Method and system for determining position of cannula tip

Abstract

The present invention relates to a method for determining the position of a cannula tip in a body, comprising the steps of: coupling an ultrasound signal into the body, where the ultrasound signal is coupled into the body using an ultrasound probe resting on the body and along an imaging plane of an ultrasound imaging process, where the imaging plane extends in a depth direction and a transverse direction; receiving an ultrasonic signal, where the ultrasonic signal is received by a plurality of sensors attached to a cannula located in the body and arranged along a longitudinal axis of the cannula at a known axial distance to the tip of the cannula and to each other; comparing the received ultrasonic signals, wherein the received ultrasonic signals are compared with respect to their signal amplitude and signal waveform by means of an evaluation device connected to the sensor; a position of the catheter tip relative to the ultrasound probe is determined, wherein the position is determined by an evaluation device depending on a comparison of a signal amplitude and a signal waveform of the received ultrasound signal. The invention also relates to a medical system for carrying out the method.

Description

TECHNICAL FIELD

[0001] The present invention relates to a method and system for determining the position of a cannula tip in a body. BACKGROUND

[0002] Such a procedure and system are among others used in the field of regional anesthesia. According to the prior art, a system is known which is called "OnVision" and which is designed to display the position of a cannula tip in the image plane of an ultrasound image. This known system is fully integrated into an underlying ultrasound imaging system. SUMMARY

[0003] It is an object of the present invention to provide a method and system of the type mentioned above which offers advantages over the prior art.

[0004] This object is achieved by providing a method having the features of claim 1 and a system having the features of claim 9. Advantageous embodiments are specified in the dependent claims. The wording of the claims is incorporated by reference into the subject matter of the description.

[0005] The method according to the invention is intended for determining the position of a cannula tip within a body and comprises the following steps: coupling an ultrasound signal into the body, wherein the ultrasound signal is coupled into the body with an ultrasound probe resting on the body and along an imaging plane of an ultrasound imaging procedure, wherein the imaging plane extends along a depth direction and a lateral direction; receiving the ultrasound signal, wherein the ultrasound signal is received by a plurality of sensors attached to a cannula located in the body and arranged along a longitudinal axis of the cannula at a known axial distance to the cannula tip and to each other; comparing the received ultrasound signals, wherein the received ultrasound signals are compared with respect to their signal amplitudes and signal waveforms by means of an evaluation device connected to the sensors; optionally comparing the signal amplitudes of the received ultrasound signals with the signal amplitudes of the ultrasound signals applied to the ultrasound probe; determining the position of the catheter tip relative to the ultrasound probe, wherein the position is determined by the evaluation device depending on the comparison of the signal amplitudes and the signal waveforms of the received ultrasound signals and optionally depending on the comparison of the received signal amplitudes and the emitted signal amplitudes. The method according to the invention has the advantage that it does not need to be integrated into the ultrasound imaging procedure. The method according to the invention is therefore universally applicable. The signal amplitudes applied to the ultrasound probe, i.e. the emitted signal amplitudes, can for example be stored as nominal values in a memory unit of the evaluation device. Therefore, no connection of the ultrasound probe to the evaluation device is needed. The method according to the invention allows for at least an approximate determination of the position of the catheter tip. In one embodiment, an axial distance between the cannula tip and the imaging plane is determined. In a further embodiment, alternatively or additionally, an inclination angle of the cannula in a perpendicular plane is determined. In a further embodiment, alternatively or additionally, a depth distance between the ultrasound probe and the cannula is determined. In a further embodiment, alternatively or additionally, a rotation angle of the cannula in a horizontal plane is determined. Based on the individual, a plurality of, or all of the above-mentioned variables, the position of the catheter tip relative to the ultrasound probe can be determined at least approximately.

[0006] In one embodiment, determining the position comprises determining an axial distance between the cannula tip and the imaging plane. By means of the evaluation device, the axial distance is determined depending on the comparison of the signal amplitudes of the received ultrasound signals and the known axial distances between the sensors. By comparing the signal amplitudes of the received ultrasound signals, it is possible to determine which of the plurality of sensors is closest to the imaging plane. Positioning a sensor directly in the imaging plane is expected to result in a maximum of the received ultrasound signals. Therefore, the sensor at which there is a maximum signal amplitude of the received ultrasound signals is closest to the imaging plane compared to the other sensors. Since the axial distance between the sensor and the sensor of the cannula tip is also known, this comparison can serve as a basis for inferring the axial distance between the cannula tip and the imaging plane. The axial distance extends along the longitudinal axis of the cannula.

