test 2 - quizlet Flashcards
The total reduction in the x-ray number in the beam after penetrating tissues known as __________
attenuation
True or False: The atomic Z and the x-photons energy will dictate the process of Attenuation.
TRUE
There is no fixed number for x-rays being absorbed while passing through different medium
exponential attenuation
True or False: Half-Value layer (HVL) X-rays of any given energy( from 10 keV to 150 keV) are more penetrating in the material of low Z than in the material with high Z. In radiography the quality of the x-rays is measured by the HVL, thus the HVL is a characteristic of the useful x-ray beam
TRUE
why do we care about HVL?
We measure x-rays quality or penetrability by HVL. Why do we care? Because we care to increase the x-rays penetrability which will remove low energy x-photons and minimize patients’ exposure time and radiation dose as well as reduce undesirable scatter
True/False: HVL of added filtration reduces “pre-filtered” x-ray beam by 50% thus making it a better quality x-rays.
TRUE
what are 3 types of filtering that takes place in an x-ray?
1) The tungsten anode absorbs a large fraction of low energy photons
2) The Glass housing of the x-ray tube and dielectric oil that surrounds it filters off some more lower energy photons
3) Added filtration by placing metal of a different thickness such as aluminum 1-3 mm thick outside the tube in x-ray beam direction. Other metals can be used with equivalent rating of aluminum Al/Eq.
what is beam hardening?
filtering out the low energy photons to increase the average energy of the x ray
True/False: The tungsten anode and the glass housing of the x-ray tube are types of inherit filtration but the added filtration is a type of added filtration.
TRUE
what is a compensatory filter and what is it used for?
Occasionally we use compensating filters composed of aluminum if we need to expose the body parts with unequal thickness (e.g. foot, cervicothoracic spine etc). Adding filtration will provide for the evenness of the area exposure in order to obtain a good quality image
The ______ will attenuate the primary beam by 90% or the primary beam will be reduced to 1/10 of its original intensity
TVL (tenth value layer)
True or False: Because of the properties of the x-ray photons it is impossible to guarantee that a given material with high atomic number will reduce 100% of the primary beam.
TRUE
__________ of the material and not its thickness may adequately estimate (not guarantee) protective shielding quality
density
Scatter radiation produced by “Compton” causes “noise” reducing image contrast and contrast resolution are best cleaned up using ______
grids
what is HVL?
HVL -of an x-ray beam is the thickness of absorbing material necessary to reduce the x-ray intensity to half (50%) of its original value
scatter intensity depends on what three things?
1) kVp (Photon Energy)
2) beam field size (Collimation)
3) patient’s thickness (Mass density)
True/False:Excessive scatter especially with the LARGE angle of deflection will lower contrast, making the visible optical densities less distinct, especially the differences between the shades of the soft tissues.
TRUE
who invented the grid and when did they do it?
Gustav Bucky in 1913
Grid is designed by the series of _______strips and aluminum and plastic interspacing Thus only transmitting those x-rays whose direction is in the straight line from the source to the receptor.
lead
grid can remove as much as ___% of scatter, exhibiting a “good clean up”
80%
(High or Low) ratio grids are less permissive to scatter with larger angle than the (Higher or Lower) ratio grids.
High ratio grids are less permissive to scatter with larger angle than the lower ratio grids.
True or False: High ratio grids are more difficult to manufacture and usually a combination of reducing the width of the interspacing and increasing the height of the strips are used to achieve the desired grid ratio
TRUE
What is the Grid ratio(s) most commonly used in diagnostic imaging?
8:1 to 10:1 grid ratio is most frequently used in radiography
what is the grid ratio and how is it determined?
Grid Ratio R=h/D. Grid ratio is determined by the thickness of its strips, the width(D) of the interspace material and the grid’s height(h).
True/False: The higher the grid ratio, the higher the radiation exposure necessary to get a sufficient number of the x-rays to the image receptor
TRUE
Increase in grid ratio is associated with radiation dose (Increase or Decrease).
increase in grid ratio is associated with radiation dose increase
what is grid frequency
frequency of lead strips
True or False: An increase in grid frequency will demand an increase in the radiographic technique and patient dose. Most grids have 25-45 lines per centimetre.
TRUE
Paralleled grid. Lead strips are paralleled to the incoming x-rays. This grid is easy to manufacture however its major flaw is a production of the ______ ______-____artefact
grid cut-off artefact
______ an undesirable absorption of primary x-rays The attenuation of the primary x-ray becomes greater as they approach the edge of the grid
grid cut-off
Grid cut-off is _________ related to source image distance (SID) A decrease in SID will result with _________ in grid cut-off.
inversely
increase
what is the purpose of moving grids?
Grid lines that may be noted on the radiograph may represent a major nuisance. In 1920 Hollis E. Potter introduced a very simple idea. He moved the grid during exposure and solved the “lines issues”. Thus the device known today as Potter-Bucky system or simply Bucky
What is the main disadvantage to the Potter-Bucky system (moving grids)?
Main disadvantages of this system are the bulky mechanism which increases the object image distance (OID) and creates potential cassette motion resulting with blurring effect
what are the 5 typical problems with grids that radiologists face?
1) Misalignment
2) grid cut-off
3) Off-focused grid
4) Upside-down grid
5) bad installation with grid moving in the same direction as grid strips Stationary grids are predominantly used for portable radiography and upright horizontal views
How are grids related to the patient dose?
increasing grid ratio will increase the patient dose Using low grid ratio reduces contrast rendering the radiograph diagnostically difficult to interpret. High grid ratio will inevitably reduce (attenuate) the total number of useful x-rays, Thus creating more demand for higher exposure and an increase in the patient dose. Usually higher GR can be met by an increase in mAs
True or False: No grid vs. grid technique at 8:1 ration at 70 kVp will increase skin entrance dose from 80 mrad to 325 mrad: Despite apparent disadvantage the dose of radiation is still relatively small.
TRUE
what is the air gap technique
As an alternative to grid a clever technique of using the air-gap between the object and the image has been developed. The image receptor is at 15-20 cm distance from the patient. This allows the larger angle scattered x-rays to deflect or diverge away without reaching the film
While using the Air Gap technique, what usually needs to be increased for every centimeter of space you put between the object and the image?
Usually the mAs should be increased at approximately 10% for each cm of the air-gap
True or False: Using the Air Gap Technique No significant dose increase is noted since such an increase in mAs will be approx. the same as in 8:1 grid ratio.
TRUE
What radiographs are commonly used in the Air Gap Technique?
lateral cervical spine radiographs are commonly obtained using air-gap technique
Scatter radiation related to Compton effect will (increase or Decrease) contrast by film fogging.
decrease
______ or the difference between different shades of grey particularly of the soft tissues is important in image quality.
contrast
Under 90 kVp grids ratio are at approx.____
8:1
True/False: Air- gap technique is used successfully without the grid to reduce scatter radiation and maintain high contrast resolution
TRUE
