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Undersurface photoacoustic imaging of plane solid specimens by the use of a line laser beam

A line laser beam focused on a solid plane specimen was used to image undersurface defect by photoacoustic microscope (PAM). A CT (computed tomography) technique was applied to the imaging of a line-shape undersurface defect fabricated at the welded region of two steel plates by drilling. The diameter of welded region is about 8mm and the steel thickness is 1.5mm each. A second harmonic green laser beam of a LD-pumped YAG laser was expanded and focused on a specimen by concave and cylindrical lenses, respectively. The laser beam scanning to the specimen was achieved by rotating with a mechanical rotating stage controlled by a computer. Rotation step and the amount of rotation were 1 and 180 degrees, respectively. Laser power and modulation frequency were 28 mW and 8 Hz, respectively. The size of a lser beam on a specimen was 8 mm (length) times 1 mm (width). The photoacoustic (PA) signal shows a maximum at the defect with the ratio of 1.43 compared to the background. The measurement time used for a single scan was 7 minutes. That was about 4 times shorter than a conventional PA imaging by 2D scanning, which takes 30 minutes. This measurement is the first one in the photoacoustic tomographic trial with a line laser beam. I. INTRODUCTION In the field of photoacoustic(PA) imaging, CT(computed tomography) technique has not been applied long time since the invention of CT in 1973. Very recently three-dimensional (3D) CT technique was developed from the view point of the application to the medical imaging (1-3). The first author has found the equivalence between X-ray CT and the two-dimensional (2D) CT PA imaging of specimens with planar structure using line-focus laser beam (LFLB) (4). In this paper, the mathematical equivalence between X-ray CT and the 2D CT PA imaging with LFLB is described. The experimental apparatus and preparation of specimens are also described. At first, preliminary experiment to inspect welded steel plates by rotating a line-focus laser beam around the center of the welded point. The PA signal dependence on rotation angle is measured for inspection (5). The CT forward-projection scheme is developed for the second stage. The results are also described. II. PRINCIPLE OF PHOTOACOUSTIC SIGNAL GENERATED BY A LINE-FOCUS LASER BEEAM