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Computed Tomography (CT) |
Computed Tomography (CT) uses X-rays to capture thinly sliced images of the human body. The technology is basically the same as regular X-ray technology, using the attenuation of the X-rays as they pass through the body to generate images.
CT X-rays differ from regular X-rays by computing the value of the attenuation at each point as they pass through the body to construct image layers, or slices. |
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What is a CT value? |
A CT value is obtained by using the difference in X-ray absorption rates due to differences in density of unirradiated matter to calculate the coefficient required to recreate an image.
The CT value is described by the equation μ(object) – μ(water)/μ(water) x 1000, where μrefers to the X-ray attenuation coefficient.
Many human organs possess μat near-water levels, so the CT values for water and air are agreed to be 0 and -1000 respectively.
The displayable range of CT values are between -1000 and +2000, and the CT values at the target area are called the ‘window level’, while the range from the highest CT values at the target level to the lowest value is called the ‘window width’. When transferring the CT image to film there is a large amount of data involved, so to avoid missing any target data it is necessary to be able to delineate for easy identification of the target area, requiring alignment between the Window Level and Window Width with the target area when transferring to film. |
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Differences between medical and dental CT technology |
Item |
Medical CT |
Dental CT |
Type |
Helical |
Cone-beam |
Sensor |
Linear |
Surface |
X-ray impact type |
X-rays irradiate linear area |
X-ray irradiates conal area |
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Benefits of Finecube cone-beam technology |
・Amount of radiation
Image can be captured in a single rotation, so patient is exposed to less radiation than in helical type scanners.
Image capture image is shortened, also making the process easier for the patient.
・Image quality
Helical type scanners capture the image in spiral form, so the precision of Z-axis resolution is poor. Cone beam scanners capture precise data for both X and Y axes, enabling high-definition imaging.
・Metallic Artifacts
Image distortion caused by metal (metallic artifacts) is reduced in cone beam scanners. |
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VR (Volume Rendering) |
Volume rendering is a method of creating visual representations of 3D data that has been attracting a lot of attention in recent years. Taking large amounts of complex 3D data and rendering it in visual form, the method allows the creation of easy-to-understand 3D images. |
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Pixels and Voxels |
A CT image is created from the distributed data of X-ray absorption in the body. The smallest unit of the displayed image is called a pixel, and by assembling pixels with CT values a CT image is formed. However, CT slices have a certain thickness, and when this is factored in the unit is referred to as a Voxel. |
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Partial Volume Effect |
A CT image is an aggregate of its CT values, and the CT value of a voxel is a function of the attenuation coefficient of the various structures enclosed within it. When there is only one structure, no problem occurs, however when there are multiple structure contained in the voxel the CT displayed by the pixel is unable to display the density of certain structures. This phenomenon is called the Partial Volume Effect. |
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