Dual energy CT

Question 1

what is dual energy imaging

Answer 1

uses two separate x-ray photon energy spectra, allowing the interrogation of materials that have different attenuation properties at different energies

• by stimulating energy dependence of attenuation, you can better separate tissue types

Question 2

note: the attenuation coefficiency is different for different tissues, therefore this can be used as a function of xray energy to better distinguish them

Question 3

separation of pixel u value into linear combination of 2 independent functions by observation at 2 energies

• quality of decomposition depends on relative shift in attenuation coefficient

Question 4

electron density proportional to photoelectric effect

atomic number cubed is proportional to Compton effect

what are each of these effects

Answer 4

photoelectric effect = a phenomenon in which electrons are ejected from the surface of a metal when light is incident on it.

Compton effect = quantum theory of high frequency photons scattering following an interaction with a charged particle, usually an electron

Question 5

example of dual energy being used to identify components:

in a contrast ct scan, the heart is bright however we dont know if this is due to calcification or the iodine contrast. Using dual energy and utilising the different xray spectrums, it can depict each component as if they were subjected to specific photon energy rather than a polychromatic beam. This makes it possible to subtract materials form each other in this case, iodine from calcium

Question 6

what are the 5 hardware options for dual energy ct

Answer 6

1. dual spiral sequence
2. dual-source scanner (simultaneously get high/low kv data offset by 90 degrees)
3. rapid kip switching (rapidly alternate between high and low kv during rotation)
4. multilayer detector (dual layer system in detection (one layer detects high kv the other detects low kv)
5. photon counting (photon counting devices to detect each even individually and measure its energy)

Question 7

what is meant by ‘effective atomic number’

Answer 7

attenuation ‘signature’ for a material

• if the material scanned behaves like a periodic element of atomic number x, then the effective atomic number of that material is x