Ch11: Secondary Factors Flashcards
What sizes of filaments are there?
Large (1-1.2mm)
Small (0.5-0.6mm)
What is the purpose of the filaments in the cathode?
affects heat of tube (tube loading capacity)
affects spatial resolution/sharpness of anatomy
T/F The smaller the focal spot means better spatial resolution or recorded detail.
True
T/F Larger focal spots create heat on a larger surface which could cause heat damage to the tube.
False
Smaller focal spots disperse heat on a smaller area, creating possible heat damage to the tube
Does focal spot size have any affect on the exposure?
no
How does focal spot size affect spatial resolution?
larger size decreases spatial resolution
larger size increases the area of un-sharpness
beam divergence occurs
T/F Distance is a factor that will either increase or decrease exposure dose to people/objects.
True
As distance (from the beam) increases, the exposure dose decreases
T/F As SID increases, the exposure to the IR increases.
False
As SID increases, the amount of radiation hitting the IR will decrease. (as long as the technique remains the same)
How do you fix a low EI if the distance is too far?
Increase your mAs
SID stands for…
source to image receptor distance
SID affects… (quantity/quality)
quantity
affects amount of radiation reaching the IR/patient
What is another abbreviation for SID?
FFD
Focal Film Distance
T/F Beam intensity will change when distance changes.
True
Amount of radiation (beam intensity) on the IR will change at different distances
(quantity)
T/F Increased SID will increase exposure, intensity, and patient dose.
False
Increasing SID will decrease all those amounts
T/F Decreased SID will increase patient dose.
True
Exposure Index and intensity will also increase
Does SID and exposure index have a inverse or direct relationship?
inverse
Does SID and patient dose have a inverse or direct relationship?
inverse
Does SID and beam intensity have a inverse or direct relationship?
inverse
What is the inverse square law?
Intensity of radiation at a given distance from the source is inversely proportional to square of the distance
(Intensity 1 / Intensity 2) =
(distance2^2 / distance1^2)
If you were to double your distance (SID) from 50in to 100in, how has your intensity changed?
intensity has decreased by 1/4
What situation would you use the inverse square law formula?
finding intensity of radiation when distance changes (as long as technical factors remain unchanged)
When would you use the direct square law formula?
maintaining exposure when SID changes, calculating mAs to compensate for change in distance (as long as other factors remain the same)
T/F As distance increases (SID), the beam becomes more perpendicular to the subject and increases magnification.
False
increasing the distance will decrease magnification
the beam does become more perpendicular
As SID increases, mAs should _____ to keep the exposure the same
increase
Direct Square Law or Exposure Maintenance Formula
mas 1 / mas 2 = (SID1^2 / SID2^2)
If SID decreased, but you did not change your mAs technique, what would happen to the EI?
increase
How do you find what % of SID is OID?
OID / SID
As SID increases, while OID remains unchanged, how does that affect magnification?
decreases as SID increases
What does OID stand for?
Object to Image Receptor Distance
As OID increases, how does that affect the amount of radiation on the IR?
less radiation strikes the IR as OID increases
T/F OID creates an air gap which prevents scatter and increases contrast
True
OID increases, and magnification….
increases
Why does OID create magnification?
the exit beam diverges after leaving the patient, increasing the size appearance of an object
OID has what affect on spatial resolution?
inverse relationship
OID increases, spatial resolution decreases
OID decreases, spatial resolution increases
T/F Rule of thumb for projections is to keep the body part of interest furthest from the IR.
False
Magnification Factor Formula
MF = SID / SOD
SOD stands for….
Source to Object Distance
Why is the Magnification factor always over 1?
the object is being magnified
the factor as a percent would be over 100% because the image is larger than the initial object
Which combo will create the best spatial resolution?
40 mAs, 80 kvp, 40in SID, 4in OID
50 mAs, 70 kvp, 50in SID, 4in OID
120 mAs, 90 kvp, 40in SID, 2in OID
10 mAs, 75 kvp, 50in SID, 2in OID
10 mAs, 75 kvp, 50in SID, 2in OID
Separate the mas and kvp b/c it doesn’t matter
find the magnification factor (divide SID by SOD)
lowest mag factor = better spatial resolution
How do you calculate the object’s true size from a magnified image?
Find the SOD if not given (SID - OID)
Calculate mag factor (SID / SOD)
Calculate object size by dividing the image size by the mag factor
What types of distortion occur with Central Ray/object alignment?
elongation
foreshortening
off center/clipping
How does angulation of the ray affect the amount of exposure to the IR?
angulation increases SID, so it decreases exposure
What is the primary purpose of a grid?
absorbs scatter radiation
increase image contrast (by cleaning up fog/scatter)
Why would you use a grid?
larger anatomy creates excess scatter
used on anatomy larger than 4 inches
How do you need to adjust your technique when using a grid?
when adding in a grid, you need to increase your mAs
Why do you need to adjust technique when adding a grid?
grids will absorb scatter and radiation in general
so it will reduce the overall amount of radiation hitting the IR
you need to increase mAs to increase the quantity to maintain the EI
What does the grid ratio mean?
It is the number of lead strips per one inch
What happens to patient dose when you add a grid? Why?
dose increases b/c you have to increase your mAs
You increase mAs to maintain EI and/or have adequate exposure
increased mAs = increase pt dose
What happens if you do not adjust mAs when you add or get rid of a grid?
quantum noise/fog
T/F A smaller light field means there is more beam restriction and more collimation.
True
Why is a smaller light field preferred?
less tissue is irradiated and less scatter
better contrast
T/F A larger light field means more beam restriction and less collimation and will result in better contrast.
False
Large Light field means….
less collimation
less beam restriction
more scatter and more tissue irradiated
lower/poor contrast
A tech asks you to make your collimation bigger, what do they mean?
Open the light field
lessen beam restriction
Collimation has a larger affect on the image when you use…
no grid
high kVp
large light field
T/F Beam restriction affects the amount of scatter produced.
True
What are some secondary factors?
Generator output
Tube filtration
Compensating Filters
SID/OID
Beam Restriction
Grids
A higher frequency generator requires (more/less) exposure techniques.
less
What affect does tube filtration have?
amount of high penetrating photons
higher energy beam
more scatter
What are compensating filters?
placed in the xray tube/beam to produce more uniform exposure
requires more mAs which will increase patient dose/exposure
What are some patient factors?
body habitus
part thickness
age (pediatric pts)
pathology/replacements
For every 2 inch increase in part thickness, the mAs needs to…
almost double
increase mAs by 2
What are two patient factors that affect contrast?
thickness and atomic number (bony or replacements)
What do you need to adjust for peds patients?
fast exposure times to decrease movement
decrease kVp (b/c thickness of parts is smaller)
What are the four body habitus/types from thickest to thinnest?
hypersthenic
sthenic
hyposthenic
asthenic
Whenever possible, use a (low/high) kVp and a (low/high) mAs.
high kVp
low mAs
To consider radiation protection, what should you remember when collimating?
always collimate to include only the necessary anatomy
For radiation protection, should you always use a grid?
No
only use when necessary
built into (usually) table and wall bucky
only needed for large body parts
Because digital imaging auto-corrects images, what should you always check to avoid overexposing your patient?
Exposure Index number
