LASER THERAPY DEVICE FOR THERAPY OF A LIVING TISSUE

Abstract

A laser therapy device comprising a pulsed laser light source. Each triggering of tissue irradiation causes application of a first heating laser pulse of a first power and a first pulse duration and at least a second heating laser pulse of a second power and a second pulse duration to the tissue. Changes in volume resulting from the rising and the decreasing power gradient of the first laser pulse are detected. The therapy device, on the basis of the measured values relative to the change in volume and taking into consideration at least the predetermined rises of the power gradients of the first heating laser pulse, determines an estimated value for the temperature increase in the tissue during irradiation of the first heating laser pulse, and generates, from the estimated value, a command that causes adjustment of the second power and/or the second pulse duration of the second laser pulse.

Claims

1. A laser therapy device for the therapy of a living tissue comprising a pulsed laser light source which emits laser light with an emission power in the range of 5 W to 100 W, a device for guiding the laser light into an applicator, a trigger device for triggering irradiation of the tissue by means of the applicator, a detection device for measuring the time-dependent changes in volume resulting from laser absorption in the tissue, an arithmetic unit for evaluating the detected volume changes and outputting control commands to a control device for controlling the power of the laser light irradiated into the tissue, wherein a. each triggering of tissue irradiation causes a first heating laser pulse with a first power and a first pulse duration and at least a second heating laser pulse with a second power and a second pulse duration to be applied to the tissue at a predetermined temporal pulse interval, wherein b. the detection device optically or acoustically records the volume changes resulting from the rising power gradient of the first laser pulse and supplies the measure values to the arithmetic unit; c. the arithmetic unit, based on the measured values of the volume change and taking into account at least the predetermined rises of the power gradients of the first heating laser pulse, determines an estimated value for the temperature increase in the tissue during irradiation of the first heating laser pulse, and d. from the estimated value, the arithmetic unit generates a command to the control device, which causes the control device to adjust the second power and/or the second pulse duration of the second laser pulse so that the irradiation of the second laser pulse heats the tissue to a predetermined target temperature.

2. The laser therapy device according to claim 1, wherein the control device is set up so that the pulse durations of all heating laser pulse applied when triggered are the same.

3. The laser therapy device according to claim 1, wherein the detection device records the changes in volume caused by the rising and fall power gradient of the second heating laser pulse and supplies the measured value to the arithmetic unit and the arithmetic unit determines and displays and logs an estimated value for the temperature increase during the irradiation of the second laser pulse.

4. The laser therapy device according to claim 1, wherein means for time recording of the emitted laser power are provided, which detect at least one portion of the laser light and supply measured values of the laser power to the arithmetic unit, whereby for the heating laser pulse the arithmetic unit determines the rises in the power gradients.

5. The laser therapy device according to claim 1, wherein the detection device is designed for recording thermally triggered pressure waves in the irradiated tissue, and comprises at least one ultrasonic transducer that records a pressure transient and emits this as measured values.

6. The laser therapy device according to claim 1, wherein the detection device is designed for the optical recording of thermally triggered movements of light-diffusing or reflecting tissue layers in the irradiated tissue, and comprises an interferometer as well as at least one photodetector, which records time-variable light intensity and emits this as measured values.

7. The laser therapy device according to claim 1, wherein the arithmetic unit is designed to determine and log, as of the start of the second heating laser pulse, the difference between the time of onset of a volume change on the rising gradient of the heating laser pulse and the next time of onset of a volume change.

8. The laser therapy device according to claim 1, wherein an operating device for user inputting of irradiation parameters into the arithmetic unit is designed to show the user selectable intervals for irradiation parameters, wherein the interval limits hierarchically depend on user inputs.

9. The laser therapy device according to claim 8, wherein the operating device is designed to select the power of the first heating laser pulse based on the user input of the pulse duration in such a way that the first heating laser pulse does not cause any lasting tissue damage.

