CHALLENGES WITH SINGLE ENERGY CT
Lesion detection (small lesion)
Image noise and quality at low kV scanning
Complex scanning protocols
ATTENUATION ON CT
Material differentiation on CT is based on x-ray
Attenuation is caused by absorption and scattering
Two main mechanisms: compton scatter and
Compton effect is energy independent
Photoelectric effect is strongly energy dependent and its
likelihood increases as the energy of incident photon
approximates the K-shell binding energy of an electron.
Changing the kV setting results in an alteration of
BASIS OF DECT
Attenuation is energy dependent- at 80 kVp, iodine signal is
2X of 140 kVp
CT numbers donot vary with beam energy for soft tissues but
do for high “z” materials
By analyzing absorption properties of different material at
different x-ray energies, materials can be distinguished. (basis
Human body is made up of many different elements- mainly
carbon, oxygen , hydrogen , nitrogen, phosphorous and
calcuim- 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.
The settings of 80 and 140
kVp are generally used
because they provide
maximum difference and
least overlap between the
spectra with standard tubes.
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 behavior of linear
attenuation co-efficient as a function of energy for different materials.
TECHNICAL APPROACHES TO DECT
Five approaches : (currently only former three are
Rapid voltage switching
Layer detectors and
Energy resolving or quantum counting detectors
Requires least hardware effort.
Achieved either as two subsequent helical scans or
as a sequence with subsequent rotations at
alternating tube voltages and stepwise table feed.
Disadvantage is rather long delay between both
acquisitions, causing artifacts from cardiac or
respiratory motion or changes in CM opacification.
But, a viable option for clinical applications w/o CM
such as metal artifact removal or kidney stone
RAPID VOLTAGE SWITCHING
Tube voltage alternates between a high value (140kV)
and a low value (80 kV) and data are collected twice for
Rotation speed of system must be reduced to account
for the acquistion of these additional projections and rise
and fall times of voltage modulation.
To account for the higher tube output at 140 kVp, the
exposure time ratio is varied between the 80-kVp and
the 140-kVp acquisitions to maximize the contrast-to-
noise ratio: typically, 65% of the exposure time is used
for the 80-kVp acquisition, and 35%, for the 140-kVp
A detector with a fast response and a data acquisition
system with a fast sampling capability are used to
capture the alternating high- and low-energy data.
The advantages of dual-energy CT with fast kilovoltage
switching are good temporal registration between high-
and low-energy datasets, which are obtained nearly
simultaneously, and the availability of the full 50-cm
field of view for use in image analysis.
Limited photon output at low voltages which results in
high noise and necessity to choose a relatively high
current and consequently high dose.
So requires additional dose because other dose
reduction features(ATCM) or optimized filtration are not
However, because a single x-ray source is used,
individual modification of the high- and low-energy x-ray
beams is difficult (not yet possible on commercially
available scanners), and spectral overlap is increased.
DUAL SOURCE CT
Two tubes running at different voltages and corresponding
detectors mounted orthogonally in one gantry.
High-energy scans are obtained at 120 or 140 kVp, and low-
energy scans are obtained simultaneously at 80 or 100 kVp.
One detector somewhat smaller than other, resulting in 33 cm
Nearly twofold investments in hardware but offers important
advantages for DECT:
Voltage, current and filter can be chosen independently for both
tubes to achieve an optimal contrast .
Although there is an angular offset between both spiral paths,
there is no temporal offset in data acquisition because equivalent
z-axis positions are scanned at same time.
Disadvantage is cross-scatter radiation, which partially hits the
noncorresponding orthogonal detector and requires
Uses an energy resolving detector with
polychromatic spectrum of one tube.
The sensitivity of two layers is determined by the
scintillator material : for eg: consisting of ZnSe or
CsI in the top layer and Gd2O2S in the bottom
The scintillator materials determine the spectral
resolution and sensitivity profiles of available
materials have a rather broad overlap.
Therefore the contrast of spectral information is
limited or requires high additional dose.
Not yet available for clinical use.
