WO1995019138A1

WO1995019138A1 - Blood flow measurement device

Info

Publication number: WO1995019138A1
Application number: PCT/EP1995/000134
Authority: WO
Inventors: Branko Breyer; Bozidar Ferec-Petric
Original assignee: Pacesetter Ab
Priority date: 1994-01-14
Filing date: 1995-01-13
Publication date: 1995-07-20
Also published as: JPH09511411A; US5785657A; EP0739183A1; HRP940025A2

Abstract

A blood flow velocity measurement device is devised where there exist such entities within the fluid which are detectable when axially flowing (passing by) an appropriate detector (35) of known and well defined dimensions mounted onto a catheter (11). The said entities produced for instance by a generator (34), when flowing by the said detector (35), induce a known single response, the response bearing direct correlation to the flow velocity (V), in the form of direct reciprocity to the said velocity and direct proportion to the length of the sensitive length of the said detector. Autocorrelation of the function obtained by the overlap and pile-up of successive events is calculated and from the characteristic points in the said autocorrelation function the axial velocity is inferred. The measurement is best when the probing beam is perpendicular to the flow.

Description

Blood flow measurement device

Field of invention

The invention pertains to medical technology. It relates to fluid flow measurement in the field of medicine, and in particular to a blood flow measurement device.

Background and prior art

Fluid flow measurement plays an important part in modern medicine. This applies both to pure diagnostic procedures as well as to direct control of disease treatment. The various fluids which are, in this sense, of interest in the human body are very different in properties and include blood, pus, urine, inhaled and exhaled air, drug infusions, etc. In all the cases there exists a possibility of measurement of the flow using the Doppler effect provided there exist a wave reflecting or transmitting entity within the said liquid. A basic property of the Doppler effect is that it takes place if there exists a velocity along the line connecting the wave transmitter and its receiver. For plane waves, the Doppler effect equals zero if the probing waves are at 90° to the direction of movement of the said fluid. This means that only the velocity component projected to the line connecting the wave transmitter and its receiver is measured. This limitation applies strictly only to plane waves, while for bundled waves (beams) one can theoretically obtain a low frequency Doppler effect due to the fact that the wave fronts in a beam are not plane (Censor D., W.L. Newhouse, T. Wonz, H.V. Ortega: Theory of Ultrasound Doppler Spectra Veloci- metry for Arbitrary Beam and Flow Configurations, IEEE Trans, on Biomedical Enginee¬ ring, 35 (1988)740). This, however, is a part of frequency spectrum which is fully within the Doppler shift frequency spectrum of other movements in the body, such as the move¬ ment of the blood vessel walls, movement of the liver due to the heart beats, breathing movements, etc. This makes the cited phenomenon hard (and often impossible) to use, and in particular to use when the measurement process is unattended by interpreting humans (i.e. is used for some automatic regulation). Cross correlation techniques for velocity measurement exist in the form of wave transmitter- receiver array systems where the correlation among different lines-of-sight is used to obtain the velocity information. Due to the intrinsic physical impossibility to reduce the size of such a system the measure- ment is slowed down, thus correctly yielding only the low range of velocities encountered in the body. In addition, to the said velocity limitation the said cross correlation method is limited to pulsed wave mode in the case when velocities axial to the measurement device are measured. (Kasai,C , K. Namekawa: Real-time two-dimensional blood flow imaging using an autocorrelation technique, Proc. IEEE Ultrasonic Symposium 1985, IEEE, p. 953). While this approach yields Doppler equivalent results when used along the scanner line-of-sight and can even measure slow perpendicular velocities, it always requires full arrays of multiple transducers and is problematic in the measurement of the high velocities perpendicular to probing beams. In EP 228 070 assigned to Aloka Co. Ltd. the problem of the knowledge of the angle has been partially solved by using two probing beams under slightly different angles. The method requires fairly elaborate electronic systems and is usable only when the acoustic window is large enough to accommodate for the said two beams. In EP-144 968 and JP-228 330 the time domain calculation is adjusted for simpli¬ fied calculation of the effects along the line-of-sight, yielding a Doppler-like result. The time domain calculation method has further been described in EP-92841, JP-070479 and US-4573477 with the advantage of increasing and optimizing the sampling rate of the Doppler equivalent on-line measurement.

