WO2010004333A1

WO2010004333A1 - Method and apparatus for ultrasound imaging using multiple transmit pulses at different center frequencies

Info

Publication number: WO2010004333A1
Application number: PCT/GB2009/050812
Authority: WO
Inventors: Tony Whittingham
Original assignee: Newcastle Upon Tyne Hospitals NHS Foundation Trust
Current assignee: Newcastle Upon Tyne Hospitals NHS Foundation Trust
Priority date: 2008-07-09
Filing date: 2009-07-09
Publication date: 2010-01-14
Anticipated expiration: 2011-01-09
Also published as: GB2461710A; GB0812502D0

Abstract

A method ofgenerating an ultrasound image of a target object, said method comprises the steps of: transmitting a first ultrasound pulse and at least one further ultrasound pulse in sequence along a predetermined scan line, each said transmitted ultrasound pulse having an energy spectrum with at least one peak, each said peakhaving a predetermined centre frequency,the predetermined centre frequency of at least one peak of one saidtransmitted ultrasound pulse beingdifferent from the predetermined centre frequency of at least one peak of at least one other transmitted ultrasound pulse; generating at least two respective ultrasound echo signals derived from the respective reflections of said first and at least one said further transmitted ultrasound pulses from a target object; and summing at least two said ultrasound echo signals to generate a resultant echo signal having a resultant echo signal energy spectrum.

Description

METHOD AND APPARATUS FOR ULTRASOUND IMAGING USING MULTIPLE TRANSMIT PULSES AT DIFFERENT CENTER FREQUENCIES

The present invention relates to an improved method and apparatus for ultrasound imaging of a target object.

Ultrasound imaging is used in numerous diagnostic procedures because of its noninvasive nature, relatively low cost and lack of exposure of the patient to harmful ionising radiation.

Ultrasound images are typically produced by generating an ultrasonic sound wave and transmitting the sound wave in a pre -determined direction along a scan line and towards a target object. Subsequently to being transmitted along the scan line, the sound wave is reflected by an interface between regions of differing characteristic acoustic impedence, which could be regions of differing density at the interface of the target object for example. The echo created when the sound wave is reflected is observed, and the position of the interface can be calculated by measuring the elapsed time between the transmission of the sound wave and the reception of the echo. Many scan lines, for example one hundred, may be interrogated in this way, until a complete image of the target object is generated.

It is to be appreciated that a single transmission of a sound wave can yield information about a plurality of interfaces along one particular scan line.

A continuous sound wave (that is, a sound wave represented by an infinite number of sinusoidal cycles) can be characterised by a single frequency. However, ultrasound transmissions by their very nature must have a beginning and an end and may therefore be considered to be pulses, which are instead represented by a finite number of cycles and can be characterised by a range of frequencies. The relative energy of the pulse at each frequency can be represented by an "energy spectrum," which is typically characterised by the presence of at least one peak. The centre frequency of a pulse can be defined as the mean of the two -3dB frequencies at either side of the energy spectrum.

For a symmetric energy spectrum, in the case where the energy spectrum of the pulse has a single peak, the centre frequency of the pulse is equivalent to the centre frequency of the peak itself. In the case where the energy spectrum of the pulse has a plurality of peaks however, the maximum energy of each peak of the energy spectrum of the pulse occurs at a different frequency. The centre frequency of any one peak of the energy spectrum can then be determined in a similar fashion to the centre frequency of the pulse except that a peak of an energy spectrum having more than one peak may not drop by 3dB on either side of its peak value before merging with an adjacent peak. In this case, the centre frequency of each peak is defined as the mean of the two -ndB frequencies at either side of the energy spectrum of the peak. The value of n is selected such that the energy may drop by ndB from its peak value without the possibility of merging with an adjacent peak.

The bandwidth of the energy spectrum of the pulse at half of its maximum amplitude (hereinafter referred to as the "bandwidth at half maximum") provides an indication of the range of frequencies of the pulse. For a symmetric energy spectrum, in the case where the energy spectrum of the pulse has a single peak, the bandwidth at half maximum of the pulse is equivalent to the bandwidth at half maximum of the peak itself.

The bandwidth of a pulse at half maximum is inversely proportional to the length of the pulse. For example, a pulse consisting of two cycles will have a bandwidth at half maximum of approximately half the centre frequency of the pulse, whilst a pulse consisting of four cycles will have a bandwidth at half maximum of approximately one quarter of the centre frequency of the pulse.

The spatial resolution of an ultrasound image is a measure of the extent to which fine detail may be distinguished on the image, and in ultrasound imaging, it is desirable to achieve a high spatial resolution. It is common practice to divide spatial resolution into two types, namely lateral resolution and axial resolution.

Lateral resolution can be described as the smallest separation that two reflecting or scattering interfaces, situated side by side at the same distance along two adjacent scan lines, can have if their echo pulses are to be resolved separately. Good lateral resolution requires that the width of the ultrasound beam is small. This, in turn requires that the centre frequency of the pulse is high.

Axial resolution can be described as the smallest separation that two reflecting or scattering interfaces, situated one behind the other on a single scan line, can have if their echo pulses are to be resolved separately. If the pulse length is more than twice the distance between the two interfaces then the leading part of the echo pulse from the deeper interface will overlap the tail of the echo pulse from the more shallow interface, and the two echo pulses will be merged on the display. Hence, good axial resolution requires that the length of the pulse transmitted along a scan line is short. An approximate indication of the pulse length can be obtained by dividing the number of cycles in the pulse by the centre frequency of the pulse. Limitations in transducer manufacturing technology mean it is difficult to reduce the number of cycles to much below two. Therefore in practice, the principal means of reducing the pulse length is to make the centre frequency of the pulse high.

In view of this, in order to achieve good axial and lateral resolution, and thereby achieve good image detail, the centre frequency of the pulse should be high.

Notwithstanding this, if a pulse with a high centre frequency is transmitted along a scan line, echo pulses from deep interfaces will tend to be undesirably attenuated. This is because material such as tissue absorbs and scatters both the transmitted pulses and the returning echo pulses to an extent that increases with the centre frequency of the pulse. Although the echo pulses can be amplified, an amplifier will also amplify the noise that may be present and may additionally itself introduce further noise. If the signal to noise ratio is too low, the echo pulse will be lost in noise, irrespective of how much the echo pulse is amplified.

To elaborate, the magnitude of the noise accompanying the amplified pulse is proportional to the bandwidth of the amplifier (the range of frequencies it can amplify). On account of the requirement that the amplifier bandwidth needs to be at least as wide as the bandwidth of the pulse, there will always be a poorer signal to noise ratio for short pulses (that is, pulses having a wide bandwidth) after amplification than there will be for longer pulses (that is, pulses having a narrower bandwidth) after amplification.

In order to achieve a satisfactory signal to noise ratio, the bandwidths of the transmitted pulses and receiver amplifiers used for deeper targets are often narrower than those used for shallower targets.

Attempts to increase the signal to noise ratio by means of using pulses having a greater amplitude, are of limited value. This is because, at large amplitudes, ultrasound pulses become distorted due to non-linear propagation, and much of the energy of the pulse is converted into higher frequency harmonics which then get strongly absorbed and scattered. This produces a phenomenon known as saturation, whereby increasing the amplitude of a pulse fails to produce any increase in pulse amplitude deep in the propagating medium (which could be tissue, for example). Moreover, in medical ultrasonic imaging, safety regulations restrict the maximum amplitude of transmitted pulses to levels that are only just above, or equal to, those of current practice.

