WO2003063696A1

WO2003063696A1 - Apparatus and method for distinguishing tissue types

Info

Publication number: WO2003063696A1
Application number: PCT/GB2003/000227
Authority: WO
Inventors: Norman James Geddes; Kevin Roy Welford; David Vedmore Willetts
Original assignee: Qinetiq Limited
Priority date: 2002-01-30
Filing date: 2003-01-21
Publication date: 2003-08-07
Also published as: GB0202066D0; GB2384851A

Abstract

Apparatus (50) for distinguishing tissue types in a region (80) internal to a human or animal body, comprises a source (52, 53) of at least partially coherent radiation, an optical fibre (55) for delivering a portion of the at least partially coherent radiation to the region and means for positioning the fibre's distal end (55A), characterised in that the apparatus further comprises means (56, 57) for imparting mechanical vibration to at least one of the tissue types, a radiation detector (74) and interfering means (61, 62, 64, 66, 68, 70, 71, 72, 73) for interfering radiation delivered by the optical fibre to the region and subsequently reflected by tissue therein with radiation from the source at the detector to generate a characteristic signal at an output thereof, the characteristic signal being characteristic of a tissue type at which the fibre's distal end is positioned. The apparatus may be comprised in endoscopic laser ablation apparatus.

Description

APPARATUS AND METHOD FOR DISTINGUISHING TISSUE TYPES

This invention relates to an apparatus and a method for distinguishing tissue types within a human or animal body, particularly, although not exclusively, for distinguishing bone tissue from adjacent nerve tissue, and to endoscopic laser ablation apparatus.

Internal examination of human or animal bodies is frequently carried out using an endoscope, or fibrescope, which delivers light to an internal region under examination and receives light therefrom by means of optical fibres. A user of an endoscope observes images of a region under investigation directly by means of an eyepiece, or alternatively images displayed on a visual display unit. Although an endoscope is a generally useful tool for internal examination it has the disadvantage that in certain circumstances the images provided are inadequate to clearly distinguish different tissue types. Also, some types of endoscope have fibre bundles which contain very few individual optical fibres; such low-diameter endoscopes are useful for accessing restricted spaces (for example within the human spine) but generate images of poorer quality than those generated by endoscopes having a larger number of individual optical fibres within a fibre bundle. Thus, a practitioner using an endoscope to carry out internal examination can encounter difficulty in clearly distinguishing tissue types (for example bone tissue and nerve tissue within a spinal region) when using an endoscope. If the practitioner intends to carry out a surgical procedure in the region on the basis of images produced by. an endoscope, there is a risk of surgical error. In the case of spinal laser ablation surgery, in which bone tissue causing pressure on nerve tissue is ablated, accidental damage to nerve tissue can have catastrophic consequences, for example paralysis.

Furthermore, it is useful in certain circumstances to have an electrical or optical signal generated, which signal is characteristic of a tissue type under examination. Such a signal may for example be used in automated procedures for mapping a region under investigation on the basis of tissue type.

It is an object of the present invention to provide an apparatus which allows improved discrimination between tissue types in a region internal to the human or animal body and which delivers an electrical signal indicative of a tissue type under examination.

According to a first aspect of the present invention, this object is achieved by apparatus for distinguishing tissue types in a region internal to a human or animal body, the apparatus comprising a source of at least partially coherent radiation, an optical fibre for delivering a portion of the at least partially coherent radiation to the region and means for positioning the optical fibre's distal end, characterised in that the apparatus further comprises means for imparting mechanical vibration to at least one of the tissue types, a radiation detector, and interfering means for interfering radiation delivered by the optical fibre to the region and subsequently reflected by tissue therein into the optical fibre's distal end with radiation from the source at the detector to generate a characteristic signal at an output thereof, the characteristic signal being characteristic of a tissue type at which the optical fibre's distal end is positioned. Conveniently, the means for imparting mechanical vibration comprises an oscillator and a probe for contacting a tissue type, the probe incorporating a transducer for converting electrical signals from the oscillator into mechanical vibration.

Preferably, the apparatus further comprises a frequency demodulator arranged to receive a characteristic signal generated at the output of the detector and generate a demodulated signal in response thereto, and a phase-locked loop arranged to receive the demodulated signal and a signal from the oscillator and produce a characteristic signal with reduced noise at an output of the phase-locked loop in response to input of the demodulated signal and the signal from the oscillator.