A larger number of prior inventors concern themselves with using the in-phase and qua¬ drature detection components for further correlation calculations with improved results in Doppler-like procedures. This applies to EP-447597 which improves the measurable velocity by using multiple measurements and particular inter correlation calculations. In FR-2551213 the two quadrature demodulation components are used with two auxiliary oscillators to obtain a Doppler equivalent result. In EP-266998 and US-4790323 the quadrature detection components are on-line compared and yield a better turbulence estimation in Doppler measurements. Yet other methods which use correlation calculations have been used for different purposes and ways. For example in DE-3544477 and GB-2170972 the autocorrelation calculation between the two Doppler shift components are used to reduce noise. Quadrature detectors and mixing oscillators in conjunction with autocorrelation calculation are used in EP- 140726 to optimize the number of samples and adjust them to flow velocity.

Calculations and measurements including transit time may be used to obtain the Doppler measured velocity profile as in EP- 150672 where two beams at different angles are used to ascertain the position of the measurement volume and profile assessment. Other principles, such as a heating and cooling measurement system as in WO-9215239 are used for instantaneous velocity and viscosity measurement (this is, however, a fairly slow process).

Another possibility, to measure fluid velocity is the transit time method which does not rely on the scatter of ultrasound from red blood cells, but on the fact that ultrasound pro¬ pagates through moving medium at a different speed compared to the speed in a still me¬ dium. Instruments based on this phenomenon have been described in a number of publis¬ hed work on flow measurement devices ( e.g. potentially implantable devices like one des¬ cribed by Franklin, D.L., Baker, D.W., Rushmer, R.F. : Pulsed ultrasonic transit time flowmeter, IRE Trans. Bio-Med. Electron. 9: p.44, 1962; dual frequency devices, e.g. Noble F.W. : Dual Frequency Ultrasonic Fluid Flowmeter, The Rev. of Scientific Instru¬ ments, 39, no. 9, (1968), p. 1327 and intravascular, implantable devices have been des¬ cribed using the same principle Plass,K.G. : A New Ultrasonic Flowmeter for Intravascular Applications, IEEE Trans, on Bio-Med. Eng. , BME-20, no. l, (1973), p.154, and finally, particular types of interferometry , the phase-shift of upstream and downstream propagating ultrasound has been described in ZarnstorffW.C , Castillo, C.A. , Crumpton, C.W. : A Phase-Shift Ultrasonic Flowmeter, IRE Trans, on Bio-Med. Electronics, 9, (1962), pp 199-203). This approach in spite of being essentially independent on the red blood cell density, has been abandoned when the sensitivity of ultrasound transducers and preampli- fiers was improved enough in order to use the Doppler effect. The main reason was apparently that the said transit time devices always had to use two opposite positioned transducers, making them less practical for implantation and usually useless for application from the body surface. Although this approach is now very rarely used, its potentials wait to be reevaluated with the improved materials technology. Some of such a development can be seen in US-4227407 to Drost who uses the previously described transit-time phe¬ nomena for interferometric blood flow measurement, with the disadvantage of having to either exactly know the dimensions of the blood vessel or to have two implanted transdu¬ cers.

Therefore it is an object of the present invention to provide an improved blood flow measurement device particularly suitable for cases when the angle between the flow and the wave beam or other sensing directivity functions used for the measurement approaches 90° or is exactly 90° and when the velocity direction is known or immaterial but the measurement of high velocities is important.

This object is attained by the blood flow measurement device according to claim 1. Preferred embodiments are described in the dependent claims.