QUANTUM COUNTING DETECTOR
Resolve the energy of each individual impacting
Used to differentiate more than two photon
energies and is very quantum efficient.
But detector materials eg; CdZnTe get saturated
rather quickly, resulting in a rapid drift of the
Thus used to scan small animals, but cannot
handle a photon flux required for clinical CT.
Direct comparison of different DECT approaches should
include both spectral contrast and dose optimally quantified as
CNR per dose.
Comparison for different scanner setups is made based on
Currently a maximum spectral contrast is achieved with a
DSCT with optimized voltage, current and filtration.
Technologic strategies that allow dose reduction include; tube
current modulation, iterative reconstruction techniques and
new detector application- specific integrated circuits (ASICs)
integrating photodiode and analog digital converters.
These features offer special benefits for DECT because CNR
is improved in both half-dose acquisitions with the two energy
spectra so the gain in dose efficieny is even greater than
single energy CT for DSCT. 20
1. Data-domain or projection-space decomposition:
To subtract equivalent projections and apply filtered
back projection to reconstruct the difference as
i.e. dual energy data sets are reconstructed before
images are reconstructed from high- and low-energy
Used for material analysis of dual-energy CT images
obtained with fast kilovoltage switching.
Projection-space decomposition is preferred because
it enables greater flexibility in material decomposition
and permits the preprocessing correction of data to
minimize beam-hardening artifacts 21
2. Image-domain decomposition:
First reconstruct standard CT images consisting of
voxels in HU and then to use postprocessing
algorithms to extract specific spectral information from
the difference between the corresponding voxels.
Dual-energy datasets are processed after the
reconstruction of high- and low-energy images.
Used for material analysis of images obtained with a
dual-source or dual-layer dual-energy CT scanner
IRS provides low and high kV images and a series of
weighted average images. The average series
integrates both acquisitions in a low noise image for
DE analysis is then performed on the dual kV series
using imaging based algorithms.
3 main types of algorithm are in use:
Optimization : First type optimizes images
Results in altered gray level CT images
Monoenergetic images in which the density for each voxel is
extrapolated to a certain energy from the two density values at
the acquired photon energies, and non linear blending algorithms
which combine high iodine contrast and low noise referred to as
Differentiation : Second type identifies or differentiates
Define a slope between the density values at both acquired
spectra and differentiate material on the basis of the
photoelectric effect within a certain density range- that is, colors
are assigned on both sides of the slope.
Another possibility is to eliminate certain substances from a
dataset by identifying the substance and then filling in for
example air density for the corresponding voxels. 23
• Quantification: Third type quantifies a substance in the
• Use a three material decomposition, quantifying one of three
• A slope is defined by the density of two basic components and a
second slope is defined by the photoelectric effect of the contrast
material being quantified.
• The density values measured at both energies are then interpreted
as a displacement from the first slope along the second one.
• This enhancement is then color coded or is also subtracted from the
In fast kV switching, 140 kV images are reconstructed
immediately at the scanner console, and are only used
to verify the adequacy of anatomic coverage.
To facilitate workflow, 80 kV images are not routinely
Calibration correction are applied to the high and low
energy datasets, which are aligned in projection space
and transformed into material basis pair projections,
which are then used to reconstruct two main types of
Material density images, which provide material specific
Monochromatic images which provide energy selective
Generation of material density images is based on the
theory of basis material decomposition, which proposes
that the attenuation coefficients of any material can be
computed as a weighted sum of the attenuation
coefficients of two basis materials as long as the k-edge
of the material is not within the evaluated energy range.
The two basis materials should have substantially
different mass attenuation coefficients; thus the material
pairs chosen usually differ greatly in efffective atomic
Substances other than chosen basis materials are
considered to contain combinations of both selected
Two sets of material density images are then generated
each demonstrating the presence or absence of each
basis material selected.
Two commonly selected dual energy CT basis material
Iodine and calcium and
Iodine and water
when iodine (high atomic number) is paired with water
(low atomic number), two separate sets of images (a set
of iodine density images and a set of water density
images) are generated.
