CA2076667A1

CA2076667A1 - Multiprobes with thermal diffusion flow monitor

Info

Publication number: CA2076667A1
Application number: CA 2076667
Authority: CA
Inventors: Alexandros D. Powers
Original assignee: Individual
Current assignee: Individual
Priority date: 1990-03-02
Filing date: 1991-01-16
Publication date: 1991-09-03
Also published as: WO1991012765A1; AU7253591A; JPH05508328A; EP0517734A1; EP0517734A4

Abstract

A multiprobe with thermal diffusion flow monitor (hereinafter known as "MPTDFM") that has improved reliability, smaller overall size, simpler method of sensor positioning, better compatibility and capability to monitor blood flow, pressure and other critical physiological parameters is provided. The MPTDFM is formed by the combination of a thermal diffusion flow monitor (20), a pressure monitor (22), a multiple parameter monitor (24) and a support structure (26).

Description

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S ~ ~J~N 139 SINGLE PROBE WITH TMERMAL DIFFUSION FLOW MONITOR

sackqround of the Invention 1. Field of the Invention.

The present invention ralates to devices which -measure tissue blood flow, particularly those based on the thermal diffusion flow concept, measure tissue pressure and can also assess the function of human tissue through the simultaneous monitoring of critical - 10 physiological parameters.

2. Description of the prior art.

The original reports using the thermal ~;
diffusion flow monitor concept appeared in the late 1960's. The work was done by Carter et al. (Carter L.P., Atkinson J.R. "Cortical blood flow in controlled hypotension as measured by thermal diffusion", J. ; --Neurol. Neurosurq. Psychiatry, Vol. 36, pp. 906-913, 1973) using a Peltier stack. In order for the Peltier -~
stack to be able to detect flow (as determined by the 20 rate of cooling) the tissue of interest needed to be ~-~
~ exposed and uniform contact between the sensor and the tissue surface was required. Although Peltier stacks are widely used, the system suffers from its relatively large size of the sensor and the variability of its outpu~.

Later, in the 1980's, improvements including signal processing to stabilize the sensors output, simplified desig~ of the sensor using a two-point system, where one point is a heat source and the other point a temperature sensor being positioned a short ~, .

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distance away, were made. Still because of its large size, need for visual placement of the probe on the surface of the tissue and unreliable readings due to loose contact of the sensor tip with the surface of the tissue continue to limit i1:s application.

Summary of the Invention A single probe with thermal-diffusion flow monitor (hereinafter known as "SPTDFM") that has improved reliability, smaller overall size, simpler method of sensor positioning, better compatibility and capability to monitor blood flow, pressure and other ~
critical physiological parameters is provided. ~ ~-The SPTDFM of the present invention uses an anemometer and is to be placed into the substance of the tissue itself through a very small surgical opening and - ~`
does not require visual positioning. The delivery ~ :
system for the SPTDFM is one that is commonly used in medicine for the placement of various types of monitors, such as pressure monitors~ It i.nvolves a small skin ! incision of approximately l cm, followed by opening of the connective tissue, such as by drilling a hole in bony coverings as would be required for access to the brain, and finally passage of the SPTDFM through this opening into the substance of the tissue. In this manner, the device can be placed quickly at the patient's bedside and a large operative procedure for ~ ~
visual positioning is not required. ~ -In addition, the thermal diffusion flow ~ ~-monitor has better compatibility wi~h pressure monitors as well as multiple parameter monitors in formin~ the SPTDFM for the detection and mo~itoring of biochemical substances.
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Finally, use of an anemometer in the SPTDFM
has the advantage of unexpected electrical properties.
Specifically, the anemometer can ~e operated at a constant temperature mode with electrical current being supplied to the sensor. Thus, as the sensor tip of the anemometer is cooled by the blood flowing in the surrounding tissues, electricity will flow to the sensor to adjust it automatically. These changes in electrical current are then directly measured to produce a read-out. This direct measurement of the electrical current eliminates the need for additional circuitry that is required by other thermal monitoring designs which`~
measure the temperature difference between a heat source and a temperature monitor.

