Subscribe to RSS
DOI: 10.1055/s-0035-1569253
Dual-Energy CT: Basic Principles, Technical Approaches, and Applications in Musculoskeletal Imaging (Part 1)
Address for correspondence
Publication History
Publication Date:
22 December 2015 (online)
Abstract
In recent years, technological advances have allowed manufacturers to implement dual-energy computed tomography (DECT) on clinical scanners. With its unique ability to differentiate basis materials by their atomic number, DECT has opened new perspectives in imaging. DECT has been used successfully in musculoskeletal imaging with applications ranging from detection, characterization, and quantification of crystal and iron deposits; to simulation of noncalcium (improving the visualization of bone marrow lesions) or noniodine images. Furthermore, the data acquired with DECT can be postprocessed to generate monoenergetic images of varying kiloelectron volts, providing new methods for image contrast optimization as well as metal artifact reduction. The first part of this article reviews the basic principles and technical aspects of DECT including radiation dose considerations. The second part focuses on applications of DECT to musculoskeletal imaging including gout and other crystal-induced arthropathies, virtual noncalcium images for the study of bone marrow lesions, the study of collagenous structures, applications in computed tomography arthrography, as well as the detection of hemosiderin and metal particles.
#
Keywords
dual-energy computed tomography - crystal-induced arthropathies - metal artifact reduction - bone marrow edema - ironThe concept of spectral imaging, or dual-energy computed tomography (DECT), is an old one, described by Godfrey Hounsfield in one of the first descriptions of CT in 1973.[1] This technique had already been suggested as a tool to differentiate tissue materials by using different X-ray spectra. The first applications of DECT to the musculoskeletal system date back to the late 1970s and early 1980s.[2] [3] [4] However, due to technological limitations, its implementation on clinical scanners was delayed until more recent years, particularly when dual-source CT was introduced in 2006.[5] [6] [7] [8]
In the first part of this review article, we explain how DECT works including the different technical approaches currently available on the market, as well as radiation dose considerations.
Dual-Energy CT: How Does It Work?
DECT is based on the principle that the attenuation of tissues (reflected by their CT attenuation number in Hounsfield units [HUs]) depends on their density, but also on their atomic number Z, as well as on the energy of the photon beam. To understand how DECT works, we first review the basics of the interactions of the X-ray beam with tissues. The attenuation of tissues in CT is mainly due to two types of interactions between the X-ray beam and tissues: the photoelectric and the Compton effects. The Compton effect strongly depends on the electron density (ρe) of the material, which is correlated with mass density, but in the small range of photons' energies used in CT, the Compton effect does not depend on photon energy ([Fig. 1]). The Compton effect is the main determinant of soft tissue contrast. The photoelectric effect, in contrast, strongly depends on the effective atomic number (Zeff) of the material as well as on the energy of the photons. Lighter atoms, such as most atoms in soft tissues and water, do not present much of a photoelectric effect in the range of energies used in clinical CT scans. Calcium and iodine, however, are susceptible to the photoelectric effect at lower energy levels, which can be exploited to differentiate those materials.
