Igal Ladabaum

Microfabricated ultrasonic transducers monolithically integrated with high voltage electronics

Capacitive microfabricated ultrasonic transducers (cMUTs) recently produced clinical-quality images, and have the potential to enable true 3D ultrasound. Much has been written about the value of integrating cMUTs with electronics, and some early work has been published (Eccardt, P.-C. et al., 1996; Noble, R.A. et al., 2002). The paper describes monolithic integration of high-quality imaging cMUTs with analog switching electronics. The switching circuitry used has the same voltage limitations as commercial multiplexing chips (200 V/sub PP/). Water tank results demonstrate that cMUTs integrated with controlling switches are fully-functional when the capacitance and resistance of the switch are inserted into the cMUT model. The results further demonstrate no measurable effect of the switch on the radiation pattern of the acoustic elements. This verifies the viability of aperture control and channel multiplexing with monolithically integrated electronics. In the short term, such integration can allow for reconfigurable apertures, arrays with many elevation rows, and optimal preamplification. it also represents a step towards a practical 2D matrix transducer, due to the density of its integration and the freedom to optimize separately the cMUT and the integrated electronics.

cMUTs and electronics for 2D and 3D imaging: monolithic integration, in-handle chip sets and system implications

Capacitive microfabricated ultrasound transducers (cMUTs) have been shown to be practical for medical imaging. Breakthrough performance requires combining these MEMS transducers with electronics. This paper explores synergies between cMUTs and electronics for 2D and 3D imaging. For example, low-noise receive signal conditioning improves tissue penetration, while transmitters capable of arbitrary waveforms minimize clutter. Bias control circuitry can create simple multi-row arrays for improved 2D contrast resolution. It also can enable 3D scanning with less complexity than current alternatives. Matrix transducer elements for 3D present challenges due to their high impedance, number and density. Monolithically integrated cMUTs can offer unique solutions to these problems, enabling isotropic 3D imaging from fully sampled arrays.

Silicon substrate ringing in microfabricated ultrasonic transducers

Experimental and theoretical evidence of silicon substrate ringing in microfabricated ultrasonic transducers is presented. This ringing is clearly observed in immersion transducers with a 650 /spl mu/m thick substrate at 7 MHz and harmonics. An analytical model of the ringing is introduced, and simulations based on the model are shown to agree with experimental observation. Experimental results are further compared to simulations carried out in time-domain, large-scale PZFlex models and qualitative agreement is demonstrated. The insights gained from the simulations and experiments are used to design and fabricate a device whose ringing mode is eliminated with a backing layer.

System and method of operating microfabricated ultrasonic transducers for harmonic imaging

A capacitive microfabricated ultrasonic transducer (cMUT) is operated to improve its performance during harmonic imaging in non-linear media, such as in contrast agents or in human tissue. The cMUT is operated by inverting the transmit waveform to adjacently spaced azimuth elements, and combining at least two additional firings without adjacent inversion, for each transmit vector, thereby canceling the second harmonic generation of the cMUT; and thus, the performance of harmonic imaging using the cMUTs can achieve improvement.

Time-domain models of MUT array cross-talk in silicon substrates

Micromachined ultrasonic transducer (MUT) arrays are topical because, among other opportunities, they may supplant piezoelectric arrays in ultrasound medical imaging. However, imaging system companies need to explore performance and design issues thoroughly before making a serious commitment to these silicon-based electrostatic devices. To this end, a fully nonlinear, 3D MUT virtual prototyping capability was recently added to PZFlex. It is described here and applied to medical-type arrays. Simple representations of MUT plate vibration, including acoustic loading and radiation, are presented. Array cross-talk through the surface structure and silicon substrate is quantified using large-scale models. Additionally, an intuitive 1D electromechanical MUT model is developed for modeling and design guidance.

Elevation beam profile control with bias polarity patterns applied to microfabricated ultrasound transducers

In contrast to PZT probes, capacitive microfabricated ultrasonic transducers (cMUTs) require a DC bias. We investigate elevation beam profile shaping by spatially varying this bias polarity. Such a scheme introduces a 180/spl deg/ phase shift in the devices impulse response. A Fresnel zone plate is realized which is capable of generating tight elevation foci using a large aperture. This paper presents measurements from a prototype Fresnel cMUT probe, and matching simulation results. The Fresnel probe is capable of improved slice thickness compared with both a conventionally lensed probe and a multi-row design. Also, combinations of bias polarity control and multiple firings can enable harmonic imaging without pre-distortion of the transmit signal. Multiple firing schemes further optimize slice thickness, and allow beam steering in elevation.

5G-1 Two Approaches to Electronically Scanned 3D Imaging Using cMUTs

Capacitive micromachined ultrasonic transducers (cMUTs) introduce new degrees of freedom in transducer design. For example, it is easy to make elements of any size, and electronics may be integrated directly under the transducer element. These characteristics derive from lithographic manufacturing on a silicon substrate, and the use of a low-temperature surface micromachining process. They are particularly helpful in creating 2D arrays for electronically scanned volume imaging. This paper describes two contrasting ways to achieve volume data acquisition. In bias voltage scanning, we create a Fresnel elevation focus using a crossed electrode design with traditional azimuth beam formation. Then using the bias voltage dependence of kT in a cMUT, N2 element connections can be reduced to 2N. Such a scheme is especially useful in high frequency linear scanning, where N can exceed 200 elements. Measurements and images from a prototype probe are presented. We also describe a method to achieve fully sampled 2D receive apertures with autonomous elements using monolithically integrated electronics. This achieves spatial Nyquist sampling in both array dimensions, with arbitrary element delay and amplitude control. It allows much more of the information in the field returning to the probe to be retrieved by the imager, causing improvements in image quality and diagnostic confidence