Looking at all medical scientific literature in MedLine, there are about 200 articles on dual energy CT, about three quarters of which were published in the last three years. These studies mostly report new clinical applications of dual energy CT and their diagnostic value. Although the field is only emerging, the research activity is so extensive that it is getting difficult to keep an overview. Also, it may be difficult to estimate the actual clinical practicability and value, even if there are articles published on a certain technique or application.

Therefore, the purpose of this review is to provide an overview of all the relevant applications of Dual Energy CT reported so far and to give account of the actual value in clinical practice.

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Download in pdf format: Dual Energy CT review article (3.6 MB)

Dual Source CT: clinical applications

The ability to map iodine content in soft tissue organs can be used to study the contrast enhancement of focal lesions, e.g. in the liver or kidney. The CT scan is obtained in normal venous phase. The iodine-related enhancement is color-coded in the image and superimposed with the normal CT image.
Additionally, a virtual non-contrast image can be derived from the contrast picture. It looks very similar to a true non-contrast image except that image noise is higher and spatial resolution is reduced. These images are mostly sufficient to determine the contrast enhancement of focal lesions. Because enhancement is determined in one dataset, misregistration, e.g. due to different respiratory positions, is impossible. An initial clinical study has shown that measured density values of virtual non-contrast images agree well with true non-contrast images, although the noise is somewhat higher (1).

Pulmonary Perfusion and Ventilation Imaging

If a three material decomposition is applied for iodine, soft tissue, and air, the perfusion of lung parenchyma can be mapped by iodine content. Shortly after arrival of the contrast bolus, the Dual Energy technique enables acquisition of a pulmonary angiogram and assessment of perfusion of the lung parenchyma in the same dataset. Initial studies have confirmed that occlusive embolism causes segmental perfusion defects, and that patchy perfusion patterns are observed in recurrent pulmonary embolism (2). Also, there is good agreement between perfusion scintigraphy with technetium-labeled macroaggregated albumin and Dual Energy perfusion maps (3).
If xenon gas is administered instead, ventilation of the lung parenchyma can be visualized. A short inhalation with a limited concentration avoids narcotic effects, but monitoring is necessary. Combining ventilation and perfusion imaging with information about the morphology and structure of the lung parenchyma may make it possible to use Dual Energy CT to perform a comprehensive workup of complex pulmonary diseases.

Dual Energy in Angiography

Another option is to use the spectral properties of iodine to differentiate it from other dense materials in the dataset. In angiography, an image can be acquired rapidly and easily with maximum intensity projections (MIPs), similar to magnetic resonance angiography (MRA).
With Dual Energy CT, it is possible to identify bone by its spectral behavior and to erase it from an angiogram. Then, the iodine in the vessels remains the only dense material in the dataset and a MIP can be calculated from a CT angiogram to closely resemble an MRA. Additionally, it is possible to detect those voxels that contain both calcium and iodine and add them back to the dataset. Calcified plaques of atherosclerotic vessels can thereby be switched on and off in the dataset to visualize both the residual lumen and the plaque distribution. This works very reliably both in supraaortic and runoff angiograms, providing an excellent overview of the vasculature and making it fast and easy to exclude relevant stenoses in a single image (4, 5).

Differentiation of Kidney Stones

An application that does not require contrast material is the differentiation of renal calculi. The three most frequent and clinically relevant types of renal stones are: calcified stones (74%), uric acid (15%) and struvite stones (11%).
Calcium and struvite stones can only be removed mechanically or crushed by an extracorporeal shock wave lithotripsy (ESWL), while uric acid calculi can be dissolved with Allopurinol and urine alkalization. While calcium and struvite (i.e. magnesium ammonium phosphate) contain ions with spectral properties, the spectral behavior of uric acid is rather weak.
A clinical in vitro and in vivo study has demonstrated the reliability of differentiating stones using this approach (6). Thus, it is possible to reliably differentiate uric acid stones from other types of renal calculi and to plan treatment accordingly without extracting the stone. It is even possible to detect uric acid in gout tophi (7).

