3D imaging using a bias-sensitive crossed-electrode array

Abstract

A method and system for imaging a sample uses a 2D array of bias-sensitive, ultrasound transducers arranged in first and second strips, and a source of radiation to stimulate the sample to be imaged. The second electrode strips are sequentially biased according to sequential biasing patterns of voltages that correspond to rows or columns of an invertible matrix. For each biasing pattern, signals are measured from the first electrode strips to detect return signals from the sample that result from the sample being stimulated. A dataset is calculated based on the measured signals, the dataset comprising an effective signal for each of a plurality of transducer elements in the array. An image of the sample is generated based on the dataset.

Claims

1. A method of imaging a sample, comprising the steps of: providing a 2D array of bias-sensitive, ultrasound transducers, each ultrasound transducer having first and second electrodes on opposite sides of a transducer element, the respective first electrodes being connected in plural first electrode strips, and the respective second electrodes being connected in plural second electrode strips, the plural first electrode strips being oriented at an angle to the plural second electrode strips, the angle being substantially different from zero; stimulating the sample to be imaged using an incident radiation source; sequentially biasing the second electrode strips according to sequential biasing patterns of voltages that correspond to rows or columns of an invertible matrix; for each biasing pattern, measuring signals from the first electrode strips to detect return signals from the sample that result from the sample being stimulated; calculating a dataset based on the measured signals, the dataset comprising an effective signal for each of a plurality of transducer elements in the array; and generating an image of the sample based on the dataset.

2. The method of claim 1, wherein calculating the dataset comprises applying the invertible matrix to the measured signals.

3. The method of claim 1, wherein the incident radiation source is electromagnetic.

4. The method of claim 1, wherein the transducer elements comprise an electrostrictive, piezoelectric, electrostrictive relaxor or piezoelectric relaxor material, and one or more biasing patterns comprise both positive and negative voltages.

5. The method of claim 1, wherein the array of ultrasound transducers comprises capacitive micromachined ultrasound transducers.

6. The method of claim 1, wherein the incident radiation source is ultrasonic.

7. The method of claim 6, wherein the ultrasound transducers are connected to transmit and receive ultrasonic signals.

8. The method of claim 7, wherein the polarity and amplitude of the emitted signals from each transducer element are dependent on the polarity and strength of the respective bias voltages.

9. The method of claim 1, wherein the 2D array defines a planar or curved surface.

10. The method of claim 1, wherein measuring the signal comprises decoupling a received AC signals from the bias.

11. The method of claim 1, wherein the second electrode strips are biased with the biasing pattern when the return pulses are measured.

12. The method of claim 1, wherein the sensitivity of the ultrasound transducer is related to the bias voltage.

13. The method of claim 1, wherein the matrix is a Hadamard matrix or an S-matrix.

14. An imaging system comprising: a 2D array of bias-sensitive ultrasound transducers, each ultrasound transducer having a first electrode on a first side of a transducer element and a second electrode on a second side of the transducer element, the respective first electrodes being connected in plural first electrode strips, and the respective second electrodes being connected in plural second electrode strips, the plural first electrode strips being oriented at an angle to the plural second electrode strips, the angle being substantially different from zero; a source of radiation that is incident on a sample to be imaged and is configured to cause the sample to generate an ultrasonic response; a controller connected to the first and second electrode strips, the controller being programmed to: sequentially bias the second electrode strips according to a sequential biasing pattern of voltages that correspond to rows or columns of an invertible matrix; for each biasing pattern, measure signals from the first electrode strips to detect return signals from the sample that result from the sample being stimulated; calculate a dataset based on the measured signals, the dataset comprising an effective signal for each of a plurality of transducer elements in the array; generate an image of the sample based on the dataset.

15. The system of claim 14, wherein the controller is programmed to calculate the dataset by applying the invertible matrix to the measured signals.

16. The system of claim 14, wherein the incident radiation source is electromagnetic.

17. The system of claim 14, wherein the transducer elements comprise an electrostrictive, piezoelectric, electrostrictive relaxor or piezoelectric relaxor material and one or more biasing patterns comprise both positive and negative voltages.

18. The system of claim 14, wherein the array of ultrasound transducers comprises capacitive micromachined ultrasound transducers.

