The invention relates to a laser system for ablative treatment of body tissue.

In the field of dentistry or the like, lasers are used for removal of hard body tissue such as dental enamel, dentine or bone material. It is well known that an Er:YAG laser is a perfect tool for drilling in materials that contain even small amounts of water. Usually, the laser operates by a pulse regime. A precise cut can be achieved by moving the laser beam from one position to the next with adjustable overlap, resulting in a desired tissue reshaping. However, reshaping by moving the laser light applicator in freehand operation is a demanding task.

A scanner device can be used in this situation; it is linked to the laser system and synchronized in such a way that it covers the desired area with a predetermined overall energy dose such that the corresponding tissue removal depth as a function of x and y coordinates will result. It is however difficult to predict the tissue removal depth by knowing only the energy density delivered to certain xy coordinates. In particular when the tissue structure is inhomogeneous , as in the case of bone, such prediction becomes virtually impossible. It is an object of the present invention to provide a laser system for ablative treatment of body tissue which facilitates achieving a certain ablation profile within the body tissue and which increases the precision of the ablation profile .

This object is solved by a laser system with the features of claim 1.

A laser system for ablative treatment of body tissue is proposed which comprises a laser source for generating a laser beam, a scanner for the laser beam, a control unit for the laser source and the scanner, a handpiece and at least one sound receiving device mounted on the handpiece for receiving sound that is generated by the laser beam when it impinges on the body tissue. The sound receiving device and the control unit are connected to form a control circuit for generating a predetermined ablation profile in the body tissue .

The invention is based on the utilization of opto-acoustic effects in that the laser beam that impinges on the body tissue not only causes ablation but also generates sound. The sound that is emitted at the point of impact of the laser beam at the treatment location is received by the at least one sound receiving device and is evaluated in the control unit. By measuring the time it takes for the sound to travel, optionally supplemented by further evaluations such as spectral analysis of the received sound or the like, a distance and thus depth measurement is carried out. Based on the laser beam that scans across the target surface and the respective point of impact of the laser beam as a sound source, an actual depth profile of the ablated treatment surface that has been scanned by the laser beam is determined in the control unit and compared to a nominal depth profile. In the control circuit according to the invention, based on the deviation between the nominal and the actual ablation depth profile, a control parameter is determined by means of which the laser source and/or the scanner is controlled or readjusted in such a way that the predetermined ablation profile is achieved.

The design according to the invention avoids the disadvantages according to the prior art and enables the user and/or the control unit to exactly know the ablation progress at any point of time. Locally deviating ablation advancements that are caused by inhomogeneities in the body tissue or other disruptive effects such as cooling medium effects, ablated tissue material or the like are compensated. Consequently, in a simple way a precise ablation depth profile according to the parameters set by the user is achieved.

In a preferred embodiment of the invention several sound receiving devices for simultaneous reception of the sound are provided that are arranged at different locations. In particular they are not positioned on a common straight line, i.e. their positions form a triangle or polygon preferably on a common flat plane. The precision of the afore described opto-acoustic depth measurement for generating a 3D ablation profile is improved in this way. In an advantageous embodiment of the invention the sound receiving device is formed by an acoustic sensor that directly receives the sound, for example, in the form of a microphone. Disruptive effects in sound reception can thus be reduced .

Alternatively it can be expedient that the sound receiving device is formed by an end of a sound conduit (wave guide) which end is provided on the handpiece; the sound conduit (wave guide) is connected to an acoustic sensor. The sound conduit (wave guide) and in particular its free end that is facing the treatment location can be designed to be of a very small volume while the large-volume acoustic sensor is mounted at a suitable location remote from the treatment location where the sensor is not in the way. Thus, the handpiece together with the attached sound receiving devices can be designed to be compact and of a minimal size; this improves its handling, in particular in spatially tight conditions. The acoustic sensor is not subjected to ablation debris an other influences. The need for sterilization of the acoustic sensor is eliminated.