In view of the attenuation and noise considerations discussed above, pulses having a low centre frequency and a long length are typically used to image deep target objects. However, as a consequence, lateral and axial resolution is reduced when imaging deeper target objects. The aim of the present invention is to overcome, or at least alleviate, at least the above mentioned disadvantages of the prior art and in particular to provide a method and apparatus for improving the lateral and axial resolution of ultrasound images of deeper target objects.

In accordance with a first aspect of the present invention, there is provided a method of generating an ultrasound image of a target object, said method comprising the steps of: -

(i) transmitting a first ultrasound pulse and at least one further ultrasound pulse in sequence along a predetermined scan line, each said transmitted ultrasound pulse having an energy spectrum with at least one peak, each said peak having a predetermined centre frequency, wherein the predetermined centre frequency of at least one peak of one said transmitted ultrasound pulse is different from the predetermined centre frequency of at least one peak of at least one other transmitted ultrasound pulse;

(ii) generating at least two respective ultrasound echo signals derived from the respective reflections of said first and at least one said further transmitted ultrasound pulses from a target object; and

(iii) summing at least two said ultrasound echo signals to generate a resultant echo signal having a resultant echo signal energy spectrum.

It is to be appreciated that although the predetermined centre frequency of at least one peak of one said transmitted ultrasound pulse is different from the predetermined centre frequency of at least one peak of at least one other transmitted ultrasound pulse, the centre frequencies at the peaks of the energy spectra can change as the ultrasound pulse travels through the tissue and towards the target object.

Each said transmitted ultrasound pulse may have an energy spectrum with a single peak. In this case, the predetermined centre frequencies of the peaks of each said transmitted ultrasound pulse may be different from each other.

Alternatively, each said transmitted ultrasound pulse may have an energy spectrum with a plurality of peaks, each peak having a different predetermined centre frequency.

In this case, the predetermined centre frequencies of each peak of each said transmitted ultrasound pulse may be different from each other.

At least one of said first or further transmitted ultrasound pulses may be a chirp.

Each of said transmitted ultrasound pulses may be chirps.

This provides the advantage of further improving the signal to noise ratio.

The ultrasound echo signals generated as a result of the reflection of said chirps from said target object may be compressed prior to being summed to generate said resultant echo signal.

The ultrasound echo signals generated as a result of the reflection of said chirps from said target object may be amplified prior to being summed to generate said resultant echo signal.

Preferably, each peak of the energy spectra of each said transmitted ultrasound pulse may have a predetermined bandwidth at half the maximum amplitude, wherein the bandwidth at half the maximum amplitude of the peak of said resultant echo signal energy spectrum is greater than the bandwidth at half the maximum amplitude of any of the peaks of the energy spectra of any of said first or further transmitted ultrasound pulses.

This provides the advantage of achieving a resultant echo signal having a wide bandwidth at half maximum, without the associated poor signal to noise ratio normally associated with using a pulse having a wide bandwidth at half maximum. In this way, the present invention allows for improved imaging of deeper target objects, such as the retina when imaging the eye.

In other words, summing at least two ultrasound echo signals each having an energy spectrum with one or more peaks with narrow bandwidths produces the wide bandwidth ultrasound echo signal that it would have been desirable to obtain by transmitting a single wide bandwidth pulse, were it not for the associated problem of a low signal to noise ratio.

It is to be appreciated that the ultrasound echo signal is derived from the reflection of the ultrasound pulse. For example, the term "ultrasound echo signal" can be understood to be an electronic manifestation of the ultrasound echo pulse, and comes into existence as a result of an ultrasound echo pulse received from a target object. For example, an ultrasound echo signal may come into existence as a result of the conversion of pressure from the ultrasound echo pulse into a voltage by a transducer.

It is to be understood however, that the ultrasound echo signals could be processed in some way before being summed. For example, the ultrasound echo signals could be amplified, filtered or, in the case of chirps for example, compressed, prior to being summed.

The improvement in the signal to noise ratio, for a given peak amplitude in the transmitted pulse, is on account of the narrow bandwidths of the peaks of the energy spectra of the transmitted ultrasound pulses and hence the associated amplifiers and/or filters used in reception. It is to be appreciated that the amplifier used to amplify the ultrasound echo signals should have a bandwidth at half maximum that is no greater than that necessary in order to benefit from the advantages of the present invention in terms of the improvement to the signal to noise ratio. For example, if an ultrasound echo signal has an energy spectrum with a single peak having a centre frequency of 2 MHz and a bandwidth at half maximum of 2 MHz, then the amplifier might have a response between 0.5 MHz and 3.5 MHz.

Preferably, the method may comprise summing each said ultrasound echo signal.

Preferably, the method may further comprise amplifying at least one said ultrasound echo signal prior to generating said resultant echo signal.

Preferably, the method may comprise amplifying at least two said ultrasound echo signals prior to generating said resultant echo signal, wherein each said amplified ultrasound echo signal is amplified by a different amount.

The method may comprise amplifying each said ultrasound echo signal, storing each said ultrasound echo signal, and then summing each said ultrasound echo signal to generate said resultant echo signal.

Alternatively, the method may comprise amplifying each said ultrasound echo signal, storing each said amplified ultrasound echo signal apart from the final one, and then summing each said ultrasound echo signal to generate said resultant echo signal.

In other words, it may not be necessary to store the final amplified ultrasound echo signal, whereby the summing process may commence as soon as the final amplified ultrasound echo signal is generated.

The amplifications provided to the ultrasound echo signals may be such that the resultant echo signal energy spectrum is substantially Gaussian in nature. However, it is to be appreciated that resultant echo signals having various different energy spectra, such as a top hat, may alternatively be generated. The method may comprise summing each said ultrasound echo signal as it is generated. In this way, it is not necessary to wait until all of the ultrasound echo signals have been generated, before any of the ultrasound echo signals are summed.

The method may further comprise providing at least one said ultrasound echo signal with a time shift.

This step may be carried out in order to correct for the differences in time delay which may be imposed on at least some of the ultrasound echo signals on account of their passing through an amplifier or other electronic circuit.

The method may further comprise providing at least one said ultrasound echo signal with a time shift so that at a time substantially halfway through the duration of the ultrasound echo signal, all of the ultrasound echo signals are substantially in phase with each other. It is to be appreciated that the time shift could alternatively be such that at any time through the duration of the ultrasound echo signals, for example at their beginnings, all of the ultrasound echo signals are substantially in phase with each other.

This time shift might, for example, be necessary to correct for any differences in the time between a transmission trigger, synchronised with the capture and storage of ultrasound echo signals, and a fiduciary point on the various transmitted ultrasound pulses. The fiduciary point on a transmitted ultrasound pulse is that point which would have to be time aligned with the fiduciary points of all the other transmitted ultrasound pulses in order that the ultrasound echo signals resulting from all the transmitted ultrasound pulses would sum to give a resultant echo signal of the desired waveform and bandwidth.

The method may further comprise providing said resultant echo signal with a time shift.