19. The system of claim 14, wherein the incident radiation source is ultrasonic.

20. The system of claim 14, wherein the ultrasound transducers are connected to transmit and receive ultrasonic signals.

21. The system of claim 20, wherein the polarity and amplitude of the emitted signals from each transducer element is dependent on the polarity and strength of the respective bias voltages.

22. The system of claim 14, wherein the 2D array defines a planar or curved surface.

23. The system of claim 14, wherein the first and second electrodes comprise top and bottom electrodes or bottom and top electrodes.

24. The system of claim 14, further comprising bias tees for decoupling a received AC signal from the bias for each measured signal strength.

25. The system of claim 14, wherein the matrix is a Hadamard matrix or an S-matrix.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) These and other features will become more apparent from the following description in which reference is made to the appended drawings, the drawings are for the purpose of illustration only and are not intended to be in any way limiting, wherein:

(2) FIG. 1 is a schematic view of an imaging system.

(3) FIG. 2 is a perspective view of a transducer array.

(4) FIGS. 3 and 4 are to plan views of biasing patterns for an array of transducers.

(5) FIG. 5 is a schematic of an experimental setup used to image the crossed-wire phantoms.

(6) FIG. 6 compares X-axis point spread function of crossed-wire phantoms imaged using (a) single-column biasing and (b) Hadamard-encoded biasing.

(7) FIG. 7 compares XZ-plane reconstructions of the crossed-wire phantoms using (a) single-column biasing and (c) Hadamard-encoded biasing, and YZ-plane reconstructions using (b) single-column biasing and (d) Hadamard-encoded biasing.

(8) FIG. 8 depicts a reconstruction of the cross-wires in scattering medium using a Hadamard-encoded bias sequence.

DESCRIPTION OF PREFERRED EMBODIMENTS

(9) There will now be described a 3D imaging technique that utilizes bias encoding to allow for a transducer array to receive signals in a way that may allow for an improved SNR. In the discussion below, rows and columns of the matrix are identified as a matter of convenience or convention. However, what is considered a row or column may be interchangeable, and more importantly, the purpose of each of the lines of transducers, either in rows or columns may be interchanged in the process described herein.

(10) The imaging technique described below may be applied to photoacoustic imaging, where the sample is stimulated using electromagnetic radiation, or ultrasonic imaging, where the sample is stimulated using ultrasonic energy. As will be recognized and based on known imaging strategies, various imaging approaches may be used to stimulate the sample and detect ultrasound energy using a 2D array.

(11) In general, the method and system used herein includes a 2D array of bias-sensitive, ultrasound transducers 14, as shown in FIG. 2, where the array is indicated generally by reference number 12. Array 12 is shown as being planar, but it may also be curved, such as parabolic in one or two dimensions, or semi-spherical, as it known in the art. Transducers 14 may be capacitive micromachined ultrasound transducers or other types of transducers. Transducers 14 may be made from piezoelectric material, electrostrictive material, piezoelectric or electrostrictive relaxor material, or any other suitable material that may be known in the art. Another example of a 2D array used for ultrasonic imaging is described in U.S. patent application Ser. No. 15/792,422 (Zemp) entitled “System and method for ultrasound imaging”, the teachings of which are incorporated herein by reference.

(12) Each ultrasound transducer 14 has first and second electrodes 16 and 18 on opposite sides of a transducer element 20. The respective first electrodes 16 are connected in plural first electrode strips 17, and the respective second electrodes 18 are connected in plural second electrode strips 19. The first strips 17 are oriented at an angle to the second strips 19 at an angle that is substantially different from zero. Typically, this will be perpendicular, or near perpendicular.

(13) The arrangement of transducer array 12 relative to a radiation source, sample, and the corresponding electronics may be constructed and defined according to common design principles as known by those skilled in the art.

(14) Referring to FIG. 1, an example of a setup is shown that was used to obtain experimental results described below. As shown, array 12 is placed on an interface PCB 32 adjacent to a sample 22 to be imaged using an incident radiation source 24. This may be a laser as depicted, but may be other radiation sources, as discussed above. In the depicted example, sample 22 is located in an intralipid solution 34 to simulate scattering that would be encountered in a typical sample, but would not be present in actual applications. Also as shown, radiation source 24 is opposite array 12 relative to sample 22. However, it will be understood that radiation source 24 may be on the same side as, or space laterally from, sample 22.

