Download Dual Energy Imaging : Clinical applications for musculoskeletal

Dual Energy Imaging : Clinical applications for
musculoskeletal imaging.
Poster No.:
C-2561
Congress:
ECR 2013
Type:
Educational Exhibit
Authors:
C. Phan, A. Miquel, C. Pradel, M. KARA, L. Arrive, Y. Menu; Paris/
FR
Keywords:
Artifacts, Computer Applications-Detection, diagnosis, CT,
Musculoskeletal bone, Metabolic disorders
DOI:
10.1594/ecr2013/C-2561
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Learning objectives
Explain the principles of dual energy imaging.
Understand how multi-energy spectral analysis can be performed.
Highlight clinical applications for musculoskeletal imaging and limitations of the
technique.
Describe beam hardening caused by metal and how to reduce it.
Background
Background
There had been attempts to utilize spectral information for tissue characterization in
Computed Tomography in the late 1970s (1-4) but only recent advances in CT technology
had allowed Dual Energy CT to achieve a significant role in clinical radiology.
At that time two separate scans were acquired. However, the long scan times leading
to patient motion artifacts, the limited spatial resolution, and the difficulty of postprocessing were the main reasons why this technique remained unachieved. Since
2006, with recent advances in CT technology, Dual Energy CT had experienced
new developments with well-established clinical applications (5-7). Among these are
renal stone characterization, bone removal for carotid evaluation or peripheral runoff CT-angiography, virtual noncontrast imaging, myocardial and pulmonary perfusion,
quantification of iodine enhancement in lesions, identification of gout arthropathy (8), and
optimization of contrast image (9).
Principles of dual energy imaging
Attenuation (measured in Hounsfield Units) is caused by interaction between incoming
radiation and tissue. It is affected by the X-ray tube energy level and the tissue's physical
and chemical properties (such as thickness, atomic number and density). Photoelectric
effect and Compton Scattering account for the majority of attenuation encountered at the
energy levels used in diagnostic radiology.
At low energy level, Photoelectric effect absorption is more important, attenuation is
higher and related to both material density and atomic number. At high energy level,
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Compton Scattering predominates, attenuation is lower and related to the material's
density.
With single kVp imaging, it is not always possible to differentiate two materials with similar
CT attenuation. Clinical discriminations between iodinated contrast and hemorrhage;
uric acid and calcium oxalate; monosodium urate (MSU) deposition and calcification are
currently face in daily practice.
When two different X-ray tube voltages are used (dual energy at 80kVp and 140kVp), the
material attenuation values at low and high energy differ (Figure 1). According to their
atomic number, curves of material decomposition have been built showing the specificity
and the behavior of attenuation for a given material (Figure 2). One can determine the
amount of each material present in each voxel (Figure 3).
Thus, Dual Energy CT has the capability to characterize the chemical composition of a
material according to the differential x-ray photon -dependent energy attenuation of the
compound being studied at two different energy levels.
How does Dual Energy CT work?
Currently, Dual Energy CT data can be obtained using different techniques, including
dual-source CT scanning (dual-source, Siemens), rapid kVp switching (single source,
GE) every 0.3/0.5 ms, and multilayer "sandwich" detectors (Philips). Dual- and singlesource dual energy CT scanners are the most widely used scanners in research or clinical
practice.
In all cases, dual energy CT scanner enables acquisition of two data sets, one at high
(140kVp) and one at low energy (80kVp) level. Through the use of attenuation value
curves, the elementary chemical composition of a scanned tissue can be identified,
providing the ability to generate material decomposition images. The post-processing
differs depending on the constructor.
For the gout application, a three-material decomposition is applied with dual-source CT
(Figure 4).
Post-processing using dual energy single-source CT is based on paired-material (ex: uric
acid / calcium). Specific extraction of an elementary material's attenuation enables the
user to reconstruct images, deleting or enhancing one of its components. For example,
the two image sets could be obtained including virtual non-uric acid images, and virtual
non-calcium images. The comparison with hyperattenuation distribution on single-energy
image will allow differentiating monosodium urate (MSU) deposition seen in gout from
other crystal deposition arthropathy (Figure 5).
From the material decomposition images, a monochromatic spectral image can be
generated, showing how the material would look if the X-ray tube produced photons at
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a single energy level, ranging from 40 keV up to 140keV or 190keV depending on the
constructor.
