PATHOLOGICAL TISSUE ANALYSIS AND REMOVAL SYSTEM

Abstract: A system (100) for distinguishing among a plurality of materials is disclosed. The system comprises a laser (101) for producing substantially monochromatic electromagnetic energy (103), which impacts a substance (105). The substance (105) generates a pressure wave (107) characteristic of the identity of the substance in response to the impact. An analyzer (109) is operatively oriented to receive the pressure wave (107) and identifies the substance in response to the pressure wave.

Description: PATHOLOGICALΗSSUEANALYSISANDREMOVALSYSTEM Field of the Invention The invention relates generally to a system and method utilizing a laser for substance analysis and, in particular, to a system and method utilizing a laser for analyzing and removing blockage in a blood vessel. Background of the Invention One of the most frequent causes of death is cardiovascular diseases and the single biggest cause of cardiovascular disease is atherosclerosis. Atherosclerosis is the buildup of fatty and fibrous deposits, that may or may not contain calcium, in an artery. Eventually the buildup may entirely block the artery. Moreover, if the artery is a coronary artery, the buildup may cause a heart attack. Treatment of atherosclerosis requires that either a new path for blood flow be provided, or that the blockage be removed. There are two widely used methods for treating coronary atherosclerosis. First, bypass surgery may be used to reroute blood around a section of blocked heart artery. Second, percutaneous transluminal coronary angioplasty (PTCA) , widens an existing path of blood flow. PTCA, also called balloon angioplasty, involves passing a catheter through the skin into a blood vessel and threading the catheter through the cardiovascular system to the partially blocked artery. A balloon is attached to the catheter and when the balloon is positioned in the partially blocked vessel, the balloon is inflated and compresses the atherosclerotic plaque, opening the artery. Thus, the vessel's interior diameter is widened to permit sufficient blood flow. Similar techniques are used for other arteries in the body. Coronary bypass surgery has some limitations. It is difficult, expensive and, in some circumstances, has a relatively high mortality rate. It is the only procedure useful for clearing occluded arteries. A third method of clearing plaque is to use a tiny auger type device. Such a device is threaded to the blockage in a manner similar to that used in PCTA, but clears the blockage mechanically by grinding it away. The use of balloon angioplasty (PTCA) and the auger method also have their limitations. The bends in arteries and the irregular configurations of plaque deposits make it difficult to reach every diseased area. Balloon angioplasty may knock pieces of plaque loose, which then flow through the vascular system. As an artery narrows, the plaque will eventually block the vessel, causing an obstruction. Moreover, in vessels that are totally obstructed it is not possible to penetrate the deposits in order to use the balloon. Finally, in many of the cases where balloon angioplasty is performed, some narrowing of the artery recurs within three to six months of the procedure. The use of lasers to clear blocked arteries is also known. One known technique involves the use of an optical fiber coupled to a laser, and a small metal cylinder tip near the tip of the optical fiber. The fiber is attached to a catheter which is threaded into the blocked vessel. A laser beam is generated by the laser and transmitted to the metal cylinder tip by the optical fiber. The cylinder tip then heats up and burns away the adjacent plaque. However, the plate will also burn through the vessel wall if it is positioned too closely to the wall. Others have suggested using a laser beam to directly destroy plaque. See, for example. Effects of Carbon Dioxide, Nd-YAG, and Argon Laser Radiation on Coronary Atheromatouε Plagues , Abella, et al., American Journal of Cardiology, Vol. 50, p. 1199. Abella describes the effectiveness of several different types of lasers to remove plaque from blood vessels that were removed from the body. Another system using lasers to remove plaque from a totally blocked vessel is described in Laser Angioplasty of Totally Occluded Coronary Arteries and Vein Grafts : Preliminary Report on a Current Trial , The American Journal of Cardiology, Volume 63, Page 9F, and includes combining the use of a laser with balloon angioplasty. Specifically, this system includes an optical fiber which passes through a balloon catheter. Energy is transmitted by the laser through the optical fiber to the blocked vessel. The laser beam impacts and destroys the plaque. When the blocked blood vessel is sufficiently cleared to allow the balloon catheter to be inserted into the blocked area, the balloon is inserted and inflated. Another known example of the use of lasers to remove plaque is described in Plague Busters , Bronson, Forbes Magazine, July, 28, 1986. Bronson describes the use of excimer lasers to remove plaque from blocked walls. Excimer lasers are chemical lasers that utilize gasses that are poisonous and corrosive. This system directs the laser energy through optical fibers attached to a catheter, to the blood vessel in very short (250 nano seconds) pulses of energy in an attempt to avoid destroying the vessel wall. Very little heat is generated because the bursts of energy are so brief, and only the matter upon which the laser beam impacts is destroyed, and the surrounding tissue will not be damaged. The physician using the system monitors the process through a camera connected to lens-tipped fibers, which are also attached to the catheter and transmit the image at the tip of the laser to the camera, to avoid impacting and destroying the vessel wall. In the event that damage to the wall occurs, or will occur, because of the position of the optical fiber, the physician repositions the optical fiber so that the laser beam is directed toward the plaque to be removed. In the prior art methods and devices the problem of avoiding damaging the vessel wall is not adequately solved. Systems that utilize a metal cylinder tip provide virtually no control and can burn through vessel walls as easily as through plaque. Systems and methods that rely on visual detection of the position of the optical fiber require expensive monitoring systems, will not necessarily provide adequate response time to prevent damage to the vessel wall, and during the actual laser pulse a plasma is created that prevents transmission of a meaningful image by such systems. Also, systems using excimer lasers require gasses that have the dangerous combination of being poisonous and corrosive. Generally, the prior art systems do not adequately provide a way to automatically or manually detect whether the laser is positioned so that it will not damage the vessel wall, nor do they provide for a way to automatically or manually disable the laser if the vessel wall will be damaged. Moreover, the prior art systems do not provide a way for the laser to be self- positioning such that it is properly directed to avoid vessel wall damage and to enhance block removal. Accordingly, the need exists for a laser system and method for removing blockage from a coronary or other artery which is relatively