Summary of the invention

A blood flow velocity measurement device is devised where there exist such entities within the fluid which are detectable when axially flowing (passing) by an appropriate detector of known and well defined dimensions. The said entities, when flowing by the said detector, induce a known single response, the response bearing direct correlation to the flow velocity, in the form of direct reciprocity to the said velocity and direct proportion to the length of the sensitive length of the said detector. Autocorrelation of the function obtained by the overlap and pile-up of successive events is calculated and from the characteristic points in the said autocorrelation function the axial velocity is inferred. The measurement is best when the probing beam is perpendicular to the flow. An undirectional (single-side-band) device for measuring fluid flow velocity at 90° has been invented. The device applies to any fluid which contains elements (further called scatterers) which can be detected as distinctive entities by some method with well defined directivity characteristics. The device consists of a detector (or detector set) capable of detecting the said entities as they enter the sensitivity volume (within the sensitivity characteristic) as well as the moment when they leave the sensitivity volume (further called beam). The velocity of the fluid which passes through the sensitive volume (crossing it by its width) is the ratio of the sensitivity volume width and the time spent within it. For the normal case when the number of scatterers is so large that their crossing times overlap, the resulting signal is a combination of overlapped single crossing characteristics. The infor¬ mation about the crossing velocity is extracted by on-line calculation of the autocorrelation function of the described function of the overlapped crossing signals of a multitude of scatterers. The information of the single crossing time across the beam is, depending on the signal processing type, the delay time at the autocorrelation function zero crossing or its negative minimum (for positive crossing signals). The type of scatterers and the type of beam are basically irrelevant, but include pulsed wave and continuous wave ultrasound with ultrasound scatterers, light waves and light scatterers, detectors of ion clusters and ion clusters in liquid, magnetism detectors (inductive or otherwise) with ferromagnetic particles dispersed in the liquid, etc.

Short description of figures

Fig. 1 represents a general situation where a sensing beam is transmitted into flowing fluid at a general angle and where the same transmitter is used for reception of the reflected waves.

Fig. 2 represents a block diagram of a device capable of using the data from the waves incident from flowing fluid for flow measurement as per Fig. 1.

Fig. 3 represents the information flow diagram implemented when all or a part of the functions are performed with a computer (digital or analogue).

Fig. 4 represents a perspective illustration of an ultrasonic transmitter receiver trans¬ ducer mounted onto an intraluminal catheter.

Fig. 5 represents an illustration of two piezoelectric transducers mounted onto a catheter.

Fig. 6 represents an illustration of paired wave (piezoelectric or light) transducers mounted onto a catheter.

Fig. 7 represents a perspective illustration of a general generating device means mounted on a catheter means capable of generation and/or injection into flowing fluid of entities.

Fig. 8 represents a three projection illustration of the device illustrated in Fig. 7 with connection leads means.

Fig. 9 is an illustration of one embodiment of a device capable of detecting entities with magnetic polarization properties.

Fig. 10 represents an illustration of a light transmitting- receiving matrix device.

Fig. 11 shows the heart having implanted a lead comprising the aortic flow measure- ment assembly.

Fig. 12 shows the caudal view on the heart illustrating the aortic flow measurement.

Detailed description of the embodiments Referring to Fig. 1 we can describe the basic principle of the velocity measurement device as follows. The beam 1 shown here is in fact a sensing directivity characteristic of a general probing device 3 which may be attained with or without the use of any sort of waves. A detectable entity (e.g. a scatterer of ultrasound waves) passes the area occupied by the sensitivity characteristic 1, e.g. an ultrasound beam, at the point where the width of the beam equals d. The scatterer passes at a general angle φ. The velocity V of the scatterer can be divided into components V_h and V_v. The velocity component V_h equals to the width d divided by the time the scatterer spends within the width d. The time spent within the beam can be measured by electronic circuits which detect the presence or the absence of the scatterer within the beam. The velocity component V_v can not be measured in this way. Let it be noted that this measurement method yields a means of measuring the velocity component perpendicular to what is possible with the Doppler method, i.e. the Doppler effect for plane waves equals zero if the velocity is purely V_h. Unless the scatterer velocity is exactly along the beam axis, the total velocity can be calculated from the, thus measured, velocity V_h by division with the sine of the angle φ. It is important to note that in case of one single beam the direction of the, thus measured velocity V_h can not be inferred from this measurement. On the other hand this method yields the absolute value of the said velocity component in the physically fastest possible way.

