Radiographic quality depends on what three factors?
1. Radiogrphic Density 2. Radiographic Contrast 3. Radiographic Geometric Detail
To produce a certain number of x-rays (mAs) you can either increase __ and decrease ___, or decrease __ and increase ___. Which way is better?
-mA -seconds -since we always want the shortest exposure time possible when we want to increase mAs we do it by increasing mA rather than seconds, if possible
What is radiographic density?
the degree of the blackness of the radiograph
What does a radiograph with the proper amount of density look like?
-it provides visualization of all the structures of interest and not be too dark or too light
The more x-rays that strike the film the ____ the radiograph, therefore _____ is dependent on the number of x-rays reaching the film
What are the factors that affect radiographic density? (5)
Exposure Settings: 1. mAs 2. kVp Developing: 3. Time and Temperature 4. Subject thickness and density 5. SID (source image distance)
How does mAs factor into radiographic density?
-As mAs is increased, intensity of the primary beam increases and more x-rays reach the subjectwhich means more will emerge from the subject and strike the film making it blacker and therefore increasing the density
How does kVp factor into radiographic density?
-As the kVp is increased the penetrating power of the x-rays increases so more go through the subfect and strike the film making it blacker and therefore increasing the density
How does time and temperature of development factor into radiographic density?
-As either/both of these is increased, unexposed silver halide crystals are turned into metallic silver (when they shouldn't be) which results in a darker radiograph = more density
How does subject thickness and density factor into radiographic density
The thicker and denser the subject (e.g bone v.s. soft tissue), the more x-rays it will absorb/ block from reaching the film resulting in a whiter/greyer radiograph or decreased density
How does SID factor into radiographic density?
-As the distance of the x-ray machine to the film increases, the number of x-rays striking the film decreases, resulting in decreased density
What is radiographc contrast?
-the visible difference in density between two adjacent areas on a radiograph
What does it mean for a radiograph to have high contrast? Low contrast?
-High contrast means big differences in density within the radiograph (lots of black and white) -Low contrast means little difference in density within the radiograph (lots of shades of grey)
What kind of contrast (range of densities) does a good quality radiograph have? Why?
-a range of densities (black, white and shades of grey) -so that all the anatomical details are easily distinguished
What is the most common cause of poor contrast?
-inappropriate exposure factors (mAs and kVp)
How does mAs factor into radiographic contrast?
-Only affect contrast if its significatnly too high or low -Significantly too low mAs means a big decrease in density resulting in the entire radiograph being too pale which lowers contrast -Significantly too high mAs means a big increase in density resulting in the radiograph being too dark although the contrast isn't altered much
What affects contrast more way too high mAs or way too low?
Way too high mAs, too low mAs doesn't alter contrast much
How does kVp factor into radiographic contrast?
-too little kVp results in low energy x-rays with little penetrating power making the exposed parts of the film dark and the patient structures all white -too much kVp results in high energy x-rays with lots of penetrating power resulting in a radiograph thats dark with all the patients structures being dark with low contrast
T or F kVp affects both contrast and density
What is the most important exposure factor controlling contrast? WHy?
-kVp -because it affects x-ray penetration of the structures which should be varied to give a detailed image -it also controls scatter radiation production which exposes inappropriate areas of the film
What is the primary function of mAs?
to produce the desired film density (blackness)
What is the primary function of kVp?
to produce the desired scale of contrast within the recorded image (detailed structures)
Scatter radiation is also known as ..
What is the effect of scatter radiation on contrast? How does this happen?
-It decreases contrast by exposing inappropriate areas of the film -Too high kVp increases scatter radiation
The larger the subject being x-rayed the ____ the kVp that must be used and the ____ amount of scatter radiation which makes it dificult to do what?
-higher -greater -to obtain a radiograph with the desired scale of contrast
In order to radiograph thick body parts without producing excessive scatter we do what five things?
1. Use aluminum filters in the tube window and lead backed cassettes 2. Collimate 3. Compress the subject 4. Use the lowest kVp possible 5. Use grids
What colors would predominate in a radiograph with low contrast?
shades of grey
What is a grid?
a device placed between the patient and film designed to absorb scatter radiation
When should grids be used?
On any body part that measures greater then 10 cm (that will require a higher kVp setting)
What is a grid made of?
-alternating strips o f lead (0.5 mm thick) and spacer material in a protective case
Why are the lead strips in a grid aligned a certain way?
so that the majority of the primary beam can pass through while x-rays not travelling in the beam direction (scatter) are absorbed
Grids have been designed to be used in certain locations (3)
1.On top of the cassette 2. Built into the cassette 3. Built into the x-ray table (Bucky)
In what six ways do grids vary?
1. Size 2. Number of lead strips per cm 3. Grid ratio 4. Grid focus 5. Pattern 6. Stationary vs. moving (bucky)
What is grid ratio? How does it affect the grids absorption of scatter?
