Mechanisms For X-ray Production, Radiography And Health Risks

Mechanisms for X-ray production using electrons

There are two mechanisms of producing X-rays through electrons. First, when a high-speed electron is rapidly decelerated as it approaches the nucleus’ electric field. As a result, the electron is deflected thereby emitting a photon of energy know as x-radiation. Often, this type of radiation is referred to bremsstrahlung and for any given source of an electron, a spectrum of these radiations would be produced until the electrons reach their maximum energy. Therefore, it is common to produce x-rays whenever high-speed electrons are suddenly decelerated. For instance, when the earth’s magnetic field traps the solar wind in the “Van Allen Radiation Belts.” (Moody, 2011).

Additionally, black holes in the universe produce x-rays. Matter that falls into the black hole experience acceleration as a result of the magnetic field existing in the black hole. The particles of matter falling into black-holes bombard each other on their way down resulting to inelastic collisions. Since we know that inelastic collision lead to emission of energy, this bombardment lead to emission of photons of energy. Due to the fact that these collisions happen at a very high speed, the energy of the emitted photons lie in the x-ray region of the electromagnetic spectrum.  

Secondly, X-rays are produced when electrons transition from one atomic orbit to another. Through such transitions, electrons are able to navigate from outer orbits to spaces left within the inner orbits. The result of the transitions is the emission of photons of energy in form of x-radiations. Notably, these radiations emit different levels of discrete energy defined starting band endpoints of the transitions. X-rays produced through these mechanisms are known as characteristic x-rays because they are distinctive for a given element and a specific transition (Nasseri, 2016).

X-ray radiation is formed by converting electron energy into photons with appropriate energies. The heat produced is undesirable hence the x-ray is designed in such a way that it maximizes x-ray production and dissipate the heat as fast as possible (Poludniowski, 2012). The x-ray has a simple construction with the cathode and the anode as the two principle elements.

Figure 1 shows the schematic diagram of an X-ray tube. When a current is passed, the tungsten filament heats up increasing the energy of electrons. As a result, electrons are dislodged from the filament surface through thermionic emission. Then, since unlike charges attract, these electrons are attracted towards the positively charged anode. They bombard the tungsten target with the maximum energy that is determined by the voltage applied to the tube. Consequently, bremsstrahlung radiations and characteristic interactions 99% of the energy into heat and 1% of the energy into x-ray photons. Radiographers can manipulate the x-ray by adjusting the tube current or voltage to produce low energy or high energy x-rays for different applications (Lucas, 2015).

Process of X-ray production in an X-ray tube

Additionally, an x-ray has a glass envelope that encloses the anode and the cathode. Thus, it insulates the anode and cathode assemblies as well as maintain a vacuum in the tube.

In an X-ray tube, approximately 90% of the radiation energy is converted to heat whereas only 1% is converted to an X-ray beam. If the huge amount of heat produced at the anode keeps on bombarding at the same spot for a longer time, the anode surface could easily deform. Consequently, the X-ray beam angle could shift reducing the overall beam efficiency and tube image quality. However, in case of a rotating anode tube, heat is evenly dispersed as the anode rotates (Poludniowski, 2012). This reduces anode heating hence can be used for performing longer scans even at higher voltages.

The fact that X-rays have been applied for a long period of time to diagnose a medical issue, monitor treatment progress or treat some conditions shows how fundamental they are. However, exposure to high levels of radiations can have far-reaching consequences on human health. According to research, high radiation levels have short time effects such as vomiting, bleeding, hair loss, fainting, and loss of skin (Eisenberg et al., 2011).

Besides, research reveals that X-rays have the ability to cause DNA mutations increasing the risk of cancer later on in life. That’s why the World Health Organization (WHO) in collaboration with the US government classified X-rays as a carcinogen. However, many radiologists argue that X-ray benefits outweigh potential undesirable effects (Lukas, 2015).

Also, exposure to x-ray radiations can damage egg and sperm cells. With the damaged DNA, babies with defects may be born. This can go to an extent of developing tumors or abnormal growths in body cells leading to leukemia.