10. The laser therapy device according to claim 8, wherein operating device designed only to allow as user inputs for the temporal pulse interval between the first and the at least second heating laser pulse, values that are greater than a thermal relaxation time of the tissue predetermined from at least from the user input for the first pulse duration.

Description

[0055] The invention will be described in more detail below by way of the figures. In these:

[0056] FIG. 1 shows a sketch of the laser system with ab arithmetic unit and detector for volume change signals;

[0057] FIG. 2 shows experimental data on the gradients rises of the laser light at various pulse durations and power using the example of a diode laser;

[0058] FIGS. 3a-3c show experimentally measured (time-shifted) pressure transients for heating laser pulses of comparable pulse durations with different powers.

[0059] Seen in FIG. 1 is a sketch of the laser device according to the invention in an embodiment for the therapy of the retina. In the sketch, the light guide means, the applicator as well as the trigger for the applicator are not shown.

[0060] A laser source (4) emits heating laser pulses (1) onto tissue (2). A detection device records the change in volume of the tissue and generates electrical signals which are supplied to an arithmetic unit (6). The arithmetic unit (6) can be a programmable microprocessor or personal computer. It processes the raw data arriving from the detection device (3)and, if required, stores these in a non-volatile mannerand on the basis of the processing results determines control commands to the laser (4). A control device for the laser (4) is not shown separately, but instead, is usually integrated into the laser (4) in a structural unit. If, as preferably assumed here, the operating current strength of the laser (4) is varied to control the laser power, the control device can be designed as a simple analogue circuit which directly transfers voltage values originating from the arithmetical unit (6) in currents. Alternatively, the control device can also be integrated into the arithmetic unit (6). The control device can also be designed to digitally receive metacommands of the arithmetic unit (6) such as, for example, values for pulse duration and pulse power and to convert these into analogue signals for controlling the laser (4).

[0061] According to the invention the laser (4) has an emission power of the order of 5 W to 100 W. Here, as an example, integrated into the laser (4) and not shown separately are preferably means for the time recording of the emitted laser power of at least one portion of the laser light (1). The detected measured values of the laser power are supplied to the arithmetic unit (6), wherein the arithmetic unit (6) determines the rises of the power gradients for the laser pulses. Such irradiation monitoring is current state of the art. In the case of very stable laser light sources, the compilation of a table with gradients for laser pulses of different pulse durations and power can be considered instead of constantly repeated measurements.

[0062] An operating device (5) intended for the user input of irradiation parameters into the arithmetic unit (6) is preferably configured to show the user selectable intervals for irradiation parameters, wherein the interval limits depend hierarchically on user inputs. This means that the operating device (5) can be in a position to adhere to certain safety rules and to decline entries that contradict these rules. When using a power laser with a high potential for causing damage, particularly in retinal therapy, incorrect parameter settings by the user can therefore be averted at an early stage. As an example, and preferably, the operating device (5) can be designed to select the power of the first heating laser pulse based on the user's input of the pulse duration in such a way that the first heating laser pulse does not cause any lasting tissue damage. It is also advantageous if the operating device (5) is designed only to allow, as user inputs for the temporal pulse interval between the first and the at least second heating laser pulse, values that are greater than at least one thermal relaxation time of the tissue predetermined from the user input for the first pulse duration. Here, hierarchical dependency means that some of the user's selection options are given priory in implementation while other selection options are considered as subordinate and, subject to predetermined, for example, programmed, safety rules are appropriately restricted depending on the higher-priority user inputs.

[0063] In this respect, the operating element (5) can comprise its own processor for data processing and a data memory for non-volatile data storage.

[0064] The operating element (5) can be designed in such a way that envisaged as user inputs are at least the target temperature and laser pulse durationas priority for exampleas well as optionally and subordinately the temporal laser pulse interval and the first power of the first heating laser pulse. In addition, the processor of the operating element (5) can perform an encryption algorithm in order to encrypt data transmitted by the arithmetic unit (6) together with the user inputs and thereby save them in an inseparable and unmodifiable manner. Such secured data are subsequently only readable together and can be used as an authenticable treatment protocol (also with a time stamp).

