RADPRO MODUE 7 and 8 Flashcards
The x-ray beam is characterized by
quantity and quality
The intensity of the x-ray beam of an x-ray imaging system is measured in
milligray in air/mGya
miliroentgen
qunaitiy
Another term for x-ray intensity
radiation exposure
is a measure of the number of ion pairs produced in air by a quantity of x-rays.
mGya (mR)
Ionization of __ increases as the number of x-rays in the beam increases.
air
Most general-purpose radiographic tubes, when operated at approximately ___-
70 kvp
effect of mas in quantity
increased
effect of kvp to qunatitiy
The change in x-ray quantity is proportional to the square of the ratio of the kVp;
note
that by increasing kVp and reducing mAs so that image receptor exposure remains constant, the patient dose is reduced significantly.
note
There is no change in contrast when using digital image receptors.
relationship between quantity and distance
X-ray intensity varies inversely with the square of the distance from the x-ray tube target. This relationship is known as the inverse square law.
note
When SID is increased, mAs must be increased by SID² to maintain constant exposure to the image receptor.
The purpose of these is to reduce the number of low-energy x-rays.
filtration
Disadvantage of x-ray beam filtration:
Reduced image contrast when using screen film caused by x-ray beam hardening. X-ray beam hardening increases the number of high energy x-rays in the beam by removing the lower-energy non-penetrating x-rays.
As the energy of an x-ray beam is increased, the penetrability is
increased
one description of the ability of an x-ray beam to pass through tissue.
Penetrability
___ and ___do not affect radiation quality; they do affect radiation quantity.
mas and distance
note
Factors that affect x-ray beam quality also influence radiographic contrast when screen film is the image receptor.
Whereas 100-keV x-rays are attenuated at the rate of approximately
3%/cm of soft tissue
10-keV x-rays are attenuated at approximately
15%/cm of soft tissue
In radiography, the quality of x-rays is measured by
HVL
In radiography, the quality of x-rays is measured by
3 to 5 mm Al or 3 to 6 cm of soft tissue.
NOTE
To improve image contrast, it is also desirable to remove x-rays with energies above a certain level. Unfortunately, such removal of regions of an x-ray beam is not normally possible.
NOTE
As filtration is increased, so is beam quality, but quantity is decreased
the glass or metal enclosure of an x-ray tube filters the emitted x-ray beam
INHERETN FILTER
The inherent filtration of a general purpose x-ray tube is approximately
0.5 mm Al equivalent.
thin sheet of Al positioned between the protective x-ray tube housing and the x-ray beam collimator is the usual form of added filtration.
ADDED FILTRATION
2 Sources of Added Filtration:
1.1-mm or more sheets of Al
Collimator contributes an additional 1 mm Al
compensates for differences in subject radiopacity.
compensating filters
X-ray interact with matter in the following five ways
coherent scattering, (2) Compton scattering, (3) photoelectric effect, (4) pair production, and (5) photodisintegration.
on __ and __ are important in making an x-ray image.
compton and pohotelectric
low-energy x-rays tend to interact with whole atoms, which have diameters of approximately
10−9 to 10−10 m
moderate energy x-rays generally interact
electron
high-energy x-rays generally interact with
nuclei
low-energy x-rays tend to interact with
whole atoms
X-rays with energies below approximately __keV interact with matter by coherent scattering,
10
the incident x-ray interacts with a target atom, causing it to become excited
Coherent Scattering
measurement of wavelength in coherent scattering
equal to that of the incident x-ray (λ = λ′) and therefore of equal energy
Note
in cohenrent direction of the scattered x-ray is different from that of the incident x-ray.
There is no energy transfer and therefore no ionization.
Coherent Scattering
Most coherently scattered x-rays are scattered in the
forward direction
X-rays throughout the diagnostic range can undergo an interaction with outer-shell electrons
Compton Scattering
the ejected electron in compton scattering is called
compton electron
what happen to the x-ray ejected in compton
The x-ray continues in a different direction with less energy
X-rays in the diagnostic range also undergo ionizing interactions with inner-shell electrons.
Photoelectric Effect
The x-ray is not scattered, but it is totally absorbed
Photoelectric Effect
photoelectric effect and earned Albert Einstein the __ Nobel Prize in physics in
1921
kinetic energy equal to the difference between the energy of the incident x-ray and the binding energy of the electron.
Photoelectric Effect
NOTE
For higher atomic number target atoms, electron binding energies are higher (37 keV for barium K-shell electrons). Therefore, the kinetic energy of the photoelectron from barium is proportionately lower.
NOTE
For low atomic number atoms, such as those found in soft tissue, the binding energy of even K-shell electrons is low (e.g., 0.3 keV for carbon). Therefore, the photoelectron is released with kinetic energy nearly equal to the energy of the incident x-ray.
RELATIONSUIP OF PHOTOEELCTRIC EFFECT TO ENERGY
The probability of the photoelectric effect is inversely proportional to the third power of the x-ray energy (1/E)³
The interaction between the x-ray and the nuclear field causes the x-ray to disappear, and in its place, two electrons appear, one positively charged (positron) and one negatively charged.
Pair Production
The positron unites with a free electron, and the mass of both particles is converted to energy in a process called
annihilation radiation.
pair production involves only x-rays with energies greater than
1.02 MEV
X-rays with energy above approximately ___ can escape interaction with electrons and the nuclear field and be absorbed directly by the nucleus.
10 MeV
the nucleus is raised to an excited state and instantly emits a nucleon or other nuclear fragment.
photodisintegration
contributes no useful information
Compton-scattered x-rays
provide diagnostic information. (radiopaque)
Photoelectric x-rays
produce dark areas in the radiograph. (radiolucent)
Transmitted x-rays
The total reduction in the number of x-rays remaining in an x-ray beam after penetration through a given thickness of tissue is called
attenuation.
the product of absorption and scattering.
attenuation.
an all-or none condition for x-ray interaction.
Absorption
An interaction such as the photoelectric effect is called an
Absorption process