In the US, it is estimated that CT scans contribute to 0.4% of cancer cases. Scientists expect this to increase with the increased use of CT scans in various medical procedures. Moreover, the study reveals that by the age of 70 years, X-ray is expected to increase the risk of cancer to approximately 1.8%. That is, the risks will still be minimal compared to the benefits. Each medical procedure poses a different risk that depends on the part of the body being scanned and the type of x-ray (Baskar, Lee, Yeo, & Yeoh, 2012).

 Part b: How X-ray Exposures are minimized in Practice

Due to its increased application in medical imaging, the safety of X-ray exposures is of great concern. The designers of these machines always ensure they are up to standard as well as offer training to radiologists in hospitals to ensure safe handling. Also, the designers ensure that the machines are well shielded prevent any possible leak of radioactive elements (Lee, 2016).

Attenuation of X-rays

Besides, advancement in medical imaging technology has increased the efficiency of X-ray machines. As such, researchers argue that the low levels of X-ray exposure carry no risk. Moreover, any damages caused by the low dose radiations to the human body are easily repaired and do not leave any lasting mutations. The threshold of radiations that can cause serious damages is far much higher than the standard X-ray dose (Baskar et al., 2012).

Overall, the safety of these procedures lies in the importance of making the right diagnosis as well as administering the right treatment for any medical application.

Diagram A corresponds to characteristic X-rays which are produced when electrons transition from one atomic orbit to another. The two peaks shown in diagram A denote gaps in the n=1 shell or n=k shell of the atom. As a result, electrons transition to fill the voids. Transitions from n=2 to n=1 result to K_α X-rays whereas transitions from n=3→1 are referred to as K_β X-rays (Hubbell, 2004). The Bremsstrahlung continuum region on the left of the two sharp peaks forms the base the two peaks.

Diagram B corresponds to bremsstrahlung X-rays which are produced when a high-speed electron is rapidly decelerated as it approaches the nucleus’ electric field. From diagram B, the bombarding energy increases with the increase in the applied voltage. Consequently, the radiation emitted lies within the x-ray region of the electromagnetic spectrum. The region is denoted by a continuous spreading of radiations which become more intense when the energy of the bombarding electrons increases. Thus, the bremsstrahlung lines show the relative intensity of the radiations when different levels of voltages are applied. It is evident that, the increase in the applied voltage increases the relative intensity of the radiation (Dyson, 2014). That is why a 50kV input peak is larger than a 20kV peak.

The attached publisher file shows the poster where attenuation has been defined and the four methods of attenuation explained to details. That is, Thomson scatter, Compton scatter, pair production and photo-electric effect.

Attenuation refers to the gradual loss of any signal strength as it passes through a medium. X-rays, just like any other signal gets attenuated depending on the photon energy and the distance traveled.

When X-rays pass through matter, they are attenuated. The intensity of the radiation decrease as it penetrates matter. Since each atom of matter through which a radiation travels through removes an x-ray from the beam, the beam density decreases with the distance. Hence, attenuation increases with the increase in the distance traveled (Haghighi, Chatterjee, & Thulkar, 2011).

Uses of X-rays in radiography and radiotherapy

On the other hand, attenuation can occur in form of signal absorption, electron emission, pair production or re-emission of radiation. The attenuation of x-rays follows the inverse square law. That is, their intensity drops exponentially from the equation,  

Where,  is the linear attenuation coefficient and x is the thickness of the material that the x-ray penetrates (Wang, Zhong, Sun, & Qin, 2017).

Total attenuation decreases with increase in the energy of X-ray photons as shown by the black line on the graph below.

Exposure to high levels of X-rays can cause DNA mutations thus triggering off cancer. For this reason, x-ray equipment needs to be well shielded to prevent possible leaking of the radiations from the x-ray tube. Therefore, the x-ray tube is enclosed in a housing that shields and absorbs radiations. From the table, the various media shown attenuate the x-ray beam differently depending on their density.

As it can be seen, a perfect vacuum allows x-rays to travel indefinitely. The x-ray travels the shortest distance in lead, showing that lead can offer enough attenuation to the x-ray signal and prevent it from penetrating deeper. The most effective cladding material from the table is lead because it offers sufficient shielding to prevent any radiation leak. That is why the housing of an x-ray tube is made up of lead.

