector 38. Such maximum
 energy frequency level will correspond to the most suitable resonant
 frequency .omega..sub.R of the stent 32, typically, and thus the circuit
 52 ascertains the resonant frequency .omega..sub.R.
 Next, in step 102, the circuit 52 causes the source/detector 38 to excite
 the stent 32 with a brief burst of acoustic energy at or near the resonant
 frequency .omega..sub.R. The signal received from the stent 32 is input to
 the circuit 52 from the conditioning circuit 58. The circuit 52 then
 proceeds to take a time series measurement of the reradiated acoustic
 energy signal from the stent 32 as represented in step 104.
 Next, the circuit 52 takes the Fourier transform of the time series data in
 step 106. The Fourier transform yields, among other things, the energy
 components of the reradiated acoustic energy at the frequencies
 .omega..sub.R. Using a lookup table based on an empirically determined
 curve like that shown in FIG. 7a, for example, the circuit 52 determines
 the damping coefficient "a" in step 108. The circuit 52 then compares the
 value of the damping coefficient "a" with a table stored in memory
 representing the graph of FIG. 7b, for example. Based on the value of "a",
 the circuit 52 estimates the degree of restenosis as represented in step
 110. The circuit 52 may then provide an output on the display 44 or the
 like indicating such estimate. Moreover, the circuit 52 may store such
 information in memory for future use in trending or the like.
 In an alternate embodiment, the circuit 52 may use other known data
 analysis techniques to analyze the frequency content of the acoustic
 energy reradiated from the stent 32. For example, wavelet transformations
 and/or neural network techniques may be employed by the control circuit.
 Moreover, such techniques may be modified to account for different
 conditions in taking the measurements such as large muscle mass, nearby
 bone structures, etc.
 Additionally, the circuit 52 may employ such techniques as pattern
 recognition to analyze the reradiated acoustic energy. For example, the
 circuit 52 may be programmed to carry out pattern recognition to analyze
 the class of resonant frequencies exhibited by the stent 32 in response to
 the acoustic excitation.
 FIGS. 8 and 9 illustrate a specially designed acoustic reradiating stent
 120 which can be substituted for the otherwise conventional stent 32
 described above. The stent 120 is made up of two hollow concentric
 cylinders 122 and 124 which are mechanically connected so that the entire
 structure has a pronounced mechanical resonance at a resonant frequency
 .omega..sub.R within the acoustic frequency band. The outer cylinder 122
 and the inner cylinder 124 are each made of a biocompatible material such
 as stainless steel, plastic, etc.
 The outer cylinder 122 is mechanically connected to the inner cylinder 124
 by resilient connecting members 126. The connecting members 126 are made
 of a resilient material such as rubber or plastic. Each member 126 is
 sufficiently rigid to maintain generally a physical separation between the
 two cylinders, yet is sufficiently resilient to allow for relative
 movement between the cylinders 122 and 124 at the resonant frequency
 .omega..sub.R. In the exemplary embodiment, the connecting members 126 are
 equally spaced around the circumference of the cylinders. However, it will
 be appreciated that other configurations are also possible.
 The stent 120 further includes a seal ring 128 at each end which seals off
 the circumferential area between the two cylinders 122. The seal rings 128
 prevent blood from entering the area between the cylinders. The seal rings
 128 are made up of a resilient material such as rubber or plastic similar
 to the connecting members 128.
 Hence, the stent 120 will exhibit a pronounced mechanical resonance based
 on the relative motion which can occur between the two concentric
 cylinders.
 The stent 120 may be utilized in accordance with the system 30 as described
 in relation to FIG. 6. In particular, the damping coefficient may be
 determined from a curve like that shown in FIG. 7a and used to estimate
 occlusion as described above. In an alternative embodiment, however, the
 degree of restenosis may be estimated using a different, albeit related,
 criteria.
 For example, FIG. 10 illustrates represents a configuration of the system
 30 in which the decay time of the reradiated acoustic energy is utilized
 to estimate restenosis. More particularly, the stent 120 is excited at or
 near its resonant frequency or frequencies .omega..sub.R in a manner
 similar to that described above in steps 100 and 102 in FIG. 6. Upon
 switching the switch 68 from excite mode to receive mode, the array 60 is
 then used by the circuit 52 to detect the acoustic energy reradiated from
 the stent 120 at the resonant frequency(s) .omega..sub.R.
