Paul van Neer

Acoustic Sizing of an Ultrasound Contrast Agent

Because the properties of ultrasound contrast agent populations after administration to patients are largely unknown, methods able to study them noninvasively are required. In this study, we acoustically performed a size distribution measurement of the ultrasound contrast agent Definity(®). Single lipid-shelled microbubbles were insonified at 25 MHz, which is considerably higher than their resonance frequency, so that their acoustic responses depended on their geometrical cross sections only. We calculated the size of each microbubble from their measured backscattered pressures. The acoustic size measurements were compared with optical reference size measurements to test their accuracy. Our acoustic sizing method was applied to 88 individual Definity(®) bubbles to derive a size distribution of this agent. The size distribution obtained acoustically showed a mean diameter (2.5 μm) and a standard deviation (0.9 μm) in agreement within 8% with the optical reference measurement. At 25 MHz, this method can be applied to bubble sizes larger than 1.2 μm in diameter. It was observed that similar sized bubbles can give different responses (up to a factor 1.5), probably because of shell differences. These limitations should be taken into account when implementing the method in vivo. This acoustic sizing method has potential for estimating the size distribution of an ultrasound contrast agent noninvasively.

Optimization of a phased-array transducer for multiple harmonic imaging in medical applications: frequency and topology

Second-harmonic imaging is currently one of the standards in commercial echographic systems for diagnosis, because of its high spatial resolution and low sensitivity to clutter and near-field artifacts. The use of nonlinear phenomena mirrors is a great set of solutions to improve echographic image resolution. To further enhance the resolution and image quality, the combination of the 3rd to 5th harmonics - dubbed the superharmonics - could be used. However, this requires a bandwidth exceeding that of conventional transducers. A promising solution features a phased-array design with interleaved low- and high-frequency elements for transmission and reception, respectively. Because the amplitude of the backscattered higher harmonics at the transducer surface is relatively low, it is highly desirable to increase the sensitivity in reception. Therefore, we investigated the optimization of the number of elements in the receiving aperture as well as their arrangement (topology). A variety of configurations was considered, including one transmit element for each receive element (1/2) up to one transmit for 7 receive elements (1/8). The topologies are assessed based on the ratio of the harmonic peak pressures in the main and grating lobes. Further, the higher harmonic level is maximized by optimization of the center frequency of the transmitted pulse. The achievable SNR for a specific application is a compromise between the frequency-dependent attenuation and nonlinearity at a required penetration depth. To calculate the SNR of the complete imaging chain, we use an approach analogous to the sonar equation used in underwater acoustics. The generated harmonic pressure fields caused by nonlinear wave propagation were modeled with the iterative nonlinear contrast source (INCS) method, the KZK, or the Burgers equation. The optimal topology for superharmonic imaging was an interleaved design with 1 transmit element per 6 receive elements. It improves the SNR by ~5 dB compared with the interleaved (1/2) design reported in literature. The optimal transmit frequency for superharmonic echocardiography was found to be 1.0 to 1.2 MHz. For superharmonic abdominal imaging this frequency was found to be 1.7 to 1.9 MHz. For 2nd-harmonic echocardiography, the optimal transmit frequency of 1.8 MHz reported in the literature was corroborated with our simulation results.

Estimating acoustic peak pressure generated by ultrasound transducers from harmonic distortion level measurement.

Pressure amplitude measurement is important for general research on ultrasound. Because it requires high accuracy, it is usually done using a hydrophone calibrated by an accredited laboratory. In this paper, a method is proposed for estimating the pressure amplitude in the ultrasound field using an uncalibrated single-element transducer and Khokhlov-Zabolotskaya-Kuznetsov simulations of the ultrasound field. The accuracy of the method is shown to be better than 20% for slightly focused and nonfocused transducers. Extending the method to a pulse-echo setup enables pressure measurement of a transducer without the need for an extra transducer or hydrophone.

A NEW TRANSESOPHAGEAL PROBE FOR NEWBORNS

Current transesophageal probes are designed for adults and are used both in the operating theatre for monitoring as well as in the outpatient clinic for patients with specific indications, like obesity, artificial valves, etc. For newborns (<5 kg), transesophageal echocardiography (TEE) is not possible because the current probes are too big for introducing them into the esophagus. There is a clear need for a small probe in newborns that are scheduled for complicated cardiac surgery and catheterization. We present the design and realization of a small TEE phased array probe with a tube diameter of 5.2mm and head size of only 8.2-7 mm. The number of elements is 48 and the center frequency of the probe is 7.5 MHz. A separate clinical evaluation study was carried out in 42 patients (Scohy et al. 2007).

Imaging beyond aliasing

A sufficiently high spatial sampling is critical for high quality imaging. If the sampling criterion is not met, artifacts appear in the image generally referred to as grating lobes. Probes with a large aperture provide a large field of view, which allows for more efficient inspection. On the other hand this leads to an increase in the number of element to obey the sampling criterion. We have developed a method that reconstructs sparsely sampled data without assuming anything about the medium. The reconstruction method involves an iterative scheme using wave field extrapolation. After the reconstruction an aliasing free dataset is obtained which can be imaged properly. Aliased and non-aliased datasets were modeled based on point diffractors and reflectors with an increasing width. The datasets were imaged using a mapping in the wavenumber-frequency domain. Up to a factor four of under-sampling can be tolerated, providing the same image quality as a properly sampled dataset.