This provides the advantage that electronic focussing is achievable. The method may further comprise filtering at least one said ultrasound echo signal prior to generating said resultant echo signal.

The filtering provided to the ultrasound echo signals may be such that the resultant echo signal energy spectrum is substantially Gaussian in nature. However, it is to be appreciated that resultant echo signals having various different energy spectra, such as a top hat, may alternatively be generated.

Preferably, the predetermined bandwidths at half the maximum amplitude of each peak of the energy spectra of said first and each said further transmitted ultrasound pulse may be substantially equal.

This provides the advantage that the method is simplified in as much as it is easier to achieve a particular shape of resultant echo signal energy spectrum.

Preferably, the sum of the predetermined bandwidths at half the maximum amplitude of each peak of the energy spectra of said first and each said further transmitted ultrasound pulse may be greater than the bandwidth at half the maximum amplitude of the resultant echo signal energy spectrum.

In this way, the energy spectra of said first and each said further transmitted ultrasound pulse overlap with each other on the frequency scale. This in turn provides the advantage that the resultant echo signal energy spectrum is free from gaps and can have a smooth envelope. In having a resultant echo signal energy spectrum which is free from gaps and which has a smooth envelope, this provides the advantage that there are less range lobes which could otherwise reduce the contrast resolution of the image.

Alternatively, the sum of the predetermined bandwidths at half the maximum amplitude of each peak of the energy spectra of said first and each said further transmitted ultrasound pulse may be less than or substantially equal to the bandwidth at half the maximum amplitude of the resultant echo signal energy spectrum. This provides the advantage that a higher signal to noise ratio can be obtained.

A first and eight further ultrasound pulses may be transmitted in sequence along said predetermined scan line.

This provides the advantage of achieving a reasonable compromise in some circumstances between the number of transmitted ultrasound pulses (frame rate penalty) and the reduction in the individual pulse bandwidth (lower noise).

However, it is to be appreciated that any number of ultrasound pulses greater than one could be transmitted in sequence along saidpre-determined scan line.

Said method may be repeated on said pre-determined scan line.

This provides the advantage that the ultrasound echo signals generated on a single scan line may be monitored over time. This is particularly advantageous in the event that the target object is for example, a beating heart.

This provides the further advantage that different parts of the target object can be quickly examined. To elaborate, this provides the advantage that a probe used to carry out the process can be quickly moved over the body, in order to interrogate each scan line.

Said method may be repeated on at least one further scan line adjacent to saidpre- determined scan line.

This provides the advantage that a complete ultrasound image, either two dimensional or three dimensional, may be generated. For example, when a plurality of different scan lines are interrogated in this way, a complete image of the target object may be created. It is to be appreciated that the method could first be carried out on said pre-determined scan line and then each said further scan line, and at least one further time on said predetermined scan line and each said further scan line.

This provides the advantage that the two dimensional or three dimensional image generated may be monitored in a real time fashion.

Said step of repeating said method on at least one further scan line adjacent to said predetermined scan line may comprise firstly, transmitting said first ultrasound pulse and each said further ultrasound pulse in sequence along said predetermined scan line, and then transmitting a further said first ultrasound pulse and further said ultrasound pulses in sequence along each said further scan line in sequence.

Alternatively, said step of repeating said method on at least one further scan line adjacent to said predetermined scan line may comprise simultaneously transmitting said first ultrasound pulses along said predetermined and at least one said further scan line, and then transmitting first said further ultrasound pulses simultaneously along said predetermined and at least one said further scan line, and repeating this process until a first and each said further ultrasound pulse have been transmitted along said predetermined and at least one said further scan line.

This provides the advantage that the total time taken to interrogate all of the scan lines of interest is reduced. In other words, the frame rate is increased. It is to be appreciated that interrogation of a scan line can be understood as being the transmission of at least one ultrasound pulse along a scan line and the reception of at least one of the ultrasound echo pulses resulting from the reflection of at least one said ultrasound pulse from at least one interface.

For example, it would take a specific amount of time to interrogate a predetermined scan line with three ultrasound pulses in sequence, then interrogate a second scan line with three ultrasound pulses in sequence and then interrogate a third scan line with three ultrasound pulses in sequence. It would, however, only take a third of this time to simultaneously interrogate said predetermined, said second and said third scan lines with said first ultrasound pulses, then simultaneously interrogate said predetermined, said second and said third scan lines with said second ultrasound pulses and then simultaneously interrogate said pre-determined, said second and said third scan lines with said third ultrasound pulses. This increase in the frame rate is not as critical when images of shallow target objects are being generated, since the "go and return time" of the ultrasound pulse is relatively small. However, this increase in the frame rate is particularly advantageous when images of deeper target objects are being generated, or in the event that the target object is moving.

Said step of repeating said method on at least one further scan line adjacent to said predetermined scan line may comprise simultaneously transmitting said first ultrasound pulses along said predetermined and each said further scan line, and then transmitting first said further ultrasound pulses simultaneously along said predetermined and each said further scan line, and repeating this process until a first and each said further ultrasound pulse have been transmitted along each said scan line.

More preferably still, the predetermined centre frequencies of each peak of the energy spectra of the ultrasound pulses which are being simultaneously transmitted along the scan lines are different from each other.

This provides the advantage that "cross talk" between adjacent scan lines, is reduced. This is because the frequency ranges of the receiving amplifiers are different at any time from one line to the next.

In accordance with a second aspect of the present invention, there is provided an apparatus for generating an ultrasound image of a target object, said apparatus comprising: - (i) transmitting means adapted to transmit a first ultrasound pulse and at least one further ultrasound pulse in sequence along a predetermined scan line, each said transmitted ultrasound pulse having an energy spectrum with at least one peak, each said peak having a predetermined centre frequency, wherein the predetermined centre frequency of at least one peak of one said transmitted ultrasound pulse is different from the predetermined centre frequency of at least one peak of at least one other transmitted ultrasound pulse;

(ii) generating means adapted to generate at least two respective ultrasound echo signals derived from the respective reflections of said first and at least one said further transmitted ultrasound pulses from a target object; and

(iii) summing means adapted to sum at least two said ultrasound echo signals to generate a resultant echo signal having a resultant echo signal energy spectrum.

Said apparatus may comprise a digital signal processor.

In accordance with a third aspect of the present invention, there is provided a software product for generating an ultrasound image of a target object, the software product comprising a computer readable medium upon which instructions are stored, wherein the instructions, when read by the computer, cause the computer to: -

(i) transmit a first ultrasound pulse and at least one further ultrasound pulse in sequence along a predetermined scan line, each said transmitted ultrasound pulse having an energy spectrum with at least one peak, each said peak having a predetermined centre frequency, wherein the predetermined centre frequency of at least one peak of one said transmitted ultrasound pulse is different from the predetermined centre frequency of at least one peak of at least one other transmitted ultrasound pulse; (ii) generate at least two respective ultrasound echo signals derived from the respective reflections of said first and at least one said further ultrasound pulses from a target object; and

(iii) sum at least two said ultrasound echo signals to generate a resultant echo signal having a resultant echo signal energy spectrum.

The software product may be a computer software product.

Alternatively, the software product may be a digital signal processing software product.

