Acoustic Monitoring And Controlling Of Laser Angioplasty Background of the Invention 1. Field of the Invention This invention relates generally to monitoring and controlling laser ablation of plague and more specifically to alerting a physician as to whether the irradiating laser beam of a laser angioplasty apparatus is striking plague or healthy vessel wall during laser angioplasty. 2. Description of the Related Art The art described in this section is not intended to constitute an admission that any patent, publication or other information referred to herein is "prior art" for this invention, unless specifically designated as such. In addition, this section should not be construed to mean that a search has been made or that no other pertinent information as defined in 37 C.F.R. §1.56(a) exists. Several different techniques have been developed for determining whether a laser beam is irradiating diseased tissue (plague) or healthy vessel tissue during laser angioplasty. These include laser-induced fluorescence, which relies on the fact that healthy tissue fluoresces differently than plaque upon irradiation by a laser. Another technique used involves generating ultrasonic waves which reflect off targets, allowing the physician to •-•see" an image. This technique involves using ultrasound waves, but not listening for an audible "report" such as that tissue makes when ablated, rather to construct an image of body tissue and plaque. other approaches analyze the frequency and/or amplitude of reflected ultrasonic sound waves to identify various body tissues and plaque. These techniques do not rely on the presence of an audible "report" caused by the ablation of tissue. Several other approaches that can be
used are magnetic resonance imaging and computerized tonography, both of which are well known to those skilled in the art. With the exception of laser-induced fluorescence these techniques described above do not rely on the use of a laser to differentiate between healthy tissue and diseased tissue. Several articles have been written relating to ablation and photoacoustic spectroscopy. Singleton et al. Applied Physics Letters, Vol. 48, pp. 878-880 (1986) , describes experimental data relating to pulse widths and shows that ablation occurs over essentially the entire range of pulse widths between 7-300 ns with an excimer laser. This article uses these results to aid in selecting the correct excimer laser for use in recanalization. There is no teaching or suggestion of using the acoustic "report" in an actual operation to monitor the ablation process. Taylor et al, Applied Physics Letters, Vol. 50, no. 25, pp. 1779-1781 (June 22, 1987), suggests that this acoustic signal may be used to measure ablation thresholds. However, no teaching or suggestion of using the acoustic "report" or signal to monitor the ablation of a laser angioplasty procedure in real-time is disclosed. The article was written to show what level of laser energy is required for ablation. Summary of the Invention Ablation is accompanied by a distinct acoustical report, i.e., a "sound". This sound is caused by the rapid heating, vaporization of the material, and subsequent expansion of the procedure site. The present invention utilizes the phenomenon of a distinctive acoustic signal or "report" discussed above in the context of providing the physician with another tool to
aid in determining whether the laser beam is striking healthy tissue or diseased tissue (plaque) during a laser angioplasty procedure. For instances in which healthy tissue has a higher ablation threshold than plaque, a signal below a certain predetermined value indicates that plaque is not being ablated. The presence of a signal over that predetermined value indicates that plaque is being ablated. The strength of the signal gives some indication of the rate at which the ablation is occurring. The characteristics of the signal give some indication of what is being ablated. The concept embodied in this apparatus and method is to monitor the laser angioplasty process, on a shot-by-shot basis, using passive acoustical means. By mounting a sound detecting element (a microphone) at or near the distal tip of the catheter and allowing the physician (or a computer) to "hear" the sound, or positioning a microphone outside the body such that it may "hear" the sound, the ablation of plaque, the type of plaque, and the rate of ablation may be monitored. The "loudness" of the report will indicate if ablation, or heating is taking place. Above a threshold, the "loudness" of the sound will also be indicative of the rate at which ablation is proceeding, and the characteristics of the sound will be indicative of what type of tissue is being ablated. Brief Description of the Drawings A detailed description of the invention is hereafter described with specific reference being made to the drawings in which: Figure 1 is a generalized block diagram showing the component parts of the apparatus; Fig. 2 is an elevational view of a preferred embodiment of the medical device of the invention, and
- F-ig. 3 is an enlarged detail view of the distal end of the device shown in Fig. 2. Description of the Preferred nhndiments While this invention may be embodied in many different forms, there is shown in the drawings and described in detail herein a specific preferred embodiment of the invention. The present description is an exemplification of the principles of the invention and is not intended to limit the invention to the particular embodiment illustrated. One form of laser angioplasty uses an "ablative" technique to ablate well defined sections of tissue without damage to surrounding healthy tissue. This is accomplished by rapid heating of the tissue compared to the thermal relaxation time of that tissue. This ablative method is often performed using excimer or pulsed dye lasers, operating at repetition rates of 1-50 Hz, pulse lengths of less than 50 μsec, and with pulse energies in the range of 10-100 mJ. Devices such as this are described in more detail in U.S. Patent No. 4,770,653 issued Nov. 13, 1988; U.S. Patent No. 4,788,975 issued Dec. 6, 1988; and co-pending applications Serial No. 272,103 filed Nov. 16, 1988, and Serial No. 278,375 filed Dec. 1, 1988, all of which are assigned to edilase, Inc. and which are hereby incorporated by reference. The present invention is intended for use with devices such as those described above or other devices making use of laser ablation. The present invention will aid the physician in determining the type of tissue being irradiated and whether the ablation is proceeding efficiently and safely. The feedback is in real time and allows the physician or an electrical device to disengage the laser in the event that the feedback indicates that the laser beam is not ablating plaque.