In a preferred embodiment a passive sound amplifier, in particular in the form of a concavely curved sound reflector, is arranged upstream of the sound receiving device. It acts as an acoustic lens by means of which the sound propagating away from the treatment location is focussed onto the sound receiving device. The amplification that is generated in this way improves the reception quality and evaluation quality of the sound that is used for depth determination.

The control unit contains preferably a first system processor and a second processor, wherein the scanner is controlled by the first processor and the laser source is controlled by the second processor, and wherein the first processor and the second processor are synchronized . In this way, an exact synchronization of the laser activation or the laser control and of the point of impact of the laser beam on the treatment surface generated by the scanner device is ensured.

In an expedient embodiment of the invention the control unit comprises a spectral analyser for or means to perform peak to peak detection of the received sound. By said means, additional information for the control circuit can be gained. It is possible to derive information with regard to the consistency of the currently removed body tissue from the spectral analysis. In particular, it can be determined whether the current ablation is carried out at a transition from one type of tissue material to another type of tissue material, for example, from bone material to soft tissue material or to different bone material, which in particular differ in density an other mechanical properties. This information is used for obtaining redundant information about entering a critical region in the tissue. As needed, this information can be employed to readjust the laser source and/or the scanner for adjustment of the ablation profile. In an advantageous embodiment, the handpiece has a cooling medium supply for the cooling medium for cooling the body- tissue wherein the cooling medium supply and the control unit are connected such that the supply of cooling medium is interrupted during reception of the sound, in particular slightly before the laser pulse, to avoid acoustic signal generation on the spray droplets. In this way, disturbing effects (noise) of the cooling medium during sound reception and sound evaluation are prevented.

The control unit is preferably designed such that the laser beam is operated with individual measuring pulses, in particular uniformly temporally spaced sequential measuring pulses, for generating the sound to be received and evaluated. Preferably, the laser source is operated in a pulse sequence in which one or several ablation pulses are followed by one or several measuring pulses that, in comparison to the ablation pulses, have a reduced energy level. By means of individual measuring pulses that with respect to their energy are matched to the respective application, a sequence of adjusted acoustic signals is produced whose further processing in the control unit leads to an augmented precision of the measuring and control result.

Embodiments of the invention will be explained in the following with the aid of the drawing in more detail. It is shown in :

Fig. 1 in a schematic side view a laser system

embodied according to the invention comprising an individual acoustic sensor as a sound receiving unit and a control circuit for producing a defined ablation profile during use for body tissue removal ; a variant of the arrangement according to Fig.

1 with three acoustic sensors distributed about the circumference of the handpiece for increasing the measuring precision; a plan view of the handpiece according to Fig.

2 with details of the arrangement of the acoustic sensors that are not arranged on a common straight line ; a variant of the arrangement according to Fig. 1 with a sound conduit (wave guide) as a sound receiving device and an acoustic sensor arranged remote therefrom for minimizing the constructive size ; an embodiment with three sound conduit (wave guide) s connected to a common acoustic sensor; an embodiment of the arrangement according to Fig. 1 with a sound reflector that is arranged upstream of the acoustic sensor;

Fig. 7 in a schematic diagram illustration a preferred pulse sequence of the laser source when operating the arrangement according to Figs. 1 through 6.

Fig. 1 shows in a schematic side view a laser system embodied according to the invention for ablative treatment of body tissue 17. The laser system comprises a laser source 1 for generating a laser beam 2, a scanner 3 for controlled deflection of the laser beam 2, a control unit 4 for the laser source 1 and the scanner 3, as well as a handpiece 5. In the handpiece 5 the scanner 3 with a movable mirror 18 is

