WO2008020157A1

WO2008020157A1 - Ultrasonic imaging of an elongate device penetrating an object

Info

Publication number: WO2008020157A1
Application number: PCT/GB2007/002781
Authority: WO
Inventors: Francesco Simonetti
Original assignee: Imperial Innovations Limited
Priority date: 2006-08-15
Filing date: 2007-08-03
Publication date: 2008-02-21
Also published as: GB0616219D0; GB2440960A

Abstract

A device for investigating matter, the device comprising an elongate device (100), e.g. a needle, which penetrates the investigated matter and an ultrasonic transducer (110) connected to the, -non -penetrating end of the needle. The needle acts as a waveguide for ultrasonic waves produced by the ultrasonic transducer, which propagate along the needle and are radiated into the investigated matter, the wave characteristics of the ultrasonic waves being chosen such that the radiated ultrasonic waves are detectable by a detector (200) regardless of the relative orientation of the detector and the needle.

Description

ULTRASONIC IMAGING OFANELONGATEDEVICE PENETRATING

AN OBJECT

FIELD OF THE INVENTION This invention relates to the field of ultrasonic imaging. More particularly, this invention relates to using ultrasonic imaging to locate an elongate device.

BACKGROUND OF THE INVENTION

Ultrasonic imaging is a real time, safe and inexpensive technology widely used in medical diagnostic imaging, accounting for 25% of all the medical studies performed in the world. Due to the high speed at which images can be acquired, ultrasound is very attractive for image-guided intervention, whereby an ultrasonic sensor array provides the surgeon with images of the location of a needle relative to the pathology.

In this context, an area of growing interest is tissue biopsy which enables an early and accurate diagnosis of cancer. As an example, needle-localised open breast biopsy is regarded as the gold standard procedure for the diagnosis of nonpalpable breast lesions. However, this type of invasive procedure is expensive and can result in a traumatic experience for patients. This has led to the development of less invasive techniques such as Fine Needle Aspiration (FNA) and Core Needle Biopsy (CNB). FNA biopsy is performed with a fine gauge needle (20-25) which is inserted in the region containing the abnormality, e.g. cyst or tumor. Once the needle is in position, a vacuum is created by means of a syringe and a liquid sample is taken. CNB is a procedure similar to FNA biopsy, however in this case the hollow needle contains a stylet, usually triggered by a spring loaded mechanism, which cuts a tissue sample. In both FNA and CNB it is of crucial importance to locate the position of the needle relative to the lesion accurately. For this purpose x- ray and ultrasound can be employed. X-ray guided procedures use two orthogonal x-ray projections which are acquired before, during and after the needle insertion. Although this modality is very accurate, it is not real-time and concerns exist about radiation doses and the relative risks for young patients. On the other hand, ultrasound provides a real-time and safe guidance technique. However, its sensitivity strongly depends on the angle of the needle relative to the array. In order for the needle to be visible using contemporary techniques (schematically illustrated in Figure 1), it has to be parallel to the array. This is due to the fact that an ultrasound image is obtained by transmitting ultrasonic waves (I) into the tissue 30 and detecting the reflections (R) arising from variations in the tissue properties or from the needle. When the needle 10 is parallel to the sensor array 20 (illustrated in Figure l(a)) the reflected wave is perpendicular to the sensor and can easily be detected. On the other hand, if the needle is at an angle, the reflected wave, which follows the specular reflection law, can be reflected away from the sensor (illustrated in Figure l(b)) making the needle invisible. As a result, needle biopsy under ultrasound guidance is a complicated procedure which requires highly experienced radiologists.

In order to address this problem, a number of different approaches have been proposed. For instance, several needle manufacturers have tried to increase the reflectivity of the needle by surface treatments such as scoring. The reflectivity enhancement depends on the degree of surface roughness produced by the treatment, the higher the roughness, the larger the scattering angle and the more visible is the needle. However, the higher the roughness, the higher is the friction at the needle-tissue interface and the larger the load required to penetrate the needle into the tissue.

