Medical Equipment Technology Department 1 Introduction to Biomed. Imaging Systems Lecture No.5-6 Dr. Yousif Mohamed Y. Abdallah X-Ray Production Objectives: X-ray tube interactions Characteristic and Bremsstrahlung X-rays X-ray emission spectrum X-ray Imaging System PRINCIPAL PARTS Operating Console High-voltage generator X-ray tube PRIMARY FUNCTION The system is designed to provide a large number of e- at cathode with high kinetic energy focused to a small target at anode. X-ray Tube Construction Radiographic Equipment G F E D C A B How X-rays are created Power is sent to x-ray tube via cables mA (milliamperage) is sent to filament on cathode side. Filament heats up electrons are produced Negative charge How X-rays are created Positive voltage (kVp) is applied to anode Negative electrons are attracted across the tube to the positive anode. Electrons slow down and finally come to rest Electron beam is focused from the cathode to the anode target by the focusing cup The distance between filament and the x-ray tube target is 1 cm. Velocity of electron is raised from zero half the speed of light E- traveling from cathode to anode Projectile electron interacts with the orbital electron of the target atom. This interaction results in the conversion of electron kinetic energy into thermal energy (heat) and electromagnetic energy in the form of infrared radiation (also heat) and x-rays. Tube Interactions Heat (99%) x-rays (1%) X-rays = Characteristic Bremsstrahlung Heat Most kinetic energy of projectile e- is converted into heat 99% Projectile e- interact with the outer-shell e- of the target atoms but do not transfer enough energy to the outer-shell e- to ionize Outer shell electrons are simply raised to an excited/ higher energy level. Heat production Outer shell electrons immediately drop back to their normal energy level with the emission of infrared radiation. The constant excitation and return of outer shell electrons are responsible for most of the heat generation Heat is an excitation rather than an ionization Heat production Production of heat in the anode increases directly with increasing x-ray tube current Doubling the x-ray tube current doubles the heat produced Increasing kVp will also increase heat production Efficiency of x-ray production is independent of the tube current Efficiency of x-ray production increases with increasing kVp. At 60 kvp % At 100 kVp % At 20 MV % Characteristic Radiation Projectile electron interact with inner shell electron Projectile e- with energy high enough to totally remove an inner-shell electron of the target atom e.g. tungsten Characteristic x-rays are produced when outer- shell e- fills an inner-shell Only K-characteristic x-rays of tungsten are useful for imaging Bremsstrahlung Radiation Bremsstrahlung is produced by projectile e- interacting with the nucleus of a target atom Bremsstrahlung Radiation A projectile e- that completely avoids the orbital e- as it passes through a target atom may come close enough to the nucleus of the atom to come under the influence of its electric field projectile e- kinetic energy to EM energy electrostatic force Bremsstrahlung Radiations As the projectile electro passes by the nucleus, it is slowed down and changes its course, leaving with reduced kinetic energy in a different direction. This loss of kinetic energy reappears as an x- ray. Bremsstrahlung is a German word meaning slowed-down Radiation X-ray energy Characteristic x-rays have very specific energies. K-characteristic x-rays require a tube potential of a least 70 kVp Bremsstrahlung x-rays that are produced can have any energy level up to the set kVp value. Brems can be produced at any projectile e- value Discrete spectrum Contains only specific values Characteristic X-ray Spectrum Characteristic has discrete energies based on the e- binding energies of tungsten Characteristic x-ray photons can have 1 of 15 different energies and no others Characteristic x-ray emission spectrum Continuous Spectrum Contains all possible values Bremsstrahlung X-ray Spectrum Brems x-rays have a range of energies and form a continuous emission spectrum Factors Affecting the x-ray emission spectrum Tube current, Tube voltage, Added filtration, Target material, Voltage waveform The general shape of an emission spectrum is always the same, but the position along the energy