 Specifically, the circuit 52 measures the amplitude of the reradiated
 acoustic energy over time in order to determine its decay time. With no
 restenosis, and hence little or no damping, the cross section of the stent
 120 will be filled with blood 130 as represented in FIG. 12a. The
 non-occluded stent 120 will have a characteristic decay time following
 excitation as represented by curve 134 in FIG. 11a. As restenosis
 proceeds, the non-blood tissue 136 will begin to fill the cross section of
 the stent 120 as shown in FIG. 12b. Depending on the particular design of
 the stent 120, the restenosis build-up will modify the decay time of the
 reradiated acoustic energy.
 In the exemplary embodiment, the stent 120 varies in decay time as a a
 function of increasing restenosis. Thus, the decay time may decrease as
 restenosis increases as represented by curve 136 in FIG. 11b. By comparing
 the decay time of the reradiated acoustic energy from a given energy level
 I.sub.start to a second level I.sub.ref, the circuit 52 is programmed to
 estimate the degree of restenosis. Such estimate can be based on expected
 values stored in the circuit 52. In addition, or in the alternative, the
 measured decay time can be stored in memory in the circuit 52 for purposes
 of trending.
 It will be appreciated that several inventive aspects have been described
 herein with respect to a stent 32 or 120. Nevertheless, it will be further
 appreciated that the same inventive aspects apply to other medical implant
 devices such as grafts, orthopedic prostheses, orthopedic trauma implants
 and reinforcements, etc. While analyzing the acoustic energy reradiated by
 the device is described in connection with determining the amount of
 restenosis, it will be appreciated that other parameters may also be
 determined. For example, frequency content changes, variations in the
 decay time, phase shifts, etc., can be utilized by the circuit 52 to
 estimate stress, strain, boundary constraints, etc., within and on the
 device. Provided the transfer function of the device 32 can be determined
 in relation to a parameter of interest, the present invention allows such
 information to be obtained remotely from the implanted device using
 acoustic energy.
 Referring now to FIG. 13, another embodiment of a system according to the
 present invention is shown for diagnosing restenosis in a stent. However,
 it will be appreciated that the same techniques can be applied to other
 medical implant devices in the same manner referred above. The system 200
 shown in FIG. 13 comprises a conventional ultrasonic imaging apparatus 202
 in combination with an analyzer module 204 and optional display/printer
 206.
 As will be discussed in more detail, the ultrasonic imaging apparatus 202
 provides ultrasonic data to the analyzer module 204. The analyzer module
 204 captures and digitizes the data, and performs one or more analyses in
 order to determine the degree of restenosis which has built up in a stent
 32 implanted within a living body. The analyzer module 204 may actively
 control the imaging apparatus 202, or function merely to acquire the data
 and perform post-acquisition processing and analysis as is discussed
 below.
 The exemplary ultrasonic imaging apparatus 202 includes a transducer array
 211 comprised of a plurality of separately driven elements 212 which each
 produce a burst of ultrasonic energy when energized by a pulsed waveform
 produced by a transmitter 213. The ultrasonic energy reflected and/or
 reradiated back to transducer array 211 from the subject under study
 (e.g., the stent 32 located within the living body as shown in FIG. 1) is
 converted to an electrical signal by each transducer element 212 and
 applied separately to a receiver 214 through a set of transmit/receive
 (T/R) switches 215. Transmitter 213, receiver 214 and switches 215 are
 operated under control of a digital controller and system memory 216
 responsive to commands by a human operator. A complete scan is performed
 by acquiring a series of echoes in which switches 215 are set to the
 transmit position, transmitter 213 is gated on momentarily to energize
 each transducer element 212, switches 215 are then set to the receive
 position, and the subsequent echo signals produced by each transducer
 element 212 are applied to receiver 214. The separate echo signals from
 each transducer element 212 are combined in receiver 214 to produce a
 single echo signal which is employed to produce a line in an image on a
 display system 217.
 The transducer array 211 typically has a number of piezoelectric transducer
 elements 212 arranged in an array and driven with separate voltages
 (apodizing). By controlling the time delays (or phase) and amplitude of
 the applied voltages, the ultrasonic waves produced by the piezoelectric
 elements 212 (transmission mode) combine to produce a net ultrasonic wave
 that travels along a preferred beam direction and is focused at a selected
 point along the beam. By controlling the time delays and amplitude of
 successive applications of the applied voltages, the beam with its focal
 point can be moved in a plane to scan the subject. Likewise, by
 controlling the time delays, etc., the beam in accordance with the present
 invention can be directed at different angles and depths relative to the
 living body in order to focus the ultrasonic radiation on a particular
 object, namely the stent 32.