[0065] The variation of the ratio of the gradients rise ?P.sub.2/?P.sub.1 of the heating laser pulse has been experimentally investigated with variation in the pulse duration and laser power. FIG. 2 shows measuring values for pulse durations of 5 ?s and 50 ?s at powers up to 14 W solely as an example illustration. It is shown the ratios of the gradients rise of the pulseas well as the gradients rise themselveschange with the applied power through changed stimulation conditions (incl. pump flow) of the laser. It is clear that the pulse-wise measurement of the gradient rises is expedient for the evaluation described here, if one does not want to carry out laborious calibration of the laser light source. Furthermore, ageing effects of the laser are possible so that one-off ex-works calibration is not necessary always sufficient.

[0066] FIGS. 3a-3c show three examples of measured pressure transients at different laser powers. The pressure transients are in each case the lower curves, the derivations of the heating laser pulses the top curves. For the sake of presentability of the comparison in all plots they are standardised in such a way that the first maximum of the first pressure transient in terms of the numerical value corresponds to the maximum gradient ?P1 of the rising gradient of the first heating laser pulse. It can easily be seen how high the contribution of a temperature increase is to a pressure transient at the end of the heating laser pulse. The acoustic duration from the sample to the sensor was removed for better presentation, i.e. the pressure transients were displaced by the duration.

[0067] FIG. 3a show pressure transients in the case of irradiation with 614 mJ/cm.sup.2: the irradiated pulse of 35 ?s pulse duration has two gradient rises ?P1 and ?P2 which each trigger a pressure transient A1 and A2. Here, the pressure transients are shown on the same time axis as the heating laser pulses, i.e. displaced by the constant signal duration from the retina to the ultrasonic transducer for better assignment. Energy measure figures dD1 and dD2 are assigned to the pressure transients, which here, for example, at determined from the maximum amplitudes of the signals A1 and A2. From this the ratio ? is calculated and T.sub.Heiz as the temperature at the end of pulse is 55? C. Such a temperature increase for a short time does not generally damages living retinal tissue. The shown curves represent an example of the first heating laser pulse of the method according to the invention.

[0068] FIG. 3b shows pressure transients in the case of irradiation with 1245 mJ/cm.sup.2: when applying a pulse with double the power compared to FIG. 3a, the same signals are seen with now increased values. The laser pulse gradients rise more quickly and the amplitudes of the pressure transients are clearly increased. With the same calculations as for FIG. 3a, this results in a temperature of 82? C. at the end of the pulse which is already in the usual range for the target temperature of photocoagulation. FIG. 3b thus represents a possible second heating laser pulse according to the invention which directly results in a therapeutic effect.

[0069] FIG. 3c show pressure transients in the case of irradiation with 1488 mJ/cm.sup.2: this irradiation leads to bubble formation in the tissue. The second pressure transient already occurs before the laser power decrease. It is thus not based on thermoelastic relaxation on cooling of the tissue, but on material evaporation. The double arrow 1 shows the time difference of onset of the acoustic signal and the end of irradiation. The modified scales are compared with FIGS. 3a and 3b. The amplitude, increased by a factor of 10, is also an indicator of microbubble formation and confirms the assumption. As the power of the irradiation as well as the time to microbubble formation are known, it can be determined with which irradiation strength the microbubble formation temperature over approximately 140? C. can be achieved. The duration of thermo-mechanical disruption up to the end of the heating laser pulse can also be used as a dose measure and, possibly, though adjusting the laser, be adapted and optimised for further laser pulses.

[0070] It should also be noted here, that the curves of FIGS. 3a-3c were not recorded at the same laser spot. Therefore the absorption coefficients of the tissue differ between the experiments.