Create
Learn
Share

DI Physics 2

rename
drhartz's version from 2017-10-11 16:01

Xray Interactions with Matter

Question Answer
what is differential absorption?xray absorption variation dependent on tissue type
amount a substance absorbs xrays is dependent on?1. tissue type thickness 2. specific gravity (density) 3. atomic number
how is a fucking radiograph even formed?xrays pass through object reaching and reacting with FILM underneath causes film to turn black
1. xray that reach the film cause film to look what color?
2. xrays that do not reach the film cause film to look what color?
1. pass through black
2. dont pass through white
radio-OPAQUE =white color
radio - LUCENT =black color
5 opacities that are recognized on a radiograph from black to white ? gas/black - fat/black- soft tissu/fluid/grey - bone/white - metal/white --> gas blubber blood bones bullet
what is mA? what is the s? Combined what do they affect? (mAs)mA: amount of xrays produced
s: time of xray emission
mAs: Density of film
what is Density?degree of blackening on radiograph
film blackening is directly related to (3)1. mAs (amount of xrays striking film)
2. FFD
3. kVp (energy of the xrays - more likely to penetrate patient)
Prefer (high or low) mA with a (short or long) exposure time? Why?high mA with a short exposure time …movement affects quality
kilovoltage (kV) dictates ____ of electrons, _____ / penetrating power of xrays , ______ xray beams ?Speed of electrons , energy/penetrating power of xrays, quality of beams
film blackening aka density is greater when kVp is high or low?high
need HIGHER kV for what type of tissue? (2)1. THICK tissue 2. high atomic number / specific gravity
increasing the kV increases xray ____ and also _____ of xrays that penetrate patient and reach the film1. energy 2. number of xrays that penetrate px
if KV is inc. by 10 how do you adjust mAs to produce the same density?divide mAs by 2
if mAs is doubled how do you adjust the kV to create the same density?decrease kV by 10
memorize

Inverse Square Law

Question Answer
inverse square law (film focus distance)exposure/intensity/density is inversely proportional to --> SQUARE OF THE DISTANCE (d^2) between xray SOURCE and imaging system/FILM
the further away the source of an xray causes what to image generation?lower exposure
if the focus film distance (FFD) is increased what must you INCREASE? why?Must increase mAs!!!!!!
now need MORE xrays to penetrate film & cause blackening
if you decrease the FFD what can you afford to decrease?the mAs
when is decreasing the FFD done?radiographing large animals when a higher mAs is not possible bc of low output machines
at twice the distance there is _____ of the xrays available1/4
decrease in xray beam intensity/density/exposure is proportional to what distancethe square of the distance from the source
what is the equation of the inverse square law?(i**/ i) = (d^2 /d**^2) (distance & intensity)
if you change the focal film distance what must you adjust? what is the equation to adjust it?mAs
mAs(1)/mAs(2) = d(1)^2 / d(2)^2
memorize

Image Generation

Question Answer
what is film density?the degree of film blackness
overexposed film can be due to what 3 things mA, kV, or time
what color is an overexposed filmblack/dark
what color is an underexposed film light
film contains an emulsion with _____ that exposure turns black and is washed off leaving white if unexposed ?silver crystals/bromide
photographic film is more sensitive to light or xrays? light
photographic film is more sensitive to light – what is used to transform incoming xrays into light?intensifying screens
intensifying screens transform xrays to light how does this help make a radiograph?reduces the amount of xrays needed to make a radiograph
2 intensifying screens are on each side of the film and must be in close contact with the film to reduce what? light diffusion
what are the 2 types intensifying screens 1. Detailed Slow screen 2. Fast screen
Speed of intensifying screen increases as crystal size increases or decreases?increased crystal size
FAST SCREENS (large crystals) have (inc. or dec.) resolution? & NEED (inc. or dec.) exposures/# of xrays decreased resolution need decreased exposure
SLOW SCREENS (small crystals) have (inc. or dec.) resolution & need (inc. or dec.) exposures/ # of xrays higher resolution/detail need higher exposures
which screens have a higher xray to light conversion which reduces exposurerare earth screens
Fast screens minimize exposure time – used for what tissue types?thorax
Slow screens give great detail – used for what tissue types?bone
Fast screens minimize what artifact?blurring … due to short time/quick
What keeps film and intensifying screens in close contact/ houses them?cassettes
memorize

Contrast

Question Answer
what is contrast? - range of shades of grey in a radiograph (white vs black) – difference in density between adjacent areas
Contrast depends on? (4)1. thickness of tissue/ physical density/atomic number
2. xray beam Energy / kVp
3. Scatter
High contrast image has (high or low) mAs & kVp High mAs and Low kVp
Low contrast image has (high or low) mAs & kVp Low mAs and High kVp
which xray interaction predominates a high contrast image?photoelectric effect (maximal differential absorption)
which xray interaction predominates a low contrast image?compton effect (low differencial absorption)
which tissues types need a high contrast image?low contrast tissues muscles, skeletal, & abdomen
which tissue types need a Low contrast image? high contrast tissues: everything in thorax ...need small structures to become visible like metastasis
High image contrast = inc or dec shades of grey?few shades of grey aka = short grey scale
Low image contrast = inc or dec shades of grey?many shade of grey aka = long grey scale
Low kV high or low contrast image?High
High kV high or low contrast image?Low
memorize

