WO2005051190A1

WO2005051190A1 - Blood flow monitoring equipment

Info

Publication number: WO2005051190A1
Application number: PCT/GB2004/004911
Authority: WO
Inventors: Anthony J. Strong; Roger Spink
Original assignee: Kings College Hospital Nhs Trust
Priority date: 2003-11-21
Filing date: 2004-11-22
Publication date: 2005-06-09
Also published as: GB0327100D0

Abstract

In or for a device for monitoring blood flow in an area during a surgical procedure in which an operating microscope allows a surgeon to observe the surgical procedure; and in which a laser light source illuminates the area, speckle laser light from the area being monitored is fed by a diversion means, to a processing means which converts the speckle laser light to an image representing blood flow, and display means to display the resultant image to the surgeon and/or his assistant; the improvement comprising so arranging said diversion means that said speckle light is divided into a first tranche feeding said processing means and at least a second tranche transmitting the image of the observed area to the surgeon through the microscope, thereby allowing the surgeon - or the surgeon and his assistant between them - a substantially unimpeded view of both the observed area under operation and the blood flow image created.

Description

BLOOD FLOW MONITORING EQUIPMENT

Field ofthe Invention

This invention relates to blood flow monitoring equipment and specifically to a device for measuring blood flow in surgical procedures.

Review ofthe Art known to the Applicant

Certain neurosurgical procedures, especially those involving surgery upon or close to major blood vessels supplying or draining the brain, carry risks of temporary or permanent damage to these vessels, or of stimulating spasms (narrowing) of these vessels. In addition, there is evidence that the capillary microcirculation of the brain may be unstable and vulnerable to surgical manipulation, especially after haemorrhage affecting the area. A monitoring method is therefore needed that will promptly provide the surgeon with information on the state of perfusion in as much as possible of the territory at risk during the surgical procedure, with minimal interference with surgery.

There is no widely established method for accomplishing this goal because the current methods are either indirect, requiring knowledge of the relationship between the variable being monitored and cerebral perfusion, or they are subject to noise due to lack of sensitivity, or due to vulnerability to inevitable mechanical disturbance of the sensor during surgical manoeuvres (hence resulting in false positive or negative data) or they are confined to sampling only a small area of the territory at risk. In addition, all existing methods require placement of the sensor within the operative field, which obstructs the surgeon's free access to the area which is being sampled.

The currently existing methods for monitoring blood flow in an operative area, comprise heat clearance probe, laser doppler flow probe and tissue oxygen probe. Of these the use of a heat clearance probe suffers from the problems of (a) noise due to lack of sensitivity, (b) vulnerability to inevitable mechanical disturbance of the sensor during surgical manoeuvres, hence resulting in false, positive or negative data, and (c) the method is confined to sampling only a small area of the territory at risk. However, the surgeon can make an assessment of the risk to tissue viability from the values obtained from thermal clearance (from public knowledge ofthe critical level of absolute perfusion — required to ensure tissue viability), since the values obtained are in these absolute units, and hence universally applicable.

The problems associated with the laser doppler flow probe are the same as those associated with the heat clearance probe but additionally this monitoring method is an indirect method, detecting change in perfusion but not in absolute units. Thus the surgeon does not know how close perfusion has fallen to the viability threshold.

The problems associated with the use of tissue oxygen probe methods are the same as for laser doppler flow probes.

Another method that has been used for monitoring cerebral blood flow in animal metabolic studies is 'laser speckle'.

When a biological surface is illuminated by coherent (laser) light, reflectance and scattering of light at a given point varies inherently, resulting in appearance of flickering or 'speckle'. The degree of this variation is inversely proportional to the motility of particles (in this case red cells in capillaries) at the given point. By imaging the surface through appropriate optics (in which aperture setting is important) and processing a sequence of images, for example 10 at a frequency of 25 per second, it is possible to express the degree of variability as 'laser speckle contrast'. The application of this principle to imaging of perfusion within the rat brain surface was demonstrated by Dunn et al in 2001 (Journal of Cerebral Blood Flow and Metabolism, volume 21 page 195 to 201). An image of the brain is generated representing the degree of variability in the reflectance at each pixel in the image. Thus the grey level in a single pixel in this image is inversely proportional to red cell movement, or perfusion, in the brain in that pixel. This technology cannot be used in operative situations (as opposed to experimental animal metabolic studies) as the technique requires the use of both a laser to illuminate the surface and the use of a CCD camera as a means of gathering a resulting optical pattern over the brain surface, prior to its analysis by a processing means. The equipment required results in obstruction ofthe area in which the surgical procedure is being carried out. The current invention aims to address this problem.

The standard operating microscope consists of a main surgeon's eye piece pair placed on top of a beam splitting mechanism which in turn is placed over the objective lens system. The beam splitting mechanism shares light to the principal surgeon's right eye with one side piece and light for his/her left eye with the opposite side piece. Conventionally, one side piece is adapted such that it can supply an image to the surgeon's assistant and the other side piece is adapted to supply an image to a standard colour video monitoring camera which displays the image on a monitoring screen for observation and training for other personnel in the operating theatre.

