Computed Tomography Overview
Important Terminology
| Term | Definition |
|---|---|
| Tomography | Creating a virtual cross section through a human body or other solid object using any kind of penetrating wave (typically X-rays or ultrasound) |
| CT | Computed Tomography or Computed Axial Tomography (CAT scan): the process of using multiple X-ray images taken across multiple angles to produce tomographic slices. Computations are used to generate the tomographic slices |
| Radon Transformation | Used to compute the linear projections of an image matrix along specified directions. |
| Sinogram | A visual representation of the raw data obtained in a computed axial tomography (CT) scan |
| Filtered Back projection | The process of deblurring and reconstructing a 2D image from the sinogram captured during the CT scanning process |
Illustration of a tomograph. S1 and S2 are the tomographic cross-sections. P is the projection
Important Resources
CT Sinogram
CT captures radon transform data which is often called a sinogram because the Radon transform of an off-center point source is a sinusoid. Each horizontal line of a sinogram is a (1D) projection of that (2D) slice in that direction.
For each angle, a projection (or line integral) is captured. This line integral represents the sum of the density of the object in a given direction. Therefore, brightness corresponds with the sum of the radiographic density of objects being scanned
To visualize the scanned cross-section, the data must be computationally reconstructed into an image from the sonogram. This process is known as a back projection (or inverse radon transformation).
Final 2D recreation of the object from the sinogram
Hounsfield units
The Hounsfield unit is a dimensionless unit used in computed tomography (CT) to measure the radiodensity of a tissue. The units are calibrated so that air is -1000 and water is 0, using the following formula:
\(HU=\frac{(\mu _{material}-\mu _{water})}{\mu _{water}}\times 1000\)
Where:
- \(\mu _{material}\) is the linear attenuation coefficient of the tissue being measured.
- \(\mu _{water}\) is the linear attenuation coefficient of distilled water.
Values typically range from -1000 to 1000. Higher Hounsfield values correspond to denser materials, making them appear brighter on a CT scan, while lower values appear darker. HU values are widely used in clinical practice for tissue differentiation, helping radiologists distinguish between different types of tissue and pathology.
The following is a table HU for common tissues and substances:
| Substance | HU |
|---|---|
| Air | −1000 |
| Lung | −500 |
| Fat | −100 to −50 |
| Water | 0 |
| CSF | 15 |
| Kidney | 30 |
| Blood | +30 to +45 |
| Muscle | +10 to +40 |
| Grey matter | +37 to +45 |
| White matter | +20 to +30 |
| Liver | +40 to +60 |
| Soft Tissue, Contrast | +100 to +300 |
| Bone (cancellous) | +700 |
| Bone (cortical) | +3000 |