This provides a characteristic signal having a greater signal-to-noise ratio than a corresponding signal generated at the output of the detector.

In order to positively identify two or more tissue types, the oscillator may be arranged to generate a signal comprising two or more frequencies each suitable for inducing mechanical vibration in a corresponding tissue type, and to pass the signal to the probe.

The apparatus may further comprise an output device arranged to receive characteristic signals from the output of the detector, or as the case may be the output of the phase-locked loop, and output a visible or audible signal characteristic of a tissue type at which the optical fibre's distal end is located. This provides a user of the apparatus with a signal which not only indicates tissue type but which is also readily comprehended. Preferably the source of at least partially coherent radiation is laser, as a laser's highly coherent output results in an easily detected interference signal at the detector. Preferably the laser is a semiconductor laser as such a laser is compact, robust and susceptible of control by simple electronic means.

According to a second aspect of the present invention, there is provided endoscopic laser ablation apparatus for performing laser ablation surgery in a region internal to the human or animal body, the apparatus comprising an ablation laser and an optical fibre for delivering radiation output by the ablation laser to the region, characterised in that the apparatus further comprises apparatus according to any one of claims 1 to 4 for distinguishing tissue types within the region, the distal ends of the optical fibres being co-located.

Prior art endoscopic laser ablation apparatus is typically positioned within a human or animal body using real-time X-ray imaging apparatus. Real-time X-ray imaging allows positioning of the apparatus to an accuracy within 1 mm. However, in certain surgical procedures, much greater positioning accuracy is required. For example in spinal disc laser. ablation surgery, the distal end of a laser ablation apparatus has to be positioned to within 500 μm in order to avoid causing damage to nerve tissue when ablation is carried out. Such damage may have catastrophic consequences, e.g. paralysis. Endoscopic laser ablation apparatus of the invention has the advantage that it may be positioned more accurately than prior art endoscopic laser ablation apparatus, and hence reduces incidence of surgical error when carrying out ablation surgery. Endoscopic laser ablation apparatus of the invention preferably further comprises an output device arranged to receive characteristic signals from the output of the detector, or as the case may be from the output of the phase-locked loop, and output a visible or audible signal characteristic of a tissue type at which the distal ends of the optical fibres are located. This provides a user of the apparatus with a signal which not only indicates tissue type but is also readily comprehended, thus assisting a user of the apparatus in carrying out ablation surgery.

Alternatively, the apparatus may further comprise an endoscope so that a user is additionally provided with an image of the internal region in which ablation surgery is to be carried out. If the endoscope incorporates a visual display unit, the visual display unit may be arranged to receive characteristic signals from the output of the detector, or as the case may be, from the phase-locked loop, and display a visible signal characteristic of a tissue type at which the distal ends of the optical fibres are located. Thus the visual display unit may be arranged to display both an image of an internal region and simultaneously a visible signal indicating tissue type at which the distal ends of fibres are located. For example, the visual display unit may be arranged to display a false colour image which corresponds to a particular tissue type.

The optical fibres delivering the at least partially coherent radiation and radiation from the ablation laser may be one and same fibre, the proximal end of which is coupled both to the source of at least partially coherent radiation and to the ablation laser. This results in an apparatus which is less invasive in use. The source of at least partially coherent radiation ideally a laser, preferably a semiconductor laser, for reasons explained above. Alternatively, the optical fibres delivering the at least partially coherent radiation and radiation from the ablation laser may be one and same fibre, with the ablation laser acting as the source of at least partially coherent radiation; in this case the apparatus further comprises means for controlling the output power of the ablation laser. Such an arrangement reduces the invasiveness of the apparatus in use, and also simplifies the apparatus, the ablation laser serving to provide both radiation for distinguishing tissue types and radiation for ablation.

According to a third aspect of the present invention, there is provided a method of distinguishing tissue types in a. region internal to a human or animal body, the method comprising the step of delivering at least partially coherent radiation from a source thereof to the region, characterised in that the method further comprises the steps of imparting mechanical vibrations to at least one tissue type within the region and interfering a portion of the at least partially coherent radiation which is reflected by a tissue type with radiation from the source at a detector to generate a characteristic signal at an output thereof, the characteristic signal being characteristic of the tissue type from which radiation is reflected.

