What does DECT allow for?
Separate out two tissues in a voxel with same LAC value by exposure to two different incident photon energies, exploiting the PEE.
What is fast kVP switching?
Quickly switch between higher and lower energies in same rotation
Pros of fast kVP switching?
Lower dosage (one scan)
Cons of fast kVP switching?
1) Needs very fast data sampling
2) Might get overlap between high and low energies
What is single source sequential DECT?
Each rotation (each slice) is performed at a higher then a lower tube potential
Pros of single source DECT?
Less motion artefact
Cons of single source DECT?
1) Higher dose form two scans
2) Poor temporal resolution as scanned twice
What is single source twin beam DECT?
Single xray tube, two material filter to spit the xray beam into higher and lower energies before reaching patient
Pros of single source twin beam DECT?
Quicker acquisition and excellent temporal resolution
What is dual source DECT?
Two xray tubes of different voltages with two sets of detectors. Placed at 90 degrees to each other.
Pros of dual source DECT?
1) Simultaneous acquisition and processing means quicker acquisition, good temporal resolution and less motion artefact
2) Can independently optimise SNR for each tube-detector pair
Cons of dual source DECT?
1) Increased dose
2) Spectral overlap (scattered radiation from one tube may be detected by dectector for other tube)
What is detector based spectral CT?
One energy and single detector but that detector is made of two layers that simultaneously detects two different energies
Pros of detector based spectral DECT?
1) Dual energy analysis can be performed on every data set aquired
2) Simultaneous acquisition and processing means quicker acquisition, good temporal resolution and less motion artefact
Give seven applications of DECT
1) Virtual unenhanced images by removing iodine contrast
2) Detect silicon leakage from breast implants
3) Virtual non-calcium images (remove calcium to see iodine update in bone eg) BM oedema
4) Better visualise hepatic and renal cysts
5) Atherosclerotic plaque removal
6) Can use lower contrast dose
7) Artefact reduction (reduce beam hardening for iodine/metal and photon starvation
Name 3 patient based artefacts
1) Motion artefact
2) Metallic artefacts via beam hardening and photon starvation
3) Incomplete projections (high attenuation objects like arms outside FOV create streak artefact in some projections)
4) Transient interruption of contrast (CTPA if increased venous return IVC (pregnant, big breath) contrast pushed out of SVC.
Name 3 physics based artefacts
1) Beam hardening artefact
2) Photon starvation
3) Partial volume artefact
What is beam hardening artefact?
Xray beam is not monoenergetic so when passing through denser tissue, average energy increases because lower energy photons moved preferentially. This higher beam energy is wrongly interpreted as due to it going through through a less attenuating area, given a lower HU and looks more black.
What effects does beam hardening artefact produce?
1) Streak artefact of lower attenuation especially when two dense structures next to each other.
2) Cupping because beam travelling through centre harder than at periphery so get darker, lower HU in centre.
Four ways to correct for beam hardening artefact?
1) Increase kVP
2) Filtration (pre-harden beam to remove low energy)
3) DECT with virtual monoenergetic beam
4) Normalisation algorithm
What is photon starvation?
In a broad part eg) across shoulders or a big patient, photon fluence (number and average energy) is too low to properly penetrate before reaching detector therefore more noise, less signal (reduced SNR)
Three ways to correct for photon starvation?
1) Adaptive filtering
2) Reduce pitch <1 so slice overlap so twice as many xrays in this part
3) Automatic mA modulation (high tube current at thicker parts) form scout image
What is partial volume artefact?
Only part of the structure is contributing the the attenuation profile of the entire width of detector beneath. This attenuation is averaged out amongst its surroundings and attenuation is reduced from what it should be.
How to correct for partial volume artefact?
Reduce slice thickness or reduce detector width
