|Fig. 1: X-ray machine. (Source: Wikimedia Commons|
Advances in miniaturizing electron linear accelerators hold much promise for the goal of one day enabling portable medical radiology devices, such as portable X-ray machines. Nuclear medicine is an established field in medicine, where multiple radiological techniques exist to image and treat the body, from X-ray imaging, to gamma ray cancer ablation. I will explore how X-ray machines work, how they are able to image the body, and how advances in miniature electron linear accelerators hold the promise of one day having portables X-ray systems. Work at Stanford by Professor Byer on laser drive dielectric microstructures have enabled potential miniaturization of electron accelerators. This serves as an appealing new direction for nuclear medicine.
X-rays are used for medical diagnostic procedures or for research purposes to give one an internal view of the body. (Fig. 1) X-ray images are useful as they give one an image of the internals of the body, allowing a medical personnel to differentiate bone from tissue, giving one an internal anatomical view of the body. This is useful as a common use of X-ray is to assess whether bones have been broken, as bones show up as bright white on X-rays. This is because X-rays penetrate the materials of the body differently, depending on the density, which is why for example it can easily penetrate through skin but not denser bone, and thus one can see bone unimpeded on a X-ray scan. X-rays are a form of electromagnetic radiation that is much more powerful than visible light, and is the second highest energy form of radiation after gamma rays. Thus given its higher energy, X-rays can pass through most objects and materials like the body. By capturing the x-rays on the other side of the patient's body using a photodiode image detector, one can capture the "shadows" formed by the objects inside the body and thus develop an x-ray image. As X-rays pass through the body, they are absorbed in different amounts by different tissues depending on the radiological density of the tissue they pass through, and thus why one can see bones, not see skin tissue, etc. Radiological density is determined by both the density and atomic number of the material being imaged. For example this is why bones which contain calcium, which has a higher atomic number than much other tissue, produces such high contrast on an X-ray image as it gets absorbed, while x-rays otherwise pass through quite easily through fat, muscle, and air cavities like the lungs, and thus are displayed as light shades of gray on the x-ray image. 
An X-ray generator is a device that produces X-rays to be used for example in medical x-ray imaging. An X-ray machine is fundamentally able to generate X-rays by accelerating electrons from a cathode into anode. The cathode often consists of a heated filament that emits electrons by thermionic emission. The electrons produced from the cathode are then accelerated by very high voltages, allowing them to collide with the metal anode target with tremendous energy. Upon collision with the anode atoms, the electrons are suddenly decelerated upon collision and produce x-rays, and this x-ray radiation produced is called "bremsstrahlung" or "braking radiation". This happens as the accelerated electrons are able to knock an electron out of an inner shell of the target metal anode atoms. When an electron from a high enough state drops down to fill the newly created vacancy, an x-ray photon is then emitted. This is called a characteristic x-ray. However as the majority of the energy from the accelerated bombarded electrons is converted into heat at the anode, usually tungsten is used at the anode to handle the high heat. A heat dissipation system involving oil is usually used to stop the anode from melting as well. 
The high voltage required to accelerate the electrons means x-ray machines require quite large sized power electronics to produce the high voltage. Thus finding a way to accelerate electrons without large volume sized power electronics systems would be a positive step in finding ways to make x-ray imaging more portable. A promising way to accelerate electrons without large scale electronics is described by Professor R.L. Byer of Stanford in his 2013 paper, "Demonstration of electron acceleration in a laser-driven dielectric microstructure." Micro-fabricated dielectric laser accelerators (DLAs) use commercial lasers as the power source with which to accelerate the electronics, and thus this commercial available laser is smaller than the high voltage power electronics. Cost can also be reduced as DLAs can be created by low-cost lithographic techniques that are suitable for mass used production. By using optical grating structures, these small DLA structures can accelerated electrons past 250 MeV m-1, using a "silica grating structure and an 800nm Ti: sapphire laser." For context, normal linear accelerators are able to accelerate an electron by 10-30 MeV per meter, while this DLA is able to accelerate 250 MeV per meter, thus in a very small distance, and thus small volume, be able to bring the electrons to the very high energy needed to produce X-rays. Given the strength of the gradient, it is able to accelerate electrons at very small distances, boding well for a reduced size, potentially portable x-ray machine. Future work by Byer states that "[this work] set the stage for the development of future multi-staged DLA devices ... [that] would enable compact table-top accelerators on the MeV-GeV scale for security scanners and medical therapy ... and portable medical imaging devices." 
X-ray imaging is an invaluable medical diagnostic tool and central part of nuclear medicine as it allows one to image the internal bones, fat, muscle, and other associated tissues of the body with varying levels of contrast. X-rays are currently produced using high voltage electron acceleration, which require large sized power electronics. However, recent advances in microstructure dielectric laser accelerators (DLAs) allow for very high electron acceleration in short distances, and only involving a laser as the power source, have the potential to produce much smaller, maybe even portable x-ray imaging systems. 
© Anjan Katta. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
 W. Huda, Review of Radiologic Physics, 4th Ed. (Lippincott, Williams, and Wilkins, 2016).
 E. A. Peralta et al., "Demonstration of Electron Acceleration in a Laser-Driven Dielectric Microstructure," Nature 503, 91 (2013).
 B. Hasegawa, Physics of Medical X-Ray Imaging (Medical Physics Pub Corp, 1987).