X-Ray Photons and UV Radiation - Essay Example

X-rays are electromagnetic waves lying between UV radiation and -rays. They were discovered by German physicist, Wilhelm Rntgen, and have wavelength 0.01-10.0 nm and frequency 30-30,000 1015 Hz). Main physical properties of X-rays could be described by the experiments where beam of x-ray photons interact with matter. Monochromatic beam of X-ray photons is directed perpendicularly to the electric and magnetic fields. For simplicity they often consider the electric field only and neglect the magnetic field. From a quantum mechanical perspective a monochromatic beam is viewed as being quantised onto photons each having an energy and momentum. The intensity of beam if then given by the number of photons passing through a given area per unit time. As the intensity is also proportional to the square of the electric field it follows that magnitude of the field is quantised (Dendy & Heaton, 1999). Thus A beam of x-ray photons is heterogenous and presents both fields: electrical and magnetic. Because X-ray beam is not originated from a point source it's divergent by its nature. In this way the magnification of the image could be achieved by the increase of the focus distance (direct dependence). Because of the existence of two different types of photon interaction within the x-ray tube there is important what peak voltage is used. The energy of electrons depends on the voltage between the anode and cathode. Higher peak voltage produce photons with higher energy (Aichinger et al., 2003). Thus measurement of the practical peak voltage is used for the quality control of X-ray units (Ramrez-Jimnez et al., 2004). There are some types of X-ray photons dependently on their origin and type of interaction with target atom. If the projectile electron interacts with an inner-shell electron of the target atom rather than an outer-shell electron than characteristic x-radiation can be produced (Christensen et al., 1979;). Contrarily to the characteristic interaction Bremsstrahlung (braking) x-radiation occur when the projectile electrons lose their kinetic energy in the interaction with the nucleus of a target atom (Aichinger et al., 2003). Actually, X-ray tube is very ineffective device - only small part of energy is transformed in X-ray beams while the rest just produce heat (Aichinger et al., 2003). Only small part of the anode surface is involved in x-ray production. This area is called as the "focal spot". There is known that smaller focal spots is more useful for imaging purposes because they generate less blurring and provide better visibility of image details. Thus X-ray tubes with small focal spots are useful for decrease of radiation loading (Aichinger et al., 2003). X rays interact with irradiated substances in form of photoelectric effect, dispersion and forming of pairs "electron-positron" (Aichinger et al., 2003) They have high penetrating capacity, its property is described by formula: I=I0e-rt, where I0 - is intensity of the beam of X-rays, - attenuation coefficient (Aichinger et al., 2003). The depth of penetrating depends on the half-value thickness, i.e. the thickness of an absorbing substance that reduces the intensity of x-ray beam to half its value (i.e. I0/2). The half-value thickness depends on the attenuating properties of the substance itself and the penetrating power of the radiation incident upon it. This property is used for such radiographic tests as roentgen-densitometry when density of tested object is compared with etalon, e.g. aluminium equivalent (Wells & Ryan, 2000). Another practical application of the half-value thickness knowledge is shielding/filtration. Protective equipment is an example of added filtration. Filtration allows increasing the average energy of X-ray beams and reduce amount of low-energy photons. It's helpful for radiation dose control. Minimum adequate filtration of the x-ray beam is achievable at the half value thickness which is equal or more than 2.5 mm of aluminium (Bushberg et al., 1994). Both Bremsstrahlung and characteristic radiation are emitted in the anode of X-ray tube. Because X-ray photons are irradiated in all directions from the focus point they use collimator to provide the continuous-wave beam directed to patient, which is used for imaging. The attenuation coefficient is the fractional change in the intensity of a parallel beam of monoenergetic radiation per unit thickness of substance (Christensen et al., 1979; Keriakes & Rosenstein, 1980). The attenuation coefficient and half-value thickness are related by the following equation: . There is important that X-ray attenuation grows with atomic number increase and falls with energy increase (Aichinger et al., 2003). The index of refraction for X-rays is slightly less than one implying total external reflection at glancing angles below the critical angle (Christensen et al., 1979). A focusing X-ray can be constructed by arranging that the incident angle is below the critical angle for total external reflection. At glancing angles below the critical angle the reflectivity is almost 100% and the X-ray beams only penetrate into the material as an evanescent wave with a typical; penetration depth about 10 angstrem. In this way X-rays can be a surface sensitive probe (Als-Nielsen & McMorrow, 2001). If we consider the incident