Summary ofthe Invention

According to the present invention there is provided in or for a device for monitoring blood flow in an area during a surgical procedure in which an operating microscope allows a surgeon to observe the surgical procedure; and in which a laser light source illuminates the area, laser speckle light from the area being monitored is fed by a diversion means, to a processing means which converts the speckle laser light to an image representing blood flow, and display means to display the resultant image to the surgeon and/or his assistant; the improvement comprising so arranging said diversion means that light is divided into a first tranche feeding said laser speckle light to said processing means and at least a second tranche transmitting the image ofthe observed area to the surgeon through the microscope, thereby allowing the surgeon - or the surgeon and his assistant between them - a substantially unimpeded view of both the observed area under operation and the blood flow image created.

Preferably the device comprises means for an operator to identify and set visual landmarks within the field of view such that the monitoring and analysing devices can monitor blood flow at a chosen point despite movement ofthe microscope.

Preferably the device communicates with vascular and respiratory monitors in order to correct for movements in the field of view which arise from vascular and respiratory processes.

Preferably the device further comprises means to reduce the level of non-laser illuminating light for short intervals (<0.3s) when the monitor is monitoring the diverted laser speckle light.

Preferably means are provided such that the image of blood flow can be overlaid on a normal image ofthe observed area.

More preferably means are provided such that the blood flow image is displayed through the microscope ocular.

Preferably means are provided such that, as the magnification used with respect to the microscope is changed, so changing the field of view, the laser light is focused to an area corresponding to the size ofthe new field of view.

Preferably means are provided such that the blood flow image can be overlaid in false colours in order to improve contrast between the two images. The blood flow image may be overlaid over the normal image in false colours such that the blood flow image can be clearly distinguished from the normal image by the use of colours such as greens and blues. Additionally the flow of blood to different areas may be represented such that the colour is representative ofthe blood flow rate.

Preferably means are provided such that the image from a second camera can be subtracted from the blood flow image in order to correct for disruption to the blood flow image due to visible light.

Preferably means are provided such that a surgeon may manually select when images are collected. Such means allowing the surgeon to collect an image immediately before and after any procedure is carried out such that changes in blood flow can be determined at appropriate points in the procedure.

Included within the scope ofthe invention is a blood flow monitoring device substantially as described herein with reference to and as illustrated by the accompanying drawing.

Brief Description ofthe Drawings

The invention will be described by reference to the accompanying drawings in which:

Figure 1 is a schematic illustration of the typical arrangement of the component parts forming the blood flow monitoring device.

Description ofthe Preferred Embodiment

The blood flow monitoring device in this embodiment is described in the context of neurosurgery, and is used in conjunction with the standard operating microscope previously mentioned.

The device either replaces the normally used colour video monitoring camera with a monochrome camera or is used in combination with the video camera, such that laser speckle light can be monitored and processed to generate an image of blood perfusion levels in a monitored area (e.g. ofthe brain surface), the image being shown on a display. This eliminates the need for a separate monitor to be present which monitors the laser speckle light. This has the advantage that it decreases the obstructions present in the area where the surgeon is working. This method also has the additional advantage that by monitoring brain perfusion through the operating microscope, the area of the brain monitored is the general area in which the surgeon is specifically working, as opposed to previous methods such as heat clearance probe, laser Doppler or tissue oxygen probe which monitor only a single point.

Reference will now be made to figure 1 of the diagrams wherein an illustration is provided of the typical arrangement of the component parts forming the blood flow monitoring device as generally indicated by (10). The device comprises a laser diode (11) which emits laser light (11a) suitable for use in laser speckle contrast imaging (in this case the wavelength of the light emitted is actually 785nm) the light is emitted such that it illuminates the area of the brain, in which the surgeon is operating. The laser light reflected and scattered (l ib) by the surface of the brain (12) and is then imaged through the objective lens ofthe operating microscope (13), and passes through the main aperture

(14) ofthe operating microscope. The diameter ofthe main aperture (14) is controlled by the surgeon. The reflected and scattered laser light is then directed by the beam splitting mechanism (15), towards a CCD camera (16). Positioned between the beam splitting mechanism (15) and the camera is an aperture (17). The aperture is set to a diameter, usually narrow, which is required to secure the optimum conditions under which the speckle pattern is inversely proportional to the perfusion. This aperture may not be compatible with that needed by the surgeon, and so in this case is placed in a position after the light has passed through the beam splitting mechanism (15). Alternatively, if a aperture (17) is not included after the beam splitter it may be necessary to use the main aperture (14) as a means to control the optical conditions such that a speckle image can be acquired.

Additional means may be required to direct the light from the beam splitting mechanism

(15) to the CCD camera (16), such as a prisms (18), additionally a band pass filter (19) may also be incorporated into the instrument, through which the laser speckle light must pass before entry into the monitoring CCD camera (16). The camera (16) generates an image which is analysed by a computer (20) using principles disclosed in the prior art see Dunn et al in 2001 (Journal of Cerebral Blood Flow and Metabolism, volume 21, page 195 to 201). The computer (20) then generates an image of perfusion levels in the area being monitored which is displayed on a screen. It takes approximately 0.3 seconds for the computer (20) to acquire - and in the present state of personal computing a further 11- 12 seconds to process - enough data from the CCD camera (16) to enable the generation of a suitable image. A sequence of such images is acquired at any desired interval, for example 5 minutes; this may be as small as 12 seconds if desired or as rapidly as the processing means will allow. This sequence can then be displayed as multiple images on a screen, additionally an image can be displayed which represents change in perfusion levels relative to an earlier image, making it easier to detect whether a change has occurred, and how extensive in the area ofthe surgical procedure this change is.

