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Nate Serafino

Metrology

Line-Detector Fan Beam CT vs. Flat-Panel-Detector Cone Beam CT

How to choose the right CT technology for your inspection goals

Published: Wednesday, September 1, 2021 - 12:03

Industrial X-ray and computed tomography (CT) for nondestructive testing are rapidly expanding, with new applications and inspection systems emerging all the time. With so many choices available, it is critical to match the right technology with your individual inspection goals. Understanding the tradeoffs between X-ray/CT system types and what works best for your specific applications will help save time and money for years to come.

One of the most important concepts to learn prior to selecting an industrial CT system is the difference between flat-panel detector cone beam CT systems (CBCT) and line-detector fan beam CT systems (FBCT). Both modalities are powerful solutions for CT inspections; however, significant performance and time trade-offs exist between them. In fact, many CT system manufacturers offer CT systems equipped with a flat-panel X-ray detector and a linear detector array. Both technologies enable powerful 3D visualization capabilities of internal features and defects, but each excels in different applications and use cases. The different use cases in conjunction with price and time differences between FBCT and CBCT make this concept critical for any prospective buyer to understand.

This article will outline each modality, where they excel, and which technology will enable you to meet your inspection goals in the most time- and cost-efficient manner.

Fan beam CT vs. cone beam CT

Fan beam CT with linear detector array

These systems are specifically configured to reduce scatter radiation by collimating the X-ray source to a fan beam and limiting the radiation acceptance angle at the detector with an adjustable slit entrance window. The detector is a single row of pixels called a linear detector array (LDA). (See figure 1.)

Data are collected with the LDA such that each rotation of the sample is reconstructed into a single CT slice that is one pixel tall in the z-dimension of the sample. To collect CT slices for the entire vertical length of the sample, the system must index the sample upward or downward for each CT slice. For this reason, FBCT scans can take from hours to days to collect data for the whole sample.

The main advantage to using FBCT-configured systems is the significant reduction in scatter radiation from collimation at the X-ray source and detector apertures. FBCT provides superior contrast resolution and fewer artifacts than data acquired with flat-panel detector CBCT systems. In CBCT, accurate surface definition, measurements, and quantitative analyses can be confounded by artifacts introduced by scatter radiation of dense and large parts.

One application where it is important to consider the difference between CBCT and FBCT is scanning and analysis of multimaterial parts. For this application, FBCT provides superior material separation and characterization capability compared to CBCT. For example, in cases of high-density components directly adjacent to low-density components (i.e., plastics next to metals), FBCT significantly reduces metal streak artifacts, enabling proper surface definitions of both high-density and low-density areas.

Linear detector array with narrow slit width for scatter reduction
Figure 1: Linear detector array with narrow slit width for scatter reduction. X-ray source collimated to fan beam. Illustration of fan beam CT data acquisition, courtesy of YXLON

Flat-panel detector cone beam CT (CBCT)

These systems use a wide-angle X-ray cone that illuminates a 2D pixel array, flat-panel detector. (See figure 2.) In CBCT, the entire CT dataset is generated with one full rotation of the sample, making this method substantially faster than FBCT. This feature of CBCT makes it the superior technology over FBCT for quick, qualitative visualization and localization of defects and features. It should also be noted that in scans with a high signal-to-noise ratio, low scatter, and beam hardening artifacts present, sophisticated quantitative analyses can still be performed on CBCT datasets. This is especially true with smaller or low-density samples. Some examples of samples where CBCT can provide low-noise datasets include small-to-medium aluminum castings, plastic injection molded parts, low-density composites, and low-density additive manufacturing parts (e.g., AL, TI, ABS).

Advancement in reconstruction software corrections and detector technologies have enabled large improvements in CBCT that are drastically increasing their applicability in difficult applications. In any CBCT scan, X-ray parameter optimization and reconstruction software corrections are very important for CBCT to offer the best image quality and the best chance for quantitative CT data analysis. Still, there exist several cases where FBCT is the only option for full quantitative analysis. As X-ray source and detector hardware and reconstruction algorithms become more advanced, this may change in the future.

 

Flat-panel detector with 2D pixel array capturing entire sample in field of view
Figure 2: Flat-panel detector with 2D pixel array capturing entire sample in field of view. X-ray source providing cone of radiation. Illustration of cone beam CT data acquisition courtesy of YXLON.

