A-level Physics medical imaging question mcq
12
A radiographer in a hospital directs a parallel beam of X-rays at the leg bone of a patient.
The attenuation (absorption) coefficient of bone is 0.7 cm −1 .
The answers below are given to one significant figure.
What is the percentage intensity of X-rays transmitted through bone of thickness 0.7 cm?
The answer is 40% I have no idea how to get that I've been using the attentuation formula but I don't get 40%
any help would be much appreciated !
The attenuation (absorption) coefficient of bone is 0.7 cm −1 .
The answers below are given to one significant figure.
What is the percentage intensity of X-rays transmitted through bone of thickness 0.7 cm?
The answer is 40% I have no idea how to get that I've been using the attentuation formula but I don't get 40%
any help would be much appreciated !
Reply 1
I don't see any answers
fraction absorbed =e^(-0.7x0.7) = 0.6 to 1 sf
fraction transmitted = 0.4
so 40%
fraction absorbed =e^(-0.7x0.7) = 0.6 to 1 sf
fraction transmitted = 0.4
so 40%
Reply 2
maruchan
OP
12
Original post by BobbJo
I don't see any answers
fraction absorbed =e^(-0.7x0.7) = 0.6 to 1 sf
fraction transmitted = 0.4
so 40%
fraction absorbed =e^(-0.7x0.7) = 0.6 to 1 sf
fraction transmitted = 0.4
so 40%
Thank you!! I kept on getting 0.6 you're a star!
Reply 3
If attenuation constant is normally in m^-1 and bone thickness is normally in metres why don’t we convert centimetres to metres? I did 0.7×10 to the -2 for both attenuation and bone thickness and I ended up getting 0.9999. 1-0.9990. = 0%
Reply 4
I am pretty sure 40% is a wrong answer. This is one of mistakes made by OCR paper maker. I = I_0 Exp(-ux) gives the transmitted intensity not the absorbed one.
Reply 5
Original post by newcastletutor
I am pretty sure 40% is a wrong answer. This is one of mistakes made by OCR paper maker. I = I_0 Exp(-ux) gives the transmitted intensity not the absorbed one.
I agree
Reply 6
Original post by Annayg
If attenuation constant is normally in m^-1 and bone thickness is normally in metres why don’t we convert centimetres to metres? I did 0.7×10 to the -2 for both attenuation and bone thickness and I ended up getting 0.9999. 1-0.9990. = 0%
It doesnt really matter as long as you use the same units cm^-1 and cm for thickness as the units cancel and become unitless so as long as both are in the same units of cm then you are fine. I am not sure what you ended up doing.
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