Now, if there are more than one scatterers passing through the beam, and if their appea¬ rance is dense enough, the signals which signal their presence within the beam may over¬ lap. In fact, if there is a large multitude of such scatterers, the individual features of the signal as individual scatterers enter and leave the beam will apparently be lost within the pile-up variable signal.

However, the autocorrelation function of such a pile-up process must by mathematical laws contain a characteristic point, a discontinuity or extreme at the autocorrelation delay time equal to the time needed for individual scatterers to pass across the beam width d. The autocorrelation function is defined as the stochastic expectation of the values of a random process that are the delay time T apart and, which are multiplied by each other. (A. Papoulis: Probability, Random Variables and Stochastic Processes; McGraw Hill Inc. , 1965. , page 359 ).

Therefore, referring to Fig. 2, the signals detected from the flow of a scattering medium are fed to an autocorrelation function calculator (digital or analogue). The time period until the occurrence of the characteristic point, normally the first minimum of the autocor¬ relation function is taken to be of the value equal to the time needed by die said scatterer to cross the distance d. Velocity is calculated by division of the width d of the said beam 1 with the thus measured time of occurrence of the characteristic point.

1 These and the following calculations and evaluations may be carried out in a suitable in¬ formation processing device and be implemented by hard- and/or software, or by hardware alone.

Referring to Fig. 3, a flow diagram for the calculation is devised as follows: The signal reflected from the flow area of interest in the flow 2 is input in real time. The signal is preprocessed, e.g. differentiated, frequency filtered, etc. Autocorrelation or autocovariance are calculated on the inflowing data. A timer circuit induces evaluation of thus gathered autocorrelation function and the search for the characteristic point, i.e. the first disconti- nuity which occurs at the point where the delay T is equal to the transition time of a single scatterer across the beam 1 of Fig. 1. The beam width d is divided by the resultant time which yields the required velocity component V_h. For single scatterer signal starting with a positive rise and ending with a decrease to initial level, the said characteristic delay appears at the delay point where the negative slope of the autocorrelation function changes the slope from the initial negative slope to zero or positive slope. Thus the characteristic delay can be detected by detecting this change in the slope of the autocorrelation function.

Referring to Fig. 4 showing the preferred embodiment of the invention (best mode), a de¬ vice 101 generating a sensitivity area 102 is mounted onto a catheter means 11, the said sensitivity area having a characteristic axial (for the catheter) dimension d. The sensitivity characterisitc is for simplicity shown as square, although it can have any physical form characteristic of wave beams created by directional antennas. The catheter means 11 can have a lumen 12 to accommodate for different functions including that of bringing to and taking away of signals for the device 101. The device 101 comprises an ultrasonic trans- mitter receiver transducer which can be used for echo detection of particles flowing parallel to the catheter axis and crossing the sensitivity area 102. The same said device serves both as the transmitter and as the receiver of the waves. Width d is the characteris¬ tic dimension which appears in the velocity calculations.

Fig. 5 represents an illustration of two piezoelectric transducers mounted with one of their dimensions, preferably with their longest dimensions parallel onto a catheter where one of them is used as continuous wave transmitter and the other as a continuous wave receiver of waves scattered from the axially flowing fluid from within the sensitive area 18 which is the cross section of the transmission characteristic 94 and reception characteristic 95 obtained by tilting the beams from the said transducers by die use of tilting devices (lenses) 97 and 98.