![](https://www.thieme-connect.de/media/smr/201505/thumbnails/10-1055-s-0035-1569253-i00861-1.jpg)
![Zoom Image](/products/assets/desktop/css/img/icon-figure-zoom.png)
As seen in [Fig. 2], the attenuation coefficient of higher Z materials such as calcium or iodine is higher for lower beam energy levels. This is due to the photoelectric effect. In [Fig. 2], when using a single-energy level, such as the effective X-ray beam energy obtained at 120 kV, it is not possible to differentiate tissues containing calcium or iodine based on their attenuation level (both show CT numbers of ∼ 1,000 HU). However, when combining the information obtained by using two different energy levels of photon spectra (i.e., at effective energies produced when applying 80 and 140 kV), this differentiation can be made (iodine has a higher Zeff than calcium, so its CT number is higher at a lower beam energy). To show graphically the influence of the Zeff on CT numbers, it is convenient to represent the CT numbers at high and low energies as shown in [Fig. 3]. Because the photoelectric effect is highly dependent on Zeff, the higher the Zeff, the steeper the slope. The slope is a characteristic of the material, and the location of the value of a given pixel along this slope depends on density. Using these properties one can:
![](https://www.thieme-connect.de/media/smr/201505/thumbnails/10-1055-s-0035-1569253-i00861-2.jpg)
![Zoom Image](/products/assets/desktop/css/img/icon-figure-zoom.png)
![](https://www.thieme-connect.de/media/smr/201505/thumbnails/10-1055-s-0035-1569253-i00861-3.jpg)
![Zoom Image](/products/assets/desktop/css/img/icon-figure-zoom.png)
-
Decompose attenuation coefficients into two basis-material components and generate basis-material density images[9] ([Fig. 4])
-
Decompose the measured attenuation coefficients into Compton and photoelectric effects[10] [11]
-
Synthesize virtual monochromatic images at the desired energy level[13] [14] [15] ([Fig. 6])
![](https://www.thieme-connect.de/media/smr/201505/thumbnails/10-1055-s-0035-1569253-i00861-4.jpg)
![Zoom Image](/products/assets/desktop/css/img/icon-figure-zoom.png)
![](https://www.thieme-connect.de/media/smr/201505/thumbnails/10-1055-s-0035-1569253-i00861-5.jpg)
![Zoom Image](/products/assets/desktop/css/img/icon-figure-zoom.png)
![](https://www.thieme-connect.de/media/smr/201505/thumbnails/10-1055-s-0035-1569253-i00861-6.jpg)
![Zoom Image](/products/assets/desktop/css/img/icon-figure-zoom.png)
To separate attenuation coefficients into two basis-material components and generate basis-material density images, we must remember that the CT number of water is not energy dependent. Thus CT numbers of soft tissues (with Zeff values comparable to that of water) remain almost constant when varying the X-ray beam energy.
An example may illustrate the principle. A voxel contains a fraction fA of material A and fraction fB of material B. The goal is to assess fA and fB. To answer this, one must solve the simple set of two equations with two unknowns:
CTHigh = fA CT(A)High + fB CT(B)High
CTLow = fA CT(A)Low + fB CT(B)Low
CTHigh and CTLow are the CT numbers of the voxel measured, respectively, at high and low energies, so CT(A)High is the CT number of material A at high energy, and so on.
To get the best material characterization, DECT should be performed in these conditions:
-
Use two monochromatic (only one X-ray energy) beams with very different energy levels
-
Acquire both data sets simultaneously
-
Get images with the same quantity of photons on the detectors
However, CT technology is based on the use of conventional X-ray tubes that produce a spectrum of photons. The production of X-rays with this quite old technology remains the state of the art, but the use of a spectrum instead of monochromatic photons is associated with several limitations. The first one is beam hardening, which introduces a variation of the effective X-ray energy that depends on the quantity and type of material the beam will have to pass through. The second major limitation is the separation of the effective energies because X-ray spectra will always contain a fraction of low-energy photons. Another limitation of DECT resides in the need for heavy postprocessing, which is being standardized but remains time consuming.[16] [18]
Three main types of algorithms are currently in use to postprocess DECT data sets[16]: (1) Image optimization algorithms usually provide three sets of images: two sets of monoenergetic images (typically at 80 or 100 keV and 140 keV), as well as an “optimum contrast” image from nonlinear blending of the low-energy images (providing high contrast) and high-energy images (providing low noise). (2) Differentiation algorithms allow the subtraction of a certain material from the data set, or the differentiation between two materials, through color coding, for example. (3) Quantification algorithms are based on three-material decompositions and for example can provide color-coded images of iodine content in postcontrast examinations.
The current spectral imaging or DECT techniques are based on the principles detailed in [Table 1]. Each approach has advantages and drawbacks that must be considered carefully. To date, no study has compared the diagnostic performance of these different techniques.
Abbreviation: CT, computed tomography.
