Computed tomography (CT) or computerized axial tomography (CAT) is a medical imaging technique for creating cross-sectional (tomographic) images by exposing a subject to X-rays. Two-dimensional X-ray attenuation images taken from different angles are computationally combined to generate the three-dimensional image, eliminating the superimposition of body structures present in the traditional X-ray images.
Voxel values in a CT image represent the relative radiodensity inside that particular volume of tissue, usually in Hounsfield units (HU). For example, HU for water is 0, for air -1000, for bone 300-2000, and for fat ∼-100.
CT angiography (CTA) is used to visualize blood vessels, with the help of injected contrast media (CM) that strongly attenuate the X-rays. This reveals irregularities in arterial and venous vasculature, including aneurysms, stenosis, and thrombi. Contrast media usually consist of one or two tri-iodobenzene rings, with variable side chains. After administration CM distribute into blood and extracellular fluids, and are excreted into urine via kidneys within a day. Chronic kidney disease is common, and because of reduced kidney function the elimination of CM is retarded, increasing the risk for contrast-induced nephropathy (CIN). CIN includes medullary ischaemia, formation of reactive oxygen species, and direct tubular cell toxicity, leading to acute kidney injury. The risk for CIN is still very small (Maaniitty et al., 2016).
Using CT images in PET
What are CT images used for?
- In attenuation correction of PET images
- As anatomical reference for accurate placement of regions of interest (ROI); MR images can also be used as anatomical reference
- In warping a subject's brain onto a template or atlas, to apply SPM or ROI templates
CT can markedly increase the radiation dose from a hybrid PET/CT study, especially in a whole body acquisition. Modern imaging systems allow management of the CT radiation dose (Inoue et al., 2022).
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Updated at: 2022-03-19
Created at: 2015-06-11
Written by: Vesa Oikonen