Differentiation of Tendons and Ligaments

Our initial experience has shown that tendons and ligaments have weak spectral properties, presumably due to the densely packed collagen. It is possible to identify thick tendons and ligaments in Dual Energy CT datasets and to display them separately, for example, to visualize the tendons of the wrist and identify ruptures. However, signal-to-noise ratio is not sufficient to depict thin ligaments; thus the clinical value of this application is limited.

Related articles: Dual Energy CT – an Introduction, Technical Implementation, Physics Background

References
1.    Graser A, Johnson TR, Hecht EM, et al. Dual-energy CT in patients suspected of having renal masses: can virtual nonenhanced images replace true nonenhanced images? Radiology 2009; 252:433-440.
2.    Thieme SF, Johnson TR, Lee C, et al. Dual-energy CT for the assessment of contrast material distribution in the pulmonary parenchyma. AJR Am J Roentgenol 2009; 193:144-149.
3.    Thieme SF, Becker CR, Hacker M, Nikolaou K, Reiser MF, Johnson TRC. Dual Energy CT for the Assessment of Lung Perfusion – Correlation to Scintigraphy. Eur J Radiol 2008; 66.
4.    Morhard D, Fink C, Graser A, Reiser MF, Becker C, Johnson TR. Cervical and cranial computed tomographic angiography with automated bone removal: dual energy computed tomography versus standard computed tomography. Invest Radiol 2009; 44:293-297.
5.    Sommer WH, Johnson TR, Becker CR, et al. The value of dual-energy bone removal in maximum intensity projections of lower extremity computed tomography angiography. Invest Radiol 2009; 44:285-292.
6.    Graser A, Johnson TR, Bader M, et al. Dual energy CT characterization of urinary calculi: initial in vitro and clinical experience. Invest Radiol 2008; 43:112-119.
7.    Johnson TR, Weckbach S, Kellner H, Reiser MF, Becker CR. Clinical image: Dual-energy tomographic molecular imaging of gout. Arthritis Rheum 2007; 56:2809.computed

For Dual Energy scanning, the tube potentials are mostly set to 140 and 80 kVp to obtain the largest spectral difference. With the additional filter on the 140 kVp spectrum, 100 kVp can be used as a lower-energy spectrum for some applications. This is especially relevant in the trunk of the body where 80 kVp tube potential can result in insufficient transmission. The protocols can be used to replace routine protocols for the respective application. Regarding radiation exposure, the tube currents are adapted so that the dose is largely equivalent to that of the routine protocol. This implies that the individual 80 and 140 kVp images are noisier than a normal CT image obtained at 120 kVp with the same dose. However, weighted average images are calculated by the image reconstruction system to resemble normal 120 kVp CT images in spectral properties and image noise. These images are immediately available for reading during the clinical routine.
Manufacturer-independent dose measurements with thermoluminiscent detectors in Alderson phantoms have also confirmed that the dose efficiency is very similar, with a marginal increase in noise but also an improved contrast-to-noise ratio at an equivalent dose.

Data handling

The foci of both x-ray tubes of the DSCT scanner are located in the same plane of the z-axis. Projection data are acquired in an angular offset of 90 degrees, and there are no equivalent projections at both potentials. Thus, immediate postprocessing of the projection data is not possible or would be very cumbersome. Postprocessing is therefore performed on the reconstructed images of both acquisitions. Specific Dual Energy CT convolution kernels handle the data in both the 80 and 140 kVp dataset equivalently in order to avoid density changes at object edges that are not caused by differences in spectral behavior.

Image analysis and iodine mapping

The images are further analyzed with a ‘three material decomposition’ (1) implemented in the postprocessing software. The CT numbers are analyzed to assign three constituents of an underlying tissue, e.g. in liver parenchyma, these would be fat, soft tissue, and iodine. The mean CT number at both potentials reflects the relative amount of soft tissue and fat in that voxel. If the CT number at 80 kVp is higher than at 140 kVp, this can be attributed to a certain concentration of iodine. The stronger the enhancement at 80kVp in relation to that at 140 kVp, the higher the concentration of iodine. Thus, the iodine content can be mapped in the image. Also, the iodine map can be subtracted from the normal CT image to obtain a ‘virtual non-contrast’ image.