The present invention takes advantage of the fact that irradiation of tissue above the ablation threshold produces a distinctive acoustical "report", "snapping sound" or sound signature. It is also important to note that the ablation threshold is higher for healthy tissue such as artery wall than for diseased tissue such as "yellow" arterial plaque. Ablation of tissue is characterized by an "ablation threshold" Ft. A typical value of Ft may be Ft = 994 mJ/mm2, based on a 80 J per pulse and a 320 μm quartz fiber. Lauraglia, Lasers in Surgery & Medicine, Vol. 8, pp. 18-21 (1988), describes with more detail the effect of pulse duration on ablation of normal intima. If tissue is irradiated with a pulse of light of fluence F > Ft (in a sufficiently short pulse) material will be ablated. Furznikov, IEEE J. Quantum Electronics, Vol. QE-23, no. 10 pp. 1751-1755 (October 1987), gives the theoretical limits on pulse widths. Singleton, Applied Physics Letters, Vol. 48 pp. 878-880 (1986), provides experimental data showing that ablation thresholds for pulse widths in the range 7-300 ns with an excimer laser are essentially equal. Data for a pulsed dye laser shows 50 μsec to be below the tissue's thermal relaxation time. If a fluence F < Ft is used, no ablation occurs. Instead the tissue is only heated. It has been shown (Furznikov and Prince et al) that if the pulse widths are below a predetermined minimum, then the thermal relaxation time of the tissue is such that the heat caused by each pulse of laser energy is conducted away from the tissue, thereby preventing damage to healthy tissue. The absence of ablation also implies that no acoustical "report" or signal will be given off (there may be a very faint sound) . It is this lack of an
acoustical "report", or a signal below a predetermined threshold value that is utilized in the present invention. Ablation thresholds depend on wavelength and the type of tissue being irradiated. For example. Prince et al, IEEE J. Quantum Electronics, Vol. QE-23, no. 10 pp. 1783-6 (October 1987) , shows that the ablation threshold of inti a at 480 nm is approximately twice the value as for yellow plaque. At a wavelength of 308 nm however, plaque generally has a higher threshold than healthy intima. The present invention is primarily intended to be used in cases where the healthy tissue ablation threshold exceeds the threshold of the diseased tissue to be ablated. For cases where plaque's ablation threshold F is lower than healthy artery's F (as with pulsed lasers at 480 nm) and where a fluence F < F < F* is used, the presence of a signal indicates that plaque is being irradiated, while the absence of a signal indicates that plaque is not being ablated. Referring now to Figure 1, a generalized block diagram of a system according to the invention is shown. A computer is shown at 10., The computer 10 is used to operate a laser 12 via a laser controller 14. In the preferred embodiment the laser beam is directed into an optical fiber which in turn extends through to the distal end of a catheter 22, shown in Figures 2 and 3. Further detail on the construction of such catheters is provided in the above mentioned patents and applications assigned to Medilase, Inc. Of course, it is contemplated that this invention may be used with other medical devices and for other types of operations involving the use of a laser beam to ablate tissue. A sound detecting element 16 such as a microphone or ultrasound transducer is positioned to pick up the acoustic "report" or sound signature as the laser *
beam irradiates the interior of a vessel. Sound detecting element 16 may be miniaturized and incorporated into the distal end of the catheter 22, may be fed down to the distal end of the catheter via one of the conduits or channels, or may even be external to the body. In the latter case, miniaturization is not required. It is only necessary that sound detecting element 16 be able to pick up an audible "report" if one is caused by a pulse of laser energy during the laser angioplasty procedure. The output of sound detecting element 16 is directed to electronics 18 where the output is processed. Electronics 18 produces a signal which is proportional to the "loudness" of the "report". The processing may include a preamplifier, amplifier and signal processing such as