Dual Energy CT Flashcards
What is the goal of dual energy methods?
To simulate the energy dependence of attenuation to better describe it.
What can Dual energy potentially do?
Remove beam hardening artefacts
Completely describe material sin the AOI.
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
What technology has Dual energy also been used in?
Airport scanners
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
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.
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
What is electron density proportional to ?
Photoelectric effect
What is atomic number ^3 proportional to?
Compton effect
What enables solutions for concentration?
Knowledge of attenuation as functions of energies enables solutions for Concentration(n)
What does FBP stand for?
Filtered back projection
What energy level does Compton scattering have?
High energy
What energy level does photoelectric effect have?
Low energy
What formula models the photo electric effect?
1/E^3
What is the photoelectric coefficient?
Ap - energy independent coefficient that is unique to each material and density
What is the photoelectric base?
Up - photoelectric energy dependant function
Independent of material
Up = 1/E^3
What is the Compton base?
UAC - Compton energy dependant function
Dependant on material
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.