inexpensive, safe and not too difficult to perform. Such a system and method should be able to be used with or without opening the chest. Also, the system and method should be able to penetrate totally occluded arteries. The method and system should remove plaque such that the plaque will not cause an embolus nor will the process of removing it cause a coronary infarct. The system and method should also provide control so that only the plaque is removed and the vessel walls will not be perforated. There exists in the prior art methods and devices used to analyze samples. One such method and device is disclosed in U.S. Patent 3,937,955, issued to Comisarow, et al., February 10, 1976. Comisarow describes an FT-ICR spectrometer that is used to analyze a sample, also called an analyte. FT-ICR spectrometry must be performed at low pressure and in a high magnetic field. In FT-ICR spectrometry a sample is ionized and subject to a multi-frequency electromagnetic pulse. The pulse excites ions to orbit at frequencies related to the mass of the ion, and the orbiting ions are detected as they pass a conductive plate. The signal induced on the conductive plate is comprised of many signals, each having a frequency equal to the cyclotron frequency of an ion in the sample, and a magnitude corresponding to the abundance of that ion. The signal induced on the plate is fourier transformed to provide information regarding the presence and abundance of the ions created. Also known in the prior art is infrared (IR) spectroscopy which uses the well-known principle that the IR energy that a compound or element will absorb is frequency dependent. To detect the presence of a compound using IR spectroscopy a sample is subjected to IR radiation having the frequency that excites the compound. The degree to which the sample absorbs the IR radiation is dependent on the abundance of the sample. The absorption is determined by monitoring the radiation that passes through the sample without being absorbed and comparing it to the amount of input radiation. In fourier transform IR spectroscopy (FTIR) multi-frequency IR radiation is applied to the sample, and many different compounds will be excited. The output radiation is again compared to the input signal. Another prior art method and apparatus for analyzing a sample called photo-acoustic spectroscopy is described in an application note of Brϋel and Kjaer, Weingarten, Oκygraphy - A New Dimension In Patient Monitoring, (publication date unknown) and in Weingarten, Respiratory Monitoring of Carbon Dioxide and Oxygen : A Ten-Year Perspective, Jour, of Clinical Monitoring, Vol. 6, No. 3, July, 1990. Weingarten describes the analysis of gasses using IR, alternating magnetic fields, and acoustic monitoring. Another photo-acoustic spectrometer is described in U.S. Patent No. 4,871,916, entitled Sensing of Methane, issued to John C. Scott, October 3, 1989. In photo-acoustic spectroscopy the IR radiation of a given frequency is modulated at a second frequency. If the analyte absorbs energy at the IR frequency acoustic waves of the modulating frequency are created and detected. In such a case the acoustic signal simply indicates that the analyte responds to the IR frequency. In order to detect different compounds using IR spectroscopy or FTIR an IR source which provides radiation at all frequencies of interest must be provided. Also, because the power of IR radiation decreases rapidly with distance and sufficient power must be provided to the analyte to excite the analyte, IR spectroscopy does not perform well when the analyte is not very close to the IR source. The need also exists for a system and method to analyze a sample which provides broadband excitation with a single frequency electromagnetic signal. There is also the need for a system using an electromagnetic signal to excite a sample which provides for the analysis of the sample at a location remote from the source of the electromagnetic signal. SUMMARY OF THE INVENTION According to a first aspect of the invention a system for distinguishing among a plurality of material includes a laser for producing substantially coherent electromagnetic energy. The electromagnetic energy impacts a substance which generates a pressure wave in response to the impact. The pressure wave is characteristic of the identity of the substance. An analyzer, which is operatively oriented to receive the pressure wave is provided for analyzing the substance in response to the pressure wave. According to a second aspect of the invention a system for removing a blockage in a blood vessel comprises a laser for producing and directing a laser beam into the blood vessel, wherein the laser beam impacts a substance in the vessel and the substance generates a pressure wave in response to the impact. The pressure wave is characteristic of the identity of the substance. An analyzer, which is operatively oriented to receive the pressure wave is provided for analyzing the substance in response to the pressure wave. An attenuator for selectively attenuating the laser beam is also provided. The attenuator has a first selectable mode of operation in which it transmits a first level of energy of the laser beam. The attenuator also has a second selectable mode of operation in which it blocks a second level of energy of the laser beam. The attenuator is operatively coupled and responsive to the analyzer. According to a third aspect of the invention a method for distinguishing among a plurality of substances comprises the step of directing substantially monochromatic electromagnetic energy to a sample, wherein the electromagnetic energy impacts the sample and the sample generates a pressure wave characteristic of the identity of the sample in response to the impact. The steps of monitoring the pressure wave and analyzing the sample in response to the pressure wave are also performed. According to a fourth aspect of the invention a system for analyzing a target comprises an electromagnetic broadband exciter optically coupled to the target and a sound detector acoustically coupled to the target. According to a fifth aspect of the invention a system for removing a blockage in a blood vessel comprises a laser, an optical coupler coupled to the laser and capable of directing a laser beam into the blood vessel, an analyzer acoustically coupled to the substance, and an attenuator having a first selectable mode of operation in which it transmits a first level of energy of the laser beam and further having a second selectable mode of operation in which it blocks a second level of energy of the laser beam. The attenuator has a control input for selecting one of the modes of operation, wherein the control input is operatively coupled and responsive to the analyzer. According to a sixth aspect of the invention an opto-acousto eter is provided. According to a seventh aspect of the invention a method of directing a laser beam carried by a fiber optic to blockage within a blood vessel comprises the step of pulsing the laser beam at a rate sufficient to center the fiber optic in the blood vessel. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a block diagram of a system for analyzing a sample in accordance with the present invention; Figure 2 is a block diagram of a system for removing blockage in a blood vessel made in accordance with the present invention; Figure 3 shows one embodiment of the controller used to control the optical coupler shown in Figure 2, constructed according to one embodiment of the present invention; Figure 4 is a detailed diagram of one embodiment of the optical coupler shown in Figure 2, constructed in accordance with the present invention. Figure 5 is a sound signature obtained from plaque; and Figure 6 is a sound signature obtained from vessel wall. Figure 7 is a fiber optic centered in a blood vessel. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Before explaining at least one embodiment of the invention in detail it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of the components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments or being practiced or carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein is for the purposes of description and should not be regarded as limiting. The invention may be generally described as an opto-acoustometer. An opto-acoustometer takes advantage of what is believed to be the hitherto unrecognized principle of nature that a substance, material, sample or target will produce a characteristic shock or acoustic wave when impacted by electromagnetic energy of sufficient power. More specifically, an opto- acoustometer is a spectrometer that provides an electromagnetic broadband excitation and the acoustical detection of at least one analyte, such as an element or a composition. Electromagnetic broadband excitation is excitation by a monochromatic electromagnetic signal capable of exciting more than one analyte. In other words the excitation of an analyte does not require a particular frequency. Furthermore, an electromagnetic broadband excitation is one in which the excitation of more than one analyte by the single electromagnetic signal does not depend on the existence of a relationship between the analytes that are excited, such as having the same or similar resonant frequencies or the same or similar chemical characteristics. In an opto- acoustometer the electromagnetic signal may be a continuous wave, a pulse, or an electromagnetic signal modulated by any envelope. Moreover, in an opto- acoustometer the detection may be limited to just one analyte. In an opto-acoustometer the shock wave created will be comprised of many frequency components and the composition of the wave uniquely identifies the analyte. Thus, the shock wave is referred to as a "sound signature" by the applicants. An opto-acoustometer has some similarity to an FT-ICR spectrometer which involves the formation of ions, multi-frequency excitation of ions, and broadband detection of ions. However, in an opto-acoustometer the step of forming ions need not be performed. In some applications of an opto-acoustometer the step of exciting the analytes includes the step of forming ions, but the ions are not necessary for detection. Rather only a pressure wave created by the impact of the electromagnetic energy or the analyte is necessary for detection. Also unlike FT-ICR spectrometers, opto- acousto eters will operate at atmospheric pressure and do not need strong magnetic fields to excite and detect the analytes. Opto-acoustometers also have some similarity to photo-acoustic spectrometers which involve the excitation of a gas using IR radiation modulated by a predetermined frequency and the detection of-acoustic signals at the predetermined frequency. However, in an opto- acoustometer analytes within the sample that will be excited are not limited to those which absorb energy in the IR frequencies transmitted as in photo-acoustic spectroscopy. Also, in an opto-acoustometer the location of the sample may be remote from the source of electromagnetic energy because monochromatic coherent light may be used, which does not decrease in power over distance as quickly as multiple frequency IR radiation does. Moreover, in photo-acoustic spectroscopy in order for the excitation signal to excite analytes having different resonant frequencies the excitation signal must actually be a composite of many different signals having different frequencies. This is different from an electromagnetic broadband exciter in which a single frequency will excite many different analytes having different frequencies of response. Of course, an electromagnetic broadband exciter may be comprised of multiple frequencies, but each frequency will excite a spectrum. Also, in photo-acoustic spectroscopy the detected signal is a single frequency signal and is not a sound signature which may directly identify the analyte. In the opto-acoustometer described in detail below the acoustic detection includes the detection of a pressure wave created when monochromatic coherent electromagnetic energy (a laser beam) impacts a sample, in other words a laser is optically coupled to the sample. The impact of the laser beam with the sample creates a plasma which propagates away from the point of impact. As the plasma propagates a pressure wave, which is characteristic of the sample, is created in the surrounding matter and is transmitted to a transducer. The transducer creates an electrical signal which is also characteristic of the sample. The electrical signal is then digitized and digitally processed to determine the identity of the sample. Referring to Figure 1 a block diagram of an opto-acoustometer 100 constructed in accordance with the present invention includes a source 101 of monochromatic electromagnetic energy and an analyzer 109. Also shown is a substance or sample 105 which may be a gas, solid or liquid and is the analyte. In operation energy source 101 generates a beam of monochromatic electromagnetic radiation (light) or energy 103, and directs beam 103 to sample 105. In one embodiment energy source 101 is preferably a 2.1 micron wavelength Ho-YAG laser, and beam 103 is a laser beam. Beam 103 impacts sample 105 and causes sample 105 to generate a pressure wave 107. Pressure wave 107 is a plasma shock wave created when beam 103 impacts sample 105. Pressure wave 107 may also be referred to as a shock wave or a sound wave. Pressure wave 107, which is characteristic of sample 105, travels away from sample 105 and is received by analyzer 109, which is acoustically coupled to sample 105. Analyzer 109 receives pressure wave 107 and, because pressure wave 107 is characteristic of sample 105, analyzes and identifies sample 105 in response to pressure wave 107. Analyzer 109 may include a microphone and an FFT computer, as will be described below. In alternative embodiment energy source 101 provides electromagnetic energy that is not coherent and monochromatic, but nonetheless delivers power to sample 105 sufficient to cause a shock wave to be generated by the impact of the electromagnetic radiation on sample 105. Such a system for analyzing a sample based on its sound signature may be used in a wide range of applications, including detecting flaws in manufactured objects, detecting air pollutants, analyzing metals, identifying vehicles, vessels or aircraft according to the paint used on or the composition of the hull, etc. , analyzing precious stones or analyzing biological substances. For example, it is possible to distinguish between copper, iron, titanium and nickel, based on the pressure wave created when a laser beam is directed to the surface of the metal. It is also believed that the relative abundance