Scatter

Question Answer
3 types of scatterCoherent, Photoelectric, Compton
scatter increases with (2)1. size of patient (density/atomic #) 2. size of exposed area
what type of radiation is produced when xrays scattersecondary radiation
most important type of scatter formation for producing radiographs ?photoelectric effect scatter
most important type of scatter formation of film fogging and exposure to personnel ?Compton scatter
photoelectric scatter is directly proportional to ?3rd power of the atomic number (Z)
what is the basis of image production?photoelectric effect scatter is proportional to third power of ATOMIC NUMBER
Photoelectric effect scatter is inversely proportional to ?third power of PHOTON ENERGY
Compton scatter is independent ofatomic number (Z)
Type of scatter that reduces image contrast?Compton scatter
Compton scatter increases withincreasing photon energy
3 bad side effects of Compton scatterfilm fogging, exposure to personal, reduces image contrast
xray interaction that ejects inner shell electron - incoming xray PHOTON is absorbed completely/no image contribution - low energy - no scatter radiation?photoelectric effect scatter
xray interaction that ejects outer shell electron - incoming xray PHOTON is scattered and NOT absorbed Compton scatter
what causes film fogging? how is the film affectedscatter that detracts from the films contrast causing blurring
what 2 things cause increase in scatter?1. high kV 2. thick areas of the body
prevent scatter PRODUCTION by (2)Collimation by reducing light beam diaphram diaphram
2. optimum kV (high = inc scatter)
prevent scatter FROM REACHING FILM by absorbing (2)1. GRIDS! use a grid for any structure > 10cm
2. LEAD on backing to cassette & xray table
what is collimation?lead shutters that reduce volume of tissue exposed so less scattered radiation is created in patient
to prevent scatter a use a grid for any structure of what size?>10cm
where are grids positioned between?patient and cassette
when using grids what must be increased to prevent underexposure ?mAv
grid ratioheight of lead strips to width of spaces between them
grid factoramount exposure factors (mAs) must be inc to compensate for using a grid
what will a higher the grid ratio do?absorb more scatter
stationary grids produce what adverse effect (vs moving grids)grid lines!
adverse effect at film edges with a parallel grid? density decreases at the edge of the film called GRID CUT OFF
what must be adjusted to use a Focused grid?FFD
which grid is the most effective at scatter absorption?crossed grids
FFD must be correct to use what type of grid?focused grids
grid factor factor of inc in mAs/exposure --> that is needed to compensate for the radiation that is absorbed by the grid
increasing the exposure factor for a grid causes what disadvantage?inc exposure to px and personnel (& blurr bc will need inc time)
only xrays that are parallel or perpendicular to orientation of lead strips pass and reach film?parallel
what removes low energy/hazardous ionizing radiation that would otherwise have penetrated the patient?tube housing and aluminum filter
what does the collimator do?its a light beam diaphram that reduces the beam to the area to be imaged
xrays that hit the patient do what 3 things?1. absorbed 2. scatter 3. penetrate patient to hit the xray film
memorize

Artifacts

What is an artifact? :unwanted optical density in image that interferes with structure of interest
Question Answer
Overexposure or underexposure during exposure or during processing
Grid cut offduring exposure
Coarse grid linesduring exposure
Excessive scatterduring exposure
Movement blurrduring exposure
Presence of objects such as collar, dirt, wet coatduring exposure
Double exposureduring exposure
Quantum mottle.. what is it? When?underexposure- grainy appearance / during exposure
Poor washingduring processing
Chemical stainingduring processing
Streakingduring processing
Air bubblesduring processing
Light foggingduring handling or during storage
Crimpingduring handling
Finger printsduring handling
Staticduring handling
Dirty hangersduring handling
Wrong safe lightduring storage
Old filmstorage
Grid cut off – why does it happen?improper alignment of xray tube & grid .. tube should be centered to grid
Crescent moon mark on radiograph comes from what?kinking the film
Light fogging is caused by?poorly closed cassette, white light before fixing film, cassette/film storage, safelight
How do you minimize blurring ?use shorter exposure time *aka shortest possible time*,
take end-inspiratory (for thorax),
take end expiratory,
GA,
use fast screens
Causes of reduced SHARPNESS/ geometric detail? (5)increased focal spot size (want smallest)
increased OBJECT to Film Distance
object not perpendicular to xray beam
xray not centered
SHORT Focal Film Distance! (want long FFD)
Magnification artifact occurs with what?increased object and film/cassete distance OFD
Advantages of OFD (3)improved resolution of small objects, enlarged image easier to interpret, air gap eliminates some scatter
Disadvantages of OFD (3)penumbra (edge unsharpness) increases, need more output/exposure, exacerbates motion blur
To decrease DISTORTION (unequal magnification of various body parts) from increased OFD what should you do ?place px as close as possible to the film aka radiograph left limb; left limb/left side should go down closest to film
Causes of distortion?inc. px to film distance, object not perpendicular to beam/ not parallel to cassette, xray beam not centered over px
memorize