Dual Energy CT Lung Perfusion Imaging in Children

We use Dual Energy CT (DECT) for pulmonary CT angiography in children which enables the evaluation of the pulmonary arteries and lung perfusion from a single contrast-enhanced CT examination. Based upon our experience, in all types of pediatric contrast-enhanced DECT protocols (chest, abdomen, and pelvis) radiation exposure is similar to, or at times even lower than, those of conventional single-energy CT.[1]

For this article, our general DECT angiography protocol is discussed followed by a specific case report focusing on the use of DECT-based lung perfusion image analysis.

Our DECT examinations are performed on a SOMATOM Definition Flash, operating at 80/Sn140 kV, where Sn indicates tin prefiltration of the high-energy tube. Real-time anatomic exposure control (CARE Dose4D™) and sinogram affirmed iterative reconstruction (SAFIRE) techniques are used to facilitate dose reduction.

Fixed acquisition parameters include:
rotation time of 0.33 seconds, detector collimation of 128 × 0.6 mm, and pitch of 1.2. The use of a higher pitch is of critical importance in pediatric patients to decrease motion artifacts, differing from DECT in adults where a lower pitch is commonly used. In pediatric CT, unlike adult CT, tube power issues (e.g., mAs limits because of faster scan speeds) are not as relevant due to the smaller patient habitus. Similarly, with the exception of very large patients (e.g., some adolescents), pediatric patients will always completely fit within the scan field of view (e.g., 33 cm) of the second tube of the Dual Source CT scanner.

A non-ionic contrast medium (320 mg/mL) is administered with a volume of 2 mL/kg, not to exceed a total of 100 mL. A power injector is used if the catheter is in an antecubital position. The flow rate selected varies according to the catheter size. Contrast medium is injected by hand if the catheter is not located in the antecubital region.

In daily routine, we use bolus tracking for timing the injection, with the region of interest (ROI) placed in the pulmonary trunk. The scan is automatically triggered once the contrast level reaches 100 HU. A default is set at 15 seconds in case the scan does not trigger. In congenital anomalies with absent or hypoplastic pulmonary arteries, the ROI is placed over the proximal descending aorta since the pulmonary arteries will fill with the systemic arterial bolus. A test bolus and saline flush are not routinely used in pediatric CTA.

To illustrate image generation and review in DECT-based perfusion imaging of the lungs, the following case is presented: A 12-year-old boy with a history of right dominant unbalanced atrial-ventricular (AV) canal, transposition of the great vessels, absent main pulmonary artery, and Fontan palliation, had worsening congestive heart failure and was a candidate for heart transplant. A pre-operative CTA of the chest was performed to assess the pulmonary vascular anatomy and lung perfusion prior to surgery.

Following parameters were used:
80 kV, 140 kV, linear blended images at a ratio of 0.7, monoenergetic plus reconstructions at 50 keV, as well as coronal and sagittal multiplanar reformat at the console. Only the axial blended images, axial monoenergetic images, and multiplanar reformats are sent to PACS for the radiologist to interpret. The new software VA48 allows reconstruction of PACS-ready monoenergetic plus images directly at the CT console. Monoenergetic plus reconstructions at 50 keV, generated at the console or with syngo.via, are preferred over the 80 kV images since they provide better contrast.

Additionally, maximum intensity projection (MIP) images are done to optimize visualization of the pulmonary arteries in patients with congenital abnormalities of the pulmonary arteries. To further improve the iodine contrast, we use the 50 keV monoenergetic plus images to generate the MIP images. In this patient, the coronal 50 keV MIP image showed the Fontan connection (arrow) joining the superior vena cava (S) and inferior vena cava (I) at the confluence of the pulmonary arteries (Fig. 1). The main pulmonary artery was absent. Flow appeared normal in the peripheral blood arteries. Axial 50 keV MIP image showed a single atrioventricular valve (arrow), a large atrial septal defect (asterisk) and a large ventricular septal defect (double asterisk) (Fig. 2). Peripheral pulmonary blood flow again appeared normal.

Furthermore, we used syngo DE Lung PBV to create pulmonary bloodvolume maps to assess the flow to the lungs. In this patient, the coronal (Fig. 3A) and axial (Fig. 3B) PBV images suggested areas of diminished perfusion to the right-upper lobe and right-middle lobes, respectively (arrows). However, overall the lungs appeared well perfused and no acute intervention was needed. By enabling a comprehensive evaluation of vessels and parenchyma, DECT has the potential to obviate additional imaging that involves radiation, such as catheter angiography.

Prof. Marilyn Siegel, MD Mallinckrodt Institute of Radiology, Washington University School of Medicine, St. Louis, Missouri, USA