Preferred embodiments of the present invention will now be described, by way of example only and not in any limitative sense, with reference to the accompanying drawings in which: -

Figure 1 shows the energy spectra of nine ultrasound pulses transmitted along a scan line in accordance with a first embodiment of the present invention;

Figure 2 shows the waveforms of the transmitted ultrasound pulses transmitted along a scan line, in accordance with a first embodiment of the present invention;

Figure 3 shows the energy spectra of the ultrasound echo signals resulting from the transmitted ultrasound pulses of Figure 1. The amplification provided to each ultrasound echo signal is such that the ultrasound echo signals sum to form a resultant echo signal having an energy spectrum with a bandwidth at half maximum b ' greater than the bandwidths at half maximum of any of the energy spectra of the ultrasound echo signals;

Figure 4 shows the waveforms of the amplified ultrasound echo signals represented by the energy spectra of Figure 3; Figure 5 shows images of a computer model of a cyst in tissue which would be generated if a single conventional wide bandwidth ultrasound echo pulse were used, for different values of the signal to noise ratio applied to the conventional ultrasound echo signals;

Figure 6 shows images of the same computer model of a scatterer-free cyst in tissue as Figure 5 which would be generated using a resultant echo signal in accordance with a first embodiment of the present invention, for different values of the signal to noise ratio applied to the first and each said further ultrasound echo signal;

Figure 7 shows a representation of random scatterers in tissue and a cyst in the computer model used to generate the images of Figures 5 and 6;

Figure 8 shows an example of the waveform of a typical ultrasound chirp pulse;

Figure 9 shows the energy spectrum representing the ultrasound chirp pulse of Figure 8;

Figure 10 shows a compressed ultrasound echo signal resulting from the transmission of the ultrasound chirp pulse shown in Figure 8;

Figure 11 shows the waveforms of three transmitted ultrasound chirp pulses in accordance with a second embodiment of the present invention;

Figure 12 shows the energy spectra representing the transmitted ultrasound chirp pulses of Figure 11;

Figure 13 shows compressed ultrasound echo signals resulting from the transmission of the three ultrasound chirp pulses shown in Figure 11 ;

Figure 14 shows a resultant echo signal in accordance with a second embodiment of the present invention; Figure 15 shows a possible arrangement for the simultaneous transmission of ultrasound pulses along six adjacent scan lines, in accordance with a third embodiment of the present invention;

Figure 16 shows the waveforms of nine transmitted ultrasound pulses in accordance with a fourth embodiment of the present invention;

Figure 17 shows the energy spectra representing the pulses of Figure 16;

Figure 18 shows the waveforms of the amplified ultrasound echo pulses resulting from the transmission of the nine ultrasound pulses shown in Figure 16;

Figure 19 shows a resultant echo signal energy spectrum in accordance with a fourth embodiment of the present invention;

Figures 20a and 20b show the waveforms of two ultrasound echo pulses, in accordance with a fifth embodiment of the present invention;

Figure 21 shows a resultant echo signal in accordance with a fifth embodiment of the present invention;

Figure 22a shows the waveform of an ultrasound pulse transmitted along a scan line in accordance with a sixth embodiment of the present invention;

Figures 22al and 22a2 show the waveforms of the constituent transmitted ultrasound pulses forming the transmitted ultrasound pulse shown in Figure 22a; and

Figures 23a, 23b and 23c show the energy spectra of three ultrasound pulses transmitted along a scan line in accordance with a sixth embodiment of the present invention. With reference to Figures 1 to 3, nine ultrasound pulses in total are transmitted along each scan line. In other words, N = 9, where N represents the total number of pulses transmitted along a particular scan line. It is to be appreciated that in this case, the ultrasound pulses are transmitted along one scan line at time. The first ultrasound pulse transmitted along the first scan line has a symmetric energy spectrum with a single peak having a centre frequency fi and a narrow bandwidth at half maximum bi. The second pulse transmitted along the first scan line has a symmetric energy spectrum with a single peak having a centre frequency f₂ and a narrow bandwidth at half maximum bi, which is the same as that of the first ultrasound pulse transmitted along the first scan line. This process is repeated until nine pulses have been transmitted along the first scan line, whereby the energy spectra of each of the ultrasound pulses have single peaks having different centre frequencies f but the same bandwidth at half maximum bi. Ultrasound pulses are then transmitted along the second, third, fourth, fifth and sixth scan lines, in a similar fashion.

It is to be noted that the narrow bandwidths of the transmitted ultrasound pulses are as a consequence of their containing more cycles and hence being longer, than conventional transmitted ultrasound pulses having energy spectra with the same centre frequencies which typically contain only two or three cycles. In the example shown in Figure 2, all of the transmitted ultrasound pulses have the same length and hence the same bandwidth at half maximum b, but contain different numbers of cycles depending upon the centre frequency f of the peak of the energy spectrum.

A transducer converts the pressure from the ultrasound echo pulses returning from the target object into a voltage, resulting in the generation of ultrasound echo signals.

With particular reference to Figures 3 and 4, the ultrasound echo signals produced by each transmission are then amplified in a narrow passband amplifier. It is to be appreciated that the narrow passband amplifier has a bandwidth at half maximum b which is wide enough for it to be able to amplify all of the frequencies in the ultrasound echo signals, but not so wide that the signal to noise ratio is too low. In practice, the same amplifier hardware may be used for all of the ultrasound echo signals, with the centre frequency of the energy spectrum and the bandwidth at half maximum being changed by software. The amplifications provided to each ultrasound echo signal may be different.

The amplified ultrasound echo signals are then stored temporarily and summed together to generate a resultant echo signal having a wide bandwidth resultant echo signal energy spectrum A with a peak having a centre frequency f and a bandwidth at half maximum b', as shown in Figure 3. As can be seen from Figure 4, all of the ultrasound echo signals are amplified by different amounts and are in phase with one another at a time substantially halfway through their duration, corresponding to the point at which the transmitted ultrasound pulses were in phase, as shown in Figure 2. It is to be appreciated that transmitted ultrasound pulses with other waveforms may be in phase at another point during their duration, as shown in Figure 16 for example, where the ultrasound pulses are in phase at their beginnings.

Processing of the ultrasound echo signals from a given scan line before summation may include, if necessary, the introduction of time delays. These delays may be different for different ultrasound echo signals transmitted along a given scan line.

The advantage of this is that it allows for compensation of any differences that may exist in the time delays introduced by electronic circuits or ultrasound transducers to electrical transmission signals or ultrasound echo signals having energy spectra with peaks having different centre frequencies. One method of measuring any differences in such delays is by an experiment in which a fixed single interface target is interrogated by each of the transmitted ultrasound pulses in turn.

As can be seen from Figure 3, the amplification provided to each of the ultrasound echo signals is chosen such that if the ultrasound echo signals were summed, a resultant echo signal would be produced, having a spectrum either the same or substantially the same, as that of the ultrasound echo signal that would have been produced by transmitting a single wide bandwidth pulse, that it would have been desirable to transmit had it not been for the problem of poor signal to noise ratio.

Also as can be seen from Figure 3, the energy spectrum of the resultant echo signal is symmetric and has a Gaussian shape. However, it is to be appreciated that resultant echo signals having energy spectra of other shapes, such as a top hat for example, may alternatively be generated. The energy spectrum of the resultant echo signal is governed by the centre frequencies of the peaks of the energy spectra of the transmitted ultrasound pulses and also the bandwidths of the peaks of the energy spectra of the transmitted ultrasound pulses. The energy spectrum of the resultant echo signal is also governed by how much the ultrasound echo signals are amplified relative to each other before being summed.

