Exposure Conversion Problems Ch. 34 Flashcards
What are most conversion tables designed to do?
provide a method of maintaining image receptor exposure.
Are exposure factors laws or rules of thumb?
Rules of thumb
What are the clinical uses of the reciprocity law?
Change time to deal with motion or a breathing technique
Inverse Square Law - most common clinical application.
Portable CXR - it all has to do with SID
When is the 4-5 cm rule used?
When there is a change in part thickness
When is the 15% rule used?
With an increase/decrease in kVp
What is the formula for inverse square law?
I1/I2 = D22/D12
What is the formula for the 4-5 cm rule?
Double or half mAs for every 4-5 cm part size
Ex: 10 mAs for 20cm = 20 mAs for 24 cm
What is the formula for the 15% rule?
If kVp increases by 15%, half mAs
If kVp decreases by 15%, double mAs
What is the formula for GCF/Bucky factor?
mAs1/mAs2 = GCF1/GCF2
What is the speed class formula?
mAs1/mAs2 = RS2/RS1
You are performing a KUB on a 43 y/o obese M pt. who keeps moving due to exessive alcohol. The APR exp system for a large pt is 80 kVp, 300 mA, lg FSS & AEC center cell. The pt moves and causes blur. The mAs readout is 75 mAs & acceptable. How can factors be adjusted to reduce time?
300 x time = 75
1000 x 0.075 = 75
increased mA will reduce time
75 mA x 2 sec = 150 mAs
100 mA x 1.5 sec = 150 mAs
50 mA x 3 sec = 150 mAs
increase time for the swimmers
8/x = 722/502 = 8/x = 5184/2500 = 20000/5184 = 3.85 mAs
You are performing a stretcher chest i the xray room. The APR exp. system for a non-bucky AP CXR is 90 kVp, 1.5 mAs, & 40” SID. However, you are able to utilize a 72” SID. How should you adjust your exp factors?
- 5/x = 402/722
- 5/x = 1600/5184
7776/1600 = 4.86 mAs
double the mAs to 16 mAs
half mAs to 10 mAs
90 kVp & 5 mAs
increase by 15% & decrease by half
3/x = 1/3.75
x=11.25 mAs
5/x = 400/200
1000/400
x=2.5 mAs
