WO2024251915A1

WO2024251915A1 - Ultrasound imaging

Info

Publication number: WO2024251915A1
Application number: PCT/EP2024/065671
Authority: WO
Inventors: Gijs VAN SOEST; Hendrik Jacob VOS
Original assignee: Erasmus University Medical Center
Current assignee: Erasmus University Medical Center
Priority date: 2023-06-07
Filing date: 2024-06-06
Publication date: 2024-12-12
Anticipated expiration: 2025-12-07
Also published as: GB202308458D0; GB2632991A

Abstract

An aspect of the disclosure relates to an ultrasound imaging system comprising: an ultrasound imaging device comprising an ultrasound transducer configured to transmit and receive ultrasound signals; and an ultrasound receiver device that is separable from the ultrasound imaging device, the ultrasound receiver device comprising: one or more ultrasound receiver elements configured to receive ultrasound signals, in which the one or more ultrasound receiver elements and the one or more ultrasound transducers are configured to operate simultaneously; an external housing configured to separate the one or more ultrasound receiver elements from the ultrasound imaging device, in which the one or more ultrasound receiver elements are within the external housing.

Description

ULTRASOUND IMAGING

Field

The present disclosure relates to an ultrasound imaging system, a ultrasound receiver device, an acoustic coupling pad and an associated method of use.

Photoacoustic (PA) imaging combines ultrasonic resolution and optical absorption contrast. PA signals are generally broadband; the received frequency range depends on the size distribution of the optical absorbers. Typically, commercial ultrasound (US) transducers are used to acquire PA images in parallel to US pulse-echo. The frequency and bandwidth are optimized for ultrasound imaging. However, a significant part of the PA information is present outside this bandwidth, impairing PA image reconstruction. Hence there is a need to improve reception bandwidth (BW) to enhance sensitivity for PA signals. Achieving broadband reception is possible by using a polyvinylidene difluoride (PVDF) transducer, or similar piezoelectric film technology, operated off- resonance (detection of frequencies below the resonant frequency (fR) of the film). PVDF has a low acoustic impedance, close to that of water and tissue, which ensures good coupling with the media. An object of some aspects of the present disclosure is to provide a device with enhanced PA reception BW without interfering with morphological echo imaging.

The disclosure relates to an ultrasound imaging system, comprising: an ultrasound imaging device comprising one or more ultrasound transducers configured to transmit and receive ultrasound signals; and an ultrasound receiver device that is separable from the ultrasound imaging device, the ultrasound receiver device comprising: one or more ultrasound receiver elements configured to receive ultrasound signals, in which the one or more ultrasound receiver elements and the one or more ultrasound transducers are configured to operate simultaneously; an external housing configured to separate the one or more ultrasound receiver elements from the ultrasound imaging device, in which the one or more ultrasound receiver elements are within the external housing. A peak sensitivity frequency of the one or more ultrasound receiver elements may be different to a peak sensitivity frequency of the ultrasound transducer. A fundamental frequency (or centre frequency) of the one or more ultrasound receiver elements may be different to a fundamental frequency (or centre frequency) of the one or more ultrasound transducers. The one or more ultrasound receiver elements may be configured to be sensitive about a harmonic or sub-harmonic frequency of the one or more ultrasound transducers of the ultrasound imaging device. An operating frequency range of the one or more ultrasound receiver elements is different to an operating frequency range of the one or more ultrasound transducers.

The one or more ultrasound receiver elements may be configured to be sensitive to an echo-signal that exhibits a power spectral density with a peak outside a -6 dB band of the one or more ultrasound transducers. The one or more ultrasound receiver elements may have a transfer loss for acoustic waves of less than one of 3 dB, 10 dB or 20 dB.

The one or more ultrasound receiver elements may comprise a polymer piezoelectric transducer with a thickness less than 100 pm. The elements on the polymer transducer are kerfless (or patterned electrodes on a continuous, uncut polymer film).

The system may further comprise one or more acoustically transparent layers that are configured to provide acoustic impedance matching, protection and/or electrical insulation.

The external housing may be configured to provide an interface for releasably engaging with the ultrasound probe.

The external housing of the ultrasound receiver device may be configured to separably engage with the ultrasound imaging device such that the one or more ultrasound transducers and the one or more ultrasound receiver elements are arranged to image substantially the same volume of interest.

The ultrasound receiver device may further comprise an optical emitter configured to emit optical excitation signals.

