In vitro measurements of radiation exposure with different modalities (computed tomography, cone beam computed tomography) for imaging the petrous bone with a pediatric anthropomorphic phantom

Abstract

Background: Various imaging modalities, such as multi-detector computed tomography (CT) and cone beam CT are commonly used in infants for the diagnosis of hearing loss and surgical planning of implantation hearing aid devices, with differing results.

Objective: We compared three different imaging modalities available in our institution, including a high-class CT scanner, a mid-class CT scanner and an angiography system with a cone beam CT option, for image quality and radiation exposure in a phantom study.

Materials and methods: While scanning an anthropomorphic phantom imitating a 1-year-old child with vendor-provided routine protocols, organ doses, surface doses and effective doses were determined for these three modalities with thermoluminescent dosimeters. The image quality was evaluated using the signal difference to noise ratio (SDNR) and the spatial resolution of a line-pair insert in the phantom head. The dose efficiency, defined as the ratio of SDNR and effective dose, was also compared.

Results: The organ and surface doses were lowest with the high-class CT protocol, but the image quality was the worst. Image quality was best with the cone beam CT protocol, which, however, had the highest radiation exposure in this study, whereas the mid-class CT was in between.

Conclusion: Based on our results, high-end CT should be used for surgical planning because it has the lowest dose, while the image quality is still sufficient for this purpose. However, if highest image quality is needed and required, e.g., by ENT surgeons, the other modalities should be considered.

Keywords: Children; Cochlear implantation; Computed tomography; Cone beam computed tomography; Petrous bone; Radiation exposure.

Fig. 1

Schematic representation of the scan angles for the Revolution CT (left) and the DynaCT (right). The numbers around the circles represent the corresponding number of degrees. To reduce the dose to the eye lenses, the tube current is reduced to 70% of its original value in the angular scan range between 300° and 80° for the Revolution CT. For the DynaCT, the tube current is switched off in the frontal region between 300° and 100°

Fig. 2

Phantom sections in a lateral topogram image. Due to the inclination of the head, different sections are exposed in the frontal and occipital regions

Fig. 3

Example thermoluminescent dosimeter positions in section #4. The designation of each hole was specified by the manufacturer of the phantom. Correct assignment of each thermoluminescent dosimeter is important for precise determination of the organ doses

Fig. 4

Placement of the surface thermoluminescent dosimeters (black circles) on the ventral surface (5 of the 7 thermoluminescent dosimeters are seen on the picture). Seven thermoluminescent dosimeters were placed analogously on the dorsal surface of the phantom

Fig. 5

Example image (Revolution CT) with the regions of interest (black circles = bone, white circles = soft tissue) to determine the signal difference to noise ratio

Fig. 6

Surface doses for the frontal head region

Fig. 7

Surface doses for the occipital head region

Fig. 8

Organ doses for the different organs in the scan region for all modalities

Fig. 9

Calculated signal difference to noise ratio (SDNR) and effective doses for each modality

Fig. 10

Spatial resolutions of the line pair inserts for the Revolution CT, Revolution Evo CT and the DynaCT, which were assessed by the radiologists. LP line pair