Time matters

FBCT collects and reconstructs only one line at a time. This means the volumetric dataset is built up slowly as each line in the z-axis is collected and reconstructed. Scan times for these scans can range from several hours to several days.

In CBCT, anything that remains in the detector field of view throughout the rotational acquisition is reconstructed into a single volumetric dataset. Scan times for these scans can range from less than five minutes to several hours.

With these principles in mind, it is important to assess throughput and volume of scanning when making the decision. If only scanning a few samples per day, FBCT may provide the throughput needed with superior image quality to CBCT. However, if high throughput is an important requirement, CBCT may be the only choice.

Volume rendering of a 20-in, wheel scanned using an YXLON UX20 CBCT system equipped with a large flat-panel detector Low-noise volume rendering of a 20-in. wheel scanned using an YXLON CT Compact System equipped with an LDA.

Figure 3: (Left)—volume rendering of a 20-in, wheel scanned using an YXLON UX20 CBCT system equipped with a large flat-panel detector. Vertical and horizontal automatic scan extensions enabled the entire sample to be quickly scanned and visualized. (Right)—low-noise volume rendering of a 20-in. wheel scanned using an YXLON CT Compact System equipped with an LDA.

When to use flat-panel detector cone beam CT

CBCT is the best choice for low-density samples that do not generate significant scatter radiation. For these samples, full quantitative analysis and measurements are usually possible with results that are comparable to data from an LDA-equipped FBCT system. For very large or dense parts, if visualization, localization, and basic measurements are all that are required for the inspection, then CBCT is also a good option. For example, in figure 4 below, the porosity in the spoke regions of the 20-in. wheel scanned on an YXLON UX20 CBCT system is clearly visible, and individual pores can be measured using simple distance measurements.

Overall, when CBCT provides adequate image quality and capability for the inspection, it is the best choice because it saves significant time and money vs. an LDA fan-beam CT system.


Figure 4:
Cone beam CT slice of a 20-in. wheel showing porosity localized in the spokes. Sample was scanned using an YXLON UX20 CBCT system with vertical and horizontal automatic scan-field extensions enabled.

When to use fan beam CT with LDA

Fan beam CT with LDA features a large inspection envelope and significant artifact reduction from X-ray collimation at the source and detector. These features make it the best choice for in-depth quantitative analysis or measurements on large or high-density parts and multimaterial assemblies. It enables accurate analysis of otherwise challenging parts. Many applications in the aerospace and automotive industries are a perfect match for FBCT, including nickel super-alloy turbine blades, stainless steel parts, engine blocks with iron sleeves, cylinder heads, and wheels.

Fan Beam CT slice of 20-in. wheel with volumetric porosity analysis enabled
Figure 5: Fan Beam CT slice of 20-in. wheel with volumetric porosity analysis enabled. Data were collected on an YXLON CT compact LDA-equipped FBCT system.

Making the grade

The success of your inspection is based on how well you match the right CT technology with the needs of your application. In summary, if it is possible to generate artifact-free data for the entire range of samples in your production with a flat-panel CBCT system, this is the correct system to choose. However, if you require quantitative analysis, and artifacts are a hindering factor with a flat-panel system, FBCT with an LDA is your only option.

Overall, understanding the performance, time, and cost tradeoffs between these system types will help you gain a better perspective on how to choose the right technology to meet your specific goals.

To help guide you through the decision process for your next project, we’ve developed a CT decision matrix. With this tool, you can easily identify the technology required to meet the goals of your inspection:

CT system decision matrix
Figure 7: CT system decision matrix

Discuss

About The Author

Nate Serafino’s picture

Nate Serafino

Nate Serafino is an X-Ray/CT Applications Engineer for YXLON international based in Hudson, Ohio. Nate has over eight years of experience in industrial and medical CT system usage, design, and theory of operation with focus on image quality optimization/processing techniques for NDT applications in the Electronics, Aerospace and Automotive industries. He also has a Masters of Engineering degree from Case Western Reserve University located in Cleveland, Ohio.

Comments

Flat panel selection

Quite a nice article, clearly explaining what the CT systems can do with FPD or LDA. Advantages and clear merits of LDA and FPD for different applications and inspection time explained lucidly. Well done!!