CT stands for computed tomography; see Computed tomography.
Computed tomography (CT) is a method that uses a computer to generate cross-sectional images from X-ray projection images taken at several different angles. Depending on the scanning method, there are a parallel beam optical system, a fan beam optical system, and a cone beam optical system. A computer reconstructs the projected image to recover a cross-sectional image (CT slice, tomogram). The mathematical basis of CT is the Radon, and inverse Radon transforms.
A CT scan is a method of obtaining a CT image. There are two methods: one is to rotate the object and perform a CT projection, and the other is to turn the radiation source and detector around the object and perform a CT projection. In industrial CT scan systems, the object is often placed on a turntable and rotated. In medical CT scan systems, a radiation source and an x-ray detector rotate around the human body to produce a CT projection. In both cases, a set of CT projection data taken at different angles is reconstructed by a computer and converted into a CT image.
CT projection is the projection of a specimen by radiation or its imaging. Or it may also refer to its 1D or 2D projected image data.
Tomography is a method of imaging various signals. The method of imaging using a computer is called CT. Radiation tomography is well known, but there are also tomographies using sound waves, terahertz waves, and electrical capacitance.
X-ray microtomography is an X-ray CT scanner that provides a 3D model with a micrometer range of pixel (voxel) sizes. It can visualize three-dimensional (3D) structures at high resolution. X-ray microtomography is also called micro-computed tomography, micro-CT, or high-resolution CT. Two types of equipment are available: one is for scanning by rotating the sample, and the other is for scanning by rotating the X-ray tube and detector around the sample.
Applications of X-ray microtomography: Electronic materials, composite materials, pharmaceutical, biology, electronic devices, geology, food and seeds, polymeric materials, fibers, casting, additive manufacturing
Reconstruction and Scanning
Reconstruction is the process of obtaining a tomography (cross-sectional image) from image data obtained by CT projection. Reconstruction methods are broadly classified into back projection and iterative reconstruction. Filter-corrected back projection (FBP) has been the mainstream method, but sequential approximate image reconstruction methods are on the rise due to their effectiveness in reducing artifacts.
Cone Beam CT
Cone-beam CT is a type of CT scanning method in which X-rays irradiated in a conical shape are imaged using a flat panel detector or similar device and processed by a computer. This method completes the scan in a relatively short time. However, cone-beam artifacts may occur.
Fan Beam CT
Fan-beam CT is a type of CT scanning method in which X-rays irradiated in a fan shape are imaged by a line sensor or X-ray flat panel and processed using a computer. It is often compared to cone-beam CT. It provides CT data with fewer artifacts than cone-beam CT, but it takes longer to obtain multi-layered CT data.
A sinogram is an image of CT projection data arranged with the horizontal axis as channels and the vertical axis as rotation angle. The user can obtain information on the center position and tilt of the axis of rotation from the sinogram. From this information, the equipment can be calibrated and inspected.
The Feldkamp method reconstructs 3D CT data from 2D projection data obtained by cone-beam CT. The Feldkamp method is a superimposed integration method classified as a filter-corrected inverse projection method.
The normal scan is the usual method of scanning. In an industrial CT scanner, it is to rotate 360°.
The half scan is a scanning method used in industrial CT scanning, in which the specimen is rotated 180° and reconstructed. It takes less time than a full scan.
The helical scan is a spiral rotating scanning method. Medical CT systems mainly use this scanning method.
The offset scan is a scanning method with the center of rotation and the camera center offset. It allows scanning over a wider area than normal scanning.
Full scan is the same as normal scan. It is sometimes used as a comparison to a half-scan.
SID is the distance from the focus of the X-ray source to the detector. It is used to calculate geometric magnification. The following other abbreviations are also used FID：focus-to-image distance (Technical Terms：X-ray)
SOD is the distance from the focus of the X-ray source to the center of rotation of the CT object. It is used when calculating geometric magnification. The following other abbreviations are also used FOD：focus-to-object distance (Technical Terms：X-ray)
Data and Imaging
CT data is three-dimensional image data obtained by reconstructing X-ray images, also called CT images.
A CT image is three-dimensional data obtained by reconstructing an X-ray image, also called CT data.
A tomogram is an image obtained by reconstruction. The CT image is a stacked image data set of these tomographic images.
A CT slice is a cross-section of a CT image. In fan-beam CT, it is one layer of the CT image reconstructed from the projection image.
CT Gray Value
The CT gray value (gray level) is a value that represents the shade of each voxel in the CT image.
A voxel is a volume element in a CT image. A CT gray value represents the data of a voxel.
The view number refers to the number of directions for rotational imaging. In other words, it can be said to be the number of transmissive images taken at different angles. The larger the number of views, the more detailed the CT data, but the longer it takes for imaging and reconstruction. Generally, 300, 360, 600, and 720 views are commonly used.
STL is a file format for data that represents the surface of a 3D shape as a number of triangles. STL is an abbreviation of StereoLithography, also known as mesh data, and is used primarily in optical fabrication and 3D printers.
VG studio is Volume Graphics' software for displaying and analyzing CT images.
Reverse engineering is the process of converting data and numerical values from product disassembly and analysis. For example, engineers use CT scanners to compare CT data of manufactured parts with 3D model data of the design.
Volume rendering is a method of representing grayscale data in three-dimensional computer graphics in three dimensions. CT imaging software uses the CT gray values of the voxels for rendering.
Multi-Planar Reconstruction (MPR)
MPR stands for multiplanar reconstruction and is the most standard display method for CT data. In a typical MPR display of an industrial CT, the screen is divided into four sections to show the X-Y, X-Z, Y-Z, and any cross-section of the CT data.
Artifact (UK: Artefact)
Artifacts are noise that appears in CT images. You should note that artifacts result in CT images that do not correspond to the original physical shape and density.
Depending on the cause, there are artifacts called beam hardening, metal artifacts, ring artifacts, etc.
Metal artifacts are noise when imaging samples that are difficult for X-rays to penetrate, such as metals. When composite materials are CT scanned, metallic artifacts are streak-like artifacts that occur around metallic parts.
Countermeasures include reduction during CT scan imaging and reduction in image processing (reconstruction).
The following methods can be used to reduce the amount of light during shooting. Increase the X-ray tube voltage.
The irradiated X-rays are filtered with a copper plate or similar material to suppress the effects of beam hardening for projection.
Ring artifacts are ring-shaped shading irregularities that appear in CT cross-sectional images. It originates from the detector's uneven sensitivity and the irradiated X-rays' uneven luminance. You can reduce it by image processing or by calibrating the X-ray camera.
Cone Beam Artifact
Cone beam artifacts, also called Feldkamp artifacts, occur in areas of the sample horizontal to the detector. You can avoid it by skewing the object.
Beam hardening is a phenomenon in which when a continuous X-ray spectrum (X-rays with broad energy/spectrum) passes through a material, more low-energy X-rays are absorbed, and the ratio of high-energy X-rays increases. (Beam hardening = hardening of the radiation quality)
Beam hardening is a type of artifact that appears in CT images. Beam hardening causes materials of uniform density to appear as materials of different densities in CT images.