Dual Energy CT Flashcards by Lauren Knight

What is the goal of dual energy methods?

To simulate the energy dependence of attenuation to better describe it.

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What can Dual energy potentially do?

Remove beam hardening artefacts

Completely describe material sin the AOI.

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What is dual energy CT?

Attenuation of the X-ray beam within voxels
different attenuation coefficients (u) generates contrast between tissue types.
Attenuation of contrast media and bone overlaps

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What technology has Dual energy also been used in?

Airport scanners

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What does the quality of the decomposition depend on?

The relative shift in attenuation co-efficients.

For example it’s easier to separate calcium and water than fat and water

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What is the basic principle behind Dual energy CT?

Separation of each pixel value into linear combination of two independent functions by observation at two energies.

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What are the examples of observing at 2 energy levels?

1) concentration of two materials with different atomic numbers; water and calcium
2) photoelectric and Compton effect
- electron density proportional to photoelectric
- atomic number cubed proportional to Compton

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What is electron density proportional to ?

Photoelectric effect

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What is atomic number ^3 proportional to?

Compton effect

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What enables solutions for concentration?

Knowledge of attenuation as functions of energies enables solutions for Concentration(n)

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What does FBP stand for?

Filtered back projection

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What energy level does Compton scattering have?

High energy

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What energy level does photoelectric effect have?

Low energy

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What formula models the photo electric effect?

1/E^3

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What is the photoelectric coefficient?

Ap - energy independent coefficient that is unique to each material and density

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What is the photoelectric base?

Up - photoelectric energy dependant function

Independent of material

Up = 1/E^3

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What is the Compton base?

UAC - Compton energy dependant function
Dependant on material

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What is the Compton coefficient?

ac - energy independent coefficient that is unique to each material and density

What are the applications of Dual energy CT?

~ base pair images (eg iodine/water, water/ calcium, iodine/ Calcium)
~ synthesised monochromatic images
~ spectral curves/ scatter plots / concentration analysis
~ material subtraction

What are some of the potential clinical uses of Dual CT?

virtual, non-contrast studies ( iodine mathematically subtracted)
other material subtraction( e.g. calcium)
metal artefact reduction( by selection of KV synthesised images)
ROI analysis- is this lesion blood/clot/cyst?
what is the composition of renal stones?
perfusion, maps( mainly heart and lung)

What are the 3 methods of acquisition?

Dual axial rotation
Dual spiral sequence ( faster)
Single spiral sequence with alternating KV ( less delay between dual energy signals)

What are the advantages of Dual spiral sequence?

No special hardware required
Can be performed on any CT scanner

What are the disadvantages of dual spiral sequence?

Patient motion between scans may cause severe degeneration of images.

What are the advantages of a dual source scanner?

Tube current and filtration, optimised to maximise CNR and spectrum separation

What are the disadvantages of a dual source scanner?

90° phase shift between low and high data sets Detect crosstalk requiring special, scatter rejection and correction

What are the advantages of rapid Kvp switching ?

Simultaneous data acquisition between low and high KVP set data sets

What are the disadvantages rapid Kvp switching?

Relatively high overlap of the energy spectra

What are advantages of multilayer detectors?

Simultaneous data acquisition between low and high KVP data sets

What are the disadvantages of multilayer detectors?

Relatively high overlap of the energy spectra Difference in noise levels between data sets

What a re the advantages of photon counting?

Permits energy, specific noise, rejection Facilitates new imaging approaches eg : K-edge subtraction

What are the disadvantages of photon counting?

? Technological realisation

What are the practical implementations of dual CT?

- test basic function before use - eg radiation dose - Determination of effective atomic number - Kidney stone analysis - Iodine concentration for perfusion maps

How does the scanner calculate the effective atomic number?

The effective atomic number is defined in terms of the attenuation signature of a material. If the material that we scan behaves like a periodic element when an atomic number X then we say that the effective atomic number of that material is X . The scanner uses the ratio of total attenuations at 70 KEv and 120 kev It matches this identified ratio to appoint on the curve of known elements This is their used as a calibration curve to calculate the effective atomic number of unknown compounds elements and mixtures .

What are some of the material we can use?

Water Carbon Air Aluminium Chalk Iodine concentrations

What does a rapidly alternating between high and low KV require?

It requires precise control of generator and tube projection interpolation Alternate tube current as well

What can’t change with alternating Kv?

The filtration can’t be changed

What are the 2 methods of evaluation of kidney stones?

~ ROI (region of interest) Effective, Z and HU determined by ROI placement over largest area of stone ~ RC (report cursor) WW. Sat at one and measured effective Z and HU of the stone as a stone appeared when the WL was increased

What’s are the values involved in renal stone characterisation?

Z value : GSI kidney stone software HU ( Hounds field unit) : 80 kvp and 140 kvp

What conclusions did they come to about kidney stones?

Only uric acid separation possible Effective Z appears to increase separation HU at 80 and 140 KV The real difference in effective atomic number among the different stones is small. Further larger sample analysis needed

What does GSI stand for?

Gem stone imaging

What does GSI accurately quantify?

Water / iodine mixtures within a phantom

What effects does GSI appear to be free from?

Beam hardening When assessed with in this specific phantom

What do low kv mono-energetic images allow? And what is the down side of this?

Low kv images allow significant increase in contrast However At reduced signal to noise, ratio when compared to conventional polychromatic imaging .

What do you have to consider before using Dual energy CT on patients?

* you are satisfied with the existing research base is extensive enough to introduce as established use * do you understand the differential diagnosis and its influence on onward patient manage * where you are unsure of the benefit to patient care, the certainty of results or the limitations of the application. The proposed protocol is assessed as part of research trial.