In a B-scan display, the screen indication length from a discontinuity corresponds to the discontinuity's length in which direction?

• A. The discontinuity's length in the direction of the transducer level
• B. The discontinuity's thickness as measured parallel to the ultrasonic beam
• C. Both A and B
• D. None of the above

Answer: (A)

In a B-scan, the image is created by stacking many A-scans as you move the transducer along the surface. The horizontal axis represents the transducer’s position along that scan path, while the vertical axis represents depth. Because of this, the length you see of a discontinuity on the screen reflects how long that flaw is along the direction you’re scanning—the dimension parallel to the surface and to the transducer’s travel. The thickness of the discontinuity, which is a dimension along the ultrasonic beam, does not set the screen’s length; that dimension relates to depth information in the image, not the horizontal extent. So the on-screen length corresponds to the discontinuity’s length in the direction of the transducer level.