In service XR Flashcards
Threshold dose for early transient erythema
2 Gy
When does a 180 degree backscattered photon occur?
When the incident photon transfers the max amount of energy to the emitted electron. (This is a Compton interaction)
all the incident photon energy is transferred to an electron, which is ejected from an atom
photoelectric effect
The probability of this effect is maximum when:
- the energy of the incident photon is equal to or just greater than the binding energy of the electron in its shell (absorption or k edge)
- and the electron is tightly bound (as in K shell).
The electron that is removed is then called a photoelectron and the incident photon is completely absorbed in the process. Hence, the photoelectric effect contributes to the attenuation of the x-ray beam as it passes through matter.
What is the photoelectric effect?
all the incident photon energy is transferred to an electron, which is ejected from an atom
What is the maximum leakage radiation allowed at 1 meter from the xray tube source when the system is operated at the maximum continuous allowable tube current and kilovoltage?
100 mR/hr NCRP report no. 91, the allowable max limit for leakage radiation is 100 mR/hr at 1 meter when at max continuous kVP and mA
Rayleigh (Coherent) scattering
Coherent scattering (also known as unmodified, classical or elastic scattering) is one of three forms of photon interaction which occurs when the energy of the x-ray or gamma photon is small in relation to the ionization energy of the atom. It, therefore, occurs with low energy radiation.
Upon interacting with the attenuating medium, the photon does not have enough energy to liberate the electron from its bound state (i.e. the photon energy is well below the binding energy of the electron) so no energy transfer occurs. There is no energy deposition and thus no dose resulting from coherent scattering. The only change is a change of direction (scatter) of the photon, hence ‘unmodified’ scatter. Coherent scattering is not a major interaction process encountered in radiography at the energies normally used.
What is Rayleigh scattering?
“coherent scattering” an xray photon of the same energy and slightly different direction is emitted
Why do screen film detectors require a low kV?
a fixed film contrast and typically require low kV to ensure adequate subject contrast that is mapped to radiographic contrast by the film characteristic curve
What is Bremsstrahlung radiation?
Bremsstrahlung radiationis electromagnetic radiation emitted from the slowing down of electrons due to interactions between the negative charge of the electron and the positive charge of the nucleus.
The energy of the bremsstrahlung photon is equal to the energy lost by the electron after it slowed compared to its initial energy.
Threshold dose for skin necrosis
>16Gy
Digital detectors are often referred to as “_____ limited” while screen film detectors are often referred to as “_______ limited”
Digital detectors are often referred to as “ SNR limited” while screen film detectors are often referred to as “contrast limited”. Image contrast with a digital system is freely adjustable as long as there is sufficient signal to noise ratio of the statistical information in the image.
Permanent epilation occurs at what dose?
7-10 Gy
What is the maximum leakage radiation allowed outside the x-ray room from the xray tube source when the system is operated at the maximum continuous allowable tube current and kilovoltage?
2 mR/week
When do effects of early transient erythema begin?
at 2 Gy, within hours after irradiation and peak at about 24 hrs
an electron is emitted and the incident photon is scattered at some angle with reduced energy.
Compton interaction
In digital radiography, what determines spatial resolution?
detector element area and sampling pitch (distance between detector elements)
In which Xray interaction does back scattering occur?
Compton interaction. For a compton interaction, an electron is emitted and the incident photon is scattered at some angle with reduced energy.
an xray photon of the same energy and slightly different direction is emitted
Rayleigh scattering
Threshold dose for dry desquamation
14
>16 Gy skin entrance dose will result in what skin injury?
skin necrosis
6 Gy skin entrance dose will result in what skin injury?
Main erythema
14 Gy skin entrance dose will result in what skin injury?
dry desquamation
Increased dynamic range in digital mammography as compared to screen film mammography results in:
increased contrast resolution. different exposure levels mapping to a wider range allows for increased contrast in a digital image
It is the main cause of scattered radiation in a material. It occurs due to the interaction of the photon (x-ray or gamma) with free electrons (unattached to atoms) or loosely bound valence shell (outer shell) electrons.
The resultant incident photon is scattered (changes direction) and imparts energy to the electron (recoil electron). The scattered photon will have a different wavelength (observed phenomenon) and thus a different energy (E=hc/λ). Energy and momentum are conserved in this process.
The Compton effect is a partial absorption process and as the original photon has lost energy, known as Compton shift (i.e. a shift of wavelength/frequency).
The wavelength change of the scattered photon can be determined by 0.024 (1- cos θ), where θ is scattered photon angle. Thus, the energy of the scattered photon decreases with increasing scattered photon angle.
Threshold dose for main erythema
6