(15) As will be discussed below, as sample 22 is stimulated, second electrode stripes 19 are biased and signals are measured from the first electrode strips 17 to detect return signals from sample 22 that result from the sample being stimulated. The signals are routed through a bias tee 26, which is connected to a DC bias control 28 and a controller 30. These signals are then processed to calculate an image of the sample. In the event that the system is used in an ultrasound imaging system, transducers 14 may be connected to both transmit and receive ultrasonic signals, as will be known in the art. For example, the polarity and amplitude of the emitted signals from each transducer element may be dependent on the polarity and strength of the respective bias voltages applied to each transducer 14. Typically, the second electrode strips 19 will be biased with the biasing pattern when the return pulses are measured, and measuring the signal may involve decoupling a received AC signal from the bias, depending on how transducers 14 are biased. Generally speaking, the sensitivity of the ultrasound transducer is related to the bias voltage, which allows the measured signal to be varied based on a bias pattern applied to array 12, as is described below.

(16) The present method and system uses a different approach to biasing and receiving signals using array 12. Rather than receive along a single biased column of elements j to obtain signals x.sub.0j(t), x.sub.1j(t), . . . x.sub.ij(t), . . . x.sub.ij(t), where i is the row number, all columns are biased using a bias pattern chosen from the rows of an invertible matrix, such as a Hadamard matrix, consisting of ±1s. Each bias pattern is applied to column biases and the row electrode information (or vice versa) is measured providing the dataset W(t):

(17) $W\left(t\right)=\left[\begin{array}{cccc}{w}_0^0\left(t\right)& w_1^0\left(t\right)& \mathrm{.Math.}& w_N^0\left(t\right)\\ w_0^1\left(t\right)& w_1^1\left(t\right)& \mathrm{.Math.}& w_N^1\left(t\right)\\ \mathrm{.Math.}& \mathrm{.Math.}& & \mathrm{.Math.}\\ w_0^B\left(t\right)& w_1^B\left(t\right)& \mathrm{.Math.}& w_N^B\left(t\right)\end{array}\right]$

(18) N is the total number of rows while B is the number of bias patterns, equal to the number of rows in the invertible matrix. Each measured signal w.sub.i.sup.b(t) for row i and bias pattern b consists of the superimposed signals of all elements in row i multiplied by the respective column bias pattern value h.sub.j(b).
w.sub.i.sup.b(t)=x.sub.i0(t)h.sub.0(b)+ . . . x.sub.iN(t)h.sub.B(b)

(19) Referring to FIG. 3, examples of biasing patterns are shown, which allows for all elements in an array to acquire signals for every imaging event. As shown, rather than biasing a single column at a time as in FIG. 1, a bias pattern consisting of +/− is applied across columns. These patterns are selected from the rows of an invertible matrix. Typically, the number of detection events and bias patterns applied will correspond with the number of rows or columns in the invertible matrix, such that, when the mathematical operations are performed, a value for each transducer may be obtained. However, in comes cases, values for only a subset of transducer elements may be sought, or some values may be predetermined or approximated, in which case the number of detection events may be reduced.

(20) The dataset may also be characterized by the following formula:

(21) $W\left(t\right)=\left[\begin{array}{ccc}{w}_1^1\left(t\right)& \mathrm{.Math.}& w_N^1\\ \mathrm{.Math.}& \ddots & \mathrm{.Math.}\\ w_1^N& \mathrm{.Math.}& w_N^N\end{array}\right]=\left[\begin{array}{ccc}{x}_1^1\left(t\right)& \mathrm{.Math.}& x_N^1\left(t\right)\\ \mathrm{.Math.}& \ddots & \mathrm{.Math.}\\ x_1^N\left(t\right)& \mathrm{.Math.}& x_N^N\end{array}\right]\left[\begin{array}{ccc}{h}_1^1& \mathrm{.Math.}& h_N^1\\ \mathrm{.Math.}& \ddots & \mathrm{.Math.}\\ h_1^N& \mathrm{.Math.}& h_N^N\end{array}\right]+N\left(t\right)$
where w.sub.j.sup.i(t) is the signal trace measured by row i during transmit event j, x.sub.j.sup.i(t) is the signal from row i when only a single bias is applied at column j, h.sub.j.sup.i is the bias weight applied to element i during transmit event j, and N(t) is the noise associated with each channel.