 The same principles apply when the transducer array 211 is employed to
 receive the reflected sound (receiver mode). That is, the voltages
 produced at the transducer elements 212 in the array 211 are summed
 together such that the net signal is indicative of the sound reflected
 from a single focal point in the subject e.g., the location of the stent
 32 in accordance with the present invention). As with the transmission
 mode, this focused reception of the ultrasonic energy is achieved by
 imparting separate time delays (and/or phase shifts) and gains to the
 signal from each transducer array element 212. In addition, to reduce side
 lobes in the receive beam the amplitude of each transducer element signal
 is modified in accordance with a window function prior to summation into
 the focused beam. Suitable ultrasonic imaging apparatuses 202 are
 described in more detail in U.S. Pat. No. 5,345,939, for example, the
 entire disclosure of which is incorporated herein by reference.
 In the exemplary embodiment, the receiver 214 provides an RF output signal
 on line 220 which represents the net signal indicative of the sound
 reflected from the single focal point. Thus, when the ultrasonic beam is
 properly focused on the stent 32 by virtue of a doctor, nurse or medical
 assistant positioning the hand-held sized transducer array 211 outside the
 body of the patient 34 in close proximity to the implant device 32 and
 adjusting the position and focus of the beam, the signal on line 220
 represents the ultrasonic signal reflected back and/or reradiated by the
 stent 32. Likewise, when the ultrasonic beam from the transducer array 211
 is focused on another portion of the living body (e.g., the heart), the
 signal on line 220 represents the acoustic energy reflected or reradiated
 by that particular portion of the body.
 The RF output signal on line 220 is input to the analyzer module 204 as
 shown in FIG. 13. The analyzer module 204 captures the received ultrasonic
 signal and digitizes the signal to produce data which is then processed in
 order to evaluate predefined parameters associated with the stent 32 such
 as the amplitude, frequency response, decay times, etc. The analyzer
 module 204 uses the measured parameters to determine the degree of
 restenosis experienced by the stent 32 based on predefined conditions, a
 neural network, expert system, or the like programmed into the analyzer
 module 204 via software, etc. The result(s) of the diagnos(es) are then
 provided by the analyzer module 204 to the display/printer 206 so that
 they may be observed or recorded by the operator. In addition, or in the
 alternative, the results of the analysis may be stored in memory by the
 analyzer module 204 together with the received data itself, for example,
 for future reference, trending, etc.
 In the exemplary embodiment, the analyzer module 204 is coupled to the
 digital controller 216 via an optional interface connection represented by
 phantom control bus 222. As will be discussed below in relation to FIG.
 14, the analyzer module 204 includes an interface which allows the
 analyzer module 204 to control the ultrasonic imaging apparatus 202
 remotely with respect to parameters such as frequency, amplitude and
 location of the ultrasonic beam transmitted/received by the transducer
 array 211. This allows the analyzer module 204 to adjust automatically
 such parameters when interrogating the stent 32. Alternatively, the
 analyzer 204 may be programmed to output instructions on the display 206
 to prompt an operator to provide various adjustments of the ultrasonic
 beam with respect to frequency, amplitude, location, etc. via the controls
 provided with the conventional apparatus 202.
 Turning now to FIG. 14, the analyzer module 204 is shown in detail. The
 analyzer module 204 includes a controller 230 which is programmed to carry
 out and/or coordinate performance of the various functions described
 herein. In addition, the analyzer module 204 includes an analog-to-digital
 (A/D) converter 232 which receives the RF output signal on line 220 and
 digitizes the signal for subsequent processing. Data storage or buffer
 memory 234 such as a hard drive or the like is provided for storing the
 digitized data received via the RF output signal. The analyzer module 204
 further includes a digital signal processor (DSP) 236 for carrying out
 high speed math computations such as FFTs, wavelet transforms, etc. in
 order to analyze the data stored in the data storage memory 234.
