INTRO: Spectral CT Flashcards
(32 cards)
What are the two dominant phenomena for X-ray attenuation in biological tissue at CT energies?
Compton scatter and Photoelectric effect. Both have dependencies on energy and Effective Atomic Number (Zeff).
Which attenuation phenomenon dominates at lower vs higher atomic numbers?
Compton scatter dominates at lower atomic numbers, while the photoelectric effect dominates at higher atomic numbers.
How does energy-dependent attenuation vary with effective atomic number (Zeff)?
Materials with larger Zeff (photelectric) tend to show increased attenuation at lower energies,
while materials with smaller Zeff (Compton) tend to show relatively constant attenuation across the energy range. This is due to the photoelectric and Compton phenomena.
What is Spectral CT?
An advanced form of CT that acquires X-ray data at multiple energy levels (typically 2), allowing differentiation of materials based on their energy-dependent attenuation properties.
What are the key capabilities enabled by Spectral CT?
Material decomposition, virtual non-contrast imaging, and quantitative iodine mapping, significantly enhancing tissue characterization.
What is a good conceptual analogy for understanding Spectral CT?
Think of Spectral CT as a “chemical spectrometer for the body” - not detecting elements directly, but deducing their presence based on how they “shine” back (attenuate) at different X-ray energies.
What exactly “decomposes” in spectral CT material decomposition?
It’s not physical decomposition - nothing breaks apart. What decomposes is the measured attenuation data for each voxel. Mathematical algorithms solve for how much of each basis material is likely present in each voxel.
How does spectral CT measure attenuation for decomposition?
Spectral CT detectors measure attenuation at 2 (sometimes more) energy levels. A mathematical algorithm then solves for how much of each basis material is likely present in each voxel. will show percentage of the basis materials (2).
In two-material decomposition, what does a point between two basis material curves represent?
It represents a linear combination of the two basis materials. For example: closer to the iodine line = voxel with 87% iodine content/13% soft tissue;
closer to the soft tissue line = 62% soft tissue/38% iodine;
exactly in the middle = roughly 50% of each material.
How does material decomposition create meaningful images?
The decomposition turns attenuation data into pixel values that reflect real materials, not just grayscale attenuation. Once decomposition is done, you can create different types of meaningful images via the spectral signature.
What is a material’s spectral signature?
A material’s spectral signature is the ratio between the photoelectric effect and Compton scatter as a function of X-ray photon energy.
What does “cross-section” mean in radiation physics and spectral CT?
Cross-section is a measure of probability that a photon will interact with an atom in a specific way. In spectral CT, it’s analogous to a target size of the atom that an incoming photon will interact with. It’s not a literal surface area - it’s a quantitative probability. The larger the cross-section, the more likely that interaction will occur.
What is iodine’s K-edge energy and why is it significant?
Iodine’s K-edge = 33.2 keV. At this energy, attenuation jumps dramatically because photons now have enough energy to eject K-shell electrons, creating a spike in the photoelectric cross-section.
Spectral Signature Equation
Spectral Signature(E) = μ_photoelectric(E) / μ_Compton(E)
Where:
E represents photon energy
μ_photoelectric(E) is the photoelectric absorption coefficient as a function of energy
μ_Compton(E) is the Compton scattering coefficient as a function of energy
Why the ratio matters:
Photoelectric Effect:
Happens more with low-energy X-rays
Strongly depends on the atomic number of the material
Heavy elements (like iodine contrast) have high photoelectric absorption
Compton Scattering:
Happens more with high-energy X-rays
Less dependent on atomic number
More consistent across different materials
significance of the equtation
This equation helps identify what materials are inside your body during medical imaging (like CT scans).
It creates a unique “fingerprint” for different materials based on how they interact with X-rays
Different tissues, contrast agents, or substances have different spectral signatures
By comparing these two effects at different X-ray energies, you can:
Distinguish materials - Bone vs. soft tissue vs. contrast agent
Improve image quality - Reduce artifacts and noise
Quantify composition - Measure how much of each material is present
Enable dual-energy imaging - Use two different X-ray energies to get more information
What does the K-edge mean for iodine’s spectral signature?
The sharp rise at the K-edge defines part of iodine’s spectral signature. Spectral CT recognizes iodine by this characteristic spike. The energy level immediately above this surge gives iodine its highest level of attenuation.
How can attenuation increase when there’s a loss of a K-shell electron?
Because when photon energy exceeds the binding energy of the K-shell electron (33.2 keV for iodine), it suddenly can eject that tightly bound electron
What’s the difference between kVp and keV?
kVp = maximum energy a photon in the X-ray beam can have; keV = energy of an individual photon in that beam. They’re related but not equal.
What energy range does a 120 kVp tube produce?
A 120 kVp tube emits a range of photons, from about 15 keV up to 200 keV.
The effective energy (average or most probable) is roughly 1/3 to 1/2 of the kVp. in this case 40 -60keV is ideal energy
Why is 80 kVp optimal for iodine imaging?
At 80 kVp, the effective photon energy is 35-40 keV. This range falls just above iodine’s K-edge (33.2 keV), where iodine’s photoelectric absorption is highest, yielding: higher contrast enhancement, stronger signal on iodine maps, and better lesion conspicuity.
How does spectral CT benefit brain CT with contrast?
Small vascular structures like the Circle of Willis or leptomeningeal vessels benefit from iodine-rich signal. In stroke or tumor assessment, low kVp (80) scans provide better delineation of enhancing lesions or infarcted areas (but with increased noise).
What are the advantages for abdominal CT with contrast?
Organs like liver, kidneys, and pancreas enhance well with iodine at 80 kVp. Portal venous phase imaging is particularly crisp due to heightened iodine-to-soft tissue contrast. as gives kev of 40 , optimal for iodine
What is a Monoenergetic (MonoE) image?
A MonoE image simulates what a CT scan would look like if acquired with a pure X-ray beam at a single energy level (40, 70 or 100 keV), instead of a broad spectrum like 120 kVp. (~55-60keV). It’s not a real scan - it’s reconstructed from spectral data using mathematical modeling.
How are MonoE images created?
Every voxel has dual-energy attenuation data that are fit to two basis materials. From that fit, you can calculate what attenuation would be at any energy you choose - that’s your MonoE image at 40, 70, or 100 keV.
