Post on 24-Dec-2014
description
Imaging Test #2 LAB
KvP, mAs Reciprocity, SID
kVp- 15% rule
• To demonstrate the visible effects of the 15% rule
• 1st exposure: 100mA, 10 mAs, 50 kVp
• On 2nd exposure, mAs ↓ by ½ & kVp ↑ by 15%
kVp- 15% rule• 2nd exposure: 58 kVp, 5 mAs • 50 kV + 15%• ↑ kV by 15% &↓mAs by ½ to get
a similar image• This is the 15% rule• If your goal is to get a similar
image and you have to change one of these factors (ex: ↑ kV for a hypersthenic pt) you must change the other accordingly to get a similar image.
Area of Coverage #1
• To demonstrate the result of change in SID
• 50 kVP, 2.5 mAs, 40”SID• This is 1st exposure @
40” SID, all other factors remain constant
Area of Coverage #2
• 2nd Exposure: SID is ↓ to 30”
Area of Coverage-#3
• 3rd exposure: SID ↑ to 50”
kVp #1
• To demonstrate the general response to ↑ kVp
• 1st exposure: 100 mA, 5mAs, 40kVp
• All other factors constant as kVp is increased for repeat exposures
• Note that very few shades of gray are visible on 1st image
kVp #2
• This should be the 3rd image• 60 (maybe 70 kVp)• Note that almost all
densities of stepwedge are visible in shades of gray
• As kV ↑ penetration ↑• Scale of shades of gray
widens• ↑kV is necessary to
penetrate thicker parts
mAs double #1
• To demonstrate the result of change in mAs value
• 50kV constant• 1st image 1.25 mAs• All other factors remain
constant as mAs is ↑ to 2.5, 5, and 10 on repeat exposures
• mAs is therefore doubled on each subsequent exposure
mAs double #2
• Image# 4: 10mAs• As mAs ↑ brightness
(density ) ↑• As mAs doubles, LGM# ↑
by .3• Once you get to a certain
level , doubling mAs only ↑ patient dose; has no effect on image
mAs Reciprocity #1
• To demonstrate mAs reciprocity.
• 1st image: 5mAs, 50 kV, 40” SID @ 50mA setting
• All other factors remain constant as mA is ↑ to 100 then 200 on repeat exposures
mAs Reciprocity #2
• Image 2: ↑ to 100 mA
mAs Reciprocity #3
• Image #3: ↑ to 200 mA• LGM#’s should be
constant• With mAs constant, any
combination of mA & s should yield the same results (same image)
mAs visible change #1
• To demonstrate the degree of change in mAs necessary to be visible to the eye
• 70kVp 50 mAs• With all other factors
constant, make repeat exposures w/10%, 20%, 30% & 50% increase in mAs
mAs visible change #2
• This image represents the 50% increase in mAs.
• The only image in the series in which the change can be appreciated.
• Either double or ½ your mAs or don’t repeat as there is not enough of a visible difference to warrant the ↑ exposure to patient
Scatter #1
• To demonstrate the relationship between kVp & scatter
• 14x17 cassette was placed in holder on table 6” outside of light field
• Chest phantom on table collimated
• 1st exposure: 100 mA, 10 mAs, 40 kVp
Scatter #2
• Image #2 kVp ↑ to 100• All other factors remain
constant• We can conclude that ↑
kVp = ↑ scatter• This is a direct relationship
SID #1
SID #2
• SID increased to 60”• Collimation unchanged
SID #3
• SID decreased to 20”• Collimation unchanged• RESULT: As distance ↑
magnification of body part ↓
• This is an inverse relationship