of different substances in a single analyte may be determined using the opto-acoustometer disclosed herein. First, it is believed the relative magnitude of the sound signature of a substance indicates the relative abundance of the plasma created by substance. However, because the absorption will vary from substance to substance, the abundance of plasma created relative to the abundance of the substance in the analyte will vary from substance to substance. To determine the relative abundance of the substance in the analyte the relative abundance of the plasma created by ' the substance must be scaled to account for the variation in the absorption. Scaling factors for substances of interest may be tabulated and could be stored in a computer for use in determining relative abundances. It is also possible to distinguish between the wall of a blood vessel and plaque obstructing flow in the blood vessel. Another application is identifying renal, ureter tract, biliary tract or gall stones. Other uses and improvements of opto-acoustometer 100 include using opto-acoustometer 100 with absorbed agents for distinguishing between samples having similar sound signatures. For example, fat absorbs tetracycline to a greater extent than other types of tissue. Thus, even if the sound signature of a fat sample and the surrounding tissue is similar, tetracycline, which will be absorbed predominantly by the fat, can be applied to the sample area. The sound signature of the fat and absorbed tetracycline will not be similar to the sound signature of the surrounding tissue which has not absorbed tetracycline, and the fat can be more readily distinguished from the surrounding tissue. Referring now Figure 2, a block diagram of a system 110 for removing blockage in a blood vessel 115 made in accordance with the present invention includes a laser 111, a controller 112, an optical coupler 113, and a detector 114, typically a transducer. In operation laser 111, which may alternatively be a different source of monochromatic electromagnetic energy, generates a laser beam 120. Laser beam 120 is directed to optical coupler 113, which preferably includes a fused-silica optical fiber available from Ceram Optic under the name "fused silica Low OH," will be described in detail below, with respect to Figure 4. Optical coupler 113 receives laser beam 120 and transmits a laser beam 123 to blood vessel 115. A portion of optical coupler 113 and blood vessel 115 are located within the body of a patient, as represented by box 122. Optical coupler 113 also is an attenuator having two modes of operation. In a first mode of operation laser beam 123 provides sufficient energy to blood vessel 115 to destroy a blockage, and in a second mode laser beam 120 is attenuated or blocked sufficiently to insure that laser beam 123 does not perforate the walls of blood vessel 115. The laser beam impacts matter (hereinafter referred to as a sample) within blood vessel 115 and a sound signature 125 is created. The sound signature 125 is acoustically coupled by optical coupler 113 to detector 114, which may be positioned adjacent optical coupler 113. One advantage of using a fused-silica fiber as part of optical coupler 113 is that a fused-silica fiber readily transmits sound waves, particularly those created by an opto-acoustometer using a Ho-YAG laser. Detector 114 generates a signal 126, preferably an electric signal, which is responsive to sound signature 125. If there is sufficient acoustic coupling between the sample and the detector 114 such that detector 114 can detect sound signature 125 without acoustic coupling by optical coupler 113, optical coupler 113 need not be an acoustic coupler. Signal 126 is provided to controller 112 (shown in greater detail below, with respect to Figure 3) which identifies the sample upon which laser beam 123 impacted. Based on that identification, controller 112 provides a feedback signal 129 which selects one of the two operating modes of optical coupler 113 so that the proper level of energy is provided to blood vessel 115. More specifically, laser 111 is preferably pulsed so that laser beams 120 and 123 are pulsed. Because laser beam 123 is pulsed, a series of shock waves or sound signatures 125 will be produced. The shape of each sound signature is not necessarily dependent on the energy of laser 123, and will therefore generally be the same (except for amplitude) in either mode of operation of optical coupler 113. The series of shock waves travel through optical coupler 113 to detector 114. Optical coupler 113 is not specially designed to be an acoustic coupler, but, like most solids, adequately transmits sound or shock waves. Detector 114 detects each shock wave and provides a series of electric pulses 126, each having the same shape that the corresponding shock wave had. The electric pulses 126 are provided to controller 112, which analyzes each pulse and identifies the material impacted by the corresponding pulse of laser beam 123. A synchronizing signal 127 is provided from laser 111 to controller 112 indicative of the beginning of each pulse of laser beam 120 so that analysis performed by controller 112 is synchronized to the beginning of each pulse 126. Controller 112 selects the mode of operation of optical coupler 113 in response to the identification of the sample that was impacted by laser beam 123. For clarification, the sequence following the generation of a single pulse by laser 111 will be repeated. Laser 111 generates a pulse 120 which travels to optical coupler 113. Optical coupler 113 transmits a pulse 123 to blood vessel 115. Pulse 123 impacts a sample within blood vessel 115, and shock wave 125 is produced. Shock wave 125 travels to optical coupler 113 and is transmitted to detector 114. Detector 114 provides an electric pulse 126 having the same shape as shock wave 125. Controller 112 receives electric signal 126 and synchronizing signal 127 from laser 111, indicating the generation of pulse 120. Based on an FFT (as will be described below) of electric pulse 126 controller 112 determines whether pulse 123 impacted plaque or vessel wall. If controller 112 determines that pulse 123 impacted plaque, signal 129 is provided to optical coupler 113 causing optical coupler 113 to operate in the high power mode (for the next pulse 120) . Alternatively, if controller 112 determines that pulse 123 impacted vessel wall signal 129 causes optical coupler 113 to operate in the low power mode (for the next pulse 120) . Each step described in this paragraph occurs prior to the generation of the next pulse 120 by laser 111. Thus, blockage may be removed without damaging a wall of vessel 115. In the preferred embodiment optical coupler 113 initially operates in the second (low power) mode. Each pulse of sound signature 125, created when a pulse of laser beam 123 impacts the sample in vessel 115, results in an electronic pulse 126, which is analyzed by controller 112. When a sound signature pulse indicates that a laser pulse 123 impacted blockage within the blood vessel 115, controller 112 selects the second (high power) mode of operation for optical coupler 113. The selection of the high power mode is performed before the generation of the next pulse of laser 111. Controller 112 continues to analyze the sound signature pulses and, if a sound signature pulse indicates that a pulse