X-RAY PRODUCTION & X-RAY EMISSIONS RADIATION PHYSICS TUBE INTERACTIONS • Electron- Anode Interaction • Imagine the energy needed to propel electron from 0 to half the speed of light in one to three centimeters. • The electrons that travel from the cathode to the anode are called projectile electrons. X-RAY PRODUCTION • Electron- Anode Interaction • When they strike the heavy metal atoms of the anode they interact with the atoms and transfer their kinetic energy to the target. • These interactions happen at a very small depth of penetration into the target. ELECTRON INTERACTION WITH TARGET • The electrons interact with either the orbital electrons or nucleus of the target atoms. • Interaction with the outer shell electrons produce heat ELECTRON INTERACTION WITH TARGET • There is no ionization but there is excitation. • More than 99% of the kinetic energy of the projectile electron is converted to thermal energy. ELECTRON INTERACTION WITH TARGET • The production of heat increases directly with tube current. • Through the diagnostic range, heat production increases directly with the increase of kVp. CHARACTERISTIC RADIATION • When the projectile electron interacts with an inner shell electron of the target atom rather than with the outer shell electron, Characteristic X- radiation can be produced. CHARACTERISTIC RADIATION • The interaction is sufficiently violent to Ionize the target atom by removing a K shell electron. • A outer shell electron falls down to replace the lost electron. CHARACTERISTIC RADIATION • The translation from outer shell electron to fill the hole in the K shell is accompanied by the emission of an x- ray photon. • The K shell has an average energy of 69 keV. CHARACTERISTIC RADIATION • Only the K- characteristic x-rays are useful and contribute greatly to diagnostic radiographs. CHARACTERISTIC RADIATION • Characteristic x- rays are produced by transitions of orbital electrons from the outer shell to the inner shell and is characteristic of the target element. BREMSSTRAHLUNG RADIATION • Heat and Characteristic x-rays are the product of interaction with the electrons of the target atom. • There is a third type of interaction. BREMSSTRAHLUNG RADIATION • The projectile electron can also interact with the nucleus of the target atom. • The nucleus has a strong positive charge. • The projectile electron misses all of the orbital electrons. BREMSSTRAHLUNG RADIATION • And comes close to the nucleus. • The strong positive charge of the nucleus causes it to slow, lose kinetic energy and change direction. BREMSSTRAHLUNG RADIATION • The lose of kinetic results in a low energy x-ray photon. • This type of x-rays are called Bremsstrahlung X- rays. BREMSSTRAHLUNG RADIATION • Bremsstrahlung is a German word for braking. • This energy of x-ray is dependent upon the amount of kinetic energy in the interaction. BREMSSTRAHLUNG RADIATION • A 70 keV electron can lose all, none or any intermediate level of kinetic energy. • The x-ray can have an energy range of 0 to 70 keV. BREMSSTRAHLUNG RADIATION • This is different from Characteristic X-ray that have a specified energy. • Low energy Bremsstrahlung x- ray result from slight interaction with the nucleus. BREMSSTRAHLUNG RADIATION • Maximum strength Bremsstrahlung X- ray happen when the projectile electron loses all of it’s kinetic energy. CHARACTERISTIC VS. BREMSSTRAHLUNG X-RAYS. • Characteristic X-ray require 70 kVp or higher. Based upon the energy of the k-shell electron. • Bremsstrahlung X-rays can be produced at any projectile electron energy. In diagnostic radiography most of the x-rays are bremsstrahlung x-rays. X-RAY EMISSION SPECTRUM • If a relative number of x-ray photons were plotted as a function of their energies we can analyze the x-ray emission spectrum. • Understanding the x-ray emission spectra is key to understanding how changes in kVp, mA, time and filtration affects the optical density and contrast of the radiograph. DISCRETE X-RAY SPECTRUM • Characteristic x-rays have a precisely fixed or discrete energies. • These energies are characteristic of the differences between electron binding energies of a particular element. • For tungsten you can have one of 15 energies . DISCRETE X-RAY SPECTRUM • There are 15 energies • There are 5 vertical line representing K x- rays. • 4 representing L x- rays. • Remaining represent lower energy outer shell electrons. DISCRETE X-RAY SPECTRUM • K x-rays are the only characteristic x-rays of tungsten that have sufficient energy to be of value in radiography. CONTINUOUS X-RAY SPECTRUM • The Bremsstrahlung x-ray energies range from zero to a peak and back to zero. • This is referred to as the Continuous X- ray Spectrum. CONTINUOUS X-RAY SPECTRUM • The majority of the useful x-rays are in the continuous spectrum. • The maximum energy will be equal to the kVp of operation. • This is why it is called kVp (peak). FOUR FACTORS INFLUENCING THE X-RAY EMISSION SPECTRUM • 1. The electrons accelerated from the cathode do not all have the peak kinetic energy. Depending upon the type of rectification and high voltage circuits, many electrons will have very low energy that produces low energy x-rays. FOUR FACTORS INFLUENCING THE X-RAY EMISSION SPECTRUM • 2. The target is relatively thick. Many of the