 A system interface 238 enables the controller 230 to communicate with the
 digital controller 216 via control bus 222 in the preferred embodiment. As
 noted above, the interface 238 in the preferred embodiment allows the
 controller 230 within the analyzer module 204 to control the ultrasonic
 imaging apparatus 202 remotely with respect to parameters such as
 frequency, amplitude and location of the ultrasonic beam
 transmitted/received by the transducer array 211. A memory 240 is included
 in the analyzer module 204 for serving as working memory as well as
 storing computer programming code designed to be executed by the
 controller 230 and/or DSP 236 for carrying out the operations described
 herein. The particular programming code can be written in any of a variety
 of conventional programming languages by those having ordinary skill in
 the art based on the disclosure provided herein. Accordingly, further
 detail on the particular programming code is omitted for sake of brevity.
 The memory 240 may include random access memory together with non-volatile
 memory. The memory 240 may include more permanent storage such as a hard
 drive, disc drive, etc., as will be appreciated. The program for carrying
 out the functions described herein is stored in computer readable format
 within the memory 240 and is accessed and executed by the controller 230
 and/or DSP 236 in order carry out such functions.
 A signature database 242 is also included in the analyzer module 204. The
 signature database 242 stores signature data associated with one or more
 known medical devices such as commercially available stents. The signature
 data may include data describing the mechanical transfer function of the
 respective stents in relation to their response to ultrasonic radiation of
 the type provided by the system 200. For example, the signature database
 242 may include frequency response information for different type stents
 as discussed below in relation to FIG. 19. Such signature data may be
 obtained empirically, based on modeling, etc. The signature data can be
 stored with respect to different types of stents which are free of
 occlusion. In addition, the signature data may include data for each stent
 representing different degrees of occlusion, for example.
 It will be appreciated that the analyzer module 204 may easily be
 incorporated into a personal computer or other device which is coupled to
 the ultrasonic imaging apparatus 202. Hence, with the addition of a
 relatively small amount of additional hardware and software programming
 running within the analyzer module 204, the system 200 of the present
 invention can make use of existing ultrasonic imaging apparatus equipment.
 This allows hospitals and other healthcare facilities to maximize use of
 their available resources. In the alternative, it will be appreciated that
 the system 200 could be configured and sold as an integral unit without
 departing from the scope of the invention.
 Referring now to FIG. 15a, provided is an example of how the amplitude of
 the reflected ultrasonic signal from a stent 32 varies as a function of
 the amount of restenosis which has built up within the stent. FIG. 15a
 represents data which was obtained at an ultrasonic frequency of 2
 megahertz (MHz) for a 2.5 millimeter (mm) stent in an uninjured artery
 from a pig. The vertical axis represents the amplitude of the reflected
 signal (arbitrary units). The horizontal axis represents time in
 microseconds following the respective stents being excited by an
 ultrasonic pulse at 2 MHz.
 Line 250 in FIG. 15a illustrates the response of a clean stent 32. Line 252
 represents the response of the same type stent 32 which has incurred a
 buildup of thrombus in which blood flow was completely blocked after 16
 minutes. As is shown in FIG. 15a, the amplitude of the ultrasonic signal
 received from the stent 32 is significantly reduced by the thrombus. FIG.
 15b illustrates the FFT of each of lines 250 and 252 (designated 250F and
 252F, respectively). As can be seen, the FFTs differ markedly for the two
 states.
 Similar information is illustrated in FIG. 15c for a 2.5 mm NIR stent 32 at
 various degrees of restenosis. As the amount of restenosis increases, the
 amplitude of the response signal tends to decrease.
 Information such as that shown in FIGS. 15a, 15b and 15c is programmed into
 the analyzer module 204 in order to diagnose the amount of restenosis
 experienced by a stent 32 under study. Such information may include
 absolute or relative amplitudes with respect to time, frequency, etc.,
 decay times as is discussed above in connection with the previous
 embodiment, harmonics, etc. Stored within the analyzer module 204 is a set
 of rules, predefined conditions, etc. against which the ultrasonic data
 received by the analyzer module 204 from the stent 32 under test can be
 compared and the analyzer module 204 compares the data so as to reach a
 conclusion. For example, if the relative amplitudes at different times for
 a particular type of stent 32 change by a predetermined fraction, the
 analyzer module 204 concludes that the stent 32 has undergone an X%
 occlusion due to restenosis. Alternatively, if the frequency components of
 the received ultrasonic signal at one or more excitation frequencies
 change by a predefined amount, the analyzer module 204 concludes that
 there is Y% occlusion, for example. Generally speaking, the analyzer
 module 204 extracts the parameters of interest from the received signal
 and calculates appropriate figures of merit which correlate with clinical
 evidence of restenosis. Such information can then be displayed via the
 display 206 or the like.