of laser 123 impacted a wall of vessel 115, controller 112 selects the low power mode of operation of optical coupler 113. The low power mode is selected before the generation of the next pulse of laser 111, to prevent damage to the wall of vessel 115. The position of optical coupler 113 within blood vessel 115 is adjusted by the physician using system 110, and the process is then repeated so that the blockage may be removed without damaging the wall of vessel 115. When clearing blockage from a blood vessel using opto-acoustometer 100 it is preferable to pre-test the system to obtain sample sound signatures for both plaque and vessel wall. Such pre-testing may be done at low power to avoid penetrating the vessel wall. The sound signatures obtained during pre-testing may be compared to known data to determine which signature corresponds to vessel wall and which signature corresponds to plaque. Figures 5 and 6 show sound signatures from plaque and vessel wall, respectively. The signatures obtained in Figures 5 and 6 were obtained using a Ho-YAG laser and the optical coupler described in the preferred embodiment. The pre-test may be avoided if a sufficient library of data has been compiled. However, it should be understood that the sound signature is also dependent on the wavelength of the laser used, and the medium through which the laser beam travels, as well as the medium through which the sound signature travels prior to detection. The invention may be used with other known techniques for clearing blocked arteries. For example, the present invention may be used to clear a small opening through blockage, and then balloon angioplasty or an auger type device may be used to clear a larger opening. In an alternative embodiment, when optical coupler 113 is in the high power mode for a first pulse of beam 123 controller 112 always selects the low power mode for the subsequent pulse. Thus, optical coupler 113 will never operate in the high power mode for two successive pulses of beam 123. Also, optical coupler 113 will operate in the high power mode for a single pulse of laser beam 120 only after controller 112 determines that the last pulse of beam 123 impacted blockage in vessel 115. So long as the physician using system 110 does not change the position of optical coupler 113 within blood vessel 115, the high power pulse of beam 123 will impact blockage. The physician may adjust the position of optical coupler 113 within blood vessel 115 without danger after any high power pulse of beam 123, because the next pulse of beam 123 will be a low power pulse. Thus, the likelihood that even a single high power pulse impacts the wall of vessel 115 is reduced because a high power pulse is produced only when the laser is directed to blockage. In another alternative embodiment, laser 111 is attenuated to a plurality of levels. Controller 112 selects the level of attenuation in response to the composition of the plaque, as determined by the sound signature. If the composition indicates that the plaque is more difficult to remove, a higher transmission level (lower attenuation level) is selected. Conversely if the composition indicates that less energy is required to remove the plaque the beam is attenuated to a greater extent. In yet another alternative embodiment, the low power pulse is provided by a first laser and the high power pulse is provided by a second laser. The first laser continuously produces pulses which will not penetrate the vessel wall. The second laser is pulsed only when analysis of the sound signature created by the first laser indicates it impacted blockage. The first and second lasers may have the same or different wavelengths. According to one embodiment, laser 111 is preferably a Ho-YAG laser generating a pulsed laser beam having a wavelength of 2.1 microns, a pulse of width of about 250 microseconds, a frequency of between 1 and 11 Hz, most preferably 8 Hz, and delivering between 0.5 and 1.8 Joules per pulse, most preferably 0.8 to 1.0 Joules per pulse. These operating parameters are merely exemplary and other values, such as 2.0 Joules or 0.5 Joules may be used. Some of the factors that should be considered when selecting an operating wavelength are: the effect of pulses of that wavelength on both blockage and the wall of vessel 115; the ability of optical coupler 113 to transmit or block the selected wavelength; the energy available at the selected wavelength; and the sound signatures produced when pulses of the operating wavelength impact the wall of vessel 115 and impact blockage. When selecting a pulse width the energy available and the sound signature produced are among the factors that should be considered. The energy transmitted by pulses 123 should be sufficient to remove blockage from vessel 115 but low enough so that a single pulse will not perforate the wall of vessel 115. These levels may be predetermined before each procedure, or when opto-acoustometer 100 is constructed. The frequency of the pulses should be slow enough to allow controller 112 to analyze the sound signature before the next pulse is provided, and fast enough to effectively remove blockage. In an alternative embodiment, rather than using a pulsed laser, a continuous beam laser may be used. Such a laser will produce a sound signature that is a continuous sound. If a continuous beam laser is used controller 112 should analyze the sound signature fast enough to switch into the low power mode before the wall of vessel 115 is perforated. Of course, the selection of one parameter can affect the selection of another parameter. For example, if the energy per laser pulse is such that no significant damage will be done to a wall of vessel 115 unless at least four successive pulses impact the wall of vessel 115, controller 112 need only sample every fourth shock wave and the interval between successive pulses may be four times the length of time controller 112 needs to analyze the sound signature of the target. In such a case, the wall of vessel 115 will not be damaged because controller 112 will recognize that the wall of vessel 115 is being impacted by the laser before damage occurs. In one embodiment detector 114 is a microphone, preferably a Calrad 10-10 Super Cardioid shotgun microphone. In an alternative embodiment detector 114 includes a microphone and an amplifier, or anything that is capable of providing a signal responsive to the sound signature of the target. Also, detector 114 could be optically coupled to blood vessel 115 by something other than optical coupler 113. For example, detector 114 could include a stethoscope placed on the skin or the esophagus of the patient near blood vessel 115. Alternatively, detector 114 could be operatively coupled to laser 111 or to an electronic switch to control laser 111. The actual mechanism to which detector 114 is coupled is not important, so long as detector 114 is able to detect the sound signature produced by the laser beam. In another embodiment optical coupler 113 includes an optical fiber which is inserted into blood vessel 115. In this embodiment one advantage of using a pulsed laser, preferably in the 1-20 Hz range, and particularly in the 8-10 Hz range, is optical coupler 113 will tend to self-steer. Referring briefly to Figure 7, in the frequency range described above a plasma, indicated by 705, created by each pulse tends to