Related articles: Dual Energy CT – an Introduction, Physical Background, Clinical Applications

References:
1.    Johnson TR, Krauss B, Sedlmair M, et al. Material differentiation by dual energy CT: initial   experience. Eur Radiol 2007; 17:1510-1517.

Different photon energies are required for Dual Energy CT. In reality, the photons produced by the X-ray tubes of a Dual Energy CT scanner have various energies, not only two distinct energy levels, as the term may suggest. The X-ray sources available today that can produce enough quanta for diagnostic imaging are tubes with rotating anodes. They have polychromatic spectra consisting of a continuous spectrum of the Bremsstrahlung, interrupted by characteristic lines of the tungsten material of the anode. If both tubes are run with the largest possible difference, i.e. at 140 and 80 kVp, the resulting mean photon energies are 76 and 56 keV, so the difference is smaller than one may expect (1). Second generation Dual Source CT systems have an additional 0.4 mm tin filter that eliminates the 140 kVp spectrum of low-energy quanta. This decreases overlap of the spectra and increases hardness of the spectrum to a mean photon energy of 92 keV (2).

Attenuated energy

Furthermore, the investigated material must have some spectral properties, i.e. differences in X-ray attenuation at different photon energies. Only then does differentiation from other material or quantification become possible. The X-ray attenuation at relevant photon energies is caused by the Compton Effect, coherent scatter, and photo effect. The first two show only small differences in different atoms. However, the photo effect shows a strong relationship with the atomic number, i.e. with the weight of the atomic nucleus of the material. Most of the atoms in the human body, i.e. hydrogen, carbon, nitrogen or oxygen, have a rather weak photo effect. Some ions such as calcium or magnesium have a somewhat stronger effect, while the photo effect of iodine is comparatively very strong. This difference can be observed in CT images obtained at lower tube potential, in which vascular enhancement from iodine contrast material is much stronger compared to the density at high voltages. This is the reason why angiographic exams are partially obtained at 80 kVp. This difference in spectral behavior can be used to detect and semi-quantify iodine in CT images.

Related articles: Dual Energy CT – an Introduction, Technical Implementation, Clinical Applications

References:
1.    Johnson TR, Krauss B, Sedlmair M, et al. Material differentiation by dual energy CT: initial experience. Eur Radiol 2007; 17:1510-1517.
2.    Petersilka M, Bruder H, Krauss B, Stierstorfer K, Flohr TG. Technical principles of dual source CT. Eur J Radiol 2008; 68:362-368.

Step-and-Shoot coronary angiography
Various strategies have been developed to reduce radiation exposure. The most important one is the prospectively electrocardiography (ECG)-gated CT coronary angiography, also called step-and-shoot (SAS) mode. The X-ray exposure time of this technique is short, and thus, is associated with low radiation doses ranging between 1.2 to 4.3 mSv  [1]. Most importantly, the SAS method is accurate [2] with a similar performance as the retrospectively ECG-gated spiral cardiac CT protocols.

The high-pitch-mode – the entire heart in a single heart cycle
With the recent advent of the second generation dual-source CT scanner equipped with two 128-slice acquisition detectors (Somatom Definition Flash; Siemens Healthcare, Forchheim, Germany), another low-dose technique, i.e., the high-pitch mode was introduced for cardiac imaging. With this mode, data acquisition is also prospectively triggered with the ECG of the patient. However, the data now is acquired in a spiral mode while the table runs with a very high pitch of 3.4, equaling a table feed of 43 cm/s. Using this high-pitch mode, the entire heart can be scanned within one single cardiac cycle, usually during diastole.

Conclusions
First reports on phantoms, animals, and humans have shown the ability of this scan mode to deliver images of diagnostic quality at a low radiation dose [3, 4, 5]. Most importantly, a recent study has shown that this high-pitch technique of the Flash-Scanner is highly accurate [6]. A prerequisite of this technique is a regular heart rate equal to or below 60–63 beats per minute. Effective radiation dose of a CT coronary angiography study employing the high-pitch mode is on average below 1 mSv. Thus, the high-pitch mode, as compared to the SAS mode, further lowers down the effective radiation doses while maintaining the high image quality and excellent performance characteristics.

