Computed Tomography

Computed Tomography (CT) is a powerful nondestructive evaluation (NDE) technique for producing 2-D and 3-D cross-sectional images of an object from flat X-ray images. Characteristics of the internal structure of an object such as dimensions, shape, internal defects, and density are readily available from CT images. Shown below is a schematic of a CT system.

In computed tomography, a test component is rotated on a platform while being exposed to an x-ray beam. The results go to a computer that manipulates the images to create a 2-dimensional radiographic image of the test component.

The test component is placed on a turntable stage that is between a radiation source and an imaging system. The turntable and the imaging system are connected to a computer so that x-ray images collected can be correlated to the position of the test component. The imaging system produces a 2-dimensional shadowgraph image of the specimen just like a film radiograph. Specialized computer software makes it possible to produce cross-sectional images of the test component as if it was being sliced.

How a CT System Works

The imaging system provides a shadowgraph of an object, with the 3-D structure compressed onto a 2-D plane. The density data along one horizontal line of the image is uncompressed and stretched out over an area. This information by itself is not very useful, but when the test component is rotated and similar data for the same linear slice is collected and overlaid, an image of the cross-sectional density of the component begins to develop. To help comprehend how this works, look at the animation below.

Pull the rings to reveal the density data.

In the animation, a single line of density data was collected when a component was at the starting position and then when it was rotated 90 degrees. Use the pull-ring to stretch out the density data in the vertical direction. It can be seen that the lighter area is stretched across the whole region. This lighter area would indicate an area of less density in the component because imaging systems typically glow brighter when they are struck with an increased amount of radiation. When the information from the second line of data is stretched across and averaged with the first set of stretched data, it becomes apparent that there is a less dense area in the upper right quadrant of the component's cross-section. Data collected at more angles of rotation and merged together will further define this feature. In the movie below, a CT image of a casting is produced. It can be seen that the cross-section of the casting becomes more defined as the casting is rotated, X-rayed and the stretched density information is added to the image.

As the system collects x-ray images of the test component at different angles, the 2-dimensional shadowgraph is reconstructed.

In the image below left is a set of cast aluminum tensile specimens. A radiographic image of several of these specimens is shown below right.

Aluminum tensile specimens a few inches long were scanned using CT. The CT scan pores in the specimens that were approximately 1 mm in diameter.

CT slices through several locations of a specimen are shown in the set of images below.

Cross-sections of the CT images show the pores in the aluminum parts clearly.

A number of slices through the object can be reconstructed to provide a 3-D view of internal and external structural details. As shown below, the 3-D image can then be manipulated and sliced in various ways to provide thorough understanding of the structure.

The 2-dimensional images can be manipulated to show a 3-dimensional shadow image of the part.