[0007] In a further embodiment, the comparison of the signal amplitudes comprises determining the sensor at which the largest signal amplitude is received compared to the other sensors. This determination is also carried out by means of the evaluation device. Alternatively, the sensor at which the smallest signal amplitude is received compared to the other sensors can be determined. The sensor with the smallest signal amplitude will typically have the largest distance to the imaging plane along the longitudinal axis of the cannula.

[0008] In a further embodiment, the determination of the position comprises determining an angle of inclination of the cannula, wherein the angle of inclination is projected onto a perpendicular plane extending along a depth direction and a longitudinal direction, and wherein the angle of inclination is determined by the evaluation device depending on a comparison of signal waveforms of the received ultrasound signals and a known axial distance between the sensors. The perpendicular plane extends along the depth direction of the imaging plane and is orthogonal to the imaging plane. The angle of inclination is determined depending on a comparison of signal waveforms of the received ultrasound signals and a known axial distance between the sensors. For example, a phase shift and / or a propagation time difference between the received signals can be determined based on the comparison of the signal waveforms. This phase shift allows to draw conclusions about the propagation time difference of the emitted ultrasound signals until the individual sensors. For a given frequency of the ultrasound signals, depending on the phase shift between the received signals of two or more sensors, their distance and / or distance difference to the ultrasound probe can be inferred. Alternatively or additionally, the propagation time difference is directly evaluated. The distance or distance difference and / or the propagation time difference together with the known axial distance between the sensors and the cannula tip allow to determine the angle of inclination, for example, based on simple geometric relations, in particular a triangular relation.

[0009] In a further embodiment, the determination of the angle of inclination comprises determining a propagation time difference of the received ultrasound signals of at least one first sensor and one second sensor of the plurality of sensors, wherein the propagation time difference is determined as a function of a comparison of signal waveforms of the first sensor and the second sensor. In this embodiment of the invention, the angle of inclination is determined as a function of the propagation time difference and a known axial distance between the first sensor and the second sensor. The propagation time difference depends on a comparison of signal waveforms of two sensors or even a plurality of sensors. The propagation time difference can be determined, for example, as a function of a phase shift of the ultrasound signal received by the first sensor and the ultrasound signal received by the second sensor. Alternatively or additionally, the propagation time from the ultrasound probe to each sensor can be measured. For this purpose, the ultrasound imaging system can be connected to the evaluation device. Alternatively, the propagation time between the ultrasound probe and the sensors can be determined by one or more further sensors which are arranged directly on the ultrasound probe or at least in the vicinity of the ultrasound probe.

[0010] In a further embodiment, the method comprises the step of comparing the signal amplitude of the received ultrasound signal with the signal amplitude of the ultrasound signal applied to the ultrasound probe by means of the evaluation device, wherein determining the position further comprises determining a depth distance between the ultrasound probe and the cannula. The depth distance extends in the imaging plane along a depth direction. Depending on the comparison of the signal amplitude of the emission of the ultrasound signal and the signal amplitude of the reception of the ultrasound signal, the depth distance is determined by means of the evaluation device. The greater the difference between the signal amplitude of the emission and the signal amplitude of the reception of the ultrasound signal, the shorter the depth distance between the cannula and the ultrasound probe. Conversely, the greater the difference between the signal amplitude of the emission and the signal amplitude of the reception, the greater the depth distance. The ultrasound signal is attenuated within the body. This attenuation causes the signal amplitude to decrease with increasing distance from the ultrasound probe in the depth direction. This circumstance is exploited by this embodiment of the present application. The signal amplitude of the emission can be stored as a nominal value in a memory unit of the evaluation device. Alternatively, the signal amplitude of the emission can be measured. Alternatively, the ultrasound probe can be connected to the evaluation device for transmitting the signal amplitude of the emission. In a further embodiment, the depth distance is determined as a function of the propagation time. For this purpose, the time of emission at the ultrasound probe must be known. The time of emission can be determined, for example, by means of a sensor in close proximity to the ultrasound probe. Alternatively or additionally, the ultrasound probe can be connected to the evaluation device in order to transmit the time of emission to the evaluation device.

