Operational and Dosimetric Aspects of Pediatric PET/CT

Abstract

No consistent guidelines exist for the acquisition of a CT scan as part of pediatric PET/CT. Given that children may be more vulnerable to the effects of ionizing radiation, it is necessary to develop methods that provide diagnostic-quality imaging when needed, in the shortest time and with the lowest patient radiation exposure. This article describes the basics of CT dosimetry and PET/CT acquisition in children. We describe the variability in pediatric PET/CT techniques, based on a survey of 19 PET/CT pediatric institutions in North America. The results of the survey demonstrated that, although most institutions used automatic tube current modulation, there remained a large variation of practice, on the order of a factor of 2-3, across sites, pointing to the need for guidelines. We introduce the approach developed at our institution for using a multiseries PET/CT acquisition technique that combines diagnostic-quality CT in the essential portion of the field of view and a low-dose technique to image the remainder of the body. This approach leads to a reduction in radiation dose to the patient while combining the PET and the diagnostic CT into a single acquisition. The standardization of pediatric PET/CT provides an opportunity for a reduction in the radiation dose to these patients while maintaining an appropriate level of diagnostic image quality.

Keywords: PET/CT; computed tomography; dosimetry; pediatric.

FIGURE 1.

Example of AEC. The underlying image is a CT topogram of a chest CT. The white profile indicates the mAs as a function of z-axis position. The mAs is higher through the shoulders due to the increase in transverse dimension and attenuating structures (bone), then drops significantly over the lungs and rises again over the abdomen when the more attenuating solid organ viscera are encountered.

FIGURE 2.

Visual representation of the 5 imaging scenarios for a 10-y-old child (red = Dx CT; pink = non-Dx CT). Scenario 1 is torso Dx CT in addition to torso non-Dx CT. Scenario 2 is torso Dx CT only. Scenario 3 is torso non-Dx CT only. Scenario 4 is Dx abdominal CT (from just above the diaphragm to just above the pelvis) in addition to torso non-Dx CT. Scenario 5 is multiseries CT with non-Dx in head/neck, chest, and legs while acquiring an abdominal Dx CT. Note that “whole body” in the figure is synonymous with “torso.”

FIGURE 3.

Attenuation correction (non-Dx) and Dx CT ED. ED estimates (in mSv) for Dx and non-Dx CT for various ages from a 1-y-old to a medium adult (MA).

FIGURE 4.

ED estimates (in mSv) from CT for a 10-y-old under the various scenarios illustrated in Figure 2.

FIGURE 5.

(A) Dx head/neck (top) and chest (bottom) CT (DLP = 139 mGy-cm; estimated ED = 2.2 mSv) acquired before the PET/CT scan. (B) Multiseries PET/CT. Left panel shows the composite coronal CT image generated from the multiseries CT acquisition performed as part of the PET/CT acquisition. The head/neck and chest portions were acquired as non-Dx CT, the abdomen as Dx CT, and upper legs as non-Dx. The red arrows indicate transition between the different series, and different noise levels reflecting different CT techniques. The values in the 3 boxes are DLP (mGy-cm) and, in parentheses, the estimated ED (mSv) for each phase of the CT scan. Upper right panel is coronal image from resulting PET using the multiseries CT for attenuation correction. No transition in quality is discernable. Lower right panels show transverse CT (top) and fused PET/CT (bottom) demonstrating a low-attenuation lesion in the posterior right kidney and the corresponding ¹⁸F-FDG uptake associated with the lesion. These latter findings could not have been made using noncontrast non-Dx CT.