WO2005085903A1

WO2005085903A1 - Phased array imaging system

Info

Publication number: WO2005085903A1
Application number: PCT/GB2005/000868
Authority: WO
Inventors: Mansour Ahmadian
Original assignee: ELEOTECH Ltd
Current assignee: ELEOTECH Ltd
Priority date: 2004-03-05
Filing date: 2005-03-07
Publication date: 2005-09-15
Anticipated expiration: 2006-09-05
Also published as: GB0405105D0

Abstract

A phased array imaging system is provided which comprises at least one transducer enabled to receive at least one incoming signal; a memory enabled to save the signals received by the transducers; a simulated wave propagation means, wherein the incoming signal is revers ed with respect to time and, subsequently, simulated emitting from at least one source thereby generating a plurality of signal powers; and an image gene rating means wherein an image is generated from the plurality of signal powers. The system can also be implemented via a computer program product.

Description

Phased Array Imaging System

The present invention relates to a phased array imaging system and particularly to a phased array imaging system using simulated wave propagation.

Many imaging systems rely on the propagation and reflection of waves in a medium. For example, ultrasound waves are commonly used in medical applications to image the foetus or internal organs of a human being.

Generally, when waves propagate from one medium to another, some of the wave is reflected and some of the wave continues to propagate. The amount of reflection is dependent on the mediums involved. If the reflected signal is detected the strength of the signal can be converted into an image intensity. By targeting pulses at particular areas and detecting the reflected and propagated waves, an image can be created of the area of interest. Phased array systems are used in many applications, most notably in ultrasound imaging. A phased array system normally has an array of transducers which allow the transmission of a pulse signal and reception of an incoming signal. However, phased array systems allow the direction of the pulse signal to be controlled through the use of a technique called "steering in sending". This is particularly useful as the array of transducers can be smaller than the target area and the target area can be scanned as necessary.

Figure 1 shows how the direction of the pulsed signal is controlled. If the array of transducers transmit the pulse signal at the same time the outputted signal is perpendicular in direction in comparison with the array of transducers (Fig. la) . To enable directional control the pulse signal is delayed with respect to one side of the array (Fig lb and Fig lc) .

The delay between triggering transducers to transmit the pulse signal is related to a target point within a medium of interest. Using this technique, it is possible to obtain images as if viewing the medium of interest from a window. This is because it is possible to steer the generated pulse signal to target points which are not in front of the array of transducers. In addition, each target point within the medium must be scanned independently. This requires a pulse for each target point and therefore takes a relatively long time. A technique called "delay and add" improved on the steering in sending technique. In delay and add, a powerful pulse signal is directed towards the medium of interest. The incoming signal detected by each of the transducers in the array is saved. Then, for each target point a delay, which would have been used in steering in sending, is used to delay the incoming signal accordingly for each transducer. The signals for each transducer are then added together to generate an associated signal power. This signal power is similar to the signal that would have been received from the target point through steering and sending but is achieved from a single pulse. The. signal powers are then converted to image intensity.

According to a first aspect of the present invention, there is provided a phased array imaging system comprising: at least one transducer enabled to receive at least one incoming signal; a memory enabled to save the signals received by the transducers; a simulated wave propagation means, wherein the incoming signal is reversed with respect to time and, subsequently, simulated emitting from at least one source thereby generating a plurality of signal powers; and an image generating means wherein an image is generated from the plurality of signal powers. Preferably, the at least one transducer is enabled to emit at least one outgoing signal.

Alternatively, the system further comprises at least one emitter enabled to emit at least one outgoing signal.

Further preferably, the at least one incoming signal is at least a partial reflection of the at least one outgoing signal.

Preferably, the simulated wave propagation means performs the steps of: reversing the at least one incoming signal with respect to time; assembling a medium simulation to approximate a target medium; selecting at least one source in the medium simulation; emitting the reversed at least one incoming signal from the at least one source; and processing a wave propagation algorithm in the medium simulation. Preferably, the simulated wave propagation means is an Inverse Transmission Line Matrix (ITLM) wave propagation simulation and the wave propagation algorithm is a Transmission Line Matrix algorithm (TLM) . Alternatively, the wave propagation algorithm is one of the following: transmission line matrix, finite difference method, finite element method, moments method or integral equation method.

