Spectra Tutorial

Question 1

How does filtration affect a beam spectra? (2 marks)

Answer 1

• Preferentially absorbs lower energy photos which contribute more skin dose
• Shifts the spectrum to the right as a result of less lower energy photons in the beam

Question 2

What common anodes are used in mammo? (3 marks)

Answer 2

Molybdenum, Rhodium and Tungsten.

Question 3

What common filters are used? (2 marks)

Answer 3

Mo and Rho.

Question 4

For a mammo procedure, assume a Mo anode is used, what effect does a Mo filter have on the spectrum? Why might Rh be used instead? (5 marks)

Answer 4

• The spectrum starts at 10 keV and finishes around 30 keV
• Mo filter is used as it has a k-edge around 20 keV and therefore reduces the number of higher energy photons which may improve contrast
• A Rh filter has a higher k-edge around 23 keV therefore results in a harder beam, which may be used for better penetration e.g. larger breasts

Question 5

Why is Erbium used in coronoary angiography procedures? (5 marks)

Answer 5

• Erbium is used as a filter as it has a k-edge around 60 keV therefore makes the beam softer
• It is useful with iodine as this has a k-edge around 33 keV which means it preferentially absorbs around this energy
• Using these two in combination means the contrast can be improved without increasing the dose