X-ray Tube and Factors controlling production Flashcards
Review of key ideas
-Atomic number (Z) = # of protons –> # of electrons
–> Increase Z = increase Eb
–> Increase distance from nucleus = decrease Eb
-Radiation = transmission of energy through space +
matter
–> x-rays, gamma-rays = electromagnetic radiation
–> wave-particle duality is the inverse relationship between energy and wavelength
–> Ionizing Vs non-ionizing radiation -differ in ability to eject electron from atom
-X-rays produced when there is:
1) Deceleration of high speed electrons by a metal target
2) Electron cascade due to removal of orbital electron from metal target
Bremsstrahlung
-“braking radiation”
-Primary source of photons
-energy of brems photon dependent on:
–> Z^2 of target
–> distance of incident electron from nucleus
–> speed of incident electrons
-Continuous spectrum of energies
Characteristic
-“fingerprint” of target
-Energy of photon dependent on:
–> Location of orbital shell vacancy
–> Z of target which influences electron binding energy
-Specific amount of energy
Three conditions must exist for x-ray production:
1) Source of free electron –> Cathode
2) High voltage to accelerate e- –> voltage
3) Metal target capable of stopping e- –> Anode
Dental intraoral X-ray machine parts:
-Control panel –> regulates x-ray production
-Tube head –> contains x-ray tube which produces x-rays
-Extension arm or bracket –> enables positioning of tube head
-Wall bracket –> Contains power supply for tube head
Tube head of x-ray machine
-Contains: x-ray tube (cathode, anode, glass tube)
-Metal housing to protect Dr. from stray radiation
-insulating oil
-Unleaded glass window of x-ray tube
-lead collimator (concentrates the beam to be narrowed)
-Position indicating device (PID)
-Aluminum disks
-tube head seal
-filament circuit
X-ray Tube
-Composed of cathode + Anode within evacuated glass envelope/ tube
-Source of electrons (cathode) and x-ray photons
X-Ray Tube: CATHODE
-Cathode filament: Produce electrons. Tungsten, W74, filament. Thermionic emission = “boiling off electrons”
-Focusing cup: Focuses electron. Molybdenum, Mo42. Parabolic shape condenses electron cloud into beam
X-Ray Tube: ANODE
-Metal target insert embedded in copper that dissipates heat
-Converts KE of e- to x-ray photons **
-99% KE converted to heat; 1% KE converted to x-rays mostly by bremsstrahlung interaction
Anode: Tungsten Target
-type of metal used dependent on x-ray energy spectrum needed
–> Dental diagnostic = Tungsten (74W) target
Why? 1) high atomic number (super efficient)
2) high melting point (3422 degree C) take on high temperature
3) High thermal conductivity (turns KE to heat and x-rays)
4) Low vapor pressure
Anode: Focal Spot
-Focal spot = area on target to which focusing cup directs electrons and from which x-rays are produced
-INFLUENCES image quality and heat generating
-Anode angle = Angle of target surface with respect to central ray of x-ray beam produced. Typically, 20 degrees to create narrowed central x-ray beam (effective focal spot size) relative to area of electron impact on target (actual focal spot size)
Anode designs
1) Stationary (dental intraoral units) –> Tungsten target embedded in copper block. Heat absorbed by copper
2) Rotating (medical units, some panoramic and CBCT units) –> Beveled tungsten disc mounted on rotor assembly to allow rotation during x-ray production. Distributes heat generated
X-Ray Tube: Evacuated Glass Chamber
-Removes obstructions to e-
-Allows maximum KE of electrons traveling cathode to anode
-prevents oxidation of filament
Power Supply: functions of x-ray unit power supply
1) Provide low-voltage current (10V) to heat cathode filament
2) Generate high potential difference (50-120 kV) between anode and cathode
-Filament transformer heats cathode filament and high voltage transformer accelerates cathode electrons
Filament transformer = Low voltage transformer
-Reduces voltage of incoming AC current in filament circuit in order to heat filament
-thermionic emission is directly dependent to incoming voltage
-Rate of e- production is controlled by filament current
-hotter filament = more electrons per unit time = more x-rays per unit time
High voltage transformer
-produce potential difference between anode and cathode
-Boosts voltage of incoming line current (volts to kilovolts)
-accelerates e- by generating high potential difference
-set by kilovoltage peak (kVp) setting
Electron Volt (eV)
Definition: amount of energy acquired by one electron as it traverses an electrical potential difference of 1 volt in a vacuum
-unit of ENERGY
–> kVp setting determines maximum energy of photon in 1:1 ration –> kvp= keV
-dental units: 50-120 kVp. Maximum energy of x-ray photons = 50-120 kev
Electrical current and X-ray generation
-Alternating polarity and intensity of incoming AC produces “bursts” of radiation
-referred to as half-wave rectified or self-rectified generators
Practical implication: -x-ray production during 1/2 cycle = longer exposure time required = higher patient dose
-wide energy spectrum = higher patient dose because low energy photons absorbed by patient
Constant potential generators (most dental x-rays now)
-DC (direct current) facilitates steady production of x-rays of consistent intensity for the majority of each cycle
practical implications: x-ray production during entire cycle = shorter exposure time
-Consistent and reliable x-ray intensity
-higher mean energy (bc able to maintain anode voltage) at lower kVp because fewer low energy photons produced
-Narrow energy spectrum (able to bring anode voltage up) = lower patient dose because fewer low energy photons produced
Timer
-Controls duration x-rays are produced
-two timing applications:
1) Heat cathode filament (~0.5 sec)
2) Create potential difference (variable)
Two calibrations of time:
a) seconds
b) pulses
time = number of pulses. 60 Hz –> 60 cycles/sec