(22) In order to retrieve a dataset consisting of only the individual element signals X(t), the bias-encoded dataset W(t) may be decoded by multiplying it with the inverted matrix H
X(t)=W(t)H.sup.−1

(23) The received element data in X(t) can then be used to reconstruct a 3D image using dynamic receive beamforming. These techniques are known in the art, and will not be described further.

(24) Imaging is done using known imaging equipment and arrangements. The actual equipment used will depend on the type of imaging (e.g. photoacoustic vs. ultrasound imaging), and the sample being imaged. For example, for an electromagnetic source of radiation, it is common to use a laser with a desired wavelength,

EXAMPLES

(25) In one example, an experiment was conducted using a 64×64 element relaxor array which was developed into a 1-3 composite 10-MHz transducer using a dice-and-fill approach. The experimental setup is shown in FIGS. 1 and 5. Referring to FIG. 1, array 12 was wire bonded to a PCB 32, which was then interfaced to a bias tee 26, connecting a relay-based bias switching board 28 and a controller 30, which includes receive preamplifiers, and a Verasonics experimental ultrasound system for performing analog-to-digital conversion. A glass tank 36 was attached to the PCB with two 17.8 μm-diameter aluminum wire phantoms 22 attached in a cross shape with a 1 mm spacing at the intersection and a 12.8 mm spacing between the first wire and the array. The tank was filled with a 1% intralipid solution 34 to simulate optical scattering medium such as tissue. The bottom wire was 12.8 mm above the array surface. Radiation source 24 was a Nd:YAG laser 532 nm pulsed beam with a 79.2 mJ/cm.sup.2 fluence, directed towards the wire intersection 22.

(26) Imaging sequences were conducted using a single biased column stepped across the array and acquiring signals across all rows, i.e. equivalent to replacing the invertible matrix with an identify matrix in the equations above, as well as using Hadamard-encoded bias patterns. Bias patterns were programmed using a microcontroller which switched an array of relays between ±50 V for each column electrode. Images were then reconstructed by using delay-and-sum beamforming in 3D.

(27) FIG. 6 depicts the X-axis point spread function of wire using a single-column biasing strategy, as shown in graph (a) compared to a Hadamard-encoded biasing strategy, as shown in graph (b). The Hadamard-encoded bias scheme demonstrated an azimuthal Full width at half maximum (FWHM) resolution of 369 μm while the single-column bias technique had a FWHM resolution of 396 μm.

(28) FIG. 7 shows reconstructed images of the crossed-wire phantoms. FIGS. 7a and 7b show the XZ-plane reconstructed images of the crossed-wire phantoms using single-column biasing and Hadamard-bias-encoding, respectively, while FIGS. 7c and 7d show the YZ-plane reconstructions using single-column biasing and Hadamard-encoded biasing, respectively. The Hadamard-encoded bias scheme demonstrated an azimuthal Full width at half maximum (FWHM) resolution of 369 μm while the single-column bias technique had a FWHM resolution of 396 μm. The signal-to-noise ratio of the Hadamard-encoded bias reconstruction was 24.6 dB while the single-column biasing reconstruction had an SNR of 15.9 dB.

(29) The imaging performance of both biasing schemes is summarized in Table I below:

(30) TABLE-US-00001 TABLE I Summary of Imaging Results Identity Matrix Bias Hadamard Matrix Bias Pattern Encoding Axial Resolution (μm) 250 220 Elevational Resolution 396 369 (μm) Azimuthal Resolution 487 311 (μm) SNR (dB) 15.9 24.6

(31) By reconstructing with 61 azimuthal lines and 31 elevational lines, a 3D reconstruction of the crossed wires was obtained, as shown in FIG. 8.

(32) The use of Hadamard-bias encoded imaging as described in this example allows for acquisition across the whole array during the entire imaging sequence while also acquiring signals from all individual elements through the use of mathematical decoding. As a result, substantial SNR and resolution benefits were observed when compared with the single-column biasing approach.

(33) In other examples the imaging speed may be increased by using solid-state switching circuitry for bias sequence programming rather than relays. In addition, larger TOBE arrays may be used without a substantial increase in channel count.