The system may comprise an analysis module. The analysis module may be coupled to the one or more ultrasound receiver elements and/or the one or more ultrasound transducers. The analysis module may be configured to process ultrasound signals simultaneously obtained by the one or more ultrasound receiver elements and the one or more ultrasound transducers.

The ultrasound receiver device may not comprise an ultrasound transmitter element. The ultrasound receiver device may be configured to receive-only.

The one or more ultrasound receiver elements may be provided as an array of receiver elements. The ultrasound receiver device further may comprise one or more ground planes parallel to the array of receiver elements.

The one or more ultrasound receiver elements may be arranged in a curved structure and/or configured to create a geometric focus.

The system may further comprise an acoustic coupling pad, the pad comprising a body with: a first portion; a second portion, in which the second portion has greater opacity or scatter than the first portion; a first surface configured to receive a transducer; and a second surface configured to be placed on a tissue under investigation.

The the second portion may be a cone-shaped or pyramidal volume within the first portion.

According to a further aspect of the disclosure there is provided an ultrasound receiver device for an ultrasound imaging system comprising an ultrasound imaging device having an ultrasound transducer configured to transmit and receive ultrasound signals, the ultrasound receiver device comprising: one or more ultrasound receiver elements configured to receive ultrasound signals, in which the one or more ultrasound receiver elements is configured to operate simultaneously with the ultrasound transducer; and an external housing configured to separate the one or more ultrasound receiver elements from the ultrasound imaging device, in which the one or more ultrasound receiver elements are within the external housing.

The ultrasound receiver elements may comprise a plurality of electrodes in a receiver electrode array. The ultrasound receiver elements may comprise a first ground plane electrode adjacent to the receiver electrode array. The ultrasound receiver elements may comprise a second ground plane electrode adjacent to the receiver electrode array. The receiver electrode array may be provided between the first and second ground plane electrodes. The first ground plane electrode may be parallel to the receiver electrode array. The second ground plane electrode may be parallel to the receiver electrode array.

According to a further aspect of the disclosure there is provided a method of obtaining information from a body using an ultrasound system. The method comprises: transmitting an ultrasound pulse signal into the body using the ultrasound imaging device; detecting an echo of the ultrasound pulse signal from the body using the ultrasound imaging device; and simultaneously with the detection using the ultrasound imaging device, detecting a receive ultrasound signal from the body using the one or more ultrasound receiver elements of the ultrasound receiver device.

According to a further aspect of the disclosure there is provided an acoustic coupling pad comprising a body with : a first portion; a second portion, in which the second portion has greater opacity than the first portion; a first surface configured to receive a transducer; and a second surface configured to be placed on a tissue under investigation.

The acoustic coupling pad may comprise one or more areas through which light can be administered to the tissue and provide an optically clear path for light delivery from the first surface to the tissue via the second surface.

The second portion may comprise an optically scattering, opaque, part which shields the transducer from the administered light by at least partially blocking transmission of light from the second surface to the first surface.

The pad may have one or more side surfaces connecting the first surface to the second surface. The one or more side surfaces may be configured to receive light to be administered to the tissue.

The second portion may be a cone-shaped or pyramidal volume within, or surrounded by, the first portion. The one or more ultrasound transducers may be configured to be operable at a frequency range which encompasses a lower receive frequency at least as low as 1 MHz and / or a higher receive frequency at least as high as 40 MHz. In some examples the frequency of the one or more ultrasound transducers can range from between 1 MHz to 30 MHz or preferably from 1 Mhz to 20 MHz. In a further example, the frequency range can be between 5 MHz and 15 MHz, which is suitable for vasculature ultrasound imaging. The receiver device may be configured to operate at the other of the lower receive frequency or the higher receive frequency. In some examples the receiver device can be sensitive to a frequency band that is either lower or higher than the frequency of the one or more ultrasound transducers transducers. In some examples the higher band could operate center frequency of at least double that of the one or more ultrasound transducers. In some examples the frequency range of one or more ultrasound transducers and respective receiver may be the following: transmission frequency range of around 2 - 5 MHz, and the corresponding received frequency range can either be higher than this range (i.e. 6 MHz - 20 MHz), or the received frequency range can be lower than the transmitted frequency range, such as up to half of the transmitted frequency range (i.e. 1 MHz - 2.5 MHz). The one or more ultrasound transducers may comprise a first transducer and a second transducer. The first transducer may comprise a piezoelectric polymer transducer or a piezoelectric composite transducer and the second transducer may comprise a piezoelectric ceramic transducer or a piezoelectric composite transducer. The polymer transducer may be a PVDF transducer or a PVTrF transducer. The one or more ultrasound transducers may comprise a first transducer operable in a lower frequency range and a second transducer operable in a higher frequency range.