On the water density image, voxels that show an energy
dependent change in attenuation resembiling the
attenuation of iodine are removed and represented
instead on the iodine density image.
The virtual unenhanced images thus generated from CE
dual energy data set can provide information equivalent
to that obtainable from unenhanced images.
Iodine density images are used to asses structures for
areas of enhancement. Only lesions that contain iodine
will show higher density. 27
DECT images can be processed to obtain images
at any specified single photon energy. Such images
are called monochromatic images.
Monochromatic projections can be generated
during the processing of material density image
data by calcualting the linear attenuation coefficinet
of an object.
Because they are generated from projection space
data, monochromatic images are less affected by
beam hardening artifact and provide more accurate
CT numbers than do standard CT images.
These advantages can improve the characterization
of renal lesions by decreasing pseudoenhancement
in simple renal cysts. 29
With DSDECT, images are generated by linear or
nonlinear sigmoidal blending of high and low
energy image datasets.
Material specific images are generated by
measuring the differences in attenuation and either
highlighting the pixels corresponding to the selected
material or subtracting the pixels corresponding to
the other materials.
For abdominal applications, a three material
decomposition (soft tissue, fat and iodine) process
is usually used to generate the virtual unenhanced
A color coded overlay image also can be generated
which enables visualization of the distribution of a
selected material across entire CT volume.
Neurological applications permit the
generation of virtual non-contrast
images for the detection of brain
hemorrhages in patients who undergo
Also allow the removal of bone and
calcium from the carotid and brain
Possible to detect a brain hemorrhage
on virtual non-contrast images.
Comparison of dual energy bone
removal to digital subtraction CTA and
automatic bone removal reveals
superiority of dual energy technique,
particularly at the level of skull base.
Radiation dose was also reduced as
compared to digital subtraction CTA.
Lack of misregistration and visualization of lung perfusion and
Misregistration avoided due to simultaneous acquisition of 80
and 140 kVp images.
In pt.s with pulmonary thromboembolism, DECT may allow the
detection of subtle emboli by revealing perfusion defects.
Assessment of lung perfusion can allow the visualization of
pathologies that have been perviously unknown, particularly in
pt.s with ILDs, emphysema, asthma or chronic
thromboembolic disease and in patients with tumors.
Xenon DECT enables collection of ventilation perfusion CT
acquisitions and in future may replace ventilation perfusion
Superior registration of DECT may demonstrate the presence
or lack of enhancement of sub-centimeter and solitary lung
Dual energy perfusion with or without the adenosine
stress test, viability imaging and cardiac iron detection.
To decrease image noise, 100 and 140 kVp acquisitions
Addition of an iterative reconstruction technique may
allow a better image quality for cardiac applications.
Can be difficult in patients with elevated heart rates due
to a worsening of the temporal resolution from 83 to 165
ms in DSCT.
Also for the characterization of plaques, for calcific
plaque removal from coronary arteries and for
evalutation of coronary stents.
Imaging protocol for aortic aneurysms consists of
unenhanced, arterial and venous phase images, which
exposes the patient to a high radiation burden because
it is repeated every six months after aortic stent grafting.
DE aortic stent graft protocol may obviate the need for
unenhanced CT and iodine map images may facilitate
the recognition of endoleaks.
Can also permit the faster removal of calcific plaques in
large arteries and bony structures in cranial region.
However the use of this technique seems difficult for
small sized arteries.
Also no sytdy has demonstrated any advantage of dual
energy CTA for peripheral artery applications.
GI AND ABDOMINAL APPLICATIONS
The use of dual energy has revealed that 80 kVp
images demonstrate a better contrast than do 140
Useful for evaluation of enhanced liver lesions in
the arterial phase in HCC and in hypervascular liver
Also used for the diagnosis of liver iron and fat,
which display opposite spectral patterns.
The measured denisty of a liver increases with
decreasing kVp and keV in patients with hepatic
iron overload, but the measured density of a liver
decreases with decreasing kVp and kev in patients
with steatosis. 44
DE cholangiography may facilitate the detection of
bile ducts and the measurement of biliary segment
Invitro studies have been performed to characterize
the dual energy of gall stones and a similar protocol
may be used for patients with biliary dilation and a
suspected choleduct stone.