X-ray as a beam of photons than we can suppose that electron is initially at rest and it is free. In a collision energy will be transferred from the photon to the electron with the result that the scattered photon has a lower energy than that of the incident one. This process is called "Compton scattering" (Als-Nielsen & McMorrow, 2001). This effect is used in digital radiography because in the absence of contrast media, the x ray contrast depends on Compton effect for soft tissue and a combination of the Compton effect and photoelectric effect for bone (Bansal, 2005). Compton scatter reduces the contrast of medical image, and, consequently, worsen its quality (Als-Nielsen & McMorrow, 2001). To avoid this they use various approaches (Als-Nielsen & McMorrow, 2001) including the use of the collimator, and/or antiscaterring grid, applying air gap technique, compression of the object. Low voltage theoretically could be useful for reducing scattering but this method is not applicable because of high risk of radiation injury (Als-Nielsen & McMorrow, 2001). There is known that the photoelectric effect predominates for iodinated contrast media (Bansal, 2005). Consequently, plain radiographs have excellent spatial resolution and they are still widely used in medical imaging. The principle of their work is based on the photoelectric effect also: x-ray beam is accepted on silver bromide plate. The reaction could be described as follows (Aichinger et al., 2003): AgBr + photon -> Br + Ag Br- + photon -> Br + e- Ag+ + e- -> Ag Image quality in screen-film radiography (i.e. image contrast, image sharpness and image noise) depends on the energy of X-ray photons, the spectral characteristics of X-radiation (Aichinger et al., 2003). In the last years the dominance of conventional roentgenography (or screen-film radiography) is rapidly declining (Bansal, 2005). This circumstance is related to some properties of screen-film detection including fixed non-linear gray scale response, fixed dose latitude and limited potential for reducing dose to the patient. Furthermore, film is expensive, contains toxic hazardous materials, its storage and recovery is difficult (Aichinger et al., 2003). The image quality in a digital system depends on the quality of x ray equipment, applied dose, and additionally on pixel size, pixel depth, signal to noise ratio, and dynamic range (Bansal, 2005). The interaction of X-radiation with biologic tissues has three stages (Robinson et al., 2005; Huda et al., 1997): Physical - absorption of ionizing radiation energy in described interactions. It takes only 1 10-19 s. It's able to destroy cells if energy of radiation is enough high. Intracellular water absorbed 50% of energy and organelles and biomolecules take 50% too. (Aichinger et al., 2003) Physical-chemical - production of active radicals. The main process is water radiolysis, the process takes about 10-11 s. secondary radicals with biosubstances. The most sensitive process is oxidative phosphorilation; the enzyme dysfunction expresses in damage of ATP production system. Protein molecules and dezoxiribonucleinic complexes (DNA in complexes with proteins, RNA and enzymes). Proteins change their configuration, it takes place aggregation of proteins because disulfide links are forming. The protein denaturation is the result of deep changing in its structure (Aichinger et al., 2003). Dose about 5-10 Gy realized in transformation of mucopolysaccharides. Peroxidative oxidation of lipids (POL) which has a character of chain reaction occurs. The cells with haploid chromosome set (spermatozoids, ovocytes) are more sensitive to x-ray action than diploid cells. Cells especially sensitive for radiation in anaphase and kataphase of mythosis Helliwell et al., 2005; Qi & Leahy, 2006). Hypoxy decreases the tissue sensitivity for ionizing radiation (Aichinger et al., 2003; Hall & Giaccia, 2005). The biologic effects of ionizing radiation is divided into three groups: somatic, somato-stochastic and genetic {Hall & Giaccia, 2005). The first group is presented by deterministic effects, includes acute and chronic ray disease, ray combustions, alopecea, ray cataract, clinical registered frustration of hemopoesis, temporary or constant sterility. This effects have close link with radiation dose and they forms in short time after action of radiation factor. For example the dose of 1-2 Sv makes the light form of acute ray disease but the dose of 10 Sv is absolutely mortal. The threshold character of the effects is used for prognosis and for reconstruction of the doses. The probability of their exceeding in the modern radiology is quite low because of the restrictions required by the modern guidelines (Hall & Giaccia, 2005). Nevertheless there were described some cases of mild radiation injuries related to the security risks (Archer, 2005) in radiology. Well known cases of mass radiation injuries in Hiroshima, Nagasaki and Chornobyl are also presented by the somatic effects mainly (Cardis, 1996). Somato-stochastic effects are more applicable to low doses used in diagnostic imaging include effects manifesting in the life of the individual (from the moment of fertilisation to death). There are cancerogenic (cancer causing) and teratogenic (causing congenital malformation) effects (Aichinger et al., 2003). For calculation of cancer