A three-dimensional reflection and diffraction tomography scanner

ABSTRACTS,

ULTRASONIC

IMAGING

AND

TISSUE

(Abstract only); Acoustical Imaging, (Plenum Press, NY, 1981, in press).

CHARACTERIZATION

Vol.

10,

A.

SYMPOSIUM

F.

Metherell,

ed.

A THREE-DIMENSIONAL REFLECTION AND DIFFRACTION TOMOGRAPHY SCANNER, Steven A. Johnsonl, Michael J. Berggrenl, Douglas A. Christensenl, Frank Stenger2, James 13a113, and Calvin Wilcox2, Departments of IBioengineering, 2Mathematics and 3Physics, University of Utah, Salt Lake City, UT 84112. We report progress in construction and operation of a hybrid mechanical and electronic ultrasound scanner for producing synthetic focus (i.e., reflection tomography) and diffraction tomographic images. Unlike many previous tomographic scanners, this scanner has provisions for mounting receiving transducer arrays and transmitting transducers on the outside of the water tank through flexible rubber couplings and thereby avoids waterproof electronic housings. Angular scanning is achieved by rotation of the water tank and transducer assembly. The transducers are moved through a small horizontal and vertical displacement in order to adequately sample the imaging aperture. This system provides both planar and cylindrical apertures. A large piston transducer (either l- or 3-inch diameter) is fitted with an asperhical lucite lens to produce diverging spherical waves from a virtual point source interior to the lens-transducer assembly. These transducers may also be used as point receivers for reflection tomography. Two multiplexed sparse receiving arrays each of two rows by 32 columns are used. Cross talk and losses in the array are minimized by using a new differential preamplifier and multiplexer circuit. A new 3-D synthetic focus software package will be interfaced to the scanner through a PDP-11/34A and Analogic AP400 array processor. 2-D refraction corrected Several new diffraction synthetic focusing software may also be used. tomography algorithms based on spherical and plane wave insonification have been developed for use with the scanner. These algorithms provide solutions

for

c and p in the

Helmholtz

equation

V2p + (u2/c2)p

more general equation V2p - V(an p) * Vp + (lo2/c2)p the latter equation gives both Snell's law and the tion coefficients.

= 0. correct

= 0 and the

It is shown that angular reflec-

A NEW TECHNIQUE FOR RECONSTRUCTION FROM SCATTERING FIELDS, D. Nahamoo and A. C. Kak, School of Electrical Engineering, Purdue University, West Lafayette, IN 47907. The theory for a new ultrasonic inverse scattering transmission imaging configuration for three-dimensional and cross-sectional reconstructions is presented. The technique is based on a small perturbation solution of the wave equation. It is a generalization of the existing diffraction techniques in the sense that it permits the use of any type of insonification, as opposed to only plane-wave insonification. Therefore, one can use existing transducers and not have with the proposed technique, to generate plane-waves, the latter being difficult to achieve experimentally. Another advantage of the new technique is its data collection configuration. For a three-dimensional reconstruction, the transmitting and For receiving transducers are moved independently in two parallel planes. the two-dimensional case of cross-sectional reconstruction, they are moved independently on two parallel axes. In both cases only two rotational positions of the object are required, these being 90' apart. The data are collected for all the possible locations of the transducers in the parallel planes for the three-dimensional case, or on the parallel axes for the two-dimensional case, for both rotational positions of the object. Therefore, the requirement in the existing diffraction techniques that the object

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