Define attenuation coefficient.

The graph shows how the attenuation coefficient μ of muscle varies with photon energy E.

In X-ray imaging, photons of energy less than 20 ke V are filtered out of the beam.

(i) Explain, with reference to the graph, why this does not significantly affect the quality of the X-ray image produced.

(ii) State the advantage to the patient of filtering out the low energy photons from the X-ray beam.

(iii) Calculate the fraction of the intensity transmitted through 3.0 mm of muscle for X-rays of energy 50 ke V.

the probability per unit length that a particular X-ray photon will be absorbed; 
or
I = I₀e^-μx; (with symbols defined)

(i) the attenuation coefficient is large which means that these low energy photons will be mostly absorbed (by muscles);
and so will not contribute to the imaging process; 

(ii) the patient avoids unnecessary harmful radiation; 

(iii) the linear attenuation coefficient is μ=3.0 cm^-1 =0.30 (mm^-1) (from graph);
and so fraction of intensity transmitted is e^-0.30x3.0=0.41

(a) Very few could define attenuation coefficient. Far too many candidates tried to define it in terms of half value thickness. If a defining equation is stated marks are only awarded if the meaning of every symbol is given.

(b)(i) proved difficult for a candidate who did not realize that almost all of the low energy X-rays would be attenuated inside the body and so would never even reach the sensor. Even so they could answer (b)(ii) well in terms of less exposure to harmful X-rays. Arithmetic errors in (b)(iii) were common unless they were fortunate enough to work in cm. There were very few who did. An attenuation coefficient of 3 cm⁻¹ was often mistakenly converted to 30 mm⁻¹ .