1. Stolzmann et al., Radiology 2008;249:71
2. Scheffel et al., Heart 2008;94:1132-7), -80
3. Lell et al., Eur Radiol 2009;19:2576-83;
4. Hausleiter et al., JCCT 2009;3:236-242;
5. Achenbach et al., JCCT 2009;3:117-121
6. Leschka et al., Eur Radiol 2009; Sep 16. (Epub ahead of print)

Furthermore, it was evident that it would be possible to run both x-ray tubes at different potentials to obtain different x-ray spectra. It was conceivable that this would make further material differentiation possible versus that achieved with x-ray attenuation in Hounsfield units.

The idea of Dual Energy CT was not entirely new when DSCT became available. Already in the late 1970s there had been attempts to exploit the spectral information of CT scans acquired at different tube potentials(5-7). However, because it was necessary to scan the object twice, the distribution of iodine would change between both acquisitions, making detection of iodine in the dataset impossible. With DSCT, simultaneous acquisition has become feasible, opening the door to a multitude of new applications.

By now, Dual Energy CT is ready for routine clinical use. Several scan protocols offer additional clinically relevant information without additional contrast material or dose. Clinical applications include, for example, the assessment of contrast enhancement in focal organ lesions, pulmonary perfusion and ventilation imaging, angiography with bone removal and display of plaque distribution, kidney stone differentiation, and visualization of tendons and ligaments.

If this article was interesting for you, read other articles from our Dual Energy series by Thorsten Johnson:

• Technical Implementation of Dual Energy CT
• Physics of Dual Energy CT
• Clinical Applications of Dual Energy CT

References:

1. Flohr TG, McCollough CH, Bruder H, et al. First performance evaluation of a dual-source CT (DSCT) system. Eur Radiol 2006; 16:256-268.
2. Johnson TR, Nikolaou K, Wintersperger BJ, et al. Dual-source CT cardiac imaging: initial experience.Eur Radiol 2006; 16:1409-1415.
3. Johnson TR, Nikolaou K, Busch S, et al. Diagnostic accuracy of dual-source computed tomography in the diagnosis of coronary artery disease. Invest Radiol 2007; 42:684-691.
4. Leber AW, Johnson T, Becker A, et al. Diagnostic accuracy of dual-source multi-slice CT-coronary angiography in patients with an intermediate pretest likelihood for coronary artery disease. Eur Heart J 2007.
5. Avrin DE, Macovski A, Zatz LE. Clinical application of Compton and photo-electric reconstruction in computed tomography: preliminary results. Invest Radiol 1978; 13:217-222.
6. Chiro GD, Brooks RA, Kessler RM, et al. Tissue signatures with dual-energy computed tomography. Radiology 1979; 131:521-523.
7. Millner MR, McDavid WD, Waggener RG, Dennis MJ, Payne WH, Sank VJ. Extraction of information from CT scans at different energies. Med Phys 1979; 6:70-71.

No hollow promises

Marketing claims accompanying the launch of a new high-end imaging system are always to be taken with a grain of salt. In the case of coronary CT angiography with 2nd generation dual-source CT, however, the emerging literature and our own experiences using this system largely support the pre-launch projections. These include technical performance, radiation exposure, as well as novel applications.

The established advantages of 1st generation dual-source CT have been consolidated, expanded, and further enhanced with the latest edition of dual-source CT technology. 2nd generation dual-source CT remains a full-body imaging system, albeit with very special capabilities for the assessment of the heart and vascular system.

Further decreased radiation exposure

In our experience, radiation exposure at cardiac CT and image acquisition times are drastically decreased with 2nd generation dual-source CT. This has been made possible by the doubling of detector elements which enables much higher table speeds. With 1st generation dual-source CT, retrospective ECG-gating and prospective ECG-triggering were the traditional cardiac CT image acquisition methods. The latest iteration of this technology now offers a unique scan mode that allows acquiring the entire cardiac anatomy within a quarter of a second, i.e. a single diastolic phase. Combined with 100kV tube potential, the radiation exposure of such a prospectively ECG-triggered, high-pitch spiral acquisition reliably remains under 1 mSv.

