• In contemporary scanners, the gantry can rotate continuously thanks to (contactless) slip-ring technology. Slip-ring technology allows fast transfer of power and data to and from the gantry without the need of power cables. Power cables would require unwinding every few turns, which would prevent continuous rotation and require reversal of gantry rotation. Cables can be replaced by brush technology that is in permanent electrical contact with the gantry and allows continuous rotation.

• The switch-mode power supply allows construction of a small and light but efficient power supply that can be housed in the gantry while generating very high voltages with limited heat production. In general, this works by converting alternating current (AC) to direct current (DC) using a switch circuit. The DC current is reconverted to AC at a higher frequency.

• The gantry rotation time is a key determinant of temporal resolution, a paramount scanner requirement for cardiac CT (discussed in Chapter 1.2). In a single-source (one X-ray tube) scanner, approximately half a revolution is needed for the acquisition of data required to reconstruct one image (half-scan algorithm). A single-source CT scanner with a rotation time of 300 ms can sample data for one image in 150 ms, which is the temporal resolution of this scanner.

• 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.