#
Radiation Dose Considerations in DECT
The radiation exposure required for DECT depends on the technology used. As discussed by Henzler et al, multiple studies have shown that dual-source DECT does not lead to increased radiation dose compared with conventional single-energy multidetector computed tomography. These data were mainly gathered from cardiac and chest applications.[19] This seems to also hold true for musculoskeletal applications. It has been shown that detection of bone marrow lesions in the knee and the ankle with dual-source DECT, using the virtual noncalcium technique, could be performed with a dose-neutral protocol (compared with conventional CT), taking advantage of the small cross-sectional scan area of the knee, compared with body applications.[20] [21] For peripheral joints, a slight increase in radiation dose might not be a problem because the effective dose to these small anatomical structures, away from any radiosensitive organs, is negligible.[22]
With dual-source DECT, radiation dose reductions strategies include tube current modulation and the use of tin filters with the high-energy spectrum (to get rid of lower energy quanta and optimize the separation between the high- and low-energy spectra). The increased contrast of the lower energy spectrum can also be used to compensate for increased noise (keeping contrast-to-noise ratio levels constant).
For single-source DECT with rapid kilovolt switching, the radiation dose is usually higher than conventional monoenergetic CT, with ratios up to three times more radiation.[23] When matched for image quality assessed by low-contrast detectability, the radiation dose remains roughly 22% and 14% higher with single-source DECT with rapid kilovolt switching compared with conventional monoenergetic CT (evaluated for head and body examinations).[24] This is at least partly due to the nonavailability of tube current modulation with the rapid kilovolt switching technique.
For both dual-source and rapid kilovolt switching techniques, further radiation dose reduction is possible in various musculoskeletal conditions with the association of iterative reconstruction techniques with newer generation scanners[16] [25] [26] [27] (see the dose optimization articles by Omoumi et al in this issue).[28] [29] For other techniques of DECT, there is no or only scarce literature addressing radiation dose issues.
#
Conclusion
With its unique ability to differentiate basis materials by their atomic number, DECT has opened new perspectives in imaging. Used appropriately, it should not lead to an increase in radiation dose. Several image reconstruction algorithms have been developed to improve image contrast (i.e., by generating monoenergetic images), to characterize or subtract certain materials in the image, as well as to perform automatized volumetric measurements.
#
#
* F. Becce and F.R. Verdun contributed equally to this work.
-
References
- 1 Hounsfield GN. Computerized transverse axial scanning (tomography). 1. Description of system. Br J Radiol 1973; 46 (552) 1016-1022
- 2 Fleischmann D, Boas FE. Computed tomography—old ideas and new technology. Eur Radiol 2011; 21 (3) 510-517
- 3 Genant HK, Boyd D. Quantitative bone mineral analysis using dual energy computed tomography. Invest Radiol 1977; 12 (6) 545-551
- 4 Kan WC, Wiley Jr AL, Wirtanen GW , et al. High Z elements in human sarcomata: assessment by multienergy CT and neutron activation analysis. AJR Am J Roentgenol 1980; 135 (1) 123-129
