User Manual

Page 52

Figure 61. Invasive imaging probe

ULTRASOUND APPLICATIONS

The wide frequency response and physical attributes of its polymeric construction makes piezo film a

material of choice in certain medical probes and in nondestructive testing applications. Additionally, the

film sensors are found in applications in ultrasonic based sensing devices, like air-ranging ultrasound for

distance measurement, in fluid level sensors, and in-flow measurement instruments using doppler shift

of sound velocity perturbations which are proportional to fluid flow.

Medical Imaging Ultrasound

Piezoelectric ceramic materials are used in medical imaging transducers because of their high sensitivity

and broad bandwidth. The d

constant, strain developed for an applied voltage, is about an order of

magnitude higher for piezo ceramics than for piezo polymer. A disadvantage of piezo ceramic is its high

acoustic impedance, about 30 MRayls (1 MRayl = 10

kg/m

s) in contrast to about 1.5 MRayls for body

tissue. This impedance mismatch can be compensated by quarter wavelength matching layers, but these

can degrade the ultrasonic pulse due to adhesive layers and construction methods. The acoustic

impedance of piezo film is about 4 MRayls, a much better match. Additionally, in higher frequency

applications requiring very thin piezo elements, ceramics are too fragile, and cannot be shaped to

desired geometries.

Invasive imaging requires lower powered devices than external

probes. Resolution of the image is considerably improved at the

higher frequencies of invasive catheters. A medical imaging

company has developed an invasive imaging probe with piezo

film for a therapeutic laser prostate catheter (Figure 61 ). The

piezo film sensor is about 30 microns thick, and is located near

the catheter tip. The unit operates at frequencies of 7 MHz and

higher.

Steered in-vivo phased-array images using piezo polymer film have

been produced for the first time by researchers at Duke

University. A 32 element array of 11 mm x .56 mm elements was

fabricated and tested with a well matched circuit designed to

optimize the transducer. The result was 28 dB lower sensitivity

than PZT transducers at 2.5 MHz operating frequency.

However, the piezo film array had improved axial resolution,

better angular response (6 dB pulse-echo response at 30 degrees), and a low interelement cross-coupling

of -35 dB. It is exceptionally difficult to diamond blade saw PZT ceramic into these small elements;

while, for piezo film, complex patterns are readily etched into the surface gold electrode. PZT must be

diced due to the severe interelement coupling problem. Duke University researchers plan to improve

the polymer probe by expanding the number of array elements to 128.

Very high resolution arrays have been traditionally formed by etching an electrode pattern on the

surface of a piezo film. Newer techniques include deposition of the copolymer directly onto silicon

wafers. The wafers are etched to minimize interelement coupling, then the copolymer is applied by spin-

coating, followed by poling. Then a top ground electrode is applied and inter-connections made. This

advance results in a very high resolution imaging. Capacitively coupling copolymer film to a dense array

of conductive traces on a PCB has achieved remarkable performance as a Tx/Rx array.