Filling the x-ray tube where the x-rays are produced

The inside of an x-ray tube requires a material or medium that allows electrons to move freely and easily from the cathode to the anode. We can see from the table that a vacuum allows x-rays to travel freely without any attenuation (Maxwell, Lowe, & Shah, 2016). Thus, a perfect vacuum is more appropriate when filling the x-ray tube where the x-rays are produced.

According to X-ray Physics, an X-ray image refers to a map of X-ray attenuation that depends on the thickness and density of tissues. Specifically, the radiographic image is a composition of X-rays that have freely passed through the body or rather have been absorbed or scattered by body structures. That is, the denser the tissues, the more the degree of attenuation of X-rays. As a result, radiographic images have lighter and darker patches to signify the level of attenuation. For instance; air, fat, metal, bone, and soft tissue are discriminated by patches of different levels of black or white.

Part a

Before their invention in 1971, there were many technological advances and discoveries that paved the way for CT scanners from the beginning of the 20^th Century.

Time	Invention
In the 1900s	Alessandro Vallebona, a renown Italian radiologist invented tomography. It used a radiographic film to observe a single section of the body. However, during its inception period, it was considered ineffective when it came to imaging of body tissues.
In the 1960s	Increased power and the computer invention led to more research in tomography.
In 1967	Godfrey Hounsfield invented the first CT scanner using X-ray technology
In 1971	The first CT scan was successfully performed on a patient’s brain.

Health risks associated with exposure to X-ray radiation

In 1973, CT scanners were installed in the US and by 2005, over 68 million CT scans were used globally. Up to date, technological advancement and biomedical engineering research are continuously improving the CT scanners (Nordqvist, 2017).

Computerized tomography (CAT or CT scan) is an example of X-ray imaging that gives a much more detailed image than a usual radiograph. Its anatomy consists of a rotating X-ray source and a ring of detectors that surround the patient (Jennings & Austin, 2010). Additionally, it has a motorized platform where the patient lies. During CT imaging, multiple images are captured from different angles and then stored in a computer for analysis.

In conventional x-rays, the body structures under examination may overlap. For instance, the ribs overlap the heart and the lungs. Usually in an x-ray, structures that require medical attention are obscured by bones or other organs making diagnosis impossible. However, in CT imaging, the rotary x-ray eliminates overlapping hence making diagnosis more apparent. CT images enable radiologists to identify and observe internal body structures and analyze their shape, size and texture. The detailed information is essential in determining whether there is a medical problem, its location and the extent to which it has developed. With modern CT scanners, this information is obtained in seconds (Jennings & Austin, 2010).

Actually, CT scanners have brought many benefits in the medical field. For instance, their fast response enables them to be used in emergency rooms. Patients are scanned quickly providing adequate time for doctors to access the patient’s medical condition. Besides, in case of internal bleeding CT images show surgeons the exact point to operate on. Thus, a surgery can be greatly compromised in without CT image information.

During the scan day, the patient lies down an examination table. The table is motorized and can slide into the CT scanner. Often, the patient lies by the back. After taking an image, the table moves slightly, enabling the machine to take an image from another part until all parts are examined. The images will then be kept for analysis (Nordqvist, 2017).

Radiation therapy or radiotherapy uses X-rays or gamma rays to kill cancerous cells. These high energy radiations may be administered through external beam radiotherapy or internal radiotherapy. The radiations damage DNA of cancer cells directly or through free radicals in the cells that damage the DNA. As a result, the cells whose DNA is damaged stop diving and die. They are then broken down and eliminated from the body (Gani, 2017.

Patients are given radiation therapy alone or in combination with chemotherapy or surgery with the hope that it will cure cancer. That is, inhibit cancer recurrence, eliminate a tumor, or both. Alternatively, radiation therapy may be given as a palliative treatment to reduce the suffering that cancer causes by relieving pain (Mehta, 2011).

