Volume 12 Issue 9 - September 2014
                       
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Dual-energy CT in the Chest
     
  Dual-energy CT (DECT) has applications in thoracic imaging. It can provide standard anatomic CT images in addition to physiological information from iodine enhancement maps and virtual non-contrast images

  Iodine enhancement maps can show characteristic pulmonary blood volume deficits due to pulmonary embolism and other lung pathologies such as emphysema

  Because iodine enhancement is greater at lower keV, a lower dose of iodine contrast material can be used when imaging with DECT, if medically indicated

  Beam hardening and streak artifacts from high density materials can be decreased  with DECT

  The radiation dose from contrast enhanced DECT is equal to or lower than standard single-energy CT


Pulmonary Embolism
Pulmonary Nodules
Routine Chest Imaging
Arteriovenous Malformations
Limitations of DECT
Scheduling
Further Information
References

Dual-energy CT (DECT) is accomplished by near simultaneous imaging at two energy levels, typically 80 kV and 140 kV, using scanners that either employ rapid kV switching or have two orthogonal sets of X-ray tunes and detectors mounted in the gantry. Attenuation properties differ at the two energy levels, which allow the DECT to differentiate and quantify certain substances such as water, calcium, and iodine (from CT contrast medium).  The low energy settings (lower monoenergetic level) of the DECT can provide excellent contrast enhancement while the higher energy setting can help obtain high signal-to-noise ratios if the penetration of X-rays is not sufficient at low energy levels.

Figure 1
Figure 1. Comparison of radiation doses between DECT and single energy CT (SECT) for both routine chest and pulmonary embolism (PE) protocols in different weight groups ("d60 Kg, 61-90 Kg, > 91 Kg).


Complex material differentiation algorithms have been developed to analyze DECT data and create multiple images. For example, iodine enhancement maps highlight the distribution of elements such as iodine from contrast material. Alternatively, the data can be post-processed to remove the contributions of iodine to create virtual non-contrast (VNC) images. Similarly, removal of calcium enables clear visualization of vasculature that would otherwise be obscured by bone or calcified plaque. Consequently, a single dual-energy CT scan can be employed instead of two or more consecutive standard CT scans while maintaining image quality and reducing radiation exposure by 30–40% (Figure 1).

There are several chest imaging protocols that may benefit DECT. For example, iodine enhancement maps provide physiological information relating to perfusion of iodine in the lung parenchyma, which may be helpful in diagnosing perfusion deficits due to pulmonary emboli or other conditions such as emphysema. Initial studies suggest that quantifying the degree of iodine enhancement may be helpful in differentiating between benign and malignant lung nodules. DECT may also be beneficial for patients who are unable to raise their arms above heads because post-processing of DECT images can limit artifacts due to beam hardening caused by the arms. Similarly, streak artifacts due to metal prostheses in, for example, the shoulder or the spine, can be reduced with DECT. Moreover, because iodine enhancement is greater at lower keV, a lower dose of iodine contrast material can be used when imaging with DECT (Figure 2), if medically indicated.

At the Mass General Hospital, DECT of the chest can be performed for both routine chest and pulmonary embolism protocols.

Click here to enlarge
Figure 2. Patient with a single kidney and shortness of breath had scattered emphysema and atelectasis on chest radiograph. Patient underwent a DECT pulmonary embolism protocol CT with only 25 cc of contrast material (generally 80-100 cc of contrast material is used). Standard CT images at 60 Kev (a and b) demonstrated excellent contrast enhancement in pulmonary arteries (main, lobar, segmental, and subsegmental). Pulmonary blood volume image (C) shows scattered defects (arrows) consistent with areas of emphysema (these defects would have reduced the sensitivity and specificity of nuclear V:Q scan)(Click on image to enlarge)


Pulmonary Embolism
CT angiography of the pulmonary arteries (CTPA) is a rapid and sensitive method for detecting emboli in the pulmonary arteries. It has replaced conventional scintigraphic methods for ventilation/perfusion imaging in a vast majority of patients. An optimal single-energy CTPA requires close attention to the timing of image acquisition after contrast administration. Inappropriate timing of image acquisition can result in a non-diagnostic exam, which is much less common with DECT than with single energy CTPA. In addition, contrast enhancement in smaller branches of the pulmonary arteries is often better on DECT than on single-energy CTPA.

Post processing of DECT pulmonary angiography can generate virtual non-contrast images, standard CPTA images, and an iodine distribution map that can show if there are any perfusion deficits. Deficits due to pulmonary embolism are typically wedge-shaped. Therefore, DECT has the potential to increase the diagnostic accuracy for pulmonary emboli. In addition, the size of the perfusion deficit is prognostic as larger perfusion deficits are associated with poorer patient outcomes.