According to the present invention, a single --probe system useful for monitoring blood flow at a tissue is provided. The single probe system comprises a i;
support structure having a first: side surface and a ~-second side surface opposite to said first side surface, an anterior and a posterior surface connected to said first and second side surfaces; thermal diffusion means housed in said support structure near said first side -~
surface, said thermal diffusion means further comprises 25 a conical hot film probe with a sensor tip contacting ;
the tissue, wherein said sensor tip comprises a metal - thin layer; a ~acking material onto which said metal thin layer is deposited; and a protective coating deposited over said metal thin layer: control means to 30 head said sensor tip as a heat source while ;~
simultaneously measuring a temperature correlating to a blood flow at the tissue; pressure monitor means housed in said support structure near said anterior end and adjacent to said thermal diffusion means to monitor pressure at the tissue; and mul~iple parameter means housed in said support structure near said second :
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F~llJ~ q~ 2 t~ 6JIIN 1 surface and adjacent to said pressure monitor means to monitor the oxygen content, temperature and potential of the tissue.

Brie~ Description of the Drawinqs The novel features which are believed to be ~.
characteristic of the invention are set forth with particularity in the appended claims. The invention 10 itself, however ~oth as to its organization and method ~, of operationj may best be understood by reference to the following description taken in connection with the accompanying drawings, wherein similar character refer to similar elements throughout and in which:
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FIGURE l illustrates a common thermal diffusion ~lood flow monitor mounted on a support structure;

FIGURE 2 illustrates an embodiment of the SPTDFM. ;~

FIGURE 3 illustrates a typical conical hot film probe used in the SPTDFM;

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~US ql/~2 lPI~u~ 3992 FIGURE 4 illustrates one embodiment of the placement of multiple, single-point sensor tips onto a single probe;

FIGURES 5a-5c illustrate emhodiments of temperature gradient monitoring by multiple single-point sensor tips in a singl~ probe;

FIGURE 6a-6c illustrate embodiments to measure tissue pressure;
, FIGURE 7 illustrates one embodiment of the positioning o f the SPTDFM probe into the substance o~
brain tissue; ``

FIGURE 8 illustrates one embodiment of the -~
SPTDFM with a side port placed on a cathetPr, and :
FIGURE 9 illustrates an embodiment of the SPTDFM using an introducer.

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Description of the Preferred Embodiment Referring now to FIG. 1, a common thermal .
di~fusion blood flow probe/pressure monitor 2 mounted on a flexible support structure 3 is shown. The monitor 2 is based on a two-point system where a point 4 is a heat source ant another point 6 measures the temperature of the tissue. Together, these two points, 4 and 6, form the sensor tip 8. The sensor tip 8 is placed on the sur~ace of the tissue to be monitored, with ~oth points 4 and 6 requiring intimate contact. The hsat source 4 .
i5 then activated to a set temperature, generally 41~ C, which is higher than the ambient temperature of the ~;
underlying tissue. As blood flows past this region, it ~

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cools the heated tissue. Thus, the temperature drop between the two points can be correlated with the rate of regional blood flow. For example, if the measured temperature is 41 C, there is little or no blood flow S through the tissue, while a reading of 35 c means that there is significant blood flow with a high degree of cooling. Since the sensor tips are relatively large, so is the resulting thermal probe having dimensions typically 7 mm (width) by 5 mm (height), including the sensor tip, support sitructure and wiring. Generally, length of the probe is not o~ significance as the end of any probe must exit through the skin to be connected to a monitor by current connector 10.

The common thermal diffusion blood flow probe 2 described above can also be used to measure tissue pressure based on the transmission of pressure waves along a tube 12 filled with fluids. One end 14 of the tube 12 is positioned such that it is surrounded by the natural fluids of the tissue. Pressure changes from the 20 tissue are transmitted through the natural fluids, which ~-~
then are directly transmitted to the fluids at the end of the tube. High tissue pressure causes the natural fluids to flow into the end of the tube, while low tissue pressure will extract fluids out of the end of the tube. This pressure differential causes a displacement of the fluids in the tube, which is then -transmitted along the entire length of the tube. By monitoring the opposite end 16 with a pressure transducer, or by measuring the changes in height of the fluid column, a direct pressure is determined.
~easurements determined by this method, however, are subject to significant error if the measuring/monitoring ;
end 14 is obstructed with tissue. This is because although solids transmit pressure waves very well, the ~
35 volume of solid tissue remains fairly constant over a ~ ;