Recently, as an alternative to surface treatments, a new polymer coating with a bubbling agent has been proposed (WO-A-00/66004). The agent develops micro-bubbles, which increase the scattering angle from the device so reducing the directionality of the reflections. Although friction is no longer an issue, it is not well understood if the presence of micro bubbles in the tissue could lead to side- effects, and clinical trials are to be performed. Moreover, it is not clear if the rate at which the micro bubbles are formed is enough to track the needle movement in real-time.

US-A-4,249,539 discloses a miniaturised hydrophone installed on the tip of the needle which acts as a point source and which can be imaged by the array. There are technological complications related to the manufacturing of such a highly miniaturised hydrophone and its placement at the tip of the needle, requiring special wiring, which might be damaged as the needle penetrates into the tissue. Moreover, with this technique, there are concerns for patient safety as there is a risk that the transducer might be damaged and lost within the tissue.

US-A-5 ,095,910 describes a system in which a transponder is installed on the needle and excites low frequency, large amplitude vibrations along the entire needle. The imaging array transmits ultrasonic waves which upon arrival on the needle are reflected at a different frequency, being Doppler-shifted by the low frequency motion of the needle. This Doppler-shifting does not, however, overcome the problem of the reflection directionality.

US-A-5,425,370 discloses a system using low frequency flexural vibrations excited along the length of the needle. In this case, the imaging array acquires two images at two instants in time which depend on the low frequency vibration of the needle. The two images are subsequently subtracted and the difference indicates the position of the needle, provided that the rest of the tissue did not move during the acquisition of the two images. This method is still sensitive to the orientation of the needle relative to the imaging array.

US-A-5,294,861 discloses an ultrasonic probe wherein sonic waveguides transmit sonic energy along the probe from ultrasonic trasducers at one end to ultrasonic directional elements at the other end. Pure modes of vibration are used to maximise transmission along the waveguide and avoid dispersion.

US-A-5,329,927 discloses a vibrating mechanism coupled to a needle to provide flexural vibrations to move the needle and allow detection of the position of the needle. This method is still sensitive to the orientation of the needle relative to the imaging array.

A number of robotic systems have also been developed. So-called passive position systems provide image guidance to assist the physician in orienting the biopsy needle. The PinPoint system developed by Marconi Medical Systems is an example of passive position. There are also semi-autonomous robotic systems which position, drive, and guide the biopsy needle under remote physician control, examples being the Mammotome- developed by Ethicon Endo Inc. and the ABBI system manufactured by the United States Surgical Corporation. The major issue with these techniques is the registration of the position of the device (obtained with encoding mechanisms) relative to the image of the pathology obtained with ultrasound or X-rays.

SUMMARY OF THE INVENTION

Viewed from one aspect the present invention provides an ultrasonic imaging apparatus comprising: an elongate device having a proximal end and a distal end, said distal end in use penetrating an object to be imaged; and an ultrasonic transducer array for at least receiving ultrasonic waves from said object; wherein an ultrasonic transducer is coupled to said proximal end of said elongate device and is arranged to generate ultrasonic waves matched to said elongate device such that said elongate device acts as a waveguide for said ultrasonic waves with said ultrasonic waves propagating along said elongate device whilst radiating therefrom into said object, said ultrasonic transducer array is arranged to detect said ultrasonic waves radiated from said elongate device; and said ultrasonic waves are radiated from said elongate device over a solid angle such that said elongate device can be imaged substantially independently of a relative orientation of said ultrasonic transducer array and said elongate device.

