Technical background, patient preparation, and quality and safety

Chapter 1.1 Key hardware components of a cardiac-enabled CT scanner

Sebastian Vandermolen, Marcel van Straten, and Francesca Pugliese


The gantry is the ring-shaped structure that houses the X-ray tube and the detector array.

X-ray tube

The X-ray tube is the component where X-ray generation occurs (Figure 1.1.1).

A tungsten filament is heated by current and emits electrons (thermionic emission).

By applying a potential difference (kilovoltage, kV) between the cathode and the anode, the electrons are accelerated towards a positively charged anode.

The resulting flow of electrons represents the tube current, measured in milliAmperes (mA).

The electrons gain energy proportional to the voltage applied (kV).

The electron beam hits the focal spot of the anode. About 4% of the energy of the beam leads to the generation of X-ray photons; the rest is dissipated as heat.

The emitted X-ray beam displays a range of different energies (polychromatic X-ray spectrum), from a few kiloelectron volts (keV) to the nominal value of the applied tube voltage (discussed further in Chapter 1.3, Figure 1.3.1).

If the applied voltage is 100 kV, the average energy of the X-ray beam is 50–60 keV.

Lower-energy X-rays are removed from the X-ray beam by the tube housing and by filtration: this is because energies at the lower end of the spectrum would otherwise be absorbed by tissue before reaching the detector and would contribute to patient dose but not to image formation.

The X-ray tube voltage determines the average energy of the X-ray beam; the X-ray tube current determines how many X-ray photons the X-ray beam is comprised of, without affecting their energy.

The intensity of the X-ray beam decreases with the inverse of the distance from the source squared.

When traversing tissue, the intensity of the X-ray beam decreases as X-ray photons interact with atoms (Chapter 1.3). The transmitted intensity depends on the initial intensity, the thickness of the tissue/material traversed, and their linear attenuation coefficient. The latter depends not only on the atomic number of the tissue/material, but also on photon energy and is generally higher at lower energy.

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Aug 22, 2023 | Posted by in CARDIOLOGY | Comments Off on Technical background, patient preparation, and quality and safety

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