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erroneous pressure determination at the monitoring end 16 of the tube. In contrast, the SPTDFM used in the ~
present invention minimizes and/or eliminates completely ~-this uncertainty in tissue pressure measurements and ~
monitoring. `;
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FIG. 2 shows an embodiment of the SPTDFM 18 of the presen' invention. The SPTDFM 18 is formed by the combination of a thermal diffusion flow monitor 20, a pressure monitor 22, a multiple parameter monitor 24 and a support structure 26. ~
~, ' ' '' The use of an anemometer to measure the rate of cooling of a solid has not been described before and the anemometer is generally used only when there is a continuous flow of material past the area of measurement .
(i.e., flowing fluid or stream of air). In addition, the combined use of an anemometer with different modalities in a single monitoring probe has not been -h commercially produced or experimentally described.
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The thermal diffusion flow monitor 20 can be ;
in the form of a conical hot film or hot wire probe 28 mounted on a catheter 30 with a diameter of 2 mm (FIG.
3). The probe 28 is a single-point sensor which acts as both a heat source and a temperature monitor. The `~
single polnt design reduces the size of the sensor tip 32 and also permits multiple sensor tips to be placed onto a single probe (FIG. 4). The sensor 34 of the conical hot film probe is usually made of nickel or ~;
platinum deposited in a thin layer onto a backing material 36, such as quartz, and connected to the electronic package by leads 38 attached to the end of ~IJ''~.TUT~.S~1E~T ~

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the film. Double quartz protective coatings 40 are deposited over the thin film to prevent damage by abrasion or chemical reaction.

In another embodiment, shown in FIG. 4, a single probe 42 with support structure 44 is shown to have multiple sensor tips 46 to simultaneously monitor `
blood flow at different tissue sites. In addition, the array 48 of sensor tips 4~ will more accurately reflect tissue blood flow by minimizing the sampling error associated with measurements made at a single site.
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Referring to FIGS. 5a-c, embodiments are shown in which temperature gradients are monitored by periodically altering the function of the single-point ~;~
sensor tips 46 in the array 48. In one embodiment (FIG.
5a), a single-point sensor tip 46 functions as a heat source. The remaining single-point sensor tips 50 then function as temperature monitors and are used to measure the temperature drop as distance of multiples of d increases from that heat source 46 and correlating the temperatures to blood flow at the tissue. In another t embodiment (FIG. 5b), several single-point sensor tips 52-56 function as heat sources. The remaining sensors 58 are used to measure the temperature drop over the intervening distances of d2 or multiples thereof and correlating the temperatures to blood flow at the tissue. In yet another embodiment (FIG. 5c), the entire array 48 of single-tip sensors are periodically heated. `~
Thus, the array 48 functions as if it were a wire which 30 is being heated and is capable of monitoring its own -~
rate of cooling which correlates to the blood flow at ~-the tissue.

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To measure tissue pressure (see FIG. 6a), a pressure transducer 22 with a movable diaphragm and strain gauge 60 is placed in contact with the tissue.
5 As pressure changes are transmitted through the tissue, ~:
they will cause a displacement of this diaphragm and strain SU13S'I'~IlJTI~ S~EET`

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lpr~ s gauge 60 from its neutral position. The degree of change is then measured by one of two basic methods. In one method (FIG. 6b), a pneumatic circuit 62 connected to the movable diaphragm and strain guage which pumps air into the pressure transducer 22 is used to counterbalance the tissue pressure causing the diaphragm and strain gauge to return to its neutral position. The pressure used to obtain this equilibrium is measured and directly correlated to the tissue pressure. In the second method (FIG. 6c), a fiber optic cable 64 and a photodetector 66 are used. Initially, when the diaphragm and strain gauge system i5 in the neutral position, light 61 emitted from a source at one end of 15 fiber optic cable 65, after incidence on a reflective ~
surface 63 connected to the diaphragm 60, is perfectly ~ ~ -aligned with the photodetector 66. As tissue pressure changes, the diaphragm 60 is displaced, altering the ~ ~
alignment of the reflective surfacè 63 and thus the ~ -reflected light beam 67 with the photodetector 66. The change in tha light intensity measured at the photodetector 66 is transmitted through the sensor cable 64 to a readout. The readout is directly related to tissue pressure. Thus, the thermal diffusion monitors of the present invention can be made fully compatible with various types of pressure monitor systems. For fluid filled cavities, such as ventricles of the brain, it may be advantageous to use a pressure monitor with a fluid filled column.