The purpose of this invention is to improve the visibility and localisation of elongate devices (such as biopsy needles for medical procedures) under ultrasound image guidance. The elongate device can take many other forms, such as a catheter, en endoscope, a flexible elongate device, etc. The elongate device is used as a waveguide and guides ultrasonic waves along its length. When the device is in contact with surrounding material (such as human tissue) part of the energy of the guided "waves leaks into the surrounding tissue. This leakage results in the radiation of ultrasonic pressure waves over a wide range of solid angle from the lateral surface of the device and its tip. In this way-the device itself becomes an active source of ultrasonic waves. The modes of these ultrasonic waves are chosen to enhance the multi-directionality of the radiated pressure waves. The pressure waves are detected by a conventional imaging ultrasonic transducer array, working in a passive mode, to produce an image of the device substantially regardless of its orientation relative to the array. Advantageously no particular surface treatment or use of chemical agents is required, providing a safer device for use in medical procedures. Furthermore, unlike some prior art techniques, the present technique does not require the transducer to be in contact with the tissue, and there is no need for it to be miniaturised, therefore it is safer and cheaper than these prior art techniques.

In preferred embodiments the array is further operable to transmit further ultrasonic waves into said object and to detect said further ultrasonic waves reflected from within said object. In this way, the array can be used to create an image not only of the elongate device, but also of the material into which the elongate device has been inserted.

In preferred embodiments the system is arranged such that distinction between the ultrasonic waves originating from the elongate device and the further ultrasonic waves originating from the array can be made. This may be achieved by various techniques, such as the control device being operable to cause the ultrasonic waves and the further ultrasonic waves to be produced sequentially, such that the origin of ultrasonic waves detected by said array may be determined by their timing. Alternatively the ultrasonic waves and the further ultrasonic waves may be produced at different frequencies, such that the origin of ultrasonic waves detected by the array may be determined by their frequency. -

In preferred embodiments the control device is operable to generate an image of the elongate device from the detected ultrasonic waves radiated from the elongate device and to generate an image of the investigated matter from the detected further ultrasonic waves reflected from within the investigated matter. By combining these two images the user may advantageously be presented with a view showing the relative position of the interventional device within the investigated matter.

In preferred embodiments the elongate device is hollow, advantageously providing a passage there through which either fluids, for example, may be delivered or through which a slender tool may reach. In further preferred embodiments the elongate device comprises an internal stylet operable to take a sample of said investigated matter at a distal end of said elongate device. Thus, the elongate device may be accurately guided to a particular location in the object, by means of the ultrasonic waves it radiates, at which point a sample of the object may be taken.

In preferred embodiments proportions of the elongate device and a mode of operation of the ultrasonic transducer are chosen such that a plurality of ultrasonic wave modes propagate along the length of the elongate device. These multiple modes of the ultrasonic waves advantageously lead to a multi-directionality of the radiated waves, such that the radiated waves may be detected in may different directions from the elongate device. In other preferred embodiments the distal end of said elongate device forms a fine point, which causes the radiated waves emanating therefrom to propagate approximately spherically, also leading to a wide range of possible detection angles i.e. over a large solid angle.

In preferred embodiments the elongate device is covered by a sleeve, such that, within the object, only the distal end of said elongate device is not covered by the sleeve, the sleeve inhibiting radiation of the ultrasonic waves from the elongate device. By this arrangement, the ultrasonic waves can be advantageously restricted to being radiated from a short portion of the distal end of the elongate device, deep in the object, improving the multi-directionality of the radiation. This sleeve may be advantageously provided by the hollow elongate device and the ultrasonic waves propagate along the stylet. This arrangement is particularly suitable when the elongate device is a biopsy needle, since advantage may be taken of the operation of the present technique, without significantly increasing the construction of the biopsy needle.

Viewed from a second aspect the present invention provides a method of ultrasonic imaging comprising the steps of: inserting a distal end of an elongate device into an object; generating at a proximal end of said elongate device ultrasonic waves matched to said elongate device so as to propagate along said needle as a waveguide and to radiate therefrom into said object; detecting, with an ultrasonic transducer array, ultrasonic waves radiated into said investigated matter from said elongate device; and forming an image of said elongate device within said object from said ultrasonic waves radiated into said object, wherein said ultrasonic waves are radiated from said elongate device over a solid angle such that said elongate device can be imaged substantially independently of a relative orientation of said ultrasonic transducer array and said elongate device.