Subject to regular heart rates

Not all patients are eligible for this acquisition mode. Successful application of this method requires slow (i.e. < 65 bpm) and regular heart rates to ensure a sufficiently long duration and predictability of the diastolic phase, during which the scan is performed. This prompted us to re-institute the use of beta-blockade in patients with faster heart rates, a practice which we had abandoned with 1st generation dual-source CT. However, in our opinion the ability of reducing radiation to a bare minimum greatly outweighs the very minor inconvenience associated with pharmaceutical rate control.

Also, the method finds its limitation in severely obese individuals, requiring us to fall back onto the traditional methods of prospective or retrospective ECG synchronization in some patients Regardless of the acquisition protocol, radiation doses are substantially lower across the board, compared with other scanner platforms.

Accordingly, 2nd generation dual-source CT and the availability of other low dose image acquisition methods should effectively eliminate prevailing concerns over radiation exposure from this non-invasive coronary CTA.

Integrative imaging of coronary heart disease – Convincing advantages

In addition, the introduction of 2nd generation dual-source CT has brought about several improvements for integrative imaging of coronary heart disease: i.e. the ability to assess coronary artery anatomy, function, perfusion, and viability with a single, stand-alone modality. Dual-energy CT, based on dual-source CT has shown great promise for this purpose, as it enables analyzing the iodine (and thus blood) distribution within healthy and diseased myocardium.

While first generation dual-source CT was limited to a temporal resolution of 165 msec in dual-energy mode, the full temporal resolution of 75 msec for motion-free imaging of the coronary arteries can now also be realized for dual-energy cardiac examinations. Advanced selective energy filtering techniques have improved the discriminatory power of dual-energy CT image data. This allows better characterization of tissues and of iodine distribution within the myocardium.

Lastly, while dual-energy CT statically images the myocardial blood supply during first pass arterial perfusion, technical advancements now also enable the rapid, time-resolved, quantitative assessment of myocardial perfusion during rest and stress.

Outlook

The efforts to refine CT into an instrument for integrative imaging of coronary heart disease are likely to continue and intensify. Whether these efforts will eventually challenge the role of traditional physiological testing remains to be seen. However, the signs bode well for CT: Patient evaluation with a single, non-invasive modality is likely safer, cheaper, and has less radiation than the routine combination of nuclear myocardial perfusion imaging and invasive coronary catheterization, which is ordinarily needed to obtain comparable complementary information.

References

Achenbach S, Marwan M, Schepis T, Pflederer T, Bruder H, Allmendinger T, Petersilka M, Anders K, Lell M, Kuettner A, Ropers D, Daniel WG, Flohr T. High-pitch spiral acquisition: a new scan mode for coronary CT angiography. J Cardiovasc Comput Tomogr. 2009;3:117-21.

Leschka S, Stolzmann P, Desbiolles L, Baumueller S, Goetti R, Schertler T, Scheffel H, Plass A, Falk V, Feuchtner G, Marincek B, Alkadhi H. Diagnostic accuracy of high-pitch dual-source CT for the assessment of coronary stenoses: first experience. Eur Radiol. 2009. [Epub ahead of print].

Achenbach S, Marwan M, Ropers D, Schepis T, Pflederer T, Anders K, Kuettner A, Daniel WG, Uder M, Lell MM. Coronary computed tomography angiography with a consistent dose below 1 mSv using prospectively electrocardiogram-triggered high-pitch spiral acquisition. Eur Heart J. 2009 Nov 5. [Epub ahead of print]

Lell M, Marwan M, Schepis T, Pflederer T, Anders K, Flohr T, Allmendinger T, Kalender W, Ertel D, Thierfelder C, Kuettner A, Ropers D, Daniel WG, Achenbach S. Prospectively ECG-triggered high-pitch spiral acquisition for coronary CT angiography using dual source CT: technique and initial experience. Eur Radiol. 2009;19:2576-83.

Bastarrika G, Lee YS, Huda W, Ruzsics B, Costello P, Schoepf UJ. CT of coronary artery disease. Radiology. 2009;253:317-38.