Capacitive micromachined ultrasonic transducers (CMUTs) present advantages such as wide frequency bandwidth, which could be further developed for nonlinear imaging. However, the driving electrostatic force induces a nonlinear behavior of the CMUT, thus generating undesirable harmonic components in the generated acoustic signal. Consequently, the use of CMUT for harmonic imaging (with or without contrast agents) becomes challenging. This paper suggests 2 compensation approaches, linear and nonlinear methods, to cancel unwanted nonlinear components. Furthermore, nonlinear responses from contrast agent were evaluated using CMUT in transmit before and after compensation. The results were compared with those obtained using a PZT transducer in transmit. Results showed that CMUT nonlinear behavior is highly influenced by the excitation to bias voltage ratio. Measurements of output pressure very close to the CMUT surface allow the estimation of optimal parameters for each compensation approach. Both methods showed a harmonic reduction higher than 20 dB when one element or several elements are excited. In addition, the study demonstrates that nonlinear approach seems to be more efficient because it is shown to be less sensitive to interelement variability and further avoids fundamental component deterioration. The results from contrast agent measurements showed that the responses obtained using CMUT elements in transmit with compensation were similar to those from PZT transducer excitation. This experimental study demonstrates the opportunity to use CMUT with traditional harmonic contrast imaging techniques.

A bslract-Plate waves inside thc piezoelectric layer arc much involved in the elements cross-coupling in transdnccr arrays for medical imaging. In this work, such waves are analyzed in 1-3 piceocompositc materials on the basis of conventional guidecl modes formalism in which the piesocomposite is considered as a homogeneous medium. Crosscoupling measurements have been madc on two different transducer arrays using nctwork analyzer and a lascr interferometric probe. It is shown how the analysis in terms of symmetrical Lamb waves gives an intcrcsting qualitative interpretation, explaining most of the cross-coupling amplitude variations with frequency. Results show that the 0th and 3rd symmctrical Lamb waves are mainly involvcd in coupling inside composite plates. The So modo is responsible for the inter-clement coupling, whereas the Ss mode widens the cffective width of the excited element. I. INTROUIJCTION OUELINC and cliaracterizatioii of 1-3 piezoelectric M cornpositc imterials [or tlic conslriictioii of I -D, 1.5-D, and 2-D arrays arc tlie subjects of exlcnsivo worlcs for morc: than 15 yr. The first lhcorctical arid cxpcriiiicntal stiltlies havc csscmtially iricntionctl tlirnc: main iric:c:lianical modes of vitxatioris: the 1,liicltriess m o c k [I], wliich is iisctl for the generation arid the dctcctioii of t,hc iiltrasonic hcain; the radial niodcs [SI; and tlic lateral or spnrioiis rnodcs caiisod by tlie Bragg diffraction of transverse waves in lhc periodic iiiicroslriicture [2] [4]. l'licse rcsoiiaiices liniil the upper frequeiicy of good iisc of a composite pattcrii. Over the past, few years, tlic dcvclopnicnt of coinplete finite clerncnt 3-D niodcls, with tinic or freqiicncy doinwin analysis, ha,s ~llowed the extonsion of those studies to the modeling of clcctro-acoustic perforniunccs of 1-3 piezoclcctric composite arrays [6]-[8], taltirig into accoiirit the periodicity of the material, t,hc arrangc:irient of ceramic rock in tlie composite niatrix, the korf geometry l~etweeii array elements, the propertlies of kerf material filler, and all other coristrnclion paranietcrs of thc device. In this work, we will pay attention to tho 1,arnh wt~vcs in a piezocomposite plate becaiisc these modes are at l,hc origin of the acoustical cross-coupling iriside an array. The iiifliielice of cross-coiipling on tlic clcctro-acoustic heliavior of multi-eleniont transducers ha,s previoiisly been disciissed by several aiit,hors [9] ~ [ll]. It is inairily responsible for Mmuscript received .Jiily 26, 1999; acr:optcd lviay 12, 2000.

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