Viewed from a third aspect the present invention provides an elongate device adapted for ultrasonic imaging comprising: an elongate device body having a distal end for penetrating an object and a proximal end; and an ultrasonic transducer connect to said proximal end of said elongate device and adapted to generate ultrasonic waves matched to said elongate device body such that said elongate device body acts as a waveguide for said ultrasonic waves with said ultrasonic waves propagating along said elongate device body whilst radiating therefrom into said object, said ultrasonic waves being radiated from said elongate device body over a solid angle such that said elongate device body can be imaged substantially independently of a relative orientation of said ultrasonic transducer array and said elongate device body.

DESCRIPTION OF THE DRAWINGS Embodiments will now be described, by way of example only, with reference to the accompanying drawings in which:

Figures Ia and Ib schematically illustrate conventional ultrasound imaging;

Figure 2 schematically illustrates one example of the system of the present technique;

Figure 3 illustrates universal phase velocity dispersion curves for ultrasonic modes which can propagate in circular steel rods;

Figure 4 illustrates attenuation dispersion curves for the modes illustrated in Figure 3, when the rod is immersed in water;

Figure 5 schematically illustrates pressure waves radiating into tissue;

Figures 6a and 6b schematically illustrate the 3 -dimensional radiation of pressure waves;

Figure 7 illustrates phase velocity and attenuation dispersion curves for a steel tube immersed in water;

Figures 8a and 8b illustrate the directionality due to diffraction of the pressure field radiating into tissue from a finite aperture;

Figure 9 schematically illustrates waves radiating from the tip of an elongate device;

Figure 10 schematically illustrates reflected guided waves radiating pressure waves;

Figure 11 schematically illustrates an external sleeve preventing direct acoustic coupling between an elongate device and the surrounding tissue;

SUBSTHTOTE SEEET (RiULE M Figure 12 schematically illustrates the integration of the present technique with a conventional phased array imaging system;

Figure 13 schematically illustrates using a bandpass and notch filter to separate elongate device and tissue images;

Figure 14 illustrates experimental images of the tip of a rod immersed in water for three different orientations of the rod relative the imaging array; and

Figure 15 schematically illustrates an experimental rod, showing the position of the sleeve and the ultrasonic transducer.

DESCRIPTION OF EMBODIMENTS Figure 2 schematically illustrates an example of a system of the present technique. This example uses a needle as the elongate device. It will be appreciated that the elongate device could take many other forms, such as a catheter or an endoscope. The elongate device may also be flexible and form a flexible elongate device along which and from which ultrasonic waves propagate. The needle (interventional device) 100 has an ultrasonic transducer 110 at its proximal end, the distal end being inserted into the object investigated material 300. Ultrasonic waves 150, generated by the ultrasonic transducer 110 are guided along the needle 100 towards its distal end. When the device is in contact with the object, part of the energy of the guided waves leaks into the surrounding material. Radiated ultrasonic waves R, propagate into the object 300 in multiple directions (i.e. over a large solid angle), such that they can be detected by the array 200, regardless of the relative orientation of the interventional device 100 and the array 200.

The multi-directional radiation can be achieved by a careful choice of the ultrasonic guided waves, the frequencies at which they are excited and some geometrical parameters of the needle. Ultrasonic guided waves differ from standard pressure or distortional waves, which travel in the bulk of a material, because they interact strongly with the boundaries of the structure in which they are propagating. Such an interaction results in the guidance effect which is analogous to that obtained in optical fibers with light waves. Clearly the characteristics of the guided waves depend on the geometry of the waveguide and the properties of the material surrounding the waveguide.

