David T. Wilson

Properties of PVDF Polymer for Sonar

Polyvinylidene fluoride (PVDF) polymer is evaluated for application in large sonar arrays. The piezoelectric coefficients and elastic constants of thick (550 urn) PVDF produced at Raytheon are measured, and the apparatus for making the measurements is described. It is shown, both theoretically and experimentally, that electrodes deposited on the surface can stiffen the polymer and significantly alter the observed properties. The free-f ield frequency response for an experimental hydrophone is shown over a broad frequency range, f ee of spurious resonances. Stability to relatively high temperature (90°C) and to high static pressure (6.9 MPa = 1000 ps 1 are shown from experimental data.

The use of half-tone screens in Fresnel zone plate imaging of incoherent sources

Abstract An off-axis Fresnel zone plate may be used as a large-area “coded” aperture for imaging incoherent sources. The zone plate is sufficiently coarse that only shadow-casting, and not diffraction, is involved. It is shown here that the usefulness of this system can be greatly increased by using a half-tone screen to introduce a spatial carrier frequency and heterodyne the object spectrum into the passband of the zone plate.

Fresnel Zone Plate Imaging in Radiology and Nuclear Medicine

X-ray tubes and gamma ray cameras have traditionally involved trade-offs between spatial resolution and radiation flux. Recently we have shown that the use of a Fresnel zone plate as a spatially-coded source or aperture can avoid this trade-off. In radiology, this technique can eliminate the need for a rotating anode and give higher resolution, while in nuclear medicine it can be used either to decrease patient dose or exposure time, or to increase resolution and greatly simplify the apparatus. With a coded source or aperture, the image is also coded, like a hologram and can be reconstructed optically. The system is tomographic with information about all planes contained in a single film.

PVF 2 Polymer Microprobe

This paper describes the design and performance of a PVF2 microprobe for use in mapping the pressure field from arrays. For medical applications, the probe must have short pulse response (i.e., broad, flat bandwidth) in addition to high sensitivity, low noise, and a wide acceptance angle. The factors that influence critical parameters, such as ringing and delayed reverberations, are specifically discussed. The probe has a center frequency of about 3 MHz. It consists of a 1.5 mm diameter disc of PVFZ polymer (30 pm thick) operated well below its resonance to obtain very broad frequency response. The PVFz is completely encased in a metal cylinder to shield it from spurious electromagnetic interference and to protect the polymer from possible corrosion by external fluids. An FET amplifier is integral to the probe housing. The experimental performance of the probe is compared with theoretical predictions, acoustically and electrically. It is also compared with the performance of other probes, including a laser pellicle system, for pulse fidelity and signal-to-noise ratio.

Coded Apertures Derived from the Fresnel Zone Plate

Mertz and Young introduced the idea of using a Fresnel zone plate as a shadow-casting reticle, or coded aperture, in X-ray astronomy. More recently, considerable progress has been made toward using the zone-plate aperture for gamma-ray imaging in nuclear medicine. The most successful configuration has used an off-axis section of a zone plate in conjunction with a halftone screen. In this paper, we discuss a variety of closely related coded apertures, including an annulus, an inverted zone plate, a spiral zone plate, and the Girard grill. In most cases, the technique of grid-coded subtraction is used to suppress the zero-order (DC) background light usually associated with zone-plate imaging. The first application of this technique, reported by Stoner et al., used a sequence of two to four on-axis zone plates. In the present paper it is shown that the method can be extended to other apertures and is also very useful in synthesizing the spatial filters for optical decoding.

A spatially-coded x-ray source.

Abstract A new type of x-ray tube which permits three-dimensional imaging on a single film and high spatial resolution in spite of a large focal spot is described. The resolution is approximately the reciprocal of the spatial frequency of a half-tone screen.

Transmitters and Receivers for Medical Ultrasonics

Wide bandwidth transducers are essential for good range resolution in pulse-echo medical ultrasound. Within this bandwidth objective, however, the criteria that determine whether a given piezoelectric material is a good transmitter are, in general, different from those that determine a good receiver. estimating the merits of a particular piezoelectric as a transmitter or receiver, and the advantages that may be gained by separating these roles in a medical ultrasound system are also discussed. Although some materials are better suited for receivers than for transmitters, this is shown to be consistent with the reciprocity principle.

A new configuration for coded aperture imaging

Abstract It is shown that two properly-scaled Fresnel zone plates in series constitute a useful coded aperture for imaging X-rays, gamma rays or other incoherent radiation.

Fresnel zone plate imaging in radiology and nuclear medicine

X-ray tubes and gamma ray cameras have traditionally involved trade-offs between spatial resolution and radiation flux. Recently we have shown that the use of a Fresnel zone plate as a spatially-coded source or aperture can avoid this trade-off. In radiology, this technique can eliminate the need for a rotating anode and give higher resolution, while in nuclear medicine it can be used either to decrease patient dose or exposure time, or to increase resolution and greatly simplify the apparatus. With a coded source or aperture, the image is also coded, like a hologram and can be reconstructed optically. The system is tomographic with information about all planes contained in a single film.

Point-Source Artifacts in Fresnel Zone Plate Imaging

When a Fresnel zone plate is used as a coded aperture to image large-area sources of X-rays or gamma rays, it is necessary to use a half-tone screen. It is shown here that small-area sources, with such a system, will always produce strong ghost images or artifacts.