arranged and connected by a control line 23 to the control unit 4. The mirror 18 can be tilted by means of externally arranged control unit 4 about two axes. For simplifying the drawing, in the side view according to Fig. 1 only one of two degrees of freedom of movement of the mirror 18 in accordance with double arrow 26 is illustrated. The laser beam 2 that is emitted by the laser source 1 that is arranged together with the control unit 4 externally in a base unit is applied by means of an articulated arm with mirrors or a flexible light guide (not illustrated) to the handpiece 5 and impinges therein on the mirror 18 of the scanner 3. On the mirror 18 it is reflected, passes through lens 20 and a protective window 19 of the handpiece 5 and is then guided to a point of impact 29 on the surface of the body tissue 17. The scanner 3 is controlled by means of a control unit 4 by connecting line 23 in such a way that the mirror 18 is tilted cyclically in accordance with double arrow 26 and in an additional degree of freedom of movement that is perpendicular to the double arrow movement. As a result of this, the point of impact 29 moves in accordance with double arrow 21 and additionally also perpendicular to the drawing plane in such a way that the treatment surface of the body tissue 17 is scanned completely by the point of impact 28 within a predetermined contour .

The laser source 1 comprises a laser that is suitable for removal of body tissue 17. Preferably, for this purpose an Er : YAG, Er, Cr : YSGG or C0 ₂ laser is used. The body tissue can be tooth enamel, dentine, bone or any other hard body tissue. However, soft body tissue such as skin or the like can also be treated. The treatment results in the body tissue 17 being removed by the impinging laser beam 2 within the contour scanned by the scanner 3 in a direction of depth. For cooling the treatment surface, a cooling medium supply 13 is arranged on the handpiece 5 and by means of connecting line 25 connected to the control unit 4. By means of the cooling medium supply 13 the cooling medium 14, for example, in form of water, is supplied onto the treatment surface of the body tissue 17.

As the laser beam 2 impinges on the body tissue 17 at the point of impact 29 sound 7 is generated that, starting from the point of impact 29 inter alia propagates in the direction toward the handpiece 5. For receiving the sound 7 that is generated by the laser beam 2 at the point of impact 29, the handpiece 5 according to the invention is provided with at least one sound receiving device 6. In the embodiment according to Fig. 1, for this purpose a single sound receiving unit 6 in the form of an acoustic sensor 8 for immediately receiving the sound 7 in the form of a microphone is provided. The sound receiving unit 6 or the acoustic sensor 8 is connected by means of connecting line 22 to the external control unit 4 and is interconnected to form a control circuit in a way to be described in the following.

The control unit 4 comprises a processor 27 and optionally a spectral analyser 28 wherein the spectral analyser 28 may also be integrated in the processor 27. The sound that is received at the handpiece 5 by the sound receiving device 6 is supplied as a signal by means of connecting line 22 to the control unit 4 and therein supplied to the processor 27 and also the spectral analyser 28. In practice a part of the control unit 4 containing a special second processor 30 will be located in the vicinity of the scanner 3 for controlling the scanner 3 and analysing the acoustic signal of the sound 7. There will be a fast communication line 31 between this second processor 30 and the system processor 27. Instead of a spectral analysis there may be employed simpler (numerically less intensive) methods to analyse the acoustic signal. As a replacement for the spectral analyser 28, e.g. means 32 to perform peak to peak detection of the acoustic signal of the sound 7 may be provided and in particular integrated into the part of the control unit 4 containing a special second processor 30, or will be integrated into the special second processor 30 itself. The results are then evaluated in such a way that the distance between the sound receiving device 6 and the point of impact 29 is determined. This distance determination is realized either continuously or in a fine grid during advancing movement of the point of impact 29 so that, based on the gained plurality of distance data, an ablation depth profile in the form of a 3-D model of the removed treatment surface is determined as an actual value. Moreover, in the control unit 4 a nominal value of the ablation depth profile is saved. Such an ablation depth profile can be selected or preset by the user depending on the treatment task wherein such a profile may comprise a certain outer contour and a certain 3-D depth profile. The entire nominal value can be of a simple nature with a constant depth and a simple circumferential contour. However, it is also possible to input a geometrically complex 3-D nominal profile .