The system may include a charge readout circuit mounted on the common substrate. At least one of the transducers may comprise a PVDF transducer and further include a charge readout circuit, the PVDF transducer and the charge readout circuit being disposed on a common substrate. The charge readout circuit may comprise a current- to-voltage converter on the common substrate. The charge readout circuit may have an output coupled to a coaxial cable. The output of the charge readout circuit may include a source follower configured to be biased by a current source coupled to the coaxial. The charge readout circuit may further include a DC current mirror supplied by the current source and providing a bias supply to a transimpedance amplifier in the current to voltage converter. The charge readout circuit and the PVDF transducer may both be powered by a current source by way of the coaxial cable. According to a further aspect of the disclosure, there is provided an ultrasound imaging system comprising: an ultrasound imaging device comprising a pulse-echo transmit/receive transducer; and an ultrasound receiver device that is separable from the ultrasound imaging device, the ultrasound receiver device comprising: an external housing configured to separate the one or more ultrasound receiver elements from the ultrasound imaging device; one or more ultrasound receiver elements within the external housing.

While the disclosure is amenable to various modifications and alternative forms, specifics thereof have been shown by way of example in the drawings and will be described in detail. It should be understood, however, that other embodiments, beyond the particular embodiments described, are possible as well. All modifications, equivalents, and alternative embodiments falling within the spirit and scope of the appended claims are covered as well.

The above discussion is not intended to represent every example embodiment or every implementation within the scope of the current or future Claim sets. The figures and Detailed Description that follow also exemplify various example embodiments. Various example embodiments may be more completely understood in consideration of the following Detailed Description in connection with the accompanying Drawings.

One or more embodiments will now be described by way of example only with reference to the accompanying drawings in which :

Figure 1 illustrates a stacked transducer arrangement;

Figure 2 illustrates a system with an ultrasound receiver to be used in conjunction with a transducer;

Figures 3a and 3b illustrate an implementation of the ultrasound system of

Figure 2;

Figure 4 illustrates an ultrasound receiver device comprising an electrode array on a polymer film substrate;

Figure 5a illustrates an implementation of an ultrasound imaging system;

Figure 5b shows typical photoacoustic signals produced by the system of Figure 5a;

Figures 6a shows typical photoacoustic signals produced by the system of Figure Figure 6b shows a combined representation of the signals in Figure 6a;

Figure 6c shows frequency spectra of the signals in Figures 6a and 6b,

Figures 7a and 7b illustrate corresponding pulse-echo and photoacoustic images obtained using the system of Figure 5a;

Figures 7c and 7d illustrate corresponding pulse-echo and photoacoustic images obtained using the system of Figure 5a;

Figure 7e demonstrates that the PVDF receiver is acoustically transparent.

Figure 8 illustrates another example ultrasound receiver device;

Figure 9 illustrates an imaging system comprising an acoustic coupling structure;

Figure 10 illustrates a 4-element PVDF receiver on a quartz substrate, with amplifiers directly adjacent to the array;

Figure 11 illustrates a photoacoustic signal recorded with the array of Figure 10, showing a bandwidth of 0.4-8.5 MHz.

Figures 12a and 12b illustrate systems with a receive array and one or more ground plane electrodes;

Figures 13a and 13b show receiver devices having concave receive arrays; and

Figure 14 illustrates example receive arrays including ID, 1.5D, 1.75D, 2D receive arrays.

Ultrasonic imaging technologies find many applications in medicine. Imaging technologies that rely on a different contrast mechanism than linear acoustic scattering benefit from flexibility in the frequency range or array structure of the receive transducer, relative to the transmitter. The present disclosure relates to a receive transducer device which may work in conjunction with another transmit or transmit/receive transducer. In some arrangements, the device provides benefits for contrast-enhanced ultrasonic imaging, photoacoustic imaging, or harmonic ultrasound imaging.

