X-ray Production. Target Interactions. Principles of Imaging Science I (RAD119) X-ray Production & Emission

Transcription

1 Principles of Imaging Science I (RAD119) X-ray Production & Emission X-ray Production X-rays are produced inside the x-ray tube when high energy projectile electrons from the filament interact with the atoms of the anode Conditions necessary: Source of electrons Target (anode) High potential difference Sudden deceleration of projectile electrons Target Interactions All occur within 0.25 to 0.5 mm of target surface Heat production Bremsstrahlung interactions Braking or slowed-down Characteristic interaction Target material 1

2 Heat Production 99.8 percent of incident electrons kinetic energy converted to heat Incident electrons transfer kinetic energy to outer shell electrons of target atoms Causes them to emit infrared radiation Heat (heat units = joules) X-ray Production Most of the kinetic energy of projectile electrons is converted to heat by interactions with outer-shell electrons of target atoms. These interactions are primarily excitations rather than ionizations Bremsstrahlung Radiation Projectile electron enters an atom in the metal of the anode and does not strike any of the electrons It may continue toward the center of the atom and come near the nucleus due to the electrostatic attraction. This attraction slows the electron down as it passes the nucleus and alters the direction of the projectile electron. 2

3 Bremsstrahlung As incident electrons get closer to nucleus, the following occurs: Photon energy increases Photon energy dependent on how close electron comes to nucleus Due to larger deflection of incident electron Bremsstrahlung Direct interaction between nucleus and incident electron Possible, but not probable Maximum energy photon Bremsstrahlung Radiation 3

4 Video Characteristic Radiation Characteristic of the target element Projectile electron strikes an atom and knocks a K shell electron out of its orbit. This leaves the atom in an unstable state Tube potential must be at least 70 kvp to eject a tungsten K shell electron K shell binding energy is 69 kev Only electron that drops into K-shell will contribute to beam Characteristic Radiation An electron from a higher orbit moves down to the hole Called a K characteristic x-ray X-ray photons are produced when the electron changes orbital shells. Cascade effect 4

5 Characteristic Radiation Characteristic x-rays are produced after the ionization of a K-shell electron. When an outershell electron fills the vacancy in the K shell, an x- ray is emitted. The energy is calculated by the difference between the electron orbits Video Characteristic Radiation Calculate the the energy of a K characteristic photon with the transition from the L shell. M shell? Calculate the energy of an L characteristic photon with the transition from the M shell. N shell? Graphed as discrete spectrum (measurable) 5

6 X-ray Production (Mostly Brems) First, only a high-energy projectile electron has enough energy to knock a K-shell electron out of its orbit to produce a characteristic x-ray. At kvp settings lower than 70, all brems.. Second, the projectile electron is more likely to miss the K-shell electron of the target atom than it is to hit it due to atom s open space Off-Focus Radiation Effect on Image Emission Spectrum General form of an x-ray emission spectrum. Characteristic radiation Bremsstrahlung radiation 6

7 Emission Spectrum Graphical representation of characteristic (discrete) and bremsstrahlung radiation (continuous) Y axis = x-ray quantity Height of the curve or bar graph Change in amplitude = change in quantity X axis = x-ray quality (kev) Shown on horizontal axis Change in position horizontal axis = change in quality Emission Spectrum: Brems Radiation Tube potential based on manufacturer specs kvp range < Graphed as continuous spectrum (wide range of energies) Selected kvp will determine maximum kev possible for any photon Minimum kev could be just above zero Factors Affecting the Emission Spectrum Milliamperage (ma) Quantity, number of photons Amplitude of continuous and discrete spectra are affected No change in position Milliamperage-seconds (mas) ma X time Change in ma results in a proportionate change in the amplitude of the x- ray emission spectrum at all energies. 7

8 Factors Affecting the Emission Spectrum Kilovoltage (kvp) Quality, penetrability Amplitude and position of continuous spectrum are affected Amplitude of discrete spectrum is affected Change in kvp results in an increase in the amplitude of the emission spectrum at all energies, but a greater increase at high energies than at low energies. Therefore, the spectrum is shifted to the right or highenergy side. Emission Spectrum Factors Affecting the Emission Spectrum Anode atomic number Slight change in Amplitude of continuous spectrum Amplitude and position of discrete spectrum is affected Discrete emission spectrum shifts to the right with an increase in the atomic number of the target material. The continuous spectrum increases slightly in amplitude, particularly to the high-energy side, with an increase in target atomic number. 8

9 Factors Affecting the Emission Spectrum Voltage Waveform Amplitude and position of continuous spectrum is affected Amplitude of discrete spectrum is affected Three-phase and high-frequency operation are considerably more efficient than single-phase operation. Both the x- ray intensity (area under the curve) and the effective energy (relative shift to the right) are increased. Shown are representative spectra for 92-kVp operations. Filtration Process of eliminating undesirable low-energy x-ray photons by insertion of absorbing materials into primary beam Allows radiographer to shape emission spectrum Factors Affecting the Emission Spectrum Filtration Inherent Window of x-ray tube O.5 mm Al equivalent Added Aluminum added between tube housing and collimator 1.0 mm Al equivalent Total Filtration = Inherent + Added 2.5 mm Al equivalent 9

10 Filtration Filtration Hardening of beam Removes low energy soft photons Increases average beam energy Soft tissue penetration requires approximately kiloelectronvolt (kev) photons Filtration Low energy photons cannot penetrate the part Only contribute to patient dose 10

11 Factors Affecting the Emission Spectrum Purpose of added filtration is to remove low energy, long wavelength photons Amplitude and position of continuous spectrum is affected Amplitude of discrete spectrum is affected Adding filtration to an x-ray tube results in reduced x-ray intensity but increased effective energy. The emission spectra represented here resulted from operation at the same ma and kvp but with different filtration. Measurement Aluminum Standard filtering material Filtration expressed as Al/Eq Half-value layer (HVL) Filtration needed to reduce beam to one half of its original intensity Types of Filtration Inherent filtration Added filtration Compound filtration Compensating filtration Total filtration 11

12 Filtration Types Inherent 0.5 mm Al equivalent X-ray tube design Added 1.0 mm Al equivalent Any filtration outside x-ray tube and housing Silver on collimator mirror Thin layers of aluminum or copper permanently added between the collimator and protective housing Filters may be changed Glass or metal envelope Dielectric oil bath Glass window of housing Inherent Filtration Inherent Filtration Tube aging increases inherent filtration Vaporized tungsten coats tube window HVL testing important 12

13 Total Filtration = Inherent + Added Filtration Does not take into account any compound or compensating filtration Effect on Tube Output Ideally, filtration would only remove lowenergy photons Some high energy photons are removed Results in decrease in radiographic density that must be compensated for with increase in technique Compound Filtration K-edge filters Two or more materials Each layer absorbs characteristic photons created in previous layer 13

14 Compensation Filtration Evens radiographic density with parts that have uneven tissue thickness or densities E.g., wedge for foot or T-spine, trough for CXR Compensating Filters Compensation Filtration Applications 14