bremsstrahlung x-ray emitted result from multiple interactions of the projectile electrons. • Each successive interaction results in less energy. FOUR FACTORS INFLUENCING THE X-RAY EMISSION SPECTRUM • 3. Low energy x-rays are more likely absorbed by the target. • 4. External filtration is always added to the tube assembly. This added filtration serves to selectively remove the lower energy photon. MINIMUM WAVELENGTH • As a photon wavelength increases, the photon energy decreases. Therefore the maximum x-ray energy is associated with the minimum x-ray wavelength. • Since the minimum wavelength of x-ray emissions corresponds to the maximum photon energy, the maximum photon energy is equal to the kVp. INTEGRATION • The total number of x-rays emitted from an x-ray tube could be determined by adding the number of x-rays emitted at each energy level over the entire spectrum. • This is referred to as integration. FACTORS AFFECTING THE SIZE AND RELATIVE POSITION OF THE X-RAY EMISSION SPECTRA. • Tube Current (mA) effects the amplitude • Tube Voltage effects the amplitude and position. • Added Filtration effects Amplitude most effective at low energies. FACTORS AFFECTING THE SIZE AND RELATIVE POSITION OF THE X-RAY EMISSION SPECTRA • Target material effects spectrum and position of the line spectrum. • Voltage waveform effects the amplitude, most effective at high energies INFLUENCE OF TUBE CURRENT • A change in mA or mAs results in a proportional change in the amplitude of the x- ray emission spectrum at all energies and the intensity of the output. INFLUENCE OF TUBE POTENTIAL • Unlike tube current, a change in kVp affects both the amplitude and the position of the x-ray emission spectrum. INFLUENCE OF TUBE POTENTIAL • When kVp increases the relative distribution of emitted photons shifts to the right or to higher energies. • 15% increase in kVp is equivalent to doubling the mAs. INFLUENCE OF ADDED FILTRATION • Adding filtration to an x-ray machine has an effect on the relative shape of the spectrum similar to that of increasing the kVp. INFLUENCE OF ADDED FILTRATION • Added filtration effectively absorbs more low energy x- ray than high energy x-rays, therefore the spectrum is reduced more to the left. X-RAY FILTRATION • Filtration of the x-ray beam has two components: • Inherent Filtration • Added Filtration • Filtration is required by law. • Aluminum is most common material. FILTRATION AFFECTS THE BEAM SPECTRUM • Filtration removes the lower energy photons that do not contribute to image production. • Added filtration results in an increased half value layer or higher quality beam. INFLUENCE OF ADDED FILTRATION • The overall result is an increase in the effective energy of the beam • The discrete and maximum energy of the x-ray spectrum is not effected. INFLUENCE OF TARGET MATERIAL • As the atomic number of the target material increases, the efficiency of the continuous spectrum x-rays increase. • The discrete spectrum also shifts to the right representing higher energy characteristic radiation. • Tungsten is used for general radiography. INFLUENCE OF TARGET MATERIAL • Some specialty tube use gold. • Molybdenum is used for mammography. It has a lower atomic number so the discrete spectrum is of a lower energy. This is ideal for soft tissue studies such as mammography. INFLUENCE OF VOLTAGE WAVEFORM • As the voltage across the x-ray tube increases for zero to its peak, the intensity and energy increase slowly at first and then rapidly as the peak voltage is obtained. INFLUENCE OF VOLTAGE WAVEFORM • The x-ray intensity is not proportional to the voltage. • The intensity is much higher at peak voltage than at lower voltages. TYPE OF X-RAY VOLTAGE • High frequency or three phase voltage waveforms will result in considerably more intense x-ray emission. TYPE OF X-RAY VOLTAGE • Operation on three phase equipment is equivalent to a 12% increase over single phase equipment. • High Frequency is a 16% increase. SINGLE-PHASE TO HIGH FREQUENCY • With the spectrum shifted to the right or higher intensity, the change in mAs for this conversion is to reduce mAs by 50%. • 30 mAs single phase = 15 mAs High Frequency or Three Phase. X-RAY EMISSION • The output intensity is measured in roentgens ( R) or milliroentgens (mR) and is termed the X-ray Quantity. • Radiation Exposure is often used instead of x-ray intensity or X-ray Quality. • The number of x-rays in the useful beam is the Radiation Quantity. ESTIMATING X-RAY INTENSITY • Using a nomogram, we can estimate the exposure output over a wide range of technical factors. • Important factors are: • Filtration • kVp ESTIMATING X-RAY INTENSITY • Exposure is expressed as mR/mAs. • With 3mm of Al filtration at 70 kVp the output is about 5 mR/mAs • At 100 mAs, the exposure would be 500 mR. FACTORS AFFECTING X-RAY QUANTITY • A number of factors affect X-ray Quantity. Theses same factors also control radiographic film density: • Milliamperage- Seconds • kVp • Distance • Filtration MA X TIME (S)= MAS • The X-ray quantity is directly proportional to the mAs. If we double the mAs, the number of electrons striking the target is doubled. • 300 mA @ 1/30 second = 10 mAs • 200 mA @ 1/20 second = 10 mAs • 100 mA @ 1/10 second = 10 mAs KILOVOLTAGE • X-ray quantity varies rapidly