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Imaging Lecture 2


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copy deck
What are 3 functions of the radiographic cassette?
1. provide close, tight contact between film & intensifying screen
2. provide a light proof container for film
3. protect screen & film from physical damage
Describe the construction of a radiographic cassette
Plastic or aluminum front (doesn't block xrays)w/ lead patch for the photoimprinter label, metallic back lined w/ lead to prevent scatter, front & back hinged together.
Are radiographic cassettes very sturdy?
No, they are easily damaged if dropped or stepped on.
Where are the intensifying screens? Purpose?
They line the front and back of the cassette. They emit light when exposed to radiation and thus decrease the amount of radiation needed to expose the film.
What are intensifying screen made of?
Plastic sheet coated w/ phosphor material (crystals that emit light)suspended in a binding material.
What determines a screen speed?
The size of the phosphor crystal.
High speed screens - made of what and what quality of image produced?
Made w/ lg crystals that produce more light thus less radiation needed.
Produces an image w/ poor resolution and more graininess
Slow speed screens - made of what and what quality of image is produced?
Made of small crystals that produce less light, thus more radiation is needed. Produces a better quality image w/ better resolution and less graniness
What are 3 different types of intensifying screens?
Rare earth, Hi-plus, Ultra detail
Describe a rare earth screen - speed, crystals, features
High speed screen w/ large crystals made of rare earth elements. Cost is high from separating the elements from their source. These are an attempt to combine high speed w/ good resolution.
What is Quantum Mottle? What causes it?
A mottling that can be seen on the film when using rare earth screen and vy low exposures. This is due to the extreme sensitivity of the rare earth phosphors.
What type of screen is used in most clinics?
Hi-Plus screens
Describe a Hi-Plus screen
Medium speed and medium crystals made of calcium tungstate. Avg. resolution - used for general x-rays
Describe an Ultra Detail screen
Slow speed w/ small crystals. Excellent resolution - often used in orthopedics.
Describe screen care
1. Clean at least monthly - keep log of when cleaned
2. Use commercial screen cleaner or mild soap & water
3. Check screen adhesion to cassette
Describe the construction of film
Silver halide crystals suspended in gelatin on both sides of a plastic acetate base that is covered w/ a protective coating
What is a latent image?
Imagine on film before processing
How is a latent image produced?
X-rays hit screen crystals which produce light. The light from the screen causes a change in the silver halide on the film which produces the image.
How is a latent image made visible?
Name two types of film
Screen & non screen
Which type of film is more sensitive to visible light?
Screen film
Which type of film is more sensitive to x-ray radiation?
Non-screen film
Which type of film produces higher detail?
Which type of film is common for dental and orthopedic use?
Name three film speeds
Fast, medium, and slow
Fast speed film is also called what?
Describe fast speed film - crystals, exposure, and detail produced.
lg crystals, needs less exposure and produces an image w/ less detail
Describe medium speed film - crystals, exposure needed, and detail produced.
Medium crystals, needs medium exposure, produces and image w/ moderate detail
Other names for med speed film?
Par film & standard film
What is the most common speed of film?
Slow speed film is also called what?
High Detail Film
Describe slow speed film - crystals, exposure, and detail
Smaller crystals need more exposure but produce greater detail
4 important things re: film care
1. Store vertically
2. Keep in cool are a w/ low humidity
3. Avoid chemicals and vapors
4. Check expiration date
What are grids made of?
Thin lead strips and spacers
What do grids do?
Absorb x-rays that are not parallel to the primary beam.
What is a grid ratio?
height of lead strip and distance between - 12:1 grid is 12mm tall w/ 1mm spacing between
The higher the grid ratio, the more _____ the grid
The higher the grid ratio, the more primary beam that is also absobed so you need to _______ the exposure to compensate.
Name three types of grids.
Parallel, focused, crisscross
Describe a parallel grid
lead strips are perpendicular to the table and going in a single direction
Describe a focused grid
The lead strips are perpendicular to the natural divergence of the primary beam, slightly angled in relation to the table.
Describe a crisscross grid
2 parallel grids superimposed on eachother producing a cross-hatch pattern
What is grid cut off?
Absorption of part of the primary beam due to the improper use of the grid
4 causes of grid cut off
1. focus grid upside down
2. primary beam not centered over a focus grid
3. either type of grid not parallel to tube
4. focus grid used w/ incorrect SID
How is a stationary grid used and what does it show?