- 5 Van Abbema JK, Van der Schaaf A, Kristanto W, Groen JM, Greuter MJW. Feasibility and accuracy of tissue characterization with dual source computed tomography. Phys Med 2012; 28 (1) 25-32
- 6 Chinnaiyan KM, McCullough PA, Flohr TG, Wegner JH, Raff GL. Improved noninvasive coronary angiography in morbidly obese patients with dual-source computed tomography. J Cardiovasc Comput Tomogr 2009; 3 (1) 35-42
- 7 Kalender WA, Buchenau S, Deak P , et al. Technical approaches to the optimisation of CT. Phys Med 2008; 24 (2) 71-79
- 8 Flohr TG, McCollough CH, Bruder H , et al. First performance evaluation of a dual-source CT (DSCT) system. Eur Radiol 2006; 16 (2) 256-268
- 9 National Institute of Standards and Technology (NIST). NIST XCOM: Photon Cross Sections Database. Available at: http://www.nist.gov/pml/data/xcom/ . Accessed December 3, 2015
- 10 Manohara SR, Hanagodimath SM, Thind KS, Gerward L. On the effective atomic number and electron density: a comprehensive set of formulas for all types of materials and energies above 1 keV. Nucl Instrum Methods Phys Res B 2008; 266 (18) 3906-3912
- 11 Yang M, Virshup G, Clayton J, Zhu X, Mohan R, Dong L. WE-C-BRB-01: In vivo measurement of proton stopping power ratios in patients using dual energy computed tomography. Med Phys 2009; 36: 2757
- 12 Yang M, Virshup G, Clayton J, Zhu XR, Mohan R, Dong L. Theoretical variance analysis of single- and dual-energy computed tomography methods for calculating proton stopping power ratios of biological tissues. Phys Med Biol 2010; 55 (5) 1343-1362
- 13 McCullough EC. Photon attenuation in computed tomography. Med Phys 1975; 2 (6) 307-320
- 14 Hawkes DJ, Jackson DF. An accurate parametrisation of the x-ray attenuation coefficient. Phys Med Biol 1980; 25 (6) 1167-1171
- 15 Henson PW. Determination of electron density, mass density and calcium fraction by mass of soft and osseous tissues by dual energy CT. Australas Phys Eng Sci Med 1989; 12 (1) 3-10
- 16 Johnson TRC. Dual-energy CT: general principles. AJR Am J Roentgenol 2012; 199 (5, Suppl): S3-S8
- 17 Omoumi P, Verdun F, Guggenberger R, Andreisek G, Becce F. Dual-energy CT: basic principles, technical approaches, and applications in musculoskeletal imaging (Part 2). Semin Musculoskelet Radiol 2015; 19 (5) 438-445
- 18 Schoepf UJ, Colletti PM. New dimensions in imaging: the awakening of dual-energy CT. AJR Am J Roentgenol 2012; 199 (5, Suppl): S1-S2
- 19 Henzler T, Fink C, Schoenberg SO, Schoepf UJ. Dual-energy CT: radiation dose aspects. AJR Am J Roentgenol 2012; 199 (5, Suppl): S16-S25
- 20 Pache G, Bulla S, Baumann T , et al. Dose reduction does not affect detection of bone marrow lesions with dual-energy CT virtual noncalcium technique. Acad Radiol 2012; 19 (12) 1539-1545
- 21 Guggenberger R, Gnannt R, Hodler J , et al. Diagnostic performance of dual-energy CT for the detection of traumatic bone marrow lesions in the ankle: comparison with MR imaging. Radiology 2012; 264 (1) 164-173
- 22 Biswas D, Bible JE, Bohan M, Simpson AK, Whang PG, Grauer JN. Radiation exposure from musculoskeletal computerized tomographic scans. J Bone Joint Surg Am 2009; 91 (8) 1882-1889
- 23 Ho LM, Yoshizumi TT, Hurwitz LM , et al. Dual energy versus single energy MDCT: measurement of radiation dose using adult abdominal imaging protocols. Acad Radiol 2009; 16 (11) 1400-1407
- 24 Li B, Yadava G, Hsieh J. Quantification of head and body CTDI(VOL) of dual-energy x-ray CT with fast-kVp switching. Med Phys 2011; 38 (5) 2595-2601
- 25 Tobalem F, Dugert E, Verdun FR , et al. MDCT arthrography of the hip: value of the adaptive statistical iterative reconstruction technique and potential for radiation dose reduction. AJR Am J Roentgenol 2014; 203 (6) W665-W673
