Steven A. Johnson

Algebraic Reconstruction of Spatial Distributions of Acoustic Velocities in Tissue from Their Time-of-Flight Profiles

Two-dimensional distributions of acoustic velocities were measured in transverse sections through intact isolated organs, using reconstruction techniques. Profiles of time-of-flight (TOF) of 10 MHz pulses through the specimen were obtained by rectilinearly scanning two opposing transducers along either side of the specimen in the plane of interest. The received pulses were digitized at a rate of one 8-bit sample-per-10 nanosec, for 512 samples and were analyzed with a computer algorithm which calculated the TOF of the pulse to within ± 10 nanosec. Typically, 256 measurements of TOF were made in each profile scan for each of 37 angles of view separated by 5°, TOF’s through tissue, normalized by TOF through water, were used to calculate velocity within the specimen, using an algebraic reconstruction technique (ART), Images obtained represented acoustic velocities in individual cross sections within the tissue specimen with a resolution of 64 by 64 elements (< 2 mm square), The disadvantage of TOF reconstruction is that transmission scanning is required. Advantages over B- and C-scan imaging are: 1) dynamic changes in gain of receiver are not required, 2) attenuation occurs on only one traversal through tissue, and 3) the absolute value of an important acoustic parameter (velocity) is determined, which may have significant diagnostic value.

Algebraic Reconstruction of Spatial Distributions of Acoustic Absorption within Tissue from Their Two-Dimensional Acoustic Projections

It has been known for many years that three-dimensional information concerning the spatial distribution of energy absorbers within an object could be obtained from two-dimensional shadow projections of the energy absorption of the object (1). Two-dimensional projections or shadows of the absorption of an object can be obtained using many forms of energy such as light, x-radiation, electrons, or sound. The problem was treated in abstract mathematics as early as 1917 by Radon (2). The first practical solution of this problem was obtained by Bracewell in 1954 (3) who applied the technique to radioastronomy. The first application of these kinds of techniques to biology were probably done by DeRosier and Klug who obtained the cross-sectional structure of the tail of a bacteria phage from one-dimensional projections obtained with electrons in an electron microscope (4).

Measurement of spatial distribution of refractive index in tissues by ultrasonic computer assisted tomography

Abstract Computerized tomography is used to calculate two independent images representing distributions of refractive index ( n ) and acoustic attenuation (α) within 1–3 mm thick cross-sections through excised organs, including canine hearts and human breasts. Values of n and α are calculated from profiles of propagation times and amplitudes, respectively, of digitized acoustic pulses obtained by rectilinear transmission scans of the tissue at multiple angles of view. Images of the local speed of ultrasound show high values in regions of muscle, breast parenchyma, medullary carcinoma and connective tissue and show low values in regions of fat. Images of acoustic attenuation show high values in regions of connective tissue and borders of scirrhus carcinoma and low values in regions of fat.

Three-dimensional visualization of the intact thorax and contents: A technique for cross-sectional reconstruction from multiplanar X-ray views☆

Abstract A method is described for obtaining the three-dimensional spatial distribution of roentgen opacity within the intact canine thorax. Video recordings of multiple roentgenographic projections are digitized during rotation of the dog in the X-ray field to provide the data required for the reconstruction of a sequence of parallel spatially adjacent cross-sectional images of the thorax or heart over their entire anatomic extent at 1 60 second intervals in time. The accuracy of the technique is demonstrated with test objects and excised canine hearts, and the capability for high temporal resolution is illustrated by the cross-sectional reconstruction of a metabolically supported, isolated working canine left ventricle at different phases of the cardiac cycle. These data, along with simultaneous measurement of intracardiac and transmural pressures, provide the determinants required for estimations of dynamic myocardial length/tension relationships that are necessary for the quantitative assessment of cardiac contractility and reserve.

An Operator-Interactive, Computer-Controlled System For High Fidelity Digitization And Analysis Of Biomedical Images

High-fidelity digitization, communication, and display equipment and the development of computer programs to manipulate and analyze the high volumes of biomedical image data generated by video roentgenographic and closed-circuit television systems have become vitally necessary in order to fully exploit the potentialities of multiplanar roentgenographic and conventional video techniques. Although a great deal of information has been gained in the past by both simple and sophisticated analyses of these data(Ref. 1,2,3,4,5),additional objective and time economical measurements, determinations and estimates are required to extend understanding of the physiological and biological phenomena studied and to provide the insight necessary to properly resolve some of the problems concerned.

