1244 IEEE SENSORS JOURNAL, VOL. 9, NO. 10, OCTOBER 2009

Polyimide-Based Conformal Ultrasound Transducer Array for Needle GuidanceMartin O. Culjat, Member, IEEE, David B. Bennett, Michael Lee, Elliott R. Brown, Fellow, IEEE,

Hua Lee, Fellow, IEEE, Warren S. Grundfest, Member, IEEE, and Rahul S. Singh, Member, IEEE

Abstract—A conformal ultrasound imaging system has beendeveloped that may potentially decrease the user variabilityassociated with operation of rigid transducer-based medicalultrasound systems. Conformal transducer arrays were built andcharacterized, featuring bulk piezoelectric elements mountedon an array of silicon islands, connected with flexible polyimidejoints. The joints successfully supported electrodes while bending,and were sufficiently soft to enable both conformation to bodysurfaces and needle insertion for ultrasound guidance procedures.A signal-to-noise ratio (SNR) of 39 dB was achieved from aneedle target in a soft tissue phantom. A space–time backwardpropagation image reconstruction algorithm was simulated todemonstrate needle detection using a conformal transducer withan arbitrary array architecture.

Index Terms—Conformal, flexible structures, polyimide film,transducer array, ultrasound imaging system.

I. INTRODUCTION

T HE high variability among users and long learning curvesassociated with ultrasound scanning and image interpre-

tation have limited broader acceptance and further expansionof medical ultrasound. Ultrasound guidance procedures, suchas needle biopsy and central line placement, require additionalskill to precisely position a needle or catheter, while simultane-ously holding a transducer.

Conformal ultrasound transducer arrays may provide analternative to rigid medical transducers [1]. These transducersmay potentially improve acoustic signal acquisition by wrap-ping around curved body surfaces, providing increased angularcoverage around objects within the body. An additional ad-vantage is that multiple view angles can be obtained with thetransducer fixed in a single position, potentially eliminatingthe expertise required for handheld mechanical scanning ofrigid ultrasound transducers. The ease of use may be especiallybeneficial to ultrasound guidance procedures by eliminating the

Manuscript received February 22, 2009; revised April 19, 2009; acceptedJune 05, 2009. Current version published September 02, 2009. This work wassupported in part by the Telemedicine and Advanced Technology ResearchCenter/Department of Defense under Grant W81XWH-07-1-0672 and GrantW81XWH-07-1-0668. The associate editor coordinating the review of thispaper and approving it for publication was Prof. Kiseon Kim.

M. O. Culjat, D. B. Bennett, M. Lee, and W. S. Grundfest are with the Centerfor Advanced Surgical and Interventional Technology (CASIT) and the HenrySamueli School of Engineering and Applied Science, University of Californiaat Los Angeles, Los Angeles, CA 90095-1600 USA (e-mail: mculjat@mednet.ucla.edu).

E. R. Brown, H. Lee, and R. S. Singh are with the Center for Advanced Sur-gical and Interventional Technology (CASIT), University of California at LosAngeles, Los Angeles, CA 90095-1600 USA and the Department of Electricaland Computer Engineering, University of California at Santa Barbara, SantaBarbara, CA 93106 USA.

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/JSEN.2009.2030270

Fig. 1. Illustration of conformal transducer cross section and top view.

need to position a transducer during needle insertion, therebydecreasing user variability.

Flexible ultrasound transducers have been proposed bya small number of groups, with the emphasis primarily onnondestructive evaluation [1]. Prior fabrication strategies havefocused on attaching bulk piezoelectrics to flexible printed cir-cuits, embedding piezoceramic rods within flexible polymersby “dice and fill” techniques, transferring piezoelectrics frompolydimethyl siloxane (PDMS) templates to flexible substrateswith adhesive films, mounting transducers onto spring-loadedprobe matrices, trench-refilling with PDMS on capacitivemicromachined ultrasound transducer (cMUT) substrates, andthe use of flexible polyvinylidene fluoride (PVDF) materials[1]. Many of these prior transducers had limited flexibility,limited scalability, electrodes that were beset by cracking orpeeling, or inefficient piezoelectric materials, resulting in lim-ited sensitivity and penetration depth. This paper describes anovel conformal ultrasound transducer with bulk piezoelectricelements mounted on etched silicon islands connected bypatterned flexible polyimide joints, as well as a demonstrationof needle guidance with an arbitrarily arranged conformaltransducer using a simulated image reconstruction.

II. TRANSDUCER DESIGN AND CHARACTERIZATION

The conformal ultrasound transducer design is illustratedin Fig. 1. The transducers featured bulk lead zirconate ti-tanate (PZT) piezoelectric elements mounted on silicon islandsformed by deep reactive ion etching (DRIE) and a backsiderelease etch. The silicon islands were connected by flexiblepolyimide joints, and the transducer was reinforced with twoparylene coats. The silicon islands served as the mechanicalsubstrate for the piezoelectric elements and, in combinationwith the parylene, achieved acoustic impedance matching tosoft tissue. A two-layer electrode network was patterned using arow-column addressing technique, with a lower Al ground run-ning perpendicular to upper Au signal lines. A tungsten-loadedepoxy acoustic backing layer was applied to the PZT to broadenthe bandwidth.