Conveniently, the mechanical vibrations are imparted by the steps of applying an oscillating signal to a transducer and placing the transducer in contact with a tissue type in the region.

A characteristic signal with reduced noise may be obtained by the steps of frequency demodulating the characteristic signal to produce a demodulated signal and inputting the demodulated signal and the oscillating signal to a phase-locked loop. In order to make the characteristic signal, or as the case may be the characteristic signal with reduced noise, readily comprehensible, it may be input to an output device to generate a characteristic visible or audible signal.

Embodiments of the invention are described below, by way of example only, with reference to the accompanying drawings in which:

Figure 1 shows apparatus known in the prior art, viz, an endoscope or fibrescope; Figure 2 shows apparatus of the invention; Figure 3 shows prior art endoscopic laser ablation apparatus; and Figure 4 shows endoscopic laser ablation apparatus of the invention.

Referring to Figure 1 , there is shown a prior art apparatus, namely an endoscope, indicated generally by 10. The endoscope 10 comprises a flexible bundle 12 of individual optical fibres (not shown) contained within a flexible sheath 14. Each of the fibres terminates at proximal and distal ends 13, 11 of the bundle 12, except for one fibre 24 which is coupled to a light source 32, for example a light-emitting diode or an incandescent lamp. The endoscope further comprises an optical detector 18, such as an array of light-sensitive pixels, and a monitor 30.

In medical use of the endoscope 10, the distal end 11 of the bundle 12 is inserted into a patient's body to view an internal region thereof. Light from the source 32 is delivered to the internal region via the fibre 24, reflected by the internal region under investigation and received by remaining fibres in the bundle 12. . Received light is conveyed to the proximal end 13 of the bundle 12. An image of the proximal end 13 of the bundle 12 is formed on the detector 18 by a lens 16, and an image of the internal region under investigation is displayed on a monitor 30.

An alternative prior art endoscope, is like to the endoscope 10, except that the lens 16 is an eyepiece allowing direct viewing of the region at the distal end 11 of the bundle 12. This alternative endoscope does not require an optical detector or a monitor.

Another prior art endoscope is like to the endoscope 10, except that the bundle 12 has a large enough diameter to allow an image to be formed on the distal end thereof by a lens which is comprised in the endoscope. The lens provides improved image quality.

Referring now to Figure 2, there is shown an apparatus of the invention, indicated generally by 50. The apparatus 50 comprises a laser 52 (for example a laser diode), optical fibres 53, 55, an optical interference unit (OIU) 54, a signal conditioning unit (SOU) 59 comprising a frequency-demodulator 58A and a phase- locked loop 58B, and an ultra-sonic oscillator 56 having a probe 57 which incorporates a transducer (not shown). The SCU 59 is electrically connected to an electrical output 75 of the OIU 54 and to the oscillator 56 by electrical connections 51 A, 51 B. The apparatus 50 further comprises mechanical means (not shown) for manipulation of the end 55A of the fibre 55 by a user (i.e. a practitioner). The apparatus 50 has an electrical output 60.

The OIU 54 comprises lenses 61 , 70, 73, a beam-splitter 62, a half-wave plate 64 and a quarter-wave plate 68 each having its optic axis at 45° to the plane of the paper, a Brewster plate 66, a mirror 71 , a beam-recombiner 72 and an optical detector 74 (e.g. a photodiode) having an output 75.

In Figure 2, the apparatus 50 is shown arranged for investigation of an internal region 80 of a patient. The region 80 comprises a bone region 84 surrounded by a nerve tissue region 82. The nerve tissue region 82 meets the bone region 84 at an interface 83. In use of the apparatus 50, a practitioner, desiring to accurately locate the position of the interface 83, moves the end 55A of the fibre 55 in the vicinity of the interface 83 whilst the probe 57 is maintained in contact with the bone region 84. When the end 55A of the fibre 55 is positioned over the bone region 84 a characteristic electrical signal is generated at the output 60.

A variety of output devices may be connected to the output 60. For example, devices such as a moving coil voltmeter or a loudspeaker may be employed to give a visual or an audible indication to the practitioner when the distal end 55A of the fibre 55 is located over a particular type of tissue. Alternatively, a chart recorder may be connected to the output 60 via suitable circuits to provide mapping of the interface region 83 if the distal end 55A of the fibre 55 is moved across the interface 83 in a systematic way.

