exposure factors in practice (secondary factors)

Question 1

what are the secondary factors that affect exposure

Answer 1

focus to fil distance (FFD)
object to film distance (OFD)
focal spot
image distrortion

Question 2

what does these acronyms stand for:
FFD/SID
FOD
OFD

Answer 2

FFD is the focus to film distance and thisa is the distance from the xray beam source to the image receptor

FOD stands for the focus to object distance and this is the distance from the xray bream source to the patient/object

OFD stands for the object to film distance and this is the distance from the object/patient to the image receptor

Question 3

what is the inverse square law (SID RELATED)

Answer 3

the intensity of radiation emited from an isotropic source is inversely proportional to the distance of the source squared as long as there is no absorption or scatter

Question 4

when would you apply the inverse square law

Answer 4

photon energies above 50kvp

Question 5

what is the effect of SID on exposure usin the inverse square law

Answer 5

if the distance from the source to receptor increases by a factor of 3 then that means that the radiatrion intensity will decrease by a factor of 3 squared

Question 6

how does xray source distance affect density and patient dose

Answer 6

as the xray source distance increases then the patient does aned the density of the xray image will decrease

Question 7

if the source distance is increased how would you maintain the same intensity

Answer 7

by increasing the mAs

Question 8

how would you know how much to increase the mAs by

Answer 8

new mAs = (new ffd2/ old ffd2) x old mAs

Question 9

how does OFD affect distortion

Answer 9

having an OFD increases magnifiaction which also increases distortion of the image
body parts that are further away from the image receptor will experience more distortiuon that the body parts closer the the image receptor

this is why it is important that the body parts are as close to the image receptor as possible

Question 10

how do we compensate for magnification

Answer 10

we must increase our FFD

magnification factor = FFD /FOD

new FFD = Magnification factor x old FFD

Question 11

HOW DOES IMAGE RECEPTOR AND OBJECT ALLIGNMENT AFFECT DISTORTION

Answer 11

if the object and the image receptor are not parraler to eachother then the image may be distorted

for example if the object or image receptor is held at an angle then the image will be distorted

however sometim,es we do this on purpose for example when viewing an oblique hand