The image from a CT study is a representation of the total attenuation per voxel within the imaged subject. Dual-energy CT (DECT) utilises the photoelectric effect to separate out different materials within the voxel based upon their different attenuations at different beam energies.
The photoelectric effect, as we’ve mentioned before, is the ejection by a photon of an electron from the innermost K-shell of an atom. An electron from the next shell fills the empty space. The energy from this is released in the form of a photoelectron. The photoelectric event can only occur if the incident photon has enough energy to overcome the binding energy of the electrons within the K-shell.
| Substance | K Edge (keV) | Atomic Number (Z) |
| Hydrogen | 0.01 | 1 |
| Carbon | 0.28 | 6 |
| Nitrogen | 0.40 | 7 |
| Oxygen | 0.53 | 8 |
| Calcium | 4.00 | 20 |
| Iodine | 33.20 | 53 |
| Barium | 37.45 | 56 |
| Gadolinium | 50.20 | 64 |
When the incident photon has an energy just above the K-shell binding energy there is a sudden jump in attenuation (K-edge) because of the increased photoelectric absorption. The K-shell binding energy and, therefore, the K-edge, depends on the element and it increases as the atomic number increases.
We can analyse the attenuation of material over different beam energies to determine the constituents of that material. As an example, if we have an element with a K-edge at 70 keV and one at 130 keV:
Techniques
There are several ways to perform dual-energy CT.
Dual-source DECT
- Two x-ray tubes of different voltages with two sets of detectors paired to the x-ray tubes. The x-ray tubes are at 90° to each other.
- Pros:
- Simultaneous acquisition and processing at the two energies leads to quicker acquisition, good overlap of images acquired at the two voltages.
- Can independently optimise the signal-to-noise ratio for each x-ray tube-detector pair
- Cons:
- Increased dosage (two scans acquired)
- Scattered radiation from one tube may be detected by the detector for the other tube
Single-source DECT
- Single x-ray tube and two detectors
- Several methods:
- Consecutive: two helical scans acquired consecutively at different tube potentials. Motion can lead to poor overlap of each image. Increased dosage (two scans acquired)
- Sequential: each single axial rotation performed at high- and low-tube potential. Increased dosage (two scans acquired) but lower likelihood of motion creation poor overlap.
- Fast kVp switch: x-ray tube switches rapidly between high- and low-tube potential multiple times within the same rotation. Lower dosage (one scan acquired). Requires very fast data sampling and may result in large overlap between high and low energies.
Detector-based spectral CT
- Single x-ray tube with a single high tube potential beam
- Single detector but made of two layers (sandwich detector) that simultaneously detects two energies.
- Dual-energy analysis can be performed on every data set acquired
Applications of DECT
Separate out materials
- Perfused blood volume / blood pool imaging e.g. pulmonary embolus and myocardial ischaemia
- Create virtual unenhanced images by removing iodine
- Atherosclerotic plaque removal
- Virtual non-calcium images – remove calcium to identify iodine uptake in bones i.e. bone marrow oedema
- Detect silicon from breast implant leakage
Hepatic
- Detection and characterisation of hepatic lesions – much more sensitive to enhancement within small hepatic lesions
Renal
- Distinguish hyperattenuating renal cysts from enhancing renal cell carcinoma
- Identify renal calculi within contrast-filled renal collecting system
- Characterise composition of renal calculi – specifically differentiating between uric acid containing and non-uric acid containing stones
Oncology
- Increased lesion conspicuity = better delineation of margins = more accurate size measurements
- Iodine maps can differentiate bland thrombus from tumour
- Better assessment of response to treatment
Vascular imaging
- Lower kVp is closer to K-edge of iodine than the 120 kVp used in normal imaging i.e. can use lower dose of contrast and maintain quality of imaging
- Create virtual unenhanced images
- Atherosclerotic plaque removal
Artifact reduction
- Reduce beam-hardening artifacts from metal and iodine
- Reduce photon starvation artifacts
Written by radiologists, for radiologists with plenty of easy-to-follow diagrams to explain complicated concepts. An excellent resource for radiology physics revision.
Σ Summary
- Dual-energy CT utilises photoelectric effect
- Different materials attenuate individually along the beam energy spectrum – use this to separate out materials based on different attenuation at a high (140 kVp) and low (80 kVp) potentials
Techniques:
- Dual-source: two x-ray tubes and two detectors
- Simultaneous acquisition = faster, good overlap
- Independently optimise tube-detector pair
- Two scans = higher patient dose
- Scatter from one system may be detected by other
- Single-source: single x-ray tube and two detectors
- Consecutive
- Sequential
- Fast kVp switch
- Detector-based spectral CT: single z-ray tube, single sandwich detector made of two layers to detect two energies
Applications:
- Create virtual unenhanced images
- Use lower contrast dose
- Aterhosclerotic plaque removal
- Virtual non-calcium images for MSK
- Detect silicon from breast implant leakage
- Better visualisation and characterisation of renal and hepatic lesions
- Artifact reduction