It is placed between film and patient on top of cassette. Film shows grid lines when processed
Where is the Potter-Bucky diaphragm that hold the grids?
Inside the table
What is grid factor?
Fact that a portion of the primary beam is absorbed by any grid.
How do you compensate for grid factor?
Double the MAS and adding a grid factor value to the KVP
What is the process of turning a latent image into the visible image on the film called?
Film processing
It is estimated that 90% of poor quality radiographs can be contributed to what?
Poor processing
What are the 5 general steps in film processing?
develop, rinse, fix, wash, dry
4 darkroom essentials
1. Keep clean
2. Keep organized & uncluttered
3. Light-proof
4. Well ventilated
A darkroom should be organized into what two distinct areas?
Wet side & dry side
What is a safelight?
A 15 watt red light used to produce enough light to work efficiently w/o fogging the film
How far away should you keep the safelight from the film?
at least 4 feet
What does the developing step of processing do?
reduces the exposed silver halide crystals to black metallic crystals
What does the rinse step of film processing do?
removes the developer and reduces the carry over of the developer into the fix
What does the fix step of film processing do?
Removes the unexposed silver halide crystals
What does the wash step of film processing do?
removes the fixing agent
4 advantages of automatic film processing
1. faster
2. fewer artifacts
3. more consistent
4. less expertise required
2 disadvantages of automatic film processing
1. higher initial cost
2. periodic maintenance needed
________ can be added to the developer and fixer to return them to their original strengths
Why is silver recovery used?
to allow *legal* disposal of corrosive processing chemicals
What are 3 *legal* methods of film identification?
1. lead letters placed on film at time of exposure
2. lead labeling tape
3. photo imprinter in darkroom - it is after exposure but before processing
In order to be legal, film identification must be _____ & _____
permanent & unalterable
5 things that should be on the film id?
name of hospital, date, name/id of patient, owner's name, x-ray view or orientation
Legally, how long must x-rays be kept?
Minimum of 7 years
Radiographic quality is based on what three things?
1. Density
2. Contrast
3. Definition
What is film density?
degree of blackness on x-ray
What is film contrast?
visible difference between two densities
What is definition?
Ability to differentiate body parts from eachother
A diagnostic radiograph will have sufficient amounts of what 4 characteristics?
density, definition, contrast, and penetration
What is radiographic penetration?
provides internal detail, not just tissue silhouettes
What three factors influence x-ray penetration?
wavelength, subject composition, and tissue thickness
Re: film density - many blacks = ____ density
Re: film density - many whites - ____ density
Film density is controlled directly by ____
Radiolucent shows as black on film which means that _____ of the radiation being produced is hitting the film
Radiopaque shows as white on film which means that _____ of the radiation being produced is hitting the film
What are 4 variables that can affect film density
KVP, SID, processing, film brand
The greater the tissue density, the ____ the film density.
lesser (more dense tissues show as white on film)
What are 5 patient densities of concern in radiology?
gas, fat, water, bone, and metal
Radiographic contrast is mostly controlled by what?
Describe high contrast. What is is also called? Used mostly in what field?
Abrupt changes of density tomes from black to white - few grays. Also called "short scale contrast" and is used mostly in orthopedics
What is low contrast? Also called? Used for what?
Gradual changes of density tones from black to white - many shades of gray. Also called "long scale contrast", is is used primarily for soft tissues.
Re: film contrast, describe wide latitude film
Needs large changes in exposure to change visible density - is less sensitive to radiation changes.
Re: film contrast, describe short latitude film
Needs only minor changes in exposure to show density change - is more sensitive to radiation changes.
Three thing that definition or image sharpness is affected by
motion, geometric unsharpness, material unsharpness
What is radiographic distortion?
size or shape of image distorted
What is radiographic magnification and what is it caused by?
Object appears larger than it is - caused by image not being as close to film as possible
What is radiographic elongation and what is it caused by?
The object appears longer than it is - caused by the tub not being perpendicular to the object
What is radiographic foreshortening and what is it caused by?
Object appears shorter than it is w/ the elevated portion magnified - caused by the object not being parallel to the film.
What two questions should be asked when critiquing the quality of a radiograph?
1. Is the film too light or too dark?
2. Is the density suitable?
What are some other things that should be looked at when critiquing a radiograph?
Positioning, collimation, proper directional label, and is the film labeled?

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