Ultrasonic array probes for pulse echo imaging are ubiquitous in healthcare centres worldwide, and used routinely for many applications. Among these are imaging of the heart, liver and peripheral vessels. Each application has its optimal frequency range for pulse-echo imaging, to which transducers are optimized, leading to specific (e.g. vascular, cardiac, abdominal) transducers. The frequency response of these devices is tuned by the build-up of the so-called "acoustic stack", which is an electrically driven mechanically resonant structure comprising an active (usually piezoelectric) layer, and usually a backing and one or more matching layers. This stack structure is characterized by many design parameters, which can be influenced by layer thicknesses and material choices. Transducer elements can be arranged laterally, forming arrays or matrices for 2D or 3D imaging.

The important imaging performance characteristics of a transducer are acoustic sensitivity and bandwidth. Since both transmission and reception are typically achieved by a piezoelectric resonator, a very sensitive transducer can be made by making a highly resonant stack structure. Such a device will necessarily have a narrow bandwidth, however. As a result, bandwidth and sensitivity are traded off to find an optimum for each application, and it is not possible to realize an infinite bandwidth while maintaining adequate transmit and receive sensitivity. Typical modern commercial array transducers may have fractional bandwidths up to Af/fo « 80% for, where fo denotes the center of the transducer response as a function of frequency, and Af is the bandwidth measured at the -6 dB level.

Advanced ultrasonic imaging technologies may generate signals that are shifted to lower or higher frequencies compared to the "normal" frequency range used for a particular application, necessitating a greater receive bandwidth. The reason for this may be intrinsic to the contrast mechanism or be given by the physical mechanism of the imaging technology. Imaging ultrasound harmonics, aided by administered contrast or generated by the tissue itself, requires receive sensitivity at higher (second, third etc. harmonics) or lower (subharmonic) frequencies. Particularly with the use of contrast bubbles, the detected frequencies may be 3-5 times as large as the exciting frequency, and the sensitivity of the transmitting elements for receiving acoustic signals will be poor in that band.

Alternatively, in photoacoustic (PA) imaging, the received signal frequency cannot be controlled. The frequency content of the generated signal depends on the size of the acoustic emitter. Larger objects emit lower frequencies. Larger objects also occupy a larger volume from which ultrasound is emitted, and this emits waves with a higher acoustic pressure, than a small volume, all other factors being equal. The result is that PA signals are biased towards lower frequencies. PA imaging is often combined with echo imaging, which uses specific frequency ranges and transducers for specific applications. In combined PA and echo imaging of carotid artery plaque, the preferred ultrasound frequency is in the order of 8—10 MHz, and the peak of the PA signal power spectral density generated in the plaque is at 2—4 MHz, which is out of the band of conventional transducers used for vascular imaging. In assessing the relative sensitivities at relevant frequencies for these advanced forms of image contrast, it is important to realize that the signals due to harmonic imaging or PA imaging are weaker by more than an order of magnitude compared to the (fundamental) pulse-echo signal.

Solutions that have been proposed for applications that require wide-band or multiband sensitivity may incorporate specific acoustic designs that provide the desired transmit and/or receive frequency responses, as required by the application. A dual- resonant acoustic stack with a single active layer has been proposed. Alternatively, multiple active layers or stack designs with complementary response can be combined. The transducers or transducer elements can be arranged in different ways. Examples have been stacked transducers, where elements with low frequency sensitivity have been mounted on the high-frequency ones, or vice versa. Narrow-band pulse-echo transducers have been stacked with broadband receivers in an integrated device.

Figure 1 illustrates a stacked transducer arrangement in which a series of transducer elements 102, 104, 106 and associated respective matching layers 108, 110, 112 are sandwiched between a PVDF transducer front layer 114 and a backing layer 116. The front later 114 faces the external environment.

These realizations provide dual-frequency response in a single acoustic stack; jointly optimizing the performance in all desired frequency bands is a challenge in acoustic design and materials choice. The different transducers can also be mounted alongside each other, which yields greater freedom in the acoustic design but can offset the responses spatially. A third example of an arrangement of transducer elements with different frequency response has been in an interleaved array. Many of these integrated designs are difficult to fabricate because of the required precision in layer thicknesses, alignment, and element positioning, and because of the complex electroacoustic response of multilayer stacks.

The present disclosure relates to an alternative solution that decouples the complementary frequency receiver from the pulse-echo transmit/receive transducer.

