Diapositive 1

Chap.2 Radiography1X-radiationX-radiation is a form of electromagnetic radiation. X-rays have a wavelength in the range of 0.01 to 10 nanometers, corresponding to frequencies in the range 31016 Hz to 31019 Hz and energies in the range 100 eV to 100 keV.

2The wavelengths are shorter than those of UV rays andlonger than those of gamma rays.X-rays with photon energies above 5-10 keV are calledhard X-rays, while those with lower energy are calledsoft X-rays. X-radiationDue to their penetrating ability hard X-rays are widelyused to image the inside of objects (ex. Medical radio-graphy and airport security).

3X-ray photons carry enough energy to ionize atoms anddisrupt molecular bonds. This makes it a type of ionizingradiation and thereby harmful to living tissue.

The ionizing capability of X-rays can be utilized in cancertreatment to kill malignant cells using radiation therapy.X-radiationX-rays are the second most commonly used medical tests,after laboratory tests. This application is known asdiagnostic radiography4Medical diagnostic radiography is undertaken by a speciallytrained professional called a diagnostic radiographer in theUK, or a radiologic technologist in the USA.Since the body is made up of various substances withdiffering densities, X-rays can be used to reveal the Internal structure of the body on film by highlighting thesedifferences using attenuation, or the absorption of X-rayphotons by the denser substances (like calcium-rich bones).DosesA medical imaging exam that involves exposure to ionizingradiation must be justified on the basis of benefit to thepatient. No practice involving exposure to ionizing radiationshould occur unless it produces sufficient benefit to theexposed individual5The Sievert (symbol: Sv) is the International System ofUnits (SI) of equivalent radiation dose. Quantities that aremeasured in sieverts are designed to represent the biological effects of ionizing radiationOne sievert equals 100 rem, an older unit of measurementstill in widespread useDosesThe sievert unit is based on the measurement of theconcentration of ionizing radiation absorbed per unit of amaterial's mass (joules of absorbed ionizing energy per kilogram). 6One sievert carries with it a 5.5% chance of eventuallydeveloping cancer1Sv = 1J/kg equivalentDoses greater than 1 sievert received over a short timeperiod are likely to cause radiation poisoning, possiblyleading to death within weeks.Dose from one x-ray7

The International Commission on Radiological Protection (ICRP) recommendslimiting artificial irradiation of the public to an average of 1mSv (0.001Sv) of effective dose per year.Production of x-rayX-rays are generated by an x-ray tube, a vaccum tube thatuses high voltage to accelerate the electron released by ahot cathode8

Production of x-rayEmission occurs when filament is heated by passing currentthrough it. When the filament is hot enough, the electronsobtain thermal energy sufficient to overcome the energybinding the electron to the metal of the filament.

9After accelerated the high velocity electrons collide with ametal target (anode), creating the x-ray by photoelectriceffect.

Production of x-rayAn incoming high-energy electron dislodges a k-shellelectron in the target, leaving a vacancy in the shell10An outer shell electron then jumps to fill the vacancy byemitting x-ray.X-ray energy = energy change in the jump.Interaction with matterDirect transmission: X-ray photons that traverse through the body without interacting and reach the image receptor11Scattering: occurs when a photon interacts with an outerorbital electron. The incident photon is then deflected by itsinteraction and is scattered from the site of the collision.Scattered photons travel in all directions.Photoelectric absorption: occurs when an incident photoncollides with an inner-shell electron resulting in totalabsorption and the incident photon ceases to exist. Theelectron is ejected from its shell, resulting in ionization. AttenuationThe x-ray beam leaving the patient carries absorption depending on the thickness and composition of the body12If all the photons posses the same energy (i.e., the beam ismonoenergetic) and if the photons are attenuated under conditions of good geometry (i.e., the beam is narrow and the transmitted beam contains no scattered photons), then the number of photons penetrating a thin slab of matter of thickness x is:

Half value layer The thickness of a slab of matter required to reduce theintensity (or exposure rate) of an x-ray beam to half is thehalf-value layer (HVL) or half value thickness (HVT) for thebeam. 13The HVL describes the quality or penetrating ability of thebeam.The HVL of a monoenergetic beam of x rays in any mediumIs:

ContrastContrast is determined by:The thickness of structure (t)Difference in linear attenuation coefficients 1 and 2 14

Contrast Large contrast between bone and muscle (decreases withincreasing energy: absorption in bone mush larger at low energies)15Little contrast between muscle and soft tissue (small difference in density)Large difference between air and tissue (owing to differencein density)Scatter reduction Scattered radiation acts like a veil over the image reducingthe contrast 16Scattered reduction could be achieved using a grid

X-ray filterAn X-ray filter is a device to block or filter out some or allwavelengths in the X-ray spectrum.17X-ray filters are used to block low-energy X-rays duringmedical x-ray imaging (radiography). Low energy X-rays aremore likely to be absorbed by the patient's soft tissues anddo not contribute to image quality.Suitable for Radiography:AluminiumCopperSilverRhodiumX-ray detection18

1- X-ray filmX-ray film is a photographic film, and this is film which willundergeo chemical changes when exposed to light (imageformation)The film is positioned in such a way that is behind thesubject to be x-rayedX-ray detection19The source emits the x-rays, they pass through the objectand through the filmThe film, which has been exposed to the x-rays, is thendeveloped to yield image2- X-ray detectorModern imaging is moving to a digital x-ray with eliminationof the film and the substitution of an x-ray detector behindthe subjectThis makes images quickly available and allows forimmediate digital storage

X-ray detection20This instrument works on the principle that as radiationpasses through air or a specific gas, ionization of themolecules in the air occur.X-ray detection21When a high voltage is placed between two areas of thegas filled space, the positive ions will be attracted to thenegative side of the detector (the cathode) and the freeelectrons will travel to the positive side (the anode). These charges are collected by the anode and cathodewhich then form a very small current in the wires going to the detectorA computer system is then used in order to controls theoperation of reconstruction and storage of acquiredimages.