The options for further processing of the resultant echo signal are the same as those available for processing an ultrasound echo signal that would have resulted from the transmission of a single wide bandwidth ultrasound pulse in the conventional manner.

For example, the resultant echo signal could be processed by means of undergoing time gain compensation, which is sometimes referred to as swept gain.

Alternatively, the resultant echo signal may be processed by means of either dynamic range compression, digitising, or edge enhancement.

It is to be appreciated that ultrasound images may be generated by processing more than one resultant echo signal from each scan line. For example, a plurality of resultant echo signals may be generated and processed for each scan line.

It is also to be appreciated that although the ultrasound echo signals used in this embodiment have symmetric energy spectra having a Gaussian shape, it is envisaged that ultrasound echo signals having energy spectra with different shapes may alternatively be used. For example, it is also envisaged that ultrasound echo signals having energy spectra having a top hat shape could be used.

The improvement to be expected by using the method of the present invention was investigated using a computer generated representation of a target object in the form of a cyst having a diameter of twelve times the wavelength of the transmitted ultrasound pulse, as shown in Figure 7. Tissue was represented by randomly placed scatterers, whilst the cyst itself contained no scatterers. For simplicity, all of the scatterers were assumed to return ultrasound echo pulses having equal amplitudes.

Figures 5 and 6 compare the images obtained by transmitting a conventional single pulse having a wide bandwidth at half maximum along each scan line, with those obtained by way of utilising resultant echo signals generated as described above, in respect of each scan line.

As can be seen from Figures 5 and 6, with no noise added to the ultrasound echo signals, the image generated by transmitting a conventional single ultrasound pulse having a wide bandwidth is very similar to that obtained by way of utilising a resultant echo signal generated in accordance with the present invention.

When noise having a root mean square (rms) amplitude equal to the rms amplitude of the ultrasound echo pulse is added to the ultrasound echo signals (that is, the signal to noise amplitude ratio is 1), the image generated by transmitting a conventional single ultrasound pulse having a wide bandwidth is less clear that that obtained by way of utilising a resultant echo signal generated in accordance with the present invention.

When noise having a rms amplitude equal to twice the rms amplitude of the ultrasound echo pulse is added to the ultrasound echo signals (that is, the signal to noise amplitude ratio is VL), the image generated by transmitting a conventional single ultrasound pulse having a wide bandwidth is significantly less clear than that obtained by way of utilising a resultant echo signal generated in accordance with the present invention, with the cyst image being almost indistinguishable from the speckle produced by the tissue.

Even when noise having a rms amplitude equal to four times the rms amplitude of the echo pulse is added to the ultrasound echo signals (that is, the signal to noise amplitude ratio is ¹A), the image generated by way of utilising a resultant echo signal generated in accordance with the present invention continues to be discernable, albeit with some internal noise. In contrast, the image generated by transmitting a conventional single ultrasound pulse having a wide bandwidth becomes lost in noise.

It is only when noise having a rms amplitude equal to eight times the maximum amplitude of the ultrasound echo pulse is added to the ultrasound echo signals (that is, the signal to noise amplitude ratio is 1/8) that the image generated by way of utilising a resultant echo signal in accordance with the present invention becomes vague.

The amplitude signal to noise ratio at which the method of the present invention fails to detect the cyst is therefore just over four times smaller than that at which the conventional single wide bandwidth ultrasound pulse method fails.

A second embodiment of the present invention, which provides further advantages, will be described below.

With reference to Figures 8 to 14, it will be understood by those skilled in the art that an ultrasound chirp pulse is an ultrasound pulse that is modulated in amplitude over time and has a frequency which progressively increases or decreases with time. Figure 8 shows the waveform of a conventional ultrasound chirp pulse having a Gaussian amplitude modulation and a frequency sweep from 1 MHz to 5 MHz. Figure 9 shows the energy spectrum corresponding to the conventional ultrasound chirp pulse of Figure 8.

Ultrasound chirp pulses may be used in order to improve the signal to noise ratio of ultrasound echo signals. After transmission, the ultrasound chirp pulses are reflected from the interfaces of the target objects to form ultrasound chirp echo pulses. The ultrasound chirp echo pulses have largely similar waveforms to the transmitted ultrasound chirp pulses but with smaller amplitudes. A transducer then converts the pressure from the ultrasound chirp echo pulses into voltage, resulting in the generation of ultrasound chirp echo signals, which are then compressed. Figure 10 shows the ultrasound echo signal obtained by compressing the ultrasound chirp echo signal resulting from the conventional ultrasound chirp pulse of Figure 8. It is to be appreciated that compression of an ultrasound chirp echo signal increases its amplitude, which improves the signal to noise ratio. Moreover, compression of the ultrasound chirp echo signal reduces the pulse length, which is important if the image is to have good axial resolution.

In this embodiment, it is to be appreciated that the transmission of the ultrasound chirp pulses and therefore the generation of a resultant echo signal, is carried out in a similar fashion to that described with reference to Figures 1 to 7, albeit with some differences as will be described below.

In particular, in this embodiment, three ultrasound chirp pulses are transmitted along each scan line, one scan line at a time. The first ultrasound chirp pulse transmitted along the first scan line has a frequency sweep from say, 1.7 MHz to 3.3 MHz, and has a symmetric energy spectrum having a single peak having a centre frequency of 2.5 MHz. The second ultrasound chirp pulse transmitted along the first scan line has a frequency sweep from say, 2.2 MHz to 3.8 MHZ and has a symmetric energy spectrum having a single peak having a centre frequency of 3.0 MHz and a bandwidth at half maximum which is the same as that of the first ultrasound chirp pulse transmitted along the first scan line. The third ultrasound chirp pulse transmitted along the first scan line has a frequency sweep from say 2.7 MHz to 4.3 MHz and has a symmetric energy spectrum having a peak having a centre frequency of 3.5 MHz and a bandwidth at half maximum which is the same as that of the first and second ultrasound chirp pulses transmitted along the first scan line. Figure 11 shows the waveforms of each of these three ultrasound chirp pulses, and Figure 12 shows the energy spectra of these three ultrasound chirp pulses. The three compressed ultrasound echo signals that are generated as a result of reflection of the three transmitted ultrasound chirp pulses from a target object on the first scan line in this example are represented in Figure 13.

The compressed ultrasound echo signals produced by each transmission are then amplified and stored temporarily, given small time shifts if necessary to bring them into phase at their centres, and then summed to produce a resultant echo signal.

Three ultrasound chirp pulses are then transmitted along the second and each further scan line in turn and in a similar fashion.

Figure 14 shows a resultant echo signal in which the amplifications provided to each of the compressed ultrasound echo signals shown in the example of Figure 13 have been chosen in order to give the resultant echo signal a waveform that is substantially the same as the wide bandwidth compressed ultrasound echo signal shown in Figure 10. The resultant echo signal of Figure 14 also has an energy spectrum having a single peak having substantially the same centre frequency as that of the energy spectrum of Figure 9, and a bandwidth at half maximum that is substantially the same as the bandwidth at half maximum of the energy spectrum shown in Figure 9.