with changes in kVp. • The change in quantity is proportional to the square of the ratio of the change. • If the kVp is doubled, the intensity would increase by a factor of four. KILOVOLTAGE • What really happens when the kVp is increased? • When kVp is increased, the penetrability of the x- rays is increased and relatively fewer x-rays are absorbed in the patient. • More rays pass through the patients to interact with the film. KILOVOLTAGE • To maintain a constant exposure of the film, an increase of 15% in kVp should be accompanied by a reduction of one half the mAs. DISTANCE • Radiation intensity from an x-ray tube varies inversely with the square of the distance from the target. This is referred to as the inverse square law. • It is the same for any type of electromagnetic energy. FILTRATION • X-ray machines have metal filters inserted into the useful beam. • The primary purpose is the remove the low energy beam that reach the patient and are absorbed superficially. FILTRATION • These low energy photons contribute nothing to the formation of the radiographic image. • Filters therefore reduce patient exposure. FILTRATION • Calculation of the amount of exposure reduction requires a knowledge of the Half-Value Layer. X-RAY QUALITY • As the effective energy of the beam is increases, the penetrability is also increased. • Penetrability refers to the range of beam in matter; high energy beams are able to penetrate matter farther than low energy beams. • Beams with high penetrability are referred to as hard. X-RAY QUALITY • Beams of low quality are called soft beams. • X-ray quality is identified numerically by HVL. • The HVL is affected by the kVp of operation and the amount of filtration in the useful beam. • X-ray quality is influenced by the kVp and filtration. HALF-VALUE LAYER (HVL) • Half-value layer is the thickness of absorbing material needed to reduce the intensity to one half of its original value. • HVL is a characteristic of the x-ray beam. • A Diagnostic x-ray beam usually has an HVL of 3 to 5 mm Al. DETERMINING THE HVL • An exposure is made without filtration and the intensity is measured. • Different thickness of filtration is added and intensity is measured. • Results are graphed. DETERMINING THE HVL • From the graph, the HVL can be determined. • The established standard for filtration is 2.5 mm Al for tube operated above 70 kVp. HALF-VALUE LAYER • HVL is the best method for specifying x-ray quality. • Variations of kVp and filtration are not simple relationships. A tube with 2mm Al operated at 90 kVp may have the same HVL as when operated at 70 kVp with 5 mm AL. HALF-VALUE LAYER • The penetrability remains constant as does the HVL. FACTORS AFFECTING X-RAY QUALITY • Kilovoltage. As the kVp is increased, so is beam quality and therefore HVL. • An increase in kVp results in a shift of the x-ray emission spectrum towards the higher energy side. • This increases the effective energy of the beam, making it more penetrating. RELATIONSHIP BETWEEN KVP AND HVL WITH 2.5 MM AL • kVp • HVL ( mm AL) • 50 • 1.9 • 75 • 2.8 • 100 • 3.7 • 125 • 4.6 FACTORS AFFECTING X-RAY QUALITY • Filtration. The primary purpose of adding filtration to the x-ray beam is to remove low energy x-rays that have no chance of getting to the film. FACTORS AFFECTING X-RAY QUALITY • As filtration is increased, so is the beam quality, but quantity is decreased. TYPES OF FILTRATION • There are three types of filtration: • Inherent Filtration: Glass envelope window equals about 0.5mm Al • Added Filtration: Added in collimator • Compensating Filtration: Used to improve image quality or radiation reduction INHERENT FILTRATION • The glass envelope of the tube filters the emerging beam. In diagnostic x-ray tubes the glass is equal to about 0.5 mm Al. • As tube ages and more tungsten is vaporized, tungsten will build up on the inside of the tube that will add more filtration. ADDED FILTRATION • One or two mm of aluminum is added filtration placed in the collimator. This filtration is generally placed on the mirror of the collimator. • This filtration attenuates x-rays of all energies emitted from the tube. This shifts the spectrum to the high side. ADDED FILTRATION • This shift in the emission spectrum results in a beam with higher effective energy, greater penetrability and higher quality. • This results in an increased half value layer. • The minimum filtration for tube operated above 70 kVp is 2.5 mm Al equivalence. COMPENSATING FILTERS • Compensating filters are added to the beam by the operator to compensate for differences in subject tissue density or type. COMPENSATING FILTERS • In areas of the body where there are great differences in tissue density, compensating filters are used to reduce exposure in the area of less density. • This reduced patient exposure and improves image quality. The thoracic spine and full spine x-rays need filtration. COMPENSATING FILTERS • This is the 40” Cervicothoracic Compensating Filter. • It may be called the thyroid filter as it reduces the exposure to the upper thorax. COMPENSATING FILTERS • This heart shaped filter is used to reduce exposure to the ovaries of females of child bearing age. • It reduces exposure by about 85%.
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