[0011] In a further embodiment, determining the position comprises comparing an axial distribution of signal amplitudes received over the length of the cannula with reference data. Axial means along the longitudinal axis of the cannula. The reference data represent at least one distribution of signal amplitudes of the ultrasound signals over the longitudinal direction and thus orthogonal to the depth direction and to the transversal direction of the imaging plane. The longitudinal direction is not to be confused with the longitudinal axis of the cannula. Determining the position further comprises determining a rotation angle of the cannula. The rotation angle is projected onto a horizontal plane extending along the longitudinal direction and the transversal direction. The horizontal plane is thus orthogonal to the imaging plane and to the vertical plane related to the determination of the tilt angle. Depending on the comparison between the axial distribution of signal amplitudes received and the reference data, the rotation angle in the horizontal plane is determined by means of the evaluation device. This embodiment of the invention is based on the consideration that the signal amplitudes of the ultrasound signals decrease with increasing orthogonal distance from the imaging plane, i.e. along the longitudinal direction. If the rotation angle is 0°, the cannula extends longitudinally in the imaging plane. Assuming that the tilt angle is also 0°, the signal amplitudes of the ultrasound signals received over the length of the cannula, i.e. along the longitudinal axis, will be approximately the same. When rotation occurs in the horizontal plane, the distance from the sensor to the imaging plane will vary along the longitudinal direction. This also changes the distribution of the signal amplitudes of the ultrasound signals received over the longitudinal axis of the cannula. By means of said comparison between the axial distribution of signal amplitudes received and the reference data, which represent at least a distribution of signal amplitudes of the ultrasound signals over the longitudinal direction, an inference can be drawn about the rotation angle of the cannula in the horizontal plane. The reference data are preferably stored in a memory unit of the evaluation device. In one configuration, the reference data can be selected from a reference data set containing different data for ultrasound probes with different specifications. The selection can be made by a user. In a further embodiment, the reference data are obtained by reference measurements. Alternatively or additionally, the reference data can represent a curve of signal amplitudes as a function of the tilt angle and / or the rotation angle.

[0012] In a further embodiment, the reference data also represent a distribution of signal amplitudes of the ultrasound signals along the depth direction within the imaging plane, wherein the rotation angle is also determined as a function of the determined tilt angle and / or the determined depth distance. By including the tilt angle and / or the depth distance in the determination of the rotation angle, it is generally possible to determine the rotation angle and thus the position of the cannula tip more accurately.

[0013] A medical system according to the invention is configured for determining a position of a cannula tip in a body. The medical system according to the invention has an optional ultrasound probe, a cannula and an evaluation device. The optional ultrasound probe is configured to couple ultrasound signals into the body. The cannula comprises a cannula tip and is equipped with a plurality of sensors. The sensors are each configured to receive an ultrasound signal. The sensors are arranged along a longitudinal axis of the cannula at a known axial distance from the tip of the cannula and from each other. The evaluation device is connected to the sensors. This connection can be wireless or wired. The evaluation device is configured for comparing the received ultrasound signals, in particular at least with respect to signal amplitudes and / or signal waveforms of the received ultrasound signals. Optionally, the evaluation device is further configured for comparing the signal amplitudes of the received ultrasound signals with the signal amplitudes of the ultrasound signals present at the ultrasound probe. The evaluation device is further configured for determining the position of the catheter tip relative to the ultrasound probe depending on the comparison of the signal amplitudes and the signal waveforms of the received signals and optionally depending on the comparison of the received signal amplitudes and the transmitted signal amplitudes. In other words: The system is configured to carry out the method according to the invention and embodiments thereof. In one embodiment, the medical system comprises the optional ultrasound probe already mentioned. In a further embodiment, the medical system does not have such an ultrasound probe. In this case, the ultrasound probe is not part of the medical system, but instead is part of an ultrasound imaging system. Embodiments of the medical system derive from the features of embodiments of the method according to the invention, in particular with respect to any additional units of the evaluation device.

[0014] In a further embodiment, the evaluation device is further configured for comparing the signal amplitudes of the received ultrasound signals with the signal amplitudes of the ultrasound signals present at the ultrasound probe and for determining the position of the catheter tip relative to the ultrasound probe depending on the comparison of the received signal amplitudes and the transmitted signal amplitudes. The signal amplitudes applied to the ultrasound probe can also be referred to as transmitted signal amplitudes.