Figure 2 illustrates a system 200 with an ultrasound receiver 218 to be used in conjunction with, but not permanently fixed relative to, a pulse-echo transducer 220. The frequency responses of the different transducers 218, 220 may be different. The acoustic sensitivity range of the receiver 218 is either at lower or higher frequencies, or both (offering greater bandwidth) than that of the pulse-echo transducer 220. The extension of the frequency response allows for high-quality registration of signals that exhibit a power spectral density with a peak outside the -6 dB band of the pulse-echo transducer. (A low frequency receiver would for instance efficiently receive signals at a frequency of at most half the centre frequency of the pulse-echo transducer. A high frequency receiver would for instance efficiently receive signals at a frequency of at least double the centre frequency of the pulse-echo transducer.) The pulse-echo and acoustic receive apertures are at least spatially partially overlapping by transmitting through the receive layer. This means that the imaged volumes overlap, and receiving harmonics of the transmitted ultrasound is possible.

In a preferred embodiment, the receiver could be a detachable receive array, consisting of a piezoelectric polymer layer such as PVDF or PVDF-TrFE with a patterned electrode array. Due to the polymer being thin, and of low acoustic impedance, the polymer layer does not pose a significant acoustic barrier to the transmitted signal from the pulseecho probe. This layer is acoustically comparable to, for example, a sterile transducer bag and thus does not impede pulse-echo imaging, or may be compared to a membrane hydrophone.

The piezoelectric polymer layer may be further coated or covered by an acoustically transparent film or layer, on one or either side of the piezoelectric polymer layer, to protect the surfaces from wear, chemical interactions, damage, or similar detrimental causes on the film rigidity and functioning. Moreover, the coating or cover may be made of electrically insulating material to avoid electrical contact between the tissue and the electronics that are part of the device, and/or the electronics that are part of the ultrasound receive system. The acoustic properties of this additional film may be chosen to provide an acoustic matching layer between the piezoelectric polymer layer and the material surrounding the device.

Figures 3a and 3b illustrate an implementation of the ultrasound system of figure 2. Figure 3a illustrates an expanded configuration and Figure 3b illustrates the system in an in-use configuration.

The ultrasound imaging system 300 comprises an ultrasound imaging device 320 and an ultrasound receiver device 318. The ultrasound imaging device 320 comprises one or more ultrasound transducers configured to transmit and receive ultrasound signals. The ultrasound receiver device 318 that is separable from the ultrasound imaging device. The ultrasound receiver device 318 comprises, in this example, a plurality of ultrasound receiver elements configured to receive ultrasound signals. As discussed in regard to the device of figure 2, the one or more ultrasound receiver elements and the one or more ultrasound transducers are configured to operate simultaneously.

The ultrasound receiver device 318 has an external housing configured to separate the ultrasound receiver elements from the ultrasound imaging device 320. The ultrasound receiver elements are within the external housing.

The polymer film of the receiver device 318 can be suspended on a holder that has been made to fit the housing of the ultrasound imaging device 320.

Acoustic coupling is achieved by filling the inside of the holder with ultrasound gel, between the ultrasound imaging device 320 and the ultrasound receiver device 318, prior to inserting the pulse-echo probe.

Figure 4 illustrates an ultrasound receiver device 318 comprising an electrode receive array 326.

In order to achieve sufficient sensitivity, front-end receive electronics 328, such as low-noise transimpedance amplifiers, can be integrated in the holder as shown in Figure 4. A parallel microcoaxial cable 332 similar to one used by a conventional pulse echo transducer may be used to connect the assembly to a parallel ultrasound receive system or an empty port on the ultrasound imaging system.

Figure 5a depicts an implementation of an ultrasound imaging system 500. The system has an 8-element PVDF receive array, through which a conventional pulse-echo probe can make echography images.

Figure 5b shows typical photoacoustic signals produced by the system of Figure 5a.

Figure 6a shows typical photoacoustic signals from a black tape sample, produced by the system of Figure 5a, received simultaneously by the ultrasound transducer and the receiver device. Both signals can be combined quantitatively, shown in Figure 6b, by calibrating them to a common reference such as a hydrophone. Figure 6c shows frequency spectra of the signals in Figure 6a, demonstrating a large effective bandwidth of the combined received signals (black curve). The reference hydrophone response is shown as the green curve. Briefly, the calibration is performed by computing the frequency spectra of the signals produced by the receiver device and the ultrasound transducer, as well as the frequency spectrum of a broadband device with a known response that covers the receive bandwidths of both the transducer and the receiver, such as a calibrated hydrophone. Compensation factors for the signals produced by the transducer and the receiver are computed by their ratio to the reference signal. The signals are multiplied by those compensation factors and summed.

