High Resolution Ultrasonic Transducers

The imaging performance of an ultrasonic transducer is related to the lateral resolution, the axial resolution and the depth of field of the transducer. The lateral resolution (Riat) is the width of the ultrasonic beam in the focal region. The axial resolution (Rax) is determined by the pulse bandwidth. The depth of field (DOF) is the length of the focused region of the beam. These values can be approximated for a spherical radiator as [2]  

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Axial resolution is defined as the capability of separating two point reflectors in an axial direction from the ultrasonic transducer. The best possible axial resolution can therefore come about by using as short a pulse as possible. This is illustrated in Figure 14, where the best axial resolution is achieved for a high-frequency, highly damped ultrasonic transducer. Short noise-free pulses in a composite material are difficult to generate because of ply echoes and individual fiber and matrix echoes. 

The design, fabrication and characterization of novel micromachined focused ultrasonic transducers for minimally invasive medical imaging procedures have been demonstrated. Transducers were fabricated using a membrane deflection technique to produce spherical sections in a piezoelectric polymer film. Pulse echo responses showed minimal ringing and wide bandwidths characteristics of 80-110 %. Axial resolution of 50 pm and lateral resolution of 51-92 pm were achieved. A preamplifier circuit incorporated into a hybrid package with the ultrasonic transducer exhibited thermal nmse of 5.3 nV/Hz. The transducers were used to image human cadaveric aorta to reveal high-resolution subsurface structures. 

Ultrasonic microscopes consist of a spherically focused transducer submerged in an incompressible medium (water, for example) and attached to precision positioners. Resolution is limited by spherical aberration caused by diffraction at the medium/ sample interface and by the precision of the positioners. Standard resolution is on the order of 100 ftm. Finer focus enhances resolution, but drastically increases the time required to get data. Fine-focus acoustic microscopy may be used to find critical flaws in high-stress portions of a device, but even reducing the volume of... 

The ease of fabricating into any shape makes it passible to focus the ultrasonic wave to one point without the help of a lens. High positioa and directioa resolution am be achieved. In many cues, the use of a focused transducer provides an increase in resolution, u the acoustic power is concentrated at the focal point. 

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