As an example, Figure 3 shows the dispersion curves (phase velocity versus frequency) of some of the guided modes which can propagate in a steel circular rod. The dispersion curves depend on the frequency-diameter product only. The modes labelled L(O₉I)₅ L(0,2), etc. are characterised by an axi-symmetric displacement field. On the other hand, the second family of modes F(I₅I), F(1, 2) etc. are bending modes whose angular distribution of the displacement field is described by a sinusoid of period 2π. Similarly the families F(N₅I), F(N,2) etc. (not shown in Figure 3 for clarity) are characterised by sinusoids of period 2Nπ. Figure 4 shows the attenuation of the modes shown in Figure 3 as they propagate along the rod when it is immersed in water, the acoustic properties of water being similar to those of human tissue. At each frequency, two or more modes can propagate in the rod. Different modes are characterised by different phase velocities and attenuations. The phase velocity determines the angle θ between the propagation direction of the pressure wave R radiating away from the rod 100 and the axis of the rod as shown in Figure 5 according to the approximate expression cos(θ) ~ cj/Cph (Equation 1), where c\ is the speed of pressure waves in the tissue and c_ph is the mode phase velocity. As a result, the faster the guided mode the larger θ.

It has to be emphasised that the diagram shown in Figure 5 represents the phase wavefronts of the radiated field over a plane containing the waveguide axis. As this plane rotates around the axis, the propagation direction describes a conical surface 400 in the case of longitudinal modes L(0,n) and a helicoid 410 in the case of fiexural waves F(m,n) as shown in Figure 6. In the following the axial plane representation is used with reference to both cases. The guided wave attenuation accounts for the energy radiated into the tissue, the higher the attenuation the larger the amount of radiated energy. However, the higher the attenuation the shorter the maximum length that the guided mode can travel along the device. The radiating field, which is due to the leakage of guided wave energy, can be thought of as the field produced by a continuous distribution of "virtual" point-sources along the device surface, at each point the strength of the source being proportional to the amplitude of the surface vibration at that point. In the present technique it is proposed to build an image of the device by imaging the virtual point-sources. For this purpose, it is important that the guided mode maintains a large amplitude along the length of the device to be imaged. Therefore, the task is to identify a suitable attenuation so as to increase the radiation energy, which improves the strength of the signal reaching the imaging array, without causing an excessive attenuation of the guided wave amplitude along the length of the device to be imaged.

The suitable attenuation can be achieved by selecting the excitation frequency of guided mode and by choosing suitable geometrical parameters for the device. The effect of frequency and geometry (diameter) on the attenuation of the modes of a rod immersed in water can be deduced from Figure 4. As an example, if the diameter is doubled and the frequency is reduced by half, i.e. the frequency- diameter ratio is constant, the attenuation is reduced by half. The value of the attenuation can also be changed by maintaining the same frequency and diameter, but using an hollow cylinder rather than a rod. As an example Figure 7 shows the dispersion curves for a tube of lmm outer diameter and 0.6mm inner diameter. Note that the curves now depend on both inner and outer diameters.

In practice, the radiation length of an interventional device will be limited to a few centimetres (~3cm for breast biopsy). Therefore, due to the phenomenon of diffraction the radiation is a strongly directional field rather than a plane wave insonifying all the tissue uniformly. As an example, Figure 8(a) shows the field produced by a limited radiating length (or aperture) when all the points vibrate in phase. If the length were infinite, the radiated field would consist of two plane waves propagating in the directions indicated by the arrows. Instead due to the finite aperture, the radiation pattern is characterised by two main lobes perpendicular to the aperture - outside the lobes the wavefield decays rapidly. Similarly, Figure 8(b) shows the radiation pattern when the phase of the radiating sources vary linearly along the aperture, as it would happen in the case of a guided wave propagating along the aperture. Each lobe forms an angle with the aperture which is given by Equation (1).

As a result, the response of the imaging system would still be sensitive to the direction of the device relative to the imaging array. However, the very nature of guided waves offer a series of degrees of freedom which can be tailored to obtain an multi-directional radiation pattern. One possibility is to excite modes with different phase velocities simultaneously.

