Question

(b) Calculate the percentage transmission for: (i) 20 keV X-rays passing through: 15 mm soft tissue, 15 mm bone and 15 mm soft tissue 100 keV X-rats passing through: 150 mm soft tissue Compare the answers to (i) and (i) (i) (ii) The data required is given on the Figure 1 (40 marks) 100 lodine Calcium Water 10 O.14 10 20 30 40 50 60 70 80 90 100 110 Energy (keV) Rajah 1 Figure 1 Mass Attenuation Coefficient (cm1g)

Answer 1

The transmission percentage through a material of thickness x is given by:

I/I=e o

where $\mu$ is mass attenuation coefficient & $\rho$ is mass density.

The soft tissues are mostly composed of water. Density of water: 1g/cc

For 20keV x rays,

и— 10ст?/9

Thickness, x=15mm

Then,

e 15-3.06 x 107 I/I,= e 10cm2/gx1g/cmx15x10-cm

Transmission %age:

T I/Io x 100= 3.06 x 10

Bones mainly consist of Calcium. Tyically, Density of bones:

3 p1500kg/m= 1.5g/cm2

Thickness: 15mm

500cm2/g

Using these values,

I/Ie-500cm2 /gx1.5g/cm x 15x0.1cm -1125

%age transmission:

I /I, x 100 e-1125 x 100

At 100keV, for water,

0.03cm2/g

Using this, transmission % at 100keV:

-0,03x1x 15 -0.45 I/I, x 100 0.6376 x 100 63.76 X X e

Comparison at 20keV and 100keV:

At 20keV, transmission %age is of the order of , this means most of the radiation gets absorbed at this energy. This is due to high attenuation coefficient at this eneryg. But at 100keV, attenuation coefficient decreases drastically. So, most of the radiation goes unabsorbed and hence, the transmission %age increases at that energy.

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