Dual energy CT uses two x-ray spectra to distinguish materials based on their differential attenuation properties. This allows reconstruction of various image sets and material-specific images without contrast. Key applications include bone removal, virtual non-calcium imaging, uric acid stone differentiation, gout detection, perfusion imaging, and differentiating enhancing lesions from calcification. Dual energy CT provides material-specific information useful for diagnosis and treatment planning in various clinical contexts.

1. DUAL ENERGY CT
1
Dual energy CT, also known as spectral CT, is a computed tomography
technique that uses two separate x-ray photon energy spectra, allowing the
interrogation of materials that have different attenuation properties at
different energies.
Whereas conventional single energy CT uses single X-ray photon energy
spectra and produces a single image set, dual energy data (attenuation
values at two energy spectra) can be used to reconstruct numerous image

2. SINGLE ENERGY CT
2

3. The settings of 80 and 140 kVp are generally used because
they provide maximum difference and least overlap between
the spectra with standard tubes.

4. BASIS OF DECT
○ Attenuation is energy dependent- at 80 kVp, iodine signal is
2X of 140 kVp
○ CT numbers do not vary with beam energy for soft tissues but do for
high “z” materials
○ By analysing absorption properties of different material at different x-
ray energies, materials can be distinguished. (basis material
decomposition)
○ Human body is made up of many different elements- mainly carbon,
oxygen , hydrogen , nitrogen, phosphorous and calcium which are
arranged in many different combinations.
○ Hydrogen, carbon, nitrogen and oxygen have similar k- edges which
are well below the energies currently used in DECT (80 kVp and 140
kVp), thus not well appreciated.
○ The k edges of calcium and iodine are higher than those of soft
tissues and although they are lower than those, they are
distinguished from soft tissues at DE imaging.

5. PRINCIPLE OF DECT
○The principle of dual energy CT is based on differential absorption of
energy(Linear attenuation coefficient) at variable KVP settings.
○For example: Let us consider a substance (A)with K-edge at 60 kv another
(B)with K-edge 130 kv, if we imagine multiple combination of A and/or B at 80KVp
and 140KVp ,there will be differential attenuation at both these energy settings.
○The object containing larger amount of substance A will show higher
attenuation at 80KVp and lower attenuation at 140 KVp, whereas object
containing larger amount of substance B will show higher attenuation at 80KVp
and at 140KVp as well.
○The CT number of blood mixed iodine becomes 1550HU similar to the value of
bone at 140 kvp. so it is nearly impossible to determine weather the object of interest
is bone or iodine blood mixed structure.
○If we take another scan of same object with on 80kvp setting , the measures CT
number will be different .if the object of interest is made with bone the CT number
will be 2200HU and the object is mixture of iodine and blood , the CT number will be
2800HU due to different behaviour of linear attenuation co-efficient as a function of
energy for different materials.

6. Energy-dependent x-ray absorption behavior of different materials in the body, plotted as Hounsfield unit
values using low (y-axis) and high (x-axis) energy x-ray spectra.Water and air are calibrated to 0 and −
1000HU, respectively, at all kVp, and thus lie on the identity line, as do many of the soft tissues in the
body. Iodine and calcium (yellow and red lines and regions of interest on the low and high energy
images) demonstrate higher HU values at lower energy, with measured attenuation increasing as their
concentrations increase, along lines with a characteristic slope, the“dual energy ratio.” Fat (purple) and
uric acid (light green) fall below the identity line, as they demonstrate lower attenuation at lower x-ray
energy. Any materials that lie substantially away from the identity line are good targets for DECT material
characterization techniques

7. Applications in Clinical
Practice
Bone removal in angiography
As DECT has the ability to differentiate different materials
from one another, calcium in the bones is readily separated
from iodine in the contrast media. Hence the bones in
any angiography study can be easily removed.
• Similarly, calcified plaques can
be removed for better visualisation of the lumen

8. A routine cerebral CT angiography study (A) shows
carotid and cerebral arteries overlapped by bones making it difficult
to visualise them. With DECT bone removal tool, all the bones are
effectively and selectively removed allowing clear visualisation of the
arteries with AVM (arrow) as shown in this MIP image (B). Similarly
the arteries with AVM (arrow) can also be evaluated in VRT image
(C) with bone removal

9. As DECT can differentiate calcium from other elements, it can also
selectively remove calcium from the region of interest. This is called
virtual non-calcium (VNC) technique and it allows visualisation of bone
marrow edema.
Multiple studies have validated this technique for bone marrow edema
detection.
Bone marrow edema

10. (A-D): CT image in coronal reconstruction (A) and 3D
reconstruction (B) shows fractures (arrows) involving the neck and the
greater tuberosity of the right humerus. With DECT, the bone marrow
edema (arrows) can be seen in the the neck and the greater tuberosity
of the humerus as seen in DECT coronal reconstruction (C) and DECT