According to a second aspect of the present invention, there is provided a method of obtaining an image from a phased array the method comprising the steps of: (i) emitting a pulse signal from at least one transducer; (ii) receiving a return signal; (iϋ) inverting the return signal with respect to time to create an inverted return signal; (iv) generating at least one simulated source; (v) simulating wave propagation of the inverted return signal from the at least one simulated source; and (vi) generating an image from the simulated wave propagation.

Preferably, simulating wave propagation generates a plurality of signal powers and an image is generated from the plurality of signal powers.

Preferably, simulating wave propagation includes the use of a Transmission Line Matrix simulation.

Alternatively, simulating wave propagation includes the use of one of the following simulation techniques: transmission line matrix, finite difference method, finite element method, moments method or integral equation method. According to a third aspect of the present invention, there is provided a computer program product directly loadable into the internal memory of a digital computer comprising software code portions for performing the steps of the second aspect of the invention when said product is run on a computer.

Preferably, the computer program product is enabled to run on a multi-processor computer.

Embodiments of the present invention will now be described, by way of example .only, with reference to the accompanying drawings, in which:

Fig. 1 shows a known technique called steering in sending for a phased array;

Fig. 2 shows a known Transmission Line Matrix model of wave propagation;

Fig. 3 shows a known modelling of a node in the Transmission Line Matrix of Figure 2;

Fig. 4 shows a known non—homogeneous modification to the modelling of the node of Figure 3;

Fig. 5 shows a flow diagram representing an Inverse Transmission Line Matrix phased array imaging system; and Fig. 6 shows an ultrasonic tomography system arrangement.

Transmission Line Matrix Modelling

About 300 years ago, Christian Huygens published his principle which is: All points on a wave front serve as point sources of spherical secondary wavelets. After a time T the new position of the wave front will be the surface of tangency to these secondary wavelets.

Later, this principle was modelled by sampling the space and representing it with a mesh of passive transmission line components. He modelled the wave propagation as voltage and current travelling in this mesh. Time was also sampled and the relationship between ΔT, the sample interval and ΔL, the sample space, is:

ΔL = ΔTC

Where C is the wave speed in the medium. Fig. 2 shows wave propagation in a two dimensional TLM mesh.

Assume that at time zero, an impulse is incident to a middle node N₀(Fig. 2a) . Node N₀ scatters a wave front W to its 4 neighbouring nodes Ni. The scattered wave W reaches the neighbouring nodes at time = ΔT (Fig. 2b) . Now these 4 nodes Ni scatter waves S to their neighbouring nodes N₂. At time = 2ΔT the wave front W can be found by finding waves scattered from points in Fig. 2b as shown in Fig 2c At each time step, each node receives an incident wave from its neighbours and scatters it to its neighbours. By repeating the above calculation for each node, the wave distribution in the medium can be calculated.

A node N in a TLM mesh is represented as shown in Fig. 3a. A propagating wave can travel along any transmission line 1, 2, 3 or 4 from the node N. The node N is modelled as shown in Fig. 3b. Voltages V¹ represent incident wave voltages and voltages V^s represent scattered wave voltages. The relationship between the incident wave voltages and scattered wave voltages is:

where K and K+l are arbitrary consecutive time steps separated by the time interval ΔT . Based on this relationship, if the magnitude of a wave (or voltage in the TLM model) is known at any time KΔT, then the magnitude of the wave at time (K+1)ΔT can be found. By repeating this for each time step wave propagation is modelled.

As the TLM model above is an iterative algorithm, it can be modelled in terms of a number of digital filters. Each node can be modelled as a digital filter with four inputs and four outputs. The digital filter model modifies the matrix relationship above into the following equations

As the above TLM model is a numerical model, it can be implemented in hardware or software. A hardware implementation would enable the algorithm to complete in a shorter length of time but a software implementation is quicker to create the model itself.