Referring to Fig. 5, we can define a sensitive area in the sense of the previous text by ap¬ plication of two cylindric or otherwise formed piezoelectric transducers 91 and 92 which have glued on or otherwise fixed ultrasound beam tilting lenses 98 and 97 respectively which tilt the directivity characteristics 94 and 95 respectively, of the two devices in such a way as to overlap within an area 18. This area 18 has the properties of the generally outlined area 102 from Fig. 4 or the generalized beam of Fig. 1. The sensitivity area can be obtained by continuous transmission of waves from one of the said transducers (e.g. 91) and reception of the scattered waves by the other of the transducers (e.g. 92). The transmission and reception can be continuous or synchronized pulsed transmission and reception. The electrical signals needed to actuate the said transducers are fed to them and taken from them to appropriate electronic circuits by conductors built into the catheter means 11 body (not shown). Fig. 6 represents an illustration of paired wave (piezoelectric or light) transducers (14 and 15 in case A and of a different form 16 and 17 in case B) mounted onto a catheter, arranged differently from those from Fig. 5, where one of them is used as continuous wave transmitter and the other as a continuous wave receiver of waves scattered from the axially flowing fluid. The directivity characteristics in case A are designated by 25 and 24 and in case B by 26 and 27, while their intersections are designated by (18) and (19) respectively. The two sets (embodiments) of transmitters and receivers of waves (light or ultrasound) are positioned in such a way as to enable one of the devices to transmit the said waves into d e scattering medium and the other device to receive the scattered waves from the area where the two directivity functions overlap.

Referring to Fig. 6 more in detail, we see an illustration of a catheter means 11 with a lumen 12 with two embodiments of light transmitter- receiver sets, A and B. In embodi¬ ment A the transmitter and the receiver of light waves 14 and 15 respectively are prefera- bly focussed and their directivity characteristics 24 and 25 respectively overlap at an area 18. In the embodiment B the wave transmitter 16 and the receiver 17 have been shown of a square form with directivity characteristics 26 and 27 overlapping within an area 19 of an essentially square form. The essentially square form of the sensitivity area has the property of having equal transit lengths for all the scatterers flowing within the fluid around the said cati eter means in axial direction relative to the said cad eter means. The light transmitter and receiver have the property of being able to transmit and receive the light waves.

Referring to Fig. 7, there is an illustration of the said cadieter 11 having mounted thereon a general property generator 34 capable of generation and/ or injection into flowing fluid of entities (e.g. ions or magnetic dipoles) detectable within sensitivity characteristics 45 of reception devices means 35 and connected to outside circuits via leads put into me lumen 12 of the catheter means. This Fig. represents an illustration of an apparatus in which one device generates a detectable property in the liquid, (e.g. ionizes it), and the other detector means measures the amount of the axially passing detectable entities (e.g. ion clusters) using the method of extracting the velocity data outlined in Fig. 1. The generator and the two general property detectors 35 have respective directivity functions 44 and 45. The property (e.g. ionization, magnetization or the like) is imposed onto the particles flowing in the liquid at velocity V and detected within the sensitivity area 45 of the property detectors. The device principle outlined in the description of Fig. 1 to 3 is then applied to d e signal thus obtained.

Referring to Fig. 8, which is a three projection illustration of the device from Fig. 7, illu¬ strating that the said generation means 34 and the said reception means 35, respectively shall be connected to the proximal side of the said catheter 11 using general conductors 51, 52, 53 led through the lumen 12 and connected or connectable to electronic circuits performing the operations outlined in Fig. 2 and 3.

Fig. 9 is an illustration of one embodiment of a device capable of detecting entities with magnetic polarization properties flowing axially with regard to the catheter means 11 by virtue of orienting the said magnetic dipoles between me south S and north N pole of a magnet with a gap 75 between them and of a length L and by detecting the change in magnetization of the dipoles by sensing coils 71 and 72.

