# Synchrotron X-ray Radiation (SXR) in Medical Imaging: Current Status and Future Prospects
## 2 Overview of Synchrotron  
A synchrotron primarily consists of an electron gun, linear accelerator (linac), circular shape vacuum tubes with circumferences of up to several hundred meters (booster and storage rings), beamline and end station. Once the electrons are generated by the electron gun, they go through the acceleration process in the linac to attain the speed close to the speed of light. These electrons are then transferred to the booster ring, where their energies are increased before transferring to the storage ring. A series of magnets separated by straight sections allows the electrons to circulate around the storage ring. Electromagnetic radiation (synchrotron light with broad-spectrum rays covering from microwaves to hard X-rays) is generated as a result of deflection of the electrons in the magnetic field created by the bending magnets. The intensity of the synchrotron light can be significantly increased by placing an insertion device (wiggler or undulator) in the straight sections of the ring. When the high intensity synchrotron light exits the storage ring through a guided shielded end, it is modulated by slits, filters and different X-ray optical components called monochromator devices, silicon crystals with focusing bent elements mirrors and refraction elements. The user can select the wavelength required for experiment either with a suitable monochromator or adjusting the emission wavelength of the insertion devices. There is also a cooling system to regulate the high heat load during experiment. A simplified layout of a synchrotron to produce monochromatic X-ray radiation is shown in Figure 1.  
In a synchrotron, a beam of emitting electrons with rest mass \(m\) and speed \(v\) is first accelerated to acquire the speed of light \(c\) (\(v\approx c\)). The beam is then guided, bent and focused through a series of powerful magnets to emit electromagnetic radiation. The radiation pattern is compressed into a narrow cone in the region of the X-ray spectrum but  
Figure 1: A simplified layout of a synchrotron to produce monochromatic X-ray radiation. Reproduced with modification from [4]. is millions of times brighter than the conventional X-ray in the direction of motion, resulting in an emission tangential to the particle orbit. This radiation is known as synchrotron X-ray radiation (SXR). An example comparison of the SXR spectrum with conventional X-ray tube radiation is shown in Figure 2.  
The opening angle \(\psi\) is measured in milliradian (mRad) and given by the following equation [3]  
\[\psi=\frac{mc^{2}}{E}=\gamma^{-1} \tag{1}\]  
In Equation (1), \(\gamma=1957E(\mathrm{GeV})\) for electrons and positrons. For a storage ring of energy \(E=1\mathrm{GeV}\), synchrotron radiation is highly collimated with \(\psi\approx 0.5\) mRad \(\approx 0.029^{\circ}\) and can provide extremely high fluxes on very small areas even at a distance of tens of meters away from the storage ring. Two types of SXR sources are used: bending magnets and insertion devices. When the bending magnet is used as a source, a very stable beam over a broad spectrum is produced. On the other hand, when the insertion device is the source (i.e., an array of magnets inserted into the straight sections of the storage ring), a more intense, tunable light is produced and optimized to specific energy (e.g., 3 GeV) [1]. Brightness or brilliance \(B\) is proportional to the total emitted power \(P\), the emitting source area, \(A\) and the angular divergence of the emission \(\Omega\) and can be represented by the following equation [8]:  
\[B\propto\frac{P}{A\Omega} \tag{2}\]  
Figure 2: (**A**) SXR spectrum from the X17 superconducting wiggler beam port at the National Synchrotron Light Source (NSLS) in comparison to a high-power conventional X-ray tube source. Reprinted with permission from [5]. (**B**) Brilliance of emitted photons (photons/sec/mrad\({}^{2}\)/mm\({}^{2}\)/0.1%) vs. energy of the emitted photon (KeV) for different sources. U in the figure refers to an undulator.