The apparatus 50 operates as follows.^" Radiation from the laser 52 is coupled into the optical fibre 53 and passed to the OIU 54. Radiation exiting the fibre 53 is linearly polarised in the plane perpendicular to the plane of the paper and is collimated by the lens 61 and directed towards the beamsplitter 62. The beamsplitter 62 divides the radiation into two beams of approximately equal power. One beam passes through the half-wave plate 64 (after which it is polarised in the plane of the paper), through the Brewster plate 66 and quarter- wave plate 68 and is coupled^' by the lens 70 into the fibre 55. The other beam is directed to the detector 74 via the beam-recombiner 72 and the lens 73. Radiation coupled into the fibre 55 passes to the distal end 55A thereof and illuminates the internal region 80 under investigation; a portion of this radiation exits the distal end 55A and is reflected by matter in the vicinity thereof back into the fibre 55. The reflected radiation passes back to the OIU 54 along the fibre 55. On exiting the fibre 55 the reflected radiation is collimated by the lens 70 and passes through the quarter-wave plate 68, after which a non-depolarised component thereof is linearly polarised in the plane perpendicular to the plane of the paper in Figure 2. The reflected, non-depolarised radiation is reflected by the Brewster plate 66 and the mirror 71 and passes to the beam-recombiner 72 and thence to the detector 74 via the lens 73. The reflected, non-depolarised radiation is thus interfered with radiation directly received by the detector 74 from the laser 52.

Ultra-sonic energy is applied to the bone region 84 by means of the ultra-sonic oscillator 56 and probe 57. The oscillator 56 generates a signal of ultra-sonic frequency which is received by the probe 57. The probe 57 comprises a transducer (not shown) which is placed in contact with the bone region 54 when the apparatus 50 is used. The bone region 84 therefore undergoes ultra-sonic vibration, however the adjacent nerve tissue region 82 does not as it lacks sufficient mechanical rigidity.

When the distal end 55A of the fibre 55 is located over the bone region 84, radiation reflected back into the fibre 55 from the bone region 84 is phase modulated due to vibrations induced therein as a result of the action of the ultra- sonic transducer comprised in the probe 57. As a result, radiation incident on the detector has a modulated intensity, and an electrical signal having an ac component is generated at the output 60.

In contrast, when the distal end 55A of the fibre 55 is located over the nerve tissue region 82, radiation reflected back into the fibre 55 is not phase modulated because the nerve tissue region 82 does not undergo vibration. In this case a dc signal is produced at the output 60.

Signals generated at the output 75 of the OIU 54 are passed to the SOU 59 together with a signal from the oscillator 56. When phase-modulated light is received by the fibre 55, a signal with an improved signal-to noise ratio is generated at the output 60, compared to a corresponding signal generated at the output 75.

Thus bone tissue 84 may be distinguished from nerve tissue 82 by monitoring the status of an output device connected to the output 60 as the distal end 55A of the fibre 55 is moved in the vicinity of the interface 83.

Some vibration may occur in the nerve tissue 82, depending on its exact mechanical properties. The magnitude of any such vibration will be less than that of vibrations induced in the bone tissue 84 and such vibration may have phase shift with respect to vibration induced in the bone tissue 84. In this case a signal having an ac component is generated at the output 60 both when the distal end 55A of the fibre 55 is located at bone tissue 84 and when it is locate at nerve tissue 82. However, in this event tissue types may still be distinguished on the basis of the amplitudes and relative phases of signals generated at the output 60.

In order to maximise signals generated at the outputs 60, 75 the frequency of the signal output by the oscillator 56 may be adjusted to optimise acoustic resonance in the bone region 84.

The ultra-sonic oscillator 56 may be arranged to generate an ultra-sonic signal consisting of two frequencies, one of which is selected for optimal acoustic coupling to bone tissue 84 and the other being selected for optimal acoustic coupling to adjacent nerve tissue 82. In this case, when the apparatus 50 is used a modulated signal is produced at the outputs 60, 75 the frequency of which corresponds to the tissue type over which the fibre end 55A is located. Positive identification of both bone and nerve tissue may thus be achieved. Each frequency may be adjusted to optimise acoustic resonance in a corresponding tissue type. Frequencies other than ultra-sonic frequencies may be used depending on the mechanical properties of one or more of the tissues types under investigation.