[0015] Further advantages and features of the invention result from the claims and the following description of preferred exemplary embodiments of the invention, which are explained with reference to the drawings. BRIEF DESCRIPTION OF DRAWINGS

[0016] Figure 1 An embodiment of a method for determining a position of a cannula tip in a body according to the invention is illustrated in a schematic block diagram, Figure 2 An embodiment of a medical system according to the invention, which is configured to carry out the method according to Figure 1 is illustrated in a schematic block diagram, Figure 3 An exemplary use case of a medical system according to Figure 2 is illustrated in a schematic simplified diagram, Figure 4 a diagram with exemplary signal waveforms is shown which are evaluated for determining a position of a cannula tip, Figure 5 a further exemplary use case of the medical system according to Figure 2 is shown, Figure 6 a further diagram with exemplary signal waveforms is shown, Figure 7 , Figure 8 , Figure 9 a further schematic representation of an exemplary use case of the medical system according to Figure 2 is shown, Figure 10 a diagram with further exemplary signal waveforms is shown, Figure 11 a further exemplary use case of the medical system according to Figure 2 is shown in a schematic representation, and Figure 12 a diagram with further exemplary signal waveforms is shown. DETAILED DESCRIPTION

[0017] According to Figure 1 , a method 100 for determining a position P of a cannula tip 31 within a body K (see Figure 3 , Figure 5 , Figure 7 , Figure 8 , Figure 9 , Figure 11 ) is provided. The method 100 can be carried out with the medical system 1 shown in Figure 2 , in particular in the context of regional anesthesia. Exemplary use cases of the medical system 1 during the execution of the method 100 are shown in Figure 3 , Figure 5 , Figure 7 to Figure 9 and Figure 11 .

[0018] The medical system 1 has an optional ultrasound probe 2, a cannula 3, a plurality of sensors 4, an evaluation device 5, and an optional display device 6. The optional ultrasound probe 2 and the optional display device 6 are not present in all embodiments.

[0019] The step 110 of the method 100 provides for coupling an ultrasound signal B into the body K. The ultrasound signal B is emitted by the ultrasound probe 2 and coupled into the body K. In use, the ultrasound probe 2 rests on the body K. Alternatively, the ultrasound probe 2 can also be inserted into an opening of the body K and arranged within the body K. The ultrasound signal B is coupled into the body K along an imaging plane E. The imaging plane E extends along a depth direction Z and a lateral direction Y.

[0020] In the embodiment shown in the drawing, the ultrasound probe 2 is a component of a separate ultrasound imaging system. The imaging plane E in question is the plane in which the ultrasound images of the ultrasound imaging process are generated / displayed. The ultrasound signals B can also be referred to as beams when the signals are coupled in. The term "beam" is known to the person skilled in the art.

[0021] Step 120 of the method 100 provides for receiving ultrasound signals B. The ultrasound signals B are received using a plurality of sensors 4. The plurality of sensors 4 is attached to the cannula 3 and arranged along the longitudinal axis L of the cannula 3 at a known axial distance to the cannula tip 31 and at a known distance to each other.

[0022] The plurality of sensors 4 is each configured for receiving ultrasound signals B and can also be referred to as ultrasound microphones.

[0023] In the embodiment shown, the plurality of sensors 4 comprises a first sensor 41, a second sensor 42 and a third sensor 43. The first sensor 41 is distally arranged in the region of the cannula tip 31. The second sensor 42 is arranged along the longitudinal axis L between the first sensor 41 and the third sensor 43 and can also be referred to as an intermediate sensor. The third sensor 43 is proximally arranged in the direction of the cannula proximal end of the cannula 3. In this case, the cannula proximal end is not shown.

[0024] In the present case, the number of three sensors is purely exemplary. There can also be more than three sensors, which is indicated by the further sensors not provided with reference signs, which are drawn in the drawing in dashed lines. Figure 2 The further sensors not provided with reference signs are illustrated in the drawing in dashed lines.

[0025] In the exemplary use case shown, the sensors 41, 42, 43 receive the emitted ultrasound signals B. The ultrasound signals S41, S42, S43 applied to the sensors 41, 42, 43 are also referred to in the following as first ultrasound signals S41, second ultrasound signals S42 and third ultrasound signals S43. The first ultrasound signals S41 are applied to the first sensor 41. The second ultrasound signals S42 are applied to the second sensor 42. The third ultrasound signals S43 are applied to the third sensor 43. The ultrasound signals S41, S42, S43 differ with regard to their signal amplitudes and / or their signal waveforms. These differences depend on the respective distance of the associated sensors 41, 42, 43 from the ultrasound probe 2 and thus from the ultrasound source. An evaluation of the signals S41, S42, S43 thus allows a determination of the position P, in which the known distances between the sensors 41, 42, 43 and to the cannula tip 31 are also included in the calculation.

