JDMS 20:395–405 November/December 2004 39510.1177/8756479304269943ARTICLEJOURNAL OF DIAGNOSTIC MEDICAL SONOGRAPHY November/December 2004 VOL. 20, NO. 6THE SILENT REVOLUTION / Powis, MooreJDMS 20:395–405 November/December 2004JDMS 20:395–405 November/December 2004The SilentRevolution:Catching Up Withthe ContemporaryComposite TransducerRAYMOND L. POWIS, PHD, FAIUM*G. WAYNE MOORE, BSC, MA†Although a remarkable signal-processing evolu-tion has been going on in the sonograph, a moresilent revolution has been going on within thescanhead with the development of compositemultielement transducers (CMETs). This articleexamines the current state of CMET develop-ment and explores what these changes mean tobasic beam formation and focusing. The discus-sion reveals some of the technologies that arepart of contemporary CMET design, includingtransducer poling, connectivity, and coupling.Inaddition, this discussion includes some qualityassurance (QA) techniques that can test thesenew transducers for proper function and deter-mine what is happening beneath the scanheadhousing. Finally, the authors propose a list ofmeasurements needed to test CMETs, includingelement sensitivity, element center operatingfrequency, element fractional bandwidth, pulseshape, pulse duration, and element and cable ca-pacitance, and they provide a QA testing proto-col for the CMET.Key words: ultrasound, transducer, multi-element, composite, quality assurance, poling,connectivity, coupling, beam forming, beamsteeringThe process of keeping track of technical devel-opments in diagnostic sonography and image qual-ity has been one of witnessing steady refinementsin technology punctuated by sudden improvementsin signal-processing architecture. Most of the re-cent changes, both large and small, have been theconsequence of that great phrase “going digital,”that is, applying established digital signal process-ing (DSP) to the formation and enhancement of thegray-scale image. DSP has also made it possible todepict blood flow patterns within the cardiovascu-lar compartments in real-time color. These imagingchanges have been so remarkable and so successfulthat, in a way, it has been like a magician’s sleight-From*Greeley, Colorado, and†Sonora Medical Systems,Longmont, Colorado.Correspondence: Raymond L. Powis, Ultrasound Consultant, 120343rd Avenue, Greeley, CO 80634. E-mail: [email protected]: 10.1177/8756479304269943In accordance with ACCME Standards, authors are required todisclose any commercial affiliations or financial interests that might beperceived as a real or apparent conflict of interest related to the contentof their JDMS CME article. The author, Dr. Raymond Powis, is aconsultant to Sonora Medical Systems.of-hand trick. We have been visually drawn towatching the more colorful moving hand (digi-tal signal processing) while missing the move-ments of the other hand (continued transducerdevelopment).These transducer changes have not been as obvi-ous as, say, a new and unique circuit board designor the real-time image of blood flow in color. Yet,we have witnessed a steady increase in transducerflexibility and capability along with a proliferationof new transducer designs that are specialized to-ward explicit scanning tasks. A silent revolutionhas been going on inside that scanhead housing.The transducer is no longer a simple, single-elementdevice that either works or does not. Instead, cur-rent transducers have become more discrete in adifferent sense. The transducer is now a set ofsmall, discrete elements that are electronically or-chestrated into various wavefront geometries toform, focus, and steer an ultrasound beam. In thisexpanding mechanical complexity, current trans-ducer designs seem to have surged ahead of ourtechnical abilities to test the element arrays. Also,many subtleties of failure that may have an impacton the clinical study are not to be found with theconventional tissue-mimicking phantom (TMP).This article examines the current state of trans-ducer development and explores what thesechanges mean to basic beam formation and focus-ing. The discussion reveals some of the technolo-gies that are part of the contemporary compositemultielement transducer (CMET). In addition, thisdiscussion includes some quality assurance tech-niques that can test these new transducers forproper function and determine what is happeningbeneath the scanhead housing. Finally, we proposea listing of measurements needed to test CMETsand pose a testing protocol to provide CMETquality assurance.We begin with a set of useful definitions shownin Table 1.Essential TransductionAt the functional center of any scanhead is thetransducing material that converts energy from oneform into another. For diagnostic sonography, it isa piezoelectric (PZ) substance organized to convertmechanical energy (pressure) into electrical energy(echo signal) and vice versa. This property emergesfrom the internal molecular organization of the PZmaterial. Applying pressure to the material causesthe internal electric dipoles to reorganize, creatingan electric field across the transducing element.1These electric dipoles also respond to an externalelectric field. As a result, a PZ material will changeshape in response to an externally applied electricfield.1For diagnostic ultrasound, the organization anddirection of the dipoles or poling is typically alongthe z-axis (Fig. 1), which is the same direction asthe mechanical vibration of the transducer. Thenatural resonant frequency of a single-elementtransducer is set by its thickness, t, which is ob-tained by lapping the transducer to λ/2, where λ =c/f; c is the characteristic, acoustic propagation ve-locity for the PZ material, and f is the center operat-ing frequency (COF) in Hz. The functional goal indesigning for a natural vibration is to have thetransducing wafer act as if it were a perfect piston.We expect to witness the natural resonant fre-quency of a transducer when it is shocked into nat-ural vibration by a sharp electrical pulse rather thandriven by a pattern of reversing voltages andcurrents (AC).Common Events for TransducersThe basic role of the scanhead in a pulse-wave(PW) sonograph is to transmit a burst of ultrasoundand receive ultrasonic echoes on a cyclic basis. Theburst of ultrasound is amplitude modulated by thetransmitter excitation technique and any internaltransducer damping used to shorten the burst dura-tion. The result is a fractional bandwidth (FBW) offrequencies