In diis embodiment of a magnetic detector of magnetizable entities flowing axially with the liquid parallel to dimension L in Fig. 9 such magnetizable entities (e.g. particles) abruptly change their magnetic orientation when entering the magnetic field between the south S and north N pole of a magnet where the distance among me said pole pieces is known and defined 75. The sudden change of the magnetic polarization and depolarization at the entrance and at the exit can be detected by said detection coils 71 and 72 and fed to signal processing systems according to Fig. 2 and 3 in order to calculate the axial velocity of the said flowing particles.

Fig. 10 represents an illustration of a light transmitting- receiving matrix device where on a support plate 201 light transmitters 202 and light receivers 203 are arranged in a dense way such as to yield a homogeneous light field in front of the device and the scattering of the light waves by scatterers flowing in front of the device are detected continuously by receivers 202 which jointly act as one receiver of the length L and width W. Distances 205 and 206 can be made as small as necessary.

Referring to Fig. 10 more in detail, die composite light transmitter-receiver device as seen from the front, comprises light transmitting devices 202, e.g. LEDs and light reception devices 203, e.g. photo transistors packed densely enough (with distances 205 and 206 small enough) to act as a joint transmitter-receiver device detecting light scatterers as they pass in front of the device. The devices 202 and 203 are preferably defocussed in such a way as to create a quasi continuous field of light in front of the whole device. If the flow is parallel to the dimension L men this is the characteristic dimension in me sense of calculation outlined in Fig. 2 and 3, and the same relates to dimension W if the flow is parallel to it.

Fig. 11 and 12 disclose the principle of aortic flow measurement by means of the transdu¬ cer assembly disclosed in previous Fig. and relevant description. The aortic flow measu¬ rement can be utilized for cardiac electrotherapy control. Disclosed measurement system is feasible to be incorporated within an implantable electrotherapy device.

As it is known in the art, every ventricular contraction produces the aortic flow wave. Accordingly, the stroke volume can be calculated from the waveform of the aortic flow, assuming that the aortic cross-section area is a known previously measured parameter. Therefore this device can be used for rate variation in rate responsive pacing, pacing cap- ture monitoring, tachycardia detection as well as differentiation, ventricular fibrillation de¬ tection, and left ventricular systolic myocardial function estimation. These physiological principles were disclosed in numerous prior art of cardiology and echocrdiography.

Fig. 11 shows the heart opened at the right atrial appendage 161. There are tricuspid valve 162, fossa ovalis 163, coronary sinus valve 164 and crista terminalis 165 within the right atrium. The vena cava superior 166 and the vena cava inferior 167 as well as the pulmonary artery 168 and the aorta 169 with truncus pulmonalis 170 are disclosed. The left atrium 171 with right superior pulmonary vein 172 as well as with right inferior pulmonary vein 173 are shown. The right ventricular apex 174 is disclosed as well as the residue of the pericardium 175. The pacemaker lead 176 is implanted through the vena cava superior 166 and right atrial cavity through the tricuspid valve 162 in the right ventricle with its tip (not shown) in the area of apex 174. The lead 176 comprises an ultrasonic transducer assembly 177 which produces the measurement ultrasonic field 178 directed towards the aortic arch 179.

Fig. 12 shows the caudal view on the heart having the analogous designations for same elements which are shown on previous Fig. 11. The lead 176 is implanted through the superior vena cava 166 as disclosed on this axial view. As it is clearly demonstrated in this projection, the ultrasonic measurement field 178 produced by transducer assembly 177 is directed towards the aortic arch 179. Ideal situation is disclosed, whereby ultrasonic beam is perpendicular towards die aortic flow.