There is little difference in implementation for software and hardware. Firstly, the output voltages are calculated based on the above equations and then the calculated outputs are distributed to their neighbouring nodes ready for the next step of calculation.

The TLM model described above is in two dimensions. A one or three dimensional model can also be constructed by altering the dimensions of the TLM mesh. The equations described in the two dimensional model change mainly due to the different number of input and output voltages. Furthermore, if the simulation can be carried out in real time or near real time then a time dimension could also be included enabling real time imaging.

A three dimensional model can require a great many calculations and a large memory. However, the TLM model can be used with multi-processor computers. The medium may be broken down into sub-sections allowing a processor to process each of the sub- sections. At the interface between sub-sections a buffer can be used to store data until it is required.

The above TLM models do not take account of the properties of the medium and therefore is only suitable for homogeneous mediums.

The model can be further developed for a non- homogeneous medium. To represent the properties of a two-dimensional non-homogeneous medium the node N (Fig 3a) is modelled as shown in Fig. 4a. Inductors L are placed in parallel with the transmission lines and are connected by a capacitor C to the node N. Furthermore, if there are losses associated with the medium, a resistor R is placed in parallel with the capacitor C, as shown in Fig. 4b. Once again, the non-homogeneous TLM model, as described above, can be adapted to other multi- dimensional models. A TLM wave propagation model can also be used to predict the origin of a wave rather than the waves propagation. This technique is known as Inverse Transmission Line Matrix (ITLM) modelling. In ITLM, a wave front received by an array of transducers is inverted in time before simulation of the inverted signal from a simulated array of transducers arranged similarly to that of the actual array of transducers.

Furthermore, TLM is an iterative technique which analyses each node in the mesh individually and therefore can be modelled as a digital filter.

Referring now to Fig. 5, a phased array imaging system 10 has a transducer array 12 and a pulse control 14. The pulse control 14 communicates with the transducer array 12 such that a pulse signal 16 is emitted. Reflection caused by the interface of two different mediums generates a reflection signal 18. The reflection signal 18 is received by the transducer array 12. A memory 20 saves the reflection signal 18 with, respect to time. A time inversion module 22 then inverts the reflection signal 18 with respect to time.

In this embodiment, an ITLM model of wave propagation method is used to analyse the reflection signal 18. It will be appreciated that any of the following techniques (and their variations) , amongst others, could be used for wave propagation simulation: FD (Finite difference method) , FE (Finite Element), MM (Moments Method) or IE (Integral Equation) . A TLM Mesh Assembly module 24 generates a TLM mesh that represents the target medium. A TLM Source Selector 26, based on the properties of the medium and the wave properties of the pulse signal 16 as well as the specification of the transducer array 12, selects appropriate sources. Normally, the selected sources would represent the transducer array 12.

A TLM simulator 28 then reads in the TLM mesh as TLM sources as well as the received signal 18 which has -been inverted in time. A TLM simulation is then performed by emitting the inverted received signal from the selected sources. The TLM simulation will, in effect, then translate the received signal 18 back to the medium transition which generated the reflection signal. The TLM simulation will in fact assign a signal power to each node of the TLM mesh. A converter module 30 reads the signal powers of the nodes of the TLM mesh and converts them to an equivalent brightness intensity. An Image 32 is created by each nodes brightness intensity being represented by a pixel in the image 32. The image creation by TLM phased array imaging is not related to the topology of the transducer array. For this reason several new imaging modes are available, for example: Tomography Current tomography technology uses X-rays as they propagate in a straight line. The same technology can not be used to develop ultrasound tomography imaging as ultrasound does not propagate in straight lines. As The TLM phased array imaging technique is not related to the topology of the transducers, the transducers can be re-organized to develop an ultrasonic tomography system. For example, a transducer set-up is shown for ultrasound tomography in Fig. 6. Transducers 50 are placed around an imaging area 52. An object 54 is placed within the imaging area 52 and the transducers 50 emit and receive ultrasound pulses enabling an image of the object 54 or the inside of the object 54 to be created.