Since the radiation field is the superposition of the fields radiated by each guided mode separately, a wide radiation angle can be achieved (as the phase velocity increases the radiation direction tends to the normal to the device). Moreover, for a single mode a wide band excitation in a dispersive region of the mode (i.e. in a frequency range where the phase velocity changes with frequency) results in pressure waves radiated in a range of directions. Once a guided wave reaches the tip of the device it is scattered in pressure waves radiating into the tissue and guided modes reflected back along the device. The radiation from the tip depends on the tip geometry, the frequency and the type of incident guided mode. In general, if the size of the tip is smaller or in the same order of magnitude as the wavelength of the pressure waves in the tissue it will act as an approximately omnidirectional point source (see Figure 9). Therefore, radiation from the tip improves multi-directionality. Guided waves reflected by the device tip also contribute to the radiation of pressure waves in a similar fashion to the incident ones. As an example, if a mode is reflected in the same mode (i.e. no mode conversion occurs) the radiation angle of the reflected mode is (π-θ) where θ is the radiation angle associated with the incident guided wave (Figure 10).

Finally another means to modify the radiation pattern of a guided wave is to modify the length of device which radiates into the tissue, for instance by using an external sleeve which prevents acoustic coupling between the device and the surrounding tissue as shown in Figure 11. A thin air gap 500 or other low acoustic impedance material between the needle 100 and the sleeve 510 prevents the energy leakage. This can easily be achieved in the case of core needle biopsy, by attaching the guided wave sensor on the stylet (which is a circular rod), and using the hollow needle as the sleeve. As explained before, the radiating length of the device affects the directionality of the radiated field in the same way as the diameter of a transducer determines the divergence angle in the Fraunhofer zone, i.e. the shorter the radiation length the more multi-directional is the radiated pressure field.

The present technique can be integrated with any of the commercially available ultrasound imaging systems. As an example, Figure 12 shows such an integration for phased array systems. ¹In the case of phased array imaging, an imaging control 600 calculates the distribution of time delays to be applied to the pulses fed into each array element via pulse transmit logic 620 so as to focus the acoustic beam at a prescribed point in the tissue. Time delays are also applied to the received signal so as to maximise the signal reflected by the same point in the tissue. By changing the time delay logic the imaging point can be steered within the tissue. The present technique can be combined with standard phased array systems by means of a transmit switch 610 which controls whether the array or the guided wave transducer are transmitting, hi the latter case, the guided wave transducer 110 (driven by function generator 660 and amplifier 670) excites guided waves which radiate into the tissue; the radiating pressure waves are subsequently detected by the array 200. The signals received by the array 200 via pulse receive logic 630 can be processed to build an image of the device 100 by using conventional algorithms for passive arrays. One possibility is to apply time delays to the received signals according to the standard algorithms used in phased arrays but using a speed of sound which is half that used in such algorithms so as to take into account the fact that the signal has travelled from the needle to the array only once.

The array 200 and the guided wave transducer 110 are excited sequentially. When the array is excited, an image of the tissue 300, and the needle if it is parallel to the array is produced. This may then be displayed on display device 650. On the other hand, if the guided wave transducer is excited, only an image of the device is obtained. The device image can be superimposed onto the tissue image in order to provide a representation of the position of the device relative to the tissue. A second possibility for the integration is based on a narrowband excitation of the guided wave sensor and suitable digital filtering, shown schematically in Figure 13. In this case, the imaging array and the guided wave sensor are excited simultaneously, the resulting pressure field being detected by the array 700. However, while the imaging array is excited with conventional broadband signals and standard time delay logic, the guided wave sensor is excited with a narrowband signal centred at a frequency different from the imaging array centre frequency. As in the previous approach, two separate images of the tissue and the device are produced. The device image 720 is obtained by applying a band pass filter 710, which removes the information contained outside the frequency band of the narrowband signal, to the data recorded by the imaging array.