position the tip 703 of optical coupler 113 in the center of blood vessel 115. The center-positioning results because a low pressure region exists in the center of the vessel. A circular cross section of the fiber optic aids centering because the pressure will be distributed symmetrically about the fiber. Other cross sections which are generally symmetric about the axis will also aid in centering, whereas after centering the laser beam pulses will be directed to the center of vessel 115 where the blockage is, rather than to the walls of blood vessel 115. The plasma will focus the tip of optical coupler 113 until the plasma dissipates, generally in the order of tenths of a second. The frequency of pulsed laser 111 should preferably be selected so that the plasma does not entirely dissipate prior to the next pulse. In this event the tip of optical coupler 113 will always be self- steering, to some extent. The operating parameters discussed above also affect the ability of optical coupler 113 to self-steer. Referring now to Figure 3, one embodiment of controller 112 is shown. In this embodiment, controller 112 includes an audio amplifier 201, an analog-to-digital converter 202, a Fast Fourier Transform (FFT) computer 204, and a personal computer 206. Audio amplifier 201 is connected to detector 114 (shown in Figure 2) and receives from detector 114 analog electric signal 126, which has the same shape as sound signature 125. Audio amplifier 201 amplifies the sound signature and provides an analog signal 203 to analog-to-digital converter 202. In one embodiment audio amplifier 201 is preferably a FANON power plus 35 amplifier, available from Allied Electronics. Any amplifier having a frequency response appropriate for amplifying the sound signature may be used. Analog-to-digital converter 202 digitizes the amplified sound signature and provides a digital time domain signal 205 indicative of the sound signature to FFT computer 204. In one embodiment FFT computer 204 is preferably a Compaq 286 model 101709 computer, which runs Rapid System FFT software, using a digital oscilloscope and R340 board. This FFT computer performs the FFT routine on the digitized data 205 provided by analog to digital converter 202. FFT computer 204 provides as an output a digital frequency domain signal 207, i.e. digital data regarding the magnitude of various preselected frequency components of the input time domain signal. FFT computer 204 also receives a synchronizing signal 127 from laser 111, which indicates the beginning of each pulse of laser 111, and allows FFT computer 204 to collect data at the beginning of each pulse of sound signature 125. FFT computer 204 is synchronized by a signal generated when laser 111 is pulsed. One known method of generating a sync signal is to wrap a wire around the strobe used in laser 111. An electrical signal will be induced in the wire when the prepulse discharge occurs. The FFT computer 204 is then synchronized to collect data at a predetermined time delay after the prepulse discharge. The time delay will depend in part on the length of time the laser pulse takes to reach the target, the time for the shock wave to reach the detector, and the time for the sensing of the prepulse discharge. Digital frequency domain data 207 of sound signature 125 is provided to personal computer 206 which pre erably has a library of frequency domain data of known sound signatures stored therein. Personal computer 206 executes a routine which finds a match, within predetermined limits, to the input data, from its library of frequency domain data. Personal computer 206 then determines whether the last pulse of laser beam 123 impacted blockage or the wall of vessel 115, depending on the data that matched the acquired sound signature data. In the preferred embodiment the searching routine is available on the Compaq 286. Detector 114 and controller 112 may cooperate to form analyzer 109 of Figure 1. In the embodiment of Figure 1 computer 206 provides digital data indicative of the identity of sample 105, rather than control signal 129. If the sound signature data indicates that laser pulse 123 impacted plaque or other blockage, a digital signal 129 is provided by personal computer 206 to optical coupler 113 (shown in Figure 2) which causes optical coupler 113 to operate in the high power mode. On the other hand, if the sound signature indicates the laser pulse impacted the wall of vessel 115, personal computer 206 provides a digital signal 129 to optical coupler 113 which causes optical coupler 113 to operate in the low power mode. Accordingly, controller 112 controls the mode of operation of optical coupler 113 and attenuates laser beam 120 so that laser beam 123 will not supply energy that will damage the walls of vessel 115. In an alternative embodiment the attenuation of the laser beam may be accomplished by reducing the power of the beam generated, rather than by reducing the transmission of the beam through an optical coupler. In an alternative embodiment FFT computer 204 and personal computer 206 are replaced by a single microprocessor which performs the required analysis and matching procedures. Moreover, the FFT routine may be replaced by other data analysis routines such as a masking or double masking routine, other transforms, determining the maximum time domain amplitude, determining the length of the time domain signal, determining the decay of the time domain signal, or any data analysis technique that adequately distinguishes between the sound signature of blockage and the sound signature of the wall of vessel 115 for the laser, pulse width, optical coupler and power selected. Also, the data analysis routine should be fast enough to avoid damage to the walls of vessel 115. Other alternative embodiments of controller 112 include analyzing the sound signature data with an analog circuit, particularly if the analysis is relatively simple, such as maximum magnitude determination, or providing a digital-to-analog converter for converting the digital control signal generated by personal computer 206 and provided to optical coupler 113 into an analog signal. Such an analog control signal could be used for providing a continuum of operating modes of optical coupler 113, with varying degrees of transmission. Other embodiments include providing the analog signal from detector 114 to a digital oscilloscope which digitizes the signal and performs the necessary analysis on the sound signature. The operator may monitor the frequency domain signal displayed on the oscilloscope and manually control optical coupler 113. Alternatively, the analog signal from detector 114 may be provided to an analog oscilloscope which displays a time domain signal. The user may then determine, based on visual inspection of the time domain signal, whether the laser is impacting the wall of vessel 115 or blockage within vessel 115. In some applications, it may be possible for the user to directly analyze a target by listening to the signal detected by detector 114. For example, the sounds produced when the sound signatures of Figures 5 and 6 were recorded were readily distinguishable by a listener (other applications may result in sound signatures having more subtle differences however) . In applications where the difference between sound signatures may be audibly detected the operator can