[0026] The method 100 provides, in a step 130, a comparison of the received ultrasound signals S41, S42, S43. This comparison is carried out by means of the evaluation device 5. For this purpose, the evaluation device 5 is connected to the sensors 4 attached to the cannula 3 by means of the signal connections 7. In the embodiment shown, the signal connections 7 are wired connections. In an embodiment not shown in the drawing, the signal connections are wireless connections. The signals S41, S42, S43 received by the sensors 41, 42, 43 are compared by means of the evaluation device 5 with regard to their signal amplitudes and signal waveforms. The signal amplitudes of the received signals S41, S42, S43 are denoted in the following with a prime and are therefore referred to as S'41, S'42, S'43.

[0027] In a step 140, the method 100 provides a comparison of the signal amplitudes S'41, S'42, S'43 of the received ultrasound signals S41, S42, S43 with the signal amplitude B' of the ultrasound signal B applied to the ultrasound probe 2. In one embodiment, the signal amplitude B' of the emitted ultrasound signal B is measured. In a further embodiment, the ultrasound probe 2 transmits the signal amplitude B' to the evaluation device 5, for example via a further signal line 8. This further signal line 8 is optional and is therefore shown in the drawing in dashed lines. In a further embodiment, the signal amplitude B' of the coupled-in ultrasound signal B is given as a nominal value of the ultrasound probe 2. This nominal value can be entered into the evaluation device 5, received from the evaluation device 5 or stored in the evaluation device 5, for example. Figure 2

[0028] The method 100 provides, in a step 150, a determination of the position P of the catheter tip 31 relative to the ultrasound probe 2. Depending on the comparison of the signal amplitudes S'41, S'42, S'43 and the signal waveforms of the received ultrasound signals S41, S42, S43, and depending on the comparison of the received signal amplitudes S'41, S'42, S'43 with the emitted signal amplitude B' of the ultrasound signal B, the position P is determined. This determination is carried out by means of the evaluation device 5.

[0029] In the embodiment shown, the evaluation device 5 has a processor unit 51 and a memory unit 52. The processor unit 51 is designed to carry out the aforementioned comparison 130, 140 and the actual determination 150 of the position P. In the embodiment shown, the nominal value of the signal amplitude B' of the ultrasound probe 2 is stored in the memory unit 52. This nominal value forms a kind of reference value.

[0030] The determined position P can be displayed graphically or in different forms by means of the display device 6. For example, the display can be output relative to the already mentioned imaging plane E. An actual ultrasound image can also be displayed by means of the display device 6.

[0031] ​Alternatively or additionally, the display device 6 can be designed to provide an audible perceptible display or output of the determined position P. To this end, the display device 6 can have a loudspeaker unit. In this case, the determined position P or a change thereof can be output in a similar manner as the parking distance control in a car, for example by means of an audio signal sequence, an audio signal volume, an audio signal level, etc.

[0032] The method 100 and the medical system 1 allow for at least an approximate determination of the position P. Approximate means that it is not necessary to determine the position P uniquely with respect to all coordinate axes. For example, the method 100 and the medical system 1 can only be used to determine an axial distance A (see Fig. 1) between the cannula tip 31 and the imaging plane E. Figure 3 Alternatively or additionally, a depth distance T (see Fig. 1) can be determined. Figure 5 As a further alternative or in addition, an inclination angle a (see Fig. 1) can be determined. Figure 7 , Figure 8 As a further alternative or in addition, a rotation angle b (see Fig. 1) can be determined. Figure 9 , Figure 11 Preferably, all of the above-mentioned variables A, T, a, b are determined such that the position P is determined as accurately and / or unambiguously as possible.

[0033] The axial distance A between the cannula tip 31 and the imaging plane E is determined depending on a comparison of the signal amplitudes S'41, S'42, S'43 of the received ultrasound signals S41, S42, S43 and the known axial distances between the sensors 41, 42, 43 and with respect to the cannula tip 31. This determination is made by means of the evaluation device 5. The determination of the axial distance A is based on the assumption that the sensor which is positioned at the shortest distance from the imaging plane E delivers the largest signal amplitude compared to the other sensors. In the exemplary case shown in Figs. 1 and 2, the first sensor 41 is positioned closest to the imaging plane E. Therefore, the first signal amplitude S'41 is the largest compared to the signal amplitudes of the other sensors (in particular the signal amplitudes S'42, S'43). In other words: In order to determine the axial distance A, the longitudinal position of the cannula 3 at which the beam B is positioned is determined. The axial distance A extends along the longitudinal axis L of the cannula 3. Figure 3 and Figure 4 In the exemplary case shown in Figs. 1 and 2, the second sensor 42 is positioned directly in the imaging plane E. Therefore, the second signal amplitude S'42 is the largest compared to the signal amplitudes of the other sensors (in particular the signal amplitudes S'41, S'43). In other words: In order to determine the axial distance A, the longitudinal position of the cannula 3 at which the beam B is positioned is determined. The axial distance A extends along the longitudinal axis L of the cannula 3. Figure 3 In the exemplary use case shown in Figs. 1 and 2, the longitudinal axis L extends parallel to a longitudinal direction X which is oriented orthogonal to a depth direction Z and a transversal direction Y of the imaging plane E. In the exemplary use case shown in Figs. 1 and 2, the depth direction Z is oriented along the depth distance T. In the exemplary use case shown in Figs. 1 and 2, the transversal direction Y is oriented along the lateral distance L. Figure 4In the exemplary diagram of the signal amplitudes S'41, S'42, S'43 of the received ultrasound signals S41, S42, S43, exemplary distributions of the signal amplitudes B' of the (transmitted) ultrasound signals B over the longitudinal direction X are also plotted. The transmitted signal amplitudes B' decrease as the distance from the imaging plane E increases. This is due to the directivity of the beam B and / or an attenuation caused by the properties of the body K.