freguency shifting, low-pass, high-pass or band-pass filtering. Such processing procedures are well known in the art. Electronics 18 is in turn connected to computer 10 and/or to a speaker 20. The speaker is used to provide an audible version of the aforementioned distinctive acoustical "report" or signal. In one embodiment of the inventive apparatus and method a physician 21, listening to the output of the speaker 20, will be able to determine when the laser beam is not ablating plaque. Physician 21 will also be able to determine the type of tissue that is being ablated by listening to the characteristics of the sound output of speaker 20. For instance, the absence of a signal, or a signal below a predetermined threshold value, while the laser beam is irradiating tissue may indicate that - healthy tissue is being irradiated, rather than diseased tissue being ablated as is desired. The physician may . then manually turn the laser 12 off via the laser controller 14 or re-target it. To a physician experienced with the apparatus, the "loudness" and
characteristics of the signal may also be used to indicate the nature of the material and the rate at which the ablation is occurring. A more preferred embodiment of the inventive apparatus and method utilizes computer 10 to monitor the signal and an associated computer program to turn off or otherwise control laser 12 via laser controller 14 in response to the signals produced by electronics 18. The computer is programmed to monitor the signal for a series of possible responses corresponding to the audible "report" indicative of plaque ablation. If those responses do not occur (such as a signal below a predetermined threshold value) then the computer turns off. Using the computer to control the laser beam has the advantage that the laser may be turned off much faster, thereby improving the safety of the procedure. Referring now to Figures 2 and 3 of the drawings, a catheter device used with the present invention in one embodiment comprises an elongated catheter, generally designated 22, having a working distal end generally designated 24. The device is adapted to be inserted into a patient and remote control means 28 is attached at a proximal end 26 for manipulation and control by a physician. The catheter is flexible and generally comprises an extruded solid plastic body 30. Body 30 may consist of a single, soft, solid, extruded plastic material or it may consist of a plastic composite reinforced with plastic or metal braided filaments, such as Dacron® polyester fiber or stainless steel. Plastics such as polytetrafluoroethylene, polyester, polyethylene and silicone may be used. When using the catheter in a vessel which contains an opaque fluid such as blood, it is often necessary to remove the opaque fluid and flush the area
with a clear fluid such as saline solution to provide a viewable work area. To accomplish this, catheter body 30 may include conduits 32 and 34, which open at distal end 24 and which are respectively connected to tubes 36 and 38 at the proximal end. Conduits 32 and 34 may be formed during extrusion of body 30. Tubes 36 and 38 include appropriate connector fittings 40 and 42, which will be familiar to those of ordinary skill in the art. Conduits 32 and 34 may thus function as suction tubes, fluid flushing tubes, supply tubes or for receiving a guide wire, in the already known manner. Laser transmitting fiber 44 extends through conduit 46 and terminates near or at the distal end of the catheter 24. Additionally, a bundle of very flexible and very small diameter optical fibers or imaging bundle 48 including a lens, as well as illumination fibers 50 may be included and will also extend through catheter 30. In the preferred embodiment the sound detection element 16, which may be a miniature microphone or a transducer, is positioned at the distal end of the catheter 30. Of course, it is also contemplated that the sound detection element 16 may also be placed outside the body, as long as it is positioned to pick up the audible "report" referred to above. This completes the description of the preferred embodiment of the invention. Those skilled in the art may recognize other equivalents to the specific embodiment described herein which equivalents are intended to be encompassed by the claims attached hereto.