Subsequently the same imaging algorithms as those used in the previous approach are used. On the other hand, the image of the tissue 740 is produced by applying a notch filter 730, which removes the frequencies associated with the narrowband signal, and applying the conventional imaging algorithms used in the previous approach. The two images 720 and 740 are then combined by combiner 750 into a single combined image.

Figure 14 shows preliminary experimental results for a lmm circular steel rod immersed in water obtained with a 32-element array. Both the longitudinal and flexural guided modes (see Figures 3 and 4) were excited at a centre frequency of 1.77MHz by means of a piezoelectric crystal 800 glued on one side of the rod and partially embedded in it (see Figure 15). The radiating length of the rod was limited to 3mm by means of an external sleeve 510 as shown in Figure 15. The images of Figure 14 refer to three different orientations of the needle relative to the array. The tip is detected regardless of the orientation of the rod with respect to the array. The images were built with the Kirchhoff migration technique for passive arrays.

Although a particular embodiment has been described herein, it will be appreciated that the invention is not limited thereto and that many modifications and additions thereto may be made within the scope of the invention as defined in the appended claims.

Claims

1. An ultrasonic imaging apparatus comprising: an elongate device having a proximal end and a distal end, said distal end in use penetrating an object to be imaged; and an ultrasonic transducer array for at least receiving ultrasonic waves from said object; wherein an ultrasonic transducer is coupled to said proximal end of said elongate device and is arranged to generate ultrasonic waves matched to said elongate device such that said elongate device acts as a waveguide for said ultrasonic waves with said ultrasonic waves propagating along said elongate device whilst radiating therefrom into said object, said ultrasonic transducer array is arranged to detect said ultrasonic waves radiated from said elongate device; and said ultrasonic waves are radiated from said elongate device over a solid angle such that said elongate device can be imaged substantially independently of a relative orientation of said ultrasonic transducer array and said elongate device.

2. An ultrasonic imaging apparatus as claimed in claim 1, wherein said ultrasonic transducer array is arranged to transmit further ultrasonic waves into said object and to detect said further ultrasonic waves reflected from within said object so that said object may be imaged.

3. An ultrasonic imaging apparatus as claimed in claim 2, wherein said ultrasonic transducer and said ultrasonic transducer array are adapted to generate said ultrasonic waves and said further ultrasonic waves sequentially, such that an origin of ultrasonic waves detected by said ultrasonic transducer array may be determined by their timing.

4. An ultrasonic imaging apparatus as claimed in claim 2, wherein said ultrasonic transducer and said ultrasonic transducer array are adapted to generate said ultrasonic waves and said further ultrasonic waves at different frequencies, such that an origin of ultrasonic waves detected by said ultrasonic transducer array may be determined by their frequency.

5. An ultrasonic imaging apparatus as claimed in any one of claims 2 to 4, comprising an imaging processor responsive said ultrasonic waves and said further ultrasonic waves to generate a combined image of said elongate device and said investigated matter.

6. An ultrasonic imaging apparatus as claimed in any of the preceding claims, wherein said elongate device is hollow.

7. An ultrasonic imaging apparatus as claimed in claim 6, wherein said elongate device comprises an internal stylet operable to take a sample of said object at a distal end of said elongate device.

8. An ultrasonic imaging apparatus as claimed in any one of the preceding claims, wherein said distal end of said elongate device forms a fine point.

9. An ultrasonic imaging apparatus as claimed in any one of the preceding claims, wherein said elongate device is covered by a sleeve, such that, within said object, only said distal end of said elongate device is not covered by said sleeve, said sleeve inhibiting radiation of said ultrasonic waves from said elongate device.

10. An ultrasonic imaging apparatus as claimed in claim 7 and 9, wherein said sleeve is formed by said hollow elongate device and said ultrasonic waves propagate along said stylet.

11. An ultrasonic imaging apparatus as claimed in any one of the preceding claims, wherein said elongate device is one of: a needle ; a biopsy needle a catheter; an endoscope; and a flexible elongate device.