[0034] The depth distance T extends in the imaging plane E along the depth direction Z (see Figure 5 ). The depth distance T is determined by means of the evaluation device 5 by means of a comparison 140 of the above-mentioned transmitted signal amplitudes B' and the received signal amplitudes S'41, S'42, S'43. The determination of the depth distance T is based on the assumption that the signal amplitudes B' of the beam B decrease as the depth along the depth direction Z increases. This decrease is shown in a simplified schematic form in the exemplary diagram according to Figure 6 , wherein the exemplary representation is logarithmic. In the embodiment shown, the course of the signal amplitudes B' over the depth direction Z is stored in the memory unit 52 of the evaluation device 5 in the form of reference data. The reference data can be determined in advance and / or only once (for example using a reference measurement on a reference measurement setup).

[0035] If the distribution of the signal amplitudes B' of the transmitted ultrasound signals B over the depth direction Z is known, the depth distance T can be inferred using a comparison with the received signal amplitudes S'41, S'42, S'43. For example, if the second sensor 42 (as in the exemplary use case) is located directly in the imaging plane E, the depth distance T along the depth direction Z between the second sensor 42 and the ultrasound probe 2 can be determined by a simple comparison between the second signal amplitude S'42 and the distribution of the signal amplitudes B' over the depth direction Z.

[0036] If the cannula 3 is to be tilted at an angle (for example, see Figure 8 ), the depth distance of the cannula tip 31 can be determined via simple trigonometric relationships and on the basis of the known distances between the sensors 41, 42, 43 and the cannula tip 31.

[0037] The angle of inclination a of the cannula 3 is projected onto a vertical plane V extending along the depth direction Z and the longitudinal direction X (see Figure 7 , Figure 8 ). The angle of inclination a is determined depending on the comparison of the signal waveforms of the received ultrasound signals S41, S42, S43 already mentioned and the known axial distances between the sensors 41, 42, 43 and relative to the cannula tip 31. The angle of inclination a is also determined by means of the evaluation device 5.

[0038] The determination of the inclination angle a is based on the assumption that depending on the distance of the sensors 41, 42, 43 from the ultrasound probe 2, different propagation times of the ultrasound signals B are obtained. For the first sensor 41 and the third sensor 43, these different propagation times are schematically indicated in the form of arrows in Figure 7 and Figure 8 . The propagation time of the ultrasound signals B between the ultrasound probe 2 and the first sensor 41 is designated as t41. The propagation time assigned to the third sensor 43 is designated as t43. In the exemplary use case according to Figure 7 , the longitudinal axis L is oriented parallel to the longitudinal direction X and the two sensors 41, 43 are positioned at the same axial distance along the longitudinal axis L with respect to the imaging plane E. The propagation times t41, t43 are thus identical. The distances between the ultrasound probe 2 and the two sensors 41, 43 are also identical. These distances are marked in Figure 7 with the reference signs d41, d43 and are obtained purely by calculation from the propagation times of the emitted ultrasound signals B and their frequency. The frequency can be determined, for example, by means of the evaluation device 5 on the basis of the received signals S41, S42, S43. Alternatively or additionally, the frequency of the ultrasound signals B can be stored as a nominal value in a memory unit 52 of the evaluation device 5. Thus, in the exemplary use case shown in Figure 7 , there is no phase shift between the signal waveforms of the received ultrasound signals S41, S43.