The apparatus 50 may be used to distinguish n different tissue types, provided at least n-1 are capable, by virtue of their mechanical properties, of supporting vibrations. If the different tissue types are connected such that vibrations may be transmitted to each from one of the tissue types, only a single probe is required. If this is not the case, more than one probe may be required to set up vibrations in various tissue types. Positive identification of the n tissue types may be achieved using one or more signals having n different frequencies, provided all n tissue types are capable of supporting vibrations.

An alternative apparatus of the invention is like to the apparatus 50 except that it has no SOU, the output of the OIU being connected directly to an output device. The alternative apparatus has a simpler construction and may be employed in circumstances where a relatively poor sign-to-noise ratio is tolerable in the output of the OIU.

In the apparatus 50, the laser 52 could alternatively be a source of partially coherent radiation (for example an LED), although a laser provides a larger interference signal at the detector 74 because a laser's output is highly coherent. A semiconductor laser is preferred as such a laser is compact, robust and susceptible of control by simple electronic means.

Referring now to Figure 3, there is shown a prior art endoscopic laser abalation apparatus, indicated generally by 100, which may be used to perform, inter alia, spinal disc laser ablation surgery, i.e. to remove vertibral bone causing pressure on nerve tissue within a human or animal spine. The apparatus 100 comprises an aim laser or LED 102 (for example a He-Ne laser) having visible output, an ablating laser 104 (for example a Ho:YAG laser), a saline system 106 for delivery of saline solution, a vacuum system 108 for removing debris from a point of surgery, a white light source 132 (for example an incandescent lamp) and a monitor system 130. The laser or LED 102 and the laser 104 are coupled to a single optical fibre 116. The saline system 106 and the vacuum system 108 are coupled to flexible tubes 118, 120. The white light source 132 is coupled to an optical fibre 124. The monitor system 130 comprises a lens, an optical detector and a visual display unit (VDU, not shown), and is coupled to a fibre bundle 122. The white light source 132, the monitor system 130, the fibre 124 and the fibre bundle 122 together form an endoscope 1 10, or fibrescope, like to that shown in Figure 1. The fibres 116, 124 are held together with the tubes 118, 120 and the fibre bundle 122 within a flexible sheath 114. Distal ends of the fibres 116, 124, the tubes 118, 120 and the fibre bundle 122 are delivered to an internal region 180 of a human or animal body, for example a spinal region. The apparatus 100 further comprises mechanical means (not shown) allowing a practitioner to manoevre the distal tip 125 of the apparatus 100.

In use of the apparatus 100, a practitioner observes that part of the internal region 180 located near the tip 125 of the apparatus 100 by means of the monitor system 130, and moves the tip 125 adjacent a region of bone to be ablated using the mechanical means. The practitioner operates the aim laser 102 to ensure that the tip 125 is in a position such that, when the ablation laser 104 is operated, bone tissue 184 is ablated rather than adjacent nerve tissue 182. When the practitioner has positioned, the tip 125 correctly, the ablation laser 104 is operated to ablate offending bone tissue. The saline and vacuum systems 106, 108 may be operated to clean a point at which surgery is carried out.

Referring now to Figure 4, there is shown an endoscopic laser abalation apparatus of the invention, indicated generally by 200. The apparatus 200 has a construction like to that of the apparatus 100; parts of the apparatus 100, 200 which are equivalent are labelled by reference numerals which differ by a value of 100. The apparatus 200 comprises an aim laser 202 coupled to an optical interference unit (OIU) 254 via an optical fibre 253, a signal conditioning unit (SCU) 259 incorporating a frequency de-modulator and a phase-locked loop (not shown) and having an output 260, an ultra-sonic oscillator 256 having a probe 257 which incorporates a transducer (not shown), and an optical fibre 255 for coupling a portion of the radiation output by the aim laser 202, via the OIU 254, to a region 280 within a human or animal body in which laser ablation surgery is to be carried out. The apparatus 200 further comprises an ablation laser 204 coupled to the fibre 255 by an optical fibre 216. Parts 202, 253, 254, 255, 256, 257, 258 and 260 constitute apparatus for distinguishing bone tissue 284 from nerve tissue 282, this apparatus being like to that shown in Figure 2.-

Instead of being coupled to the fibre 255, the fibre 216 may extend to the distal end 225 of the apparatus 200. However the use of a single fibre 255 for delivery of both ablating radiation and radiation from the aim laser 202 keeps the diameter of the sheath 214 to a minimum.