In the control unit 4 the currently determined ablation profile is compared with the preset nominal ablation profile and, based on the difference, a parameter for the laser source 1 and/or the scanner 3 is derived. The intensity of the laser source 1 and its pulse sequence are readjusted as is the scanner 3 in such a way that by means of the laser beam 2 and by means of the point of impact 29 that scans the treatment surface the desired nominal ablation profile is achieved. The laser source 1 that is connected by means of the connecting line 24 with control unit 4 and the scanner 3 connected by connecting line 23 with the control unit 4 may be controlled by the same primary system processor 27 of the control unit 4. In the shown embodiment, they are controlled by two different processors, i.e. the laser source 1 is controlled by the primary system processor 27, while the scanner 3 is controlled by the second processor 30. However, the two processors 27, 30 have to be well synchronized, as shown by the fast communication line 31. In this way, syn- chronization of the control of the laser source 1 and of the scanner 3 is ensured. As a whole, in this way a closed control circuit for obtaining the desired ablation profile in the body tissue 17 is provided.

The depth or advancing measurement of the ablation process may be done exclusively by measuring the time it takes for the sound 7 to travel between the point of impact 29 and the sound receiving device 6. In addition, the signal of the received sound 7 can be subjected to a spectral analysis in the spectral analyser 28 or can be subjected to a peak to peak detection or evaluation in the means 32. The spectral analysis or the peak to peak detection provide inter alia information with regard to the properties of the body tissue 17 currently being removed. In particular, the spectral analysis or the peak to peak detection can provide information whether the point of impact 29 is within a first type of tissue or a second type of tissue different from the first type. In this way it can be detected whether a tissue layer has been penetrated and a different tissue layer positioned underneath has been reached. This information not only can be employed for readjusting the laser source 1 and/or the scanner 3 but also for a demand-dependent adaptation of the nominal ablation profile.

As a result of spectral analysis, also inhomogeneities in the body tissue 17 can be recognized. The information that is derived therefrom can be used for an improved adaptation of the control properties. However, the detection of undesirable or unexpected inhomogeneities can be used also for a read- justment of the ablation process or for stopping the treatment .

Fig. 2 shows a variant of the arrangement according to Fig. 1 in which several sound receiving devices 6 are arranged at different locations on a handpiece 5. As in the embodiment according to Fig. 1 the sound receiving devices 6 are in the form of acoustic sensors 8 in the form of microphones that directly receive the sound 7. The bottom view of Fig. 3 of the arrangement according to Fig. 2 shows that a total of three sound receiving devices 6 are uniformly distributed about the handpiece 5. However, an asymmetric distribution may be expedient also. Moreover, it can be advantageous to have two or more sound receiving devices 6. When looking both at Fig. 2 and Fig. 3, it is apparent that the sound receiving devices 6 define a triangle, i.e., they are not positioned on a common straight line. They are all connected by connecting lines 22 to the control unit 4 (Fig. 1) and are provided for simultaneously receiving the sound 7. This results in an improvement of the measuring precision and the derived three- dimensional actual ablation profile. The illustration of Fig. 2 also shows that all sound receiving devices 6 have the same spacing to the surface of the body tissue 17. However, it may also be expedient to provide different spacings in order to obtain from the differing distances and the resulting different travel times of the sound 7 additional information for controlling or governing the ablation process.

Fig. 4 shows a variant of the arrangement according to Fig. 1 in which an individual or single sound receiving device 6 is provided in the form of an end 9 of a sound conduit (wave guide) 10 in the form of a hollow guide arranged at the hand piece and facing the body tissue 17. The end 9 of the sound conduit (wave guide) 10 is open toward the body tissue 17 but can also be covered by a membrane or the like and receives the incoming sound 7. From here, the sound 7 is guided through the sound conduit (wave guide) 10 to a remotely positioned acoustic sensor 8 whose signal is supplied by connecting line 22 to the control unit 4 (Fig. 1) . The acoustic sensor 8 can be attached at a suitable location of the handpiece 5. For an appropriate configuration of the sound conduit (wave guide) 10 however an arrangement of the acoustic sensor 8 remote from the handpiece 5, for example, on the base unit of the laser system is possible also.