Claims

1. Blood flow measurement device for measuring a flow (V_h) perpendicular to a probing beam, comprising a catheter means (11) which can be inserted into blood vessels in the body, at least one detecting device means (101) mounted onto the cadieter means at some position along its length in such a way as not to impede its indwelling into the blood vessel, said detecting device means having a defined and limited directivity characteristic capable of detecting entities flowing within the said directivity characteristic (102), and generating detection signals corresponding to a detection, electronic circuitry adapted to measure and perform time autocorrelation calculations on the signals generated by said detecting device means, and evaluation means responsive to calculation data signals output by said electronic circuitry and adapted to determine a velocity of said flow therefrom.

2. Device according to claim 1, wherein said detecting device means comprises at least one out of an ultrasonic transducer (101) capable of pulse-echo operation, an ultrasonic transducer pair (91, 92) capable of continuous wave operation, ion generator means (34) capable of generating ions within the flowing blood, ion detector means (35) capable of detecting the density of ions in front of itself, and magnetic polarization detector means (61, 62; 71, 72) capable of detecting magnetically polarized entities in front of itself.

3. Device according to claims 1 or 2, wherein said catheter means comprises electrical conductors (51-53) running along said catheter means to the proximal side thereof.

4. Device according to claim 3, wherein said conductors are terminated with electrical connector means at the proximal side of said catheter means.

5. Device according to claim 2, wherein said ultrasonic transducer means includes piezoelectric transducers of different geometrical forms which yield a well defined directivity characteristic (102), said ultrasonic transducer means being mounted onto the catheter (11).

6. Device according to claim 5, wherein the piezoelectric transducer directivity characteristic axis runs essentially perpendicular to said catheter means axis.

7. Device according to claim 2, characterized by one or a multitude of piezo¬ electric transducers capable of pulse-echo operation sensitive enough to detect ultrasound scattered from erytrocites.

8. Device according to claim 2, characterized by one or a multitude of paired piezoelectric transducers (91, 92) whose directivity characteristics (94, 95) overlap and cross within a limited and well defined volume (18) outside of the said cadieter means, and with one of their dimensions parallel to the said cadieter means.

9. Device according to any of claims 1 to 8, characterized by one or a multi¬ tude of paired light transducers (14, 15; 16, 17) whose transmission and reception directi¬ vity characteristics (24, 25; 26, 27) respectively overlap and cross within a limited and well defined volume (18; 19) outside of the said cadieter means.

10. Device according to any of claims 1 to 9, characterized by a ferromagnetic particle detection device of a well defined lengd capable of detecting presence or absence of one or a multitude of ferromagnetic particles in front of it.

11. Device according to claim 10, wherein said ferromagnetic particle detection device contains a magnetic polarization north-south piece pair (62, 61, S, N) creating a magnetic field (75) and a pair of detection windings (71, 72) which detect a change in the magnetic field when me said magnetizable (ferromagnetic) particles enter and leave the region of the said magnetic field, respectively.

12. Device according to any of claims 1 to 11, characterized by a surface (201) light transmission-detection device comprising a dense multitude of light sources (202) and detectors (203) with directivity characteristic overlapping at a defined height above the said surface, and each having essentially an inverse square sensitivity characteristic with distance.

13. Device according to any of claims 2 to 12, wherein said electronic circuitry is arranged at the proximal side of said catheter means and is adapted to generate electric signals which activate the respective transducers as well as to detect electric signals generated by said transducers.

14. Device according to claim 13, wherein said evaluation means is arranged at the proximal side of said catheter means.

15. Device according to any of claims 1 to 14, wherein said evaluation means is adapted to detect a predetermined feature (e.g. minimum, zero crossing, inflexion, etc.) in said calculation data signals, thus obtøining a characteristic delay time within an autocorrelation function.

16. Device according to claim 15, characterized in that said evaluation means comprises dividing means adapted to essentially divide a sensitivity field lengdi of said detecting device means (101) with the characteristic delay time.

17. Device according to claim 16, characterized by the display and/or transmit¬ ting means for displaying or otherwise further conveying a result of the division for further use as a number proportional to the velocity of the medium flowing acmally along the catheter means.