Figures 7a and 7b illustrate corresponding pulse-echo imaging 700a and photoacoustic imaging 700b that have been acquired simultaneously, in the same plane, by the system of Figure 5a. Figures 7c and 7d illustrate further examples of corresponding pulse-echo imaging 700c and photoacoustic imaging 700d that have been acquired simultaneously, in the same plane, by the system of Figure 5a. Figures 7a an 7c, and 7b and 7d, are illustrated using the same respective amplitude scales. In each case the pairs of pulse-echo and photoacoustic signals have been acquired through the acoustic receive layer. Figure 7e demonstrates that the PVDF receiver is acoustically transparent, showing equivalence of the photoacoustic signals received with and without the PVDF layer.

Figure 8 illustrates another example ultrasound receiver device 800. The device differs from that described previously with respect to Figure 4, for example, in that , for PA imaging, the holder assembly may also include an illumination port, for instance consisting of a fibre bundle 333 which is mounted on the side of the receive transducer layer 326 or behind it to illuminate through the layer. Actual light sources, such as light emitting diode bars, may also be included in the device 800.

Figure 9 illustrates an imaging system 900 comprising an acoustic coupling structure 950. The acoustic coupling structure 950 may be coupled to the receiver device 918 and may include an optically scattering portion 952 and a relatively optically clear portion 954.

In this way, the acoustic coupling structure 950 provides an acoustic coupling pad comprising a body with a first portion 952 and a second portion 954. The second portion 954 has greater opacity than the first portion 952. A first surface 956 of the structure 950 is configured to receive a transducer. A second surface 958, on an opposed face of the structure 950, is configured to be placed on a tissue 960 under investigation. The second portion comprises a cone-shaped or pyramidal-shaped volume within the first portion so that light can be introduced by one or more further surfaces provided between the first and second surfaces 956, 958.

Light can be administered to the tissue 960 via the first portion, which provides an optically clear path for light delivery to the tissue via the second surface. The second portion 954 provides an optically scattering, opaque, part which shields the transducer from the administered light by at least partially blocking transmission of light to the first surface.

The optically scattering section 952 shields the receiver device 918 and the transducer from excitation light that may be fed in separately. Optical exposure of a piezoelectric material can generate a strong interfering signal that can saturate the amplifiers and limit the visibility of short range (short acoustic delay time) objects. Optically shielding the transducer can minimize this undesired effect and preferentially direct the light towards the tissue. The acoustic coupling structure 950 may be used with or without the receiver device that is the main topic of this disclosure.

The part of the assembly that fits around a conventional ultrasound probe may be customized to fit specific transducer types, as these have different outer dimensions that are independent of the polymer film receiver properties.

Active electronics may be used to electrically match receiver elements to the coaxial or twisted pair cables connecting the receiver to a (multichannel) data acquisition mainframe. The mismatch in electrical impedance may otherwise increase the sensitivity to electronic noise from the environment or associated instruments. These interface electronics may include signal amplification, filtering and other signal conditioning. For harmonic imaging applications, the transmitted pulse may generate a strong signal in the receiver if its frequency lies within the receiver bandwidth. Filters may be implemented before the amplification stage to reject the transmitted frequency in the signal coming from the receiver. In one embodiment, a digital front end can be included in the assembly, realizing improved noise immunity and potentially channel multiplexing, data compression, or digital filtering of the received signals.

Figure 10 illustrates a receiver that consists of, or comprises, one or more ultrasound elements. Multiple elements can be patterned in the electrode of the polymer layer or physically separated by cutting kerfs. Kerfless designs are more flexible in terms of element layout and have been found to deliver adequate element separation and acceptable receive response.

Figure 11 displays a photoacoustic signal recorded with the array of Figure 10. The photoacoustic signal profiles include a received signal 1164 (in mV, left hand axis) and a normalized amplitude signal 1166 (dB, right hand axis), showing a bandwidth of 0.4- 8.5 MHz.

Figure 12a illustrates system 1200a with a receive array 1226a as an outer electrode and a grounded inner electrode 1270 (to be positions closer to the transducer that the outer electrode).

Figure 12b illustrates system 1200b that differs from that of figure 12a in that the receive array 1226b is provided as a middle layer that is sandwiched between the grounded inner electrode 1270 and a grounded outer electrode 1272.