1530-437X/$26.00 © 2009 IEEE

CULJAT et al.: POLYIMIDE-BASED CONFORMAL ULTRASOUND TRANSDUCER ARRAY FOR NEEDLE GUIDANCE 1245

Fig. 2. (a) Eight-element linear conformal transducer array with needleinserted through a polyimide joint for ultrasound guidance procedure, and(b) wrapped around a finger. Backing layer is not included so that the elementsare visible.

Fig. 3. Representative acoustic signal received from pulse-echo measurementwith transducer wrapped around a 14 gauge needle in a phantom. Monostaticdata is shown from (1,1) transmit/receive element pair and bistatic data from(2,3) element pair.

Polyimide was selected for the joints because it is mechani-cally strong, has good thermal properties, is readily patterned byphotolithographic techniques, and is resistant to chemicals usedin microfabrication processes. Two polyimide layers were spincoated and cured onto the wafer—the first to hold the siliconislands together prior to backside etching, and the second to en-capsulate and protect the electrodes, reinforce the flexible joints,and serve as alignment trenches for the placement of piezoelec-tric elements. Bulk PZT was selected for its high piezoelec-tric coupling efficiency compared to other alternatives, such aspiezocomposites or thin-film piezoelectrics.

Eight-element linear conformal transducer arrays werefabricated (Fig. 2) using 1.8 mm 1.8 mm PZT elementsoperating in a fundamental thickness mode having a resonantfrequency of 12 MHz, a dB bandwidth of 2 MHz, and acapacitance of 230 pF. Experiments were carried out with atransducer wrapped around a 2 cm diameter cylindrical softtissue phantom with embedded 14 gauge steel needle. Tworepresentative acoustic pulses from a monostatic pair (sameelement transmits/receives) and bistatic pair (different ele-ments transmit/receive) are shown in Fig. 3. The measuredacoustic pulse width was 1.8 s in water, upon excitationwith a 200 ns, 2.5 continuous wave pulse (2.5 cycles).The operating (or sensor) signal-to-noise ratio (SNR) of themonastatic pairs (maximum SNR) was 39 dB using a low-noiseamplifier (Minicircuits ZFL-1000LN) for the receiver. Thehigh flexibility was demonstrated by bending the transducersto radii of curvature smaller than 1 cm [Fig. 2(b)]. Durabilitywas confirmed by observing that the polyimide/parylene joints

Fig. 4. Simulated image reconstruction of eight transducer elements arbitrarilypositioned around a target distribution representing a needle.

suffered no visible damage or measurable electrical deterio-ration of the interconnects following 10 000 bending cycles.The flexible polyimide/parylene joints were sufficiently soft toenable penetration with a needle [Fig. 2(a)], which will be ofpractical benefit during ultrasound guidance procedures withlarger arrays.

III. SIMULATED IMAGE RECONSTRUCTION

An image reconstruction algorithm based on space–timebackward propagation was developed and tested [2]. Thealgorithm used computer-generated models to investigatethe feasibility of the conformal ultrasound imaging systemapproach for needle guidance. Eight transducer elementswith 2 MHz bandwidth were arranged in an arbitrary non-circular conformal arrangement around a target distributionrepresenting a 14 gauge needle. A composite image clearlyvisualizing the needle target was reconstructed (Fig. 4) fromsimulated band-limited time-delay and range profiles of alltransmit-receive element pairs. This result suggests that thereconstruction algorithm and the conformal transducer arraycan be used to locate landmarks, and that fixed, organizedarrays are not essential for high-resolution imaging.

IV. CONCLUSION

The results to date have demonstrated the feasibility of theconformal ultrasound approach for procedures such as ultra-sound guidance. Further simulations are underway to examineresolving capability under various array architectures and forvarious applications. A transducer array fabrication process wasdeveloped that can be extended to two dimensions and scaled toa variety of array sizes, densities, and architectures. This scala-bility, in addition to the high flexibility, durability, and ability ofthe polyimide joints to support electrode layers during bendingand withstand needle penetration, are key advances in the trans-ducer design. Together, the advances described here may enablea new approach to ultrasound imaging, allowing high quality im-agery to be obtained with less expertise and reduced training fora variety of medical conditions.

REFERENCES

[1] D. B. Bennett et al., “A conformal ultrasound transducer array fea-turing microfabricated polyimide joints,” in Proc. SPIE Smart Mate-rials/NDE, 2009, p. 7295, 72951W.

[2] H. Lee, “Formulation of the generalized backward projection methodfor acoustical imaging,” IEEE Trans. Sonics Ultrason., vol. SU-31, no.3, pp. 157–161, 1984.