Digital Computer Simulation Study of a Real-Time Collection, Post-Processing Synthetic Focusing Ultrasound Cardiac Camera

High resolution ultrasound images (90% of Rayleigh limit at all depths) were obtained by computer analysis of digitized (at 10 and 20 points per microsecond, 8 bits per point) signals detected with a 32 element lead zirconium titanate 3.0 MHz array. Every sixth element was used as a transmitter and pulsed 31 times while the signals from the remaining 31 elements were addressed sequentially by an analog switch and their signals digitized and recorded. Each pixel in the cross-sectional image was produced by calculating the inner product of a specific window function and the ensemble of digitized signals. Thus the array is mathematically focused optimally for each pixel in the image. The acquisition and storage of all received signals allows subsequent approximate calculation of spatial distributions of such parameters as reflection, index of refraction, attenuation, etc. The extension of these techniques through the use of a fast analog and digital computer interface to obtain realtime and stop-action imaging is treated. Examples of images produced by these algorithms, the time requirements of various algorithms as determined by the computation speeds of present and anticipated digital and analog processing hardware is presented. Supported in part by NIH research grants HT-4-2904, RR-7, and HL-04664. Also supported in part by a contract from the Office of Naval Research to E. M. Eyring, with whom S. A. Johnson was a part-time research associate.

Reconstructing Three-Dimensional Fluid Velocity Vector Fields from Acoustic Transmission Measurements

A theory with supporting experimental evidence is presented for reconstructing the three-dimensional fluid velocity vector field in a moving medium from a set of measurements of the acoustic propagation time between a multiplicity of transmitter and receiver locations on a stationary boundary surface. The inversion of the integrals’relating the acoustic propagation path to the propagation time measurements is affected by linearization and discrete approximation of the integrals and application of an algebraic reconstruction technique (ART). The problem of the presence of certain invisible fluid flow functions is treated. Since this technique does not require the presence of scattering centers or the optical transparency of the medium, it may be applied in many cases (i.e., turbid, opaque, or chemically pure media) where Doppler or optical (e.g., laser holography) methods fail.

Breast Imaging by Ultrasonic Computer-Assisted Tomography

The purpose of our applying acoustic tomographic methods in the breast is directed toward the early detection of carcinoma of the breast. Our rationale is based on the principle that early detection of cancer is not only dependent upon the spatial resolution of the physical method of detection but also depends upon sensitivity for detecting changes in basic properties of tissue which can be logically related to the normal and abnormal histologic elements and spatial organization of the organ under investigation. For the breast, detection of such changes demands methods which are capable of delineating the basic elements of the normal breast such as fat, inter- and intralobar connective tissue, ducts and acinar tissue as well as the localization of common lesions of the breast such as fibrocystic disease, cysts, fibroadenomas, medullary carcinoma, and scirrhous carcinoma. Development of such capabilities of histologic and spatial tissue differentiation should allow one not only to find an established advanced lesion, but also to detect early changes in the morphology of the normal breast by detecting focal changes in the acoustic properties of normal tissue constituents which may imply the presence or the potential of development of a neoplasm.

The Television Camera in Dynamic Videoangiography

Abstract Three characteristics of conventional television cameras—image integration and storage, interlaced scanning, and image retention—make these cameras unsuitable for dynamic angiography. Use of noninterlaced scanning and synchronized 60/sec. pulsed operation of the x-ray source avoids degradation of temporal resolution caused by image integration and provides discrete video images at 60/sec. if cameras with minimal image retention are used. Image orthicon, lead monoxide vidicon, and silicon diode integrated target epicon camera tubes have less than 10% image retention after single scans and are suitable for angiography. Stop-action films of television displays confirm the poor temporal resolution of conventional cameras.

Refractive Index by Reconstruction: Use to Improve Compound B-Scan Resolution

Reconstructions of two-dimensional distributions of acoustic speed were utilized to correct digitized compound B-scan images for aberrations caused by inhomogeneous refractive index. Profiles of propagation’delays of acoustic pulses obtained during compound transmission scanning at 60 angles of view separated by 6° were used to reconstruct the distribution of acoustic speeds within a 64 × 64 element grid in the scan plane within which 8 to 16 digitized B-scans were obtained for views separated by 22.5° or less. Each B-scan contained 1000 to 1300 pulses, digitized at 10 or 20 megasamples/s. Values of calculated acoustic speed within those elements of the reconstruction grid which were intersected by the locus of each pulse trajectory were utilized to map the temporal sequence of echoes within each echo signal into the correct spatial sequence of echoes within the B-scan image. Straight line approximations to the loci of the acoustic beams were used. The set of corrected B-scans were summed to obtain high resolution compound B-scans of 128 × 128 or 256 × 256 picture elements. This method seems particularly suited to breast imaging although enhance. ment of abdominal scans may be possible as well.