manually control optical coupler 113. Additional modifications include those such as providing the output of controller 112 to laser 111 and controlling the output of laser 111 directly, rather than controlling the transmission of optical coupler 113. There are numerous other modifications which may be made to controller 112 and to system 110. Referring now to Figure 4, optical coupler 113 is shown in detail. Optical coupler 113 includes optical fibers 301 and 302, preferably comprised of fused-silica, two gimbals 303 and 304, two taper holders 305 and 306, two lenses 307 and 313, a tourmaline flat 308, a galvanometer 310, and a scatter disc 315. Detector 114, which is a microphone, is also shown. Optical fibers 301 and 302, which may be comprised of material other than fused-silica, are fixed to taper holders 305 and 306, respectively. Taper holders 305 and 306 are, in turn, held in a fixed position with respect to one another by gimbals 303 and 304, respectively, for proper alignment of optical fibers 301 and 302. Scatter disc 315 is connected to fused- silica fiber 302 and prevents scattered light waves from impacting the tip of optical fiber 302. Lenses 307 and 313 are disposed between optical fiber 301 and scatter disc 315, and tourmaline flat 308 is disposed between lenses 307 and 313. As will be described in greater detail below, tourmaline flat 308 is affixed to galvanometer 310. The orientation and position of optical fibers 301 and 302, scatter disc 315, lenses 307 and 313, and tourmaline flat 308 are such that light carried by optical fiber 301 is transmitted through lens 307, tourmaline flat 308, lens 313, and scatter disc 315 to optical fiber 302. In operation fused-silica fiber 301 is coupled to laser 111 such that beam 120 is carried by fused- silica fiber 301. Laser beam 120 passes through optical fiber 301 and exits at taper holder 305. After exiting optical fiber 301 at taper holder 305, laser beam 120 diverges and impacts lens 307. Lens 307 focuses or causes further divergence of laser beam 120, depending upon the power of laser 111 and the power needed to destroy blood vessel 115, which passes through lens 307 to tourmaline flat 308. Specifically, lens 307 must provide sufficient focusing or divergence of the laser beam so that when optical coupler 113 operates in the high power mode, blockage will be removed, and when optical coupler 113 operates in the low power mode, the wall of vessel 115 will not be destroyed. If fiber optic 301 is sufficiently close to tourmaline flat 308, and the proper amount of power is thus transmitted to tourmaline flat 308, it will not be necessary to use lens 307. Tourmaline flat 308 is a tourmaline crystal such as that which may be readily purchased from most gem dealers, and is preferably colorless and polished so that it has two substantially flat parallel surfaces. Tourmaline flat 308 is oriented so that the laser beam impacts a first of the two parallel flat surfaces and exits the tourmaline at the second surface. Tourmaline flat 308 is rotatably attached to galvanometer 310 in the same fashion as a typical needle used to indicate voltage is rotatably attached to a conventional galvanometer, as indicated by positions 308B and 308C. Thus, tourmaline flat 308 is deflected at angle dependent upon the voltage applied to the positive and negative terminal of galvanometer 310, just as a needle used to indicate voltage is deflected. In the preferred embodiment tourmaline flat 308 rotates about an axis that is substantially perpendicular to the direction of the laser beam through tourmaline flat 308. Thus, the angle of incidence of the laser beam to tourmaline flat 308 is dependent upon the angle to which tourmaline flat 308 is deflected, and hence also dependent on the voltage applied to the inputs of galvanometer 310. In order to understand how optical coupler 113 is switched between the low power mode and the high power mode a brief discussion of polarization will follow. Light travelling in a given direction (for convenience we will call the direction of travel the Z direction) is comprised of electric signals transverse to the direction of travel. These signals may be considered as having two components at directions that are right angles to one another. Because these directions are also transverse to the direction of travel we will call them the X and Y directions. The intensity of the light is dependent on the magnitude of the X and Y components. Plane polarization of light is causing one of the X or Y components to be absorbed or reflected and the other to be transmitted. Plane polarization reduces the intensity of light because one component is absorbed. One way to plane polarize light is to pass it through a polarizing substance, such as tourmaline. A second way to polarize light is to pass it through an etalon. A polarizing substance absorbs all of one component and will completely plane polarize the transmitted light, regardless of the angle of incidence of the light. Moreover, the plane of polarization (whether X or Y components are transmitted) may be controlled by rotating the polarizer about the axis of the incident light. If light passes through two polarizers that are oriented to plane polarize in the same plane (e.g. the X component is absorbed) the second polarizer does not reduce the intensity of transmitted light, since the light incident upon the second polarizer only has Y components, which are transmitted by the polarizer. Conversely, if a first polarizer is oriented to plane polarize in one plane (e.g. the X component is absorbed) and a second polarizer is oriented to plane polarize in the other plane (the Y component is absorbed) no light will be transmitted by the second polarizer because the light incident upon the second polarizer only has Y components. As the orientation of the second polarizer relative to the first polarizer is varied from the same direction to a perpendicular direction the amount of light transmitted will vary from the maximum to none. It is also possible to partially plane polarize (more X component is transmitted than Y component for example) transmitted light by causing it to be incident upon a substance at an angle other than ninety degrees. The angle at which the transmitted light is plane polarized to the greatest extent is called the Brewster angle. The more the angle of incidence varies from the Brewster angle, the less the transmitted light is plane polarized, i.e. less X component is transmitted and more Y component is transmitted. Unlike a polarizing substance, the light that is not transmitted is not absorbed, but rather is reflected by this process. To act as a selective transmitter tourmaline flat 308 is oriented so that if the plane polarization due to it inherently being a polarizer substance transmits only X components, the angle of incidence partial plane polarization transmits more X component light than Y component light. At the Brewster angle most of the light that is not reflected is in the X direction, and tourmaline flat 308 absorbs very little light, Thus most of the light not reflected is transmitted. However, as the angle varies from the Brewster angle the portion of light not reflected that is in the Y direction increases, and tourmaline flat 308 will absorb more light, thus reducing the amount of light transmitted. In the preferred embodiment