AbstractThis paper describes the method of narrow aperture phased array ultrasonic computed tomography (NAPA‐CT), conceived for three‐dimensional imaging with increased resolution and speed in the field of clinical diagnosis. By reducing the size of the transducer in the thickness direction, a beam is formed such that the focal region of the phased array is a circular arc. This arc focus is scanned in a sector and is further rotated about the sound axis to acquire an echo. Then, the scatterer distribution on a hemisphere containing the arc as its part can be reconstructed by the inverse Radon transform. The feasibility of the proposed method is investigated by simulation and experiment. © 2002 Wiley Periodicals, Inc. Electron Comm Jpn Pt 3, 86(2): 72–81, 2003; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/ecjc.10029