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The ACTA (Automatic Computerized Transverse Axial) tomographic scanner is designed for the radiological delineation of tissue abnormalities through the whole body, accurately portraying in pictorial and quantitative form both the nature of the lesion and its precise three-dimensional location in the body. The nature of the lesion is identified by means of the relative absorption coefficient (for X-rays), and the location of the lesion is given precisely by its position in the picture and location of its cross section. The concept of computerized transverse axial tomography involves the production of X-ray absorption profiles made at different angles in the same cross-sectional plane. A highly columnated X-ray beam scans the cross section in two composite modes of motion - a translation, or scan-pass, and a rotation. The pencil-thin beam of X-rays passes through the body and is detected by a sodium iodide crystal, whose scintillation is measured by a photomultiplier tube. The X-ray profiles are sampled, digitized, and fed into a computer, which synthesizes them into a picture giving the relative absorption coefficients of the body cross section in the plane of the scan. The cross section picture can have as many as 160 X 160 elements, each representing 1.5 mm x 1.5 mm of cross-sectional area. Two adjacent cross sections can be scanned each time (about 4 1/2 min.), each of which has a thickness of 7.5 mm and with a 3 mm distance between them. The picture is reconstructed immediately upon completion of the scan and is instantaneously displayed on a color TV console and two black-and-white consoles. The color TV display shows at a glance the areas of different ranges of absorption coefficients as areas of different colors which, in turn, will facilitate the clinical diagnosis. The picture can also be stored for permanent record.
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