Thus the resultant ultrasound echo signal obtained by transmitting three narrow bandwidth ultrasound chirp pulses can be made to be substantially the same, as that of the ultrasound echo signal that would have been produced by transmitting a single wide band width ultrasound chirp pulse. However, the narrower bandwidths of the three transmitted ultrasound chirp pulses allows the bandwidth of the amplifiers to be reduced, meaning that the signal to noise ratio of the resultant echo signal is greater than that that which would have been possible had a single wide bandwidth ultrasound chirp pulse been transmitted. In this example, the waveform and the bandwidth at half maximum of the resultant echo signal is similar to that of the ultrasound echo signal that would have been produced by transmitting the single 1 MHz to 5MHz conventional ultrasound chirp pulse of Figure 8.

It is noticeable that there are low amplitude range lobes at the extreme ends of the resultant echo signal shown in Figure 14. These range lobes can in some circumstances reduce the contrast resolution of the image, but can be reduced by means of increasing the frequency sweeps and hence the bandwidths of the transmitted ultrasound chirp pulses (but not to such a degree that the signal to noise ratio is decreased to an unacceptable level) or by increasing the total number of ultrasound chirp pulses having energy spectra with peaks having different centre frequencies transmitted along a particular scan line.

In order to reduce the time it takes to transmit all of the ultrasound pulses (either ultrasound chirp pulses, or ordinary ultrasound pulses as described with reference to Figures 1 to 7) along each of the scan lines, at least two scan lines may be interrogated simultaneously. It is to be appreciated that although all of the scan lines of interest may be interrogated simultaneously, it is alternatively envisaged that only some of the scan lines of interest may be interrogated simultaneously.

In the case where at least two of the scan lines of interest are interrogated simultaneously, individual amplifiers having different frequency passbands could be arranged in parallel for each of the scan lines of interest to amplify the ultrasound echo signals returning from the interfaces along these scan lines. It is however to be appreciated that alternatively, only one amplifier could be used in which the frequency passbands can be altered electronically, as opposed to employing a plurality of amplifiers arranged in parallel. Where M represents the total number of scan lines to be simultaneously interrogated (which may or may not be equal to the total number of scan lines being interrogated), once the first ultrasound pulses have been transmitted simultaneously along the M scan lines, further ultrasound pulses are transmitted simultaneously along each of the M scan lines, and this process is repeated until N pulses in total have been transmitted along each of the M scan lines. In order to minimise the degree of cross-talk between scan lines, the M scan lines should be spaced as far apart as reasonably possible within the field of view.

A third embodiment of the invention will now be described with reference to Figure 15, which shows a possible arrangement for the simultaneous transmission of ultrasound pulses along six scan lines as described above (in this case, M=6). At one instant, six ultrasound pulses, each having an energy spectrum with a single peak having a different centre frequency (fi, f₂, f₃, U, fs and f₆ respectively) are being transmitted simultaneously along their respective scan lines. If the centre frequency of the peak of the energy spectrum of the first ultrasound pulse transmitted along a first scan line is fj, it could be f₁₊i for the next transmission along the same scan line, f₁₊₂ for the next, and so on up to f^ (where N is the total number of pulses transmitted along a scan line), after which it would be fi then f₂ etc. up to f_i, by which time all N pulses would have been transmitted along the first scan line. For example, if ten pulses were to be transmitted along each scan line, the centre frequencies of the peaks of the energy spectra of the transmitted ultrasound pulses for the first of the six scan lines would be in the sequence fi, f₂, f₃, U, fs, fβ, fz, fs, fθ and fio. The centre frequencies of the peaks of the energy spectra of the transmitted ultrasound pulses for the next scan line being interrogated could be in the sequence f₃, U, fs, f₆, f₇, f₈, {9, fio, fi and f₂. This arrangement ensures that at any time, ultrasound pulses with adjacent centre frequencies of the peaks of their energy spectra (i.e. fi and f₂, or fs and f₆) are well separated spatially from each other, thereby reducing the extent of crosstalk between scan lines.

Referring now to Figures 16 to 19, a fourth embodiment of the invention is described, whereby ultrasound pulses having a sine waveform with an exponentially decaying amplitude are used as the transmitted ultrasound pulses. It is to be appreciated that the procedure of ultrasound pulse transmission, generation of ultrasound echo signals, and summation of the ultrasound echo signals is the same as that described with respect to the Gaussian ultrasound pulses of Figures 1 to 4. Referring now to Figures 20 and 21, a fifth embodiment of the present invention, which provides further advantages, will be described below.

This embodiment utilises the second harmonic frequency components that are acquired by the ultrasound pulses as they propagate through the tissue being imaged. The second harmonic frequency components of the ultrasound echo pulses can then be used to generate respective ultrasound echo signals, which are then summed to generate the resultant echo signal in a similar fashion to that described with respect to the previous embodiments.

The images generated using the resultant echo signals of this embodiment have less acoustic noise and greater lateral resolution than images generated using resultant echo signals made from the fundamental frequency components of the ultrasound echo pulses.

One way of extracting the second harmonic frequency components of ultrasound echo pulses is by a method known as pulse inversion. This involves interrogating each scan line with two transmitted ultrasound pulses, whereby the first transmitted pulse has an inverse waveform to that of the second transmitted pulse. It is to be appreciated that the energy spectra of the first and second transmitted ultrasound pulses each have a single peak having the same centre frequency. Two ultrasound echo signals resulting from the reflection of the first and second transmitted pulses from the target object are then generated. These two ultrasound echo signals are filtered and then summed, with the result that the fundamental frequency components are cancelled out and a second harmonic component is extracted.

In this embodiment, N interrogations of each scan line are carried out using N narrow bandwidth transmitted ultrasound pulses each having symmetric energy spectra with single peaks having different centre frequencies. In this embodiment however, a further N interrogations of each scan line are carried out using N narrow bandwidth transmitted ultrasound pulses, which are inverted versions of the transmitted ultrasound pulses used in the first N interrogations. Each ultrasound echo signal generated from a narrow band ultrasound echo pulse is filtered by a band pass amplifier centred on a frequency equal to twice that of the transmitted ultrasound pulse and a bandwidth which is no greater than that needed to pass the narrow band second harmonic spectrum. All of the 2N filtered ultrasound echo signals are summed, thereby producing a resultant echo signal in which the fundamental frequency components are cancelled out, leaving behind only the second harmonic frequency components. The resultant echo signal generated has an energy spectrum with the same wide bandwidth as the resultant echo signal that would have been obtained by summing the N non-inverted transmitted ultrasound pulses.

It is to be appreciated that the transmitted ultrasound pulses initially ideally only have fundamental frequency components, with the second harmonic frequency components being generated as the ultrasound pulse propagates in the tissue.

