Philip E. Bloomfield

Experimental study of the acoustical properties of polymers utilized to construct PVDF ultrasonic transducers and the acousto-electric properties of PVDF and P(VDF/TrFE) films

Several acoustic transmission and reflection technique measurements were carried out to determine mechanical properties (acoustic attenuation and velocity) versus frequency of polyvinylidene-fluoride (PVDF) and six other polymers. Acoustic measurements (0.5 to 12 MHz) included time-delay spectrometry (TDS; in which separate transmitting and receiving transducers utilize a swept frequency signal) and two pulse-echo methods (short tone burst echoes utilizing transducers with different center frequencies and Fourier analysis of echoes sent and received by damped transducers operating in the broadband pulse mode). Electrical impedance measurements of piezoelectric thin films of PVDF and P(VDF/TrFE) yielded comparable high frequency mechanical parameters. Of the seven acoustically examined polymers, PVDF had the greatest acoustic impedance, lowest acoustic velocity, and greatest mechanical loss (13.4 dB/cm per MHz). Polymethyl-methacrylate (PMMA; lucite) and polydimethyl-pentane (TPX) had the lowest loss. PMMA had the highest acoustic velocity, and TPX had the lowest acoustic impedance and a velocity almost identical to that of PVDF. These data are useful in the design of backing, matching, and lens materials to be used in association with PVDF transducers.

Multilayer transducer transfer matrix formalism

A complete formulation of direct and inverse 4 /spl times/ 4 transfer matrices for parallel and series, electrically connected, mechanically stacked, 1-D thickness mode multilayer piezoelectric transducers is presented. Complex coefficients account for the mechanical, dielectric, and piezoelectric losses. The direct or inverse 4 /spl times/ 4 transfer matrix transfers quantities at the top surface into their values at the bottom surface or vice versa, respectively. The 4 /spl times/ 4 transfer matrices derive from the 3 /spl times/ 3 transfer matrices, which follow from the 3 /spl times/ 3 matrix for the general three-port. For both parallel and series connections, the 3 /spl times/ 3 and 4 /spl times/ 4 direct and inverse transfer matrices are interrelated through transformation symmetries; also, the inverse matrix can be obtained from the direct matrix by changing the sign of both the piezoelectric coefficient and the explicitly occurring complex variable, j. For the electrically parallel connected case, explicit voltage orientation reversals occur at successive piezoelectric layers. Cascading the 4 /spl times/ 4 matrices yields the sum of the currents through the piezoelectric layers for the electrically parallel-connected case and the sum of the voltage differences across the layers for the electrically series-connected case. The resultant matrices are calculated for the cascading of n identical piezoelectric layers connected in parallel and series.

Enhanced bandwidth ultrasound transducers with multiple piezoelectric polymer layers

A Barker-coded multiple layer piezopolymer transducer design is described. The design is intended to improve the transmitting sensitivity of a PVDF transducer while retaining PVDF materials wide bandwidth properties. Computer simulations of the coded multilayered PVDF transducer were carried out and several prototypes of the transducers were built and tested. The results of comparisons between the measurements and theoretical predictions are presented. Advantages and disadvantages of the coded multilayer approach to bandwidth enhancement of ultrasound transducer are also discussed. Barker-coded transducers hold promise to emerge as a new class of ultrasound imaging transducers having a sensitivity on par with that offered by presently used PZT or composite transducers but with significantly wider bandwidth.

Membrane hydrophone phase characteristics through nonlinear acoustics measurements

This work considers the need for both the amplitude and phase to fully characterize polyvinylidene fluoride (PVDF) membrane hydrophones and presents a comprehensive discussion of the nonlinear acoustic measurements utilized to extract the phase information and the experimental results taken with two widely used PVDF membrane hydrophones up to 100 MHz. A semi-empirical computer model utilized the hyperbolic propagation operator to predict the nonlinear pressure field and provide the complex frequency response of the corresponding source transducer. The PVDF hydrophone phase characteristics, which were obtained directly from the difference between the computer-modeled nonlinear field simulation and the corresponding measured harmonic frequency phase values, agree to within 10% with the phase predictions obtained from receive-transfer-function simulations based on software modeling of the membranes physical properties. Cable loading effects and membrane hydrophone resonances were distinguished and identified through a series of impedance measurements and receive transfer function simulations on the hydrophones including their hard-wired coaxial cables. The results obtained indicate that the PVDF membrane hydrophones phase versus frequency plot exhibits oscillations about a monotonically decreasing line. The maxima and minima inflection point slopes occur at the membrane thickness resonances and antiresonances, respectively. A cable resonance was seen at 100 MHz for the hydrophone with a 1-m cable attached, but not seen for the hydrophone with a shorter 0.65-m cable.