The apparatus 200 further comprises a light source 232 coupled to an optical fibre 224, a monitor system 230 (comprising a lens, a detector and a visual display unit (VDU) which are not shown) coupled to a fibre bundle 222. These parts comprise an endoscope 210 or fibrescope like to that shown in Figure 1.

In use, a practitioner positions the distal end 225 of the apparatus 200 approximately over a region of bone to be ablated using the mechanical means and the endoscope 210. By receiving visible or audible signals from an output device to which signals at the output 260 (or as the case be the output 275) are input, the practitioner may position the distal end 225 of the apparatus 200 very accurately over a region of bone to be ablated, and much more accurately than is possible merely by use of the endoscope 210 alone. Once the practitioner has accurately positioned the distal end 225 of the apparatus 200, ablation is performed by operating the ablation laser 204.

If means are provided to control the output power of the ablation laser 204, this laser may be used for aiming the apparatus 200 as well as for effecting ablation.

In this case the ablation laser 204 is coupled to the OIU 254 via the fibre 253. The aim laser 202 and the fibre 216 may then be dispensed with.

An alternative endoscopic laser ablation apparatus of the invention is like to the apparatus 200, except that signals from the output 260 are passed to the monitor system 230 and are used to give a visual indication on the VDU of the monitor system corresponding to a tissue type over which the distal end of the apparatus is positioned.

A further alternative endoscopic laser ablation apparatus of the invention is like to the apparatus 200, except that signals from an OIU are passed directly to an output device, or as the case may be to a monitor system, and a SCU is not employed. This provides a simpler apparatus in circumstances where a reduced signal-to-noise ratio is tolerable in the output of the OIU.

Another alternative endoscopic laser ablation apparatus of the invention comprises an ablation laser coupled to an optical fibre (for delivery of ablating radiation) and the apparatus of Figure 2, but does not comprise an endoscope or fibrescope. Although the foregoing description describes use of apparatus of the invention for distinguishing different tissue types, e.g. bone tissue and nerve tissue, the apparatus may also be used to distinguish regions of the same biological tissue type, provided such regions have mechanical or other properties such that these regions respond differently to an applied vibration. For example, a diseased region of nerve tissue has a different density and/or different elasticity compared to a healthy region of nerve tissue. Therefore, when a mechanical vibration is applied to bone tissue adjoining nerve tissue, signals generated at points 75, 60 of the apparatus 50 when the end 55A is positioned over healthy nerve tissue will differ from signals at those points generated when the end 55A is positioned over diseased nerve tissue.

Another use of apparatus of the invention is in distinguishing regions consisting of the same biological material, all of which are healthy but which are in different biological states such that the regions have different responses to an applied vibration, due to differing mechanical or other properties within the regions. For example, in human or animal fertility treatment, it is necessary to identify eggs in the female which are in a sufficiently high state of fertility to allow their use in fertility treatments. The fertility of an egg is related to the viscosity of its surrounding follicular fluid, and therefore a mechanical vibration applied to a female ovary produces different vibrational responses within follicular fluid surrounding eggs in differing states of fertility. Signals generated at points 60, 75 of the apparatus 50 when the distal end 55A of the apparatus 50 is positioned over follicular fluid are therefore indicative of the fertility of an egg within that fluid.

Claims

1. Apparatus (50) for distinguishing tissue types in a region (80) internal to a human or animal body, the apparatus comprising a source (52, 53) of at least partially coherent radiation, an optical fibre (55) for delivering a portion of the at least partially coherent radiation to the region and means for positioning the optical fibre's distal end (55A), characterised in that the apparatus further comprises means (56, 57) for imparting mechanical vibration to at least one of the tissue types, a radiation detector (74), and interfering means (61 , 62, 64, 66, 68, 70, 71 , 72, 73) for interfering radiation delivered by the optical fibre to the region and subsequently reflected by tissue therein into the optical fibre's distal end with radiation from the source at the detector to generate a characteristic signal at an output (75) thereof, the characteristic signal being characteristic of a tissue type at which the optical fibre's distal end is positioned.