The multiple parameter monitors usè-d in the -present SPTDFM are well-known, and some modification thereof might be utilized without ma~erial effect upon ~-the principle of the present invention. It should '~ " r`' ^` T'TI~T~

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suffice to indicate that the types of multiple parameter monitor utilized in preferred embodiments of this invention include those temperature, oxy~en and -~ -potential s.ensors (TOP Cat. No. M11199-19 probe) .'' , ST'T1JTE S~IE~T

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produced by OttoSensors Corporation, 11000 Cedar Avenue, Cleveland, Ohio 44106.

The support structure 26 for the SPTDFM 18 must be flexible, thermally inert, of a small size while still supporting the placement of multiple sensors and nonallergenic to biological tissues. Materials for the fabrication of the support structure 26 are well-known ;
and include various silicone based materials such as those used for medical catheters.

In operation in one embodiment as shown in FIG. 7, the SPTDFM probe 18 of the present invention is placed into the substance of the tissue (brain) 70 instead of onto the surface of the tissue. The tissues 72 covering the organ-brain are the skin and the bones of the skull. Hollow bolts 74 are used to hold the SPTDFM 18 in a stationary position in the tissue and permit exit of elsctrical wires 38 for attachment to external electrical components. In other embodiments involving only connective tissues, after the SPTDFM
probe 18 is inserted, the tissue can be closed around , the probe to provide anchoring instead of the use of ~.
bolts 74. Having the probe placed inside the tissue permits an easier method of sensor positioning and also - !
25 dramatically reduces the chance of a poor contact ;~
between the thermal sensor tip 32 and the tissue 70, which is the major source of error with the earlier surface devices.

To provide more accurate readings, by ~-eliminating chemical and thermal interferences from the support structure such as the catheter 30 and to permit the monitoring of a larger amount of tissue, a sensor tip 32 may be advanced through a side port 76 of a suitable catheter 30 (FIG. 8). In this design the g~ H~

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-10- , sensor tip 32 is in a measuring position, completely surrounded by tissue.

During insertion of the SPTDFM 18 into the tissue, tissue injury may occur as a result of the physical deformity which takes places as the SPTDFM 18 passes through the organ. This injury will cause the body to mount a localized increase in blood flow, which may lead to inaccurate blood flow determination. To minimize and possibly eliminate this potential error in measurement, an introducer 78 may be used (FIG. 9). The introducer 78 is essentially a cylindrical structure of greater diameter than the SPTDFM 18 itself. The -~
introducer 78 is first inserted into the tissue. Once ~ i 15 the introducer 78 is in position, the SPTDFM can be ~ -inserted through the center of the introducer. The introducer can then be removed and as the tissue returns ~-~
to its original position, the SPTDFM sites on the probe will be surrounded.

Those skilled in the art will fully appreciate that the preferred embodiment shown and desirable to illustrate the prPsent invention is exemplary only and that the same principles may be employed in providing a SPTDFM to monitor blood flow and simultaneously monitor pressure and critical physiological parameters. It will be further appreciated that various other modifications or changes, particularly with respect to probe construction, might be made without departing from the gist and essence of the invention. Accordingly, it ~`
should be further understood that the invention should be deemed limited only by the scope of the claims which follow and should be interpreted as encompassing all system constructions fairly regardable as functional equivalents of the subject matter to which claims are directed.

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Claims

Having described our invention, what we claim and desire to secure by letter patent is:

1. A single probe system useful for monitoring blood flow at a tissue comprises:
a support structure having a first side surface and a second side surface opposite to said first side surface, an anterior and a posterior surface connected to said first and second side surfaces;
thermal diffusion means housed in said support structure near said first side surface, said thermal diffusion means further comprises a conical hot film probe with a sensor tip contacting the tissue, wherein said sensor tip comprises a metal thin layer;
a backing material onto which said metal thin layer is deposited; and a protective coating deposited over said metal thin layer;
control means to heat said sensor tip as a heat source while simultaneously measuring a temperature correlating to a blood flow at the tissue;
pressure monitor means housed in said support structure near said anterior end and adjacent to said thermal diffusion means to monitor pressure at the tissue; and multiple parameter means housed in said support structure near said second surface and adjacent to said pressure monitor means to monitor the oxygen content, temperature and potential of the tissue.

2. The single probe system of claim 1, wherein further said pressure monitor means further comprises a pressure transducer having a movable diaphragm and strain gauge that displaced in accordance with the pressure exerted by the tissue.