The type of radiotherapy prescribed depends on factors such as the type of cancer, the size and location of the tumor, the health status of the patient, how far into the body the x-ray has to penetrate, and other factors such as patient’s age.

Besides all these factors, external-beam radiation therapy is the most common type of radiation therapy. It is delivered in terms of x-ray beams from a LINAC machine which uses electrical energy to give the high-energy radiations. Patients usually receive treatments depending on the dose prescribed. An example of a LINAC machine is the 3-D conformal radiation therapy (3D-CRT). These machines use sophisticated and advanced computer software to deliver precise radiation therapy to shaped target parts (Nordqvist, 2017).

References

Baskar, R., Lee, K. A., Yeo, R., & Yeoh, K. (2012). Cancer and Radiation Therapy: Current Advances and Future Directions. International Journal of Medical Sciences, 9(3), 193-199. doi:10.7150/ijms.3635

Dyson, N. A. (2014). Characteristic X-rays. X-Rays in atomic and nuclear physics, 62-135. doi:10.1017/cbo9780511470806.005

Eisenberg, M. J., Afilalo, J., Lawler, P. R., Abrahamowicz, M., Richard, H., & Pilote, L. (2011). Cancer risk related to low-dose ionizing radiation from cardiac imaging in patients after acute myocardial infarction. Canadian Medical Association Journal, 183(4), 430-436. doi:10.1503/cmaj.100463

Gani, C. (2017). SP-0431: Radiomics in radiotherapy. How is it used to personalise treatment and to predict toxicity and/or tumour control. Radiotherapy and Oncology, 123, S228. doi:10.1016/s0167-8140(17)30873-3

Haghighi, R. R., Chatterjee, S., & Thulkar, S. (2011). X-ray attenuation coefficient of mixtures: Inputs for dual-energy CT. Medical Physics, 38(10), 5270-5279. doi:10.1118/1.3626572

Hubbell, J. H. (2004). Photon cross sections, attenuation coefficients, and energy absorption coefficients from 10 keV to 100 GeV. doi:10.6028/nbs.nsrds.29

Jennings, J. T., & Austin, A. D. (2010). Novel use of a micro-computed tomography scanner to trace larvae of wood boring insects. Australian Journal of Entomology, 50(2), 160-163. doi:10.1111/j.1440-6055.2010.00792.x

Lee, C. I. (2016). Low-Dose Ionizing Radiation from Medical Imaging. Oxford Medicine Online. doi:10.1093/med/9780190223700.003.0050

Lucas, J. (2015, March 12). What Are X-Rays? Retrieved from https://www.livescience.com/32344-what-are-x-rays.html

Maxwell, A., Lowe, G., & Shah, A. (2016). Commissioning the CT part of a PET-CT scanner for use as a radiotherapy CT scanner. Physica Medica, 32(2), 421. doi:10.1016/j.ejmp.2015.07.033

Mehta, A. (2011). Chapter-08 Rational Uses of Chemotherapy, Radiotherapy and Hormone Therapy. Changing Paradigm in Breast Cancer Management, 53-68. doi:10.5005/jp/books/11247_8

Moody, M. F. (2011). Part II. Optics. Structural Biology Using Electrons and X-rays, 123. doi:10.1016/b978-0-12-370581-5.00027-6

Nasseri, M. M. (2016, June). Determination of Tungsten Target Parameters for Transmission X-ray Tube: A Simulation Study Using Geant4. Retrieved from https://www.sciencedirect.com/science/article/pii/S1738573316000152

Nordqvist, C. (2017, January 26). CT Scan or CAT Scan: How Does It Work? Retrieved from https://www.medicalnewstoday.com/articles/153201.php

Poludniowski, G. G. (2012). Calculation of x-ray spectra emerging from an x-ray tube. Part II. X-ray production and filtration in x-ray targets. Medical Physics, 34(6Part1), 2175-2186. doi:10.1118/1.2734726

Wang, Y., Zhong, F., Sun, X., & Qin, X. (2017). Realistic image composite with best-buddy prior of natural image patches. 2017 IEEE International Conference on Image Processing (ICIP). doi:10.1109/icip.2017.8296687.