2. Apparatus according to claim 1 wherein the means for imparting mechanical vibration comprises an oscillator (56) and a probe (57) for contacting a tissue type, the probe incorporating a transducer for converting electrical signals from the oscillator into mechanical vibration.

3. Apparatus according to claim 2 further comprising a frequency demodulator (58A) arranged to receive a characteristic signal generated at the output of the detector and generate a demodulated signal in response thereto, and a phase- locked loop (58B) arranged to receive the demodulated signal and a signal from the oscillator and produce a characteristic signal with reduced noise at an output (60) of the phase-locked loop in response to input of the demodulated signal and the signal from the oscillator.

4. Apparatus according to claim 2 or claim 3 wherein the oscillator is arranged to generate a signal comprising two or more frequencies each suitable for inducing mechanical vibration in a corresponding tissue type, and to pass the signal to the probe.

5. Apparatus according to any preceding claim further comprising an output device arranged to receive characteristic signals from the output of the detector, or as the case may be the output of the phase-locked loop, and output a visible or audible signal characteristic of a tissue type at which the optical fibre's distal end is located.

6. Apparatus according to any preceding claim wherein the source of at least partially coherent radiation is a laser.

7. Apparatus according to claim 8 wherein the laser is a semiconductor laser.

8. Endoscopic laser ablation apparatus (200) for performing laser ablation surgery in a region (280) internal to the human or animal body, the apparatus comprising an ablation laser (204) and an optical fibre (255) for delivering radiation output by the ablation laser to the region, characterised in that the apparatus further comprises apparatus according to any one of claims 1 to 4 for distinguishing tissue types within the region, the distal ends of the optical fibres being co-located.

9. Apparatus according to claim 8 further comprising an output device arranged to receive characteristic signals from the output of the detector, or as the case may be from the output of the phase-locked loop, and output a visible or audible signal characteristic of a tissue type at which the distal ends of the optical fibres are located.

10. Apparatus according to claim 8 further comprising an endoscope (210).

11. Apparatus according to claim 10 wherein the endoscope incorporates a visual display unit for displaying images of the region.

12. Apparatus according to claim 1 1 wherein the visual display unit is arranged to receive characteristic signals from the output of the detector, or as the case may be, from the output of the phase-locked loop, and display a visible signal characteristic of a tissue type at which the distal ends of the optical fibres are located.

13. Apparatus according to any one of claims 8 to 12 wherein the optical fibres are one and the same optical fibre, the proximal end of which is coupled both to the source of at least partially coherent radiation and to the ablation laser.

14. Apparatus according to claim 13 where the source of at least partially coherent radiation is a laser.

15. Apparatus according to claim 14 wherein the laser is a semiconductor laser.

16. Apparatus according to any one of claims 8 to 12 wherein the optical fibres are one and the same optical fibre and the source of at least partially coherent radiation is the ablation laser and wherein means are provided for controlling the output power of the ablation laser.

17. A method of distinguishing regions of tissue, or other biological material, in or from a human or animal body, the method comprising the step of delivering at least partially coherent radiation from a source thereof to a first region, characterised in that the method further comprises the steps of imparting mechanical vibrations to at least one of said regions and interfering a portion of the at least partially coherent radiation which is reflected by said first region with radiation from the source at a detector to generate a characteristic signal at an output thereof, the characteristic signal being characteristic of the first region of tissue, or other biological material, from which radiation is reflected.

18. The method of claim 17 wherein the regions of tissue consist of different tissue types.

19. The method of claim 17 wherein the regions of biological material consist of follicular fluid within a human or animal ovary.

20. A method according to any one of claims 17 to 19 wherein the mechanical vibrations are imparted by the steps of

(i) applying an oscillating signal to a transducer, and (ii) placing the transducer in contact with a region of tissue or other biological material. ^'

21. A method according to claim 20 further comprising the steps of:

(i) frequency demodulating the characteristic signal to produce to produce a demodulated signal, and

(ii) inputting the demodulated signal and the oscillating signal to a phase- locked loop to produce a characteristic signal with reduced noise.

22. A method according to any one of claims 17 to 21 further comprising the step of inputting the characteristic signal, or as the case may the characteristic signal with reduced noise, to an output device to generate a characteristic visible or audible signal.