Temporal Bone CT: Improved Image Quality and Potential for Decreased Radiation Dose Using an Ultra-High-Resolution Scan Mode with an Iterative Reconstruction Algorithm

Abstract

Background and purpose: Radiation dose in temporal bone CT imaging can be high due to the requirement of high spatial resolution. In this study, we assessed whether CT imaging of the temporal bone by using an ultra-high-resolution scan mode combined with iterative reconstruction provides higher spatial resolution and lower image noise than a z-axis ultra-high-resolution mode.

Materials and methods: Patients with baseline temporal bone CT scans acquired by using a z-axis ultra-high-resolution protocol and a follow-up scan by using the ultra-high-resolution-iterative reconstruction technique were identified. Images of left and right temporal bones were reconstructed in the axial, coronal, and Poschl planes. Three neuroradiologists assessed the spatial resolution of the following structures: round and oval windows, incudomallear and incudostapedial joints, basal turn spiral lamina, and scutum. The paired z-axis ultra-high-resolution and ultra-high-resolution-iterative reconstruction images were displayed side by side in random order, with readers blinded to the imaging protocol. Image noise was compared in ROIs over the posterior fossa.

Results: We identified 8 patients, yielding 16 sets of temporal bone images (left and right). Three sets were excluded because the patient underwent surgery between the 2 examinations. Spatial resolution was comparable (Poschl) or slightly better (axial and coronal planes) with ultra-high-resolution-iterative reconstruction than with z-axis ultra-high-resolution. A paired t test indicated that noise was significantly lower with ultra-high-resolution-iterative reconstruction than with z-axis ultra-high-resolution (P < .001), with a mean noise reduction of 37% (range, 18%-49%).

Conclusions: The ultra-high-resolution-iterative reconstruction scan mode has similar or slightly better resolution relative to the z-axis ultra-high-resolution mode for CT of the temporal bone but significantly (P < .01) lower image noise, which may enable the dose to be reduced by approximately 50%.

Fig 1.

Spatial resolution scores for images in the axial, coronal, and Poschl planes, averaged across individual structures in each imaging plane. The scale assessed UHR-IR images relative to zUHR images: 1 = inferior resolution with degraded visualization, 2 = slightly inferior resolution without affecting visualization; 3 = equivalent, 4 = slightly superior resolution without affecting visualization, 5 = superior resolution with improved visualization. The means are shown as lined bars, with the value above each bar. Statistical significance was determined with the Wilcoxon signed rank test.

Fig 2.

Comparison of spatial resolution of the round window. Representative axial CT images of the round window of the same patient scanned with the zUHR technique (A) and UHR-IR technique (B). The UHR-IR technique produced superior spatial resolution and lower image noise.

Fig 3.

Comparison of the spatial resolution of the incudomallear joint. Representative axial images of the incudomallear joint of the same patient scanned with the zUHR technique (A) and UHR-IR technique (B). The UHR-IR technique produced superior spatial resolution and lower image noise.

Fig 4.

Lower image noise in images acquired with UHR-IR. Image noise was measured at the posterior fossa in axial images from each of the 13 datasets by using the zUHR and UHR-IR techniques.

Fig 5.

Spatial resolution and image noise in images from a representative patient. Axial (A and B) and coronal (C and D) CT images of the same patient scanned with the zUHR technique (A and C) and UHR-IR technique (B and D). Substantial noise reduction was achieved by using the UHR-IR technique.