-the relationship of the height of the lead strips to te distance between them -the higher the ratio the more scatter radiation absorbed
In order to be used effectively grids must be _______ correctly
What two positioning factors affect grid effectiveness?
1. Focal point: the center point where the beam must be centered 2. Grid Focus: the distance of the grid from the x-ray tube
What is grid cutoff due to?
A misalignment of the grid lines and the primary beam, resulting in too much of the primary beam being absorbed by the grid
What does a radiograph with grid cutoff look like?
it appears as a light underexposed film with distinct white lines
What are four common causes of grid cutoff?
1. Improper centering of the primary beam and grid (Improper Focal Point) 2. Decreased distance between the anode and the grid (Improper Grid Focus) 3. Tilting the tube or grid laterally 4. Focused grid upside down
How do grids factor into the exposure settings (mAs and kVp)? What changes are made?
-Because some of the primary beam is absorbed by the grid, the exposure factors need to be increased. -Usually mAs is increased by doubling the exposure time - kVp is also increased by an amount that depends on the grid ratio
What are the four types of grid patterns?
1. Linear 2. Crossed 3. Parallel unfocused 4. Focused
What type of grid pattern do most table-type x-ray machines have? What is the advantage to this pattern?
-Linear -can angle tube along length of the grid for oblique views
Which grid pattern has the maximum scatter absorption? What is the disadvantage to this pattern?
-Crossed -Cannot use for oblique views
Which kind of grid pattern allows for diverging x-rays to pass through, has a very specific focus and is expensive?
-Focused (progressively angled lead strips)
Which grid pattern can only be used with small fields or cutoff occurs?
What are stationary grids?
Grids which doesn't move during exposure
T or F with stationary grids even with proper use, grid lines will be visible
What are moving grids? How do they produce no grid lines? Another name?
-a linear grid placed so the lead strips run parallel to the legnth of the table -the grid moves back and forth during exposure so grid lines are blurred and cannot be seen on the radiograph -aka Bucky (refers to the tray where the grid is held under the table)
What are the disadvantages to bucky grids? (2)
1. can break down 2. cannot be used with portable equipment
How do you change the mA settings when you need to use a grid? the kVp?
-double the exposure time -increase by about 10
What is radiographic detail?
-image sharpness and clarity, how clear the edge of anatomical structures appear
With underpenetration, the outlines of the ____ are not ______ and the silhouette appears all _____. What should be changed?
-structures -visible -white -the kVp should be raised 10-15%
When the film is too light but there is still adequate penetration what can still be seen? What should be changed?
-The outlines of the structures can still be seen even though they are all too light -the mAs should be raised by 30-50%
When the film is too dark but still has adequate penetration the ____ ____ still looks white. What should be changed?
-bone tissue -lower the mAs 30-50%
When the film is too dark but is overpenetrated the ____ tissue looks _____. What should be changed?
-bone tissue -grey -lower kVp 10-15%
What should you consider when assessing a radiograph for regarding detail? (2)
1. Check for image clarity (is it blurred?) 2. Check for geometric distortion
What is the most important source of film fog? How is it identified?
-Scatter -grey areas on the unexposed area of a processed radiograph
What are five sources of film fog other than scatter?
1. Exposure to x-rays (other than scatter) 2. Chemical Fog (fumes) 3. Exposure to light 4. Storage Fog (too hot or humid) 5. Age Fog (expired)
What are artifacts on a radiograph? Why are they undesirable?
-anything that should not be on the radiograph -they can make diagnosis impossible or lead to a misdiagnosis
When artifacts occur consistently what needs to be checked?
-cassettes and screens need to be checked for cleanliness and damage
What are nine commonly occurring artifacts? What does each look like?
1. Static electricity: a black tree 2. Pressure on the film: black areas 3. Splashed drops of liquid: a white area 4. Contrast media: white blotches 5. Scratches: white lines 6. Portion of the Film is unexposed: white area 7. Stain: any discolouration 8. Failure to prep subject: e.g. collars 9. Crinkle marks: curved black or white lines
How can stains on a radiograph be avoided?
-using fresh chemicals and developing according to a standardized time-temperature table
What causes a portion of the film to be unexposed (artifact)?
the beam not being perpendicular to the subject or the cassette not being centred under the subject/beam
What is the rule of thumb for getting better tisue differentiation?
-decreasing the kV by 5 and doubling the mAs
Why is tight collimation important?(2)
1. reduces secondary radiation (scatter) 2. results in better radiographs
T or F An increase in the temperature of the processing solutions will result in a darker radiograph.
The source image distance (SID) is the distance from the x-ray machine to...
Grids should be used when any body part measures greater than:
Radiographic _______is how clear the edge of the anatomical structures appear.
T or F You note that your radiograph once developed is too dark due to over penetration, you would compensate by decreasing the kVp by 10 - 15%.
If the cassette is not centered under the subject or the primary beam, how will the film look?