Abstract

Case: Dual Energy Lung Perfusion CT in Children: Feasible, Useful and Officially Recognized with the Walter E. Berdon Award

posted by Hyun Woo Goo, M.D. | Nov 22, 2011

The assessment of lung perfusion is now possible with CT examinations using Dual Energy CT (DECT). At Asan Medical Center, Korea, these examinations have been performed since 2008 and many of the research results have been published in various scientific journals. Recently, one of these articles regarding DECT in children, entitled, “Initial experience of dual energy lung perfusion CT using a dual source CT system in children,” was selected for the Walter E. Berdon Award as the most outstanding basic science paper appearing in Pediatric Radiology in 2010[1].

New applications with DECT
Since the introduction of Dual Source CT, novel applications of DECT have been reported in scientific literature. This new and further developing CT technology provides a new horizon of functional and material imaging, such as lung perfusion (iodine) and ventilation (xenon) imaging or uric-acid imaging, beyond conventional anatomic imaging. At Asan Medical Center, pioneering clinical research on qualitative and quantitative assessment of regional lung perfusion and/or ventilation using DECT have been actively performed in both children and adults.[1-7] It is of critical importance that imaging protocols should be optimized for children depending on body size and study indications.
These optimized DECT protocols are available in previous studies.[1-7] Because CT is the primary imaging study of pulmonary thromboembolism, DECT could be easily implemented in clinical practice for this diagnostic task without additional radiation dose. The identification of lung perfusion defects in addition to pulmonary arterial thromboemboli is an unprecedented benefit of Dual Energy lung perfusion CT and may facilitate not only accurate diagnosis but also accurate prognostic assessment of pulmonary thromboembolism.

Study Indications and High-risk Patients
Although pulmonary thromboembolism is relatively rare in children, this potentially fatal, acute disease should be diagnosed accurately and promptly. It should be noted that the risk of pulmonary thromboembolism is high in certain pediatric patient groups, such as congenital or valvular heart disease, coagulation abnormality (e.g., antiphospholipid syndrome), deep-vein thrombosis, and dehydration. At Asan Medical Center, DECT is routinely performed in children who are suspected of having pulmonary thromboembolism. Free-breathing conditions with variable degrees of respiratory difficulty, common in pulmonary thromboembolism, should not be considered as a contraindication of Dual Energy lung perfusion CT. On the other hand, metallic instruments in the thorax may substantially degrade the image quality of DECT lung perfusion images. Therefore, conventional pulmonary CT angiography is generally preferred to DECT in those cases.

Radiation Dose
At Asan Medical Center, in order to minimize radiation exposure from diagnostic imaging in children, pediatric CT radiation dose is tailored depending on body weight[8] or cross-section area and mean density of the body.[9] Similar dose-reducing strategies can be applied to DECT.[1-7] As a result, the mean radiation dose of pediatric Dual Energy lung perfusion CT scans was 2.6 mSv, which is dose-neutral.[1] Recently introduced iterative reconstruction techniques may further reduce radiation dose of DECT without degrading image quality.

Scan Modes
In Dual Energy scan mode of SOMATOM Definition, two tube voltages, usually 80 kV and 140 kV in children, are used with approximately four-times greater tube current for 80 kV. In thick children, 100 kV may be used instead of 80 kV to improve image quality. Narrower beam collimation, i.e., 14 x 1.2 mm, provides better image quality because scattered radiation correction function is available in the beam collimation.
Several upgrades in DECT scanning have been made in the SOMATOM Definition Flash. First, a higher x-ray spectral energy level of 140 kV is achieved by using a tin filter, which results in greater x-ray spectral separation between two kV levels of DECT. Second, scattered radiation correction function becomes available in full beam collimation. Therefore, the full beam collimation, i.e., 64 x 0.6 mm, can be used in children to reduce scan time and motion artifacts.

Contrast Injection
An optimal contrast injection method is essential to obtain peak lung parenchymal enhancement on DECT. A bolus tracking method or a test injection method is usually preferred to an empirical fixed delay method since pulmonary hemodynamics are commonly out of normal ranges in this population with pulmonary thromboembolism. Another important thing is the minimization of perivenous streak artifacts from undiluted contrast agent in the thoracic systemic veins because DECT images are fairly vulnerable to these streak artifacts. Caudo-cranial scan direction and saline chaser may be used to mitigate these artifacts.

Table 1: Examination protocol

Table 1: Examination protocol

1. 7-year-old child with familial dilated cardiomyopathy and right ventricular thrombi. 1. Axial DECT PBV image shows perfusion defect in the left dorsal lung. Small right pleural effusion is noted.
2. Sagittal DECT PBV image also shows perfusion defect in the left dorsal lung. Large thrombi (arrows) are seen in the right ventricle. Pericardial effusion is noted.
3. VRT view of pulmonary vessels shows no evidence of thromboembolism in the major pulmonary arteries but decreased visualization of small peripheral pulmonary arterial branches in both lower lung fields.
4. DE VRT view of the lung shows decreased perfusion in both lower lungs, greater on the left at a glance.

References

  1. Goo HW. Initial experience of dual-energy lung perfusion CT using a dual-source CT system in children. Pediatr Radiol. 2010 Sep;40(9):1536-44.
  2. Goo HW. et al. Xenon ventilation CT using a dual-source dual-energy technique: dynamic ventilation abnormality in a child with bronchial atresia. Pediatr Radiol. 2008 Oct;38(10):1113-6.
  3. Chae EJ. et al. Xenon ventilation CT with a dual-energy technique of dual-source CT: initial experience. Radiology. 2008 Aug;248(2):615-24.
  4. Chae EJ. et al. Xenon ventilation imaging using dual-energy computed tomography in asthmatics: initial experience. Invest Radiol. 2010 Jun;45(6):354-61.
  5. Goo HW. et al. Xenon ventilation CT using dual-source and dual-energy technique in children with bronchiolitis obliterans: correlation of xenon and CT density values with pulmonary function test results. Pediatr Radiol. 2010 Sep;40(9):1490-7.
  6. Goo HW, et al. Collateral ventilation to congenital hyperlucent lung lesions assessed on xenon-enhanced dynamic dual-energy CT: an initial experience. Korean J Radiol. 2011 Jan-Feb;12(1):25-33.
  7. Goo HW, et al. Redistributed regional ventilation after the administration of a bronchodilator demonstrated on xenon-inhaled dual-energy CT in a patient with asthma. Korean J Radiol. 2011 May;12(3):386-9.
  8. Yang DH, et al. Pediatric 16-slice CT protocol: radiation dose and image quality. J Korean Radiol Soc. 2008;59(5):333-47.
  9. Goo HW. Individualized volume CT dose index determined by cross-sectional area and mean density of the body to achieve uniform image noise of contrast-enhanced pediatric chest CT obtained at variable kV levels and with combined tube current modulation. Pediatr Radiol. 2011;41(7):839-47.

This article was first published on Somatom Sessions online.



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