12. A method of ultrasonic imaging comprising the steps of: inserting a distal end of an elongate device into an object; generating at a proximal end of said elongate device ultrasonic -waves matched to said elongate device so as to propagate along said elongate device as a waveguide and to radiate therefrom into said object; detecting, with an ultrasonic transducer array, ultrasonic waves radiated into said investigated matter from said elongate device; and forming an image of said elongate device within said object from said ultrasonic waves radiated into said object, wherein said ultrasonic waves are radiated from said elongate device over a solid angle such that said elongate device can be imaged substantially independently of a relative orientation of said ultrasonic transducer array and said elongate device.

13. A method as claimed in claim 12, comprising the steps of: transmitting further ultrasonic waves into said object from said, ultrasonic transducer array; and detecting with said ultrasonic transducer array said further ultrasonic waves reflected from within said object so that said object may be imaged.

14. A method as claimed in claim 13, wherein said ultrasonic waves and said further ultrasonic waves are produced sequentially, such that an origin of, ultrasonic waves detected by said ultrasonic transducer array may be determined by their timing.

15. A method as claimed in claim 13, wherein said ultrasonic waves and said further ultrasonic waves are produced at different frequencies, such that an origin of ultrasonic waves detected by said ultrasonic transducer array may be determined by their frequency.

16. A method as claimed in any one of claims 12 to 15 wherein said elongate device is hollow.

17. A method as claimed in claim 16, wherein said elongate device comprises an internal stylet operable to take a sample of said object at a distal end of said elongate device.

18. A method as claimed in any one of the preceding claims, wherein said distal end of said elongate device forms a fine point.

19. A method as claimed in any one of the preceding claims, wherein said elongate device is covered by a sleeve, such that, within said object, only said distal end of said elongate device is not covered by said sleeve, said sleeve inhibiting radiation of said ultrasonic waves from said elongate device.

20. A method as claimed in any one of claims 12 to 19, comprising the further step of: generating a combined image of said elongate device and said object.

21. A method as claimed in any one of claims 12 to 20, wherein said elongate device is one of: a needle ; a biopsy needle a catheter; an endoscope; and a flexible elongate device.

22. An elongate device adapted for ultrasonic imaging comprising: an elongate device body having a distal end for penetrating an object and a proximal end; and an ultrasonic transducer connect to said proximal end of said elongate device and adapted to generate ultrasonic waves matched to said elongate device body such that said elongate device body acts as a waveguide for said ultrasonic waves with said ultrasonic waves propagating along said elongate device body whilst radiating therefrom into said object, said ultrasonic waves being radiated from said elongate device body over a solid angle such that said elongate device body can be imaged substantially independently of a relative orientation of said ultrasonic transducer array and said elongate device body.

23. An interventional device as claimed in claim 22, wherein said elongate device body is hollow.

24. An interventional device as claimed in claim 23, wherein said elongate device comprises an internal stylet operable to take a sample of said object at a distal end of said elongate device.

25. An interventional device as claimed in any of claims 22 to 24, wherein proportions of said elongate device body and a mode of operation of said ultrasonic transducer are chosen such that a plurality of ultrasonic wave modes propagate along the length of said elongate device body.

26. An interventional device as claimed in any of claims 22 to 25, wherein said distal end of said elongate device body forms a fine point.

27. An interventional device as claimed in any of claims 22 to 26, wherein said elongate device body is covered by a sleeve, such that, within said object, only said distal end of said elongate device body is not covered by said sleeve, said sleeve inhibiting radiation of said ultrasonic waves from said elongate device body.

28. An interventional device as claimed in claim 24 and claim 27, wherein said sleeve is formed by said hollow elongate device body and said ultrasonic waves propagate along said stylet.

29. An interventional device as claimed in any of claims 22 to 28, wherein said elongate device is one of: a needle ; a biopsy needle a catheter; an endoscope; and a flexible elongate device.