the block circuit diagram in fig1 shows the detection part of a device for embolism detection in accordance with a first preferred embodiment of the invention ( best mode ). the arrangement has two mutually independent probe channels ( indicated in the figure by the indices “ a ” for the first channel and “ b ” for the second channel ) which respectively generate separately in themselves an evaluatable ultrasonic doppler reception signal , which is then used in the procedure according to the invention for embolism detection or distinction . a first probe 10 a is connected on the transmitting side to a first multiplexing and transmitting unit 12 a which is designed for current frequencies of ultrasonic sonography , for example 2 , 4 , 8 or 16 mhz , and which is connected to an oscillator module 14 a disposed upstream thereof . on the receiving side an output signal of the first probe 10 a is applied to a first multiplexing and pre - amplifier unit 16 a , is subsequently processed by a first mixer and demodulator unit 18 a , and is passed to a first ( programmable ) high pass filter 20 a . in the arrangement shown in the block circuit diagram , the filter 20 a is followed by a first adjustable amplifier 22 a (“ programmable gain ”) whose output signal is processed by a first sample - and - hold ( s & amp ; h ) circuit 24 a . the signal produced in that way is then applied on the one hand to a first audio filter 26 a for signal output for an embolism signal which is to be acoustically evaluated , and on the other hand a common fft - processing unit 28 ( fft = fast fourier transformation ) receives the output signal of the first sample - and - hold circuit 24 a for processing of the visual representation of the doppler signal as a spectral representation in relation to time . the fft - processing unit 28 has a common detector unit 30 connected downstream thereof . the parallel second channel which is connected to the second probe 10 b is of a structure corresponding to the above - described first channel , more specifically by means of a second transmitting unit 12 b which is also controlled by the oscillator 14 , and on the receiving side by means of multiplexer / pre - amplifier unit 16 b , mixer 18 b , high pass filter 20 b , amplifier 22 b and sample - and - hold circuit 24 b whose signal is again passed on the one hand to a second audio filter 26 b and on the other hand is also applied to the common fft - processing unit 28 . a common control unit 32 which is implemented by means of a microcontroller provides on the one hand for control of the first and second high pass filters 20 a , 20 b , the first and second intermediate amplifiers 22 a , 22 b and the two sample - and - hold circuits 24 a , 24 b and in addition provides for control of the oscillator unit 14 . the latter in turn controls the mixer units 18 a , 18 b provided for quadrature demodulation . in addition , associated with each probe 10 a , 10 b is an identification signal line 34 a , 34 b for the control unit 32 . with the exception of the common fft - processing unit 28 and the detector unit 30 , the circuit which has been described hereinbefore is a current product , as is offered by the applicants for example under the name “ multi - dopp ” as a two - channel system for medical ultrasonic diagnostics . in a manner which is advantageous in accordance with the invention however , the invention provides that drawing a distinction between an embolism and an interference signal to be suppressed , for example an artefact , is effected in the detector unit 30 , by evaluation of a respective reception signal — in the illustrated embodiment , after a fourier transformation operation , that is to say in relation to a signal in the frequency region . in the manner which is advantageous according to the invention , the detector unit 30 is designed as an electronic unit for digital signal processing , in such a way that an artefact which is expressed in both channels as a substantially simultaneous , marked rise in amplitude of the reception signal , is recognised as such by virtue of the relationship in respect of time between the channels and consequently no embolism detection signal is outputted on a detector signal output 36 . to put this more precisely , for example a movement of the probes which can be secured by means of a suitable support unit to the human body , for example on the head , results in an unwanted , high - level reception signal , caused by such movement , in both probes , occurring practically simultaneously ( or within a time interval of less than 3 ms ). the reception signal in both channels , which is further processed in accordance with the processing units in the block circuit diagram of fig1 is then to be identified as simultaneous by the common detector unit , whereupon then the conclusion is drawn that an artefact is involved . in comparison for example the occurrence of an embolism in a detection region of the first probe ( which is suitably directed for example onto a region of a blood vessel ) would result in a ( high - level ) signal while the second probe which is directed onto another region of the blood vessel , onto a region of tissue or bone outside or however onto another vessel ( or which for example in the case of measurement on the head is disposed on the opposite end ) would not detect the embolism detected in the first channel . for that reason , a detection signal of high amplitude admittedly occurs in the first channel but not in the second channel ( or , in the case of spatial displacement of the two detection regions of the probes in the same vessel , at a time interval corresponding to the transportation speed in the flow of blood ). in a corresponding manner it is possible by means of the detector unit to establish that a characteristic rise in signal has not occurred at the same moment in time in both channels so that it is possible to conclude that an embolism is involved . as fig1 shows fft - outputs 38 a , 38 b are provided for subsequent visual signal representation of the transformed signals of the signal channels which are designed in the known and usual manner , and in addition an operator handling the device can acoustically monitor the respective channel by way of audio outputs 40 a , 40 b of the first and second audio filters 26 a , 26 b respectively . an additional gate and multiplexer unit 42 which is shown in broken line and which is connected downstream of the second pre - amplifier unit 16 b implements a development of the embodiment shown n fig1 : more specifically , the additional unit 42 , as indicated by the further line shown in broken form , also receives the output signal of the first pre - amplifier 16 a , triggering the function of the gate and multiplexer unit 42 . in accordance with the output signal of the first pre - amplifier 16 a , the unit 42 produces a certain time delay in regard to sampling for the second channel ( b ) whereby — due to the different transit time — another detection region ( more precisely : another detection depth ) is afforded for signal preparation in the second channel . the reference signal formed in that way ( considered as the “ gate ” in accordance with the time displacement ) is then , in the described manner , along the second channel , mixed and demodulated ( 18 b ), filtered ( 20 b ), amplified ( 22 b ) and sampled ( 24 b ) so that the output signal of the probe 1 occurs at the common fft - processing unit in duplicate — produced on the one hand by the first channel and on the other hand produced by the second channel , displaced by the gate spacing . the output signal of the second channel therefore corresponds to a displacement in respect of location of the detection depth with respect to the first channel , wherein that displacement — depending on the respective setting of the multiplexer / gate unit 42 — can lead or trail the first channel , in regard to a depth of penetration . in particular the gate provided in that way can also be set in such a fashion that the ( reference ) gate signal produced by means of the multiplexer / gate unit 42 does not fall into the vessel which is being monitored in the first channel , but for example on a bone or a piece of tissue adjacent to the vessel . in that case no high - level detection signal will be outputted in the second channel which is additionally used as a reference , if an embolism in the blood vessel produces a signal change in the first channel , so that then the existence of an embolism can be correspondingly reliably established . fig4 shows a usual transit time or depth setting which is suitable in particular also for operation of the device shown in fig1 . while both the detection region for the first channel a ( uppermost signal in fig4 ) and also for the second channel b ( second signal in fig4 ) are put to a depth of between about 40 and 45 mm from the probe surface — in the illustrated embodiment measurement is effected by way of the channels a and b on both sides of the head — the reference signal in the first channel a ref is put to a depth region of between 30 and 35 mm , when carrying out measurements on the head near the skull bone of the patient and in dependence on the respective anatomy involved . as the trigger or clock signals in fig4 show therefore in the case of an embolism — which would not be detected in the reference gate as with positioning on the bone the latter is outside the vessel — only the first channel would generate an embolism detection signal but the reference channel and the second channel would not . in comparison an artefact would occur equally in the signal channel a and in the reference channel a ref . the gate therefore produces a delay corresponding to a depth of penetration , which is different in relation to the signal channel , due to the varying transit time . fig2 illustrates a block circuit diagram showing a simplified embodiment which is slightly modified in relation to the embodiment of fig1 . corresponding functional elements are denoted by references corresponding to those used in fig2 fig2 having a first complete signal channel which at the end in turn has an fft - output 38 and an audio output 40 while the reference gate is not in the form of a complete signal channel but only in the form of a mixer / gate unit connected downstream of the pre - amplifier 16 , corresponding to the unit 42 in fig1 . this simplified doppler channel does not require the further , downstream - disposed signal preparation operation for it is only required for detection and distinction of an artefact ( and not for example for further calculation and representation of a fourier - transformed spectral imaging ). accordingly the detector module 44 detects both the reception signal of the first channel and also the signal of the reference channel , which is outputted by the gate unit 42 , and produces therefrom a detector output signal for the detector signal output 36 when in the above - described manner a characteristic amplitude signal occurs at