CT scan Overview
A Computed Tomography (CT) scan has proved to be a revolutionizing technology, for medical and industrial applications. Allowing users access to reconstruct and inspect an object internally and externally in 3D, without destroying the object being scanned, provides limitless opportunities. Not only has it allowed users to diagnose patients and treat them accordingly to save lives, it has allowed industry professionals to be pro-active in identifying and resolving internal failures for industrial parts. Industry leaders often turn to outsourced labs with qualified Nondestructive Testing (NDT) personnel to qualify and validate part components, easily and accurately.
What is a CT scan?
A Computed Tomography (CT) scan, also known as CAT scan or X-ray CT, is the process of collecting 2D cross sectional slices, also known as tomographic images, to form a 3D model for internal and external inspection, with the assistance of a computer program or software. Commonly used for medical and industrial applications, computed tomography is a nondestructive means of accumulating internal data for inspection purposes.
Computed Tomography is based on radiographic technology, which uses a digital detector panel to capture 2D x-ray images on an axis of rotation. For medical applications, the x-ray source rotates, whereas for industrial applications, the subject being scanned is placed on a rotary table.
How to conduct CT scans
In order to conduct a Computed Tomography scan, a radiographer must be present to prepare the subject for scanning and calibrate the system in accordance to the purpose of the scan. Although there are many steps that are subject to the part being scanned, the following are common steps a radiographer takes to conduct a CT x-ray or CAT scan:
Match the part size and material to the appropriate x-ray source with high or low energy exposure depending on the purpose of the scan.
Place the part on the rotary table – located between the digital detector panel and x-ray radiation source. Ensure that the system has been closed shut before beginning CT scanning process.
Begin the radiation exposure process. As the part rotates 360 degrees, the x-ray source penetrates through the part. The varying density of the part absorbs varying amounts of radiation. The remaining radiation travels to the detector panel, which captures a 2D x-ray image. This process is repeated to capture hundreds to thousands of 2D x-ray images.
Several hundred to thousand 2D x-ray images which are captured are reconstructed mathematically to develop a 3D rendering or a 3D model of the part being scanned. This model is used for further internal and external analysis of the part, whether it is to identify internal failures or to conduct a dimensional analysis.
Analyzing CT scan results
The resulting scan is a set of voxel based data portraying the varying density of the part, in varying grayscale value. Density of a part absorbs the radiation which is exposed to the part being scanned. Since the cracks or voids do not absorb any radiation, they will appear with a higher density on the resulting image – darker on the greyscale value. The areas with high density material tend to absorb more radiation, thus, appearing lighter on the greyscale value. The images are analyzed similar to any x-ray or radiographic testing technique. For X-ray CT scanning, a 3D model is reconstructed, which can be further analyzed for internal failures, measurements and wall thickness, to name a few. The results can be color coordinated according to project requirements to ease the visual of analysis.
Applications of CT scan or CAT scans
X-ray CT scans were implemented in the medical field for healthcare applications, in the 1970’s. Today, the technology has evolved with the technological advancements, allowing users to access CT scans for industrial purposes. The following are common ways CT x-ray can be used for industrial applications:
- Failure analysis
- Pre-production evaluation, verification and qualification of industrial parts
- Production analysis for repeatability and consistency
- Reverse engineering applications
- Part to CAD comparison/ Part to Part comparison
- Wall thickness analysis
- PPAP – Automotive applications
- AS9102 – Aerospace applications
- GD&T part programming
- Defect location and analysis
- Fiber orientation
Industrial image quality of CT scan
The quality and accuracy of a CT scan is dependent on many different features of the scan. These different aspects play a significant role in determining the quality of the scan:
Size of the part – The size of the part being scanned must fit the detector panel size range. If the part is larger than the size of the detector panel, the resulting scan will not reflect the part in its entirety. It is always beneficial to pair a part with a CT system that consists of a detector panel slightly larger than the size of the part. If the part is too large for the scan, the part can be scanned in sections and analyzed independently.
Material of the part – Density of a part plays a significant role in determining the accuracy of the resulting scan. If a part consists of high dense material, more x-ray energy is required to penetrate through the material. If the part consists of low dense material, it requires less energy to penetrate through, for an accurate result. If too much energy penetrates through a material, the results can display scattered data. If not enough energy penetrates through any given material, the resulting scan may not provide enough data or accuracy.
Resolution desired – Depending on the resolution desired by the user to evaluate internal features of the scan, a pre-determined amount of 2D x-ray slices are captured. The more slices that are captured, the higher the accuracy of the reconstructed 3D model. If extremely high resolution is desired, but the density of the part is too high, it becomes difficult to retrieve high accuracy.
Benefits of CT scan
There are numerous benefits of utilizing Industrial CT for internal and external part inspection. Most common benefits include the following:
- Nondestructive means of inspection
- Cut costs and reduce time to production
- Accurate and precise means of measurement
- Access multiple different types of analysis with one CT dataset
- Non-invasive and non-intrusive method of inspection
- Research and development tool
- 3D visual and representation of part, internally