The dual-energy CT scanner has no increased radiation dose compared to a singleenergy CT scanner at 120kVp.
How to reduce beam hardening?
CT plays a key role in evaluating orthopedic implants after surgery due to its high spatial
resolution and 3D reconstruction. Metal hardware causes beam-hardening of the X-ray
and photon starvation effects leading to dark bands or streak artifacts on CT images
that impact on orthopedic implant analysis, metallic-bone interfaces and adjacent tissue
analysis. These artifacts are related to tube voltage, current, image reconstruction, kernel,
hardware composition geometry and body region (10). Beam-hardening artefacts result
from the x-ray polychromaticity. Low-energy x-rays of the polychromatic x-ray beams are
preferentially attenuated through metallic prosthesis, which leads to an increase in the
average energy of the beam. A monochromatic x-ray does not lead to an increase in the
average energy and reduces the metallic artifacts.
Beam-hardening artifacts could be reduced by different approaches with acquisition of
image data at two different energy spectra with reconstruction of monochromatic image
or monoenergetic extrapolations that remove CT numbers shifts due to beam hardening.
Several studies (10-14) have demonstrated by iterative monoenergy reconstructions
that they decreased while image quality improved with increasing tube voltage.
Recommended monoenergies depending on the type of metal orthopedic devices and
the body regions vary from 105 KeV to 130 keV (14) and from 123 to 141 kev for posterior
spinal implants (13) (Figure 6).
The metal artefacts reduction software (MARs) is specific of CT device capable of
Gemstone Spectral Imaging (GSI). In this method, metal prosthesis can be segmented
in reconstructed image based on CT number threshold, reducing metallic artifacts for a
better delineation of the prosthesis itself, and the periprosthetic region. The use of MARS
is specially indicated for the analysis of bone-prothesis interface or periprothetic region
analysis for large prostheses such as knee arthroplasty, uni- or bilateral hip arthroplasty
(Figures 7, 8). When using GSI-MARs we should cautious of the slight degradation in
image quality (Figure 9). The GSI-CT with or without MARS does not increase radiation
dose.
Images for this section:
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Fig. 1: Figure 1 : Attenuation of two materials at different photon energies.
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Fig. 2: Figure 2 : Plots of CT numbers for fat, muscle, bone, and iodine at different photon
energies pointing how iodine differs in attenuation compared to the tissues. Attenuation is
function of the X-ray tube energy level and the tissue's physical and chemical properties.
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Fig. 3: Figure 3 : Principle of materiel decomposition used by the dual-energy software
(Leonardo, Siemens Healthcare). The software for gout algorithm (A) splits every voxel
in 80- and 140_kV image pair into three components represented by cortical bone,
trabecular bone, and uric acid. Compounds above line represent calcium (color-coded
in blue), and compounds below line represent uric acid (color-coded in green). Colorcoded material decomposition cross sectional (B) and volume render (C) images show
decomposition of uric acid in green.
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Fig. 4: Figure 4 : Flow diagram for dual-source dual-energy CT post-processing.
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Fig. 5: Figure 5 : Flow diagram for single-source dual-energy CT post-processing.
Attenuation curves of MSU and calcium are implemented in the GSI Viewer in order to
separate those two materials. In the virtual non-calcium images the voxels containing
MSU are highlighted while the voxels containing calcium disappeared from the images.
In the virtual non-acid-uric images, the voxels containing calcium are highlighted while
the voxels containing MSU disappeared from the images. Images are also displayed as
colour-coded maps based on attenuation and effective Z values; MSU is coded in red
and calcium in blue. Note that spatial resolution could be a limitation in detecting very
small MSU deposits.
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Fig. 6: Figure 6: Metal artefacts reduction. Axial conventional image (120kV) (A) of
posterior spinal fusion implant shows metal artefact enabling visualization of central canal
of the spine. Marked metal artefact reduction is seen for monochromatic reconstruction
at 110 keV (C) compared to 70 keV (B).
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Fig. 7: Figure 7 : Metal artifact reduction using virtual monochromatic images acquired
from single-source dual-energy CT data. A, Image shows pedicle screw acquired with
single-energy scan at 120 kV. B, Monochromatic image at 110 keV. Streaking artefact
caused by metal is almost completely eliminated.