11. Using DECT, the artefacts from metal can be significantly reduced to almost nil. As
the artefacts are more at lower energy x-ray beams, the monochromatic images at
higher keV settings show significantly less artefacts.
Additionally, iterative reconstruction protocols such as iMAR (interactive metal
artefact reduction) can be used that further reduce the metal artefacts. When both
the techniques are used together, the final result shows almost nil metal artefacts.
Ideal for peri prosthetic soft tissue assesment.
Metal artefact reduction

12. metal artefact reduction in a patient with bilateral total hip replacement. Higher
keV images shows less metal artefacts and addition of iterative reconstruction
(iMAR) further reduces
the metal artefacts and when both techniques are combined, the final
image shows almost nil artefacts

13. As different types of renal calculi are treated differently, knowledge of the
composition of stones may guide management decisions and predict the
effectiveness of therapy. For example, uric acid calculi can be managed
medically with urine alkalinisation that facilitates dissolution, while non-uric acid
calculi need other lines of treatment such as lithotripsy or surgery.
Using DECT, it is possible to differentiate uric acid from non-uric acid calculi.
Keeping the dual energy ratio at 1.05, the uric acid stones lie below the line/slope
and are colored red, whereas the non-uric acid stones lie above the line/slope and
are differently color-coded (blue)
URINARY STONE

14. (A-C): Another example of urinary calculi (arrows), two calculi are seen in the
kidney in the CT scan image (A). DECT the calculus
red (B) in colour and falling below the line (C), thus confirming uric acid calculi

15. Gout
Gout is characterized by deposition of monosodium urate (MSU) crystals in the
joints and soft tissue.
Diagnosis can be made on clinical and biochemical bases, and definite diagnosis
requires microscopic demonstration of MSU crystals from the aspiration of the joint
fluid, a method that is invasive and may have false negative results.
Keeping the dual energy ratio specific to uric acid crystals,
the uric acid crystals are coloured green, whereas calcium
is coloured blue. The diagnosis of gout can be easily made,
non-invasively.

16. 35 years male presented with pain and swelling
in right foot and wrist. Serum uric acid-13 mg/dl. Radiographs of both feet, knees and wrists
(A) shows juxta-articular erosions (arrows)around right 1st metatarsophalangeal joint, right
lateral femoral condyle, right distal ulna and 5th carpometacarpal joint with soft tissue.
Diagnosis of gout was made. B. DECT (B) allows visualisation of the urate crystals seen as
green colored areas (arrows), confirming the diagnosis. Also,it shows involvement of the
asymptomatic and radiographically occult areas and displays total tophus volume

17. Pulmonary thromboembolism
The dual energy iodine maps allow us to evaluate
lung perfusion and hence are useful to demonstrate
perfusion defects in cases of pulmonary thromboembolism.

18. A-E): CT Pulmonary angiography axial
(A) and coronal
(B) images shows extensive bilateral
pulmonary thromboembolism (arrows).
The HRCT axial (C) image shows a
developing infarct (arrow)
in the posterior segment of the right upper
lobe. The DECT images(D and E) shows
perfusion defects (arrows) in the lung
parenchyma bilaterally with the infarct in
the right upper lobe
(A-E): The follow up CT Pulmonary
angiography axial (A) and
coronal (B) images, after 3 months of
anticoagulation treatment shows
complete resolution of the bilateral pulmonary
thromboembolism. The
HRCT axial (C) image shows resolution of the
infarct in the right upper
lobe. The DECT images (D and E) shows
reversal of the perfusion
defects in the lung parenchyma bilaterally

19. Lung nodule
With DECT a single contrast enhanced scan is sufficient to differentiate an
enhancing nodule from calcification with the use of virtual non enhanced image
reconstructions.
The plain HRCT (A) and contrast (B) axial images
shows an ill-defined spiculated enhancing mass (arrow) in the right
upper lobe with mediastinal lymphadenopathy (short arrow). The DECT
axial images shows the more enhancing part with higher iodine uptake
in the medial part of the mass (C), which can be targeted for biopsy,
while the lateral part (D) shows relatively less enhancement

20. 53 years female, treated case of carcinoma
breast,
came for follow up scan. The contrast
enhanced abdomen image
(D) shows a hypodense lesion (arrow) in
the left lobe of the liver.
The DECT iodine maps (A-C) shows
significant iodine uptake (arrow)
suggestive of an enhancing lesion,
worrisome for metastatic disease,
which was later proved with biopsy
A-D): The contrast enhanced abdomen image
(D) shows a
hypodense lesion (arrow) in the left lobe of the
liver. The DECT iodine
maps (A-C) shows no significant iodine uptake
(arrow) suggestive of
a simple cyst