[0039] In the use case shown in Figure 8 , the situation is different. There, starting from the horizontal orientation according to Figure 7 , the cannula 3 is inclined distally downward in the depth direction T. The third sensor 43 is thus closer to the ultrasound probe 2 than the first sensor 41. Different propagation times t41', t43' and thus different distances d41', d43' are thus obtained. In this case, the propagation times t41, t43 or t41', t43' are not determined by direct time measurement. Instead, the indirect determination is carried out by comparing the signal waveforms of the signals S41, S43 and the resulting phase shift between these two signals S41, S43. The inclination angle a can be determined on the basis of the difference between the propagation times and simple trigonometric relationships.

[0040] If the inclination angle a is known, the axial distance A and / or the depth distance T can be determined with improved accuracy and vice versa. This is again based on simple trigonometric relationships and depends on the known distances between the sensors 41, 42, 43 and their distance from the cannula tip 31.

[0041] The rotation angle b (see Figure 9) is the angle projected onto a horizontal plane H. The horizontal plane H is oriented orthogonal to the imaging plane E and to the vertical plane V. Thus, the horizontal plane H extends along the longitudinal direction X and the transversal direction Y.

[0042] The ultrasound probe 2 is elongated along the longitudinal direction X and the transversal direction Y. In other words: the dimensions along the longitudinal direction X and the transversal direction Y are significantly larger than the dimension along the vertical direction Z. Figure 9 and Figure 11 The ultrasound probe 2 is elongated along the longitudinal direction X and the transversal direction Y. In other words: the dimensions along the longitudinal direction X and the transversal direction Y are significantly larger than the dimension along the vertical direction Z.

[0043] The determination of the rotation angle β is in turn based on the assumption that the signal amplitude B’ of the in-coupled ultrasound signal B (i.e. the beam) decreases with increasing distance from the imaging plane E and thus along the longitudinal direction X. An exemplary decrease of the signal amplitude B’ is schematically shown in Figure 12 .

[0044] In an exemplary use case according to Figure 9 , the rotation angle β is approximately zero and thus the longitudinal axis L is oriented parallel to the transversal direction Y. Such a longitudinal extension of the cannula 3 within the imaging plane E is also referred to as an “in-plane” arrangement. Assuming that the signal amplitude B’ of the beam B in the longitudinal direction Y is at least constant over the length of the ultrasound probe 2, the “in-plane” arrangement also results in approximately the same signal amplitudes S’41, S’42, S’43 along the longitudinal axis L (see Figure 10 ).

[0045] With increasing rotation angle β, the difference between the signal amplitudes S’41, S’42, S’43 also increases. This difference is largest at a rotation angle β of 90° (see Figure 11 , Figure 12 , the “out-of-plane” arrangement). Thus, the rotation angle β can be determined by comparing the axial distribution of the received signal amplitudes S’41, S’42, S’43 over the length of the cannula 3 (i.e. along the longitudinal axis L) with the distribution of the signal amplitudes B over the longitudinal direction X. In the present case, the distribution of the signal amplitudes B’ of the ultrasound signal B over the longitudinal direction X is stored in the form of reference data in the storage unit 52.

Claims

1. Method (100) for determining a position (P) of a cannula tip (31) within a body (K), comprising the following steps: coupling (110) an ultrasound signal (B) into the body (K), wherein the ultrasound signal (B) is coupled into the body (K) with an ultrasound probe (2) resting on the body (B) and along an imaging plane (E) of an ultrasound imaging procedure, wherein the imaging plane (E) extends along a depth direction (Z) and a lateral direction (Y); receiving (120) the ultrasound signal (B), wherein the ultrasound signal (B) is received by a plurality of sensors (41, 42, 43) attached to a cannula (3) located in the body and arranged along a longitudinal axis (L) of the cannula (3) at a known axial distance to the cannula tip (31) and to each other; comparing (130) the received ultrasound signals (S41, S42, S43), wherein the received ultrasound signals (S41, S42, S43) are compared with respect to their signal amplitudes (S'41, S'42, S'43) and their signal waveforms by means of an evaluation device (5) connected to the sensors (41, 42, 43); and determining (150) a position (P) of the catheter tip (31) relative to the ultrasound probe (2), wherein the position (P) is determined by the evaluation device (5) depending on the comparison of the signal amplitudes (S'41, S'42, S'43) and the signal waveforms of the received ultrasound signals (S41, S42, S43).

2. Method (100) according to claim 1, wherein determining (150) the position (P) further comprises: determining an axial distance (A) between the cannula tip (31) and the imaging plane (E), wherein the axial distance (A) is determined by the evaluation device (5) depending on the comparison of the signal amplitudes (S'41, S'42, S'43) of the received ultrasound signals (S41, S42, S43) and the known axial distance between the sensors (41, 42, 43).