With particular reference to Figures 20a and 20b, the ultrasound echo pulse as shown in Figure 20a has a waveform consisting of a fundamental sinewave plus a second harmonic frequency component, all multiplied by a Gaussian envelope. The ultrasound echo pulse as shown in Figure 20b has a waveform which is the sum of an inverted version of the fundamental sine wave and the same second harmonic frequency component. The centre frequency of the peak of the energy spectrum of the ultrasound echo pulse of Figure 20b is the same as that of the ultrasound echo pulse of Figure 20a. In total, there are N ultrasound echo pulses of the type as shown in Figure 20a and N ultrasound echo pulses of the type as shown in Figure 20b, with N ultrasound echo pulse pairs being formed. Each pair comprises an ultrasound echo pulse such as that as shown in Figure 20a and an ultrasound echo pulse such as that shown in Figure 20b. Each ultrasound echo pulse in a pair has an energy spectrum with a peak having the same centre frequency as the other ultrasound echo pulse in the pair, but the peaks of the energy spectra of each pair of ultrasound echo pulses have different centre frequencies. The 2N ultrasound echo signals generated as a result of the afore -mentioned ultrasound echo pulses are then summed to produce the resultant echo signal as shown in Figure 21. It is to be appreciated that the filtering and summing can be carried out in a different order. For example, each pair of ultrasound echo signals from inverted and non-inverted transmissions can be first summed together. The summed signal from each pair is then filtered with a narrow band filter centred at twice the centre frequency of those two transmissions. All the N narrow band filtered signals are then summed to produce the resultant wide band second harmonic signal. This has the advantage of involving fewer filtering operations.

It is to be appreciated that in all of the embodiments described, the centre frequencies of the peaks of the energy spectra of the ultrasound pulses can change as the ultrasound pulse travels through tissue and towards the target object. In particular, the energy spectrum of the ultrasound pulse can lose higher frequency components as the ultrasound pulse travels deeper into the tissue. This means that the centre frequencies and bandwidths of the peaks of the energy spectrum both can reduce with increasing depth. These depth dependent changes would not, however, have an adverse effect upon the summation of the ultrasound echo signals if the fractional change in the centre frequency of the peak of the energy spectrum were the same for all of the energy spectra. Assuming that this is the case, although the bandwidth of the resultant echo signal will be slightly less than if there was no loss of higher frequency components, the bandwidth of the resultant echo signal will still be greater than the bandwidths of any of the peaks of the energy spectra of any of the constituent ultrasound echo signals.

For a Gaussian shaped transmitted ultrasound pulse having an energy spectrum with a single peak, achieving the same fractional change in the centre frequency of the peak of the energy spectrum for all of the transmitted ultrasound pulses requires that the centre frequency of the peak of the energy spectrum for each transmitted ultrasound pulse is directly proportional to the square of its bandwidth at half the maximum amplitude. The centre frequencies of the peaks of the energy spectra of the transmitted ultrasound pulses and the bandwidths of the transmitted ultrasound pulses can be arranged to satisfy this, as well as the requirement that each energy spectrum intersects its neighbours at the same fraction of the peak amplitude. For example, if nine ultrasound pulses are to be transmitted along a scan line, the normalised centre frequencies of the peaks of the energy spectra of each transmitted ultrasound pulse relative to that of the middle (fifth) transmitted ultrasound pulse could be as shown below: -

Referring now to Figures 22 and 23, a sixth embodiment, which provides further advantages, will be described below.

In this embodiment, the energy spectrum of at least one of the transmitted ultrasound pulses has two or more peaks, each peak being centred on a different frequency. This is in contrast to the previous embodiments in which the energy spectra of all of the transmitted ultrasound pulses have only one peak.

Figure 22a shows an example of the waveform of a first transmitted ultrasound pulse that is the result of summing two pulses having single peaked energy spectra with different centre frequencies, as shown in Figures 22 al and 22a2. The energy spectrum of the pulse of Figure 22a has two peaks, as shown in Figure 23a. The scan line is then interrogated with one or more further transmitted ultrasound pulses, also having double peaked energy spectra, as shown in the example of Figure 23b. For each interrogation, the ultrasound echo signals are filtered by two narrow bandwidth filters acting in parallel, each tuned to different frequencies, being the centre frequencies of the two peaks of the energy spectrum of the transmitted ultrasound pulse. The filtered ultrasound echo signals from all the transmissions are summed to produce a resultant echo signal with a bandwidth greater than that of any of the peaks of the energy spectra of the transmitted pulses.

Alternatively, one or more of the further transmitted ultrasound pulses may have an energy spectrum with a single peak, as shown in Figure 23c. The echo signals from this transmission are filtered by a narrow band filter tuned to the centre frequency of the transmitted pulse, before being summed with the filtered echo signals from the other transmissions. In this example, the centre frequency of the single peak in Figure 23c is midway between the two peaks in Figure 23 a.

It will be appreciated that ultrasound pulses having single, double or multiple peaks in their energy spectra can be transmitted in any order.

In using transmitted ultrasound pulses having energy spectra with two or more peaks, this provides the advantage that, for a given peak amplitude transmitted ultrasound pulse, there is an improvement in signal to noise ratio and resolution when compared with a single wide bandwidth pulse, in common with the transmitted ultrasound pulses used in the previous embodiments. In addition however, the scan line can be interrogated more quickly using a pulse having a multiple peaked energy spectrum when compared with using a transmitted ultrasound pulse having only a single peak in its energy spectrum. In effect two or more narrow bandwidth ultrasound pulses with different centre frequencies are transmitted simultaneously as one transmitted ultrasound pulse.

It is to be appreciated that the procedure of ultrasound pulse transmission, generation of ultrasound echo signals, and summation of the ultrasound echo signals is the same as that described with respect to the previous embodiments. It will be appreciated by persons skilled in the art that the above embodiments have been described by way of example only, and not in any limitative sense, and that various alterations and modifications are possible without departing from the scope of the invention as defined by the appended claims.

Claims

1. A method of generating an ultrasound image of a target object, said method comprising the steps of: -

(iii)summing at least two said ultrasound echo signals to generate a resultant echo signal having a resultant echo signal energy spectrum.

2. A method of generating an ultrasound image of a target object as claimed in claim

1 , wherein each said transmitted ultrasound pulse has an energy spectrum with a single peak.

3. A method of generating an ultrasound image of a target object as claimed in claim

2, wherein the predetermined centre frequencies of the peaks of each said transmitted ultrasound pulse are different from each other.

4. A method of generating an ultrasound image of a target object as claimed in claim 1 , wherein each said transmitted ultrasound pulse has an energy spectrum with a plurality of peaks, each peak having a different predetermined centre frequency.

5. A method of generating an ultrasound image of a target object as claimed in claim 4, wherein the predetermined centre frequencies of each peak of each said transmitted ultrasound pulse are different from each other.

6. A method of generating an ultrasound image of a target object as claimed in any one of the previous claims, wherein at least one of said first or further transmitted ultrasound pulses is a chirp.

7. A method of generating an ultrasound image of a target object as claimed in any one of the previous claims, wherein each of said transmitted ultrasound pulses is a chirp.

8. A method of generating an ultrasound image of a target object as claimed in any one of claims 6 or 7, wherein the ultrasound echo signals generated as a result of the reflection of said chirps from said target object are compressed prior to being summed to generate said resultant echo signal.

9. A method of generating an ultrasound image of a target object as claimed in any one of claims 6 to 8, wherein the ultrasound echo signals generated as a result of the reflection of said chirps from said target object are amplified prior to being summed to generate said resultant echo signal.

