Dual-Energy CT: New Horizon in Medical Imaging

Abstract

Dual-energy CT has remained underutilized over the past decade probably due to a cumbersome workflow issue and current technical limitations. Clinical radiologists should be made aware of the potential clinical benefits of dual-energy CT over single-energy CT. To accomplish this aim, the basic principle, current acquisition methods with advantages and disadvantages, and various material-specific imaging methods as clinical applications of dual-energy CT should be addressed in detail. Current dual-energy CT acquisition methods include dual tubes with or without beam filtration, rapid voltage switching, dual-layer detector, split filter technique, and sequential scanning. Dual-energy material-specific imaging methods include virtual monoenergetic or monochromatic imaging, effective atomic number map, virtual non-contrast or unenhanced imaging, virtual non-calcium imaging, iodine map, inhaled xenon map, uric acid imaging, automatic bone removal, and lung vessels analysis. In this review, we focus on dual-energy CT imaging including related issues of radiation exposure to patients, scanning and post-processing options, and potential clinical benefits mainly to improve the understanding of clinical radiologists and thus, expand the clinical use of dual-energy CT; in addition, we briefly describe the current technical limitations of dual-energy CT and the current developments of photon-counting detector.

Keywords: CT imaging techniques; Dual-energy CT; Effective atomic number; Material decomposition; Photon-counting detector; Spectral CT; Virtual monoenergetic imaging.

Fig. 1. Illustration of five different methods of dual-energy CT data acquisition.

1 = dual tubes with or without beam filtration, 2 = rapid voltage switching with single tube, 3 = dual-layer detector with single tube, 4 = single tube with split filter, 5 = single tube with sequential dual scans

Fig. 2. Contrast-enhanced axial abdominal CT images using rapid voltage switching with single tube.

A. Image generated immediately after dual-energy scanning by using 140 kVp projections only shows high image noise. B. Virtual monoenergetic image at 70 keV showing improved image quality needs to be additionally reconstructed for diagnostic imaging. C. Iodine map demonstrates improved iodine contrast-to-noise ratio. Of note, patient skin, cloth, and CT table appear artifactually bright on iodine map.

Fig. 3. Contrast-enhanced chest volume-rendered CT images with cropped posterior chest wall to unveil cardiovascular structures.

A, B. Compared with volume-rendered image reconstructed from linearly mixed dual-energy images with ratio of 0.8 (A), volume-rendered 40 keV virtual monoenergetic image (B) shows further increase in cardiovascular opacification, but simultaneously increased noise compromises iodine contrast-to-noise ratio and image quality. C. On volume-rendered noise-optimized 40 keV virtual monoenergetic image, image noise reduction decoupled with increased iodine contrast leads to improved iodine contrast-to-noise ratio and image quality.

Fig. 4. Coronal chest noise-optimized virtual monoenergetic dual-energy CT imaging.

Beam-hardening and/or photon starvation artifacts in thoracic inlet and shoulder pronounced in 40 keV image (A) are reduced in 60 keV images (B). Because iodine contrast is progressively reduced at higher keV images, overall optimal image quality can be achieved around 60 keV depending on patients' size as well as body region.

Fig. 5. Contrast-enhanced axial chest virtual monoenergetic dual-energy CT imaging.

A. Three round regions of interest are placed in left atrium, back muscle, and subcutaneous fat in anterior chest wall, respectively, on axial chest CT image. B. Graph illustrating changes in CT value in three regions of interest as function of energy. Iodine in blood (white line) shows higher CT values at lower keV, while fat (orange line) reveals lower CT values at lower keV. In contrast, muscle (yellow line) demonstrates almost constant CT values in range of 40–190 keV.

Fig. 6. Contrast-enhanced axial chest dual-energy CT imaging with posterior spinal fixation for scoliosis.

A. Linearly mixed image with ratio of 0.4 shows beam-hardening artifacts caused by pedicle screws. B. Beam-hardening artifacts become less prominent on 130 keV image at expense of reduced iodine enhancement in vessels.

Fig. 7. Contrast-enhanced sagittal chest dual-energy CT imaging acquired with dual-layer detector technique.

A. 70 keV image reveals subsegmental embolus (arrow) in anterior basal segment of left lower lobe. B, C. Wedge-shaped perfusion defect (arrows) is seen on iodine map (B) and more conspicuously on effective atomic number map (C).

Fig. 8. Coronal abdominopelvic dual-energy CT imaging in patient with Hodgkin lymphoma.

A, B. Linearly mixed image, iodine map. Right renal artery (long arrows), left renal vein (short arrows), inferior vena cava (asterisks) are displaced or encased by extensive, necrotic retroperitoneal lymphadenopathy. Lymphadenopathy shows subtle peripheral enhancement on iodine map (B). Multiple hypodense small nodules are noted in spleen.

Fig. 9. Axial brain dual-energy CT imaging in patient with recurrent primitive neuroectodermal tumor.

A-C. Linearly mixed image, iodine map, virtual non-contrast image. Larger anterior hyperdense lesion, pure intracerebral hemorrhage, shows no enhancement on iodine map (B) and hyperdensity suggesting recent hemorrhage on virtual non-contrast image (C). In contrast, smaller heterogenous lesion (arrows) reveals enhancing areas suggesting viable tumor on iodine map (B).

Fig. 10. Axial chest xenon-inhaled dual-energy CT imaging in patient with post-infectious bronchiolitis obliterans.

A. Linearly mixed image shows bronchial wall thickenings and mosaic lung hyperlucency in right middle and lower lobes. Collapse of anterior basal segment of right lower lobe is also noted. B. Xenon map demonstrates severely reduced xenon enhancement in right lower lobe and mildly, heterogeneously decreased xenon enhancement in right middle lobe.

Fig. 11. Dual-energy CT imaging of right foot in patient with gout.

Color-coded map (A) and volume-rendered image (B) show periarticular green foci (arrows) suggesting monosodium urate deposits and associated soft tissue swelling. False-positive artifacts are noted in typical location around nail bed and skin of great toe on volume-rendered image (B).

Fig. 12. Head CT angiographic volume-rendered imaging.

A. Three-dimensional dual-energy angiographic image after automatic dual-energy bone removal shows residual bone at skull base due to incomplete dual-energy iodine-bone separation. B. Three-dimensional dual-energy angiographic image after detailed manual bone removal improves quality of head angiography but is time-consuming.

Fig. 13. Dual-energy chest CT imaging demonstrating lung vessels analysis.

A. Axial image with lung vessels analysis shows normal enhancing pulmonary vessels in light blue in both lungs and limited dual-energy field of view (arrows) typically seen in dual-energy technique using dual X-ray tubes. B. In patient with dextrocaria, pulmonary atresia, ventricular septal defect, right aortic arch, and Eisenmenger syndrome, unobstructed pulmonary vessels in both lungs are red, secondary to very slow pulmonary circulation.

Fig. 14. Dual-energy pulmonary blood volume map demonstrating cardiac motion and beam-hardening artifacts.

A. On axial pulmonary blood volume map, cardiac motion artifacts appear as red color areas around heart as well as blue areas in right middle lobe. B. On sagittal pulmonary blood volume map, beam-hardening artifacts appear as pattern of oblique stripes parallel to ribs.