A further embodiment of the invention is illustrated in Fig. 5 according to which, as in the embodiment of Fig. 4, an individual or single sound receiving device 6 is formed by the free end 9 of a sound conduit (wave guide) 10. However, in total there are several, in this case three, ends 9 of sound conduits (wave guides) 10 provided as a sound receiving device 6, respectively, and, in an arrangement like that of Fig. 2 and Fig. 3, distributed uniformly about the circumference of the handpiece 5. All ends of the sound conduits (wave guides) 10 opposite the sound receiving device 6 are combined and connected to a single, common acoustic sensor 8. However, an embodiment may be expedient in which the number of the acoustic sensors 8 is the same as or different than the number of ends 9. In particular, it may be expedient to correlate each end 9 serving as a sound receiving device 6 with one acoustic sensor 8.

In a further variant of the invention in accordance with the illustration of Fig. 6, the sound receiving device 6 embodied in an exemplary fashion as an acoustic sensor 8 has arranged upstream thereof a passive sound amplifier 11. The passive sound amplifier 11 in the illustrated embodiment is concave and in this case a parabolic curved sound reflector 12 that focuses the sound that is emitted at the point of impact 29 onto the sound receiving device 6.

If not otherwise noted, all embodiments according to Figs. 1 through 6 correspond with one another with respect to their other features and reference numerals. Moreover, in the context of the invention, it is possible to combine different features in a suitable way with one another. For example, it can be expedient to provide several sound amplifiers 11 according to Fig. 6 for several sound receiving devices 6 according to Figs. 2, 3, and 5. Of course, in this connection, instead of an acoustic sensor 8 that immediately or directly receives the sound 7, an embodiment in accordance with Figs. 2 to 5 with a free end 9 of a sound conduit (wave guide) 10 can be expedient also.

Fig. 7 shows in a schematic diagram illustration the exemplary course of pulses with which the laser source 1 according to Figs. 1 to 6 is operated. In the diagram according to Fig. 7 the pulse intensity p over the time t is plotted. The laser beam 2 is accordingly operated with individual measur- ing pulses 15 for generating the sound (Figs. 1 through Fig. 6) for reception and evaluation. The measuring pulses 15 follow in a temporal uniformly spaced sequence relative to one another but can also be spaced at irregular time intervals .

With simultaneous reference to Figs. 1 through 7, it is apparent that an individual measuring pulse 15 on the point of impact 29 will emit a sound signal at an exactly defined and known point in time. As a result of its propagation speed the sound 7 will impinge at a later point in time on the sound receiving device 6 so that based on the temporal difference and the speed of sound the spacing between the point of impact 29 and the sound receiving device 6 can be determined and based on this the ablation progress can be determined .

The measuring pulse 15 may be selected with regard to its pulse intensity p in such a way that the desired ablation effect on the body tissue 17 will occur. In accordance with illustration of Fig. 7, between individual measuring pulses 15 there are however one or several ablation pulses 16; in an exemplary fashion, ten ablation pulses 16 are illustrated between two measuring pulses 15. The ablation pulses 16 differ with respect to pulse width and pulse intensity p from the measuring pulses 15 in such a way that the measuring pulses 15 have reduced energy in comparison to the ablation pulses 16. The energy of the measuring pulses 15 is sufficient in order to produce the sound 7 according to Figs. 1 through 6 while however the actual ablation process is carried out with increased energy by the ablation pulses 16. During an ablation pulse 16 no sound measurement is performed. However, cooling medium 14 is supplied through the cooling medium supply 13 to the body tissue 17. The cooling medium supply 13 is controlled by means of control unit 4 and connecting line 25 (Fig. 1) and is controlled in such a way that the supply of cooling medium 14 is interrupted at, in particular slightly before the point in time of the measuring pulse 15 and reception of sound 7 at the sound receiving device, to avoid acoustic signal generation on the spray droplets of the cooling medium 14. In this way it is prevented that the cooling medium 14 will generate noise in the sound 7.