 Data such as that shown in FIGS. 15a, 15b and 15c can also be stored in the
 signature database 242 as signature patterns against which the analyzer
 module 204 can compare measured ultrasonic data from a stent 32 within a
 living body. It will be appreciated that the DSP 236 may be tasked by the
 controller 230 to carry out the complex math functions (e.g., FFTs,
 pattern matching, etc.) associated with the various desired analyses at
 high speed using conventional techniques. Each of the respective
 components within the analyzer module 204 are configured to be able to
 access the appropriate data from the other components as needed again
 using conventional techniques.
 Referring now to FIG. 16, shown is a flowchart illustrating the general
 operating process of the system 200 in accordance with the present
 invention. An operator begins the procedure by placing the transducer
 array 211 on the body of the patient in proximity of the implanted stent
 32. In step 300, the precise location of the stent 32 within the body is
 determined in order to ensure that the ultrasonic beam from the transducer
 array 211 is incident thereon. Step 300 may be carried out automatically
 as described below in connection with FIG. 17, or manually as discussed
 below in connection with FIG. 18, for example.
 Upon locating the stent 32, the system 200 proceeds to step 302 in which
 the stent 32 is irradiated with ultrasonic energy from the transducer
 array 211. The reflected/reradiated energy from the stent 32 is received
 by the transducer array 211 and the resultant RF output signal is provided
 to the analyzer module 204. The analyzer module 204 may control the
 particular frequenc(ies), amplitude(s), etc. of the ultrasonic beam
 automatically via the control bus 222 (FIG. 13), or simply prompt the
 operator to set the appropriate parameters via the display 206 or the
 like. The analyzer module 204 in step 302 captures and digitizes the data
 via the A/D converter 232, and stores the data in the data storage memory
 234.
 In step 304, the analyzer module 204 performs preprogrammed routines for
 analyzing the acquired data such as taking the FFT, wavelet
 transformations, etc. The analyzer module 204 uses such information in the
 manner described above in order to assess the extent of restenosis
 experienced by the stent 32. Next, in step 306 the analyzer module 204
 outputs the diagnosis via the display 206 or the like.
 FIG. 17 illustrates an automated embodiment for locating a stent 32 within
 the body in accordance with the present invention. Once the transducer
 array 211 has been placed outside the body in close proximity to the stent
 32 by the operator in step 300, the controller 230 within the analyzer
 module 204 provides control commands to the controller 216 in the imaging
 apparatus 202 to direct and receive the ultrasonic beam to/from the
 location of the stent 32. For example, the ultrasonic beam is first set to
 an initial location (e.g., .theta.=0.degree.) as shown in step 310. The
 analyzer module 204 then acquires and analyzes the ultrasonic data
 received from such location in step 312. In step 314, the analyzer module
 204 determines whether the data acquired in step 312 includes a
 characteristic feature indicative of the presence of the stent 32. For
 example, the stent 32 may be known to exhibit a substantial resonance at a
 particular frequency, such resonance not being exhibited by other portions
 of the body.
 If in step 314 the characteristic feature is detected as determined by the
 analyzer module 204, the location of the beam is noted and fixed via the
 control bus 222 as represented in step 316. On the other hand, if the
 characteristic feature is not detected in step 314, the analyzer module
 204 proceeds to step 318 wherein it causes the controller 216 to adjust
 the location of the ultrasonic beam and the process returns to step 312.
 Accordingly, the location of the ultrasonic beam may be adjusted
 incrementally in steps 312, 314 and 318 based on a predefined pattern, for
 example, until the precise location of the stent 32 is determined.
 FIG. 18 illustrates an embodiment of step 300 which is carried out
 semi-manually. The imaging apparatus 202 is configured such that the
 ultrasonic beam position as transmitted/received by the transducer array
 211 is fixed (e.g., .theta.=0.degree.). After the operator has placed the
 transducer array 211 proximate the stent 32 on the body, the analyzer
 module 204 is configured to acquire and analyze the ultrasonic data in
 step 320 similar to step 312. Next, in step 322 the analyzer module 204
 determines if the predefined characteristic feature is present in the
 received data similar to step 314. If yes, the analyzer module 204 in step
 324 displays an acknowledgment to the operator on the display 206 to
 instruct the operator to maintain the present position of the transducer
 array 211. If no in step 322, the analyzer module 204 in step 326 displays
 a request on the display 206 that the operator adjust the location of the
 transducer array 211 by either physically moving the array 211 or changing
 the beam location by controlling the parameters of the imaging apparatus
 202 in a conventional manner.