tourmaline flat 308 is deflected so that the incident angle of the laser beam is the Brewster angle when optical coupler 113 is in the high power mode (transmission of between 0.8 and 1.0 Joules) . When optical coupler 113 is in the low power mode (transmission of approximately 0.3 Joules) tourmaline flat 308 is deflected to an angle other than the Brewster angle. Preferably galvanometer 311 is calibrated so that in the low power mode tourmaline flat 308 transmits thirty percent, plus or minus ten percent, of the light that it transmits in the high power mode. Tourmaline has the further property that it emits an electrical charge when heated. Thus, when a laser pulse passes through and heats tourmaline flat 308, an electric charge is emitted. This charge can easily be monitored, using a resistor tied to ground, e.g., and used to provide a sync signal for the FFT computer. To properly control the angle of deflection galvanometer 310 is calibrated such that upon receiving a "HIGH" signal from personal computer 206 tourmaline flat 308 will be deflected to one of the desired positions. Also, upon receiving a "LOW" signal from personal computer 206 tourmaline flat 308 will be deflected to the other desired position. In one alternative embodiment two polarizing substances are used, and their relative orientation is controlled by personal computer 206. Alternatively, a fixed polarizer and a flat piece of glass is provided. The glass is deflected in the same manner that tourmaline flat 308 is deflected, and the combined effect will be similar to deflecting a tourmaline flat. Another alternative shutter is a wedge-shaped shutter that is rotated to partially block, to a varying extent, the laser beam. Other known shutters, including acousto- optical, electro-optical, Pockels cell and polarizing cube shutters may be used as well. One skilled in the art will also recognize that, if the laser is sufficiently powerful, an angle other than the Brewster angle may be used for the second position. The only requirement is that when the tourmaline is in the second position, the power transmitted by the laser beam to the plaque should be sufficient to remove plaque. Similarly, the 30% level transmitted by the tourmaline in the low power position is also not critical, but rather, the angle must be such that the power transmitted by fiber 302 to the vessel 115 is not sufficient to damage the walls of vessel 115. It is also possible to use a substance other than tourmaline as the shutter, so long as the wavelength that the laser is operating at is compatible with the material selected and the energy transmitted at the two positions is appropriate. Referring again to Figure 4, af er passing through tourmaline flat 308, the laser impacts lens 313. Lens 313 focuses the beam towards scatter disc 315, which is connected to fused-silica fiber 302. The beam then passes through optical or fused-silica fiber 302 and is delivered to the blocked vessel 115. Detector 114 is acoustically coupled to controller 112, as described above, and controller 112 analyzes the sound signature produced when the pulse laser beam impacts the sample. As one skilled in the art should know, the angle at which tourmaline flat 308 is disposed affects the focusing of the beam on fiber optic 302 and plate 315. The extent to which the beam is focused on fiber optic 302 affects the power delivered to the sample and should thus be considered when determining the positions for tourmaline flat 308. Thus, it should be apparent that there has been provided a method and apparatus for analyzing a sample from a sound pattern generated by a laser beam impacting the sample. A particular application of the invention is to distinguish arterial plaque from vessel walls and to remove plaque. The invention may also be used to identify other substances, including compositions, elements, paints and metal alloys, removing blockage from a blood vessel that fully satisfies the aims and advantages set forth above. Although the invention has been described in accordance with specific embodiments thereof, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art. Accordingly, it is intended to embrace all such alternatives, modifications and variations that fall within the spirit and broad scope of the appended claims.

Claims: CLAIMS 1. A system for distinguishing among a plurality of materials comprising: a laser; an optical coupler coupled to said laser and capable of directing a laser beam to the materials; an analyzer acoustically coupled to the material; and an attenuator having a first selectable mode of operation in which said attenuator transmits a first level of energy of said laser beam, said attenuator further having a second selectable mode of operation in which said attenuator blocks a second level of energy of said laser beam, said attenuator further having a control input for selecting one of said first and second modes of operation, wherein said control input is operatively coupled and responsive to said analyzer. 2. The system of claim 1 wherein said analyzer includes a transducer acoustically coupled to said material. 3. The system of claim 1 wherein said laser is a pulsed Ho-YAG laser. 4. The system of claim 1 wherein said analyzer includes a fast fourier transformer in electrical communication with said transducer. 5. The system of claim 1 wherein said attenuator includes a tourmaline flat capable of being rotated about an axis perpendicular to said laser beam. 6. The system of claim 1 in which said optical coupler is capable of being inserted into a blood vessel, and further in which said first level of energy of said laser beam is capable of removing a blockage in the blood vessel and in the event that said attenuator is in said second selectable mode of operation the level of energy transmitted by said attenuator is not suf icient to damage the wall of said vessel. 7. A method for distinguishing among a plurality of materials comprising the steps of: directing electromagnetic energy to a substance wherein said electromagnetic energy impacts the substance, the substance generating a pressure wave in response to said impacting, said pressure wave being characteristic of the identity of the substance; and monitoring said pressure wave. 8. The method of claim 7 wherein said step of directing includes the step of directing coherent electromagnetic energy to the substance. 9. The method of claim 8 further includes the step of analyzing the substance in response to said pressure wave. 10. The method of claim 9 further including the step of pulsing said electromagnetic energy. 11. The method of claim 10 further including the step directing said electromagnetic energy into a blood vessel. 12. The method of claim 7 further including the step of producing and fast fourier transforming a signal responsive to said pressure wave. 13. The method of claim 7 further including the steps of providing a signal uniquely analyzing the substance; and selectively attenuating said electromagnetic energy in response to said signal uniquely analyzing the substance.

Inventors: JOHNSON W DUDLEY, MUCKERHEIDE MYRON C, JOHNSON, W., DUDLEY, MUCKERHEIDE, MYRON, C.
Applicants: JOHNSON W DUDLEY, SETON HEALTH CARE FOUNDATION I, JOHNSON, W., DUDLEY, SETON HEALTH CARE FOUNDATION, INC.
Application year: 1992
Main classification: A61B5
IPC codes: A61B5, A61B18