3. The single probe system of claim 2, wherein said pressure monitor means further comprises a pneumatic circuit connected to said movable diaphragm and strain gauge to balance the displacement of said diaphragm and strain gauge by the pressure exerted by the tissue.

4. The single probe system of claim 2, wherein said pressure means further comprises a fiber optic photodetector system connected to said movable diaphragm and strain gauge to measure pressure exerted by the tissue;
said photodetector system comprises:
a light source, a fiber optic cable to carry a light from said source onto a reflective surface connected to said movable diaphragm at a first position;
a first reflected light reflected from said reflective surface onto a photodetector; and measuring means to measure a change of light intensity at said photodetector when a second reflected light reflected from said reflective surface when said movable diaphragm is at a second position in accordance with the pressure exerted by the tissue.

5. The single probe system of claim 1, wherein said support structure further comprises a silicone based catheter.

6. The single probe system of claim 5, wherein said catheter further comprises a side port for advancing said thermal diffusion flow monitor into the tissue.

7. The single probe system of claim 1, further comprises an introducer surrounding said support structure to reduce inaccuracy in blood flow measurements.

8. A single probe system useful for monitoring blood flow at a tissue comprises:
a support structure having a first side surface and a second side surface opposite to said first side surface, an anterior and a posterior surface connected to said first and second side surfaces;
thermal diffusion means housed in said support structure near said first side surface, said thermal diffusion means further comprises a conical hot film probe with a plurality of sensor tips contacting the tissue, wherein each sensor tip comprises a metal thin layer;
a backing material onto which said metal thin layer deposited; and a protective coating deposited over said metal thin layer;
control means to heat a first sensor tip of said plurality of sensor tips as a heat source while simultaneously measure temperature drop of sensor tips other than said first sensor tip of said plurality of sensor tips at distance of multiples of d1 from said first sensor tip, and correlating said temperatures to blood flows at the tissue;

pressure monitor means housed in said support structure near said anterior end and adjacent to said thermal diffusion means to monitor pressure at the tissue; and multiple parameter means housed in said support structure near said second surface and adjacent to said pressure monitor means to monitor the oxygen content, temperature and potential of the tissue.

9. The single probe system of claim 8, wherein further said pressure monitor means further comprises a pressure transducer having a movable diaphragm and strain gauge that displaced in accordance with the pressure exerted by the tissue.

10. The single probe system of claim 9, wherein said pressure monitor means further comprises a pneumatic circuit connected to said movable diaphragm and strain gauge to balance the displacement of said diaphragm and strain gauge by the pressure exerted by the tissue.

11. The single probe system of claim 9, wherein said pressure means further comprises a fiber optic photodetector system connected to said movable diaphragm and strain gauge to measure pressure exerted by the tissue;
said photodetector system comprises:
a light source, a fiber optic cable to carry a light from said source onto a reflective surface connected to said movable diaphragm at a first position;
a first reflected light reflected from said reflective surface onto a photodetector; and measuring means to measure a change of light intensity at said photodetector when a second reflected light reflected from said reflective surface when said movable diaphragm is at a second position in accordance with the pressure exerted by the tissue.

12. The single probe system of claim 8, wherein said support structure further comprises a silicone based catheter.

13. The single probe system of claim 12, wherein said catheter further comprises a side port for advancing said thermal diffusion flow monitor into the tissue.

14. The single probe system of claim 8, further comprises an introducer surrounding said support structure to reduce inaccuracy in blood flow measurements.

15. A single probe system useful for monitoring blood flow at a tissue comprises:
a support structure having a first side surface and a second side surface opposite to said first side surface, an anterior and a posterior surface connected to said first and second side surfaces;
thermal diffusion means housed in said support structure near said first side surface, said thermal diffusion means further comprises a conical hot film probe with a plurality of sensor tips contacting the tissue, wherein each sensor tip comprises a metal thin layer;
a backing material onto which said metal thin layer deposited; and a protective coating deposited over said metal thin layer;
control means to heat at least two sensor tips of said plurality of sensor tips as heat sources while simultaneously measure temperature drop of sensor tips other than said at least two sensor tips of said plurality of sensor tips at distance of multiples of d2 from one of said at least two sensor tips;, and correlating said temperatures to blood flows at the tissue;
pressure monitor means housed in said support structure near said anterior end and adjacent to said thermal diffusion means to monitor pressure at the tissue; and multiple parameter means housed in said support structure near said second surface and adjacent to said pressure monitor means to monitor the oxygen content, temperature and potential of the tissue.