3. Method according to claim 2, wherein comparing the signal amplitudes (S'41, S'42, S'43) further comprises: determining the sensor (42) at which a maximum signal amplitude (S'42) is received compared to the other sensors (41, 43).

4. Method (100) according to any of the preceding claims, wherein determining (150) the position (P) further comprises: determining an inclination angle (a) of the cannula (3), wherein the inclination angle (a) is projected onto a vertical plane (V) extending along the depth direction (Z) and a longitudinal direction (X), and wherein the inclination angle (a) is determined by the evaluation device (5) depending on the comparison of the signal waveforms of the received ultrasound signals (S41, S42, S43) and the known axial distance between the sensors (41, 42, 43).

5. Method (100) according to claim 4, wherein determining the inclination angle (a) further comprises: determining a maximum signal amplitude (S'42) of the received ultrasound signal (S42) compared to the other received ultrasound signals (S41, S43). determining a difference in the propagation time of the received ultrasound signals (S41, S42, S43) for at least a first sensor (41) and a second sensor (43) of the plurality of sensors (41, 42, 43), wherein the difference in the propagation time is determined depending on a comparison of the signal waveforms of the first sensor (41) and the second sensor (43), and determining the tilt angle (a) as a function of the difference in the propagation time and a known axial distance between the first sensor (41) and the second sensor (43).

6. The method (100) according to any one of the preceding claims, having the additional step of comparing (140) the signal amplitudes (S'41, S'42, S'43) of the received ultrasound signals (S41, S42, S43) with the signal amplitude (B') of the ultrasound signal (B) applied to the ultrasound probe (2), wherein the comparison (140) is carried out using the evaluation device (5); wherein determining (150) the position (P) further comprises: determining a depth distance (T) between the ultrasound probe (2) and the cannula (3), wherein the depth distance (T) extends along a depth direction (Z) in the imaging plane (E), and wherein the depth distance (T) is determined by the evaluation device (5) depending on the comparison of the signal amplitude (B') applied to the ultrasound probe (2) and the received signal amplitudes (S'41, S'42, S'43).

7. The method (100) according to any one of the preceding claims, wherein determining (150) the position (P) further comprises: comparing the axial distribution of the received signal amplitudes (S'41, S'42, S'43) over the length of the cannula (3) with reference data representing at least one distribution of the signal amplitude (B') of the ultrasound signal (B) over the longitudinal direction (X); determining a rotation angle (β) of the cannula (3), wherein the rotation angle (β) is projected onto a horizontal plane (H) extending along the longitudinal direction (X) and the transversal direction (Y), and wherein the rotation angle (β) is determined by the evaluation device (5) as a function of the comparison between the axial distribution of the received signal amplitudes (S'41, S'42, S'43) and the reference data.

8. The method (100) according to claim 7 in combination with claim 4 and / or 6, wherein the reference data also represents a distribution of the signal amplitude (B') of the ultrasound signal (B) along the depth direction (Z) within the imaging plane (E), and wherein the rotation angle (β) is also determined as a function of the determined tilt angle (a) and / or the determined depth distance (T), and / or vice versa.

9. Medical system (1) for determining a position (P) of a cannula tip (31) within a body (K), comprising an ultrasound probe (2) configured to couple an ultrasound signal (B) into the body (K), A cannula (3) having a cannula tip (31) and having a plurality of sensors (41, 42, 43) configured to receive ultrasound signals (B) and arranged at known axial distances to the cannula tip (31) and to each other along a longitudinal axis (L) of the cannula (3); an evaluation device (5) connected to the sensors (41, 42, 43), wherein the evaluation device (5) is configured to compare (130) the received ultrasound signals (S41, S42, S43), in particular at least with respect to their signal amplitudes (S'41, S'42, S'43) and their signal waveforms, and determine (150) a position (P) of the catheter tip (31) relative to the ultrasound probe (2) depending on the comparison (130) of the signal amplitudes (S'41, S'42, S'43) and the signal waveforms of the received signals (S41, S42, S43).

10. The medical system (1) according to claim 9, wherein the evaluation device (5) is further configured to compare (140) the signal amplitudes (S'41, S'42, S'43) of the received ultrasound signals (S41, S42, S43) with a signal amplitude (B') of the ultrasound signals (B) applied to the ultrasound probe (2), and determine (150) the position (P) of the catheter tip (31) relative to the ultrasound probe (2) depending on the comparison (140) of the received signal amplitudes (S'41, S'42, S'43) and the emitted signal amplitude (B').

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