Determination of ultrasound hydrophone phase from Fourier-Hilbert transformed 1 to 40 MHz time delay spectrometry amplitude

A Fourier-Hilbert transform (FHT) algorithm procedure was developed to obtain the complex characteristics of different hydrophones which included the most widely used membrane and capsule types. The theory presented was experimentally verified using magnitude response of the hydrophone probes determined with the time-delay-spectrometry (TDS) technique. The TDS setup implemented was similar to one used by Wear et al. in 2011, with the frequency range being extended to 1 to 40 MHz to ensure consistency with recently published international standards covering the properties, calibration, and performance evaluation of ultrasound hydrophones. The theoretical and experimental limitations of FHT were examined and revealed that extrapolations of the available logarithmic (volts per pascal magnitude) frequency response were needed to carry out acceptable FHT calculations of the phase response. The complex-valued analytic signal Hilbert transform (HT) algorithm was less practical in developing an efficient minimum phase calculation protocol than the equivalent real-valued discrete convolution HT algorithm. Examples of measured transfer function and FHT calculated phase response spectra of representative membrane and capsule ultrasound hydrophones are presented and are shown to obey the simple rule for minimum phase presented by Heyser in 1969.

Variable-frequency multilayer PVDF transducers for ultrasound imaging

We have developed an innovative, nonresonant wide bandwidth multilayer PVDF ultrasonic transducer, well suited for both conventional clinical imaging in the frequency range 3 - 15 MHz as well as for high frequency back-scatter microscopy imaging between 20 - 100 MHz. Such operation is desirable in clinical practice, eliminating the need for the separate probes used to minimize the tradeoff between achievable penetration depths and desired image resolution. The unique properties of our transducer were achieved by stacking individual PVDF layers in a parallel or anti-parallel polarization direction following a Barker coded pattern. The thickness of a single layer determines our transducers bandwidth; its electrical properties are similar to those of conventional PZT transducers; and its overall pulse-echo sensitivity is sufficiently high for directly interfacing with a commercially available ultrasound imaging system. Using our transducer model, key parameters of the design were predicted and compared with single layer PZT and PVDF transducers in the 3 - 15 MHz and 25 - 100 MHz frequency ranges, respectively. Several prototypes of our wide bandwidth multilayer transducers were fabricated and tested in water. Agreement between experimental results and corresponding computer predictions indicate that the multilayer design outperforms the PZT transducer with respect to axial resolution and overall pulse-echo sensitivity in the frequency range 3 - 100 MHz.

The design, processing, evaluation and characterization of pyroelectric PVDF copolymer/silicon MOSFET detector arrays

We have developed a 64 element linear array of pyroelectric elements fully integrated on silicon wafers with MOS readout devices. The ferroelectric polymer film sensor deposited and polarized on the extended gate of the MOSFET results in a hybrid circuit, the pyroelectric-oxide-semiconductor field effect transistor (POSFET). The fabrication of the wafers included the design of the various masks required to produce the layers which made up the transistor array: stopper layer, active layer, poly-silicon layer, contacts layer, and bottom electrode layer. A thin film of the ferroelectric copolymer P(VDF/TrFE) was spin coated onto the wafer. Patterned gold electrodes were sputtered as the top electrode layer. The ferroelectric copolymer was hysteresis poled in situ. Tests performed included the arrays response to a CO/sub 2/ laser operating in the CW and single pulse modes at 10.6 /spl mu/m. We present details of the design, processing, and testing of the fabricated devices.

Nonlinear acoustics determination of phase characteristics of PVDF membrane hydrophones

When an ultrasonic pressure wave propagates through a nonlinear medium, the relative phasing of the generated harmonics causes a distinct asymmetry between the positive and negative pressure levels and between the rise and fall time of examined waveforms. A faithful quantitative reproduction of the source transducers pressure field requires amplitude and phase measurements by calibrated hydrophone probes. Nonlinear hydrophone calibration provides amplitude and phase information at discrete multiples of an acoustic sources fundamental frequency. Two PVDF bilaminar membrane hydrophones were first calibrated in terms of their amplitude sensitivity to the pressure levels generated by two different HIFU (High Intensity Focused Ultrasound) circular source transducers operating at 5 MHz and 10 MHz, enabling phase studies up to 105 and 100 MHz, respectively. Introducing two newly-developed phase-dispersion representations, the phase responses of the two membrane hydrophones were determined with respect to the phase of the complex frequency response extracted from the nonlinear field simulated by a semi-empirical computer model which predicts the near and the far field pressure distributions. These phase differences compared favorably with the results obtained from the commercially available PiezoCAD simulation model. The protocol for specifying the complex pressure field of source transducers through measurements using the calibrated hydrophones is described. The results obtained indicate that the membranes exhibit close to linear decay of phase against the frequency.