the output of the sample - and - hold circuit 24 , but a corresponding signal does not occur at the same moment in time at the output of the reference gate 42 . fig3 shows a further embodiment of the present invention , more specifically in the device shown in fig3 a total of up to four different probes 10 a through 10 d ( corresponding to a respective one of four channels a , b , c , d ) can be connected , which at the transmitting side are acted upon by transmitting units 12 a through 12 d and which on the receiving side are tapped off by pre - amplifiers 16 a through 16 d with mixer / demodulator units 18 a through 18 d connected on the output side thereof . connected downstream of the mixer units as a multiplex arrangement are eight gate channels each comprising a controllable gates , a high pass hp i and a sample - and - hold circuit s & amp ; h i ( in each case i = 1 . . . 8 ). control and variable , individually settable association of the respective gate channels with the four probe channels is implemented by the action of a gate control unit 46 which is in the form of a suitably designed controller . that control unit which has an oscillator portion also controls the transmitting units and the receiving units which are connected upstream of the gate units ; the multiplex audio output , comprising audio multiplexer 48 , equalizer 50 and audio processor 52 , as well as an fft - processing unit 54 for further processing and output of the signals for visual representation are supplied with control signals by the control unit 46 . connected downstream of the fft - processing unit 54 once again is an embolism detector unit 56 having a detector signal output 36 which in the above - described manner , from the fft - transformed gate signals which occur in the form of a multiplex signal , evaluates at least one of the eight gates as a reference gate and from the signal configurations concludes the occurrence of an embolism ( as distinct from an artefact ). the embodiment illustrated in fig3 permits in a highly flexible manner the use of up to four different probes , wherein , similarly to the mode of operation as described with reference to fig1 or fig2 a single probe also already permits complete embolism detection operation in accordance with the present invention , as long as at least one of the eight gate channels is set up in the above - described manner as a reference channel and in particular is directed onto a differing ( reference ) detection region outside a vessel to be monitored . the remaining gates can then monitor for example in graduated or staggered fashion different depths of a vessel . alternatively , it would be possible to envisage a configuration in which there are provided for example two probes each with four gates which are stepped in respect of depth , wherein in each case at least one of the four gates is set as a reference gate . the detector unit 56 diagrammatically illustrated in fig3 is then appropriately suitably set or programmed for signal evaluation purposes . it will be seen from the foregoing discussion that the invention can be carried into effect in various ways , in which respect in particular it is also not necessary that an fft - transformation procedure or the like operation always has to be effected with the received ultrasonic signal , prior to detection of an embolism ( linked to distinguishing it from an artefact , in accordance with the invention ). on the contrary it is certainly possible , see for example the original embodiment of fig1 to implement that distinguishing operation solely in the time domain or region of the signals which are detected or which are to be compared . a corresponding consideration applies in regard to the provision of reference gates which , by virtue of the change in accordance with the invention in the detection time of the reflected signal , reflect the state of a detection region in the body , which is a different region ( due to the transit time involved ). it is assumed that the best distinction between embolism and artefact , in accordance with the invention , is possible when the detection region for the additional reference channel is set at a location outside the blood vessel to be observed , because in that way mutual influencing of the channels is substantially avoided . in the above - indicated fashion however it is in principle also possible to provide for detecting and distinguishing an embolism if the signals to be evaluated represent two different detection locations in the same vessel . all in all therefore embolisms can thus be reliably distinguished from artefacts in the described fashion , in which respect laboratory tests have shown that — compared to known methods of embolism detection — markedly improved success rates can be achieved . if the detection region for the reference channel is set to a location outside the vessel to be observed , then in the case of an actual embolism in the vessel there is practically no signal input into the second ( reference ) channel ; any coupling or cross - talk effects are expressed at best in only slightly rising amplitudes in the reference channel . checking using measurement procedures has also shown that an artefact always occurs practically simultaneously in both channels and leads there in measurable signal changes ; at the longest , a time delay between the channels is up to three ms , and is therefore markedly below the time settings which are used by way of example in accordance with the present invention .