Although ultrasound tomography is described above, the present invention is equally applicable to other forms of tomography.

In addition, as the topology of the transducers is unrelated to image creation, radars may be improved by placing several transducers or arrays of transducers over a large area. This would allow objects which are designed to deflect radar waves away from the emitters to still be detected. The image created would be accurate enough to identify the object. Positron Emission Tomography (PET) PET is a procedure that allows a physician to examine the heart, brain, and other organs. PET images show the chemical functioning of an organ or tissue, unlike X-ray, CT, or MRI which show only body structure. PET scanners detect emission from positrons which were injected into a patient's blood. Since time, duration and shape of the emission from the positron is not known, the "delay and add" technique can not be used. Since ITLM phased array imaging is not dependent on the type and shape of the input signal, ITLM could be used for image creation in PET scanners .

Seismic data processing Since the origin of seismic signals are not known or are not well defined, it is very difficult to process them. ITLM phased array imaging can be used to analyse seismic signal and generate images of different layers of ground and rocks. This is particularly useful in identifying formations that would suggest oil reserves.

Modifications and improvements may be incorporated without departing from the scope of the invention. For example, the present invention may be used in other image processing applications involving detecting waveforms.

Claims

CLAIMS 1. A phased array imaging system comprising: at least one transducer enabled to receive at least one incoming signal; a memory enabled to save the signals received by the transducers; a simulated wave propagation means, wherein the incoming signal is reversed with respect to time and, subsequently, simulated emitting from at least one source thereby generating a plurality of signal powers; and an image generating means wherein an image is generated from the plurality of signal powers.

2. A system as claimed in claim 1, wherein the at least one transducer is enabled to emit at least one outgoing signal.

3. A system as claimed in claim 1, further comprising at least one emitter enabled to emit at least one outgoing signal.

4. A system as claimed in claim 2 or claim 3, wherein the at least one incoming signal is at least a partial reflection of the at least one outgoing signal.

5. A system as claimed in any of claims 1 to 4, wherein the simulated wave propagation means performs the steps of: reversing the at least one incoming signal with respect to time; assembling a medium simulation to approximate a target medium; selecting at least one source in the medium simulation; emitting the reversed at least one incoming signal from the at least one source; and processing a wave propagation algorithm in the medium simulation.

6. A system as claimed in any of claims 1 to 5, wherein the simulated wave propagation means is an Inverse Transmission Line Matrix (ITLM) wave propagation simulation and the wave propagation algorithm is a Transmission Line Matrix algorithm (TLM) .

7. A system as claimed in any of claims 1 to 5, wherein the wave propagation algorithm is one of the following: transmission line matrix, finite difference method, finite element method, moments method or integral equation method.

8. A method of obtaining an image from a phased array the method comprising the steps of: (i) emitting a pulse signal from at least one transducer; (ii) receiving a return signal; ( ϋi ) inverting the return signal with respect to time to create an inverted return signal ; ( iv) generating at least one simulated source ; (v) simulating wave propagation of the inverted return signal from the at least one simulated source ; and (vi ) generating an image from the simulated wave propagation .

9. A method as claimed in claim 8, wherein simulating wave propagation generates a plurality of signal powers and an image is generated from the plurality of signal powers.

10. A method as claimed in claim 7 or claim 8, wherein simulating wave propagation includes the use of a Transmission Line Matrix simulation.

11 . A method as claimed in claim 7 or claim 8 , wherein simulating wave propagation includes the use of one of the following simulation techniques : transmission line matrix, finite difference method, f inite element method, moments method or integral equation method .

12 . A computer program product directly loadable into the internal memory of a digital computer comprising software code portions for performing the steps of any of claims 8 to 11, when said product is run on a computer.

13. A computer program product as claimed in claim 10, wherein the computer program product is enabled to run on a multi-processor computer.