axis can change Quality The farther to the right the higher the effective energy or quality Quantity The more values in the curve, the higher the x- ray intensity or quantity mAs A change in mA results in the amplitude change of the x-ray emission spectrum at all energies The shape of the curve will remain the same mA increase from 200 to 400 kVp A change in voltage peak affects both the amplitude and the position of the x-ray emission spectrum Filtration Adding filtration is called hardening the x-ray beam because of the increase in average energy Filtration more effectively absorb low-energy x- rays than high energy x-rays Characteristic spectrum is not affected & the maximum energy of x-ray emission is not affected Filtration Adding filtration to the useful beam reduces the x-ray beam intensity while increasing the average energy (higher quality) Lowering the amplitude and shifting to the right What A does this graph indicate? Target Material The atomic number of the target affects both the quantity and quality of x-rays Increasing the target atomic number increases the efficiency of x-ray production and the energy of characteristic and bremsstrhlung x-rays Target material Voltage Waveform 5 voltage waveforms: half-wave rectification, full- wave rectification, 3-phase/6-pulse, 3-phase/12- pulse, and high-frequency. Maintaining high voltage potential Voltage generators Factors affecting X-Ray beam quality and quantity An increase inResults in Current(mAs)An increase in quantity; no change in quality Voltage (kVp)An increase in quantity and quality Added filtrationA decease in quantity and an increase in quality Target atomic number(Z)An increase in quantity and quality Voltage rippleA decrease in quantity and quality kVp KINETIC ENERGY OF ELECTRONS CATHODE MADE OF TUNGSTEN + 1%-3% THORIUM TUNGSTEN Z # 74 MELTING POINT- 3,410 DEG. CELSIUS THORIUM Z # 90 THERMIONIC EMISSION CATHODE HEATED UP TO AT LEAST 2,200 DEG. CELSIUS ANODE TUNGSTEN TARGET TUNGTEN AS TARGET HIGH Z# EFFICIENCY OF X-RAY PRODUCTION HIGH MELTING POINT 3,410 C TARGET HEATED TO 2,000 C X-RAY PRODUCTION BREMSSTRAHLUNG RADIATION CHARACTERISTIC RADIATION BREMSSTRAHLUNG RADIATION If an incoming free electron gets close to the nucleus of a target atom, the strong electric field of the nucleus will attract the electron, thus changing direction and speed of the electron. The Electron looses energy which will be emitted as an X- ray photon. The energy of this photon will depend on the degree of interaction between nucleus and electron, i.e. the passing distance. Several subsequent interactions between one and the same electron and different nuclei are possible. X-rays originating from this process are called bremsstrahlung. Bemsstrahlung is a German word directly describing the process: "Strahlung" means "radiation", and "Bremse" means "brake Bremsstrahlung Radiation BREMS DIFFERENT DEGREES OF DECCELERATION X-RAYS HEAT BREMS RADIATION IS: POLYENERGETIC ! 90% OF X-RAYS ARE PRODUCED THROUGH BREMS INTERACTIONS WHEN KVP APPLIED BREMS EMISSION- CONTINUOUS The high energy electron can also cause an electron close to the nucleus in a metal atom to be knocked out from its place. This vacancy is filled by an electron further out from the nucleus. The well defined difference in binding energy, characteristic of the material, is emitted as a monoenergetic photon. When detected this X-ray photon gives rise to a characteristic X-ray line in the energy spectrum. Characteristic X-rays Characteristic Radiation KE OF PROJECTILE ELECTRON > BINDING ENERGYORBITAL ELECTRON CHARACTERISTIC CASCADE TUNGSTEN-74 K-70 KEV L-12 KEV M-2.8 KEV BINDING ENERGIES OF DIFFERENT SHELL ELECTRONS CHARACTERISTIC X-RAYS L K = 58 keV MK 70-3 = 67 keV M L 12-3 = 9 keV TRANSFER OF ELECTRONS BETWEEN OUTER SHELLS RESULTS IN: HEAT PRODUCTION ! EACH CHARACTERISTIC RADIATION ( ex. K TO L TRANSFER) IS: MONOENERGETIC ! THERE ARE MANY CHARACTERISTIC RADIATION PRODUCED IN ONE ATOM THEREFORE CHARACTERISTIC RADIATION IS ALSO POLYENERGETIC ! CHARACTERISTIC EMISSION- LESS POLYENERGETIC! X-RAYS PRODUCTION AN INEFFICIENT PROCESS 80 KVP 99.4% HEAT 0.6% X-RAYS % EFFICIENCY OF X-RAY PRODUCTION % EFFICIENCY= K * Z * kVp K = 1 * 10 -4 X-RAY EMISSION ISOTROPICAL- IN EVERY DIRECTION USEFUL RADIATION PROJECTED TOWARD THE PATIENT LEAKAGE RADIATION HOUSING OFF-FOCUS RADIATION USEFUL OFF-FOCUS 73