In this way, a double layer of receive polymer is provided between the receive array and the respective grounded electrodes 1270, 1272 in order to double the sensitivity, measured in pV/Pa, prior to amplification, and provide shielding of the device by sandwiching the hot electrode in between the layers.

The shape and size of the receiver elements cab be optimized depending on the chosen frequency of the receiver. For example, the elements can be patterned in the electrode material by laser ablation or chemical etching. The definition of the receive aperture can thus be freely chosen, independent of the pulse-echo transducer that is to be used in conjunction with the receiver. The number of elements in an array can be up to 64, 96, 128, or more, mostly limited by the number of channels that can be accommodated by the cable, or the number of channels that can be simultaneously received by the mainframe. As an example, in a linear array used for vascular imaging, the -6 dB bandwidth may be 5-12 MHz, with 192 elements in a 38 mm array. A receive array that adds low frequency response, centred at 3 MHz, can thus consist of 64 elements at triple the spacing of the pulse-echo transducer. In total, 256 channels need to be connected for pulse-echo and low-frequency receive.

The acoustically translucent active receive layer may be made of various thicknesses, such that the receive operates either in a resonant mode with relatively high sensitivity but a bandwidth that is limited by the mechanical resonance in the layer. Alternatively, the layer may be chosen to have a thickness such that the resonant frequency is higher or lower than the desired frequency response, resulting in an essentially flat frequency response. The reduced sensitivity can potentially be restored by the integrated signal amplifier. A thick receive film will likely be a more significant obstacle in the pulse-echo image; using a film thickness that is mechanically resonant with the applied transmit frequency of the pulse-echo probe is likely to interfere more strongly with the ultrasound image, generating artefacts and attenuation, and thus is not preferred.

In order to further improve acoustic sensitivity and reduce the dispersion of the frequency response in the elevational direction (which defines the resolution perpendicular to the image plane when using a linear array), the receive transducer may be shaped to realize an acoustic receive focus. The layer may be curved, for instance in the elevational direction, by attaching the layer on a shaped, acoustically translucent support (e.g., a stiff gel pad with one flat side and one concave side, which faces forward) realizing a focus.

Figures 13a and 13b show a receiver devices 1300a, 1300b that each have a concave receive array 1326a, 1326b. The elements of the receive array 1326a in Figure 13a run along the concave surface and the elements of the receive array 1326b in Figure 13b run transverse to the concave surface.

For PA imaging, it has been demonstrated that a curved, concave, receive array strongly improves image fidelity, by suppression of prominent (limited-view) artefacts. Alternatively, a transmissive lens may be placed between the receive array and the imaged volume.

The in-plane image focus can be achieved either by beamforming the received data, aided if possible by transmit focusing (if harmonics of the transmitted field are detected). If transmit focusing is employed, it may be possible to read out only a subset of receive channels simultaneously, aided by a front-end multiplexer. This can reduce the cable count and thus simplify the device. In the case of PA imaging, it will be most advantageous to read out all elements simultaneously.

Figure 14 illustrates example receive arrays including ID, 1.5D, 1.75D, 2D arrays, in periodic, apodized or aperiodic patterns. In general, the receive array may be n- dimensional, where n is in the range 1 to 2). It is not necessary to cover the entire aperture of the assembly, or that of the pulse-echo transducer.

Claims

1. An ultrasound imaging system comprising: an ultrasound imaging device comprising one or more ultrasound transducers configured to transmit and receive ultrasound signals; and an ultrasound receiver device that is separable from the ultrasound imaging device, the ultrasound receiver device comprising: one or more ultrasound receiver elements configured to receive ultrasound signals, in which the one or more ultrasound receiver elements and the one or more ultrasound transducers are configured to operate simultaneously; an external housing configured to separate the one or more ultrasound receiver elements from the ultrasound imaging device, in which the one or more ultrasound receiver elements are within the external housing.

2. The system of claim 1, in which a peak sensitivity frequency of the one or more ultrasound receiver elements is different to a peak sensitivity frequency of the one or more ultrasound transducers.

3. The system of claim 2, in which a fundamental frequency of the one or more ultrasound receiver elements is different to a fundamental frequency of the one or more ultrasound transducers.

4. The system of claim 3, in which the one or more ultrasound receiver elements is configured to be sensitive about a harmonic or sub-harmonic frequency of the one or more ultrasound transducers of the ultrasound imaging device.

5. The system of any of claims 2 to 4, in which an operating frequency range of the one or more ultrasound receiver elements is different to an operating frequency range of the one or more ultrasound transducers.

