What Is a Helical Scan?

Helical CT is a type of 3-D CT for non-destructive testing in which the X-ray source describes a helical (or spiral) trajectory relative to the object while a two-dimensional array of detectors measures the transmitted radiation on part of a cone of rays emanating from the source. Both rotation and vertical movements are executed simultaneously; together, they make a helical effect. (See Figure 1.) This technique has a variety of advantages.

Figure 1.

For example, it avoids the creation of ring artifacts (a CT phenomenon resulting from miscalibration or failure of one or more detector elements in a CT scanner) by providing the same resolution across the object being scanned, and eliminates some of the blur that might occur with a normal cone beam CT at large beam angles, also known as Feldkamp artifacts. (See Figure 2.) It is especially suitable for tall parts because it eliminates the need to “stitch” images together.

Figure 2.

Advancements in Helical Scanning

In most helical CT implementations, it is very difficult to measure the top view and the bottom view of the article under inspection. YXLON's HeliExtend, our brand name for helical CT, not only avoids the creation of so-called Feldkamp artifacts, but it simplifies the capture of images using helical scans.

As realized in the FF20 CT and FF35 CT systems, helical scans are automated using a  built-in wizard that allows the user to define the bottom area of the part and the top area of the part, which allows the system to gather more information during rotation as the beam moves. As anyone who's ever used a CT system knows, one can spend a lot of time trying to find the right position in the CT scanner. With HeliExtend, the user simply defines one area, rotates 90 degrees, and defines the other area. The system knows the center of rotation. Another feature of these automatic calculations is that the system can be programmed to focus on various regions of interest (ROI). The user can scan them separately without repositioning the part.

Benefits

One would be correct in assuming that a helical scan can take more time (as much as approximately 20% more) to acquire than the same part run in a non-helical CT X-ray configuration. However, there are two major benefits of using this method, especially for vertically elongated parts:

• 1. No need to stich images
• 2. Greater magnifcation with greater detail

With this method, the system can scan the complete volume of the part, with the highest magnification possible. With the added element of rotation, long, complex parts can be accurately examined. Also, while the overall acquisition time for helical scans may be slightly longer than cone beam CT, time is saved in post-image processing because there is no need to use software to stich together a series of images. With helical, the data set is complete.

Learn More

For more information about the various features of CT X-ray systems, watch the 31-minute video webinar to learn about:

• Applications and markets for FF20 CT and FF35 CT
• Mechanical design specifications needed for high resolution image capture and metrology requirements
• Super-accurate Nano- and μ-Focus Tubes
• Software interfaces
• Software that can create inspection sequences and prevent collisions inside the X-ray cabinet
• Helical Scans and Virtual Rotation Axis Scans
• System monitoring and system integration with leading Metrology software

Watch the video to learn more.

If you have any questions about how non-destructive computed tomography could be applied in your manufacturing or research operation, please contact Dirk Steiner at dirk.steiner@yxlon.com or 770-289-7708.