10. A method of generating an ultrasound image of a target object as claimed in any one of the previous claims, wherein each peak of the energy spectra of each said transmitted ultrasound pulse has a predetermined bandwidth at half the maximum amplitude, wherein the bandwidth at half the maximum amplitude of the peak of said resultant echo signal energy spectrum is greater than the bandwidth at half the maximum amplitude of any of the peaks of the energy spectra of any of said first or further transmitted ultrasound pulses.

11. A method of generating an ultrasound image of a target object as claimed in any one of the previous claims, comprising summing each said ultrasound echo signal.

12. A method of generating an ultrasound image of a target object as claimed in any one of the previous claims, further comprising amplifying at least one said ultrasound echo signal prior to generating said resultant echo signal.

13. A method of generating an ultrasound image of a target object as claimed in any one of the previous claims, further comprising amplifying at least two said ultrasound echo signals prior to generating said resultant echo signal, wherein each said amplified ultrasound echo signal is amplified by a different amount.

14. A method of generating an ultrasound image of a target object as claimed in any one of the previous claims, comprising amplifying each said ultrasound echo signal, storing each said amplified ultrasound echo signal, and then summing each said ultrasound echo signal to generate said resultant echo signal.

15. A method of generating an ultrasound image of a target object as claimed in any one of claims 1 to 13, comprising amplifying each said ultrasound echo signal, storing each said ultrasound echo signal apart from the final one, and then summing each said ultrasound echo signal to generate said resultant echo signal.

16. A method of generating an ultrasound image of a target object as claimed in any one of claims 1 to 13, comprising summing each said ultrasound echo signal as it is generated.

17. A method of generating an ultrasound image of a target object as claimed in any one of claims 12 to 16, wherein the amplifications provided to the ultrasound echo signals are such that the resultant echo signal energy spectrum is substantially Gaussian in nature.

18. A method of generating an ultrasound image of a target object as claimed in any one of the previous claims, further comprising providing at least one said ultrasound echo signal with a time shift.

19. A method of generating an ultrasound image of a target object as claimed in any one of the previous claims, further comprising providing at least one said ultrasound echo signal with a time shift so that at a time substantially halfway through the duration of the ultrasound echo signal, all of the ultrasound echo signals are substantially in phase with each other.

20. A method of generating an ultrasound image of a target object as claimed in any one of the previous claims, further comprising providing said resultant echo signal with a time shift.

21. A method of generating an ultrasound image of a target object as claimed in any one of the previous claims, further comprising filtering at least one said ultrasound echo signal prior to generating said resultant echo signal.

22. A method of generating an ultrasound image of a target object as claimed in claim 21 , wherein the filtering provided to the ultrasound echo signals is such that the resultant echo signal energy spectrum is substantially Gaussian in nature.

23. A method of generating an ultrasound image of a target object as claimed in any one of the previous claims, wherein the predetermined bandwidths at half the maximum amplitude of each peak of the energy spectra of said first and each said further transmitted ultrasound pulse are substantially equal.

24. A method of generating an ultrasound image of a target object as claimed in any one of the previous claims, wherein the sum of the predetermined bandwidths at half the maximum amplitude of each peak of the energy spectra of said first and each said further transmitted ultrasound pulse are greater than the bandwidth at half the maximum amplitude of the resultant echo signal energy spectrum.

25. A method of generating an ultrasound image of a target object as claimed in any one of claims 1 to 23, wherein the sum of the predetermined bandwidths at half the maximum amplitude of each peak of the energy spectra of said first and each said further transmitted ultrasound pulse are less than or substantially equal to the bandwidth at half the maximum amplitude of the resultant echo signal energy spectrum.

26. A method of generating an ultrasound image of a target object as claimed in any one of the previous claims wherein a first and eight further ultrasound pulses are transmitted in sequence along said pre-determined scan line.

27. A method of generating an ultrasound image of a target object as claimed in any one of the previous claims, wherein said method is repeated on said predetermined scan line.

28. A method of generating an ultrasound image of a target object as claimed in any one of the previous claims, wherein said method is repeated on at least one further scan line adjacent to said pre-determined scan line.

29. A method of generating an ultrasound image of a target object as claimed in claim 28, wherein said step of repeating said method on at least one further scan line adjacent to said predetermined scan line comprises firstly, transmitting said first ultrasound pulse and each said further ultrasound pulse in sequence along said predetermined scan line, and then transmitting a further said first ultrasound pulse and further said ultrasound pulses in sequence along each said further scan line in sequence.

30. A method of generating an ultrasound image of a target object as claimed in claim 28, wherein said step of repeating said method on at least one further scan line adjacent to said predetermined scan line comprises simultaneously transmitting said first ultrasound pulses along said predetermined and at least one said further scan line, and then transmitting first said further ultrasound pulses simultaneously along said predetermined and at least one said further scan line, and repeating this process until a first and each said further ultrasound pulse have been transmitted along said predetermined and at least one said further scan line.

31. A method of generating an ultrasound image of a target object as claimed in claim 30, wherein said step of repeating said method on at least one further scan line adjacent to said predetermined scan line comprises simultaneously transmitting said first ultrasound pulses along said predetermined and each said further scan line, and then transmitting first said further ultrasound pulses simultaneously along said predetermined and each said further scan line, and repeating this process until a first and each said further ultrasound pulse have been transmitted along each said scan line.

32. A method of generating an ultrasound image of a target object as claimed in any one of claims 30 or 31, wherein the predetermined centre frequencies of each peak of the energy spectra of the ultrasound pulses which are being simultaneously transmitted along the scan lines are different from each other.

33. An apparatus for generating an ultrasound image of a target object, said apparatus comprising: -

(i) transmitting means adapted to transmit a first ultrasound pulse and at least one further ultrasound pulse in sequence along a predetermined scan line, each said transmitted ultrasound pulse having an energy spectrum with at least one peak, each said peak having a predetermined centre frequency, wherein the predetermined centre frequency of at least one peak of one said transmitted ultrasound pulse is different from the predetermined centre frequency of at least one peak of at least one other transmitted ultrasound pulse;

34. An apparatus for generating an ultrasound image of a target object as claimed in claim 33, wherein said apparatus comprises a digital signal processor.

35. A software product for generating an ultrasound image of a target object, the software product comprising a computer readable medium upon which instructions are stored, wherein the instructions, when read by the computer, cause the computer to: -

(i) transmit a first ultrasound pulse and at least one further ultrasound pulse in sequence along a predetermined scan line, each said transmitted ultrasound pulse having an energy spectrum with at least one peak, each said peak having a predetermined centre frequency, wherein the predetermined centre frequency of at least one peak of one said transmitted ultrasound pulse is different from the predetermined centre frequency of at least one peak of at least one other transmitted ultrasound pulse;

(ii) generate at least two respective ultrasound echo signals derived from the respective reflections of said first and at least one said further ultrasound pulses from a target object; and

36. A software product for generating an ultrasound image of a target object as claimed in claim 35, wherein the software product is a computer software product.

37. A software product for generating an ultrasound image of a target object as claimed in claim 35, wherein the software product is a digital signal processing software product.

38. A method of generating an ultrasound image of a target object substantially as hereinbefore described with reference to the accompanying drawings.

39. An apparatus for generating an ultrasound image of a target object substantially as hereinbefore described with reference to the accompanying drawings.

40. A software product for generating an ultrasound image of a target object substantially as hereinbefore described with reference to the accompanying drawings.