- 26 Omoumi P, Verdun FR, Ben Salah Y , et al. Low-dose multidetector computed tomography of the cervical spine: optimization of iterative reconstruction strength levels. Acta Radiol 2014; 55 (3) 335-344
- 27 Becce F, Ben Salah Y, Verdun FR , et al. Computed tomography of the cervical spine: comparison of image quality between a standard-dose and a low-dose protocol using filtered back-projection and iterative reconstruction. Skeletal Radiol 2013; 42 (7) 937-945
- 28 Omoumi P, Becce F, Ott J, Racine D, Verdun F. Optimization of radiation dose and image quality in musculoskeletal CT: emphasis on iterative reconstruction techniques (Part 1). Semin Musculoskelet Radiol 2015; 19 (5) 415-421
- 29 Omoumi P, Verdun F, Becce F. Optimization of radiation dose and image quality in Musculoskeletal CT: emphasis on iterative reconstruction techniques (Part 2). Semin Musculoskelet Radiol 2015; 19 (5) 422-430
Address for correspondence
-
References
- 1 Hounsfield GN. Computerized transverse axial scanning (tomography). 1. Description of system. Br J Radiol 1973; 46 (552) 1016-1022
- 2 Fleischmann D, Boas FE. Computed tomography—old ideas and new technology. Eur Radiol 2011; 21 (3) 510-517
- 3 Genant HK, Boyd D. Quantitative bone mineral analysis using dual energy computed tomography. Invest Radiol 1977; 12 (6) 545-551
- 4 Kan WC, Wiley Jr AL, Wirtanen GW , et al. High Z elements in human sarcomata: assessment by multienergy CT and neutron activation analysis. AJR Am J Roentgenol 1980; 135 (1) 123-129
- 5 Van Abbema JK, Van der Schaaf A, Kristanto W, Groen JM, Greuter MJW. Feasibility and accuracy of tissue characterization with dual source computed tomography. Phys Med 2012; 28 (1) 25-32
- 6 Chinnaiyan KM, McCullough PA, Flohr TG, Wegner JH, Raff GL. Improved noninvasive coronary angiography in morbidly obese patients with dual-source computed tomography. J Cardiovasc Comput Tomogr 2009; 3 (1) 35-42
- 7 Kalender WA, Buchenau S, Deak P , et al. Technical approaches to the optimisation of CT. Phys Med 2008; 24 (2) 71-79
- 8 Flohr TG, McCollough CH, Bruder H , et al. First performance evaluation of a dual-source CT (DSCT) system. Eur Radiol 2006; 16 (2) 256-268
- 9 National Institute of Standards and Technology (NIST). NIST XCOM: Photon Cross Sections Database. Available at: http://www.nist.gov/pml/data/xcom/ . Accessed December 3, 2015
- 10 Manohara SR, Hanagodimath SM, Thind KS, Gerward L. On the effective atomic number and electron density: a comprehensive set of formulas for all types of materials and energies above 1 keV. Nucl Instrum Methods Phys Res B 2008; 266 (18) 3906-3912
- 11 Yang M, Virshup G, Clayton J, Zhu X, Mohan R, Dong L. WE-C-BRB-01: In vivo measurement of proton stopping power ratios in patients using dual energy computed tomography. Med Phys 2009; 36: 2757
- 12 Yang M, Virshup G, Clayton J, Zhu XR, Mohan R, Dong L. Theoretical variance analysis of single- and dual-energy computed tomography methods for calculating proton stopping power ratios of biological tissues. Phys Med Biol 2010; 55 (5) 1343-1362
- 13 McCullough EC. Photon attenuation in computed tomography. Med Phys 1975; 2 (6) 307-320
- 14 Hawkes DJ, Jackson DF. An accurate parametrisation of the x-ray attenuation coefficient. Phys Med Biol 1980; 25 (6) 1167-1171
- 15 Henson PW. Determination of electron density, mass density and calcium fraction by mass of soft and osseous tissues by dual energy CT. Australas Phys Eng Sci Med 1989; 12 (1) 3-10
- 16 Johnson TRC. Dual-energy CT: general principles. AJR Am J Roentgenol 2012; 199 (5, Suppl): S3-S8