Mode vibrations of a matrix transducer for three-dimensional second harmonic transesophageal echocardiography

Transesophageal echocardiography (TEE) uses the esophagus as an imaging window to the heart. This enables cardiac imaging without interference from the ribs or lungs and allows for higher frequency ultrasound to be used compared with transthoracic echocardiography (TTE). TEE facilitates the successful imaging of obese or elderly patients, where TTE may be unable to produce images of satisfactory quality. Recently, three-dimensional (3-D) TEE has been introduced, which greatly improves the image quality and diagnostic value of TEE by adding an extra dimension. Further improvement could be achieved by optimizing 3-D TEE for harmonic imaging. This article describes the optimal geometry and element configuration for a matrix probe for 3-D second harmonic TEE. The array concept features separated transmit and receive subarrays. The element geometry was studied using finite element modeling and a transmit subarray prototype was examined both acoustically and with laser interferometry. The transmit subarray is suitable for its role, with a 3 MHz resonance frequency, a 40%-50% -3 dB bandwidth and crosstalk levels <-27 dB. The proposed concept for the receive subarray has a 5.6 MHz center frequency and a 50% -3 dB bandwidth.

A dual pulse technique for improving the point spread function of superharmonic imaging systems.

Nonlinear propagation causes the generation of higher harmonics of the emitted fundamental spectrum. Nowadays, medical echography employs reflections of the second harmonic, because this yields improved axial and lateral resolutions, and less reflections from nearby artifacts and grating lobes, as compared to fundamental imaging. To further exploit the benefits of higher harmonic imaging while keeping sufficient signal strength for detection, superharmonic imaging combines reflections of the third, fourth, and fifth harmonics. A drawback of adding harmonic reflections is the possible occurrence of ripples in the point spread function (PSF). Recently, a dual pulse technique was proposed for avoiding these ripples. This technique uses the emission of two consecutive pulses with a slightly different frequency, and performs imaging after summation of both superharmonic reflections. In this presentation, it is theoretically explained why this approach yields a better PSF than single pulse superharmonic imaging...

Data interpolation beyond aliasing

Proper spatial sampling is critical for many applications. If the sampling criterion is not met, artifacts appear for example in images. Last year an iterative approach was presented using wave field extrapolation to interpolate spatially aliased signals. The main idea behind this approach is that after inverse wave field extrapolation the signal is concentrated in a small region with a high amplitude, while the aliasing artifacts are spread-out through the domain. Inverse wave field extrapolation focusses optimally at one depth, making the performance of the reconstruction depth dependent. Obviously the method can be repeated for several depths. This year we show an alternative approach using an imaging/inverse-imaging approach. The demonstration of this approach is extended to 2D-arrays where the sampling limitations are even more critical. Moreover we show in this paper that the interpolation approach is not limited to near-field data, but can also be used on far-field data (plane waves). The Radon transform can be used for plane waves to focus the data. The approach is demonstrated using modeled and measured data for linear and 2D arrays.

Design and characterization of a sensitive optical micro-machined ultrasound transducer

Novel 3D intravascular or transesophageal ultrasound approaches require transducer arrays containing many small elements. Conventional piezo-electric techniques face fabrication challenges due to narrow kerfs and dense wiring. Micro-machined alternatives like CMUTs and PMUTs lack either sensitivity or bandwidth to fully compete. Therefore we developed an opto-mechanical ultrasound sensor. The absence of wiring makes it MRI compatible. The developed OMUT contains integrated photonics, which is fabricated using standard silicon-on-insulator technology, providing a small footprint and enabling mass production and ease of integration. The sensor consists of a straight waveguide and a micro-ring resonator integrated on a 124 μm wide, 2.7 μm thick acoustical membrane. Light, passing the waveguide, is partly coupled into the ring resonator. A dip appears in the spectrum of the transmitted light at the resonance wavelength of the micro-ring. If the acoustical membrane and hence the micro-ring is deformed due to a...

Design of a Matrix Transducer for Three-Dimensional Second Harmonic Transesophageal Echocardiography

Three-dimensional (3D) echocardiography visualizes the 3D anatomy and function of the heart. For 3D imaging an ultrasound matrix of several thousands of elements is required. To connect the matrix to an external imaging system, smart signal processing with integrated circuitry in the tip of the TEE probe is required for channel reduction. To separate the low voltage integrated receive circuitry from the high voltages required for transmission, our design features a separate transmit and receive subarray. In this study we focus on the transmit subarray. A 3D model of an individual element was developed using the finite element method (FEM). The model was validated by laser interferometer and acoustic measurements. Measurement and simulations matched well. The maximum transmit transfer was 3 nm/V at 2.4 MHz for both the FEM simulation of an element in air and the laser interferometer measurement. The FEM simulation of an element in water resulted in a maximum transfer of 43 kPa/V at 2.3 MHz and the acoustic measurement in 55 kPa/V at 2.5 MHz. The maximum pressure is ~1 MPa/120Vpp, which is sufficient pressure for second harmonic imaging. The proposed design of the transmit subarray is suitable for its role in a 3D 2H TEE probe.