risk they use laws of "dose-effect" interaction in the low doses. There are simple statistic models (linear, square, linear-square). The preference is given back to the last. The question about threshold level for cancerogenesis is unclear now. Probably some processes of oncogen activation and cancare developing could be radiation dose-dependent but there are no evidence about the intensity of such exposure. If these thresholds will be found, all the system of radiation security should be changed (Hall & Giaccia, 2005). Genetic effects include genetic mutation and chromosome aberration. At present the most of specialists consider teratogenic effects can be dose-dependent (Hall & Giaccia, 2005). The action of the ionizing radiation in the early stages of embriogenesis and gestation (first three months of the pregnancy) has as the threshold absorbed dose of 0.1 Gy (ibid). The most radiosensitive organs are presented by gonads and marrow. Embrion is very sensitive to X-ray radiation thus its diagnostic method should be restricted in pregnant women and women of fertile age. Lens, endocrinic, muscular, parenchimatous, and lymphoid tissue are also sensitive to the action of X-ray beams but less than tissues with higher mitotic index (Hart et al., 1994). The most resistant to the action of X-ray are bones, fibrous connective tissue (Hall & Giaccia, 2005). Typical doses in the different X-ray examination are following: for chest X-ray - 200 Sv, for brain x-ray - 250 Sv, abdomen - 5.5 mSv, for dental radiography - 100 Sv, for mammography - 500 Sv and for computer tomohraphy - 100 mSv (Hall & Giaccia, 2005). Conclusion: X-ray electromagnetic waves are widely used in medicine. The diagnostic value of routine roentgenographic examination is still outstanding, nevertheless there are some problems related to the physical properties of X-radiation. First of all, X-ray tube is very ineffective device - only small part of energy is transformed in X-ray beams while the rest just produce heat. The problem of filtration and collimation is important for reducing amount of scattered radiation. X-ray radiation is hazardous for radiologist and for patient. It can cause cataract and other radiation related disease and require appropriate radiation protection, e.g. shielding, decrease of exposure time, decrease of radiation intensity and increase of the distance between radiation source and exposed person (Bushberg et al., 1994). The choice of contrast medium should be based on the physical properties of the substance, particularly, on its half-value thickness. Thus for colon and bowel we can use barium contrast whereas for angiography we use iodine-containing contrast and for lungs - xenon. Routine screen-film radiography have limited capacities to determine defects and/or pathological changes in the soft tissues while this method seems to be a "golden standard" for bones. Implementation of digital technologies and, particularly, different schemes of computer tomography (CT) is very promising; nevertheless the radiation load is very high in CT. References: 1. Aichinger H., Dierker J., Joite-Barfu S., Sbel M. Radiation Exposure and Image Quality in X-Ray Diagnostic Radiology: Physical Principles and Clinical Applications Springer; 1 ed. 2003 - 212 S. 2. Als-Nielsen J, McMorrow D. Elements of Modern X-ray Physics John Wiley & Sons; 1 edition - 2001 336 p. 3. An introduction to the physics of diagnostic radiology by Christensen E., Curry T., Dowdey J. 1979 4. Archer BR. Recent history of the shielding of medical x-ray imaging facilities. Health Phys. 2005 Jun;88(6):579-86 5. Bansal G.I. Digital radiography. A comparison with modern conventional imaging. PMJ. 2005 6. Bushberg JT, Seibert JA, Leidholdt Jr. EM, Boone JM. The essential physics of medical imaging. Lippincott, Williams & Wilkins: NY; Chapters 4 and 9; 1994. 7. Cardis E. Epidemiology of accidental radiation exposures. Environ Health Perspect. 1996 May;104 Suppl 3:643-9 8. Dendy & Heaton Physics for Diagnostic Radiology. Institute of Physics; 2 edition 1999 - 446 p. 9. Hall E., Giaccia A. Radiobiology for the Radiologist Lippincott Williams & Wilkins; 6th edition , 2005 656 P. 10. Hart D, Jones DG and Wall BF Estimation of effective dose in diagnostic radiology from entrance surface dose and dose-area product measurements, Report NRPB-R262 (HMSO, London). 1994 11. Helliwell M, Jones RH, Kaucic V, Logar NZ. The use of softer X-rays in the structure elucidation of microporous materials. J Synchrotron Radiat. 2005 Jul;12(Pt 4):420-30 12. Huda W, Atherton JV, Ware DE, Cumming WA. An approach for the estimation of effective radiation dose at CT in pediatric patients. Radiology 203(2):417-22; 1997. 13. J. G. Keriakes and M. Rosenstein; CRC Handbook of Radiation Doses in Nuclear Medicine and Diagnostic X-rays; CRC Press, Inc.; Boca Raton, FL; 1980 14. Qi J, Leahy RM. Iterative reconstruction techniques in emission computed tomography. Phys Med Biol. 2006 Aug 7;51(15):R541-78. 15. Ramirez-Jimenez FJ et al. Considerations on the measurement of practical peak voltage in diagnostic radiology. // Br J Radiol. 2004 Sep;77(921):745-50 16. Robinson S, Suomalainen A, Kortesniemi M. Mu-CT. Eur J Radiol. 2005 Nov;56(2):185-91. 17. Wells J, Ryan PJ. The long-term performance of DXA bone densitometers. Br J Radiol. 2000 Jul;73(871):737-9. Read More