- 17 Omoumi P, Verdun F, Guggenberger R, Andreisek G, Becce F. Dual-energy CT: basic principles, technical approaches, and applications in musculoskeletal imaging (Part 2). Semin Musculoskelet Radiol 2015; 19 (5) 438-445
- 18 Schoepf UJ, Colletti PM. New dimensions in imaging: the awakening of dual-energy CT. AJR Am J Roentgenol 2012; 199 (5, Suppl): S1-S2
- 19 Henzler T, Fink C, Schoenberg SO, Schoepf UJ. Dual-energy CT: radiation dose aspects. AJR Am J Roentgenol 2012; 199 (5, Suppl): S16-S25
- 20 Pache G, Bulla S, Baumann T , et al. Dose reduction does not affect detection of bone marrow lesions with dual-energy CT virtual noncalcium technique. Acad Radiol 2012; 19 (12) 1539-1545
- 21 Guggenberger R, Gnannt R, Hodler J , et al. Diagnostic performance of dual-energy CT for the detection of traumatic bone marrow lesions in the ankle: comparison with MR imaging. Radiology 2012; 264 (1) 164-173
- 22 Biswas D, Bible JE, Bohan M, Simpson AK, Whang PG, Grauer JN. Radiation exposure from musculoskeletal computerized tomographic scans. J Bone Joint Surg Am 2009; 91 (8) 1882-1889
- 23 Ho LM, Yoshizumi TT, Hurwitz LM , et al. Dual energy versus single energy MDCT: measurement of radiation dose using adult abdominal imaging protocols. Acad Radiol 2009; 16 (11) 1400-1407
- 24 Li B, Yadava G, Hsieh J. Quantification of head and body CTDI(VOL) of dual-energy x-ray CT with fast-kVp switching. Med Phys 2011; 38 (5) 2595-2601
- 25 Tobalem F, Dugert E, Verdun FR , et al. MDCT arthrography of the hip: value of the adaptive statistical iterative reconstruction technique and potential for radiation dose reduction. AJR Am J Roentgenol 2014; 203 (6) W665-W673
- 26 Omoumi P, Verdun FR, Ben Salah Y , et al. Low-dose multidetector computed tomography of the cervical spine: optimization of iterative reconstruction strength levels. Acta Radiol 2014; 55 (3) 335-344
- 27 Becce F, Ben Salah Y, Verdun FR , et al. Computed tomography of the cervical spine: comparison of image quality between a standard-dose and a low-dose protocol using filtered back-projection and iterative reconstruction. Skeletal Radiol 2013; 42 (7) 937-945
- 28 Omoumi P, Becce F, Ott J, Racine D, Verdun F. Optimization of radiation dose and image quality in musculoskeletal CT: emphasis on iterative reconstruction techniques (Part 1). Semin Musculoskelet Radiol 2015; 19 (5) 415-421
- 29 Omoumi P, Verdun F, Becce F. Optimization of radiation dose and image quality in Musculoskeletal CT: emphasis on iterative reconstruction techniques (Part 2). Semin Musculoskelet Radiol 2015; 19 (5) 422-430
![](https://www.thieme-connect.de/media/smr/201505/thumbnails/10-1055-s-0035-1569253-i00861-1.jpg)
![Zoom Image](/products/assets/desktop/css/img/icon-figure-zoom.png)
![](https://www.thieme-connect.de/media/smr/201505/thumbnails/10-1055-s-0035-1569253-i00861-2.jpg)
![Zoom Image](/products/assets/desktop/css/img/icon-figure-zoom.png)
![](https://www.thieme-connect.de/media/smr/201505/thumbnails/10-1055-s-0035-1569253-i00861-3.jpg)
![Zoom Image](/products/assets/desktop/css/img/icon-figure-zoom.png)
![](https://www.thieme-connect.de/media/smr/201505/thumbnails/10-1055-s-0035-1569253-i00861-4.jpg)
![Zoom Image](/products/assets/desktop/css/img/icon-figure-zoom.png)
![](https://www.thieme-connect.de/media/smr/201505/thumbnails/10-1055-s-0035-1569253-i00861-5.jpg)
![Zoom Image](/products/assets/desktop/css/img/icon-figure-zoom.png)
![](https://www.thieme-connect.de/media/smr/201505/thumbnails/10-1055-s-0035-1569253-i00861-6.jpg)
![Zoom Image](/products/assets/desktop/css/img/icon-figure-zoom.png)