Systems which generate images from laser speckle light are vulnerable to problems caused by visible light which disrupts the formation of an image from laser speckle light. As such the system may incorporate a means to reduce the level of normal surgical illumination while the perfusion images ofthe brain surface are being obtained using laser speckle imaging. This does not cause problems for the surgeon as it only takes approximately 0.3 of a second for the CCD camera (16) to collect enough data to generate an image. The device may additionally emit a warning tone to indicate that the computer is about to acquire data from the CCD camera (16) which will result in a temporary reduction in the level of surgical illumination. In response to this the surgeon may either allow the computer (20) to acquire its image, or if the surgeon wishes to avoid a decrease in the level of surgical illumination he can be provided with means to allow him to delay the image acquisition either for a pre-determined interval set by the surgeon or until such a point as the surgeon requests a further image to be taken.

The surgeon views the area in which he is operating by means ofthe eyepieces (21). The surgeon is normally provided with a stereoscopic view. The surgeon's assistant views the area ofthe operation by means of beam splitter (15a), beam splitter (22) and the eyepieces (23) and is normally provided with a view with restricted stereo depth.

It will be appreciated by the person skilled in the art that from time to time the surgeon will be required to re-position the microscope. Thus in a sequence of speckle perfusion images ofthe brain surface the field of view may vary, negating the value of computerised sequential difference imaging. This problem however can be overcome by the use of software that can register the image series to a single set of co-ordinates, permitting correct difference imaging. The surgeon can aid this process by identifying and using visual landmarks in their field of view, such that it is not necessary for the re-orientating of the microscope into its original position when a speckle perfusion image is to be acquired. The software will automatically identify and acquire an image from the appropriate area. Alternatively the surgeon will be able to make comparisons manually by viewing the set of unregistered perfusion images on a single screen, and discerning for him/herself unique anatomical patterns of folding visible at different locations in the images.

An additional issue associated with the use of such technology is that there are sometimes fine movements of the brain synchronous with the cardiac and respiratory cycles, the amplitude being dependent on anaesthetic conditions. This can result in small changes in registration during the 0.3 second interval required for acquisition of the raw speckle images prior to processing, and hence introduce variability in the calculated images. This problem can be negated by feeding the vascular and respiratory signals from the anaesthetic monitor to the computer, such that the computer can compensate for these variations. Alternatively the software can avoid such variations by only sampling the monitored area at points where movement of the cardiac and respiratory cycles are at a minimum or are non-existent.

Claims

1. In or for a device for monitoring blood flow in an area during a surgical procedure in which an operating microscope allows a surgeon to observe the surgical procedure; and in which a laser light source illuminates the area, laser speckle light from the area being monitored is fed by a diversion means, to a processing means which converts the laser speckle light to an image representing blood flow, and display means to display the resultant image to the surgeon and/or his assistant; the improvement comprising so arranging said diversion means that light is divided into a first tranche feeding said laser speckle light to said processing means and at least a second tranche transmitting the image of the observed area to the surgeon through the microscope, thereby allowing the surgeon - or the surgeon and his assistant between them - a substantially unimpeded view of both the observed area under operation and the blood flow image created.

2. The improvement of claim 1 additionally comprising means for an operator to identify and set visual landmarks within the field of view such that the monitoring and analysing devices can monitor blood flow at a chosen point despite movement ofthe microscope.

3. The improvement of any preceding claim wherein the monitoring and analysing device communicate with vascular and respiratory monitors in order to correct for movements in the field of view which arise from vascular and respiratory processes.

4. The improvement of any preceding claim further comprising means to reduce the level of non-laser illuminating light for short intervals (<0.3s) when the monitor is monitoring the diverted laser speckle light.

5. The improvement of any preceding claim wherein means are provided such that the image of blood flow can be overlaid on a normal image ofthe observed area.

6. The improvement of claim 5 wherein means are provided such that the blood flow image is displayed through the microscope ocular.

7. The improvement of any preceding claim wherein means are provided such that, as the magnification used with respect to the microscope is changed, so changing the field of view, the laser light is focused to an area corresponding to the size of the new field of view.

8. The improvement of any preceding claim wherein means are provided such that the blood flow image can be overlaid in false colours in order to improve contrast between the two images.

9. The improvement of any preceding claim wherein means are provided such that the image from a second camera can be subtracted from the blood flow image in order to correct for disruption to the blood flow image due to visible light.

10. The improvement of any preceding claims wherein means are provided such that a surgeon may manually select when images are collected.

11. A blood flow monitoring device substantially as described herein with reference to and as illustrated by the accompanying drawing.