16. The single probe system of claim 15, wherein further said pressure monitor means further comprises a pressure transducer having a movable diaphragm and strain gauge that displaced in accordance with the pressure exerted by the tissue.

17. The single probe system of claim 16, wherein said pressure monitor means further comprises a pneumatic circuit connected to said movable diaphragm and strain gauge to balance the displacement of said diaphragm and strain gauge by the pressure exerted by the tissue.

18. The single probe system of claim 16, wherein said pressure means further comprises a fiber optic photodetector system connected to said movable diaphragm and strain gauge to measure pressure exerted by the tissue;
said photodetector system comprises:
a light source, a fiber optic cable to carry a light from said source onto a reflective surface connected to said movable diaphragm at a first position;
a first reflected light reflected from said reflective surface onto a photodetector; and measuring means to measure a change of light intensity at said photodetector when a second reflected light reflected from said reflective surface when said movable diaphragm is at a second position in accordance with the pressure exerted by the tissue.

19. The single probe system of claim 15, wherein said support structure further comprises a silicone based catheter.

20. The single probe system of claim 19, wherein said catheter further comprises a side port for advancing said thermal diffusion flow monitor into the tissue.

21. The single probe system of claim 15, further comprises an introducer surrounding said support structure to reduce inaccuracy in blood flow measurements.

22. A thermal diffusion means useful for monitoring blood flow at a tissue comprises:
a conical hot film probe with a sensor tip contacting the tissue, wherein said sensor tip comprises a metal thin layer;

a backing material onto which said metal thin layer deposited;
a protective coating deposited over said metal thin layer; and control means to heat said sensor tip as a heat source while simultaneously measure a temperure correlating to a blood flow at the tissue;

23. A thermal diffusion means useful for monitoring blood flow at a tissue comprises:
a conical hot film probe with a plurality of sensor tips contacting the tissue, wherein each sensor tip comprises a metal thin layer;
a backing material onto which said metal thin layer deposited; and a protective coating deposited over said metal thin layer;
control means to heat a first sensor tip of said plurality of sensor tips as a heat source while simultaneously measure temperature drops of sensor tips other than said first sensor tip of said plurality of sensor tips at distance of multiples of d1 from said first sensor tip, and correlating said temperatures to blood flows at the tissue.

24. A thermal diffusion means useful for monitoring blood flow at a tissue comprises:
a conical hot film probe with a plurality of sensor tips contacting the tissue, wherein each sensor tip comprises a metal thin layer;
a backing material onto which said metal thin layer deposited; and a protective coating deposited over said metal thin layer;

control means to heat at least two sensor tips of said plurality of sensor tips as heat sources while simultaneously measure temperature drop of sensor tips other than said at least two sensor tips of said plurality of sensor tips at distance of multiples of d2 from one of said at least two sensor tips, and correlating said temperatures to blood flows at the tissue.

25. A method of monitoring blood flow at a tissue comprising the steps of:
cutting a skin incision of about 1 cm in a protective covering of the tissue;
inserting a single probe system of claim 1 into the tissue;
heating said sensor tip as a heat source while simultaneously measuring a temperature of the tissue and correlating said temperature to a blood flow at the tissue.

26. A method of monitoring blood flow at a tissue comprising the steps of:
cutting a skin incision of about 1 cm in a protective covering of the tissue;
inserting a single probe system of claim 8 into the tissue;
heating a first sensor tip of said plurality of sensor tips as a heat source while simultaneously measuring temperature drops of sensor tips other than said first sensor tip of said plurality of sensor tips at distance of multiples of d1 from said first sensor tip and correlating said temperatures to blood flows at the tissue.

27. A method of monitoring blood flow at a tissue comprising the steps of:
cutting a skin incision of about 1 cm in a protective covering of the tissue;
inserting a single probe system of claim 15 into the tissue;
heating at least two sensor tips of said plurality of sensor tips as heat sources while simultaneously measuring temperature drops of sensor tips other than said at least two sensor tips of said plurality of sensor tips at distance of multiples of d2 from one of said at least two sensor tips;, and correlating said temperatures to blood flows at the tissue.