Analysis and Design of High-Frequency 1-D CMUT Imaging Arrays in Noncollapsed Mode

Abstract

High frequency ultrasound imaging arrays are important for a broad range of applications, from small animal imaging to photoacoustics. CMUT arrays are particularly attractive for these applications as low noise receiver electronics can be integrated for an overall improved performance. In this paper we present a comprehensive analysis of high frequency CMUT arrays based on an experimentally verified CMUT array simulation tool. The results obtained on an example, a 40 MHz 1-D CMUT array for intravascular imaging, are used to obtain key design insights and tradeoffs for receive only and pulse-echo imaging. For the receiver side, thermal mechanical current noise, plane wave pressure sensitivity, and pressure noise spectrum are extracted from simulations. Using these parameters, we find that the receiver performance of CMUT arrays can be close to an ideal piston, independent of gap thickness and applied DC bias, when coupled to low noise electronics with arrays utilizing smaller membranes performing better. For pulse-echo imaging, thermal mechanical current noise limited SNR is observed to be dependent on the maximum available voltage and gap thickness. In terms of bandwidth, we find that Bragg resonance of the array, related to the fill factor, is a significant determinant of the high frequency limit and the fluid loaded single membrane resonance determines the lower limit. Based on these results, we present design guidelines requiring only fluid loaded single membrane simulations and membrane pitch to achieve a desired pulse-echo response. We also provide a design example and discuss limitations of the approach.

Lumped-Parameter Equivalent Circuit Modeling of CMUT Array Elements

“…Likewise, each configuration is designed to have periodically spaced columns. It is worth mentioning that the choice of a periodic layout is the most suitable to address all the imaging methods, in particular plane wave techniques [67]. The thickness-size pairs which give an immersed element central frequency of 5 𝑀𝐻𝑧, as well as the required inter-column spaces associated to each pair, are plotted Fig.…”

cMUT technology developments

“…For high-frequency (20–40 MHz) imaging applications, piezoelectric transducer arrays are difficult to fabricate, while cMUTs have inherent advantages [64] . As a bulk resonator, the piezoelectric plate thickness must be approximately half the acoustic wavelength in the material; high-frequency piezoelectrics are thus extremely thin, fragile, and difficult to process (all other acoustic stack and bonding layer thicknesses have tight tolerances as well).…”

Dual-modality rigid endoscope for photoacoustic imaging and white light videoscopy

“…First, for high resolution three-dimensional (3D) PAT, a high-density two-dimensional (2D) ultrasound array that provides fine spatial sampling (<100 μm) and wide detection bandwidth (tens of MHz) is required but is challenging to achieve with conventional piezoelectric detection technology 12 . Capacitive micromachined ultrasonic transducers (CMUT) fabrication technology can produce high-density arrays 13 – 16 but lacks the large detection bandwidth for high-resolution PAT imaging. Second, efficient delivery of the excitation light to the tissue requires delivering the widefield illumination through the detector to the tissue.…”