Dual-energy subtraction (DES) radiography is an advanced projection X-ray technique designed to reduce one of the biggest limitations of conventional radiography: anatomical overlap.
On a standard chest radiograph, ribs and clavicles overlap the lungs. A small pulmonary nodule may therefore be partially hidden by bone. DES uses X-ray information at different energies to separate bone and soft-tissue information, producing images that can make certain findings easier to visualize.
But does DES actually improve diagnosis? Does it increase radiation dose? And what changes for the radiographer?
The answer depends on the DES system and the clinical task.
Dual-energy subtraction radiography can produce:
DES is particularly associated with chest radiography because removing overlapping ribs and clavicles can improve visualization of the lungs.
Importantly, these are processed representations of the same anatomy. DES does not reveal anatomy that was never exposed to the X-ray beam; it uses differences in energy-dependent attenuation to separate information contained in the acquisition.
The physics behind DES comes from a familiar radiography principle: bone and soft tissue do not attenuate X-rays in exactly the same way at different photon energies.
At lower photon energies, photoelectric absorption becomes more important. Because calcium-containing bone has a higher effective atomic number than surrounding soft tissue, the difference between bone and soft-tissue attenuation is greater at lower energies.
Two-shot DES systems therefore commonly use approximately:
After acquisition, the system performs weighted image processing. By selecting an appropriate weighting, the system can suppress the contribution from bone to create a soft-tissue-selective image, or suppress much of the soft-tissue contribution to create a bone-selective image.
A two-shot system makes two rapid exposures: a low-energy exposure followed by a high-energy exposure. One published prospective protocol used approximately 150 milliseconds between exposures.
The high-energy exposure may also serve as the conventional radiograph.
Although the interval is short, movement can occur. If structures change position between exposures, they may not align perfectly during subtraction, producing misregistration artifact around structures such as the heart borders, diaphragm, ribs, clavicles, and other moving tissues.
Modern single-exposure systems can use a dual-layer flat-panel detector.
Instead of making separate low- and high-kVp exposures, the system makes one polychromatic X-ray exposure. The detector layers capture different energy-dependent information from that same exposure, allowing the system to generate conventional, bone-selective and soft-tissue-selective images.
Because both energy components are obtained during the same exposure, there is no inter-exposure interval, eliminating the characteristic inter-exposure misregistration problem of two-shot DES.
| Feature | Two-Shot DES | One-Shot DES |
|---|---|---|
| Acquisition | Two exposures | One exposure |
| Energy separation | Separate low- and high-energy exposures | Energy-dependent detector response |
| Typical two-shot technique | ~60 + ~120 kVp | Single polychromatic exposure |
| Time between energy images | Often ~100–150 ms | None |
| Inter-exposure motion artifact | Possible | Not applicable |
| Dose consideration | Second exposure adds dose | Avoids the separate second exposure |
DES is still projection radiography. A PA chest remains a PA chest. Patient positioning, centering and collimation generally follow the department's established protocol for the requested projection — see our chest X-ray positioning guide.
With a two-shot system, movement between exposures can create subtraction artifacts.
Before making the exposure:
With a one-shot dual-layer system, there is no motion between two separate energy exposures, although ordinary patient motion can still reduce image sharpness.
Dose depends on the DES architecture and protocol.
A two-shot system makes an additional exposure. In one published optimized protocol, the low-energy exposure was approximately 50% of the entrance surface dose of the high-energy exposure.
Example:
1.5× is not a universal DES dose multiplier. Dose depends on equipment design, patient size, detector technology, grid use, exposure settings, dose allocation, and local optimization.
A single-exposure dual-layer system avoids the separate second exposure required by two-shot DES. That does not mean every one-shot DES examination has an identical dose to every conventional chest radiograph. Dose comparisons must use the actual equipment and acquisition protocol.
The evidence is indication-specific.
A 2025 systematic review and meta-analysis evaluated 23 studies published between 1994 and 2022.
Reported pooled results included:
Removing overlapping bone can help make pulmonary nodules more conspicuous. However, DES remains projection radiography and does not replace CT.
A 2011 study found fracture sensitivity of 34.3% with conventional radiographs versus 33.5% with energy-subtracted bone images, without a significant improvement.
Readers nevertheless rated the energy-subtracted images higher for image quality.
Better image appearance does not necessarily equal better diagnostic performance.
A 2023 study involving 155 nodules in 139 patients found that adding one-shot DES soft-tissue images improved accuracy for all five readers when distinguishing calcified from non-calcified nodules, although the magnitude and statistical significance varied by reader.
A 2021 intra-individual comparison using CT as the reference standard reported:
The findings reinforce that additional processed images do not automatically improve every diagnostic task.
A 2024 Ontario Health technology assessment found the available evidence for outcomes such as line and tube tip visibility, diagnostic confidence and review time to be very uncertain.
Think of the conventional image as the anatomical anchor and use processed images as supplementary information.
Look for:
Look for:
An apparent abnormality on a processed image should be correlated with the conventional radiograph rather than interpreted in isolation.
Not necessarily. On a two-shot system, cardiac or respiratory motion can create misregistration resembling a discontinuity.
Not demonstrated. The cited rib-fracture study found improved image-quality ratings without improved fracture sensitivity.
Not necessarily. Two-shot DES adds another exposure. Single-exposure systems avoid that separate exposure, but actual dose still depends on the system and protocol.
Incorrect. The bone has been mathematically suppressed. Its absence on a soft-tissue-selective image does not establish that the bone itself is normal.
Dual-energy X-ray absorptiometry (DXA/DEXA) is primarily designed to measure bone mineral density. DES is an image-subtraction technique used in projection radiography. See our guide to the DEXA scan.
Dual-energy CT uses CT acquisition and reconstruction techniques to characterize materials using energy-dependent attenuation. DES remains projection radiography. For CT-related background, see CT vs MRI.
Tomosynthesis uses multiple projections over a limited angular range to reconstruct sectional information. DES separates information based on differences in X-ray energy.
Temporal subtraction compares a current radiograph with a previous radiograph to highlight interval change. Dual-energy subtraction uses energy-dependent information from an examination.
Work through the physics that decides dose: use the exposure and dose calculators to see how kVp and mAs interact, and revisit kVp and mAs explained for the underlying exposure factors.
Dual-energy subtraction radiography is an X-ray technique that uses energy-dependent differences in tissue attenuation to generate conventional, bone-selective and soft-tissue-selective images.
Ribs and clavicles can obscure pulmonary structures. Suppressing bone can make some underlying soft-tissue abnormalities easier to visualize.
It depends on the system. Two-shot DES adds a second exposure. Single-exposure dual-layer systems obtain energy-dependent information from one exposure and avoid the separate second exposure.
No. DES is projection radiography; dual-energy CT is a CT acquisition and reconstruction technique.
No. Research suggests DES can improve pulmonary-nodule detection compared with conventional radiography, but it remains a projection technique and should not be considered a replacement for CT.
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