Pulmonary Nodules
Initial research studies have reported that a single phase DECT acquired 100 seconds after administration of contrast material may help to distinguish benign and malignant nodules. The selected region of interest in DECT is identical in each of the of the post-processed images but may not be so for single energy dynamic CT. Iodine enhancement at 80 kV is approximately twice that at 140 kV, which is typically used for single energy CT. Iodine distribution maps give a quantitative assessment of iodine uptake. For all these reasons, DECT has the potential to outperform single-energy CT for diagnostic accuracy for malignancy in lung nodules while reducing radiation dose.

Routine Chest Imaging
Chest CT examinations are performed for a number of reasons, including cancer staging, assessing response to therapy and surveillance, characterization of radiographic abnormalities, and for symptomatic patients with normal radiographic images. In addition to providing standard CT images obtained from routine chest single-energy CT, a contrast enhanced routine DECT enables simultaneous assessment of the pulmonary arteries and provides almost the same information as that obtained through a pulmonary embolism protocol. Iodine enhancement maps provide physiological information and make it easier to recognize pathologies such as pulmonary infarct (Figure 3), pneumonia and atelectasis. In addition, characteristic patterns of perfusion deficits have been associated with other lung pathologies, such as emphysema and air trapping.

Click here to enlarge
Figure 3. Standard CT images (A and B) at 60 kev demonstrate occlusive filling defect (arrows) within the right interlobar pulmonary artery and consolidation in the right lower lobe. Pulmonary blood volume image (C) demonstrates an area of reduced iodine uptake (arrows) which is larger than the size of consolidation. This opacity is consistent with pulmonary infarction. (Click on image to enlarge)


DECT has similar advantages for cancer staging, response to therapy, and disease monitoring. Vascularity, and therefore iodine uptake, can diminish in tumors that are responding to therapy before any tumor shrinkage occurs. In addition, DECT can differentiate between iodine enhancement and calcification, which may be observed in lymph nodes in non-contrast images and may be a source of confusion in single-energy CT.

Arteriovenous Malformations
DECT angiography can also be helpful to image pulmonary arteriovenous malformations (AVMs) with a single phase contrast-enhanced DECT examination. Because of greater iodine enhancement with DECT, it is possible to identify the nidus of the AVMs from surrounding hemorrhage and infection. Additionally, pulmonary blood volume images can provide an estimate of any associated perfusion defects from the untreated and coiled AVMs.

Limitations of DECT
In CT scanners with dual detectors, the field of view of the smaller detector is limited (34 cm) and in some patients may not be large enough to image the entire thorax. In most cases, it is possible to perform a scout scan and then position the patient to include the entire lungs. The limitation of smaller field of view does not apply to CT scanners at Mass General Hospital which operate with rapid KV switching technique.

Scheduling
Three CT scanners on the main campus of Mass General Hospital are capable of DECT (out of a total of 18) and a fourth scanner will be installed in the Emergency Department in the near future. Referring physicians may request DECT by specifying it in the instructions associated with the scan, or radiologists can choose it depending on the patient’s circumstances. Appointments can be made through ROE (inside Partners network) or ROE Portal (outside Partners network) or by calling 617-724-XRAY (9729).

Further Information
For further information on DECT of the chest, please contact Mannudeep Kalra, MD or Subba R. Digumarthy, MD, Thoracic Imaging Division, Department of Radiology, Massachusetts General Hospital, at 617-724-2275.

We would like to thank Alexi Otrakji, MD, Mannudeep Kalra, MD, and Subba R. Digumarthy, MD. Thoracic imaging, and Venkatesh Arumugan Murugan, MD, Department of Radiology, Massachusetts General Hospital, for their assistance and advice on this issue.



References

Bauer RW, et al. (2011). Dual energy CT pulmonary blood volume assessment in acute pulmonary embolism - correlation with D-dimer level, right heart strain and clinical outcome. Eur Radiol 21:1914-21

Chae EJ, et al. (2008). Clinical utility of dual-energy CT in the evaluation of solitary pulmonary nodules: initial experience. Radiology 249:671-81

Geyer LL, et al. (2012). Imaging of acute pulmonary embolism using a dual energy CT system with rapid kVp switching: initial results. Eur J Radiol 81:3711-8

Kim BH, et al. (2012). Analysis of perfusion defects by causes other than acute pulmonary thromboembolism on contrast-enhanced dual-energy CT in consecutive 537 patients. Eur J Radiol 81:e647-52

Kim EY, et al. (2014). Assessment of perfusion pattern and extent of perfusion defect on dual-energy CT angiography: correlations between the causes of pulmonary hypertension and vascular parameters. Korean J Radiol 15:286-94



©2014 MGH Department of Radiology

Janet Cochrane Miller, D. Phil., Author
Raul N. Uppot, Editor


 

 

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