Computed Tomography (CT) overview
Computed Tomography was developed for medical applications in the early 1900’s, and was effectively in use by the 1970’s. With the advancements in x-ray sources, computer technology and software, industrial applications for computed tomography have become more prevalent in recent years. Having the ability to digitally capture and reconstruct any object for internal and external evaluation in 3D, computed tomography has provided users with limitless opportunities. Nondestructive testing labs are available to provide easy access and a quick and accurate source for part analysis and evaluation.
What is Computed Tomography (CT)?
Computed Tomography, also called X-ray computed Tomography (CT), 3D computed Tomography or Computerized Axial Tomography (CAT) scan, is based on a radiographic testing technique utilizing x-ray technology to view and analyze the internal structure of an object.
Computed Tomography is a nondestructive testing (NDT) method which captures several 2D x-ray images, while a test subject rotates, producing cross sectional slices.
These cross sectional (tomographic) slices are reconstructed into a 3D model with the assistance of specialized software. The viewer has the option of evaluating images in 2D x-ray cross sectional slices to analyze a certain area of the object or evaluate the object in complete 3D form. Computed Tomography is commonly used for medical applications to diagnose and analyze internal bodily functions and industrial applications to test and inspect parts for quality control purposes.
How X-ray Computed Tomography works
Using an x-ray source, geometry is captured and digitally processed to develop a three dimensional rendering for the internal and external features of the scanned object. There are three main components necessary for a computed tomography process to take place:
X-ray source – The x-ray source is the heart of the computed tomography (CT) process.. An x-ray source is placed on the opposite side of the detector panel. The source exposes radiation on to an object to be scanned and analyzed. X-ray energy sources are measured in electronvolts (KeV), which range from low energy to very high energy sources, applying to metal and nonmetallic based components.
Detector panel – Detector panel is what captures two dimensional radiographic images, while the part is on an axis of rotation. These captured images are used to produce cross sectional slices also known as tomographic images. The data and cross sectional slices are transferred on to a software for the final step. As oppose to conventional radiographic testing techniques utilizing film radiography, CT uses digital detector panels to capture data.
Software for means of reconstruction – The cross sectional tomographic images are processed sliced by slice to re-develop and re-construct data into a 3D model. This 3D model is used to analyze part internally and externally. Collectively, this process is completely nondestructive and allows users to utilize the part after the testing process is complete.
What determines X-ray Computed Tomography image quality?
There are many factors in the computed tomography process that determine the quality and sharpness of the resulting image:
Size of the part being tested – a detector panel is used to capture all the 2D x-ray images, while the part is on an axis of rotation. If the object being scanned does not fit the detector panel, the resulting image will not reflect the object in its entirety. It is extremely important for radiographers to ensure the object being inspected is within the detector panel size range. If the part is too large, the resulting image will not be representative of the entire part. The radiographer may need to utilize a system with a larger detector panel or scan the object as many times as required to capture the entire object and analyze results individually. However, if only a certain area of the testing subject needs to be inspected, it may not be necessary for the subject to fit the detector panel size range, given the area of interest fits within detector panel size range.
Orientation of the x-ray source subject to the part – The distance of the x-ray source and the part plays a significant role in determining the accuracy and the quality of the image. The part, depending on the size and the purpose of the scan, can be placed closer to the x-ray source to detect a certain area or can be closer to the detector panel to capture part in its entirety with enough exposure of radiation.
Material of the part being scanned – Density of a part determines the quality of the resulting scan. The higher the density, the more exposure of radiation required to evaluate internal features. With lower density objects, less exposure of radiation is required. If too much exposure is passed through a low density material, the resulting scan will display scattered data. If not enough exposure is passed through a high density material, viewers are not able to retrieve accurate data.
Amount of 2D cross sectional slices developed – Depending on the resolution required for the purpose of a scan, the amount of 2D cross sectional slices can be determined. If not enough cross sectional slices are captured, the resulting image will not display a highly accurate result. If more 2D cross sectional slices are captured, higher accuracy of the resulting scan is possible, for the reconstructed 3D model.
When is Computed Tomography necessary?
Computed tomography is necessary when a user is looking to evaluate, analyze or test the internal and/or external features of a component without destroying the object. Industrial Computed Tomography is generally necessary during five different manufacturing stages:
- Pre-production – design and analysis
- Production – evaluate consistency
- Failure investigation – locate defects
- Lot inspection – repeatability test
- Reverse engineering – design adjustment or research & development
Benefits of Industrial Computed Tomography
Computed Tomography is very useful for medical and industrial purposes. A few common benefits regarding industrial applications include the following:
- Nondestructive testing method
- Quick and accurate resulting data
- Accessible with the use of outsourced NDT labs
- Saves manufacturing cost (NDT)
- Reverse engineering tool
- Quality management resource
- Research and development
- Comparison tool to determine consistency and repeat-ability
Uses of Computed Tomography
Computed Tomography, as mentioned above, has many different types of uses and applications. For industrial applications, computed tomography can commonly be used for the following types of analysis:
- Locate, measure and retrieve the orientation of internal porosity
- Identify, locate and measure cracks and internal defects such as inclusion
- Measurement plan tool
- First Article Inspection
- PPAP tool – Automotive standard requirements
- AS9012 – Aerospace standard requirements
- GD&T Part Programming
- Part to CAD comparison
- Part to Part comparison
- Development of a CAD file – Reverse Engineering
- Wall thickness analysis
- Failure analysis
- Forensic analysis
- Fiber analysis