The use of 80 kvp data obtainde by DECT may
permit a better distinction of pancreatic
adenocarcinoma from the adjacent normal
parenchyma by increasing the conspicuity of the
Obviates the non-contrast prone images from
diagnostic CT colonoscopy protocols.
Colonic polyps and masses are enhanced approx.
40-50 HU on post contrast images. Thus the
enhancement of colonic masses can be
differentiated from stool through the use of iodine
DECT images and non cathartic DECT colonoscopy
may be feasible, esp. in elderly patients.
Most useful MSK application is the differentiation of
gout and pseudogout via the diagnosis of uric acid
and calcium crystals in the joint space.
Tendon and ligament visualization has been
proposed by CT vendors but few studies have
evaluated this application.
Metal artifact reduction has been proposed for the
fast kVp switching technique, wich emplyos images
with a low keV.
Renal mass characterization:
Paired iodine and water density images obtained at contrast enhanced
DECT can help distinguish hyper attenuating cysts from enhancing
masses without the need for an unenhanced scanning phase.
The greater brightness of renal masses on iodine density images helps
distinguish them from non enhancing cysts even when their attenuation
measurements on CECT images are similar.
Color coded image overlays in each pixel allow easy visualization of
presence or absence of enhancement.
Also water density images may be used as virtual unenhanced images
to allow the detection of calcification within a renal mass.
Limitations: Virtual unenhanced images are noisier than real
Because of material decomposition algorithms used, calcification in
renal lesions is less conspicuous on virtual unenhanced images than on
Smaller amounts of fat in renal masses are difficult to detect on virtual
images and can be measured only on conventional thin section CT
Characterization of renal stones:
Different types of renal calculi; calcium based stones- 70-
80%,struvite stones- 5-15%, uric acid stones 5-10%, and
cystine stones 1-2.5%.
Knowledge about the composition of stones may guide
decisions about theire management and shape
expectations concerning the effectiveness of therapy.
Uric acid calculi can be managed with oral medications
that facilitate dissolution, struvite calculi are amenable to
ESWL, whereas calcium oxalate monohydrate and
cystine calculi are resistant to fragementation with
at dual energy, the change in attenuation between high
and low energy scans can be used to differentiate types
of calculi, some of which might have a similar attenuation
when scanned at single energy level.
With the ability to generate virtual unenhanced
images from contrast enhanced dual energy CT
image datasets, dual energy CT urography can
lessen the need for an unenhanced scanning
But small stones may be less well depicted on
virtual images than on actual unenhanced images.
WHAT WE HAVE
application in syngo.via
Restriction of FOV ( with DSCT)
High radiation dose
Addition of a tin filter helps to decrease
Increase in radiation dose can be justified when
unenhanced images are eliminated from protocols,
which may result in dose saving.
Noise in the 80 kVp images
Evaluations of patients with a high BMI.
Noise constraints may result in a suboptimal image
Volumetric DECT acquisitions enables the emergence of
new applications with potential benefits.
Major advantages of DECT are material decomposition,
separation of iodine from the image and prevention of
misregistration particularly in thorax and abdomen, and
renal mass and stone characterization in the urinary
The use of iterative reconstruction techniques can
facilitate the wider use of DECT applications by
decreasing the noise and the radiation dose.
These technologies may eventually lead to better
detection and characterization of lesions in the body and
objective evaluation of iodine uptake by various organs,
leading to DECT becoming an alternative of PET-CT.
FIND MORE AT:
Johnson TR; Dual energy CT: general principles
Muşturay Karçaaltıncaba, Aykut Aktaş; Dual-energy
CT revisited with multidetector CT: review of
principles and clinical applications
Johnson TR, Kraus B, et.al.; Material differentiation
by Dual energy CT: initial experience
Kaza RK, Plat FJ et.al.; Dual-Energy CT with
Single- and Dual-Source Scanners: Current
Applications in Evaluating the Genitourinary Tract