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Fig. 8: Figure 8 : Metal artefact reduction using virtual monochromatic images acquired
from single-source dual-energy CT. Conventonial image (A) demonstrates metallic
artefacts that is markedly reduced on GSI-Mars image (B) allowing visualisation of intraarticular fluid in the left hip arthroplasty.
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Fig. 9: Figure 9 : Sagittal reconstruction at 110 kev (A) of posterior spinal fusion
implants of lombar spine. GSI-Mars image (B) shows degradation in image quality for the
evaluation of the L4-L5 intersomatc device.
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Imaging findings OR Procedure details
Dual energy CT (DECT) has recently shown to reliably identify monosodium urate (MSU)
deposition seen in gout from other crystal deposition arthropathy. This ability allows early
noninvasive diagnosis of gout and discrimination from other diseases that can mimic or
coexist with gout.
In case of suspicion of gouty arthropathy, areas of hyperattenuation on single-energy
images are identified as MSU deposit or calcium using image post-processing based
on a material decomposition and color code. This application is illustrated with various
clinical cases.
Example 1: 68-year-old patient with proved multifocal gout arthritis complaining for the
first time of midfoot pain (figure 10). Dense deposits are seen on monochromatic sagittal
image (A) in tarsal tunnel, talo-crural joint, and Lisfranc joint. There is an erosion (arrow)
adjacent to the hyperattenuation foci. Dense deposits are visible on MSU image (B) and
disappear on virtual non-calcium image (C). These findings allow concluding to articular
MSU crystal deposits characteristic of erosive gout arthritis also well depicted on STIR
sagittal image (D).
Example 2: 53-year-old woman with suspected CPPD disease of the knee (figure 11).
Monochromatic coronal image (A) demonstrates dense deposits in the cartilages with a
mean attenuation of 300 UH. Dense deposits disappear on virtual non-calcium image
(B), are visible on virtual non-MSU image (C), and are coded in blue on colour-coded
image (D), confirming CPPD disease.
Example 3: 50-year-old man with elbow bursitis (Figure 12). Hyperattenuation foci are
seen on single energy image within the tricipital tendon and olecranon bursae. Postprocessing (Leonardo, Siemens Healthcare) provides 2D and volume-rendered images
and confirms hyperattenuation foci as MSU deposits color-coded in green.
Images for this section:
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Fig. 10: Figure 10 : 68-year-old patient with proved multifocal gout arthritis complaining
for the first time of midfoot pain. Dense deposits are seen on monochromatic sagittal
image (A) in tarsal tunnel, talo-crural joint, and Lisfranc joint. There is erosion (arrow)
adjacent to the hyperattenuation foci. Dense deposits are visible on MSU image (B) and
disappear on virtual non-calcium image (C). These findings allow concluding to articular
MSU crystal deposits characteristic of erosive gout arthritis also well depicted on STIR
sagittal image (D).
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Fig. 11: Figure 11 : 53-year-old woman with suspected CPPD disease of the knee.
Monochromatic coronal image (A) demonstrates dense deposits in the cartilages with a
mean attenuation of 300 UH. Dense deposits disappear on virtual non-calcium image
(B), are visible on virtual non-MSU image (C), and are coded in blue on colour-coded
image (D), confirming CPPD disease.
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Fig. 12: Figure 12 : 50-year-old man with elbow bursitis. Hyperattenuation foci are seen
on single energy image within the tricipital tendon and olecranon bursae. Post-processing
(Leonardo, Siemens Healthcare) provides 2D and volume-rendered images and confirms
hyperattenuation foci as MSU deposits color-coded in green.
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Conclusion
DECT is a reliable technique that identifies MSU deposits within tophi and allows
evaluation to response to urate-lowering therapy. DECT with specific postprocessing
reduces MA and improves delineation of prosthesis and periprosthetic region. GSICT with or without MARs does not increase the radiation dose. However, potential
overcorrection when using GSI-Mars occurs.
References
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12. Lee et al. Metal artecat reduction in gemstone spectral imaging dual-energy CT with
and without metal artefact reduction software. Eur Radiol (2012) 22:1331-1340
13. Guggenberger et al. Metallic artefacts reduction with monoenergetic dual-energy CT:
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Personal Information
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