 In an even more manual approach, the operator in step 300 observes a full
 ultrasound scanned image initially obtained, and visually identifies the
 characteristic feature of interest. Such feature will occur at one or more
 lines of the scanned image, and represents the response of the stent 32
 within the image. The operator identifies the respective line or lines of
 the scanned image and enters such information into the analyzer module
 204. The data from those respective lines is then analyzed in step 304.
 FIG. 19 represents the manner in which different stents and/or types of
 stents can exhibit different signatures with respect to frequency response
 over a predefined band or another predefined parameter, for example. As is
 shown in FIG. 19, stents 1 thru 3 may exhibit different amplitudes of
 reflected/reradiated energy across a frequency band f1 to f2. This
 information is stored in the signature database 242 based on empirical
 measurements, modeling, etc., for example.
 FIG. 20 illustrates a process by which the analyzer module 204 is
 programmed to utilize signature recognition as part of the analysis step
 304 in FIG. 16. For example, the analyzer module 204 in step 330 acquires
 from the data storage memory 234 data meeting a predefined criteria. Such
 data may be frequency response data across the frequency band f1 to f2
 similar to that shown in FIG. 19. Next, in step 332 the DSP 236 is
 employed to attempt to match the data obtained in step 330 with one of the
 patterns stored in the signature database 242 using known matching
 techniques. In step 334, the analyzer module 204 determines if the
 acquired data matches within a predetermined degree one of the patterns
 stored in the signature database 242. If yes, it is concluded that
 information pertaining to the stent 32 under study is available. Such
 information may be prestored together with the signature data in the
 database 242. In step 336, the analyzer 204 utilizes such information to
 facilitate the diagnosis. For example, such information may be helpful in
 normalizing the acquired data or choosing the particular evaluation
 criteria to be applied to the data obtained from the stent 32. Also, by
 being able to differentiate between different stents non-invasively, the
 present invention is particularly useful with respect to patients for whom
 there are no records of the particular stent which has been implanted. If
 in step 334 the analyzer module 204 is unable to match the acquired data
 to a signature stored in the database 242, the analyzer module may be
 programmed to proceed with a standard default analysis, for example.
 It will therefore be appreciated that the present invention provides a
 means for early detection of restenosis within a stent. By detecting
 restenosis early, a patient can be placed on preventative drug therapy, an
 exercise regimen, etc., and possibly avoid surgery in the future.
 Moreover, the present invention allows such procedures to be carried out
 using predominantly existing equipment so as to help minimize costs
 associated with healthcare.
 The present invention is not limited only to the aspect of non-invasive
 early detection of restenosis in stents, but also may include the
 additional steps of treating the restenosis. Since the invention provides
 for early detection, non-invasive and/or less invasive methods of
 treatment may be employed. For example, the present invention includes the
 additional steps such as radiation treatment, photodynamic therapy via a
 catheter, mechanical removal of the restenosis via catheter, etc.
 Furthermore, drug based treatments such as subcutaneous angiopectin
 treatment may be employed based on early detection in accordance with the
 present invention. The stent site is perfused with the drug to
 prevent/slow the restenosis process. See, e.g., M. K. Hong et al.,
 "Continuous Subcutaneous Angiopectin Treatment Significantly Reduces
 Neointimal Hyperplasia in a Porcine Coronary In-Stent Restenosis Model",
 Circulation, 95:2, 1997.
 Although the invention has been shown and described with respect to certain
 preferred embodiments, it is obvious that equivalents and modifications
 will occur to others skilled in the art upon the reading and understanding
 of the specification. For example, while the present invention has been
 described primarily in the context of an implant device which is a stent,
 other type devices can also be used. In addition, while particular
 existing ultrasonic imaging apparatuses are mentioned, the present
 invention has utility with other existing and future ultrasonic
 apparatuses. For example, the present invention also contemplates the use
 of future ultrasonic techniques such as nondiffracting X waves presently
 being discussed in the literature. Moreover, a technique such as
 modulation of an ultrasonic carrier signal at or near the resonances of
 the implant device can be utilized to improve signal-to-noise ratios.
 Also, time-reversal techniques may be employed to the ultrasonic signals
 transmitted into and received from the body to minimize the effects of
 noise, energy losses, etc. The present invention includes all such
 equivalents and modifications, and is limited only by the scope of the
 following claims.