6. The system of any of claims 2 to 5, in which the one or more ultrasound receiver elements is configured to be sensitive to an echo-signal that exhibits a power spectral density with a peak outside a -6 dB band of the one or more ultrasound transducers.

7. The system of any preceding claim, in which the one or more ultrasound receiver elements have a transfer loss for acoustic waves of less than 20dB.

8. The system of any preceding claim, in which the one or more ultrasound receiver elements comprises a polymer piezoelectric transducer with a thickness less than 100 pm.

9. The system of claim 7, in which the elements on the polymer transducer are kerfless.

10. The system of claim 8-9, comprising additional acoustically transparent layers that are configured to provide acoustic impedance matching, protection and/or electrical insulation.

11. The system of any preceding claim, in which the external housing is configured to provide an interface for releasably engaging with the ultrasound probe.

12. The system of any preceding claim, in which the external housing of the ultrasound receiver device is configured to separably engage with the ultrasound imaging device such that the one or more ultrasound transducers and the one or more ultrasound receiver elements are arranged to image substantially the same volume of interest.

13. The system of any preceding claim, in which the ultrasound receiver device further comprises an optical emitter configured to emit optical excitation signals.

14. The system of any preceding claim, comprising an analysis module coupled to the one or more ultrasound receiver elements and the one or more ultrasound transducers, in which the analysis module is configured to process ultrasound signals simultaneously obtained by the one or more ultrasound receiver elements and the one or more ultrasound transducers.

15. The system of any preceding claim, in which the ultrasound receiver device does not comprise an ultrasound transmitter element.

16. The system of any preceding claim, in which the one or more ultrasound receiver elements is provided as an array of receiver elements, the receiver device further comprising one or more ground planes parallel to the array of receiver elements.

17. The system of any preceding claim, in which the one or more ultrasound receiver elements are configured in a curved structure, creating a geometric focus.

18. The system of any preceding claim, further comprising an acoustic coupling pad, the pad comprising a body with : a first portion; a second portion, in which the second portion has greater opacity or scatter than the first portion; a first surface configured to receive a transducer; and a second surface configured to be placed on a tissue under investigation.

19. The system of claim 18, in which the second portion is a cone-shaped or pyramidal volume within the first portion.

20. An ultrasound receiver device for an ultrasound imaging system comprising an ultrasound imaging device having an ultrasound transducer configured to transmit and receive ultrasound signals, the ultrasound receiver device comprising: one or more ultrasound receiver elements configured to receive ultrasound signals, in which the one or more ultrasound receiver elements is configured to operate simultaneously with the ultrasound transducer; and an external housing configured to separate the one or more ultrasound receiver elements from the ultrasound imaging device, in which the one or more ultrasound receiver elements are within the external housing.

21. The ultrasound receiver device of claim 20, wherein the ultrasound receiver elements comprise a plurality of electrodes in a receiver electrode array, and a first ground plane electrode adjacent to the receiver electrode array.

22. The ultrasound receiver device of claim 21, wherein the ultrasound receiver elements comprise a second ground plane electrode adjacent to the receiver electrode array, wherein the receiver electrode array is provided between the first and second ground plane electrodes.

23. A method of obtaining information from a body using the ultrasound system of any of claims 1 to 22, comprising: transmitting an ultrasound pulse signal into the body using the ultrasound imaging device; detecting an echo of the ultrasound pulse signal from the body using the ultrasound imaging device; and simultaneously with the detection using the ultrasound imaging device, detecting a receive ultrasound signal from the body using the one or more ultrasound receiver elements of the ultrasound receiver device.

24. An acoustic coupling pad comprising a body with : a first portion; a second portion, in which the second portion has greater opacity than the first portion; a first surface configured to receive a transducer; and a second surface configured to be placed on a tissue under investigation.

25. The acoustic coupling pad of claim 24, wherein the first portion is an area through which light can be administered to the tissue, which provides an optically clear path for light delivery to the tissue via the second surface.

26. The acoustic coupling pad of claim 25, wherein the second portion is an optically scattering, opaque, part which shields the transducer from administered light by at least partially blocking transmission of light to the first surface.

27. The acoustic coupling pad of any of claims 24 to 26 in which the body has one or more further surfaces connecting the first and second surfaces and configured to receive light.

28. The acoustic coupling pad of any of claims 24 to 27, in which the second portion is a cone-shaped or pyramidal volume within the first portion.